clarke, chris-science of ice cream-royal society of chemistry (2012)

The Science of Ice Cream2nd Edition

Chris ClarkeUnilever, Colworth Science Park, Bedford, UKE-mail: [email protected]

ISBN: 978-1-84973-127-0

A catalogue record for this book is available from the British Library

r Chris Clarke 2012

All rights reserved

Apart from fair dealing for the purposes of research for non-commercial purposes orfor private study, criticism or review, as permitted under the Copyright, Designs andPatents Act 1988 and the Copyright and Related Rights Regulations 2003, thispublication may not be reproduced, stored or transmitted, in any form or by anymeans, without the prior permission in writing of The Royal Society of Chemistry orthe copyright owner, or in the case of reproduction in accordance with the terms oflicences issued by the Copyright Licensing Agency in the UK, or in accordance withthe terms of the licences issued by the appropriate Reproduction Rights Organizationoutside the UK. Enquiries concerning reproduction outside the terms stated hereshould be sent to The Royal Society of Chemistry at the address printed on this page.

The right of Chris Clarke to be identied as the author of this work has beenasserted by him in accordance with the Copyright, Designs and Patents Act 1988.

The RSC is not responsible for individual opinions expressed in this work.

Published by The Royal Society of Chemistry,Thomas Graham House, Science Park, Milton Road,Cambridge CB4 0WF, UK

Registered Charity Number 207890

For further information see our web site at www.rsc.org

Printed and bound in Great Britain by CPI Group (UK) Ltd, Croydon,CR0 4YY, UK

Preface to the Second Edition

The second edition of The Science of Ice Cream incorporates themain developments in the ingredients and techniques used in icecream that have taken place since the rst edition was publishednearly eight years ago. Chapter 7 has a new section which dis-cusses the topic which has probably received the most attentionin recent years: the challenge of reducing the fat, sugar andcalorie contents of ice cream whilst maintaining its taste andtexture. I have also added some extra material as a result ofsuggestions made by reviewers of the rst edition. I wouldparticularly like to thank Tom Coultate, Loyd Wix and JeUnderdown for their help with the new material, and my family now enlarged by the arrival of Oliver for their support.

The Science of Ice Cream, 2nd Edition

Chris Clarker Chris Clarke 2012

Published by the Royal Society of Chemistry, www.rsc.org

v

Contents

Glossary xv

Symbols xix

Chapter 1The Story of Ice Cream 1

1.1 What is Ice Cream? 1

1.2 The History of Ice Cream 4

1.3 The Global Ice Cream Market 11

1.4 Selling Ice Cream: Fun, Indulgence and Refreshment 13

References 13

Chapter 2Colloidal Dispersions, Freezing and Rheology 15

2.1 Introduction 15

2.2 Colloidal Dispersions 152.2.1 Emulsions 162.2.2 Sols 192.2.3 Foams 202.2.4 Coarsening of Colloidal Dispersions 21


Chris Clarke

r Chris Clarke 2012


ix

2.3 Freezing 232.3.1 Supercooling and Nucleation 232.3.2 Growth 272.3.3 Freezing Point Depression 272.3.4 The SaltWater Phase Diagram 292.3.5 The SucroseWater Phase Diagram 312.3.6 Newtons Law of Cooling 33

2.4 The Rheology of Solutions and Suspensions 342.4.1 The Rheology of Solutions of Small Molecules 352.4.2 The Rheology of Polymer Solutions 362.4.3 The Rheology of Suspensions 40

References 40

Chapter 3Ice Cream Ingredients 41

3.1 Introduction 41

3.2 Milk Proteins 43

3.3 Sugars 453.3.1 Glucose 463.3.2 Fructose 473.3.3 Sucrose 483.3.4 Lactose 483.3.5 Corn Syrup 493.3.6 Sugar Alcohols 49

3.4 Oils and Fats 503.4.1 The Chemistry of Oils and Fats 503.4.2 Milk Fat 523.4.3 Vegetable Fats 53

3.5 Water 54

3.6 Emulsiers 543.6.1 Mono-/diglycerides 543.6.2 Egg Yolk 56

3.7 Stabilizers 563.7.1 Sodium Alginate 573.7.2 Carrageenan 583.7.3 Locust Bean Gum 583.7.4 Guar Gum 593.7.5 Pectin 603.7.6 Xanthan 60

x Contents

3.7.7 Sodium Carboxymethyl Cellulose 603.7.8 Gelatin 61

3.8 Flavours 613.8.1 Vanilla 623.8.2 Chocolate 633.8.3 Strawberry 64

3.9 Colours 64

3.10 Other Components 653.10.1 Chocolate 653.10.2 Fruit 663.10.3 Nuts 673.10.4 Bakery Products 68

Reference 68

Chapter 4Making Ice Cream in the Factory 69

4.1 Introduction 69

4.2 Mix Preparation 704.2.1 Dosing and Mixing of Ingredients 704.2.2 Pasteurization and Homogenization 71

4.3 Ageing 75

4.4 Freezing 77

4.5 Hardening 874.5.1 Low-temperature Extrusion 92

4.6 Water Ices 93

References 93

Chapter 5Product Assembly 94

5.1 Introduction 94

5.2 The Physical Properties of Chocolate and Couverture 96

5.3 Cones and Sandwiches 98

5.4 Stick Products, Bars and Tubes 1015.4.1 Moulding 1015.4.2 Extrude and Cut 104

xiContents

5.4.3 Dipping 1065.4.4 Enrobing 1085.4.5 Dry Coating 109

5.5 Desserts 109

5.6 Packaging 111

5.7 Cold Storage and Distribution 113

Reference 114

Chapter 6Measuring Ice Cream 115

6.1 Introduction 115

6.2 Visualization and Characterization of

Microstructure 1166.2.1 Scanning Electron Microscopy 1166.2.2 Transmission Electron Microscopy 1186.2.3 Optical Microscopy 1216.2.4 Confocal Laser Scanning Microscopy 1236.2.5 Measurement of the Fat Structure 1246.2.6 Image Analysis and Microstructural

Characterization 1256.2.7 Computer Simulations of Microstructure 126

6.3 Mechanical and Rheological Properties 1286.3.1 Mechanical Properties 1296.3.2 The Three-point Bend Test 1306.3.3 The Compression Test 1316.3.4 The Hardness Test 1326.3.5 Rheological Properties 133

6.4 Thermal Properties 1356.4.1 Calorimetry 1356.4.2 Thermal Conductivity 1366.4.3 Thermal Mechanical Analysis 1376.4.4 Meltdown 137

6.5 Microbiological Properties 138

6.6 Sensory Properties 1396.6.1 Analytical Methods 1416.6.2 Consumer Methods 145

References 145

xii Contents

Chapter 7Ice Cream: A Complex Composite Material 146


7.2 The Four Components 1487.2.1 Ice 1487.2.2 Matrix 1557.2.3 Fat 1597.2.4 Air 162

7.3 Ice Cream as a Composite Material 166

7.4 Ice Cream as a Complex Fluid 171

7.5 Microstructure Breakdown during Consumption 173

7.6 Linking Microstructure to Texture 174

7.7 Improving the Nutritional Prole whilst Maintaining the

Sensory Properties 1767.7.1 Reducing the Total Fat Content 1767.7.2 Reducing the Saturated Fat Content 1787.7.3 Reducing the Sugar Content 180

7.8 Summary 182

References 182

Chapter 8Experiments with Ice Cream and Ice Cream Products 184


8.2 Experiments 184Experiment 1: Mechanical Refrigeration 184Experiment 2: Stabilizing Emulsions and Foams 185Experiment 3: Whipping Cream 186Experiment 4: Freezing Point Depression 186Experiment 5: Supercooling and Nucleation 187Experiment 6: Supersaturation, Nucleation and Latent Heat 188Experiment 7: Viscosity 189Experiment 8: Making Ice Cream Mix 191Experiment 9: Freezing with Ice and Salt 193Experiment 10: Freezing Ice Cream in your Freezer 193Experiment 11: Domestic Ice Cream Makers 194Experiment 12: Making Ice Cream with Liquid Nitrogen 194Experiment 13: Slush- and Quiescently Frozen Water Ices 195

xiiiContents

Experiment 14: Measuring Overrun 196Experiment 15: Measuring Hardness 198Experiment 16: Sensory Evaluation 199Experiment 17: The Eect of Temperature on FlavourIntensity 199

References 200

Subject Index 201

xiv Contents

CHAPTER 1

The Story of Ice Cream

1.1 WHAT IS ICE CREAM?

Ice cream is an enormously popular food. The term ice cream inits broadest sense covers a wide range of dierent types of frozendessert. The main ones are:

Dairy ice cream a frozen, aerated mixture of dairy ingre-dients, sugars and avours.

Non-dairy ice cream made with milk protein or otherproteins and vegetable fat.

Gelato an Italian-style custard-based ice cream which con-tains egg yolks.

Frozen yoghurt which may contain lactic acid organisms, orsimply yoghurt avour.

Milk ice similar to ice cream, but unaerated and containingless dairy fat.

Sorbet fruit-based, aerated, sugar syrup which containsneither fat nor milk.

Sherbet similar to a sorbet, but containing some milk orcream.

Water ice frozen sugar syrup with avour and colour, suchas an ice lolly.

Fruit ice similar to water ice, but made with real fruit juice.




1

What these all have in common is that they are sweetened,avoured, contain ice and, unlike any other frozen food, arenormally eaten in the frozen state.The legal denition of ice cream varies from country to country.

In the UK ice cream is dened as a frozen food product con-taining a minimum of 5% fat and 2.5% milk protein, which isobtained by heat-treating and subsequently freezing an emulsionof fat, milk solids and sugar (or sweetener), with or without othersubstances. Dairy ice cream must in addition contain no fatother than milk fat, with the exception of fat that is present inanother ingredient, for example egg, avouring, or emulsier.1

In the USA, ice cream must contain at least 10% milk fat and20% total milk solids, and must weigh a minimum of 0.54 kg perlitre.2

Ice cream is often categorized as premium, standard or economy.Premium ice cream is generally made from the best quality ingre-dients and has a relatively high amount of dairy fat and a lowamount of air (hence it is relatively expensive), whereas economyice cream is made from cheaper ingredients (e.g. vegetable fat) andcontains more air. However, these terms have no legal standingwithin the UK market, and one manufacturers economy ice creammay be similar to a standard ice cream from another.Most people are very familiar with the appearance, taste and

texture of ice cream and there are many recipes for making it incookery books. However, few people know why certain ingredientsand a time-consuming preparation process are required. Theanswer is that ice cream is an extremely complex, intricate anddelicate substance. In fact, it has been called just about the mostcomplex food colloid of all.3 The science of ice cream consists ofunderstanding its ingredients, processing, microstructure and tex-ture, and, crucially, the links between them. This requires a wholerange of scientic disciplines, including physical chemistry, foodscience, colloid science, chemical engineering, microscopy, mate-rials science and consumer science, shown in Figure 1.1.The ingredients and processing create the microstructure which

is shown schematically in Figure 1.2. It consists of ice crystals, airbubbles and fat droplets in the size range from 1 mm to 0.1 mm anda viscous solution of sugars, polysaccharides and milk proteins,known as the matrix. The texture we perceive when we eat icecream is the sensory manifestation of the microstructure. Thus

2 Chapter 1

microstructure is at the heart of the science of ice cream, and formsthe central theme running through this book.In order to describe the science of ice cream, it is rst necessary

to describe some of the physical chemistry and colloid science that

Ingredients

Processing

Microstructure Texture

DairyScience

FoodScience

ChemicalEngineering

PhysicalChemistry

Microscopy

ColloidScience

MaterialsScience

ConsumerScience

Figure 1.1 The sciences of ice cream.

Matrix

Air bubbles

Ice crystals

Fat droplets:on the air bubble surface and in the matrix

0.1 mm

Figure 1.2 Schematic diagram of the microstructure of ice cream.

3The Story of Ice Cream

underpins it; these are laid out in Chapter 2. Chapters 3 and 4 coverthe ingredients and the ice cream making process respectively.Chapter 5 focuses on the production of various types of ice creamproduct. The physical and sensory measurements used to quantifyand describe it are discussed in Chapter 6, and the microstructure,as well as its relationship to the texture, is examined in Chapter 7.Finally, Chapter 8 describes a number of experiments that illustratethe science of ice cream which may be performed in the laboratory,classroom or kitchen. We begin, however, by looking at whereand when ice cream was invented, and how it has evolved into thehuge range of products eaten by billions of people all aroundthe world today.

1.2 THE HISTORY OF ICE CREAM

Ice cream as we recognize it today has been in existence for at least300 years, although its origins probably go much further back intime. The history of ice cream is full of myths and stories, whichhave little real evidence to support them. A typical history beginswith the Roman Emperor Nero (AD 3768) who is said to haveeaten fruit chilled with snow brought down from the mountains byslaves. Elsewhere, Mongolian horsemen are reputed to haveinvented ice cream. They took cream in containers made fromanimal intestines as provisions on long journeys across the Gobidesert in winter. As they galloped, the cream was vigorously sha-ken, while the sub-zero temperature caused it to freeze simulta-neously. The expansion of the Mongol empire spread this ideathrough China, from where Marco Polo reputedly brought the ideato Italy when he returned from his travels in 1295. It has beenclaimed that ice cream was introduced to France from Italy whenthe 14-year-old Catherine de Medici was married to the DucdOrleans (later Henri II of France) in 1533. Her entourageincluded Italian chefs who brought the recipe for ice cream withthem. The secret of making ice cream remained known to only afew. So precious was it that Charles I of England is said to haveoered his French chef a pension of d500 per year to keep his recipesecret.However, historical research has found little factual evidence to

support any of these stories. The only mention of ice in connectionwith Nero comes from Pliny the Elder in the rst century AD, who

4 Chapter 1

records the discovery that water which has been boiled freezesfaster and is healthier. There is no mention of ice cream in any ofthe manuscripts describing Marco Polos travels. Indeed, modernhistorians doubt that he even reached China. It is unlikely thatCatherines chefs knew how to make ice cream since, at that time,the method of refrigeration by mixing ice and salt was known inEurope only to a handful of scientists. Nor is there any doc-umentary evidence for Charles chef.We cannot be absolutely sure of exactly who invented ice cream,

or where and when. In reality, the history of ice cream is closelyassociated with the development of refrigeration techniques andcan be traced in several stages coupled with these:

1. Cooling food and drink by mixing it with snow or ice.2. The discovery that dissolving salts in water produces cooling.3. The discovery (and spread of the knowledge) that mixing salts

and snow or ice cools it even further.4. The invention of the ice cream maker in the mid-19th century.5. The development of mechanical refrigeration in the late 19th

and early 20th centuries.

Concurrent with these a vast range of recipes has been devel-oped, spanning the spectrum from chilled fruit juices to what weunderstand as ice cream today. The evolution of ice cream has beendescribed in detail in the two excellent histories listed under Fur-ther Reading (Section 1.6) and the following summary owes muchto their researches.Ice has been used to chill food and drink for at least 4000 years in

many dierent parts of the world. Ice cellars dating back to 2000BC have been discovered in Mesopotamia (present-day Iraq).Records from the Zhou dynasty in China ca. the 11th century BCdescribe the role of the court iceman, who had a large staresponsible for harvesting ice every winter and storing it in cellarsto be served with drinks in the summer. An ice-cooled dessert,made from water bualoes milk mixed with our and camphor isrecorded in the Tang dynasty (AD 618907). In Greece, drinkswere served with snow in about 500 BC and a Roman cookerybook dating from the 1st century AD includes recipes for sweetdesserts that are sprinkled with snow before serving. Sweet drinkscooled with ice are recorded in Persia in the 2nd century AD.


Rather than harvesting ice during the winter, the Persians exploitedthe cold desert nights to freeze water that had been placed inshallow pits.The rst signicant step forward was the discovery that water is

cooled when salts are dissolved in it, such as common salt (sodiumchloride), saltpetre (potassium nitrate), sal-ammoniac (ammoniumchloride) or alum (a mixture of aluminium sulphate and potassiumsulphate). When the crystals dissolve, the strong bonds between theions are broken, extracting heat from the surrounding water, so thetemperature drops. Adding a mixture of 5 parts ammoniumchloride and 5 parts potassium nitrate to 16 parts water at 10 1Ccauses the temperature of the mixture to drop to about 12 1C,sucient to freeze a vessel of pure water immersed in it. Thisphenomenon is rst recorded in an Indian poem from the 4thcentury AD, and described in detail in an Arabic medical textbookfrom 1242. Another book in Arabic containing sorbet recipesappeared at about the same time.These methods were known in the West by the early 16th cen-

tury. Soon thereafter, in 1589, Giambattista Della Porta, a scientistfrom Naples, made the breakthrough that paved the way formaking ice cream as we know it today. He reported the discovery,possibly based on Arabic knowledge of cooling techniques, thatmuch greater cooling could be achieved by mixing ice and salt.(This eect is explained in Chapter 2.) Della Porta used this dis-covery to freeze wine in a glass placed in a mixture of ice and salt.Others became aware of the phenomenon, and the rst reports oficed desserts produced using this method of freezing began toappear in the 1620s. These appear to have been water ices, ratherthan ice cream. Water ices were served at banquets in Paris, Naples,Florence and Spain during the 1660s.The earliest evidence for ice cream in England comes from a list

of the food that was served at the feast of St George at Windsor inMay 1671; ice cream was served, but only at King Charles IIstable. Techniques and recipes then developed, notably in France.In 1674 Nicholas Lemery published a recipe for water ice and twoyears later Pierre Barra described freezing a mixture of fruit, creamand sugar by using snow and saltpetre. L. Audiger noted theimportance of whipping the mixture during freezing to break uplarge ice crystals in 1692. This technique still forms the basis of icecream making today.

6 Chapter 1

More recipe books appeared in the 18th century that helped towiden knowledge of how to make water ices and ice creams whichbecame a luxury served by the aristocracy at banquets. This wasaccompanied by the production of special moulds, ice pails (forkeeping ices cold) and serving cups. The extent of the spread of icecream can be seen by the orders for the utensils associated with itsproduction and serving. Those who are recorded as having suchservices include Louis XV of France, Gustaf III of Sweden andCatherine the Great of Russia. Ice cream remained the preserve ofroyalty and aristocracy in Europe, and of high-ranking ocials inthe USA, where in 1744 the governor of Maryland is one of the rstpeople recorded to have served it. A presidential connection withice cream began with George Washington, who often served it atocial functions, as did the third president, Thomas Jeerson.After his time as American envoy in France, Jeerson brought backan ice cream recipe from his French chef. Ice cream was served atthe inaugural dinner for the fourth president, James Madison, andsubsequently became a regular feature of the White House menu.By the beginning of the 19th century, ice cream had started to

move from the tables of the aristocracy into restaurants and cafesthat served the well-o middle classes. This was accompanied by anincreasing interest in good food, reected in the appearance ofmany books on cooking. Ice cream was made by hand, by placing abowl containing ice cream mix in a barrel lled with ice and salt,and using a scraper to remove growing ice crystals from the sides ofthe bowl until in the 1840s Nancy Johnson of Philadelphia inventedthe rst ice cream making machine. Her invention comprised twospatulas that tted tightly into a long cylindrical barrel. The spa-tulas contained holes and were attached to a shaft that could berotated with a crank. The outside of the cylinder was cooled with amixture of salt and ice. The holes made it easier to rotate thespatulas in the mix and simultaneously scrape ice crystals o theinner wall of the cylinder. This invention simplied ice creamproduction and ensured a more uniform texture than had pre-viously been possible. At about the same time, people in Europewere coming up with other ideas for ice cream making machinesand a large number of patents were published both in Europe andthe USA in the following decades. Mechanization meant that icecream could be produced more cheaply and in much largervolumes. Jacob Fussell, a dairy farmer from Baltimore, USA, is


commonly considered to be the founder of the modern ice creamindustry. Faced with a surplus of milk during the summer, hedecided to sell it as ice cream. He built the rst ice cream factory in1851, and subsequently expanded to Washington, Boston and NewYork, where he sold ice cream at a price which ordinary peoplecould aord.In England, ice cream became available to the masses towards

the end of the 19th century. This was due in part to the emigrationof Italians, many of whom became ice cream vendors in citiesaround the world. Of the approximately ten thousand Italiansliving in England and Wales in at the time of the 1890 census, justunder one thousand listed their occupation as street vendors, mostof whom sold ice cream. They sold it in small, thick-walled glasses,known as penny-licks. These were usually wiped with a cloth andre-used, and were thus a considerable health hazard, particularly forchildren. The street vendors would drum up business by calling outecco un poco, Italian for here is a little. The words ecco unpoco became hokey pokey, now meaning either poor quality icecream, or deception/trickery.4 Poor hygiene standards necessitatedthe introduction of regulations around the turn of the century.One person who was not impressed with the quality of street-sold

ice cream was Agnes Marshall (18551905). She was a celebritycook with an interest in new technology. As well as inventing an icecream maker (Figure 1.3), she wrote several books, including twodedicated to ice cream. She toured extensively, lecturing anddemonstrating her techniques to large audiences and campaignedfor better standards of food hygiene. She can also claim to haveinvented the ice cream cone in 1888, since a recipe for cornets withcream appears in one of her books. However, the cone really tooko at the 1904 St Louis Worlds Fair, when a stall selling ice creamran out of dishes in which to serve it. Ernest A. Hamwi, a waevendor in the next-door stall, had the bright idea of rolling up hiswaes as cones instead. There were tens of thousands of visitors atthe Fair, so the idea then spread rapidly throughout the USA.Ice creammaking required a source of ice. Starting in the rst half

of the 19th century, ice was collected from rivers and lakes inNorway, Sweden, Canada and the northern USA and then shippedto cities in Europe, the USA and even as far away as Calcutta!However, physicists and engineers were developing techniques forarticial refrigeration based on the liquefaction of gases such as

8 Chapter 1

propane and ammonia. The gas is compressed until it liquees,during which process it heats up. It is then allowed to cool down tonear ambient temperature whilst still pressurized. Finally the pres-sure is released, which allows the liquid to expand and evaporateextracting heat from the surroundings in the process, hence pro-viding refrigeration (Experiment 1 in Chapter 8 demonstrates thisprinciple). A number of cooling machines were invented, but Carlvon Lindes invention, demonstrated at the 1873 Worlds Fair inVienna, was the rst really successful one. Using ammonia gas in aclosed circuit, he could rapidly produce substantial quantities of ice.Eventually, articial production took over from harvesting as themain source of ice.In the rst few decades of the 20th century ice cream production

expanded and became industrialized. The major reason for this was

Figure 1.3 Mrs Marshalls ice cream maker (courtesy of the London CanalMuseum, www.canalmuseum.org.uk).


technological developments in the production and transport of icecream. The development of mechanical refrigeration allowed theuse of chilled brine (a concentrated salt solution which freezes wellbelow 0 1C) as the refrigerant instead of salt and ice. This greatlyincreased the rate of heat transfer between the ice cream mixand the refrigerant, and hence the speed of production. In 1927Clarence Vogt invented the continuous freezer, with a horizontal(rather than a vertical) cylinder. Mix was pumped in at one end andice cream pumped out from the other, so that ice cream could bemade continuously, rather than in batches. The modern ice creamfactory freezer had arrived. (Note the distinction between thefreezer in the factory, which converts ice cream mix into frozen icecream, and the domestic or shop freezer in which you keep it coldbefore consumption. Throughout this book the term factoryfreezer is used for the former and freezer for the latter.) Thesedevelopments were accompanied by the introduction of pasteur-ization, which reduced concerns over the safety of ice cream, andhomogenization, which produced a smoother, creamier product bybreaking the fat into tiny droplets. Finally, better transportthrough the railways and the automobile made the supply ofingredients and the distribution of products possible over muchgreater distances than before.Many of the names which are familiar to consumers around the

world today have their origins in this period. In the USA, Breyers,which had consisted of a number of shops in the 1870s, built newfactories and expanded its annual production of ice cream to nearlyfour million litres by 1914. In the UK, Walls set up its rst icecream factory in Acton, London, in 1922. Ice cream was sold fromtricycles, and the phrase Stop me and buy one became veryfamiliar to ice cream consumers. Production came to a halt duringthe SecondWorld War due to shortages of ingredients and the needto convert the factories to producing essential foods, such asmargarine. When production resumed after the war, rationing wasstill in place, and it was forbidden to use cream to make ice cream.Manufacturers therefore switched to using vegetable fats and milkpowder. By the time rationing ended in 1953, the British publichad become accustomed to the taste of ice cream produced withvegetable fat. For this reason, and also because it is cheaper, asubstantial amount of ice cream in the UK is still made withvegetable fat today. Partly as a result of a shortage of manpower

10 Chapter 1

after the war, the main sales outlet changed from tricycles tofreezers in corner shops. As their businesses grew, large companiessuch as Walls, which became part of Unilever in 1929, and LyonsMaid, which was bought by Nestle in 1992, set up research anddevelopment departments to study ice cream and its manufacture.One of the scientists who conducted research for Lyons in the1950s was a young chemist, Margaret Roberts, who later becamebetter known under her married name of Thatcher. In recentyears many other companies have joined the long-establishedmanufacturers. Two of the best known of these are HaagenDazs and Ben and Jerrys. Reuben Mattus founded HaagenDazs in New York in 1960. He chose the (meaningless) namebecause it sounded Danish and was therefore associated withdairy produce. This tted well with his new product, a high-quality,high-price ice cream. Ben Cohen and Jerry Greeneld set up anice cream parlour in an abandoned petrol station in Burlington,Vermont, in 1978. Their all-natural ice cream company with astrong social mission became famous after winning a major courtbattle with Haagen Dazs (owned at the time by Pillsbury). Ben andJerrys was acquired by Unilever in 2000 and is developing into aworld-wide business.

1.3 THE GLOBAL ICE CREAM MARKET

Ice cream is made and eaten in almost every country in the world.The total worldwide production of ice cream and related frozendesserts was 14.4 billion litres in 2001, i.e. an average of 2.4 litresper person, worth d35 billion.5 Unilever and Nestle are the largestworldwide producers with about one third of the market betweenthem. A huge range of dierent avours is available, includingsavoury ones. Dierences in culture and climate produce widevariations in the amounts, types and avours of ice cream pro-duced and consumed in dierent countries.The USA is the largest producer of ice cream (about 6 billion

litres per annum) and has a per capita annual consumption ofabout 22 litres; only New Zealanders eat more, with an averageconsumption of 26 litres. 9% of all the milk produced in USA isused to make ice cream, and more than 90% of US households buyit. It is often eaten as a snack, much as biscuits are eaten in theUK. Sales of ice cream in the US in 2000 were about $20 billion


(d13 billion). Approximately two thirds of this was sold in scoopshops, restaurants, retail outlets etc. and eaten out of the home.One third was sold in supermarkets, grocery shops etc., mostly ashalf-gallon (2.2 litre) tubs.More than half of the sales were premiumice cream; low-fat ice cream, frozen yoghurt, and sherbet accountfor smaller (o10%) but signicant proportions of the market.Vanilla is the most popular avour, accounting for about a quarterof sales, followed by chocolate. Ice cream with pieces of othercomponents (known as inclusions), such as cookie dough, marsh-mallows, fruit chunks, nuts, chocolate, toee or fudge, is becomingincreasingly popular, and now accounts for nearly a quarter of sales.The European per capita ice cream consumption gures are

surprising at rst sight. One might expect that more ice creamwould be consumed in hot southern European countries, such asSpain (about 6 litres per person per year) and Portugal (4 litres)than in cold northern European countries such as Sweden (12litres) and Germany (8 litres). However, the reverse is true. Themain reason for this is that northern Europeans are used toconsuming lots of milk, cheese, butter etc. whereas the southernEuropean diet contains much less dairy produce. Another factorthat inuences consumption is whether households own a largefreezer in which to keep quantities of ice cream. This in turn may beinuenced by local building regulations! The exception to thisnorthsouth divide is Italy (9 litres), where there is a great traditionof making and eating ice cream. In many European countries barsand stick products are more important than tubs.The UK falls roughly in the middle of the list of European

countries, with a per capita consumption of about 7 litres, andannual sales of about d1.5 billion. A small number of large com-panies, such as Walls (Unilever), Mars and Richmond Foods(which produces ice cream for Nestle, and several supermarketsown brands) have substantial market shares, but about half istaken by the several hundred small independent companies. Thesemostly employ fewer than ten people, and sell only locally. Themost popular avours in the UK are vanilla, chocolate andstrawberry and, like the US, ice cream often contains inclusions toprovide greater interest and variety for the consumer. Magnum isthe largest single brand 41% of adults in UK have bought one.6

In other parts of the world, the market is very dierent. Forexample, in south-east Asia, the largest demand is for refreshing

12 Chapter 1

products, such as water ices. Ice cream comes in avours thatseem very strange and exotic to Western palates for examplegreen tea and red bean ice cream in Japan, sweet corn ice creamin Malaysia, chilli ice cream in Indonesia and sesame seed icecream in Korea.

1.4 SELLING ICE CREAM: FUN, INDULGENCE ANDREFRESHMENT

Two factors that have a major eect on the sales of ice creamproducts are the weather and advertising. Ice cream sales are veryseasonal, peaking in the summer. For example, in France, 65% ofsales are made between June and September, and in Italy theaverage consumption per capita per month is 0.1 litres in Januaryand 1.3 litres in July. The weather can have a substantial impact onsales, especially at particular times such as bank holiday weekends.While this is beyond anyones control, the large companies try toincrease their sales by spending several million pounds each year onpromoting their products. Ice cream advertisements and televisioncommercials are often based on one of three themes: fun, indul-gence or refreshment. A fun image is typically used to promoteproducts aimed at children, or families. The advertising forpremium products (which may well use an element of sexualattraction) usually aims to project an indulgent image. Water iceproducts, for example ice lollies, are frequently marketed on theirability to cool down and refresh the consumer. Whether a productis indulgent or refreshing largely depends on its ingredients and theprocessing method by which it is produced. We will look at these inChapters 3, 4 and 5.

REFERENCES

1. The Food Labelling Regulations 1996, Statutory Instrument1499, www.legislation.gov.uk.

2. United States Code of Federal Regulations 21 CFR Chapter 1,y135.110.

3. E. Dickinson, An Introduction to Food Colloids, Oxford Uni-versity Press, Oxford, 1992.

4. New Shorter Oxford English Dictionary, Clarendon Press,Oxford, 1993.


5. Global and US market information from the InternationalDairy Foods Association (www.idfa.org), and Dairy Ind. Int.,2002, 67, 27.

6. UK market information from Ice Creams and Frozen Desserts:Key Note Market Report Plus, ed. E Clarke, Hampton, 7th edn,2000, and M. Stogo, Ice Cream and Frozen Deserts: a Com-mercial Guide to Production and Marketing, Wiley, New York,1998.

FURTHER READING

C. Liddell and R. Weir, Ices: The Denitive Guide, Hodder &Stoughton, London, 1993.

P. Reinders, Licks, Sticks and Bricks A World History of IceCream, Unilever, Rotterdam, 1999.

14 Chapter 1

CHAPTER 2

Colloidal Dispersions, Freezingand Rheology

2.1 INTRODUCTION

A typical ice cream consists of about 30% ice, 50% air, 5% fat and15% matrix (sugar solution) by volume. It therefore contains allthree states of matter: solid ice and fat, liquid sugar solution andgas. The solid and gas are in the form of small particles icecrystals, fat droplets and air bubbles in a continuous liquid phase,the matrix. In order to understand the creation of the micro-structure during the manufacturing process we must rst introducesome concepts from the physical chemistry of colloids, freezing andrheology (the study of the deformation and ow of materials).

2.2 COLLOIDAL DISPERSIONS

Colloidal dispersions consist of small particles of one phase (solid,liquid or gas) in another continuous phase. The particle size mayrange from nanometres to tens of microns. There are eight dierenttypes of colloidal dispersion, summarized in Table 2.1.Colloidal dispersions have a very large surface area for their

volume. Therefore the surface properties of the phases have a largeinuence on the properties as a whole. Ice cream is simultaneouslyan emulsion (fat droplets), a sol (ice crystals) and a foam (air




15

bubbles), and also contains other colloids in the form of caseinmicelles, other proteins and polysaccharides in the matrix.

2.2.1 Emulsions

Emulsions are dispersions of droplets of one liquid in another.Many foods, for example mayonnaise, vinaigrette salad dressing,milk, cream and ice cream are oil-in-water emulsions, i.e. the oil isdispersed as droplets in a continuous aqueous phase. Low-fatmargarines are water-in-oil emulsions, i.e. the water is dispersed ina continuous oil phase.A familiar property of liquids is that they behave as if they have

an elastic skin which holds the liquid molecules together, and triesto minimize its surface area. This property is the surface tension(for a liquid surrounded by gas). The surface tension is responsiblefor many well-known properties of liquids, for example the bulgeof liquid (the meniscus) above a cup that has been overlled andthe fact that at stones can be bounced o the surface of a lake.Just as the surface of a liquid has a surface tension, the interfacebetween two immiscible liquids, such as oil and water, has aninterfacial tension. This arises because water molecules prefer to besurrounded by other water molecules rather than oil molecules.If oil and water are vigorously mixed together the oil can be

dispersed as an emulsion of small droplets. Small droplets have alarge surface area to volume ratio. Consider a test tube containingan emulsion of oil droplets (radius r, total oil volume Voil) in water(Figure 2.1).

Table 2.1 The classication of colloidal dispersions.

Continuousphase

Dispersedphase Name Examples

Solid Solid Solid sol Ruby, glass, composites, ceramics,bone

Solid Liquid Solid emulsion Bitumen, asphalt, opal, pearl, jellySolid Gas Solid foam Expanded polystyrene, pumiceLiquid Solid Sol Ink, paint, blood, toothpaste, mudLiquid Liquid Emulsion Milk, mayonnaise, creamLiquid Gas Foam Head on beer, bubble bathGas Solid Aerosol Smoke, dustGas Liquid Aerosol Mist, fog, clouds, deodorant

16 Chapter 2

The number of droplets (n) is given by the total volume of oildivided by the volume of an individual drop.

n VoilVdroplet

Voil4p3r3

2:1

The total interfacial area (Ai) is obtained by multiplying n by thesurface area of a droplet.

Ai n 4pr2 Voil4p3r3 4pr2 3Voil

r2:2

Thus as the droplets get smaller, the oil/water interfacial areagets larger. This is plotted in Figure 2.2, for Voil 1 cm3.Very small droplets have a very large oil/water interfacial area.

This means that many oil/water contacts are created, and theinterfacial energy, Ei, is large.

Ei gAi 2:3where g is the interfacial tension. Emulsions are inherently unstablebecause they can reduce their energy by reducing the interfacialarea, e.g. by coalescence of small droplets into large ones. Thus,

Figure 2.1 Schematic diagram of an oil-in-water emulsion (light droplets, oil;dark continuous phase, water).

17Colloidal Dispersions, Freezing and Rheology

after some time, a vinaigrette dressing will separate into an oillayer and an aqueous layer. However, emulsions can be stabilizedby surface active molecules. These consist of a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail (Figure 2.3aand b). The hydrophilic part of the molecule is attracted to thewater and the hydrophobic part is attracted to the oil. Theonly way to satisfy both parts of the molecule simultaneously is forit to be located at an oil/water interface (Figure 2.3c). This reducesthe interfacial tension and makes the emulsion more stable.Experiment 2 in Chapter 8 demonstrates this.Most food emulsions are stabilized by proteins and/or emulsi-

ers which slow down or prevent the separation of the oil andthe water. Proteins are built from amino acids, some of which arehydrophilic and some of which are hydrophobic. Thus certainproteins, such as casein from milk, have hydrophilic and hydro-phobic regions which makes them surface-active. Emulsiers,for example mono- and diglycerides, also contain hydrophilicand hydrophobic regions. Some emulsions, known as Pickeringemulsions, are stabilized by solid particles which adsorb onto the

Droplet radius (m)

Tota

l sur

face

are

a (m

2 )

0 20 40 60 80 100

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Figure 2.2 Plot of the total surface area of an emulsion as a function of dropletsize.

18 Chapter 2

interface between the two phases. Homogenized milk, in whichthe fat droplets are in part stabilized by casein micelles, is anexample of Pickering stabilization.

2.2.2 Sols

Sols are dispersions of solid particles in a continuous liquid phase,for example the ice crystals in ice cream, pigment particles in inkand paint, soil particles in mud, and platelets in blood. The solidphase is often signicantly denser than the liquid phase, so it isliable to sediment out. Brownian motion helps to keep small par-ticles in suspension, so large particles are most likely to sediment.

C

CH2

H2C

CH2

CH2

H2C

CH2

H2C

H2C

CH2

H2C

CH2

CH2

H2C

CH2

H2C

H2C

CH3O

O

H2C

HC OH

H2C OH

Hydrophobic tailHydrophilic head

Water

Air

(a)

(b)

(c)

Figure 2.3 (a) The molecular structure of the emulsier glycerol monostearate;(b) a schematic diagram of a surface active molecule; and (c) aschematic diagram of surface active molecules at the interfacebetween water and air or oil.


They can be easily re-suspended by stirring or shaking. This is whyyou stir a tin of paint before painting, or shake a carton of orangejuice before opening it. When the particles are less dense than theliquid, they tend to oat to the top of the liquid. This is known ascreaming, and is more often observed in foams and emulsions,e.g. full fat milk where the cream rises to the top of the bottle.

2.2.3 Foams

A foam is a dispersion of gas bubbles in a relatively small volumeof a liquid or solid continuous phase. Liquid foams consist of gasbubbles separated by thin liquid lms. It is not possible to make afoam from pure water: the bubbles disappear as soon as they arecreated. However, if surface-active molecules, such as soap,emulsiers or certain proteins, are present they adsorb to the gas/liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread,sponge cake or lava, have solid walls between the gas bubbles.Liquid foams have unusual macroscopic properties that arisefrom the physical chemistry of bubble interfaces and the structureformed by the packing of the gas bubbles. For small, gentledeformations they behave like an elastic solid and, whendeformed more, they can ow like a liquid. When the pressure ortemperature is changed, their volume changes approximatelyaccording to the ideal gas law (PV/T constant). Thus foamsexhibit features of all three fundamental states of matter. In icecream, the gas-phase volume is relatively low for a foam (about50%), so the bubbles do not come into contact with each otherand are therefore generally spherical. Some foams, for examplebubble bath, contain so much gas that the bubbles are in closecontact, and form a polyhedral structure.The amount of air incorporated in a foam is often reported

in terms of the overrun. The overrun is the ratio of the volumeof gas (Vgas) to the volume of liquid (Vliquid), expressed as apercentage, i.e.

overrun VgasVliquid

100 Vfoam VliquidVliquid

100: 2:4

Thus a foam that has twice the volume of the liquid from whichit was made has an overrun of 100%.

20 Chapter 2

There are several dierent means of making foams. Whipping isnormally used in the kitchen e.g. for foaming cream or egg white.During whipping, large bubbles of air are entrained in a viscousliquid, elongated by stresses due to the vigorous agitation, and as aresult break down into smaller ones. The viscosity of the liquid isimportant. If the liquid is too viscous, it is dicult to beat andtherefore to incorporate the air; if it is not viscous enough, the lmbetween the air bubbles rapidly drains, and the bubbles coalesce.The overrun achieved typically increases with whipping speed, untila plateau is reached when equilibrium is established between therate of bubble formation and the rate of break up. Industrialprocesses often need to be more reproducible and controllable thanwhipping. For example bubbles may be formed at an orice, e.g.sparging air into the liquid. (In ice cream manufacture, air isinjected as large bubbles which are reduced in size by beating.)Bubbles can also be formed in situ, e.g. in carbonated soft drinks,where bubbles of dissolved gas come out of solution when there is achange of pressure; or in bread, where carbon dioxide is generatedby yeast during baking.Liquid foams, like emulsions, have a tendency to separate into

distinct gas and liquid phases in order to decrease the total inter-facial area. They may exist for a few seconds (e.g. champagnebubbles) or months (e.g. ice cream, provided it is kept frozen),depending on the properties of the liquid and the surface-activemolecule. Creaming in foams is accompanied by drainage of theliquid from between the bubbles. Since the matrix is very viscous,creaming and drainage are very slow in ice cream.

2.2.4 Coarsening of Colloidal Dispersions

Physical systems tend towards the state of lowest energy. We haveseen in the section on emulsions above that the interfacial area, andhence the interfacial energy, of a dispersion increases as the par-ticles get smaller. Colloidal dispersions (such as the three in icecream) therefore have an inherent tendency, driven by surfacetension, to separate into distinct bulk phases. While the totalvolume of the dispersed phase is constant, the size of the particlesincreases and their number reduces, thus decreasing the totalinterfacial energy. This is known generally as coarsening, orrecrystallization in the context of ice crystals.


There are two main coarsening mechanisms common to dierenttypes of dispersion (though they are known by dierent names).The rst is called coalescence in the context of emulsions andfoams, and accretion in the context of dispersions of ice crystals.This is the joining together of two or more adjacent particles toform a single, larger one (Figure 2.4, left). The second mechanismis called Ostwald ripening when referring to ice crystals andemulsions, and disproportionation when referring to foams. Thistakes place by the transfer of individual molecules from smallparticles to larger ones by diusion through the continuous phase(Figure 2.4, right). Coalescence/accretion is the dominant processfor some dispersions, often at high dispersed phase volume,whereas Ostwald ripening/disproportionation is more important inothers, such as low dispersed phase volume.The recrystallization of ice crystals and the coarsening of air

bubbles both lead to deterioration in the texture of ice cream, and anumber of methods are employed to slow them down. In contrast,the coalescence of fat droplets is deliberately promoted during icecreammanufacture, and is crucial in making good quality ice cream.The means used to control the coarsening processes of the ice, fatand air dispersions in ice cream are discussed in Chapters 4 and 7.

Figure 2.4 (left) Coalescence/accretion and (right) Ostwald ripening/disproportionation.

22 Chapter 2

2.3 FREEZING

Ice is a crystalline solid, in which the molecules are held in a hexa-gonal lattice by intermolecular forces. This hexagonal symmetry isthe reason why snowakes have 6 sides or points. The moleculesvibrate, but stay xed in their positions in the lattice (Figure 2.5a).If the temperature is raised, these vibrations become larger until, atthe melting point (0 1C for pure water at atmospheric pressure), themolecules have enough energy to escape from their xed positions,i.e. the ice melts. In the liquid state, the molecules can move pasteach other, although they remain in close contact (Figure 2.5b).

2.3.1 Supercooling and Nucleation

Figure 2.6 (top) shows how the temperature of a beaker of crushedice, initially at 6 1C, changes as it warms up in a laboratory atroom temperature. Heat enters the beaker from the surroundings,slowly raising the temperature of the ice. After about 10 min, thetemperature reaches 0 1C and the ice begins to melt. At this point,the temperature stops rising and remains constant. This is becausethe heat that enters the beaker is used up in breaking the inter-molecular forces in the ice lattice as the ice melts, rather thanraising the temperature. Eventually, when all the ice has melted, thetemperature begins to rise again. The heat that is needed to change

(a) (b)

Figure 2.5 The arrangement of H2O molecules (a) in solid ice and (b) in liquidwater; oxygen atoms are grey and hydrogen are black.


the ice to water is known as the latent heat. (The heat that causesthe temperature of a substance to change is known as the sen-sible heat, because it can be sensed as a temperature change.)Experiment 6 in Chapter 8 demonstrates latent heat.Now observe what happens when we reverse the process. Figure

2.6 (bottom) shows how the temperature of a beaker of water,initially at 5 1C, changes on cooling to below 0 1C. As heat owsout of the beaker into the surroundings, the temperature falls: themolecules have less thermal energy and so move around lessquickly. If freezing were the exact reverse of melting, we wouldexpect that when the temperature reaches 0 1C, ice would begin toform. This would release the latent heat, so that the temperaturewould then remain constant, until all the water had turned into ice.In fact, the temperature continues to fall below 0 1C, without theformation of any ice, in this case to about 2 1C. This phenomenon

0 10 20 30 40 50

0 10 20 30 40 50

Time (minutes)

Time (minutes)

Tem

per

atu

re (

oC

)T

emp

erat

ure

(oC

)

0

2

4

6

2

4

6

6

4

2

0

2

4

6

Figure 2.6 Plot of the temperature as a function of time for (top) ice meltingand (bottom) water freezing.

24 Chapter 2

is called supercooling. At about 20min, the temperature risessharply, reaching 0 1C, where it again forms a plateau, beforecooling down again.To understand why supercooling occurs, we need to understand

what is taking place at a molecular level. When ice crystals melt, themolecules at the crystal surface switch from the ordered lattice intothe liquid one at a time. The reverse process occurs when an icecrystal grows: individual water molecules from the liquid join ontothe ice crystal lattice. However, if there are no ice crystals in thebeaker, the supercooled water does not have a lattice to join ontoso it cannot freeze one molecule at a time. Before freezing canoccur, tiny embryonic ice crystals (nuclei) have to form. In theliquid state, nuclei form when the random motion of the watermolecules brings a small number of molecules together in a crystal-like arrangement. When a nucleus is formed, an interface is createdbetween the nucleus and the water. There is an energy gain informing a nucleus because below 0 1C ice has a lower energy thanwater. However, there is also an energy cost, due to the creation ofthe new surface. The net energy E of forming a spherical nucleusof radius r is the sum of the energy gain due to change of water toice and the energy cost due to formation of the surface (eqn (2.5)).

E LT TmT

4p3r3 g 4pr2 2:5

L is the latent heat per unit volume, T the absolute temperature,Tm the melting point and g is the surface tension. Figure 2.7 plotsthis equation for three dierent temperatures. Below the maximumin the curves, the energy cost of increasing in size is greater than theenergy gain. Therefore small nuclei rapidly break up after theyhave formed. The maximum in the curves determines the criticalradius (r*) above which nuclei can lower their energy by growing.As the temperature decreases from 10 1C to 30 1C (i.e. thesupercooling increases) the critical radius decreases from 5 to1.5 nm. The height of the maximum, i.e. the energy barrier tonucleation, also decreases. Thus, the greater the supercooling, themore likely it is that ice crystals will form.Since nucleation relies on the random clustering of water mole-

cules it is a random process and does not occur at a xed tem-perature. Absolutely pure water can theoretically be supercooled to


40 1C. However, in practice nucleation occurs at higher tem-peratures (2 1C in Figure 2.6, bottom). This is because tiny par-ticles in the water (e.g. dust) or the walls of the beaker can act astemplates on which the water molecules can begin to cluster in acrystalline arrangement. This means that fewer water molecules areneeded to form a stable nucleus, and therefore the supercooling isreduced. Some particles are particularly eective at causingnucleation, for example silver iodide crystals, or a protein producedby the bacterium, Pseudomonas syringae, that grows naturally onplants. In fact, these are often added to the water used in snow-making machines in ski resorts, to ensure that the water dropletswhich are sprayed into the cold air freeze before they hit theground. Experiment 5 in Chapter 8 demonstrates supercooling andnucleation.As we have seen, energy (latent heat) is needed to overcome the

intermolecular forces to convert a crystal lattice into a liquid.Similarly, the reverse process, freezing, gives out latent heat. Thelatent heat of water is very large because of the strong forcesbetween the water molecules: the heat released when 1 kg of icefreezes is sucient to raise the temperature of 1 kg of water from0 to 80 1C. The release of the latent heat after nucleation causes therapid rise in temperature that is seen in Figure 2.6 (bottom). Oncethe temperature reaches 0 1C, it remains constant until all the waterhas frozen. At this point the temperature begins to fall again.

0 2 4 6 8 10

En

erg

y (J

) 0.00E+00

5.00E18

1.00E17

5.00E18

Radius (nm)

30 oC

20 oC

10 oC

Figure 2.7 The energy required to form a ice crystal nucleus plotted as afunction of ice crystal size at 10, 20 and 30 1C.

26 Chapter 2

2.3.2 Growth

While nucleation requires several degrees of supercooling, crystalgrowth (sometimes called propagation) is possible with very littlesupercooling (o0.01 1C). This is because molecules can join onto analready existing crystal, rather than having to form a completelynew one. Crystal growth therefore begins as soon as nucleation hasoccurred, and continues until equilibrium is reached, i.e. thesupercooling has been removed. The competition between nuclea-tion and crystal growth determines the characteristics of the icecrystals formed. Rapid freezing (i.e. large degrees of supercooling/low temperature) produces many nuclei, and allows little time forgrowth, so a large number of small ice crystals are formed. Slowfreezing (low degrees of supercooling/relatively high temperature)on the other hand produces fewer, larger crystals. Fast and slowfreezing processes are used in water-ice manufacture to producedierent-sized ice crystals, and hence dierent textures. This isdiscussed in Chapters 4 and 7.When all the water has frozen, or when there is no longer any

supercooling, growth ceases and the amount of ice does notincrease any further. However, the system is not necessarily atequilibrium. Small ice crystals have a greater surface area (for agiven volume) than large ones, which costs energy. The energy canbe lowered if the ice crystals undergo recrystallization.

2.3.3 Freezing Point Depression

The equilibrium freezing point of pure water at atmosphericpressure is 0 1C. If a solute, (e.g. sugar or salt) is present, the solutemolecules do not t comfortably into the ice crystal lattice. Theyeectively get in the way of the water molecules trying to join ontothe crystal, so that it is harder for the water to freeze. This results ina lower freezing point. For example salt is put on roads in winterbecause it lowers the freezing point to below the temperature of theroad, so that the ice melts.Figure 2.8 shows the same cooling experiment as Figure 2.6, but

using a 12% sucrose solution instead of pure water. Supercoolingtakes place until nucleation occurs, and the temperature rises to thefreezing point (in this case, 0.77 1C). Unlike the graph for purewater, the plateau does not remain at for long. The formation of


ice leads to an increase in the concentration of the sucrose solution,and hence a further decrease in the freezing point.The amount by which the freezing point changes is known as

the freezing point depression, and depends on the number ofsolute molecules present, but not their type. It can be shown that(for low solute concentrations) the freezing point depression, DT, isgiven by

DT Kx; 2:6

where K is a constant (known as the cryoscopic constant) and x isthe mole fraction of solute, i.e. the number of solute moleculesdivided by the total number of molecules (water+solute). Since thefreezing point depression depends on the number of molecules, thesmaller the molecular weight of the solute, the more eective it willbe at depressing the freezing point on a weight for weight basis.Thus, 1 g of salt (molecular weight 58.5) causes a larger freezingpoint depression than 1 g of sucrose (molecular weight 342). Infact, determining the freezing point depression caused by a knownmass of a compound was historically an important method formeasuring its molecular weight. The antifreeze used in cars usuallycontains ethylene glycol to depress the freezing point. Alcoholicdrinks have freezing points below 0 1C, because alcohol (ethanol)lowers the freezing point. This is why vodka can (and indeed

0 20 40 60 80 100

Time (minutes)

Tem

per

atu

re (

oC

)

5

4

3

2

1

0

1

2

3

4

Figure 2.8 The temperature of a 12% sucrose solution plotted as a function oftime during freezing.

28 Chapter 2

should) be served straight from the freezer. Experiment 4 inChapter 8 demonstrates freezing point depression.

2.3.4 The SaltWater Phase Diagram

We can plot the freezing point of a solution as a function of itsconcentration. (Throughout this book concentrations are expressedas % weight/weight, abbreviated % w/w, unless otherwise stated.)This is shown for salt (sodium chloride) solutions in Figure 2.9 asthe line separating the saltwater and ice+saltwater regions.The solubility of salt in water, i.e. the maximum concentrationthat can be dissolved at, as a function of temperature is also shownas the line separating the saltwater and NaCl crystals+saltwaterregions. These two curves, together with the horizontal linethrough their intersection, form the phase diagram for salt solutions.Mixtures of salt and water can take dierent forms, or phases, i.e.ice, salt crystals or salt solution. The phase diagram is essentially amap that shows which phase(s) are present at any given temperatureand composition. (However, they do not indicate how the phase isdistributed, e.g. a few large ice crystals or many small ones.)

Tem

pera

ture

(o C

)

NaCl concentration (%w/w)23.3 100

NaCl Crystals+ Saltwater

Ice + Saltwater

Ice + NaCl Crystals

Saltwater

A CB

D

E

0

0

21.1

Figure 2.9 The phase diagram for sodium chloride solutions. Reprinted withpermission from IOP Publishing Ltd.1


At warm temperatures and low salt concentrations all the saltcrystals dissolve in the water: for example a 15% salt solution at10 1C (the point marked A). If salt is added while the temperatureis kept constant (arrow ABC) the salt concentration increasesuntil, when the solubility line is reached (B), no more salt can bedissolved; the solution is said to be saturated. If any further salt isadded it does not dissolve, but remains as crystals, so that thesystem is a mixture of saturated salt solution and salt crystals (C).Now consider what happens if we start at A and reduce thetemperature without adding or removing salt (arrow AD). As thesolution is cooled, nothing happens until the freezing point curveis reached at D. If the temperature is lowered below D, ice formsafter supercooling and nucleation. The salt is excluded from theice crystals so the concentration of salt in the solution increases;this is known as freeze-concentration. The freezing point isdepressed further and the solution moves down the freezing pointcurve as more ice is formed. When the freezing point curvemeets the solubility curve, the eutectic point is reached (E). Thiscorresponds to 76.7% ice, 23.3% sodium chloride and 21.1 1C.The solution cannot be concentrated any further because the limitof solubility of the salt has been reached. If a solution with theeutectic composition is cooled, nothing happens until thewhole solution freezes at the eutectic temperature. A solid withthe eutectic composition melts at the lowest temperature ofany composition; hence the name eutectic, from the Greek foreasily melted.When ice and salt at 0 1C are mixed some of the ice melts because

the salt depresses the freezing point. In order to melt, the ice mustabsorb the latent heat of fusion from the surroundings. In theexample mentioned above, the latent heat is absorbed from theroad. Since the road is large, the heat it gives up only causes itstemperature to drop by a tiny amount. However, if this is done forexample in an insulated bowl the heat can only come from the icesalt mixture itself. The removal of this heat causes a drop in thetemperature. The latent heat of melting of ice is large compared tothe amount of heat required to cause a change in temperature of1 1C, so this causes a signicant drop in the temperature. Theeutectic temperature can be reached by mixing salt and ice in thecorrect proportions. This eect was the chief method of refrigera-tion for producing ice cream in the nineteenth century, and can be

30 Chapter 2

used to make ice cream at home without using a freezer or in aclassroom (see Experiment 9 in Chapter 8).

2.3.5 The SucroseWater Phase Diagram

The phase diagram for sucrose and water (Figure 2.10) is ratherdierent from the saltwater one, and is particularly important inice cream making. When a 30% sucrose solution (point A) is cooled,the freezing point curve is reached at B (the freezing point depres-sion deviates signicantly from eqn (2.6) for sucrose concentrations

Sucrose concentration (% w/w)

Temperature (C)

0

120

40

Sucrose solution

Ice + Solution

Ice + Glass

Glass

Supersaturated Solution

A

B

CD

E

0 20 40 60 80 100

Figure 2.10 The phase diagram for sucrose solutions.


above about 10%). Ice then forms, the solution freeze-concentratesand the freezing point is depressed further (BC).At any temperature below the freezing point, (e.g. 10 1C) there

is a certain amount of ice in equilibrium with concentrated sucrosesolution. The amount of ice can be determined from the phasediagram. At C, the sucrose concentration is 57%. Let us supposethat we started with 1 kg of solution, i.e. 300 g sucrose and 700 gwater. The sucrose concentration is given by the mass of sucrosedivided by the mass of solution, i.e.

300

1000mice ; 2:7

where mice is the mass of ice present at 10 1C (in g). We know fromthe phase diagram that the sucrose concentration is 57%. Therefore

mice 1000 3000:57

470 g; 2:8

i.e. a 30% sucrose solution contains 47% ice at 10 1C.In fact, we can do this calculation to obtain the amount of ice at

any temperature, provided we know the freezing point curve. Thisis important when formulating ice cream recipes, in order to ensurethat the ice cream contains the right amount of ice at its servingtemperature. This is often called the ice curve. Figure 2.11 showsthe ice curve for a 30% sucrose solution between 0 and 25 1C. Thetemperature at which the rst ice begins to form (2.7 1C) is thefreezing point depression.

60

50

40

30

20

10

0

Ice

cont

ent (

% w

/w)

Temperature (oC)

0 5 10 15 20

Figure 2.11 The ice content of a 30% sucrose solution as a function oftemperature.

32 Chapter 2

So far, the behaviour is the same as with salt solutions. However,unlike salt, sucrose crystals do not form readily, and the solutioncan become supersaturated, i.e. the solute concentration increasesbeyond the point at which it should precipitate out of solution (cf.supercooling) and the freezing point curve can be extended beyondthe theoretical eutectic point, D (63% sucrose, 13.7 1C). Thesolution passes D and continues along the freezing point curve untilit meets the glass transition line (E). As the temperature is reducedand the solution becomes more concentrated, its viscosity increases.Eventually the viscosity becomes so large that the solution eec-tively becomes a solid. However, unlike a crystal, the molecules arenot ordered on a lattice but have a liquid-like structure, althoughthey are not free to move past each other. The solution forms a solidstate known as a glass (ordinary glass has this type of disorderedstructure). The change to a glassy solid is known as the glass tran-sition. Unlike freezing, for example, it is not a true phase transitionbecause it is a kinetic, not a thermodynamic phenomenon. None-theless a curve representing the glass transition temperature as afunction of concentration can be added to the phase diagram.(Technically, it should now be called a state diagram, because itincludes a non-equilibrium state). No more changes happen whenthe solution is cooled further. In practice it is experimentally dicultto reach the maximum freeze-concentration at E because themolecular motion becomes very slow before this point is reached.

2.3.6 Newtons Law of Cooling

Newtons law of cooling states that the temperature dierencebetween a refrigerant (Tr) and (for example) an ice cream mix(Tmix) determines the rate at which it cools down (eqn (2.9)).

dTmix

dt/ Tr Tmix 2:9

Thus colder refrigerants cool faster. The coldest refrigerantavailable to the Victorians was ice and salt at about 20 1C. Todayice cream factories typically use liquid ammonia at 30 1C, and icecream making is much faster. In fact, Newtons law of coolingexplains why the world record for the fastest ice cream ever madeused liquid nitrogen at 196 1C.2 This is demonstrated in Experi-ment 12 in Chapter 8.


2.4 THE RHEOLOGY OF SOLUTIONS ANDSUSPENSIONS

The study of the ow properties of liquids is called rheology.One way of characterizing the thickness or runniness of liquids isby their viscosity. Consider a liquid between two parallel surfaces(with area A). The bottom surface is xed and the top one moves ata constant velocity (v). This type of deformation is known as shear.A force (F) is applied to the top plate to keep it moving. The liquidis dragged along with the moving plate due to its viscosity witha velocity which is largest close to the top plate and decreaseswith the distance from it. Thus there is a velocity gradient in theliquid (v/h), where h is the distance between the plates as shownin Figure 2.12.Newton suggested that the velocity gradient is proportional to

the shear stress s (i.e. the force applied to keep the plate moving perunit area).

s FA Z v

h Z _g 2:10

The constant of proportionality in eqn (2.10) is the viscosity ofthe liquid (Z). Some uids, such as water, olive oil and sucrosesolutions obey this equation and are said to be Newtonian. Theirviscosity does not depend on the velocity gradient, i.e. how fast theliquid is sheared known as the shear rate, _g. More complex uids(e.g. solutions of polymers) have a viscosity that does depend onthe shear rate. Such uids are called non-Newtonian. Manycomplex uids, for example tomato ketchup and ice cream mix,become less viscous when they are sheared and are described asshear-thinning. Tapping the bottom of the bottle applies shear tothe ketchup, which becomes less viscous and ows more easily onto

v

h Gradient = v/h

Figure 2.12 The ow of a Newtonian liquid between parallel surfaces.

34 Chapter 2

your plate. Others, such as a concentrated solution of cornstarch orquicksand, become more viscous (i.e. they are shear-thickening).Experiment 7 in Chapter 8 gives some examples of non-Newtonianuids. A single viscosity is not sucient to describe the owproperties of non-Newtonian liquids and if a viscosity is stated, theshear rate at which it was measured must also be given.The rheology of ice cream is much more complex than that of a

simple liquid. The continuous liquid phase is a solution of small(sugar) and large (stabilizer) molecules, in which particles of otherphases (ice crystals, fat droplets and air bubbles) are suspended.We must rst look at the eects of each of these and the eect oftemperature in order to understand the rheology of ice cream.

2.4.1 The Rheology of Solutions of Small Molecules

The molecules in a liquid are in close contact (Figure 2.5b).The ease with they can move past each other depends on thetemperature. The lower the temperature of the liquid, the moreslowly the molecules move, and the harder it is for them to over-come the forces between them and move past each other. The resultof this at the macroscopic level is that the whole liquid ows lesseasily, i.e. its viscosity increases. Conversely, as the temperatureincreases the molecules move faster and more freely so the viscositydecreases. This is illustrated in Figure 2.13, which shows theviscosity of a 20% sugar solution as a function of temperature.

Temperature (oC)

Vis

cosi

ty (

mP

a s)

10 20 30 40 50 60 70 80

3

2.5

2

1.5

1

0.5

0

Figure 2.13 The viscosity of a 20% sucrose solution as a function oftemperature (data from ref. 3).


Figure 2.14 shows that the viscosity of sucrose solutionsincreases as a function of the solute concentration.One way to understand this is by considering the sucrose mole-

cules (which are signicantly larger than the water molecules) asparticles in suspension in the water. Einstein showed that theviscosity of a dilute suspension of non-interacting sphericalparticles (Z) in a liquid of intrinsic viscosity (Zl) increases with thefraction of the volume they occupy (j).

Z Zl1 2:5j 2:11

Einstein applied eqn (2.11) to sucrose solutions and, by com-bining this with data on diusion, was able to deduce the size of thesucrose molecule.4 For high concentrations (such as those typicalof ice cream mixes) the interactions between the particles furtherincrease the viscosity; this can be accounted for by adding termsproportional to j2, j3. . . to the right hand side of eqn (2.11).Experiment 7 in Chapter 8 describes a simple method for com-paring the viscosity of solutions of dierent concentrations.

2.4.2 The Rheology of Polymer Solutions

A polymer is a molecule that consists of a long chain of smallmolecules (monomers) that are linked together. The stabilizers usedin cream are mostly polysaccharides, i.e. polymers of sugar mole-cules. Stabilizers have a number of functions, one of which is to

Vis

cosi

ty (

mP

a s)

1000

100

10

110 20 30 40 50 60 700

Concentration (weight %)

Figure 2.14 Plot of viscosity against sucrose concentration at 20 1C (data fromref. 3).

36 Chapter 2

increase the viscosity of ice cream mixes. Figure 2.15 shows theviscosity of a typical polymer solution as a function of con-centration. The polymer increases the solution viscosity at lowconcentrations and above a certain concentration the viscosityincreases even more rapidly.Figure 2.16 shows why this happens. At very low concentrations,

each polymer chain takes the form of a separate coil and can movewithout interference from the others (Figure 2.16a). The coiloccupies a larger volume than the total volume of the monomers;this is why the viscosity is higher for than small molecule solutes.However, above a certain concentration (known as the entangle-ment concentration), the polymer chains begin to overlap with eachother and become entangled (Figure 2.16b). This means that it ismuch harder for the chains to move past each other, and theviscosity increases rapidly. A bowl of spaghetti demonstrates thiseect: it is dicult to pick up one piece without also taking several

Viscosity

Concentration

Entanglementconcentration

Figure 2.15 Schematic graph of the viscosity of a polymer solution as a func-tion of concentration.


others because they are entangled. The onset of entanglements canbe seen in Figure 2.15 as the increase in the slope at the entan-glement concentration. The stabilizers in the matrix of ice creamare usually above the entanglement concentration. The method ofExperiment 7 in Chapter 8 can also be used to measure the viscosityof polymer solutions.If very high shear is applied to an entangled polymer solution

the polymers can be pulled apart and disentangled. The results ina decrease in the viscosity at high shear rates, known as shear-thinning. Figure 2.17 shows shear-thinning in a solution of guargum, a natural polymer commonly used as a stabilizer in ice cream.Entanglements are purely topological. However, there can also

be specic chemical interactions between polymer chains. Forexample there is hydrogen bonding between polymers in guar gumsolutions, which results in a higher viscosity than would be expectedfor purely topological entanglements. Very strong interactions canresult in the formation of a cross-linked polymer network, i.e. a gel(Figure 2.18). Because the cross-links are xed, the polymers cannotmove past each other. Therefore, instead of owing like a liquid,the gel returns to its original shape when the shear is removed, i.e.it is elastic. (The analogy in this case is spaghetti that has beenknotted together, so that the strands cannot be separated.)A number of stabilizers can form gels under certain conditions,e.g. locust bean gum, pectin, sodium alginate and k-carrageenan.

(a) (b)

Figure 2.16 (a) Low concentration: the coils are separate; (b) above the criticalconcentration, they overlap and entangle.

38 Chapter 2

Shear rate (s1)

Vis

cosi

ty (

0.1

Pa

s)

103

102

101

100102 101 100 101 102 103

Figure 2.17 The viscosity (Z) of a guar solution as a function of shear rate ( _g).Reprinted from ref. 5, copyright (1982) with permission fromElsevier.

Figure 2.18 A polymer gel, with permanent cross-links between the polymerstrands.


2.4.3 The Rheology of Suspensions

Ice crystals increase the apparent viscosity because they are solidparticles suspended in the matrix (cf. eqn (2.11)). However, becausethe ice crystals can interact with each other during ow (since theirvolume fraction is quite high) and because they are not spherical,the viscosity is greater than eqn (2.11) predicts. Similarly, fatdroplets in emulsions and gas bubbles in foams with a relativelylow gas-phase volume can also be considered as suspended parti-cles. Foam rheology becomes more complex at high gas-phasevolumes because bubbles interact with each other. Air bubbles andliquid fat droplets are not rigid and therefore can deform in theow. At very high shear, they can be broken up. This is importantat two points in the ice cream manufacturing process: the break-upof the fat emulsion in the homogenizer and the break-up of the airbubbles in the factory freezer (see Chapter 4).The combination of the eects of temperature, small and large

molecules in solution, and hard and soft suspended particles(whose number and size change through the manufacturing pro-cess) makes the rheology of ice cream extremely complex. This is anarea which has been, and continues to be, the subject of muchresearch. The rheology of ice cream and the techniques used tomeasure it are described in Chapters 6 and 7.

REFERENCES

1. C. J. Clarke, Phys. Educ., 2003, 38, 248.2. Guinness World Records 2002, ed. M. C. Young, Guinness

World Records, London, 2002.3. M. Mathlouthi and J. Genotelle, in Sucrose: properties

and applications, ed. M. Mathlouthi and P. Reiser, BlackieAcademic & Professional, London, 1995.

4. A.Einstein,Ann.Phys., 1906, 19, 289 andAnn.Phys., 1911, 34, 591.5. G. Robinson, S. B. Ross-Murphy and E. R. Morris, Carbohydr.

Res., 1982, 107, 17.

FURTHER READING

E. Dickinson, An Introduction to Food Colloids, Oxford UniversityPress, Oxford, 1992.

J. W. Mullin, Crystallization, Butterworth-Heinemann, Oxford,3rd edn, 1997.

40 Chapter 2

CHAPTER 3

Ice Cream Ingredients

3.1 INTRODUCTION

In this chapter, we look at the ingredients used in ice cream pro-ducts. These can be classied into three groups:

Major ingredients, present in signicant quantities (at least afew % by weight), such as milk protein, sugar, fat and water.

Minor ingredients, present in small quantities (less than about1% by weight), such as emulsiers, stabilizers, colours andavours.

Components such as chocolate, biscuits, wafers, fruit piecesand nuts that are combined with ice cream to make products.

Most ice creams also contain a signicant proportion (byvolume) of air, although this is not usually thought of as aningredient. The ingredients can be obtained from various rawmaterials: for example, milk protein and fat (and some water)could be supplied together in the form of milk or cream; alter-natively they could come from separate raw materials, i.e. skimmedmilk powder and butterfat or vegetable fat. This choice largelydepends upon the type of product required, cost and availability ofraw materials and the scale of production. Table 3.1 gives a typicalice cream composition (or formulation).




41

The total solids is the sum of all the ingredients other than water.In general, high total solids formulations give better quality icecream. In an all-natural ice cream, articial emulsiers, coloursand avours are avoided. Products such as sorbets, milk ices orwater ices contain a subset of the ingredients of ice cream. Forexample, water ices do not usually contain milk protein or fat. Atypical water ice formulation is given in Table 3.2. Concentratedfruit juice (added at a few percent) may be used instead of thecolours, avours, acid and some of the sugar.Table 3.3 gives a nutritional analysis of a 100 ml serving of a

typical ice cream. The main contributors to the energy content arethe fat (9 kcal g1), protein (4 kcal g1) and sugar (also 4 kcal g1).

Table 3.1 A typical ice cream formulation.

Ingredient Amount (% weight)

Fat 715%Milk protein 45%Lactose 57%Other sugars 1216%Stabilizers, emulsiers and avours 0.5%Total solids 2840%Water 6072%

Table 3.2 A typical water ice formulation.

Ingredient Amount (% weight)

Sugars 1424%Stabilizers, colours and avours 0.5%Citric acid 0.5%Total solids 1525%Water 7585%

Table 3.3 Nutritional analysis of a typical ice cream.

Ingredient Amount per 100mL ice cream

Total fat 7 gSaturated fat 5 gCarbohydrate 14 gSugars 13.5 gProtein 1.8 gFibre 0.5 gEnergy 530 kJ (125 kcal)

42 Chapter 3

So while the ice cream contains roughly twice as much sugar asfat, the fat provides more than half of the energy content. The fat,sugar and protein make ice cream a high energy density food.However, when consumed in typical per capita amounts (e.g. 10litres per year), it provides only a relatively small proportion of thetotal energy in the overall diet.The amounts of the ingredients vary substantially between

dierent ice cream products. Low fat (B5%) ice cream containssimilar amounts of fat, sugar and protein to a typical avouredyoghurt, whereas a high-fat, low-overrun premium ice cream mayhave substantially higher fat and energy contents. Other compo-nents, such as chocolate, wafer cones, biscuit, fruit pieces, saucesand nuts, can have a large impact. For example, a typical coneproduct contains around 60 g of ice cream, 16 g wafer, 12 g choc-olate, 5 g sauce and 5 g hazelnuts. The non-ice cream componentstogether provide about 60% of the products fat and energy.Ice cream is a good source of essential amino acids, such as

tryptophan and lysine from the milk proteins. It provides vitamins,such as vitamins A, D, E and K from milk and may also containvitamin C from fruit. Furthermore it contains minerals, in parti-cular calcium and phosphorus which are important for buildingteeth and bones, and also magnesium, potassium, sodium and zinc.Ice cream products containing specic ingredients which provide

nutritional benets have been developed in recent years. Theseinclude ice cream enriched with vitamins and/or minerals, espe-cially calcium; prebiotic or probiotic ice cream that promotesbenecial bacteria in the intestinal tract; and ice cream that con-tains polyunsaturated fats (e.g. omega-3 fats) or phytosterols.

3.2 MILK PROTEINS

Cows milk contains about 87% water. The remainder consists oflactose (4.8%), fat (4%) present as small globules, and proteins(3.5%) which are in three forms: on the fat globule surface,clustered into colloidal particles and as globular proteins in theaqueous phase, together with small quantities of inorganic salts,notably calcium and phosphate (0.29%). The exact compositiondepends on the breed of cow, the diet and the season. Fat and lactoseare discussed below in the sections on fats and oils, and sugars,respectively. The components of milk other than fat and water are

43Ice Cream Ingredients

collectively known asmilk solids non-fat (MSNF). These are usuallysupplied together in milk or skimmed milk powder.Milk contains two main types of protein: casein (80%) and whey

proteins (20%). Casein and whey proteins are distinguished bytheir solubility at pH 4.6 (at 20 1C): caseins are insoluble, whereaswhey proteins are soluble. There are 4 main casein proteins: as1-,as2-, b- and k-casein. Most of the casein proteins are presentas colloidal particles, typically 100 nm in size, known as caseinmicelles. Their natural function is to carry the insoluble calciumphosphate which is needed by mammalian young. Casein micellesscatter light; this accounts for the opacity of milk. The caseins arerelatively small protein molecules. They are very surface-activebecause one end of the molecules consists mostly of hydrophilicamino acids (such as serine and glutamic acid) whereas the otherconsists mostly of hydrophobic ones (for example leucine, valineand phenylalanine). Caseins are quite stable to heat denaturation,but they can be denatured by excessive heat, leading to aggregationand precipitation. There are also 4 types of whey protein: lactoglo-bulin, lactalbumin, bovine serum albumin and immunoglobulins.These are globular proteins which are also surface-active and arepresent in milk as a colloidal solution, 36 nm in size. They are moreheat-sensitive than the caseins, and lose their surface activity onheat denaturation. Milk also contains one further type of protein:enzymes. These are a group of proteins that have the ability tocatalyze specic chemical reactions.Milk proteins have two important functions in ice cream. Firstly,

they can stabilize water-continuous emulsions and foams becausethey are surface-active. We will see in Chapter 4 that this hasimportant consequences for the formation and stability of the airbubbles in ice cream. Secondly, they contribute to the characteristicdairy avour. Milk proteins for ice cream manufacture areobtained from several dierent raw materials:

Milk (concentrated, skimmed or whole). Skimmed milk powder. Whey powders. Buttermilk or buttermilk powder.

To produce concentrated or powdered skimmed milk, whole milkis rst pasteurized and then separated into skimmed milk and creamwhich contain 0.1% and 4850% fat respectively. The skimmedmilk

44 Chapter 3

is then concentrated to a dened composition by evaporation. Thiscan be spray-dried to produce skimmed milk powder.Whey is a cheap source of milk protein since it is a by-product of

cheese manufacture. It is often used in a powdered form. However,it has some disadvantages in ice cream manufacture. Firstly, it canincrease the amount of lactose in the formulation. At high lactoseconcentrations, the lactose can come out of solution and formcrystals which produce a sandy texture in ice cream (see Chapter 7).For this reason, it is preferable to use whey powders in which thelactose content has been reduced. Secondly, as mentioned above,whey is less heat-stable than casein and can be denatured duringthe manufacturing process, reducing its functionality.Buttermilk is a by-product of butter manufacture. Pasteurized

cream is cultured and then churned to produce butter and butter-milk. It has approximately the same composition as skimmed milk,and can also be concentrated and spray-dried. Buttermilk providesa distinctive fresh avour.The choice of source of milk protein is based on availability,

convenience and cost. Liquid products oer ease and speed oftransfer and weighing, whereas powders do not need chilled sto-rage, have a more consistent composition and also have lowertransport costs.Soy protein can be used instead of milk protein to produce ice

creams which are suitable for people who do not eat dairy pro-ducts, for example because they are lactose-intolerant.

3.3 SUGARS

Sugars are used in all types of ice cream and water ice. A wholerange of dierent molecules, such as glucose, fructose (the sugar infruit), sucrose (the sugar you have in your kitchen) and lactose (thesugar in milk) are covered by the term sugar. Monosaccharidesare the simplest group of sugars, and conform to the chemicalformula (CH2O)n. The most important group of monosaccharidesare the hexoses, i.e. those for which n 6. There are many dierentmolecules with this formula (isomers), of which the naturallyoccurring ones are glucose, fructose, galactose and mannose.When two monosaccharides are joined together, a disaccharide isformed. Naturally occurring disaccharides include sucrose, themajor soluble energy reserve in plants; trehalose, which has a similarfunction in fungi, yeasts, lichens and insects; and lactose, which, as

45Ice Cream Ingredients

has already been mentioned, is present in mammalian milk. Higheroligosaccharides (for example ranose) are formed from three ormore monosaccharides. When more than about 10 monosaccharidesare joined together, the resulting polymer is known as a poly-saccharide. Most stabilizers (which are discussed below) are poly-saccharides. The chemistry of saccharides is complex because as wellas the naturally occurring molecules there are many other isomers.Some of these are chiral, i.e. two molecules have the same structureexcept that the

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