polysaccharides 1 1. introduction those carbohydrates that consist of more than 10 (sometimes...

PolysaccharidPolysaccharides 1es 1

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IntroductionIntroductionThose carbohydrates that consist of more than

10 (sometimes defined as >20) monosaccharide units linked via a glycosidic bond

Glycosidic bond

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IntroductionIntroductionThose carbohydrates that consist of more than 10

(sometimes defined as >20) monosaccharide units linked via a glycosidic bond

Also called glycansThey are classified as:

◦ Homo-polysaccharides Glycans composed of a single monosaccharide unit with one

or more type of glycosidic linkages Example: Starch

◦ Hetero-polysaccharides Glycans composed of more than one type of monosaccharides Examples: Gums, dietary fibers, pectinic substances,

hemicellulose

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StarchStarchSecond largest biomass on earth, after celluloseThe predominant food reserve substance in

plants where it occurs as starch granules1. Seeds (e.g. peas, grains, beans) ?2. Roots, tubers , stems (e.g. potato, tapioca, pineapple) ?3. Fruits (e.g. plantain, bananas) ?4. Leaves (e.g. tobacco) ?

One of our main carbohydrate source (Two more?)

Primary use in foods◦ Vast number of uses and functionalities◦ Naturally present in plant derived foods

E.g. contributes to the texture of bread, cooked rice, pasta and potatoes, etc.

◦ Added to foods for viscosity, gelling, moisture retaining, stabilizing, texturizing, thickening, anti-staling, film-forming, binding, foaming applications, etc. etc.

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Uses of Starch

Canning filling viscosity aid suspension aid for particulates opacity agent body for soups, sauces, puddings and gravies aseptically canned products beverages such as coffee, teas or chocolate

Cereals and Snacks hot extruded snacks chips, pretzels, etc. extruded and fried foods ready-to-eat cereals

Cooked Meat Binder water binder for formed meat smoked meats, low-fat meats pet foods (dried and canned)

Flavors and Beverage Clouds encapsulation of flavors, fats, spray dried flavors for dry beveragebeverage emulsions liquid and powdered non-dairy creamers

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Uses of Starch

Bakery pies, tarts fillings, glazes custards and icings cakes, donuts

Dressings, Soups and Sauces mayonnaise-type pourable salad dressings (high shear) spoonable dressings instant dry salad dressing mixes low-fat dressing canned gravies and sauces frozen gravies and sauces soups and chowders

Frozen Foods fruit fillings meat pies dairy products soups, sauces entrees cream-based products

Confectionery dusting powder, licorice jelly gums, hard gums panned candies

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AMYLOSE : Large linear molecule with 1-4 glycosidic bonds-Some may have a few branches

CHEMICAL STRUCTUREPolymer of D-glucose unitsTwo major forms:

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AMYLOSE : Large linear molecule with 1-4 glycosidic bonds-Some may have a few branches

Importance of structure: Determines its packing ability e.g. Retrogradation

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Linear, helical structureStrong helical complex

Glucose units

AMYLOPECTIN: Highly branched and very large molecule-Branches linked to the main chain via 1-6 bonds-Branches very close giving amylopectin a cluster structure


a (1-6) linkage

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6

9


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Helical branches

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StarchStarchSummary of the different properties of amylose and amylopectin

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StarchStarch

The structure and ratio of amylose/amylopectin varies with starch variety and age of plant

Starch granule morphology also varies with plant source

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StarchStarch

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StarchStarchDifferent sizes and shapes of starch granules from different plant sources

(A)Corn starch (B) Wheat starch (C) Potato starch (D) Rice starch

Polarized light micrographs of potato starch

“Maltese cross”

10 μm

Amylose and amylopectin inside the granules are tightly packed via extensive H-bonding-Crystalline property

Granules are insoluble in cold water

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StarchStarchDifferent starch granules have also different x-

ray diffraction patterns: A, B and C (combination of A+B)

X-ray diffraction studies show that the starch granule is a mixture of crystalline and amorphous regions◦ The crystalline region consists of starch in the form of a

double helix – very stable Hydrophobic and H-bonds Insoluble in water at room T; need energy to break the

bonds

Starch double helix

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Crystalline and Amorphous

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Maltese cross18

Mutations can induce dramatic changes in starch granule morphology and thus modified function

Pea starch granules

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StarchStarchThe aforementioned variations lead to different

functional properties, e.g.:◦ More H-bonds and packing higher gelatinization temps.

Need more energy to disrupt the double helix◦ More phosphate groups lower gelatinization T, higher

viscosity and clarity of starch paste Electrostatic repulsion due to charged P-group

◦ Amylose/ Amylopectin ratio Great effect on gelation and texture of starch pastes Waxy starch: 0-8% amylose

Disperses more easily, produces a clear viscous paste rather than gels

Normal starch: 20-30% amylose High tendency to form gels

High amylose starch: >50% amylose Very high tendency to form gels

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Gelatinization of Starch

Raw Starch Room Temp (27oC) Heating (40oC)

Swelling (50oC) Swelling (60oC) Rupturing (65oC)

Gelatinization and pasting (70-90oC)

Imploding (90oC) 21

StarchStarchGELATINIZATION When starch is heated

sufficiently (to break the hydrophobic and H-bonds) in the presence of water the granule begins to swell (loses order):◦ Granule imbibes water◦ Viscosity is increased◦ Clarity is increased◦ Some amylose may leach

out◦ Get loss of birefringence

(“Maltese cross” disappears) Starch goes from “glassy”

to rubbery state (Tg)◦ E.g. uncooked vs. cooked

spaghetti The Tg varies depending on

e.g. %H2O and starch type

Starch Gelatinization T

Corn 62-70ºC

Waxy corn

62.5-72ºC

Sorghum 68-75ºC

Potato 59-67.5ºC

Tapioca 58.5-70ºC22

StarchStarchPASTING Second stage swelling occurs

when starch is heated above the gelatinization T◦ This is needed to obtain

maximum viscosity More amylose is leached out

and it eventually gels (retrogradation – on cooling)

Eventually the starch granule will completely disrupt

The end point T needed to obtain the maximum viscosity is starch dependent

Starch End Point T

Corn 95ºC

Waxy corn 75ºC

Sorghum 95ºC

Potato 70ºC

Tapioca 85ºC

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A cereal starch gel showing how water is being trapped by the gel network

Amylose(linear) Amylopectin

(branched)

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Corn starch granules Wheat starch granules

Starch was heated at 80°C for 30 min

Amylose Amylose

Starch granule

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StarchStarch Changes in starch

viscosity are evaluated using a Brabender amylograph◦ Starch slurry is heated

(and cooled) at a constant rate with constant stirring and viscosity measured

◦ Can get a lot of useful information from the amylographs E.g if starch has high peak

viscosity then you know they need more energy to stir and manipulate

By comparing different starches you can pick the one just right for your application

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Peak viscosity

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StarchStarch

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Each type of starch has a characteristic gelatinization/pasting profile

StarchSTARCH GELATINIZATION

Gelatinization only occurs with heat and water(Dry heating = Dextrinization)

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When heating begins, water absorbed on granule surface; granules still clumped together (30oC)

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At 40oC, more water absorbed and granules start separating

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Starch

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STARCH GELATINIZATION

At 50oC, more water absorbed and granules start swelling; H-bonds within granule broken; Amylose leak out


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At 65oC, more water absorbed and granules start rupturing;


At 70oC, more rupturing and leakage, Gelatinization: Loss of birefringence (Maltese cross disappears), more viscous, translucent, enzyme action; more clarity

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At 90oC, optimum gelatinization; granules may disintegrate or implode, Overcooking could decrease viscosity !!

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Starch

On cooling, gelatinized starch retains some of its crystal structure –Retrogradation

Starch paste on cooling form intermolecular bonds – gel

Gel structure mostly from Amylose bound to each other and from slight interaction with Amylopectin molecules.

This phenomenon is called retrogradation

Critical for ultimate rigidity of final product

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StarchRetrogradation and staling

Starch granule

After gelatinization, amylose is removed from the granule

On cooling, amylose line-up and form H-bonds. They form crystals- Retrogradation

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Viscosity Gelation

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StarchStarch Ratio of amylose to amylopectin and their size is important

for the different behaviors seen:◦ Amylose

swelling greater loss of peak viscosity

◦ Amylose swelling stronger gels

◦ Larger amylose/amylopectin molecules More viscosity Weaker gels

Chemical groups and interactions with other ingredients also play an important role◦ E.g. potato starch and phosphate groups◦ Cereal starches and phospholipids

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Starch

Uses of High Amylose starch Stronger gels High gelling capacity useful for making

candies – decreases hardening times from 72 h to 24 h – reduces cost

Firm, crisp, crunchy films – coating battered products such as French fries, frozen fish, poultry, vegetables

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StarchUses of low Amylose starch

Viscous pastes with stringy texture Stabilizers and all-purpose thickeners for

food products – particularly those which undergo large temperature changes during processing

Waxy starch do not lose water during freezing and thawing – frozen food products

Emulsifier for salad dressings

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StarchStarchExamples of starch properties and

functionCereal starches (corn, wheat, rice

etc.)◦A type (low mol. weight amylose and

amylopectin with short branches) High degree of packing/crystallinity Higher gelatinization/pasting T Low swelling power (10-20 g H2O/g dry starch)

◦Viscous, short bodied paste◦Opaque gels on cooling

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StarchStarch

Examples of starch properties and function

Root and tuber starches◦B type (high mol. weight amylose and

amylopectin with long branches) Low degree of packing/crystallinity Lower gelatinization/pasting T High swelling power (400-1000 g H2O/g dry

starch)

◦Highly viscous, long bodied paste◦Weak clear gels

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StarchStarch

Examples of starch properties and function

Waxy starches (waxy corn)◦Genetically modified (bred) to contain

only amylopectin Less order and less packing (why? AP?) Higher swelling power than cereal starches

◦Very heavy bodied and stringy◦No or poor gelation

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StarchStarchRetrogradation and stalingWhen a hot starch is cooled it generally forms a

viscoelastic, firm and rigid gel or precipitate ◦ starch molecules are realigning and forming double helixes

that then will aggregate starch becomes progressively less soluble

◦ Applies to the linear portion of the starch moleculesThis phenomena is called RETROGRADATIONHappens far more rapidly with amylose vs.

amylopectin◦ During heating amylose leeches out of the starch granule

and gels on cooling High mol. weight amylose less prone to escape High conc. of amylopectin, e.g. in waxy maize, leads to

less/slower retrogradation

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StarchStarchRetrogradation and stalingRetrogradation is the cause behind firming of

bread as it cools after baking◦ Causes bread to become STALE on storage◦ First event: during baking amylose leeches out and

retrogrades almost fully on cooling positively affects texture by giving an elastic and tender crumb

◦ Second event: On storage amylopectin branches slowly associates crumb and bread hardens = STALING

◦ Happens faster during refrigeration but is inhibited (slowed down during freezing)

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StarchStarch

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Some factors affecting retrogradation1. Size of linear starch molecule

Smaller amyloses have more tendency than large to closely associate into “crystal precipitates”

2. Amylose/amylopectin ratio More amylose = more and faster retrogradation

(Why?) The branches of amylopectin participate in

retrogradation at later stages

3. Rate of cooling Fast Gel Slow Crystal precipitates

(retrogradation)

StarchStarch Some factors affecting

retrogradation4. Temperature

Cooling accelerates Freezing stops Heating reverses (e.g. bread)

5. Interfering molecules Fatty acids

Form insoluble complex with starch and interfere with retrogradation

Emulsifiers/surfactants (both hydrophilic and hydrophobic nature) Inhibit crystallization by complexing with helical amylose Commonly used in dough for bread and other baked goods to

increase shelf life

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StarchStarch

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Interactions of starch with other molecules◦ Iodine

I3- complexes with both amylose (blue color) and amylopectin (red-purple color – branches are too short) helixes

Used to determine amount of amylose in starch

Has to be linearOver 45 unitsUnder 12 units – no color

StarchStarch Interactions of starch with other molecules

◦ Fatty acids and polar lipids (e.g. emulsifiers) Bind tightly into the hydrophobic region of the helix Interfere with gelatinization and pasting

Entangle starch molecules and lead to less swelling, more opacity and “short” viscosity

Inhibit crystallization (retrogradation) Surface active lipids can coat the starch granule and make

it “waterproof”◦ Water

If too low no gelatinization (need >60%)◦ Sugars

Compete for water retard/delay gelatinization (Tg)

◦ pH Acid can cleave the glycosidic bonds less

swelling/viscosity

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StarchStarch

Starch modificationNecessary for many food applicationsNative starch is very sensitive to harsh processing and

storage conditions:◦ Low pH◦ High and low Temperature◦ Pumping◦ Storage and transportation

Modifications can be chemical, physical and enzymaticLimited number of modified starches approved by FDA

◦ Have to be labeled as “modified food starch”

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StarchStarchStarch modification1) Hydrolysis (Hydro - lysis)

◦ Use acid to hydrolyze starch into smaller units Need high heat (hot acid sprayed over starch) Limited to non-crystalline region

◦ Dark pigments and off-flavors can develop◦ Changes induced by decreased chain length

1. Sweetness increases (Why?)2. Increased reactivity as reducing sugar (Why?)3. Decreased viscosity and low peak viscosity (Why?)4. Increased solubility (Why?)5. Increased fermentability6. Less swelling on heating

◦ Get an increase in Dextrose Equivalence (DE) with increased hydrolysis DE=100/DP (Measure of reducing group) DP = Degree of Polymerization

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StarchStarch

Starch modification (Hydrolysis cont.)

◦Products of acid hydrolysis Thin boiled starch

Starch heated and sprayed with hot acid (or treated with hot HCl gas) Starch neutralized Starch washed and dried

Starch granule remains “intact” Granule breaks easily when heated in water Clearer and stronger gels than native starch

Gum candy (jelly beans, jujubes) Films and adhesives (candy and nut coatings) Processed cheese

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StarchStarchStarch modification (Hydrolysis cont.)

◦ Can also use enzymes to hydrolyze starch a – amylase

Endo-enzymes = cleave the glycosidic bond within starch in the non-crystalline regions (cleaves a- 1-4 linkage)

Products are DEXTRINS (small starch units), MALTOSE and MALTOTRIOSE

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a -amylase

StarchStarchStarch modification (Hydrolysis cont.)

◦ Can also use enzymes to hydrolyze starch -Amylase

Exo-enzyme = hydrolyzes starch at the non-reducing end

Product is MALTOSE (cleaves a- 1-4 linkage)

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b-amylase

StarchStarch


Amyloglucosidase Exo-enzyme = hydrolyzes starch at the non-

reducing end at -1-4 and -1-6 linkages Product is D-GLUCOSE (low calorie beer)

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Amyloglucosidase

StarchStarch


Iso-amylase & Pullulanase -> hydrolyze -1-6 bonds (remove branches)

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Iso-amylase

StarchStarch


◦Products of enzymatic hydrolysis Dextrins

Heated starch treated with enzymes (can also use acid)

Can produce a range of partially hydrolyzed starch polymers

Maltodextrin <20 DE Bland flavor, no sweetness Good bulking agents Good humectants/water absorbers at ~20 DE Fat-like properties at ~5 DE 1-4 kcal/g

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StarchStarch

Starch modification ( Hydrolysis cont.)

◦Products of enzymatic hydrolysis Corn syrups

A combination of enzymes used to produce corn syrups Corn syrup solids

Hot starch hydrolyzed with enzymes and then dried 20-60 DE

High Fructose Corn syrup (liquid) Gelatinized starch + enzyme dextrose (liquid glucose)

glucose isomerase Fructose (42%) + Glucose (58%) Cation exchange column high fructose corn syrup (>55% fructose) e.g. Colas

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Starch modification1) Cross linked starch2) Derivatized starch or substituted starch

Anionic (negative charges) Cationic (positive charge) Non-ionic (hydroxy alkyl starch)

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StarchStarch

Starch modification2) Cross-linked starches

◦ Most common chemical modification method◦ Can react starch with a variety of bi-functional cross-

linking reagents E.g. sodium trimetaphosphate at alkaline pH followed by

drying Get phosphate diester bond (only need a few)

◦ Results resistance to granule rupture and degradation Firmer texture and viscosity Can tolerate harsh processing conditions

Acid, shear and thermal treatment (e.g. canning) Very stable on processing and storage

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Degree of cross linking Application

Lightly cross linked Neutral and slightly acidic foods

Medium cross linked High acid foods;delay gelation for some canned foods; foods stored at low temperature;

Highly cross linked High shear, high temperature

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StarchStarch

Starch modification 3) Derivatized or substituted starches

◦ Can react OH groups of starch with various compounds to change function

A) Oxidized starches React starch with hypochlorite at pH 9-9.5 Increased whiteness and partial hydrolysis More electrostatic repulsion clearer pastes and

less retrogradation and more stability Cake flour and frying batter

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StarchStarchStarch modification (Derivatized starches cont.)

B) Phosphorylated starches Treat starch with phosphates or polyphosphates Starch gets a negative charge and thus there is

more electrostatic repulsion between the starch molecules

Lower Tg, viscosity + clarity + stability (low retrogradation)

Can control viscosity by salt addition Add salt and viscosity drops (“screens” electrostatic

repulsion)

This starch is useful in products that are to be frozen and thawed and it is an excellent thickening agent

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StarchStarch

Starch modification (Derivatized starches cont.)

C) Hydroxypropylated starches Treat starch with propylene oxide (Starch-

O-CH2-CHOH-CH3) at alkaline pH Great cold-storage and freeze-thaw

stability = clear paste with no retrogradation used in frozen foods and desserts as thickening agent

Also smooth texture and often used to improve viscosity under acidic conditions

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StarchStarch

Starch modification (Derivatized starches cont.)

D) Acetylated starches Starch treated with acetic anhydride at alkaline pH Can only be used at very low acetylation levels in foods Low Tg, improved freeze-thaw stability, low retrogradation

and good clarity

E) Succinated starches (Butanedioic acid ) Starch treated with succinic anhydride at alkaline pH Get a negative charge = repulsion (similar to

phosphorylated starch) solubility and viscosity

Can make by treating with octenyl succinic anhydride (has a hydrophobic part) Now have a hydrophilic and hydrophobic starch Good emulsifier (stabilize both water and lipid phase)

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StarchStarch

Starch modification4) Physically modified starches

A) Pre-gelatinized starch Starch paste heated in a drum dryer where

granules almost instantaneously gelatinizes and pastes paste dries off and is made into a powder

The powder is now soluble in cold water! Provides instant viscosity upon addition (no heat needed)

Applications Instant soups/puddings Pizza toppings Instant cakes etc.

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StarchStarch

Starch modification (Physically modified starches cont.)

B) Cold water soluble/swelling starch Starch (corn) heated in 70-90% ethanol

(alcohol) Swells extensively in cold water Dispersible in sugar solutions (when

stirred rapidly) Pour in molds and you get gum candy on setting Used in muffin batters to hold particles (e.g.

berries)

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StarchStarch

Starch modification (Physically modified starches cont.)

C) Resistant starch Highly crystalline starch formed by

repeated freezing and thawing (retrogradation) of high amylose starch

Non-crystalline regions removed by enzymes and the rest is resistant starch

Used as dietary fiber (non-digestive)

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polysaccharides 1 1. introduction those carbohydrates that consist of more than 10 (sometimes...

Documents

glycosidic bonds

sauces soups

bonds branches

large molecule branches

monosaccharide units

large linear molecule

sauces frozen gravies

canned flavors