preservation by use of high temperature

7/30/2019 Preservation by Use of High Temperature

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MA. EDELWINA M. BLAS PhD.


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Heat is one of the oldest methods of destroyingmicroorganisms in food processing andpreservation.

The greatest advance in food hygiene wasinadvertently made when humans discovered the

advantage of boiling, roasting, baking and otherheat treatments of food, hence preserving thefood for longer periods.

Food is also rendered safe by the application ofheat because most pathogenic microorganisms

are comparatively heat-sensitive. Some of the methods of heat treatment used for

food preservation are discussed below.


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The processing of foods by heat (or heattreatment) is the most importantconservation technique of long duration.

It aims to destroy or partially or totally inhibitenzymes and microorganisms, whosepresence or proliferation could alter the foodin question or make it unfit for human

consumption.


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The effect of heat treatment is related to thecouple time / temperature.

In general, the higher the temperature is highand the longer the term, the greater theeffect will be important.

However, we must also take into account thethermal resistance of microorganisms and

enzymes and is highly variable.


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Heat may be used either for processing or conversion offoods or simply as a means or preserving the food.

In heat processing or conversion the application of heat isused primarily to effect chemical changes in food.

Cooking, frying and baking involve both processing andpreservation operations.

Cooking makes food palatable and tender and also destroysa large proportion of microorganisms and natural enzymes.

Cooked foods can be stored for several days provided theyare protected from recontamination.


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The type of heat treatment will depend on thekind of organism to be killed, otherpreservative methods to be employed and theeffect of heat on the food.

The use of heat also affects the foodadversely and hence it is necessary to useonly mild heat treatment that ensuresfreedom from pathogens and enzyme activityand enhance the self life of the food.


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There are several techniques of food preservation byheat treatment such as sterilization, pasteurization,thermisation, cooking and blanching.

The heat sterilization involves exposing food to atemperature generally exceeding 100 C for a period

sufficient to inhibit enzymes and all forms ofmicroorganisms, including bacteria spore. Sterilization of a food is not sufficient in itself for its

long-term conservation. A subsequent contamination of food by

microorganisms environment might occur. Toaddress this, we proceed to the sterilization ofcontainer (container) and content (the food), thecontainer must be watertight and microorganisms tonot recontamination after sterilization.


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The sterilization of the food and its containercan be achieved in two ways:

The first is a simultaneous sterilization of the

container and the content (appertization). The second is to separate sterilization of the

container and the content followed (asepticpackaging).


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Appertization: Sterilization of both the containerand contents The appertization is a process of conservation is to

be sterilized by the heat of perishable goods in

containers (cans, jars, etc..) Tightly closed. Its discovery dates back to the 1790s. Nicolas Appert was the first who developed the

method of sterilization by heat in a tightly closed.

The appertization is widely used today for long termfood of animal or vegetable origin. The shelf life of foods appertiss is several months

to several years.


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Sterilization of the container and separate the content In this case, the food product (content) is sterilized by heat

treatment before being enclosed in its container. The latter is also sterilized by heat or by other processes

(by ultra-violet, for example), but prior to contain theproduct.

Then, the content is sterilized sealed in its packaging(container), and sterilized.

The process of conditioning takes place in an enclosurewhich prevents the contamination of the product by themicroorganisms in the environment:

This is theaseptic packaging.

This technique is generallyused for the conservation of liquid products (milk, juice,etc..)

In packages that can not stand the appertization as plasticbags and cartons Tetra Brik type.


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When the sterilization of the product iscarried out at high temperature (135 C to150 C) for a short duration (15 sec. To 1sec.).

We talk about sterilization UHT (Ultra HighTemperature). This technique has the advantage of

preserving the organoleptic and nutritional

quality of the product sterilized. However, it can not be used in the case of

liquid products like milk.


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Pasteurization is a heat treatment sufficientto moderate and the destruction ofpathogenic microorganisms and a largenumber of microorganisms of alteration.

The temperature of treatment is generallyless than 100 C and lasts a few seconds toseveral minutes.

The following table provides illustrativeexamples of scales ofpasteurization.


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Sample Schedule of pasteurization

Products Pasteurization temperature Duration treatment

Fruit juices (pH 4.5)

65 C 30 min

71 C 10 min

80 C 15 s


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All microorganisms are not eliminated bypasteurization, the heat treatment must befollowed by an abrupt cooling.

Pasteurized foods are usually kept in cold (4 C)

to slow the development of germs still present.Their shelf life is so limited.

Outside of refrigeration, other means ofconservation can be used in parallel to counter

the growth of microorganisms survivors, such asthe addition of chemical preservatives, reducingwater activity (aw), acidification, etc..


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Pasteurization is named after its inventor,Louis Pasteur, a French chemist.

Pasteurization is a process of heat treatmentof milk, beer and some other beverages. Itrequires sufficient holding time to assure thethermal destruction of pathogens andorganisms responsible for spoilage, withoutaltering the nutritional value. It involvesheating the food to a specific temperature fora specific time and then cooling rapidly.


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Pasteurisation kills most but not all of the microorganismspresent It is a very useful method when more rigorous heat treatment

could harm the quality of the product, as in the case of milk, andwhen the aim is to kill only the pathogens that are not very heat-resistant.

The temperature applied and the holding time of pasteurisationvary with the equipment available and the type of food product.

In milk pasteurisation, the time-temperature combination isselected on the basis of the thermal death time of the mostresistant pathogens (TB bacilli) that may be present in raw milk,and the maximum temperature and time at which the taste,palatability and nutritive value of milk are maintained.

Normally milk is pasteurised at 62.8C for at least 30 minutes orat 71.7C for at least 15 seconds, or, if using ultra-hightemperature (UHT), at 135C for 12 seconds.

UHT milk is sterilised, meaning all forms of life are destroyed.This extends its storage time but does affect the taste.


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Thermisation is a heat treatment applied to raw milk.The latter should be brought to a temperature of atleast 63 C for 16 seconds.

Thermisation is a weakened form of pasteurization. Its main objective is the destruction of pathogenic

bacteria that may be found in milk, all withoutaltering its technological characteristics. However, this treatment may decrease the microbial

load banal milk (lactic acid bacteria, bacteria refining,etc.)..

That charge is generally used for the preparation ofraw milk cheese. In this case, we must appeal tobacteria grown in the laboratory to replace thosemissing.


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Is a heat treatment of a few minutes at 70 Cto 100 C to destroy the enzymes that mayaffect the vegetables or fruit before furtherprocessing (freezing, drying, etc.)..

In fact the destruction of enzymes is oneobjective among many others and the role ofblanching as a pretreatment before drying,lyophilization, or freezing appertization.


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Before drying or freeze-drying, blanching is used as apretreatment for the destruction of enzymes responsible fororganoleptic changes such as changes in flavor and / orcolor.

The treatment also allows the reduction of microbial load andfacilitates rehydration.

Before appertization, blanching is not intended to destroy theenzymes when the scales applied widely enough for theirdestruction. In this case,it has other roles:

Removal of occluded gas in the tissues before casing,otherwise their presence leads to an internal overpressure,with a risk of bending or flochage boxes.

Fill hot: It generally takes the precaution to fill the hot juiceboxes so that crimping (or capping) will take place inatmosphere of steam and the air is so driven.

Blanching can follow the hot vegetables to prevent a coolingof the juice in contact with vegetables which are cold.


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Blanching Control The effectiveness of blanching can be controlled

by testing the inactivation or the presence of twoenzymes widely distributed in plants: catalase

and peroxidase. Catalase is an enzyme oxydative resulting in theformation of oxygen from hydrogen peroxide:

2 H2O2 2 H2O + O2 The activity of catalase is highlighted by grinding

the product can be milled and mixed withcalcium carbonate and hydrogen peroxide: anoff-gas (O2) indicating a catalase activity.


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Peroxidase is an enzyme that catalyzes theoxidation with hydrogen peroxide of varioussubstrates:

H2O2 + 2 AH 2H2O + 2 A

The activity of peroxidase is revealed by theappearance of a brown color instead of blackwhen guaiacol and hydrogen peroxide in contactwith the product milled.

Peroxidase is considered the most heat-and the

effectiveness of blanching can be evaluatedbased solely on the test inactivation or presenceof this enzyme.


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Elimination of false taste: Cauliflowers notblanched retains a bitter taste intolerable.

Elimination disorders: Blanching helpseliminate troubles such as those made byextracting starch.

Before freezing, in addition to thedestruction of enzymes, the removal of

occluded gas in the tissues and consequentlylimit the phenomena of oxidation.


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Cooking is a heat treatment of foods to make

them consumable. Its main objective is the development of the

organoleptic characteristics of the product:

Improving the taste, odor, color and texture. According to the scales used, the cooking may be

associated with a substantial reduction or evenelimination of the microbial load present on theproduct.

Cooked products can be stored in therefrigerator for a few days, and in the freezer fora few weeks.


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There are five ways of cooking basic foodscan be dipped in a liquid such as water, broth(flavored or not) or wine (poaching, boilingbroth or stew), immersed in animal fat or oil

(frying), exposed to steam (steam cookingand to some extent, braising) or dry heat(roasting, baking, cooking on the grill) oreven stoves in small quantities of hot fat(skipped).


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Although the spores of Clostridium botulinum, whichcauses botulism, are extremely heat-resistant, the toxinproduced by this organism is readily destroyed by boiling.

However, some toxins produced by other bacteria such asstaphylococci are not easily inactivated.

Thermophilic (heat-loving) organisms may survive the

effects of boiling and can cause food spoilage ifenvironmental conditions are favourable for them. Bacterial destruction by heat is affected by time and

temperature variation. The higher the temperature, the more rapid is the

destruction. On the other hand, as the temperature is lowered, the

time of exposure (holding time) needs to be longer.


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However, cooking will not sterilize a product.

Cooking also destroys the toxin formed by Clostridiumbotulinumduring a ten minute exposure of the food to moistheat at 100 degree C.

Thus cooking provides a final measure of protection forconsumer form food borne diseases.

The killing of microorganisms by heat is due to thermaldenaturation of protein and enzymes of the microorganismrequired for its metabolic activity and growth.

The heat treatment necessary to kill the organisms or sporesvaries with the kind of organism, its state and theenvironment during heating.


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Cooking can have some disadvantages. It can damage the foods appearance, texture

and flavour, and may also destroy someimportant vitamins.

Nevertheless, the advantages of cookingoutweigh the disadvantages because itinhibits spoilage and possible disease

transmission.


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Boiling is the process of applying heat to water untilthe temperature reaches about 100C. Boiling foods in water cannot completely destroy all

microorganisms, but the vegetative cells of bacteria,yeasts and moulds are generally quickly destroyed at

temperatures of 100C or above. Spores of some bacteria are extremely resistant to

heat and are not killed at this temperature, althoughtheir growth is prevented.

For this reason, boiling food can rarely be relied uponto ensure complete destruction of all organisms.

However, most pathogens are killed, provided thatsufficient exposure time is maintained.


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Canning is one of the most widely used modern

methods of processing and preserving food. It involves the careful preparation of food packed

into a sealed tin, glass or plastic container which

is subjected to defined high temperatures (above100C) for an appropriate period of time, andthen cooled.

Following the thermal (heat) processing, thesealed container must be cooled immediately to a

temperature of about 38C to preventunnecessary adverse effects of heat on thetexture, flavour or colour of the food.


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The canning method involves the following steps:sterilising the food to be canned, packing it insterile, air-tight stainless metal, glass or plasticcontainers, and then hermetically sealing (i.e.with a complete, airtight seal) the containers to

prevent contamination during handling andstorage. In the heat process, all vegetativebacteria are destroyed and spores cannot grow.

Any can that is damaged or swollen should notbe used. A swollen, bulging can indicates that

gas is being produced on the inside anddemonstrates there is microbial activity in thefood, so it would not be safe to eat.


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Canning is the process of applying heat tofood thats sealed in a jar in order to destroyany microorganisms that can cause foodspoilage.

Proper canning techniques stop this spoilageby heating the food for a specific period oftime and killing these unwantedmicroorganisms.

During the canning process, air is driven fromthe jar and a vacuum is formed as the jarcools and seals.


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Water-bath canning: This method, sometimesreferred to as hot water canning, uses a largekettle of boiling water.

Filled jars are submerged in the water andheated to an internal temperature of 212degrees for a specific period of time.

Use this method for processing high-acidfoods, such as fruit, items made from fruit,pickles, pickled food, and tomatoes.


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Pressure canning: Pressure canning uses alarge kettle that produces steam in a lockedcompartment.

The filled jars in the kettle reach an internal

temperature of 240 degrees under a specificpressure (stated in pounds) thats measuredwith a dial gauge or weighted gauge on thepressure-canner cover.

Use a pressure canner for processingvegetables and other low-acid foods, such asmeat, poultry, and fish.


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Dont confuse a pressure canner with a pressurecooker, which is used to cook food quickly. Apressure cooker does not have adequate roomfor both the canning jars and the water needed tocreate the right amount of pressure to preserve

foods. Older canning methods are unreliable and, forthat reason, arent used or recommended todayfor home-canning. Occasionally, these methodsare revived as being faster and easier than

water-bath or pressure canning, but using othermethods is like playing Russian roulette withyour food safety.


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preservation by use of high temperature

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