Low Temperature Food Preservation

8/3/2019 Low Temperature Food Preservation

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Preservation is based on the fact that activities

of the microorganism can be slowed down at

temp above freezing and generally stopped at

subfreezing temp.

As a rule of thumb, for every 10oC temperature

change, the rate of reaction changes by a factor

of 2 to 3.


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Low temperature food

preservation


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Definitions

Psychrophies : grow b/w 0-20C , optimum

range of 10-15C.

Psychrotroph: grow at 5C or below

Three distinct temperature ranges for low

temp.stored foods:

Chilling temperature: 5-7C to 10-15C,suitable for storage of certain vegetables and

fruits


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Refrigerator temperature: 0-7C

Freezing temperature: at or below -18C

Chilling for preservation is aimed to extend the shelf-life or keeping quality of a living produce, e.g., fruitsand vegetables.

This is mainly due to reduction of metabolic activity atlow temperature.

Chilling may also be used to extend the storage life ofproducts such as pasteurized and sterilized cannedfoods.


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Chilling for development of desired biologicaland biochemical processes includesfermentation of beer and wine, meat ageing,

and processing of many dairy products such ascheeses, etc.

Temporarily changing certain physico-chemicalproperties to facilitate some processingtreatments is used in the processing of bakeryproducts, chocolate, butter, margarine etc.


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Refrigeration is the most common method of chilling orlow temperature storage of fruits and vegetables.Refrigerated storage helps to retard the spoilage inperishable crops in the following manner:

aging due to ripening, softening, textural and colourchanges;

undesirable metabolic changes and respiratory heatproduction;

moisture loss and the wilting; spoilage due to invasion by bacteria, fungi, and yeasts;and

undesirable growth, such as sprouting of potatoes.


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One of the most important functions of refrigeration is tocontrol the respiration of crops

Respiration generates heat as sugars, fats, and proteins inthe cells of the crop are oxidized.

The loss of these stored food reserves through respirationmeans decreased food value, loss of flavour, loss of saleableweight, and more rapid deterioration.

The respiration rate of a product strongly determines itstransit and post-harvest life.

The higher the storage temperature, the higher will be therespiration rate.

Apart from temperature, humidity and concentration ofgases are also important.


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. The principal refrigeration requirements of fruits and

vegetables are controlled low temperature, air circulationand humidity.

Relative humidity (RH): Most food stores are best atrefrigeration temperatures when the relative humidity ofair is between 80 and 95 per cent. This is generally relatedto moisture content of foods. Celery, spinach and severalother crisp leafy vegetables require 90-95% RH.

Controlled low temperature: Fruits and vegetables arehighly perishable because they respire and produce heatat varying rates. They, therefore, need to be stored at lowtemperatures.

Refrigeration is the gentlest method of extending the shelflife of fruits and vegetables. By and large, it has relativelyfew adverse effects upon taste, texture, nutritive valueand overall changes in food.


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Domestic refrigerators usually run at 4.47.2C (40-45F).

Properly designed refrigerators, refrigerated storage roomsand warehouses provide sufficient refrigeration capacityand insulation to maintain the room within about 1C.

Proper insulation is must to maintain the optimum storagetemperature. Refrigeration system must be of appropriatesize to handle maximum expected heat load as anundersized system will allow temperature to rise duringpeak heat load conditions.

Different fruits and vegetables evolve different heat duringrespiration. green beans, broccoli, sweet corn, green peas,spinach and strawberry evolve high heat during respiration.These products are difficult to store.


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Based on their respiration rate, the storagerequirement and shelf-life of fruits and vegetables isalso different

While temperature is the primary concern in the

storage of fruits and vegetables, relative humidity isalso important.

The relative humidity of the storage unit directlyinfluences water loss in produce.

Water loss can severely degrade quality-for instance,wilted greens may require excessive trimming, andgrapes may shatter loose from clusters if their stemsdry out.

Water loss means saleable weight loss and reduced

profit.


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Most fruit and vegetable crops retain betterquality at high relative humidity (80 to 95%), butat this humidity, disease growth is encouraged.

The cool temperatures in storage rooms help toreduce disease growth, but sanitation and otherpreventative methods are also required.

Maintaining high relative humidity in storage is

complicated by the fact that refrigerationremoves moisture.

Humidification devices may be used.


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Air circulation: Air must be well circulated in the coldstorage rooms.

This will help more heat away from food surface to coolingcoil.

Air, that is, circulated within a cold storage must not be toomoist or too dry.

If it is high in humidity, moisture will condense on surfaceof cold foods and moulds will grow on these surfaces.

On the other hand, if air is too dry, it will cause excessive

drying out. Most food stores are best at refrigerationtemperatures when the relative humidity of air is between80-95% for prolonging the storage period.

Foods can be protected from losing excessive moisture byusing several packaging methods. This forms a barrier for

migration of moisture from food to storage temperature.


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Preparation of foods for freezing


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Blanching: two methods, immersion of foods intohot water, use of steam.

Functions of blanching:

Inactivation of enzymes Enhancement or fixing of green color of

vegetables

Reduction of m.o. Facilitating the packing of leafy vegetables

Displacement of entrapped air in plant tissues


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Freezing of foods and its effects

Quick freezing: is the process by which the

temperature of foods is lowered to about -

20C within 30 min.

May be achieved by direct immersion or

indirect contact of foods with the refrigerant.

Slow freezing : process whereby desired

temperature is achieved with in 3 to 72 hrs


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Indirect contact freezers

immersion freezer

Direct contact freezer.


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Slow freezing favors

large extracellular

crystals

Cause cell damage bydisrupting cell

membrane, cell walls

and internal structures

to the point that afterthawing product is

different from original


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Storage stability of frozen foods

For any type of food preservation method, theretention of nutritional components is aconcern, but freezing is probably the least

destructive when properly done Fruits - Most frozen fruits will maintain a high

quality for 8 to 12 months.

Vegetables - Most vegetables will maintain ahigh quality for 12 to 18 months at -18 C orlower.


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. One of the most common forms of quality

degradation due to moisture migration infrozen foods is freezer burn, a conditiondefined as the glassy appearance in somefrozen products produced by ice crystalsevaporating on the surface area of a product

The grainy, brownish spots occurring on theproduct cause the tissue to become dry andtough and to develop off-flavors.

This quality defect can be prevented by usingheavyweight, moisture proof packagingduring the freezing process


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Microorganisms differ significantly in their sensitivity tofreezing, thus the main concern about the microbiologicalaspects of freezing is the growth of organisms duringthawing rather than during freezing

the freezing process does not significantly destroy themicroorganisms that may be present in fruits andvegetables

sufficient number of survivors are still present that canmultiply and cause spoilage of the product during thawing.

Fluctuation in storage temperature is one of the mainreasons for microbial deterioration of frozen productsduring storage.

Thus, a careful inspection of frozen products is essential toensure proper freezing storage with constant temperatures


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Effect of freezing on microbial cell

Water that freezes is called free water. Free waterforms ice crystals. Slow freezing- extracellular, fast-intracellular,depletion of free water-dehydration

Inc. in viscosity of cellular matter

Loss of cytoplasmic gases

Changes in pH,

conc. Of electrolytes

Alteration of colloidal state

Denatuartion of protein

Temp. shock to m.o.

SOME CHARACTERISTICS OF


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SOME CHARACTERISTICS OF

PSYCHROTROPHS AND

PSYCHROPHILES There is an increase in unsaturated fatty acidresidues. The usual lipid content of mostbacteria is between 2% and 5%, most or all of

which is in the cell membrane. Many psychrotrophs synthesize neutral lipids

and phospholipids containing an increasedproportion of unsaturated fatty acids whengrown at low temperatures compared withgrowth at higher temperatures.


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The widespread occurrence of low-temperature-induced changes in fatty acid composition suggeststhat they are associated with physiological

mechanisms of the cell. It is known that an increase in the degree of

unsaturation of fatty acids in lipids leads to adecrease in the lipid melting point.

Psychrotrophs synthesize high levels ofpolysaccharides.

Pigment production is favored.

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Low Temperature Food Preservation