What is freezing?
Freezing of fruits and vegetables with the help of freezing to prevent microorganism growth and therefore the action of food enzymes greatly simple-minded.
Freezing as a preservation technique most likely was observed by prehistoric people throughout cold weather; and, till frozen storage cabinets were developed within the late 1800s, present snow and ice were wont to freeze foods outside. So we discuss about the Freezing of fruits and vegetables in this article.
With the event of mechanical refrigeration and of quick-freezing technical school niques, the frozen foods business has expanded rapidly. Even in homes, state change of foods has currently become common as a result of home deep-freezers are pronto available in a position.
Under the same old conditions of storage of frozen foods microorganism growth is prevented fully and therefore the action of food enzymes greatly simple-minded.
The lower the storage temperature the slower are the speed of a chemical or protein reaction, however most of them can still continue at any temperature.
There are varied strategies of freezing
(1) Sharp state change (Slow freezing)
This technique, 1st used in 1861, involves state change by circulation of air. Either naturally or with the help of fans. The temperature might vary from -15 to -29°C and state change might take from 3 to 72 hours.
The ice crystals shaped are large and rupture the cells. The thawed tissue cannot be ready to again its original water content. the primary products to be sharp frozen were meat and butter.
Today electric refrigerator rooms are maintained at -23 to -29°C or perhaps lower, in distinction to the earlier temperature of -18°C.
(2) Fast freezing used as freezing of fruits and vegetables
In this method the food attains the temperature of most poudrin formation (0 to -4°C) in 30 minutes or less. Such a speed ends up in formation of terribly little ice crystals and thus minimum disturbance of cell structure. Most foods are fast frozen by one in every of the subsequent 3 methods:
(A) By direct immersion
Since liquids are smart heat conductors, food is frozen apace by direct immersion in a very liquid like brine or sugar answer at coldness. Berries in sugar answer, packed fruit juices and concentrates area unit frozen during this manner.
The refrigeration medium should be edible and capable of remaining liquescent at -18°C and slightly below. Direct immersion equipments like Ottesen Brine freezer, Zarotschenzeff “Fog” freezer, T.V.A. Freezer, Bartlett electric refrigerator, etc., of business importance earlier, aren’t used these days.
(i) There’s excellent contact between the refrigerant medium and therefore the product, thence the speed of warmth transfer is very high.
(ii) Fruits are frozen with a coating of syrup that preserves the color and flavour during storage.
(iii) The frozen product isn’t a solid block as a result of each piece is separate.
(1) Brine could be a sensible refrigerant medium however it can not be used for fruits.
(ii) It’s tough to create a syrup that may not become viscous at low temperature.
(iii) The refrigeration temperature should be carefully controlled, as at high temperature the medium can enter the product by diffusion and at low temperature the medium might freeze solid.
(iv) It’s terribly tough to keep up the medium at an exact concentration and additionally to stay it free from dirt and contamination.
(B) By indirect contact with refrigerant
Indirect phase change could also be defined as phase change by contact of the product with a metal surface that is it self cooled by phase change brine or different refrigerant media.
This is often an previous method of phase change in which the food or package is unbroken up-to-date with the passage through which the refrigerant at -18 to -46°C flows.
Know Automatic Package deep freezer, Patterson Continuous Plate deep freezer, FMC Continu ous will deep freezer and Birdseye Freezers are supported this principle.
(C) By air blast (Air blast freezing)
This refers to vigorous circulation of cold air so as to freeze the product. Temperature reduction is completed by inserting the foodstuffs on trays or on a belt that are then passed slowly through an insulated tunnel containing air in it.
Here the air tem perature is just about -18 to -34°C or perhaps lower. This method is economi cal and a spread of sizes and shapes will be accommodated.
(a) Fluidized bed freezing
This is a modification of air blast freezing. The foodstuff is fluidized to make a bed of particles, so frozen. Air is forced upward through the belt to part raise or suspend the particles.
If the air is appropriately cooled, drying will be done quite rapidly. The depth of the bed of particles are various with the product. Solid food particles of the dimensions of peas upto strawberries will be frozen with a depth of 1 to degree Celsius.
Peas and whole kernel corn are simply fluidized particles and therefore the bed depth used is slightly over one in green beans/french beans square measure part fluidizable products, and therefore the depth is to 10 inches.
Fluidized bed freezing has bound advantages: it offers additional sufficient heat transfer and additional fast rates of freezing. Extent to that the product gets dehydrated is a smaller amount, and defrosting of equipment is needed less often.
(b) Plate freezing
In this method, food products are placed in-tuned with a chilly surface. The cooling temperature of the metal surface area is maintained by using cold brine or vaporizing refrigerants.
This method is suitable for packaged food products which can rest on/slide against or be ironed between cold metal plates. The method is additionally appropriate for unpacked foodstuffs, e.g., shrimps, which might be frozen by freeze adhesion to a slowly rotating cold drum.
(3) Refrigerant freezing used as freezing of fruits and vegetables
Although most foods retain their quality once fast frozen by the higher than ways, a couple of (mushrooms, sliced tomatoes, whole strawberries and raspber ries) need ultrafast temperature reduction.
Such materials are subjected to refrigerant temperature reduction that is outlined as temperature reduction at terribly coldness (below -60°C). The refriger ants used at this time in refrigerant temperature reduction square measure nitrogen and liquid greenhouse emission.
Within the former case, temperature reduction is also achieved by
(i) immersion within the liquid,
(ii) spraying of liquid, or
(iii) circulation of its vapour over the product to be frozen.
This is a method wherever temperature reduction is preceded by partial dehydration. in case of some fruits and vegetables regarding 50 per cent of the wet is removed by dehydration before freezing.
This has been found to boost the standard of the food. Dehydration doesn’t cause deterioration and dehydro-frozen foods are comparatively a lot of stable.
(5) freeze-drying used as freezing of fruits and vegetables
In this method food is 1st frozen at -18°C on trays within the lower chamber of a freeze drier and also the frozen material dried (initially at 30°C for 24 hours then at 20°C) under high vacuum (0.1 mm Hg) within the higher chamber.
Direct sublima tion of the ice takes place while not passing through the intermediate liquid stage. the product is highly absorbent , glorious in style and flavour and might be reconstituted promptly.
Mango pulp, fruit juice concentrate, edible fruit juice and guava pulp are dehydrated by this methodology.
Changes during freezing
Quick-freezing rapidly slows down chemical and catalyst reactions in foods and stops microbic growth. the same result is made by sharp temperature reduction, however less rapidly. The physical effects of temperature reduction are of nice importance.
There’s an enlargement in volume of the foodstuff and ice crystals kind and grow in size. These crystals are larger in slow temperature reduction than in fast temperature reduction and a lot of ice accumulates between tissue cells and should crush the cells.
Throughout temperature reduction, water is decentralized in food by the formation of ice crystals.
Changes during storage
Chemical and physical reactions that decrease overall quality continue even in foods hold on at 0°F (-18°C). The losses that occur throughout a traditional storage amount sometimes exceed any harm that happens to the food throughout the initial freez ing or thawing.
(A) Physical changes occur during freezing of fruits and vegetables
Fluctuation in storage temperature leads to a rise within the size of ice crystals leading to physical harm to the food.
Recrystallization may be a natural process within which several little ice crystals com bine to create a smaller range of enormous crystals. Temperature fluctuations throughout storage and longer storage times enhance recrystallization.
Recrystallization additionally happens within the early stages of thawing wherever it usually damages plant cells that were left intact throughout the initial phase change method. the top result’s cut quality of product that had been properly frozen.
Desiccation of the food at its surface is probably going to require place throughout storage once water goes from the solid to the vaporized state while not passing through the liquid part as against the means ice would commonly soften.
If placed during a glass of water, it’s referred to as sublimation. fridge|electric refrigerator|fridge} burn is caused by sublimation of ice from the surface of the food into the air within the freezer.
Protein alterations liable for a decrease in solubility are jointly referred to as denaturation. because the ionic concentration will increase in phase change, through a discount in water content
The proteins type bonds with one another rather than water, and insoluble complexes result. this transformation (protein denaturation) is also liable for magnified toughness in frozen foods.
(B) Chemical changes occur during freezing of fruits and vegetables
Microbial growth ceases at temperatures below 28°F (-2°C) for many foods, however chemical reactions continue in foods even at 0°F (-18°C). Such reactions embrace lipoid oxidization.
Maillard and catalyst browning, flavour deterioration, macromolecule insolubilization, and degradation of chlorophyl and vitamins. Quality changes, not safety (microbial) changes, are the explanation for food product discard.
Plant and animal tissues and gels-this encompasses most foods-thaw a lot of slowly than they freeze. It takes longer to thaw a food than to freeze it as a result of the heat transfer is by physical phenomenon.
Throughout phase change, the outer surfaces contain ice crystals initial, followed by the middle parts. In thawing, the outer parts initial modification from solid ice to liquid water followed by melting of the central parts.
At first the temperature rise is fast, however this solely happens before abundant of the outer surface has modified to the liquid state; most foods stay solid till 23°F.
Nonflowable water may be a higher dielectric than ice. A upland stage follows that is that the results of this unflowering water impairing heat transfer.
Once thawing is completed with microwaves, physical phenomenon isn’t the main technique of warmth transfer and this scenario doesn’t apply.
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