CRYOGENICS IN FOOD PRESERVATION

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Transcript of CRYOGENICS IN FOOD PRESERVATION

ROHIT POLAS 071020045

Food Classifications1. Perishable foods spoil fastest without processing or preservation tend to be potentially hazardous foods 2. Semi-perishable foods longer shelf/storage times when held at optimum conditions

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Non-perishable foods most resistant contamination normally from improper handling

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Reasons: Microbial activities caused by yeast, fungi, bacteria etc.. Chemical and biochemical changes in food ow do the micro-organisms s oil the food? The micro-organisms are toxin in nature. They roduce toxin chemicals when they act on the food.

Low Temperature Food Preservation Reasons for use 1) slow growth / reproduction of organism 2) reduced chemical and enzymatic activity 3) easy to perform

Low Temperature Food Preservationy Chilled storage: 50F (10C) to 59F (15C) y Refrigerated storage: 32F (0C) to 45F (7C) y Freezer storage: 0F (18C) or below

Chillingy Chilling is a short term process of preservation.

Chilled foods are prepared foods which, for reasons of safety or quality are designed to be stored at or below 8C for their entire life (e.g.salads).y The optimum temperature for storage is 5C .

y Cooked-Chilled roducts re dishes which re

cooked irst, r idl chilled within inutes then stored. The roduct needs to e reheated rior to eating.y Cook-chilled roducts can onl

e stored or days & this includes the day of anufacture .

y Good for fruits & vegetables

Refrigerationy

Refrigeration slows down the biological, chemical , and physical reactions that shorten the shelf life of food.

All perishable foods should be refrigerated as so as possible, preferably during transport, to

prevent bacteria from multiplying.

Shelf-life in RefrigerationCommodity0-5 C meat 7-10 days fish 2-5 days 1 day 8 hours

Temperature20-22 C 35-38 C

1 day

5-6 hours

milk 14-21 days Fruits(most) 1 week to year Leafy veggies 3-20 days 1-7 days 1-3 days 1-20 days 8 hours 4-6 hours 1-7 days

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Freezingof preserving food. Freezing is a longer-term methodFreezing uses two controls to prevent the growth of bacteria. The liquid is converted to a solid (ice) and is therefore unavailable to bacteria. The low temperatures reduce the rate of bacterial growth. Together these controls allow food to be stored for long periods. It is important to remember that the bacteria have not been destroyed. On thawing they will become active again

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y Freezing makes water unavailable to

microorganisms.y The chemical and physical reactions leading to

deterioration are slowed by freezing.y Freezer burn: White or grayish patches on frozen

food caused by water evaporating into the package s air spaces.

y Before freezing, anti-o idants may be added to

foods to prevent fats present in the products from becoming rancid as this chemical reaction is not prevented by freezing.y During freezing the time factor is critical. If

freezing is slow, large ice crystals will form, damaging the cell walls of the food

Quick Freezingy Food quickly frozen from 0 > -18C in

appro imately 12 minutes. Creates smaller ice crystals less damage to the food. The food retains its shape texture with this method.y There is little change in the nutritional value of

foods in some cases the vitamin content could be higher than in fresh foods. Vegetables are frozen very quickly after harvesting, while fresh fruit vegetables lose ascorbic acid (vit. C) during transportation and storage

Slow freezing = large crystals

Quick freezing = small crystals

Freezing methods1. 2. 3. 4.

Freezing by contact with a cooled solid, Freezing by contact with a cooled liquid, Freezing by contact with a cooled gas, Two-phase freezing.

Freezing by contact with a cooled solid :used for foods with regular geometry (rectangular cartons, slabs such as fish sticks and fillets, meat fillets, ice cream in carton boxes, frozen chocolate bars and like)

Freezing by contact with a cooled liquid :y highest heat transfer due to better contact

of unregular surfaces.

y Possible to freeze foods individually (IQF,

individually quick frozen) rather than in the form of blocks(BF block frozen). (strawberries, peas, carrot cubes, corn etc.) much better preserved. from off-flavors, taste etc. (salt brines, syrups, glycol)

y Specific shapes of fruits and vegetables

y The heat transfer fluid must be edible, free

Freezing by contact with a cooled gasy y

used successfully for foods of irregular shape. ideal for IQF freezing in fluidized beds. Air at 20 to 40oC is used at a velocity of 6m/s for fluidization. Freezerburn (dehydration of food surface) should be avoided. Fluidized bed freezers are widely used for peas, berry fruits, cherries, potato slices, pepper-, carrot cubes, green beans and many more fruits and vegetables.

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Two-phase freezing :y

uses cryogens, a subliming solid (CO2) or a boiling liquid (CO2, N2-BP 196oC), as a cooling medium. The equipment are called cryogenic freezers. Heat transfer by phase change causes very fast freezing. Method effectively applied to foods that are difficult to freeze otherwise ( eggs, mushrooms, shrimps, prawns)

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Freezing timey Two approaches to definition of freezing time; one relevant to the quality of the product, the other to the throughout of the freezing plant. y Nominal freezing time is the time that elapses from the instant the surface of the food body reaches 0oC to the instant that the thermal center reaches a temperature 10o colder than the temperature of initial ice formation at this point. y Effective freezing time is the time the food material will spend in the freezing equipment

Factors that affect the quality of frozen foodsy

Rate of freezing: If slow, large ice crystals will form. Large ice crystals may puncture and disrupt tissue cells resulting in loss of water and structure. If rapid, ice crystals formed are small and have a fine texture. Storage conditions: Fluctuating temperatures may cause crystal growth and damaged tissues.

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Increased electrolyte concentration: May cause irreversible changes in colloidal structures and emulsions. Enlarged volume: Increase in volume during freezing should be taken into consideration when rigid packages are used. Frozen chain: Frozen state of the material should be protected until it reaches the final customer.

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Freezersy Frozen food should be trans orted home uickly, using a cool box to revent defrosting. They can be stored using the star markings on the freezer, which are a guide to the length of time they may be storedy * = one week

The freezer star ratings are as follows * Ice Box -6C * * Ice Box -12C * * * Ice Box 18C * * * * Freezer 18C

y ** = one month y *** = 3 months

Temperature Length of storage

Storage Life Of Frozen Foods

Immersion Freezingy

In immersion freezing, food is placed in a refrigerant prior to freezing. Brine is often used for fish, and a sugar solution for fruits. This provides a layer which protects the food from the dry atmosphere of the freezer. The product is immersed in this solution or sprayed while being conveyed through the freezer, resulting in fast temperature reduction through direct heat exchange .

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Direct immersion of a product into a liquid refrigerant is the most rapid way of freezing since liquids have better heat conducting properties than air. Immersion freezing systems have been commonly used for shell freezing of large particles due to the reducing ability of product dehydration when the outer layer is frozen quickly.

Cryogenic Freezingy Uses li uid nitrogen which is very cold (-1 C) Food asses through a tunnel where nitrogen gas is s rayed downwards. beefburger will be frozen in 1 minute at these extreme temperatures.y This produces small crystals, and little moisture loss.

y This method is used in freezing prawns. The prawns are first dipped in li uid nitrogen to freeze the outside layer. This prevents the prawns sticking together and from sticking to the freezer belts.

y The refrigerant consumption is in the range of

1.2-kg refrigerant per kg of the product. Typical food products used in this system are, fish fillets, seafood, fruits, berries.

Plate Freezingy Ideal for thin, flat foods such as steak, fish

fillets or burgers.y The food is pressed between hallow metal

plates, either horizontally or vertically, with a refrigerant circulating inside the plates. Pressure is applied for good contacty This speeds up the freezing process

freezing occurs evenly throughout the food

Accelerated Freeze Drying (AFD)y This is a combination of freezing and drying. Foods are quickly frozen which produces small ice crystals in the cells. The food is then placed in a vacuum under reduced pressure and heated gently.

y In these conditions the ice crystals change directly to water vapour. This is known as sublimation. y Products treated this way weigh very little, are porous and look like fresh products. They can be easily reconstituted with water y Coffee, tea, shellfish and food for camping expeditions are treated in this way

Spiral freezery Continous flexible mesh belt formed into

spiral tie carries food up through a refrigerated chamber. Spiral freezers require small floor space have high capacity(up to 3000kJ/kg ) .y Low maintaince cost

flexibity to freeze a

wide range of foods

Tunnel freezersIn tunnel freezers, the products on trays are placed in racks or trolleys and frozen with cold air circulation inside the tunnel. In order to allow air circulation, optimum space is provided between layers of trolley, which can be moved continuously in and out of the freezer manually or by forklift trucks. This freezing system is suitable for all types of products, although there are some mechanical constraints including the requirement of high manpower for handling, cleaning, and transportation of trays.

Air blast freezery commonly used due to its temperature stability and versatility for several product types. y air is used as the freezing medium in the freezing design, either as still air or forced air. y Freezing is accomplished by placing the food in freezing rooms called sharp freezers. Still, air freezing is the cheapest way of freezing and has the added advantage of a constant temperature during frozen storage, which allows usage for unprocessed bulk products like beef quarters and fish.

y slowest method of freezing due to the low surface heat transfer coefficient of circulating air inside the room. y Freezing time in sharp freezers is largely dependent on the temp. Of freezing chamber type initial temperature size of product. y Air blast freezers grouped in two categories depending on the mode of process, as either inline or batch. y Continuous freezers are the most suitable systems for mass production of packaged products with similar freezing times, in which the product is carried through on trucks or on conveyors.

Referancesy Google search y Wikipedia.org y www.fao.org y Vadlo.com

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