FOOD PRESERVATION

COMMERCIAL PRESERVATION

© PDST Home Economics

COMMERCIAL FREEZING

• Blast Freezing: Food passed through tunnel on conveyor belt and cold air (-30ºC) is blown over it. Suitable for most food.

• Plate or Contact Freezing: Food is pressed between 2 cold metal surfaces. Used for thin foods e.g. burgers, fish fillets or fingers, chops.

                                                   

COMMERCIAL FREEZING

• Flow Freezing: Food passes through a tunnel on a perforated conveyor belt. Freezing air is blown from underneath and freezes each piece of food separately so they don’t stick together e.g. peas, berries etc.

COMMERCIAL HEAT TREATMENTS

• Canning/ Bottling, Pasteurisation, Sterilisation and UHT

CANNING / BOTTLING

• Fruit, vegetables, meat, fish, soup, stew.• Principles: (a) Heat destroys enzymes and microbes (b) Sealing in an airtight container prevents

recontamination.• Method: Preparation of food - Vegetables blanched &

meat cooked. Cans filled with food + liquid (syrup, brine, sauce). Air removed. Air tight seal. Sterilised. Cans cooled & labelled.

CANNING / BOTTLING

• High acid foods: e.g. Fruit need fast heat treatment - 100°C for <30 min.

• Low acid foods: e.g. Meat need higher temp and longer time - 115 °C for >30 min.

• Cans are lacquered on inside to prevent reaction between food and metal.

• Aseptic canning :Food and cans are sterilised separately at ultra high temperatures. The cans are filled, sealed, cooled and labelled. High temp means less time involved so less damage to food. Also used for large cans of food.

CANNING / BOTTLING

• Don’t buy bulging, dented or rusted tins.

• Remove food from can once opened and use quickly.

• Bottling same as canning except glass containers used e.g. ketchup, sauces, mayonnaise.

COMMERCIAL CHEMICAL PRESERVATION

• Chemical preservation means the addition of preservatives to food to prevent enzyme and microbial activity.

• Chemicals used are: salt, sugar, sulphur dioxide, sorbic acid, antioxidants, alcohol and acid e.g. vinegar.

PRINCIPLES OF CHEMICAL PRESERVATION

• Most chemicals work by dissolving in water of the food cells forming a concentrated solution. Microbial cells are dehydrated by osmosis and die.

• Alcohol denatures the protein in the microbial cells and destroys them.

• Acids lower the pH of the food making it an unsuitable environment for enzyme or microbial activity.

COMMERCIAL DEHYDRATION• Principle: Microbes need water. Dehydration

removes the water from the food stopping microbial growth.

• Methods: Sun drying e.g.tomatoes, Spray drying

e.g.milk, Roller drying e.g. breakfast cereals, Accelerated freeze drying (AFD)

AFD=Food frozen @ low temp. Tiny ice crystals then evaporated in vacuum chamber (sublimation).

Advantages: Lightweight foods. Less heat needed so better flavour & nutritive value. Longer shelf life e.g. coffee, fruit, vegetables, meat.

(H.L) IRRADIATION Radiation is used to destroy micro-

organisms in food

Advantages

• Destroys microbes and other parasites.

• Longer shelf life• Slows ripening and

sprouting• No effect on colour,

flavour, texture• No need for chemicals

Disadvantages

• Not suitable for high fat food- rancidity

• Loss of vitamins• High levels of

radiation needed for some microbes

• Public are concerned• May be used to

disguise poor products

(H.L) EFFECTS OF PRESERVATION

Freezing• Microbes and enzymes

inactivated• Some loss Vit. B1 and C

on thawing• Loss of texture

especially if frozen slowly

• Possible freezer burn

Canning• Microbes and enzymes

destroyed• Vit. B and C destroyed

by heat• Food cooked• Loss of colour, flavour &

texture• Canning liquid affects

nutritive value- salt, fat or sugar content

EFFECTS OF DRYING ON FOOD

• Microbes destroyed

• Water removed

• Water soluble Vit. B & C lost

• Changes in shape, weight, colour, texture of food