LESSON 7 Flashcards
Refrigeration temperatures
-2°C to 16°C.
Effects of 10°C drop in temperature
slows down rate of senescence by 2 to 3 times and microbial growth by 3 to 6 times.
Controlled atmosphere
The atmosphere surrounding a food product is different from that of the normal atmosphere, and the composition of the atmosphere around the product is constantly monitored and maintained at preset levels.
Freezer temperatures
Commercial freezing requires a minimum of -18°C. Household freezers only reach temperatures of -12 to -14°C
Freezing principles
- lower temperatures used (remember that microorganisms can not grow well at temperatures below -9.5°C)
- lower water activity (by freezing the “free” water present in the food)
Changes in food during freezing and thawing
- Chemical changes such as oxidative rancidity or oxidation of flavour components, pigments and vitamins.
- Enzymatic reactions such as enzymatic browning or lipolytic rancidity.
- Meats become tougher due to protein denaturation by chemical effects and cell breakage by ice crystals
Five factors of freezing rate
1 Food composition: Some food components such as proteins and fats act as insulators. Presence of these components slow down the freezing.
2 Temperature difference: the greater the temperature difference between the food and the refrigerant, the faster the freezing rate.
3 Product thickness/geometry and heat transfer rate: the thinner the food piece or greater the heat transfer rate, the faster the freezing rate.
4 Air velocity: the greater the velocity of refrigerated air or circulating refrigerant, the faster the freezing rate.
5 Degree of contact: the more contact between the food and the cooling medium, the faster the freezing rate.
These five factors are given great attention in the
Air blast freezing
Air blast freezing is a moderately fast freeze because of vigorous circulation of cold air. The product is placed on trays or mesh belts and passed slowly through an insulated tunnel. In different systems the temperature may range from -18°C to -34°C, with an air velocity of 100-3500 lineal feet per minute, with a counter current air flow.
Fluidized-bed freezers
frozen as individual pieces, not stuck together. solid particles ranging in size from peas to strawberries are being exposed through a movement of the cold air (-20 to -34°C) at high velocity as they pass along a conveyor belt. This will impart a vibratory motion to food particles, accelerating the freezing rate. The cold air being forced upward through the bed lifts and suspends the food particles, thus fluidization occurs.
Indirect contact freezing
food is placed on belts or trays and a refrigerant circulates through a wall beside the food. As the food comes into “contact” with the cold wall, it quickly cools down and freezes.
Immersion freezing
the immersion of packaged or un-packaged food products directly in a non-toxic refrigerant fluid. The refrigerant fluids commonly used are propylene glycol, glycerol, sodium chloride, calcium chloride, and mixtures of salt and sugar.
Cryogenic freezing
Un-packaged or thinly packaged foods are exposed to extremely cold freezant. In contrast to the liquid immersion freezing, heat removal is accomplished during a change of state by the freezant.
Advantages and disadvantages: Air blast freezing, IQF
PRO: Economical, Can freeze various sizes and shapes of food, IQF has more efficient heat transfer, increased rate of freezing.
CON: Possible excess dehydration (freezer burn),
Undesirable bulging of the packages (by expansion of the product) may occur, Non-uniform products can not be fluidized (IQF) easily
Advantages and disadvantages: Indirect contact freezing
PRO: Economical Minimal dehydration Minimal package bulging CON: Slow freezing process Products must be of uniform thickness
Advantages and disadvantages: Immersion/cryogenic freezing
PRO:
Rapid freezing process
Almost no dehydration
Oxygen is excluded, decreasing oxidative spoilage
Individual freezing pieces have less freezing damage
CON: Difficult to find suitable freezants
Cost of operating is high
Below -9.5°C, there is…..
no significant growth of spoilage or pathogenic organisms
Freezing refers to temperatures below the freezing point of water. In the food industry, a minimum of ___ is required.
-18°C
