Fish Microbiology and Quality Assurance (2+2)

Unit 4 - Food preservation techniques and microorganisms

4.1.1.b. Preservation by Freezing
Freezing involves lowering of temperature of food to -20^oC and storage at same temperature. At this temperature the water in food as well as in microorganism is converted to ice crystals which affect fluidity of cell. This ensures prolonged shelflife as microbial activity is completely stopped at this temperature condition.

Freezing is achieved by

• Quick freezing: where temperature is lowered to -20^oC within 30 min.
• Slow freezing: where temperature is lowered to -20^oC within 3~72 hours.

Quick freezing is more advantageous than slow freezing in achieving product quality. During freezing water in food is converted to ice crystals of variable size. Freezing also brings about changes in properties of food such as pH, titratable acidity, ionic strength, viscosity, osmotic pressure, freezing point, O/R potential etc. These changes along with non-availability of water make the environment unsuitable for microbial growth and activity.

Comparison of effect of freezing methods on microorganisms

Quick freezing	Slow freezing
Small ice crystals formed	Large ice crystals formed
Suppresses microbial metabolism	Break down of metabolic rapport and causes cell damage
Brief exposure to adverse conditions	Longer exposure to injurious factors
No adaptation to low temperature	Gradual adaptation
Causes thermal shock to microbes	No thermal shock effect
No protective effect	Accumulation of concentrated solutes with beneficial effects.
Drip loss is less	Drip loss is more

Shelf life of frozen foods
Frozen foods can be stored indefinitely without microbial spoilage, but not done so as they lose original flavour and texture after thawing. Thus frozen foods are assigned freezer life. Freezer life for frozen foods is determined based on texture, flavor, tenderness, colour and nutritional quality upon thawing and cooking. Freezer life of frozen stored food does not depend on microbiology of frozen foods.

Effect of freezing on microorganisms
Freezing causes sudden mortality immediately on freezing, varying with microbial species. The number of surviving microorganisms after freezing die gradually when stored in frozen condition. Decline in microbial number is rapid at temperature below freezing point (-2^oC) than at lower temperature, and is slow below -20^oC. Bacteria differ greatly in their capacity to survive during freezing. Generally cocci are more resistant than Gram negative bacteria. Food poisoning bacteria are less resistant to freezing while, microbial endospores and toxins are not affected by freezing.