Fish Microbiology and Quality Assurance

Unit 4 - Food preservation techniques and microorganisms

4.1.1.a. Chill storage of fish
Chill storage is a process by which temperature of fish is reduced close to freezing point of water (0^oC). This delays both biochemical and bacteriological processes, thus prolongs shelf life. Deteriorative changes are retarded as long as low temperature is maintained. This ensures preserving natural nutritional and functional properties of food.

Factors influencing quality of chilled food

• Quality of chilled food depends upon factors such as;
• Raw material quality
• Method and duration of chilling
• Efficiency of storage method

Chilling can be achieved by use of ice (crushed/flake ice) and use of homogenous coolant (cold air or cold liquid), and refrigerated temperature. Use of cold liquid may be in the form of chilled freshwater for light chilling or refrigerated seawater/brine to attain temperature of 0-1^oC

Bacteria associated with low temperature storage
Several microorganisms are capable of surviving and growing at low temperature and cause spoilage. Bacteria capable of a growing at or below 7^oC are widely distributed. Gram negative bacteria are more common than Gram positive. Psychrotrophs grow well at this temperature. Growth at temperature below 00C is caused mainly by yeasts and molds than bacteria because of low water activity. The lowest recorded temperature for growth of microorganisms in food is -34^oC, by yeast.

The composition of microorganisms associated with fish changes during chill storage. Proportion of mesophiles decrease and psychrophiles dominate. Common bacterial genera associated with chilling temperature condition of foods are;
Acinetobacter, Aeromonas, Enterococcus, Pseudomonas, Vibrio, Erwinia, Moraxella, Enterobacter, Achromobacter, Flavobacterium, Micrococcus etc.

Extension of shelf life varying from 6 days to 30 days for different fish species has been reported by ice storage.