Fish Microbiology and Quality Assurance

Unit 8 - Spoilage of fresh and processed fish and fishery products

8.2.3. Microbiology and spoilage of frozen fish
The bacteriological quality of frozen products depends on the bacterial load of the raw material, contamination during handling and processing and extent of removal of these contaminants during processing. The freezing and storage under frozen condition has detrimental effect on surviving microorganisms and reduction in count is highly variable.

Growth and survival of microorganisms associated with frozen foods
The psychrotrophic bacteria in fish are sensitive to freezing stress, and the sensitivity is strain dependent. Spoilage microorganisms generally grow and cause spoilage when the raw fish product is held long time before freezing, frozen at a very slow rate (slow freezing conditions), thawed too slowly or held under thawed condition for a long time. However, the ultimate activity of microorganisms depends on the time duration and temperature of holding the product. As most microorganisms are unable to grow below -10 ºC or -12 ºC, increase in temperature above this limit results in dramatic increase in growth rate. The changes in microflora and biochemical alterations observed in frozen fish products are similar to that of raw chilled fish.
Seafood held at elevated frozen storage temperatures (-10 ºC to -5 ºC) are likely to support mold growth though at very slow rate. Some molds and yeasts can grow in that range. Though bacteria do not grow in frozen foods, they are able to survive freezing and frozen storage to a certain extent, so that thawed fish spoils about as fast as fish that has never been frozen. Gram negative bacteria are more readily killed by the freezing conditions than Gram positive bacteria while, spores are least sensitive and thus show better survival.
Microbial analysis of frozen product gives some information about the quality of the fish before it was frozen. However, all strains of microorganisms are to some extent killed by freezing and frozen storage. Hence, the bacterial load in frozen fish is always lower than that observed before the product was frozen. For frozen foods use of E. coli as indicator of sanitary quality during processing is not suitable, and instead fecal streptococci are preferred owing to their greater ability to survive freezing.

Pathogenic microorganisms and frozen foods
Among human pathogenic microorganisms, Vibrio parahaemolyticus is encountered in small numbers in frozen foods since it is quite sensitive to conditions of freezing and thawing. However, these potentially dangerous bacteria get destroyed on cooking, but pose threat only under the conditions of recontamination of the cooked food and abuse of holding time and temperature.
The potentially toxigenic Clostridium botulinum is not affected by freezing conditions and presents no hazard unless conditions for its outgrowth and toxin production are provided. However, toxins that might be present in the raw product or produced as a result of bacterial growth in seafood prior to freezing would not be inactivated by the freezing process. Generally, conditions permitting the development of large populations of spoilage bacteria also favour toxigenesis of C. botulinum. The poor temperature control which is commonly encountered during the distribution of frozen seafood favours bacterial growth leading to spoilage. Staphylococci are reasonably resistant to the effects of freezing hence their load in frozen food gives an indication on the extent of handling/human contact that the food had received prioer to freezing.

Prevention of spoilage
Foods frozen and held at appropriate temperature conditions are free from microbial spoilage unless the conditions of freezing and thawing had permitted growth of associated microorganisms as in slow freezing and slow thawing. Therefore, temperature control is the principal means to stop microbial activity in frozen seafood involving rapid freezing, holding at or below -18 ºC and rapid thawing.