Freezing Technology

5.2.1 Microbial growth at low temperature

                 Freezing of fish is done at – 40⁰C and the frozen fish is further stored at – 18⁰C.  During freezing, 80 to 90% of the Gram-negative bacteria die out and the residual bacteria cannot grow in the temperature of frozen storage.  So, during freezing preservation of fish, there is no bacterial spoilage.  But, before cooking, the frozen fish has to be thawed.  During the thawing process the residual bacteria, which are predominantly Gram positive, can cause spoilage of the thawed fish.  Hence, frozen fish will have to be thawed within the shortest possible time.   The relation between microbial growth and temperature is still considered as complex phenomena governed by inter-related factors such as substrate composition, freezing rate, microbial type. 

As the temperature falls, bacterial growth rate is reduced and the lag period is extended until the minimum temperature limit is reached when growth ceases.  Some microbes will cease growing at 0⁰C or even higher while others continue to grow below the freezing point of food.   Growth rate below 0⁰C is very slow. The reported minimum growth of microorganism varying from 10⁰C to - 10⁰C.  But for practical cases, the lower limit of growth for bacteria, yeast or moulds can take as -7⁰C.

            Most food stuffs contain low molecular weight compounds such as salts, sugars, etc. which decrease the freezing point of water and lower the water activity.  As more and more water is converted to ice with the fall in freezing point, the water activity in the remaining unfrozen water is reduced.  Hence, microorganisms as they approache their minimum growth temperatures are subjected to two factors mainly, i.e. (i) the concentration of the solutes in the unfrozen water which destroys the osmotic equilibrium of cell membrane leading to metabolic diffusion and (ii) strong inhibitory action due to low water activity.  Additional stress imposed on the cell prior to freezing causes changes in the enzyme activities of microbial populations and reduces the efficiency of their metabolic pathways and makes them more susceptible to low temperatures. This is of particular importance to food such as fish, which is chilled prior to freezing.  Hence, the ultimate effect of freezing is thought to be due to ice crystal formation and solute concentration caused by the damage to the semi permeable properties of the cell membrane.  At temperature above -10°C, freezing occur only externally and a cell that could make osmotic adjustments and escape death.  On the other hand, below -10°C, cell membrane fails to act as an osmotic barrier to the proliferation of ice already formed outside the cell resulting in intracellular ice formation and death.  Damage to DNA is also involved.

            When a food product having a mixed flora is exposed to low temperature, a small change in temperature leads to substantial variation in the relative growth rates of the micro-organisms present.  It has been conclusively proved that Gram-positive bacteria particularly Gram-positive cocci are most resistant to deleterious action of freezing.  Yeasts and molds that are able to grow at relatively lower water activities are also a common component in frozen foods. Cells those are not killed by freezing may be sub-lethally injured and such injury is reversible.  The repaired cells regain all the normal characteristics of the original cell within a few hours and this happens in the lag period. Injured cells often exhibit increased nutritional needs, impaired membrane permeability, reduced resistance to environmental stress (e.g. low pH) and increased sensitivity to inhibitory agents.

Qualitative changes in bacterial flora during freezing

            Freezing imparts a selective action on the bacterial flora of fish and various bacterial species are affected at different levels.   But Gram-positive bacteria are more resistant to freezing and frozen storage than Gram negatives.  But among Gram positives, differing sensitivity to freezing is noticed.  Bacillus, Lactobacillus and Micrococcus are more susceptible to freezing than ‘Coryneform’ bacteria.  Among the gram negatives, sensitivity to freezing is almost comparable except the case of Vibrio species that are very much sensitive.  Pseudomonas and Aeromonas, Moraxella and Flavobacterium species are more resistant to the lethal action of freezing.

Effect of thawing

            Generally frozen sea foods are considered highly stable and less prone to bacterial decomposition.  But they can never be considered as sterile and completely free from bacteria.  If there is no contamination after thawing the micro-flora on the thawed seafood will have only the bacterial population consisting of the freezing resistant species only.  This dormant population is inactive during the frozen storage period.  But when the product is defrosted (thawed) surviving bacteria are liberated and immediately begin multiplying resulting in the chemical breakdown of the product.

            During thawing process, bacteria start multiplying within the temperature limit characteristic for the different species.   The higher the external temperature the more favorable for outgrowth of most of the bacteria.  Hence, defrosting at a lowered temperature ensures a lower multiplication rate.  Multiplication rate of bacteria will be slower at low temperatures of thawing. As in the case of defrosting temperature, defrosting rate is also found to have a decisive role in bacterial multiplication and the resulting spoilage.  A slow rate of thawing allows the bacteria to multiply considerably causing the spoilage of the product.  Compared to other foodstuffs, there are greater chances of spoilage of seafood during and after thawing because fish harbours a psychrophilic bacterial flora even from the beginning.  Though freezing and immediate storage kill as much as 50% of the psychrophilic organisms, sufficient numbers remain to promote spoilage of thawed frozen product.  When frozen fish is defrosted and kept in the refrigerator, they undergo spoilage similar to that of unfrozen fish.