Microbial growth at low temperature

The relation between microbial growth and temperature is phenomena governed by 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. (i) the concentration of the solutes in the unfrozen water which destroys the osmotic equilibrium of cell membrane leading to metabolic diffusion (ii) strong inhibitory action due to low water activity. Below -10⁰C, 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. 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.

Quality changes is bacterial flora during freezing Gram-positive bacteria are more resistant to freezing and frozen storage than Gram negatives. Pseudomonas Aeromonas, Moraxella and Flavobacterium species are more resistant to the lethal action of freezing.

Effect of thawing Defrosting temperature,defrosting rate is also found to have a decisive role inbacterial multiplication and the resulting spoilage. A slow rate of thawing allows the bacteria to multiply considerably causing the spoilage of the product.

Role of Poly phosphate in minimizig drip loss: Water holding capacity of tissues comes down due aggregation and protein denaturation and hence results in increased drip loss. Importatn reasons for drip loss are Freezing changes ionic environment of fish muscle Reduction in pH due to production of lactic acid Degradation of nucleotide which contains active phosphorus. Formation of actomysin from actin and myosin Poly phosphate: Chelate proxidant metal ions, minimizing catalytic action of metal ions in fat oxidation.

Use of Polyphosphate in freezing of fish Dip treatment in phosphate solution prior to freezing gives better results. In prawns, in addition to dip treatment in phosphate solution , it may be mixed with glaze water. Poly PO4 is used to increase the water holding capacity of muscle tissues effectively. Tri poly phosphate is extensively used but its effect depends on the species of fish.

Removal of pro-oxidant chemical compounds Removal of pro-oxidant chemical compounds. Enzymes responsible for fat oxidation may be inhibited. Glaze containing ascorbic acid and citric acid mixture is effective against fatty fish and onset of rancidity can be delayed for a long time.