Fish Microbiology and Quality Assurance (2+2)

Unit 2 - Growth and survival of microorganisms in food

2.1.2 pH and buffering capacity
The acidity and alkalinity of an environment affects growth and metabolism of microorganisms as the activity and stability of macromolecules, enzymes and nutrient transport is influenced by pH. Generally, bacteria grow fast at pH 6-8. But bacteria that produce acids have optimum pH between pH 5 and 6 (Ex: Lactobacillus and Acetic acid bacteria). Yeast grows best at pH 4.5-6.0 and Fungi at 3.5 – 4.0.

In low pH foods (Ex. Fruits), spoilage is mainly by yeasts and fungi than bacteria. Fishes with pH around neutrality (6.5-7.5) favour bacterial growth and spoil rapidly than meat (pH: 5.5 – 6.5). Ability of low pH to restrict microbial growth has been employed as a method of food preservation (Ex: use of acetic and lactic acid).

Buffering capacity refers to the ability of foods to withstand pH changes. Microorganisms have ability to change pH of the surrounding environment to their optimal level by their metabolic activity. Decorboxylation of aminoacids releases amines which increases surrounding pH. Deamination of aminoacids by enzyme deaminases release organic acids causing decease in pH. Thus, protein rich foods like fish and meat have better buffering capacity than carbohydrate rich foods.