Fish Microbiology and Quality Assurance (2+2)

2.2.3. Presence and concentration of gases in the environmen

Unit 2 - Growth and survival of microorganisms in food

2.2.3. Presence and concentration of gases in the environment
Exposure of foods to gases in the storage environment (gaseous environment) affects growth and survival of microorganisms in foods. Since exposure of food to oxygen favors growth of aerobic microorganisms, gaseous environment need to be modified to ensure reduced microbial activity and resultant spoilage. This approach is commonly employed in the preservation of fruits and vegetables.

Gases used to control microorganisms in foods
Carbon dioxide, ozone and nitrogen are most important gases used to control microorganisms in food. Several ready to eat foods are packed in the presence of these gases to reduce microbial activity and extend shelf life of packed foods. Such foods are referred to as Modified Atmosphere Package (MAP) foods.

Carbon dioxide is single most important atmospheric gas used to control microorganisms in foods and is used in varying concentration depending on the type of food. Carbon dioxide in elevated pressure is also used in carbonated water and soft drinks. Molds and Gram negative microorganisms are more sensitive to CO₂ than Gram positive bacteria. Lactobacilli are resistant to CO₂.Yeasts show considerable resistance and tolerate high CO₂ level but can cause spoilage of carbonated beverage (Ex. Brettanomyces sp.)

Mechanism of inhibition
Carbon dioxide mainly acts as bacteriostatic agent. But some microorganisms are killed by prolonged exposure.Mechanism of inhibition of CO₂ is due to the formation of carbonic acid which lowers pH. Lowered pH affects physical properties of plasma membrane of microorganisms and affects solute transport, inhibits key enzymes, and reacts with amino group of proteins causing changes in their property and activity.

Ozone (O₃) is also has antimicrobial properties and extends shelf life of certain fruits and vegetables foods. O₃ concentration of 0.15-5 ppm is known to double the shelf life by inhibiting spoilage bacteria and yeast.