Fish Microbiology and Quality Assurance

Unit 4 - Food preservation techniques and microorganisms

4.4.2. Effect of radiation on food quality
Irradiation helps to improve the shelflife of food, but often it can bring about undesirable changes in irradiated food caused directly by irradiation and indirectly by post irradiation reactions. The free radicals that are produced during radiation process in some foods can bring about oxidative changes which can result in product discoloration, tissue softening (in fruits) due to degradation of pectin and cellulose, and development of rancidity in high fat products due to the production of carbonyl and peroxide radicals during radiation and subsequent storage in presence of oxygen. Product discoloration and rancidity can be reduced by giving radiation process at low temperature in the absence of oxygen.

Foods permitted for irradiation
Irradiation is permitted in many countries for several foods. These include,

• Inhibition of sprouting of potatoes, onion, garlic , mushrooms etc.
• Decontamination of food ingredients (species), and insect disinfection in cereals and grains.
• Destruction of parasites in meat
• Inactivation of Salmonella in poultry, eggs, shrimps, frog legs.
• Delay in fruit maturation (strawberries, mango, papaya).
• Mould and yeast reduction in many foods

Radiation resistance of microorganisms
Microbicidal effect of irradiation is due to the direct interaction of radiation with key molecules within the microbial cell as well as inhibitory effect of free radicals (H⁺, OH^- ions) produced by the radiolysis of water. Resistance to radiation varies with different microorganisms. Among microorganisms, Gram negative bacteria are more sensitive than Gram positive followed by spores.
Many spoilage bacteria of seafoods are Gram negative, hence are least resistant to irradiation. E. coli being highly sensitive not useful as indicator of fecal contamination in irradiated foods. Gram positive bacteria like S. aureus, Micrococcus, Bacillus and Clostridium are more resistant. Viruses are extremely resistant to irradiation.

Radiation resistant bacteria
Among microorganisms, Deinococcus radiophilus, a Gram positive, non-spore former is most resistant to radiation and can survive radiation of 15 K Gy. However, D. radiodurans is most well studied and first radio-resistant organism isolated.
The actual mechanism is of resistance is not clearly known, but it is attributed to its unusual cell wall composition (absence of techoic acid), presence of outer membrane and pigmentation besides other mechanisms.