Fish Microbiology and Quality Assurance (2+2)

Unit 4 - Food preservation techniques and microorganisms

4.3.2. Effects of drying on microorganisms
Drying reduces the microbial activity in foods to a large extent, and depends on the moisture level attained after drying. Thus the storage stability (spoilage) of dried foods depends mainly on the aw of food. Generally, bacteria require high aw for growth (> 0.90) compared to yeasts and molds.

• At a_w of 8.0~0.85: Spoilage of dried foods is caused by a variety of fungi and spoilage occurs in 1-2 weeks period.
• At a_w of 0.75: Delayed spoilage caused by few types of organisms (fungi).
• At a_w of 0.70: Spoilage greatly delayed, may not occur during prolonged storage.
• At a_w of 0.65: Very few organism can grow, and spoilage is unlikely for 2 years.

Growth of most organisms is prevented at aw <0.65, but some molds (Ex: Aspergillus echinulatus, Zygosaccharomyces rouxii) are able to grow and cause spoilage. Some of the molds involved in the spoilage of dried foods are Candida, Botrytis, Rhizopus, Mucor, Saccharomycopsis, Alternaria, Aspergillus, Zygosaccharomyces etc.
Though drying destroys some microorganisms, endospores of bacteria, yeasts, molds and certain bacteria survive drying process. Microbial spoilage of dried foods can be prevented by storing at low Rh condition. Storage at high Rh condition enables absorption of moisture from the atmosphere by dried foods until equilibrium is reached leading to spoilage beginning from the product surface.

Microorganisms associated with dried fish
Spoilage of dried fish during storage can result from bacteria and fungi.

Bacterial spoilage
In salt dried fish (marine) bacteria are capable of growing at varying concentration of salt and can cause spoilage. These include;

• Slightly halophilic bacteria: These grow at 2~5 % salt. Ex: Many marine bacteria.
• Moderately halophilic bacteria: These grow at 5-20 % salt content. Ex: Many bacteria such as Pseudomonas, Vibrio, Moraxella, Micrococcus, Acinetobacter, Flavobacterium etc.
• Exstermely halophilic bacteria: These tolerate 20-32 % salt. Ex: Halobacterium, Micrococcus, Sarcina etc. Exstremely halophilic bacteria cause intense red/pink pigmentation in dried food.
• Halophic bacteria are capable of growing at low RH (75%) unlike other bacteria. The main source of these bacteria to fish is through the salt used for salting of fish. These grow on surface of salt dried fish resulting in red coloration / pigmentation.

Fungal spoilage
Fungi are the common cause of spoilage of salt dried fish. Being aerobic and requiring low aw these grow on surface. Several species such as Aspergillus, Penicillium, Fusarium, Paecilomyces etc have been recorded from dried fish in India.