food preservation by drying. fish drying small fishes - dried whole; large fish - dried by cutting...
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Food preservation by drying
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Fish drying
• Small fishes - dried whole; large fish - dried by cutting open or cutting to small pieces to ensure faster drying
• Fish may be salted or not salted before drying
• After drying, fish are packed and stored for later consumption as human food
• The storage stability or shelflife of dried fish depends on - quality of the raw material used, extent of drying and storage conditions to which fish is exposed after drying
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Categories of dried foods
• Dried foods are generally divided in to 2 categories based on moisture content-
• Low moisture foods: These foods do not contain moisture level more than 25%, and have aw of 0.6 or less. These food have better keeping quality and shelflife
• Intermediate moisture foods (IMF): These contain 15-50% moisture and aw
between 0.60 and 0.85.Ex: Dried fruits, cakes etc.
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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
• The storage stability (spoilage) of dried foods depends mainly on the aw of food
• Bacteria require high aw for growth (> 0.90) compared to yeasts and molds
• At aw. 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 aw of 0.75: Delayed spoilage caused by few types of organisms (fungi).
• At aw of 0.70: Spoilage greatly delayed, may not occur during prolonged storage.
• At aw of 0.65: Very few organism can grow, and spoilage is unlikely for 2 years.
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• 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
• Molds involved in the spoilage of dried foods are Candida, Botrytis, Rhizopus, Mucor, Saccharomycopsis, Alternaria, Aspergillus, Zygosaccharomyces etc.
• Endospores of bacteria, yeasts, molds and certain bacteria survive drying process
• Microbial spoilage of dried foods - 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
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Microorganisms associated with dried fish:
• Spoilage of dried fish during storage can result from bacteria and fungi.
Bacterial spoilage:
• Bacteria growing in salt dried fish (marine) are:
- 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.
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• 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
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Fungal spoilage:
• Fungi are the common cause of spoilage of salt dried fish
• Being aerobic and requiring low aw these grow on surface
• Ex: Aspergillus, Penicillium, Fusarium, Paecilomyces etc
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