Food Microbiology & Safety
Helen Billman-Jacobe
Sources of microorganisms in food_Fish
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Ray and Bhunia Chapter 3
Intended learning outcomes
Differentiate between normal flora in plant and animal derived foods, contaminants, spoilage organisms, pathogens and functional organisms
Relate the intrinsic and extrinsic factors of food substances to how microorganism can grow and survive
Intrinsic factors
•may be up to 25% lipid
•Low amount of connective tissue •15 – 20% protein
•<1% carbohydrate
•The intrinsic factors on species and rearing environment
Microbes in fish
Muscle essentially sterile
Skin, gills and gut carry large numbers of microbes
Fish are cold blooded and the microbes that live on them reflect the normal body temperature of the fish
• Temperate waters -2 – 12°C
Bacteria associated with marine fish are likely to be salt tolerant .~ 2-3% salt
Skin ~102 – 107 cfu/ml
Alimentary canal 103 – 109 cfu/ml Gills 103 – 109 cfu/ml
Farmed vs wild
The flesh of molluscs differs from vertebrate fish and crustaceans •It may contain up to 3% carbohydrate in the form of glycogen •Molluscs are filter-feeders
•The filter small particles from large volumes of water
•They can concentrate microbes including viruses
Oyster farming
FSANZ Microbiological limits
n=number of samples, m=acceptable microbiological limit
Non-microbial spoilage: Fatty fish containing large amounts of polyunsaturated fats may go rancid
Most spoilage is of microbial origin
Very little carbohydrate in fish has 2 consequences
• there is little chance of acidification of the muscle ~pH6.5
• Bacteria on the fish have to use nitrogenous compounds as an energy source → off odors
Trimethylamine oxide is an osmoregulator in fish
TMAO can be used by some bacteria i.e Shewanella putrifaciens under microaerophiic and anaerobic conditions
Shewanella putrifaciens
TMAO Trimethylamine
TMAO has a high redox potential
Off odors and flavours
•Products of protein decomposition
•hydrogen sulphide •Methyl mercaptan •Dimethyl sulphide •Skatole (from tryptophan)
TMAO and near neutral pH means that MAP does not improve keeping quality
Gaseous atmosphere
Moderate pH6.5
Extrinsic factors
temperature
Catch, gut then chill asap
Intrinsic factors
Water activity
pH and buffering
High due to TMAO
Redox potential Antimicrobial factors
Nutrient content
Fish protein rich
Shellfish carbohydrate rich
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