W2 L2: Microbiology of meat & fish

Composition of meat:

• Most B vitamins in useful quantities
• Water activity (aw ): ~0.99
• Redox potential (Eh): mostly facultative anaerobic or anaerobic
• pH: 5.2 to 7 (~5.6)
• Slaughter procedures, hygiene interventions and the addition of hurdles is important for shelf life and safety

Production of meat

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Farm→ transport→ Lairage→ slaughter→ dressing→ chilling→ cutting & boning → packaging or processing

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• Maintains reserve of animals for processing so the abattoir can operate at a constant speed
• Dressing→ head, the legs below the knees, tail, skin and ,with the exception of kidneys, all the viscera are removed. Pigs de-haired, for poultry feathers are removed after scalding but the skin is left

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Intrinsic bacteria

• Commensal bacteria naturally present in muscular tissues and blood vessels

  

Extrinsic bacteria

• Faeces
• Hide contact
• Spilling of body fluids
• Aerosols and sprays
• Contaminated hands or equipment
• Abattoir workers
• Inappropriate preservation methods

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Spoilage or pathogenic bacteria that contaminate meat during slaughter, processing and storage

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Common meat spoilage bacteria

Facultative anaerobes

• LAB → natural gut microflora (Lactobacillus, Enterococcus, Pediococcus)
• Staphylococcus (hide)
• Shewanella (poultry)

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Aerobes

• Pseudomonas
• Acinetobacter
• Moraxella
• Micrococcus
• Fungi & yeasts also contaminate the meat surface

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Meat spoilage

Carbohydrates

• glycogen & glucose easily taken up by some bacteria producing organic acids - giving off sour flavour

Protein

• collagen, actin, myosin, myoglobin (Short peptides, amino acids, amines)
• Gram -ve aerobes Pseudomonas →Production of extracellular polysaccharides (EPS): slime → bitter/ putrid

Lipids

• Aldehydes Ketones Short-chain fatty acids

• Fruity Sweet

  

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Preservation of meat

• freezing
• Canning (clostridia)
• modified atmosphere packaging → LAB
• vacuum packing → LAB
• Aerobic chill storage → Pseudomonas, Listeria monocytogenes
• Fermented meats → LAB
• cured meats → LAB

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Lactic Acid Bacteria (LAB)

• Lactobacillus
• Enterococcus
• Pediococcus

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Fish consumption

• Seafood is a generic term to cover all commercial fish & shellfish
• Fish
  • Round white fish: cod, haddock, whiting
  • Flat white fish: plaice, halibut
  • Fatty fish: herring, mackerel
  • Freshwater/Seawater fish: trout, salmon
  • Sharks and rays: dogfish, skates
• Crustaceae → lobsters, crabs, shrimps, prawns
• Molluscs → mussels, oysters, clams

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microflora of fresh fish

• Skin ~ 103 – 105 cfu/cm2
• Gills ~ 103 – 104 cfu/g
• Intestines ~ 103 – 109 cfu/g

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• Lower counts on fish from clean, cold waters

• Higher counts on fish from tropical, sub-tropical and polluted waters

• But more psychrotrophs on fish from cold waters

• Most bacteria are Gram -ve

• Important genera→ Pseudomonas, Acinetobacter, Aeromonas, Shewanella, Vibrio, Flavobacterium & Cytophaga

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Spoilage of fresh fish vs fresh meat

• Like meat fish is nutritious & thus perishable, so spoilage will begin immediately after death
• pH is higher, so pH is less restrictive to bacteria
• There is less carbohydrates as glycogen is used by fish during catching, so protein/AAs are used by bacteria earlier on → results in more putrid volatiles
• In cold water fish → higher proportion of psychrotrophs that multiply during chill storage, accounting for > 80% of the spoilage population

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Microbial spoilage in fish

1. Indole production→ spoilage bacteria convert tryptophan into indole, which has an intense faecal odour
2. Formation of ammonia→ spoilage bacteria deaminate free AAs to release ammonia
   • Even worse in sharks & rays as they contain high levels of urea that can also be converted into ammonia
3. Formation of TMA→ spoilage bacteria convert trimethylamine N-oxide (TMAO) & odourless osmolyte - into the foul smelling Trimethylamine (TMA)

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Preservation of fish

• Cooked products
• Canned fish→ Clostridia
• Vacuum packed
• Marinated fish
• Smoked fish
• Fermented fish - Baltic sea herring → Halanaerobium sp (High pH, high salt concentration, acetic acid, formic acid)
• Dried
• Frozen products - Anisakis

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Fish borne disease

• Vibrio cholerae
• Vibrio parahaemolyticus
• Vibrio vulnificus
• Enteric viruses (Norovirus)
• Scombroid fish poisoning→ If fish such as mackerel, tuna, bonito are not chilled properly after being caught → bacteria begin to convert histidine into histamine
• Shellfish poisoning→ bivalve molluscs are filter feeders ∴ accumulate toxins produced by microscopic algae & cyanobacteria

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Considerations of Bivalve molluscs

Particular problems:

• Filter feeders → conc. pathogens & toxins
• Grow in inshore & perhaps sewage-polluted waters
• Freq. eaten raw or lightly cooked
• Freq. consumed whole (w/ gut)
• Scallops are safer: offshore, less polluted waters, eviscerated and cooked

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Conclusions:

• Bacteria are naturally present in meat & fish
• The predominant natural bacteria found in meat are Gram +ve, facultative anaerobes (LAB)
• The most abundant natural bacteria present in fish are Gram -ve aerobes
• Natural bacteria are involved in spoilage & fermentation (also disease)
• Spoilage bacteria & pathogens contaminate meat & fish
• Most pathogens are controlled by heating

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