Food spoilage

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Food spoilage

• What causes it
• How to minimize it

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You know it when you see it Or smell it Or taste
it
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What are the steps of food spoilage?

• Introduce microbes to food
• Food environment is favorable for growth
• Food is stored at a temperature that favros
  growth
• Enough time elapses
• Thermoduric microbes survive heat treatment
• Heat-stable enzymes can degrade food
• Bacteria, molds, and yeasts cause most food
  spoilage

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Microbes and food

• Most nonsterile foods contain many types of
  microbes
• Spoiled foods have one or a few- that outgrew the
  others (much more slowly than in laboratory
  conditions!)
• Aerobic foods Pseudomonas
• Anaerobic foods Lactobacillus or Leuconostoc

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Animal muscle tissue contains few bacteria

• Hide, hair, hooves, GI tract
• Hide removal
• Breaching GI tract
• Processing environment and tools
• Staphylococcus, Micrococcus, Pseudomonas

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Poultry

• Skin, feathers, and feet
• Feces and litter from coops
• Potable water for chilling

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Finfish and shellfish

• Water temperature and feeding patterns
• Warm vs cold
• Psychrotrophic vs mesophilic microbes
• Bottom feeders, filter feeders (molluscs)
• Harvesting methods
• Trawling vs line caught
• Storage

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What kinds of microbes cause meat spoilage?

• Common bacterial species

• Mechanisms

• Pseudomonas
• Acinetobacter
• Moraxella
• Micrococcus
• Staphylococcus
• Shewanella in poultry
• Fungi
• Among others

• Adherence (pili)
• Formation of glycocalyx
• Motility
• Adaptation to temperature, pH

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Recall types of microbes

• Oxygen requirements

• Temperature optima

So different types of microbes can grow as
conditions change
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Growth under storage conditions

• Microbes were intrinsic or introduced by
  processing
• Generally few species grow
• Aerobic conditions Pseudomonas favored
• Vacuum packed lactobacilli if low pH
• Bacillus and Shewanella can grow at higher pH
• Lowering aw reduces microbial growth

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Meat, poultry, and fish are good food sources for
microbes

• High aw
• Protein gt lipids gt carbohydrates
• Microbial metabolism will lower pH
• Slow cooling may favor the growth of anaerobes in
  deep tissue
• Fungi may grow if surface gets dry
• Fish vary in lipid content
• Molluscs have higher carbohydrate content and are
  spoiled by fermenters

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Spoilage factors are diverse like the food
environment

• Carbohydrates metabolized first, then lipids,
  then proteins (as microbial count increases)
• Products from
• Carbohydrates- carbon dioxide or fermentation
  products
• Lipids- aldehydes, ketones, short-chain fatty
  acids
• Proteins- amino acids, amines, short peptides
• Nonprotein nitrogenous compounds (usu. breakdown
  products from lysed cells)

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How does microbial metabolism adversely affect
food?

• Volatile end products produce odor
• Oxidation of pigmented products can change color
• Breakdown of tissues by degradative enzymes can
  change texture
• Production of dextran or sheer numbers can
  produce slime
• Water can be released (purge)

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Specific spoilage organisms meats

• High protein, low carbohydrate
• High aw, pH tends to be acidic
• Aerobes Pseudomonas (grows fast), expends
  glucose
• Acintobacter and Moraxella prefer to utilize
  amino acids
• Facultative anaerobes and anaerobes if oxygen is
  limited (vacuum-packed meats)
• Comminuted (ground) meats spoil faster due to
  increased surface area

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Different issues with processed
meats Heat-resistant organisms Introduced by
handling Preservatives often added Lactobacillus
Leuconostoc Amino acid metabolism Putrefaction,
odor, sliminess
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Eggshells do not protect against microbial
infection! Eggs do have natural
protection lysozyme, alkaline pH, chelators,
protease inhibitors Gram-negative motile
rods green, black, red rots Dried eggs not
susceptible to spoilage
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Milk and milk products

• Whats in milk?
• Protein
• Casein, lactalbumin, amino acids
• Carbohydrate
• lactose
• Lipids
• Degraded by milk lipases into butyric, capric,
  caproic acids
• Minerals

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Pasteurization does not kill everything

• Micrococcus, Enterococcus, and others can survive
• Pseudomonas, spore formers, and others can be
  introduced afterward
• UHT (ultra high temperature, 150oC for a few
  seconds) is essentially sterilized
• Concentrated milk products are heat treated
• Butter tends to be contaminated by yeasts and
  molds

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Fruits and vegetables Vary in carbohydrates, prot
eins, pH What sorts of organisms would spoil
them? Innate or introduced?
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Fermented foods are not immune to spoilage
Generally yeasts and acidophilic bacteria
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Canned foods
Heat treated to kill microbes Low acid kill
most spore formers flat sour- no
gas thermophilic anaerobe-gas sulfide stinker-
gas and discoloration High acid all vegetative
bacteria
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Refrigerated foods

• Psychrophilic and psychrotrophic microbes
• Handling introduces microbes
• Some pathogens can grow at low temperatures
• With long storage, microbes can increase to
  disease causing levels
• Competitive advantages adaptation to cold. Low
  O2, production of bacteriocins
• Clostridium grow in vacuum-packed foods

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Summary

• Why are different foods spoiled differently?
• Available nutrients
• Capability for rapid growth of microbes
• End products organics, inorganics, gases?
• Enzyme activity?