Mod 4 A

• Module 4: The Microbiota of Food Products
• Intrinsic Food Properties

  • Intrinsic Parameters
  • pH
  • Moisture content
  • Oxidation–reduction potential
  • Nutrient
  • Antimicrobial constituents
  • Biological structures
• Intrinsic Parameters

  • Inherent part of the tissues
  • Natural preservation
  • Prediction of the general types of microorganisms that are likely to grow
  • Overall stability of the food
  • pH
  • Moisture content
  • Oxidation–reduction potential
  • Nutrient
  • Antimicrobial constituents
  • Biological structures
• Optimum pH for Most Microorganisms
• Arrange the following in terms of acid tolerance: Molds > yeast > LAB > S. aureus > Vibrio
• LAB is more acid tolerant in HCl than in acetic acid
• Which will spoil first? Meat from a fatigued animal spoils faster than meat from a well-rested animal
• Food with Biological Acidity

  • Fermented milk
  • Sauerkraut
  • Pickles
• Acidity or pH due to the actions of certain microorganisms
• Buffered Food: Food that tends to resist changes in pH
• How pH affects Microorganisms: Optimal pH for proper enzyme function, Optimal pH for proper nutrient transport, Morphology changes, Longer lag phase if pH is nonoptimal, Internal pH of almost all cells is near neutrality
• Adaptation to Low pH: Maintain cytoplasmic pH values nearer to neutrality, Highly impermeable cell membrane, Pump protons out of their cells at a relatively high rate, High H+ for
• pH


  Longer lag phase if pH is nonoptimal
• Internal pH of almost all cells is near neutrality
• Adaptation to Low pH


  1. Maintain cytoplasmic pH values nearer to neutrality

  2. Highly impermeable cell membrane (Restricts H+ influx)

  3. Pump protons out of their cells at a relatively high rate

  4. High H+ for the stability of the cytoplasmic membrane

  5. Increased expression of amino acid decarboxylase

  6. Consumes cytoplasmic protons through catalytic reactions
• Adaptation to High pH

  1. Maintain cytoplasmic pH values nearer to neutrality

  2. Sodium motive force (Active regulation of pH)

  3. Coupling Na+ expulsion to electron transport
• Intrinsic Parameters

  • pH
  • Moisture content
  • Oxidation–reduction potential
  • Nutrient
  • Antimicrobial constituents
  • Biological structures
• Water Activity (aw) is the ratio of the vapor pressure of food to the vapor pressure of pure water at the same temperature
• Water Activity (aw)

  • Pure water (aw=1)
  • Most fresh food (aw=0.99)
• Bacteria

  Greater than fungi
• Yeast and molds

  Wider range than bacteria
• Gram negative bacteria

  Greater than Gram positive bacteria
• Relationship of Temperature, Nutrition, and pH to aw

  • Low aw reduces ability to growth at any temperature
  • Wider range of aw leads to optimum temperature and nutrient
  • Increased range of aw increases nutrient availability
  • Higher minimum aw requirement leads to nonoptimal pH
• Effects of Low aw on Microorganisms

  1. High osmotic stress

  2. Increased lag phase

  3. Decreased growth rate

  4. Decreased population size

  5. Adverse effect on all metabolic activities
• Adaptations to Low aw

  1. Intracellular accumulation of compatible solutes

  2. Soluble molecules that have no net charge at physiological pH

  3. Do not adhere to or react with intracellular macromolecules

  4. e.g. carnitine, glycine betaine, and proline

  5. Keep the cell membrane fluid
• Adaptations to Extremely Low aw

  1. Ability to concentrate salts, polyols, and amino acids to sufficient levels

  2. Prevent the cells from losing water

  3. Allow the cell to extract water from the water-depressed external environment

  4. Osmophiles accumulate polyhydric alcohols to a concentration commensurate with their extracellular aw

  5. Halophiles maintain osmotic equilibrium by maintaining the concentration of KCl in their cytoplasm equal to the environment
• Oxidation–Reduction Potential (O/R) is related to the ease with which
• Intrinsic Parameters

  • pH
  • Moisture content
  • Oxidation–reduction potential
  • Nutrient
  • Antimicrobial constituents
  • Biological structures
• Oxidation–Reduction Potential (O/R) is the ease with which the substrate loses or gains electrons
• Oxidation–Reduction Potential (O/R): 'Expressed by the symbol Eh'
• Positive Eh values (Oxidized)

  Aerobes
• Negative Eh values (Reduced)

  Anaerobes
• Zero Eh


  Concentration of oxidant and reductant is equal
• Pre-rigor meat has high Eh, while post-rigor meat has low Eh
• Substances that Maintain Reducing Conditions

  • -SH groups in meats and ascorbic acid
  • Reducing sugars in fruits and vegetables
• Aerobes can lower Eh of their environment

  • O2 in the medium is depleted
  • Growth of cells not slowed
  • Ability to use O2-donating substances in the medium
  • Medium becomes poorer in oxidizing and richer in reducing substances
• Anaerobes cannot lower Eh of their environment

  Metabolic byproducts (e.g. H2S) can lower Eh
• Sources of Energy


  • Sugars, alcohols, and amino acids
  • Complex carbohydrates
  • Fats
• Primary Nitrogen Sources

  • Amino acids
  • Nucleotides
  • Peptides and proteins
• Vitamins and Minerals

  • Low quantities
  • Almost all natural foods have an abundant quantity for those organisms that are unable to synthesize their essential requirements
  • Need to be supplied to Gram positive bacteria
  • Can be synthesized by Gram negative bacteria and molds
  • Fruits have lower B vitamins than meat