Intrinsic Parameters (Food-Related Factors)

Intrinsic factors are inherent properties of food that influence microbial growth.

1. Nutrient Content

• Like all living organisms, microorganisms require nutrients for growth and metabolism.
• Essential nutrients include:
  • Carbon source: Sugars, starch, and cellulose are common energy sources.
  • Nitrogen source: Amino acids, peptides, and proteins are primary nitrogen sources.
  • Minerals and vitamins: Microorganisms need trace minerals like magnesium, iron, and calcium, along with vitamins for enzymatic functions.
• Nutritional preference: Molds have the lowest nutrient requirements, followed by yeasts, Gram-negative bacteria, and Gram-positive bacteria.

2. Water Activity (aW)

• Water activity (aW) represents the availability of water for microbial growth. Formula:
• aW = Vapour pressure of food / Vapour pressure of pure watera.
• The water activity scale ranges from 0 (bone dry) to 1.0 (pure water).
• Microbial growth is possible only above certain water activity thresholds:

• Bacteria: aW > 0.91
• Yeasts: aW > 0.80
• Molds: aW > 0.60

• Food preservation methods like drying, freezing, and adding solutes (e.g., sugar and salt) reduce water activity.

3. pH and Buffering Capacity

• The pH of food affects microbial growth by influencing enzyme activity, nutrient uptake, and cell membrane function.
• Microorganisms are classified based on their pH preference:
  • Acidophiles: Thrive at pH 1.0–5.0 (e.g., Lactobacillus).
  • Neutrophiles: Prefer pH 6.0–8.0 (e.g., E. coli).
  • Alkalophiles: Grow at pH 8.0–11.0 (e.g., Bacillus spp.).
• Most bacteria prefer a pH range of 6.0–7.5, while yeasts and molds can grow in more acidic conditions.

4. Redox Potential (Eh)

• Redox potential represents the electron transfer potential of food, measured in millivolts (mV).
• Aerobic microorganisms (e.g., Pseudomonas) require positive Eh values, while anaerobic microorganisms (e.g., Clostridium botulinum) thrive under negative Eh conditions.
• Factors influencing redox potential include pH, oxygen availability, and the presence of oxidants and reductants.

5. Antimicrobial Constituents

• Certain foods naturally contain antimicrobial compounds that inhibit microbial growth:
  • Spices: Eugenol (cloves), allicin (garlic), cinnamaldehyde (cinnamon), thymol (sage).
  • Milk: Lactoferrin, lactoperoxidase, lysozyme.
  • Eggs: Ovotransferrin, avidin, lysozyme.
  • Fruits and Vegetables: Hydroxycinnamic acid derivatives (e.g., ferulic, chlorogenic acids).
• These compounds act as natural preservatives, prolonging food shelf life.

6. Biological Structures (Physical Barriers)

• The natural structure of food can protect it from microbial invasion:
  • Outer coverings: Shells of eggs and nuts, fruit peels, and seed coats.
  • Animal tissues: Hide of animals.
• Once the protective layer is damaged (e.g., peeling, cutting), microorganisms can invade and multiply.

Extrinsic Parameters (Environmental Factors) Extrinsic factors refer to external conditions surrounding the food that affect microbial growth.

1. Temperature of Storage

• Temperature significantly influences microbial growth, with microorganisms classified into three groups:
  • Psychrophiles: Grow at 0–20°C (e.g., Listeria monocytogenes).
  • Mesophiles: Grow at 20–45°C, including most pathogens (e.g., Salmonella, E. coli).
  • Thermophiles: Thrive at 45–70°C (e.g., Bacillus stearothermophilus).
• Freezing (-18°C or below): Stops microbial growth but does not kill all microorganisms.
• Refrigeration (4°C): Slows microbial growth but allows psychrotrophs to survive.
• Pasteurization (60–85°C): Kills most non-spore-forming bacteria.
• Sterilization (>121°C): Kills all microorganisms, including spores.

2. Relative Humidity (RH)

• RH refers to the moisture content of the storage environment.
• High RH promotes surface microbial growth, while low RH prevents it.
• Even foods with low water activity can absorb moisture from a humid environment, increasing susceptibility to spoilage.

3. Gas Composition

• The concentration of gases like oxygen, carbon dioxide, and nitrogen affects microbial growth:
  • Oxygen: Required by aerobes (e.g., Pseudomonas), while anaerobes (e.g., Clostridium) grow in its absence.
  • Carbon dioxide: Inhibits microbial growth by lowering pH and permeability of cell membranes.
  • Modified Atmosphere Packaging (MAP): Reduces oxygen and increases CO₂ to extend shelf life.

4. Presence of Competing Microorganisms The presence of beneficial microorganisms can inhibit pathogens through competition for nutrients and production of antimicrobial substances (e.g., bacteriocins).

Preservation Methods Based on Growth Factors

Food preservation techniques leverage intrinsic and extrinsic factors to prevent microbial growth: