Food Microbiology Study Notes

Food Microbiology

Water

• pH

• Conditions for Spoilage

  • Physical structure

  • Oxygen

  • Temperature

Foods that spoil quickly:

• Moist foods

  • Examples: Ground or sliced meat, milk, yogurt

• Neutral foods at unrefrigerated temperatures (25°C)

Foods that resist spoilage:

• Dry foods

  • Examples: Flour

• Acidic foods

  • Examples: Refrigerated whole meat

Microorganism Growth in Foods

• Controlling Factors:

  • State 1

  • State 2

  • Intrinsic Factors: Composition (microorganisms present, physical and biological state)

  • Extrinsic Factors: Temperature, relative humidity, gases, contaminating microorganisms

Extrinsic Factors:

• Temperature: Lower temperatures delay microbial growth

  • Psychrophiles and psychrotrophs can grow at low temperatures

• Atmosphere: Oxygen promotes growth

  • Modified Atmosphere Packaging (MAP): Use of shrink wrap and vacuum technologies to package food in controlled atmospheres

• Relative Humidity: High levels promote microbial growth

  • Lower water activity inhibits microbial growth. Water activity is reduced by drying, addition of salt or sugar

• Osmophilic microorganisms prefer high osmotic pressure.

• Xerophilic microorganisms prefer low water activity.

Intrinsic Factors:

• Composition

• pH: Impacts microbial community makeup; chemical reactions occur when microbes grow in food.

• Oxidation-reduction potential: Altered by cooking.

• Physical Structure: Grinding and mixing increase surface area and distribute microbes, promoting microbial growth. The outer skin of vegetables and fruits slows microbial growth.

• Presence of Antimicrobial Substances:

  • Coumarins in fruits and vegetables

  • Lysozyme in cow’s milk and eggs

  • Aldehydic and phenolic compounds (aroma to prevent bacterial growth) in herbs and spices

  • Allicin in garlic

  • Polyphenols in green and black teas

Microbial Growth and Food Spoilage

• Food Spoilage: Results from growth of microbes in food, altering food visibly and rendering it unsuitable for consumption.

  • Involves predictable succession of microbes.

  • Different foods undergo different types of spoilage processes.

  • Toxins: Sometimes produced (e.g., algae toxins contaminating shellfish and finfish).

• Shelf Life Categories:

  • Non-perishable foods: e.g., pasta

  • Semi-perishable foods: e.g., bread

  • Perishable foods: e.g., eggs, milk

Food-Borne Diseases

• Two Primary Types:

  • Food-Borne Infections: Microbes are transferred to consumer.

  • Food Intoxications: Result from the consumption of toxins in foods where microbes have grown.

• Examples:

  • Staphylococcal food poisoning

  • Botulism

  • Clostridium perfringens food poisoning

  • Bacillus cereus food poisoning

Detection of Food-Borne Pathogens

• Must be rapid and sensitive:

  • Culture techniques (too slow).

  • Immunological techniques (very sensitive).

  • Molecular techniques (sensitive and specific).

Major Food-Borne Infectious Diseases (Table 41.6)

Microbiology of Fermented Foods

• Fermentation: Partial breakdown of carbohydrates occurring in the absence of oxygen.

• Microorganisms Used: Bacteria, yeasts, and fungi are used as animal and human food sources.

Major Fermentations:

• Lactic: Dairy products (e.g., cheese, buttermilk, kefir, yogurt).

• Propionic: Fermented foods.

• Ethanol: Alcoholic beverages.

Probiotics

• Definition: Live microorganisms intended to provide health benefits when consumed, generally by improving or restoring the gut flora (bacteria).

Major Categories of Fermented Milk Products (Table 41.7)

I. Lactic Fermentations

• Mesophilic: Buttermilk, cultured buttermilk.

• Thermophilic: Yogurt, laban, zabadi, labneh, skyr.

• Therapeutic Examples: Biogarde, Bifighurt, Acidophilus milk, yakult, Cultura-AB ®, Kefir, koumiss, acidophilus-yeast milk.

Other Fermented Foods

• Table 41.9: Fermented Foods Produced from Fruits, Vegetables, Beans, and Related Substrates:

Conclusion

• Importance: Understanding food microbiology is crucial for ensuring food safety and quality. Various preservation techniques mitigate spoilage and food-borne illnesses while promoting beneficial fermentation processes.