Microorganisms in Food

Microorganisms in Food - Tiny Food Frenemies

• Major Types & Examples:

  • Bacteria: Salmonella, E. coli, Listeria, Clostridium, Lactic Acid Bacteria (LAB)
  • Fungi: Yeasts (Saccharomyces), Molds (Aspergillus, Penicillium)
  • Viruses: Hepatitis A, Norovirus
  • Protozoa: Giardia, Cryptosporidium

• Key Roles in Food:

  • Spoilage: Cause undesirable changes (e.g., Pseudomonas)
  • Pathogenic: Lead to foodborne illness (e.g., Salmonella, Norovirus)
  • Beneficial: Used in fermentation (yogurt, kefir), probiotics (e.g., Lactobacillus)

⭐ Clostridium botulinum toxin is heat-labile; proper cooking destroys it. Spores survive boiling, requiring autoclaving (121°C, 15 psi, 15-20 min) for inactivation in canning.

Microorganisms in Food - Spoilage Scoundrels

• Food spoilage: Undesirable microbial-induced changes in appearance, odor, flavor, or texture.
• Key Spoilage Agents:
  • Bacteria: Pseudomonas (slime, off-odors in meat/milk, esp. psychrotrophs), Bacillus (ropy bread, flat sour), Clostridium (gas, putrefaction), Lactobacillus (souring, greening).
  • Molds: Aspergillus, Penicillium, Rhizopus (visible growth, off-flavors, mycotoxins).
  • Yeasts: Saccharomyces, Zygosaccharomyces, Candida (fermentation, slime, off-odors/flavors).
• Manifestations: Proteolysis (putrid), lipolysis (rancid), saccharolysis (sour, gas e.g., $CO_2$, $H_2S$).

⭐ Pseudomonas aeruginosa can produce pyoverdin (fluorescent green pigment) causing visible spoilage in refrigerated foods like meat and eggs, often accompanied by fruity or putrid odors.

Microorganisms in Food - Pathogen Parade

• Bacterial Pathogens:
  • Salmonella spp. (non-typhoidal): Poultry, eggs. Gastroenteritis. Invasive potential.
  • Escherichia coli O157:H7: Undercooked beef, raw produce. Hemorrhagic colitis, HUS. Shiga toxin.
  • Listeria monocytogenes: Deli meats, soft cheeses. Listeriosis, meningitis in vulnerable. Grows at 4°C; intracellular.
  • Staphylococcus aureus: Salads, custards, meats. Rapid onset vomiting (preformed, heat-stable enterotoxin).
  • Clostridium botulinum: Improperly canned foods, honey (infants). Botulism (flaccid paralysis, neurotoxin). Spore-former.
  • Clostridium perfringens: Reheated meats, gravy. Watery diarrhea, cramps (enterotoxin produced in gut). Spore-former.
  • Campylobacter jejuni: Undercooked poultry, raw milk. Bloody diarrhea; Guillain-Barré syndrome. Microaerophilic.
  • Bacillus cereus: Fried rice (emetic toxin, heat-stable); meats/stews (diarrheal toxin, heat-labile). Spore-former.
• Viral Pathogens:
  • Norovirus: Shellfish, ready-to-eat foods. Acute gastroenteritis (vomiting dominant). Highly contagious, low infectious dose.
  • Hepatitis A Virus: Contaminated water, shellfish. Jaundice, fever, liver inflammation.

⭐ Staphylococcus aureus food poisoning is characterized by a short incubation period (1-6 hours) due to ingestion of pre-formed, heat-stable enterotoxin.

Microorganisms in Food - Microbe Management

• Growth Factors:
  • 📌 FATTOM: Food, Acidity (pH), Time, Temperature, Oxygen, Moisture ($a_w$).
  • Intrinsic: pH, water activity ($a_w$), redox potential, nutrients, antimicrobials.
  • Extrinsic: Temperature, gaseous atmosphere (O₂, CO₂), relative humidity (RH).

• Control (Hurdle Technology): Combining preservation methods.
  • Temperature: Refrigeration (<5°C), Freezing (<-18°C), Pasteurization (LTLT 63°C, 30 min), Sterilization.
  • ↓ Water Activity ($a_w$): Drying, salting, sugaring (target $a_w$ <0.85 (bacteria)).
  • pH Control: Acidification (e.g., lactic, acetic acid).
  • Modified Atmosphere Packaging (MAP).
  • Chemical Preservatives: Sorbates, benzoates, nitrites.

⭐ Most foodborne pathogens are mesophiles, growing optimally between 20-45°C. The food "Danger Zone" is 5-60°C (41-135°F), where rapid growth occurs.

High‑Yield Points - ⚡ Biggest Takeaways

• Key pathogens: Salmonella, E. coli O157:H7 (HUS), Listeria (listeriosis), Campylobacter, S. aureus (preformed enterotoxin).
• Spore-formers: Bacillus cereus (fried rice, toxins), Clostridium botulinum (canned food, neurotoxin), C. perfringens (meat, enterotoxin).
• Mycotoxins: Aflatoxin (Aspergillus, hepatocarcinogenic), Ochratoxin A (nephrotoxic).
• Spoilage: Pseudomonas, yeasts, molds cause undesirable sensory changes.
• Beneficial: Lactic Acid Bacteria (LAB) in fermentation (yogurt, idli), probiotics.
• E. coli: Standard indicator of fecal contamination in food/water.
• Preservation: Pasteurization (reduces pathogens), sterilization (kills spores).