Food Microbiology Notes

Food Microbiology

Food Fermentation

  • Enhances taste and flavor.
  • Bacteria produce acid, but can also cause spoilage.

Fermentation

  • Recycles NAD+NAD^+. Pyruvate accepts electrons (not oxygen).
  • Pyruvate converts to lactic acid.
  • Fermented foods mainly involve bacteria and sometimes fungi.
  • Lactic acid bacteria can grow at pH 3.5, preventing pathogen growth.

Food Preservation

  • Coverings (rinds + shells) prevent spoilage.
  • Antimicrobials in foods (e.g., lysozyme in egg white, benzoic acid in cranberries, allicin in garlic).
  • Temperature affects bacteria growth:
    • Refrigeration slows growth.
    • Freezing stops growth.
  • Oxygen affects growth (e.g., C.botulinumC. botulinum is anaerobic).

Fermented Foods Examples

  • Yogurt, cheese, sour cream, pickled vegetables, wine, beer.
  • Fermentation preserves food and can be an acquired taste.

Lactic Acid Fermentation

  • Acidic taste in sharp cheeses.
  • Involves Lactobacillus, Lactococcus, and others.
  • These are obligate fermenters, making lactic acid.
  • Milk spoilage: bacteria contaminate milk, acid coagulates proteins, causing curdling.

Cheese Making

  • Milk is fermented, proteins coagulate into curd, liquid (whey) is removed.
  • Curds are salted, pressed, shaped, and ripened with microbes to change texture and flavor.
  • Longer ripening increases acidity and sharpens taste.

Swiss Cheese

  • Nutty flavor from propionic acid produced by Propionibacterium.
  • Holes are from CO2CO_2 byproduct of fermentation.

Yogurt

  • Milk is evaporated and concentrated at 40°C.
  • Thermophilic bacteria (Streptococcus, Lactobacillus) produce lactic acid and other flavor molecules.

Pickling Vegetables

  • Preserves and provides acquired taste.
  • Sauerkraut: cabbage is shredded, salt is added to inhibit some bacteria, packed tight to remove oxygen, and bacteria provide acid.

Salami and Pepperoni

  • Sugar for fermentation, salt for flavor, nitrite as a preservative.
  • Smoked and heated to kill bacteria, then dried.

Alcoholic Fermentation

  • Uses yeast.
  • Wine: grapes are crushed to get juice/sugar, brewer’s yeast (S. cerevisiae) is added, SO2SO_2 stops oxidation.
    • Sugar converts to ethanol and CO2CO_2.
    • Solids are removed, aged in oak barrels, filtered, CO2CO_2 is released, and wine is bottled.
    • Alcohol content is <14%. Sweet wine has more residual sugar.

Sake

  • Japanese wine involving several fermentation steps.
  • Rice starch is cooked, fungus (Aspergillus) produces amylase to convert starch to sugar, S. cerevisiae converts sugar to alcohol and CO2CO_2, lactic acid bacteria add flavor.

Beer

  • Grain starch (barley) is malted/germinated to make enzymes that convert starch to sugar.
  • Milled and mashed with water, solid is removed, liquid (wort) is boiled to inactivate enzymes and microbes, hops are added for bitter flavor and antimicrobials.
  • Transferred to fermentation tank, yeast converts sugar to alcohol and CO2CO_2, aged and bottled (3-6% alcohol).

Distilled Spirits

  • Scotch, whiskey, gin: similar to beer initially.
  • Wort is not boiled, enzymes continue degrading starches after fermentation, ethanol is distilled.

Vinegar

  • Oxidation of alcohol: sugar → ethanol → acetic acid (4% acid).

Bread

  • Baker’s yeast (S. cerevisiae) mixed with flour and sugar for fermentation.
  • Produced CO2CO_2 makes bread rise and become spongy.
  • Alcohol evaporates. Sourdough bread uses yeast and lactic acid bacteria for sour flavor.

Food Spoilage

  • Microbes cause bad smell and taste due to metabolites.
  • Generally not pathogens.
  • Need 23°C (room temp).
  • Most pathogens need 37°C (body temp).
  • Need nutrients not in foods.

Foodborne Illness

  • Food poisoning can be caused by a toxin (intoxication) or a pathogen (infection).

Intoxication

  • Caused by toxins (e.g., exotoxin from S. aureus, neurotoxin from C. botulinum).
  • S. aureus: food poisoning symptoms (nausea, vomiting) occur 4-6 hours after ingestion.
  • C. botulinum: neurotoxin causes paralysis; associated with improper home canning; bulging/damaged cans should be discarded.

Infection

  • Caused by consuming live microbes (e.g., Salmonella, E. coli).
  • Incubation period of 1 day, symptoms include diarrhea.
  • Thorough cooking and avoiding cross-contamination are crucial.
  • Steaks: cook exterior, Ground meat: cook interior.

Food Preservation Methods

  • Canning: autoclave to kill endospores, store at room temperature.
  • Pasteurization: preserves taste, but not for endospores.
  • Cooking: even heating, but not for endospores.
  • Refrigeration: slows growth.
  • Freezing: stops growth, but sharp ice crystals can damage some cells.
  • Drying: removes water.
  • Adding salt/sugar.
  • Adding acids: lowers pH.
  • Benzoic acid: stops fungal growth.
  • Nitrates: used in cured meats to inhibit C. botulinum.
  • Sulfur dioxide: used in wines and juices to stop vinegar formation.
  • Gamma irradiation: used on meats and spices, doesn’t change flavor or texture.