Food Microbiology Notes

Food Microbiology

Food Fermentation

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

Fermentation

• Recycles $NAD^+$. 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., $C. botulinum$ 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 $CO_2$ 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, $SO_2$ stops oxidation.
  • Sugar converts to ethanol and $CO_2$.
  • Solids are removed, aged in oak barrels, filtered, $CO_2$ 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 $CO_2$, 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 $CO_2$, 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 $CO_2$ 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.