Unit 4: Food and Industrial Microbiology

1. General Account of Microbiology of Foods

Food is a rich source of nutrients, making it an ideal medium for the growth of microorganisms. Food microbiology studies the microbes that inhabit, create, or contaminate food.

Microbes in food can be:

• Beneficial (Food Production): These are used in fermentation to produce new food products, enhance flavor, and preserve food.
  • Bacteria: Lactobacillus (yogurt, cheese), Acetobacter (vinegar).
  • Fungi (Yeasts): Saccharomyces cerevisiae (bread, beer, wine).
  • Fungi (Molds): Penicillium (blue cheese), Aspergillus (soy sauce).
• Harmful (Food Spoilage): These microbes grow in food and degrade it, causing undesirable changes in odor, taste, texture, and appearance.
  • Molds (e.g., Rhizopus on bread), yeasts (e.g., on fruit), and bacteria (e.g., Pseudomonas on refrigerated meat).
  • Spoilage reduces shelf life and causes economic loss.
• Dangerous (Food-borne Pathogens): These microbes, when ingested, cause illness or "food poisoning."
  • Bacteria: Salmonella, E. coli O157:H7, Listeria, Clostridium botulinum.
  • Viruses: Norovirus, Hepatitis A.

2. Food Poisoning

Food poisoning (or food-borne illness) is any illness resulting from the consumption of food contaminated with pathogenic microbes, toxins, or chemicals.

There are two main types of microbial food poisoning:

1. Food Infection

• Cause: Ingesting food that contains living pathogenic microorganisms. The microbes establish themselves in the gut and multiply, causing illness.
• Onset: Usually slower (12 hours to several days), as the microbes need time to grow.
• Symptoms: Often involves fever, diarrhea, abdominal cramps.
• Example: Salmonella (from undercooked chicken or eggs).

2. Food Intoxication (Poisoning)

• Cause: Ingesting food that contains pre-formed toxins produced by microbes *in the food*. The live microbe does not need to be present.
• Onset: Usually rapid (a few hours), as the toxin is already present and acts quickly.
• Symptoms: Often involves vomiting, nausea. Fever is less common.
• Example 1: Staphylococcus aureus. This bacterium grows in food (e.g., cream-filled pastries, potato salad) and produces a heat-stable enterotoxin.
• Example 2: Clostridium botulinum. This anaerobic bacterium produces botulinum toxin (a deadly neurotoxin) in improperly canned foods.

3. Pasteurization of Milk

Definition: Pasteurization is a process of applying mild heat to a liquid (like milk) for a specific time to kill most pathogenic (disease-causing) microbes and reduce the number of spoilage microbes.

• Important: Pasteurization is NOT sterilization. It does not kill all microbes, especially heat-resistant spores. This is why pasteurized milk still needs to be refrigerated and will eventually spoil.
• Purpose:
  1. Safety: Kills harmful pathogens commonly found in raw milk (e.g., Listeria, Salmonella, E. coli, Mycobacterium tuberculosis).
  2. Shelf Life: Reduces the number of spoilage organisms, extending the milk's shelf life.
• Common Methods:
  • High-Temperature, Short-Time (HTST): This is the most common method. Milk is heated to 72°C for 15 seconds, then rapidly cooled.
  • Ultra-High Temperature (UHT): Milk is heated to 138-150°C for 1-2 seconds. This sterilizes the milk, which can then be stored at room temperature for months (e.g., in "Tetra Paks").
  • Low-Temperature, Long-Time (LTLT): An older method. Milk is heated to 63°C for 30 minutes.

4. Fermentation

Definition (Industrial): In an industrial context, fermentation refers to *any* large-scale microbial process (aerobic or anaerobic) used to produce a desired product, whether it's the microbial cells themselves (biomass) or a product they make (metabolite).

Definition (Biological): In biology, fermentation is a metabolic process that converts sugar to acids, gases, or alcohol in the absence of oxygen (anaerobic). It is a way for a cell to produce ATP without an electron transport chain.

Example: Yeast fermentation (C6H12O6 → 2 C2H5OH + 2 CO2)

5. Solid-state and Liquid-state Fermentations

This classification is based on the amount of free-flowing water in the system.

Solid-State Fermentation (SSF)

• Description: Microbes grow on a solid substrate (e.g., grain, bran, straw) with very little or no free water. The moisture is absorbed *by* the solid.
• Organisms: Primarily used with filamentous fungi (molds), which are adapted to grow on surfaces and in low-moisture conditions.
• Advantages:
  • Simpler, lower-cost reactors.
  • Higher product concentration.
  • Lower energy requirement (less water to heat/cool).
• Examples: Production of tempeh, blue cheese, traditional soy sauce, and some enzymes (e.g., amylase from Aspergillus on wheat bran).

Liquid-State (Submerged) Fermentation (SmF)

• Description: Microbes grow submerged in a liquid nutrient broth inside a large vessel called a fermenter or bioreactor.
• Organisms: Used for bacteria, yeasts, and some fungi.
• Key Features: Requires continuous agitation (mixing), aeration (sparging with sterile air, if aerobic), and strict control of pH, temperature, and dissolved oxygen.
• Advantages:
  • Easier to control environmental parameters (pH, temp).
  • Easier to scale up.
  • Easier to extract the product from the liquid broth.
• Examples: Production of antibiotics (Penicillin), most enzymes, alcohol (beer/wine), and organic acids.

6. Batch and Continuous Fermentations

This classification is based on how the fermenter is operated.

Batch Fermentation

• Description: A closed system.
• Process:
  1. The fermenter is filled with sterile nutrient broth (media).
  2. It is inoculated with the microorganism.
  3. The fermentation runs for a set time (e.g., 2-7 days). Nothing is added or removed (except air for aerobic processes).
  4. The microbial growth follows a classic growth curve (lag, log, stationary, death phases).
  5. At the end, the entire batch is harvested, the product is extracted, and the fermenter is cleaned and sterilized for the next batch.
• Use: Very common. Used for products made during the stationary phase (e.g., most antibiotics, which are secondary metabolites).

Continuous Fermentation

• Description: An open system.
• Process:
  1. The fermentation is set up and inoculated.
  2. Once the microbes reach the log (exponential) phase, a continuous feed of fresh sterile media is pumped *in*.
  3. At the *same time*, an equal volume of used media and product is pumped *out*.
• Goal: To maintain the culture in a steady state of continuous, exponential growth (a chemostat).
• Use: More efficient for products made during the log phase (e.g., ethanol, or microbial biomass itself). It is harder to maintain sterility for long periods.

7. Industrial Production

Alcohol (Ethanol)

• Product: Ethanol (C2H5OH).
• Microorganism: The yeast Saccharomyces cerevisiae (Brewer's/Baker's yeast).
• Process: Anaerobic fermentation of sugars (e.g., glucose from corn, grapes, or barley).

  Equation: C6H12O6 (Glucose) → 2 C2H5OH (Ethanol) + 2 CO2 (Carbon Dioxide)

• Uses: Beverages (beer, wine), biofuels, industrial solvent, antiseptic.

Enzymes (Amylase)

• Product: Amylase (an enzyme that breaks down starch into simple sugars).
• Microorganisms:
  • Fungus: Aspergillus oryzae (often in Solid-State Fermentation).
  • Bacterium: Bacillus subtilis (often in Submerged Fermentation).
• Uses: Brewing (to convert barley starch), baking (to feed yeast), detergents (to remove starch stains), making corn syrup.

Antibiotics (Penicillin, Streptomycin)

Antibiotics are secondary metabolites, meaning they are produced by the microbe during the stationary phase of growth, not the log phase. This is why they are almost always produced using batch fermentation.

• Penicillin:
  • Microorganism: The fungus Penicillium chrysogenum.
  • Process: Aerobic, submerged batch fermentation in a large fermenter fed with a medium like corn steep liquor.
• Streptomycin:
  • Microorganism: The bacterium (actinomycete) Streptomyces griseus.
  • Process: Aerobic, submerged batch fermentation.

Organic Acids