Microbiology of Milk

The microbiology of milk involves the study of microorganisms present in milk, their sources, roles, and impacts on quality, safety, and dairy product development. Fresh milk is a nutrient-rich medium, making it an ideal environment for microbial growth. Below is a detailed discussion of the microbiology of milk, focusing primarily on cow milk (the most studied), with notes on variability across species and conditions.

Overview of Milk Microbiology

• Microbial Load: Freshly drawn milk from a healthy udder contains very few microbes (<10²-10³ CFU/mL), but contamination during milking, handling, or storage increases this to 10⁴-10⁶ CFU/mL in raw milk.

• Types: Bacteria (predominant), yeasts, molds, and occasionally viruses or protozoa.

• Significance: Microbes affect milk safety (pathogens), spoilage (souring, off-flavors), and processing (fermentation for cheese, yogurt).

Sources of Microorganisms in Milk

1. Udder Interior:

   • Low numbers of bacteria (e.g., Streptococcus, Micrococcus) naturally present in healthy udders.

   • Mastitis introduces pathogens (e.g., Staphylococcus aureus, Escherichia coli).

2. Udder Exterior:

   • Skin contaminants like Staphylococcus, Corynebacterium, and fecal bacteria (e.g., Enterobacter).

3. Milking Environment:

   • Air, dust, and equipment introduce Pseudomonas, Bacillus, and molds.

4. Feed and Water:

   • Silage or contaminated water can add Listeria, Clostridium, or coliforms.

5. Human Handling:

   • Hands or improperly sanitized equipment contribute Staphylococcus or Lactobacillus.

Major Microbial Groups in Milk

1. Lactic Acid Bacteria (LAB)

• Examples: Lactobacillus (e.g., L. delbrueckii), Streptococcus (e.g., S. thermophilus), Leuconostoc, Lactococcus (e.g., L. lactis).

• Characteristics: Gram-positive, ferment lactose to lactic acid, non-spore-forming.

• Role:

  • Beneficial: Used in yogurt, cheese, and kefir production (lower pH, preserve milk).

  • Spoilage: Cause souring in raw milk if uncontrolled.

• Growth Conditions: Thrive at 20-40°C, pH 4-7.

• Concentration: Low in fresh milk; dominate post-fermentation.

2. Psychrotrophic Bacteria

• Examples: Pseudomonas (e.g., P. fluorescens), Acinetobacter, Flavobacterium.

• Characteristics: Gram-negative, aerobic, grow at low temperatures (0-7°C).

• Role:

  • Spoilage: Produce proteases and lipases, causing bitterness, rancidity, and off-flavors in refrigerated milk.

• Significance: Major issue in pasteurized milk stored cold; heat-resistant enzymes persist post-pasteurization.

• Source: Contaminated water, equipment.

3. Coliforms

• Examples: Escherichia coli, Enterobacter, Klebsiella.

• Characteristics: Gram-negative, lactose-fermenting, fecal indicators.

• Role:

  • Spoilage: Produce gas, acid, and off-odors.

  • Pathogenic: Indicate contamination (e.g., E. coli O157:H7 causes illness).

• Significance: High counts signal poor hygiene; rare in healthy udder milk.

4. Pathogenic Bacteria

• Examples:

  • Staphylococcus aureus: Toxin-producing (mastitis).

  • Listeria monocytogenes: Psychrotrophic, causes listeriosis.

  • Salmonella spp.: From fecal contamination.

  • Campylobacter jejuni: From raw milk consumption.

  • Mycobacterium bovis: Tuberculosis (historical concern).

• Characteristics: Vary (Gram-positive/negative), some heat-resistant spores.

• Role: Health hazard in raw milk; pasteurization eliminates most.

• Significance: Rare in modern dairy with good practices; critical in raw milk debates.

5. Spore-Forming Bacteria

• Examples: Bacillus (e.g., B. cereus), Clostridium (e.g., C. botulinum, C. tyrobutyricum).

• Characteristics: Gram-positive, form heat-resistant spores.

• Role:

  • B. cereus: Spoilage (bitterness), occasional pathogen.

  • C. tyrobutyricum: Late blowing in cheese (gas production).

• Significance: Survive pasteurization; issue in UHT milk or cheese.

6. Yeasts and Molds

• Examples: Candida, Geotrichum (yeasts); Aspergillus, Penicillium (molds).

• Characteristics: Eukaryotic, slow-growing, some tolerate low pH.

• Role:

  • Spoilage: Off-flavors, sliminess in raw or poorly stored milk.

  • Beneficial: Used in some cheeses (e.g., Penicillium roqueforti in blue cheese).

• Source: Air, equipment, feed.

7. Other Microbes

• Examples: Micrococcus, Corynebacterium (non-pathogenic udder flora); bacteriophages (attack LAB in cheese-making).

• Role: Minor contributors to microbial load; phages disrupt fermentation.

Factors Affecting Microbial Content

1. Udder Health: Mastitis increases pathogens and somatic cells.

2. Hygiene: Poor milking practices (dirty equipment, hands) raise counts.

3. Temperature:

   • <4°C: Psychrotrophs dominate.

   • 20-40°C: LAB and mesophiles grow rapidly.

4. Time: Fresh milk spoils within hours at room temperature without cooling.

5. Species: Human and horse milk have more natural antimicrobials (e.g., lysozyme), reducing initial load.

6. Processing: Pasteurization (72°C, 15s) kills most vegetative cells; UHT (135°C, 2-5s) sterilizes.

Microbial Dynamics in Milk

• Fresh Milk: Low microbial load (10²-10³ CFU/mL) from udder; mostly harmless Micrococcus or Streptococcus.

• Raw Milk Storage:

  • Cold (4°C): Psychrotrophs (Pseudomonas) multiply, spoiling milk in days.

  • Warm (20-30°C): LAB ferment lactose, souring milk in hours.

• Post-Pasteurization: Surviving spores (Bacillus) or post-process contamination (Pseudomonas) cause spoilage.

• Fermentation: Controlled LAB growth produces yogurt, cheese, lowering pH to ~4-5, inhibiting pathogens.

Impact on Milk Quality and Safety

1. Spoilage:

   • Acid production (LAB): Souring, curdling.

   • Proteolysis/lipolysis (psychrotrophs): Bitterness, rancidity.

   • Gas production (coliforms, Clostridium): Bloating, off-odors.

2. Safety: Pathogens like Listeria, Salmonella pose risks in raw milk; rare in pasteurized milk.

3. Shelf Life: High initial counts or poor refrigeration shorten usability.

4. Flavor: Microbial metabolism (e.g., Clostridium in cheese) adds desirable or undesirable notes.

Control Measures

• Hygiene: Clean udders, sanitized equipment, filtered milk.

• Cooling: Immediate refrigeration (<4°C) slows growth.

• Heat Treatment:

  • Pasteurization: Eliminates vegetative pathogens/spoilage bacteria.

  • UHT: Destroys spores, extends shelf life.

• Preservatives: Natural (e.g., lactoperoxidase system) or added (e.g., CO₂ in some systems).

• Testing: Somatic cell counts, total bacterial counts (e.g., <20,000 CFU/mL for Grade A milk).

Species Variability

• Cow Milk: High LAB and psychrotrophs in raw state; rumen pathogens (e.g., Mycobacterium) possible.

• Goat/Sheep Milk: Similar to cow but often higher initial counts due to manual milking.

• Horse Milk: Lower counts, more lysozyme, suited for fermentation (e.g., kumis).

• Human Milk: Rich in antimicrobials (lactoferrin, lysozyme), minimal pathogens in healthy conditions.