Raw milk spoilage and preservation

Raw Milk Spoilage

Raw milk spoilage refers to the deterioration of milk quality due to microbial activity, enzymatic reactions, and chemical changes that occur in unprocessed milk after milking. As a nutrient-rich medium (containing water, lactose, proteins, fats, and minerals), raw milk is highly susceptible to spoilage if not properly handled or refrigerated. Below is a detailed discussion of the mechanisms, causative agents, types, and factors influencing raw milk spoilage.

Overview of Raw Milk Spoilage

• Definition: Loss of sensory quality (taste, smell, texture) and safety due to microbial growth or biochemical reactions.

• Initial State: Freshly drawn milk from a healthy udder has a low microbial load (<10²-10³ CFU/mL) and is sterile in the udder, but contamination during milking increases this rapidly.

• Spoilage Onset: Without refrigeration, spoilage begins within hours; even when chilled, it occurs over days.

Mechanisms of Spoilage

1. Microbial Metabolism

Microorganisms ferment or degrade milk components, producing undesirable compounds:

• Lactose Fermentation:

  • Lactic acid bacteria (LAB) like Lactococcus lactis and Lactobacillus spp. convert lactose into lactic acid, lowering pH and causing souring.

  • Coliforms (e.g., Escherichia coli) produce acid and gas (CO₂, H₂), leading to curdling and bloating.

• Protein Degradation:

  • Psychrotrophs (e.g., Pseudomonas fluorescens) and spore-formers (Bacillus cereus) secrete proteases, breaking proteins into peptides and amino acids, resulting in bitterness and putrid odors (e.g., ammonia, sulfur compounds).

• Fat Breakdown:

  • Lipases from Pseudomonas or endogenous milk lipase hydrolyze triglycerides into free fatty acids (e.g., butyric acid), causing rancidity and soapy tastes.

2. Enzymatic Activity

• Native Enzymes:

  • Lipase: Naturally present in milk, activated by agitation (e.g., pumping), hydrolyzes fats into rancid compounds.

  • Proteases (e.g., plasmin): Degrade caseins, softening texture and releasing bitter peptides.

• Significance: Enzymatic spoilage occurs even with low microbial counts if milk is mishandled.

3. Chemical Reactions

• Oxidation: Oxygen exposure oxidizes unsaturated fatty acids in milk fat, producing aldehydes and ketones (metallic, cardboard-like off-flavors).

• Light-Induced: UV light degrades riboflavin and lipids, forming sulfur compounds (e.g., methanethiol) with a burnt or cooked odor.

Types of Spoilage

1. Souring:

   • Cause: LAB ferment lactose to lactic acid (pH drops from ~6.7 to <5.0).

   • Symptoms: Sour taste, acidic smell, curdling (casein precipitates at pH 4.6).

   • Conditions: Warm temperatures (20-40°C), common in unrefrigerated milk.

2. Rancidity:

   • Cause: Lipolysis by microbial or native lipases releases short-chain fatty acids (e.g., butyric, caproic).

   • Symptoms: Sharp, soapy, or cheesy odor; unpleasant taste.

   • Conditions: Agitation or psychrotrophic growth at 4-7°C.

3. Bitterness:

   • Cause: Proteolysis by Pseudomonas or Bacillus breaks proteins into bitter peptides.

   • Symptoms: Bitter aftertaste, often with a putrid smell.

   • Conditions: Prolonged cold storage (psychrotrophs dominate).

4. Putrefaction:

   • Cause: Protein degradation into amines, ammonia, and sulfur compounds by spoilage bacteria (e.g., Clostridium, Pseudomonas).

   • Symptoms: Foul, rotten egg-like odor.

   • Conditions: Advanced spoilage, warm storage.

5. Gas Production:

   • Cause: Coliforms or Clostridium ferment lactose or proteins, producing CO₂ and H₂.

   • Symptoms: Bloating, frothing, or explosive curds.

   • Conditions: Poor hygiene, warm temperatures.

6. Ropiness/Sliminess:

   • Cause: Exopolysaccharides from Pseudomonas, Alcaligenes, or Leuconostoc.

   • Symptoms: Thick, sticky texture; stringy when poured.

   • Conditions: Cold storage with specific contaminants.

7. Color Changes:

   • Cause: Pigments from Pseudomonas (e.g., blue-green pyocyanin) or molds.

   • Symptoms: Discoloration (blue, yellow, gray).

   • Conditions: Rare, linked to heavy contamination.

Microorganisms Involved in Spoilage

1. Lactic Acid Bacteria (LAB):

   • Lactococcus, Lactobacillus, Streptococcus.

   • Souring via lactic acid; rapid at 20-40°C.

2. Psychrotrophs:

   • Pseudomonas fluorescens, P. putida, Acinetobacter.

   • Proteolysis and lipolysis; thrive at 0-7°C (refrigerated milk).

3. Coliforms:

   • E. coli, Enterobacter, Klebsiella.

   • Gas, acid, off-odors; indicate poor hygiene.

4. Spore-Formers:

   • Bacillus cereus, Clostridium spp..

   • Bitterness, gas; spores survive mild heat.

5. Yeasts and Molds:

   • Candida, Geotrichum, Aspergillus.

   • Off-flavors, sliminess; slow growth in raw milk.

Factors Influencing Spoilage

1. Temperature:

   • Warm (20-40°C): LAB and mesophiles dominate, rapid souring (hours).

   • Cold (0-7°C): Psychrotrophs grow slowly, spoilage in days (bitterness, rancidity).

2. Time:

   • Unrefrigerated: Spoils in 6-12 hours.

   • Refrigerated: Spoils in 3-7 days, depending on initial load.

3. Initial Microbial Load:

   • Poor hygiene (e.g., dirty equipment) increases starting CFU/mL, accelerating spoilage.

   • Healthy udder milk (<10³ CFU/mL) spoils slower.

4. Hygiene:

   • Contamination from udder, hands, or milking gear introduces spoilage organisms.

5. Oxygen Availability:

   • Aerobic bacteria (e.g., Pseudomonas) need O₂ for lipolysis/proteolysis; anaerobic Clostridium produce gas in sealed conditions.

6. Milk Composition:

   • High fat (e.g., buffalo milk) increases rancidity risk; high lactose fuels fermentation.

7. Health of Animal:

   • Mastitis raises counts of Staphylococcus or coliforms, hastening spoilage.

Sensory and Physical Changes

• Taste: Sour, bitter, rancid, or putrid.

• Odor: Acidic, cheesy, rotten, or metallic.

• Texture: Curdled, slimy, or gassy (bloated).

• Appearance: Clots, discoloration, or froth.

Raw Milk Preservation

Raw milk preservation involves techniques to extend its shelf life, maintain quality, and ensure safety by slowing microbial growth, enzymatic activity, and chemical degradation without heat treatment (e.g., pasteurization). Since raw milk is unprocessed and highly perishable, preservation methods focus on inhibiting spoilage while retaining its natural properties. Below is a detailed discussion of raw milk preservation methods, their mechanisms, effectiveness, and considerations.

Overview of Raw Milk Preservation

• Objective: Prevent spoilage (souring, rancidity, bitterness) and pathogen growth (Listeria, Salmonella) in milk straight from the udder.

• Challenges: Nutrient-rich composition (lactic acid bacteria, psychrotrophs, enzymes) and initial microbial load (~10²-10⁵ CFU/mL) make raw milk prone to rapid deterioration.

• Shelf Life: Unpreserved raw milk spoils in 6-12 hours at room temperature or 3-7 days refrigerated; preservation extends this significantly.

Methods of Raw Milk Preservation

1. Refrigeration (Cold Storage)

• Mechanism: Lowers temperature to 0-4°C, slowing microbial metabolism (e.g., Pseudomonas, LAB) and enzymatic activity (lipase, protease).

• Process: Milk is cooled immediately after milking using bulk tanks or ice baths.

• Effectiveness:

  • Extends shelf life to 5-10 days (depending on initial microbial load).

  • Psychrotrophs (Pseudomonas) still grow slowly, causing bitterness/rancidity over time.

• Considerations:

  • Most common and practical method.

  • Requires consistent cold chain; ineffective against spores (Bacillus).

• Limitations: Does not kill microbes; spoilage resumes if temperature rises.

2. Freezing

• Mechanism: Reduces temperature to -18°C or lower, halting microbial growth and enzymatic reactions by freezing water into ice crystals.

• Process: Milk is stored in sealed containers (to prevent ice crystal damage) in freezers.

• Effectiveness:

  • Extends shelf life to weeks or months (e.g., 1-6 months).

  • Preserves nutrients and flavor better than heat treatment.

• Considerations:

  • Used for small batches or colostrum storage.

  • Texture may change (fat separation, protein denaturation) upon thawing.

• Limitations: Impractical for large volumes; thawing reintroduces spoilage risk.

3. Lactoperoxidase System (LP System)

• Mechanism: Activates milk’s natural antimicrobial enzyme, lactoperoxidase, by adding thiocyanate (SCN⁻) and hydrogen peroxide (H₂O₂), producing hypothiocyanite (OSCN⁻), which inhibits bacteria.

• Process:

  • Add ~10-15 ppm SCN⁻ and H₂O₂ to raw milk.

  • Naturally present in milk but enhanced externally.

• Effectiveness:

  • Extends shelf life by 12-24 hours at 30°C or 3-5 days at 4°C.

  • Effective against Gram-negative (Pseudomonas, E. coli) and some Gram-positive bacteria.

• Considerations:

  • Approved by FAO/WHO for use in warm climates lacking refrigeration.

  • Safe, leaves no harmful residues.

• Limitations: Temporary; ineffective against spores or high microbial loads.

4. Carbon Dioxide (CO₂) Addition

• Mechanism: Dissolved CO₂ lowers pH slightly (e.g., from 6.7 to 6.0-6.4) and creates an anaerobic environment, inhibiting aerobic spoilage bacteria (Pseudomonas).

• Process: Inject food-grade CO₂ (0.5-1 g/L) into milk under pressure; store sealed.

• Effectiveness:

  • Extends refrigerated shelf life by 2-5 days.

  • Reduces psychrotrophic growth; less effect on LAB or anaerobes.

• Considerations:

  • Used commercially in some regions.

  • Minimal flavor impact at low levels.

• Limitations: Requires equipment; high CO₂ may alter taste (fizzy, acidic).

5. Microfiltration

• Mechanism: Filters milk through membranes (1-2 µm pores), removing bacteria, somatic cells, and spores while retaining proteins, fats, and lactose.

• Process: Milk is passed through ceramic or polymer filters under pressure.

• Effectiveness:

  • Reduces microbial load by 99.9% (e.g., from 10⁵ to <10² CFU/mL).

  • Extends shelf life to 2-3 weeks refrigerated (often paired with mild heat).

• Considerations:

  • Retains raw milk’s sensory and nutritional qualities.

  • Used in premium raw milk markets.

• Limitations: Expensive; does not remove enzymes or dissolved contaminants.

6. High-Pressure Processing (HPP)

• Mechanism: Applies pressure (300-600 MPa) to disrupt microbial cell membranes and inactivate enzymes without heat.

• Process: Milk is sealed in flexible containers and subjected to pressure for 3-5 minutes.

• Effectiveness:

  • Reduces bacteria by 4-6 log cycles (e.g., 10⁶ to 10¹ CFU/mL).

  • Extends shelf life to 3-6 weeks refrigerated.

• Considerations:

  • Preserves raw milk’s flavor and nutrients better than pasteurization.

  • Emerging technology for niche markets.

• Limitations: Costly; incomplete spore inactivation (Bacillus, Clostridium).

7. Addition of Natural Preservatives

• Examples:

  • Nisin: Bacteriocin from Lactococcus lactis, inhibits Gram-positive bacteria.

  • Essential Oils: E.g., oregano or thyme oil, with antimicrobial properties.

• Mechanism: Disrupt microbial cell walls or metabolism.

• Effectiveness:

  • Extends shelf life by 1-3 days (nisin) or variably (oils).

  • Targets specific bacteria (e.g., Staphylococcus, LAB).

• Considerations:

  • Nisin is GRAS (Generally Recognized As Safe); oils less studied in milk.

  • May alter flavor (herbal notes from oils).

• Limitations: Limited spectrum; regulatory approval varies.

8. Controlled Atmosphere Storage

• Mechanism: Stores milk in low-oxygen or nitrogen-rich environments to inhibit aerobic bacteria (Pseudomonas).

• Process: Milk is sealed in containers flushed with N₂ or CO₂.

• Effectiveness:

  • Extends shelf life by 2-4 days refrigerated.

  • Reduces oxidation and rancidity.

• Considerations: Experimental; not widely adopted.

• Limitations: Impractical for large-scale use; anaerobes (Clostridium) unaffected.

9. Aseptic Collection and Storage

• Mechanism: Minimizes initial contamination using sterile milking equipment and containers, reducing starting microbial load.

• Process: Closed milking systems, sanitized udders, and immediate sealing.

• Effectiveness:

  • Keeps counts <10³ CFU/mL, delaying spoilage by 1-2 days.

  • Enhances other methods (e.g., refrigeration).

• Considerations: Standard in high-quality raw milk production.

• Limitations: Does not stop growth; requires strict hygiene.

Factors Influencing Preservation Success

1. Initial Microbial Load: Milk with <10³ CFU/mL (healthy udder, clean milking) preserves better than contaminated milk (>10⁵ CFU/mL).

2. Temperature: Consistent cold (0-4°C) is critical; fluctuations accelerate spoilage.

3. Storage Time: Longer preservation (e.g., freezing, HPP) suits delayed use; short-term methods (e.g., LP system) suit immediate transport.

4. Milk Composition: High-fat milk (e.g., buffalo) is more prone to rancidity; high-lactose milk (e.g., horse) fuels fermentation.

5. Pathogen Presence: Methods like HPP or microfiltration address safety better than refrigeration alone.