Titratable Acidity of Milk

The determination of titratable acidity (TA) of milk is a widely used method to assess its freshness, quality, and suitability for processing. It measures the total acid content (primarily lactic acid from bacterial fermentation) by titrating milk with a base, typically sodium hydroxide (NaOH), using an indicator like phenolphthalein. Here’s a detailed explanation:

Purpose

• Freshness: Detects acid development from microbial activity, indicating spoilage.

• Processing Suitability: High acidity can destabilize proteins during heat treatment (e.g., pasteurization, UHT).

• Quality Control: Ensures milk meets standards for consumption or product manufacture (e.g., cheese, yogurt).

Principle

• Milk naturally contains weak acids (e.g., lactic, citric, phosphoric) and buffering components (e.g., proteins, phosphates). Fresh milk has a low titratable acidity due to minimal lactic acid. As bacteria (e.g., Lactobacillus, Streptococcus) ferment lactose into lactic acid, acidity increases, lowering pH and raising TA.

• TA is expressed as a percentage of lactic acid (% LA), the dominant acid in spoiled milk, by titrating with NaOH until a neutral pH (~8.3) is reached, indicated by a color change.

Procedure

1. Sample Preparation:

   • Take a well-mixed milk sample (e.g., 10 mL or 9 mL, depending on protocol) at room temperature (~20°C/68°F).

   • Use fresh or refrigerated milk; avoid shaking excessively to prevent fat separation.

2. Equipment:

   • Burette (e.g., 10 mL or 25 mL) with 0.1 N NaOH (standardized sodium hydroxide).

   • Pipette or measuring cylinder.

   • Conical flask or beaker.

   • Phenolphthalein indicator (1% in ethanol).

   • White background for color observation.

3. Steps:

   • Pipette 10 mL of milk into a flask.

   • Add 3-5 drops of phenolphthalein indicator; milk stays white if acidic.

   • Fill the burette with 0.1 N NaOH.

   • Titrate slowly, swirling the flask, until the first persistent faint pink color appears (endpoint, pH ~8.3).

   • Record the volume of NaOH used (e.g., 1.5 mL).

4. Calculation:

   • Formula:
     TA (% lactic acid) = (Volume of NaOH in mL × Normality of NaOH × 0.009 × 100) / Volume of milk in mL.

   • Simplified (for 10 mL milk, 0.1 N NaOH):
     TA (%) = Volume of NaOH (mL) × 0.009.

   • Example: 1.5 mL NaOH used → TA = 1.5 × 0.009 = 0.0135% LA.

Interpretation

• Fresh Milk:

  • TA: 0.13-0.16% LA (pH 6.6-6.8).

  • Indicates natural acidity from casein, phosphates, and citric acid.

• Slightly Acidic:

  • TA: 0.17-0.20% LA (pH ~6.4-6.6).

  • Early bacterial activity; may still be usable but monitor stability.

• Spoiled Milk:

  • TA: >0.20% LA (pH <6.4).

  • Significant lactic acid from fermentation; unsuitable for fluid use, fails COB/alcohol tests.

• Mastitis Milk:

  • TA may be normal or slightly elevated, but pH shifts (e.g., >6.8) due to ionic changes.

Factors Affecting Titratable Acidity

1. Microbial Activity:

   • Lactic acid bacteria increase TA over time, especially if milk is stored >4°C (39°F).

2. Storage Time/Temperature:

   • Fresh milk at 4°C holds TA steady for days; at 20°C, TA rises within hours.

3. Milk Type:

   • Buffalo milk (~0.15-0.18% TA) slightly higher than cow’s milk due to higher solids.

   • Colostrum has higher TA (~0.2-0.3%) from elevated proteins.

4. Adulteration:

   • Water dilutes TA; added acids (rare) increase it artificially.

5. Cow Health:

   • Mastitis may lower TA slightly due to reduced lactose, but ionic shifts dominate.

Applications

• Platform Testing: Screens incoming milk; >0.20% TA often leads to rejection.

• Processing: High TA milk curdles during heating, affecting cheese or UHT yield.

• Shelf Life: Tracks spoilage progression in raw or pasteurized milk.

• Regulatory: Ensures compliance (e.g., India FSSAI: TA <0.17% for fresh milk).

Advantages

• Simple: Basic lab equipment (burette, indicator) suffices.

• Rapid: Takes 5-10 minutes.

• Reliable: Correlates well with microbial spoilage and stability.

Limitations

• Buffering Effect: Milk’s natural buffers (e.g., phosphates) mask small acid increases; TA lags pH drop initially.

• Subjectivity: Endpoint (faint pink) can vary by observer; over-titration inflates results.

• Non-Specific: Measures total acidity, not just lactic acid; other acids (e.g., citric) contribute.

Correlation with Other Tests

• pH: TA 0.13-0.16% ≈ pH 6.6-6.8; TA >0.20% ≈ pH <6.4.

• COB/Alcohol Tests: TA >0.17-0.20% often predicts protein instability.

• Resazurin: High TA aligns with faster dye reduction (high bacterial activity).

Practical Example

• Scenario: 10 mL milk titrated with 0.1 N NaOH; 1.6 mL used.

  • TA = 1.6 × 0.009 = 0.0144% LA.

  • Interpretation: Fresh milk, suitable for processing.

• Alternative: 2.5 mL NaOH used.

  • TA = 2.5 × 0.009 = 0.0225% LA.

  • Interpretation: Spoiled, likely to clot on boiling.