Temperature measurement is a straightforward but essential physical test to assess milk’s storage conditions, freshness, and safety. It is typically conducted at various stages of the dairy supply chain, including collection, transport, storage, and processing. The goal is to ensure milk is maintained at safe temperatures (e.g., below 4°C for raw milk, 4–7°C for pasteurized milk) to prevent microbial proliferation and spoilage.
1. Equipment for Temperature Measurement
The choice of equipment depends on the setting (e.g., farm, collection center, dairy plant) and required precision. Common tools include:
Digital Thermometers: Portable, battery-operated devices with a probe for quick and accurate readings (accuracy: ±0.1°C).
Infrared Thermometers: Non-contact devices that measure surface temperature, suitable for rapid checks but less accurate for bulk milk.
Glass Mercury or Alcohol Thermometers: Traditional, less common due to safety concerns (mercury) but still used in some rural settings (accuracy: ±0.5°C).
Thermocouples: Advanced sensors used in automated dairy systems for continuous monitoring (accuracy: ±0.1°C).
Data Loggers: Devices that record temperature over time, used in transport or storage tanks to ensure compliance with cold chain standards.
Automated Inline Sensors: Integrated into milking machines or processing lines for real-time temperature monitoring in large dairies.
Indian Context: Digital thermometers and thermocouples are widely used in cooperatives (e.g., Amul, Mother Dairy) and FSSAI-accredited labs, while glass thermometers are still prevalent in rural collection centers due to cost-effectiveness.
2. Procedure for Measuring Milk Temperature
The procedure varies slightly based on the milk source (e.g., cans, tanks, retail packages) and equipment used. Below is a standardized method for measuring temperature using a digital thermometer, the most common tool in India.
a. Preparation
Calibrate the Thermometer: Verify the thermometer’s accuracy by placing the probe in a known temperature standard (e.g., ice water at 0°C or boiling water at 100°C, adjusted for altitude). Calibration should be done monthly or as per FSSAI guidelines.
Sterilize the Probe: Clean the thermometer probe with 70% ethanol or a food-grade sanitizer and rinse with sterile water to prevent contamination.
Sample Collection:
Collect a representative milk sample (e.g., 100–500 mL) using proper sampling techniques (e.g., single or composite sampling, as described in previous responses).
For bulk milk (e.g., tanks), agitate the milk mechanically for 5–10 minutes to ensure uniform temperature distribution.
Environment: Conduct measurements in a clean area to avoid external heat sources (e.g., direct sunlight) affecting readings.
b. Measurement Process
Insert the Probe:
For small containers (e.g., cans, bottles): Insert the digital thermometer probe at least 5–10 cm into the milk, ensuring it is submerged in the center of the sample.
For bulk tanks: Use a long probe to reach the middle of the tank, avoiding contact with tank walls, which may have different temperatures.
For retail packages (e.g., pouches): Open the package aseptically and insert the probe, or use an infrared thermometer for surface measurement (less accurate).
Stabilize the Reading:
Wait 10–30 seconds for the digital thermometer to stabilize and display a consistent reading (most modern devices beep when stable).
Record the temperature in degrees Celsius (°C) to one decimal place (e.g., 4.2°C).
Repeat for Accuracy: Take 2–3 readings from different parts of the sample (e.g., top, middle, bottom) and calculate the average to account for temperature gradients.
Inline Measurement (for automated systems):
Use thermocouples or inline sensors installed in milking machines, transport tanks, or processing lines to continuously monitor temperature.
Record data via a digital interface or data logger for compliance documentation.
c. Interpretation
Expected Temperatures (per FSSAI and Codex standards):
Raw Milk: Below 4°C immediately after milking and during transport/storage to prevent microbial growth.
Pasteurized Milk: 4–7°C during storage and retail to maintain shelf-life.
UHT Milk: Room temperature (20–25°C) if unopened, but refrigerated (4–7°C) after opening.
Abnormal Readings:
Above 4°C for raw milk: Indicates inadequate cooling, risking bacterial proliferation (e.g., coliforms, psychrotrophs).
Above 7°C for pasteurized milk: Suggests cold chain failure, reducing shelf-life.
Below 0°C: Indicates freezing, which may damage milk’s protein structure and texture.
Action: If temperatures exceed safe limits, reject the milk for processing or conduct further tests (e.g., total plate count, titratable acidity) to assess spoilage.
3. Precautions and Considerations
Sterility: Always sterilize the probe before and after use to avoid introducing microbes or contaminants, critical for microbiological safety in India’s warm climate.
Calibration: Regularly calibrate thermometers to ensure accuracy, as per FSSAI’s Manual of Methods of Analysis of Foods: Milk and Milk Products (2025).
Probe Placement: Avoid contact with container walls or air pockets, which may give false readings.
Rapid Measurement: Measure temperature immediately after sampling to prevent warming or cooling of the sample during handling.
Cold Chain Monitoring: Use data loggers or inline sensors for continuous monitoring during transport, especially in India’s rural areas with limited refrigeration.
Documentation: Record temperature readings with sample details (e.g., date, time, source) for traceability, as required by FSSAI.
Environmental Factors: Avoid measuring near heat sources (e.g., machinery, sunlight) or in humid conditions that may affect equipment performance.
Equipment Maintenance: Store thermometers in protective cases and replace batteries regularly to ensure reliability.
Indian Challenges:
Limited access to advanced equipment (e.g., thermocouples) in rural areas necessitates reliance on portable digital thermometers.
Frequent power outages in India require backup cooling systems to maintain safe temperatures during measurement and storage.
4. Applications in India
Milk Collection Centers: Temperature checks are mandatory to accept raw milk from farmers, ensuring it is below 4°C to prevent spoilage (common in cooperatives like Amul).
Transport and Storage: Data loggers monitor milk temperature during transport to dairy plants, addressing India’s cold chain gaps.
Processing: Inline sensors verify milk temperature during pasteurization (e.g., 72°C for 15 seconds) and cooling (below 4°C).
Regulatory Compliance: FSSAI’s 2018 National Milk Safety and Quality Survey used temperature measurements to assess cold chain adherence.
Public Health: Proper temperature control prevents microbial growth, critical in India where 41% of milk samples were non-compliant in the 2018 FSSAI survey.