Methods for Measuring Temperature in Milk Processing

Temperature measurement is one of the most critical operations in any milk processing facility. Whether it’s pasteurization, chilling, or storage, even a slight deviation in temperature can compromise product safety and quality. The U.S. Food and Drug Administration mandates strict time-temperature combinations at every control point – for instance, HTST pasteurization requires heating milk to exactly 72°C (161.6°F) for 15 seconds. Too low, and harmful pathogens like Salmonella and E. coli may survive. Too high, and you risk destroying valuable nutrients. That’s why choosing the right temperature measurement instrument – and understanding how it works – matters so much in dairy operations.

Why accurate temperature measurement matters in milk processing 🔗

Milk is a highly perishable product. Its high moisture content, rich nutrients, and near-neutral pH make it an ideal breeding ground for bacteria. Contaminated milk can lead to serious foodborne diseases such as brucellosis, listeriosis, and tuberculosis. This is why every stage of milk processing – from collection and transport to pasteurization and cold storage – depends on precise temperature control.

According to dairy industry data, the average temperature of incoming raw milk at a processing plant is around 6.18°C, and after initial cooling it drops to about 4.74°C. Maintaining these narrow temperature windows requires reliable instruments that can deliver consistent, accurate readings throughout the processing chain. A failure in temperature monitoring doesn’t just affect product quality – it can trigger costly recalls and regulatory penalties.

Understanding temperature scales used in dairy processing 🔗

Before we discuss specific instruments, it’s worth understanding the three temperature scales commonly encountered in milk processing.

Celsius (°C) 🔗

Celsius is the most widely used scale in dairy processing worldwide. It is based on two fixed reference points: the freezing point of water at 0°C and the boiling point of water at 100°C, both at standard atmospheric pressure. Most pasteurization guidelines, storage requirements, and processing standards are specified in Celsius, making it the default scale for dairy professionals across most countries.

Fahrenheit (°F) 🔗

Fahrenheit remains the standard in the United States dairy industry. The Pasteurized Milk Ordinance (PMO) established by the FDA specifies many of its temperature requirements in Fahrenheit. For example, raw milk must be maintained within 32-40°F (0-4.4°C) during transport and storage. To convert between the two scales, you can use the formula: °F = (°C × 9/5) + 32.

Kelvin (K) 🔗

The Kelvin scale is primarily used in scientific research and thermodynamic calculations related to dairy processing. It is an absolute temperature scale that starts at absolute zero (−273.15°C), the point at which molecular motion theoretically stops. While you won’t find Kelvin on a pasteurizer display, it plays a role in research and development work within the dairy sector, especially in studies involving gas behaviour and heat transfer calculations.

Glass thermometers 🔗

Glass thermometers are among the oldest and most familiar temperature measurement devices. Despite the rise of electronic alternatives, they remain in use across many dairy operations for their simplicity and reliability.

How mercury glass thermometers work 🔗

A mercury glass thermometer consists of a sealed glass tube with a mercury-filled reservoir (bulb) at one end, connected to a very narrow channel called the capillary bore. The working principle is straightforward: as temperature rises, the mercury inside the bulb expands and rises up the calibrated scale etched on the stem. As temperature drops, the mercury contracts and moves back down. This direct, predictable relationship between temperature and mercury volume provides a reliable reading.

Mercury thermometers can measure a wide range of temperatures, roughly from −37°C to 356°C. This range can be extended even further – up to 800°C – by filling the space above the mercury with an inert gas like nitrogen, which raises the boiling point of the mercury, and by using fused quartz glass instead of standard Pyrex.

Advantages and limitations 🔗

The key strength of mercury thermometers is their accuracy and stability. They don’t require batteries, they don’t suffer from electronic drift, and when properly calibrated, they can provide readings accurate to within 0.1°C. Dedicated dairy mercury thermometers with specific temperature ranges (such as 0-220°F) have been standard equipment in milk collection centres and quality control laboratories for decades.

However, mercury thermometers come with significant drawbacks. Mercury is toxic, and a broken thermometer releases harmful vapours. This has led to a growing international movement to phase them out. The U.S. Environmental Protection Agency actively discourages their use, and at least 13 U.S. states have passed laws restricting the sale of mercury thermometers. In dairy applications, this has accelerated the shift toward electronic alternatives, though mercury thermometers are still permitted in some industrial testing contexts.

Pressure-spring thermometers 🔗

Pressure-spring thermometers – also called Bourdon tube thermometers – are mechanical devices widely used in dairy and other food processing industries, especially where remote temperature reading is needed or where electrical instruments aren’t practical.

Working principle 🔗

The thermal system of a pressure-spring thermometer consists of three main components: a metal bulb, a capillary tube, and a receiving element (the Bourdon tube or pressure spring). The metal bulb is filled with a thermometric fluid – a liquid, gas, or liquid-vapour combination – and is placed at the measurement point. When temperature changes, the fluid expands or contracts, altering the pressure within the closed system. This pressure change travels through the capillary tube to the Bourdon tube, a C-shaped metal tube with an oval cross-section. As internal pressure changes, the tube straightens or curves, and this movement is transferred through a mechanical linkage to a pointer on a calibrated dial.

Types of pressure-spring thermometers 🔗

There are three main types, classified by the fluid used inside:

Liquid-expansion type: These use mercury, ethanol, or toluene as the filling liquid. They work similarly to liquid-in-glass thermometers but operate a Bourdon tube for readings. Mercury is the most common choice due to its wide temperature range (approximately −35°C to 540°C). Ethanol and toluene have a thermal expansion rate about six times greater than mercury, which can be advantageous for sensitivity.

Gas-expansion type: These use an inert gas, typically nitrogen, under high initial pressure. They operate based on the ideal gas law – at constant volume, pressure is directly proportional to temperature. Gas-filled thermometers offer a good linear relationship between temperature and pressure, with ranges typically from −130°C to +540°C.

Vapour-pressure type: These contain both a liquid and its vapour within the system. The pressure reading depends on the saturated vapour pressure at the bulb temperature, making them independent of ambient temperature variations in the capillary and Bourdon tube. However, the pressure-temperature relationship is non-linear, meaning the scale divisions are uneven.

Advantages and limitations 🔗

Pressure-spring thermometers are valued for their fundamental simplicity. They require no external power, are robust enough for harsh industrial environments, and can transmit temperature readings over distances – some systems work with capillary lengths of up to 100 metres. They are also relatively inexpensive and provide good accuracy, typically within ±1°C.

On the downside, their bulb size may be too large for tight spaces, their maximum temperature range is more limited than electronic systems, and they need proper calibration of all three components (bulb, capillary, and Bourdon tube). Elevation differences between the bulb and gauge can also introduce pressure errors that affect accuracy.

Electrical resistance thermometers (RTDs) 🔗

For applications demanding the highest accuracy – such as pasteurization control – electrical resistance thermometers, commonly known as Resistance Temperature Detectors (RTDs), are the preferred choice. These are electronic sensors that determine temperature by measuring the change in electrical resistance of a metal element.

Working principle 🔗

RTDs operate on a well-established physical property: the electrical resistance of pure metals increases predictably as temperature rises. A small, constant electric current is passed through a sensing element – a fine wire or thin film made of a pure metal. As the temperature changes, the resistance of this element changes proportionally. This resistance is measured (typically using a Wheatstone bridge circuit) and then converted into a temperature reading using a known calibration curve.

The most widely used RTD is the Pt100 – a platinum element with a resistance of 100 ohms at 0°C. Its resistance changes at a rate of approximately 0.385 ohms per degree Celsius. This nearly linear relationship between resistance and temperature makes platinum RTDs exceptionally easy to calibrate and highly predictable over a wide temperature range.

Why platinum is the preferred material 🔗

Platinum dominates the RTD market for several reasons. It can be manufactured to very high purity, which ensures consistent and repeatable resistance-temperature characteristics. It is chemically stable, resistant to corrosion, and does not oxidize easily. Its melting point is 1,772°C, far above the temperatures encountered in dairy processing. When properly calibrated, a platinum RTD can achieve reproducibility to less than a thousandth of a degree Celsius, making it one of the most accurate temperature sensors available.

Other metals used in RTDs include nickel (less expensive but limited to about 300°C and less stable over time) and copper (excellent linearity but prone to oxidation).

RTD construction types 🔗

RTDs are available in three main construction styles:

Wire-wound RTDs: These consist of fine platinum wire coiled around a ceramic or glass core. They provide the highest accuracy and are suited for demanding applications, including dairy pasteurization systems.

Thin-film RTDs: A thin layer of platinum is deposited onto a ceramic substrate. These are more compact, less expensive, and have a faster response time than wire-wound types, making them popular in many industrial settings.

Coiled-element RTDs: The resistance wire is loosely placed within a ceramic form filled with non-conductive powder. This design allows the wire to expand and contract freely, minimising strain-induced errors while improving heat transfer and response time.

Wire configurations for accuracy 🔗

One practical consideration with RTDs is that the lead wires connecting the sensor to the measurement instrument also have resistance, which can affect the reading. To address this, RTDs come in different wiring configurations:

2-wire configuration: The simplest setup, but lead wire resistance can introduce errors. Suitable only for low-accuracy applications.

3-wire configuration: The most common in industry. A third “dummy” wire compensates for lead resistance, significantly improving accuracy.

4-wire configuration: Provides the best accuracy by completely eliminating lead wire resistance effects. This is the standard in laboratory and calibration settings.

Advantages and limitations of RTDs 🔗

RTDs offer several compelling benefits: high accuracy (typically ±0.1°C), excellent long-term stability, good linearity, and repeatable measurements over years of use. They are the go-to sensor for critical control points in dairy processing where even a small temperature error can have serious safety implications.

Their limitations include a relatively slower response time compared to thermocouples, the need for an external power source, and higher cost – particularly for platinum wire-wound types. They are also less suited for temperatures above 660°C, though this is well beyond the range needed in dairy applications.

Choosing the right thermometer for dairy applications 🔗

The choice of temperature measurement instrument depends on the specific requirements of each processing stage. For critical control points like pasteurization, where precision and fast response are essential, RTDs are the clear winner. Modern dairy facilities often use self-calibrating hygienic temperature sensors that can read milk temperature in real time during pasteurisation and ensure the temperature stays constant within the desired range.

For general monitoring tasks – such as checking storage tank temperatures or incoming milk temperature – pressure-spring thermometers can offer adequate accuracy at a lower cost, especially in locations where electrical power isn’t available. Glass thermometers, while increasingly being phased out, may still appear in some quality control laboratories for calibration verification or specific testing protocols.

In practice, modern dairy plants often use a combination of these instruments. Automated temperature monitoring systems with electronic sensors are becoming the norm, especially as the industry invests heavily in new processing capacity. However, understanding the fundamental principles behind each type of thermometer – thermal expansion, fluid pressure, and electrical resistance – gives dairy professionals the knowledge to select, use, and troubleshoot their equipment effectively.

What do you think? How important is it for dairy professionals to understand the working principles behind temperature instruments, rather than simply relying on digital readouts? And with mercury thermometers being phased out globally, what challenges do smaller dairy operations face in upgrading to electronic alternatives?

References

1. https://www.fda.gov/
2. https://www.who.int/news-room/fact-sheets/detail/food-safety
3. https://www.advancedenergy.com/en-us/about/news/blog/how-to-automatically-record-milk-temperatures-in-the-dairy-industry-datalogging-thermometers-for-p/
4. https://en.wikipedia.org/wiki/Mercury-in-glass_thermometer
5. https://www.epa.gov/mercury/mercury-thermometers
6. https://blog.wika.com/en/knowhow/bourdon-tube-pressure-gauge-functional-principle/
7. https://www.forbesmarshall.com/instrupedia/pressure-spring-thermometers/
8. https://www.sciencedirect.com/topics/engineering/pressure-thermometer
9. https://en.wikipedia.org/wiki/Resistance_thermometer
10. https://www.electrical4u.com/resistance-temperature-detector-or-rtd-construction-and-working-principle/
11. https://www.sciencedirect.com/topics/engineering/platinum-resistance-thermometer
12. https://www.prelectronics.com/the-fundamentals-of-rtd-temperature-sensors/
13. https://www.elpro.si/en/the-importance-of-measurement-in-the-dairy-industry/