Key Engineering Units in Food Processing: SI, Imperial, and More

Every food product you pick up from a supermarket shelf – whether it’s a 500 g packet of flour or a 1-litre carton of milk – carries measurements. These measurements are not random; they follow carefully defined engineering unit systems. In food processing, accurate measurement of temperature, pressure, mass, volume, and energy is non-negotiable. A slight error in unit conversion during pasteurization or sterilization can compromise food safety. That is why understanding engineering units is foundational knowledge for anyone working in food science or food technology.

What are engineering units? 🔗

Engineering units are standardized quantities used to express and measure physical properties such as length, mass, time, temperature, force, and energy. They provide a common language for scientists, engineers, and technicians across the globe. Without standardized units, a food engineer in India and a machine manufacturer in Germany would have no reliable way to communicate equipment specifications, processing parameters, or product requirements.

Over centuries, different regions developed their own measurement systems based on local conventions. As international trade and scientific collaboration expanded, the need for a unified measurement framework became clear. Today, multiple unit systems still coexist, though one – the SI system – has emerged as the global standard. Let’s look at each system and its relevance to food processing.

The Imperial (English) system 🔗

The Imperial system, also known as the British Imperial system or the FPS (foot-pound-second) system, originated from historical British measurement conventions. Its three fundamental units are the foot for length, the pound for mass, and the second for time.

In food processing, the Imperial system still appears in countries like the United States, where oven temperatures are set in Fahrenheit, liquids measured in gallons and fluid ounces, and ingredient weights given in pounds and ounces. If you’ve ever seen a recipe calling for “350°F” or “2 lbs of sugar,” that’s the Imperial system at work.

However, the Imperial system has a significant drawback: conversions between units are not decimal-based. There are 12 inches in a foot, 3 feet in a yard, 5,280 feet in a mile, and 16 ounces in a pound. These irregular conversion factors make calculations cumbersome and error-prone, especially in industrial food processing where precision is critical.

Common Imperial units in food processing 🔗

Length: inches, feet, yards. Used for equipment dimensions and facility layouts in countries that follow this system. Mass: ounces (oz), pounds (lb), tons. Commonly used for ingredient quantities and product packaging. Temperature: degrees Fahrenheit (°F). Used in baking and cooking applications, particularly in the US. Volume: fluid ounces, pints, quarts, gallons. Used for liquid ingredient measurement and storage tank capacities. Pressure: pounds per square inch (psi). Frequently encountered in steam boiler and pressure vessel specifications.

The CGS system (centimetre, gram, second) 🔗

The CGS system was one of the earliest coherent metric systems. It was proposed in 1832 by the German mathematician Carl Friedrich Gauss, who suggested basing a system of absolute units on three fundamental quantities: length, mass, and time. The specific units chosen were the centimetre (cm) for length, gram (g) for mass, and second (s) for time.

In food science, the CGS system finds limited but specific applications. It is sometimes used in laboratory settings for small-scale measurements – for instance, measuring particle sizes of powdered ingredients in centimetres or weighing small samples in grams during quality control testing.

Key CGS units 🔗

Force: dyne (1 dyne = 1 g·cm/s²). This is an extremely small unit of force, rarely practical for industrial food processing. Energy: erg (1 erg = 1 dyne·cm). Again, a very small unit; one joule equals 10 million ergs. Pressure: barye (1 dyne/cm²). Not commonly encountered in modern food engineering.

The main limitation of the CGS system is that its units are too small for practical industrial use. When dealing with factory-scale food processing – large batches of dough, high-pressure sterilization, or industrial refrigeration – CGS units result in unwieldy, very large numbers. This practical inconvenience led to the development of the MKS system.

The MKS system (metre, kilogram, second) 🔗

The MKS system addressed the CGS system’s limitations by using larger, more practical base units: the metre (m) for length, the kilogram (kg) for mass, and the second (s) for time. In 1901, Italian physicist Giovanni Giorgi proposed extending the MKS system with a fourth electrical unit, and this idea eventually gained acceptance internationally. The MKS system became the direct foundation of the modern SI system.

For food processing, MKS units are far more convenient than CGS units. Equipment dimensions are measured in metres, ingredient batch sizes in kilograms, and processing times in seconds or minutes. The derived unit of force in the MKS system is the newton (1 N = 1 kg·m/s²), and the unit of energy is the joule (1 J = 1 N·m) – both of which are practical sizes for real-world engineering calculations.

The SI system (Système International d’Unités) 🔗

The International System of Units (SI) is the most widely accepted measurement system in the world today. It is the modern, refined version of the metric system and serves as the global standard for scientific, engineering, and commercial measurements. According to the International Bureau of Weights and Measures (BIPM), all SI units have been defined in terms of fundamental physical constants since May 2019, ensuring long-term stability and precision.

The National Institute of Standards and Technology (NIST) describes the SI as resting on a foundation of seven defining constants, including the speed of light, the Planck constant, and the Avogadro constant. These constants anchor every SI unit to unchanging properties of nature rather than physical artefacts.

The seven SI base units 🔗

The SI system is built on seven base units, each representing a fundamental physical quantity:

1. Metre (m) – Length: The metre is the SI unit of length. In food processing, it is used for measuring equipment dimensions, conveyor belt lengths, pipe diameters, and facility layouts. For example, the length of a continuous pasteurization tunnel or the diameter of a mixing vessel would be expressed in metres.

2. Kilogram (kg) – Mass: The kilogram is the SI unit of mass. It is the most frequently used base unit in food processing – for measuring raw material quantities, batch sizes, product weights, and packaging specifications. An industrial dough mixer, for instance, might handle batches of 200-500 kg.

3. Second (s) – Time: The second is the SI unit of time. It is fundamental for defining process durations, reaction rates, and equipment operating cycles. Pasteurization holding times, for example, are specified in seconds (such as 72°C for 15 seconds in HTST pasteurization).

4. Ampere (A) – Electric current: The ampere measures electric current. In food processing plants, it is relevant for specifying motor ratings, electrical equipment loads, and power consumption of processing machinery.

5. Kelvin (K) – Thermodynamic temperature: The kelvin is the SI unit of temperature. While Celsius (°C) is more commonly used in everyday food processing, kelvin is essential for scientific calculations involving thermodynamic properties, heat transfer, and thermal processing design.

6. Mole (mol) – Amount of substance: The mole quantifies the amount of a chemical substance. It is used in food chemistry calculations, such as determining concentrations of preservatives, acidity levels, or nutrient content during formulation.

7. Candela (cd) – Luminous intensity: The candela measures luminous intensity. While less directly used in food processing, it is relevant in designing quality inspection systems, optical sorting equipment, and proper lighting for food production facilities.

Derived SI units important in food processing 🔗

Beyond the seven base units, the SI system includes derived units that are formed by combining base units mathematically. These derived units measure more complex physical quantities encountered daily in food engineering operations.

Newton (N) – Force: One newton equals one kilogram-metre per second squared (kg·m/s²). Newtons are used to measure forces in mixing, compression, extrusion, and structural loading on equipment. For example, the force required to extrude pasta dough through a die or the compressive force in a tablet press for food supplements.

Pascal (Pa) – Pressure: One pascal equals one newton per square metre (N/m²). Pressure measurement is critical in steam-based sterilization (retort processing), vacuum packaging, and high-pressure processing (HPP). In practice, kilopascals (kPa) and megapascals (MPa) are more commonly used because one pascal represents a very small amount of pressure.

Joule (J) – Energy: One joule equals one newton-metre (N·m). Energy calculations in food processing involve heating, cooling, drying, and mechanical operations. For instance, calculating how much energy is needed to heat 1,000 litres of milk from 4°C to 72°C requires working with joules (or kilojoules).

Watt (W) – Power: One watt equals one joule per second (J/s). Watt is used to rate motors, heating elements, refrigeration compressors, and other equipment. A commercial food dryer might consume 50 kW of power.

Degree Celsius (°C) – Temperature: Although not a base unit, Celsius is a derived SI unit widely used in food processing for specifying cooking, chilling, freezing, and storage temperatures.

Supplementary and special units 🔗

The SI system also recognizes certain supplementary measurements. The radian (rad) measures plane angles, and the steradian (sr) measures solid angles. While these are not commonly encountered in food processing, they can be relevant in fields like optical sensing and radiation-based food preservation techniques.

Additionally, some non-SI units are still accepted for use alongside SI because of their deep-rooted practical importance. These include the minute, hour, and day for time; the litre (L) for volume; the tonne (t) for mass (1 t = 1,000 kg); and the bar for pressure. In food processing, you will encounter these units regularly – a batch processing time of 2 hours, a storage tank of 10,000 litres, or a boiler pressure of 3 bar.

Why the SI system dominates food processing globally 🔗

There are several compelling reasons why SI has become the universal standard in food science and engineering:

Decimal-based conversions: All SI units scale by powers of ten, using prefixes like kilo-, milli-, and micro-. Converting between milligrams, grams, and kilograms is straightforward – just move the decimal point. This eliminates the kind of conversion errors that plague non-decimal systems.

Global acceptance: The SI is recognized and used by virtually every country in the world for scientific and industrial purposes. When a food company in India exports products to Europe, both sides use the same measurement language.

Regulatory compliance: International food safety standards, including those set by Codex Alimentarius (a joint FAO/WHO body), specify limits and parameters in SI units. Nutritional labelling, contaminant limits, and processing requirements all rely on SI measurements.

Scientific consistency: Research papers, technical manuals, and engineering handbooks in food science overwhelmingly use SI units. This consistency ensures that knowledge is transferable and reproducible across laboratories and production facilities worldwide.

Unit conversion: a practical necessity 🔗

Despite SI’s dominance, food processing professionals frequently need to convert between unit systems. Imported equipment from the United States might have specifications in Imperial units. Older research literature may use CGS units. Being able to convert accurately is a practical skill.

Here are some common conversions relevant to food processing: 1 inch = 2.54 cm; 1 pound = 0.4536 kg; 1 gallon (US) = 3.785 litres; 1 psi = 6.895 kPa; °F to °C: subtract 32, then multiply by 5/9; 1 BTU (British Thermal Unit) = 1,055 joules; 1 calorie = 4.186 joules.

A common real-world scenario: if a US-manufactured retort specifies operating pressure in psi and your plant’s gauges read in kPa, you need to multiply the psi value by 6.895 to get the correct kPa reading. Getting this conversion wrong could mean under-processing (risking microbial survival) or over-processing (degrading product quality).

Dimensional consistency in food engineering calculations 🔗

One of the most important principles when working with engineering units is dimensional consistency. Every term in an engineering equation must have compatible units. If you are calculating heat energy using the formula Q = m × c × ΔT (where Q is heat energy, m is mass, c is specific heat capacity, and ΔT is the temperature change), all values must be in consistent units – typically joules, kilograms, J/(kg·K), and kelvin (or °C, since the interval is identical).

Mixing units from different systems in a single calculation without proper conversion is one of the most common sources of error in food engineering. Ensuring dimension and unit consistency in equations is a fundamental practice in food process design.

SI prefixes: scaling units for practical use 🔗

SI prefixes allow the same base or derived unit to be scaled up or down to suit the measurement at hand. This is one of SI’s greatest strengths. Here are the prefixes most commonly used in food processing:

Mega (M) = 10⁶: Megapascal (MPa) is used in high-pressure food processing, where pressures can reach 400-600 MPa. Kilo (k) = 10³: Kilogram (kg), kilojoule (kJ), kilopascal (kPa), and kilowatt (kW) are everyday units in food plants. Centi (c) = 10⁻²: Centimetre (cm) is used for smaller measurements like product dimensions or film thickness. Milli (m) = 10⁻³: Milligram (mg) and millilitre (mL) are standard in food chemistry and nutritional analysis. Micro (μ) = 10⁻⁶: Micrometre (μm) is used for measuring particle sizes in powdered foods, and microgram (μg) for trace nutrient or contaminant analysis.

Bringing it all together 🔗

Engineering units are not just an academic topic – they are the backbone of every measurement, calculation, and specification in food processing. The evolution from region-specific systems like the Imperial and CGS to the globally accepted SI system reflects the food industry’s increasing need for precision, standardization, and international compatibility. Whether you are calibrating a thermocouple, sizing a pump, or labelling a product for export, a solid understanding of these unit systems will serve you well throughout your career in food technology.

What do you think? How often do unit conversion challenges come up in your day-to-day food processing or laboratory work? And with the SI system being nearly universal, do you think the Imperial system will eventually phase out completely in the food industry?

References

1. https://www.sciencedirect.com/topics/engineering/mks-system
2. https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units
3. https://en.wikipedia.org/wiki/MKS_units
4. https://www.bipm.org/en/measurement-units
5. https://www.nist.gov/pml/owm/metric-si/si-units
6. https://www.fao.org/fao-who-codexalimentarius/en/
7. https://www.sciencedirect.com/science/article/pii/B9780128184738000074