Utilizing Radiation for Heat Transfer in Food Processes

Radiation is one of the three fundamental modes of heat transfer, alongside conduction and convection. But unlike the other two, radiation doesn’t need any physical medium to carry energy – it travels through electromagnetic waves. In food processing, this characteristic makes radiation uniquely powerful. From the browning of bread in an oven to the sublimation stage in freeze-drying, radiative heat transfer plays a critical role in operations that demand precise surface heating and energy efficiency. Understanding how radiation works – and how to calculate it – is essential for anyone studying food processing and engineering.

What is radiation heat transfer? 🔗

Radiation is the transfer of thermal energy through electromagnetic waves, primarily in the infrared spectrum. Every object above absolute zero emits some thermal radiation, and the warmer the object, the more energy it radiates. In food processing, the infrared portion of the spectrum (wavelengths from about 0.75 µm to 1000 µm) is the most relevant because food components – water, proteins, fats, and sugars – absorb infrared energy efficiently.

What sets radiation apart from conduction and convection is that it requires no contact and no medium. Heat travels directly from the source to the food surface at the speed of light. This is why radiation is considered the fastest mode of heat transfer. When you feel the warmth of a fire without touching it, that’s radiant energy at work. In an industrial oven, much of the energy reaching the food surface comes not from circulating hot air alone, but from radiant heat emitted by the oven walls and heating elements.

The Stefan-Boltzmann law 🔗

The mathematical foundation for calculating radiative heat transfer is the Stefan-Boltzmann law. This law states that the total energy radiated by a surface per unit time is proportional to the fourth power of its absolute temperature. The equation is:

q = σ × ε × A × T⁴

Where:

q = heat transfer rate (W), σ = Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴), ε = emissivity of the surface (dimensionless, ranging from 0 to 1), A = surface area (m²), and T = absolute temperature in Kelvin.

The fourth-power relationship is what makes this law so significant in practice. A relatively small increase in temperature produces a dramatic increase in radiated energy. If you double the absolute temperature of a surface, the radiant energy output increases sixteen-fold. This explains why high-temperature processes like broiling and grilling can rapidly brown and cook food surfaces.

Net radiative heat transfer between two surfaces 🔗

In real food processing scenarios, radiation doesn’t just flow from a hot surface into empty space. There’s always a surrounding environment radiating energy back. The net rate of radiative heat transfer between a food surface at temperature T₁ and its surroundings at temperature T₂ is given by:

q_net = σ × ε × A × (T₂⁴ − T₁⁴)

When T₂ is greater than T₁, the food absorbs more energy than it emits – it heats up. When the food is hotter than its surroundings, it loses heat by radiation. This net exchange equation is fundamental to designing ovens, dryers, and freezers where precise thermal control matters. According to the Stefan-Boltzmann law formulation, the emissivity value in this equation pertains to the object alone, regardless of the surroundings’ surface properties.

The role of emissivity 🔗

Emissivity (ε) measures how effectively a surface emits or absorbs thermal radiation compared to an ideal blackbody, which has an emissivity of 1. A perfect reflector, like a polished metal surface, has an emissivity close to 0. Real materials fall somewhere in between.

In food processing, emissivity is a highly practical concept. Most food surfaces – bread crust, meat, vegetables, dough – have emissivities in the range of 0.8 to 0.95, making them excellent absorbers and emitters of infrared radiation. This is one reason why radiant heating is so effective for food: food surfaces naturally absorb a very high proportion of the infrared energy directed at them.

By contrast, shiny metallic surfaces like polished aluminium reflect most of the incident radiation and have low emissivities (around 0.1). This property is deliberately used in food processing equipment design. For instance, reflective aluminium linings inside infrared dryers bounce radiation back toward the food, improving energy efficiency.

Why emissivity matters in equipment design 🔗

Understanding emissivity helps food engineers select the right materials for heating chambers, trays, and reflectors. In infrared heating systems, ceramic emitters and gas-fired radiators are chosen based on the wavelength and intensity of radiation they produce. The food’s emissivity, combined with the emitter’s characteristics, determines how efficiently energy transfers to the product. Poor matching between emitter wavelength and food absorption characteristics leads to wasted energy and uneven heating.

How temperature differences drive radiative heat transfer 🔗

The fourth-power dependence on temperature means that the temperature difference between the heat source and the food is the most influential variable in radiation-based food processing. Consider an oven heating element at 500°C (773 K) and a food surface at 100°C (373 K). The difference in T⁴ values is enormous, producing a very high rate of energy transfer to the food surface.

This sensitivity also demands careful control. In industrial baking or broiling, even small fluctuations in the heating element temperature can cause significant changes in radiant output. Modern food processing systems often use feedback control based on the food’s surface temperature rather than just the heater temperature. This approach provides more consistent results, especially in processes like browning where surface temperature directly governs the Maillard reaction and crust formation.

Applications of radiation in food processing 🔗

Baking 🔗

Baking is one of the most established applications of radiative heat transfer. Inside an oven, heat reaches the dough through a combination of radiation, convection, and conduction. Radiation is particularly responsible for crust formation and browning. Infrared ovens transfer heat rapidly to bread surfaces, achieving penetration depths of 2-3 mm. This rapid surface heating triggers starch gelatinisation, protein denaturation, and the Maillard reaction – the chemical process responsible for the golden-brown crust. Research published in Molecules notes that infrared baking can reduce processing time while producing results comparable to or better than conventional ovens.

A notable development is the combination of infrared and microwave heating for baking. In this approach, microwaves heat the interior of the product quickly, while infrared radiation handles surface browning and crisping. This combination addresses the common problem of microwave baking, which tends to produce soft, pale surfaces lacking the appeal of traditionally baked goods.

Drying and dehydration 🔗

Infrared radiation is widely used for drying agricultural products and food materials. Water molecules strongly absorb infrared energy, particularly at wavelengths around 2.7, 6.0, and beyond 12.5 µm. When infrared radiation strikes a moist food surface, the O-H bonds in water molecules begin to vibrate at the frequency of the incoming radiation, converting radiant energy into heat and driving evaporation.

Infrared drying offers several advantages over conventional hot-air drying. According to a review in Foods, IR drying provides higher heat transfer rates, shorter drying times, improved energy efficiency, and better retention of product quality. One study combining infrared with convective drying of vegetables found a 48% reduction in drying time and 63% savings in energy consumption compared to conventional methods alone.

However, because infrared has limited penetration depth – typically just a few millimetres – it works best for thin layers of food material. For thicker products, infrared is commonly used in combination with hot air or vacuum drying to achieve both surface and internal moisture removal.

Freeze-drying 🔗

Freeze-drying (lyophilisation) is a process where water is removed from frozen food by sublimation under vacuum. In this low-pressure environment, convection is negligible due to the very low air density. This means heat reaches the product primarily through conduction and radiation.

Radiation becomes especially significant for vials or trays located near the edges and corners of freeze-dryer shelves, where they receive additional radiant energy from the chamber walls. This variation in radiation exposure is a major source of product non-uniformity in freeze-drying. Some freeze dryers use infrared sources positioned above flat trays to deliver uniform radiant heating to the product surface. Infrared-assisted freeze drying has shown promise for producing high-quality instant foods while reducing the traditionally long drying times and high energy costs associated with freeze-drying.

Roasting and surface treatments 🔗

Infrared radiation is used commercially for roasting nuts, browning meats, and surface-treating various foods. The rapid, controlled surface heating that radiation provides is ideal for browning reactions without overheating the food interior. For example, infrared roasting of cashew nuts has been shown to improve the oxidative stability of their oil, likely due to Maillard reaction products that have antioxidant properties.

Infrared is also applied for microbial decontamination of food surfaces. The rapid surface temperature increase inactivates bacteria, spores, and moulds without prolonged heat exposure that would degrade the product interior. Carbon infrared systems have been used to reduce mould formation on bread surfaces before packaging, extending shelf life by several days.

Advantages and limitations of radiation in food processing 🔗

Radiation-based heating offers several clear advantages in food processing. It provides rapid heat delivery directly to the food surface without requiring a heating medium like water or air. Equipment tends to be compact, energy-efficient, and easy to control. The technology preserves vitamins better than many conventional methods and causes minimal flavour loss.

On the other hand, the shallow penetration depth of infrared radiation limits its effectiveness with thick or bulky food items. Surface overheating is a risk if temperature is not carefully controlled, especially given the fourth-power relationship in the Stefan-Boltzmann law. For thick products, radiation must be combined with conduction, convection, or other methods to ensure uniform heating throughout.

Practical considerations for food engineers 🔗

Designing an effective radiation-based food process requires attention to several factors. The geometry of the heating system – the distance between emitter and food, the angle of incidence, and the shape of the heating chamber – all influence how uniformly radiation reaches the product. Surface characteristics of the food, including colour, moisture content, and texture, affect its emissivity and therefore how efficiently it absorbs radiant energy.

The choice of infrared emitter also matters. Electric infrared emitters produce radiation at wavelengths ranging from near-infrared (0.75-1.4 µm) to far-infrared (3-1000 µm), depending on the filament temperature. Gas-fired emitters tend to produce medium to long wavelengths. For food drying, ceramic emitters operating in the far-infrared range are common because their emission wavelengths align well with water’s absorption bands. For baking and browning, near-infrared sources that penetrate slightly deeper into the food surface are preferred.

Temperature measurement and control are critical. Since radiant output changes dramatically with small temperature changes, industrial systems typically use infrared thermometers or thermal imaging to monitor the food surface temperature in real time and adjust emitter power accordingly.

What do you think? How might the combination of infrared radiation with newer technologies like pulsed electric fields or ultrasound change the future of food processing? And in your experience with baking or drying, have you noticed how surface characteristics of food – its colour, moisture, or texture – affect how quickly it heats up?

References

1. https://courses.lumenlearning.com/suny-physics/chapter/14-7-radiation/
2. https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC6891297/
4. https://www.sciencedirect.com/topics/food-science/infrared-heating
5. https://www.mdpi.com/1420-3049/24/22/4125
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC11675892/
7. https://en.wikipedia.org/wiki/Freeze_drying
8. https://www.intechopen.com/online-first/1219200