Infrared Heating

When radiant electromagnetic energy impinges upon a food surface, it may induce

changes in the electronic, vibrational, and rotational states of atoms and molecules. As food is

exposed to IR radiation, it is absorbed, reflected, or scattered. IR waves are the part of

electromagnetic spectrum with wavelengths varying form 300 GHz (1 mm) to 400 THz (750

nm). It can be divided into three parts (i) near IR 120 to 400 THz (2,500 to 750 nm), (ii) Mid-IR,

30 to 120 THz (10 to 2.5 μm) and (iii) far IR 300 GHz to 30 THz (1 mm to 10 μm)

(Krishnamurthy et al., 2008). Radiant energy belonging to IR region on impingement on the

biomaterials is absorbed on the surface and increases the heat flux (Parroufe et al., 1992), but

Dutta and Ni (2002), in their work, postulated that IR waves are also capable of internal heating,

depending on their penetrating depth. The food substances absorb FIR energy most efficiently

through the mechanism of changes in the molecular vibrational state, which can lead to radiative

heating. Water and organic compounds such as proteins and starches, which are the main

components of food, absorb FIR energy at wavelengths greater than 2.5 μm (Sakai & Hanzawa

1994), with most food falling in the high transmissivities (low absorptivities) 2.5-3.0 μm (Sandu,

1986) region. IR energy is more effective in surface moisture removal as compared to convective

hot air, as the radiant energy has higher heat flux. The waves directly heat the product without

heating the ambient air (Jones, 1992), through internal heat generation as well as absorbance of

radiant energy, which in turn converts to heat energy (Ginzburg, 1969 cited by Sharma et al.,

2005a). IR has excellent radiation characteristics and high energy conversion rates can be

achieved. Since radiant energy very quickly generates heat within the material, lowering the

temperature gradient quickly, energy consumption reduces considerably. IR energy can be

achieved by using ceramic coated radiators (Mongpraneet et al., 2002). Since the penetration

depth is not much as compared to other electromagnetic waves such as MW, RF; IR can only be

employed in cases where the sample thickness is very small (about 1 mm) and surface heating

governing the process as it is the case of conductive or convective heating. This mode of energy

transfer is quite energy efficient and not environmentally hazardous (van der Drift et al., 1997).

Because of its low penetration depth, IR energy has not been used extensively as a thermal

processing mode for blanching fruits and vegetables. Research findings suggests that IR heating

causes surface cell damage limited to less than a millimeter distance and has better texture

compared to hot water blanching process (Galindo et al., 2005). Simultaneous blanching and

partial dehydration of apple slices has been attempted by Zhu et al. (2010) and it was found that

PPO and POD inactivation was better in thinner slices and with good visual quality.