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FIR energy penetrates very little, almost all the energy being converted to heat at the surface of the food,
NIR wavelength region (wavelength < 1.25 µm), approximately 50% of the radiation is reflected back, while less than 10% radiation is reflected back at the FIR wavelength region.
However NIR penetrates deeper.
A study using NIR-I imaging reported a maximum penetration depth through tissue of ~3.2 cm. (1.25") [Deep-tissue optical imaging of near cellular-sized features | nature.com]
Table 4—. Penetration depth of NIR (0.75 to 1.4 μm) into food products.
|Product||Spectral peak (μm)||Depth of penetration (mm)|
|Dough, wheat||1.0||4 to 6|
|Bread, wheat||1.0||11 to 12|
|Bread, biscuit, dried||1.0||4|
|Tomato paste, 70% to 85% water||1.0||1|
|Dry potatoes||0.88||15 to 18|
A Comprehensive Review on Infrared Heating - Applications in Food Processing