The detection of foreign bodies in foods is essential in quality control. Some of the most widely used methods include metal detection, X-ray techniques and visual inspection. However, they all have disadvantages that can limit their widespread application in the food industry.

There are several methods based on different types of electromagnetic or acoustic techniques that can be applied to foodstuffs. The use of signals in the infrared (IR) wavelength range has been widely implemented in the medical imaging field and near-IR (NIR) spectroscopy has found application in the food industry, mainly for monitoring structural properties and changes in structure.

NIR imaging is able to image the internal structure of different types of food materials and is sensitive to many non-metallic materials. It is also non-ionizing and potentially low cost, so offers advantages over X-ray inspection and ultrasonic methods in certain applications such as in porous and layered foods.  However, little work has been reported on the actual imaging process in NIR techniques.

A study by Pallav et al.¹ evaluated a through-transmission NIR imaging system using a single NIR-wavelength of 850 nm for imaging different types of food materials. The technique utilizes a modulated source and lock-in amplifier detection circuit to provide high sensitivity of changes in transmission signal levels. Examples are presented of the detection of metallic and non-metallic foreign bodies in different foods including dough-based samples, a block of cheese, a layered meat sample, a chicken drumstick, lean pork meat and fat, a chocolate bar and cherries. This technique was successfully used to image a wide range of food materials and to detect different foreign materials.

¹ Pallav, P; Diamond, GG; Hutchins, DA; Green, RJ; Gan, TH (2009). A near-infrared (NIR) technique for imaging food materials. Journal of Food Science 74 (1) E23–E33.

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