Near‐Infrared Spectroscopy in Food Analysis

Abstract

Near‐infrared (NIR) spectroscopy is based on the absorption of electromagnetic radiation at wavelengths in the range 780–2500 nm. NIR spectra of foods comprise broad bands arising from overlapping absorptions corresponding mainly to overtones and combinations of vibrational modes involving CH, OH and NH chemical bonds. The concentrations of constituents such as water, protein, fat and carbohydrate can in principle be determined using classical absorption spectroscopy. However, for most food samples, this chemical information is obscured by changes in the spectra caused by physical properties such as the particle size of powders. This means that NIR spectroscopy becomes a secondary method requiring calibration against a reference method for the constituent of interest. As a consequence of the physics of diffuse transmittance and reflectance and the complexity of the spectra, calibration is normally carried out using multivariate mathematics (chemometrics). NIR spectroscopy is used routinely for the compositional, functional and sensory analysis of food ingredients, process intermediates and final products. The major advantage of NIR is that usually no sample preparation is necessary, hence the analysis is very simple and very fast (between 15 and 90 s) and can be carried out on‐line. One of the strengths of NIR technology is that it allows several constituents to be measured concurrently. In addition, for each fundamental vibration there exists a corresponding series of overtone and combination bands with each successive overtone band approximately an order of magnitude less intense than the preceding one. This provides a built‐in dilution series which allows several choices of absorptions of different intensity containing the same chemical information. Finally, the relatively weak absorption due to water enables high‐moisture foods to be analyzed. The major limitation of NIR spectroscopy in food analysis is its dependence on less‐precise reference methods.