In-line colour monitoring during food extrusion: Sensitivity and correlation with product colour

During food extrusion the degree of cooking and the quantity of expensive colorants used, both influence colour and the aesthetic appeal of the product. Thus, the use of a fiber optic equipped spectrophotometer for colour monitoring inside the extruder holds great potential for product characterization and effective quality control. However, this approach presents two difficulties: (1) inadequate sensitivity of the spectrophotometer to dark colours; and (2) the need to correlate the colour monitored inside the extruder (in-line) with that of the final product (off-line). In this paper, a method of spectrophotometer calibration, termed the “virtual white reference”, is shown to overcome the sensitivity and reproducibility problems related to dark-colour measurements. In addition, to predict the final-product colour based on in-line measurements, different artificial neural networks were tested. A multilayer feedforward network with three hidden neurons provided an acceptable correlation between the in-line and off-line colour measurements.