A knowledge of the composition of processed foods is important for public health and for legal requirements such as food labelling. Chemical analyzes are normally used to obtain accurate determination of food composition, but such analytical methods are expensive, time consuming, and not suitable for rapid in-line measurement of bulk samples. There is a critical demand for the rapid, low-cost determination of food composition to provide a responsive control mechanism and an efficient quality control system in food processes. A non-destructive microwave transmission line technique is used here to measure the relative complex permittivity (epsilon*) of food samples. It is demonstrated that this microwave technique can be calibrated to measure a specific food component. Experiments have been conducted in measuring the content of resistant starch and brewer's spent grain in extruded food samples. With appropriate calibration the proposed method has potential to be employed as a non-destructive technique for dietary fibre measurement in food products. (c) 2007 Elsevier Ltd. All rights reserved.