Improving syneresis control in cheese making may require the development of an optical sensor technology to measure whey fat content during processing. Light sidescatter and transmission responses as a function of-whey fat concentration (0-0.9%) were measured using a fibre optic spectrometer (300-1100 nm) to determine if an optical correlation could be developed. Normalized spectral responses versus fat concentration followed a power function. Waveband ratio combinations were calculated and tested for prediction of whey fat concentration using two prediction models. The best regressions were found to use sidescatter ratios as predictors and typically contained numerator and denominator ratio wavebands in the ranges 725-1025 and 375-475, respectively. S-875/425, the ratio calculated by dividing the normalized sidescatter intensity at waveband 875 by that at 425 nm, was found to be related to whey fat concentration (R-2 = 0.99) using an exponential model. These results suggest that whey fat concentration during syneresis could be measured with an inline fibre optic sensor. (c) 2004 Elsevier Ltd. All rights reserved.