Possible unexpected peaks from oil drop size measurements in milk

Abstract: The shelf life of milk is amongst others dependent on the size of the oil droplets. These droplets can be measured using the laser diffraction (LD) technique. LD is a popular technique to measure particle size distributions (PSD) that has an outstanding broad measuring range capability, is easy-to-use and has a high reproducibility. While this technique is extremely popular, there are some conditions under which LD shows unexpected results in the form of unexpected peaks. The purpose of the current study was to determine under which conditions unexpected peaks appear or do not appear while measuring the PSD of oil droplets on LD equipment. Samples of pasteurized milk were measured on two versions of LD equipment to investigate and identify the conditions under which the LD gives these unexpected peaks, including the variables: pretreatment, sample concentration (i.e. obscuration rate) and dispersing unit stirring speed. A more simplified system (i.e. model emulsion) and a complementary technique (dynamic light scattering) were used to further investigate the unexpected peaks. Results showed that the right sided unexpected peaks are believed to be caused by the incorporation of air. Right sided unexpected peaks can also be caused by dirt in the equipment and a too low sample concentration (i.e. obscuration rate). The second finding of this study is focused on left sided unexpected peaks. These peaks are believed to be caused by a too high sample concentration, which results in the effect of multiple scattering. Another type of left sided unexpected peaks is an increased standard deviation caused by using unoptimized variables of the LD equipment (e.g. too high stirring speed or too low sample concentration). As a result of the increased sensitivity in one of the models of the LD equipment, it was found that it is even more crucial to introduce the adequate optical parameters values (e.g. absorption index). When increasing the absorption index blue light, the discrepancies between different models can be reduced. However, more research needs to be done to confirm this theory. All in all, unexpected peaks can be caused by air, sample concentration, dirt and differences between LD equipment.