Towards in-flow monitoring of fat content and fluid composition of dairy milk using microfluidic confocal Raman spectroscopy

Dairy milk composition analysis is critical for quality control and pricing purposes. Particularly, fat content in dairy milk determines its market value, and free fatty acids can be used as a proxy for milk quality. Milk with high fat contents (4% and above) is progressively skimmed to obtain a chain of products (butter, cheese, milk, powder milk, etc.), increasing the profitability in a traditionally low-margin industry. Besides total fat percentage, the composition of fatty acids is also relevant, as C14, C16 and C18 free fatty acids can be used to monitor cattle food intake. Established chemical separation techniques to measure fat content, such as the Gerber method, are off-line, manual, labor intensive, cannot distinguish fats and have a high error margin. We present a monitoring system concept, free of labelling and sample processing, for fat milk content using Raman spectroscopy in flowing microfluidic channels. The technique has been tested using several microfluidic flow rates (20 μl/min to 1000 μl/min) with Raman measurements of 5 seconds of exposure time and with a single accumulation. We show that Raman spectra remain the same even after continuously refreshing the fluid during measurements at increasing flow rates. Proof of concept Raman measurements at the region of interest for undiluted whole (3.5% of fat), semi-skimmed (1.5%) and buttermilk (0.5%) are presented under different microfluidic flows, showing the potential of the technique. The technique can also be used to identify different fats, proteins, and minerals.