Holographic sensor for the rapid detection of milk adulteration

Milk adulteration through dilution and the addition of nitrogen-rich chemicals is a persistent issue in the dairy industry, affecting product quality and consumer safety. Current monitoring techniques often rely on protein nitrogen content, which can be misrepresented by these additives. We report a reflective holographic sensor that can directly detect diluted milk by monitoring the shrinkage of the holographic grating, which induces a rapid and reversible blue shift of 34 nm across milk dilutions ranging from 10 to 100 vol%. The holographic milk sensor demonstrates high selectivity, remaining unaffected by variations in fat content, ionic strength, or pH. Testing with various casein suspensions reveals that the shrinkage effect is specifically triggered by calcium caseinate micelles, in marked contrast to free casein slurries in water. Moreover, adding melamine to artificially compensate for the nitrogen loss in diluted milk results in swelling rather than contraction. This holographic sensor offers a reliable and effective tool for quality control in the dairy industry.