Surface acoustic wave resonators for milk analysis

The analysis of milk samples includes both identification and quantification of specific components and the analysis of liquid properties. The former covers both the normal ingredients of milk and unwanted components, such as veterinary drug residues and pathogenic microorganisms, while the latter includes conductivity and viscosity measurements. Taken together, the parameters allow a comprehensive statement about the quality of the milk tested. Acoustic sensors, such as quartz crystal microbalances and surface acoustic wave (SAW) sensors, can in principle be used for both types of analysis, whereby the actual application depends on the surface coating and functionalization. The viscosity of milk is typically used as indicator of quality and freshness, since it is influenced, among others, by protein aggregation. However, the addition of some drugs, such as ampicillin (a penicillin antibiotic), may also influence the viscosity of milk, since ampicillin is known to form polymers and conjugates with proteins. Measurements with SAW resonators were applied to distinguish between milk samples with and without ampicillin. The sensors were coated with a hydrogel layer to reduce interfering effects from non-specific mass adsorption, similar to the use of shielding layers in biosensor applications.