Urea is an adulteration risk in milk, particularly in areas of the world with less systematic or industrialised supply chains. It is already a natural component of milk but is also cheaply and widely available. Adding urea to milk increases the nitrogen content, hence increasing the apparent protein, enabling the milk to be diluted with water. It is not a sophisticated fraud.
Researchers at Baba Mastnath University have developed and published (open access) a simple onsite sensor to measure urea in milk that can be used in the field. It is based on urease enzymes on a nylon membrane attached to an ammonium ion-selective electrode. The biosensor gave a rapid 20-second response time at pH 5.5, detecting urea concentrations between 0.001 and 0.80 mM. The authors validated with recovery experiments from milk spiked with urea (recoveries were above 97%) and went on to characterise the typical natural urea background concentration in milks from different regions of Northern India. They found that their sensor compared favourably with other urea potentiometric biosensors and with other laboratory-based test methods.
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