Researchers have invented a biosensor that can test the quality of milk by measuring the levels of a specific enzyme found in raw milk1. This portable biosensor could potentially be used for monitoring milk quality in rural-area kitchens.
Milk is rich in carbohydrate, fat, minerals and vitamins. However, disease-causing bacteria often degrade the quality of milk, increasing the risks of food-borne diseases. Alkaline phosphatase (ALP), a heat-resistant enzyme, is commonly used as an indicator for such microbial contamination. Existing techniques for measuring the levels of this enzyme are complex and time-consuming.
In their search for a fast and cheap sensor, scientists from the Indian Institute of Technology in Guhawati, India, made the biosensor by attaching anti-ALP, specific antibodies to ALP, onto a filter paper. The team, led by Pranjal Chandra, then tested the efficiency of the biosensor in detecting and measuring the levels of ALP in raw milk collected from villages and commercially available milk samples.
After exposing the biosensor to different concentrations of ALP-containing milk, it was dipped in a solution of a fluorescent molecule. The ALP bound to anti-ALP antibodies on the sensor surface broke down the fluorescent molecule, generating a blue-green colour. This colour change indicated the presence of ALP in milk.
Increases in ALP concentration increased the intensity of colour change. The biosensor retained 98 per cent of its original sensitivity for two weeks. It was able to selectively detect ALP even in the presence of interfering biomolecules such as citric acid, lactose, casein, vitamins and various ions that are usually found in milk.
1. Mahato, K. et al. Paper-based miniaturized immunosensor for naked eye ALP detection based on digital image colorimetry integrated with smartphone. Biosens. Bioelectron.128, 9-16 (2019)