A paper sensor that can detect freshness of milk

To test the freshness of milk and to estimate how well the milk has been pasteurized the scientists at the Indian Institute of Technology (IIT), Guwahati, have developed a simple paper kit. This kit has been aided with a smart phone and it can help to ensure that the milk is consumed before it turns too sour.

Dr Pranjal Chandra and Kuldeep Mahato

Dr Pranjal Chandra and Kuldeep Mahato

Milk being highly perishable commodity and it is prone to action of enzymes and microorganisms. The safety of milk has always been a matter concern. Although there are so many techniques to prevent spoilage of milk such as pasteurisation, freezing and preservation using additives but still there is no way to know if the milk is fresh or stale or how effective is the pasteurisation.

The researchers have used ordinary filter paper to prepare the detector. The filter paper was cut into small discs using office punch and impregnated with chemical probes that preferentially react with Alkaline Phosphatase (ALP).  The ‘probes’ used are antibodies that specifically bind to ALP. When ALP comes into contact with the probe, it turns white paper disc into a coloured one.

Dr Pranjal Chandra said, “We soaked paper discs in 4-carboxybenzene diazonium solution and then chemically treated to expose-COOH groups on the diazonium. The -COOH groups then attach to NH2 groups on anti-ALP probe molecules. Thus the anti-ALP probes are fixed on paper. When a drop of milk is poured on the tiny paper disc, the ALP in milk reacts with probes, resulting in change of colour.” The team also confirmed that colour is only due to ALP and not due to interference of vitamins, other proteins and minerals in the milk.

Dr Chandra also added that the sensor works in both qualitative and quantitative modes. “No separate reader is required for qualitative analysis as it works just like pregnancy test strips. While colour change shows ALP’s presence, the exact amount of ALP is determined using a smartphone.”

As per the researchers the present cost is around Rs 80 to Rs 125 per kit and it could come down when it will be manufactured on large scale. The kit could come handy in milk bars, large kitchens and at milk collection centres where freshness of milk is a concern. It can find other applications too.

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