New sensor to test milk

Researchers at the Indian Institute of Technology Guwahati (IITG) have developed a simple paper-based sensor to assess the freshness of milk, and the method is instantaneous. Commonly used tests to study the effectiveness of pasteurisation such as the Methylene Blue Dye Reduction Test are time-consuming. It can take hours for the colour changes that indicate the presence or absence of microbes to occur. Commercial phenol-based tests require sophisticated spectrophotometers and involve multi-step procedures.

A research team led by Pranjal Chandra of the Department of Biosciences and Bioengineering at IITG developed a simple visual detection technique to detect the quality of milk without the need for special equipment or instruments. Their work was recently published in the journal “Biosensors and Bioelectronics”.

Alkaline phosphatase (ALP), an enzyme present in raw milk, is destroyed at high pasteurisation temperature and is therefore an important biomarker in the quality control of milk. Detection of ALP in milk can thus point to inadequate pasteurisation and/or contamination with raw milk. According to Pranjal Chandra, despite ALP’s recognisable detection potential in native milk, the multi-step nature of and the requirement of sophisticated bulky analytical instruments and trained personnel to detect ALP with the currently used methods limit their use as a sensor of milk quality in remote settings and in home kitchens. Such testing, he pointed out, would require easy-to-operate portable detection kits.

The researchers took simple filter paper, chemically modified it and loaded it with the anti-ALP compound 5-bromo-4-chloro-3-indolyl phosphate, which captures the ALP present in the milk by forming a blue-green precipitate. The intensity of the colour indicates the amount of ALP present. In the absence of ALP, there will be no colouration. The team used a smartphone to capture the image of the colour and used the RGB (Red Green Blue) filter in the phone to profile the colour obtained, which could be correlated to the ALP concentration in the test sample.

“Our sensor takes merely 13 minutes to detect ALP, and hence it can be applied for quick onsite analysis,” Pranjal Chandra said. The researchers successfully tested milk obtained from villages and commercially available milk samples using their kit and found that they could detect down to 0.87 units of ALP per millilitre of milk to 91-100 per cent accuracy. This detection limit and accuracy make it possible to discriminate raw milk from pasteurised/boiled milk. The team has developed a miniaturised detection kit. Kuldeep Mahato, Ashutosh Kumar and Buddhadev Purohit of the research team have also developed an advanced version of the paper-based sensor with improved accuracy using a label-free bioelectronic chip. The researchers plan to commercialise both variants of the kit.