Milk spoilage triggered by changes in pH contributes to the waste of billions of liters worldwide, while a cost-effective spoilage tracker for household environments has been hard to find.
The best before date on the milk bags, a standard indicator, is not always a meaningful way to judge the freshness of the milk in real time.
Two researchers from Christ (Deemed to be University), Bengaluru have combined insights from existing research on pH-responsive tracking methods using hibiscus flowers and an Android application to develop an easy-to-use indicator that visually monitors the freshness of milk.
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The researchers – Chaithra KP and Vinod TP – rubbed coated hibiscus petals onto paper, which was then used to test the milk droplets (fresh, packaged and powdered) on it. The color change of the milk-treated paper from green to purple to pink indicated the nature of the samples—fresh, spoiled, or spoiled.
The pH of fresh milk is estimated to be between 6.5 and 6.7, which decreases as it spoils. Milk samples adjusted with lactic acid to achieve different pH levels (6.68, 6.5, 6.0, 5.5, 5.0 and 4.4) were added to the indicator to help distinguish between of fresh, spoiled and spoiled milk. Real-time spoilage sampling was also performed on milk stored in refrigerators and kept at room temperature for different periods of time.
After sampling, the researchers photographed the color changes and analyzed their RGB (red, green, and blue) indices using an Android app, Color Grab. They used chromatic redshift to quantify these changes, with closer proximity to red indicating higher acidity (and lower pH).
Understand color changes
Vinod, Associate Professor, Department of Chemistry at Christ, said the science – which involves the use of halochromic materials that change color in response to different pH levels – is well known, but what’s new is the benefit of analyzing with it naked eye and the applicability of the sensor in resource-constrained settings.
“The anthocyanins (plant pigments) contained in the hibiscus form the basis of this sensor. It could be a cost-effective alternative for real-time milk freshness monitoring. The expiration date on packs of milk isn’t always accurate and only applies to milk when it’s in the pack,” Vinod told DH.
The fact that the indicator does not require the extraction of anthocyanins from natural products or laboratory expertise, and works with readily available materials, makes it an ideal starting point for broader food quality monitoring research, according to the researchers.
According to Vinod, the scope of the indicator could be significantly expanded with a mobile app that directly calculates and displays milk freshness. “In its current form, the indicator takes a few minutes to complete the evaluation,” he said.
Researchers have also proposed the indicator for potential extension to quality control of other foods. Their results were recently published in the journal ChemistrySelect, which is published on behalf of Chemistry Europe, a coalition of chemical societies from 15 European countries.