Nanomaterial-based sensor for the detection of milk spoilage

• Silver nanoparticles biofunctionalized with cysteine are used for milk spoilage detection.
• Cysteine acts as a conjugating agent for nanoparticles and lactic acid.
• Various stages of milk spoilage gave specific color based on their pH/acidity.

The present study shows a simple, rapid; highly sensitive and economical assay for gradation of milk quality based on lactic acid in milk using cysteine modified silver nanoparticles. Cysteine helps in the binding of silver nanoparticles (AgNPs) and lactic acid (LA) leading to the aggregation of nanoparticles resulting in visible color change. With the increase in the acid content, the increase in aggregation of the nanoparticles leading to the distinct change in color was observed. Milk spoilage was determined by analyzing the pH, acidity, lactic acid concentration, Lactic Acid Bacteria (LAB) count, alcohol test and Clot on Boiling (COB) test. Based on these analyses, milk samples were categorized as fresh, pre-spoiled and spoiled. The UV-Visible spectrum, Dynamic Light Scattering, and Raman spectroscopy show spectral shift and aggregation of the synthesized AgNP-cysteine conjugate with pre-spoiled and spoiled milk and resulting in corresponding color change. In the presence of Lactic Acid, Cysteine induces the aggregation of AgNPs leading to a change in yellow color of the AgNP to orange - red -purple. The control milk, pre-spoiled milk, and spoiled milk shows the color change from yellow to red – orange to grey-blue respectively.