Longitudinal Study of Microbial Load and Chemical Quality Degradation in Pasteurized Milk during Refrigerated Storage

Abstract

Background: Pasteurized milk, widely consumed in India, requires rigorous monitoring due to frequent microbial contamination and quality concerns during storage. Aim: To assess changes in microbial dynamics and chemical quality of pasteurized milk from various brands during refrigerated storage by analyzing pH, acidity, clot on boiling, and microbial counts, to compare shelf life and safety compliance. Study Design: The study used an experimental longitudinal design, analyzing four pasteurized milk brands at multiple refrigerated storage intervals for chemical and microbiological changes. Place and Duration of Study: The study was conducted in Tirupati, India, using milk samples collected from the local market, and lasted for a storage period of up to four days. Methodology: The study analyzed four pasteurized milk brands (A–D) stored at 4°C. Chemical parameters (pH, acidity, clot-on-boiling, alcohol test) followed IS:1479-1 (1960). Microbiological analysis included total bacterial count (Standard Plate Count Agar), coliform count (Violet Red Bile Agar), and methylene blue reduction time per BIS standards. Results: Chemical parameters (pH, acidity, clot-on-boiling, alcohol test) and microbiological parameters (total bacterial count, coliform count, methylene blue reduction time) were evaluated. Statistical analysis using ANOVA (P = 0.05) revealed no significant differences (P > 0.05) among brands for pH (6.6–6.8 on day 0; 6.6–6.7 on day 3) or acidity (0.16% lactic acid on day 0; 0.17–0.18% on day 3). Microbiologically, all brands showed non-significant differences (P > 0.05) in total bacterial count (TBC: 2.65–3.78 log₁₀ CFU/mL on day 0; 4.30–4.48 log₁₀ CFU/mL on day 3) and coliform count (0–1 log₁₀ CFU/mL). However, brand D exhibited significantly higher TBC (4.48 log₁₀ CFU/mL, p < 0.05) and coliform presence (1 log₁₀ CFU/mL) by day 3, exceeding BIS standards. Methylene blue reduction time declined from 4–5 hours (day 0) to 0.5–2.5 hours (day 3), indicating reduced quality. All brands remained chemically stable for 3 days, but microbiological spoilage occurred by day 4. Conclusion: All four pasteurized milk brands met BIS standards for chemical and microbiological quality up to three days at 4°C, but spoilage increased after.