Effect of storage methods and duration of storage on the bacteriological quality of processed liquid milk post-opening

Abstract

This research aimed at determining the quality of processed liquid milk post-opening as influenced by methods and duration of storage. Four storage methods were employed post-opening for 7 days using 2 highly patronized processed liquid milk types (M1 and M2) i.e. 1. milk refrigerated with its tin in a closed storage bowl, 2. milk refrigerated with its tin inside a sealed rubber, 3. milk refrigerated directly in its tin, and 4. milk in its tin placed in a cup containing water at room temperature, all of which are practiced by the wider public. The bacteriological enumeration of M1 showed minimum to maximum colony-forming unit (CFU) of 6.6–12.1 log10 CFU ml−1, 7.5–14.3 log10 CFU ml−1, 6.3–14.4 log10 CFU ml−1 and 7.0–17.7 log10 CFU ml−1 for storage methods 1 to 4 respectively, while M2 recorded 6.9–9.5 log10 CFU ml−1, 9.2–14.7 log10 CFU ml−1, 5.3–14.7 log10 CFU ml−1 and 6.0–21.0 log10 CFU ml−1 also for storage methods 1 to 4 respectively. Highest levels of bacteria growth were recorded from day 4 to day 7 of storage for all the milk types and storage methods applied. The bacteriological quality of all the milk types prior to storage did not meet the 6.3 log10 CFU ml−1 limit set by the Ghana Standard Board - the least counts of 6.79 log10 CFU ml−1 and 8.01 log10 CFU ml−1 were recorded for M1 and M2 respectively. Among the bacteria identified were Pseudomonas species with 71% growth for M1 and 70% growth for M2 whereas Bacillus species were least prevalent with no record for M1 but 2% for M2. A pH of 6 was recorded for both M1 and M2 immediately after opening the tins. However, M1 dropped in pH by one unit under storage methods 3 and 4, whilst that of M2 remained constant in the four storage methods over the 7-day period. Milk refrigerated with its tin in a closed storage bowl recorded the least bacteria growth in both milk types over the 7-day duration of storage, hence, the best storage method for maintaining the quality of processed liquid milk within a week post-opening.