Effect of temperature-time combination and carbon dioxide concentration on inactivation of microorganisms in chilled pasteurized cow milk

Abstract

The main objective o f this study was to find out the effect o f temperature - time (TT) combination and carbon dioxide concentration on inactivation o f thermoduric and psychrotrophic microorganisms in chilled pasteurized cow milk. Raw cow milk obtained from the university farm was standardized to 3.25% fat, homogenized and HTST pasteurized using different TT combinations such as 72°C/15 sec (TTi), 74.5°C/13 sec (TT2), 77°C/12 sec (TT3), 79.5°C/10 sec (TT4), 82°C/8 sec (TT5). Pasteurized milk was bottled and chilled at 4 °C. Phosphatase test was conducted to detect the efficiency o f pasteurization. In a separate study pasteurized milk was treated with carbon dioxide and sensory threshold was determined using ASTM (American Society for Testing and Materials) E 679 Ascending Concentration Series Method o f Limits to select the suitable carbon dioxide level that can be added into milk. Pasteurized bottled milk was treated with 3 levels o f carbon dioxide (0, 7 and 15 mM), chilled at 4 °C and analyzed using standard microbiological methods for thermoduric and psychrotrophic counts. Sensory threshold ~ level o f carbon dioxide was 15.61 mM. A significant (P<0.05) TT combination x dissolved carbon dioxide concentration x storage time interaction was detected for psychrotrophic and thermoduric counts (log cfu/ml) in pasteurized carbon dioxide added to cow milk. Each TT combination had different psychrotrophic killing effects. TT5 showed significantly higher (P<0.05) psychrotrophic count on day 1. Therefore, this TT combination was not effective in killing psychrotrophs when compared with other TT combinations. A significant reduction o f psychrotrophic counts was observed with the increase o f carbon dioxide concentration in most o f the TT combinations. It clearly indicated that the carbon dioxide has an effect on the destruction o f psychrotrophs. TTj showed the highest increase o f psychrotrophic count with the storage period involving the lowest temperature and highest time. There was no clear relationship between each TT combination and between each carbon dioxide concentration for thermoduric counts. Thermodurics increased with storage period in all treatment combinations. In most o f the cases, this increment was significant. The above study revealed that the application o f small amounts o f carbon dioxide could effectively control the growth o f psychrotrophic microorganisms in refrigerated pasteurized cow milk without detrimental effects to the product quality.