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.
