Combined effects of carbon dioxide addition and barrier films on microbial and sensory changes in pasteurized milk

The growth of psychrotrophic microorganisms is an important factor in the deterioration of refrigerated pasteurized milk. Dissolved CO2 inhibits certain spoilage microorganisms in foods provided that the packaging offers a sufficient barrier to CO2 evolution. The objectives of this work were, first, to estimate the sensory threshold for dissolved CO2 in 2% milk and, second, to determine the relationship between microbial growth and package barrier properties for pasteurized milk to which CO2 had been added at concentrations near the flavor threshold. The sensory threshold, as determined by a trained panel, for CO2 in 2% pasteurized milk was > 2.8 mM and < 9.1 mM. Pasteurized milk was inoculated with a cocktail of spoilage microorganisms, packaged in different barrier film pouches, and stored at 6.1 degrees C for up to 28 d. The addition of CO2 at concentrations of 8.7 and 21.5 mM increased the time needed to reach 10(6) cfu/ml from 6.4 d (no CO2) to 8.0 and 10.9 d, respectively, in low barrier pouches. In high barrier pouches, the time needed to reach 10(6) cfu/ml was increased to 9.7 and 13.4 d, respectively, at CO2 concentrations of 8.7 and 21.5 mM. This increase represents an increase in shelf-life of approximately 25 to 200%. Microbial counts had longer lag times and lower growth rates and took longer to reach stationary growth as the concentration of CO2 increased in all films than did the control milk. The control milk curdled in less than 17 d, but the test milk in the high barrier packaging had not curdled at 28 d. These data suggest that the shelf-life of pasteurized refrigerated milk could be extended by at least 25 to 200% at CO2 concentrations near the sensory threshold. The major variables in shelf-life are the amount of added CO2 and the barrier properties of the package.