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Effects of Carbon Dioxide on Bacterial Growth Parameters in Milk as Measured by Conductivity

Department of Food Science, Cornell University, Ithaca, NY 14853

Corresponding author:
J. H. Hotchkiss; e-mail:
jhh3{at}cornell.edu.

Inhibition of bacterial growth by dissolved carbon dioxide (CO₂) has been well established in many foods including dairy foods. However, the effects of dissolved CO₂ on specific growth parameters such as length of lag phase, time to maximum growth rate, and numbers of organisms at the stationary phase have not been quantified for organisms of concern in milk. The effect of dissolved CO₂ concentrations of 0.6 to 61.4 mM on specific bacterial growth parameters in raw or single organism inoculated sterile milk was determined at 15°C by conductance. Commingled raw or sterile milks were amended to a final concentration of 0.5 mg/ml each of urea and arginine HCl. Sterile milks were inoculated singly with one of six different microorganisms to a final concentration of approximately 10² to 10³ cfu/ml; raw milk was adjusted to a final indigenous bacterial population of approximately 10³ cfu/ml. Conductivity of the milk was recorded every 60 s over 4 to 5 d in a circulating apparatus at 15°C. Conductivity values were fit to Gompertz equations and growth parameters calculated. Conductance correlated with plate counts and was satisfactory for monitoring microbial growth. Data fit the Gompertz equation with high correlation (R² = 0.96 to 1.00). In all cases, dissolved CO₂ significantly inhibited growth of raw milk bacteria, influencing lag, exponential, and stationary growth phases as well as all tested monocultures.

Key Words: milk bacteria • carbon dioxide • conductance • Gompertz model

Abbreviation key: SPC = standard plate counts, TSA = tryptic soy agar

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