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Milk pH as a Function of CO₂ Concentration, Temperature, and Pressure in a Heat Exchanger¹

Northeast Dairy Foods Research Center, Department of Food Science, Cornell University, Ithaca, NY 14853

Corresponding author: D. M. Barbano; e-mail address: dmb37{at}cornell.edu.

Raw skim milk, with or without added CO₂, was heated, held, and cooled in a small pilot-scale tubular heat exchanger (372 ml/min). The experiment was replicated twice, and, for each replication, milk was first carbonated at 0 to 1°C to contain 0 (control), 600, 1200, 1800, and 2400 ppm added CO₂ using a continuous carbonation unit. After storage at 0 to 1°C, portions of milk at each CO₂ concentration were heated to 40, 56, 72, and 80°C, held at the desired temperature for 30 s (except 80°C, holding 20 s) and cooled to 0 to 1°C. At each temperature, five pressures were applied: 69, 138, 207, 276, and 345 kPa. Pressure was controlled with a needle valve at the heat exchanger exit. Both the pressure gauge and pH probe were inline at the end of the holding section. Milk pH during heating depended on CO₂ concentration, temperature, and pressure. During heating of milk without added CO₂, pH decreased linearly as a function of increasing temperature but was independent of pressure. In general, the pH of milk with added CO₂ decreased with increasing CO₂ concentration and pressure. For milk with added CO₂, at a fixed CO₂ concentration, the effect of pressure on pH decrease was greater at a higher temperature. At a fixed temperature, the effect of pressure on pH decrease was greater for milk with a higher CO₂ concentration. Thermal death of bacteria during pasteurization of milk without added CO₂ is probably due not only to temperature but also to the decrease in pH that occurs during the process. Increasing milk CO₂ concentration and pressure decreases the milk pH even further during heating and may further enhance the microbial killing power of pasteurization.

Key Words: carbon dioxide concentration • pH in heat exchanger • temperature • pressure