Dairy Foods: Processing

Our objective was to determine the temperature and vacuum relationships for efficient removal of added CO2 from raw milk. Added CO2 in milk and dairy products inhibits the growth of spoilage organisms during refrigerated storage and shipping. As milk and milk concentrates are shipped over longer distances, the use of CO2 is becoming more common to maintain raw milk quality. In most cases, some or all of the CO2 needs to be removed from the milk before further processing. Raw milk before CO2 addition typically contained about 100 to 200 ppm CO2 and had a pH of about 6.9 at 4°C. In the present study, about 1600 kg of 4°C raw milk was continuously injected with CO2 with a holding time of 15 s at 172 kPa to achieve a concentration of 2000 ppm. Milk containing added CO2 was stored at 4°C for 24 h before initiating the CO2 removal process. The cold milk containing CO2 was pumped through a plate heat exchanger at 24.5 kg/min directly into the spray nozzle inlet of a pilot-scale APV vacuum deaerator system equipped with a spray nozzle that produced a cone shaped spray pattern into a vacuum chamber. Six different milk temperatures (51.5, 57.5, 63, 68, 74, and 79.5°C) and 5 different vacuum levels at each temperature (in the range of 621 to 1293 mmHg Torr) were evaluated. The CO2 content (Mocon CO2 analyzer) of milk and milk pH were measured for each treatment and control milk. The experiment was replicated 3 times with a new milk source each time. The CO2 content of milk decreased and pH increased with increasing temperature and vacuum. As temperature increased, less vacuum was required to achieve complete CO2 removal. Milk pH was highly correlated with CO2 removal and pH can be used as a rapid measurement method to determine if CO2 removal is near completion. A linear regression model (r-square 0.98) was developed that defines the relationship between temperature and vacuum required to reduce the CO2 content of the milk to a level not significantly different form the milk before CO2 addition.