Preservation of the microbiological and biochemical quality of raw milk by carbon dioxide addition: A pilot-scale study

Abstract

Carbon dioxide treatment of refrigerated raw milk was evaluated as a method for extending storage life by inhibiting growth of psychrotrophic bacteria and other bacterial groups in raw milk. The effect of CO2 acidification followed by degasification and pasteurization on biochemical and microbiological properties of cold stored milk was studied on a pilot scale. Two CO2 treatments (acidification to pH 6.2 and to pH 6.0) were compared with a control (untreated) milk during 4 days of storage at 4°C. Total bacterial counts in the categories of milk established in this study were mainly determined by the proteolytic psychrotroph levels. The inhibitory capability of CO2 was greater in the low-quality than in the high-quality milk category. Acidification at pH 6.0 was more inhibitory than that at pH 6.2, especially against proteolytic psychrotrophs. Neither caseins nor whey proteins were affected by CO2 treatment and pasteurization. Organic acid (orotic, citric, uric, formic, acetic, propionic, and hippuric) concentrations did not change after CO2 treatment, cold storage, or the pasteurization process: the lactic acid content of CO2-treated milks remained constant during the refrigeration time but increased slightly in the control. In general, lower amounts of volatile compounds were produced in CO2-treated milks during refrigeration than in control milk. Ethanol and 2-propanol levels were most affected by desgasification and pasteurization. Sensory evaluation revealed no significant differences between CO2-treated milk after degasificalion and pasteurization and the untreated milk used as control. It was concluded that degasification and pasteurization on a pilot scale eliminated CO2 from milk with minimum detrimental effects on its biochemical and sensory properties, making this process acceptable for milk preservation.