PRESERVATION OF THE MICROBIOLOGICAL AND BIOCHEMICAL QUALITY OF RAW-MILK BY CARBON-DIOXIDE ADDITION - A PILOT-SCALE STUDY

Carbon dioxide treatment of refrigerated raw milk was evaluated as a m ethod for extending storage life by inhibiting growth of psychrotrophi c bacteria and other bacterial groups in raw milk. The effect of CO2 a cidification followed by degasification and pasteurization on biochemi cal 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 sto rage at 4 degrees C. Total bacterial counts in the categories of milk established in this study were mainly determined by the proteolytic ps ychrotroph 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 pro teolytic psychrotrophs. Neither caseins nor whey proteins were affecte d by CO2 treatment and pasteurization. Organic acid (erotic, citric, u ric, formic, acetic, propionic, and hippuric) concentrations did not c hange after CO2 treatment, cold storage, or the pasteurization process ; the lactic acid content of CO2-treated milks remained constant durin g the refrigeration time bur increased slightly in the control. In gen eral, lower amounts of volatile compounds were produced in CO2-treated milks during refrigeration than in control milk. Ethanol and 2-propan ol levels were most affected by degasification and pasteurization. Sen sory evaluation revealed no significant differences between CO2-treate d milk after degasification and pasteurization and the untreated milk used as control. It was concluded that degasification and pasteurizati on on a pilot scale eliminated CO2 from milli with minimum detrimental effects on its biochemical and sensory properties, making this proces s acceptable for milk preservation.