Effect of addition of CO2 to raw milk on quality of UHT-treated milk

The objective of this study was to evaluate the effect of addition of CO2 to raw milk on UHT milk quality during storage. Control milk (without CO2 addition) and treated milk (with CO2 addition up to pH 6.2) were stored in bulk tanks at 4°C for 6 d. After storage, both samples were UHT processed using indirect heating (140°C for 5 s). Samples were aseptically packed in low-density polyethylene pouches and stored in the dark at room temperature. Raw milk was evaluated upon receipt for physicochemical composition, proteolysis, lipolysis, standard plate count, psychrotrophic bacteria, and Pseudomonas spp. counts, and after 6 d of storage for proteolysis, lipolysis, and microbial counts. After processing, UHT milk samples were evaluated for physicochemical composition, proteolysis, and lipolysis. Samples were evaluated for proteolysis and lipolysis twice a month until 120 d. Peptides from pH 4.6-soluble N filtrates were performed by reversed-phase HPLC after 1 and 120 d of storage. A split-plot design was used and the complete experiment was carried out in triplicate. The results were evaluated by ANOVA and Tukey's test. After 6 d of storage, CO2-treated raw milk kept its physicochemical and microbiological quality, whereas the untreated milk showed significant quality losses. A significant increase in proteolysis occurred during 120 d of storage in both treatments, but the increase occurred 1.4 times faster in untreated UHT milk than in CO2-treated UHT milk. In both UHT milks, the proteolysis was a consequence of the action of plasmin and microbial proteases. However, the untreated UHT milk showed higher microbial protease activity than the treated UHT milk. The addition of CO2 to the raw milk maintained the quality during storage, resulting in UHT milk with less proteolysis and possibly longer shelf life, which is usually limited by age gelation of UHT milk.