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Effect of CO₂ Addition to Raw Milk on Proteolysis and Lipolysis at 4°C¹

Y. Ma^*, D. M. Barbano^* and M. Santos

^* Northeast Dairy Foods Research Center, Department of Food Science, Cornell University, Ithaca, NY 14853
Departamento de Nutrição e Produção Animal-VNP, Universidade de São Paulo, Pirassununga, SP, Brazil

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

Fresh raw milks, with low (3.1 x 10⁴ cell/ml) and high (1.1 x 10⁶ cells/ml) somatic cell count (SCC), were standardized to 3.25% fat, and from each a preserved (with 0.02% potassium dichromate) and an unpreserved portion were prepared. Subsamples of each portion were carbonated to contain 0 (control, pH 6.9) and 1500 (pH 6.2) ppm added CO₂, and HCl acidified to pH 6.2. Milk pH was measured at 4°C. For the preserved low- and high-SCC milks, two additional carbonation levels, 500 (pH 6.5) and 1000 (pH 6.3) ppm, were prepared. Milks were stored at 4°C and analyzed on d 0, 7, 14, and 21 for microbial count, proteolysis, and lipolysis. The addition of 1500 ppm CO₂, but not HCl, effectively delayed microbial growth at 4°C. In general, in both the low- and high-SCC unpreserved milks, there was more proteolysis and lipolysis in control and HCl acidified milks than in milk with 1500 ppm added CO₂. Higher levels of proteolysis and lipolysis in the unpreserved milks without added CO₂ were related to higher bacteria counts in those milks. In preserved low- and high-SCC milks, microbial growth was inhibited, and proteolysis and lipolysis were caused by endogenous milk enzymes (e.g., plasmin and lipoprotein lipase). Compared with control, both milk with 1500 ppm added CO₂ and milk with HCl acidification had less proteolysis. The effect of carbonation or acidification with HCl on proteolysis in preserved milks was more pronounced in the high SCC milk, probably due to its high endogenous protease activity. Plasmin is an alkaline protease and the reduction in milk pH by added CO₂ or HCl explained the reduction in proteolysis. No effect of carbonation or acidification of milk on lipolysis was observed in the preserved low- and high-SCC milks. The CO₂ addition to raw milk decreased proteolysis via at least two mechanisms: the reduction of microbial proteases due to a reduced microbial growth and the possible reduction of endogenous protease activity due to a lower milk pH. The effect of CO₂ on lipolysis was mostly due to a reduced microbial growth. High-quality raw milk (i.e., low initial bacteria count and low SCC) with 1500 ppm added CO₂ can be stored at 4°C for 14 d with minimal proteolysis and lipolysis and with standard plate count <3 x 10⁵ cfu/ml.

Key Words: carbon dioxide • raw milk storage • proteolysis • lipolysis

Abbreviation key: CC = coliform count, CHM = chloroform-heptane-methanol, CN/TP = casein as a percentage of true protein, , LPL = lipoprotein lipase, NCN = noncasein nitrogen, NPN = nonprotein nitrogen, PBC = psychrotrophic bacterial count, PMO = Pasteurized Milk Ordinance, SPC = standard plate count, TN = total nitrogen, TP = true protein

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