The effect of the occurrence of psychrotrophic bacteria on sialic acid content of milk

Abstract

The sialic acid content of the caseinoglycopeptide released from casein during proteolysis was measured in cold stored raw milks, reconstituted milks that were inoculated with a proteolyic strain of Pseudomonas fluorescens at different levels (103, 104, 105 and 10 7cfu/mL), thermised milks and sterilized milks. The amount of soluble sialic acid increased with the growth of psychrotrophic bacteria in all milk samples during cold storage. While the highest increasing was observed (226.04%) in the reconstituted milk samples inoculated with highly proteolytic strains (107cfu/mL) the lowest increasing occurred in sterilized milks (35.46%) at the end of storage. It can be claimed that the increases in soluble sialic acid content during cold storage of milk can be used as an indicator of proteolytic action by psychrotrophic bacteria. During the storage the coagulation times of all samples decreased. The shortest coagulation time was seen in control sample (2.3 min) while the highest was found in sterilized milk (7.05 min). Depending on storage time and type of milk, the acidities of the samples changed importantly during storage (p<0.05).