Relationship between milk stability and physico-chemical characteristics of raw milk of different breeds during storage at ambient temperature

This study investigated the relationship between milk stability and physico-chemical characteristics of raw milk of four cow breeds in Sri Lanka: Sahiwal, Friesian Cross, Jersey Cross, and Australian Friesian Sahiwal. This study examined the changes in electrical conductivity (EC), pH, turbidity, and total soluble solids (TSS%) during an 8 h storage period at ambient temperature (28±1 oC) and their correlation with the alcohol test results as a platform test. Results indicated that EC did not significantly change in the raw milk samples of Sahiwal, Friesian cross, or Australian Friesian Sahiwal breeds. Milk pH of all samples decreased with the storage period; however, significant (P<0.05) reductions of pH were observed in the Jersey cross and Australian Friesian Sahiwal breeds, compared to their initial pH of the raw milk samples 6.10±0.2 and 6.36±0.2, respectively.  The turbidity of raw milk samples of the four cow breeds was significantly increased with the storage period (P<0.05). Australian Friesian Sahiwal breeds showed the highest turbidity development (241.0±3.7 NTU) while Jersey Crossbreeds showed the significantly (P<0.05) lowest turbidity development of 191.2±11.3 NTU. Both Sahiwal and Jersey Cross breeds significantly decreased (P<0.05) TSS content from 9.35 to 7.10% and 9.25 to 7.25% respectively with the storage period. The chi-square results indicated that there is no significant association (P>0.05) between alcohol test and the milk samples obtained from the different breeds. The significant negative correlation of turbidity (ρ>-0.8; P<0.05) and TSS% (ρ>-0.4; P<0.05) showed with the alcohol test in all cow breeds. Binary logistic regression results showed that the significant correlation between alcohol test and milk turbidity (Wald X² = 4.44, P<0.05). These findings highlight measuring the turbidity of raw milk would be a critical parameter for assessing milk stability and development of an in-situ instrument to quantitatively detect raw milk turbidity instead of the qualitative alcohol test.