Volatile organic compounds and D-amino acids in ewes' milk as affected by temperature treatments and total microbial count

The aim of the study was to approach the analytical determination of chemical markers able to define the quality of sheep milk. Gas chromatographic determination of free amino acid enantiomers and headspace solid-phase microextraction, gas chromatography mass spectrometric detection of volatile compounds were applied to ewes’s milk samples characterized by different total microbial counts (TMC) and temperature treatments. Twenty bulk raw milk samples were obtained from Sardinian ewes and treated, before the final freeze-storage at -80° C, as follows: 1) raw milk; 2) raw milk refrigerated at 4°C for 48 h; 3) raw milk treated at 37°C for 3 h; 4) raw milk treated at 37°C for 3 h and refrigerated at 4°C for 48 h. Correlations between TMC and D-amino acids or volatile compounds were evaluated by Pearson’s analysis. Student-Newman-Keuls was used as post hoc test for comparison of the means among different groups, P value of ≤ 0.05 was considered as significant. Positive correlations between TMC and D-Ala (P<0.05) and between D-Ala and D-Asp (P<0.05) were found. D-amino acids contents in milk groups did not produce any significant difference. D/D+L amounts of D-Ala, D-Pro and D-Asp were lowest in group 1. D-Ala was highest in group 3 and 4, whereas D-Asp was highest in group 2. Changes in D-Met were opposite to D-Ala and D-Asp variations, with the highest values observed in group 1 and the lowest ones in group 4. Concerning the volatile fraction, ketones resulted the most abundant class of compounds, followed by aldehydes and alcohols. Sulfur and acid compounds were less represented. No correlations were observed between TMC and volatile compounds in milk; similarly, comparison of means of volatile compounds within groups was not significant.