Changes of some pesticide residues in milk during production and storage of kefir and their effect on quality properties of kefir

In this study, the method was developed for multi residue analyses of pesticides using gas chromatography mass spectrometer (GC-MS) in milk and kefir matrices and validation parameters were studied to provide validity for the developed method. Within the scope of the study, different concentrations (0.1 and 1.0 μg/mL) of pesticide-added milks which contained fat in different proportions (%0.1 and %3.0) were used. A total of 32 types of kefir were produced from milks by applying heat treatment at different norms (5 min at 90°C and 15 min at 90°C), with double stage (150/50 bar) homogenization process or without homogenization process, and using different starter cultures (kefir grains and commercial kefir starter culture). The appropriate process parameters for the production of kefir, which contained the lowest level of pesticides, were determined by taking the fat amounts of the milk into consideration. The changes in physicochemical and microbiological properties and pesticide amounts of the kefir samples produced by using the determined parameters were detected during storage period at 4°C for 30 days. The applications of homogenization process heat treatment at 90°C 5 min and the use of commercial kefir starter culture in kefirs produced from 0.1 μg/mL of a pesticide mixture-added milk with a fat content of 3.0% resulted in more reduction of the pesticides amount in milk. In kefirs produced from 0.1 μg/mL of a pesticide mixture-added milk with a fat content of 0.1%; it was confirmed that without applying homogenization process and heat treatment at 90°C 15 min, provided more reduction in pesticide concentrations; however, the effect of the type of starter culture used the reduction in pesticide concentrations was not significant. The concentration of pesticides in the kefirs produced from milk containing pesticides at a concentration of 0.1 μg/mL, was predicted to fall below the detection limits before the 30 day storage period was completed and the high concentration content (1.0 μg/mL) was taken as a basis for proper assessment. Processing factors were calculated in order to determine the effect of process parameters applied to milk samples containing pesticide mixture on pesticide concentrations. Pesticide content was found to be effective on the pH, titration acidity, syneresis, apparent viscosity, consistency coefficient and thixotropy values of the kefir samples produced from milk with 3.0% fat. It was determined that in the kefir samples produced from milks with 0.1% and 3.0% fat contents, the pH values decreased but titratable acidity and syneresis values increased during storage period. In microbiological iv analysis, while the pesticide content was found to be effective on number of total mesophilic aerobic bacteria, lactobacilli, lactococci, acetic acid bacteria and yeast, there was no significant effect on the number of leuconostoc bacteria. In all kefir samples during storage period it was determined that the number of yeasts increased but the number of total mesophilic aerobic bacteria, lactobacilli, lactococci, leuconostoc and acetic acid bacteria decreased. It was generally shown that the amount of pesticides reduced in kefir samples during storage; however, the amount of the reduction varied according to the type of pesticide and the amount of fat in kefirs. The amount of pesticide in kefir samples during the passage through dynamic in vitro gastrointestinal model reduced in different rates depending on the amount of fat, the concentration and the type of pesticide. KEYWORDS: Gastrointestinal model, GC-MS, processing factor, kefir, microbiology, pesticide