Background: In many developing countries, milk chilling facilities are not available on the farm where milk is produced, rather these are located at the distance of 10-12 km. After milking, it takes about 2-3 h to reach milk to the chilling facilities. The milk is then chilled and transported to the milk processing plants for thermal processing and value addition. In developing countries, shelf life of pasteurized milk is only 3 days, as compared to 7-10 days in developed countries. The factors which are responsible for the shorter shelf life of pasteurized milk should be discovered for the improvement of dairy sectors of these countries. The magnitude of chemical changes which takes place in un-chilled milk and their effect on fatty acids profile, antioxidant status and lipid oxidation is not previously studied.
Methods: Raw milk samples of the same farm were either rapidly chilled to 4 °C immediately or held at room temperature (35 ± 2 °C) for 2 h followed by rapid chilling to 4 °C. Immediately and delayed chilled raw milk samples were stored at 4 °C for 72 h. Both milk samples were pasteurized at 65 °C, filled in 250 ml transparent PET bottles and stored at 4 °C for 6 days. Fatty acid profile, selenium, zinc, total antioxidant capacity, total flavonoid content and 1, 1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity, free fatty acids, peroxide value and anisidine value were determined at different stages of the experiment. This experiment was repeated with milk of same farm for at least five times.
Results: Storing raw milk at ambient temperature (35 ± 2 °C) significantly influenced the pH and lactose content of milk. The loss of short-chain fatty acids in delayed chilled milk was 1.19%, 3.27% and 1.60%, as compared to immediately chilled raw milk. In delayed chilled milk, loss of C18:1 and C18:2 after 3 days of storage period was 6.67% and 01.22. In delayed chilled milk after 6 days of storage, loss of C18:1 and C18:2 was 7.7% and 1.39%, respectively. In immediately chilled milk loss of C18:1 and C18:2 after 3 days of storage was 3.48% and 0.64%. In immediately chilled milk loss of C18:1 and C18:2 after 6 days of storage was 4.57% and 0.9%. Almost 41% vitamin E was lost when raw milk was stored at ambient temperature for 2 hrs. About 21% and 7% vitamin E was lost in delayed and immediately chilled milk, when samples were analyzed immediately after pasteurization. Loss of selenium and zinc contents after 2 h of ambient storage of raw milk were 0.43 and 224 μg/100 g. After 2 h of storage of milk at ambient temperature, free fatty acids increased by 0.03% (p < 0.05). After 6 days of storage, rise of free fatty acids in immediately and delayed chilled milk was 0.06% and 0.14%, respecitively. Rise of 0.13(MeqO2/kg) was recorded, when un-chilled raw milk was stored at ambient temperature for 2 h. After 3 and 6 days of storage, peroxide value of pasteurized milk (delayed chilled) was 0.88 and 1.56 (MeqO2/kg). After 3 and 6 days of storage, peroxide value of pasteurized (immediately chilled) was 0.39 and 0.42(MeqO2/kg). After 2 hrs of ambient storage, 18.41% flavonoids were lost. After 2 hrs of ambient storage of raw milk, loss of total antioxidant capacity and DPPH free radical scavenging activity was 29.31% and 44.53%. After 6 days of pasteurization, loss of total antioxidant capacity and DPPH free radical scavenging activity in delayed chilled raw milk was 72.1% and 89.57%.
Conclusions: The findings of this investigation showed that delayed chilling of raw milk leads to several undesirable chemical changes in lipid fraction of milk.
Keywords: Delay chilling; Lipids characterization; Pasteurization; Raw milk; Storage.