Department of Dairy Technology, University of Veterinary and Animal Sciences, Lahore, Pakistan.
Institute of Home and Food Sciences, Faculty of Life Sciences, Government College University, Faisalabad, Pakistan.
Lipids Health Dis. 2018 Aug 17;17(1):190. doi: 10.1186/s12944-018-0843-0.
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.
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.
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 C and C after 3 days of storage period was 6.67% and 01.22. In delayed chilled milk after 6 days of storage, loss of C and C was 7.7% and 1.39%, respectively. In immediately chilled milk loss of C and C after 3 days of storage was 3.48% and 0.64%. In immediately chilled milk loss of C and C 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(MeqO/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 (MeqO/kg). After 3 and 6 days of storage, peroxide value of pasteurized (immediately chilled) was 0.39 and 0.42(MeqO/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%.
The findings of this investigation showed that delayed chilling of raw milk leads to several undesirable chemical changes in lipid fraction of milk.
在许多发展中国家，牛奶冷却设施并不设在产奶的农场，而是设在距离农场 10-12 公里的地方。挤奶后，牛奶需要大约 2-3 小时才能到达冷却设施。然后将牛奶冷却并运送到牛奶加工厂进行热处理和增值。在发展中国家，巴氏杀菌奶的保质期只有 3 天，而在发达国家则为 7-10 天。应该发现导致巴氏杀菌奶保质期较短的因素，以改善这些国家的乳制品行业。以前没有研究过未冷却牛奶中发生的化学变化的程度及其对脂肪酸谱、抗氧化状态和脂质氧化的影响。
同一农场的生奶样本要么立即快速冷却至 4°C，要么在室温（35±2°C）下放置 2 小时，然后快速冷却至 4°C。立即和延迟冷却的生奶样本在 4°C 下储存 72 小时。立即和延迟冷却的生奶样本在 4°C 下储存 72 小时。立即和延迟冷却的生奶样本在 4°C 下储存 72 小时。立即和延迟冷却的生奶样本在 4°C 下储存 72 小时。立即和延迟冷却的生奶样本在 4°C 下储存 72 小时。生奶样本在 65°C 下巴氏杀菌，装在 250 毫升透明 PET 瓶中，在 4°C 下储存 6 天。在实验的不同阶段测定脂肪酸谱、硒、锌、总抗氧化能力、总黄酮含量和 1,1-二苯基-2-苦基肼（DPPH）自由基清除活性、游离脂肪酸、过氧化物值和茴香胺值。至少用同一农场的牛奶重复了五次该实验。
将生奶在环境温度（35±2°C）下储存会显著影响牛奶的 pH 值和乳糖含量。延迟冷却的生奶中短链脂肪酸的损失分别为 1.19%、3.27%和 1.60%。在延迟冷却的生奶中，储存 3 天后 C 和 C 的损失分别为 6.67%和 01.22。在储存 6 天后的延迟冷却生奶中，C 和 C 的损失分别为 7.7%和 1.39。在立即冷却的生奶中，储存 3 天后 C 和 C 的损失分别为 3.48%和 0.64%。在储存 6 天后的立即冷却的生奶中，C 和 C 的损失分别为 4.57%和 0.9%。当生奶在环境温度下储存 2 小时时，约有 41%的维生素 E 流失。巴氏杀菌后立即和延迟冷却的生奶中，维生素 E 的损失分别约为 21%和 7%。在环境温度下储存生奶 2 小时后，硒和锌含量分别损失 0.43 和 224μg/100g。在环境温度下储存牛奶 2 小时后，游离脂肪酸增加了 0.03%（p<0.05）。在储存 6 天后，立即冷却和延迟冷却的生奶中游离脂肪酸的上升分别为 0.06%和 0.14%。当生奶在环境温度下储存 2 小时时，记录到 0.13(MeqO/kg)的上升。在储存 3 和 6 天后，巴氏杀菌奶（延迟冷却）的过氧化物值分别为 0.88 和 1.56（MeqO/kg）。在储存 3 和 6 天后，巴氏杀菌（立即冷却）奶的过氧化物值分别为 0.39 和 0.42（MeqO/kg）。在环境温度下储存 2 小时后，有 18.41%的类黄酮流失。在环境温度下储存生奶 2 小时后，总抗氧化能力和 DPPH 自由基清除活性损失 29.31%和 44.53%。巴氏杀菌后 6 天，延迟冷却的生奶中总抗氧化能力和 DPPH 自由基清除活性损失分别为 72.1%和 89.57%。
本研究结果表明，生奶的延迟冷却会导致牛奶脂质成分发生多种不良的化学变化。
