人乳脂肪球(human milk fat globule,HMFG)膜结构的完整性对于乳脂的消化和吸收有积极的影响,该文主要探究不同储存条件下HMFG微观结构的变化及其对人乳风味的影响。将新鲜人乳混匀后进行分装,分别于-18 ℃冻藏和4 ℃冷藏,检测人乳中的菌落总数、脂肪及非酯化游离脂肪酸(non-esterified free fatty acid,NEFA)含量、pH值、HMFG粒径及其微观结构、风味等指标随储存时间(2、4和7 d)的变化,新鲜人乳指标于采样当日完成检测。结果表明,人乳的菌落总数在4 ℃冷藏7 d时显著增加,超过人乳库制定的标准(1×105 CFU/mL),其余储存条件下无显著变化。-18 ℃冻藏7 d,人乳的脂肪含量、NEFA含量和pH值无显著变化;4 ℃冷藏人乳的脂肪含量在7 d时显著降低,在2 d时NEFA含量显著增加,而pH值显著降低。-18 ℃冻藏人乳,在4 d时HMFG膜即出现破损,7 d时HMFG堆积融合,粒径增加,粒径分布范围增大;而4 ℃冷藏人乳的HMFG在4 d之内无显著变化,7 d时粒径分布变宽。GC-MS和电子舌分析人乳的风味(气味和滋味)显示,-18 ℃冻藏人乳的风味物质含量变化较小,4 ℃冷藏人乳的酸类物质含量显著升高;电子舌味觉信号的主成分分析(principal component analysis,PCA)表明,相较于4 ℃冷藏,-18 ℃冻藏人乳的滋味与新鲜样品较为相似。研究表明,-18 ℃短期冻藏后人乳风味与新鲜样品更为相似;4 ℃冷藏则更有利于保持HMFG膜微观结构。
The structural integrity of the human milk fat globule (HMFG) membrane could positively affect the digestion and absorption of milk fat. This work was to investigate the microstructure changes of HMFG and its effect on the human milk flavor under different storage conditions. Fresh human milk was mixed and repacked under -18 and 4 ℃, respectively. The total bacterial count, the content of non-esterified free fatty acid (NEFA), pH value, particle size and microstructure of HMFG, and flavoring components in human milk were determined during storage (2, 4 and 7 d). Fresh human milk was detected on the day of sample collection. The total bacterial colony count of human milk significantly increased after 7 d cold storage at 4 ℃, which exceeded the standard (1×105 CFU/mL) set by the human milk bank. However, it was no significant changes under the remaining storage conditions. The fat content, NEFA content and pH value of human milk at -18 ℃ frozen storage for 7 d showed insignificant changes. Moreover, under 4 ℃ cold storage, the fat content of human milk significantly decreased after 7 d, while the NEFA content significantly increased and the pH value significantly decreased after 2 d. The membrane of HMFG was punctured and leaked under -18 ℃ for 4 d, and subsequently, their accumulation and fusion caused the particle size distribution of HMFG to increase after storage of 7 d. But no significant changes were observed for those stored at 4 ℃ within 4 d and wider distribution was only observed after 7 d. The flavors (odor and taste) of human milk were analyzed by GC-MS and electronic tongue respectively. The results showed that limited changes were observed for the flavor substances content in frozen human milk stored at -18 ℃, while that of samples stored at 4 ℃ increased significantly. The principal component analysis of electronic tongue taste signals showed that, compared with samples stored at 4 ℃, the taste of human milk stored at -18 ℃ was more similar to that of fresh human milk. The results showed that the short storage of human milk at -18 ℃ made the flavor more similar to that of the fresh samples. The microstructure of the HMFG membrane could be well maintained under 4 ℃ cold storage.
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