Department of Molecular Biology, Center for Translational and Advanced Research, Graduate School of Medicine, Tohoku University, Sendai 980-8575, Japan.
J Dairy Sci. 2011 Dec;94(12):5857-63. doi: 10.3168/jds.2011-4704.
With the widespread consumption of milk, the complete characterization of the constituents of milk and milk products is important in terms of functionality and safety. In this study, a novel nonreducing carbohydrate was separated from powdered skim milk and was identified using electron spray ionization-mass spectrometry (m/z 385.1[M + H(+)]), ¹H, ¹³C, ¹H¹H-correlation spectroscopy, and heteronuclear single quantum-nuclear magnetic resonance spectra. The carbohydrate was identified as a lactose derivative of urea, N-carbamoyl-o-β-D-galactopyranosyl-(1-4)-D-glucopyranosylamine (lactose ureide, LU). For the HPLC analysis of LU in milk and milk products, benzoylated LU, hepta-o-benzoyl lactose ureide (melting point 137-139°C; m/z 1,113 [M + H⁺]; wavelength of maximum absorption, λ(max), 229 nm; molar extinction coefficient, ε, 8.1037 × 10⁷), was used as a standard. The crude nonreducing carbohydrate fraction from raw milk, thermally processed milk, and milk products such as powdered milks were directly benzoylated and subjected to HPLC analysis using an octadecylsilyl column to determine the quantity of LU. The content of LU in 10% solutions of powdered skim milk and powdered infant formula (5.0±1.1 and 4.9±1.5 mg/L, respectively) were almost 3-fold higher than that of UHT milk (1.6±0.5 mg/L) and higher than that of low-temperature, long-time-processed (pasteurized at 65°C for 30 min) milk (1.2±0.3 mg/L) and the fresh raw milk sample (0.3±0.1 mg/L). A time-course of the LU content in raw milk during heating at 110°C revealed that LU increased with time. From these results, it is likely that LU is formed by the Maillard-type reaction between the lactose and urea in milk and milk products. Because the concentration of LU in milk increased with the degree of processing heat treatment, it could serve as an indicator of the thermal deterioration of milk. Although it is known that the human intestine is unable to digest LU, the gastrointestinal bacteria in human subjects are able to digest and utilize urea nitrogen in formation of essential amino acids that are available to the host human. These findings suggest that LU in milk might have a functional role in human health.
随着牛奶的广泛消费，牛奶及其制品中成分的全面特性对于其功能和安全性而言非常重要。在这项研究中，从奶粉中分离出了一种新型非还原碳水化合物，并通过电子喷雾电离-质谱（m/z 385.1[M + H(+)]）、¹H、¹³C、¹H¹H 相关光谱和异核单量子-核磁共振谱进行了鉴定。该碳水化合物被鉴定为尿素的乳糖衍生物，N-氨基甲酰-o-β-D-半乳糖基-(1-4)-D-葡萄糖基胺（乳糖脲，LU）。为了在牛奶和奶制品中分析 LU，使用了苯甲酰化的 LU，即七-O-苯甲酰乳糖脲（熔点 137-139°C；m/z 1,113 [M + H⁺]；最大吸收波长，λ(max)，229nm；摩尔消光系数，ε，8.1037×10⁷）作为标准。对生奶、热处理奶以及奶粉等奶制品中的粗非还原碳水化合物进行直接苯甲酰化处理，并使用十八烷基硅烷柱进行 HPLC 分析，以确定 LU 的含量。在 10%的奶粉和婴儿配方粉（分别为 5.0±1.1 和 4.9±1.5mg/L）的溶液中，LU 的含量几乎是超高温处理奶（1.6±0.5mg/L）的 3 倍，高于低温长时间处理（在 65°C 下巴氏杀菌 30min）奶（1.2±0.3mg/L）和新鲜生奶样本（0.3±0.1mg/L）的含量。对 110°C 加热过程中生奶中 LU 含量的时间进程进行研究，结果表明 LU 含量随时间而增加。从这些结果来看，LU 可能是牛奶及其制品中的乳糖和尿素之间的美拉德型反应形成的。由于 LU 浓度随加工热处理程度的增加而增加，因此它可以作为牛奶热劣化的指示物。虽然已知人肠无法消化 LU，但人体胃肠道细菌能够消化和利用尿素氮，形成可被宿主人类利用的必需氨基酸。这些发现表明，牛奶中的 LU 可能对人类健康具有功能作用。
