Wigertz K, Hansen I, Høier-Madsen M, Holm J, Jägerstad M
Department of Applied Nutrition and Food Chemistry, Lund University, Sweden.
Int J Food Sci Nutr. 1996 Jul;47(4):315-22. doi: 10.3109/09637489609041031.
The main objective of this study was to investigate the effects of pasteurisation, UHT processing and fermentation on the concentration of folate-binding proteins (FBP) and their folate binding capacity in comparison with the retention of the most predominant folate from, 5-CH3THF. The amount of folate-binding protein (FBP) was analysed using enzyme-linked immunosorbent assay (ELISA). Unprocessed milk and pasteurised milk were found to contain similar amounts, 211 and 168 nmol/l, of FBP, respectively. UHT-processed milk and Yoghurt naturelle, both processed at temperatures above 90 degrees C, contained only 5.2 and 0.2 nmol/l FBP, respectively. As an indication of the protein-binding capacity free and protein-bound folates were analysed after charcoal treatment using the radio-protein binding assay method (RPBA). These results indicated that all folates in unprocessed milk and pasteurised milk were protein-bound, while folates in UHT-processed milk and Yoghurt naturelle occurred freely which is supported by our findings on FBP. High-performance liquid chromatography analysis indicated that unprocessed milk, pasteurised milk, UHT-processed milk and Yoghurt naturelle contained 44.8 +/- 2.1 (n = 10), 41.1 +/- 0.9 (n = 10), 36.1 +/- 1.8 (n = 10) and 35.6 +/- 9.1 micrograms/l (n = 10) 5-methyltetrahydrofolates (5-CH3THF), respectively, after deconjugation. Corresponding values for total milk folates analysed using radio-protein binding assay were 80.4 +/- 0.9 (n = 10), 64.2 +/- 2.7 (n = 10), 48.2 +/- 1.8 (n = 10) and 54.0 +/- 8.2 micrograms/l (n = 10), respectively. Hence, both methods indicated significant (P < 0.05) losses of 5-CH3THF as a result of pasteurisation, UHT processing and fermentation, compared with unprocessed milk. In spite of apparent discrepancies in folate concentrations obtained using the two different methods, these results support the equimolar ratio of FBP and folates in unprocessed and pasteurised milk when data on 5-CH3THF, obtained using HPLC were corrected for differences in recovery. Thus, heat processing of milk not only reduced the amount of 5-CH3 THF significantly, but also changed the concentration of FBP and the folate-binding capacity of FBP, which may have implications on the bioavailability of milk folates.
本研究的主要目的是调查巴氏杀菌、超高温瞬时灭菌处理和发酵对叶酸结合蛋白（FBP）浓度及其叶酸结合能力的影响，并与5-甲基四氢叶酸（5-CH3THF）这种最主要叶酸的保留情况进行比较。采用酶联免疫吸附测定法（ELISA）分析叶酸结合蛋白（FBP）的含量。发现未加工的牛奶和巴氏杀菌牛奶中FBP的含量分别为211和168 nmol/L，二者含量相似。超高温瞬时灭菌处理的牛奶和天然酸奶均在90摄氏度以上的温度下加工，其FBP含量分别仅为5.2和0.2 nmol/L。作为蛋白质结合能力的一个指标，使用放射性蛋白质结合测定法（RPBA）在经过活性炭处理后分析游离叶酸和与蛋白质结合的叶酸。这些结果表明，未加工牛奶和巴氏杀菌牛奶中的所有叶酸均与蛋白质结合，而超高温瞬时灭菌处理的牛奶和天然酸奶中的叶酸则以游离形式存在，这与我们对FBP的研究结果相符。高效液相色谱分析表明，经过解共轭后，未加工牛奶、巴氏杀菌牛奶、超高温瞬时灭菌处理的牛奶和天然酸奶中5-甲基四氢叶酸（5-CH3THF）的含量分别为44.8±2.1（n = 10）、41.1±0.9（n = 10）、36.1±1.8（n = 10）和35.6±9.1 μg/L（n = 10）。使用放射性蛋白质结合测定法分析的总牛奶叶酸的相应值分别为80.4±0.9（n = 10）、64.2±2.7（n = 10）、48.2±1.8（n = 10）和54.0±8.2 μg/L（n = 10）。因此，两种方法均表明，与未加工牛奶相比，巴氏杀菌、超高温瞬时灭菌处理和发酵均导致5-CH3THF显著损失（P < 0.05）。尽管使用两种不同方法获得的叶酸浓度存在明显差异，但当对使用高效液相色谱法获得的5-CH3THF数据进行回收率差异校正后，这些结果支持未加工和巴氏杀菌牛奶中FBP与叶酸的等摩尔比。因此，牛奶的热处理不仅显著降低了5-CH3THF的含量，还改变了FBP的浓度以及FBP的叶酸结合能力，这可能会对牛奶叶酸的生物利用度产生影响。
