Acid-induced gelation of enzymatically cross-linked caseinate in different ionic milieus
Acid-induced gelation of enzymatically cross-linked caseinate in different ionic milieus
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DOI：
10.1016/j.foodhyd.2018.01.037
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发表时间：
2018-02
影响因子：
10.7
通讯作者：
Norbert Raak;C. Schöne;H. Rohm;D. Jaros
中科院分区：
文献类型：
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作者：
Norbert Raak;C. Schöne;H. Rohm;D. Jaros
用酸性酪蛋白粉末制备具有不同离子环境的酪蛋白酸盐溶液(27 g/kg)：分别用氢氧化钠和氢氧化钙中和得到酪氨酸钠(NaCN，I∼0.015 /L)和酪氨酸钙(CaCN，I∼0.03 /L)，并溶解在磷酸盐缓冲液中，得到高离子强度的酪蛋白酸盐溶液(CN-PB，I∼0.16 /L)。用微生物谷氨酰胺转氨酶(MTGase)处理一定的孵育时间会导致不同程度的酪蛋白交联，其特征是尺寸排除层析和N-ε-(γ-谷氨酰基-赖氨酸异肽含量(IC)。当∼的酪蛋白聚合率达到90%时，聚合物尺寸达到最大值，并且按NaCN <； CN-PB <； CaCN的顺序增大。然而，进一步的酶处理增加了IC，指向了现有聚合物中的交联链。我们认为，聚合物的最大尺寸是由酪蛋白颗粒在溶液中自组装产生的尺寸决定的，mTGase最好作用于同一颗粒内的分子。因此，在较高离子强度(CN-PB)或存在二价阳离子(CaCN)时增强缔合作用可能会导致更大的共价交联酪蛋白聚合物。此外，振荡流变仪还表明，葡萄糖酸-δ-内酯酸化凝胶的硬挺度与酪蛋白交联度的关系取决于离子环境。而对于NaCN，G‘MAX随着交联时间的延长而增加，离子的存在导致中等交联强度下的G’MAX最高。当在交联型NaCN中加入氯化钠时，也观察到了这一点。结果表明，凝胶形成过程中的静电引力受到离子的干扰，在广泛交联的酪蛋白颗粒情况下，不能通过重排来补偿。
Acid casein powder was used to prepare caseinate solutions (27 g/kg) with different ionic milieus: sodium caseinate (NaCN,I∼0.015 mol/L) and calcium caseinate (CaCN,I∼0.03 mol/L) were obtained by neutralisation with NaOH and Ca(OH)2, respectively, and dissolving in phosphate buffer resulted in a high ionic strength caseinate solution (CN-PB,I∼0.16 mol/L). Treatment with microbial transglutaminase (mTGase) for defined incubation times lead to different extents of casein cross-linking, which were characterised by size exclusion chromatography and the N-ε-(γ-glutamyl)-lysine isopeptide content (IC). Maximum polymer size was reached at ∼90% casein polymerisation, and increased in the order NaCN < CN-PB < CaCN. Further enzyme treatment, however, increased the IC, pointing to cross-links within existing polymers. We suggest that the maximum polymer size is determined by the size of casein particles resulting from self-assembly in solution and that mTGase preferably acts on molecules within the same particle. Enhanced association at higher ionic strength (CN-PB) or in the presence of bivalent cations (CaCN) may therefore result in larger covalently cross-linked casein polymers. Furthermore, oscillation rheometry revealed that the relationship between casein cross-linking and stiffness of gels acidified with glucono-δ-lactone depends on the ionic milieu. While for NaCN G'MAXincreased with the time of cross-linking, the presence of ions resulted in the highest G'MAXat moderate cross-linking intensities. This was also observed when NaCl was added to cross-linked NaCN. The results suggest that electrostatic attraction during gel formation are interfered by ions and cannot be compensated by rearrangements in case of extensively cross-linked casein particles.