Hassan A N, Ipsen R, Janzen T, Qvist K B
Center for Advanced Food Studies, Department of Dairy and Food Science, Royal Veterinary and Agricultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark.
J Dairy Sci. 2003 May;86(5):1632-8. doi: 10.3168/jds.S0022-0302(03)73748-5.
Yogurt was made using an exopolysaccharide-producing strain of Streptococcus thermophilus and its genetic variant that only differed from the mother strain in its inability to produce exopolysaccharides. The microstructure was investigated using confocal scanning laser microscopy, allowing observation of fully hydrated yogurt and the distribution of exopolysaccharide within the protein network. Yogurt made with the exopolysaccharide-producing culture exhibited increased consistency coefficients, but lower flow behavior index, yield stress, viscoelastic moduli and phase angle values than did yogurt made with the culture unable to produce exopolysaccharide. The exopolysaccharides, when present, were found in pores in the gel network separate from the aggregated protein. These effects could be explained by the incompatibility of the exopolysaccharides with the protein aggregates in the milk. Stirring affected the yogurt made with exopolysaccharide differently from yogurt without exopolysaccharide, as it did not exhibit immediate syneresis, although the structural breakdown was increased. The shear-induced microstructure in a yogurt made with exopolysaccharide-producing culture was shown to consist of compartmentalized protein aggregates between channels containing exopolysaccharide, hindering syneresis as well as the buildup of structure after stirring.
酸奶是用一种能产生胞外多糖的嗜热链球菌菌株及其遗传变体制作而成的，该遗传变体与母菌株的唯一区别在于其无法产生胞外多糖。使用共聚焦扫描激光显微镜对微观结构进行了研究，从而能够观察到完全水合的酸奶以及胞外多糖在蛋白质网络中的分布情况。与用不能产生胞外多糖的培养物制作的酸奶相比，用能产生胞外多糖的培养物制作的酸奶表现出更高的稠度系数，但流动行为指数、屈服应力、粘弹性模量和相角值更低。当存在胞外多糖时，它们存在于凝胶网络中与聚集蛋白质分开的孔隙中。这些影响可以用胞外多糖与牛奶中的蛋白质聚集体不相容来解释。搅拌对用胞外多糖制作的酸奶和不含胞外多糖的酸奶的影响不同，因为前者不会立即出现乳清分离现象，尽管其结构破坏有所增加。用能产生胞外多糖的培养物制作的酸奶中，剪切诱导的微观结构显示为由含有胞外多糖的通道之间的分隔蛋白质聚集体组成，这既阻碍了乳清分离，也阻碍了搅拌后结构的形成。
