TI Food and Nutrition, 6709 PA, Wageningen, the Netherlands; Systems Biology Lab, Vrije Universiteit Amsterdam, 1081 HZ, Amsterdam, the Netherlands.
TI Food and Nutrition, 6709 PA, Wageningen, the Netherlands; Health Department, NIZO Food Research, 6718 ZB, Ede, the Netherlands.
J Dairy Sci. 2021 Aug;104(8):8530-8540. doi: 10.3168/jds.2020-20108. Epub 2021 Apr 30.
The turbidity of milk prohibits the use of optical density measurements for strain characterizations. This often limits research to laboratory media. Here, we cleared milk through centrifugation to remove insoluble milk solids. This resulted in a clear liquid phase, termed milk serum, in which optical density measurements can be used to track microbial growth until a pH of 5.2 is reached. At pH 5.2 coagulation of the soluble protein occurs, making the medium opaque again. We found that behavior in milk serum was predictive of that in milk for 39 Lactococcus lactis (R = 0.81) and to a lesser extent for 42 Lactiplantibacillus plantarum (formerly Lactobacillus plantarum; R = 0.49) strains. Hence, milk serum can be used as an optically clear alternative to milk for comparison of microbial growth and metabolic characteristics. Characterization of the growth rate, specific acidification rate for optical density at a wavelength of 600 nm, and the amount of acid produced per unit of biomass for all these strains in milk serum, showed that almost all strains could grow in milk, with higher specific acidification and growth rates of Lc. lactis strains compared with Lb. plantarum strains. Nondairy Lc. lactis isolates had a lower growth and specific acidification rate than dairy isolates. The amount of acid produced per unit biomass was relatively high and similar for Lc. lactis dairy and nondairy isolates, as opposed to Lb. plantarum isolates. Lactococcus lactis ssp. lactis showed slightly lower growth and acidification rates when compared with ssp. cremoris. For Lc. lactis strains a doubling of the specific acidification rate occurred with a doubling of the maximum growth rate. This relation was not found for Lb. plantarum strains, where the acidification rate remained relatively constant across 39 strains with growth rates ranging from 0.2 h to 0.3 h. We conclude that milk serum is a valuable alternative to milk for high-throughput strain characterization during milk fermentation.
牛奶的浊度妨碍了使用光密度测量来进行菌株特征描述。这通常将研究限制在实验室培养基中。在这里，我们通过离心去除不溶性牛奶固体来澄清牛奶。这导致了一个清澈的液相，称为牛奶血清，可以在达到 pH 值 5.2 之前使用光密度测量来跟踪微生物生长。在 pH 值 5.2 时，可溶性蛋白质发生凝固，使培养基再次变得不透明。我们发现，牛奶血清中的行为可预测牛奶中的行为，对于 39 株乳球菌（R = 0.81）和在较小程度上对于 42 株植物乳杆菌（以前称为植物乳杆菌；R = 0.49）菌株。因此，牛奶血清可以用作与牛奶相比的光学透明替代品，用于比较微生物生长和代谢特征。在牛奶血清中对所有这些菌株的生长速率、600nm 波长下的特定酸化速率以及单位生物量产生的酸量进行特征描述表明，几乎所有菌株都可以在牛奶中生长，与植物乳杆菌菌株相比，乳球菌菌株的特定酸化和生长速率更高。非乳制品乳球菌分离株的生长和特定酸化速率低于乳制品分离株。单位生物量产生的酸量相对较高，且与植物乳杆菌分离株相比，乳制品和非乳制品乳球菌分离株相似。与乳脂亚种相比，乳脂亚种的生长和酸化速率略低。当特定酸化速率增加一倍时，乳球菌菌株的生长和酸化速率增加一倍。在 39 株生长速率从 0.2 h 到 0.3 h 不等的植物乳杆菌菌株中，未发现这种关系，其中酸化速率相对恒定。我们得出结论，牛奶血清是在牛奶发酵过程中进行高通量菌株特征描述的有价值的牛奶替代品。
