A centrifugation-based clearing method allows high-throughput acidification and growth-rate measurements in milk

摘要

The turbidity of milk prohibits the use of opticaldensity measurements for strain characterizations.This often limits research to laboratory media. Here,we cleared milk through centrifugation to remove insolublemilk solids. This resulted in a clear liquid phase,termed milk serum, in which optical density measurementscan be used to track microbial growth until a pHof 5.2 is reached. At pH 5.2 coagulation of the solubleprotein occurs, making the medium opaque again. Wefound that behavior in milk serum was predictive ofthat in milk for 39 Lactococcus lactis (R~2 = 0.81) andto a lesser extent for 42 Lactiplantibacillus plantarum(formerly Lactobacillus plantarum; R~2 = 0.49) strains.Hence, milk serum can be used as an optically clearalternative to milk for comparison of microbial growthand metabolic characteristics. Characterization of thegrowth rate, specific acidification rate for optical densityat a wavelength of 600 nm, and the amount of acidproduced per unit of biomass for all these strains inmilk serum, showed that almost all strains could growin milk, with higher specific acidification and growthrates of Lc. lactis strains compared with Lb. plantarumstrains. Nondairy Lc. lactis isolates had a lower growthand specific acidification rate than dairy isolates. Theamount of acid produced per unit biomass was relativelyhigh and similar for Lc. lactis dairy and nondairy isolates,as opposed to Lb. plantarum isolates. Lactococcuslactis ssp. lactis showed slightly lower growth and acidificationrates when compared with ssp. cremoris. ForLc. lactis strains a doubling of the specific acidificationrate occurred with a doubling of the maximum growthrate. This relation was not found for Lb. plantarumstrains, where the acidification rate remained relativelyconstant across 39 strains with growth rates rangingfrom 0.2 h~(−1) to 0.3 h~(−1). We conclude that milk serumis a valuable alternative to milk for high-throughputstrain characterization during milk fermentation.