An automatic turbidimetric method to screen yeast extracts as fermentation nutrient ingredients

Abstract Yeast extracts (YEs) are frequently used as fermentation nutrient ingredients. However, lots from the same manufacturing process gave biomass and growth rate ( μ ) levels that could vary by almost 50%. Establishing growth curves with shake flasks, or in fermenters under external pH control, is tedious and labour-demanding. An automated turbidimetry (AT) system (Bioscreen™) was thus used with the aim of screening various YEs in a growth medium optimization process. Biomass production of Lactobacillus plantarum MA18/5-U was evaluated by optical density (OD) at 600 nm. Media destined for the AT system were diluted 20 fold, and the data obtained were multiplied by the dilution factor, while media in flasks and fermenters were not diluted for the fermentation, but samples were diluted for the biomass measurement. Biomass data of the AT system underestimated the levels obtained in shaker flasks and pH-controlled fermenters by 30 to 50%. However, good correlations were obtained between the various systems ( R 2 values of between 0.94 and 0.96). Therefore, AT was found to be useful in predicting the potential biomass level in this particular YE-supplemented medium. The μ max data obtained in the AT unit were approximately half of those obtained in flasks, and a third of those in fermentations conducted under external pH control (fermenters). A good correlation between AT and flask μ max data was obtained ( R 2 =0.98). However, a rather low correlation ( R 2 =0.45) was reached between AT and fermenter μ max data. The AT system could be adapted to effectively replace shake flask cultures in the evaluation of the biostimulatory activity of YEs on L . plantarum MA18/5-U. It could also be useful as a YE screening tool in the development of media destined for fermentations under external pH control.