ABSTRACT
Kefirs are fermented beverages containing yeast and bacteria produced by the fermentation of water or milk with kefir grains. Because microorganism density may influence a product's health benefits, label accuracy regarding viable bacterial density and taxonomy of fermented foods is important. In this study, the microbiota of 5 commercial kefir products were measured quantitatively using standard plating techniques and characterized using high-resolution, long-read 16S rRNA gene amplicon sequencing. To enumerate viable lactic acid bacteria, 2 lots of each product were plated on de Man, Rogosa and Sharpe agar upon opening and following 14 d and incubated under anaerobic and aerobic conditions. Results revealed that 66% of products with a guaranteed count of colony-forming units per gram overstated microorganism density by at least 1 log, with only product E exceeding 1 × 109 cfu/g. Sequencing results demonstrated moderate product label accuracy in regard to taxonomy, yet several products contained bacterial species above the minimum detectable threshold (0.001% relative abundance) that were not included on the labels (e.g., Streptococcus salivarius, Lactobacillus paracasei). Our results demonstrate a moderate level of labeling accuracy for commercial kefir products intended for human consumption. Regulatory agencies and consumers must continue to scrutinize these products and demand a higher level of accuracy and quality.
ABSTRACT
Bioethanol fermentation is continuously exposed to contamination by Lactic Acid Bacteria (LAB). In this study, the effect of adding nisin (250 mg l-1 ) to the acid wash on the viability of five bacterial contaminants were evaluated both alone and in co-incubation with Saccharomyces cerevisiae. Additionally, fed-batch fermentation was performed using an acid or acid/nisin wash for S. cerevisiae alone and cocultured with the LAB strains. Parameters such as ethanol production, sugar consumption and lactic acid production were monitored. Four model LAB were more susceptible to the acid/nisin wash than the acid wash, and were most susceptible when incubated with yeast. A fifth model LAB was very sensitive to both treatments regardless of the presence of yeast. The addition of nisin to the acid wash lowered the required time for adequate washing and resulted in a higher ethanol production (54·5 ± 0·1 g l-1 ) than the acid wash alone (52·6 ± 0·1 g l-1 ) in a subsequent fermentation. These results indicate the potential benefits of supplementing with nisin to improve the acid wash step of bioethanol fermentations. SIGNIFICANCE AND IMPACT OF THE STUDY: Acid washing by the bioethanol fermentation industry reduces yeast efficiency and selects for contaminant bacteria that are resistant to acid treatments. This study demonstrates that the incorporation of nisin into the acid wash step results in a more potent removal of lactic acid bacteria while significantly shortening the length of time needed for the acid wash.
