Fermentation of African kale (Brassica carinata) using L. plantarum BFE 5092 and L. fermentum BFE 6620 starter strains

• African kale could be fermented with starter cultures without additional sugar
• L. plantarum and L. fermentum starter cultures were well suited for this fermentation
• DGGE and metagenomics showed that without added starters, a reliable fermentation did not take place
• Water soluble vitamins decreased but appreciable amounts of vitamin C were still present

Vegetables produced in Africa are sources of much needed micronutrients and fermentation is one way to enhance the shelf life of these perishable products. To prevent post-harvest losses and preserve African leafy vegetables, Lactobacillus plantarum BFE 5092 and Lactobacillus fermentum BFE 6620 starter strains were investigated for their application in fermentation of African kale (Brassica carinata) leaves. They were inoculated at 1 × 107 cfu/ml and grew to a maximum level of 108 cfu/ml during 24 h submerged fermentation. The strains utilized simple sugars (i.e., glucose, fructose, and sucrose) in the kale to quickly reduce the pH from pH 6.0 to pH 3.6 within 24 h. The strains continued to produce both d and l lactic acid up to 144 h, reaching a maximum concentration of 4.0 g/l. Fermentations with pathogens inoculated at 104 cfu/ml showed that the quick growth of the starters inhibited the growth of Listeria monocytogenes and Salmonella Enteritidis, as well as other enterobacteria. Denaturing gradient gel electrophoresis and 16S rRNA gene (V3-V4-region) amplicon sequencing showed that in the spontaneous fermentations a microbial succession took place, though with marked differences in biodiversity from fermentation to fermentation. The fermentations inoculated with starters however were clearly dominated by both the inoculated strains throughout the fermentations. RAPD-PCR fingerprinting showed that the strains established themselves at approx. equal proportions. Although vitamins C, B1 and B2 decreased during the fermentation, the final level of vitamin C in the product was an appreciable concentration of 35 mg/100 g. In conclusion, controlled fermentation of kale offers a promising avenue to prevent spoilage and improve the shelf life and safety.