Fermentation of Youlk, an Australian Native Root Vegetable, Using Defined Lactic Acid Bacterial Strains

Youlk (Platusace deflexa) is a native root vegetable in Australia that is primarily consumed fresh, baked, or roasted. This study aimed to produce a fermented product using youlk with added lactic acid bacteria (LAB), in comparison with spontaneous fermentations. Three LAB strains were selected for their high in vitro antioxidant activity, leading to eight formulations that featured individual strains or a 1:1:1 combination. The strains were Latilactobacillus sakei F1, Lacticaseibacillus paracasei D2 and Lacticaseibacillus rhamnosus JL, all previously isolated from fermented vegetables. Two salt levels (2% and 4%) were used, together with the addition of 1% Australian native pepper (Tasmannia lanceolata). LAB counts, pH, titratable acidity, organic acid content, and volatile organic compounds (VOCs) were monitored over a three-week period. LAB viable numbers (7.5-9.3 log CFU/g), lactic acid content (1770-2740 ppm) and titratable acidity (0.72-0.89%) were significantly higher and pH was lower (<4.0 for the combined strains) in the LAB-inoculated group compared with the spontaneously fermented control group. The opportunistic pathogen Pluralibacter gergoviae was present in spontaneously fermented youlk but not in the LAB-inoculated group, likely due to lower pH. A total of 21 VOCs were detected, with α-pinene being the most abundant. Aroma-enhancing compounds like acetoin and linalool increased significantly by day 14. Principal component analysis (PCA) indicated that fermentation duration influenced VOC production more than the LAB strains. These results suggest that youlk is a promising candidate for fermented food production, with LAB strains enhancing fermentation and providing advantages over spontaneous fermentation.