Location: Food Science ResearchTitle: Content of xylose, trehalose and L-citrulline in cucumber fermentations and utilization of such compounds by certain lactic acid bacteria
Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2020
Publication Date: 10/1/2020
Citation: Ucar, R.A., Perez Diaz, I.M., Dean, L.L. 2020. Content of xylose, trehalose and L-citrulline in cucumber fermentations and utilization of such compounds by certain lactic acid bacteria. Food Microbiology. 91:103454. https://doi.org/10.1016/j.fm.2020.103454.
Interpretive Summary: Cucumber fermentation depends on the utilization of sugars by the lactic acid bacteria naturally present in the fruit. Fermented cucumbers can undergo a secondary fermentation as a function of time should the needed nutrients and microbes come together. This research aimed at studying a potential cause of fermented cucumbers spoilage. We investigated if fresh and fermented cucumbers naturally contain free L-citrulline, a non-protein amino acid, and the sugars trehalose and xylose. These three molecules are known to be present in plant material. If these molecules are present, it was of interest to understand they could be utilized as energy source by the same bacteria that spearhead cucumber fermentations and a bacterium known to cause spoilage of fermented cucumbers. We found that trehalose and xylose were occasionally present in cucumber fermentaions and are readily utilized by the bacteria spearheading primary fermentation. L-citrulline, in the other hand, was detected in all the samples tested and was uniquely utilized by the spoilage associated bacterium, L. buchneri. L-citrulline was produced by the bacteria involved in cucumber fermentation. The metabolic fate of L-citrulline is dependent on the presence of the primary sugar, glucose, in a medium. The amino acid serves as a nutrient for growth for bacterial cells and can contribute to increase the characteristic acidic pH in a fermentation, which is advantageous for the survival of bacterial cells. It was found that the use of L. pentosus, L. brevis and L. buchneri in a mixed starter culture aids in the removal of alternate energy sources, including L-citrulline, and the generation of a stable FrCJ fermentation for 55 days under anaerobiosis. Further studies will define if the use of a tripartite starter culture in cucumber fermentation enhances microbial stability during long-term bulk storage.
Technical Abstract: This research determined the concentration of trehalose, xylose and L-citrulline in fresh and fermented cucumbers and their utilization by Lactobacillus pentosus, Lactobacillus plantarum, Lactobacillus brevis and Lactobacillus buchneri. Targeted compounds were measured by HPLC and the ability of the lactobacilli to utilize them was scrutinized in fermented cucumber juice. Fresh cucumber juice was supplemented with trehalose, xylose and L-citrulline to observed mixed culture fermentations. Changes in the biochemistry, pH and colony counts during fermentations were monitored. Trehalose, xylose and L-citrulline were detected in fermentations to15.51 ± 1.68 mM, a fresh cucumber sample at 36.05 mM and in fresh and fermented cucumber samples at 1.05 ± 0.63 mM, respectively. Most of the LAB tested utilized trehalose and xylose in FCJM at pH 4.7. Lcitrulline was utilized by L. buchneri and produced by other LAB. L-citrulline (12.43 ± 2.3 mM) was converted to ammonia (14.54 ± 3.60 mM) and the biogenic amine ornithine (14.19 ± 1.07 mM) by L. buchneri at pH 4.7 in the presence of 0.5 ± 0.2 mM glucose enhancing growth by 0.5 log CFU/mL. The use of a mixed starter culture containing L. buchneri aided in the removal of L-citrulline and enhanced the fermentation stability. The utilization of L-citrulline by L. buchneri may be a cause of concern for the stability of cucumber fermentations at pH 3.7 or above. This study identifies the use of a tripartite starter culture as an enhancer of microbial stability for fermented cucumbers.