Inhibition of Lactobacillus fermentum SHY10 on the white membrane production of soaked pickled radish

Abstract

The formation of white bio-membrane (shenghua) on the surface of pickle leads to uneatable and spoiled products, which has been the key problem restricting the development of Sichuan pickle industry. In this study, the 17 microorganisms in the white membrane of pickled radish were screened and identified, of which Candida parapsilosis was the main strain causing "shenghua". The membrane-forming ability of Candida parapsilosis was determined by crystal violet staining to explore its adaptability to the fermentation environment concerning temperature and oxygen. It was found that Candida parapsilosis had the strongest membrane-forming capacity under the aerobic condition at 37°C, with the highest OD_{595 nm} value reached to 3.473 ± 0.07 at 72 h post inoculation. This research identified Lactobacillus fermentum SHY10 to be the inhibitor of the membrane production of Candida parapsilosis via the Oxford cup method on a Petri dish, and via co-inoculation with Candida parapsilosis in pickles. Furthermore, this study specified that the cell-free supernatant (CFS) of L. fermentum SHY10 had the most significant inhibitory effects and likely to result from protein substances in the CFS. Proteases treated CFS had significantly reduced inhibitory effects against membrane formation, which confirmed that the active component was protein substances. Overall, this study identified a functional LAB strain with significant inhibitory effects against the white membrane formation in pickles, which provide a safe and consumer-friendly solution for the membrane problem in the fermented vegetable industry.

Keywords: Inhibition mechanism; Lactobacillus fermentum; bio‐membrane; pickled radish; white film.

FIGURE 1

Screening test results of membrane‐producing strains. Note: From left to right: Pediococcus ethanolidurans; Lactobacillus plantarum; Enterococcus faecium; Candida parapsilosis; Bacillus altitudinis; Klebsiella aerogenes; Paenibacillus barcinonensis

FIGURE 2

OD values of Candida parapsilosis during 24‐h incubation at 37°C

FIGURE 3

Membrane formation in pickles with/without inoculation of Candida parapsilosis

FIGURE 4

Appearance of pickles inoculated with membrane‐producing yeast (a) and pickles inoculated with both membrane‐producing yeast and Lactobacillus fermentum SHY10 (b)

FIGURE 5

Inhibitory effect of Lactobacillus fermentum SHY10 on Candida parapsilosis on plates

FIGURE 6

Effect of proteases on the inhibitory effects of supernatant of Lactobacillus fermentum SHY10 against Candida parapsilosis