We developed a pH-controlled batch fermentation process with separately immobilized cells of the protective coculture of Lactobacillus paracasei subsp. paracasei SM20 and Propionibacterium jensenii SM11 in supplemented whey permeate medium yielding cell-free supernatants
with high antiyeast activity against Candida pulcherrima and Rhodotorula mucilaginosa. The antiyeast compounds were resistant to proteinase K and pronase E treatments and showed high heat resistance (121°C for 15 min). Diafiltration (1,000-Da cutoff) revealed that the inhibitory
metabolites have low molecular weights. Partial purification of active compounds was achieved by a microplate bioassay controlled procedure with solid-phase extraction (C18) followed by (i) gel filtration chromatography or (ii) semipreparative reverse-phase high-performance liquid chromatography
(C18). In addition to propionic, acetic, and lactic acids, 2-pyrrolidone-5-carboxylic acid, 3-phenyllactic acid, hydroxyphenyllactic acid, and succinic acid were identified by chromatography and mass spectrometry. Accurate quantifications revealed only low concentrations (up to 7 mM) of 2-pyrrolidone-5-carboxylic
acid, 3-phenyllactic acid, and hydroxyphenyllactic acid produced during fermentation in contrast to relatively high MICs (50 to more than 500 mM) determined at different pH values (4.0, 5.0, and 6.0). Succinic acid was present at higher concentrations (29 mM) in cell-free supernatants but
with comparable high MICs (200 to more than 500 mM and pH 4.0, 5.0, and 6.0). Although none of these compounds was the main substance responsible per se for suppression of yeast growth, our study revealed a complex antiyeast mechanism with putative synergistic effects between several low-molecular-weight
Document Type: Research Article
Laboratory of Food Biotechnology, Institute of Food Science and Nutrition, ETH Zentrum, 8092 Zurich, Switzerland 2:
Laboratory for Mass Spectrometry, Institute of Organic Chemistry, University of Zurich, 8057 Zurich, Switzerland
Publication date: December 1, 2008
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