71A-40 |
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N. A. Kocaoglu-Vurma and P. D. COURTNEY. Dept. of Food Science and Technology, The Ohio State Univ., 2015 Fyffe Ct., 110 Parker Food Science Bldg., Columbus, OH 43210-1007 The characteristics of starter and nonstarter bacteria are known to affect cheese flavor and quality. Citrate, when not already metabolized by starter culture bacteria during cheese production, can be utilized by nonstarter bacteria during cheese ripening. Citrate metabolism later in Swiss cheese ripening may adversely affect cheese quality by producing cracks. The objective was to evaluate citrate metabolism by nonstarter Swiss cheese Lactobacillus isolates. Twenty-three nonstarter Lactobacillus strains were previously isolated from domestic and European Swiss cheeses. Stationary phase cultures were spot inoculated on citrate differential medium and grown at 37 °C for 24 hr to qualitatively evaluate citrate utilization. For quantitative assessment, strains were grown in MRS broth supplemented with 10 mM MgCl2 and lacking acetate, citrate, and beef extract. Glucose or citrate was added to a concentration of 30 mM. Bacterial growth was evaluated after 24 hr of anaerobic incubation at 37 °C by measuring absorbance at 600 nm. The test strain was considered positive for citrate utilization when absorbance obtained with citrate as the sole carbon source was at least 30% of the absorbance in media containing glucose as the sole carbon source. Citrate differential medium is valuable in rapid assessment of citrate utilization of lactobacilli. On this medium, L. helveticus, L. gasseri, and L. delbrüeckii strains did not metabolize citrate, while L. casei, L. fermentum, and L. rhamnosus strains utilized citrate. However, L. rhamnosus strains were weak citrate utilizers. Percent relative growth on modified MRS broth indicated that L. delbrüeckii and L. helveticus strains cannot metabolize citrate as the sole carbon source. Among the other strains tested, L. casei strains were strong citrate utilizers, followed by L. rhamnosus, L. fermentum, and L. gasseri strains. Improved understanding of citrate metabolism in lactobacilli will assist in starter and adjunct culture selection for consistent, high-quality cheese production.
Session 71A, Dairy Foods: General
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |