IDENTIFICATION OF STARTER CULTURES IN THERMALLY TREATED PLAIN YOGURT USING GENE PROBES AND POLYMERASE CHAIN-REACTION

Plain yogurts (set type) fermented by three different starter cultures were subjected to different heat treatments (20 s at 72, 80 or 100 de grees C, or 30 min at 60, 70, 80 or 100 degrees C). DNA was extracted from each yogurt and analysed by agarose gel electrophoresis, by speci es-specific hybridization and by primer-specific polymerase chain reac tion (PCR). Obvious degradation of DNA could be observed after heating at 80 degrees C for 30 min and at 100 degrees C for 20 s and 30 min. Identification of Lactobacillus delbrueckii using a species-specific h ybridization fragment could be carried out in yogurt treated at 100 de grees C for 20 s or for 30 min at lower temperatures. After treatment for 30 min at 100 degrees C identification by hybridization was no lon ger possible. However, under the same conditions identification of sta rter microorganisms was still possible by primer-specific PCR, and thi s was demonstrated as an example for Streptococcus thermophilus.