Characterisation of starter culture for yoghurt design

Abstract

Yoghurt is produced by the fermentation of milk by two primary thermophilic lactic acid bacteria - Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These two bacteria are the most essential components of the starter culture that makes up this dairy product. The thesis work represents part of a larger project, where a number of yoghurt strains are characterized as an input to a modeling approach for genotype-to-phenotype predictions for the selection of the best consortia. In the current study, batch experiments in anaerobic bottles were carried out for one strain of each microorganism and these two strains in a consortium. Growth curves were collected by measuring the optical density of samples over time and extracellular metabolites were analysed using High-Performance Liquid Chromatography. As results, the growth of the strains is compared as monoculture and co-culture and a qualitative analysis of metabolites is performed. The specific growth rate of the co-culture is higher than that of the mono cultures. Compounds contributing to the flavour of the yoghurt are detected amongst others. In estonian: Jogurtit toodetakse kääritamise meetodil, kus põhikomponendiks on kaks primaarset termofiilset piimhappebakterit - Streptococcus thermophilus ja Lactobacillus deldrueckii subsp. bulgaricus. Need kaks bakterit on antud piimatoote koostises oleva algkultuuri kõige olulisemad komponendid. Lõputöö on osa suuremast projektist, kus kirjeldatakse mitmeid jogurtitüvesid, kui sisendit genotüübi ja fenotüübi ennustamise modelleerimisele, parima konsortsiumi valimisel. Selles uuringus tehti anaeroobsetes pudelites partiikatseid, iga mikroorganismi ühe tüve ja nende kahe tüve kohta konsortsiumis. Kasvukõverad koguti, mõõtes proovide optilist tihedust aja jooksul ja rakuväliseid. Metaboliite analüüsiti kõrgtehnoloogilise vedelikkromatograafia abil. Tüvede kasvu võrreldakse monokultuuri ja ühiskultuurina ning viiakse läbi metaboliitide kvalitatiivne analüüs. Kopokultuuri erikasvumäär on kõrgem kui monokultuuridel. Muu hulgas tuvastatakse jogurti maitset soodustavad ühendid.