Interaction between dairy yeasts and lactic acid bacteria strains during milk fermentation

Twelve yeast strains of eight different species and four lactic acid bacteria (LAB) strains belonging to three species were inoculated into UHT skimmed milk separately and as co-cultures to determine their growth, activities and interactions. pH was recorded at the end of fermentation, and both yeasts and LAB enumerated by selective culturing. Organic acids and volatile compounds were analysed by HPLC and gas chromatography–mass spectrometry, respectively. Both alone and in co-culture, LAB strains reached higher cell densities than yeast strains. In most co-culture combinations, LAB and yeasts increased or decrease their numbers over those obtained in single cultures in a strain–strain dependent manner. LAB strains produced high levels of lactic acid (up to 930 mg per 100 ml−1) and decreasing quantities of propionic, acetic (except Lactococcus lactis strains), butyric, and pyruvic acids. In contrast, lactic acid was only produced by Geotrichum candidum isolates, and most yeast species did not form acetic and butyric acids. Organic acid profiles of the co-cultures mostly reflected the acid produced and utilized by the LAB/yeast pair. The production of volatile components varied widely between single and co-cultures, with no clear tendency observed. Characteristically, diacetyl was mostly produced by LAB strains, while yeasts were responsible for producing a majority of maltic compounds. The production of other volatile compounds by either LAB or yeasts was strain dependent.