The essential oils from aerial parts of Melissa officinalis , Lavandula angustifolia , Salvia officinalis , and Mentha piperita were analyzed by gas chromatography and gas chromatography–mass spectrometry. Their antimicrobial activities were evaluated against five food spoilage yeasts, Torulaspora delbrueckii , Zygosaccharomyces bailii , Pichia membranifaciens , Dekkera anomala , and Yarrowia lipolytica . Saccharomyces cerevisiae was also used as a reference. The oils were preliminarily screened by a disc diffusion technique, with the most active being the oil from M. officinalis . MICs were determined by the broth dilution method, and the main components of the oils were also tested by this method. The essential oil of M. officinalis at 500 μg/ml completely inhibited the growth of all yeast species. The main component of the oil of M. officinalis is citral (neral plus geranial) (58.3%), which showed a marked fungitoxic effect, contributing to its high activity.
Twenty-one strains of commercial wine yeasts and 17 non- Saccharomyces species of different provenance were surveyed for their ability to produce hydrogen sulphide in synthetic grape juice medium indicator agar with different nitrogen sources, as well as in natural grape juice. Bacto Biggy agar, a commercially available bismuth-containing agar, was used to compare our results with others previously reported in the literature. Under identical physiological conditions, the strains used in this study displayed similar growth patterns but varied in colony color intensity in all media, suggesting significant differences in sulphite reductase activity. Sulphite reductase activity was absent for only one strain of Saccharomyces cerevisiae . All other strains produced an off-odor to different extents, depending significantly ( P <0.05) on medium composition. Within the same species of some non- Saccharomyces yeasts, strain variation existed as it did for Saccharomyces . In natural musts, strains fell into three major groups: (i) nonproducers, (ii) must-composition-dependent producers, and (iii) invariable producers. In synthetic media, the formation of sulphide by strains of S. cerevisiae results from the reduction of sulphate. Therefore, this rapid screening methodology promises to be a very useful tool for winemakers for determining the risk of hydrogen sulphide formation by a given yeast strain in a specific grape juice.
A collection of yeasts, isolated mostly from spoiled wines, was used in order to develop a differential medium for Zygosaccharomyces bailii . The 118 selected strains of 21 species differed in their origin and resistance to preservatives and belonged to the genera Pichia , Torulaspora , Dekkera , Debaryomyces , Saccharomycodes , Issatchenkia , Kluyveromyces , Kloeckera , Lodderomyces , Schizosaccharomyces , Rhodotorula , Saccharomyces , and Zygosaccharomyces . The design of the culture medium was based on the different ability of the various yeast species to grow in a mineral medium with glucose and formic acid (mixed-substrate medium) as the only carbon and energy sources and supplemented with an acid-base indicator. By manipulating the concentration of the acid and the sugar it was possible to select conditions where only Z. bailii strains gave rise to alkalinization, associated with a color change of the medium (positive response). The final composition of the mixed medium was adjusted as a compromise between the percentage of recovery and selectivity for Z. bailii . This was accomplished by the use of pure or mixed cultures of the yeast strains and applying the membrane filtration methodology. The microbiological analysis of two samples of contaminated Vinho Verde showed that the new medium can be considered as a differential medium to distinguish Z. bailii from other contaminating yeasts, having potential application in the microbiological control of wines and probably other beverages and foods.