Manfred Grossmann

Impact of different malolactic fermentation inoculation scenarios on Riesling wine aroma

During malolactic fermentation (MLF), lactic acid bacteria influence wine aroma and flavour by the production of volatile metabolites and the modification of aroma compounds derived from grapes and yeasts. The present study investigated the impact of different MLF inoculation strategies with two different Oenococcus oeni strains on cool climate Riesling wines and the volatile wine aroma profile. Four different timings were chosen for inoculation with bacteria to conduct MLF in a Riesling must/wine with a high acidity (pH 2.9–3.1). Treatments with simultaneous inoculation showed a reduced total fermentation time (alcoholic and malolactic) compared to the sequential inoculations. No negative impact of simultaneous alcoholic and malolactic fermentation on fermentation success and on the final wine volatile aroma composition was observed. Compared to sequential inoculation, wines with co-inoculation tended to have higher concentrations of ethyl and acetate esters, including acetic acid phenylethylester, acetic acid 3-methylbutylester, butyric acid ethylester, lactic acid ethylester and succinic acid diethylester. Results of this study provide some alternatives to diversify the number of wine styles by safely conducting MLF in low-pH, cool-climate white musts with potential high alcohol content.

Influence of grape treatment on the wine yeast populations isolated from spontaneous fermentations

Aim:  To study the influence of different methods of grape treatment in wineries on the diversity of the yeast species in spontaneous fermentations.

The use of glucose oxidase and catalase for the enzymatic reduction of the potential ethanol content in wine.

Due to the increase of sugar levels in wine grapes as one of the impacts of climate change, alcohol reduction in wines becomes a major focus of interest. This study combines the use of glucose oxidase and catalase activities with the aim of rapid conversion of glucose into non-fermentable gluconic acid. The H2O2 hydrolysing activity of purified catalase is necessary in order to stabilize glucose oxidase activity. After establishing the adequate enzyme ratio, the procedure was applied in large-scale trials (16L- and 220L-scale) of which one was conducted in a winery under industrial wine making conditions. Both enzyme activity and wine flavour were clearly influenced by the obligatory aeration in the different trials. With the enzyme treatment an alcohol reduction of 2%vol. was achieved after 30h of aeration. However the enzyme treated wines were significantly more acidic and less typical.

Genetically modified wine yeasts and risk assessment studies covering different steps within the wine making process

The use of gene technology to modify the genome of wine yeasts belonging to the species Saccharomyces cerevisiae began in the early 1990s. From a purely scientific point of view, many yeast constructs [genetically modified organisms (GMO)] have been made so far, covering more or less all stages of the wine making process in which microorganisms or commercial enzymes play a key role. The range of theoretical applications is summarised in this report. So far, only two wine-producing countries worldwide allow the use of engineered wine yeasts; the changing situation in Germany regarding consumers’ attitudes towards gene technology, and foodstuffs thus produced, will be outlined here. Experiments at the Geisenheim Research Center have highlighted the essential stages of the wine making process where yeasts are involved by using engineered wine yeasts in comparison with non-engineered yeast strains. Greenhouse simulations revealed the persistence of genetically modified (gm) yeasts when these were used as fertilizers, as vintners do with yeast lees after the fermentation process. Furthermore, the persistence of engineered yeast was also monitored in fermentations, after bottling, and after biological treatment of winery waste water. It turned out that engineered wine yeast strains behave like non-engineered wine yeasts. They also persist in the winery interior and installations as well as becoming part of the yeast flora on grape vines in a vineyard with annual fluctuations in the composition of the yeast populations.

Yeast protein extracts: an alternative fining agent for red wines

Wine producers have to label the use of certain animal proteins for fining of must and wine since 2012 [regulation (EC) No. 579/2012] under specific conditions, which has encouraged the search for alternative fining agents produced out of plants or yeasts. Yeast proteins are naturally present in wine and first studies of yeast protein extracts (YPE) for the fining of wine have been published, which were mainly focused on the technical properties of the YPE as fining agents. The present study used YPE of different production techniques for fining of red wine and evaluated their technical properties. Furthermore, the sensory effect of the new fining agents, their protein composition and the presence of glycans and glycoproteins were examined. The YPE showed differences in their protein profile and in protein glycosylation, but all YPE that were investigated were successfully applied for fining of red wine. Mannoproteins were detected in all YPE, and they did not prevent the mutual precipitation of proteins and tannins that is necessary for successful fining as reported earlier in case of glycans and glycoproteins. The chemical parameters of the red wine were modified by fining with YPE in a way comparable to fining with gelatine, a traditional fining agent used as reference, and the initial quality of wine flavour was maintained during the treatments.

Glycolytic functions are conserved in the genome of the wine yeast Hanseniaspora uvarum and pyruvate kinase limits its capacity for alcoholic fermentation

ABSTRACT Hanseniaspora uvarum (anamorph Kloeckera apiculata) is a predominant yeast on wine grapes and other fruits and has a strong influence on wine quality, even when Saccharomyces cerevisiae starter cultures are employed. In this work, we sequenced and annotated approximately 93% of the H. uvarum genome. Southern and synteny analyses were employed to construct a map of the seven chromosomes present in a type strain. Comparative determinations of specific enzyme activities within the fermentative pathway in H. uvarum and S. cerevisiae indicated that the reduced capacity of the former yeast for ethanol production is caused primarily by an ∼10-fold-lower activity of the key glycolytic enzyme pyruvate kinase. The heterologous expression of the encoding gene, H. uvarumPYK1 (HuPYK1), and two genes encoding the phosphofructokinase subunits, HuPFK1 and HuPFK2, in the respective deletion mutants of S. cerevisiae confirmed their functional homology. IMPORTANCE Hanseniaspora uvarum is a predominant yeast species on grapes and other fruits. It contributes significantly to the production of desired as well as unfavorable aroma compounds and thus determines the quality of the final product, especially wine. Despite this obvious importance, knowledge on its genetics is scarce. As a basis for targeted metabolic modifications, here we provide the results of a genomic sequencing approach, including the annotation of 3,010 protein-encoding genes, e.g., those encoding the entire sugar fermentation pathway, key components of stress response signaling pathways, and enzymes catalyzing the production of aroma compounds. Comparative analyses suggest that the low fermentative capacity of H. uvarum compared to that of Saccharomyces cerevisiae can be attributed to low pyruvate kinase activity. The data reported here are expected to aid in establishing H. uvarum as a non-Saccharomyces yeast in starter cultures for wine and cider fermentations.