The effect of pH (1.7-3.2) and sugar concentration (64-68 °Brix) on the growth of Zygosaccharomyces rouxii MC9 using response surface methodology was studied. Experiments were carried out in concentrated grape juice inoculated with Z. rouxii at isothermal conditions (23 °C) for 60 days. pH was the variable with the highest effect on growth parameters (potential maximum growth rate and lag phase duration), although the effect of sugar concentration were also significant. In a second experiment, the time for spoilage by this microorganism in concentrated grape juice was evaluated at isothermal (23 °C) and non-isothermal conditions, in an effort to reproduce standard storage and overseas shipping temperature conditions, respectively. Results show that pH was again the environmental factor with the highest impact on delaying the spoilage of the product. Thereby, a pH value below 2.0 was enough to increase the shelf life of the product for more than 60 days in both isothermal and non-isothermal conditions. The information obtained in the present work could be used by producers and buyers to predict the growth and time for spoilage of Z. rouxii in concentrated grape juice.
Zygosaccharomyces rouxii is the most frequent spoilage yeast species detected in concentrated grape juice. In order to reduce Z. rouxii populations and consequently extend the microbiological shelf life of this product, different programs of thermal pasteurization and high hydrostatic pressures processing were evaluated. Results showed that pasteurization temperatures higher than 75 °C are necessary to reduce Z. rouxii population in concentrated grape juice. Reduction of 7 logarithms can be reached after 90 s at 75 and 80 °C, and 5 s at 85 °C of pasteurization treatment. High hydrostatic pressure treatment above 500 MPa for 2 min are necessary to reduce 7 logarithms of Z. rouxii population and to significantly extend the shelf life of concentrate grape juice. Extension of holding times from 3 to 5 min, at the different high hydrostatic pressures evaluated, did not improve the Z. rouxii population reduction, nor the shelf life extension of concentrate grape juice. In conclusion, thermal pasteurization and high hydrostatic pressure could be suitable treatments to achieve the reduction of Z. rouxii population below the recommendation limit (10 2 CFU/g) and extension of the microbiological shelf life of concentrate grape juice.
The yeast Saccharomyces cerevisiae is the main species responsible for the process that involves the transformation of grape must into wine, with the initial nitrogen in the grape must being vital for it. One of the main problems in the wine industry is the deficiency of nitrogen sources in the grape must, leading to stuck or sluggish fermentations, and generating economic losses. In this scenario, an alternative is the isolation or generation of yeast strains with low nitrogen requirements for fermentation. In the present study, we carry out a genetic improvement program using as a base population a group of 70 strains isolated from winemaking environments mainly in Chile and Argentina (F0), making from it a first and second filial generation (F1 and F2, respectively) based in different families and hybrids. It was found that the trait under study has a high heritability, obtaining in the F2 population strains that consume a minor proportion of the nitrogen sources present in the must. Among these improved strains, strain “686” specially showed a marked drop in the nitrogen consumption, without losing fermentative performance, in synthetic grape must at laboratory level. When using this improved strain to produce wine from a natural grape must (supplemented and non-supplemented with ammonium) at pilot scale under wine cellar conditions, a similar fermentative capacity was obtained between this strain and a widely used commercial strain (EC1118). However, when fermented in a non-supplemented must, improved strain “686” showed the presence of a marked floral aroma absent for EC1118 strain, this difference being probably a direct consequence of its different pattern in amino acid consumption. The combination of the capacity of improved strain “686” to ferment without nitrogen addition and produce floral aromas may be of commercial interest for the wine industry.
The current market requirements to obtain wines with enhanced complexity and varietal character propose the finding of new microorganisms capable of driving the malolactic fermentation (MLF) and contributing to new and distinctive sensory profiles in wines. During MLF, malic acid naturally present in the must is converted into lactic acid by a decarboxylation reaction driven by lactic acid bacteria. Recently, a new strain of Lactobacillus plantarum suitable for MLF in high-pH wines was selected. This study evaluates the capacity of the L. plantarum V22 starter culture to complete MLF at laboratory and semi-industrial scale in different Malbec musts from Argentina. Malbec musts were co-inoculated with S. cerevisiae D254 and L. plantarum V22 to conduct alcoholic fermentation and MLF respectively. Two groups of Malbec musts were identified according to the ability of L. plantarum to consume malic acid. A first group, called "High Feasibility for MLF", is characterised by the high viability of L. plantarum and the completion of MLF. On the other hand, a second group was identified, termed "Partial Feasibility for MLF", and is characterised by a decreased viability of L. plantarum and incomplete MLF. This study shows that the capacity of L. plantarum to conduct MLF in Malbec musts is dependent on must composition, further suggesting that the evolution of pH during the fermentation process, rather than initial pH, is a critical factor affecting the successful completion of MLF.
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