Acids present in wines and their perceived taste

Organic acids are a significant constituent of wines. They present a sour/acid taste, and influence wine stability, color, pH and have a significant effect on the final wine mouthfeel quality. The acid composition and concentration of wines is influenced by many factors, from grape variety and climatic conditions to fermentation process and yeast. Some of the acids are formed in the grape-berries, during berry development, others are formed during alcoholic fermentation (AF) and malolactic fermentation (MLF). During these biological processes, it is prudent to monitor the concentration of organic acids, and a distinction is made between acids directly produced in grapes (tartaric, malic, and citric) and those originating from AF and MLF (succinic, lactic, and acetic acids, among others), Table 1.

Non-Saccharomyces yeasts and wine acidity modulation

The main volatile compounds and the organic acids formed/consumed by non-Saccharomyces yeasts during grape-must fermentation are described in Table 2.

Table 2. Main volatile compounds produced and effect on wine acidity of seven non-Saccharomyces yeasts. Adapted from Vilela.

Save View full size Expand inline Collapse inline

During MLF, the metabolism of Oenococcus oeni can improve the wine sensory characteristics by decreasing acidity. However, the success of MLF is influenced by enological parameters, such as temperature, pH, alcohol content and SO₂ concentration, among others. An alternative to MLF was studied by Benito and coworkers¹ to be used, particularly, in warm viticulture regions such as the south of Spain where the risk of suffering a deviation during the MLF process increases due to the high must pH which may contribute to produce wines with high volatile acidity and biogenic amine values. The mentioned authors used two non-Saccharomyces yeasts as an alternative to MLF. Malic acid was consumed by Schizosaccharomyces pombe by converting it to ethanol and CO₂ (Figure 1), whereas Lachancea thermotolerans produces lactic acid in order to maintain wine acidity balance, especially in wines produced from low acidity musts.

Figure 1. Schematic representation of the maloalcoholic fermentation performed by Schizosaccharomyces pombe. Adapted from Vilela.

Save View full size Expand inline Collapse inline

Another malic acid consuming strain, not yet commercialized or produced at an industrial level, is Issatchenkia orientalis/Pichia kudriavzevii. In 2014, del Mónaco and coworkers² found a P. kudriavzevii isolate that was able to degrade l-malic acid in microvinification assays, increasing the pH by 0.2–0.3 units. Furthermore, this yeast strain produced low levels of ethanol and significant levels of glycerol.

L. thermotolerans can produce lactic acid and glycerol during fermentation, and all these interesting features can be a way to address the problems of increased alcohol content/reduction in the total acidity of wines, associated with global climate changes. Moreover, L. thermotolerans in the presence of some O₂, by shifting its metabolism more towards respiration than fermentation, is also capable of decreasing wine volatile acidity³.

At the beginning of AF, many species can grow at the same time in the grape-must and Candida stellata spp. have been described in this stage of fermentation. An interesting C. stellata feature is its ability to form succinic acid⁴. Succinic acid could positively influence the sensory/mouthfeel profile of wines with insufficient acidity. Nevertheless, due to its ‘salt-bitter-sour’ taste, excessive levels would negatively influence wine quality.

Another interesting yeast is Torulaspora delbrueckii. An indication of the positive impact of T. delbrueckii activity on wine quality was demonstrated by Bely and coworkers⁵ when fermenting botrytized musts and, afterward, by Azzolini and coworkers⁶ in Vino Santo, a sweet wine, due to its low production of acetic acid. Moreover, T. delbrueckii in the production of Amarone wine (Amarone della Valpolicella), a high-alcohol dry red wine obtained from withered grapes, was able to promote the formation of alcohols, fermentative esters, fatty acids, and lactones, which are important in the Amarone wine flavor.

S. pombe and L. thermotolerans are presently produced at an industrial level by biotechnological companies. T. delbrueckii is commercialized in the form of a pure culture, selected for its properties such as increase in aromatic complexity, mouthfeel, low production of volatile acidity and high resistance to initial osmotic shock. According to the supplier, it is highly recommended for the fermentation of late harvest wines in sequential culture with S. cerevisiae.

Zygosaccharomyces spp. are well known for their ability to spoil food and beverages. The fermentative nature of Zygosaccharomyces metabolism produces CO₂ that is of concern if the food product is in a sealed container, as the increase in gas can cause the container to leak or, in extreme cases, explode. Haze or sediment can occur in beverages as cellular biomass accumulates, and in some cases, surface biofilms can be formed. The aroma and taste modification can occur by the production of secondary metabolites with sensory impacts on the food or beverage and include acetic acid, esters, and higher alcohols. Nevertheless, some strains are used in the industrial production of balsamic vinegar and other food products. Recently, Lencioni and coworkers⁷ studied the possibility to decrease wine volatile acidity of high-sugar musts, by using Zygosaccharomyces florentina and Starmerella bacillaris in multistarter fermentations with S. cerevisiae. Independently of fermentation temperature, the mixed fermentations performed best to reduce volatile acidity, therefore being a valuable tool for performing fermentation of high-sugar musts.

Final remarks

In conclusion, many non-Saccharomyces yeasts present interesting enological properties and when used in single or mixed cultures with S. cerevisiae, these yeast species can modulate wine acidity and increase the production of some interesting compounds, such as polysaccharides, glycerol, and volatile compounds, such as 2-phenyl ethanol and 2-methyl 1-butanol.

Nevertheless, if the use of some of the yeast species cited could modulate wine acidity, some can also act as spoilage, through the production of off-flavors, haze or sediment, depending on the environmental conditions. So, a deeper study of the enological traits of these yeasts will provide new data for consideration in the modulation of wine acidity.

Acknowledgments: We appreciate the financial support provided to the Research Unit in Vila Real (PEst-OE/QUI/UI0616/2014) by FCT—Portugal and COMPETE. Additional thanks to the Project NORTE-01-0145-FEDER-000038 (I&D INNOVINE&WINE—Innovation Platform of Vine & Wine).