Introduction
Merlot is known for its supple texture and ripe fruit flavours, however, it can be a challenging variety to work with as it is known for high sugar and low to medium acidity musts, especially in warmer grape-growing climates.1 Its tendency to overripen in warm areas is further exacerbated through accelerated phenological development in the context of climate change.2 Consequently, Merlot wines often contain overly high ethanol levels but lack acidity.
Optimal harvest timing is critical when working with Merlot, and winemakers often adjust their grape growing and winemaking regimes to try and minimise the potential alcohol content and improve the acidity. These interventions can be costly, complicated and detrimental for wine quality and sensory perception.
A specific yeast strain, Lachancea thermotolerans (LT) is an acidifying, lower ethanol yielding yeast and is, therefore, a fitting solution for the high sugar/low acidity conundrum. This yeast is naturally found on grapes and in wines and has been investigated for its application in oenology for many years already.3,4
A comprehensive study5 aimed to determine the performance of five LT strains using Merlot musts with high sugar/low acidity content compared to control treatments. The fermentation performance as well as the chemical and sensory analyses of the various treatments were reported. The original publication focussed on the extent of acidification, the production of primary and secondary metabolites and the sensory rating by wine experts. Only the main findings of the study will be reported in this article.
Lachancea thermotolerans capabilities
Before we dive into the main findings of the above-mentioned study, it is important to understand the advantages and properties of LT yeast in winemaking:
- Acidifier
The major metabolic contribution of LT is the partial conversion of sugars to lactic acid (as opposed to ethanol) during alcoholic fermentation. LT yeast is extremely efficient in converting sugars to lactic acid: the maximum reported concentration of lactic acid formed during LT wine fermentation is 16.6 g/L.7 By comparison, Saccharomyces cerevisiae yeasts produce very little, if any lactic acid.8 The advantage of lactic acid is that it is an effective acidulant and it is stable (physiochemically and microbially), while other permitted acidulants might not be.6
- Lowers the alcohol content
Depending on the strain and the conditions, reports describe either similar or about 1% v/v lower ethanol concentrations in wines co-fermented with LT compared to their respective Saccharomyces cerevisiae fermentations.9–12
- Other properties
Other compositional alterations in LT wines include increases in glycerol, decreases in acetic acid and partial degradation of malic acid.10,11,13–18 LT contributes to the production of wines with enhanced mouthfeel, floral notes, fruitiness and freshness.19
Materials and methods
Merlot grapes (high sugar/low acidity) were handpicked from Australian vineyards during the 2019 vintage. Juice analyses showed the juice had approximately 26° Brix/Balling and a pH of 3.9. Five different LT strains were used in this study (LT1, LT2, LT3, LT4, LT5). Of the commercially available strains, LT2 has been identified as Laffort’s ZYMAFLORE® OMEGALT.20
LT strains can ferment to about 10% v/v ethanol21 and therefore require simultaneous or sequential addition of another co-starter to complete fermentation. In the current study, all LT strains’ performance was tested by doing a co-inoculation and a sequential inoculation with a Saccharomyces cerevisiae yeast strain. These fermentations and wines were compared to a treatment where the chosen Saccharomyces cerevisiae (SC) was used alone to ferment the juice as well as an uninoculated treatment (UN).
For the sensory evaluation, 47 experienced wine tasters were asked to evaluate the intensities of certain sensory attributes using a seven-point scale where 1=extremely low, 4=moderate intensity and 7=extremely high. Assessors were also asked to indicate which attribute best described the wine acidity profile by selecting one or more of the following profiles: “flat/flabby”, “crisp/fresh/bright”, “sour/tart” and “harsh/acrid”.
Results
Saccharomyces cerevisiae (control)
| Ethanol % v/v | pH | TA (g/L as tartaric acid) | Lactic acid (g/L) |
| 16.5 | 3.86 | 5 | 0.4 |
- resulted in high-alcohol and low acidity wines
- sensory panel rated the wine as the least ‘acidic’
- sensory panel rated the wine high in ‘sweetness’, ‘hotness’ and ‘body’ (probably due to the lower acidity as these attributes did not correspond with the residual sugar levels)
- high abundance of ethyl esters of medium-chain fatty acids however, the compounds failed to enhance the fruity character of the wines
- approximately 50% of the sensory panel described the wine as ‘flat/flabby’
Uninoculated
| Ethanol % v/v | pH | TA (g/L as tartaric acid) | Lactic acid (g/L) |
| 16.2 | 3.89 | 4.7 | 1.7 |
- resulted in high-alcohol and low acidity wines (comparable to the control)
- only treatment that completed spontaneous malolactic fermentation (MLF)
- sensory panel rated the wine as the least ‘acidic’ (comparable to the control)
- sensory panel rated the wine high in ‘sweetness’, ‘hotness’ and ‘body’ (probably due to the lower acidity as these attributes did not correspond with the residual sugar levels)
- the wines had higher intensity ratings for the sensory attributes ‘VA’ and ‘oxidised’ and had relatively higher acetic acid and ethyl acetate concentrations
- the wines had lower intensity ratings for the sensory attributes ‘red fruit’ and ‘herbaceous’
- approximately 50% of the sensory panel described the wine as ‘flat/flabby’ (comparable to the control)
Lachancea thermotolerans
There was variability in the capacity of the five different LT modalities to alter some of the properties. For instance, the behaviour of the low lactate producing strain LT3 was in stark contrast to the LT1 and LT2 strains, which were much more efficient in acidifying the fermenting must. For the purpose of this article, only the results from LT1, LT2, LT4 and LT5 will be discussed. More details can be obtained from the original publication.5
| Ethanol % v/v | pH | TA (g/L as tartaric acid) | Lactic acid (g/L) |
| 15.0 – 16.3 | 3.36 – 3.71 | 6.2 – 11.1 | 1.8 – 8.1 |
The table above shows the range of concentrations for the eight different LT treatments (four strains x two inoculation regimes) where the improvement from the SC control and the UN treatment is evident.
Acidification
Both the strain as well as the inoculation regime (co-inoculation vs sequential inoculation) had a significant effect on lactic acid concentration, pH and titratable acidity (TA). In general, the delay in SC inoculation (sequential) allowed the LT yeasts to contribution more in terms of lactic acid production and acidification.
- The LT treatments resulted in wine acidification (drop in pH and increase in TA) when compared to SC and UN.
- Wines with more lactic acid also contained lower alcohol content.
- LT1 and LT2 (both sequential) showed remarkable acidification properties delivering the wines with the highest TA and lowest pH values.
- The acidification potential of LT4 and LT5 was intermediary (but still a significant improvement from SC and UN).
- Deliberate MLF was not conducted in the experimental wines so as to better understand the impact of yeast treatments alone, however it should be noted that the concentrations of lactic acid above 4 g/L can have an inhibiting effect on MLF.
Alcohol
- LT2 (sequential) resulted in the wine with the lowest ethanol content (15.6% v/v) amongst the dry wines.
- The average decrease in ethanol for the dry fermented co-inoculations was 0.4% v/v, while the average decrease in ethanol for the dry fermented sequential inoculations was 0.9% v/v.
The lower alcohol content reported can be attributed to the partial conversion of sugar to lactic acid and not ethanol in the LT treatments, the extent of which varies between the strains and inoculation regimes.7,15
Sensory evaluation
- The TA and lactic acid content corresponded well with the sensory panel’s intensity rating of ‘length of acidity’ and ‘perception of acidity’. As a result, the LT samples (especially sequential inoculations) scored higher in these attributes.
- The LT wines scored lower in ‘hotness’, ‘bitterness’ and ‘body’ when compared to SC and UN.
- The flavour profile of the LT wines was largely shifted towards the ‘red fruit’ spectrum.
- The reference to ‘flat/flabby’ was significantly less for the LT wines (especially LT1 and LT2) when compared to SC and UN.
Conclusion
This study demonstrated the capabilities of Lachancea thermotolerans (in co-cultures with Saccharomyces cerevisiae) to acidify wines while reducing the ethanol content. This is a valuable tool in situations where musts have high sugar and low acid content. For the production of quality Merlot wines (especially in warming climates), the use of Lachancea thermotolerans can not only effectively modulate wine acidity and ethanol, but also add flavour balance (‘red fruits’) to the wine while steering it away from being described as ‘flat/flabby’.
It is important to note that certain parameters were more affected by the specific Lachancea thermotolerans strain and other parameters by the inoculation regime. It seems that strain LT2, commercially known as Laffort’s ZYMAFLORE® OMEGALT, were particularly well suited for the production of quality Merlot wines.5,20 Strains differed significantly in their capabilities and sound scientific results (such as the peer-reviewed article summarised here) should be consulted to ensure the right strain is chosen to achieve the desired effects.
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