Many producers say, "You cannot make a wine that exceeds the potential of the grapes used as raw material."
We often hear the phrase "Good wine comes from good grapes...", and it seems that how the grapes turn out is indeed a crucial factor in winemaking.
However, even when hearing "good grapes for winemaking," some people might not readily picture what makes grapes good or bad.
Here, referencing Toru Okuda's "Quality of Raw Material Grapes for Wine Production," we will examine the key points regarding the quality of wine grapes.
Four Key Points to Note in Wine Grape Quality
It is not simply a matter of wine grapes being moderately ripe at harvest time.
In Japan, it is said that culled scrap grapes used to be sent for winemaking in the past, but in modern Japanese wine, paying careful attention to the quality of the raw material grapes has become fundamental.
Here are the four points to pay attention to regarding the quality of wine grapes.
- Sugar content
- Acidity
- Nitrogen
- Aroma precursors
Naturally, there are many more aspects to consider, but because these are areas of grape quality that producers pay particular attention to, let us briefly explain each of them.
Sugar Content
The sugar contained in grapes is converted into alcohol through fermentation.
Therefore, the sugar content of the grapes used as the raw material is a very important point in winemaking.
The alcohol concentration of wine is generally 10 to 13% (v/v), and to achieve this concentration, it is said that raw material grapes need a sugar content of about 20 to 23 °Brix.
Brix is a physical quantity used as a measure of sugar content to gauge the amount of sugar present, utilizing the property that the refractive index changes depending on the concentration of sugar dissolved in water.
It is frequently used as an indicator of fruit sweetness, but since it is somewhat complex, those who want to know more in detail may want to check here.
The Relationship between International Sugar Scale and Brix – HORIBA
If the sugar content contained in grapes is insufficient, the alcohol content of the resulting wine drops, and risks such as microbial contamination increase.
However, if the sugar content is too high, the alcohol content of the resulting wine becomes excessively high, raising concerns about upsetting the balance of the wine's flavor (a problem in wine-producing regions in arid zones, etc.).
Therefore, determining at what sugar content to harvest becomes important for producers to craft high-quality wine.
Incidentally, many Japanese wines tend not to reach very high sugar levels in their raw material grapes due to the growing environment, and it is said that current circumstances often leave no choice but to rely on chaptalization.
However, some producers have succeeded in raising the grape sugar content by managing the cultivation environment, while others intentionally pursue pure winemaking without forcing up the alcohol level; thus, philosophies regarding sugar content vary.
When you pick up a bottle of wine, it might be interesting to imagine the sugar content of the raw material grapes based on its alcohol percentage.
Acidity
Wine is an alcoholic beverage characterized by high acidity.
About 90% of the acids in wine are the organic acids tartaric acid and malic acid, and their content is believed to affect not only flavor differences but also resistance to microbial contamination.
In recent years, with natural wines and elegant styles becoming global trends, conscientious producers seem to have become extremely sensitive to the acidity of their raw material grapes.
By their nature, grape sugar content and acidity (tartaric and malic acids) gently rise alongside ripening, but after a certain point, sugar levels surge and malic acid drops sharply.
If organic acids decrease because sugar content was prioritized, the risk of microbial contamination increases; conversely, harvesting too early risks ending up with intense, searingly acidic wine without adequate sugar levels...
Having to harvest grapes at a time that carefully balances sugar and acidity is considered a difficult challenge for producers.
Incidentally, an organic acid concentration of around 0.6–0.8% in tartaric acid equivalent is said to be ideal, but the reality is that growing grapes that satisfy this condition is by no means easy.
Though not all, quite a few Japanese wines undergo chaptalization or acidification to achieve balance.
However, most producers agonize over deciding the harvest date for their grapes.
Chaptalization and acidification are not inherently bad, and it is best to view them as one method for crafting sound wine.
Nitrogen
In alcoholic fermentation for wine, yeast is essential.
For yeast to proliferate, it is said that substances required for protein production such as nitrogen, phosphoric acid, vitamins, minerals, long-chain fatty acids, and sterols are necessary.
Among these, nitrogen is especially important in winemaking.
Because grapes used for winemaking are often deficient in nitrogen, depletion of nitrogen sources during fermentation can lead to stuck fermentations (fermentation arrests).
Although there are additives to prevent stuck fermentations, finishing fermentation naturally without sticking is of course ideal.
In recent years, as grape quality improvements have been advocated in Japanese wine as well, the "YAN concentration" of raw material grapes is measured to avoid stuck fermentations.
The compounds contained in grapes that can be assimilated by yeast as nitrogen sources are mainly amino acids and ammonia, and the nitrogen contained in these compounds is known as YAN (Yeast Assimilable Nitrogen).
The YAN concentration of raw material grapes is said to significantly impact the quality of the finished wine, including avoiding stuck fermentations and influencing wine aromas.
Aroma Precursors
Something that has also gained attention in Japanese wine in recent years is the presence of aroma precursors that contribute to wine aromas.
Compounds related to wine aromas include alcohols, terpenes, phenols, and norisoprenoids, and grapes contain "aroma precursors" where sugars and other molecules are bound to these.
These are said to exist abundantly in grape skins and other parts, but they have almost no aroma in their raw grape state.
These "aroma precursors" turn into aroma compounds when the sugars are hydrolyzed during wine fermentation by glucosidase derived from the grapes or yeast.
An example is 3MH, which contributes to Koshu's grapefruit-like aroma.
It exists in grapes as an "aroma precursor" and is known to reveal its aroma after turning into wine.
The more "aroma precursors" a grape has, the more aromatic the wine becomes, but it has been suggested that the content varies depending on harvest timing and the time of day.
In other words, some producers harvest while also considering how much of these "aroma precursors" they can retain at the grape stage.
It Is Also Interesting to Learn About Producers' Devotion to Grapes
Producers use raw material grapes that suit the wine they each aim to create.
As introduced here, while some producers strive to harvest well-balanced grapes, others intentionally maintain high acidity, or delay harvest to balance sugar and phenolic levels—approaches vary widely.
There are also producers who do not dwell on minor details at all, harvesting purely based on how the grapes taste.
If you have an opportunity to speak with producers in the future, it would be fascinating to ask about their philosophy on grapes.
Please take an interest not only in wine, but also in the grapes that serve as its raw material.
References
Quality of Raw Material Grapes for Wine Production — Toru Okuda
The Relationship between International Sugar Scale and Brix – HORIBA