Recently, I had the opportunity to taste wines at a winery located in Enzan, Yamanashi Prefecture. During the tasting of several wines from their lineup, one particular characteristic struck me strongly: the taste of iron.
When I asked the winemaker about this, they explained that the vineyard used for the wines with this distinctive taste is located near a former iron mine. The soil at the bottom of a nearby river is so rich in iron that it displays a distinctly red color.
The vineyard soil contains abundant iron content, and this taste can be clearly detected in the wines.
In such circumstances, it's natural to hypothesize that the vine roots absorb iron from the soil, resulting in iron accumulation in the grape juice, which then manifests in the wine's flavor. However, according to current scientific consensus, this explanation is incorrect.
Yet it remains an undeniable fact that wines made from grapes grown and harvested in iron-rich soil exhibit a distinct iron taste. How does this mechanism work?
This article will explore this phenomenon in detail.
Note: Before proceeding with this article, I strongly recommend reading "Understanding Soil" to better understand the terminology and concepts discussed here.
Minerals in Wine
In wine tasting, we frequently encounter expressions like "mineral notes." While there is considerable debate about "minerals" in the tasting context, this article focuses purely on the chemical perspective of "minerals" contained in wine.
To state the conclusion upfront: wines contain multiple types of substances referred to as "minerals" in varying degrees.
More precisely, these "minerals" refer to electrically charged metal ions. Examples found in wine include:
- Potassium
- Magnesium
- Calcium
- Sodium
- Iron
- Aluminum
- Zinc
Minerals in Soil
Meanwhile, the vineyard soil where grapes are grown also contains "minerals." As discussed in the "Understanding Soil" article, these are derived from the geological formations underlying the soil, with over 100 different substances identified.
Since geological distribution is not uniform, there are variations in the distribution of geology-derived minerals by location. Commonly found substances include:
- Silica
- Aluminum
- Iron
- Magnesium
- Calcium
- Potassium
- Sodium
"Minerals" Don't Always Come from Roots
The substances listed above are merely examples. Nevertheless, we can see that potassium, magnesium, calcium, sodium, aluminum, and iron are present in both wine and soil.
Given this overlap, it's tempting to assume these substances are absorbed from the soil through the roots.
Indeed, potassium is heavily distributed in grape skins and clearly accumulates during grape development. In such cases, we can reasonably conclude that these substances are likely absorbed and accumulated through the roots.
However, we must also consider the possibility of contamination.
Heavy Metals Come from Contamination
Among the minerals listed as wine components above, iron, aluminum, and zinc are classified as heavy metals.
These heavy metals are not absorbed through the roots.
As detailed in the "Wine and Heavy Metals" article, these substances primarily enter wine through contamination pathways. A prominent example is contamination from materials used in winery equipment.
Potential sources include tanks and their associated valves that may come into direct contact with wine, or indirect factors such as iron, aluminum, and zinc used in water faucets and surrounding areas. These metals can contaminate wine through direct contact or via water used during cleaning processes that adheres to tank surfaces and subsequently mixes with the wine.
Where Did the "Iron" in the Tasted Wine Come From?
So where did the "iron" taste in the Yamanashi wines I mentioned at the beginning come from? It's clear that this likely wasn't contamination from winery equipment as described above.
If this iron content originated from winery equipment, basically all wines produced there should exhibit similar characteristics. While it's possible that tank-specific equipment could affect only wines stored in that particular tank, the iron notes were far too pronounced for such an explanation.
Additionally, iron corrodes more easily than aluminum and other metals, making it rarely used in tanks and associated equipment, which supports this assessment.
The two wines in which I strongly detected iron were both made from grapes grown in the same vineyard block.
That vineyard block was the one "containing so much iron that the soil surface appears red."
As previously established:
- Vine roots do not absorb heavy metals and accumulate them in fruit
- The source was not winery equipment contamination
Given these points, only one possibility remains: this iron content resulted from surface contamination through adhesion.
Conclusion: Wine is Made Without Washing Grapes
Wine is fundamentally produced by immediately processing harvested grapes. Crucially, this process typically does not include a grape washing stage.
This is because washing grapes could potentially compromise grape quality, and it's prohibited by legal requirements in many regions.
Consequently, virtually everything that adheres to grape surfaces in the vineyard is brought directly into the winery.
Consider the analogy of automobile dirt. A freshly washed car gleams beautifully, but after some time, dust and other particles accumulate and dirty the surface. This dirt cannot be removed by light rain alone.
The same phenomenon occurs with grapes. Various dust particles lifted from the soil surface by wind adhere to grape surfaces. Light rain cannot wash away these adhered materials. The primary substances that adhere are those abundantly present in that vineyard's topsoil.
In other words, in vineyard blocks where the topsoil contains enough iron to turn red, substantial amounts of iron will adhere to grape surfaces.
The substances that can enter wine through this pathway are not limited to iron. Whether they reach concentrations sufficient to be tasted varies, but the possibilities are truly diverse for each vineyard.
When focusing on "soil," we tend to concentrate solely on absorption through "roots," but external influences on vines like this can also be considered soil-based effects in a broader sense.
Conversely, some argue that substances not originally present in grapes constitute "contamination."
Substances incorporated through surface adhesion to grapes are not in ionized states and differ in nature from other substances naturally present in grapes, making direct comparison impossible. Understanding these distinctions is essential when evaluating how to assess such wines' flavors.
