Preferences in wine vary widely, but few people seem to dislike the profound sweetness that characterises dessert wines. Even setting aside Château d’Yquem, the mere thought of a Riesling Trockenbeerenauslese from an exceptional vintage is enough to bring a smile.
Dessert wines, however, encompass several distinct categories. Production methods differ across types, and with them, flavour and aromatic profiles diverge substantially. Comparing wines of different types, one may well question whether they can reasonably be grouped under a single designation. The presence or absence of red colour is one of the defining characteristics that differentiates these types.
What Constitutes a Dessert Wine
The term “dessert wine” is widely used, yet carries no formal regulatory definition. The name derives from the common practice of serving these intensely sweet wines alongside or after dessert.
Some online sources apply the term to sweet wines in general. It is certainly accurate that dessert wines are sweet wines; however, the threshold at which a wine qualifies as a “dessert wine” is inherently subjective, and defining the category by residual sugar alone is somewhat reductive.
Nonetheless, a working definition is necessary. The author’s own experiential benchmark, framed within the German classification system, is that wines at or above the Auslese (literally “selection”) level qualify as dessert wines. Within the same late-harvest category, Spätlese (“late harvest”) falls, in most cases, just short of this threshold.
Naturally, exceptions exist — Spätlese wines with unusually high residual sugar are treated as dessert wines accordingly. For those who find the distinction between Spätlese and Auslese unconvincing, a direct comparative tasting of both would likely make the intuitive basis for this boundary self-evident.
It must be emphasised that the above is based specifically on the German classification framework. Dessert wine is a global category that cannot be defined by any single national standard. A commonly cited tripartite classification organises dessert wines as follows: fortified wines (wines to which spirits are added to arrest fermentation, retaining residual sugar while raising alcohol content), straw wines and passito-style wines (in which grapes are dried on straw mats or racks to concentrate juice through water evaporation), and late-harvest wines (including noble rot wines as a subcategory). Within the late-harvest category, whether to further distinguish Spätlese from Auslese varies by context and classification system.
Dessert Wines With and Without Red Colour
As noted above, colour — or its absence — is one of the characteristics that differentiates dessert wine types. Searching any online wine retailer for intensely sweet red wines will immediately reveal a consistent pattern: results are limited almost exclusively to fortified wines, in which fermentation is arrested through the addition of spirits, or to straw wines in the broad sense, where concentration is achieved through freezing or desiccation.
A fortified wine is produced by adding a neutral spirit (spirits) during fermentation to halt yeast activity, thereby retaining substantial residual sugar while simultaneously increasing alcohol content. A straw wine achieves concentration by removing water from the grape juice — either through artificial freezing or drying on straw — thereby elevating sugar density.
With limited exceptions, producing a deeply coloured red dessert wine is effectively impossible without deliberate human intervention to raise or preserve sugar levels through such methods.
The underlying reason involves Botrytis cinerea (hereinafter Botrytis) — the filamentous fungus responsible for noble rot.
It should be noted that where only healthy, uninfected grapes achieve full physiological ripeness or undergo natural freezing, late-harvest wines and ice wines made from red varieties can indeed yield deeply coloured red dessert wines. In exceptional cases, grape varieties with particularly dense pigmentation in the skins may retain comparatively intense colour even when infected with Botrytis, occasionally producing wines that appear red to the eye. Conversely, because Botrytis plays a central role in the biochemical processes that concentrate sugar in grape juice, dessert wines produced without Botrytis tend to achieve somewhat lower levels of sweetness.
Botrytis and Wine Colour
Wines produced from Botrytis-infected grapes are, as a general rule, not made as red wines — regardless of whether the wine is intended to be sweet. Two distinct reasons account for this.
Reason 1: Winemaking Practice — Avoidance of Fungal Off-Character Transfer
For a red wine to develop its characteristic colour, the grape skins must be macerated in the juice for an extended period, allowing the anthocyanin (anthocyanin) pigments contained in the skins to be extracted into the must. In Botrytis-infected grapes, the surface of each berry’s skin is colonised by fungal mycelium. Macerating mould-covered skins in the juice — even for the purpose of colour extraction — inevitably transfers fungal flavours and aromas into the must, an outcome that is firmly unacceptable in professional winemaking.
To prevent such transfer, contact time between the skins and the juice must be minimised as far as possible. Eliminating this contact also eliminates colour extraction, which is why the resulting wine does not develop red colour.
Reason 2: Biological Mechanism — Botrytis-Mediated Anthocyanin Degradation
Red wine owes its colour to the presence of anthocyanins — glycosides in which anthocyanidins (anthocyanidin) serve as the aglycone, bound to one or more sugar residues. Multiple anthocyanin species exist, distinguished by differences in their glycosidic sugar chain composition, and their chromatic expression varies accordingly. While the chromophore (chromophore) resides in the aglycone moiety, the nature of the sugar chain is critical in determining the specific colour expressed.
The critical problem is that Botrytis — in common with many other fungal organisms — exhibits high glucosidase (glucosidase) activity. Glucosidase is an enzyme that catalyses the hydrolytic cleavage of glucosidic bonds (glucosidic bond). In the berry skin tissue of Botrytis-infected grapes, glucosidase produced by the fungus hydrolyses the glucosidic bonds within anthocyanin molecules. As anthocyanins are glycosides in which sugars or sugar chains are attached to the anthocyanidin aglycone, cleavage of these bonds causes anthocyanins to lose their structural integrity and, with it, their capacity for chromatic expression.
Research conducted across multiple countries and regions on compositional changes in Botrytis-infected black-fruited grape varieties has consistently reported substantial reductions in the content of anthocyanins and other phenolic compounds (phenolic compounds) in the berry skin following fungal infection.
Through this mechanism, Botrytis infection causes the progressive loss of pigmentation in the berry skin. Consequently, even if maceration is attempted with the intent of extracting colour, sufficient pigment is simply no longer available.
In summary, the two reasons why Botrytis infection precludes red wine production are, respectively: a winemaking decision based on wine quality considerations, and an unavoidable biological constraint inherent to the fungal infection itself.
Summary: Producing Red Dessert Wine from Botrytis-Infected Grapes
Dessert wines encompass types from which deeply coloured red wines can be produced, and types from which they cannot.
If the objective is to produce a red dessert wine, Botrytis infection must be avoided. However, in botrytised wines — where Botrytis infection is the fundamental prerequisite of the production method — as well as in late-harvest styles such as ice wine and Auslese where delayed harvest increases the likelihood of infection, red wine production is effectively precluded. It is worth noting that it is precisely this infection that enables the production of wines with the extreme sweetness characteristic of the highest dessert wine categories.
That said, it would be incorrect to state categorically that red wine cannot be produced from any Botrytis-infected grapes.
While some research suggests that Botrytis infection progresses on a bunch-by-bunch basis, at least in the early stages of infection, individual berries within a single bunch present differently — some infected, others not yet. This heterogeneity opens a narrow but real winemaking possibility.
Where the proportion of infected berries within a bunch remains low, berry-by-berry sorting allows infected berries to be pressed immediately, yielding high-sugar juice, while uninfected berries are directed to maceration for colour extraction. The pressed juice is introduced directly into the maceration vessel, and from that point the two components are vinified together. This constitutes the viable approach for producing a red wine with elevated sugar content from partially infected fruit.
This method is nonetheless subject to significant constraints. Because juice from Botrytis-infected berries is incorporated, some degree of Botrytis-derived aroma will be present in the finished wine. Furthermore, since the majority of juice originates from uninfected berries, the overall must sugar concentration will be lower than that achievable when a high proportion of berries are fully infected. The resulting reduction in sweetness tends to place such wines in an ambiguous position within the dessert wine category. Given the considerable labour investment relative to the difficulty of achieving commensurate commercial value, whether this approach is worth pursuing ultimately rests with the individual winemaker’s priorities and judgement.
