The assessment of potential damage to a wine collection due to heat exposure is an inherently complicated issue, in large part because the process of chemical change during wine maturation is also very complicated. To explain in brief without entering too far into the chemistry behind it, the aromas in a wine come from volatile compounds. Some of these are innate in the must (unfermented grape juice), while others are produced by the action of the yeast the during the fermentation process, and yet more are created after fermentation through winemaking processes.
After fermentation wine can undergo maturation in tank or barrel, and it ages further in bottle. Existing flavor compounds within the wine interact, and the combination of acids and alcohols create new volatile compounds. Not every wine, however, improves with age, and it is not possible to anticipate precisely how a wine will evolve. It is universally agreed, though, and supported by substantial literature, that the rate of evolution will differ based on storage conditions. Ideal storage temperature has long been considered to be 55º, which is, perhaps not coincidentally, the temperature below six feet underground and, hence, of an underground cave or cellar. At this temperature wine changes at a rate which historically is considered optimal, generally producing flavors that are considered desirable and give complexity to the wine.
Higher temperatures accelerate the rate of change, not only of the desirable but also undesirable compounds. The higher the temperature the faster the rate of acceleration. According to one scholarly article, wine ages two to eight times faster at 73º than at 55º, and at a considerably greater rate at 91º. Moreover, many of the less favorable compounds that can form in wine form slowly at low temperatures but develop more rapidly at elevated storage temperatures. As a result, high storage temperatures can lead to an imbalance of favorable versus unfavorable flavors. Lastly, there are compounds, considered unfavorable except in the case of certain fortified wines, that form only in the presence of high temperatures, conditions which also destroy a wine’s more volatile, and often desirable, elements. This explains the phenomenon of “cooked” wine, whereby wine subjected to high temperatures develops brown hues, suffers a loss of freshness and delicacy and sees formation of compounds not created at lower temperatures, all of which gives the wine stewed flavors that may dominate all others.
Given the increasing acceleration of premature maturation as temperature increases, the amount of time required for wine to suffer adverse alteration decreases as temperature increases. Estimates and assessments vary and, to an extent, are subjective, but it is widely held that at temperatures in excess of 86º wine can become cooked within 24 hours, and this time frame can be compressed to just 6 hours if temperatures exceed 100º. But at even lower temperatures wine can suffer damage beyond premature maturation. At temperatures above 82º the seal of a cork can become compromised, permitting the ingress of oxygen, which degrades the wine, flattening its flavors and leading to the creation of harsh acetic acid. Oxidation can also be caused by rapid fluctuations in storage temperature. Heat causes the wine and air bubble in the bottle to expand, and should temperatures drop rapidly the air bubble, as it contracts, can create a vacuum that draws air into the bottle. So even if temperatures do not reach extremes, oxidation of wines can still be caused by storage conditions.
At this point it should be noted that, provided wines do not freeze, there is no evidence for wines being damaged by temperatures below 55º. While storage at low temperatures will retard the maturation of a wine, consistent low temperature storage should not damage the wine. There are, however, a number of inherent risks regarding low temperatures that makes such storage ill-advised. First, should temperatures drop below 25º the wine can freeze, and the resulting expansion can raise the cork, compromising its seal, and possibly break the bottle. Either situation will render a wine valueless. In addition, low temperatures are often not constant but, rather, occur as part of rapid temperature fluctuation. As noted above, such fluctuations can draw oxygen into the bottle and cause degradation of the wine. Lastly, low temperature air retains less moisture than warmer air, and low temperature storage risks equally low humidity that can cause corks to dry and contract, thereby allowing for ingress of harmful oxygen. Accordingly, storage at temperatures much below 45º is not advised.
Up until now we have been discussing the aromatic evolution of a wine. Beyond that, however, the structure of wine evolves with age, altering its texture, body and balance. These elements depend not on the volatile compounds but on the dry extract, phenolic compounds (anthocyanins/color compounds and tannins), alcohol, glycerol, and acidity. Of particular importance is the polymerization of the phenolic compounds, which contribute to a wine’s astringency. Polymerization here is the gradual combination of the color compounds and the tannins and is a key component of wine aging. As the molecules combine, they become insoluble and fall to the bottom of the bottle as sediment, resulting in a silkier and less astringent wine. This process is also accelerated at higher temperatures.
While it is not always possible to detect heat damage to wine from visual inspection, there are some conditions that can raise a strong presumption that it has occurred. Protruding corks provide some of the most likely evidence. Temperatures above 90º cause sufficient expansion of the wine that the ensuing pressure on the cork can force it to rise and protrude from the bottle neck, although, as noted above, this can also be caused by the wine having expanded due to freezing. Active seepage is another strong indicator of heat damage. This can be caused when heat causes a cork’s seal to fail and the wine to expand and leak past the cork, but it can also be caused by cork failure even in the absence of heat. Signs of old seepage, where the wine rivulets have dried and, sometimes, the label shows wine stains aligned with the rivulets, can indicate older exposure to excessive heat but can, as with active seepage, also be caused by cork failure unrelated to heat. Exposure to heat can also be detected in color changes to the wine, with whites becoming gold, then amber, and even brown. Reds will take on garnet, then brick, then brown hues. Additionally, sediment that is uncharacteristically heavy for the age of the wine can point to heat exposure.
Additional data can be gained if possibly affected wines are sampled and compared either to control samples or similar wines as previously experienced by the inspector. Even before the inspector tastes the wine being sampled additional information can be gleaned. Wine stains visible up the sides of an extracted cork indicate a compromised cork seal, which, as discussed above, can result from heat exposure or a faulty cork. Upon tasting the wine, accelerated aging—wines that taste more mature than their age would warrant—coupled with documented exposure to high heat, provides strong evidence of heat damage to wine. To gauge the measure of the damage, different types of wine would be sampled, ranging from the lighter and more delicate to the robust. Perceptible changes in only the more delicate wines would indicate a low level of damage, while changes to the sturdier wines would indicate a higher degree.
Because of the complexity of the issues at hand it is not possible to assign absolutely a percentage of damage to every bottle in a potentially affected collection. However, since exposure to extreme heat should be disclosed to potential buyers or consignors of a collection, this would likely impact the eventual merchantability of the collection. Such an incident would render a collection difficult, if not impossible, to consign to a first-tier auction house. It would also render it more difficult to sell through other channels, such as second-tier auction houses, retail consignment and private transaction.
 Pandell, Alexander J. “How Temperature Affects the Aging of Wine.” The Alchemist’s Wine Perspective. March 2, 2019, re-published from The Alchemist’s Wine Perspective, Issue One, November 1996. https://www.wineperspective.com/how-temperature-affects-wine-aging/.