During hot summer months, specifying refrigerated delivery for wine shipments has become almost standard practice. For wines delivered to homes, storing them in dedicated wine cellars whenever possible is widely recommended.
The reason these temperature-related handling methods and precautions are frequently emphasized is that wine is an exceptionally temperature-sensitive beverage. Temperature sensitivity specifically means the following: exposure to high temperatures can easily alter wine's taste and aroma. Since these effects tend to be undesirable in most cases, wines affected by heat are sometimes described as having undergone heat deterioration.
When drinking wine that has been transported by regular truck on hot summer days or stored in a closet throughout an entire summer, you may notice a distinctly different taste. While such experiences teach us that wine should not be kept in hot places, explaining exactly what happens to cause these changes in taste and aroma is not straightforward.
This article focuses on explaining the changes that occur within wine when it is transported or stored under high-temperature conditions.
The Ideal Wine Storage Temperature is 11°C
We're told that wine should not be kept in high-temperature environments, but it's often unclear exactly what temperature becomes problematic.
The appropriate temperature for wine storage has been specified by various experts with slightly different temperature ranges. Reviewing this information reveals that most recommendations fall within 10-18°C, with the more ideal range being 10-15°C centered around 11°C. While temperatures up to 20°C are unlikely to cause fatal damage, temperatures exceeding 25°C are noted to affect the volatile compounds in wine, causing changes in aroma.
Aged wines become increasingly sensitive to temperature. For older vintage wines, continued storage above 18°C significantly increases the risk of accelerated aging. Even when using dedicated cellars, maintaining perfectly constant storage temperatures is difficult, but it's advisable to set temperatures on the lower side, accounting for temperature increases during opening and closing.
Temperature Sensitivity Varies by Wine
For consumers, being given detailed specifications can be troublesome, so having a simple directive like "store below X degrees" would certainly be more convenient. In this sense, the aforementioned 10-15°C serves as a useful guideline. Typical household refrigerators maintain temperatures of 2-6°C, with vegetable compartments around 7°C. Both of these are somewhat too cold for wine storage.
However, wine is primarily affected when temperatures are high. While extremely low temperatures are also said to be problematic, German verification studies reported no impact on wine quality even after long-term transportation at 0°C. One might think, "Then storing in a household refrigerator should be fine," but the problem here lies in the expanded temperature differential with ambient air due to overcooling and the range of temperature fluctuation.
Multiple studies have confirmed that storage in environments prone to frequent temperature variations, even with lower base temperatures, has a greater impact on wine than storage at somewhat higher but consistent temperatures. Household refrigerators are opened frequently, leading to significant variations in internal temperature. Some studies indicate that leaving a refrigerator open for 10 seconds raises the internal temperature by 1°C. While how seriously to take this depends on one's perspective, it's undeniable that refrigerators are not well-suited for wine storage.
What further complicates matters is the variation in sensitivity among different wines.
The way temperature affects wines is completely individual to each wine. Classification by region, grape variety, or even wines of the same region, vintage, and variety cannot predict temperature sensitivity. While it's clearly established that white wines are more sensitive than reds, that's about the extent of reliable categorization. Beyond the red versus white distinction, sensitivity varies completely from wine to wine. This means systematic judgment based on wine categories is virtually impossible.
Given these circumstances, the approach to storage and transportation handling becomes one of eliminating any potential risk factors. This means maintaining stable storage at low temperatures with minimal temperature fluctuation. For this reason, household refrigerators and environments prone to overcooling are unsuitable.
Complete Protection of Wine from Heat Effects is Nearly Impossible
While wine's exposure to heat is often thought to occur only during home or shop storage and post-purchase transportation, this is not actually the case. Wine is frequently exposed to potential heat effects.
The first opportunity occurs within the winery itself.
During winemaking processes such as alcoholic fermentation, wine temperatures are usually managed quite strictly. However, once fermentation is complete and the wine enters the aging period, it is very commonly left to ambient temperatures. Ambient temperature is simply air temperature or room temperature.
Naturally, air and room temperatures vary significantly with seasonal changes and differ greatly by country and region. Even in Europe, which was previously cooler, global warming has led to increasing numbers of days when external temperatures approach 40°C during summer. When external temperatures approach 40°C, indoor temperatures can easily exceed 25°C in non-air-conditioned spaces. While this doesn't immediately translate to wine liquid temperature, the temperature of stored wine will gradually rise.
An even greater risk exists in the storage environment of bottled wines awaiting shipment.
The larger the winery, the greater the number of bottles in storage. These bottles awaiting shipment are typically stored stacked in high-ceiling warehouses. Temperatures near the ceiling can be significantly higher than at ground level.
While wineries in originally hot regions may have fully air-conditioned warehouses, most wineries worldwide lack such facilities. Even when they do, temperature management often focuses on ground-level areas, with upper warehouse temperatures frequently unconsidered.
Bottles shipped from wineries reach shops and consumers through land and sea transportation. Non-temperature-controlled transportation can see cargo hold temperatures approach 50°C, which is why wine transportation typically uses temperature-controlled services to prevent wine temperature increases. However, even during supposedly temperature-controlled transportation, temperature increases can occur due to container positioning and transportation conditions. Cases of temporary temperature increases to around 40°C have been documented.
In these various scenarios, wine may be subjected to unrecognized heat effects. Eliminating all such exposure is virtually impossible.
The Highly Complex Effects of High Temperature
When wine is exposed to hot environments at some point and suffers some impact, this is commonly described as heat deterioration. Those hearing this term understand it vaguely as wine being damaged by heat. However, the reality is not so simple. When wine is exposed to heat, it undergoes extremely complex, multifaceted effects involving various interrelated factors.
Heat Accelerates Wine Aging
Heat deterioration in wine is sometimes understood as accelerated aging. Wine aging is one aspect of oxidation reactions. Oxidation reactions are chemical reactions occurring within wine, and their reaction rates are influenced by heat. Higher temperatures increase reaction rates.
In this sense, when wine is exposed to heat, so-called aging also progresses more rapidly. However, heat effects also cause changes in wine that don't occur during normal aging. Heat deterioration is not simply premature aging.
Volume Expansion Leads to Oxidation and Microbial Contamination
One temperature-related effect is physical changes, including volume changes.
The wine itself and the air in the headspace—the air layer between the liquid surface and cork interior—expand when temperature increases. Very roughly calculated, a 10°C temperature increase changes bottle volume by more than 15ml. This volume increase creates pressure that pushes up the cork.
If the cork is actually pushed up, the effect is visually apparent. However, even when the cork appears unaffected, spaces sufficient for air escape may form at the interface between cork and bottle wall. When this happens, the bottle is no longer properly sealed. Oxygen begins flowing into the bottle through these spaces.
Furthermore, liquid may enter these gaps and penetrate the cork from the sides, potentially weakening cork structure and increasing the cork's oxygen permeability. Liquid leakage can also become a source of mold and other microbial contamination.
Changes in bottle internal pressure may also affect the rates of various chemical reactions.
Both liquids and gases constantly expand and contract according to temperature. Therefore, when wine bottle storage temperatures become unstable, internal bottle pressure fluctuates dramatically accordingly. In bottles where expansion has loosened cork interfaces, subsequent low-temperature storage causes volume contraction that draws external air into the bottle. This increases oxygen quantity in the headspace, leading to oxidation reactions based on that oxygen.
While these effects might seem limited to corks, even screw caps can develop gaps between cap liners and bottle mouths due to upward cap pressure. While the effects are certainly less pronounced than with corks, they are not entirely absent.
Chemical Changes Directly Altering Aroma and Taste
Wine condition is typically managed using several numerical values: alcohol content, pH, residual sugar, total acidity, and extract content. Each of these parameters has a direct relationship with wine taste. If these values change, wine taste will feel significantly different. Therefore, wine changes are often thought to result from changes in these various parameter values.
However, heat has been confirmed through multiple studies to cause no significant changes in any of these parameters. Instead, what is affected are free and bound SO₂ content, phenolic compounds, and the types and concentrations of volatile compounds.
Simply stated, the following changes occur in wines stored at high temperatures: free and bound SO₂ content decreases, 420nm absorbance increases, and concentrations of norisoprenoids such as TDN and vitispirane increase, while concentrations of ester compounds with fruity aromas, including isoamyl acetate, decrease.
Heat effects are not limited to reductions in wine components. Increases in certain ester compounds have also been confirmed. Many of these increased volatile compounds cause unpleasant odors in wine. The increased compounds included hydrogen sulfide and methanethiol, which cause sulfur odors known as reductive off-flavors.
The patterns of chemical changes occurring in wine vary significantly by wine type. While heat effects are generally greater in white wines than red wines, the reactions occurring within wine are far more complex in red wines than in white wines.
Summary | Temperature Effects Differ Completely Between Short-term and Long-term Exposure
Wine importers using temperature-controlled reefer containers during long ocean shipping periods and wine shops or consumers using refrigerated delivery for summer transportation are motivated by essentially the same reason: preventing wine exposure to high temperatures.
However, examining the actual effects on wine reveals that the only common factor between these two situations is effects based on volume expansion. This is because chemical effects within wine do not occur during domestic delivery periods lasting only a few days at most.
Various components in wine require considerable time to begin chemical reactions. The time required for this is not measured in hours or days, but in months, as reported in verification studies.
Nevertheless, this doesn't mean wine undergoes no changes during short-term transportation or storage periods. Regardless of the nature of changes, wine transportation without refrigerated delivery during summer should be avoided, and wine storage should be in locations where temperature is consistently maintained at low levels. This principle is not disputed.
What should be understood is the following: changes occurring over months-long periods such as ocean shipping or long-term storage differ in nature from short-term changes over several days. Furthermore, focusing only on physical properties like wine alcohol content or residual sugar when examining such changes may lead to the mistaken understanding that heat has no effect on wine—a conclusion divorced from reality.
