Light-struck flavor, or "nikkoushuu" in Japanese. The name might evoke images of freshly laundered white sheets fluttering under a clear blue sky, or the pleasant scent of sunshine when wrapped in those sheets. Unfortunately, that is not what this term refers to.
Light-struck flavor is a type of wine fault known in English as Light-Struck Taste (LST) or Light-Struck Flavor (LSF), and in French as Goût de Lumière. While this off-flavor is frequently identified in beer and milk, it is by no means foreign to sake or wine.
Light-struck flavor is primarily a defect that develops during storage. As its name suggests, it occurs when wine or beer is exposed to sunlight or other light sources. Because the mechanism causing this defect is well understood, its occurrence can be prevented with considerable reliability by using darker-colored bottles that block light transmission.
On the other hand, from a wine producer's perspective, there is often a desire to use clear bottles to showcase the wine's color—particularly for rosé wines. Recently, clear bottles incorporating UV-absorbing materials or surface coatings have entered the market. Nevertheless, compared to darker-colored bottles, light transmission remains a concern even with these products.
This article will explain what light-struck flavor is and how it can be prevented.
Multiple Compounds Cause Light-Struck Flavor
When examining the compounds responsible for light-struck flavor or LST in detail, we find that multiple types of chemical substances are involved.
Consider beer, for example. LST in beer is sometimes described as "skunky." The compound responsible for this defect in beer is MBT, or 3-methylbut-2-ene-1-thiol. This substance is formed when isohumulone, a bitter compound derived from hops, is exposed to light and reacts with other substances present in the beer.
In wine, however, the compounds primarily responsible for light-struck flavor are methanethiol (MeSH) and dimethyl disulphide (DMDS). These substances produce aromas reminiscent of garlic, onions, or boiled cabbage. While MBT, MeSH, and DMDS are chemically distinct compounds, they are all classified as highly volatile organic sulfur compounds.
Although the specific compounds responsible for light-struck flavor vary depending on the type of beverage, they are fundamentally sulfur-based off-flavors related to the smell associated with rotten eggs.
The Cause of Light-Struck Flavor: UV Light and Vitamin B2
While isohumulone from hops plays a role in the development of light-struck flavor in beer, wine does not contain this compound. The key player in LST development in wine is vitamin B2, also known as riboflavin.
UV light is the primary driver of LST formation. In environments devoid of UV radiation, LST does not occur in wine, beer, or milk. However, asking whether UV light alone causes LST yields an answer of "sometimes yes, sometimes no." Research has revealed that there are two distinct pathways through which LST develops.
LST Development Pathway 1: Direct Effects of UV Light
The first pathway involves the degradation of certain wine components due to UV exposure. In this pathway, direct UV effects on components combine with the generation of reactive oxygen species and other radicals to lead to LST formation.
An important point often misunderstood in various explanations: these radicals are generated when alcohol-containing aqueous solutions are exposed to UV light. This means that any beverage containing alcohol—not just fermented beverages—can be affected by light and potentially develop LST.
LST Development Pathway 2: The Riboflavin-Mediated Pathway
The second pathway involves riboflavin, or vitamin B2. This is the primary route for LST formation. Notably, the isohumulone-related LST development in beer also occurs through this pathway.
In this pathway, riboflavin itself does not directly generate LST. Rather, riboflavin works in conjunction with other components to produce MeSH and DMDS, the actual odor-causing compounds of LST.
The Mechanism: Off-Flavors Created by Vitamin B2 and Amino Acids
The essence of light-struck flavor is the sulfur-based odor perceived in onions or boiled cabbage.
For such sulfur-based aromas to develop, sulfur must be supplied from somewhere as a raw material. In wine, the source of this sulfur is certain amino acids present in the wine.
Multiple types of amino acids exist. Approximately 500 amino acids have been discovered in nature, 22 of which are protein building blocks, and humans are said to be composed of 20 of these types.
Among these numerous amino acids, several contain sulfur in their molecular structure and are called sulfur-containing amino acids. Sulfur-containing amino acids are also present in grapes, and when these amino acids undergo Strecker degradation mediated by riboflavin, they produce MeSH and DMDS, the compounds problematic in LST. Among sulfur-containing amino acids, methionine and cysteine are particularly important in this process. Methionine is said to occupy a central position in LST formation.
Strecker degradation is a chemical reaction that occurs as a side reaction of the Maillard reaction. Specifically, α-dicarbonyl compounds, which are intermediate products of the Maillard reaction, react with α-amino acids and undergo oxidative decomposition, producing an aldehyde with one fewer carbon atom than the original amino acid, along with carbon dioxide.
The Chemical Process Leading to LST Formation
When riboflavin absorbs light, it acts as a photosensitizer. Light in the wavelength range of 370–450 nm has a particularly significant effect.
Under the influence of excited riboflavin, the sulfur-containing amino acids present in grapes undergo Strecker degradation. As a result, methionine is converted to methional. Subsequently, volatile sulfur compounds such as methanethiol, methyl disulfide, dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide are generated. For sulfur-containing amino acids other than methionine, cysteine produces hydrogen sulfide.
Tryptophan, an amino acid known to be present in milk, is also known to be affected by photoexcited riboflavin. Tryptophan is also present in grapes, and when this tryptophan is degraded, it produces indolelactic acid. From there, it further degrades into aminoacetophenone, which is also known as a causative compound of UTA (untypical aging).
The generation of aminoacetophenone during the LST formation process is thought to be one reason why LST is sometimes not clearly distinguished from UTA.
Additionally, skatole is also produced from the degradation of indolelactic acid.
The combination of these various chemical substances is what is recognized as LST, or light-struck flavor.
Light-Struck Flavor Also Occurs in Sparkling Wine
As might be predicted from the fact that it is problematic in beer and milk, light-struck flavor as an off-flavor is largely indiscriminate. It occurs in white wine, rosé wine, and sparkling wine alike.
Originally, attention was drawn to light-struck flavor in relation to wine because this defect became problematic in Champagne, one of the world's premier sparkling wines.
However, the LST formation pathway primarily affected by riboflavin is due to a type of oxidation that can also be called photo-oxidation. For this reason, wines with relatively high oxidative resistance, such as red wines, are less prone to developing LST—though they are not completely immune.
Wine Damaged in Just Minutes
When wine does not have particularly high resistance, light can damage wine at an astonishing speed. That speed is, remarkably, just a few minutes.
There have been experimental cases where wine was exposed to light and changes in wine components were measured. According to these results, MeSH was generated in the wine just 2.3 minutes after light exposure began. DMDS was detected after 3.5 minutes, and methional was detected after 5.3 minutes. In other words, the wine's aroma changed after being exposed to light for merely 5–6 minutes.
Consider the following scenarios: A purchased bottle was placed near a sunny window for just a brief moment before being stored in a cool, dark place. During stock replenishment at a store, the wine was exposed to sunlight for just a few minutes. A corner of a store shelf receives sunlight for a few minutes each day. Or, due to product display arrangements, bottles are constantly illuminated by downlights.
In these seemingly trivial moments that we might not normally pay attention to, wine may have already lost its original aroma.
Notably, there exists a pathway for MeSH generation that proceeds directly from methionine without going through methional. The detection of MeSH at a much earlier stage than methional in this validation experiment is likely due to this direct generation pathway.
Summary | How to Prevent Light-Struck Flavor
Measures for Producers
The most important and straightforward method to prevent light-struck flavor is to adopt bottles that do not transmit UV light.
Previously, only colored bottles possessed such functionality. However, recently, bottles that almost completely prevent UV transmission have become available even in clear glass, through coating techniques applied to the glass surface or by incorporating UV-absorbing materials at the material level. When wineries want to showcase the beautiful color of rosé wines, or when they want to use clear bottles for light sparkling wines like pétillant naturel to convey a light and refreshing image, it is advisable to actively adopt such UV-blocking bottles.
Measures for Consumers
From the consumer's perspective, however, it is impossible to determine whether a given bottle has UV-blocking capabilities. If the manufacturer clearly indicates this on the label, a judgment can be made, but otherwise, there are not many options available.
The most reliable approach is to avoid wines in clear bottles with unknown levels of UV protection. When this is not practical, choosing bottles placed at the back of store shelves—locations that appear to have been exposed to less light—can serve as a modest defensive measure, though its reliability is not particularly high.
Additionally, if you know in advance which wine you want to purchase, you can discuss with the store to have a bottle set aside separately rather than displayed on the shelf. Most wines are not exposed to sunlight during transport. However, an important caveat here is that storage period includes storage time at the winery. If the winery stored bottled wines together in large boxes without light protection, the symptoms may have progressed during that period. While most fluorescent lights do not have the intensity of sunlight, they do emit light in the UV range.
The Final Option
The last resort is to drink it while recognizing it as a wine with that particular taste profile. Sulfur-based aromas are odors that can occur during the winemaking process, not just from light-struck flavor. The same applies to UTA—just because such aromas are detected does not necessarily mean it is light-struck flavor.
Calling sulfur aromas a characteristic of wine seems somewhat forced. However, some producers speak positively of such aromas as distinctive qualities.
In one validation study, light-struck flavor was detected in nearly 40% of the wines examined. For wines bottled in clear glass even a decade ago, it is not particularly rare—in a sense, it is a "common off-flavor." How to deal with this unpleasant aroma, including one's own purchasing behavior, is best left to consumers to decide for themselves.
