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Blighted by the lights?

Published:  23 July, 2008

Light is well known to promote chemical changes in foodstuffs. Brewers have known about light's flavour impact on beer for years. It's widely reported that the product goes skunky' when it's exposed to light - even just in the time it takes you to nurse your beer on a lazy summer afternoon.
Beer developing off flavours in the presence of light was noted as long ago as 1875, which is why, until the marketeers got involved, almost all beer was bottled in dark glass, amber having been found to be the best at filtering most of the harmful wavelengths.

It is much more an issue for white and ros wines. In reds, the high tannin content offers a protective veil, binding riboflavin (see below). And while the issue can virtually be avoided by using amber bottles, many considerations are important with the positioning of any product, not least of all the packaging. Consequently we are seeing an increasing number of packaging options, including clear glass, in the drinks aisles.

Light produces a photochemical reaction in beer and wine. The responsible wavelengths are between 350 and 500 nanometres (nm; equivalent to one billionth of a metre) but activity is noted to peak between 370nm and 440nm, in the

near-ultraviolet and part of the visible wavelengths. Amber bottles filter up to 98% of these wavelengths, which is why they're the most efficient.

Dark green glass filters just 63% of these critical wavelengths, while white flint (clear) glass filters a meagre 10%. So it's only when we put beer and wine into clear glass (either bottles for the shop or pints and wine glasses for the garden) that light degradation becomes an issue.

Light penetrating the glass excites riboflavin (vitamin B2, present in beer in concentrations around 1mg/litre, and in wine at concentrations of 0.4mg/litre), which acts as a catalyst, cleaving the compounds responsible for beer's classic bitter flavours.

Wine naturally contains between 1g/l and 4g/l of cysteine and methionine (amino acids). Riboflavin, found naturally in must, reacts with these acids, forming hydrogen sulphide and mercaptans - notably 3-methyl-2-butene-1-thiol (MBT). MBT is the skunky thiol (mercaptan), smelling pungently of leek, onion, cooked cabbage, wet wool or soy. It is highly flavour- and aroma-active and humans can smell it at concentrations similar to TCA, namely about 4 nanograms (ng)/l. In pale beer, concentrations as low as 1ng/l can make the beer unpalatable.

Paul Hegarty, head of communications for Coors brewers, has had 15 years' technical experience where light-struck issues were high on the agenda. He is prosaic about the issue, claiming that light-struck beer tastes skunky and rubbery', and even likens it to drinking a pint outside on a sunny day. Experts might define it as a fault,' he says, but consumers might quite like it. It reminds them of sunny holidays.'

Dr Ellen Norman, head of analytical development at Brewing Research International, says: Research has suggested that consumers do not agree on whether light-struck is a negative flavour in beer - some actually prefer it. We have also seen studies where consumers are unable to pick up differences between a beer before and after the beer has sat in the light.'

However, hop cones can be extracted using supercritical CO2 (renowned in the wine industry for reducing TCA levels in cork granules). The MBT-forming compounds undergo another process to make them light-stable, enabling the resulting beer to be stored safely in clear glass. It is thought that the number of brands produced using these processed hop products is relatively low.

Mercaptan marvel

But wine has never suffered from a skunky aroma. Peter Godden, group manager of industry development and support at the Australian Wine Research Institute (AWRI), says: The formation of light-struck flavour is exacerbated by low oxygen in both beer and wine, where the wine already has a propensity to develop reductive characters. But the reasons for this are different because the compounds thought to be responsible in beer do not exist in wine. However, lees-aged sparkling wine, which contains more methionine and other compounds than other wine styles, may be susceptible to the formation of light-struck flavour through a mechanism that is similar to that in beer.'

We know that some grape varieties and wine styles are more susceptible to reductive characters than others; not every wine reacts the same way. It also depends on how the wine is made, and how many amino acids it contains. Given the right (or wrong) light conditions, mercaptans develop. Gerd Stepp, winemaker for Marks & Spencer, explains: Mercaptans are relevant in winemaking. A lot of the precursors in wine are hydrogen disulphides (H2S). H2S has a high threshold, but it can continue to develop mercaptans, and these have a low threshold [4ng/l]. So tiny, almost undetectable, amounts of H2S [can go into the bottle] but once the bottle is closed, the wine can develop hints of mercaptan.'

In the case of light-struck, cysteine and methionine amino acids come from yeast autolysis (the time spent on lees) thus it is not so common to find Champagne in clear glass. For wine with low amino acid content - therefore without lees contact - the chance for wine to get light-struck is much smaller. But Stepp adds: You taste it much more easily in aromatic or neutral white wine; onion flavours come through. With red wines there is more air, oak, open rackings, so H2S is lower.'

Paolo Bisol of Ruggeri says: Our ros has been in clear glass since 1950, but Pinot Bianco and Pinot Nero are less sensitive. It might be better in dark glass, but you can see the colour. However, it is important for Prosecco to be in dark glass because Prosecco is sensitive to light. It is light, fresh, young and fruity.'

Cristal clear

Wines such as traditional-method sparklers, with their long lees ageing and consequently high amino acid content, are susceptible to light-struck. Jean-Baptiste Lcaillon, chef de caves at Champagne Louis Roederer, says: Amino acids such as cysteine are very important in wine as they are responsible for the aromatic structure. In the case of sparkling, a large part of the aromatic profile is due to autolysis of yeast proteins. During the autolysis, some enzymatic reactions separate the proteins into thousands of amino acids that are aromatic and responsible for the yeasty, nutty characters.'

He offers a number of solutions. Our only focus is to protect the wine from UV,' he says. You can do it by using green bottles that filter 92% of the UV, which is enough to protect the wine during reasonable sun exposure. A strong sun exposure would be bad for any compounds of the wine... so it is to be avoided. For Cristal, we protect the clear bottles with coloured cellophane. This cellophane is specially designed to filter 98% of the UV, which means, if the bottles stay wrapped, it is better protected than a green bottle. This is why we add a leaflet in all our cases of Cristal which recommends keeping the cellophane on until the last moment.' Of course, this doesn't mean the retailer or consumer will keep the cellophane on. The bottle looks stylish without the foil, even with a warning note.

Marks & Spencer uses no clear glass at all for its wine range. Five or six years ago, all the wines were moved to green or amber glass. Stepp says: If wine is in a presentation box then it might be in clear glass, like Champagne, but the box protects the wine.' It's not always easy to get the most protective colour of glass. He adds: UVAG green has a higher UV protective quality; it uses a different oxide for colouring. But it is limited by availability, so we have to see if we can find it on the local market, and if not, we go with the normal green.' He says it costs too much to transport empty UVAG bottles to the country of bottling.

The M&S policy makes it difficult to draw attention to its ros wines, which many other retailers and brand owners market in clear glass to draw attention to their attractive colour and mood suggestions. The M&S technical team insists on green glass to protect the wine, and this outweighs any marketing considerations. To signify ros to consumers, the bottles all have overtly pink labels and distinctive pink capsules of the same hue to illustrate the wine's tint.

Stepp says: Commercially and presentationally, it is quite a challenge. The wine's colour is less visible, and the colour is one of the key features of selling a ros. Green glass is less attractive.'

Fresh coat of plastic

There has been some experimentation with coatings over clear glass which filter the UV and allow the presentational benefits of clear glass. Peter Godden says: In 2001, we conducted a trial using a clear plasticised coating for the outer surface of clear glass bottles. Its purpose was to block UV light from damaging wine. A Chardonnay was packaged in coated bottles, and also in the same bottle without the coating. The bottles were then exposed to UV light (approximately 50cm from the light source of approximately 360nm) for 21 days.

During sensory evaluation, the wine in the coated bottles was rated higher for freshness and for fruit characters, and lower for a whole range of reductive descriptors (reduced sulphur, smoky, burnt rubber, bacon, burnt match, gunsmoke). The results were highly statistically significant - the coating appeared to work extremely well.'

Stepp explains why M&S did not pursue this option: A UV protective coating can be applied once the bottle is finished. The coating has to be completely transparent on clear glass, otherwise it defeats the purpose. But if the coating is 100% clear, its protection only goes to 400nm, so the 440nm peak was not covered. And to get protection over 400nm, the protective coating becomes coloured.'

Greater UV protective capabilities give a cloudy quality to the film, which may explain a lack of progression with this technology. Stepp suggests that this avenue may still be worth exploring if improvements were made to bottle coatings currently available in the industry.

In a store environment, wine in clear glass can develop noticeable degradation flavours under some types of fluorescent lighting after just a few hours. Stepp says: Green glass protection is not 100%, so store lighting is also important, as is stock rotation; it doesn't take long for mercaptans to form. Generally the quality degradation of on-shelf wines in clear glass would be quite high. Ros is slightly more unstable because colour can turn. We've done a lot of work on store lighting, such as tests with filters on certain light. We are using lights now that are better. And this is also relevant for other products, such as chocolate, which is sensitive to light.'

Howard Winn, quality manager for beers, wines and spirits at Sainsbury's, says: We have a few wines in flint glass - the usual suspects like ros and sweet Bordeaux. It is an ongoing project to move away from fluorescent lighting, reducing lighting in the beer, wine and spirit aisles, and moving to lower-energy lights positioned away from the fixtures. Deeper steel shelves have also helped reduce the exposure of bottles to light. Our lighter Sherry styles have recently been moved to different glass, which offers higher UV protection. Similarly, our ros Champagne moved to green glass a few years ago. It's all about protecting the wine.'

Lcaillon is optimistic about the trade's level of knowledge: There is the question of bottle shops and fridges. It is obvious that if the wine is under full light for a while, there is an important risk of light-struck, but I believe the trade is aware of that problem and will take all necessary measures to protect the wine.'

With all the right precautions, light-struck need not be a big issue, but the creative tension between brand image and positioning, and technical precision, means the issue is still relevant. Stepp offers yet another option, saying: There is quite a bit of work from our side on the wine making. If we did progress on clear glass with ros, we would want to make wines that are less prone to mercaptans, for example, have less of the amino acids cysteine and methionine, and have a slightly faster fermentation, focusing on fresh fruit flavours with less lees contact. Our experiments are ongoing.'