Turning White Wine into Red

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You Asked: Should we worry about arsenic in wine?

wineA story about arsenic-laced wine is panicking a lot of people. It’s all about a lawsuit brought against the producers of some wines claiming they contain unsafe amounts of arsenic. As far as I can tell, the lawsuit is an attempt at money grab by a company that performs analyses for substances such as arsenic in beverages. The idea seems to be to cash in on the public fear generated by the lawsuit. People will clamor for the testing of wines, a service the company provides. Any story about arsenic, the fabled “widow maker,” is guaranteed to trigger publicity. Witness Dr. Oz’s shows on arsenic in apple juice and wine.

According to the lawsuit, some cheaper wines contain up to five times as much arsenic as is allowed in tap water, which is 10 ppb. Anything that grows in soil will have some arsenic because arsenic compounds occur naturally. The 10 ppb limit in water has a large safety factor built in, but more importantly, people do not drink as much wine as water. And if they do, they need to worry about the alcohol content far more than about the arsenic content.

 Joe Schwarcz

Soft drink bottles are made of a plastic called polyethyleneglycol terephthalate, or PET. While this plastic is fine for storing soft drinks, why is it not recommended for storing home-made wine?

While PET has a very low permeability when it comes to carbon dioxide, it readily allows oxygen to pass through. And oxygen is the enemy of wine! When we talk about storing soft drinks, permeability to carbon dioxide is the critical factor. A beverage that loses carbonation loses its appeal. In this case oxygen permeability is not an issue. While oxygen passing into a plastic soft drink bottle from the air may react with some of the flavor components, the effect would be minor given that we don’t store soft drinks for extended periods. But of course we do store wine to age it. And this is where oxygen becomes a problem. Grape juice contains a variety of compounds called polyphenols which can react with oxygen and produce a variety of colors and flavors. This really is the same chemistry that occurs when an apple is cut and exposed to the air. Reaction between polyphenols and oxygen produces the brown discoloration. Not only will the apple slices look different, they will also taste different. The same thing can happen with wine. White wine is more susceptible to such changes because it lacks some of red wines colored compounds, the anthocyanins, which can act as antioxidants. Sulfur dioxide is also an effective antioxidant, which explains why compounds such as sodium bisulfite are used to preserve wine. Burning sulfur inside wine barrels to produce sulfur dioxide is an age-old method of preservation. Now back to our plastic bottles. As we have seen, empty soft drink bottles are too permeable to oxygen and are not appropriate for storing wine. But wine can be purchased in plastic containers, although I suspect a true eonophile would look warily upon this method of marketing. So how do the marketers solve the problem of oxygen permeability? By sandwiching a layer of an oxygen-impermeable plastic between layers of food-grade polyethylene or polypropylene. Ethylene-vinyl alcohol copolymer is ideal for this purpose since it allows very little oxygen to pass through. A bit of ingenious chemistry. So while it is not a good idea to store your wine in old soda bottles, it is quite acceptable to purchase wine in plastic containers.

Joe Schwarcz

What are “Breathable Wine Glasses?”

wineI had heard about them for a few months already. There is a new variety of wine glasses made out of “breathable glass” produced by the German company “Eisch Glasskultur”. According to the advertising a wine poured in one of these glasses will, in the space of 2 to 4 minutes, exhibit a degree of aeration similar to that observed for the same wine in a traditional decanter after 1 to 2 hours. Sceptical, I communicated with the importer who was kind enough to send me a “breathable glass” accompanied, for control purposes, with a regular “non-breathable” glass but otherwise in all points identical. I then set up the experiments according to the well established “double blind” scientific standard. I asked my wife to mark anonymously without my knowledge the two glasses. Then I chose a full-bodied red wine which I poured in equivalent amounts in both glasses. After 4 minutes I tasted the wine in both glasses. The results were unequivocal. In the glass marked “A”, the “breathable glass”, the nose was more developed and more complex. In terms of taste it was softer and rounder. Having convinced myself I repeated the blind experiment with four of my colleagues. It was unanimous; it does seem to make a difference. I would very much like to discover the technique used to produce these glasses. It is of course a secret but the company mentions …”a proprietary oxygenation treatment”. It is possibly the same type of phenomenon found in sport clothes made out of “Gor- Tex” which also “breathe”. The textile contains micro-pores that are small enough to allow gases to escape but blocs the larger water drops. Whatever it is I raise my glass to the “breathable” wine glass as another giant step in scientific discovery!

Ariel Fenster

The Atom Bomb and the Age of Wine

The purveyors of fake wines probably never dreamed that they could be caught by a carbon atom. A special kind of carbon atom. One that in addition to the six protons in its nucleus has eight neutrons instead of the usual six. It’s called carbon 14. It’s very rare but can be a powerful tool in analytical chemistry. Let’s start at the beginning. In fact, the very beginning, the Big Bang. Over 13 billion years ago the universe began to expand from an extremely dense and hot core, spewing out the basic units of matter that would eventually form the chemical elements. These in turn would become the building blocks of everything in the universe. (more…)

What is “Bordeaux Mixture?”

It isn’t a blend of wines from the Bordeaux region of France to sell to undiscerning customers. Bordeaux mixture is a fungicide made of copper sulphate and calcium hydroxide, the latter better known as lime. Yeasts, moulds and mushrooms, all of which are classified as fungi, can present a nightmare for farmers. They can destroy a potato crop (potato blight), damage fruit (apple scab) or sicken grape vines, which then affects the yield and quality of the grapes. Since copper ions inactivate the enzymes that fungal spores need to germinate, spraying with Bordeaux mixture can prevent fungal infestation. This technique has been used around the world since its discovery by the French botanist Pierre- Marie-Alexis Millardet in the 1860s. It was back then that French vines were first affected by a fungal disease known as powdery mildew. The French blamed the Americans, and were probably right. American grape vines had been introduced into Europe to see how they would grow there, and to investigate whether grafting them onto existing vines might lead to new varieties of grapes. Unfortunately, some of these vines were infested with insects and fungi which had not been a problem in America because the vines had evolved a resistance to them. But it was a different story in France.

The Great French Wine Blight was caused by an American aphid that destroyed vines by injecting a toxin as it sucked their sap. Somewhat less devastating, but still with a huge economic impact, was the fungus known as downy mildew. It was an infestation with downy mildew led to Millardet’s classic observation. Walking by a vineyard in Bordeaux, he noted that vines close to a road that ran through the vineyard were not affected while others were. Millardet learned that the vintner had had a problem with passersby pilfering his grapes and decided to fight back. He sprayed the vines with the copper sulphate-lime mix because it tasted bitter and left a visible residue that he hoped would deter the grape thieves. Millardet figured that it was also this mix that must have deterred the fungus. And he soon proved that it was. Bordeaux mixture took its place as a significant weapon in farmers’ arsenal in the battle against fungi. And not just conventional farmers, but those abiding by “organic” farming methods as well. Since both lime and copper sulphate occur in nature, Bordeaux mix can be used by organic farmers, perhaps to the surprise of people who believe that organic agriculture uses no pesticides. In fact a number of pesticides, ranging from pyrethrins isolated from chrysanthemums to copper sulphate are allowed. Are they necessarily safer than the demonified synthetic pesticides? Not necessarily. Bordeaux mixture’s copper can be harmful to fish, livestock, earthworms and even humans. But it can prevent downy mildew. You can drink to that.


Joe Schwarcz

What does “blanc de noirs” mean when referring to wine?

By removing the grape skins after red grapes are pressed. Many people believe that red wine is made from red grapes and white wine from green grapes. They are wrong. When grapes are pressed, the juice is white, no matter what variety of grape is used. But if the juice is left in contact with red grape skins, pigments leach out of the skins and color the wine. These pigments belong to a family of compounds called anthocyanins which are responsible for the coloration of many fruits and vegetables. If red grapes are pressed and the juice is allowed to ferment after being separated from the skins, the result is white wine, referred to as “blanc de noirs” meaning a white wine from dark grapes. A well-known example is a variety of champagne produced from the Pinot Noir grapes. White wines can also be made from white grapes with the skins being removed before fermentation starts. Since there is no need for color, there is no point in having the juice be in contact with the skins. Such contact would only serve to leach out tannins, compounds that add astringency and a bitter flavor to the wine. In the case of red wines, contact with the skins of course is necessary to infuse the fermenting juice with the colored anthocyanins. But this means that tannins are also being leached out. So a prime question in red wine production is when to transfer the wine from the fermentation vessel where it is contact with skins to barrels where it is not. Leaving the juice in contact with the skins after color has fully developed can yield too high a tannin content and is a no-no.

Now, tannins are not totally undesirable. They add body to the wine and a certain “pucker factor” that contributes to the wine-drinking experience. Tannins are complex polymers of molecules called phenols which have the capability of increasing the viscosity of saliva by linking together some of its naturally occurring proteins. Indeed, tannins extracted from plants are used to “tan” leather, a process that cross-links proteins, turning soft animal skin into a substance tough enough for shoes, belts and furniture. Vintners know that color is a marker of the fermentation process and have traditionally used its development to judge how long to let grape juice sit in contact with skins. Early experiments to put such decisions on a scientific footing resulted in the production of “wine color cards” that featured a series of circles with different red hues for comparison with the color of a fermenting batch of wine. The colors were based on experiments that had shown the optimal time for transfer of the fermenting juice to barrels. Today various sophisticated scientific instruments are available to help vintners produce high quality wine of consistent flavour. Spectrometers can measure subtle differences in color and pressure transducer sensors can monitor the extent to which sugar has been converted to alcohol. Amazingly, though, the best instrument for distinguishing the subtleties of wine is still the human nose which can detect the presence of some chemicals in concentrations that defy detection by the most sensitive instrumental techniques.


Joe Schwarcz

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