[MUSIC] Welcome back. In our previous lesson, we discussed commonly occurring odor faults including oxidation faults. In this lesson, we will turn our attention to the discussion of vinegar. Our next fault is also a type of oxidation but is caused by microbes usually vinegar bacteria. Wine makers and tasters call this odor volatile acidity or you'll hear them just refer to it as VA, standing for Volatile Acidity. What they're talking about here is the smell of vinegar or sometimes, a very volatile scent of nail polish remover, a solvent like smell. The reason I say that this is also an oxidation smell is that acetobacter aceti, which is the bacterium that causes the smell are oxidizing ethanol to acetic acid. And this requires the presence of oxygen. So, this can happen, of course, in the winery when wine is being aged in barrels or tanks that are not full, but it can also happen in bottles of wine that have vinegar bacteria contained in the wine, and extra oxygen may be seeping in through the cork. These bacteria can become active and start converting that ethanol in the wine to VA, to acetic acid. Our threshold for vinegar, for acetic acid in wine is about 900 parts per million. That's a fair amount that is necessary for us to smell vinegar. Kind of an interesting side not for you, all wines have vinegar in them, have acetic acid in them. In fact, they all have acetic acid in them from the moment they've finished fermentation. White wines, very, very low amounts, typically red wine's little bit more. But since our threshold is 900 parts per million or 900 milligrams per liter, if you find 50 parts per million by analysis in a white wine or 150 parts per million by analysis in a red wine, you can't smell it. It's below threshold. It's sub threshold. So, if we can't smell something that's in a wine, but we know it's there from laboratory analysis, does it really matter? No, it doesn't really matter. It only matters if, as we're following the life of the wine, as the wine is aging away in our winery, if the amount through laboratory analysis is shown to be increasing gradually, gradually, that's an indication that these vinegar bacteria are alive and working in our wine and we may want to do something to put a stop to that. So the level doesn't come up to threshold or exceed threshold before we bottle the wine. And if it does, we're going to be bottling vinegary wine, and nobody will buy it. Vinegar bacteria are literally ubiquitous, they're everywhere and we can't escape them. But it's not that big of a deal, they're very easy to control by good winery sanitation, by a judicious use of sulfur dioxide in wine which everyone in the world uses. So, it's really not a problem. There are some areas in the world where vinegar is a little more prominent than others. And I'm thinking of maybe some sites in the Old World where the vinegar In wines may be a little bit more apparent. I'm reminded of a story of an Italian professor who came to UC Davis to do a post doc. And one of our professors invited students and faculty over for dinner, and the visiting professor had brought some wines from his hometown in Italy to contribute to the dinner. So, as eager students we all arrived early and we opened the wines just to get them ready. And we we're horrified because every single one of these red wines from this professor's village had some noticeable VA, some noticeable vinegary smells. So, we were pretty concerned, we didn't want to embarrass anyone who was coming over for dinner, so we talked to our host professor and he said, well listen, let's not really worry about this. I'll tell you what we'll do. Bring these wines out after everybody is seated at the table and we have passed the food around and everyone has started to eat, then come out pour the wines. And so, we did as we were told. People started eating all of their American-style Italian food for dinner. And we went out and brought the wine back in and poured a little bit for everybody and put extra bottles on the table. Not a single person noticed the VA in the wine. The wine was compatible with the food. In fact, we were serving very spicy, very savory foods. The wine was actually perfect with the food. And here's a case where that little bit of extra volatile acidity elevated the acid flavor in the wine, the acid basic taste in the wine and made it go better with the food than it might have had it not had that VA. So, an example of when a local flavor, a local idiosyncrasy of a wine, may be actually be to its benefit, but when taken out of context by eager young students on a different continent, we might misunderstand these wines, and not understand their usage. So, when acetic acid is developing in a wine due to the activity of these vinegar bacteria, from time to time, there may be an ester formation between acetic acid and ethanol. Ethanol, as you know, is the normal alcohol that we find in wines, the normal alcohol that was made by yeast during primary fermentation. But, if acetic acid is being formed later on in the wine, sometimes a molecule of ethanol will join with a molecule of acetic acid and make a very volatile smell known as ethyl acetate. It's just simply a molecule of ethanol joined in an ester formation with a molecule of acetic acid. And these smells we describe as nail polish remover or solvent like. The sad thing is that the threshold for these smells is around a 100 milligrams per liter or 150 maybe milligrams per liter, parts per million. Far lower than the 900 part per million threshold for acetic acid alone. So, you could envision a situation where acetic acid all by itself is sub threshold. We don't smell it. We don't even realize it's there. But if some of it were to form this ethyl acetate formation, all of a sudden it jumps out of solution and tweaks our nose and we say, my gosh, I think this wine smells like a nail salon. That wraps up our discussion of vinegar in this lesson. In our next lesson, we will discuss reduction faults.
