We enjoyed a superb bottle of pinot noir and bought another a few days later because we liked it so much. However, when we decanted the second bottle there was persistent foam on the decanter glass, which we had not experienced with the first bottle. The foam was not from soap because we washed the decanter with plain water, followed by several rinses. The cork was fine, but the wine smelled more of alcohol (almost like cognac) than it did of pinot noir. Further, the flavor had only a barely perceptible hint of what the other bottle tasted like. Do you have any ideas what caused this unpleasant and unexpected experience?

When your pinot froths like a pilsner, you know you’ve got a problem. Finding the cause requires a bit of detective work.

The fact that we’re dealing with pinot noir is an important clue. Look at the label. My hunch is that you’ll see the words “unfiltered” or “non-filtre.” This means the wine was not run through filtering pads before being bottled to clarify it and to remove excess grape solids.

Although filtration can be beneficial, if overused, it strips color and fruit and makes the wine taste thin and stringy. Because pinot noir is naturally prone to such deficiencies, wine makers are increasingly wary of filtering it and are avoiding filtration (and a related technique called fining) entirely.

Doing without filtration often yields a better wine with a fuller bouquet and more robust flavors. But it’s also a risk. What is overlooked is that since the time of Louis Pasteur, filtration has been used to make wine more stable by removing microorganisms that can cause a secondary fermentation in the bottle. Wines that go through “malo in the bottle” (malolactic fermentation) taste yucky and have lots of malodorous little gas bubbles. In other words, they’re just like your disappointing second bottle.

It should be noted however, that bubbles do not always spell trouble. Wine makers sometimes leave in small amounts of the natural carbon dioxide from fermentation to add a little zing to young wines intended for casual quaffing. Although I personally could do without this prickly sensation on my palate, it’s harmless, and couple of brisk swirls in the glass or in the decanter will blow this off with little or no effect on taste or bouquet.

If you should ever have another case of malo in the bottle, return it to the store. Since it is a clear defect, you’re entitled to a full refund.