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Question: My beer often has a buttery taste and smell. What is it? How do I make it go away?
Answer: A buttery taste, alternately described as butterscotch, comes from a chemical compound called diacetyl. Diacetyl is a natural byproduct of fermentation and occurs to some extent in just about all beers. Too much diacetyl can overwhelm a beer's flavor profile. A small amount of diacetyl is considered appropriate for a couple of ale styles such as Scottish ales and American Amber. Smaller amounts are usually found in other ales as well. Lagers, in contrast, should never exhibit diacetyl.
Diacetyl can be produced a couple of different ways. Normally, it is formed in the early stages of fermentation by yeast action. The level builds up as primary fermentation proceeds. High temperatures lead to higher levels. Then, during secondary fermentation, yeast consumes diacetyl and the level drops again. Higher temperature leads to a faster rate of reduction.
You can control the amount of diacetyl by manipulating fermentation temperature. If your ales have too much diacetyl, try keeping the primary fermentation a bit cooler. Aim for about 63°F. Then, once the beer is in secondary, boost the temperature a bit. Aim for about 68°F. This will minimize the amount initially formed, and then more quickly reduce the level. Also, once you bottle or keg your beer, give it enough time to age. Diacetyl levels will drop with longer ageing. Don't refrigerate your beer too soon, since this will cause the yeast to go dormant and inhibit the reduction process.
If you're making a lager, try giving your beer a diacetyl rest. Usually, lagers are kept cold throughout fermentation process. However, if you warm up the beer towards the end of secondary fermentation, the diacetyl level will drop pretty fast. Bring the beer up to the low sixties for 48 hours. Then drop the temperature back down to lager temperature by about 2 degrees per day.
Diacetyl can also be formed from some types of bacterial action or wild yeast. Also, if the wort doesn't have enough protein, diacetyl reduction will be impaired. This can occur if you use high levels of refined sugar or adjuncts in you recipe. If temperature control doesn't reduce your diacetyl level, check into these possibilities.
Not sure what diacetyl actually tastes like? Try a bottle of Hidden River Pale Ale from the Old Fort Brewing Company, Cold Springs Minnesota (Albertson's carries it). I've found it invariably has a large dose. If you check the label, you can see why. It proudly claims, "Aged in cold cellars". If you ask me, they put it in the cellar way too early.