Sometimes too much of a compound is the cause of a fault. Other times, the definition of whether its a fault depends on the beverage. Diacetyl (C4H6O2) is an example of such a compound. In most beers, it’s considered a fault but, in most California chardonnays, it’s desired. Butter is the aroma you get from diacetyl. In fact, it’s used in butter substitutes like margarine, as well as some microwaveable popcorn to provide a buttery aroma. In cider, beer, and wine, the aroma threshold can vary but is usually defined between 2-3 mg/l(1).
Diacetyl can be created during fermentation by yeast but, it is more commonly formed during Malolactic Fermentation (MLF) which is a biodeacidification process breaking down stronger acids like malic into weaker acids like lactic. Diacetyl is also a precursor of acetoin, which can be further reduced to 2,3-butanediol. MLF usually occurs post fermentation during the aging or maturation of cider. Aging cider on the lees will help enable MLF because as yeast die, they breakdown. This is a process called autolysis. When this happens, the yeast release nutrients that can be used by the LAB to perform the biodeacidification process.
If you want the buttery aroma and taste that diacetyl provides, you want to encourage MLF. If you don’t want diacetyl, you want to prevent MLF. Note that yeast can create diacetyl but, yeast will also absorb diacetyl. If you are fermenting at cooler temperatures, 60F (16C) or below, the yeast is slow to absorb the diacetyl so you need to raise the temperature and let it rest a couple days after fermentation is complete. This is often called a diacetyl rest and is used by lager beer brewers who ferment at cool temperatures. It allows the yeast to reabsorb the diacetyl. Fermenting at temperatures above 65F (18C) will naturally allow the yeast to absorb the diacetyl it produces so the rest period is not as critical.
To encourage MLF and the production of diacetyl, you should consider aging on the lees. Fines lees are recommended versus the gross lees. Fine lees are the yeast that settle out after the first racking. Gross lees are the sediments at the bottom of the fermentor after primary fermentation completes. It contains not only yeast but also apple pulp, pectin, and many other components. The yeast will help provide nutrients to the LAB. If you want to ensure MLF you should add a lactic acid bacteria culture to your cider. This will help ensure the process occurs more quickly. A natural MLF reaction can take months to occur.
If you don’t want diacetyl, you can use sulfites to kill the LAB and prevent MLF. You should also avoid aging on the lees. Just remember that adding sulfites can also have negative impacts. Most decisions are a trade off but two good rules for home cider makers are to 1) practice good sanitation of your equipment and 2) avoid oxygen once fermentation starts. There are always exceptions but these will help minimize cider faults.
(1) B. Martineau and associates, Effect of wine type on the detection threshold for diacetyl, Food Research International, Vol.28, No. 2, pp. 139-143, 1995
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