Acetaldehyde is one of the several precursor compounds produced by yeast during fermentation. It has a green apple-like aroma and flavors and is common in young beer and mead.
Glucose >> Pyruvic Acid >> Acetaldehyde >> Ethanol
Although Acetaldehyde is a natural precursor to ethanol formation, its residual levels in the finished brew is not desirable. At excessive levels, it might even give a paint solvent/ nail polish smell. Here are the few causes for this off-flavor:
Insufficient healthy yeast count. Many home brewers end up with old or improperly stored yeast. Also to save cost and time some brewers sprinkle the dry yeast on their wort. Making a vigorous starter batch and ensuring adequate oxygen and nutrients reach the yeast. Fast Pressure Brewing. A brewery under pressure to churn batches as quickly as possible tries to ferment alcohol as fast as possible. This does not give yeast enough time to metabolize the precursor compounds completely. This problem is larger for lagering than for ale. Another reason for acetaldehyde is fermenting inverted sugar instead of natural unpasteurized honey during mead making. Adulterated packaged honey is therefore avoided by mead makers.
Remedy
48 hours of Diacetyl rest for lagers and ales. Slightly warm conditioning and patience are all it takes for the yeast to complete its task. In mead tasting like acetone: Allow it to age for at least 6 months. High sugar and natural compounds in honey prevent yeast activity. Hence it takes it longer to clear up.