If the fermentation is at the point where the producer wishes to finish it, a combination of refrigeration and/or accelerated clarification is normally used. The time taken to cool the ferment sufficiently should be taken into account and the ferment stopped earlier than the actual sugar level required. Clarification gets rid of the active yeast and allows other methods to work. The addition of SO2 by itself to stop fermentation does not work in all cases, as the products formed during the fermentation (predominantly acetaldehyde) may bind and inactivate the SO2 before it has had a chance to act against the yeast. If the number of suspended yeast cells has been lowered by clarification and activity reduced by cooling there is more chance it will be successful. It may still be necessary to add a larger than usual amount of SO2 to ensure cessation of activity by the yeast. It is vital to keep a close check on the product towards the end of fermentation to see whether it is developing detectable levels of hydrogen sulfide. This is particularly true in the case of wines undergoing lees contact, which should be stirred through and, once the lees have settled, tasted. If there are high levels of hydrogen sulfide, the wine should be immediately racked and aerated or a copper treatment may be necessary, either by passing the wine over copper chips or by adding 0.2–0.4 mg/l copper sulfate (Morgan et al., 2006).