Pectin, which is present in many of the ingredients that we use in winemaking, can prevent finished wines from clearing. 

To stop this the ingredients should be treated with the pectin detroying enzyme, known as pectolase. 
This will be most effective if added to the pulped fruit in the bucket, along with the sulphite, before the addition of the remaining ingredients. 
Pectolase works best on the fruit itself and is not inhibited by the presence of sulphur dioxide.

Pectin is more soluble at higher temperatures, which is one reason for not heating ingredients when preparing a must. 
Additionally, when heating or boiling a fruit containing pectin, any pectin destroying enzymes naturally present are destroyed, making the use of pectolase even more essential. 
It is always beneficial to add the enzyme before the fermentation starts, as the action of enzymes is inhibited by the presence of alcohol. 
If in any doubt it is advisable to treat all fruits and any juice that is not clear.