Skim the scum off the jam and let it cool.

撇去果酱上的浮沫，让它冷却。

《牛津词典》

The circular movement of the cooking foods also meant that scum or skin formed neatly in the center, making it easier to skim the residue off the top.

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Nothing is more of a turn off than a bathroom where the grout is dirty or moldy or full of soap scum.

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