scum [skʌm]

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n.槽粕，渣滓，卑贱之人；v.形成浮渣，去除浮渣

a layer of unpleasant or unwanted material that has formed on the top of a liquid

Skim off any scum.撇净浮沫。
Skim the scum off the jam and let it cool.撇去果酱上的浮沫，让它冷却。