Should it be skimmed milk? Or skim milk? Here are two cartons of the stuff I bought in the supermarket. They don't seem to be able to decide – the one on the left has an -ed suffix, while the one on the right does not.Actually, the dropping of the -ed suffix is a regular process: ice-cream was once iced cream, while mincemeat was once minced meat.
What is happening here is that a final /t/ or /d/ gets deleted if it is surrounded by two consonants. And this can occur in many different contexts, so world cup usually has no /d/ and best man has no final /t/. And this occurs whether the /t/ or /d/ is a suffix or not.
In fact, the milk on the right, from 'Greenfields', is produced in Indonesia, while that on the left, from 'Table Cape', is from Tasmania in Australia. It seems that native speakers are quite happy to drop the -ed suffix, while those from elsewhere might be more concerned to retain more traditional forms. (Though, of course, we need more data to check this hypothesis.)
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