The role of a marinade is threefold: to protect food from drying out, especially in the fierce heat of a grill or barbecue; to tenderise; and to flavour.
Oil based marinades are the protective ones, good for grilling fish and meat with little natural fat, while acidic marinades based on wine, vinegar and fruit juices perform a tenderising function and also counteract fat in oily foods such as pork, herring or mackerel.
Yogurt tenderises, too, and is the basis of many Indian dishes, especially Tandoori ones.
The time you marinate foods depends on the effect you want to create.
To flavour and protect, allow about four hours at room temperature or overnight in the fridge; for tenderising, you can double this time.
Fish and shellfish deteriorate quickly and should be left only for an hour at room temperature or two hours in the fridge.
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