Almost every salad recipes I've read needs some oil. What's its effect on vegetables? How does it change the flavor?
If oil is not used for the taste, why is it necessary for salads?
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Here are a couple reasons why (for which I know) oil is used on salads:
And as already noted, it is not necessary for salads, you can use just lime juice (like in some thai salads), boiled dressing (that is thickened with starch), stock reduction, fruit/vegetable purees, yoghurt dressings or something else that does not require oil (e.g. I have made slightly pickled japanese salads that only required some vinegar and brine and the main aromas came from the veggies).
Also, I found this to answer "What's its effect on vegetables?":
McGee, on Food&Cooking says:
Oil seeps through the waxy leaf cuticle and spreads into the leaf interior, where it displaces air and causes the leaf to darken and its structure to collapse.
All food needs some presence of fat in order to be have a desirable taste. Meats don't need any added fat but green leafy vegetables have almost no fat. Green vegetables are rich in vitamin A and K which are only soluble in fat. So it stands to reason that the combination of olive oil with lettuce would have a pleasant taste.