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If I want to make a shrimp salad, I put the lettuce and other raw vegetables on the plate, cook the shrimps and put them on the veggies. However, after a minute or so, the lettuce becomes soft, like it's wilting, probably because of the heat.

Is there a way to slow this process down? It's not easy to make a simple salad for multiple persons that looks nice when you deliver it.

I'm pretty sure this also applies to other vegetables and other cooked food. If not, please mention so in your answer.

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Shrimp salad is typically served with cold shrimps in a sauce. Though some types of lettuce taste quite nice when gently cooked this way –  TFD Jun 8 '12 at 23:21
    
How do you call this then? farm5.staticflickr.com/4064/4521793568_c4a5a79d53_z.jpg –  Mien Jun 8 '12 at 23:28
    
Salad with shrimps on top? Are they meant to be served hot? –  TFD Jun 9 '12 at 1:25
    
Yes, they are served hot. –  Mien Jun 9 '12 at 5:37
    
I've always enjoyed lightly wilted greens. Romaine especially is tasty when lightly cooked - grilled or stir fried. For my breakfast, I often put lettuce in between an english muffin and a fried or poached egg, and enjoy the slightly wilted quality. –  Jonathan Jun 10 '12 at 19:19
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2 Answers

up vote 7 down vote accepted

Wilting in greens is triggered by temperature, pH, and salt content. To reduce wilting, you can cool the vegetables or shrimp, make the vegetables more acidic*, or decrease their salt content.

My suggestion would be to cool the shrimp with an ice water bath or cold running water. This is the most traditional approach for shrimp salad. Alternately, you could apply an acidic dressing before topping, but wait to salt it until the shrimp have cooled to 140F.

Alternately, wilting of vegetables adds a rather interesting variation in flavor or texture to a salad. This technique has been desirable and in vogue at various times.

Why does this all happen?

Plant cells wilt when cooking breaks apart the cellulose-based cell walls, and allows water to escape, causing them to soften. To quote On Food and Cooking (pp 282):

When the tissue reaches 140F/60C, the cell membranes are damaged, the cells lose water and deflate, and the tissue as a whole goes from firm and crisp to limp and flabby.

Acids will impede wilting, because the cell walls are held together with hemicellulose, and it is "not very soluble in an acid environment" (pp 282). Table salt is a problem, because "its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruits and vegetable walls, thus breaking the cross-links and helping dissolve the hemicelluloses" (pp 283). Calcium has the opposite affect, so if you can use hard water or add calcium salts, do so.

*Acidic solutions will reduce green colors(On Food and Cooking, pp 280-281), but preserve texture (282).

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It is unlikely that many cells will reach 60°C. If it did, the lettuce would taste cooked, not wilted. There is a simpler explanation: warmth speeds up the lettuce's metabolism a lot. It ages many times faster. Also, this accelerated aging is more damaging than a normal aging, because there is not enough oxygen to sustain its accelerated breath rate, and there are no nutrients and water comming from the (now severed) root. It loses water when breathing, but doesn't get any new water, so it loses turgor -> wilts. –  rumtscho Jun 13 '12 at 11:28
    
@rumtscho: While I follow your logic, I don't agree. Accelerated aging wouldn't occur in a matter of minutes, however heat transfer from hot shrimps would. Consider if you will: the shrimps will be at nearly 100C, or even hotter on the outside. They are mostly water, by weight, as is lettuce, so specific heats are comparable. The surface layer of greens will be insulated from the rest by air, and the shrimp may outweigh it by 10:1 or so. These are at 20C. Thus, when they reach thermal equilibrium, assuming 90C initial shrimp temp it will be at 83C, causing wilting. –  BobMcGee Jun 14 '12 at 15:16
    
@rumtscho: I ran out of characters for comment, but I should conclude by saying that while this is a very rough approximation, it is sufficient to demonstrate the point. The point still holds even if shrimp is only cooked to 150F/63C. Light wilting of greens, like blanching, does not change the flavor greatly, so I don't agree that you'll get a cooked taste to it. Obviously the degree of wilting will depend on how much shrimp you put it the salad, and how hot they are, but even for small amounts it should cause localized heat-based wilting. –  BobMcGee Jun 14 '12 at 16:22
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Besides BobMcGee's answer, which is quite thorough, one option would be to avoid using lettuce in your salad. For example, you could make a salad of wild rice, and sauteed asparagus and mushrooms. Placing hot shrimps on top won't wilt anything, and you can garnish with sprigs of parsley or other herbs that will remain nice and crisp because they don't have a large surface area in contact with the shrimp.

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