Washing Lettuce and Mushrooms

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What's worse: not washing bagged lettuce or not washing mushrooms?

Not washing mushrooms.
Mareya Ibrahim, founder of Eat Cleaner food wash products, says most bagged lettuce is washed in a chlorine bath, but she recommends rinsing it anyway "as a preventative precaution." Without question, though, mushrooms should be thoroughly cleaned before cooking, for two reasons. Firstly, they're usually grown in soil using fertilizers and may carry bacteria like E. coli. Secondly, the handling and transportation of the mushrooms can contaminate these little sponges even further. Use wipes or a damp paper towel or kitchen towel to clean them.
Lemon juice

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What's worse: using bottled lemon juice or none at all (if you don't have fresh lemon juice)?

None at all.
You might think bottled lemon juice is an unacceptable substitute for the real deal, and it is true that once any kind of citrus juice comes in contact with oxygen, it starts losing its flavor and gets more acidic, which means it doesn't taste quite as lemony, limey or orangey as fresh would. And many commercial brands contain controversial additives. Still, using bottled lemon juice is better than none at all—provided you use a high-quality brand. Sabrina Sexton, a chef instructor at the Institute of Culinary Education in New York, uses Santa Cruz Organic Pure Lemon Juice or Nellie & Joe's Key West Lemon Juice for cocktails. If you're caught without a good quality bottle of lemon juice, consider adding a splash of orange juice or even vinegar (not for drinks, though), which will add the burst of acidity and freshness lemon juice usually brings to a dish.
Fresh peas frozen peas

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What's worse: frozen peas or fresh peas?

In most cases, fresh peas.
Frozen peas are better than most of the fresh peas you'll see at your grocery store, says Sexton. There is such a short window of time for fresh ones to be delicious, that usually, by the time they get from the farm to your store, they're starchy and flavorless. Frozen ones are sweet, not mealy, and a useful ingredient to keep on hand.
Fresh tomatoes and canned tomatoes

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What's worse: cooking with canned tomatoes or with fresh tomatoes?

Fresh tomatoes.
Canned tomatoes, whether they're whole or chopped, have a much more concentrated flavor than fresh tomatoes do. If you are making a tomato sauce and want it to have a deep, rich tomato flavor, "it's almost impossible to achieve it with fresh," says Sexton.
Fried fish and steamed fish

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What's worse: frying fish or steaming it, if you want to avoid a fishy odor in your kitchen?

Frying.
If you want to avoid the house-permeating fishy aroma of cooked seafood, don't fry it. In his book On Food and Cooking, Harold McGee explains that frying (as well as broiling and baking) fish "propels fishy vapors into the kitchen." Enclosing fish in a covered pan, thereby steaming it, will reduce the exposure of fish's surface to the air (and the fishy smell). Also, let the fish cool down a bit before taking the lid off; that will decrease the "volatility" of the vapors that do escape.
Chili and wasabi

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What's worse: the pain from a mouthful of food overdosed with chili, or too much horseradish or wasabi?

Too much horseradish or wasabi.
McGee knows how much a jalapeño pepper's heat can burn your mouth but says it isn't as startling as the punch you get from too much horseradish or wasabi, whose irritants can quickly get into the airstream and cause a bout of coughing or even choking. If you accidentally go too heavy on the wasabi next time you're eating sushi, breathe out through your mouth and in through your nose to avoid drawing the irritants from your mouth into your lungs.
Cucumbers

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What's worse: washing vegetables before putting them in the fridge, or storing them "dirty"?

Washing them first.
Even though cold temperatures slow down most bacterial growth, some bacteria can multiply in the fridge, especially if they're in a moist environment. You can try to dry produce well, but still, Ibrahim says, "It's best to wait until you're ready to eat produce to wash it, due to water introducing mold, decay and potential bacteria growth."