What causes apples to brown?

Online Answer
This unappetizing phenomenon is actually due to a chain of biochemical reactions known as "enzymatic browning." When an apple is injured (or cut into pieces), the plant tissue is exposed to oxygen. ... This results in new chemicals (o-quinones), which then react with amino acids to produce brown-colored melanins..
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Another way to use apple cider vinegar when cooking is to make a marinade. In fact, apple cider vinegar is a popular ingredient in many steak marinades, as it gives the meat a nice sweet and sour flavor. Combine it with wine, garlic, soy sauce, onion and cayenne pepper to give your steak a delicious flavor.
I use it to make veggie pasta salad/slaw, but it can be used to make zucchini pasta. Simply "peel" the zucchini with the peeler and voila – little veggie "pasta" strips!.
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Answer: It the apple's apperance will change,brownish discolartion occurs if exposed to oxygen for a certain time and temperature.
Heat a saucepan with water (at least 3 inches of water so that the poached egg doesn't get stuck on the bottom of the pan). Add a small amount of apple cider vinegar or lemon juice to the water (the egg white becomes firmer faster if you add vinegar, but it's not essential if you're not making runny poached eggs)..