Chemical Engineering in Daily Life – Question 23: Why do the vegetable leaves in green vegetable soup turn yellow sometimes? Standard answer already exists
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This post was last edited by yjqin1 on 2015-9-8 at 18:31. Do you take care of your wife at home? Anyway, I have to take care of it. For example, you prepare delicious steamed white rice for your wife before she gets home from work in the evening; you heat up some meat dishes and some vegetable dishes, plus one cold meat dish and one cold vegetable dish, and you’re also making a soup with vegetables and shrimp flakes. . . . . . With a ding, the wife returned home; you quickly turned off the stove and went to greet her, handing her her bag and slippers. . . . My wife enjoyed eating happily while you asked her how she was, listening to various interesting stories from work; when she was about 70% full, she firmly put down her chopsticks and ordered, \"Bring the soup.\" You rush over, prepare a bowl of vegetable soup that is neither too cold nor too hot, place a spoon in it, bend down to hand it to your wife at eye level, and say respectfully, “My dear, please have some soup.” You wait humbly for her praise before going on to carry out the sacred task of washing the dishes. Suddenly, a lion’s roar was heard: “Dareful wretch! How dare you turn the vegetable soup into such a color? You’ve angered this great sovereign!” (The following 50 characters are omitted). As a result, the rubbing board came in handy again. This is the background. Have you noticed, dear reader, that when making bok choy soup, after the water comes to a boil, the bok choy is added to the soup, the lid is placed on the pot, it is left to simmer for a while before the heat is turned off and the lid is removed? By then, the soup is at the perfect temperature to drink, and the bok choy retains its green color, preserving both its appearance and flavor. But if you turn off the heat right after bringing the pot to a boil without lifting the lid, and then open the lid after a while only to find that the leaves have changed color, turning into a dull yellow. May I ask why? It needs to be explained from a chemical and chemical engineering perspective.
Chlorophyll is destroyed at high temperatures, and lutein shows its color.
This post doesn’t have many followers either. Don’t you find it very interesting? I’ll give the answer in a few days.
It seems it was Mom who taught me that you shouldn’t cover the vegetables while cooking them; otherwise they turn yellow and look like pigweed–that’s what she said
It is said that adding a little oil prevents vegetables from changing color; the chef says it prevents oxidation by keeping air out
Teacher, don’t rush; take your time. First, establish your own brand, and then more people will be willing to participate. But the teacher shouldn’t make things too complex, as that will result in only a few people being able to keep up
Why doesn’t the teacher comment on my replies?
To be honest, I’m struggling with this. Generally speaking, when I post the exam questions in the form of posts, the answers are already available. After all, some of these issues have been on my mind for half a lifetime; it’s also been 30 years since I graduated from college. I generally don’t immediately refute or explain the wrong answers given by netizens. That’s equivalent to giving the answer early on. The difficulty is that some netizens’ answers differ from my own, and such answers might be more correct, with simpler and more flawless explanations. This time, that’s exactly what happened. In this case, it is necessary to look up some information.
Standard answer: The green color of vegetables generally comes from chlorophyll, a compound containing magnesium ions. Chlorophyll is unstable and tends to decompose under the influence of acids, bases, heat, and oxidizing agents, turning brown, yellow, or colorless. Taking stir-frying or making soup as examples, heating is certainly involved in the stir-frying process; during quick stir-frying or stewing, the surface temperature of the food is very high, the vapor pressure of water is high, which means the oxygen concentration is relatively low. Therefore, during short- to medium-term heating alone, chlorophyll will not be destroyed. However, if the lid is on and heating is stopped after that, the temperature of the food or soup in the pot gradually decreases, as does the vapor pressure. At this point, air begins to enter the pot; since the lid is in place, evaporation of water is not significant, so the temperature of the food or soup drops more slowly. In this way, under the combined effect of a higher oxygen partial pressure and a higher temperature, more chlorophyll in the vegetables is oxidized, resulting in the most noticeable color change. If you turn off the heat as soon as the soup is prepared or the dishes are cooked, and leave the lid off the pot, even if the pot remains on the stove, the temperature of the food or soup drops rapidly due to water evaporation (give it a try yourself; if you don’t believe it, use a thermometer to measure how the temperature of the soup in the pot changes over time) ； Do not use your hands; even hands with thick skin cannot withstand heat above 45 degrees Celsius, not even for a minute, as the oxidizing effect of oxygen becomes less strong in such conditions.