Article from WeChat public account:Shell (ID:Guokr42), author: Qian Cheng, from the title figure: Oriental IC
Things have to start a few days ago.
It was late that day, and there was only one bowl of sour soup in the refrigerator – this was the takeaway that had not been eaten the day before.
In the principle of “Don’t waste and balance the diet”, I bought some vegetables on the Internet and cooked them with the sour soup.
The strange thing happened: I didn’t cook for a long time, I found that the color of the green vegetables has turned into a dark yellow-green, and the green color of the green vegetables just after the pot is completely gone! Although the taste is good, but after all, the color is not good, the selling has become so bad that the mood of eating is greatly affected.
This is not my dinner that day, but the visual effect is almost
I remember that I would have had this experience before. For example, the old vinegar mixed with a spinach, the rice did not finish the leaves first yellow; in the hot and sour powder, the green vegetables were turned into a yellowish brown in a short time; the fire of the fried blue saffron was not mastered, and the result was fried into a smashing west. yellow flower.
In order to determine this, we also made a special experiment: buy spinach, half mixed vinegar, half did not mix. As a result, it can be clearly seen that the vinegar-added spinach is obviously changed.Yellow.
The spinach on the left Vinegar, no added on the right, start color change for 45 minutes
After a night, the vinegar-added spinach turns yellow.
Why is green leafy vegetables so yellowed?
Too long to read version:
1. The reason why vegetables change color is because chlorophyll turns into chlorophyll “pheophyllin” under acidic conditions or when heated.
2. Discoloration does not affect food safety and slightly affects nutrition, but don’t worry too much.
3. Reduce cooking time, avoid acidic environment, and avoid discoloration reaction.
4. In the industry to prevent vegetable discoloration, an additional green food coloring, chlorophyll copper sodium salt will be added.
unstable chlorophyll
Everyone knows that green leafy vegetables can be presentedGreen is because it contains chlorophyll. With chlorophyll, plants are able to perform photosynthesis and get the energy necessary for survival.
However, chlorophyll is a failure to use as a food coloring because it is too unstable. Just have a little bit of acid, or add a little heat, and change it to you in minutes.
The core part of chlorophyll is a ring structure called a porphyrin ring with a magnesium ion in the center of the ring. But magnesium is easily replaced by other elements, such as hydrogen ions, which become pheophytin. The pheophytin is brown. When most of the chlorophyll in green leafy vegetables is converted to pheophytin, the original bright green color of green leafy vegetables will disappear and become darker olive green.
Molecular model of chlorophyll, green represents magnesium ion | Wikimedia Commons
Chlorophyll has protein protection in living cells and is relatively stable; but once plant cells die, chlorophyll is more susceptible to release from proteins.
The sour soup beef, which is often eaten, has two characteristics of acidity and heating in the production process. This environment is very unfriendly. For chlorophyll, it is the ultimate survival mode.
First of all, the production process of sour soup is either by fermentation to produce organic acid or by adding a large amount of vinegar, so the concentration of free hydrogen ions is very large and the pH is very low, generally less than 4.5. However, as long as the chlorophyll is left in the environment with a pH lower than 6, a very obvious color change reaction occurs.
Moreover, the sour soup dishes need to be cooked for a relatively long time, and the temperature is around 100 °C. However, as long as the chlorophyll is in an environment of 60 ° C or higher, a significant color change reaction occurs.
For chlorophyll, sour soup is the ultimate survival mode
After all, really can’t blame the environment is too harsh, or chlorophyll itself is too fragile!
How to prevent discoloration from happening?
The home wants to prevent discoloration of green leafy vegetables. The practice is simple: shorten cooking time and reduce acid contact.
In the industry, in order to maintain the green color of cooked vegetables, there is another idea: to make vegetables “specimen”.
Yes, I am serious. There is a classic experiment in high school or college students to make plant specimens. Leaf specimens are usually treated with acetic acid and then soaked with copper sulfate (or copper acetate) solution so that the green color of the leaves can be preserved for a long time.
The chlorophyll present in the chloroplast under the microscope | Fabelfroh / Wikimedia Commons
The principle of this method is to first convert the chlorophyll in the leaves into pheophytin with acetic acid, and then put it into the copper solution, so that the copper ions enter the center of the porphyrin ring to form chlorophyll copper; the chlorophyll copper is more than the original magnesium. Chlorophyll is much more stable and the color is brightGreen – in this way, plant specimens can play the role of “green”.
However, chlorophyll copper is insoluble in water and is difficult to add directly to vegetables, so the application is still very limited. At present, for the green protection of green leafy vegetables, a green food coloring called “chlorophyll copper sodium salt” is usually added. It is another modified version of chlorophyll, which is very water soluble, so it is widely used today, and it can be used in some canned foods, fruit and vegetable juices, and pastries.
Sometimes may also use chlorophyll copper sodium salt |
In addition to this, there are many ways to protect the green, such as making the whole food system alkaline – chlorophyll will become more stable in the environment with pH greater than 7. There are also methods for protecting green with zinc salts and protein buffers, but the current application is not particularly extensive.
However, when cooking at home, these food colors and food additives are not easy to buy. Turning the food into alkaline will destroy the flavor of the food Also don’t drink sour soup). Therefore, if you want to keep green, what you can do now is to minimize cooking time and avoid acidic environment.
Can vegetable color change still eat? Still nutrition?
Someone might ask if there is a food safety risk if the vegetables turn yellow brown. Will there be any change in nutritional value?
Color-changing vegetables can be safely eaten because it is only a reaction involving chlorophyll and does not produce any toxic or harmful substances.
Although the color is not good, it will not produce toxic substances
With nutrition, it is a bit complicated to say.
First, vegetable discoloration involves the reaction of chlorophyll. In this process, magnesium is removed from chlorophyll and released into the external environment. Therefore, the magnesium in the vegetables may be partially transferred to the soup. If you finish the soup, it will not be a loss for you. But if the soup is not drunk, the magnesium that may be ingested is slightly less.
Secondly, causes the yellowing of green leafy vegetables due to acidic conditions and high temperatures. Some nutrients are not acid resistant. For example, folic acid is not very stable under acidic conditions; high temperature also destroys some nutrients, such as some B vitamins and vitamin C will decompose at high temperatures.
However, the loss of these nutrients is only a small part and is hard to avoid during cooking. On the other hand, it is easy for us to make up for these losses through other foods, so there is no need to care too much. Otherwise, we can only eat salad.
References
[1] Schwartz SJ , Elbe JHV . Kinetics of Chlorophyll Degradation to Pyropheophytin in Vegetables [J]. Journal of Food Science, 1983, 48(4): 1303-1306.
[2] Weemaes C A , Ooms V , Van Loey A M , et al. Kinetics of Chlorophyll Degradation>
Article from WeChat public account:Shell (ID:Guokr42), author: Qian Cheng