How to keep edamame and vegetables green when cooking? Do you need "technology and hard work"?

Summer is the season for flowers. Eating spiced edamame and spiced peanuts at a food stall, drinking beer, and chatting with friends is one of the pleasures of life for many people.

Recently, a topic in the media has attracted a lot of attention:

Why are the edamame I cook yellow at home, but the edamame the store gives me green? Why don't they turn yellow after being in the refrigerator for several hours?

Some people say that edamame keeps its green color because it uses bleaching powder and formaldehyde. This is a rumor.

Formaldehyde and bleaching agents are used for bleaching, not for color preservation . Relevant tests have not found excessive formaldehyde in edamame, but there is a recent news report that excessive formaldehyde residues were found in shelled peanuts, which may be caused by bleaching the peanut shells. It is unlikely that the store has the motivation to bleach the edamame...

Today let’s talk about the technical issues of keeping edamame and vegetables green.

Whether it is edamame or green leafy vegetables, the green color comes from chlorophyll. Chlorophyll contains magnesium and phytol. After removing the magnesium and phytol, the color will turn into yellowish olive green. In fact, the greenness of vegetables has always been one of the biggest headaches in the processing of green leafy vegetables.

Why do green vegetables turn yellow?

There is no way, many natural pigments are unstable . They either dissolve in water and fade during cooking, such as the purple of eggplant skin and the red of red amaranth; or they change color when they encounter acid, alkali and metal ions, such as purple cabbage turning gray-purple in a frying pan; or they turn yellow and brown after being heated for a period of time, such as various green leafy vegetables.

Artificial synthetic pigments are very stable, so if you see color change during cooking, it is almost certain that it is a natural pigment and there is no need to be afraid.

What are some tips for keeping vegetables green?

There are many factors related to chlorophyll discoloration, including pH, light, heating, oxidation, enzyme action, etc. Although chefs don’t understand the scientific principles, they have their own tips for keeping vegetables green. I will briefly sort them out for you.

1. Adjust the pH.

The rate of chlorophyll demagnesiation is related to pH, and the rate of demagnesiation is fast under acidic conditions.

Generally speaking, if you boil edamame (or most green leafy vegetables) in weakly alkaline water (such as water in Beijing) for a few minutes, they will remain green instead of yellow. However, many families do not control the time when boiling edamame, often boiling for 20 to 30 minutes, and the edamame will inevitably gradually turn yellow-brown.

The water in the south is mainly acidic, so cooking vegetables and edamame will turn yellow easily . Because of this, some chefs will try to adjust the acidity and alkalinity of water. For example, adding a small amount of alkali when cooking edamame will prevent the color from turning yellow so easily. Another example is adding a little very dilute lime water (calcium hydroxide) or calcium chloride, which can not only adjust the acidity and alkalinity, but also replace part of the magnesium ions with calcium ions, making it easier for vegetables to keep green and have a crisper taste.

Calcium is an essential element for the human body, and it is needed in large quantities. Adding a small amount of calcium salt will not cause toxicity. Don't worry about adding too much, because too much will make it bitter. In fact, many pickles are made by adding calcium salt to keep them crisp, which is a traditional process.

However, adding alkaline substances also has a side effect - it will destroy vitamin B1. Edamame is a food that is very rich in vitamin B1, and it is a pity to lose it for the sake of "appearance".

2 Cook with lid open.

Many people have had this experience: when cooking green vegetables, as long as the lid is open, they will not turn yellow easily.

This is because vegetable cells contain more or less organic acids, which can evaporate along with water vapor when heated (the principle of steam distillation).

If you cook it with the lid open, most of the acid will just fly away instead of bothering the chlorophyll.

If you cook with the lid on, the organic acids cannot escape and instead enter the chlorophyll, driving out the magnesium ions, causing the leaves to turn yellow easily.

3 Add some oil.

Adding oil to protect the color is a physical effect. It is well known that water and oil are incompatible. If the surface of edamame is coated with a layer of oil, water molecules will not easily enter, and the hydrogen ions dissolved in water will enter the leaves or edamame pods more slowly. In this way, the color change will be delayed when cooking edamame.

Chefs in the southern region are well aware of this, so when they blanch green leafy vegetables, they always add some oil to the blanching water.

4 Add plenty of salt.

Sodium ions can also replace magnesium ions in chlorophyll, but at this time the chlorophyll can still remain green. The combination of sodium ions and chlorophyll is not very tight, and it is also possible to discolor and turn yellow. However, if a large amount of salt is added, it is equivalent to providing a high concentration of sodium ions. These sodium ions mainly rely on "strength in numbers" to compete with hydrogen ions for positions and maintain green.

Many chefs use this method to add some salt to the blanching water when blanching vegetables, so that the leaves appear greener. In fact, the so-called "blanching" of vegetables usually requires both oil and salt to be added to the blanching water, and the vegetables must be taken out quickly, so that the leaves will look green.

Of course, we all know that adding too much salt is not good for health. Moreover, adding too much salt will make the taste of green leafy vegetables more tough and less crisp, which is not worth the loss.

5 Blanch quickly and then freeze.

First put the edamame in boiling water and cook it quickly for a few minutes until it is half cooked. Then freeze it in time. This will allow the semi-finished edamame to remain green for a longer period of time.

There are three principles involved: first, the heating time is short, the organic acids in the cells have not been released yet, and the chlorophyll has no time to demagnesium; second, the edamame is heated quickly to kill the oxidase and enzyme that hydrolyzes phytol; these enzymes can destroy the structure of chlorophyll, so the enzyme inactivation treatment is also beneficial to slow down the yellowing rate; third, timely cooling, refrigeration, and freezing can slow down the rate of chemical reactions, which will also significantly reduce the rate of chlorophyll demagnesium.

Most restaurant chains say that’s exactly the approach they take.

They first blanch the edamame quickly, then pack the half-cooked green edamame into bags and deliver them to various stores in a frozen or refrigerated manner. After the store receives the refrigerated or frozen half-cooked edamame, it only needs to take out a portion at any time and cook it for a few minutes for the customer to eat. This ensures safety and helps maintain the fresh color.

The above methods, if used in combination, are sufficient to meet the needs of most families and restaurants.

The same goes for edamame, and the same principle applies to blanching various green vegetables. You may wish to apply what you have learned to other situations.

The blanching time must be short and the cooling speed after blanching must be fast. For example, a study found that blanching Shanghai greens only requires 43 seconds at 86°C and adding 0.6% calcium chloride to the blanching water, and the chlorophyll preservation rate is as high as 96%.

However, there are some more high-tech methods.

6. Use composite greening technology.

In vegetable processing, if zinc ions and copper ions are used to replace magnesium ions, and salt is added, magnesium ion replacement products such as zinc sodium chlorophyllin or copper sodium chlorophyllin can be formed. These products are not only green, but also have a strong color and will not turn yellow. Sodium copper chlorophyllin itself is a pigment permitted for use in food processing in China and can be classified as a "semi-natural pigment". Since zinc and copper are both essential elements for the human body, they can be used under controllable conditions in factory production.

However, this type of method must precisely control the dosage, and it is best to use different metal ions in combination . For example, when zinc ions are used for color protection, the effect is poor when the zinc ions are lower than 20 mg/kg, and when the color effect is the best, the zinc ions have exceeded the residual standard of 75 mg/kg. In the production of dried vegetables, zinc citrate, which is safer, can be used, and adding a small amount of vinegar during soaking and rehydration can dissolve excess zinc ions and reduce the zinc content when eaten.

Because the accuracy of catering operations is very low, it is difficult to control the amount of residues, and excessive intake of elements such as zinc and copper may also cause poisoning, so China does not allow this method to be used in catering food prepared on site . In addition, we eat a lot of green leafy vegetables every day. If the dishes are dyed with sodium copper chlorophyll or zinc citrate is used to protect the color, then the intake of copper or zinc will be too high.

As the catering industry moves towards prefabrication, the central kitchen will deliver all kinds of semi-finished dishes, which will inevitably need at least a few hours of color preservation, and green preservation is a big problem. A few hours is enough for all kinds of green leafy vegetables to turn yellow and taste bad, making people lose their appetite at first sight.

The good news is that the large-scale prefabrication of central kitchens has transformed the operations of small workshops with great freedom into standardized operations that can be precisely controlled by technicians. Many prefabricated products can more legally and safely use food additives to control the color and texture of food, while preventing excessive proliferation of microorganisms and ensuring food safety.

For example, China has conducted relevant research on the color preservation technology of pre-prepared dishes . A small amount of sodium bicarbonate (baking soda) is used to adjust the pH value of the blanching water, and calcium chloride and calcium ascorbate (ascorbic acid is vitamin C) are added to shorten the blanching time to less than 1 minute. This allows green leafy vegetables to remain fresh and green even after being transported at 60°C for 4 hours.

In fact, if you don’t care about the color of edamame, the easiest way is——

7 Cook and eat as you go.

Some stores keep a pot of edamame on low heat, not letting it cool down, and serve it as you like. The aroma of cooked edamame wafts through the store. This approach can keep things safe because microorganisms cannot reproduce or produce toxins at 60°C or above. As long as the cooked edamame is kept warm at at least 60°C, bacteria can be prevented from multiplying, preventing food poisoning after eating.

However, if the heating time is too long, the vitamin loss will become greater and greater, and the color will gradually turn yellow.

Even so, I still think this method is much better than leaving the edamame at room temperature for several hours. After all, food safety is the top priority , and losing some vitamins is better than vomiting, diarrhea, and abdominal pain after eating, or even being sent to the hospital due to dehydration and electrolyte imbalance.

The appearance of edamame is not that important; the most important thing is its safety after being eaten.

References:

Wang Zhiwei. A brief analysis of the storage method of blanched beans at room temperature to keep them green. Seed Science and Technology, 2020, 38(15): 39-40

Zhou Xiang, Li Simin. Research on optimization of pretreatment conditions for Shanghai green tea. Journal of Guangdong Light Industry Vocational and Technical College. 2022, 21(06): 15-20

Zhang Jinmei, Tang Yun, Yi Kuixin, et al. Application of zinc-containing regreening agent in the regreening and greening process of bamboo shoots. Food Industry, 2022, 43(4):39-43

Yu Yigang, Wu Huiyi, Hu Yuhan, et al. Optimization of green leafy vegetables preservation and crispness in meal preparation. Modern Food Science and Technology. 2020, 36(10): 190-199