Fresh bamboo shoots contain some ingredients that are not conducive to digestion and absorption, such as oxalic acid, phytic acid and tannin, protease inhibitor and coarse cellulose. Untreated fresh bamboo shoots have a fresh flavor, but they also have a strong astringency. If you cook directly, it will be very astringent. You have to soak in water first, then boil for a few minutes to remove the boiled bamboo shoots, so as to remove most of its astringency. This astringency is largely caused by oxalic acid. Oxalic acid can interact with protein of tongue surface mucosa, which makes protein coagulate and cause astringency. The heavier the astringency of vegetables, the higher the oxalic acid content.
Oxalic acid and phytic acid exist in almost all natural plant foods, but the content is different. The content of oxalic acid in daily vegetables such as Chinese cabbage, Chinese cabbage, rape, Chinese kale, Chinese flowering cabbage, Chinese flowering cabbage, cauliflower and broccoli is very low, so you can't eat obvious astringency. Oxalic acid not only harms the digestive tract of animals, but also reduces the activities of various digestive enzymes.
Of course, the oxalic acid content of bamboo shoots also varies with varieties and places of origin. High-quality bamboo shoots are less bitter and astringent, and the oxalic acid content may be lower. In addition to oxalic acid, bamboo shoots also contain high levels of tyrosine, which together with oxalic acid, phytic acid, tannin and other substances cause the bitter taste of bamboo shoots. Tannin belongs to polyphenols and tastes astringent. Phytic acid is often mentioned by people, which is the main substance that hinders the absorption of trace elements in miscellaneous grains and beans. It is said that eating miscellaneous grains causes nutritional deficiency mainly because it is afraid that too much phytic acid will affect the absorption rate of iron and zinc under vegetarian conditions. They all reduce the activity of digestive enzymes and affect the efficiency of digestion and absorption.
Tyrosine is an amino acid with phenolic hydroxyl structure. The content of tyrosine in bamboo shoots is very high, and some white substances often precipitate on the surface of cooked bamboo shoots. Its main component is tyrosine, which is an amino acid and not a toxic substance. It's very simple to tell, just taste it with your tongue. If the astringency is not obvious, it means that the oxalic acid content is not too high. On the contrary, the content is very high. However, the reason why bamboo shoots hinder digestion is not just oxalic acid. It also contains trypsin inhibitor, a protein substance. Trypsin inhibitors can inhibit human trypsin activity, thus seriously hindering protein's digestion and absorption. If people's original digestive ability is poor, then eating protease inhibitors from food will make the problem of indigestion more serious. Raw soybeans contain protease inhibitors, which are actually found in many raw foods. For example, all kinds of nuts, peanuts, beans and egg whites have protease inhibitor activity.
The trypsin inhibitor activity of most foods will be destroyed by heat, but some of them are as heat-resistant as eggs and will be inactivated at 60-70 degrees Celsius. Some are heat-resistant, for example, the trypsin inhibitor of soybean needs to be boiled for more than 8 minutes to destroy 85%. This trypsin inhibitor in bamboo shoots also belongs to the heat-resistant type.
Generally speaking, when people cook soybeans and peanuts, the heating time is longer. However, when frying vegetables, the heating time is relatively short, and the temperature in the central part of the cut vegetables is not high, often not reaching 100 degrees Celsius. Studies have shown that trypsin inhibitor extracted from bamboo shoots at 75 degrees Celsius for 5 minutes still has strong enzyme inhibitory activity.
In addition, bamboo shoots are rich in crude fiber, and the main component is cellulose. It is much thicker than the whole grains cooked after soaking, and eating too much may cause physical damage to the stomach. People with gastritis, gastric ulcer and other problems are not suitable for eating a lot of bamboo shoots. The above ingredients contained in it, collectively referred to as anti-nutritional factors, are the factors that hinder digestion and absorption. However, how to eat spring bamboo shoots, a seasonal delicacy, to reduce its adverse effects on the digestive tract?
When eating bamboo shoots daily, we should first soak and cook them, remove most oxalic acid and phytic acid, reduce tannin content and inactivate most protease inhibitors. Coupled with fat-rich pork belly, ribs and other ingredients stew, with a lot of saturated fat to soften cellulose, it tastes smooth.
In food processing, bamboo shoots are usually boiled, and citric acid and a small amount of salt are added to promote the dissolution of bitter oxalic acid, phytic acid and phenols. It was found that after adding 0.2% citric acid, about 40 mg of total phenol, about 280 mg of oxalic acid and about 180 mg of tyrosine could be dissolved per liter of water after boiling 10 minute, and the bitter taste was greatly reduced. At the same time, bamboo shoots will become softer and chewier. In daily processing, the cooking time with acid will exceed 10 minute, and another advantage of cooking with acid will also prevent the color of bamboo shoots from browning during processing. Therefore, the bagged boiled bamboo shoots sold in the supermarket will feel a little sour after being opened. It is citric acid used in processing, which is harmless to health. Of course, vitamin C will also be lost after cooking, although adding acid can reduce the loss rate, but after all, it will lose more than half.
Therefore, if you buy bagged boiled bamboo shoots sold in the supermarket, you basically don't have to worry about eating stomachache, and the texture of bamboo shoots is softened.
Another traditional practice is to make bamboo shoots into fermented vegetables, such as pickles. It was found that lactic acid fermentation could obviously reduce the content of anti-nutritional factors in bamboo shoots. After 24 hours of co-fermentation with Lactobacillus bulgaricus and Streptococcus thermophilus, the oxalic acid content decreased by about 19% and the phytic acid content decreased by about 46%.
Another concern is the nitrite content in bamboo shoots. Although higher levels of nitrite in vegetables will not cause acute poisoning, it may increase the synthesis of nitrosamine carcinogens in the stomach.
Before bamboo shoots were unearthed, the highest nitrite content was 84 mg/kg, which exceeded the nitrite standard for processed vegetables in China (less than 20 mg/kg). When the excavated height reaches 5-15cm, the content of nitrite is lower than10mg/kg, so it is safe to eat.
However, the content of nitrite can be greatly reduced by removing the boiled bamboo shoot water after boiling. According to the data in the textbook of food chemistry, blanching can remove more than 70% of nitrite at most, so those measures to remove oxalic acid, phytic acid and tyrosine are equally effective in removing nitrite.
It should be reminded that fresh bamboo shoots are rich in oxalic acid, phytic acid, tyrosine, other polyphenols and trypsin inhibitors, which will bring challenges to gastrointestinal function. At the same time, it contains more nitrate and nitrite, which may bring safety risks if it is not handled well. When eating fresh bamboo shoots, choose unearthed bamboo shoots, and the nitrite content is relatively low. The content of nitrite in bamboo shoots that have not been unearthed is high, so it needs to be boiled to remove nitrite. Eat it as soon as possible after you buy it. If you can't eat it as soon as possible, try to keep it in cold storage to delay the transformation of nitrate into nitrite. Eating bagged boiled bamboo shoots, or cutting fresh bamboo shoots and washing them with water, and adding a small amount of white vinegar to cook for 10 minute, can remove most oxalic acid, phytic acid, tyrosine and nitrite, and destroy most trypsin inhibitors.
If the gastrointestinal function is strong, but there are problems such as hypertension, hyperlipidemia and constipation, it is suitable to eat more untreated fresh bamboo shoots. If the gastrointestinal function is weak, or diarrhea often occurs, it is recommended to eat less at a time, cook it in advance, add meat, stew it slowly, or crush it with a pressure cooker.