1. Why Does Cut Burdock Turn Brown in the Air?
If you cut burdock and leave it for a while, the cut surface turns brown. Burdock contains polyphenols such as chlorogenic acid. When exposed to air after cutting, an oxidation enzyme (polyphenol oxidase) inside the vegetable causes oxidation, which changes the color [1]. Cooking science papers explain that soaking burdock in water or heating it is done to improve the taste and to prevent this browning change. Therefore, "aku-nuki" for burdock aims at both color and taste. So, what is the goal for spinach?
2. How Does Boiling Spinach Reduce Oxalic Acid?
The astringent taste in spinach is said to be related to a component called oxalic acid [2]. Since oxalic acid dissolves in water, it can be reduced by boiling or soaking in water. The Ministry of Agriculture, Forestry and Fisheries suggests putting spinach into boiling water root-side first and boiling for about 1 minute, then rinsing in running water. However, boiling too long also reduces vitamin C. An experiment by the National Agriculture and Food Research Organization put the balance of "what is removed" and "what remains" into numbers [3]. Boiling at 100°C for 120 seconds removed 67% of the oxalic acid, while 77% of a component called lutein remained. This means about one-third of the oxalic acid remained, and about one-quarter of the lutein was lost. The experiment suggests conditions can be chosen between 60°C for 30 seconds and 100°C for 120 seconds. These values are from experimental conditions and may not be the same in a home pot. Still, it shows that "aku-nuki" is not about removing everything, but choosing how much to reduce and what to keep.
3. Is the Astringent Taste of Spinach Really Caused by Oxalic Acid?
However, research suggests that the astringent taste of spinach might not be the taste of oxalic acid itself [4]. A report in the Journal of the Japanese Society of Cookery Science found that while the boiling water had less oxalic acid than the original extract, the taste of oxalic acid was stronger. It was stated that bitterness and the taste of oxalic acid came from different components. It is also suggested that calcium in saliva binding with oxalic acid may be related to the taste. Since this is an interpretation shown by one study, it does not mean the astringency is fully explained. Still, it shows that it is difficult to decide that "aku" equals "oxalic acid." The Ministry of Agriculture, Forestry and Fisheries explains "aku" in mountain vegetables as strong flavors such as astringency, bitterness, and astringent taste [5]. In mountain vegetables, "aku" is used as a term for strong flavors, not the name of a single substance. So, is there "aku-nuki" that affects the body, not just taste?
4. Why Do We Remove the Harsh Taste from Bracken Fern?
For mountain vegetables, "aku-nuki" has another role besides taste. The Ministry of Agriculture, Forestry and Fisheries explains that "aku-nuki" can also be expected to remove natural toxins [5]. Bracken fern is a representative example, containing a component called ptaquiloside. The International Agency for Research on Cancer (IARC) classified bracken fern in 1987 as "possibly carcinogenic to humans" in Group 2B [6]. Information from the Hong Kong Food Safety Centre, listed in the Food Safety Commission database, gives examples of boiling young leaves in plenty of water for 15 minutes, or steaming for 10–12 minutes and discarding the water before cooking, to reduce water-soluble toxic substances. It also indicates not eating too much. The important point here is that "aku-nuki" is a process of "reducing" natural toxins, not "eliminating" them. Traditional preparation methods can also be expected to reduce poison. Both spinach and bracken fern involve reducing water-soluble components. So, how has bamboo shoot, where the identity of "aku" seems more complex, been handled?
According to the Hong Kong Food Safety Centre information in the Food Safety Commission database, to reduce water-soluble toxic substances, young leaves should be boiled in a large amount of water for 15 minutes, or steamed for 10–12 minutes and the water discarded before cooking. It is also important not to eat too much.
5. Which Component Causes the Astringent Taste of Bamboo Shoots?
The astringent taste of bamboo shoots is said to be related to a component called homogentisic acid [7]. A study in the Journal of the Japanese Society for Horticultural Science found that when Moso bamboo was stored at 1°C, this component decreased over 9 days [8]. Since the amount of the component changes with storage temperature, the story is not simple. When boiling, cooking science literature introduces a method of boiling in rice bran water (15% rice bran to water) for 40–50 minutes until a bamboo skewer goes through. Compared to the 1–2 minutes for spinach, this is a much longer time. So far, this has been about the components of "aku." However, pre-boiling has another face: how heat penetrates.
6. Why Are Root Vegetables Boiled from Cold Water and Leafy Vegetables from Boiling Water?
According to an explanation by Tsuji Culinary Institute, root vegetables should be boiled from cold water, and leafy vegetables from boiling water [9]. This is because if root vegetables are put into hot water, the outside can soften while the inside may remain hard. Green vegetables become vivid in color when put into boiling water. Temperature also has surprising effects. An experiment in the Journal of Home Economics of Japan found that if potatoes and other vegetables are pre-heated at 55–70°C for about 100 minutes and then boiled at 100°C, the tissue becomes hard [10]. A mechanism is proposed where pectin connecting vegetable cells changes and binds with calcium, causing hardness. This is a long-time experiment and cannot be said to be the same as home pre-boiling. Still, it teaches that the hardness of vegetables changes depending on how they are heated.
7. Activity: How Can We Compare Color Changes in Cut Burdock?
Today’s activity: With an adult, cut two slices of burdock (ask an adult to use the knife). Place one slice on a plate and put the other in a cup with water. Compare the color of the cut surfaces after 5 minutes, 30 minutes, and 1 hour. The results will change depending on the burdock and the room conditions. Your answer in your own kitchen is the continuation of this article. For official information, reading the Ministry of Agriculture, Forestry and Fisheries' "How to Reduce the Astringent Taste of Spinach" and "Do not pick or eat wild plants or mountain vegetables you do not know!" provides guidelines for home use and precautions for eating mountain vegetables.
Make two slices of burdock (with an adult using the knife), place one in the air and the other in water, and compare the color of the cut surfaces after 5 minutes, 30 minutes, and 1 hour.