Do your hands itch when peeling yam? This will stop itching

Blueberry yam, candied yam, steamed yam, fried yam, yam is delicious, but it is not easy to handle.

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When peeling yam, the sticky and slippery mucus will stick to your hands and make them extremely itchy.

There are many people who like to eat yam but hate to process it. Image source: Weibo screenshot

1

What is the mucus component of yam?

As early as 1948, Japanese scholar Tsai SS discovered that yam mucilage is composed of mannan-protein molecular complexes, that is, mainly a complex of sugar and protein. Later studies found that yam mucilage also includes a small amount of allantoin, amino acids, lipids, diosgenin, catecholamines, yam hydrochloride, etc.

Image source: Photographed by the author

It is the presence of proteins and sugars that makes it sticky. The viscosity of this mucus is affected by many factors, such as concentration, ion concentration, acidity and alkalinity, and temperature. Viscosity increases with increasing concentration, decreases with increasing ion concentration, and increases at lower temperatures.

2

Why do my hands feel so itchy when they are stained with yam mucus?

Itch is an unpleasant feeling that can cause people to scratch or have a reflex, and is a kind of body protection response. Itch-causing substances are mainly divided into two categories: one of which is histamine-type itching substances. The receptors of this type of substance are mainly histamine receptors, including substances that can stimulate mast cells to produce histamine and histamine. The itching caused by mosquito bites comes from this. It can be simply considered that when redness, swelling and scars appear , it is due to the itching caused by histamine-type substances.

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The other category is non-histamine itch-causing substances, such as chloroquine, short peptide SLIGRL, bromelain, black quinoa protein, etc. Their main receptors are Mrgprs family proteins (Mas-related G protein-coupled receptors) and PAR-2 ​​receptors. Itching diseases such as chronic itching and contact dermatitis are closely related to Mrgprs.

When some people peel fresh yam, they will experience severe itching wherever the mucus comes into contact with the skin, forming contact dermatitis. This itch is different from the itch caused by mosquito bites. The former does not immediately cause local redness, swelling, blisters or rashes on the skin, and the itching area is also larger. What exactly is this non-histamine substance that causes itching?

Many people say that the itching caused by yam mucus is due to the saponins or saponins in it, but there is currently no literature reporting that the above substances are the culprits of yam itching.

In order to clarify this issue, researchers extracted a substance called allantoin from yam, which is a hydantoin derivative widely found in animals and plants and is also a metabolite of purine. Its content in yam is very small, with only about 3.567 mg of allantoin per gram of yam.

Allantoin crystal image, source: Reference [1]

Next, the researchers conducted the following experiment using allantoin. They shaved the hair on the necks and backs of mice, then divided them into two groups, injected allantoin into the necks of the mice in the experimental group and injected saline into the necks of the mice in the control group, and observed and collected the number of times the two groups of mice scratched within an hour.

The mice injected with allantoin scratched significantly more often than the control group, indicating that allantoin can induce itch in mice.

Allantoin causes itching. Image source: Reference [1]

The researchers further obtained mouse dorsal root ganglion (DRG) neuron cells and used calcium imaging technology to examine the cell activity of itch-causing substances. The results showed that about 8% of DRG neuron cells responded to allantoin. Similarly, electrophysiological experiments showed that stimulation with allantoin can also directly induce action potentials in diameter neurons. This shows that allantoin may be an itch-causing substance in yam.

3

If mucus accidentally gets on your hands, how can you relieve the itching?

Allantoin is soluble in dilute ethanol, hot water, glycerol and weak alkaline solutions, and will also decompose at high temperatures.

When handling yam, you can peel it with gloves , or throw the yam into boiling water for two minutes to allow the allantoin in the yam skin to decompose, and then peel it.

If fresh yam mucus gets on your skin, you can blow it with a hair dryer or warm yourself up by a fire next to the stove. The allantoin will be decomposed by the heat and you won't feel itchy.

In addition, washing with hot water, baking soda or sodium carbonate solution can also wash away the allantoin and have an anti-itching effect.

References:

1. Yang Y, Sun YL, Liu P, et al. Study on itch-causing substances and itch-causing mechanisms in yam[J]. Progress in Biochemistry and Biophysics, 2019 (10): 7.

2. Zhang Jiaojiao. Study on the properties of yam mucilage and enzymatic hydrolysis and flour making process[D]. Jiangnan University, 2014.

3. Jian Tunyu. Isolation and purification of pruritic active components from yam and its correlation with Mrgprs family[D]. Nanjing University of Chinese Medicine, 2014.

4. Hama H, Tanabe T, Suzuki N. A study of itch intensity in relation to two-point tactual discrimination[J]. Shinrigaku kenkyu: The Japanese journal of psychology, 1986, 57(3): 179-182.

This article is produced by Science Popularization China, produced by De novo (Dalian Institute of Chemical Physics, Chinese Academy of Sciences), and supervised by Computer Network Information Center, Chinese Academy of Sciences

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