With the arrival of midsummer, the "summer relief artifact" watermelon has received more and more attention, and a melon-related profession - "melon tester" has also entered the public eye, and related topics have also attracted heated discussions among netizens in recent days. Because one of the key actions in the work of the "melon tester" is to pat the watermelon, someone gave them an interesting nickname: "melon master".

A melon with a low voice is a good melon

According to media reports, these professional "watermelon quality inspectors" can judge whether the melon is raw or cooked, good or bad as long as they take a look at the melon, pat it, and flick it, and a series of operations take only a few seconds. During the peak period of melons, "melon testers" need to "shoot" up to tens of thousands of melons every day and select qualified individuals from them.

Some netizens said after watching the operation of the "melon tester": "Every time I buy melons, I will take a pat, in fact, I don't know why I took a shot." In fact, not only this netizen does not understand the mystery of "patting melons", many "foreigners" do not understand, a few years ago, the media reported that an Italian supermarket specially put up a prompt sign, asking customers not to shoot watermelons when picking watermelons - "because these watermelons really won't respond to you." ”

However, in fact, according to relevant research results, "listening to sound and distinguishing melons" is actually a quite scientific way to select melons.

According to the data, the selection of watermelons according to the sound of patting is based on the principle that the propagation of sound waves in the medium is affected by the density and texture of the medium. Therefore, there is a connection between the sound waves obtained by tapping the watermelon and the quality of the watermelon.

Specifically, the structure of watermelon is divided into melon skin and melon bowl. During the ripening process of watermelon, the hardness and elastic modulus of the melon skin will change, that is, the melon skin with ripe flesh is more elastic, and there will be a slight rebound feeling when patted. In addition, under the action of cellulase, the internal cellulose is degraded, the sugar content and its own structure of the watermelon also change, and the knocking sound will naturally be different.

So what sound is a melon that counts as a good melon? A study by the team from Hebei Agricultural University gives the answer. In the experiment, the researchers first collected the patting audio signal at the central equator position of the melon, and then tested the destructive ripeness of the watermelon, and divided the monitored watermelon into four grades: raw, ripe, ripe and overripe.

In the grade set by the researchers, the "raw" watermelon melon has a pale pink center and a thicker white melon on the edge, which is fleshy and not sweet; The "ripe" watermelon melon is all pink, with a sweet taste and crisp flesh; The "ripe" watermelon melon is bright red and fluffy, and you can see tiny particles, and the taste is soft and crunchy sweet or sandy; "Overripe" watermelons are fluffy, dark red, fibrous or hollow, and have a significantly spoiled taste.

The final results show that the processed audio-related frequencies between 133-160 Hz are ripe melons, while those with frequencies greater than 189 Hz are raw melons, those between 160-189 Hz are ripe melons, and those less than 133 Hz are overripe melons.

In terms of auditory sensation, the higher the hertz, the sharper the voice, and vice versa. It can be deduced that melons with a relatively low patting sound belong to ripe and palatable melons, while raw melons with a crisp patting sound are raw.

Use "light" to inspect watermelons more efficiently

In addition to manual patting "melon testing", in recent years, some non-destructive testing technologies that can be used for large-scale and rapid inspection of watermelons have gradually emerged, helping China's watermelon industry to further move towards mechanization and intelligence.

At the 6th Spectrum Network Conference held in mid-June, researcher Huang Wenqian from the Agricultural Intelligent Equipment Research Center of Beijing Academy of Agriculture and Forestry Sciences introduced an efficient watermelon detection technology based on full transmission near-infrared spectroscopy. With the help of a series of technologies, watermelons can quickly complete quality tests such as sugar content, ripeness, and hollow in the process of passing through the equipment.

In the demonstration video, it can be seen that watermelons are placed in the base of the conveyor belt, and the speed of passing through the inspection equipment is fast and smooth, and it takes almost only a few seconds to complete the quality inspection of a melon.

According to Huang Wenqian, light can enter the inside of the measured object and carry useful information, and they rely on this physical basis to independently develop a fully transmitted multi-point visible/near-infrared spectroscopy system OnlineNIR. The system can obtain information about the hydrogen-containing groups in the sample by illuminating the sample with a special incandescent lamp. With the support of this information, the establishment of a chemometric model can achieve non-destructive testing of sample Brix, acidity and internal defects.

In fact, watermelon, a common fruit, has many difficulties in non-destructive testing, including thick melon skin and uneven distribution of sugar content; The spectral signal is weak and unstable under high-speed dynamic conditions. Models are susceptible to factors such as fruit size, posture, and texture.

In order to cope with these difficulties, Huang Wenqian's team focused on optimizing the optical path structure during the development of this new spectral detection system, improving the signal-to-noise ratio of the spectral signal, and ensuring that stable spectral signals can be obtained under the conditions of low-power illumination and short integration time. Moreover, the information obtained by the team using precision positioning technology allows for a comprehensive analysis of the entire watermelon, as well as selective analysis for specific areas.

For why to continue to develop watermelon quality non-destructive testing technology, Huang Wenqian said at the spectrum network conference that quality and intensification is the future development direction of the watermelon industry, set standards for watermelons, and according to this standard watermelon classified sales, to achieve its transformation from agricultural products to commodities, can improve the added value of melons, increase farmers' income, optimize production management, has significant economic and social benefits, but at present, China's non-destructive testing technology for the internal quality of watermelons is less, and foreign related equipment is expensive.

In addition, data show that China is the world's first "watermelon country", with watermelon production reaching 2020.6024 million tons in 69, accounting for 59.29% of the world's total watermelon production. In the face of such a huge number of melons, at the peak of the watermelon market, all rely on manual inspection and sorting, which often makes the "melon inspectors" and melon farmers a little overwhelmed. In recent years, the emergence of new technologies for watermelon-quality non-destructive testing may help them reduce work pressure.

West China Metropolis Daily - cover news reporter Tan Yuqing