Ancient fruit preservation: How dare you let the queen eat rotten melons?

Produced by: Science Popularization China

Author: Liu Jinying (Institute of Botany, Chinese Academy of Sciences)

Producer: China Science Expo

With the coming of summer, fruits are prone to spoilage, and how to keep them fresh has become a topic of concern. Nowadays, fruits are mainly eaten fresh. Since fresh fruits still undergo life activities such as respiration and metabolism after being picked, and fruits have strong regional and seasonal characteristics, improper storage and transportation will cause fruits to wilt and rot, thus losing their edible value, causing huge economic losses every year. Therefore, preservation technology is particularly important in the post-harvest processing of fruits.

Various fresh fruits

(Photo source: veer)

In ancient times when materials were scarce and technology was limited, people used their diligence and wisdom, relied on the natural environment and physical means to create many ingenious methods that are conducive to fruit preservation, including waxing, cellaring, refrigeration, and mixed storage. These methods effectively extended the shelf life of fruits and reduced food waste. The ancients' seemingly simple fruit preservation methods contain great wisdom, and many of these methods have been improved and are still in good use today.

The wisdom of ancient preservation

Waxing preservation method

The wax preservation technology can be traced back to the Sui Dynasty. "New Theory of the Five Dynasties" said: "Emperor Wen of the Sui Dynasty loved oranges. He picked yellow oranges in Shu and sealed their stems with wax. They remained fresh for a long time." In the Ming Dynasty, "The Book of the Hidden Gods of the Immortal" recorded: "Grapes were wrapped in wax and placed in a jar, and then the wax was melted and sealed. They would not spoil until winter."

Wax has good waterproof and sealing properties. Wrapping fruits with wax can slow down the respiration intensity of fruits and prevent water evaporation. It effectively delays the aging of fruits and prevents the infection of pathogenic microorganisms, thus keeping the fruits fresh . The ancient practice of sealing fruits with wax is the prototype of today's coating preservation. Applying natural wax made of beeswax, shellac, palm wax, etc. on fruits can not only delay the aging of fruits and make the fruits bright and beautiful, but also prevent mechanical damage to fruits during transportation. Generally speaking, local seasonal fruits are not artificially waxed, but off-season and imported fruits are often coated with wax on the surface for long-distance transportation and storage.

Orange processing

(Photo source: veer photo gallery)

Cellar preservation method

The cellar provides an ideal storage environment for fruits. The Song Dynasty book "Gewu Cuotan" records the method of storing citrus: "Dig a cellar in the ground and cover it with straw or pine grass about an inch thick. Cut the oranges from the tree with scissors, making sure not to damage the skin, and then line them up in the cellar one by one... Seal them with wet mud all around, and they will not go bad until next year." The temperature in the cellar is relatively low, and the temperature and soil moisture are maintained at a relatively stable state, creating a closed space with "constant temperature and moisture retention".

In addition, the gas exchange between the inside and outside of the cellar is poor, and when covered with straw, carbon dioxide accumulates in the environment and the oxygen content decreases, thereby weakening the respiration of the fruit and maintaining the freshness of the fruit.

The cellar storage method is still used today, and some homes in the north still use cellars. Since fresh fruits and vegetables are hard to buy in winter, potatoes, radishes, cabbages, apples, etc. are stored in the vegetable cellar and can be stored until the beginning of the next spring. However, due to the poor ventilation of the cellar, it is very dangerous to blindly go down to the cellar. Therefore, when going down to the cellar, you should light a candle first to test the oxygen content in the cellar to prevent suffocation due to lack of oxygen.

cellar

(Photo source: veer photo gallery)

Refrigeration preservation method

Refrigeration was one of the important means of preserving food in ancient times. People learned how to store ice in ancient times. According to the earliest record in "Zhou Li·Tianguan·Lingren", there was an official named "Lingren" in the Zhou Dynasty who was responsible for managing ice policy. Lingren cut ice in December every year and stored three times the amount of ice expected to be used in the ice room. The ice stored in winter was placed in a bronze container called "bingjian", which was the ancient "refrigerator".

In the early days, ice cubes were mainly used to preserve food in the palace and to cool down the royal family. It was not until the development of ice-making technology in the Tang and Song dynasties that ice cubes were no longer exclusive to dignitaries. In the Ming Dynasty, records of storing apples in ice cellars began to appear. The "Qunfangpu" mentioned that apples stored in ice cellars "are especially delicious in summer."

Refrigeration is a relatively convenient way to preserve fruits. Low temperature is very suitable for the storage of most fruits. Under low temperature conditions, the fruit's respiration rate is reduced, enzyme activity is reduced, metabolism is reduced, and water evaporation is reduced, which effectively delays the aging of the fruit. The refrigerators we use today to preserve fruits also follow this principle.

Warring States Bronze Ice Mirror

(Photo source: China Social Sciences Network)

Sealing container preservation method

In ancient times, there was no vacuum packaging method, so the fruit was stored in a sealed manner. Lychees are delicate and have a very short shelf life. In the "Preface to the Lychee Picture" by the Tang Dynasty poet Bai Juyi, lychees were described as "one day away from the branch, the color changes, two days, the fragrance changes, and three days, the taste changes", so lychees were called "off the branch" in ancient times.

Tang Xuanzong's favorite concubine Yang Guifei loved to eat lychees, which are grown in abundance in the Lingnan region. In ancient times, when transportation was inconvenient, it would take nearly ten days to transport lychees to the capital even if they were transported day and night at full speed. So how did Yang Guifei eat fresh lychees?

The History of Lychees records that the joints of ten-year-old bamboos were opened, and lychees with branches and leaves were wrapped in wet straw paper and placed inside. Finally, the bamboo barrels were sealed with mud and transported to Chang'an over a thousand miles. The air and pathogenic microorganisms were isolated under sealed conditions, and the gas composition in the bamboo barrels changed with the respiration of the lychees. This principle is basically the same as the current controlled atmosphere storage. The lychees transported in this way were still as fresh as before and tasted delicious, which won the favor of Yang Guifei - as the saying goes, "A horse in the red dust makes the concubine smile."

Fresh lychees

(Photo source: veer photo gallery)

Mixed storage method

In the Ming Dynasty, based on the spontaneous atmosphere storage, a better and more ingenious storage method was discovered by utilizing the different respiration characteristics of fruits and vegetables. "Duoneng Bi Shi" says: "For fruits such as pears, chestnuts, and citrus, take two stone jars, fill them with river mud and well water, and sprinkle mung beans on them. Put bamboo strips in the jars, remove two inches of mud, place the fruits on top, cover them tightly with mud, and bean sprouts will grow on the jujube fruits. After years, the color and taste will remain the same as new."

The preservation method of mixing fruits and mung beans cleverly utilizes the respiratory characteristics of fruits and vegetables . When mung beans germinate, their respiration intensity increases, consuming a large amount of oxygen and releasing carbon dioxide, creating a low-oxygen, high-carbon dioxide gas condition for post-harvest fruit storage, which is very conducive to the preservation of fruits. This dynamic gas change controlled atmosphere storage method only appeared internationally between the 1970s and 1980s, and the clever ancient Chinese workers had already applied this method to their lives.

Literary Gathering by Emperor Huizong of Song

(Photo source: National Palace Museum, Taipei)

Improvement of modern preservation technology

Modern preservation technology is a microcosm of ancient technical means. With the development and progress of science and technology, domestic fruit preservation technology has gained a lot of valuable experience in long-term production practice. Now there are various preservation methods, and the preservation effect has been significantly improved. Currently, effective fruit preservation methods include: physical preservation technology, chemical preservation technology and biological preservation technology.

Physical preservation technology

Common modern physical preservation technologies include: low temperature refrigeration, controlled atmosphere storage, reduced pressure storage, irradiation treatment, etc. Among them, low temperature refrigeration and controlled atmosphere storage are very common and have been widely used since ancient times.

In 1928, the UK built the world's first controlled atmosphere warehouse, which achieved great success in the storage of apples. Controlled atmosphere storage is actually based on sealed and refrigerated storage, by controlling the ratio of oxygen, carbon dioxide and nitrogen in the warehouse to inhibit the respiration of fruits and the production of ethylene. The ancient sealed storage and mixed storage methods of preservation are based on this principle. Controlled atmosphere storage technology significantly extends the shelf life of fruits and maintains their taste and nutrients. It is particularly suitable for respiratory climacteric fruits such as bananas and kiwis.

Decompression storage is a new storage method developed after refrigeration and controlled atmosphere storage. It is called the "new preservation technology of the century" and is the third revolution in preservation technology. It solves the problem of peaches being difficult to preserve. Decompression storage refers to placing fruits and vegetables in a specific closed container, extracting part of the air in the warehouse and introducing air below atmospheric pressure to form a low-oxygen and low-pressure gas environment, while promoting the diffusion of gases such as ethylene in fruits and vegetables to the outside, achieving ultra-long-term storage and preservation.

Short-wave ultraviolet (UV-C) treatment is now gradually becoming one of the main technologies for preserving agricultural products. UV-C treatment can sterilize the surface of fruits and inhibit the infection of pathogens after fruit harvest. In addition, UV-C treatment can induce the production of secondary metabolites in fruits, such as total phenols, flavonoids, and anthocyanin content, improve the antioxidant capacity of fruits, delay fruit aging, and maintain fruit quality.

Fresh Fruit

(Photo source: veer photo gallery)

Chemical preservation technology

Compared with physical preservation, chemical preservation equipment has low investment cost and simple operation, and is widely used in post-harvest fruit processing. Chemical preservation technology includes antibacterial agents, antioxidants, growth inhibitors, etc. These chemicals can effectively inhibit the growth of microorganisms and delay fruit aging.

Commonly used antibacterial preservatives include imidazoles, benzimidazoles, and biguanide salts, which have good inhibitory effects on penicillium, green mold, etc., but frequent use will lead to drug resistance in pathogens. In addition, the use of these fungicides requires attention to the scope of use and dosage to prevent harm to human health.

Ethylene is the culprit of fruit aging after harvest, and ethylene receptor inhibitors have now effectively solved the problem of fruit ripening and rotting during transportation . Bananas will still produce "endogenous ethylene" after being picked to promote their own fruit ripening. Adding ethylene inhibitors such as 1-MCP (1-methylcyclopropene) can effectively inhibit the synthesis and release of ethylene in fruits after harvest, preventing bananas from ripening and softening prematurely.

The ethylene gas produced by bananas mainly comes from the rhizome, so in our daily life, we can wrap the banana stem with plastic wrap to extend the shelf life of bananas. Similarly, we can also do the opposite and use the characteristics of bananas or apples releasing a lot of ethylene during the ripening process to ripen other unripe fruits.

fruit

(Photo source: veer photo gallery)

Biological preservation technology

Bio-preservation refers to the use of natural substances extracted from natural biological materials or obtained through bioengineering technology to extend the shelf life of fruits. For example, natural substances such as antimicrobial proteins and enzyme inhibitors are used to inhibit the growth of microorganisms, or probiotics are used to inhibit the reproduction of harmful bacteria. This method is not only effective but also harmless to the environment and human body, and has great development potential.

Antagonistic bacteria preservation technology is more friendly to ecological development. Its principle is to use microbial populations to produce antibiotics, bacteriocins, lysozymes and other antimicrobial substances to repel pathogens in fruits, compete for space and nutrients, and thus inhibit the spoilage of fruit pathogens on fruits. For example, spraying Trichoderma before strawberry harvest can effectively reduce the incidence of post-harvest gray mold, and spraying yeast can prevent post-harvest kiwifruit gray mold and papaya anthracnose.

However, the activity of these antagonistic microorganisms is easily affected by the environment, resulting in unstable preservation effects. Therefore, enhancing the survival rate and stability of these microorganisms is crucial in practical applications, which is also a difficult problem in current research.

Chitosan coating materials have been a research hotspot in recent years . Chitosan is a natural high molecular polymer extracted from the shells of marine organisms such as shrimps and crabs after processing. This preservative is safe, non-toxic and edible. Chitosan has good film-forming properties. When applied to the surface of the fruit, it can form a film that can slow down the transpiration and respiration of the fruit, prevent water loss, and delay fruit aging. At the same time, under the protection of this film, the growth of microorganisms on the surface of the fruit is inhibited, helping the fruit resist infection by post-harvest pathogens.

Advances in genetic engineering technology may bring revolutionary changes to fruit preservation. Through gene editing technology, fruit varieties that are more resistant to storage and pests can be cultivated, fundamentally solving the problem of fruit perishability. Although this technology has great potential, its application still needs standardized management. The development of gene editing technology will provide new ideas and solutions for fruit preservation.

Fruit and technology

(Photo source: veer photo gallery)

Conclusion

Fruit preservation technology has undergone tremendous changes from ancient times to the present, from simple natural methods to complex high-tech means. Every step of development is aimed at extending the shelf life of fruits and maintaining their freshness and nutrition.

Ancient preservation methods have limited applicability and are usually only applicable to specific types of fruits and specific environmental conditions. Modern technology has a wide range of applications and can be applied to almost all types of fruits and can adapt to different storage and transportation conditions. However, modern preservation technology also faces new challenges. In the future, with the continuous advancement of science and technology and the enhancement of people's environmental awareness, fruit preservation technology will develop in a more efficient, environmentally friendly and sustainable direction, providing better protection for human health.

References

1.Zhao P, Pierre NJ, Liu X et al. Microbial Spoilage of Fruits: A Review on Causes and Prevention Methods[J]. Food Reviews International, 2022, 38(S1): 225-246.

2. Liang Pan, Li Yueyan, Huang Shaoyun, et al. Research progress on storage and preservation technology of citrus fruits[J]. Packaging Engineering, 2021, 42(13): 57-66.

3. Luo Chen, Zhou Xiaodong, Ju Xiaochen, et al. Study on the effect of reduced pressure storage and preservation technology on the preservation of fruits and vegetables[J]. Home Appliance Technology, 2021, (S1): 61-65.

4. Ma Yuting, Wang Ruihuan. Research progress of fruit preservation technology[J]. Shandong Chemical Industry, 2020, 49(20): 48-49.

5. Tang Lejin, Yang Qinzhan, Qu Jieguang. Research status and prospects of fruit storage and preservation technology[J]. Journal of Food Safety and Quality, 2019, 10(09): 2474-2480.

6. Tian Shiping, Luo Yunbo, Wang Guixi, et al. Biological basis of postharvest horticultural products[M]. Beijing: Science Press, 2011.

7. Wang Yuting, Zhou Xin, Wang Zhe, et al. Current status of tropical fruit production, preservation and processing[J]. Tropical Agricultural Sciences, 2023, 43(10): 80-88.

8. Zhu Lulu, Gao Fengyi, Li Dahu. Research progress of chitosan coating technology in fruit preservation[J]. Agricultural Products Processing, 2022, (06): 72-76.

9. Zhuang Xuzhi. On the development of citrus storage technology in ancient my country [J]. Sichuan Fruit Trees, 1994, (04): 20-23.