Frozen for ten months without rotting! What is the "technology and hard work" of this kind of lychee?

Science Times reporter Hu Lijuan

"If I can eat 300 lychees a day, I'd be willing to live in Lingnan forever." After finally waiting for the "lychee season", many consumers have found that the price of lychees has risen significantly this year, and in some areas it has even been sold for 70 yuan per pound. It is said that this is related to factors such as the "good and bad years" of lychees and the reduction in production caused by extreme weather. Can we still achieve "lychee freedom" this year?

The national lychee production may drop by 40% this year

my country is the world's largest producer of lychees, and Guangdong is the main producer of lychees in my country, accounting for about 60% of the country's total production. However, in April this year, Guangdong Province experienced the heaviest rainfall in the same period since meteorological records began, with an average monthly rainfall of 497.7 mm, 2.8 times the same period in normal years. In addition, last year's warm winter and the late spring cold at the beginning of this year disrupted the flowering rhythm of lychees, and frequent extreme weather such as hail and tornadoes also affected the fruit setting rate of lychees.

At the same time, the production of litchi has a regular performance of "big year and small year". Last year, my country's litchi production hit a record high of 3.097 million tons, so this year itself is a "small year" for litchi. According to Chen Houbin, chief scientist of the National Litchi and Longan Industry Technology System and professor of South China Agricultural University, compared with last year, the national litchi production will decrease by 45% this year, and the Guangdong litchi production will decrease by more than 50%.

Lychee hibernation is like animal hibernation

"This is a lychee that has been 'frozen' for 10 months and was picked in June last year." Liang Qinru, deputy director of the Guangdong Provincial Department of Science and Technology, recently demonstrated lychees preserved using ultra-low temperature freshness-locking freeze-sleep technology on a TV program. It is reported that the frozen lychees taste similar to ice cream when not completely thawed, and the color, aroma and taste can be restored to more than 80% of fresh lychees after thawing, which can not only alleviate this year's lychee "production crisis", but also meet consumers' demand for fresh lychees all year round.

"Ultra-low temperature freezing technology is to pre-cool, protect the color, freeze and preserve fresh lychees immediately after picking, so that they can remain fresh after long-term storage." Zhao Hongfei, associate professor of the School of Biological Sciences and Technology of Beijing Forestry University, introduced in an interview with the Popular Science Times reporter. Lychees are easy to brown, rot and have a short shelf life, with an average annual loss of more than 20%. At present, the commonly used preservation method in the market is to place ice bags in simple foam boxes. In addition, there are low-temperature combined with controlled atmosphere preservation, coating preservation, irradiation technology, etc.

The core point of ultra-low temperature cryosleep technology is to improve the heat exchange efficiency below minus 35 degrees Celsius, adjust the formation path and morphological distribution of ice crystals when lychees are frozen, and allow the lychee pulp and pulp cells to "hibernate" quickly without destroying the original structure of the pulp, just like animal hibernation.

According to reports, in order to keep the best taste of lychees, consumers need to put frozen lychees in a refrigerator at -18℃ as soon as possible after receiving them, and take them out to thaw when eating. After thawing, they need to be eaten as soon as possible within 2 hours and cannot be frozen repeatedly.

Can other fruits also be frozen?

Zhao Hongfei said that in addition to lychees, cryosleep technology can theoretically be applied to the preservation of other fruits, but some problems also need to be solved: "The freezing and thawing of fruits is a very complicated process. In addition to heat conduction, it also undergoes a series of physical and chemical changes. This may affect the quality of the fruit, including water retention, pectin decomposition, texture changes, color, and microbial conditions."

In addition, the speed of freezing and thawing and the size of ice crystals formed during the process will also affect the pulp texture. When the fruit is placed in the refrigerator, the crystallization and freezing process will start from the outside and gradually develop to the inside of the fruit. However, due to the temperature difference and the fluctuation of the freezing temperature, many large ice crystals may form inside the fruit, affecting the taste.

"Based on the considerations of saving energy and maintaining quality, new technical methods such as high-pressure freezing, ultrasonic freezing, high-voltage electrostatic field freezing, antifreeze proteins and peptides have been continuously proposed and tried," said Zhao Hongfei.