From fresh to freeze-dried, what has the fruit and vegetable crisps eaten by astronauts experienced?

When it comes to freeze-dried fruit and vegetable snacks that we often eat now, we have to mention freeze-drying technology, and this has to start with the development of aerospace technology in the last century. After Gagarin of the former Soviet Union entered space on a spacecraft in 1961, more and more astronauts began to live in outer space. Long-term life in space has put higher demands on the daily nutritional supplements of astronauts. You must know that in the outer space environment where gravity is lost, eating - especially liquid or fluid food is a troublesome and even dangerous thing. Although various fruits are rich in vitamins and fiber, they are not suitable for consumption in space because they are rich in water.

Image source: Internet

But this little problem is not difficult for scientists. Since the biggest obstacle to eating fruits in outer space is the liquid inside the fruits, why not just make the fruits "dry"? Of course, it is easy to simply dry the fruits, whether it is air-drying, baking or pickling. But the difficulty is how to retain all the original nutrients of the fruits to the greatest extent while making them "dry". So after repeated research and verification by scientists, freeze-drying technology was used in the aerospace field in the 1960s.

1. From fresh fruit to freeze-dried fruit, only three steps are needed

The principle of freeze-drying technology is actually not complicated. It is to dry vegetables and fruits in an ultra-low temperature environment. The whole process is roughly divided into three steps:

01 Quick Freeze

Through quick freezing, vegetables and fruits are quickly placed in an ultra-low temperature environment of minus 40 degrees Celsius. In such a low temperature environment, the moisture contained in vegetables and fruits will instantly solidify into a solid state.

Image source: Yitu.com

This process can "lock" all the effective nutrients in fruits and vegetables to prevent nutrient loss during the dehydration process. The reason for using quick freezing is that if the cooling speed is not fast enough, the water in the food will form large ice crystals during the solidification process, thus destroying the internal physiological structure of the food.

02 Vacuuming

After quick freezing of fruits and vegetables, a vacuuming process is required, which involves the conversion process of water from solid to gaseous state.

Dry ice sublimation | Source: Yitu.com

We know that at 1 standard atmospheric pressure, the boiling point of water is 100℃, that is, it turns into water vapor at 100℃. However, the boiling point of water is not fixed. As the air pressure decreases, the boiling point of water also decreases. When the air pressure drops to 0.006 standard atmospheric pressure, water can sublime directly from solid ice to gaseous water vapor without changing to liquid state. When we vacuum the ultra-low temperature fruits and vegetables, we are actually putting them in an extremely low pressure environment, so as to prepare for the drying of food in a low-temperature freezing environment.

03 Drying

In the low-temperature and low-pressure environment created by the first two steps, it only takes a slight increase in temperature. The solid water "frozen" in the fruits and vegetables will directly sublimate into gaseous water vapor and leave the food. Note that in this low-temperature and low-pressure environment, the fruits and vegetables only lose water, while other nutrients are almost all retained. Not only that, the cell structure of fruits and vegetables is not completely destroyed in this process, so the external "shape" of the food is also retained. The only difference is that the internal structure becomes loose and porous due to the loss of water.

Air-dried mango vs freeze-dried mango | Image source: nu1983/deposit_internal/IC, Hajai Photo/adobe/IC photo

After the three steps of processing, the fruits and vegetables lose water but retain almost all other nutrients. They are also crunchy and very suitable for astronauts to eat in space. Later, with the commercialization of this technology, freeze-dried foods have gradually been favored by food manufacturers because of their nutrition retention, ease of long-distance transportation, long-term low-temperature storage and unique taste. So today, we ordinary people also have the opportunity to taste the food that was originally only available to astronauts.

2. Freeze drying, precise lightning protection

In fact, looking back at history, in order to store food for a long time, humans have invented a variety of dehydration and drying methods for fruits and vegetables, such as pickling, air drying, and baking. However, these methods all have various defects without exception, which will lead to a large loss of nutrients. The main reason is that some characteristics of fruit and vegetable ingredients themselves create insurmountable minefields in the processing process. The main minefields are as follows:

01 High temperature denaturation

One of the main nutrients in fruits and vegetables is various vitamins, but many vitamins have poor thermal stability and are easily decomposed and destroyed by heat. Therefore, if food is dried by baking or even frying, it will inevitably lead to the loss of vitamins.

Apple pie | Source: Paixin Creative

02 Oxidation

In addition to being easily destroyed by high temperatures, vitamins are also easily decomposed by oxidation. Therefore, even if fruits and vegetables are treated by air drying or shade drying (such as raisins) without heating, a large amount of vitamins will still be lost.

Image source: Paixin Creative

03 Respiratory response

Fresh fruits and vegetables, when their cells are still active, will continue to respire all the time, which will not only consume the sugar contained in the food, but also may produce fermentation. Therefore, when air-drying or shade-drying food, if the temperature, oxygen concentration and other environmental controls are not properly controlled, the food may be deteriorated faster due to respiration and fermentation.

Image source: Paixin Creative

04 Osmosis

Preserved fruits and vegetables are pickled, which is a method of exposing fruit and vegetable cells to a high concentration of salt and sugar solution, causing the cells to lose a large amount of water due to the osmotic effect of the cell membrane. However, this method will cause a large amount of water-soluble minerals to be lost along with the water in the plant body, and at the same time, the sugar or salt in the external solution will penetrate into the cells, causing the salt or sugar nutrition of the food to exceed the standard.

Image source: fydorov/adobe/IC photo

Compared with the disadvantages of the above-mentioned traditional food drying methods, freeze-drying technology can perfectly and accurately avoid lightning - the quick freezing technology prevents nutrients from being denatured due to high temperature, while inhibiting the respiratory reaction of plants. The vacuum environment avoids oxidation, and the drying method of directly sublimating water from the solid state avoids the loss of water-soluble nutrients with water. Only a small amount of vitamins will be lost during the entire process, and other nutrients such as dietary fiber, minerals, organic acids, etc. are fully retained. More importantly, the fruit and vegetable foods that are dried and dehydrated in a rapid low-temperature environment retain some cell activity because the main internal cell structure is not destroyed (some cell walls will be destroyed during the ice sublimation process).

Image source: Paixin Creative

When the freeze-dried food is stored at room temperature, it can also partially restore its activity by reabsorbing moisture from the air. The longer it is stored at room temperature, the more water it absorbs. This is why the freeze-dried food is crispy when it is just taken out, but it will gradually become soft after being stored for a period of time.

Source: Chongqing Science and Technology Museum

Review expert: CPU, science writer.

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