The freeze-drying technology was invented at the beginning of the 20th century. After World War II, the freeze-drying technology began to be applied in food, medicine, aerospace and other fields, and was gradually popularized in industrialization. Freeze-dried food is also known as FD (Freeze Dried) food. The advantage of this kind of food is that it can maintain its own freshness and nutrients. It can be stored at room temperature for more than 5 years without preservatives. Because its finished products remove most of the moisture and have the advantages of light weight, easy to carry and transport, freeze-dried food has also begun to enter people's daily life and become a leisure and convenient health food

In developed countries such as Japan, the United States and Europe, freeze-dried food has developed to the degree of industrialization and automation. In the simple and convenient cost structure of convenience food in these countries, the proportion of freeze-dried food with good nutrition and quality is generally more than 50%, and its popularity in daily life has also reached a high level. Data shows that the freeze-dried food market in the United States and Japan is relatively mature, requiring 6 million tons and 2 million tons of freeze-dried food each year. image.png

At present, freeze-dried foods in China are still relatively conventional, such as freeze-dried fruit slices, freeze-dried vegetables, freeze-dried beef and soup products. Compared with the traditional baked, fried, puffed and honeyed foods, freeze-dried foods maintain the original natural flavor of the food. The color, fragrance, taste and shape of the ingredients do not contain any additives. They have good rehydration ability, and can be eaten immediately or in a bubble. They are time-saving, labor-saving, portable, and portable, and have attracted the attention of many consumers.

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Principle of freeze-drying technology

Freeze drying technology refers to vacuum freeze drying technology, which is different from frying or baking and spray drying when making powder. Freeze drying is to sublimate frozen food in a vacuum environment and dehydrate the food. In short, freeze-drying is actually a sublimation process of frozen products. Sublimation occurs when the frozen liquid begins to transform into gas.

The whole freeze-drying process mainly includes pre-freezing, quick-freezing and vacuum drying stages.

The basic principle of freeze-drying is based on the three-state change of water. There are three phases of water, namely, solid, liquid and gas. The three phases can be converted from each other and coexist. The three-phase change is mainly based on the pressure and temperature of water.

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Food freeze-drying is to freeze fresh food such as vegetables, meat, fruit, etc. in advance and quickly, and sublimate the water in the food from solid to gas under vacuum, and then remove some combined water through analytical drying, so as to achieve low-temperature dehydration and drying.

1. Pre-freezing and quick-freezing

The freeze-dried food must be pre-frozen below its crystallization temperature, that is, the substances (solutes) that make up the food must be frozen first. The reason for adopting quick-freezing method is that the faster the freezing speed, the smaller the ice crystals in the food, and the less the mechanical damage to the cells.

2. Vacuum drying

The drying stage of freeze-drying is generally divided into two stages: primary drying and secondary drying, also known as sublimation drying and analytical drying.

(1) Primary drying

After pre-freezing, ice must be removed from food through sublimation, which requires careful control of two parameters: temperature and pressure. When food absorbs heat under vacuum, ice crystals will sublimate into water vapor and overflow from the food surface. The sublimation rate depends on the difference between the vapor pressure of the food and the vapor pressure of the ice collector. Molecules will move from samples with high pressure to samples with low pressure. Since the vapor pressure is related to temperature, the temperature of food must also be higher than the temperature of ice collector.

(2) Secondary drying

After the first drying, all the ice in the food sublimates, but there are still some liquids in the food, so it is necessary to carry out the second drying to completely remove the excess water. This process is also called isothermal desorption. The excess water will be desorbed through high temperature.

Characteristics of freeze-dried food

1. High nutrition

Vacuum freeze-drying is a drying process carried out under the conditions of extremely low temperature and high vacuum. When processing, the food is basically in an oxygen-free and completely dark environment, with small thermal denaturation, effectively maintaining the color, aroma, taste and shape of fresh food, and maximizing the preservation of various vitamins, carbohydrates, proteins and other nutrients, chlorophyll, biological enzymes, amino acids and other nutrients and flavor substances in the food. Some research data show that the nutrition of freeze-dried food preserved for 1-2 years can completely replace fresh food.

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2. Palatability

Because the water in the ingredients will separate out in the original place during the freeze-drying process, this avoids the common drying method that the nutrients will be carried to the surface of the ingredients due to the internal water flowing and migrating to the surface, resulting in the loss of nutrients and the hardening of the surface of the ingredients and other undesirable phenomena, with ideal instant solubility and rapid rehydration.

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3. High rehydration

In the freeze-drying process, the solid ice crystals sublimate into water vapor and leave gaps in the ingredients. Therefore, the vacuum freeze-drying food has a dry spongy porous structure, and therefore has an ideal instant solubility and fast and nearly complete rehydration. As long as an appropriate amount of water is added when eating, it can restore to the original fresh taste within a few seconds to minutes.

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4. Durable and convenient to use

The freeze-dried food is completely dehydrated. Generally speaking, the residual moisture of the freeze-dried food is less than 5%, and the residual moisture in the food is evenly distributed. Therefore, under the condition of vacuum packaging, it can be stored in normal temperature for 3-5 years.

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Of course, with the development of freeze-dried technology, its cost will also decline. In addition, freeze-dried food conforms to the three major global food development trends of "green food", "convenience food" and "health food", which will be a new trend in the future food industry.