Preparation process of strawberry dried fruit without addition

Abstract

This invention discloses a process for preparing additive-free dried strawberries, belonging to the field of food processing technology. The process includes weight grading, atmospheric pressure superheated steam sterilization, differentiated gradient temperature control, moisture-proof coating, and nitrogen-filled packaging, enabling additive-free production of dried strawberries while improving product stability and extending shelf life.

Description

Technical Field

[0001] This invention relates to the field of food processing technology, specifically to a process for preparing additive-free dried strawberries. Background Technology

[0002] Strawberries are rich in nutrients and have a sweet and sour taste, making them a favorite among consumers. However, strawberries are soft, have a high water content, and are prone to spoilage after harvesting, resulting in a short shelf life that greatly limits their storage, transportation, and processing applications. Dried strawberries, as an important processed product, effectively reduce the fruit's moisture content, extend their shelf life, and retain their nutrients and flavor, leading to a growing market demand.

[0003] However, traditional methods for preparing dried strawberries, such as hot air drying, generally suffer from the following technical drawbacks: high-temperature processing leads to the degradation of heat-sensitive components in strawberries, volatilization of natural flavor substances, severe browning, and poor rehydration. The finished dried strawberries are highly hygroscopic and prone to rehydration and clumping during storage. To extend shelf life and maintain color, sulfites and synthetic antioxidants are often added.

[0004] Therefore, developing new processes for preparing dried strawberries is of great significance. Summary of the Invention

[0005] The purpose of this invention is to provide a highly stable, additive-free strawberry dried fruit preparation process, which enables additive-free production of strawberry dried fruit, while improving product stability and extending shelf life.

[0006] A process for preparing additive-free dried strawberries includes the following steps: S1: Fresh strawberries with stems removed are divided into small, medium, large and extra-large fruits according to their weight; the small fruit weighs less than 10g, the medium fruit weighs 10~15g, the large fruit weighs more than 15g but not more than 25g, and the extra-large fruit weighs more than 25g. S2: The small fruit, the medium fruit, the large fruit and the extra-large fruit are sterilized by superheated steam at 121~125℃ for 120-125 seconds, and then pre-frozen at -30℃ for 6-8 hours; S3: Place the pre-frozen strawberries into a vacuum freeze dryer that has been pre-cooled for 30 minutes, maintaining a vacuum of 10-20 Pa. Dry the strawberries using a differentiated temperature gradient based on their size until the moisture content is below 2%. The differentiated temperature gradient is as follows: for smaller strawberries, sequentially heat them at -30℃ for 0.5 hours, then increase the temperature to -25℃ at a rate of 5℃ / hour, heat them at -25℃ for 0.5 hours, then increase the temperature to -20℃ at a rate of 5℃ / hour, then heat them at -20℃ for 0.5 hours, and finally increase the temperature to -15℃ at a rate of 5℃ / hour. The process involves several steps: holding at -15℃ for 0.5 hours, increasing the temperature to -10℃ at a rate of 5℃ / h, holding at -10℃ for 0.5 hours, increasing the temperature to -5℃ at a rate of 5℃ / h, holding at -5℃ for 0.5 hours, increasing the temperature to 0℃ at a rate of 5℃ / h, holding at 0℃ for 0.5 hours, increasing the temperature to 5℃ at a rate of 5℃ / h, holding at 5℃ for 0.5 hours, increasing the temperature to 10℃ at a rate of 5℃ / h, holding at 10℃ for 0.5 hours, increasing the temperature to 15℃ at a rate of 10℃ / h, holding at 15℃ for 0.5 hours, and increasing the temperature to 20℃ at a rate of 5℃ / h. Hold at 0℃ for 1 hour, then raise the temperature to 25℃ at a rate of 5℃ / h, hold at 25℃ for 1 hour, raise the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 1 hour, raise the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 1 hour, raise the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 1 hour, raise the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 1 hour, raise the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, and then raise the temperature at a rate of 2.5℃ / h... The temperature is increased to 40℃ and held at 40℃ for 2 hours. Then, the temperature is increased to 42.5℃ at a rate of 2.5℃ / h and held at 42.5℃ for 2 hours. Next, the temperature is increased to 45℃ at a rate of 2.5℃ / h and held at 45℃ for 2 hours. Then, the temperature is increased to 47.5℃ at a rate of 2.5℃ / h and held at 47.5℃ for 2 hours. Finally, the temperature is increased to 50℃ at a rate of 2.5℃ / h and held at 50℃ for 3 hours. Then, the temperature is increased to 52.5℃ at a rate of 2.5℃ / h and held at 52.5℃ for 2 hours. Finally, the temperature is increased to 55℃ at a rate of 2.5℃ / h and held at 55℃ for 1 hour. The process involves sequentially heating the fruit at -30℃ for 0.5 hours, then raising the temperature to -25℃ at a rate of 5℃ / h, holding at -25℃ for 0.5 hours, raising the temperature to -20℃ at a rate of 5℃ / h, holding at -20℃ for 0.5 hours, raising the temperature to -15℃ at a rate of 5℃ / h, holding at -15℃ for 0.5 hours, raising the temperature to -10℃ at a rate of 5℃ / h, holding at -10℃ for 0.5 hours, raising the temperature to -5℃ at a rate of 5℃ / h, holding at -5℃ for 0.5 hours, and then raising the temperature to 0℃ at a rate of 5℃ / h. Hold at ℃ for 0.5h, then increase the temperature to 2.5℃ at a rate of 5℃ / h; hold at 2.5℃ for 0.5h, then increase the temperature to 5℃ at a rate of 5℃ / h; hold at 5℃ for 0.5h, then increase the temperature to 7.5℃ at a rate of 5℃ / h; hold at 7.5℃ for 0.5h, then increase the temperature to 10℃ at a rate of 5℃ / h; hold at 10℃ for 1h, then increase the temperature to 15℃ at a rate of 5℃ / h; hold at 15℃ for 1h, then increase the temperature to 20℃ at a rate of 5℃ / h; hold at 20℃ for 1h, then increase the temperature to 25℃ at a rate of 5℃ / h. ℃, 25℃ for 1 hour, increase temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, increase temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, increase temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, increase temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, increase temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, increase temperature to 40℃ at a rate of 2.5℃ / h, 40℃ Hold at this temperature for 2 hours, then raise the temperature to 42.5℃ at a rate of 2.5℃ / h, hold at 42.5℃ for 2 hours, raise the temperature to 45℃ at a rate of 2.5℃ / h, hold at 45℃ for 2 hours, raise the temperature to 47.5℃ at a rate of 2.5℃ / h, hold at 47.5℃ for 2 hours, raise the temperature to 50℃ at a rate of 2.5℃ / h, hold at 50℃ for 3 hours, raise the temperature to 52.5℃ at a rate of 2.5℃ / h, hold at 52.5℃ for 2 hours, raise the temperature to 55℃ at a rate of 2.5℃ / h, and hold at 55℃ for 1 hour. The large fruit was sequentially heated as follows: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. Hold at 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, hold at 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, hold at 5℃ for 1h, increase temperature to 7.5℃ at a rate of 5℃ / h, hold at 7.5℃ for 1h, increase temperature to 10℃ at a rate of 5℃ / h, hold at 10℃ for 1h, increase temperature to 15℃ at a rate of 5℃ / h, hold at 15℃ for 1h, increase temperature to 20℃ at a rate of 5℃ / h, hold at 20℃ for 1h, and increase temperature to 25℃ at a rate of 5℃ / h. Hold at 25℃ for 1 hour, then increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, then increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, then increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, then increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, then increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, then increase the temperature to 40℃ at a rate of 2.5℃ / h, and hold at 40℃. The process involves: heating at a constant temperature for 2 hours, increasing the temperature to 42.5℃ at a rate of 2.5℃ / h, holding at 42.5℃ for 2 hours, increasing the temperature to 45℃ at a rate of 2.5℃ / h, holding at 45℃ for 3 hours, increasing the temperature to 47.5℃ at a rate of 2.5℃ / h, holding at 47.5℃ for 3 hours, increasing the temperature to 50℃ at a rate of 2.5℃ / h, holding at 50℃ for 3 hours, increasing the temperature to 52.5℃ at a rate of 2.5℃ / h, holding at 52.5℃ for 4 hours, increasing the temperature to 55℃ at a rate of 2.5℃ / h, and holding at 55℃ for 2 hours.

[0007] The extra-large fruit was sequentially heated as follows: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. The process involves: holding at 0℃ for 0.5 hours, increasing the temperature to 2.5℃ at a rate of 5℃ / h, holding at 2.5℃ for 0.5 hours, increasing the temperature to 5℃ at a rate of 5℃ / h, holding at 5℃ for 1 hour, increasing the temperature to 7.5℃ at a rate of 5℃ / h, holding at 7.5℃ for 1 hour, increasing the temperature to 10℃ at a rate of 5℃ / h, holding at 10℃ for 1 hour, increasing the temperature to 15℃ at a rate of 5℃ / h, holding at 15℃ for 1 hour, increasing the temperature to 20℃ at a rate of 5℃ / h, holding at 20℃ for 1 hour, and finally increasing the temperature to 25℃ at a rate of 5℃ / h. The process involves: holding at 25℃ for 1 hour, increasing the temperature to 27.5℃ at a rate of 2.5℃ / h, holding at 27.5℃ for 2 hours, increasing the temperature to 30℃ at a rate of 2.5℃ / h, holding at 30℃ for 2 hours, increasing the temperature to 32.5℃ at a rate of 2.5℃ / h, holding at 32.5℃ for 2 hours, increasing the temperature to 35℃ at a rate of 2.5℃ / h, holding at 35℃ for 2 hours, increasing the temperature to 37.5℃ at a rate of 2.5℃ / h, holding at 37.5℃ for 2 hours, increasing the temperature to 40℃ at a rate of 2.5℃ / h, and finally reaching 40℃. Hold at this temperature for 2 hours, then increase the temperature to 42.5℃ at a rate of 2.5℃ / h, hold at 42.5℃ for 2 hours, increase the temperature to 45℃ at a rate of 2.5℃ / h, hold at 45℃ for 3 hours, increase the temperature to 47.5℃ at a rate of 2.5℃ / h, hold at 47.5℃ for 4 hours, increase the temperature to 50℃ at a rate of 2.5℃ / h, hold at 50℃ for 4 hours, increase the temperature to 52.5℃ at a rate of 2.5℃ / h, hold at 52.5℃ for 4 hours, increase the temperature to 55℃ at a rate of 2.5℃ / h, and hold at 55℃ for 3 hours.

[0008] S4: After drying, the strawberries are coated with a moisture-proof film and then packaged with nitrogen.

[0009] Beneficial effects Compared with the prior art, the beneficial effects of the technical solution provided by the present invention are specifically reflected in the following aspects: (1) Grading strawberries by weight solves the problem of poor uniformity in industrial production. Differentiated gradient temperature changes ensure that strawberries of different sizes can reach the target moisture content, avoid local overheating or insufficient drying, and effectively guarantee the overall quality of dried strawberries.

[0010] (2) Rapid sterilization with atmospheric pressure superheated steam thoroughly inactivates enzymes and kills bacteria while minimizing damage to the fruit pulp tissue, laying the foundation for the preservation of active substances in subsequent freeze-drying. Gradient heating (especially the 0℃ transition section and the relatively rapid heating before 0℃, and the relatively slow heating after 0℃) can smoothly achieve ice crystal sublimation and removal of bound water, avoiding collapse and charring caused by excessively rapid heating, which is the key to obtaining high rehydration rate (>95%) and low color difference (ΔL<3).

[0011] (3) Nitrogen packaging isolates oxygen and moisture from the outside, providing the first physical barrier. The moisture-proof coating packaging and nitrogen packaging form a protective system that takes into account both the inside and outside, which can work together to extend the shelf life at room temperature to more than 18 months.

[0012] (4) No artificial preservatives, pigments or antioxidants are added to the entire process. The preservation is achieved entirely by physical processes and the natural components of strawberries, which effectively improves the safety of dried strawberry products. Detailed Implementation

[0013] An embodiment of the present invention provides a process for preparing additive-free dried strawberries, comprising the following steps: S1: Fresh strawberries, after stem removal, are categorized by weight into small, medium, large, and extra-large. Small strawberries weigh less than 10g, medium strawberries weigh 10-15g, large strawberries weigh more than 15g but not exceeding 25g, and extra-large strawberries weigh more than 25g. Fresh strawberries are defined as those that are 80-90% ripe, undamaged, and free from pests and diseases. Grading strawberries by weight helps address the issue of inconsistent quality in industrial production, ensuring that each dried strawberry has a uniform quality.

[0014] S2: Small, medium, large, and extra-large strawberries are sterilized for 120-125 seconds using superheated steam (e.g., superheated water steam) at 121-125°C under normal pressure, followed by pre-freezing at -30°C for 6-8 hours. Pre-freezing continues until the strawberry's core temperature reaches below the eutectic point (e.g., ≤-20°C). It can be understood that under normal pressure, further heating saturated steam at 100°C (leaving the aqueous phase and preventing the generation of new steam) can raise its temperature to 121-125°C, exceeding the boiling point; this is what is meant by superheated steam at normal pressure. This invention utilizes this superheated steam at normal pressure, leveraging its high-temperature characteristics to achieve instantaneous sterilization in 120-125 seconds, balancing sterilization effectiveness with strawberry nutrient retention (instantaneous high temperature reduces damage to heat-sensitive components). In other words, rapid sterilization with superheated steam at normal pressure thoroughly inactivates enzymes while minimizing damage to the fruit tissue, laying the foundation for subsequent freeze-drying to preserve active substances.

[0015] S3: Place the pre-frozen strawberries into a vacuum freeze dryer that has been pre-cooled for 30 minutes, maintaining a vacuum of 10-20 Pa. Dry the strawberries using a differentiated temperature gradient based on their size until the moisture content is below 2%. The differentiated temperature gradient is as follows: for the smaller strawberries, sequentially heat them at -30℃ for 0.5 hours, then increase the temperature to -25℃ at a rate of 5℃ / h, heat them at -25℃ for 0.5 hours, then increase the temperature to -20℃ at a rate of 5℃ / h, then heat them at -20℃ for 0.5 hours, and finally increase the temperature to -15℃ at a rate of 5℃ / h. Hold at -15℃ for 0.5h, then raise the temperature to -10℃ at a rate of 5℃ / h; hold at -10℃ for 0.5h, then raise the temperature to -5℃ at a rate of 5℃ / h; hold at -5℃ for 0.5h, then raise the temperature to 0℃ at a rate of 5℃ / h; hold at 0℃ for 0.5h, then raise the temperature to 5℃ at a rate of 5℃ / h; hold at 5℃ for 0.5h, then raise the temperature to 10℃ at a rate of 5℃ / h; hold at 10℃ for 0.5h, then raise the temperature to 15℃ at a rate of 10℃ / h; hold at 15℃ for 0.5h, then raise the temperature to 20℃ at a rate of 5℃ / h; 20℃... Hold at ℃ for 1 hour, then increase the temperature to 25℃ at a rate of 5℃ / h, hold at 25℃ for 1 hour, increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 1 hour, increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 1 hour, increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 1 hour, increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 1 hour, increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, and then increase the temperature at a rate of 2.5℃ / h... The temperature is increased to 40℃ and held at 40℃ for 2 hours. The temperature is increased to 42.5℃ at a rate of 2.5℃ / h and held at 42.5℃ for 2 hours. The temperature is increased to 45℃ at a rate of 2.5℃ / h and held at 45℃ for 2 hours. The temperature is increased to 47.5℃ at a rate of 2.5℃ / h and held at 47.5℃ for 2 hours. The temperature is increased to 50℃ at a rate of 2.5℃ / h and held at 50℃ for 3 hours. The temperature is increased to 52.5℃ at a rate of 2.5℃ / h and held at 52.5℃ for 2 hours. The temperature is increased to 55℃ at a rate of 2.5℃ / h and held at 55℃ for 1 hour.The process involves sequentially heating the fruit at -30℃ for 0.5 hours, then raising the temperature to -25℃ at a rate of 5℃ / h, holding at -25℃ for 0.5 hours, raising the temperature to -20℃ at a rate of 5℃ / h, holding at -20℃ for 0.5 hours, raising the temperature to -15℃ at a rate of 5℃ / h, holding at -15℃ for 0.5 hours, raising the temperature to -10℃ at a rate of 5℃ / h, holding at -10℃ for 0.5 hours, raising the temperature to -5℃ at a rate of 5℃ / h, holding at -5℃ for 0.5 hours, and then raising the temperature to 0℃ at a rate of 5℃ / h. Hold at ℃ for 0.5h, then increase the temperature to 2.5℃ at a rate of 5℃ / h; hold at 2.5℃ for 0.5h, then increase the temperature to 5℃ at a rate of 5℃ / h; hold at 5℃ for 0.5h, then increase the temperature to 7.5℃ at a rate of 5℃ / h; hold at 7.5℃ for 0.5h, then increase the temperature to 10℃ at a rate of 5℃ / h; hold at 10℃ for 1h, then increase the temperature to 15℃ at a rate of 5℃ / h; hold at 15℃ for 1h, then increase the temperature to 20℃ at a rate of 5℃ / h; hold at 20℃ for 1h, then increase the temperature to 25℃ at a rate of 5℃ / h. ℃, 25℃ for 1 hour, increase temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, increase temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, increase temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, increase temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, increase temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, increase temperature to 40℃ at a rate of 2.5℃ / h, 40℃ Hold at this temperature for 2 hours, then increase the temperature to 42.5℃ at a rate of 2.5℃ / h, hold at 42.5℃ for 2 hours, increase the temperature to 45℃ at a rate of 2.5℃ / h, hold at 45℃ for 2 hours, increase the temperature to 47.5℃ at a rate of 2.5℃ / h, hold at 47.5℃ for 2 hours, increase the temperature to 50℃ at a rate of 2.5℃ / h, hold at 50℃ for 3 hours, increase the temperature to 52.5℃ at a rate of 2.5℃ / h, hold at 52.5℃ for 2 hours, increase the temperature to 55℃ at a rate of 2.5℃ / h, and hold at 55℃ for 1 hour.The large fruit was sequentially heated as follows: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. Hold at 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, hold at 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, hold at 5℃ for 1h, increase temperature to 7.5℃ at a rate of 5℃ / h, hold at 7.5℃ for 1h, increase temperature to 10℃ at a rate of 5℃ / h, hold at 10℃ for 1h, increase temperature to 15℃ at a rate of 5℃ / h, hold at 15℃ for 1h, increase temperature to 20℃ at a rate of 5℃ / h, hold at 20℃ for 1h, and increase temperature to 25℃ at a rate of 5℃ / h. Hold at 25℃ for 1 hour, then increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, then increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, then increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, then increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, then increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, then increase the temperature to 40℃ at a rate of 2.5℃ / h, and hold at 40℃. The process involves: heating at a constant temperature for 2 hours, increasing the temperature to 42.5℃ at a rate of 2.5℃ / h, holding at 42.5℃ for 2 hours, increasing the temperature to 45℃ at a rate of 2.5℃ / h, holding at 45℃ for 3 hours, increasing the temperature to 47.5℃ at a rate of 2.5℃ / h, holding at 47.5℃ for 3 hours, increasing the temperature to 50℃ at a rate of 2.5℃ / h, holding at 50℃ for 3 hours, increasing the temperature to 52.5℃ at a rate of 2.5℃ / h, holding at 52.5℃ for 4 hours, increasing the temperature to 55℃ at a rate of 2.5℃ / h, and holding at 55℃ for 2 hours.Extra-large fruits were subjected to the following temperature treatments in sequence: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. Hold at 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, hold at 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, hold at 5℃ for 1h, increase temperature to 7.5℃ at a rate of 5℃ / h, hold at 7.5℃ for 1h, increase temperature to 10℃ at a rate of 5℃ / h, hold at 10℃ for 1h, increase temperature to 15℃ at a rate of 5℃ / h, hold at 15℃ for 1h, increase temperature to 20℃ at a rate of 5℃ / h, hold at 20℃ for 1h, and increase temperature to 25℃ at a rate of 5℃ / h. Hold at 25℃ for 1 hour, then increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, then increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, then increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, then increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, then increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, then increase the temperature to 40℃ at a rate of 2.5℃ / h, and hold at 40℃. The process involves several steps: heating at a constant temperature for 2 hours, increasing to 42.5℃ at a rate of 2.5℃ / h, holding at 42.5℃ for 2 hours, increasing to 45℃ at a rate of 2.5℃ / h, holding at 45℃ for 3 hours, increasing to 47.5℃ at a rate of 2.5℃ / h, holding at 47.5℃ for 4 hours, increasing to 50℃ at a rate of 2.5℃ / h, holding at 50℃ for 4 hours, increasing to 52.5℃ at a rate of 2.5℃ / h, holding at 52.5℃ for 4 hours, increasing to 55℃ at a rate of 2.5℃ / h, and holding at 55℃ for 3 hours. This gradient heating (especially the rapid heating before 0℃ and the slower heating after 0℃) ensures smooth sublimation of ice crystals and removal of bound water, avoiding collapse and charring caused by excessively rapid heating. This is crucial for achieving a high rehydration rate (>95%) and low color difference (ΔL<3). In addition, the differentiated gradient temperature change controls the moisture content to below 2%, reducing the internal moisture of the product and providing a guarantee for subsequent preservation packaging. The low moisture content can reduce the pressure on the anti-moisture coating and at the same time reduce the occurrence of oxidation reactions in nitrogen packaging.

[0016] S4: After drying, the strawberries are coated with an anti-moisture coating and then packaged with nitrogen. The anti-moisture coating and nitrogen packaging form a comprehensive protection system that can extend the shelf life at room temperature to more than 18 months.

[0017] In some embodiments, the preparation process of dried strawberries without additives is as described in S1 to S4 above, without the need for additional artificial preservatives, pigments, antioxidants, etc.

[0018] In some embodiments, the pre-freezing time in S2 can be 6h, 6.5h, 7h, 7.5h, 8h, etc.

[0019] In some embodiments, the preparation of the coating solution used for the anti-moisture coating includes: S01: Preparation of zein solution: Zein is dissolved in an 85% (v / v) ethanol solution and magnetically stirred at room temperature for 45-60 min to ensure complete dissolution, resulting in a homogeneous zein solution. S02: Preparation of beeswax solution: Edible beeswax is dispersed in anhydrous ethanol at a mass-to-volume ratio of 1:5, heated and stirred in an 85°C water bath until the beeswax is completely dissolved, resulting in a clear beeswax solution. S03: Preparation of composite coating solution: The zein solution is kept at a temperature of 45-65°C in a water bath, and the beeswax solution is added in small amounts several times while stirring; then, stirring is continued at 1500 r / min for 40 min to promote uniform mixing of the two phases; finally, the solution is filtered through a 40-mesh filter to remove insoluble impurities, resulting in a homogeneous and stable coating solution.

[0020] It is understandable that the ethanol used in the preparation process can be effectively removed during the preparation process, thus meeting food requirements.

[0021] In some embodiments, the moisture-proof coating packaging includes: using a dryer, at an environment of 65°C, evenly spraying the coating liquid onto the dried strawberry surface, respraying every 2 minutes, and turning the fruit several times to ensure that the coating liquid forms a complete and uniform covering layer on the fruit surface, avoiding localized missed areas or uneven coating; immediately placing the fruit in a hot air environment to dry after spraying. Optionally, the number of resprays can be 1, 2, 3, 4, 5, etc. The hot air environment is 65°C.

[0022] In some embodiments, nitrogen-filled packaging includes: placing strawberries coated with a moisture-proof film inside a packaging bag, vacuuming the bag, filling it with nitrogen, and sealing it.

[0023] In some embodiments, the water vapor transmission rate of the packaging bag is <1 g / m 2 • day, oxygen permeability <5cm 3 / m 2 ·day.

[0024] The following are examples and comparative examples.

[0025] Example 1 The preparation process of dried strawberries includes: S1: Grade the fresh strawberries with stems removed according to the weight of each fruit: small (<10g), medium (10~15g), large (greater than 15g but not more than 25g), and extra-large (>25g). After grading, wash each fruit twice with deionized water and drain off the surface moisture.

[0026] S2: Sterilize small, medium, large, and extra-large fruits with superheated steam at 123℃ for 120 seconds; after sterilization, transfer them to a -35℃ freezer for 6 hours.

[0027] S3: Pre-cool the vacuum freeze dryer for 30 minutes, and put the pre-frozen small, medium, large and extra-large fruits into the dryer respectively, maintaining a vacuum of 15Pa throughout the process. The small fruit was heated sequentially as follows: -30℃ for 0.5h, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5h, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5h, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5h, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5h, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5h, then heated to 0℃ at a rate of 5℃ / h; 0℃ for 0.5h, then heated to 5℃ at a rate of 5℃ / h; 5℃ for 0.5h, then heated to 10℃ at a rate of 5℃ / h, and finally held at 10℃. 0.5h, heating to 15℃ at a rate of 10℃ / h, holding at 15℃ for 0.5h, heating to 20℃ at a rate of 5℃ / h, holding at 20℃ for 1h, heating to 25℃ at a rate of 5℃ / h, holding at 25℃ for 1h, heating to 27.5℃ at a rate of 2.5℃ / h, holding at 27.5℃ for 1h, heating to 30℃ at a rate of 2.5℃ / h, holding at 30℃ for 1h, heating to 32.5℃ at a rate of 2.5℃ / h, holding at 32.5℃ for 1h, heating to 35℃ at a rate of 2.5℃ / h, holding at 35℃ for 1h, heating to 37.5℃ at a rate of 2.5℃ / h, holding at 37.5℃ for 2h, and so on. The temperature was increased to 40℃ at a rate of 0.5℃ / h, held at 40℃ for 2 hours, increased to 42.5℃ at a rate of 2.5℃ / h, held at 42.5℃ for 2 hours, increased to 45℃ at a rate of 2.5℃ / h, held at 45℃ for 2 hours, increased to 47.5℃ at a rate of 2.5℃ / h, held at 47.5℃ for 2 hours, increased to 50℃ at a rate of 2.5℃ / h, held at 50℃ for 3 hours, increased to 52.5℃ at a rate of 2.5℃ / h, held at 52.5℃ for 2 hours, increased to 55℃ at a rate of 2.5℃ / h, and held at 55℃ for 1 hour. For the intermediate product, the temperature was successively held at -30℃ for 0.5 hours, then increased to -25℃ at a rate of 5℃ / h. ℃, -25℃ for 0.5h, increase temperature to -20℃ at a rate of 5℃ / h, -20℃ for 0.5h, increase temperature to -15℃ at a rate of 5℃ / h, -15℃ for 0.5h, increase temperature to -10℃ at a rate of 5℃ / h, -10℃ for 0.5h, increase temperature to -5℃ at a rate of 5℃ / h, -5℃ for 0.5h, increase temperature to 0℃ at a rate of 5℃ / h, 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, 5℃ for 0.5h, increase temperature to 7.5℃ at a rate of 5℃ / h, 7.5℃ for 0.5h.The process involves: heating at a rate of 5℃ / h to 10℃, holding at 10℃ for 1 hour, heating at a rate of 5℃ / h to 15℃, holding at 15℃ for 1 hour, heating at a rate of 5℃ / h to 20℃, holding at 20℃ for 1 hour, heating at a rate of 5℃ / h to 25℃, holding at 25℃ for 1 hour, heating at a rate of 2.5℃ / h to 27.5℃, holding at 27.5℃ for 2 hours, heating at a rate of 2.5℃ / h to 30℃, holding at 30℃ for 2 hours, heating at a rate of 2.5℃ / h to 32.5℃, holding at 32.5℃ for 2 hours, heating at a rate of 2.5℃ / h to 35℃, holding at 35℃ for 2 hours, heating at a rate of 2.5℃ / h to 37.5℃, holding at 37.5℃ for 2 hours, and finally heating at a rate of 2.5℃ / h to 40℃. The process involves several steps: holding at 40℃ for 2 hours, increasing the temperature to 42.5℃ at a rate of 2.5℃ / h, holding at 42.5℃ for 2 hours, increasing the temperature to 45℃ at a rate of 2.5℃ / h, holding at 45℃ for 2 hours, increasing the temperature to 47.5℃ at a rate of 2.5℃ / h, holding at 47.5℃ for 2 hours, increasing the temperature to 50℃ at a rate of 2.5℃ / h, holding at 50℃ for 3 hours, increasing the temperature to 52.5℃ at a rate of 2.5℃ / h, holding at 52.5℃ for 2 hours, increasing the temperature to 55℃ at a rate of 2.5℃ / h, and holding at 55℃ for 1 hour. For larger fruits, the process involves holding at -30℃ for 0.5 hours, increasing the temperature to -25℃ at a rate of 5℃ / h, holding at -25℃ for 0.5 hours, increasing the temperature to -20℃ at a rate of 5℃ / h, holding at -20℃ for 0.5 hours, and then increasing the temperature to -20℃ at a rate of 5℃ / h. The temperature is increased at a rate of 5℃ / h to -15℃, held at -15℃ for 0.5h, increased to -10℃ at a rate of 5℃ / h, held at -10℃ for 0.5h, increased to -5℃ at a rate of 5℃ / h, held at -5℃ for 0.5h, increased to 0℃ at a rate of 5℃ / h, held at 0℃ for 0.5h, increased to 2.5℃ at a rate of 5℃ / h, held at 2.5℃ for 0.5h, increased to 5℃ at a rate of 5℃ / h, held at 5℃ for 1h, increased to 7.5℃ at a rate of 5℃ / h, held at 7.5℃ for 1h, increased to 10℃ at a rate of 5℃ / h, held at 10℃ for 1h, increased to 15℃ at a rate of 5℃ / h, held at 15℃ for 1h, increased to 20℃ at a rate of 5℃ / h, held at 20℃ for 1h, and increased to 2... The process involves heating at 5℃ and 25℃ for 1 hour, then increasing the temperature at a rate of 2.5℃ / h to 27.5℃, holding at 27.5℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 30℃, holding at 30℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 32.5℃, holding at 32.5℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 35℃, holding at 35℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 37.5℃, holding at 37.5℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 40℃, holding at 40℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 42.5℃, holding at 42.5℃ for 2 hours, increasing the temperature at a rate of 2.5℃ / h to 45℃, holding at 45℃ for 3 hours, and finally increasing the temperature at a rate of 2.5℃ / h to 47.5℃.For extra-large fruits, the temperature was successively increased as follows: 5℃ for 3 hours, then increased to 50℃ at a rate of 2.5℃ / h, held at 50℃ for 3 hours, increased to 52.5℃ at a rate of 2.5℃ / h, held at 52.5℃ for 4 hours, increased to 55℃ at a rate of 2.5℃ / h, and held at 55℃ for 2 hours. For extra-large fruits, the temperature was successively increased as follows: -30℃ for 0.5 hours, increased to -25℃ at a rate of 5℃ / h, held at -25℃ for 0.5 hours, increased to -20℃ at a rate of 5℃ / h, held at -20℃ for 0.5 hours, increased to -15℃ at a rate of 5℃ / h, held at -15℃ for 0.5 hours, increased to -10℃ at a rate of 5℃ / h, held at -10℃ for 0.5 hours, increased to -5℃ at a rate of 5℃ / h, held at -5℃ for 0.5 hours, and increased to 0℃ at a rate of 5℃ / h. Hold at 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, hold at 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, hold at 5℃ for 1h, increase temperature to 7.5℃ at a rate of 5℃ / h, hold at 7.5℃ for 1h, increase temperature to 10℃ at a rate of 5℃ / h, hold at 10℃ for 1h, increase temperature to 15℃ at a rate of 5℃ / h, hold at 15℃ for 1h, increase temperature to 20℃ at a rate of 5℃ / h, hold at 20℃ for 1h, and increase temperature to 25℃ at a rate of 5℃ / h. Hold at 25℃ for 1 hour, then increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, then increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, then increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, then increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, then increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, then increase the temperature to 40℃ at a rate of 2.5℃ / h, and hold at 40℃. The process involves: maintaining a constant temperature for 2 hours; increasing the temperature at a rate of 2.5℃ / h to 42.5℃; holding at 42.5℃ for 2 hours; increasing the temperature at a rate of 2.5℃ / h to 45℃; holding at 45℃ for 3 hours; increasing the temperature at a rate of 2.5℃ / h to 47.5℃; holding at 47.5℃ for 4 hours; increasing the temperature at a rate of 2.5℃ / h to 50℃; holding at 50℃ for 4 hours; increasing the temperature at a rate of 2.5℃ / h to 52.5℃; holding at 52.5℃ for 4 hours; and increasing the temperature at a rate of 2.5℃ / h to 55℃; holding at 55℃ for 3 hours. The strawberries are dried until the moisture content is 1.8%.

[0028] S4: After drying, the strawberries are sequentially treated with a moisture-proof coating and then packaged with nitrogen. Preparation of coating solutions: S01: Preparation of zein solution: Dissolve zein in an 85% (v / v) ethanol solution and stir magnetically for 50 min at room temperature to ensure complete dissolution, obtaining a homogeneous zein solution. S02: Preparation of beeswax solution: Disperse edible beeswax in anhydrous ethanol at a mass-to-volume ratio of 1:5, heat and stir in an 85℃ water bath until the beeswax is completely dissolved, obtaining a clear beeswax solution. S03: Preparation of composite coating solution: Keep the zein solution in a 50℃ water bath, and add the beeswax solution in small amounts several times while stirring; then continue stirring at 1500 r / min for 40 min to promote uniform mixing of the two phases; finally, filter through a 40-mesh filter to remove insoluble impurities, obtaining a homogeneous and stable coating solution.

[0029] Using a dryer, spray the coating liquid evenly onto the dried strawberry surface at 65℃. Re-spray every 2 minutes (re-spray multiple times) and turn the fruit over several times to ensure that the coating liquid forms a complete and uniform covering layer on the fruit surface, avoiding local missed areas or uneven coating. Immediately after spraying, place the fruit in a hot air environment (65℃) to dry.

[0030] Nitrogen-filled packaging involves placing strawberries coated with a moisture-proof film inside a packaging bag, vacuum-sealing the bag, filling it with nitrogen, and then sealing it. The water vapor transmission rate of the packaging bag is <1g / m³. 2 • day, oxygen permeability <5cm 3 / m 2 • day. An externally vacuum-type nitrogen-filled packaging machine is used. First, a vacuum is drawn to -0.09MPa, then 99.9% pure food-grade nitrogen is filled to atmospheric pressure and sealed.

[0031] Comparative Example 1 The traditional hot air drying process was adopted: fresh strawberries, the same as in Example 1, were selected, the stems were removed, washed and drained, and they were not graded; they were dried with hot air at 80℃ for 8 hours until the moisture content was 1.8%; after cooling, they were coated with an anti-moisture coating and packaged with nitrogen. The difference between the anti-moisture coating and that in Example 1 is that 0.1% potassium sorbate (preservative) and 0.05% vitamin C (antioxidant) were added to the coating solution by weight of the total mixture.

[0032] Comparative Example 2 The difference from Example 1 is that: in S1, no weight is measured, and all strawberries are mixed together; in S3, a uniform freeze-drying procedure is used, and the drying procedure is the same as the medium-sized fruit drying procedure in Example 1. The remaining steps are the same as in Example 1.

[0033] Comparative Example 3 The difference between Example 1 and Example 2 is that S2 does not involve atmospheric pressure superheated steam sterilization, but only pre-freezing at -30°C for 6 hours. The remaining steps are the same as in Example 1.

[0034] Comparative Example 4 The difference between Example 1 and Example 2 is that S4 only involves nitrogen packaging and does not involve moisture-proof coating treatment. The remaining steps are the same as in Example 1.

[0035] Test case (1) Rehydration rate test: Weigh 2g of dried strawberries, put them into 50ml of 25℃ deionized water, soak for 30min, take them out and drain the surface water, weigh them after rehydration, and calculate the rehydration rate (rehydration rate = weight after rehydration / weight before drying × 100%).

[0036] (2) Color difference ΔL test: The L value of fresh strawberries and dried strawberries is measured by colorimeter, and the absolute value of the difference between the two is calculated. The smaller the ΔL, the closer the color is to that of fresh strawberries.

[0037] (3) Vitamin C content test: The vitamin C content (mg / 100g) in dried strawberries was determined by high performance liquid chromatography.

[0038] (4) Room temperature storage period test: Place all samples in a room temperature environment of 25℃ and 60% relative humidity, seal and store, observe the color, odor and shape of the samples regularly, and record the time when the samples show moisture, browning, odor and mold, which is the storage period; if the above phenomena do not appear within 18 months, the storage period is determined to be ≥18 months.

[0039] (5) Moisture regain test: After storing the sample at room temperature for 6 months, weigh 5g of the sample and determine its moisture content. Compare it with the moisture content before storage and calculate the moisture regain (moisture regain = (moisture content after storage - moisture content before storage) / moisture content before storage × 100%).

[0040] The test results are shown in the table below.

[0041]

[0042] As can be seen from the table above, the preparation process of Example 1 can effectively achieve additive-free production, the product rehydration rate is higher than 95%, the color difference ΔL is less than 3, the vitamin C content is high, the moisture regain rate is less than 0.5% after 6 months of storage, and the storage period at room temperature is ≥18 months.

[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0044] For those skilled in the art, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A process for preparing additive-free dried strawberries, characterized in that, Includes the following steps: S1: Fresh strawberries with stems removed are divided into small, medium, large and extra-large fruits according to their weight; the small fruit weighs less than 10g, the medium fruit weighs 10~15g, the large fruit weighs more than 15g but not more than 25g, and the extra-large fruit weighs more than 25g. S2: The small fruit, the medium fruit, the large fruit and the extra-large fruit are sterilized by superheated steam at 121~125℃ for 120-125 seconds, and then pre-frozen at -30℃ for 6~8 hours; S3: Place the pre-frozen strawberries into a vacuum freeze dryer that has been pre-cooled for 30 minutes, maintaining a vacuum of 10-20 Pa. Dry the strawberries using a differentiated temperature gradient based on their size until the moisture content is below 2%. The differentiated temperature gradient is as follows: for smaller strawberries, sequentially heat them at -30℃ for 0.5 hours, then increase the temperature to -25℃ at a rate of 5℃ / hour, heat them at -25℃ for 0.5 hours, then increase the temperature to -20℃ at a rate of 5℃ / hour, then heat them at -20℃ for 0.5 hours, and finally increase the temperature to -15℃ at a rate of 5℃ / hour. The process involves several steps: holding at -15℃ for 0.5 hours, increasing the temperature to -10℃ at a rate of 5℃ / h, holding at -10℃ for 0.5 hours, increasing the temperature to -5℃ at a rate of 5℃ / h, holding at -5℃ for 0.5 hours, increasing the temperature to 0℃ at a rate of 5℃ / h, holding at 0℃ for 0.5 hours, increasing the temperature to 5℃ at a rate of 5℃ / h, holding at 5℃ for 0.5 hours, increasing the temperature to 10℃ at a rate of 5℃ / h, holding at 10℃ for 0.5 hours, increasing the temperature to 15℃ at a rate of 10℃ / h, holding at 15℃ for 0.5 hours, and increasing the temperature to 20℃ at a rate of 5℃ / h. Hold at 0℃ for 1 hour, then raise the temperature to 25℃ at a rate of 5℃ / h, hold at 25℃ for 1 hour, raise the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 1 hour, raise the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 1 hour, raise the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 1 hour, raise the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 1 hour, raise the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, and then raise the temperature at a rate of 2.5℃ / h... The temperature is increased to 40℃ and held at 40℃ for 2 hours. Then, the temperature is increased to 42.5℃ at a rate of 2.5℃ / h and held at 42.5℃ for 2 hours. Next, the temperature is increased to 45℃ at a rate of 2.5℃ / h and held at 45℃ for 2 hours. Then, the temperature is increased to 47.5℃ at a rate of 2.5℃ / h and held at 47.5℃ for 2 hours. Finally, the temperature is increased to 50℃ at a rate of 2.5℃ / h and held at 50℃ for 3 hours. Then, the temperature is increased to 52.5℃ at a rate of 2.5℃ / h and held at 52.5℃ for 2 hours. Finally, the temperature is increased to 55℃ at a rate of 2.5℃ / h and held at 55℃ for 1 hour. The process involves sequentially heating the fruit at -30℃ for 0.5 hours, then raising the temperature to -25℃ at a rate of 5℃ / h, holding at -25℃ for 0.5 hours, raising the temperature to -20℃ at a rate of 5℃ / h, holding at -20℃ for 0.5 hours, raising the temperature to -15℃ at a rate of 5℃ / h, holding at -15℃ for 0.5 hours, raising the temperature to -10℃ at a rate of 5℃ / h, holding at -10℃ for 0.5 hours, raising the temperature to -5℃ at a rate of 5℃ / h, holding at -5℃ for 0.5 hours, and then raising the temperature to 0℃ at a rate of 5℃ / h. Hold at ℃ for 0.5h, then increase the temperature to 2.5℃ at a rate of 5℃ / h; hold at 2.5℃ for 0.5h, then increase the temperature to 5℃ at a rate of 5℃ / h; hold at 5℃ for 0.5h, then increase the temperature to 7.5℃ at a rate of 5℃ / h; hold at 7.5℃ for 0.5h, then increase the temperature to 10℃ at a rate of 5℃ / h; hold at 10℃ for 1h, then increase the temperature to 15℃ at a rate of 5℃ / h; hold at 15℃ for 1h, then increase the temperature to 20℃ at a rate of 5℃ / h; hold at 20℃ for 1h, then increase the temperature to 25℃ at a rate of 5℃ / h. ℃, 25℃ for 1 hour, increase temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, increase temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, increase temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, increase temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, increase temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, increase temperature to 40℃ at a rate of 2.5℃ / h, 40℃ Hold at this temperature for 2 hours, then raise the temperature to 42.5℃ at a rate of 2.5℃ / h, hold at 42.5℃ for 2 hours, raise the temperature to 45℃ at a rate of 2.5℃ / h, hold at 45℃ for 2 hours, raise the temperature to 47.5℃ at a rate of 2.5℃ / h, hold at 47.5℃ for 2 hours, raise the temperature to 50℃ at a rate of 2.5℃ / h, hold at 50℃ for 3 hours, raise the temperature to 52.5℃ at a rate of 2.5℃ / h, hold at 52.5℃ for 2 hours, raise the temperature to 55℃ at a rate of 2.5℃ / h, and hold at 55℃ for 1 hour. The large fruit was sequentially heated as follows: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. Hold at 0℃ for 0.5h, increase temperature to 2.5℃ at a rate of 5℃ / h, hold at 2.5℃ for 0.5h, increase temperature to 5℃ at a rate of 5℃ / h, hold at 5℃ for 1h, increase temperature to 7.5℃ at a rate of 5℃ / h, hold at 7.5℃ for 1h, increase temperature to 10℃ at a rate of 5℃ / h, hold at 10℃ for 1h, increase temperature to 15℃ at a rate of 5℃ / h, hold at 15℃ for 1h, increase temperature to 20℃ at a rate of 5℃ / h, hold at 20℃ for 1h, and increase temperature to 25℃ at a rate of 5℃ / h. Hold at 25℃ for 1 hour, then increase the temperature to 27.5℃ at a rate of 2.5℃ / h, hold at 27.5℃ for 2 hours, then increase the temperature to 30℃ at a rate of 2.5℃ / h, hold at 30℃ for 2 hours, then increase the temperature to 32.5℃ at a rate of 2.5℃ / h, hold at 32.5℃ for 2 hours, then increase the temperature to 35℃ at a rate of 2.5℃ / h, hold at 35℃ for 2 hours, then increase the temperature to 37.5℃ at a rate of 2.5℃ / h, hold at 37.5℃ for 2 hours, then increase the temperature to 40℃ at a rate of 2.5℃ / h, and hold at 40℃. The process involves: heating at a constant temperature for 2 hours, increasing the temperature to 42.5℃ at a rate of 2.5℃ / h, holding at 42.5℃ for 2 hours, increasing the temperature to 45℃ at a rate of 2.5℃ / h, holding at 45℃ for 3 hours, increasing the temperature to 47.5℃ at a rate of 2.5℃ / h, holding at 47.5℃ for 3 hours, increasing the temperature to 50℃ at a rate of 2.5℃ / h, holding at 50℃ for 3 hours, increasing the temperature to 52.5℃ at a rate of 2.5℃ / h, holding at 52.5℃ for 4 hours, increasing the temperature to 55℃ at a rate of 2.5℃ / h, and holding at 55℃ for 2 hours. The extra-large fruit was sequentially heated as follows: -30℃ for 0.5 hours, then heated to -25℃ at a rate of 5℃ / h; -25℃ for 0.5 hours, then heated to -20℃ at a rate of 5℃ / h; -20℃ for 0.5 hours, then heated to -15℃ at a rate of 5℃ / h; -15℃ for 0.5 hours, then heated to -10℃ at a rate of 5℃ / h; -10℃ for 0.5 hours, then heated to -5℃ at a rate of 5℃ / h; -5℃ for 0.5 hours, then heated to 0℃ at a rate of 5℃ / h. The process involves: holding at 0℃ for 0.5 hours, increasing the temperature to 2.5℃ at a rate of 5℃ / h, holding at 2.5℃ for 0.5 hours, increasing the temperature to 5℃ at a rate of 5℃ / h, holding at 5℃ for 1 hour, increasing the temperature to 7.5℃ at a rate of 5℃ / h, holding at 7.5℃ for 1 hour, increasing the temperature to 10℃ at a rate of 5℃ / h, holding at 10℃ for 1 hour, increasing the temperature to 15℃ at a rate of 5℃ / h, holding at 15℃ for 1 hour, increasing the temperature to 20℃ at a rate of 5℃ / h, holding at 20℃ for 1 hour, and finally increasing the temperature to 25℃ at a rate of 5℃ / h. The process involves: holding at 25℃ for 1 hour, increasing the temperature to 27.5℃ at a rate of 2.5℃ / h, holding at 27.5℃ for 2 hours, increasing the temperature to 30℃ at a rate of 2.5℃ / h, holding at 30℃ for 2 hours, increasing the temperature to 32.5℃ at a rate of 2.5℃ / h, holding at 32.5℃ for 2 hours, increasing the temperature to 35℃ at a rate of 2.5℃ / h, holding at 35℃ for 2 hours, increasing the temperature to 37.5℃ at a rate of 2.5℃ / h, holding at 37.5℃ for 2 hours, increasing the temperature to 40℃ at a rate of 2.5℃ / h, and finally reaching 40℃. Hold at this temperature for 2 hours, then increase the temperature to 42.5℃ at a rate of 2.5℃ / h, hold at 42.5℃ for 2 hours, increase the temperature to 45℃ at a rate of 2.5℃ / h, hold at 45℃ for 3 hours, increase the temperature to 47.5℃ at a rate of 2.5℃ / h, hold at 47.5℃ for 4 hours, increase the temperature to 50℃ at a rate of 2.5℃ / h, hold at 50℃ for 4 hours, increase the temperature to 52.5℃ at a rate of 2.5℃ / h, hold at 52.5℃ for 4 hours, increase the temperature to 55℃ at a rate of 2.5℃ / h, and hold at 55℃ for 3 hours. S4: After drying, the strawberries are coated with a moisture-proof film and then packaged with nitrogen.

2. The additive-free dried strawberry preparation process as described in claim 1, characterized in that, The preparation of the coating liquid used in the anti-moisture coating includes: S01: Preparation of zein solution: Dissolve zein in an 85% ethanol solution and stir magnetically at room temperature for 45-60 minutes to ensure complete dissolution and obtain a homogeneous zein solution. S02: Preparation of beeswax solution: Disperse edible beeswax in anhydrous ethanol at a mass-to-volume ratio of 1:5, heat and stir in an 85°C water bath until the beeswax is completely dissolved to obtain a clear beeswax solution. S03: Preparation of composite coating solution: Place the corn glycerin solution in a water bath at 45~65℃ and keep it warm. Add beeswax solution in small amounts several times while stirring. Then stir continuously at 1500r / min for 40min to promote uniform mixing of the two phases. Finally, filter through a 40-mesh filter to remove insoluble impurities and obtain a uniform and stable coating solution.

3. The additive-free dried strawberry preparation process as described in claim 2, characterized in that, The moisture-proof coating packaging includes: using a dryer, spraying the coating liquid evenly onto the dried strawberry surface at 65°C, respraying every 2 minutes, and turning the fruit several times to ensure that the coating liquid forms a complete and uniform covering layer on the fruit surface, avoiding local missed coating or uneven coating, and immediately placing it in a hot air environment to dry after spraying.

4. The additive-free dried strawberry preparation process according to any one of claims 1 to 3, characterized in that, The nitrogen-filled packaging includes: placing the strawberries coated with the moisture-proof film into a packaging bag, vacuuming the bag, filling it with nitrogen, and then sealing it.

5. The additive-free dried strawberry preparation process as described in claim 4, characterized in that, The water vapor transmission rate of the packaging bag is <1g / m 2 • day, oxygen permeability <5cm 3 / m 2 ·day.

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