Vacuum Precooling Fresh-Keeping Technology for Cut Flowers and Seed Storage Methods
Vacuum precooling fresh-keeping of cut flowers after harvest. Harvested flowers should undergo rapid cooling treatment immediately, namely precooling. There are many precooling fresh-keeping methods for cut flowers, and one of the most effective is vacuum precooling fresh-keeping. Vacuum precooling fresh-keeping is based on the property that water evaporation can be accelerated under vacuum conditions. The flowers are placed in a vacuum precooling chamber for vacuum treatment, part of the water in the flowers transpires outward, and the latent heat is released outside with the water vapor, thereby rapidly lowering the temperature of the flowers. Vacuum precooling is not only fast and relatively uniform in cooling, but also, because the air pressure is reduced, the oxygen concentration in the flower tissues is lowered and the concentration of released ethylene is also reduced, which can effectively maintain flower freshness and extend the storage period.
Process parameters of vacuum precooling fresh-keeping for cut flowers. Vacuum precooling fresh-keeping of cut flowers mainly controls temperature and humidity by controlling the degree of vacuum during the precooling process. The required level of vacuum is related to the physiological characteristics of the flowers in the precooling chamber, the sensitive quantity of the flowers, the humidity of the flowers themselves, and the performance of the vacuum equipment.
Problems and countermeasures in vacuum precooling fresh-keeping of cut flowers. The vacuum precooling fresh-keeping process can cause water loss in flowers. For approximately every 10°C drop in temperature, about 1% of water is lost. For example, if the temperature drops from 30°C to 5°C, about 3% of water is lost. If the water loss of flowers is severe, the surface stomata open, which leads to reduced freshness and affects flower quality. Therefore, suitable humidity for the flowers must be strictly controlled. In the United States, a spray vacuum precooling device has already been applied, which effectively avoids dehydration of flowers during the precooling process while also enabling rapid cooling. At present, the author is conducting vacuum precooling treatment. Water absorption refers to allowing the flowers to absorb water for a period before precooling, then placing them together with the container into the vacuum chamber for vacuum precooling. This method is mainly suitable for flowers that require a relatively low final precooling temperature (such as 1°C to 2°C). In addition, for flowers that have many flower buds after harvest, about 1% silver nitrate is added to the water in the container as a preservative. Because the container occupies a certain amount of space in the vacuum chamber, water absorption treatment affects production efficiency. Spraying treatment refers to using a humidifier to uniformly spray and humidify the flowers before precooling, so that the flowers have sufficient moisture before precooling. Practice has proved that spraying treatment is simple and applicable.
Vacuum precooling has notable features such as fast cooling time (generally only 20 to 30 minutes), high efficiency, uniform cooling, and no influence from packaging. It is especially suitable for precooling and fresh-keeping of items with a large surface area to volume ratio such as cut flowers. Vacuum precooling fresh-keeping of cut flowers still needs continued exploration and experimentation. With the continuous improvement of precooling processes and further improvement in the performance of vacuum precooling equipment, the application of vacuum precooling fresh-keeping in the storage or transportation of cut flowers will become increasingly widespread.