Vacuum pre-cooling is an efficient method for rapidly reducing the temperature of freshly harvested flowers, significantly enhancing their freshness and extending their shelf life. The storage and transportation of flowers after harvest involve complex technological processes, and the application of vacuum pre-cooling technology has now become a key practice in this field. By effectively regulating ambient temperature, it helps maintain the quality of the flowers and prolongs their storage period.
One of the most critical steps in flower preservation is cooling. After being harvested, flowers experience a rapid increase in respiration, generating a significant amount of heat known as "field heat." This heat must be removed quickly, or it can severely impact the quality and longevity of the flowers. Studies show that if flowers are left at room temperature (around 20°C) for just one day, it can reduce their storage life by 5 to 7 days under ideal conditions like 0°C. Therefore, immediate pre-cooling is essential.
Without proper pre-cooling, flowers are often placed directly into storage or transport vehicles, which are not designed to handle large amounts of field heat. This can lead to excessive condensation due to the temperature difference between the flowers and the storage environment, causing water droplets to form on the surfaces and potentially damage the flowers. Pre-cooling ensures that only a small amount of additional cooling is needed once the flowers are in storage, making the process more efficient and effective.
Among various pre-cooling methods, vacuum pre-cooling stands out as one of the most effective. It works by using the principle of evaporation under vacuum conditions. Flowers are placed in a vacuum chamber, where some of the moisture evaporates, taking away the latent heat and rapidly lowering the temperature. This method not only cools the flowers quickly and evenly but also reduces oxygen pressure and ethylene levels in the tissue, helping to maintain freshness and extend the storage period.
The success of vacuum pre-cooling depends on controlling humidity and temperature during the process, which is influenced by factors such as the type of flower, its moisture content, and the performance of the vacuum equipment. One challenge is water loss during the process—each 10°C drop can result in about 1% water loss. If this becomes severe, it can affect the quality and freshness of the flowers. To address this, methods like water absorption and spraying are used.
In the U.S., a spray vacuum pre-cooling system has been developed to minimize dehydration while ensuring fast cooling. Some growers use water absorption techniques, where flowers are soaked in water before being placed in the vacuum chamber. This method is suitable for flowers requiring lower final temperatures. For flowers with many buds, a small amount of silver nitrate is sometimes added to the water to preserve them. However, this method can be less efficient due to the space taken up by the containers.
Another common approach is water spraying, where flowers are misted with water before pre-cooling to ensure they remain moist. This technique is simple, practical, and widely used.
Vacuum pre-cooling offers numerous advantages: it cools flowers quickly (usually within 20–30 minutes), ensures even temperature distribution, and does not interfere with packaging. It is especially suitable for flowers with high surface-to-volume ratios. Although still being refined, vacuum pre-cooling is gaining popularity as the technology continues to improve. With ongoing research and development, it is expected to play an even greater role in the post-harvest handling of flowers.
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