Fresh vegetables typically contain 65% to 96% moisture, but after harvest, they gradually lose water during storage and transportation, leading to wilting, weight loss, and significant economic losses. Even a small amount of dehydration—such as 5% water loss—can cause wilting or shriveling, while in some cases, the quality is affected without visible wilting. This can lead to changes in taste, crispness, color, and overall flavor.
Mild dehydration disrupts the metabolic balance of vegetables and increases the activity of hydrolytic enzymes. For instance, sweet potatoes become sweeter when dehydrated because starch breaks down into sugar. Severe dehydration raises the concentration of cell fluids, leading to high levels of ions like ammonia and hydrogen, which can poison cells and damage the protoplasmic structure. Excessive dehydration also boosts abscisic acid, stimulating ethylene production, which accelerates aging and leaf drop. Additionally, reduced turgor pressure and altered mechanical properties can weaken the vegetable's ability to resist diseases and store for long periods. The more dehydrated the tissue, the more susceptible it becomes to microbial infection and faster the decline in disease resistance.
Therefore, controlling water loss during post-harvest handling, storage, and transport is crucial. However, there are exceptions. For example, onions and garlic benefit from proper drying before storage to promote dormancy. Chinese cabbage should be slightly dried before storage to lower its freezing point and improve cold resistance.
To prevent water loss in harvested vegetables, several measures can be taken:
First, packaging, waxing, or coating. One of the most effective ways to reduce moisture loss is by using plastic films or waterproof materials to cover the product, or placing it in bags, boxes, or cartons. Polyethylene film is an excellent choice. However, it's important to note that packaging can slow down cooling. Also, the water-absorbing properties of the packaging material should not be ignored. Using compound wax or rosin can help prevent moisture absorption. Although more expensive, this method has practical value in commercial settings. Waxing or painting the surface of the product and then sealing it in appropriate packaging can also effectively prevent water loss.
Second, increasing air humidity. Raising the relative humidity around the vegetables is another effective strategy. While high humidity may encourage bacterial growth, it can be combined with fungicides. Automated humidifiers can spray mist or steam into storage areas, or water can be sprinkled on the floor, wet straw curtains can be hung, or condensation tubes can be kept slightly cooler than the storage temperature. Maintaining around 95% relative humidity in the warehouse helps prevent moisture loss.
Third, proper ventilation. Whether using mechanical or natural ventilation, adequate airflow is essential to remove heat and prevent temperature fluctuations. However, wind speed should be controlled between 0.3 to 3 m/s, as it has minimal impact on moisture retention.
Fourth, using a sandwich-type cold storage. This system consists of two walls with cold air circulating in between. The outer wall is both insulating and moisture-proof, while the inner wall is not insulated. The evaporator is placed between the walls and transfers heat through conduction. Since the evaporator is not inside the storage area, it doesn't cause frost or capture moisture from the product, maintaining high humidity and preventing dehydration.
Fifth, using a breeze library. Cold air is delivered to the storage area through holes on the top or is humidified before being introduced. This method helps maintain optimal humidity and prevents moisture loss.
Proper preservation of fruits and vegetables is key to maintaining their quality and freshness. By adapting methods to specific conditions, producers can ensure the best possible results and extend shelf life.
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