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1. After finishing the harvesting of vegetables, the non-edible parts and the leaves, soil, etc., brought about by the harvesting from the field shall be removed in time according to the characteristics of different types. Such as: the roots of the leafy vegetables and the outer leaves of the yellow leaves, the upper leaves of the root vegetables, the short shortening stems and the outer leaves of the cauliflower.
For the purpose of storage of vegetables, its finishing is more important. After finishing, the pathogens and spores attached to the outer leaves or yellow leaves can be removed to reduce the transmission of diseases in storage. That is to say, if these broken leaves are brought into the storage environment, these near-necrotic parts are exposed to unfavorable conditions such as higher humidity and are extremely susceptible to moldy rot; at the same time, they are not conducive to the ventilation of vegetables during storage. It will bring great losses.
2. Selection and grading selection and grading are the necessary commercialization measures for vegetables entering the circulation after harvesting.
The selection is based on sorting and further eliminating products with pests and diseases, mechanical injuries and poor development. In general, the selection is often combined with grading. Operators should wear gloves. Handle them gently during the selection process to avoid new mechanical damage.
Classification is to distinguish products according to certain specifications and quality standards. It is an important measure for producers to put their products into the market, and is also the basis for operators to facilitate quality comparison and pricing. The grading should be based on pre-established quality standards. At present, China has initially established a professional standard system for vegetable products. The commodity standards for major fresh vegetables such as tomatoes, cucumbers, green peppers, and Chinese cabbage have been published by Standard Press. Most of the current standards for vegetable products are divided into three grades based on specifications and quality. Mainly based on the degree of solidity, cleanliness, freshness, uniformity, weight, color, shape, and whether there is no pest or disease or mechanical injury. The graded vegetable products have the same size, uniform specifications, and advantages and disadvantages, which increase the value of the goods and reduce the losses during storage and transportation. With the improvement of the level of vegetable production and the increasingly active market economy, the relevant departments are also formulating the quality standards for vegetable products in wholesale market transactions. After perfecting the professional standards for vegetable products, there will be a more scientific and unified basis for product classification. Adapt to the development of the modernization of the circulation of vegetable products.
3. Pre-cold harvested vegetables should be quickly removed from the field heat before storage and processing. The process of quickly cooling the temperature to the specified temperature in time is called pre-cooling. Through pre-cooling, the temperature of the storage environment caused by the heat of breathing can be prevented from rising, thereby reducing the respiratory intensity of the vegetables and reducing the post-harvest loss from the resistance. Different types and varieties of vegetables require different pre-cooling temperature conditions, and suitable pre-cooling methods are also different.
In order to allow the vegetables to be pre-chilled in time after harvest, it is best to proceed in the production area.
The pre-cooling methods for vegetables mainly include the following types:
(1) Natural cooling and pre-cooling Place the harvested vegetables in a cool and ventilated place, so that the product can be cooled naturally to achieve cooling. This method is simple, easy, and does not require any equipment. This is a more feasible method where the conditions are simple. However, this pre-cooling method is restricted by the external temperature at that time, and it is impossible to reach the pre-cooling temperature required by the product, and the pre-cooling time is long and the effect is also poor. In northern China, the storage of Chinese cabbage is generally more pre-cooled.
(2) Cold storage pre-cooling The vegetable products packed in the packing box are stacked in the cold storage, leaving a gap between the broom and the bun and the same direction as the air outlet of the cold storage ventilator, so as to ensure that the air flow can be taken smoothly. The heat of the product. In order to achieve a better pre-cooling effect, the air velocity in the library should reach 1-2 meters per second, but it should not be too large to avoid excessive dehydration of fresh vegetables. This method is currently a common pre-cooling method and can be applied to various vegetables.
(3) Forced ventilation pre-cooling (differential pressure pre-cooling) creates airflow with different pressures on the two sides of the crates where the product is installed, so that cold air is forced through the individual crates and passes around each product. , so as to take away the heat of the product. This method is about 4 to 10 times faster than the pre-cooling of cold storage, and pre-cooling with a cold store can only dissipate heat from the surface of the package. This pre-cooling method also applies to most vegetables.
There are several ways to force ventilation cooling. The tunnel cooling method was used for many years in South Africa and the United States. After years of research by scientific and technical personnel in China, we designed a simple forced-air pre-cooling facility. The specific method is: the product is placed in a uniform box with uniform ventilation holes, and the box code is placed in a rectangular box, leaving a gap in the center of the box longitudinally. At both ends of the box and the top of the box, it is covered with canvas or plastic film. Strictly sealed, one end of which is connected to the fan exhausted outwards, so that the pressure drop zone is formed at the center of the crucible, forcing the cold air on both sides of the uncovered canvas to enter the low pressure area from the ventilation hole of the packing box, The heat is brought out of the low pressure area and discharged by the fan to the enthalpy area so as to achieve the effect of precooling. This method must pay attention to the rational stacking of the packing box and the rational placement of the canvas and fan, so that the cold air only enters through the vent holes on the packing box, otherwise the pre-cooling effect cannot be achieved.
(4) Vacuum pre-cooling is to put the vegetables in a sealed container, quickly draw out the air in the container, reduce the pressure in the container, so that the product is cooled due to evaporation of surface moisture. Under normal atmospheric pressure (760 mmHg* at 101.3 kPa), water evaporates at 100°C, and when the pressure drops to 0.53 kPa, water can evaporate at 0°C. Every 5°C decrease in temperature, about 1% of the product's weight is evaporated. In order not to make the vegetables lose too much water, spray some water before the product is pre-cooled. This method is suitable for precooling leafy vegetables. In addition, vacuum pre-cooling methods can also be used for such as cypress, mushroom, Brussels sprouts, and Dutch beans.
Vacuum pre-cooling method, due to the need to have a special vacuum pre-cooling device can be implemented, and the investment is relatively large, the current domestic use of this method is mainly used for pre-cooling the export of vegetables.
(5) Cold water pre-cooling Spray the cooled water (as close to 0 °C as possible) on the vegetables, or soak the vegetables in the flowing cold water to achieve the purpose of cooling the vegetables. Since the heat capacity of water is much greater than that of air, the cold water pre-cooling method using water as a heat transfer medium is faster than the ventilation pre-cooling, and the cooling water can be recycled. However, disinfection must be applied to cold water, otherwise the product will be contaminated by microorganisms. Therefore, some disinfectants should be added to cold water, such as the addition of hypochlorite, such as sodium hypochlorite.
The cold water pre-cooling method is a chiller, which is often cleaned with water during use.
The cold water pre-cooling method can be combined with the post-harvest cleaning and disinfection of vegetables. This type of pre-cooling method is most suitable for fruit vegetables and root vegetables but not for leafy vegetables.
(6) Contact with ice pre-cooling This method is complementary to other pre-cooling methods. It is to put the fine ice or ice salt mixture on the top of the vegetable containers in the packaging containers or cars and trains. This can reduce the temperature of the product, it can also ensure the freshness of the product in the transport, while also playing the role of pre-cooling. However, this method can only be used for products that do not come in contact with ice. Such as spinach, cauliflower and radish.
Postharvest Treatment and Storage of Vegetables
Before vegetables are harvested and stored and transported, various preparatory treatments, also called post-harvest treatments, are required depending on the type, storage time, mode of transport, and sales purpose. It includes sorting, sorting, grading, pre-cooling, wound healing, drying and chemical treatment. Through the treatment, the vegetables as a product not only improve the appearance but also minimize the life activities of the vegetables, and play a great role in maintaining the freshness of the vegetables and prolonging the storage life of the vegetables. The more timely the postharvest treatment, the better the effect of storage and freshness.