A vacuum processing device that uses vacuum pre-cooling technology to extract air and water vapor in a vacuum chamber to reduce the pressure in the vacuum chamber. Under low air pressure, the boiling point of water decreases, the latent heat of evaporation increases, and the free water on the surface of the cooled object vaporizes, taking away the heat of itself and the environment, and achieving the cooling and cooling effect.
The fresh fruits and vegetables after harvesting are still living and living, and a series of physiological changes will continue. Some of the unique flavors and tastes of fruits and vegetables will be further revealed, and the color, aroma and nutrients are more suitable for people's needs. The process is called the post-ripening stage (for example, we can see that the bananas picked in the south are green, and the bananas in the north are almost ripe bananas), and the fruits and vegetables gradually begin to lose the original color and fragrance, and even soften and disintegrate. This is the aging stage.
The vacuum pre-cooling technology is to reduce the evaporation of latent heat of a part of the food in a low-pressure environment, thereby lowering the temperature of the rest of the food.
The main factors affecting the physiological changes of fresh fruits and vegetables after harvest are temperature, humidity, environmental gas, packaging and mechanical damage. These factors will cause weight loss, body shrinkage, color and gloss to fade. Generally speaking, the breathing patterns after harvesting of fruits and vegetables are divided into two types: they are climacteric and non-jumping. The former has a peak in respiratory intensity when mature, and the flavor quality of the fruit is best, and then the respiratory intensity decreases. The fruit will then age (ethylene will promote the peak of the breathing of the climacteric fruit, leading to early maturity and aging), such as apples, bananas, peaches, mangoes, etc. In the case of berries, fresh blackberries, blueberries and strawberries are very perishable crops, and they all need to be pre-cooled immediately after harvest to prevent rapid deterioration in quality. In the higher temperature environment, it will soften and rot within four hours. If they are pre-cooled, the shelf life can be extended to ten days or even longer.
The fresh fruits and vegetables after harvesting have a lot of field heat and a strong breathing heat. Generally, within a certain temperature range, the respiratory intensity increases by 1 to 1.5 times or even higher for every 10 ¡ãC increase. The greater the respiratory intensity, the greater the material consumption and the shorter the shelf life. The heat generated by the respiration of crops accelerates its maturity and even corruption, appearances, color changes and tissue softening that are unwilling to see. Breathing heat can also cause changes in metabolism, affecting the taste and nutrition of products. Fungi, bacteria and various microbial activities and reproduction are most abundant in the range of 30 ¡ã C ~ 60 ¡ã C. Fresh fruits and vegetables are generally weighed and sold, so the weight loss caused by water loss also reduces the economic value of the product.
The concept of pre-cooling is different from general cooling. ¡°Pre-cooling¡± refers to the process in which the food rapidly drops from the initial temperature (about 30 ¡ã C) to the required end temperature (0 to 15 ¡ã C), that is, the rapid cooling process before refrigeration, modified atmosphere preservation and rapid freezing is called pre-cooling. Cold, common vacuum pre-cooling, water pre-cooling, ice water pre-cooling. ¡°Pre-cooling¡± is a very effective method to quickly eliminate the field heat and respiratory heat of fresh fruits and vegetables after harvesting. It also slows down the physiological activities of fresh fruits and vegetables, inhibits the proliferation of microorganisms such as fungi or overgrowth itself (such as excessive asparagus). It will bend after growing, and broccoli will bloom. It will reduce its dry consumption (weight loss) and various losses in circulation, so that consumers can get high-quality and clean fruits and vegetables. Among them, vacuum pre-cooling is currently one of the most advanced preservation technologies in the world.
The basic principle of vacuum pre-cooling: under normal atmospheric pressure, water evaporates at 100 ¡ã C, while the atmospheric pressure is 610 Pa, and water evaporates at 0 ¡ã C. This is because the boiling point of water decreases with the decrease of ambient atmospheric pressure, which is like high People in the latitudes can find that water boiling is not 100 ¡ã C, but below 100 ¡ã C. The water boils and evaporates quickly, absorbing heat rapidly. Put the fresh fruits and vegetables in a closed container and quickly extract the air. As the pressure continues to decrease, the water in the fiber gap between the fruits and vegetables begins to evaporate. When evaporating, it will take away the latent heat due to evaporation, causing the temperature of fruits and vegetables to decrease, so it is sometimes called Pre-cooled for evaporation.
Vacuum pre-cooling is especially effective for products such as lettuce, lettuce and spinach. Products with high water permeability, such as corn and cauliflower, can also be used in this way, due to their loose structure and faster evaporation of water. The water loss in vacuum cooling is generally about 3-5%, which does not cause the phenomenon of "wilting", softening, and no freshness. During vacuum cooling, due to the pressure difference between the inside and outside of the fruit and vegetable tissue, the harmful gases and heat in the tissue are also "extracted", which can delay the arrival of the peak of the fruit and vegetable climax. Any other cooling method is to slowly "permeate" the cooling from the outer surface to the inside of the tissue, and in the case of vacuum cooling, the cooling is uniformly cooled from the inside to the outside of the tissue, which is unique to vacuum cooling, thus preserving long time. Compared with other cooling methods, the vacuum cooling method has the following characteristics:
1. Cooling speed: 20-30 minutes to reach the required refrigerating temperature.
2. Cooling evenly: Free water vaporization on the surface of the product takes away its own heat for cooling purposes, achieving uniform cooling from the inside to the outside.
3. Clean and hygienic: Under vacuum, it can sterilize or inhibit bacterial growth and prevent cross-contamination.
4. Thin layer drying effect: It has unique effects such as epidermal damage or inhibition of expansion.
5. Not subject to packaging restrictions: As long as the package has air holes, it can be evenly cooled.
6. High freshness: It can keep the original color, fragrance and taste of food and extend the shelf life.
7. High degree of automation: The pressure sensor can be used to control the pressure of the refrigeration system and the vacuum system. It is convenient to adjust the vacuum of the vacuum cooler and can be remotely controlled to facilitate monitoring of equipment operation and quick resolution of equipment failure.
8. Highly accurate: equipped with precision digital display temperature and humidity controller to accurately control vacuum and humidity.
9. Safe and stable: The electrical part adopts famous brand products to ensure the stable and long-lasting operation of the machine and safe operation.
Problems with vacuum cooler:
(1) Vacuum pre-cooling equipment has a high price and a large initial investment.
(2) For fruits and vegetables with relatively small surface area, such as cucumber and tomato, the cooling effect is not satisfactory.
10. The fresh-keeping time is long, and it can be directly transported without entering the cold room. Short-distance transportation can be done without the insulated car.
The following table compares the storage period of vacuum pre-cooling of some items with the general refrigeration (unit: day)
Type cold room refrigeration vacuum pre-cooling
Spinach ~ 8 ~ 40
Mushroom ~ 2 ~ 10
Cabbage ~ 8 ~ 40
Celery ~ 8 ~ 40
Green peas ~ 6 ~ 12
Fresh pork ~ 7 ~ 12
Compared with general refrigeration, vacuum pre-cooling has a high rate of 19.5% for fruits and vegetables; 15.51% for weight loss; and 23.5% for total sugar. Vacuum pre-cooling not only pre-cools fresh fruits and vegetables, but also applies to pre-cooled fresh cut flowers, rice, steamed products, cooked foods, etc.
The picture below shows the data collection of vacuum pre-cooling in the company's laboratory for the company's research and development personnel.
Serial number product temperature change weight change moisture content change
Before cooling (¡ãC) After cooling (¡ãC) Before cooling (g) After cooling (g) Weight loss ratio (%) Before cooling (%) After cooling (%) Change value (%)
1 sample 1627.91611516.2170.9668.102.86
2 sample 2624.12662477.1460.3558.801.55
3 sample 370.28.53613347.4871.3067.074.23
4 sample 449.89.61621544.9465.7063.122.58
The average cooling time is 12.5 min, and the average water loss rate is 2.8% (in the foregoing, the water loss due to vacuum cooling is generally about 3-5%), and the weight change is 6.44%. As can be seen from the above table, the cooling rate, the water loss rate, etc. are all within the required range, and the appearance and taste of the food maintain the original characteristics. From the aspect of cooling alone, vacuum pre-cooling is completely suitable for the cooling process in the food production process. Of course, we will further sample and compare the bacteria and microorganisms before and after the experiment (such as E. coli, etc.).
The basic components of vacuum pre-cooling equipment are: vacuum tanks, vacuum systems, water vapor traps, refrigeration systems and control systems.
Vacuum tank: The vacuum tank is generally made of carbon or stainless steel. The door has a sliding door, a hydraulic door and a manual door. For larger equipment, a conveyor belt mechanism can be formed at the bottom to facilitate loading and unloading of materials. The tank is also provided with drainage to ensure the interior is clean.
Water vapor trap: or cold tank, used to ¡°adsorb¡± moisture in the air to prevent moisture from entering the vacuum pump to emulsify the oil and damage the vacuum pump assembly.
Vacuum system: It is used to form vacuum to achieve rapid cooling. Vacuum pump is a key component of vacuum system. It is determined according to different specifications and sizes of vacuum pre-cooling device and specific conditions.
Refrigeration system: is to provide a cold source for the water vapor trap, so that the water vapor evaporated from the fruits and vegetables in the vacuum chamber is condensed and discharged on the cooling coil of the water vapor trap, so that it is not sucked by the vacuum pump and affects the pumping performance of the pump (small intermittent Refrigeration units for continuous or pre-cooled vacuum pre-cooling units often use water-cooled/air-cooled compression condensing units.
Control system: refers to the process control of the entire cooling process, which can automatically control the vacuum system, refrigeration system, etc. according to the preset program and each sensor. For example, if the pressure drop in the vacuum chamber is too low (5 mbar), it may cause frostbite in some products, so the pressure in the vacuum tank should be controlled accurately. A control system that automatically maintains pressure is a guarantee of safe cooling. Nowadays, the control system often adopts PLC or industrial process controller. The vacuum pre-cooling equipment used in the experiment above adopts the PLC control system with touch screen, which is very easy to operate. It can preset parameters such as pressure, temperature, cooling system opening point, etc., so that the cooling process can be automatically operated according to predetermined requirements.
The loading and unloading mechanism of the vacuum pre-cooling equipment is easy to handle and dispose. It is determined according to the user's loading and unloading method. There are often conveyor belts, but also forklifts or hydraulic propellers.
The tank of the vacuum pre-cooling device is often designed as a square, divided into fixed and mobile. The mobile vacuum preset is an integrated design that can be installed directly on the car to the picking site. The vacuum pre-cooling device equipped with a spray device is often used for pre-cooling of fruits and root foods with low surface moisture. Humidification during vacuum pre-cooling reduces evaporation of moisture inside the product and reduces weight loss. Appropriate humidification can also prevent the appearance of spots in the vacuum pre-cooling of the leaf products. At the same time, the humidification device can also be used for disinfection to meet the food safety requirements of users. Vacuum pre-cooling is a fast and efficient way to batch process fresh fruits and vegetables and food. Of course, vacuum pre-cooling also has its limitations. For example, the initial investment of equipment is high, and the products of sealed packaging are poor (poor evaporation of water).
Fresh fruits and vegetables are vacuum pre-cooled and then air-conditioned to increase the freshness of fresh fruits and vegetables. With such preservation technology, it is not "if you leave the branch, the day will change, the day will change, the taste will change on the 3rd, and the color and fragrance will go away on the 4th and 5th," but in the north. You can eat fresh, original fruits and vegetables like the South