CN101156623B - Combined drying method for preparing dehydrated vegetables and fruits - Google Patents

Abstract

The invention relates to a dehydration and drying method for high-grade vegetables and fruits. After the vegetables and fruits are sliced, they are dried in a microwave vacuum dryer. The pressure is between 2.5 ~ 4.5KPa (residual pressure), and the dry material can reciprocate with the drying plate in the microwave vacuum dryer. Microwave vacuum drying removes about 40-50% of the total moisture, then takes it out and repacks it into a stainless steel plate and puts it in a freeze dryer, completely freezes at -25°C and sublimates and dries. The drying time is about 7-10 hours, and the moisture drops. To 6 ~ 7% safe moisture. The dehydrated vegetables and fruits prepared by the present invention are very close to those of pure freeze-dried products in terms of color, aroma and nutrient retention, the volume is 60-70% of pure freeze-dried products, the shape and structure are also close, and the drying cost is low. It can save more than 40% energy.

Description

Combined drying method for preparing dehydrated vegetables and fruits

technical field

The invention relates to the field of food and agricultural product processing, in particular to a dehydration processing method for fruits and vegetables.

Background technique

Among many drying methods, due to high drying temperature and long drying time, hot air drying results in poor quality of dried food, large color change, large loss of color, fragrance and nutrients, hard tissue structure and poor rehydration; traditional vacuum Although the drying temperature is relatively low, since the convection heat transfer in the drying room is almost non-existent, but the heat conduction is the main method, so the heat transfer speed is slow, the drying time is still relatively long, and the retention of color, fragrance and nutrients is still limited. The tissue structure also hardens, and the rehydration property is also poor; the drying quality of freeze-drying is the best, it can basically retain the original color, aroma, taste and nutritional components or biological activity of the food, and the shape of the product after drying is basically unchanged , the tissue structure is porous, and the rehydration property is good. In the processing of high-grade dehydrated vegetables, fruits and other foods, the drying process is the key to determine the product quality. At present, high-grade dehydrated fruit and vegetable products are all produced by freeze-drying process, and various traditional drying processes cannot produce high-quality dehydrated fruits and vegetables. However, freeze-drying is not only expensive in equipment, but also high in operating costs. It needs to maintain a low temperature of -25°C, a high vacuum of 0.5Kpa, a drying time of about 20 hours, and a limited production capacity, so the production cost of the product is very high.

Contents of the invention

The purpose of this invention is to provide a new combined drying method for preparing dehydrated vegetables and fruits. The color, fragrance and nutritional components of the products dried by this method are very close to those of pure freeze-dried vegetables and fruits. The total drying cost is much lower than that of freeze drying, and it can save energy by more than 40% compared with pure freeze drying of fruits and vegetables.

The purpose of the present invention is achieved by the following technical solutions.

A combined drying method for preparing dehydrated vegetables and fruits, characterized in that:

a. Microwave drying: place slices of vegetables and fruits in a microwave drying tray, and dry them in a microwave vacuum dryer. The applied microwave power density is between 0.5 and 1.2 W/g, and the vacuum degree is between 25 and 45Kpa; Remove 40-50% of the total moisture in vegetables and fruits to form semi-dry materials;

At this time, the volume of the dried material is about 60-70% of the original volume, and no warpage occurs, and then stop the microwave vacuum drying;

b. Freeze-drying: The semi-dried material taken out of the microwave vacuum dryer is put into a freeze-drying tray and placed in a freeze-dryer, and is completely frozen at a temperature of -25°C; the frozen material is sublimated in the freeze-dryer Drying, the drying time is about 7-10 hours, and the moisture is reduced to 6-7% safe moisture, forming dehydrated vegetables and fruits.

The slice thickness of vegetables and fruits is less than 10mm. The microwave drying tray is made of tetrafluoroethylene or pure polypropylene. The freeze-drying tray is made of stainless steel.

During microwave drying, the vegetables and fruits to be dried are rotated or reciprocated in the microwave vacuum dryer to eliminate microwave hot spots generated during the drying process.

The cost of microwave drying combined with microwave system and vacuum system is much lower than that of freeze drying. Microwave vacuum drying technology combines the advantages of microwave and vacuum, that is, it not only reduces the drying temperature, but also speeds up the drying speed. Features: The drying temperature can be controlled within the range of 25-45°C, which can better retain the original color and fragrance of the dried food and other materials, and the loss of heat-sensitive nutrients or active ingredients such as vitamins is very small, and better drying can be obtained. Product quality, drying cost is much lower than freeze drying. However, the product dried by this technology has a large deformation in the later stage of drying, and there is a large gap between the appearance and shape of the freeze-dried product.

Detailed ways

Below by embodiment, the present invention will be further described.

Example 1:

Carrots are cut into slices with a thickness of 6 mm, and 2 kg of carrot slices are taken in a thin layer and placed in a tetrafluoroethylene tray, and put into a microwave vacuum dryer with a microwave output power of 2KW for drying, and the vacuum degree is 3.0Kpa (residual pressure). In 16 minutes, 712g of water was sloughed off, that is, 40.5% of the moisture in the original sample was removed (about 88% of the water content of fresh carrots, 1760g of moisture in total), and the sample was taken out and then freeze-dried for 10 hours, and the water content of the product after taking out was 6.4 % (moisture basis), measuring its volume is 68.6% of the pure freeze-dried sample, and the retention rate of carotene is determined to be 97.1%, while the retention rate of carotene in the pure freeze-dried sample is 98.5%. Therefore, the quality of the carrot slices prepared by the combination method is very close to that of pure freeze-dried carrot slices, and the difference in appearance is also very small.

Example 2:

After peeling the apples, cut them into thin slices with a thickness of 3 to 5 mm. Take 2 kg of apple slices and place them in a tetrafluoroethylene tray, and put them into a microwave vacuum dryer with a microwave output power of 2 KW for drying. The vacuum degree is 3.0 Kpa (residual press), dried for 20 minutes, and 910g of water was removed, that is, 50.5% of the moisture in the original sample was removed (fresh apples contain about 90% of moisture, and a total of 1800g of moisture), the sample was taken out and then freeze-dried for 10 hours. The water content was 6.9% (wet basis), and its volume was measured to be 62.2% of the pure freeze-dried sample, and the vitamin C retention rate was determined to be 94.6%, while the vitamin C retention rate of the pure freeze-dried sample was 97.4%. Therefore, the quality of the apple slices prepared by the combination method is very close to that of pure freeze-dried apple slices, and the color difference of the products is also very small.

Claims (5)

1. A combined drying method for preparing dehydrated vegetables and fruits is characterized in that: a. Microwave drying: place slices of vegetables and fruits in a microwave drying tray, and dry them in a microwave vacuum dryer. The applied microwave power density is between 0.5 and 1.2 W/g, and the vacuum degree is 3.0Kpa; remove the vegetables and 40-50% of the total moisture in the fruit to form a semi-dry material; b. Freeze-drying: The semi-dried material taken out of the microwave vacuum dryer is put into a freeze-drying tray and placed in a freeze-dryer, and is completely frozen at a temperature of -25°C; the frozen material is sublimated in the freeze-dryer Drying, the drying time is about 7-10 hours, and the moisture is reduced to 6-7% safe moisture, forming dehydrated vegetables and fruits. 2. The combined drying method for preparing dehydrated vegetables and fruits as claimed in claim 1, characterized in that: the slice thickness of vegetables and fruits is less than 10mm. 3. The combined drying method for preparing dehydrated vegetables and fruits according to claim 1, characterized in that: the microwave drying tray is made of tetrafluoroethylene or pure polypropylene. 4. The combined drying method for preparing dehydrated vegetables and fruits according to claim 1, characterized in that: the freeze-drying tray is made of stainless steel. 5. The combined drying method for preparing dehydrated vegetables and fruits as claimed in claim 1 is characterized in that: when carrying out microwave drying, the vegetables and fruits to be dried are rotated or reciprocated in a microwave vacuum dryer to eliminate the drying process Microwave hotspots generated in the