Apple freeze drying process

Apple freeze drying process
Apple meat is fine and brittle, juicy, aromatic, sweet and sour, nutritious, with antipyretic and clearing fire, thirst, diuretic and antidiarrheal effect [, but due to high water content, nutrient-rich, susceptible to mechanical damage and Microbial infestation and corruption, thus losing the value of goods and processing quality. Processing apples by freeze-drying technology can reduce nutrient loss and reduce tissue damage to a minimum, and maintain the original color, fragrance, taste and shape of apples to a great extent. And easy to transport and store.
Freeze drying process:
1.1 Materials
Commercially available red Fuji apple, vitamin C (analytical grade).
1.1.1 Instruments and equipment
The vacuum freeze dryer consists of a refrigeration system, a vacuum system, a heating system, an automatic control system, and a freeze/dry chamber.
1.2 Experimental methods
1.2.1 Process
Apple→Cleaning→Cutting, Color Protection→Freezing→Sublimation Drying→Analytical Drying→Packaging
1.2.2 Pretreatment
After washing the apple with detergent, rinse it with water, remove the core, cut into 3 × 2.5 × 2cm blocks, and cut into the 0.1% vitamin C solution for color protection to prevent the fruit pieces from oxidative browning. Soak for 5 minutes, then plate 2cm thick, and insert the temperature probe into the center of the fruit block, and put it into the freezer to freeze.
The freeze-dried apple adopts the experimental freeze dryer. When drying, the heat is supplied to the dried material through the shelf in the form of conduction and radiation, which is a heat transfer process; the process of eliminating steam is a mass transfer process, and the water vapor generated by sublimation needs to be The internal mass transfer process reaches the surface of the material and is transferred to the steam condenser through an external mass transfer process. The lyophilization time is related to the lyophilization temperature, the water content, the thickness of the layer and the heat transfer coefficient. In order to increase the speed, the above various parameters can be adjusted in practice.
Effect of freezing rate on the quality of freeze-dried apples The purpose of freezing is to solidify the moisture in the food and make the product have the same morphology before lyophilization to prevent it from happening due to vacuum during sublimation and desorption. Concentrated undesirable changes such as foaming and shrinkage. The key to the freezing process is to control the freezing rate of the food. The freezing rate directly affects the drying rate and product quality. When the temperature is frozen slowly, the formed ice crystal particles are larger, which is beneficial to the sublimation of ice crystals in the material, but the quality of the product is poor. In apple freeze-drying, the slow freezing makes the apple block stay in the air for too long, the enzyme in the apple browns, and the color of the fruit becomes yellow, which affects the quality of the apple, so it is necessary in the pretreatment. Apple color protection. When frozen quickly, the ice crystal particles produced are small, the sublimation is slower, and the dry product is resilience.
Apple's temperature drop has three stages:
In the first stage, the temperature of the apple is lowered from the initial temperature to the freezing point of the apple. At this time, the heat released by the apple is sensible heat. This heat is compared with the heat released during the whole freezing process, and the value is relatively small, so the cooling rate is fast.
The freezing curve is steep.
In the second stage, the temperature of the apple dropped from the freezing point of the apple to -2 °C to about -5 °C. At this time, most of the water in the apple formed ice, releasing a large amount of latent heat (when each kilogram of water was formed into ice, it was released. 334.72kJ of heat). Most of the heat of the apple is released at this stage during the entire freezing process, so the apple is slow to cool at this stage and the freezing curve is flat.
In the third stage, the temperature of the apple continues to drop from -5 °C to the final temperature. At this time, part of the heat released is due to the cooling of the ice, and the other part is due to the residual small amount of water continuing to freeze. The curve at this stage is also steep. .
Material temperature effect
During the sublimation process, the material needs to absorb a large amount of sublimation latent heat, so that the sublimation process can proceed smoothly. During the sublimation process, the temperature of the frozen apple should not exceed the eutectic temperature of the apple. The temperature of the dried layer should not exceed the disintegration temperature of the apple or the allowable temperature of 55 ° C. Otherwise, the frozen layer of the apple will be higher than the eutectic point due to the temperature. Melting, the dry layer will disintegrate, the product will appear dry shrinkage and surface hardening, preventing the sublimation process from continuing, but the temperature of the apple cannot be much lower than the temperature of the eutectic point, otherwise the sublimation rate will decrease, and the time required for the sublimation step will be lengthened. . Therefore, the temperature at which the frozen apple is controlled is lower than and close to the eutectic point temperature. In the desorption stage, there is no frozen water in the apple. The temperature of the apple is as high as possible at a temperature of 80-90 ° C. The more energy of the adsorbed water remaining in the apple is desorbed from the apple to reduce the moisture of the apple by 2%. ~5%, shorten drying time
The heat required to freeze the material comes from the shelf and the temperature of the material is controlled by controlling the shelf temperature.
  in conclusion
Through the study of the freeze-drying process of apples, the following conclusions can be drawn:
Although slow freezing can shorten the freeze-drying time, the slow freezing rate may cause the enzyme in the apple to brown, so the faster freezing speed should be adopted without affecting the quality of the apple, 0.67cm/h; Low not only prolongs the freeze-drying time, but also reduces the quality of apples. Therefore, in order to shorten the drying time and improve the product quality, it is necessary to maintain a sufficiently high vacuum in the drying chamber, 70-90 Pa; the influence of the heating plate temperature on the freeze-drying process and apple quality The heating shelf temperature of 80 ~ 90 °C can shorten the freeze-drying time under the premise of ensuring the quality of the freeze-dried product; when the condenser temperature is -40 ~ -50 °C, the freeze-drying can be carried out smoothly; the apple freeze-drying process is The heat transfer control process should use a lower operating pressure; the thickness of the material has a great influence on the freeze-drying time, and it is suitable to use a thinner material for freeze-drying.