Selection of raw materials, pre-freezing, sublimation, drying, packaging and storage.
Generally. pretreatment refers to all treatments before sublimation drying, so freezing before drying also belongs to pretreatment. Raw material pretreatment and conventional fruit and vegetable drying and frozen products are the same, such as selection, cleaning, peeling, cutting, blanching, cooling and other processing, not detailed here. When fruit or vegetable juices are freeze-dried, they are pre-concentrated using cheaper processing methods and then granular when pre-frozen.
Pre freezing is to freeze the raw materials after the pre-treatment, which is an important process of lyophilization. Due to a series of complex biochemical and physicochemical changes in frozen fruits and vegetables, the quality of freeze-dried fruits and vegetables will be directly affected by the quality of pre-freezing. During the freezing process, the effect of the freezing rate on the quality and drying time of the frozen material is mainly considered.
The differences between quick freezing and slow freezing are as follows: the ice crystals produced by quick freezing are smaller, while the ice crystals produced by slow freezing are larger. Large ice crystals are favorable to sublimation while small ice crystals are unfavorable to sublimation. Small ice crystals have less effect on cells. The smaller the ice crystals are, the more they can reflect the original structure and properties of the product after drying. But the freezing rate is high and the energy consumption is high. An optimal freezing rate should be selected to ensure the quality of freeze-dried food and minimize the freezing energy consumption.
When drying, the wet load of the lyophilizer is the drying quality of the drying board per unit area, which is an important factor to determine the drying time. The thickness of the dried food is also a factor affecting the drying time.
In freeze-drying, the drying of the material is promoted from the outer layer to the inner layer, so the drying time is longer when the drying material is thicker. In actual drying, the dried materials are cut into a uniform thickness of 15~30mm. The amount of material to be loaded by the drying board per unit area should be determined according to the heating method and various types of dry food. In the use of industrial large-scale equipment for drying, if the drying cycle is 6~8h, the dry plate material loading capacity is 5~15kg/m2.
The drying temperature
In order to shorten the drying time during freeze-drying, it is necessary to provide the heat needed for sublimation of ice crystals effectively, so various practical heating methods have been designed. The drying temperature must be controlled in such a way as not to cause the melting of ice crystals in the dried material and not to cause thermal denaturation of the dried part due to overheating. Therefore, in the single heating mode, the temperature of the drying plate should be controlled at 70~80℃ in the early stage of vigorous sublimation drying, 60℃ in the middle stage of drying, and 40~50℃ in the later stage of drying.
Judgment of drying end point
The drying end point can be determined by the following indicators: the temperature of the material and the temperature of the heating plate tend to be consistent and maintain for a period of time; The pump set (or cold trap) vacuum gauge is consistent with the drying chamber vacuum gauge, and keeps it for a period of time; The cold trap temperature of the vacuum gauge in the drying chamber basically recovers to the index of the equipment when it is unloaded and keeps for a period of time. For the lyophilizer with a large butterfly valve, the large butterfly valve can be closed, and the vacuum machine basically does not drop or rarely drops. The above four criteria can be used alone, combined or combined.
Post-processing includes unloading, semi-finished product selection, packaging and other processes. After freeze-drying, nitrogen or dry air is injected into the drying room to break the vacuum, and then the material is immediately removed in a closed environment with relative humidity below 50%, temperature 22-25 ℃ and less dust, and the semi-finished products are selected and packed in the same environment. Because the freeze-dried material has a large surface area and very strong hygroscopicity, it is necessary to complete these operations in a relatively dry environment.
After freeze-dried food has a large surface area, some ingredients in the food are directly exposed to the air, easy to contact the oxygen in the air and absorb water, resulting in the gradual deterioration of freeze-dried food. Secondly, most freeze-dried foods have natural colors, which are easily degraded by light. In the process of oxidation and pigment degradation, the temperature is also an important factor affecting these chemical reactions. Therefore, the packaging of freeze-dried food mainly considers how to prevent or mitigate the impact of the above four factors.
The porous spongy structure of freeze-dried food has both its advantages and disadvantages. First, once the product is exposed to air, it is easy to absorb moisture and oxidize and reduce quality. Therefore, freeze-dried food should be vacuum-packed, preferably packed with nitrogen. Second, the volume of freeze-dried food is relatively large, which is not conducive to packaging, transportation and sales, so freeze-dried food is often compressed after packaging. In addition, the porous structure makes the freeze-dried food easy to be broken during transportation and sales. Therefore, that freeze-dried food that is inconvenient to compress packaging, packaging materials or packaging forms with certain protective effects should be used.
Freeze-dried food commonly used packaging materials for PE bag and compound aluminum foil bag, PE bag commonly used for a large package, the compound aluminum foil bag commonly used for small packing, outer packing usually choose cowhide corrugated carton, the size of meeting the need of the container transport, use PE bag as the inner packing, to strengthen its isolated from the role of oxygen, water, steam, commonly used double, when necessary, also can use the tin can packaging, But the price is higher, the application is not very common. No matter what kind of packaging material is used, it is necessary to use vacuum nitrogen filling, and add deoxidizer and desiccant.
Freeze-dried food should be stored in a cool, dry place, if conditions, it is best placed in low temperature and low humidity environment, shelf life is usually 1~2 years, can be properly extended when using tin packaging.
1. The small chives
(1) Process flow
Raw materials are inspected, cleaned, sliced, laid out, pre-frozen, freeze-dried, discharged, semi-finished products are inspected, packaged and stored in storage.
(2) Operation points
Raw materials fresh, no disease, no yellow leaves, no damage, green color. The qualified raw materials shall be selected according to the raw material acceptance method, and the inclusion shall be removed. The cleaning shall be rinsed with flowing water and the surface sediment shall be washed.
Use a slicer to cut green Onions into 4 to 5mm segments.
Spread the chopped scallions evenly into a freeze-dried dish with a loading capacity of 8-9kg /m2.
Quick-freeze, the chives with the freeze-dried dish is placed on the special crane and pushed into the quick-freeze storage. Pay attention to placing the temperature probe. When the temperature reaches below 18℃, it can be maintained for 0.5h.
Freeze-drying, push the pre-frozen material into the prepared freeze-drying machine quickly, the faster the action, the better, quickly close the door of the drying box, and immediately start to vacuum, the time to complete the above part is generally about 10 to 15 minutes, too long may cause the melting of the material surface. After reaching the working pressure, it starts to heat according to the set heating curve. The heating curve of lyophilized chives is generally divided into 6 sections: within 30mim, the temperature is evenly heated to 120℃, and the equipment with excellent performance can reach it within 15min; The temperature was maintained at 120℃ for 3h; Within 1h, the temperature was evenly cooled to 80℃. Maintain a constant temperature at 80℃ for 2h; In 1.5h, the temperature was evenly cooled to 60℃. Maintain a constant temperature at 60℃ for 2h. The whole lyophilization cycle is about 10h. Since the performance of each dynamic drying equipment is different, the above parameters are only for reference. Whether it can be used should be determined by test first.
Discharge, Discharge should be carried out in a closed and clean area with less dust. The indoor relative humidity is below 50% and the temperature is 22-25 ℃. If there is no time for semi-finished products to be inspected after discharge, the opening should be closed in the container first.
Semi-finished products sorting - mainly pick out inclusions, bad cutting and other unqualified products, its environmental requirements and unloading the same.
Packing, According to the specifications of the contract with the customer packaging, usually 5kg/ box, 8kg/ box, 10kg/ box for choice, in order to prevent the stack of outer packing box deformation, the outer box is often designed into a high box (square bottom, the height is about two and a half times the length of the bottom edge), and loading only to 2~2.5cm from the top is appropriate.
(1) Process flow
Selection of raw materials: fruit washing, shell removing, core removing, cleaning, cutting, pre-freezing and freeze-drying packaging.
(2) Operation points
Pre-freeze the chopped lychee meat to a temperature of 30℃. When freeze-drying, the cold trap temperature should be pre-cooled to 35℃, the drying chamber door should be opened, the frozen lychees should be loaded, the chamber door should be closed, and the vacuum unit should be started to vacuum. When the vacuum degree reaches 60Pa, the heating should be started. During the heating process, the stable working vacuum degree should be guaranteed, and the maximum temperature of the article should not exceed 50℃. Freeze-dried, vacuumed and packed with nitrogen.