Production process and case sharing of vacuum freeze-dried fruit and vegetable products (freeze-dried cranberries, blueberries)

A brief description of the main equipment and production line of vacuum freeze drying

1. Main equipment

The most important equipment for freeze-dried food production is the vacuum freeze-drying unit for food, the performance, energy consumption and the degree of operation automation of the unit determine the technical level of the freeze-dried food production enterprises, and the freeze-dryer for food is divided into batch and continuous. Among them, the batch freeze dryer is composed of five parts: drying box, heating system, vacuum system, refrigeration system and control system.

(1) Drying box: there are two main forms of cylindrical and square, each with its own advantages, cylindrical manufacturing is easy, but there is more space that cannot be used; On the contrary, square shapes have a high utilization rate of space, but they are more difficult to manufacture.

(2) Heating system: most of them use radiation heat transfer, the radiant plate is made of anodic electroplated aluminum, the heat conduction medium is heat conduction oil, saturated water, secondary steam, organic solvents such as propylene glycol, glycerin, etc., and the heat source is high-pressure water vapor.

(3) Vacuum system: the use of mechanical vacuum pump, there are roots pump + oil seal pump and roots pump + roots pump + roots pump + atmospheric injection + water ring pump, the main difference between the two is that the former can not pump steam, so the efficiency of the cold trap is higher, the advantage is that the energy consumption is small, the latter is just the opposite, can pump a small amount of water vapor, the disadvantage is that the energy consumption is large.

(4) Refrigeration system: It is composed of a refrigeration unit and a cooling pipe, and is used as a water vapor trap (also known as a cold trap) in the following forms according to its relative position with the radiant plate assembly: (1) bottom-mounted type: placed at the bottom of the radiant plate assembly; (2) Side-mounted: placed on both sides of the radiant panel assembly; (3) Rear-mounted: placed behind the radiant panel assembly; (4) Alternative: placed in the radiant plate components are not in the same container, placed in another container, and there is a short and thick pipe fitting connection between the two containers.

(5) Control system: there are two categories of manual control and automatic control, and automatic control is divided into two categories: instrument automatic control and PLC programmable control.

2. Configuration of simple freeze-dried food production line

(1) Pre-treatment equipment: (1) 1 multi-purpose slicer, with a production capacity of 200~500kg/h; (2) Cooling tank: the sterilized cooling tank can be replaced by a cleaning tank, and the simple production line usually uses an oscillating drain machine for tap water cooling and draining, if possible, it is best to use a centrifuge; (3) Quick-frozen warehouse: The configuration of quick-frozen warehouse is suitable for the processing capacity of the freeze-dryer, and it is advisable to produce ingredients in daily shifts to meet the lyophilization of 2~3 shifts. It is best to set the cold source separately or semi-separately (the low-pressure circulation part is independent) to prevent the temperature of the cold trap in the freeze-drying process from being affected when the heat load of the quick-freezing warehouse is large.

(2) freeze-drying workshop: freeze-drying workshop is the key process of freeze-dried food production, its setting should also take into account the degree of product quality impact, the shorter the distance between the general quick-freezing warehouse and the freeze-drying box door, the better, and this channel is best dedicated, only as a logistics channel, not as a flow of people, the door end of the freeze-drying box is best separated from other parts of the freeze-dryer and equipment, so as to reduce the pollution of the environment and the finished product.

(3) unloading and packaging room: unloading and packaging room should be closed, the ground should be at least with terrazzo waxing, the wall should be pasted with white tiles to 1.5m high, the above part should be brushed with white waterproof coating, the workshop must be equipped with air conditioning dehumidification equipment, air conditioning refrigeration can be used with the cold storage system, dehumidification equipment should be selected rotary dehumidifier, in order to reduce the cost of dehumidification, the workshop is best windowless, the door is as closed as possible, it is best to use a special sealed door for dust-free clean room, and there should be ventilation, disinfection, sterilization and other facilities.

When packing, there should generally be a stainless steel workbench, a vacuum nitrogen filling sealing machine, a water ring vacuum pump should be used for vacuum equipment, the pumping speed is about 20L/s, and the nitrogen filling source is industrial nitrogen, which can be filled by vacuum filtration.

3. Modern assembly line configuration

Compared with the simple production line, the modern assembly line only adds automatic mechanical equipment such as cleaning, peeling, antivirus, cooling, and conveying in the pre-treatment to reduce the amount of manual work in the pre-treatment.

Production process of freeze-dried fruits and vegetables

1. Process flow

Raw material selection→ finishing→ pre-freezing→ sublimation, drying→ post-processing→ packaging and storage.

2. Operational points

(1) Pretreatment: General pretreatment refers to all treatments before sublimation drying, so freezing before drying also belongs to pretreatment. The pretreatment of raw materials is the same as that of conventional fruit and vegetable drying and fruit and vegetable quick-frozen products, and it is necessary to select, clean, peel, cut, blanch, cool and other treatments. In the case of freeze-drying of fruit or vegetable juices, the product is pre-concentrated using a less expensive processing method and then turned into granules during pre-freezing.

Pre-freezing is the freezing of raw materials after pretreatment, which is an important process of freeze-drying. Due to a series of complex biochemical and physicochemical changes in the freezing process of fruits and vegetables, the quality of pre-freezing will directly affect the quality of freeze-dried fruits and vegetables. In the freezing process, it is important to consider the effect of the freezing rate of the frozen material on its quality and drying time.

There are the following differences between quick freezing and slow freezing: the ice crystals produced by quick freezing are smaller, and the ice crystals produced by slow freezing are larger; Large ice crystals are conducive to sublimation, and small ice crystals are not conducive to sublimation; Small ice crystals have less impact on cells, and the smaller the ice crystals, the more they can reflect the original tissue structure and properties of the product after drying. However, the freezing rate is high, and the energy required is also high. An optimal freezing rate should be selected to ensure the quality of freeze-dried food while minimizing the energy consumption required for freezing.

(2) Sublimation drying: It is the core process in the production process of freeze-dried food. It is necessary to control the process conditions.

(1) Loading capacity: When drying, the wet weight loading capacity of the freeze dryer is the quality of drying on the drying plate per unit area, which is an important factor to determine the drying time. The thickness of the food being dried is also a factor that affects the drying time. In freeze drying, the drying of the material is pushed from the outer layer to the inner layer, so when the material to be dried is thicker, a longer drying time is required. In the actual drying, the dried materials are cut into a uniform thickness of 15~30mm. The amount of materials that should be loaded into the drying plate per unit area should be determined according to the heating method and the type of dry food. When the industrial large-scale device is used for drying, if the drying cycle is 6~8h, the loading capacity of the drying plate is 5~15kg/m2.

(2) Drying temperature: In order to shorten the drying time, it is necessary to effectively supply the heat required for the sublimation of ice crystals, so various practical heating methods are designed. The drying temperature must be controlled within the range of not causing the ice crystals in the dried material to melt and the dried part will not cause thermal denaturation due to overheating. Therefore, in a single heating method, the temperature of the drying plate should be controlled at 70~80 °C in the early stage of drying with vigorous sublimation, 60 °C in the middle stage of drying, and 40~50 °C in the late stage of drying.

(3) Judgment of drying end point: The drying end point can be judged by the following indications: the temperature of the material and the temperature of the heating plate tend to be basically consistent and maintained for a period of time; The vacuum gauge of the pump set (or cold trap) tends to be consistent with the vacuum gauge of the drying chamber and is maintained for a period of time; The temperature of the cold trap of the vacuum gauge in the drying chamber basically returns to the index when the equipment is unloaded and is maintained for a period of time; For the freeze dryer with a large butterfly valve, the large butterfly valve can be closed, and the vacuum machine basically does not drop or falls very little. The above four judgment bases can be used separately, or in combination or in combination.

(3) Post-processing: Post-processing includes unloading, semi-finished product selection, packaging and other processes.

After the freeze-drying velocity, nitrogen or dry air is injected into the drying chamber to break the vacuum, and then the material is immediately removed and discharged in a closed environment with a relative humidity of less than 50%, a temperature of 22~25 °C and less dust, and the semi-finished products are sorted and packaged in the same environment. Because the lyophilized material has a large surface area and is very hygroscopic, it is necessary to complete these processes in a relatively dry environment.

(4) Packaging and storage: After freeze-drying, food has a huge surface area, and some ingredients in food are directly exposed to the air, which is easy to contact oxygen and adsorption moisture in the air, resulting in the gradual deterioration of freeze-dried food. Secondly, most freeze-dried foods have natural colors, which tend to degrade when exposed to light. Temperature is also an important factor in the oxidation and pigment degradation process. Therefore, the packaging of freeze-dried food mainly examines how to prevent or mitigate the impact of the above four factors.

The porous sponge-like 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 degrade the quality, so freeze-dried food should be vacuum-packed, preferably nitrogen-filled packaging; Second, freeze-dried food occupies a relatively large volume, which is not conducive to packaging, transportation and sales, so freeze-dried food is often compressed and packaged. In addition, the porous and loose structure makes the freeze-dried food easy to break during transportation and sales, so for those freeze-dried foods that are inconvenient to compress and package, the packaging materials or packaging forms should have a certain protective effect.

Freeze-dried food commonly used packaging materials for PE bags and composite aluminum-platinum bags, PE bags are often used for large packaging, composite aluminum-platinum bags are often used for small packaging, the outer packaging is usually made of kraft corrugated cardboard boxes, its size meets the needs of container transportation, when using PE bags for inner packaging, in order to strengthen its role of isolating oxygen, water and steam, it is commonly used double-layered, if necessary, it can also be packaged in iron cans, but the price is higher and 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 oxygen scavengers and desiccant.

Freeze-dried food should be stored in a cool, dry place, if possible, it is best to place it in a low temperature and low humidity environment, the shelf life is usually 1~2 years, and it can be appropriately extended when packaged in iron cans.

Examples of freeze-dried fruits and vegetables

1. Freeze-dried cranberries

(1) Fruit selection and cleaning: select high-quality mature cranberries, wash the cranberries with clean water, spray them with a concentration of 70% ethanol solution to sterilize, and place them in a cool and ventilated place to dry after spraying;

(2) Pre-freezing: quick-freeze the dried cranberries and reduce the temperature to below -40 °C within 15min; The fruit temperature was ≤ 35 °C and continued to be frozen for 5.0h;

(3) Vacuum freeze drying: put the frozen cranberries in a microwave vacuum dryer with a low temperature control device (-10 °C——-20 °C), start the vacuum pump, after 30min, the vacuum degree reaches 450Pa, start the microwave source, set the microwave power to 8W/g, the drying time is 6h, the material temperature is controlled in the range of no more than -10 °C during microwave drying, and the final water content reaches less than 5%;

(4) Packaging: Vacuum-packed cranberries after microwave drying.

2. Freeze-dried blueberries

(1) Cleaning: take 20kg of ripe fresh blueberry fruit, remove impurities, immerse in an aqueous chlorine dioxide solution with a concentration of 75ppm for 4min, drain and set aside;

(2) Pre-freezing: Put the blueberries into a freeze-drying tray and pre-cool them in a cold storage below -40°C for 180 min.

(3) Vacuum freeze drying: turn on the refrigeration machine to make the refrigerant temperature of the water trap reach -30~-40 °C; In the process of vacuum freeze-drying, the temperature of the heating plate is heated by computer automatic control program, and the heating process of the heating plate is carried out according to the function F=Fl+k×(t/t1)2, F is the temperature of the heating plate at the time of heating, t=0~tl, and the unit is min; The total time t1=1200min, the initial temperature of the heating plate F1=34°C, and the termination temperature F2=58°C:k=24. Put the pre-cooled blueberries into the vacuum freeze dryer and turn on the vacuum pump; When the vacuum degree of the vacuum chamber is equal to 50pa, start the program heating program and start the timing; The whole drying process keeps the vacuum degree small equal to 50Pa, about 1400min after the end of the freeze-drying of blueberry fruit, after the freeze-drying of blueberry fruit, the moisture content of the material is between 5.0%~7.0%.

(4) Packaging: Quickly move the material out of the freeze dryer and put it into an aluminum film bag.