Abstract:
An apparatus  20  for microwave vacuum-drying organic materials such as foods and bioactives has a plurality of microwave generators  50  actuated so as to cause interference between their respective microwave streams and evenly distribute the microwave energy across the vacuum chamber  34.  The microwave-transparent window  36  in the chamber is arranged so the organic material to be dried is moved across it on a conveyor belt  60  and the microwave energy passing into the chamber  34  immediately encounters the organic materials, thus attenuating the energy and reducing arcing.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention pertains to apparatuses and methods for microwave vacuum-drying of organic materials, including food products and biologically-active materials such as vaccines, antibiotics, proteins and microorganism cultures. 
       BACKGROUND OF THE INVENTION 
       [0002]    Dehydration of organic materials is commonly done in the food processing industry and in the production of biologically-active materials. It may be done in order to preserve the products for storage. It may also be done to create a product that is used in the dehydrated form, for example dried herbs and various kinds of chips. Conventional methods of dehydrating organic products include air-drying and freeze-drying. Both of these drying methods have their limitations. In general terms, air-drying is slow and freeze-drying is expensive, and both methods tend to degrade the appearance and texture of the products, which is undesirable in the case of foods. 
         [0003]    Another method employed to dehydrate food products and bilogically-active materials is microwave vacuum dehydration. Examples of this in the patent literature include WO 2009/049409 A1, Durance et al., published Apr. 23, 2009, and WO 2009/033285 A1, Durance et al., published Mar. 19, 2009. Microwave vacuum-drying is a rapid method that can yield products with improved quality compared to air-dried and freeze-dried products. Because the drying is done under reduced pressure, the boiling point of water and the oxygen content of the atmosphere are lowered, so food and medicinal components sensitive to oxidation and thermal degradation can be retained to a higher degree than by air-drying. The drying process is also much faster than air- and freeze-drying. The present invention is directed to improvements in the art of microwave vacuum-drying. 
       SUMMARY OF THE INVENTION 
       [0004]    One aspect of the invention pertains to the mode of operation of microwave generators in the drying of organic materials in a vacuum chamber. The present inventors have determined that, by employing multiple microwave generators and actuating them in programmed combination and sequence, it is possible to control with precision the microwave field across a microwave transparent window. By utilizing the interference of microwaves, the invention can achieve electrically-controlled microwave random scanning so that the organic materials are treated uniformly by microwave energy. This in turn permits enhanced control of the drying process. 
         [0005]    When two or more microwave sources are operating together such that the output streams of microwaves overlap, the output streams will combine and interfere with each other in a confluent stream or beam, analogous to two rivers joining together. In the case of two microwave streams, if they are out of phase with each other, the phase shift will cause a reorientation of the confluent stream of microwaves. The inventors have determined that when multiple microwave sources are actuated and de-actuated randomly in time, the result is a random orientation of the microwave beam. This principle can be employed to randomly and evenly distribute or scan the confluent microwave stream across a defined space, without the need for mechanically reorienting the microwave generators. In the present invention the microwave stream is distributed or scanned across a microwave-transparent window for transmission of microwave radiation into a vacuum chamber containing organic material to be dehydrated. The material to be dehydrated may optionally be in movement through or within the vacuum chamber. Such movement of the material may aid in further distribution of microwave energy within the material. 
         [0006]    The microwave chamber may incorporate a matched water load designed to absorb microwave energy that has passed through the window and through the organic material being dehydrated. The water load serves to reduce reflection of excess microwave radiation within the vacuum chamber so as to control the potential for nodes of high electric field and thus reduce the potential for arcing. 
         [0007]    According to one aspect of the invention, there is provided an apparatus for dehydrating organic material, comprising a vacuum chamber, a set of two or more microwave generators, a microwave-transparent window for transmission of microwave radiation into the vacuum chamber, a microwave chamber between the microwave generators and the window, and means for controlling the operation of the microwave generators to actuate and de-actuate them in programmed combination and sequence, causing interference between the stream of microwaves from any one generator and the stream of microwaves from any other generator of the set. 
         [0008]    Another aspect of the invention pertains to reducing the arcing of microwave radiation that occurs in microwave vacuum dehydrators. Arcing can cause burning of the products being dehydrated. The inventors have determined that such arcing can be reduced by an arrangement in which the organic material is placed so that radiation passing through the microwave-transparent window of the vacuum chamber immediately encounters the organic material, before passing farther into the vacuum chamber. This has the effect of attenuating the microwave energy within the vacuum chamber and thus reducing arcing. Microwave reflections caused by the organic material go back into the microwave chamber, where they can create standing waves and hot spots. Since the microwave chamber is at atmospheric pressure, the likelihood of arcing is very low. The arrangement is accomplished by placing the organic material on or close to the window, and optionally by conveying the organic material across the window, for example on a microwave-transparent conveyor belt that is on or close to the window. 
         [0009]    According to this aspect of the invention, there is provided an apparatus for dehydrating organic material, comprising a vacuum chamber, a microwave generator, a microwave-transparent window in the vacuum chamber, a microwave chamber between the window and the generator, and optionally means for conveying the organic material across the microwave-transparent window within the vacuum chamber. 
         [0010]    Both of the aforesaid features, i.e. the control of the microwave field and the reduction of arcing, can be incorporated into a single apparatus. According to this aspect of the invention, the apparatus includes a vacuum chamber, a set of two or more microwave generators, a microwave-transparent window in the vacuum chamber, a microwave chamber between the microwave generators and the window, the window being positioned so that the organic material can be placed on or adjacent to it such that microwave radiation passing through the window immediately encounters the organic material, and means for controlling the operation of the microwave generators to actuate and de-actuate them in programmed combination and sequence, causing interference between the stream of microwaves from any one generator and the stream of microwaves from any other generator of the set. 
         [0011]    The invention further provides methods for dehydrating an organic material. Examples of materials suitable for dehydration by the invention include fruit, either whole, puree or pieces, either frozen or un-frozen, including banana, mango, papaya, pineapple, melon, apples, pears, cherries, berries, peaches, apricots, plums, grapes, oranges, lemons, grapefruit; vegetables, either fresh or frozen, whole, puree or pieces, including peas, beans, corn, carrots, tomatoes, peppers, herbs, potatoes, beets, turnips, squash, onions, garlic; fruit and vegetable juices; pre-cooked grains including rice, oats, wheat, barley, corn, flaxseed; hydrocolloid solutions or suspensions, vegetable gums; frozen liquid bacterial cultures, vaccines, enzymes, protein isolates; amino acids; injectable drugs, pharmaceutical drugs, natural medicinal compounds, antibiotics, antibodies; composite materials in which a hydrocolloid or gum surrounds and encapsulates a droplet or particle of a relatively less stable material as a means of protecting and stabilizing the less sensitive material; meats, fish and seafoods, either fresh or frozen, either whole, puree or pieces; dairy products such as milk, cheese, whey proteins isolates and yogurt; and moist extracts of fruits, vegetables and meats. 
         [0012]    One aspect of the method of dehydration pertains to the mode of operation of the microwave generators. According to this aspect, the method comprises introducing the organic material into a vacuum chamber, reducing pressure in the vacuum chamber to less than atmospheric, actuating and de-actuating a set of two or more microwave generators in programmed combination and sequence, causing interference between a stream of microwaves from any one of the generators and a stream of microwaves from any other of the generators, and applying the streams of microwave radiation through a microwave-transparent window into the vacuum chamber to dehydrate the organic material, and removing the dehydrated material from the vacuum chamber. 
         [0013]    Another aspect of the method of dehydration pertains to the reduction of the arcing of microwave radiation in a microwave vacuum dehydrator while carrying out dehydration of the organic material, such that the microwave field is attenuated by the organic material immediately upon passage through the window. According to this aspect, the method comprises introducing the organic material into a vacuum chamber, reducing the pressure to a pressure less than atmospheric, applying microwave radiation through the window to dehydrate the organic material on or close to the window, optionally conveying it through the vacuum chamber, and removing the dehydrated organic material from the vacuum chamber. 
         [0014]    Both of the aforesaid methods can be incorporated into a single method, which includes conveying the organic material across a microwave-transparent window and also actuating and de-actuating a plurality of microwave generators in programmed combination and sequence to apply microwave radiation through the window. 
         [0015]    These and other features of the invention will be apparent from the following description and drawings of the preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is an isometric view of an apparatus according to one embodiment of the invention. 
           [0017]      FIG. 2  is an isometric view from the opposite side of the  FIG. 1  view, with the output module housing and the vacuum chamber cover removed. 
           [0018]      FIG. 3  is a cutaway view on the line  3 - 3  of  FIG. 1 . 
           [0019]      FIG. 4  is a cutaway view on the line  4 - 4  of  FIG. 2 . 
           [0020]      FIG. 5  is a longitudinal cutaway of another embodiment of the apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Throughout the following description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
         [0022]    The dehydrating apparatus  20  comprises a processing unit  22 , in which organic material is microwave vacuum-dried. The unit has an input end  24  and an output end  26 , with a raw material loading module  28  at the input end and a dehydrated material unloading module  30  at the output end. The processing unit  22  is supported on a frame  32 . 
         [0023]    A vacuum chamber  34  extends the length of the processing unit  22 . A microwave-transparent window  36 , made of Teflon, forms the bottom wall of the vacuum chamber. The vacuum chamber has a cover  38  and side walls  40 , with support members  106  between the walls. Microwave chamber modules are arranged below the window  36 , there being four such modules  42 A,  42 B,  42 C,  42 D in the illustrated embodiment. Each module has a set of six microwave generators  50  and a microwave chamber  52 . Each microwave chamber has a floor  54  with recesses  56 , each accommodating a respective microwave generator  50 , and two lateral side walls  58  which flare outwardly in the upward direction. The transverse side walls  59  of the microwave chambers separate the adjacent microwave chambers. The microwave-transparent window  36  forms the top wall of the microwave chambers  52 . The microwave chambers are not sealed from the atmosphere and are thus air-filled and at atmospheric pressure. The microwave chambers are used to create interferential microwaves by means of space and distance. 
         [0024]    The embodiment of  FIGS. 1 to 4  has six microwave generators in each module, but the apparatus can alternatively have a different number of generators in each module, so long as there are at least two generators in each module, so that interference can occur between the waves generated by the respective generators. For example, each module may have two, three, four or more microwave generators. The generators within a module may be arranged in two or more rows with each row having two or more generators. For example, in the embodiment of  FIGS. 1 to 4 , there are two parallel rows (a row being oriented perpendicular to the longitudinal axis of the processing unit  22 ) with three generators per row. Alternatively, there may be a single row of generators in each module, perpendicular to the longitudinal axis of the processing unit, each row having two, three or more generators.  FIG. 5  illustrates an embodiment  200  of the apparatus in which there are three microwave generators  50  in each module, arranged in a single row. In all cases, the spacing between the generators within a module is selected to be such that interference will occur between the microwave streams generated by the respective generators. 
         [0025]    A microwave-transparent conveyor belt  60  for transport of the organic material through the vacuum chamber extends along the window  36  and is in direct contact with it. The conveyor belt extends into the loading and unloading modules  28 ,  30 , as described below, and forms a continuous loop, for example by running below the microwave generators. 
         [0026]    The loading module  28  has a raw material feed distributor  62  positioned above the conveyor belt  60  and configured to drop raw material to be dehydrated onto the conveyor belt. The belt runs over the conveyor rollers  63  in the loading module. A raw material supply tank  64  is connected via a feed conduit  66  to the raw material feed distributor  62 . A feed controller  68  controls the flow of raw material into the distributor. The supply tank  64  is at atmospheric pressure. A raw material in liquid form for dehydration can be sucked into the distributor and vacuum chamber by the pressure differential between the vacuum chamber and the supply tank. A raw material for dehydration in the form of pieces or granules, or in open containers, may be introduced into the processing unit  22  by a conduit, auger or other transfer device capable of transporting material into the vacuum chamber without substantial loss of vacuum. For example, a series of individual open containers such as glass serum vials may be introduced into the vacuum chamber and conveyed across the window, either on a microwave-transparent belt, or directly on the window. The loading module  28  has a housing  70 , fastened and sealed to the input end  24  of the processing unit. The interior of the loading module is open to the vacuum chamber and is accordingly at reduced pressure during operation of the apparatus. A viewing window  72  in the housing permits visual inspection into the vacuum chamber. 
         [0027]    The unloading module  30  has conveyor rollers  74  for guiding the conveyor belt  60 . A material collector  76  is positioned under the outward roller to receive dehydrated material that is dropped from the conveyor belt. A drive roller  78 , rotated by a motor  80 , drives the conveyor belt. The unloading module  30  includes a housing  82 , fastened and sealed to the output end  26  of the processing unit  22  and a viewing window  73 . The unloading module is open to the vacuum chamber and is thus at reduced pressure during operation of the apparatus. 
         [0028]    Optionally, and as illustrated in  FIG. 3 , the conveyor belt  60  may form a continuous loop without extending below the microwave generators. Here, the conveyor belt path is around a single roller  63  in the loading module and a single roller  74  in the unloading module, with the return path of the belt being between its forward path and the microwave-transparent window  36 . The belt in its forward path thus lies on the belt in its return path, which in turn lies on the microwave window  36 . A drive roller (not shown in  FIG. 3 ) is provided to drive the belt. 
         [0029]    An auger conveyor  84  is positioned under the material collector  76  and receives the dehydrated material from it. A vacuum seal  86  between the lower end of the material collector  76  and the auger conveyor  84  maintains the vacuum within the vacuum chamber  34  and auger conveyor  84 . The auger  84  is driven by a motor  88 . A pair of outlet valves  90 A,  90 B at one end of the auger conveyor provides for removal of the dehydrated material from the apparatus. The valves  90 A,  90 B function as an airlock to permit the removal of dried product. Vacuum-sealed containers (not shown in the drawings) to receive the dried product are attached to the valves. One valve is open at a time to permit one container to receive product from the auger, while the other valve is closed to permit removal of a filled container from that valve. The two valves are opened and closed alternately to permit the auger to run continuously. 
         [0030]    The dehydrating apparatus includes a vacuum pump  96  operatively connected via a conduit  97  to vacuum distributor  110 , which in turn connects to the vacuum chamber at vacuum ports  98  in the side walls  40  thereof. A condenser  100  is operatively connected to the refrigerator unit  102 , for condensing water vapor produced during dehydration of the organic material. Alternative, the condensor may be situated outside the vacuum chamber, connected between the vacuum chamber and the vacuum pump. 
         [0031]    The apparatus includes a refrigeration unit  102 , comprising a compressor, cooling fan and refrigerant pump, connected to convey refrigerant via a refrigerant pipe  108  to the condensor  100  and thus maintain the condensor at a desired temperature. 
         [0032]    A water load is provided at the upper part of the vacuum chamber  34  to absorb microwave energy and thus prevent reflection of microwaves in the vacuum chamber. This is accomplished by microwave-transparent water tubing  112 , shown in  FIG. 4 , under the cover  38  of the vacuum chamber. The water that is pumped through the tubing by a pump (not shown in the drawings) can be salt or fresh water. The tubing  112  may be in various sizes and configurations to match the microwave field. 
         [0033]    The apparatus  20  includes a programable logic controller (PLC), programmed and connected to control the operation of the system, including controlling the inflow of feedstock, the motors, the microwave generators, the vacuum pump and the refrigerant pump. It is programmed to actuate and de-actuate the microwave generators within each of the microwave generator modules  42 A to  42 D. Whenever a microwave generator is actuated, the microwave created by the generator will have a new phase. Because of the microwave interference, the microwave with the new phase interferes with other microwaves and creates a new microwave distribution throughout the organic materials. Although microwave generators are actuated and de-actuated via the PLC programming, microwave phases are created randomly. The microwave distribution is thus changed randomly and frequently. Over time, the average microwave energy throughout the organic material is equalized. 
         [0034]    For example, within a module, a given generator may be turned on at a set time for a duration of five seconds, then off for two seconds, then on for five seconds, and so on. It is preferred that at least two of the generators within a module are radiating at any given time so that the power output of the generators is higher and is available for product dehydration. The microwaves produced by any one generator are always out of phase with the microwaves produced by each other generator, due to the inherent random variation in the time at which the generators are actuated. The microwave streams thus interfere with each other to produce reinforced pulses of microwave energy which enter the vacuum chamber. The direction of the pulses varies randomly throughout the area in the chamber irradiated by the generators in a module. Over a period of time, all parts of that area are subjected to a substantially equal amount of energy. 
         [0035]    The dehydrating apparatus  20  operates according to the following method. The vacuum pump, refrigerant pump, water pump, microwave generators, motors  80 ,  88  and the raw material feed controller  68  are actuated, all under the control of the PLC. Pressure within the vacuum chamber may be in the range of about 0.01 to about 100 Torr, alternatively about 0.1 to about 30 Torr. The organic material to be dehydrated is fed onto the conveyor belt  60  and is carried through the vacuum chamber across the microwave-transparent window. The material is dehydrated by the radiation from the generators passing through the window. Processing time may be in the range of about 0.5 to 2 hours. The dehydrated material falls into the material collector  76 , moves into the auger conveyor  84  and is removed from the apparatus through the outlet valves  90 A,  90 B. 
       Example 1  
       [0036]    A dehydration apparatus in the form of the apparatus  20  described above has microwave generators each having a power output of 1,200 watts. The peak power within a given microwave generator module of four generators is accordingly 4.8 kilowatts. The vacuum system evacuates the apparatus to an absolute pressure of 0.1 Torr. Each microwave chamber has a height of 14.5 inches (36.8 cm), a length of 27 inches (68.6 cm) and a width (at the top) of 28 inches (71.1 cm). The spacing between the microwave generators (center to center) is 12 inches (30.5 cm) between adjacent rows and 6 inches (15.2 cm) between adjacent generators within a row. The conveyor belt is operated at a speed in the range of 0.01 to 1 centimeters per second. 
       Example 2  
       [0037]    An embodiment of the invention for drying organic materials in a batch mode was operated to dehydrate various foods, bioactive materials and materials in vials, in some cases using frozen materials. The final moisture content of the dehydrated material was measured.
       (a) Bulk samples of beef chunks, brussels sprouts, green peas and egg whites were dried at a pressure of 35 Torr. The results are shown in Table 1.       
 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Average 
                 Final 
               
               
                   
                 Drying 
                 microwave 
                 moisture 
               
               
                 Sample and Weight 
                 Time 
                 power (W) 
                 content 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Beef chunks 
                 (874 g) 
                 130 min 
                 1532 
                  1.02 +/− 0.061% 
               
               
                 Brussel sprouts 
                 (843 g) 
                 288 min 
                 973 
                 4.66 +/− 1.17% 
               
               
                 Green peas 
                 (759 g) 
                 287 min 
                 946 
                 4.59 +/− 0.61% 
               
               
                 Egg White 
                 (384 g) 
                 236 min 
                 1473 
                 1.76 +/− 0.55% 
               
               
                   
               
             
          
         
       
       
         
           
             (b) Frozen samples of bioactive materials, namely lipase and amylase, were dried at a pressure of about 0.15 Torr or less. The results are shown in Table 2. 
           
         
       
     
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Average 
                 Final 
               
               
                   
                 Drying 
                 microwave 
                 moisture 
               
               
                 Sample and Weight 
                 time 
                 power (W) 
                 content 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Lipase (15% w/v) 
                 (337 g) 
                 9.5 hours 
                 1298 
                 3.06 +/− 0.36% 
               
               
                 α-amylase 
                 (241 g) 
                  12 hours 
                 1071 
                 2.60 +/− 0.24% 
               
               
                 (15% w/v) 
               
               
                   
               
             
          
         
       
       
         
           
             (c) Samples of materials in vials were dried at a pressure of about 0.15 Torr or less. The results are shown in Table 3. 
           
         
       
     
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                   
                 Average 
                 Final 
               
               
                   
                 Drying 
                 microwave 
                 moisture 
               
             
          
           
               
                 Sample and Weight 
                 time 
                 power (W) 
                 content 
               
               
                   
               
             
          
           
               
                 
                   Lactobacillus 
                 
                 (215 vials, 
                  96 min 
                 1118 
                 2.97 +/− 0.8%  
               
               
                   
                 1 g each) 
               
               
                 Skim milk solution 
                 (220 vials, 
                 168 min 
                 1390 
                 2.05 +/− 0.01% 
               
               
                   
                 1 g each) 
               
               
                 Lipase (20% w/v) 
                 (215 vials, 
                 230 min 
                 652 
                 3.47 +/− 0.55% 
               
               
                   
                 2 g each) 
               
               
                   
               
             
          
         
       
     
         [0041]    It is to be understood that, although particular means for performing certain functions of the apparatus, or particular structures or steps, have been described above in respect of the preferred embodiments, various other means, structures and steps may be employed in the apparatus and method of the invention. Examples of this include the following.
       (i) The means for conveying the organic material across the microwave-transparent window can include means such as vibration of the window, sloping the window and using gravity, mechanical pushers, etc.   (ii) The means for reducing the pressure in the vacuum chamber can include any means for applying a vacuum to the vacuum chamber, such as connection to a central vacuum system of a plant.   (iii) The means for loading the organic material into the vacuum chamber and/or for unloading dehydrated material can be structured to facilitate the loading of various forms of material, e.g. solids, gels, etc., and the handling of materials in containers, for example vaccines contained in vials.   (iv) The dehydration apparatus may be structured to operate in batch mode as well as continuous mode. For batch mode, the apparatus may not require loading and unloading modules, or any conveyor for moving the organic materials through the vacuum chamber. Rather, the materials are placed in the vacuum chamber, which is then sealed and evacuated. After dehydration, the vacuum is broken, the vacuum chamber is unsealed and the dried materials are removed. Such operation may be mechanized or done manually by an operator.   (v) The processing apparatus may include any desired and practical number of modules, including a single module. The modules may be arranged in a stacked configuration in order to reduce the floorspace require by the apparatus.       
 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           20  dehydrating apparatus 
           22  processing unit 
           24  input end of processing unit 
           26  output end of processing unit 
           28  loading module 
           30  unloading module 
           32  frame 
           34  vacuum chamber 
           36  microwave-transparent window 
           38  cover of vacuum chamber 
           40  side walls of vacuum chamber 
           42 A-D microwave chamber modules 
           50  microwave generator 
           52  microwave chamber 
           54  floor of microwave chamber 
           56  recesses in floor 
           58  side walls of microwave chamber 
           59  end walls of microwave chambers 
           60  conveyor belt 
           62  raw material distributor 
           63  conveyor belt rollers in loading module 
           64  raw material tank 
           66  feed conduit 
           68  feed controller 
           70  housing of loading module 
           72  viewing window of loading module 
           73  viewing window of unloading module 
           74  conveyor belt rollers in unloading module 
           76  material collector 
           78  drive roller 
           80  motor for drive roller 
           82  housing of unloading module 
           84  auger conveyor 
           85  vacuum seal in unloading module 
           88  motor for auger conveyor 
           90 A, B outlet control valves 
           96  vacuum pump 
           97  vacuum conduit 
           98  vacuum ports in vacuum chamber side walls 
           100  condenser 
           102  refrigeration unit 
           106  vacuum chamber supports 
           108  refrigerant pipe 
           110  vacuum distributor 
           112  water load tubing 
           200  dehydrating apparatus