UNITED STATES DEPARTMENT OF AGRICULTURE 



MISCELLANEOUS PUBLICATION NO. 540 



Washington, D. C. 



Issued July 



VEGETABLE AND FRUIT DEHYDRATION 

 A MANUAL FOR PLANT OPERATORS 



Prepared by Bureau of Agricultural and Industrial Chemistry, Agricul- 

 tural Research Administration 



CONTENTS 



l 



Introduction 



Locating new plants... 



Plant lay-out, equipment, and capital invest- 

 ment 



Buildings and lay-out 



Preparation line 



Dehydrator. 



Packaging room 



Plant and equipment costs 



Handling capacities and utility require- 

 ments 



Storing and handling fresh fruits and vege- 

 tables 



Receiving the product 



Storage 



Transportation 



Preparation of raw materials 



Washing 



Grading 



Root peeling 



Trimming 



Cutting 



Blanching 



Adequacy of blanching as measured by 



tests for enzyme activity 



Methods of blanching 



Vegetable blanchers 



Sulfur ing vegetables 



Principles involved in the drying process 



The vaporization of water 



Properties of air and water vapor 



Volume of mixtures of air and water vapor- 

 Specific heat of air- water vapor mixtures— 



Wet-bulb temperature 



Humidity charts 



Characteristics of evaporation from a 



moist solid 



Evaporation from the surface of a wet 



material 



Removal of water from deeper layers 



Effect of thickness of piece 



"Case hardening" 



E quilibrium moisture content 



Typical drying curves, constant drying 



conditions 



Effect of wet-bulb depression 



Effect of temperature level . 



Effect of air velocity 



Manner of exposure to air stream 



Effect of thickness of layer 



Character of the support 



Manner of preparation 



Shape and size of pieces 



Nature of the fruit or vegetable 



Estimation of drying time 



Drying conditions within a dehydrator. .. 



Cooling of air due to evaporation 



Maximum evaporation capacity of a de- 

 hydrator 



Experimental determination of drying 



time 



Effects of recirculation of air 



Heat usage in a dehydrator 



Balance between efficiency and effective- 

 ness _ 



Page 

 2 

 3 



74 



Page 

 Effect of drying conditions on quality of prod- 

 uct 74 



Heat damage— scorching 



Damage to nutritive quality. 

 Damage to color and flavor. . 



Conveyor-belt dehydrators 



Tunnel-and-truck dehydrators— 



Counterflow arrangement 



Parallel-flow arrangement... 

 Combination arrangements- 

 Compartment arrangement— 

 Closed-cycle arrangement- 



s' 



C ombined blanching and predry ing 84 



Starting and stopping the tunnel drier 84 



Simple counterflow tunnel 84 



Parallel-flow tunnels in multistage driers . 87 



Counterflow tunnel as second-stage drier . 88 



Cabinet dehydrators 88 



Types of cabinet dehydrators 90 



Preparation equipment for use with 



cabinet driers 92 



Operation of the cabinet drier 92 



Continuous dehydration in cabinet driers.. 93 

 Establishing a time and temperature 



schedule for cabinet dehydration 94 



Sources of heat for dehydrators 96 



Direct-combustion heaters 96 



Indirect heating system 98 



Temperature controllers 99 



Mechanical movement of air in dehydrators ... 103 



Centrifugal or rotary impeller fans 103 



Propeller, axial-flow, or disk fans 104 



Application of fan laws in choice of fan. . . 104 



Bin-type finishing driers 106 



Types of bin driers 106 



Operation of bin driers 107 



Determining needed capacity 108 



Air desiccators 110 



Sources of heat 111 



Other types of dehydrators and their uses 111 



Spray driers 111 



Vacuum driers 112 



Rotary-drum driers 113 



Rotary driers 113 



Apple and prune kiln driers 114 



Multistage dehydration 114 



Changes in material as drying progresses . 115 



Examples of multistage systems 115 



Finishing and packaging 116 



Final inspection of the dry product 119 



Grades and specifications for fruits and 



vegetables 120 



Temperature reduction to maintain quality in 



dehydrated products 120 



Standard types of packages 122 



Packaging equipment and methods 124 



Cylinder-and-meter method 124 



Vacuum-chamber method 126 



Gassing by means of "dry ice" 126 



Sealing 128 



Packaging in cartons 128 



Labeling packages and cases 128 



128 



