The Drying Process

Obviously the first thing to be done in preparing a leaf meal is to dry the leaves. In the method of fractional drying developed at the Eastern Regional Research Laboratory, in Philadelphia, the thin and more valuable leaf-blade portion dries much more rapidly than the thicker and less valuable midribs and stems. To take advantage of these factors, the leaves are dried rapidly to the point where the blades are brittle but the stems and midribs are still tough. The material is immediately fed into a hammer mill having no screen. This breaks the brittle leaves loose from the stems. The two fractions are then separated in a current of air adjusted to blow the leaf fraction away from the heavier stems. The process is about 95 percent efficient. The wet stems can be discarded, or they can be crushed and dried separately to give a less valuable product.

In final pilot-plant studies a triple-belt, continuous drier was used. In it the air is circulated at high velocity downward through each belt individually. It is equipped for control of wet- and dry-bulb temperature, air velocity, and percentage of air recirculated. Experiments showed the following conditions to be best for this type of drier : Inlet air temperature of 240 F., rapid circulation of air through the bed of material at a rate of 175 cubic feet a minute per square foot of bed, a light loading on the first belt and heavier loading on the successive belts, and agitation of the material as it falls from one belt to the next to break up any lumps. The permissible load on the first belt varies with the kind of waste.

The actual size of the drier required would naturally depend on the quantities of waste available and hourly capacities desired, but for the purpose of general discussion and comparison of capacities for different wastes, a drier having a total drying area of 930 square feet has been selected. It would have an hourly production rate of from 300 to 1,200 pounds of dry, separated leaf meal, depending upon the waste being processed. With a belt 8 1/2 feet wide, a three-belt multiple-deck drier with this drying area would be approximately 40 feet long. A hammer mill that could handle approximately 1 ton an hour would take care of the drier output. The material then goes to an air separator consisting of a fan and large diameter pipes so arranged that the leaf tractions are discharged at the top through a cyclone separator and the stems come out at the bottom. The cleaned leaf material is then ground in a hammer mill to produce the final meal.

Waste lettuce, citrus pulp, and tomato skins and seeds are now being artificially dried for feed. In the Everglades section of Florida, where a large variety of vegetable materials are grown over a long season, utilization of waste for feeds is proving practical. Drying equipment is already installed for preparing feeds from sweetpotato vine (sweetpotatoes are grown in large quantities for the production of starch), ramie tops, which are removed before processing the plants for fiber, and a special kind of grass from which lemon oil is distilled. Machine-dried alfalfa is commonplace. Can some of the vegetable wastes we have mentioned join the ranks?

The tonnage of wastes is tremendous. Some are already accumulated in great quantities at vipers and packing sheds; others could easily be harvested mechanically, as is now done with sugar-beet leaves. Their seasonal span is favorable (beginning in June with peas, followed by various vegetables throughout the summer, lima beans in September, and broccoli in October and November). A highly nutritious leaf meal can be prepared from a number of them; they are relished and well utilized by poultry, at least. The mechanics of the drying and separating have been worked out. The over-all cost of the final leaf meal will vary with the different materials from $22 to $95 a ton.

The commercial feasibility of the proposition depends largely on two questions: Can exact locations be found where sufficient wastes are readily available for a sufficient number of successive weeks to keep the operating costs to a minimum? And will the consumer pay for the real worth of these meals, considering at least their content of protein, carotene, and riboflavin? We believe the answer to both questions is yes.

THE AUTHORS

J. J. Willaman is head of the biochemical division of the Eastern Regional Research Laboratory, Philadelphia, Pa.

R. K. Eskew is head of the chemical engineering and development division of the Eastern Regional Research Laboratory, Philadelphia.