Floating Peas To Clean Them

A. M. Neubert.

Froth flotation, a technique first used by the mining industry for concentrating and enriching ores, has been adapted to the commercial cleaning of vined green peas to be canned or frozen. It reduces labor in inspecting and sorting, improves the quality of the packaged product, and saves for market large tonnages of peas that would otherwise be discarded because of heavy contamination with foreign material.

Present methods of processing green peas exemplify the efficiency that can be achieved in the commercial production and preservation of a highly perishable food through the application of science. Advances made by the industry now permit operations on a tremendous scale. A number of processors in the Blue Mountain area of Washington and Oregon harvest 7,500 to 12,000 acres of peas annually and are equipped to process from 300 to 500 tons of shelled peas a day. Labor requirements in harvesting, preparing, Processing, and warehousing canned peas have been reduced in some cases to less than 1 1/3 man-days a ton of peas processed, or about 25 seconds of labor for each 20-ounce can.

By the proper selection of land, varieties, and planting schedules, the Processor obtains a continuous supply If green peas of prime maturity over a period of 30 to 60 days. Seeding, insect control, and harvesting are highly mechanized and provide shelled peas for processing with little hand labor.

The pea viner, a special type of thresher that removes the peas from the pods on the vine by a controlled beating action, may be considered the key to the industry. It permits the recovery of the tender green peas from the cut vines without picking and shelling the pods by hand. Processing operations consist of washing the vined peas, removing foreign material, sorting the peas by size and maturity, blanching, packaging, preserving by heat or freezing, and warehousing. Here again, mechanization and technical controls have greatly reduced the need for hand labor.

Although the pea viner makes large-scale commercial processing practical, it contributes a difficult problem to later preparation. The beating action of the viner breaks some of the more tender peas, which, with the debris from pods and leaves, contaminate the shelled peas. Other more serious contaminants are the seeds and flowers from weeds that grow in the fields and are harvested with the pea vines.

Many of the preparation steps in the processing plant were developed for the purpose of removing dirt, debris, and foreign materials from the vined peas. Preliminary sizing removes material that does not fall within the size range of the peas. Air separation removes material less dense than peas. A water wash . removes dirt and also makes a separation based on density. Stones and other heavy material are trapped in riffles, and material that will float in water is skimmed off the surface of the washer. The washed peas may be graded for size and passed through reels or shaker screens, where some additional debris is removed on the basis of size.

Those steps remove most of the debris and foreign material from the vined peas, but the burden of assuring completely clean peas rests on hand sorters. In normal operations, where peas are practically free from weed parts, hand-sorting requirements are not excessive in the larger sizes. Sorting costs are considerably higher, however, for the very small peas, which contain most of the broken peas and skins.

The sorting problem is seriously complicated when the vined peas contain weed parts that cannot be removed by the common mechanical cleaning methods, notably nightshade berries, dogfennel flowers, and tarweed seeds.

The problem of removing such material by hand becomes clearer when one realizes that a 20-ounce can will hold more than 1,500 small peas. If the peas are contaminated with only 1 percent of foreign material, for example, it becomes necessary to pick out more than 15 pieces of such material for each can. If even one piece is overlooked, the peas will not meet the rigid requirements for Fancy peas.

Because only a limited amount of foreign material can be removed by hand in commercial processing, it has frequently been necessary to slow up the sorting line or discard the contaminated peas entirely. Nightshade berries are particularly troublesome, because their resemblance to peas in size, shape, and color makes complete removal by hand difficult, even if they are present in only small amounts. Losses due to contamination with nightshade berries alone in peas grown in the Blue Mountain area of Oregon and Washington were estimated by various canners at 3 to 10 percent of the 1941 crop. It was to remove these foreign materials from the vined peas that the froth-flotation process was developed.

DIFFERENCES in size, shape, or density are the basis for the separation of foreign material from sound peas by the conventional mechanical cleaning methods. As broken peas and the troublesome weed parts do not differ sufficiently from sound peas in those properties, some other physical characteristic must be selected as the basis for separating them.

The froth-flotation process employs differences in wettability ( the property that determines how readily a solution will spread on a surface) to separate the sound peas from foreign matter. Materials of the same density, but differing sufficiently in wettability, can be mechanically separated on the basis of the property by being placed in a suitable solution. The material not easily wetted will float; that easily wetted will sink. This type of separation can be made, even though the density of the floating material greatly exceeds that of the supporting solution.

In practical applications of the principle, the materials to be separated are usually introduced into a solution containing air dispersed as fine bubbles. By control of the wetting properties of the solution, bubbles are attached to materials that are not easily wetted. The attached bubbles buoy such materials up so that they float. Easily wetted materials, however, sink through the solution and bubbles. Thus separation is effected.

In early applications of froth flotation, some success was achieved in the separation of foreign material from peas by careful control of the wetting properties of a water solution through the addition of synthetic detergents or salt.

Slight natural differences in wettability between peas and foreign material, particularly nightshade berries, were not great enough, however, to make its complete removal from peas practical solely by control of wetting properties of the separating solution. Further experiments showed that the differences in wettability could be greatly increased by conditioning the surface of the peas and contaminating materials with a light mineral oil. After such conditioning, sound peas could be made to sink in a foaming solution of controlled wetting properties, so that a sharp separation could be made from nightshade berries and other foreign materials which floated. It was possible also to float cracked and broken peas, pea skins, and other viner debris because of the attachment of bubbles to the broken or torn surfaces. These preliminary observations were adapted to large-scale pea processing by the development of suitable equipment and controls.

On the basis of laboratory and pilot-plant studies, sodium lauryl sulfate, a synthetic detergent employed extensively for washing dishes and clothing, was selected for use in controlling the wetting properties of the separating solution. A deodorized, low-viscosity mineral oil was found most suitable for conditioning the surface of the peas. The most convenient form of the oil was an emulsion, which could be prepared by adding 0.5 to 1.5 percent oil to a water solution of the detergent. In that way, the detergent served to control the wetting properties of the solution, to emulsify the solution and thus disperse the oil, and to stabilize air incorporated into the solution as small bubbles.