Making Use of Tons of Citrus Waste

Harry S. Owens, M. K. Veldhuis, W. Dayton Maclay.

More than 2 million tons of pulp, peel, and rag remain each year after citrus fruits are processed into juice, frozen concentrate, and sections. What to do with those mountains of waste?

When the industry was young, the only wastes were the culls and surplus fruit, which were dumped on wasteland or used as soil conditioners on cultivated land. Solid wastes from early canneries were handled in the same way. The liquid wastes were ponded or flushed into streams, lakes, or sewers. All such makeshifts were unsatisfactory and dangerous. A pile of rotting orange peels soon begins to stink; underground water supplies are contaminated; and the increased biochemical oxygen demand kills aquatic life or exceeds the capacities of sewage-treating plants.

Industrial, State, and Federal research organizations investigated the increasingly serious problem. From their efforts came several economically valuable products. Now 80 to 90 percent of citrus wastes are converted into usable products, such as dried pulp, molasses, pectin, essential oils, brined peel, citric acid, limonene, feed yeast, and biologically active materials.

From the waste peel, mare, and seeds that came from the processing lines, research men developed dried pulp, which is used extensively for feeding dairy and beef cattle and is suitable for feeding other animals. It contains about 8 percent moisture, 6 percent ash, 6 percent crude protein based on total nitrogen, 6 percent crude fat, 14 percent crude fiber, and 66 percent nitrogen-free extract. Although it must be supplemented with some other feed, it contains significant amounts of protein, fats, and minerals.

To make dried pulp, fresh peel is first ground in a hammer mill. One-half to 1 percent of lime is added to the peel immediately after grinding; the amount is carefully measured in order to get the best pressing characteristics. The lime neutralizes the acids and catalyzes the de-esterification of the pectin in the peel to form calcium pectate, which facilitates pressing and drying. Formerly the peel was allowed to stand in bins for about 45 minutes before pressing or drying to allow time for the lime to react. Now the time of reaction is shortened to 15 minutes or so by stirring the peel constantly as it moves slowly through a pug mill. Continuous presses remove as much liquid as possible. In some processing plants the pulp is heated by direct steam injection to about 120 F. during this step to facilitate the pressing. The weight of liquor removed is about equal to the weight of the pressed pulp. Direct-fired or steam-heated rotary kiln driers are used to remove the moisture from the pulp. In some mills the pulp is given a preliminary drying in direct-fired units and finished in steam-heated units. Careful control of the drying rates and temperatures is necessary to produce the fluffy, light-colored feeds that are considered desirable. About 1 ton of feed is obtained from 10 tons of cannery waste. In the past 10 years, production of dried peel has increased to approximately 200,000 tons a year.

Press or drain liquor from citrus peel contains 5 to 7 percent sugar and a total of 10 to 12 percent soluble solids. It cannot be flushed into sewers or ponded unless care is taken to reduce the biochemical oxygen demand or to prevent bad smells. Most of the press liquor is concentrated to produce molasses. Multiple-effect evaporation is commonly used. One plant in Texas uses direct heating with a submerged gas flame, followed by two stages of evaporation.

The first stage of a multiple-effect evaporator is operated under positive pressure (up to 26 pounds) and the last stage under negative pressure (down to 26 inches of mercury). Intermediate stages may be used to increase the number of pounds of water evaporated per pound of fuel consumed. Evaporators may be constructed of mild steel. Corrosion is something of a problem if not all of the acid is neutralized by the lime. A more serious problem is scaling of the heat-exchanger tubes in the evaporators, because the scale builds up rapidly, interferes with heat transfer and circulation, and must be removed about twice a week by boiling with lye. The exact nature of the scale has not been determined, but it probably is composed largely of calcium citrate, calcium pectate, and fibrous material. Trouble is encountered primarily in the first effect of the evaporator. Some operators give the press juice a preliminary heating to 212 F. or higher in an auxiliary heat exchanger, which can be cleaned easily. The heating precipitates some material, which is removed in a continuous clarifier, and lengthens the time the evaporators can be operated between cleanings.

In a recent installation, submerged gas burners are used to give the press juice a preliminary heating and concentration to about 22 percent solids. Carbonation by the products of combustion forms a precipitate, which is removed in continuous thickeners. Either method reduces the amount of suspended matter in the final product and improves the quality. Analyses of 13 samples of Florida citrus molasses showed an average of 71.4 percent total solids, 42 percent sugars, 3.8 percent crude protein, 1.1 percent pectin, and 4.8 percent ash. The pH value was 4.7.

Citrus molasses is dark brown and bitter. It is used mainly in cattle feeds, in which it is usually mixed with other materials, although it can be fed full-strength. Some is mixed with wet citrus pulp and then dried to make a feed. The amount of total digestible nutrients in the molasses is about 57 percent. Some is used as a fermentation substrate in the production of alcohol.

Production of citrus molasses increased from none in 1940-41 to nearly 42,000 tons in the 1949-50 season.

ANOTHER USE for press juice is in the production of yeast, particularly Torulopsis utilis, which grows rapidly and is therefore less susceptible to contamination than other yeasts. It is rich in vitamins of the B complex and is a good supplement in feeds. About half the dried yeast is crude protein. It is deficient in methionine, one of the essential amino acids, which, however, is present in cereal proteins. A ration containing this yeast with some cereal would provide all the essential amino acids.

Research workers in the Southern Regional Research Laboratory, the United States Citrus Products Laboratory in Winter Haven, Fla., and a commercial cannery worked together to develop a process for the production of feed yeast. Their experiments included the operation of a 200-gallon-per-hour pilot plant. They developed a continuous method, which gives a good yield. The juice from the feed mill is first passed through an 80-mesh screen to remove particles of pulp. Then it is diluted with water to a sugar concentration of about 2 percent, heated to 200 F. to destroy micro-organisms, cooled, and pumped continuously into the yeast propagator. A concentrated nutrient solution is metered into the propagator in proportion to the feed rate. The propagator is kept thoroughly aerated by air introduced through porous stone candles in the bottom of the tank. The product from the propagator flows continuously into a collecting tank and thence to special centrifuges which separate the yeast as a thick cream. The yeast cream is dried on a drum drier, pulverized, and packaged.

As nutrients, phosphates and nitrogen compounds must be added. Some benefit can be obtained from the phosphorus in the juice, but it must be supplemented. About 0.19 pound of ammonium sulfate, 0.045 pound of anhydrous ammonia, and 0.045 pound of 75 percent phosphoric acid are required per pound of yeast. The acidity in the propagator is easily controlled within the range of pH 4 to 4.5 by varying the ratio of ammonia to ammonium sulfate. With the method of aeration used, from 500 to 700 cubic feet of air was required per pound of yeast produced. Cooling coils were installed in the propagator to dissipate the heat of fermentation and maintain a constant temperature of 95 F.

A pure culture of Torulopsis utilis is grown in the propagator. Continuous feeding of the pasteurized press juice and nutrients is started and maintained as soon as the actively growing culture fills the propagator. No new culture is needed so long as the yeast grows rapidly. The problem of foaming is controlled by closing the top of the propagator and providing a large overflow tube to a collecting tank where the foam is broken.

Fermentation proceeds rapidly and the rate of feed an hour can equal one-third the propagator volume. This means an average retention time of only 3 hours, which is considered short for fermentations. Yields of yeast are progressively smaller with increasing sugar content of the feed. At a 2 percent concentration, the yield is 44 percent of the sugars consumed. Utilizations better than 95 percent of the sugars and two-thirds of the total organic matter were obtained. The drum-dried yeast analyzed 47 percent crude protein and 3.3 percent phosphate (as the pentoxide). The product is light in color, fluffy, and, though not washed, only slightly bitter.

THE ISOLATION OF OIL from the rind of the lemon is one of the earliest chemurgic applications of citrus fruit. Hand pressing of the peel against sponges has been practiced in Sicily since the 18th century; at one time the United States imported nearly a half million pounds of lemon oil annually. Machinery has made possible the production each year of more than 1,500,000 pounds of oils from citrus peels.

The whole fruit, waste cannery peel, or flavedo, the colored part of the peel, can be used. The material is ground and pressed in screw extractors or pressed between fluted rolls to yield an oil emulsion. A recently developed juice extractor, which presses the whole fruit, delivers separately the edible juice and an oil emulsion. The emulsion is screened and the oil separated by centrifuging. The centrifuged oil is placed in cold storage, where waxes separate, the wax-free product being known as cold-pressed oil. These methods are used particularly with lemons, oranges, and grapefruit. Less than half the peel oil in the fruit is recovered from the fruit used. These oils are sometimes concentrated under vacuum to produce concentrated oils in which a major part of the limonene has been removed.

Distilled oils also are prepared from limes, oranges, and lemons. The whole fruit, peel, or liquid effluent from cold-pressed oil preparation is subjected to steam distillation. The oil separates readily from the distillate. Distilled oils are generally inferior to cold-pressed oils. A distilled oil of excellent quality is obtained in some juice canneries by flashing the juice under vacuum. Additional distilled oil is obtained during the manufacture of molasses from the first effect or during flashing of the press juice, but it has little value for flavoring purposes.

Cold-pressed oils are used for flavoring, especially in bottled and fountain beverages, cakes, candies, and pies. Some oils, particularly the distilled, are used to perfume soaps. The amount that can be used for these purposes is limited and much less than the potential supply. Scientists are trying to find other uses in plastics and as solvents; particularly are they seeking further use for the distilled oil that is recovered during other operations. Other outlets are needed; the potential supply from waste cannery peel is about 20 million pounds annually.