Chemistry and Technology of Citrus

M. K. Veldhuis.

In the years between the First and Second World Wars, oranges ceased to be a rarity to be hung on Christmas trees or to be enjoyed only at certain seasons and became one of our most common foods.

Modern transportation has made fresh oranges and other citrus fruits available in regions far from producing centers. Developments in the technology of canning have transformed them into grocery staples. The fresh orange spoils in less than 3 weeks after picking, but its processed juice can be preserved for consumption the year around.

Five States supply all our citrus fruits. Florida grows more than half of our oranges and grapefruit and nearly all our limes. California is next in oranges and produces most of the lemons. Texas and Arizona grow large quantities of grapefruit and some oranges. Louisiana produces a small orange crop.

Only 20 years ago Florida groves were fast becoming nonproductive. Then a spectacular improvement in cultural practices the inclusion of small amounts of minor elements in fertilizers helped to rejuvenate the citrus trees there, so that production in Florida alone is as large as the entire national apple crop. The production Of oranges in Florida rose from 12 million boxes in 1931 to 57 million boxes in 1950.

Including tangerines, lemons, and limes, as well as oranges and grapefruit, the production in all producing areas in 1950 was more than 150 million boxes, against 85 million boxes in 1936.

This increased production made it necessary to can a larger part of the crop. In Florida, in 1950, 31 million boxes of oranges nearly half the crop were processed, compared to about one-eighth before 1936. In 1936, about one-third of the grapefruit was canned; now half the crop is sold as canned products. Total processing operations now consume nearly 3 million tons of citrus fruits. That means that 11 large cans of juice are available annually for each person in the United States.

The number of canneries has also increased in the past 15 years. In Florida about 50 canning plants processed citrus fruits in 1950.

The manufacture in recent years of much larger amounts of a high-carbohydrate cattle feed and a citrus-molasses feed as byproducts of cannery wastes has helped the economy of the industry.

Canning at first was a salvage operation to use fruits rejected from fresh.. shipments because of blemished skins or unpopular sizes. Now much of the fruit moves directly from the grove to the cannery. In fact, the biggest part of the crop in Florida is grown just for canning.

Fresh and processed citrus fruit compete somewhat, but because fresh citrus fruits also compete with other fresh fruits, the greater convenience to the housewife of the canned citrus may actually give the crop a means of holding its own in the competitive market.

A POPULAR NEW frozen concentrated orange juice is an example of joint research by the Florida Citrus Commission and the Department of Agriculture and illustrates the forward march of processing techniques. The new product is an orange juice of fourfold concentrated strength so like the fresh juice in flavor and taste when it is rediluted and so convenient to use that its consumption has about trebled each year it has been in production. During the 1950 season, 21,647,000 gallons of it was produced in Florida. Within 5 years of its introduction, the juice has become the most popular frozen food. It is simple to use; all one does is to empty a can of the concentrated juice into a container and add to it three times its volume of water. Not even ice is necessary because the readily diluted concentrate cools the drink. Orange juice is a good source of vitamin C, nearly all of which is retained in the concentrate.

Several other citrus products marketed in cans are in demand fruit sections, sherbet purees, beverage bases, and marmalade. Powdered orange juice has been produced, but it was not available in large quantities in 1950.

CANNED PASTEURIZED single-strength juices are the largest single type of product. Until 1943, grapefruit juice was packed in the largest amounts. Then, with advances in the technology of canning and a better understanding of what was required, canned orange juice surpassed grapefruit in volume. Blended orange and grapefruit juices are also canned in large quantities and tangerine and lime juices in smaller quantities.

Pasteurized juices retain satisfactorily the nutritive value of the original fruit. Even during storage, the rate of loss of vitamin C (ascorbic acid) is low. For instance, more than 80 percent of vitamin C is found in juice that has been stored for 6 months at a little above room temperature, provided the juice has not come in contact with copper or brass. Other components of the juice acids, sugar, and minerals are almost unaffected by processing.

Pasteurization, however, does affect the flavor. To be satisfactory, canned juices must retain their pleasing flavor after several months of storage at 70 to 90 F. The flavor of pasteurized juices differs from that of fresh juice and further changes in flavor occur later during storage. Significant off-flavors develop after about a year's storage at room temperature. With cooler storage, the juices can retain flavor longer. Grapefruit juice has the longest storage life. Blended orange and grapefruit juices come next.

Changes in fatty materials, peel oil, and other volatile or nonvolatile compounds cause off-flavors during storage. Research organizations are making progress in determining the cause of the gradual flavor deterioration and in devising means of preventing it.

The pasteurization of single-strength juices is the least expensive of the various ways of processing citrus fruits. It can be done rapidly the processes are largely automatic and are adaptable to large-scale operations. Some canneries process more than 1,500 tons a day in this manner.

METHODS of processing have been greatly improved in the past few years. Almost all equipment has been remodeled so that it is more efficient and easier to clean and maintain. Stainless-steel equipment is almost universally used.

Although nearly every plant uses a variation of the method, and employs different types of machinery, practically all include six steps in sequence. The fruit is washed and conveyed to extractors, which remove the juice. Seeds and excess pulp are screened out. The juice is passed through deaerators, which rid it of dissolved oxygen. Then it is pumped through continuous, rapid pasteurizers, where micro-organisms are destroyed and enzymes are inactivated. The hot juice flows directly to the can-filling and can-closing equipment. The closed cans are cooled rapidly by rotation under sprays of cold water.

Washing consists of soaking the fruits in large tanks that contain soaps and germicides, then passing them through a machine equipped with scrubbing brushes, and rinsing by spraying with clean water.

A conveyor usually carries the washed fruit to automatic extractors for juicing. Modern extractors are fast and deliver juice of high quality. Most types cut the fruit into halves and press or ream out the juice in much the same way as does an orange juicer in the kitchen. A recently developed extractor squeezes the whole fruit and forces the juice out through a tube inserted through one side of the orange. This juice contains pulp, seed, and peel, or other suspended solids, so it is run through a screw-type finisher or a series of vibrating or revolving screens of successively finer mesh. The screens remove all but the desired amount of solids.

THE JUICE is collected in stainless-steel tanks and mixed, and the content of acid and total solids is determined. These values provide a check on the quality of the fruit being used. If a sweeter product is desired, sugar is added at this point.

To remove dissolved oxygen, the juice is injected into a closed chamber under a high vacuum. The deaeration makes canning more efficient and may also prevent oxidative and flavor changes in the finished juices.

Because consumers may object to peel oil in the juices, some canneries use deoilers, in which a small fraction of the juice is vaporized under vacuum at a comparatively low temperature and a major part of the oil in the juice goes off with the vapor.

The next step, pasteurization by heating, is carried out rapidly in stainless-steel, tubular heat exchangers, in order to minimize changes in flavor due to the heating. Pasteurization destroys organisms that might cause the product to spoil by fermentation and inactivates enzymes that might cause undesirable changes in the juice, such as clotting of the suspended matter. Citrus juices are acid, and heating to about 160 F. is sufficient to destroy the micro-organisms, but heating to 190 or above is needed to inactivate the enzymes. The temperature required varies with the time of heating. In one type of pasteurizing equipment, the juice is heated for 40 seconds to 193 to 200 ; in another, to 240 to 265 in about 4 seconds. The juice generally is cooled to 185 to 190 before being run into cans. The filled cans are closed and cooled rapidly by spinning under sprays of cold water.

CONCENTRATED pasteurized juices are popular in the preparation of beverages, sherbets, and candies. The juice is extracted and concentrated under moderately high vacuum at a temperature of 120 F. The process reduces the volume of the juice to about one-seventh. Then the concentrate is heated to pasteurize it, run hot into cans, and rapidly cooled. The product differs from fresh juice in that it lacks aromatic and some other flavor characteristics. Just the same, it is an acceptable product and retains well the vitamins and food value of the original juice. It keeps well under refrigeration at 40 but will deteriorate in a month or two at room temperature.

FROZEN SINGLE-STRENGTH JUICES are produced in limited quantities. The process is the same as for canning, except that the juice is not usually pasteurized before being frozen. Sound, ripe fruits are selected, and great care is taken in processing to insure cleanliness because of the health hazards involved when juices are not pasteurized. The juice is run through a refrigerated heat exchanger, to lower the temperature to 30 to 35 F., and then into cans, which are sealed under a vacuum and rolled or spiraled through a conveyor filled with refrigerated brine. Some juice is frozen in air-blast tunnels. Frozen juices should be stored at 0 to 10 . An objection, which has limited the acceptance of the product, is its slowness to thaw when about to be used.

To develop the basic process for the preparation of a frozen orange concentrate, research workers in the Florida Citrus Commission and the Winter Haven, Fla., laboratory of the Bureau of Agricultural and Industrial Chemistry first centered their attention on producing a high-quality concentrate by high-vacuum evaporation. The scientists learned that if they evaporated the juice rapidly at 80 F. or somewhat below, no cooked flavor or off-flavor developed, but the product tasted flat and lacked the aroma of the fresh juice, because nearly all the volatile flavoring materials had been removed with the water vapor.