AGRICUL.TURE HANDBOOK NO. 98 NOVEMBER 1956 CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUC; AND BYPRODUCTS UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON, D. C. CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUC AND BYPRODUCTS Prepared by the AGRICULTURAL RESEARCH SERVICE AGRICULTURE HANDBOOK NO. 98 NOVEMBER 1956 UNITED STATES DEPARTMENT OF AGRICULTURE • WASHINGTON, D. C. CONTENTS Page Page Structure and composition of citrus fruits 1 Processing of citrus—Continued Macroscopic and microscopic structure 2 Frozen concenti-ates—Continued Nitrogenous constituents 3 Frozen concentrated tangerine juice 48 Enzymes G Frozen concentrated grapefruit juice 48 Organic acids 7 Frozen purees 48 Flavonoids 9 Powdered citrus juices 49 Bitter principles 11 Citrus byproducts 53 Volatile flavoring constituents 13 Pectin 53 Pigments 15 Pectin manufacture 53 Sugars 15 Use of pectic substances 56 Lipids 17 Low-methoxyl pectins 56 Pectic substances 18 Pectate pulp 57 Vitamins 20 Citric acid 57 Inorganic constituents 24 Citrus-seed oils 60 Flavonoids 61 Processing of citrus 24 Essential oils 62 Canning 24 Citrus vinegar 65 Orange juice 24 Marmalades 66 Grapefruit juice 28 Brined citrus 68 Blended juices 28 Grapefruit, lemon, and orange peel 68 Tangerine juice 29 Limes 68 Lime juice 29 Citron 69 Lemon juice 29 Candied citrus 69 Grapefruit sections 30 Grapefruit, lemon, and orange peel 69 Mandarin sections 32 Kumquats 69 Chilled orange juice 33 Citron peel 70 Sulfured citrus juices 34 Waste disposal 70 Pasteurized concentrates 35 Dilute liquid waste 70 Methods for disposal 71 Beverage bases 36 Biological treatments 72 Bottler's base for carbonated drinks 37 Citrus feed 73 Pasteurized base for noncarbonated drinks 37 Beverage bases preserved with sulfur dioxide _ 37 Citrus molasses 77 Feed yeast 78 Frozen concentrates 38 Industrial alcohol 79 Frozen concentrated orange juice 38 Lactic acid 80 Frozen concentrated lemon juice 47 Frozen concentrate for lemonade 47 Literature cited 81 Frozen concentrate for limeade 48 Index 97 P'or sale by the Superintendent of Documents, U. S. Government Printing OflBce Washington 25, D. C. - Price 50 cents CHEMISTRY AND TECHNOLOGY OF CITRUS. CITRUS PRODUCTS, AND BYPRODUCTS Prepared by the AGRICULTURAL RESEARCH SERVICE' Production of citrus in the United States partment of Agriculture, State experiment has increased from about 174 million boxes in stations, and private industry. 1943 to slightly less than 200 million boxes in In carrying out most of the operations de- 1955. Increasing attention is being given to scribed, competent technical aid is necessary, the processing and utilization of citrus other and the economic situation, both local and na- than for the fresh-fruit market in order to tional, should be adequately investigated before effectively market the ever-expanding produc- considering commercial operations. tion of citrus fruit. In the early 1930's the principal fruit used for processing consisted of culls from fresh- STRUCTURE AND COMPOSITION OF fruit packinghouses. This fruit was sound, but because of offsize or peel blemishes was not con- CITRUS FRUITS sidered suitable for the fresh-fruit market. Processing of these culls into citrus products The demand for and acceptance of citrus added to the returns of the grower. During the fruits in the daily diet of human beings is based 1953-54 season 43 percent of the oranges, 51 largely on their nutritional value, flavor, percent of the grapefruit, and 53 percent of the aroma, and other esthetic characteristics, such lemons gi'own reached the fresh-fruit market. as color and texture. These quality factors are The remaining percentage was processed by dependent directly on the structure and chem- canning or freezing, or was made into pre- ical composition of the fruit. Citrus fruits serves, citric acid, pectin, oil, or feed. In a few are a primary source of our daily requirement years processing has progressed from a means for vitamin C, and, in addition, supplementary of utilizing culls to becoming the backbone nutritional value is obtained from the amino of the citrus industry. Figure 1 shows a citrus- acids, inorganic salts, carbohydrates, and prob- products plant. ably other still unidentified factors present in The purpose of this handbook is to describe the edible portion of the fruit. The color of the latest methods for processing citrus prod- the fruit is derived from carotenoid pigments, ucts and byproducts. These methods are the chlorophyll, and possibly ñavonoids. The char- results of researches bj^ the United States De- acteristic aroma is obtained from the volatile essential oils found in the peel. All these chem- ^^ Supersedes Circular 577, Citrus Fruit Products. ical constituents plus many others present in " Contributors were E. A. Beavens, G. J. Keller, J. G. smaller quantities are the result of the com- Kirchner, R. J. McColloch, J. M. Miller, R. G. Rice, L. B. Rockland, and J. C. Underwood, chemists, and bined influences of genetic regulatory mechan- E. R. Wolford, bacteriolog-ist, of the Western Utiliza- isms and the physical, chemical, and biological tion Research Branch; 0. W. Bissett, T. J. Kew, W. C. environments to which the fruits are subjected Scott, L. J. Swift, and M. K. Veldhuis, chemists, of during growth and after harvest. Table 1 sum- the Southern Utilization Research Branch; and H. W. von Loesecke, chemist, of the former Washington marizes the chemical composition of orange Utilization Research Branch. juice. AGRICULTURE HANDBOOK 98, U. S. DEPARTMENT OF AGRICULTURE FIGURE 1.—Aerial view of a Florida citrus-products plant. (Courtesy of Minute Maid Corp., Plymouth, Fla.) TABLE l.—Approximate chemical composition iological functions without interfering or re- of orange juice acting with one another. However, when the juice is extracted from the fruit by crushing, pressing, or reaming, these multitudinous con- stituents are mixed together where they may Class of Con- Total react to bring about undesirable changes in constituents stituents soluble solids flavor, color, and aroma. In order to maintain maximum quality in processed citrus products, Number Percent it is essential that we know what chemical Carbohydrates 7 76.0 constituents are present in citrus fruits, how Organic acids 7 9.6 Amino acids, free 17 5.4 they react with each other, and how their re- Inorganic ions_ 14 3.2 actions may be controlled. Vitamins 14 2.5 Lipid constituents _ — 18 1.2 Nitrogen bases and glutathione 5 .9 Macroscopic and Microscopic Flavonoids 1 .8 Volatile constituents 33 .38 Structure Carotenoids 22 .013 Enzymes 12 (Fig. 2) Total 150 UOO Citrus fruits are covered with a rind, or peel, to protect the pulp, or edible portion, of the ' Approximate. fruit. The rind is made up of a cuticle on the outside, thinly covering an epidermal layer In fresh citrus fruits the many chemical con- (flavedo) containing numerous oil sacs, or stituents and enzyme systems, conservatively glands, filled with an aromatic essential oil estimated at several hundred, are so located in having commercial value (p. 62). In addition different cells, tissues, and other portions of the to these oil sacs, the flavedo contains coloring fruit that they are able to perform their phys- matter, which is not uniformly distributed CHEMISTRY AND TECHNOLOGY OF CITRUS, CITRUS PRODUCTS, AND BYPRODUCTS FIGURE 2.—Schematic view of the cross section of an orange: a, Oil sacs in flavedo; 6, seed; c, juice sacs; d, center core; e, albedo; /, segment; g^ segment membrane. throughout this layer but is concentrated in Nitrogenous Constituents minute bodies called chromatophores, green in young fruit and gradually turning yellow or The nitrogen content of whole citrus fruits orange as the fruit matures (291).^ varies between 0.1 and 0.2 percent on a wet- A white spongy portion of parenchymatous weight basis (5Jf,). However, the nitrogenous cells, known as the albedo, lies directly beneath constituents in citrus-fruit juices compose from the flavedo. The cells of this spongy layer are 5 to 10 percent of the total solids. Those known loosely arranged with large intercellular spaces to occur in citrus fruits include proteins, simple and are irregular in shape. This layer contains peptides, amino acids, betaines, phosphatides, approximately 20 percent of pectinous sub- and related substances. These classes of con- stances, which can be recovered in the form of stituents are normal metabolites of both plant citrus pectin (p. 53). and animal tissues, and contribute to the nutri- tional value of those plant and animal products The inner pulp or flesh of the fruit consists used for food and feed. Evidence has indicated of segments (locules), separated by a mem- that amino acid-sugar (260) interactions may brane of thin epidermal tissue and containing be important in the darkening and development numerous spindle-shaped juice sacs (vesicles) of off-flavors in citrus (iS8) and other food and seeds. The juice sacs in each segment are products (87). attached to the segment wall and in contact The proteins in citrus fruits are relatively with the peel by means of fine threads of vary- insoluble, and are found to be associated with ing length. Embedded within the cellular tis- the solid portions of the fruit, such as the seeds, sue in the central part of each juice vesicle are flavedo, albedo, chromatophores, and pulp. oil droplets (90). Yellow chromatophores of Amino acids are found in the juice of the edible crystalline origin may be observed when these portion of the fruit and in the aqueous alcohol juice sacs are crushed and examined micro- extractable fraction of the peel. scopically (277), Until recently very little systematic work The central axis (core) of the fruit is com- had been done on the determination of the posed of a white spongy tissue similar to that nature, amounts, and reactivities of the nitro- found in the albedo. The core and segment genous constituents in citrus fruits, primarily membranes are collectively called the ''rag'' of because of the unavailability of satisfactory the extracted juice.