The main functions of the packing house are: to separate the fruit to be sold fresh from the fruit to be processed, to degreen, clean, wash, disinfect, wax, sort, mark, package, and finally, to prepare the fruit for storage and transport. The harvested crop is transported to the packing house, received and degreened with ethylene if necessary. Drenching in chlorinated water is necessary for sanitizing and prevention of contamination. Processing quality fruit is removed and sent to industry. Fruit to be marketed as fresh is brush washed, dried, and waxed. There are two methods of waxing, namely, the solvent wax method and the water–wax emulsion application. The waxed fruit is graded according to quality (color, blemishes, defects) and size. Electronic sorters are available. Citrus fruit can be packaged, manually or by automatic machines, in telescopic boxes, tray boxes, wire-bound wooden crates, or net bags, depending on the subsequent mode of transport and storage.

Canned grapefruit segments, canned mandarin sections, candied peels and citrus fruit, jams, and jellies are commercial products of considerable importance. Canned grapefruit segments are probably the first industrial citrus product, developed before the expanse of processing for juice. The most frequently practiced method of production of grapefruit and mandarin segments is based on lye peeling. Candied peels are made by cooking cut peels in heavy sugar syrup and subsequent drying. They are important ingredients in bakery. Candied (glazed) kumquats are appreciated confectionery items. The major participation of citrus in the manufacture of jams and jellies is through the widespread use of citrus pectin as a gel forming agent. High methoxyl pectin is used in normal preserves. Manufacturers of low and no sugar jams make use of low methoxyl pectin. Practically, all the drying techniques have been tried with citrus juices, with meager success. The only dry citrus juice powder of some presence on the market is lemon powder containing maltodextrin as carrier.

We define “shelf life” as the postproduction length of time, until the quality and safety of a product are reduced below a specified level by spoilage. The most frequent kind of spoilage in citrus products is chemical spoilage which includes nonenzymatic browning, color change for other reasons, loss of ascorbic acid, deterioration of taste and aroma, and degradation of the bioactive principles. Kinetic models for the prediction of shelf life are available. Single strength juices and concentrates are pasteurized, chilled, aseptically filled, and stored under refrigeration in very large storage tanks (tank farms). For retail trade, concentrates are frozen, packed in small cans or plastic cups, and stored at subfreezing temperature. Citrus essential oils should be packed in metal (tinplate or aluminum) containers or colored glass bottles and stored under refrigeration for a shelf life of 1 year. Aqueous essences are refrigerated. Most other citrus products are shelf stable.

Juices and their derivatives are by far the most important products made from the major citrus varieties. Among citrus fruits, oranges are the leading variety utilized for juice production. “Single-strength juice,” also known as 100% juice, is either not from concentrate (NFC) juice, or juice reconstituted from a concentrate by dilution with water to the natural single-strength Brix. There are two fundamentally different systems of raw material procurement for the industry. In the “industry only” or the “industrial orchard” system, nearly all the crop is sent to the factory to be processed. This system accounts for a great part of citrus production in Florida, Brazil, and parts of China. The second system is the “fresh/industry mix” system, the objective of which is to maximize the proportion of crop marketed as fresh fruit, processing only the fruit that cannot be sold as such, for reasons of external quality, fruit size, trade barriers, or excess production. After harvesting and transporting to the processing plant, the fruit is graded and washed. Juice extraction systems may be separated into two groups: those where the fruit is cut into two halves before juice extraction and those where the juice is extracted from whole fruit without halving. In the first group the essential oil is separated from the whole fruit before juice extraction. In the second group the essential oil is obtained simultaneously with juice extraction. Three leading industrial systems of juice extraction are described in detail. Excess pulp is removed by different methods of screening and the juice is pasteurized to inactivate cloud-breaking enzymes and spoilage microorganisms. Deaeration and homogenization before pasteurization are frequently practiced options. Several nonthermal stabilization processes are described. The pulp wash processes for the recovery of soluble fruit solids from pulp are explained. The production of single-strength juice by dilution of concentrates and of various modified juices is discussed.

The genus “Citrus” belongs to the subfamily of Aurantioideae, family of Rutaceae, order of Geraniales and comprises numerous varieties of most popular fruits such as sweet, sour and bitter oranges, tangerines, grapefruit, lemons, limes, citrons, etc., and a very large number of hybrids and cybrids (cytoplasmatic hybrids). The cultivation of citrus trees is believed to have been practiced at least 4000 years ago in south-east Asia. Citrus fruits are grown almost in every country 30–35 degrees north and south of the Equator. Total world production of citrus fruits in the 2010–2011 season was in excess of 115 million tons, more than any other fruit crop. The leading variety is oranges (61%), followed by tangerines (22%), lemons and limes (11%), and grapefruit (6%). The most important total citrus producing countries are China (24%), Brazil (24%), and the United States (11%).

The citrus processing industry for juice and concentrate generates a huge quantity of various by-products. As a waste, these by-products constitute a serious environmental problem. Processed, they can be transformed to marketable products and generate additional revenue. According to (Kesterson, J.W., Hendrickson, R., 1958. Econ. Botany, 12, 164–185): “Proceeds from the production of by-products may spell the difference between profit and loss in the citrus canning industry.” Quantitatively, the peels and rag discarded at the juice extractors constitute the most important of citrus by-products, accounting for 50–55% of the weight of processed fruit. Fresh or dried they are chiefly utilized as animal feed. Liming and pressing liberates sugar containing fluids that are converted to molasses or fermented to alcohol or vinegar. Bases for the manufacture of citrus flavored beverages are producing by comminuting together fruit, peels, pulp, juice, etc. Colloid mills and homogenizers are utilized for their production. Excess pulp and juice sacs are recovered, frozen, and added back to juice. Pectin is a valuable by-product of the citrus processing industry. Over 80% of the pectin in the world is obtained from citrus peels. Essential oils are fractionated into terpeneless oils and d-limonene. Citrus fiber is a relatively new and profitable by-product with multiple uses. Seeds of citrus fruit have been utilized for oil and meal until the 1970s.

Citrus fruits and juices enjoy the reputation of being healthy foods, mainly as sources of vitamin C. One glass of orange juice supplies 135% of the average recommended daily intake for a male adult. However, citrus products also provide an array of other valuable nutrients. Lately, consumer interest in citrus has grown, because of the so-called bioactive constituens, mainly antioxidants and fiber. Although the quantity of these constituents ingested by normal consumption is not sufficient for therapeutic purposes, such consumption is conceived as beneficial to health. Vitamin C is a cofactor of enzymes responsible for the biosynthesis of collagen. But vitamin C is also an antioxidant. In fact, it is the most abundant water-soluble antioxidant in the human body. Strong epidemiological evidence for the protective action of vitamin C for nonhormone dependent types of cancer has been recorded. However, claims of curative effect of vitamin C on cancer were mostly rejected. Citrus fruits and their juices figure among the recommended sources of folic acid in most dietetic guides. The antioxidants in citrus are primarily phenolic compounds such as bioflavonoids. Hesperetin, naringenin, and nobiletin are specifically cited as potential drugs against neurodegeneration, dementia, as well as epilepsy and stroke. Citrus essential oils also have bioactive properties. Soluble fiber in citrus is effective in lowering blood cholesterol. Some nutritionists have accused citrus for causing dental erosion and others for inducing obesity. Baring grossly excessive consumption in unrealistic quantities, both claims are unfounded.

Quality changes in the harvested fruit can be studied from two different angles, namely, postharvest physiology and postharvest pathology. Respiration is the predominant physiological process in harvested fruits. In contrast to most other fruits, citrus fruits are nonclimacteric. Their rate of postharvest respiration is low and does not show a tendency to increase. Detached citrus fruit does not ripen. The rate of respiration is faster at higher temperature. As long as the oxygen supply is adequate, the respiration quotient is close to unity. Respiration quotient values higher than unity indicate that fermentative processes are taking place, in addition to respiration. Respiration can be depressed by decreasing the concentration of oxygen or increasing the concentration of carbon dioxide in the atmosphere surrounding the fruit. In essence, this is the technology of “controlled atmosphere.” Another important postharvest change is moisture loss due to transpiration, partially depressed by waxing. Postharvest changes in mechanical properties include softening. Postharvest deterioration of taste and aroma is particularly important in mandarins. Chilling injury is observed most frequently when fruit, previously stored at low temperature for a few weeks, is exposed to ambient temperature. The most common type of chilling injury in citrus is “pitting,” to which grapefruit is particularly susceptible. The most common postharvest pathogens of citrus fruit are fungi and particularly Penicillium digitatum and Penicillium italicum, known as the green and blue molds, respectively. The most commonly applied fungicides are sodium orthophenyl phenate and thiabendazole. Most frequently recommended storage conditions are temperatures of 5–9°C for oranges, 7–12°C for lemons, 9–12°C for grapefruit, 3–9°C for tangerines and mandarins, and 9–12°C for limes, high relative humidity and adequate aeration.

Citrus can grow in a wide variety of soils, provided that soil drainage is adequate but the optimal soils are structures deep sandy or moderately loamy structures with pH between 5.5 and 7.0. Citrus trees are most sensitive to soil salinity. Climate has the strongest effect on citrus growth and production. The commercial citrus growing regions of the world are located in a belt between 40 degrees south and north of the equator, and nowhere outside this belt. Most cultivars are sensitive to frost. Strong winds and hailstorms are additional risk factors. Citrus needs heat, sunshine, and adequate rainfall or irrigation. The commonly used unit of the heat supplied to the plant is the Growth-Degree-Day or the heat unit per day. Citrus trees reproduce by seeds, cuttings, budding (grafting), and layering, with budding being the most widespread method. The properties of the grafted tree are a combination of the characteristics of the rootstock and those of the scion. Orchard layout, that is, planting pattern and density should be planned. High planting density means more trees per hectare but less fruit per tree due to less exposure to sunlight. The importance of irrigation in citriculture cannot be overestimated. A variety of irrigation systems, such as flooding, sprinklers, and drip irrigation, are in use. Intensive culture of citrus requires fertilization. Nitrogen, phosphorus, and potassium are the principal macronutrients. Pest control, disease management, and sanitary maintenance are critical tasks. Harvesting is one of the most expensive operations in citriculture. Citrus is harvested mostly by hand and occasionally by machines.

Quality control (QC) and quality assurance (QA) are essential to the sustainability of any industrial activity, including citrus production and processing. It is important to assign QC/QA responsibility to a position as high as possible in the corporate management. The hazard analysis critical control point (HACCP) system is applied to ensure safety of the operation. The application of HACCP is enforced in many countries by national and international regulation agencies. In the United States “Juice HACCP” is regulated by the FDA. The main objective of HACCP is to predict and prevent hazards. HACCP does not exclude the use of other systems of quality assurance, such as good manufacturing practice (GMP). Serious outbreaks of Salmonella and Escherichia coli O157:H7, both attributed to citrus juice, underlined the need to consider microbiological quality control as a safety issue. An important function of QC is the prevention of product falsification (authentication of citrus origin). Development of sophisticated fraud detection methods continues to consume considerable time and money.