Production of single-strength citrus juices

Chapter

8

Production of single-strength citrus juices 8.1  INTRODUCTION AND TERMINOLOGY The citrus fruit processing industry is a multiproduct sector that utilizes the entire fruit. Fruit juices and their derivatives are by far the most important products made from the major varieties. Codex Alimentarius defines juice as “unfermented but fermentable juice, intended for direct consumption, obtained by the mechanical process from sound, ripe fruits, preserved exclusively by physical means. The juice may be turbid or clear. The juice may be at its original concentration (“not from concentrate” (NFC) juice), or it may have been concentrated and later reconstituted with water (“from concentrate” juice (FCJ)), in a proportion suitable for the purpose of maintaining the essential composition and quality factors of the juice. The addition of sugars or acids can be permitted but must be endorsed in the individual standard.” (FAO, 1992). The first commercial citrus juice was canned orange juice, produced in California in the early 20th century. Among citrus fruits, oranges are the leading variety utilized for processing (Fig. 8.1). After its extraction from the fruit, citrus juice is either marketed at its original concentration or concentrated. “Single-strength juice,” also known as 100% juice, is either NFC juice, as defined below, or juice reconstituted from a concentrate by dilution with water to the natural single-strength Brix. Both kinds of single-strength juice belong to the category of “ready to serve RTS” or “ready to drink RTD” juices. NFC juice is fresh juice extracted from the fruit and has not been concentrated. It is usually judged of higher quality than juice from concentrate and is slightly more expensive. NFC juice is, at present, the closest match to freshly squeezed juice. It meets with outstanding consumer acceptance for its flavor, convenience, and its image of a healthy food product. It commands a steadily growing market share and competes successfully with FCJ and retail frozen concentrate for home use. By far the largest proportion of RTD juices, both NFC and FGJ, are pasteurized by aseptic processing. Very large quantities are stored under refrigeration in tank farms.

Citrus Fruit Processing. http://dx.doi.org/10.1016/B978-0-12-803133-9.00008-4 Copyright © 2016 Elsevier Inc. All rights reserved.

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128 CHAPTER 8  Production of single-strength citrus juices

■■FIGURE 8.1  Citrus fruit utilization for processing, by varieties. (FAO, 2013)

Fresh, “raw,” or “freshly squeezed” unpasteurized NFC is in small but increasing demand by the nature-oriented consumers. It has a very short shelf life (Fellers, 1988) and is known to have caused illness and even fatal outbreaks due to contamination with pathogenic microorganisms (Mihajlovic et al., 2013). A version of the Codex Alimentarius standard for citrus (orange) juice is appended (see Appendix I).

8.2  PROCUREMENT OF FRUIT FOR THE PROCESSING INDUSTRY 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. The fruit for processing is separated from the main stream in the packing house, as we have seen. This system is predominant in Spain, California, Israel, and many other countries. It is also the quasi only one applied to mandarins and other easy peelers.

8.4 Reception and storage 129

8.3  HARVESTING, LOADING, AND TRANSPORTING TO THE PROCESSING PLANT The methods of collection and transport of the fruit differ among regions and according to the scale. In Florida, the fruit for processing is sent to the processing plant directly from the orchard, without passing through a packing house. The orchard is divided into blocks. Representative samples are taken from each block and the maturity index (0Bx/titrable acidity) is determined. If the index meets the standard set by the processing industry, it is decided to harvest the block. Factory maturity standards are usually higher than the minimum standard set by the authorities. By far the majority of the fruit is harvested by hand. Mechanical harvesting has been introduced but is not yet widely practiced, for economical reasons. It can be postulated, however, that more of the fruit intended for processing will be mechanically harvested in the future, if and when the cost and availability of labor will make mechanical harvesting a more profitable option. The pickers collect the detached fruit in bags, as we have seen before, and when the bag is full they empty it into pallet bins. A farm vehicle, known as the “goat,” lifts the bins with the help of a small hydraulic crane and dumps the fruit into a tub at the rear of the vehicle. When the tub is full, the goat moves near the final transport vehicle, which can be a dump truck or a tractor-driven trailer, hydraulically lifts and tilts the tub so as to transfer the fruit into that vehicle (Fig. 8.2). The fruit travels to the factory in bulk, in a truck or trailer which may have a holding capacity of 20 t or more. The proportion of fruit badly damaged in transport depends on the distance, the quality of the road, the weather, and the degree of maturity of the oranges. In the fresh/industry mixed system, the fruit culled in the packing house is sent to the processing plant in the same way.

8.4  RECEPTION AND STORAGE A flow diagram of the reception and storage operations is shown in Fig. 8.3. At the factory, the trucks or trailers are weighed, and then emptied on a conveyor or into a pit by gravity. A short roller conveyor is provided, on which most of the sand, leaves, and trash is eliminated and the fruit is given a first inspection for the removal of badly damaged fruit. It is important to remove the rotten and wounded fruit before storage to avoid contamination. The rejected material is usually returned to the truck which is weighed again at the exit from the plant area. In most plants the fruit is elevated and distributed into surge bins. A surge bin serves as a storage buffer and for delivering the fruit to the processing

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■■FIGURE 8.2  “Goat” unloading. (Courtesy: Chet Townsend UltimateCitrus.com)

■■FIGURE 8.3  Flow diagram of fruit reception and storage.

line at a regular and controlled rate. The bins are made of wood, concrete, plastic coated metal mesh, or stainless steel (Fig. 8.4a and 8.4b). The bins are equipped with inclined baffles, to make the descent of the fruit smooth and gentle, to avoid crushing by excessive pressure on the fruit at the bottom and to distribute the fruit evenly. In some models, the baffle inclination is adjustable by the user. At the bottom of the bin, an adjustable discharge gate regulates the exit rate. The gate opening may be regulated manually

8.4 Reception and storage 131

■■FIGURE 8.4  (a) Surge bin, stainless steel walls. (Courtesy: JBT FoodTech); (b) Surge bins, wooden walls. (Courtesy: Bertuzzi Food Processing SRL)

or automatically. Automatic control systems regulating the opening of the gates according to the demand are available (Fig. 8.5). These systems include level sensors placed inside the bins at high and low positions. The bins are well aerated and roofed to avoid penetration of rain. Some processors do not use surge bins but feed the incoming fruit directly to the processing line. In this case, the trailer serves as a surge bin and the rate of fruit flow is governed by the rate at which the fruit is unloaded from the trailer. Water jets are often used to unload the trailers and, at the same time give the fruit a first washing. Reception into surge bins is, of course, much more convenient.

■■FIGURE 8.5  Control of surge bin discharge rate.

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At the exit from the surge bin, a withdrawal belt takes the fruit to an elevator which delivers it to the washer. Despite precautions, the reception and storage areas are the most soiled part of the plant. It is advisable, therefore, to allow adequate distance and physical separation between this area and the following operations and to avoid, as much as possible movement of personnel and equipment between the two parts. The washing operation provides, to some extent, the limit between “dirty” and “clean.”

8.5  WASHING, INSPECTION, SIZING The fruit is washed in brush washers, already described in the chapter on packing house operations. Rotating brushed clean the fruit thoroughly while spinning it to expose the entire surface and conveying it in a direction perpendicular to the brush axis. At the same time, water jets operating at pressures in the order of 100–200 psi (7–14 bar) spray the fruit (Fig. 8.6). Detergents approved for use on foods are sometimes applied. The spray water is usually chlorinated at the rate of 100–200 ppm free chlorine. Chlorination at this stage helps reduce microbial build-up on the conveyor belts and other equipment in contact with the fruit. To permit thorough scrubbing of the surface, the residence time on the brushes must be around 30 s. There are no spray nozzles at the section of the washer near the exit, to allow removing excess water from the fruit surface. From the washer, the oranges are discharged on a roller-grader (Fig. 8.7). The roller-grader is a wide roller conveyor spinning and advancing the fruit in a single layer while workers on both sides remove manually foreign

■■FIGURE 8.6  Brush washer. (Courtesy: JBT FoodTech)

8.6 Extraction of juice and essential oil 133

■■FIGURE 8.7  Roller grader. (Courtesy: JBT FoodTech)

objects and fruit not suitable for juice extraction. The rejected material is either dropped into discharge chutes or placed on a discharge conveyor. Adequate lighting is essential for efficient sorting. It is advisable to equip the roller-grader with variable speed drive, to adjust the flow rate of fruit to the sorting capacity of the workers. From the roller-grader the fruit is conveyed to the juice extraction area. In most plant layouts the juice extraction area, also called juice room or extraction room, is an indoors area, usually on an upper floor of the plant in which case a feeding elevator is required. The reason for such a layout is sanitation and ease of conveying the extraction products and by-products for further processing by gravitation. Most juice extraction systems require presizing of the fruit. When required, the sizers are located in the juice room, close to the extractors. There are different types of sizing machines. Usually, the type of sizer used is closely related to the type of juice extractor. The fruit sizers will therefore be described in the next section on juice extraction.

8.6  EXTRACTION OF JUICE AND ESSENTIAL OIL The “heart” of the juice processing line is the “juice extractor” or the machine where juice is obtained from the fruit by pressing or by reaming. The peel oil is obtained simultaneously. There are several commercial systems of juice and oil extraction (Nelson and Tressler, 1980). They can be separated into two groups: those where the fruit is cut into two halves

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before juice extraction and those where the juice is extracted from whole fruit without halving. Selection of the type of juice extractor determines, to a large extent, the composition of the whole processing line. We shall therefore start by describing in detail three of the leading industrial juice extraction solutions commercially available. In all systems, juice production occurs simultaneously with the recovery of essential oil and peels are a by-product. In the first system the essential oil is first removed from the whole fruit, then the fruit is cut into two halves and the juice is obtained by squeezing or reaming the halved fruit. In the second system, the whole fruit is squeezed, taking care to avoid contact between the juice and the external surface of the fruit including the peel essential oil. In the third system the fruit is treated, essentially like in the first.

8.6.1  The “Indelicato” juice extraction system The Indelicato system belongs to the first group of processes. The essential oil is first recovered from the whole fruit. The de-oiled fruit is rinsed and then halved. Juice is obtained from the halved fruit by pressing or reaming. The Indelicato juice extractor is made by the Fratelli Indelicato S.r.l. Company, from Giarre (Catania, Italy). The Company was founded in 1946 by the brothers Paolo and Carmelo Indelicato. The citrus juice extractor, named Tagliabirillatrice A8, patented in 1948, had a capacity of 4,800 fruit per hour. The systems offered today have a capacity of 6 to 20 t of fruit for one machine, depending on the model selected. The first operation is extraction of the essential oil in the “Polycitrus oil extractor” (Fig. 8.8). The fruit is elevated to a hopper from which it is fed to the machine at an electronically controlled rate. The fruit falls on rotating cylinders with rasping surfaces (Fig. 8.9). The cylinders rotate along their axis which is in the direction of the fruit flow. They spin the fruit and rasp the rind from all sides, while chain-driven paddles advance the fruit toward the exit. The peel oil and rasping debris are washed away from the surface of the fruit by water sprays. The suspension/emulsion, consisting of water, debris, and oil, is collected in a tank and sent to further processing. The speed of rotation of the cylinders and the velocity of the paddles moving the fruit are controllable to assure the desired degree of rasping. The de-oiled fruit is further cleaned of residual debris and oil by a brushing machine (Fig. 8.10) and then elevated into the “Spellalbedo” juice extractor. The system does not require sizing of the fruit. The flow diagram suggested by the company does not include a step of fruit washing, presumably because the fruit is thoroughly cleaned by the rasping action. However, when green mandarins are processed, a stage of brushing–cleaning is incorporated before the oil extractor.

8.6 Extraction of juice and essential oil 135

■■FIGURE 8.8  “Polycitrus” oil extractor. (Courtesy: Fratelli Indelicato S.r.l.)

■■FIGURE 8.9  Rasping cylinders. (Courtesy: Fratelli Indelicato S.r.l.)

The Polycitrus juice extractor (Fig. 8.11) performs two operations. First, the fruit is cut into two halves by a fixed knife. Rotating discs present the fruits to the knife one by one. In the second operation, each half is pressed by rotating rasping cylinders against a perforated stainless-steel plate. The clearance between the cylinders and the perforated plate decreases as the half fruit is pressed. The juice passes through the perforations and is

136 CHAPTER 8  Production of single-strength citrus juices

■■FIGURE 8.10  Brushing machine. (Courtesy: Fratelli Indelicato S.r.l.)

collected in a tank from which it is pumped away for further processing. The peels are discarded. Since the fruit has been already de-oiled and thoroughly brushed before being halved in the Polycitrus, contamination of the juice with peel oil is efficiently avoided. A variant of the Polycitrus, the Polycitrus Spellalbedo, is equipped with an adjustable device for removing residual pulp and membranes from the peels. The clean peels suitable for the production of candied peel (Fig. 8.12) are discharged to a screw conveyor.

■■FIGURE 8.11  “Polycitrus juice extractor”. (Courtesy: Fratelli Indelicato S.r.l.)

■■FIGURE 8.12  Clean peels from “Spellalbedo” juice extractor. (Courtesy: Fratelli Indelicato S.r.l.)

8.6 Extraction of juice and essential oil 137

■■FIGURE 8.13  Screw press “Polypress”. (Courtesy: Fratelli Indelicato S.r.l.)

The pulp and membranes removed from the peels are discharged through another opening on another screw conveyor and elevated into a screw press (Polypress, Fig. 8.13) where they are pressed to recover some additional juice, termed “secondary juice.” The suspension/emulsion from the oil extractor is pumped to a finisher where the emulsion is freed of most of the solid debris (Fig. 8.14). The finisher consists of rotating screw inside a perforated cylinder. An adjustable exit gate regulates the pressure exerted on the solids. The oil emulsion

■■FIGURE 8.14  Finisher for oil-debris-water mixture. (Courtesy: Fratelli Indelicato S.r.l.)

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■■FIGURE 8.15  Pure essential oil from

centrifuge. (Courtesy: Fratelli Indelicato S.r.l.)

■■FIGURE 8.16  Rotofinisher. (Courtesy: Fratelli Indelicato S.r.l.)

is sent to the first centrifugal separator where the residual solids (frit) and some of the water are removed. The oil-rich phase is treated in a second centrifuge separator where the reminder of water is removed and clear essential oil is obtained (Fig. 8.15). The water phase is admitted in a series of decanter tanks and recycled, after clarification, back to the spray nozzles of the oil extractor. The raw juice from the Spellalbedo extractor is pumped to a two-stage screw finisher (Rotofinisher 2 SE, Fig. 8.16). Moderate pressure is applied at the first stage where the delicate juice sacs are removed. Entire floating juice sacs constitute a valuable stream as we will see later. The juice phase is sent to the second stage for the controlled separation of pulp, depending on the specified pulp content of the juice. The above description refers to oranges. The processing method for lemons is different. Because the lemons are not spherical, they cannot be efficiently rasped in the Polycitrus oil extractor. The process is inverted: the juice is extracted first and the oil is recovered after. First, the lemons are elevated to an oscillating feed hopper feeding the machine named Birillatrice-Sfumatrice (Fig. 8.17). A set of rotating disks convey the lemons, four at a time, in the correct position, to a stationary knife that cuts the fruit into two halves (Fig. 8.18). Each half falls into a plastic cup and is

8.6 Extraction of juice and essential oil 139

■■FIGURE 8.17  Birillatrice-Sfumatrice extractor. (Courtesy: Fratelli Indelicato S.r.l.)

■■FIGURE 8.18  Halving the fruit. (Courtesy: Fratelli Indelicato S.r.l.)

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■■FIGURE 8.19  Reaming heads. (Courtesy: Fratelli Indelicato S.r.l.)

pressed against a rotating reamer (Fig. 8.19). The juice and pulp extracted by reaming are collected in a tank, to be processed further, as described above for oranges. The peels (Fig. 8.20) are discharged to the oil extraction section of the machine where they are pressed by a rotating drum against a fixed surface. The pressure breaks the oil glands and liberates the oil which is washed away by water sprays. The mixture of oil and water is collected in a tank for further processing. The manufacturers claim that the essential oil extracted by the Birillatrice–Sfumatrice extractor is of superior quality, similar to the oil obtained by the ancient manual “sponge” method (Braverman, 1949). The working capacity of the machine is 15,000 lemons per hour. In summary, the Indelicato system, consisting of the oil extractor, juice extractor, brushing machine, finishers, centrifuges, buffer tanks, elevators, conveyors, pumps and controls, generates the following streams of products: ■ ■ ■ ■

■■FIGURE 8.20  Lemon peels ready to be pressed for the recovery of essential oil. (Courtesy: Fratelli Indelicato S.r.l.)

■ ■ ■

Citrus juice with specified content of pulp Juice sacs Pulp Clear essential oil Secondary pressed juice Clean peels Peels and rag for animal feeding

8.6 Extraction of juice and essential oil 141

8.6.2  The JBT FoodTech (FMC) system This widely applied system represents the second type of juice extraction technologies. The whole fruit is pressed, without halving. While being squeezed, the fruit is enclosed and supported by a pair of cups with intermeshing fingers. This prevents bursting or disintegration of the peel as a result of the pressure. The juice and essential oil are recovered separately and simultaneously. The JBT juice extractors (Fig. 8.21) are supplied by JBT FoodTech (formerly, Food Machinery Corporation FMC. JBT is short for John Bean

■■FIGURE 8.21  JBT juice extractor. View of cups. (Courtesy: JBT FoodTech)

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Technologies). They are not sold but leased, at a rate defined by the volume of juice produced. Several models varying in the size and number of “heads” (pairs of cups) are available. An early version, named Super Juice Extractor, with cups attached to a rotating circular table, was evaluated at the time, as “the most efficient and most promising of all existing automatic extractors” (Braverman, 1949). Shortly later, the rotating machine was replaced by the much more efficient “Inline” system where the cups are installed in straight line. According to JBT FoodTech, 75% of the world’s citrus juice production in over 35 different countries is based on this technology. Sizing of the fruit according to the size of the cups is important. Undersize fruit is not well squeezed. Oversize fruit is not well supported by the cups and may be disintegrated by the pressure and may also cause mechanical damage to the cups. The sizer may be physically separated from the extractors. In this case, any kind of sizer may be used and the different sizes separated by the sizer are fed separately by conveyor belts to the extractor with cups of the corresponding size. In a more efficient layout, a long beltroll sizer is installed in parallel to a battery of extractors, with the belt inclined toward the extractors (Fig. 8.22). Extractors for the smallest fruit are installed at the head end of the line. Fig. 8.23 shows the squeezing cycle of the extractor in four stages: ■

■

■

■

Stage 1: The cup is open. A feeding device places one fruit in the lower cup. Stage 2: The upper cup moves downward and squeezing begins. The sharp edge of the perforated prefinisher tube (lower cutter) cuts a circular “plug” on the peel at the base of the fruit. A circular cutter (upper cutter) cuts a similar plug on the top of the fruit. Stage 3: The downward movement of the upper cup continues, increasing the pressure on the fruit. The fruit endocarp is forced into the prefinisher tube. The pressure causes the oil glands of the peel to burst and liberate essential oil. A small quantity of water sprayed on the exterior of the fruit washes away the oil. The recommended water flow-rate is 8 gallons (about 30 L) per minute per extractor. The water–oil mixture is collected separately. The empty peel (Fig. 8.24) is discharged between the upper cutter and the upper cup. Stage 4: The orifice tube moves upward into the prefinisher tube and exerts pressure on its contents between the two plugs. The juice and juice sacs are forced out of the prefinisher tube through its perforations and into the juice manifold. The rag, membranes, and the two plugs cut from the peel are discharged through an opening in the orifice tube.

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■■FIGURE 8.22  Inclined belt of sizer feeding battery of JBT extractors. (Courtesy: JBT FoodTech)

After the squeezing cycle has been completed the upper cup retracts, a new fruit is placed on the lower cup and a new cycle of squeezing begins. The raw juice emerging from the juice manifold is sent to finishing. The oil– water mixture is treated for the recovery of oil. The peels are discarded as cattle-feed or processed further.

■■FIGURE 8.24  Orange peel discarded from JBT extractor, showing hole cut in the peel, and marks of the cup fingers.

■■FIGURE 8.23  Phases of the squeezing cycle in JBT juice extractor. (Courtesy: JBT FoodTech)

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As mentioned before, the JBT extractor comes in several models, differing in the number and size of cups. The Model 191B has eight pairs of cups and operates at a speed of 100 strokes per minute. Its processing capacity, at 100% efficiency, is therefore, 800 fruits per minute. The cup size is 60 mm and it can handle fruit with a diameter varying from 25 to 60 mm. The Model 291B/392B has five pairs of cups and does 100 strokes per minute. It can take cups of different sizes. The Model 491B has three pairs of cups and operates at a speed of 75 strokes per minute. It is usually equipped with 127 mm cups and serves to process large grapefruit. The new Model 593 (Fig. 8.25) can handle up to 600 fruits per minute, has a more sanitary structure, and is said to produce juice with a lower content of essential oil. Reducing the oil content is particularly important when producing not-from-concentrate (NFC) quality juice, due to the bitterness imparted by traces of oil and the risk of off-flavor development upon storage.

■■FIGURE 8.25  JBT Model 593 Juice Extractor. (Courtesy: JBT FoodTech)

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8.6.3  The Brown juice extraction system The Brown system, widely utilized by the citrus processing industry, belongs to the first group of juice extraction methods. The Brown juice extraction machinery is produced and marketed by the Brown International Corporation L.L.C., located in Winter Haven, Florida. Since 1947, the company has been manufacturing equipment for the citrus and vegetable processing industry. A flow diagram of the citrus juice manufacturing process is shown in Fig. 8.26 The process starts with the extraction of essential oil from the whole fruit, by the Brown Oil Extractor, abbreviated as BOE (Fig. 8.27).

■■FIGURE 8.26  Simplified flow diagram of citrus fruit processing for juice and concentrate.

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■■FIGURE 8.27  The Brown Oil Extractor BOE. (Courtesy: Brown International Corporation, LLC)

After the usual reception, surge storage, washing, and inspection operations, the fruit is elevated by a metering elevator and fed into the BOE at a controlled rate. The BOE consists of rotating rolls carrying on their surface sharp stainless-steel points. The axis of the rollers is perpendicular to the direction of the fruit flow, so that the rotation of the rolls spins the fruit and advances it at the same time. The sharp points lightly puncture the entire surface of the fruit, rupturing the oil glands and liberating the oil. Complete coverage of the fruit surface in the case of nonspherical fruits such as lemons is assured by oscillating the rolls horizontally while rotating. The oil released is washed away by water sprays. After exiting the BOE and prior to juice extraction, the fruit passes through a dryer, where rollers wipe the oil and water clinging to the surface. Unlike other systems based on rasping the peel, the BOE produces an oil–water mixture containing little debris, where most of the oil is not emulsified but free. This makes separation of the pure oil easier. The manufacturers emphasize this feature as “a significant advantage” of the BOE method of oil removal. The oil–water mixture is sent to a desludger centrifuge where the solids are rejected as sludge and most of the water is separated, This water is recycled to the BOE while the oil-rich phase goes to a polishing centrifuge where the remaining water is removed and clear oil is obtained (Fig. 8.28). The second stage is the extraction of juice. After the BOE, the fruit is classified according to size (Fig. 8.29) and fed into the hopper of one of the juice extractors corresponding to the fruit size. The sizer is of the belt-roll type. The Brown Juice Extractors are usually installed in batteries of 10–14 machines, grouped by fruit size, with the sizer running along the battery (Fig. 8.30). The working capacity of each line is 45–80 t of fruit per hour.

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■■FIGURE 8.28  Flow diagram of the separation of pure peel oil. (Courtesy: Brown International Corporation, LLC)

■■FIGURE 8.29  Sizer and feeder to the Brown Juice Extractor. (Courtesy: Brown International Corporation, LLC)

■■FIGURE 8.30  Group of Brown Juice Extractors. (Courtesy: Brown International Corporation, LLC)

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■■FIGURE 8.31  The Brown Juice Extractor. (Courtesy: Brown International Corporation, LLC)

The extractors (Fig. 8.31) can be equipped with various interchangeable parts so as to treat oranges, grapefruit, lemons, or tangerines. Cleaning is performed by a built-in clean-in-place system. From the hopper, the fruit is fed, one by one, into a conveying chain bearing opposing half cups. Each fruit, enclosed between two half cups, is presented to a knife and cut into two halves. The fruit halves are pressed against rotating reamers (Fig. 8.32). The pressure exerted on the fruit during reaming is governed by the air pressure applied to the cups. The reamed juice and pulp are collected in a tank for further processing and the half peels (Fig. 8.33) are discharged. The raw reamed juice is “finished,” that is, its pulp content is reduced to a desired level. International Brown Corporation supplies several models of paddle and screw finishers for citrus juices (Figs. 8.34 and 8.35).

8.6.4  Other juice extraction systems A juice extractor, operating by squeezing the whole fruit between pairs of fingered cups, is supplied by Bertuzzi Food Processing in Italy (Fig. 8.36). The standard machine, named “Citroevolution 3,” has three penetrating heads and handles 250 fruits, with diameters between 50 and 85–90 mm per minute.

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■■FIGURE 8.32  Rotating reamers of the Brown Juice Extractor. (Courtesy: Brown International Corporation, LLC)

■■FIGURE 8.33  Half peels from Brown Juice Extractor. (Courtesy: Brown International Corporation, LLC)

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■■FIGURE 8.34  Brown paddle finisher. (Courtesy: Brown International Corporation, LLC)

■■FIGURE 8.35  Brown screw finisher. (Courtesy: Brown International Corporation, LLC)

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■■FIGURE 8.36  The Citroevolution3 juice extractor. (Courtesy: Bertuzzi Food Processing, Italy)

A totally different method of juice extraction was proposed by Khazaei et al. (2008). A pressurized jet of air is employed for extracting juice and juice sacs from halved citrus fruits. The best route of the nozzle was found to be a figure of eight movement. Air pressures of 300 and 400 kPa were found to be operative enough to remove whole juice sacs from citrus fruits. No industrial application of this interesting process is known.

8.7 CHILLING The raw juice exiting the extractors of any kind is collected in a surge tank of appropriate size. The surge tank is essential for maintaining a constant flow of juice in the continuation of the process. The raw, yet unpasteurized juice is prone to rapid microbial, enzymatic, and oxidative deterioration. To maintain top quality, it is advisable to chill the juice to 8–10°C or less as soon as possible. Rapid chilling is achieved by pumping the juice through a heat exchanger, against mechanically refrigerated water or brine. The chilled raw juice is sent directly to screening or kept in cold jacketed (cold-wall) tanks. The heat exchangers used in citrus processing will be discussed in connection with pasteurization.

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8.8 SCREENING The raw juice obtained from all types of juice extractors contains an excessive quantity of suspended solids, consisting of pulp (see Section 2.4), juice sacs, membranes, and residual pieces of disintegrated peel. The fine particles of pulp should be kept in the juice as they provide the characteristic turbidity (cloud), color, and a substantial part of the aroma of the juice. Juice sacs are a valuable component of the suspended particles and are separated and mixed back to some types of juices to provide the “freshly squeezed” appearance. Suspended solids in excess are separated by a variety of methods, covering almost all the solid–liquid separation operations known in process engineering: screening, filtration, centrifugation, hydrocyclones, etc.

8.8.1  Vibrating screens Open vibrating screens serve to separate suspended solids by gravitation, without pressure. Rectangular or round screens are used. The most popular type is the round vibrating screen, commonly known as the SWECO screen (Fig. 8.37). The SWECO separator is a circular vibratory screening device that vibrates about its center of gravity. The machine is equipped with an upper and a lower eccentric weight. The upper weight creates vibration in the horizontal plane and imparts to the fluid on the screen movement toward the periphery. The lower weight creates vertical vibrations, avoiding clogging

■■FIGURE 8.37  SWECO® Round Vibratory Separator. (Courtesy: SWECO® Co.)

8.8 Screening 153

of the screen and facilitating flow through the perforation. The angle of lead given the lower weight with relation to the upper weight determines the velocity of the spiral flow of the fluid across the screen and is adjustable. Interchangeable screens with different mesh apertures are available. The SWECO separator can be equipped with more than one screen, one on the top of the other, on the same machine. A three-deck separator is recommended for citrus juice. The upper screen has the largest mesh opening and retains the coarser particles of rag and seeds. The second screen has intermediate mesh opening and retains the delicate juice sacs. The third screen is used to remove the undesired quantity of excess pulp. The juice, thus liberated from excess suspended solids, flows out through the bottom of the separator.

8.8.2  Cylindrical screens According to Braverman (1949), the best way to screen citrus juice is to let it run inside a long, slightly inclined, slowly revolving perforated cylindrical drum. The juice that passes through the perforation is collected underneath the drum while the retained solids are discharged at the far end of the cylinder. No pressure is applied other than the very slight centrifugal force induced by the slow revolution. This type of screening is not commonly used in industry today. In the present version, the perforated drum is stationary and the fluid is moved through the drum with the help of revolving paddles or a conveying screw. Called strainers, finishers, pulpers, crushers, or extractors, depending on the function for which they are used, these are among the most versatile machines of the fruit and vegetable processing industry. The drums, made of perforated stainless-steel plate, are usually supplied as half cylinders to be clamped together. The perforated drum is externally supported by ribs. Thicker plates are used for heavy-duty applications. Interchangeable screens with different perforation sizes are used for different applications. For citrus, JBT specifies perforation sizes of 0.020 inch (0.5 mm) for primary juice and 0.015 inch (0.375 mm) for pulp recovery. Braverman (1949) recommends 3 mm for the first screening operation, for the production of “raw juices” used in the manufacture of “squash” and down to 0.4 mm for juice destined to concentration. In the paddle finishers (Fig. 8.38), a number of paddles, attached to the revolving shaft, impart to the fluid radial and axial movement. The radial component depends on the speed of revolution and the gap between the paddles and the screen. It determines the force with which the fluid hits the perforated drum. The axial movement depends on the speed of revolution and the

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■■FIGURE 8.38  Paddle finisher, showing rotor. (Courtesy: JBT FoodTech)

lead angle of the paddles. It determines the axial velocity and therefore the residence time of the material. The standard specified speed of revolution is 100 rpm but all the operation conditions are adjustable. The separated solids, known as the pomace (like the solids separated from crushed apples in the production of apple juice), exit through a weighed gate at the far end of the machine. The adjustable parameters mentioned above determine the juice yield and the dryness of the pomace. In the screw finishers (Fig. 8.39), the rotating element inside the drum is a conveying screw. Exit of the pomace is controlled by a plug valve, loaded mechanically or by adjustable air pressure. The processing capacity of the screw finisher is lower than that of its paddle-operated counterpart. Screw finishers are usually preferred for the manufacture of NFC juice.

8.8.3 Centrifuges Centrifugal solid–liquid separation is seldom applied for juice finishing, with the possible exception of the production of clear lemon juice, to be discussed later. On the other hand, centrifuges of this kind are advantageously used in the recovery and purification of essential oils. The water–oil mixture from the juice extraction process contains substantial amounts of solids, particularly in extraction processes where the oil is liberated by rasping the peel. The first operation for the recovery of oil from such mixtures is to remove the suspended solids, called frit, debris, or crumb. Several types of solid-retaining centrifuges are available for this purpose. Centrifugal solid–liquid separation is performed by different types of centrifuges, depending on the solids content of the feed. In centrifuges with a solid-wall bowl, the liquid phases are discharged continuously but the solid

8.8 Screening 155

■■FIGURE 8.39  Screw finisher, showing rotor. (Courtesy: JBT FoodTech)

“sludge” is accumulated in the bowl. The machine is stopped periodically, the bowl is opened and cleaned by hand. Alternatively, the bowl may be equipped with nozzles through which the sludge is discharged continuously, with a certain quantity of nozzle. Nozzle centrifuges are used for treating feeds containing up to 10% solids by volume. In a variation of the self-cleaning separators, supplied by Westfalia GEA, several ports for the discharge of solids are located around the bowl periphery. The ports are periodically opened and closed by means of a sliding hydraulic piston located in the bowl bottom. The solids are ejected without stopping the machine. The centrifugal separator shown in Figure 8.40 separates the liquid feed into three phases, namely, solids, water phase, and oil-rich emulsion. The solids are retained in the bowl and removed periodically by hand or discharged continuously through nozzles. In the so-called intermittent solid discharge centrifuges, the bowl wall consists of two conical halves pressed together by hydraulic force (Fig. 8.41). When the bowl is full with solids, the hydraulic system releases the bottom half which drops slightly to leave an opening for the ejection of the solids (Berk, 2013). Intermittent discharge centrifuges can handle feeds with up to 30–40% solids by volume. Mixtures containing a higher concentration of solids require a different type of centrifuge, called decanter, not often used in the citrus industry.

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■■FIGURE 8.40  Centrifugal separator. (Courtesy: Alfa-Laval)

■■FIGURE 8.41  Intermittent centrifugal desludger. (Courtesy: Alfa-Laval)

8.9 Deaeration 157

After removal of the suspended solid particles by screeners or solids retaining centrifuges, the so-called emulsion of essential oil is sent to a polishing centrifuge where pure oil is separated from the watery phase by virtue of the difference in specific gravity between oil and water.

8.8.4 Hydrocyclones Cyclones are extremely simple devices for the separation of solid particles from gas or liquid. When used with liquid feed they are called hydrocyclones. Their simplicity, ease of operation, and lack of moving parts are important advantages. They consist of a vertical cylindrical or conical tube, with a tangential feed entry and exit ports on both ends (Figs. 8.42 and 8.43). The tangential entry imparts to the fluid a circular movement. The heavier solid particles separate and move to the periphery while the lighter liquid moves toward the central axis. The solids move down in a spiral-like travel and exit continuously through the lower port. The liquid is discharged through the upper port. The degree of separation of a given mixture in a given cyclone depends on the feed flow-rate and the diameter of the lower opening. A higher degree of clarification is achieved by using a larger bottom outlet. When using hydrocylones, it is important to assure a steady flow of juice and to avoid occlusion of air. Inserts of different sizes are used for changing the outlet diameter. When used, the hydrocyclone is the first finishing device receiving the raw juice from the extractor. The hydrocyclone is usually tuned so as to separate the juice to approximately 50% overflow and 50% underflow. The overflow is juice containing good quality pulp, ready for further processing. The underflow is screened in a paddle or screw finisher to recover more finished juice and to separate excessive and defective pulp. A flow diagram of the treatment of juice using a hydrocyclone is shown in Fig. 8.44.

8.9 DEAERATION In the course of extraction and screening, the juice is intimately exposed to air, which results in dissolution of oxygen, almost to saturation. Dissolved oxygen may be expected to enhance vitamin C oxidation, nonenzymatic browning, degradation of aroma compounds, and induction of off-flavor. Investigations carried out to test this effect produced controversial results. Rassis and Saguy (1995) did not find any significant effect of oxygen on browning of commercial citrus juices and concentrates. According to Soares and Hotchkiss (1999), the rate of ascorbic acid loss in bottled juice depends on the oxygen permeability of the package and not on the initial dissolved

■■FIGURE 8.42  Hydrocyclone. (Courtesy: JBT FoodTech)

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■■FIGURE 8.43  Flow pattern in the hydrocyclone. (Courtesy: JBT FoodTech)

oxygen content of the juice. Robertson and Samaniego (1986) also found that the initial concentration of dissolved oxygen had no effect on the rate of ascorbic acid degradation and browning of lemon juice upon storage. Nevertheless, deaeration of the juice before pasteurization and packaging is recommended by some authors (eg, Sandhu and Minhas, 2006). Deaeration

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■■FIGURE 8.44  Flow diagram of citrus juice processing, using hydrocyclone. (Courtesy: JBT FoodTech)

is generally applied before citrus juices are bulk stored in tank farms (see Chapter 11). Mannheim and Passy (1979) investigated the effect of deaeration methods on the quality of bottled orange and grapefruit juices. When applied to citrus juices, the preferred method of deaeration consists of flashing the juice into a vacuum vessel. The concentration of oxygen dissolved in the juice follows the Henry’s Law, which can be written as follows: p = kH c

where p = the partial pressure of the gas kH = Henry’s constant. It depends on the gas/liquid pair and on the temperature c = equilibrium concentration of the gas in the liquid. The constant kH rises with increasing temperature. To obtain low oxygen concentration c in the juice, therefore, one has to operate at low p (hence the use

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■■FIGURE 8.45  Deaerator. (Courtesy: Alva-Laval)

of vacuum), at a temperature high enough to make kH sufficiently high. This means that not only oxygen but also volatile aroma components are removed by vacuum flashing. According to Jordán et al. (2003), a considerable proportion of aroma components are indeed removed by deaeration of the juice. In summary, with the current systems of refrigerated commercialization of retail packaged orange juice, deaeration is not considered a requisite. Instead, careful handling of the juice to minimize contact with air during screening and in flow, as well as use of packaging materials with low permeability to oxygen and expulsion of the air from the headspace, are the ­preferred actions to minimize oxidative deterioration of the juice during storage. However, deaeration (Fig. 8.45) is practiced in the case of juice to be canned for long shelf life or bulk stored in refrigerated tank farm. Expulsion of oxygen occurs simultaneously in the course of de-oiling or essence recovery in the production of concentrates.

8.10 HOMOGENIZATION Homogenization of dispersed systems (suspensions and emulsions) refers to the reduction of the size of the dispersed particles by applying some sort of shearing process. Several homogenization methods are in use and they differ in the mechanism of applying shear to the fluid. The most frequently used method in the food industry is the high pressure homogenization whereby the fluid is forced, at high pressure, through a narrow gap. The main objective of homogenization of citrus juices is to reduce the size of the suspended pulp particles and thus to increase turbidity and intensify color.

8.10 Homogenization 161

Betoret et al. (2009) homogenized fresh orange juice at different homogenization pressures in the range of 0–30 MPa and found that the pressure had an effect on particle size, cloudiness, and color, but not on flavonoid content. Sentandreu et al. (2011) homogenized low-pulp juice at a pressure of 20 MPa and obtained a well-colored product. Juices of similar quality were obtained either when the whole juice was homogenized, or when homogenization was applied only to the pulp separated by centrifugation and followed by blending this homogenized pulp back into the low pulp fraction. Betoret et al. (2012) homogenized low-pulp mandarin juice and found that the high pressure process did not have negative effects on antiradical activity. The treated mandarin juice was used in a study of vacuum impregnation of apples. More juice was found to penetrate the apple tissue when homogenized at higher pressure. When applied, homogenization is done by high-pressure homogenizers, known as Gaulin homogenizers. Basically, these are reciprocating piston pumps that force the fluid through an adjustable narrow gap (Figs. 8.46 and 8.47). Disintegration of the particles occurs as a result of shear, friction, and cavitation. Pressure homogenization is sometimes applied to citrus pulp, with the objective of reducing the size of the pulp particles and enhancing their clouding effect when used in soft beverages. The possible use of very high-pressure homogenization as a nonthermal pasteurization process will be discussed in Section 8.12.5. Treatment with high-power ultrasonic waves is akin to homogenization, as it disintegrates solid particles in suspension. Aadil et al. (2013) investigated

■■FIGURE 8.46  Structure of high pressure homogenizing head.

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■■FIGURE 8.47  Five-head homogenizer. (Courtesy: GEA-Soavi)

the effect of ultrasonic treatment on the quality of grapefruit juice. Their results showed significant improvement in cloudiness, total antioxidant capacity, free radical scavenging activity, ascorbic acid, total phenolics, flavonoids, and flavonols in juice sonicated for 30, 60, and 90 min at 28 kHz frequency with no change in pH, acidity, and °Bx.

8.11  PULP WASH Regardless of the juice extraction method, raw citrus juices contain a more or less large excess of pulp, which is separated, classified, and incorporated into various products. Various classes of pulp are marketable items. The pulp leaving the finishers contains a considerable amount of juice at 12–13 Bx. Most finishers can be adjusted to deliver a drier pulp and a somewhat higher juice yield, but this affects the quality of the juice. Excessively tight finishing produces more viscous juice, which is objectionable, particularly if concentrates are produced. Pulp washing is a process for recovering a good part of the soluble solids in the pulp. The process consists of countercurrent multistage leaching of the pulp with water. A flow diagram of a two-stage pulp wash process is shown in Fig. 8.48. Usually, three or four stages of extraction are used. A larger number of stages do not improve yield considerably (Kimball, 1999). Each stage consists of a mixing vessel, pump, and screener. A compact pulp wash system with three stages on a skid is commercially available (Fig. 8.49).

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■■FIGURE 8.48  Flow diagram of 2-stage countercurrent pulp wash.

■■FIGURE 8.49  3-stage pulp wash unit on skid. (Courtesy: JBT FoodTech)

A water-to-pulp ratio of 1–2.5 is usually applied. Using drinking quality water is essential. About 90% of the soluble solids in the original pulp are recovered, which corresponds to an increase of about 8–10% in the total yield of soluble solids. The exhausted pulp can be dried and sold as a source of fiber or it can join the peels in the production of animal feed. The watery extract, also known as WESOS or WESGS (water-extracted soluble orange solids or water-extracted soluble grapefruit solids, respectively), contains about 5% soluble solids and can be concentrated to 60–65 Bx by evaporation. Concentrated WESOS and WESGS are highly viscous and may require enzymatic treatment for the reduction of viscosity. Pulp wash concentrate is a marketable product but its utilization is restricted by official

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regulations. Online mixing of nonconcentrated WESOS with nonconcentrated orange juice obtained from the same fruit is permitted in Florida, but adding concentrated WESOS to separately produced orange juice concentrate is prohibited. European regulations prohibit diffusional operations in citrus processing. The law requires that WESOS and WESGS be clearly labeled as such. The extracts can be freely used in the production of soft drinks (Licandro and Odio, 2002; Kimball, 1999). WESOS and WESGS are much less expensive than the corresponding juices. They are mainly used as clouding and flavoring agents in soft drinks. Their use may require enzymatic degradation of the pectic substances to reduce the viscosity (Braddock and Kesterson, 1975). Furthermore, they, and particularly WESGS, may be too bitter to be used without debittering by adsorption or ion exchange, mainly due to their high limonin content. Core wash is another water extraction operation, applicable only to the JBT method of juice extraction. In the JBT extractor, the contents of the fruit endocarp are compressed in the perforated prefinisher tube between the two round plugs cut from the peel. At the end of the compression, the material in the prefinisher tube contains the axial core, carpelar membranes, the two plugs, and some juice. When the squeezing cycle is completed, this material is ejected through a special opening. It may be discarded together with the peels or collected as a separate stream and subjected to multistage countercurrent extraction with water to recover the soluble solids, just as the pulp wash process. Fewer stages and less vigorous agitation are applied. The core wash fluid is sold or internally utilized like the pulp wash extract.

8.12 PASTEURIZATION Stabilization of citrus juices by pasteurization serves the double purpose of inactivating enzymes (mainly pectin methylesterase) and destroying pathogenic and spoilage-causing microorganisms. As explained in Chapter 2, inactivation of pectin methylesterase (PME) is essential for the stability of the uniform cloudiness of the juice and for preventing gelation in concentrates. Destruction of the microorganisms is of paramount importance for food safety and for the prevention of spoilage. Pectin methylesterases are more heat resistant than the common microflora of juices. Therefore, pasteurization protocols have always been adjusted so as to inactivate PME, assuming that any heat treatment capable of completely inactivating the enzyme is certainly capable of satisfactorily destroying the vegetative microflora of the juice. As to the spore-forming microorganisms, it has always been assumed that the low pH of the juice provides sufficient warranty against their growth in citrus juices.

8.12 Pasteurization 165

The kinetics of thermal inactivation of PME and the minimal time– temperature combinations for the stabilization of citrus juices have been extensively investigated. Some of the investigations were mentioned in Chapter 2. According to Braverman (1949), inactivation of “pectic enzymes” requires heating of at least 4 min at 85°C, or 1 min at 88°C, or a fraction of a minute at 100°C or higher. Nordby and Nagy (1980) place the temperatures of inactivation in the range of 86–99°C, stating that the time of heating and the pH must be considered in selecting the exact temperature. According to Rebeck (1995), citrus juice is flash-pasteurized at 185–200°F (85–93°C) with a holding time of 30 s. Wicker and Temelli (1988) investigated the heat sensitivity of PME in orange juice pulp and confirmed the existence of multiple isoforms. More recently, the thermal inactivation kinetics of PME was evaluated at different values of pH and processing temperature by Tribess and Tadini (2006). The enzyme was found to have several isoforms with different heat sensitivities. The thermolabile fraction of PME was more heatstable at higher pH. The established industrial practice is, apparently, heating the juice to 92–98°C for about 30 s immediately after extraction and screening. Destruction of microorganisms and inactivation of enzymes are not the only outcome of thermal pasteurization. Other effects of the heat treatment include induction of “cooked taste,” destruction of vitamins, undesirable color changes, etc. Each of these heat-induced chemical reactions has its own kinetic parameters. The activation energy of the chemical reactions (thermal damage) is lower than that of the thermal inactivation of enzymes, which, in its turn, is lower than that of the thermal killing of microorganisms. Therefore, for a giving stabilization effect, heat treatment at a higher temperature for a shorter time results in less thermal damage to quality than heat treatment at lower temperature for longer time. This is known as the “high-temperature-short-time” concept. Accordingly, citrus juices are rapidly heated in appropriate heat exchangers, held at high temperature for the specified time necessary for stabilization, and then cooled as rapidly as possible. The rapidity of the cooling step often determines the extent of thermal damage to sensory quality and nutritional value. Several types of citrus juices, produced by different thermal processing methods, are present in the market. The common method of pasteurization in industry makes use of heat exchangers for heating. Two other methods of thermal treatment are ohmic heating and microwave and RF heating. In addition, nonthermal methods of pasteurization are at different stages of investigation or initial application. These unconventional methods will be described in the next section.

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8.12.1  Canned juice Canned juices, first produced in the 1920s, were the first commercially available citrus juices. The production of canned juices declined steadily after the development of the frozen concentrate in the 1940s, but they continued to be produced in considerable quantity until the 1960s. In 1970, they represented only about 6% of the citrus juice sold in the United States and in 1980 their market share fell to less than 4%. However, they are still produced in small quantity in certain countries and in small establishments. While the US market for orange juice switched to frozen concentrate, grapefruit juice was predominantly canned. The screened juice, at the desired pulp content, is rapidly and continuously heated in a heat exchanger. The most common types of heat exchangers for juice pasteurization are tubular heat exchangers and plate heat exchangers, the later being the more widespread. Adaptation of the heat exchanger specifications to the pulp content of the juice is important to avoid accumulation of sediments and scorching. The juice is usually heated to about 92°C, using steam or pressurized hot water as the heating medium. The temperature is automatically controlled. In-flow residence time in the pasteurizer is about 40 s. The heat exchanger is usually equipped with a flow diversion valve that sends back juice that did not reach the specified temperature. The hot juice is admitted in the holding tank of the can filling machine. Rotary piston fillers (Fig. 8.50) synchronized with an automatic can seamer (Fig. 8.51) are commonly installed. These systems have capacities from 60 to 600 cans per minute. Plain tin cans are preferred despite the corrosiveness of the juice,

■■FIGURE 8.50  Piston filler.

8.12 Pasteurization 167

■■FIGURE 8.51  High capacity can seamer. (Courtesy: Angelus Co.)

because of the reducing action of tin and its beneficial effect on vitamin C loss and browning. The empty cans are flushed with air, hot water, or steam. By the time it reaches the can, the juice has cooled down to approximately 80–85°C, but this is satisfactory, as PME has been already inactivated and the temperature in the can is amply sufficient to disinfect the can and prevent recontamination with microorganisms. After filling and sealing, the cans are inverted to disinfect the lid. The cans are spin-cooled with sprays of chlorinated water. The rate of cooling is rather slow, due to relatively poor heat transfer. Some thermal damage occurs due to the slow cooling. It is advisable not to cool the cans too much but to maintain a final temperature of 35–40°C to accelerate drying of the can surface and to avoid corrosion. Hot filling and subsequent cooling of the sealed can results in reduced pressure inside the package. The vacuum inside the can is one of the indicators of the adequacy of the process. Canned juice is usually stored and marketed at ambient temperature.

8.12.2  Bottled hot-fill juice The sequence of heat–fill–seal–cool of canning can be applied to bottling. Pasteurized juices in glass bottles have been a popular item in the market, particularly outside the United States. The main advantages of glass are transparency and inertness. Its main shortcomings are fragility and weight. Lately, bottles made of the polymer polyethylene terephthalate (PET), widely used in the packaging of drinking water, successfully penetrated the juice industry, replacing glass bottles. PET bottles are fairly transparent, light weight, and impact resistant. They can withstand the filling temperature.

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They provide excellent barrier properties to oxygen and moisture. The hotfill bottling process is essentially similar to canning.

Polyethylene terephthalate

8.12.3  Aseptically processed juice It is well known that the slow cooling phase of hot-fill methods is particularly detrimental to the sensory and nutritional quality of juices. Aseptic processing technology overcomes this problem. The sequence of operations in aseptic processing is: heat–cool–aseptically fill into presterilized packaging–seal. The juice is pumped to a heat exchanger system where it is rapidly heated following the HTST principle and immediately rapidly cooled to preserve quality. The heat exchanger may have a “regenerative” section for exchange of heat between the hot fluid and the incoming cold product, whereby energy is saved. Meanwhile, the empty packages, multilayer cartons, or PET bottles are sanitized with a chemical disinfectant. In the Tetra Pak system, the packaging material in strip form passes through a bath of 25–30% hydrogen peroxide under ultraviolet light, cut and formed to produce the presterilized carton. The sanitizing agent is destroyed by heat. In the case of PET bottles, the containers are preformed or made in situ from PET powder and flakes, by injection and blow-molding. They are sanitized in inverted position, inside and out, with hydrogen peroxide or with peroxyacetic acid (also known as peracetic acid). When peracetic acid is used, sanitizer residues are rinsed away with sterile water. The caps are sterilized in the same way. Now the sterilized package and the sterile product meet in an environment kept under aseptic conditions, with the help of steam, chemical agents, ultraviolet light, and filtered air in laminar flow. The containers are cold-filled, sealed or capped, stored and marketed under refrigeration, hence the term “chilled juices” used to distinguish these products from canned or frozen juices and concentrates. A drop of liquid nitrogen is sometimes added before closing to create an inert headspace. A schematic flow diagram of aseptic processing is given in Fig. 8.52. Aseptic processing in cans was known and practiced since the 1920s, particularly with dairy products (David et al., 2013). In 1981, FDA approved the use of hydrogen peroxide as a sanitizer and opened the way for the widespread aseptic packaging of liquid and semiliquid foods in carton boxes.

8.12 Pasteurization 169

■■FIGURE 8.52  Simplified flow diagram of aseptic filling.

Considering the number of products involved, aseptic processing deserves to be qualified as the leading real innovation of the last 50 years in the food industry. One of the outstanding advances resulting from aseptic processing is the ability to utilize a vast variety of packaging materials and a wide range of container sizes that can be filled. Using aseptic processing technology, citrus juices, pulps, and concentrates can be filled not only in retail packages of different sizes, but also in sterilized bags, tots, drums, tanks, tankers, and containers for storage or transport. Aseptic filling will be discussed in more detail in the chapter on packaging and storage.

8.12.4  Heat exchangers for heating and cooling citrus juice Heat exchangers are devices for the exchange of heat between two fluids, separated by a heat conducting partition, usually made of metal (Berk, 2013). They are extensively used in the citrus processing industry for heating and cooling duties. The most frequently used types are tubular heat exchangers and plate heat exchangers. A third type, the swept-surface heat exchanger, is mainly used for slush freezing of concentrated. a. Tubular heat exchangers (Fig. 8.53): in its simplest form, the tubular heat exchanger consists of two concentric tubes (tube-in-tube). The product is pumped through the inner tube while the heating or cooling medium flows through the annular space between the tubes (Fig. 8.54). For larger heat transfer area, a triple tube exchanger (tube-in tube-in tube), in which the product is fed to the middle tube and the heating

170 CHAPTER 8  Production of single-strength citrus juices

■■FIGURE 8.53  Tubular (shell-and-tube) heat exchanger. (Courtesy: Alfa Laval)

■■FIGURE 8.54  Annular flow in tube-in-tube heat exchanger. (Courtesy: Alfa-Laval)

■■FIGURE 8.55  Tubular heat exchanger for viscous products “Visco-line”. (Courtesy: Alfa Laval)

or cooling medium to the inner and outer tubes, is available. Tubular heat exchangers are particularly suitable for highly viscous fluids or fluids containing a high concentration of particle solids or fibers, such as pulp or pulp-rich juices. A special type (Alfa Laval ViscoLine) has corrugated walls to facilitate flow and prevent stagnation (Fig. 8.55) in the case of juice with high pulp content. b. Plate heat exchangers (Fig. 8.56): plate heat exchangers consist of a stack of corrugated metal plates, pressed together and mounted on a fixed frame. The product flows over one side of the plates and the heating or cooling medium over the other side. Leakage is prevented by gaskets. The gap between the plates is made narrow so as to increase flow velocity and enhance the rate of heat exchange. However, wide gap exchangers are available for use with fluids having a high viscosity and/or a high content of solids, such as pulp, in suspension. A group of plates constituting the heating section and another group

8.12 Pasteurization 171

■■FIGURE 8.56  Plate heat exchanger. (Courtesy: Alfa Laval)



serving as the cooling section are mounted on the same frame. The heating medium is usually hot or very hot water under pressure. The cooling medium is usually mechanically refrigerated water or brine. Initially introduced for the pasteurization of milk, plate heat exchangers are now extensively used in the food industry (Berk, 2013). Their main advantages are: ■ Flexibility: their capacity can be increased or decreased by adding or removing plates. ■ Sanitation: all the heat transfer areas are easily cleaned and inspected by opening the stack. ■ Compactness: for its capacity, the plate heat exchanger occupies little floor area. ■ Efficiency: elevated heat transfer coefficients are achieved thanks to the turbulent flow. Kim et al. (1999) measured the heat transfer coefficient in a plate heat exchanger during the pasteurization of orange juice. The values found varied from 983 to 6,500 W m−2 °C, whereas the water heat transfer coefficient varied from 8,387 to 24,245 W m−2 °C which shows the effect of material properties (viscosity, suspended solids, specific gravity) on heat transfer.

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■■FIGURE 8.57  Horizontal scraped-surface heat exchanger. (Courtesy: SPX FLOW, Inc.)

c. Swept surface heat exchangers can be vertical or horizontal (Fig. 8.57). Swept (or scraped) surface heat exchanger comprises a jacketed cylinder equipped with a central rotating dasher with scraping or wiping blades (Fig. 8.58). The product is fed into the cylinder and the heating or cooling medium into the surrounding jacket. The dasher rotating at a velocity of 600–700 rpm spreads and moves the product across the heat transfer area. Swept surface heat exchangers can be horizontal or vertical. In the citrus industry, they may be used for heating or cooling but their main service is for slush freezing of concentrates.

8.12.5  Nonconventional pasteurization methods

■■FIGURE 8.58  Structure of swept-surface heat exchanger. (Courtesy: Alfa Laval)

a. Ohmic heating: ohmic heating is a method whereby a material is heated by passing an electric current through it (Joule effect).The principles and applications of ohmic heating were reviewed by Knirsch et al. (2010). The ohmic heater consists of one or several pairs of electrodes to which voltage is applied (Fig. 8.59). In the continuous mode, the fluid to be heated flows through the space between the electrode and acts as a moving electrical resistance. Heat is instantly generated internally and not transmitted through heat transfer surfaces. Ohmic processing is, therefore, particularly suitable for heating pumpable foods containing solid particles and highly viscous fluids as well as highly heat sensitive products that cannot tolerate large temperature gradients. Alternating current at normal line frequency and graphite electrodes are used in commercial units.

8.12 Pasteurization 173

■■FIGURE 8.59  Ohmic heater. (Courtesy: Alfa Laval)

Leizerson and Shimoni (2008) treated orange juice by continuous high-temperature (90, 120, and 150°C) ohmic heating. They reported complete inactivation of microorganisms and a 98% reduction of PME activity, with only 15% loss of vitamin C and no damage to sensory quality. Vikram et al. (2005) compared ohmic, microwave, and conventional heating and found that ohmic heating resulted in better retention of vitamin C in orange juice. Tumpanuvatr and Jittanit (2012) investigated the evolution of temperature in juices and purees of fruits, included oranges, subjected to ohmic heating. The authors did not find any significant difference in organoleptic quality and residual vitamin C between products pasteurized by ohmic heating and products conventionally heated at the same heating rate. Icier and Ilicali (2005) applied ohmic heating to orange juice concentrate and studied the effect of concentration on electrical conductivity. Demirdöven and Baysal (2014) worked on the optimization of PME inactivation by ohmic heating. All these investigators reported no deterioration of the sensory properties. However, despite theoretical advantages and favorable reports from investigators, the industrial application of ohmic heating to citrus products is not widespread, apparently because of cost and engineering problems yet unsolved. b. Microwave heating: the application of microwaves to the heat treatment of citrus juices attracted considerable research interest for

174 CHAPTER 8  Production of single-strength citrus juices

a long time (Copson, 1954; Nikdel and MacKellar, 1992; Villamiel et al., 1998; Cinquanta et al., 2010). Villamiel et al. (1998) evaluated the effect of continuous microwave heating of orange juice on PME inactivation, ascorbic acid, free amino acids, carbohydrates, and hydroxymethyl furfural content as well as nonenzymatic browning. The results were compared with those recorded for conventional pasteurization in a tubular heat exchanger. The microwave process was found to be efficient in PME inactivation and gave results similar to pasteurization in a tubular heat exchanger. In all other parameters, continuous microwave heating was equal to or better than the conventional process. c. Nonthermal processes: ohmic and microwave heating processes may have merits for the rapidity of heating but they are still thermal processes and do not bring solutions to the problem of quality deterioration due to the relatively slow rate of cooling. Therefore, there is strong interest in developing nonthermal or cold pasteurization processes. A number of such processes, also termed “emerging technologies,” are at different stages of development and initial industrial application (Sizer and Balasubramaniam, 1999; Cullen et al., 2012). Some of these processes are: high-pressure pasteurization (Basak and Ramaswamy, 1996; Polydera et al., 2004), pulsed electric fields (Sánchez-Moreno et al., 2005), pulsed intense light (Zhang et al., 2011), ultraviolet light (Gayán et al., 2012), and ionizing irradiation (Foley et al., 2002). “Minimal processing” is also a term used for some of the emerging technologies (Plaza et al., 2011). High-pressure pasteurization (HPP) refers to the application of hydrostatic pressure in the order of 600 MPa to packaged food in a pressure vessel for the duration of a target holding time of a few minutes. The temperature of the product rises due to compression. HPP is essentially a batch process. Significant reduction in vegetative cell counts can be achieved but HPP is practically incapable of inactivating enzymes (Zhang et al., 2011) and therefore cannot fully replace thermal processing of citrus juice. This was confirmed by Sampedro et al. (2008) who worked on orange juice–milk mixtures. The use of carbon dioxide in combination with high pressure does not enhance pectin methylesterase inactivation or microbial sterilization but may improve ascorbic acid retention (Boff et al., 2003; Truong et al., 2002). The technique of preservation with pulsed electric fields (PEF) is based on the application of short pulses (microseconds to milliseconds) of high-voltage fields (15–50 kV cm−1). This treatment,

8.12 Pasteurization 175

also named electroporation or electropermeation, is known to cause irreversible disorders in the cell membranes and resulting of death of microorganisms. Reports on the effect of PEF on enzymes in general and PME in particular are controversial (Terefe et al., 2015). Part of this effect is attributed to ohmic heating due to electric current caused by the application of voltage. Elez-Martínez et al. (2007) applied high-intensity PEF to orange juice and determined the parameters of the PME inactivation kinetics as a function of field strength, frequency, and treatment time. Espachs-Barroso et al. (2006) studied the effect of high PEF on isolated enzymes and found maximum PME inactivation of 87% at the most extreme conditions. Uemura and Isobe (2003) developed an apparatus for the inactivation of Bacilus subtilis spores in orange juice with high AC voltage under pressure. Heinz et al. (2002) formulated basic principles for the design of PEF treatment of liquid foods. Agcam et al. (2014) applied a new inactivation kinetic model to study PEF treatment of orange juice. Cserhalmi et al (2006) and Hartyáni et al. (2011) studied the physico-chemical and sensory effects of pulsed electric field on citrus juices using electronic nose and tongue. Their results did not signal any significant difference between the treated samples and the control. Ultraviolet light is not effective in turbid media such as citrus juices unless the juice is irradiated as a thin film. Gayán et al. (2012) succeeded in inactivating E. coli in orange juice using ultraviolet light and moderately high temperature (55°C) in an annular thin film reactor. Geveke and Torres obtained 5–6 log reduction of E. coli and S, cerevisiae in grapefruit juice, using centrifugal force to produce a thin film. Ionizing radiation, either with gamma rays or with electron beams, has long been tried for the enzymatic and microbiological stabilization of citrus juices (Braddock et al., 1970) and found to be unacceptable, due to flavor deterioration. Deterioration of flavor is detectable even at radiation doses far below those needed for preservation (Yoo et al., 2003). Flavor degradation is due both to the destruction of existing flavor components (Fan and Gates, 2001) and to the generation of new objectionable components, particularly sulfur-containing substances (Foley et al., 2002; Fan, 2004). The pulsed intense light process consists of exposing the food to pulses of intense light emitted by high-power lamps. This treatment has a certain potential as a surface decontamination method but is ineffective in opaque liquids such as orange juice (Ferrario et al., 2013).

176 CHAPTER 8  Production of single-strength citrus juices

8.13  SINGLE-STRENGTH JUICES FROM CONCENTRATE Considerable quantities of ready-to-serve juices are produced from concentrated juices, by diluting with water. The history of the development of frozen citrus concentrates and the processes for their production will be described in the next chapter. From the industry’s viewpoint, the production of juices from concentrate fulfills a number of desirable functions: ■

■

■

■

It expands the production season and increases plant and personnel utilization. It permits blending different concentrates and auxiliary ingredients, whereby desirable quality (Brix/acidity ratio, color, aroma, nutritional additives) and cost targets are met. It helps satisfy a sizeable proportion of the population desiring to consume a juice of acceptable quality that costs somewhat less than NFC juice. It allows internal utilization of several by-products of the industry, such as pulps, juice sacs, essences.

Concentrates may be preserved by canning, aseptic processing, chemical preservatives, or freezing. The majority of concentrates produces in Florida and Brazil are frozen. Frozen concentrated orange juice in retail packages for reconstitution at home has been the leading citrus product in the United States for a long time. For reconstitution to RTS juice in the industry, frozen concentrate in lined 55 gal (net weight of 65 Bx concentrate about 256 kg) steel drums is used mainly. The first operation is to thaw the frozen concentrate. The best procedure for thawing or rather tempering is to place the drums in a cold room at 4°C. This takes 48–60 h. Using steam or hot water to accelerate the process is not recommended due to the risk of localized overheating (Redd et al., 1992). Another way to empty the drums is to crush the concentrate using a “concentrate chopper” (Fig. 8.60). The chopper combines mechanical agitation and heat and rapidly converts the concentrate to pourable fluid. The thawed concentrate is pumped to coldwall storage tanks. Frozen pulp is handled in the same way. All the ingredients are blended according to the formulation and drinking quality water is added to dilute to the specified Bx. The next step is pasteurization of the reconstituted juice. This will be the second time that the juice is subjected to pasteurization. The primary pasteurization was carried out immediately after juice extraction, with the main purpose of inactivating the pectolytic enzymes. The concentrate and the reconstituted juice made of it are practically devoid of PME activity. The second pasteurization, carried out before filling the reconstituted juice into its final package, has the objective of killing spoilage microorganisms that may have recontaminated

8.14 Clarified juices 177

■■FIGURE 8.60  Concentrate chopper. (Courtesy: JBT FoodTech)

the material during storage, transport, and processing. Despite the fact that pathogens and microorganisms capable of spoiling juice at pH 4 and below are much more heat labile than PME, the industrial custom is to perform the second pasteurization at 95°C for 15–30 s as a margin of security. Researchers at Tetra Pak determined that pasteurization at 80°C for 15 s would be sufficient to produce a stable, commercially sterile juice. Tubular or plate heat exchangers are used for pasteurization. The juice may be hot-filled into glass or PET bottles or aseptically packaged in cartons or PET bottles. Just as NFCJ, juice from concentrate requires storage and marketing under refrigeration for the protection of quality.

8.14  CLARIFIED JUICES Clarified lemon and lime juices are used as natural acidulants, culinary ingredients and widely utilized bar supplies. They are marketed as singlestrength juices or as concentrates. Clarified lemon juice in small bottles, preserved with sulfur dioxide, is available as a culinary condiment.

178 CHAPTER 8  Production of single-strength citrus juices

After extraction, screening, and pasteurization, the juices are treated with commercial pectolytic enzymes at 35°C, then with coagulants such as gelatin and bentonite, and clarified by centrifugation and/or filtration (Uçan et al., 2014). Nearly total clarification may be achieved by membrane microfiltration. Clarification may also be achieved by adding polygalacturonic acid, without enzymes (Baker, 2006). According to the author, polygalacturonic acid, dissolved by neutralization with KOH, rapidly clarified lime, lemon, grapefruit, orange, and apple juices. The turbidity was reduced to 5% or less of its original value in orange, grapefruit, and lemon juices in 1 h. Optimum polygalacturonic acid concentration ranged from 75 to 500 ppm. The flow behavior of depectinized, clarified orange juice was studied by Ibarz et al. (1994) and found to be Newtonian.

8.15  REDUCED ACIDITY AND DEBITTERED ORANGE AND GRAPEFRUIT JUICES According to Norman (1990) “approximately 20% of the US population does not consume citrus products due to the high acidity associated with them.” Accordingly, some producers offer an acid reduced variation of their product for the consumers who prefer their juice “on the smooth side.” De-acidification is achieved by treatment with an anion exchange resin that binds the citrate anion of the juice against the hydroxyl ion of the resin. 3R − OH + H 3 Cit → R 3 Cit + 3 H 2 O

When applied to grapefruit juice this treatment has been found to reduce simultaneously the acidity and the bitterness. Due to the relatively large size of the citrate anion, macroreticular resins are used. Usually, the pulp is removed before the treatment to prevent clotting of the adsorption column and added back to the de-acidified juice. Bitter taste is a major cause for food rejection. According to Drewnowski (1997), this may be due to the perception that bitterness predicts toxicity. As mentioned before, bitterness is the main consumer objection to grapefruit juice. Debittering of grapefruit juice involves removal of bitter flavonoids and limonoids (Dekker, 1988). Matthews et al. (1990) and Guadagni et al. (1973) proposed a bitterness index connected to chemical composition, whereby one point is assigned to each ppm of naringin while 20 points are given for 1 ppm of limonin. Debittering can be achieved by absorption of the bitter substances on vinyl-dodecylbenzene resins. Sami et al. (1997) investigated the consumer acceptance of debittered grapefruit juice and found, surprisingly, that consumers prefer moderately bitter (bitterness index= 450) rather than more debittered juice. Johnson and Chandler (1985) screened a

8.16 Blended juices 179

large number of commercial resins for their acidity and bitterness-reducing capability. Hernandez et al. (1992) investigated a process of debittering grapefruit juice and grapefruit pulp, consisting of ultrafiltration followed by adsorption in a resin column. Limonin in grapefruit juice and grapefruit pulp wash was completely removed. Sensory evaluation by a taste panel indicated that debittering improved the acceptability of juice and pulp wash. However, Kranz et al. (2011) reported on the adsorption of flavor components during grapefruit juice debittering using resins. Stinco et al. (2013) investigated the effect of industrial debittering on the bioactive components and nutritional value of orange juice and found that debittering resulted in considerable reduction of ascorbic acid and antioxidant activity. A different method of debittering, avoiding the losses due to adsorption by resins, uses the enzyme naringinase (Olsen and Hill, 1964; Chien et al., 2001). Naringinase catalyzes hydrolysis of naringin to sugars and the nonbitter aglycone (see Chapter 2). An ingenious method, proposed by Soares and Hotchkiss (1988), makes use of active packaging technology whereby grapefruit juice is packaged in cellulose acetate film containing immobilized naringinase.

8.16  BLENDED JUICES Blending concentrates prepared from fruit with varying state of maturity, concentrates from various origins, or concentrates from various varieties is common practice in industry. Carter and Barros (1988) evaluated the flavor of orange juice prepared by blending frozen Florida concentrates produced with varying extraction yields. Blended juices prepared from the concentrates of two or more varieties are found in the market. Most commonly, grapefruit juice is blended with another, less acidic and less bitter citrus juice such as orange and tangerine juice. Wagner and Shaw (1978) conducted sensory evaluation tests on blends of tangerine and grapefruit juices reconstituted from concentrates. Flavor thresholds, preferences, and influences of Brix and Brix/acid ratio on tangerine–grapefruit juice blends were established. It was found that the flavor thresholds of tangerine in grapefruit juice were 14–15% and that blends at concentrations of 15–35% differed significantly from and were preferred over unblended grapefruit juice. Blends with unusual fruit juices and even with nonfruit food fluids have been tried. Inyang and Abah (1997) reported on orange juice blended with the juice of steamed cashew apple. According to the authors, the blend was stable and organoleptically acceptable. Chauhan et al. (2014) prepared a

180 CHAPTER 8  Production of single-strength citrus juices

refreshing beverage from mature coconut water blended with lemon juice. Branger et al. (1999) studied the sensory characteristics of blends of grapefruit juice and cottage cheese whey.

8.17  “RAW” OR UNPASTEURIZED JUICE The temptation to drink “fresh” or “unprocessed” juice sometimes overcomes precaution. In response to increasing demand, the production and marketing of unpasteurized juices (including but not only citrus) is a growing business. Unpasteurized juice is available in the refrigerated food section of many grocery stores. A number of industrial companies produce and bottle unpasteurized juices for local or even interstate distribution in the United States. Despite their acidity, citrus juices support growth of many kinds of microorganisms including pathogens, such as Salmonella (typhimurium and Muenchen) and E. coli O157. Frequently, outbreaks have been linked to the consumption of contaminated juices (Singh et al., 1996; Vojdani et al., 2008; Mihajlovic et al., 2013). Following a fatal case caused by raw cider, USDA ruled that packages of unpasteurized juices should bear a warning label that reads: “WARNING: This product has not been pasteurized and therefore may contain harmful bacteria that can cause serious illness in children, the elderly and persons with weakened immune systems.” Restaurants and establishments that sell juice freshly squeezed “on demand” are exempt of such warning. As to the production of unpasteurized juice in industry, it is clear that extra care should be devoted to the quality and cleanliness of the fruit and the sanitary condition of the equipment. JBT FoodTech markets a complete plant for unpasteurized bottled citrus juice. The plant comprises a fruit preparation section (unloading, brush washing, grading, sizing), juice extractors, juice finisher (optional) and chiller, peel conveyor, cold wall tanks, pumps and automatic or manual (depending on production capacity) filler. Such a plant, with one extractor, would produce in excess of 1,000 half-gallon jugs of juice per hour. Automatic juicing machines are used more and more frequently in restaurants and hotels.

8.18  FERMENTED “JUICES” The regulations in the United States and elsewhere emphasize the limitation of the term “juice” to unfermented fruit products only. Nevertheless, interesting fermented beverages can be made from citrus juices. EscuderoLópez et al. (2013) reported on a fermented orange “juice” prepared by

References 181

controlled alcoholic fermentation. Fermentation was found to cause an increase in flavanone and carotenoid content. Ascorbic acid level was not affected. The authors concluded that the fermented product had health-related advantages.

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