CHAPTER 6

GENERAL JUICE MANUFACTURE PRINCIPLES

6.1 Unit operations applied to juice

There are a number of unit operations involved in converting whole fruit to the desired juice, puree, or pulp product (Table 6.1). Fruit handling depends upon the process design. If the raw material is destined for multiple uses such as fresh market, whole piece processing, additional products and juice, the flow scheme will differ greatly from one for a juice-only plant. In some circumstances, cleaning, sorting and inspection precede in-plant storage or the operations can be reversed or repeated immediately prior to juicing. As indicated, an ounce of prevention (preventing/removing contaminants from raw material) is worth a pound of cure (trying to clean up contaminated juice).

Table 6.1: Unit operation involved in juice manufacture.

Although there are key differences in the handling of each type of fruit destined for juice (Table 6.2) and important varietal and seasonal adjustments, the generalized flow scheme in Figure 6.1 puts the operations in perspective. Normally, juice fruit does not receive the care in packing, transport or postharvest treatment reserved for fresh market and solid pack fruit (Fellows, 1997; Arthey and Ashurst, 1996). Operations such as cooling, washing, sorting and inspecting require attention to mass and heat transfer. Cooling depends upon heat transfer from fruit to air (possibly water). Cooling and cleaning can involve physical removal of surface debris by brushes or air jet separation prior to washing with water. These steps substantially decrease water use and speed up product flow. Efficient equipment minimizes cooling/heating energy and wash water use. When performed improperly the contamination level can actually be increased. Thus equipment and water sanitation is critical with chlorination and recycling usually necessary.

Figure 6.1: Generalized juice flow scheme.

Table 6.2: Fruit characteristics affecting juicing.

6.1.1 Pre-process storage

The logistics of production, harvest, transport and ripening dictate that many fruits must be held prior to juicing. The seasonal harvest window may be much shorter than the time required processing the entire annual crop and stabilizing the resulting juice and finished products. As with the aforementioned steps, careful holding is necessary to allow optimum ripening to occur or prevent spoilage and contamination. Producers and processors should have a good understanding of postharvest physiology, particularly as it relates to the species and cultivars in question.

With certain climacteric fruit, ripening and senescence can be controlled (delayed, maintained constant, accelerated, or promoted uniformly) by the judicious use of low temperature, moderate humidity and adjustment of oxygen, carbon dioxide and ethylene levels (Table 5.4). These practices termed controlled or modified atmosphere storage can greatly extend the storage life of some fruits and vegetables (Arthey and Ashurst, 1996).

With other fruit, holding is short term and serves only temporarily to prevent contamination and damage while accommodating processing flow. Some fruit can be frozen and stored for long periods. Freezing is costly, but promotes cellular breakdown and facilitates shipping and juicing. In hot climates where refrigeration or freezing facilities may be neither available nor practical for juice fruit, strategies such as just-in-time harvest, cooler night time harvest operations, rapid transportation and shady, well ventilated storage can help balance the processing load and stave off deterioration for a few critical hours. Many processors are disturbingly unaware of the importance of these procedures; but every little bit helps and the benefits add up.

Packing for transport should be gentle and sanitary. Simply throwing fruit into a bulk container and travel over rough terrain guarantees partial, unacceptable juicing during transit, prior to delivery. Moreover, if the delivery time is lengthy or if the lot must be held for more than a few hours at elevated temperature, incipient fermentation will follow. A major problem in handling and storage of fruit destined for juice is the relatively low value of the crop. Thus all post-harvest operations up to and including juicing often do not receive the attention they merit. Such produce failing to meet GAPs represents a safety as well as a quality hazard and is soon eliminated from trade in all but the poorest markets. Food safety regulations and quality standards continue to eliminate businesses employing such poor agricultural and manufacturing practices.

In some regions local processors stated this chauvinistic line, "We're doing fine, our products are inexpensive and are the consumer's only alternative." The result has been responsible for the dramatic increase in alternative beverages such as imported juices, carbonated soft drinks and other beverages. Most seriously, local fruit juices and all locally processed products have received an undeserved poor quality image. Consumers do not make subtle distinctions as an entire industry can be "tarred with the same brush".

6.1.2 Cleaning, sorting and inspection

If the fruit producer has followed GAPs and the harvest and handling have been accomplished effectively, fruit arriving at the processing plant should be reasonably sanitary and of optimum quality, thus simplifying succeeding operations. Nevertheless, quality cannot be taken for granted. In many cases the basis for payment is the condition of the fruit as received. Hence sampling and analysis for composition and quality are mandatory.

Government norms, an industry association, or agreement between producer and processor in advance of the crop purchase can dictate applicable quality standards. Some agreements are seasonal or even ready before planting or harvest. A representative sample of the shipment may be drawn according to statistical procedures, inspected for visible defects and foreign matter and then analyzed for microbial load, pathogens, pesticide residues, aflatoxin level, colour, sugar, acid, flavour, or other important safety and quality attributes.(Table 6.3). Figure 6.2a and b illustrate raw material delivery inspection procedures for apples and grapes.

Figure 6.2a and b: Delivery inspection of apples and grapes.
Analysis upon delivery is the basis for payment

Table 6.3: Some quality criteria for juicing.

Rejection at this stage is a serious matter, since considerable time, effort and expense has already been invested in the suspect lot. If the defect is correctable (cleanup, resorting and inspection) a price penalty may be imposed or alternate processing required. However, if the lot cannot be corrected, contamination (microbial or chemical) or far off-standard composition/quality disposal costs and reputation damages, result. In either circumstance process flow is disrupted and plant operations suffer.

Thus for reasons of quality and safety it is important that GAPs be in place before the harvest season, agreed to by all parties, followed strictly and documented throughout the production and delivery chain. At the juice processing facility, the GAPs interface with the processor's system (GMPs), which should be equally well designed and practiced. It is inefficient and illogical to take fruit that has been well handled up to this point and then subject it to inferior juice manufacture operations. The components of GAP and GMP systems are emphasized throughout this text.

Prior to juicing, the fruit can be washed, thoroughly inspected and sometimes sized (fruit-dependent). Inspection and removal of unsound fruit is very important, more so than in whole fruit processing. In solid packs one bad piece of fruit can cause a defect in one container, but after juicing that same piece of defective fruit can end up contaminating an entire lot of juice. In the same context, a few pieces containing microbial pathogens or toxic chemicals can and do raise havoc when juiced.

Dry pre-cleaning steps and water recycling systems may be required depending upon the availability and sanitary quality of water. GAPs should insure that dirty fruit is not delivered for processing. However, weather and delivery conditions may require the removal of dust, mud or transport-induced foreign matter. Water dumping/transport can serve to gently convey the fruit, but this is a dangerous practice, if dump water is not adequately chlorinated or otherwise treated. Maintenance of water quality is an expensive but essential feature of any quality juice processing facility. A screen shaker in conjunction with air jets and dry brushes can effectively reduce water usage, provided that the fruit can tolerate such mechanical handling.

Inspection can be manual, contingent upon workers observing and removing defects or automatic, effected by computer controlled sensors to detect off colour, shape or size (Figure 6.3). Sophisticated instrumentation operating at high speed is being increasingly employed in modern processing facilities, although the human eye, hand and mind still commonly make the final decision.

6.1.3 Crushing and juicing

There are many fruit-specific ways to extract juice. Some are well-established, large-scale procedures for commercial fruits and will be explained in detailed for those fruits. Operations range from kitchen to industrial scale depending upon volume, end use and raw material. Table 6.2 provides some general guidelines for various fruit types. The goal in juice manufacture is to remove as much of the desirable components from the fruit as possible without also extracting the undesirables. Thorough comminution maximizes the yield, but by so doing extracts substances from everything, i.e. seed, skin, core, etc.

Thus the compromise between juice yield and quality dictates the juicing and subsequent steps. Fruit with unpalatable skin and seeds must be treated more cautiously than one that can be completely pulverized. It is possible to minimize extraction of skin and seed components by a crushing regime that mashes or removes edible flesh, while sparing other portions. However, some fruit must be carefully peeled and deseeded or cored prior to juicing. Hand labour is the current alternative with many minor fruits, although there is an economic incentive to mechanize if possible.

Contour peelers, such as used with apples (Figure 6.4) can be adapted to a range of fruits that are sufficiently firm and uniform to facilitate the rotary peeling action. A battery of analogous units was the precursor of the Ginaca, used successfully for pineapple (Section 15.1). Peeling systems that are effective with some vegetables such as lye, abrasion, enzymatic, explosive, are less satisfactory with delicate fruit flesh. Nevertheless, cleverly designed machinery can greatly facilitate these labour-intensive operations. Under all circumstances, a final human inspection and piece selection/rejection step is mandatory.

Generally a whole fruit is more stable than the juice, unless rapidly preserved after extraction. So fruit should not be committed to juice until the material can rapidly be stabilized or the process go to completion. Attention to quality at the prejuicing step is extremely critical. Otherwise surface debris and portions of skin or seed can easily ruin the colour or flavour of an entire batch of juice. Of course, in a similar sense, microbial and chemical contaminants at juicing can and have caused disastrous public health consequences.

Figure 6.3: Automatic inspection/grading system.
Courtesy Key Technology, Inc. (Key, 2000)

Figure 6.4: Apple peeler/corer.
Courtesy of Basics Products, Inc. (Back to Basics Products, 2000)

a.

b.

Figures 6.5a and 6.5b: Small and large fruit crushers.
Capacity ~100 kg to 40 MT/hr.

a.                                                                b.

c.                                                                d.

Figure 6.6a. b., c. and d: Fruit pulper screw finisher, paddle pulper, paddle pulper with coarse screen, paddle pulper with brushes for soft fruit.

Juice extraction equipment ranges from hand operated crushers to tonnes/hour mechanical extractors (Figure 6.5). With soft or comminuted fruit a cone screw expresser or paddle pulper fitted with appropriate screens serves to separate the juice from particulate matter (Figure 6.6). Where skin or seed shattering is a problem, brush paddles can replace metal bars. Two pulpers in series with screens of ~1 to 0.2 mm can effectively clean up many juices and often yield a usable thick pulp by-product.

With thicker purees a pressing step may be needed and several options are available. For refractory material, pretreatment with a macerating enzyme with or without heating to ~60ºC and holding up to ~40 minutes can greatly increase yield and subsequent pressing/clarification steps. Enzyme suppliers have selections of enzyme blends with hydrolytic activity against fibrous plant materials such as pectin, hemicellulose and cellulose to match the composition of most plant material (Hohn, Somogyi, et al., 1996).

Even without macerating enzyme addition, heating to ~70ºC softens the fruit, inactivates native enzymes, reduces microbial load and affects an increase in juice yield. However, delicate flavours can be destroyed and unacceptable darkening due to enzymatic and non-enzymatic browning can occur. Rapid heating and cooling prior to juicing can overcome some of these quality problems. Traditionally mashed fruit and purees were batch heated to optimum macerating enzyme temperature (~55 to 60ºC) in open steam-jacketed kettles with stirring. This step and subsequent cooling can be accomplished by swept surface heat exchangers (Figure 8.4) or thermal screws (a hollow steam-heated auger in a steam-jacketed trough). Steam or cooling fluid flowing through both auger and trough effectively and continuously heats or cools the material. Pulpy, viscous puree cannot be passed through a plate or tube heat exchanger due to clogging problems, although, after solids reduction, these units are appropriate.

A thermal screw is a continuous system for both transporting and heating or cooling crushed fruit or whole small fruit. In a hot press regimen, the material passes directly from the crusher to the press holding tank via the screw, adjusted to deliver the crush at the proper temperature for enzyme activity, ~60ºC. In continuous pressing, the screw can even be slowed down enough to accomplish the holding time and then feed directly to the press (or to another feed/holding screw). Only reasonably small particles can be so treated and care must be taken to insure even heating and avoid scorching of the crush.

Macerating enzymes also facilitate juice clarification steps. In some cases the naturally present enzymes (primarily pectinases) can be allowed to act at ambient temperature prior to pressing. However lengthy holding may favour spoilage organisms and the natural hydrolytic activity is apt to be slower than with added industrial enzymes. For fruits with delicate flavour or those where colour extraction from seed or skin is not desired, an immediate press at ambient temperature (referred to as a cold press) with or without enzymes is favoured. Juice yield will thereby be lowered; a compromise dictated to avoid undesirable heat-induced darkening.

A word of caution regarding commercial enzymes, enzyme preparations available in powder or liquid form, have a finite shelf life and should be refrigerated, since activity decreases rapidly at ambient temperature. These processing aids are fairly expensive and should be used sparingly. The solutions and powders are warranted only when the value of the additional juice yield and its quality exceeds enzyme cost. Also, they are mixtures containing individual enzymes, some of which may promote undesirable reactions, so consultation with suppliers and pilot testing is advised.

Another means of extracting water-soluble components from fruits and plant material is infusion extraction. The flesh is comminuted with added water to dissolve solids that are then separated from the pulp as described. Multiple extractions with temperature and pH adjustment and enzyme treatment can extract practically all-soluble solids. A counter-current flow where fresh solvent (usually water) contacts the last stage of spent pulp is quite efficient (Figure 6.7) and used widely in the Citrus Industry (Section 11.3.4). In this case, subsequent concentration of the extract is necessary to return to the initial fruit ºBrix.

With soft, readily extractable fruits a steam extraction system has promise. An early example is a home extraction unit of Scandinavian origin (Figure 6.8). The fruit is in a strainer that drains concentrically away from an inner container with boiling water. Rising steam condenses in the fruit compartment or on the fruit, heating it and leaching out soluble that drain away from the pulp. A larger scale semi-continuous unit of French design is shown in Figure 6.8. Despite the use of high temperature, this system steam inactivates enzymes (blanches) and pasteurizes the fruit and excludes oxygen during extraction. Thus the juice has unusually good colour and flavour retention.

Figure 6.7: Pulp wash extraction schematic.

a.

b.

Figure 6.8a. and b: Steam extractors. Kitchen and pilot plant models.

Independent of the system used, if the extract is then concentrated back to the original solids level, it is technically juice. However, labelling as juice depends on specific country regulations. The pulp or press residue of high value fruits can be extracted in this manner with water or other solvents to yield extracts containing pigments, nutrients, nutraceuticals, essences, or other useful by-products.

6.1.4 Pressing

The pressing operation can also range from manual to mechanical (Figure 6.9) with complete automated systems common in the juice industry. Kitchen-scale juicers or food processors are effective for small quantities, but for larger multi-kilogram amounts, flow resistance and distance the expressed juice must travel to the press surface complicate pressing. The rack and cloth press increases the surface area to volume ratio and is quite effective, albeit labour intensive. Figure 6.10 shows a simple press based on a hydraulic truck jack developed by Cornell University (Downing, 1972). All supports are wood, including the racks. Tough cottons or synthetic cloths provide an inexpensive, durable batch press, albeit difficult to clean and sanitize.

Figure 6.10: Simple hydraulic rack and cloth press.
(Downing, 1972)

Figure 6.11: Bladder press, 100L capacity.

Figure 6.9: Juice presses - hand basket, rack and cloth and pneumatic bladder.

Other batch systems involve hydraulic basket presses where pistons move down into a press sack containing the crush. Better extraction is obtained by mixing the crush between multiple pressings to expose fresh surface near the cloth. This mixing step is accomplished in bladder presses by rotating a horizontal basket fitted with a chain or internal mechanism to redistribute the cake and reinflating the bladder to provide pressure on the crush (Figure 6.11). Vertical bladder presses inflated by air or water pressure are also in use. Periodic deflation of the bladder and manual cake redistribution is necessary to insure a fresh press surface.

At the other extreme are continuous screw presses capable of handling many tonnes/hour of crushed fruit. Care must be taken not to subject the press cake to excessive shear; or else seeds and other solids will contribute undesirable components to the juice. A more expensive continuous yet gentle system is a belt press where the pulp is pressed between porous belts by rollers. In most press configurations, adding several percent of a press aid to the crushed fruit can increase yield. Press aids consist of clean rice hulls or cellulose fibre that provide drain channels for the expressed juice. Multiple pressings or rotations (in bladder presses) further increase yield, but require more time and can extract undesirables if overdone. The press aid also serves to strain out particulates; juice expressed late in the cycle tends to be clearer (and darker, if browning has occurred). However, press aids can present a disposal problem, may be expensive and unless well refined, can contribute off flavours.

With some fruits allowing the crush to settle provides natural drainage. The pulpy fraction either floats or sinks for easy separation and greatly reduces the volume needing pressing. Fining with the use of bentonnesite or highly adsorbtive powder capable of flocculating colloidal material from the juice speeds up settling and is commonly used in wine clarification. Similarly, addition of protein or tannins as fining agents can not only remove suspended solid, but also form complexes with macromolecules that, if not removed, can cause haze formation in finished juice (Chapter 13).

Figure 6.12: Juice sampling after manual crushing through a nylon bag.

Table 6.4: Press factors influencing juice yield and quality.

6.1.5 Juice clarification

For more fluid juices where cloud or turbidity is not acceptable primary extracted juice must be treated further. A settling step can help, if the juice can be held refrigerated for a few hours. At ambient tropical temperatures holding is not recommended. Rapid methods such as centrifugation and filtration can produce a clear juice. A continuous or a decanting centrifuge with automatic desludging to produce a clear or nearly clear juice is quite effective. (Juices where a cloud is desired generally do not require filtration; centrifugation is adequate.) The stream should be settled or coarse strained prior to centrifugation in order to reduce the sludge load in the feed going to the centrifuge. A fine mesh shaker screen can further remove particulates (Figure 6.13). A centrifuge is a very costly item; however, it greatly simplifies subsequent filtration steps and is an essential component in many juice processing operations (Figure 6.14).

Figure 6.13a: Shaker separating screen.
Mesh size and vibration frequency can be varied to accommodate feed viscosity

Figure 6.13b: Shaker separating screen used in conjunction with a continuous centrifuge.

Figure 6.13c: Commercial dewaterer/dejuicer.
Courtesy Keys Technology, Inc.

Figure 6.14: Continuous centrifuges.
Commercial ~200-1 000 L/hr, Pilot plant ~ 5 to 20 L/hr

6.1.6 Filtration

As with pre-centrifugation, the juice stream should be cleaned up as much as possible to reduce treated volume and increase throughput. There are many filtration systems well suited to various juices. These range from plate and frame filters, fitted with porous cellulose pads, (Figure 6.15) to plastic, ceramic, or metal membranes. Diatomaceous earth mixed with the liquid serves to greatly increase the surface area and porosity of the filter bed and hence the particulate absorbing capacity of the filter.

Figure 6.15: Plate and frame and vacuum filter.

In the extreme case a filter can have small enough pores to physically remove microorganisms from the juice (sterile filtration) or even remove macromolecules such as proteins and carbohydrate polymers (ultrafiltration). These processing steps will be considered later. The production of a clear or brilliantly clear juice and the prevention of post filtration turbidity are the normal goals. Membrane clogging is a concern in any filter, so pretreatment to minimize particulates and flow patterns that self clean the membrane are essential.

The system can be made continuous by a rotary vacuum filter (Figure 6.15). A vacuum deposits fresh filter aid suspended in the juice on the drum surface. The juice passes into the rotating drum through the filter bed that is constantly renewed by scraping the spent material off the surface after a rotation. Various other filters use durable ceramic or metal porous membranes with backflush capabilities and flow patterns that minimize clogging. These systems operate in parallel to insure continuous operation and can reduce or eliminate the need for filter aid; a purchase and disposal expense.

6.1.7 Deaeration

In the operations described crushing, comminution, pressing, shaker separation, centrifugation and filtration, the fruit and juice are subjected to considerable aeration. The inclusion of oxygen can promote enzymatic browning, destroy nutrients, modify flavour and otherwise damage quality. Therefore, care should be taken to perform these steps rapidly, at low temperature and/or protect the material from oxygen, if possible. Sometimes preheating to inactivate natural and/or added enzymes is useful, provided rapid cooling follows.

Deaeration can be accomplished by either flashing the heated juice into a vacuum chamber or saturating the juice with an inert gas. Nitrogen or carbon dioxide is bubbled through the juice prior to storing under an inert atmosphere. Clearly, once air is removed or replaced by inert gas, the juice must be protected from the atmosphere in all subsequent processing steps. Deaeration, especially flashing off at high temperature, can also remove some desirable volatile aroma, another compromise facing the juice technologist.

6.1.8 A word of caution regarding processing systems

The excellent research reporting juice details - preparation, processing, composition, storage quality, etc. most likely came from academic or institutional research labs working with small, albeit representative samples of fruit. Handling is usually ideal with the major emphases on repeatability, speed and attention to detail. Rarely are the studies concerned with the logistics of handling large, i.e. commercial quantities, under industrial conditions.Thus scale-up and the attendant difficulties are not stressed. Just because a fruit can yield an outstanding juice in the lab doesn't mean it is commercially viable. Equipment from harvest to storage must be developed to efficiently perform the myriad of operations under varying and often vastly different and difficult operating conditions. Furthermore, these operations must match reasonably well the process flow scheme. Incompatibility in throughput, or highly sensitive procedures with excessive down time raise havoc with production flow and cannot be tolerated. Ingenuity and/or technology can eventually overcome these hurdles, but only with considerable patience and time.

Hand labour and care can represent a first step in scale-up and produce juice products for local niche markets. However, this is no substitute for well-designed mechanization, to replace or at least supplement human labour. Thousands of labourers cannot match several high-speed extractors for long. Some fruits mentioned here have superb eating quality, but will have scant juice (or solid pack) potential, until some degree of cost-reducing mechanization is introduced into the system.

Generally it is the inspection, cleaning, sorting peeling/coring/pulping operations that cause the greatest problem. (Once the flesh is isolated in a reasonably clean form, subsequent operations scale up fairly easily.) This is a major challenge for small processors. If they don't continually strive to improve juice manufacture efficiency, somebody else will or the fruit will remain under exploited to the detriment of the growing region and consumers.

Curiously, the awesome ingenuity and craftsmanship demonstrated by third world mechanics in repairing and maintaining complicated machinery and equipment (automotive, refrigeration, electrical) with little training or parts inventory has not been extended to food processing equipment design and development. Of course, all such equipment is kept running long after the anticipated design life, past zero depreciation. New and innovative mechanical solutions to tedious processing steps are either conspicuously absent or not shared within the local food industry. There is a critical need for such innovation, especially since existing (large scale/multinational) equipment manufacturers with the experience and talent view this as a niche market not worthy of the investment. Conversely, if appropriate small/medium scale equipment does exist, it is atrociously expensive and/or complicated to operate and maintain.