Module 4. Fruits and vegetables juice processing
Lesson 13
EQUIPMENTS AND METHODS OF EXTRACTION, CLARIFICATION AND PRESERVATION
13.1 Introduction
In order to prepare fruits and vegetable juices at commercial level, several equipments are needed. Some of the important equipments required for processing of specific fruit and vegetables are discussed herein.
13.2 Pome Fruit and Small Fruit Processing
Generally, most pome fruit (e.g. Apple, Pear, Quince) and small stone fruit (e.g. Plum, Olive, Peach, Cherry) can be used for juice extraction. No peeling is needed. Small stone fruit such as apricots and plums might have to be destoned (pitted) depending on the grinding–extraction equipment selection. Cherries, although containing a pit, may be pressed with the pit intact. Breakage of the pit will release benzaldehyde, the familiar aroma of maraschino-type cherries.
13.3 Disintegration
The juicing process starts with the crushing step to break down the cell tissue. Grain sizes of 5 - 8 mm diameter are recommended for presses, while grain sizes of 3 - 5 mm are desirable for decanters.
13.3.1 Hammer mills
These are devices used to crush the whole fruit in preparation for pressing. Hammer mills consist of heavy stainless steel bars spinning from a common axis under high-speed rotation. The fruit is disintegrated until it passes out through a screen of a specific size mounted in the bottom of the mill. With firm fruit, a small screen size should be used, and the mash will be of a finer particle size. Mash from firm fruit will press more easily, and the smaller particle size will allow greater yields.
13.3.2 Grinding mills
They offer an alternative method to disintegrate fruit. The fruit was drawn past fixed knives mounted on a rotating cylinder. Control of the grind was accomplished by adjusting the depth of the knives and, thus, the size of the cut from the fruit.
13.3.3 Grinding disk mills
They offer more flexibility and improved performance. In the Bucher-Guyer unit, the fruit is transported by a feed screw to the grinding area. The screw pressurizes the fruit against a rotating disk equipped with grinding knives in a star pattern, and the milled fruit exits via an adjustable discharge slot. The process can be controlled by adjusting the feeder speed, the rotating speed of the grinding disk, the width of the product discharge slot (up to 10 mm), or by changing the knife size. Better yield is obtained by requisite adjustment corresponding to the fruit ripeness at the time of operation.
13.3.4 Grating mills
These mills are used in small juice operations to produce uniformly sized fruit pieces. Fruit is fed to a rotating–grating disk with fixed aperture, and the shredded fruit is discharged at the bottom. Fruit must be relatively firm with small seeds or pitted.
13.3.5 Stemmer/crushers
These crushers are used in grape juice processing to remove residual stems, leaves, and petioles from grapes and to perform the initial crush of the fruit. These units are designed around a perforated rotating drum, with holes ~ 2.5 cm in diameter. While traversing the rotating drum, the grapes are caught by the perforated drum and knocked from the stems. Individual grapes are broken open or crushed in the process and dropped through the drum. Stems, leaves, etc. continue on to the center of the drum and are discharged at the end for waste. Grapes are generally put through the crusher in order to gently express the juice and free up the flesh, yet still not break the seeds. Breakage of the seeds releases increased amounts of phenolics, adding to the astringency of the juice.
13.3.6 Stoned fruit mills
Such mills are used for plums and apricots to crush the fruit without breaking the stones to avoid juice flavor changes and storage instability. Hard rubber-lobed wheels rotate simultaneously, forcing the fruit down and separating most of the flesh from the intact stone.
13.3.7 Turbo extractors
These are used for extraction of juice and puree from fruits and vegetables. The cold extractor unit has a feeding section with a variable speed screw and a cutting head; a softening section consisting of a stator and rotor (roto-pulse); and an extraction area equipped with a rotor with paddles and a perforated cylindrical screen that continuously turns the product by centrifugal force (Figure 13.1). The extractor can be adjusted by changing the feeding speed, the rotor speed, the gap between the rotor and the screen, and the screen size. The fruit can be protected from oxidation by the injection of nitrogen gas or antioxidant solution to the cutting area through built-in openings.
Fig. 13.1 Turbo extractor
13.4 Hot Break Process
In order to maximize juice yield and color-flavor extraction, a hot break process is often used. The most common use is in grape juice processing, but other fruits such as cherries, plums, and berries may also benefit. Increased interest in highly colored juices, rich in phenolic compounds with associated health benefits, is driving the development of better techniques to preserve the functional components while maximizing the extraction. Typically crushed fruit or mash passes through a large bore, tubular heat exchanger where it is heated to 50 to 60oC. This stage, known as the hot break process, is designed to extract a large amount of color and assist in maximizing the yield. To the hot fruit, a pectolytic enzyme is added, and in case of red grape juice processing, kraft (wood pulp) paper is also added prior to pressing to serve as a press aid.
The addition of press aid to the mash provides coarseness and channels for the juice to exit. Alternative press aids include rice hulls, bleached kraft-fiber sheets or rolled stock, and ground wood pulp. If the juice is going to be extracted by decanting or centrifugation, then there is no need for press aids.
13.5 Mash Enzyme Treatment
This step might not be used for the production of high quality, single-strength, cloudy and clear juices, where the preservation of the fresh flavor is imperative. Soluble pectin found in fresh juice as a result of the activity of pectolytic enzymes that are located in the fruit cell wall. The soluble pectin is the cause for difficulty in juice extraction due to increased juice viscosity and the lubrication it affords the press cake. Typically, the fruit mash is heated to 45 to 50oC followed by the addition of pectolytic enzymes. Reaction time can take up to 1 - 2 h.
De-pectinization is designed to reduce the viscosity and slipperiness of the pulp and thus permit the effective use of decanters and presses with proper press aids. It is especially useful in processing mature and stored fruit that results in low juice yield. Several depectinizing tanks are employed so that a continuous flow may be maintained to the presses or decanters. Treatment of the mash with enzymes is expected to increase the yield, reduce the processing time, and improve the extraction of valued components of the fruit.
13.6 Fruit Juice Extraction Equipments
13.6.1 Rack and frame hydraulic press
The hydraulic rack and frame press is a very common batch press system found in small juice operations (Figure 13.2). Heavy cotton or nylon cloths are filled with a set amount of mash and then folded to produce what is called a cheese. The individual cheese is stacked and separated by a wooden, stainless steel, or plastic spacer platen. The combined stack is then compressed using a hydraulic ram, during which the juice is expressed. The process delivers good yield but is labor intensive.
Fig. 13.2 Hydraulic rack and frame press
13.6.2 Horizontal piston press
One of the most successful press systems in the fruit juice market is the Bucher horizontal piston press, Switzerland. This press is capable of pressing berries, stone fruit, and vegetables. It operates in batch mode with loads of up to 14 t/filling. Flexible drainage elements covered with a nylon filter cloth carry the expressed juice out to a manifold.
The Bucher-Guyer Press is a highly automated pressing system used in a batch pressing operation. Generally, this system consists of a rotatable basket or cylinder with a hydraulic ram used for juice expression. Within the cylinder are fabric-covered flexible rubber rods with longitudinal grooves in them, that allow the juice to transport easily to the discharge port.
13.6.3 Bladder press
The Willmes Press is a commonly used system for grape juice pressing. It is a pneumatic-based system that consists of a perforated, rotatable, horizontal cylinder with an inflatable rubber tube (air bag) in the center. The cylinder is filled with grape mash through a door on the cylinder wall, which is rotated to the top position. After filling, the press is rotated to ensure even filling. During this rotation, the air bag is filled, creating the mash compression action. The bag is then collapsed, and the cylinder is rotated. The rotation and pneumatic compression of the mash is repeated many times with increasing air pressure.
13.6.4 Belt press
The continuous belt press is effective for grape and apple juice processing. In belt presses, a layer of mash is pumped onto the belt entering the machine. Press aid may be added for improved yield and reduced suspended solids. The belt is either folded over or another belt is layered on top of the one carrying the mash. A series of pressurized rollers compress the enveloped mash. Expressed juice is caught in drip pans. The cake is discharged from the last pressure roller.
13.6.5 Screw press
A typical screw press consists of a reinforced, stainless steel cylindrical screen enclosing a large bore screw with narrow clearance between the screw and the screen (Figure 13.3). Breaker bars are located between the screw intervals in order to disrupt the compressing mash. Back pressure is provided at the end of the chamber and is usually adjustable. Capacities for screw presses with diameters of 30.5 and 41.0 cm are 5,080 and 15,240 kg/h, respectively.
Fig. 13.3 Conical screw press
13.6.6 Decanter centrifuge
In addition to sieving technology, the separation of juice from the mash can be performed by sedimentation through increased gravity in a decanter. Centrifugal force is used to accelerate the settling of higher density insoluble particles present in the juice. Enzyme-treated mash is best suited for juicing by decanters, as the reduced viscosity and higher temperatures result in faster and more effective separation. The photograph of such decanter is furnished in Figure 13.4.
Fig. 13.4 Decanter centrifuge
13.6.7 Pulper cum finisher
The separation of liquid and solids is accomplished by means of paddles rotating concentrically within a cylindrical screen. The liquid and desired amount of solids passes through the screen. The balance of the solids (pomace) is discharged through a large non-plugging port. The dryness of the pomace with a given screen can be controlled by paddle speed, pitch, clearance, or feed rate. Production throughput is dependent on the type of product being prepared, screen hole size and open area, paddle speed and pitch. An inlet impellor for breaking or macerating is available as an optional accessory.
Diverse type of materials viz. apricots, tomatoes, pumpkin, pears, apple, plums, berries, prunes and figs can be satisfactorily reduced to pulp, free of seeds, skins and fiber. Products such as citrus juices, jam, soup, peanut butter, jelly and fruit nectar can be finished to uniform clarification consistency. The picture of such machine is shown in Figure 13.5.
Fig. 13.5 Pulper cum finisher
13.7 Mechanical Separation
For clarification of juice, after the enzyme (Pectinase) treatment, the sedimentable particulates are separated by mechanical means. The equipments used for such process are as follows:
13.7.1 Decanters and finishers
A high-solids stream can be partially clarified using decanters and finishers. Both pieces of equipment operate on the same principle with a spinning central cone, drum, and set of paddles pushing the juice through a screen of some type. The unit is typically mounted horizontally, and throughput is relatively high. Total suspended solids may be reduced to < 1% during operation, depending upon the characteristics of the feed stream and operating conditions of the separator.
13.7.2 Centrifugation
It is used for removal of juice-insoluble solids. A centrifuge places the juice under high gravimetric force induced by centrifugal action. This is effective in producing a juice that is opaque but free of visible solids. Modern centrifuges are highly automated and run continuously with timed solids ejection. Centrifuges with a high force of gravity are capable of producing clear juice under optimized conditions. Centrifuge must be operated in a manner to minimize the introduction of oxygen in the product. Possible remedies include the use of inert gas.
13.7.3 Pressure filtration devices
13.7.3.1 Filter press
The cost is typically lower than other types of pressure filters. The system can be dismantled easily for inspection and cleaning. Filter cakes can be easily washed from the system once disassembling has progressed. In the filter press, the amount of unfiltered liquid is relatively low once the shutdown process is terminated.
13.7.3.2 Cylindrical element filter
In this system, tubular elements are suspended vertically in a closed tank system. Juice enters from the base of the system and filters through the elements, and the filtrate exits from the top of the system. Wash down and automation of this system are relatively straightforward.
13.7.3.3 Vertical leaf filter
It is a low-cost system because of the inherent simplicity of its design. It offers an easy cake removal system and can be automated. A modified version of the leaf filter is the horizontal tank vertical leaf filter that accommodates a very large area of filtration surface, up to 2000 ft2 (180 m2). Filter leaves can easily be removed, inspected, and repaired.
13.7.3.4 Rotating leaf filter
In this filter, the filtration elements are circular leaves suspended on a central axis. The leaves are rotated only during cleaning and discharging, which allows for an automated and rapid cake removal and clean-up system.
13.7.3.5 Horizontal rotating leaf filter
It is essentially identical to the vertical rotating leaf system, except that it is available in much smaller models.
13.8 Clarification of Fruit Juice
Consumers have a strong preference for clear juices. In order to have attractive appearance of finished fruit juice, especially for beverages like fruit juice cordial, clarification of juice is highly essential. Such clarification can be done by the help of centrifugation and use of pectinase followed by decantation. Filter aids such as Infusorial earth, bentonite helps in achieving better clarification of fruit juices. Ultrafiltration (UF) and microfiltration (MF) have been used commercially for the clarification of fruit juices. After extraction, the fruit juice after depectinization is fed to UF unit for clarification. If the juice contains strong colour, microfiltration can be suitable for avoiding colour losses.
Pre-centrifugation (10,000g for 15 min) of juice (especially cherry juice) before clarification is recommended.
13.9 Preservation of Fruit Juices
Traditionally, the shelf-life stability of juices has been achieved by thermal processing. Low temperature long time (LTLT – 63-65oC/30 min) and high temperature short time (HTST – 72-90oC/15-30 sec.) treatments are the most commonly used techniques for juice pasteurization. However, thermal pasteurization tends to reduce the product quality and freshness. Therefore, some non-thermal pasteurization methods have been proposed during the last couple of decades, including high hydrostatic pressure (HHP – pressures up to 1000 MPa with or without heat), pulsed electric field (PEF), etc. These emerging techniques seem to have the potential to provide “fresh-like” and safe fruit juices with prolonged shelf-life.
Apart from thermal pasteurization, some chemical preservatives are also widely used for the extension of the shelf-life of fruit juices and beverages. Two of the most commonly used preservatives are potassium sorbate and sodium benzoate. However, consumer demand for natural origin, safe and environmental friendly food preservatives has been increasing since 1990s. Natural antimicrobials such as bacteriocins, organic acids, essential oils and phenolic compounds have shown considerable promise for use in some food products. Natural antimicrobials such as bacteriocins, lactoperoxidase, herb leaves and oils, spices, chitozan and organic acids have shown feasibility for use in some food products. Some of them have been considered as Generally Recognized As Safe (GRAS) additives in foods. Bacteriocins are series of antimicrobial peptides which are readily degraded by proteolytic enzymes in the human body. Among them, nisin is the most commonly used food preservative that has been used to preserve fruit and vegetable juices.