Module 5. Food processing equipments and unit operations

Lesson 40


40.1 Introduction

Juice extraction—the elimination of the juice from fibrous, solid particles—is a basic technological step of fruit juice production. The fruit has to be prepared prior to juice extraction, which is then followed by juice clarification and drink completion. Subsequently, the finished drink is packed and preserved.

Diffusion-based liquid extraction requires chopping to minimize the thickness of the slices and strips. In addition the size of these pieces should form channels to ensure the flow of the extraction liquid. There are different devices for the crushing of fruits. These can be specialized for a given fruit (e.g., apple crusher) or generally used as hammer- and roller based machines. Their common feature is the rotating system and the pressing, shearing, pulling, and striking forces applied.

40.2 Fruit Juice Production


Operation in Juice Extraction

This operation can be divided into two steps: fruit chopping and preparation and the separation from solid fruit particles.

40.2.1 Chopping

The aim of this step is to smash, cut the fruit, increase its surface, and launch cell-fluid elimination. However, this can lead to enzymatic reactions damaging valuable components. Therefore, the fruit has to be processed immediately after chopping. If this step is done appropriately, the fruit is not pulpy but consists of homogenous, irregular-shaped, few-millimeter-sized particles, which tend to form channels to drain the liquid when pressed .

40.2.2 Chopped Fruit Preparation

Procedures designed to prepare chopped fruits are to increase juice yield and prevent undesirable changes (chemical, biological, mechanical, etc.) to achieve better aroma, flavor, and color properties. The type of preparation will depend largely on the type of fruit and production technology.There are several methods for this operation, such as mechanical, freezing, enzymatic, vibration, ultrasonic, electro-plasmolytic, ion-radiation procedures, and heat treatment. In practice, mechanical operations, heat treatments, and enzymatic solutions are widely used .

Mechanical preparation is used to chop fruit flesh, smash the tissues, and increase the surface. Stiff raw materials (e.g., apple) are usually crushed; meanwhile, soft ones (e.g., red currant) are only cracked. Crushing opens up the tissues, damaging some of the cells as well, and the draining of cell-fluid begins.

The degree of chopping is determined by the method of juice extraction. If pressing is applied, the chopped fruit releases the juice under a relatively small amount of pressure. Appropriately prepared chopped fruits contain particles of nearly identical size, enabling channels to form for the liquid to drain. If the fruit is chopped into very fine pieces, it spreads easily, expands under pressure and does not tend to form channels to drain the juice.

40.2.3 Methods of Extraction

In recent years there has been considerable interest in using extraction instead of expression for recovering juices from fruits and vegetables. Countercurrent screw extractors, some operated intermittently, have been used to extract juice with water. In some cases this results in higher yields of good quality compared to that obtained by expression.

Centrifugation may be used for a variety of tasks in fruit juice processing. Self opening centrifuges are used to remove pulp and control the level of pulp remaining in pineapple and citrus juices. Centrifuged apple juice is cloudy but free from visible pulp particles. Tubular bowl centrifuges were originally used to clarify apple juice but more recently nozzle and self-opening machine are used. The use of hermetically sealed centrifuges prevents excessive aeration of the juice. In the production of oils from citrus fruits centrifugation is applied in two stages. The product from the extractor contains an emulsion of 0.5–3.0% oil. This is concentrated up to 50–70% oil in a nozzle or self-opening centrifuge. The concentrated emulsion is then separated in a second centrifuge to produce the citrus oil.

There are many other applications for centrifugation in food processing, e.g. tubular bowl machines for clarifying cider and sugar syrups , nozzle and self-opening machines for dewatering starches and decanting centrifuges for recovering animal and vegetable protein, separating fat from comminuted meat and separating coffee and tea slurries.

Preparation with heat treatment is mainly used prior to the pressing of berries, since it can increase juice yield by 5–10%.

40.2.4 Process Description

In this process, the liquid phase of fruits is detached from solid particles. There are different methods for this separation: pressing, diffusion, centrifugal procedures, and reverse-osmosis. The type of equipment applied depends on the fruit species, production line, and economical background. The most widely used solution is pressing. Pressing separates a food system into two phases. In this case, fruit tissues mean the solid phase, while the liquid between the particles is the liquid phase. Pressing needs outside forces to create tension in the system, drain liquid, resulting in shape modification. The equipment hinders the disposal of the solid phase and the liquid gathers in a vessel. The remaining material, with low liquid content, is called marc. The most important parameter of pressing is the liquid yield, which means the percentage of juice extracted, compared to the raw material at the beginning of the process. Juice yield is basically determined by the type of the pressing device, and the quality and preparation of the raw material.

Fruit processing industry applies continuous—such as belt- and screw-based—and intermittent—like the package and basket type—pressing machines. In addition, decanters are based on centrifugal forces.

The juice of fruits can also be detached with extraction. It means that semi permeable cell walls are made permeable following a heat treatment and the cell fluid is then dissolved with water. This process is featured by the degree of extraction, expressing the amount of extracted valuable substances, compared to the total valuable matter content of the fruit.

The amount of substances diffused is in direct proportion with the diffusion coefficient, the active surface, and the concentration gradient.

In order to increase the diffusion coefficient and the permeability of the cell walls, diffusion fluid extraction is performed at 50–70◦C. Active surface can be increased by proper chopping. The concentration gradient is determined by the stream conditions and the solvent–cell fluid ratio. However, the amount of solvent applied is limited by the concentration decrease of the liquid extracted. Diffusion juice extraction is usually carried out in double-screw extractor devices.

40.2.5 Energy optimizing

Proper juice extraction is important to optimize the efficiency of the juice production process as well as the quality of the finished drink. The latter is true because oranges have thick peels, which contain bitter resins that must be carefully separated to avoid tainting the sweeter juice. There are two automated extraction methods commonly used by the industry. In the first place the fruit between two metal cups with sharpened metal tubes at their base. The upper cup descends and the fingers on each cup mesh to express the juice as the tubes cut holes in the top and bottom of the fruit. The fruit solids are compressed into the bottom tube between the two plugs of peel while the juice is forced out through perforations in the tube wall. At the same time, a water spray washes away the oil from the peel. This oil is reclaimed for later use.

The second type of extraction has the oranges cut in half before the juice is removed. The fruits are sliced as they pass by a stationary knife and the halves are then picked up by rubber suction cups and moved against plastic serrated reamers. The rotating reamers express the juice as the orange halves travel around the conveyor line. Some of the peel oil may be removed prior to extraction by needles which prick the skin, thereby releasing the oil which is washed away. Modern extraction equipment of this type can slice, ream, and eject a peel in about 3 seconds.

40.2.6 Extraction Equipment

In the food industry solid - liquid extraction is mainly applied to relatively coarse, particulate materials, usually greater than 200 mesh in size. In many cases, the solid has a cellular structure. The equipment used includes single and multistage static tanks and continuous moving bed extractors. Vigorous movement of the solid is seldom desirable.

a) Single Stage Extractors


The simplest form of extractor consists of an open tank, fitted with a false bottom which supports a bed of the solid to be extracted. The solvent is distributed over the surface of the bed of solid, percolates down beneath the false bottom. In the food applications , extraction is often carried out at high temperatures and pressures. In the case of oil extraction the solvent is relatively volatile. For these reasons and also for hygienic considerations, vessels are usually totally enclosed and capable of with-standing the required pressure.

Provision may be made for re-circulating the outflow. A heater may be incorporated in the feed line and/or re-circulation line to enable the temperature of the solution to be maintained at the desired value. Some units are jacketed for this purpose. Such cells may be filled manually or with the aid of a conveyor or some other mechanical device. The solid residue is removed manually or dumped through an outlet in the base of the cell. Where volatile solvents are being used, it is possible to incorporate a solvent recovery and recycling system in such units. This enables a relatively concentrated overflow to be obtained.

Single stage units are used for pilot plant and small-scale commercial operations for the extraction of oil from seeds, beans and nuts, coffee solubles from ground & roasted beans and tea from dried leaves.

Many materials - fruits, vegetables and seeds contain valuable liquid constituents within the cell structure composing them. Groundnuts, coconuts, soybeans, sunflower seeds and olives for example, yield edible oils and fats, while juices from a variety of fruits are used in the manufacture of various soft drinks and wines. The cell walls normally require disruption before this liquid constituent can be separated and some form of pre- treatment such as pulping or heating is often required.

Traditionally, separation is accomplished by either solvent extraction - as in the recovery of fats from animal and vegetable materials - or by mechanical expression. Expression, the unit operation considered here, is the separation of liquids from solids by the application of compressive forces and is often used in the food and beverage industries.

The methods of expressing the liquid from the solid - liquid matrix are :

(i) Hydraulic pressing

(ii) Roller pressing

(iii) Screw pressing

Hydraulic presses are widely used in fruit juice processing whereas roller presses have not really been accepted. Roller presses are universally used in expressing juices from sugar cane.

Screw presses are being used in fruit juice expression but here they tend to be used as 'finisher' following a prior pulp thickening process using other thickening techniques. A recent process uses a basket centrifuge for pre-thickening before expression. Both hydraulic and screw presses are used in the extraction of oils and during oil milling, the oil being expressed from cleaned oil bearing seeds after cooking and conditioning in steam cookers. In general, hydraulic presses are used for bath type operations while roller and screw presses find their main application in continuous processing.

The efficiency of an expression process depends on several factors including :

(I) The yield stress of the solid phase (i.e. its resistance to deformation).

(ii) The porosity of the cake formed.

(iii) The viscosity of the liquid extracted (expressed).

(iv) The compressive force applied.

The rate of flow of liquid through the interstices of the cake of 'pomace' will depend on the thickness of the cake and on its porosity, both quantities which can vary with the degree of compressed applied.

The nature of the pulp itself depends on its type and previous history. Fruit pulp, for example, varies with the particular variety of a fruit, with the climatic and soil conditions under which it was grown, its maturity at picking and any metabolic changes occurring during the interval between pickling and processing.

b) Hydraulic Pressing


Two types of hydraulic press are in common use—

1) Plate Press : The Pulp to be expressed is placed in heavy cotton filter bags or cloths, which are placed between grooved pressure plates arranged in a vertical stack. Hydraulic pressure is applied across the ends of the stack, a pressure of 31-62 MN/m (2-4 ton/in2 ) being developed. The pulp, in the form of relatively thin layers, experiences a compressive force. Keeping the layers thin permits reasonably rapid draining of liquor through the cake interstices and across the grooved plate faces to common receivers.

Filling, pressing, opening and cleaning of these batch units requires a high labour usage. In an attempt to achieve extraction more economically, presses capable of exerting increasing pressure in several stages have been developed. In these units the assembly of plates moves beneath a series of pressure heads each exerting a higher pressure than the preceding head. The pressure should be increased gradually, since rapid changes can lead to sharp decreases in bed voidage with a consequent drop in drainage rate.

c) Continuous press : To reduce the high labour requirements associated with hydraulic expression, continuous presses have been developed.

i) Roller Press


In this type of unit, a compressive force is applied by pressing the pulp between heavy rollers. Modifications of this method have been developed to improve the separation of liquid from solid. For instance, simple crushing rolls of the type used for expressing juice from the sugar cane have drainage grooves (Menchaerts groves) on the roller surfaces. These grooves direct the liquors away from the compressed cake thus reducing re-wetting. Pulp is fed between the rollers as shown liquid is expressed from the pulp and flows to a collection launder, solids pass to the third delivery roller where it is removed by a 'doctor knife'. In other types of roller presses, drums similar in design to those used in drum filtration are used. The face of the drum is perforated and covered by a filter cloth. De- watering rollers bear against the face of the drum. Pressing takes place between the drum and rollers, liquid is drawn through the filter cloth and into the perforated drum, the interior of which may be maintained under reduced pressure to aid flow. The cake is removed by 'doctor knife' or 'string discharge'.

ii) Screw Press


In this unit, known in the oil milling industry as an expeller, fruit pulps or oil seed meals are fed to a thick walled cylinder containing a rotating polished screw having a gradually decreasing pit. Material trapped between the screw and the inside of the cylinder barrel passes through a gradually reducing flow area, experiencing and increasing compressive force. The walls of the cylinder contain fine perforations or lots covered by adjustable screens, through which expressed liquor drains from the cake. The expressed cake leaves the unit through an adjustable discharge port. Power consumption in this unit is high. Power is dissipated in friction and may raise the temperature appreciably. The risk of thermal degradation of heat sensitive materials can be reduced by the use of hollow water-cooled screws. The degree of compression achieved can be varied by adjusting the area of the discharge port and by varying the speed of rotation of the worm. Shaft speed falls in the rage of 5-500 rev/min depending on application and barrel pressures of 138-276 MN/m2 (20,000-40,000 psi) are achieved. Good separations at through inputs of up to 8500 kg/h are reported with the residual cake containing 4.5 % liquid. With certain fruits, passage of fine particles with the liquid can present problems. In such cases subsequent clarification by centrifugation may be required.

Last modified: Wednesday, 31 October 2012, 5:22 AM