Fruit and juice processing

Barry Taylor

3.1 Introduction

It could be said that freshly pressed fruit juice provides the truly natural answer to all the requirements of a soft drink: thirst quenching, fresh, healthy, flavour-some, nutritional and, of course, natural. So why should it have been necessary to look any further towards the creation of different flavour types and the myriad of drink varieties that has appeared over the years in beverage markets around the world? Necessity, being the mother of invention, has been the driving force in all this. In the early years of the industry there was a real necessity for soft drinks manufacturers, in order to stay in business, to control a major threat to their trade, that of microbial fermentation and spoilage of the bottled product.

A freshly squeezed orange or fresh pulped and strained apple would supply a fruit juice drink for immediate consumption, but to expect it to maintain its quality for even a day or two was tempting providence. Nowadays, with the benefit of ultra-high temperature (UHT) pasteurisation, aseptic packaging techniques and systems, pressed juices can be stored for extended periods with very little deterioration in quality. Previously, reliance had to be places upon the use of preservatives and 'classical' pasteurisation at lower temperatures (70°C+) and longer holding times. As businesses grew and production and filling lines enlarged and developed, so did understanding of the need for correct sanitisa-tion of the plant items. The progressive nature of the soft drinks industry has meant that throughout its history there have been many innovative developments, and in the early years these centred on the filling and packaging, or containerising, of beverages. During the second half of the twentieth century, apart from the continuous move towards more efficient means of production and marketing of bottled or canned soft drink products, there was much progress in our knowledge of the constituents responsible for perceived flavour notes. Advances in instrumental analytical techniques have made it possible to identify those chemicals in natural extracts (whether of fruit or botanical origin) that provide the characteristic flavour profile.

The advent of gas chromatography in the 1950s, its coupling with the diagnostic strength of mass spectroscopy (GC-MS) and the remarkable improvements in column sensitivity have no doubt been critical factors in the generation of the large numbers of beverage flavours and drink types available on today's market. This analytical knowledge, apart from leading the way to 'designer drinks', has also served to maintain and standardise the quality of a range of beverage types that still base their success upon traditional fruit juice systems. The majority of the active flavour components of most fruit types have been identified (TNO-CIVO) and provide the beverage technologist with a basis for the addition of certain characters in the development of a new product.

It is the emphasis placed upon certain flavour characters that can provide the drink with its identity in the marketplace. This can be achieved by the use of both natural and synthetic or nature-identical flavourings to create the desired top-note effects. A fruit-based drink will always declare as part of its list of ingredients the juice or juices (if mixed) used, at natural or 'single-strength' status. Usually appearing fairly high in order of concentration, the fruit juices will provide the generic character of the drink, with its specific identity being given by the flavouring materials chosen for a particular formulation (preferably in synergy with the flavour contribution of the juice ingredient).

The beverage technologist has a wide range of fruit types to choose from, and this chapter will investigate some of the procedures associated with the processing of these to produce fruit juices commercially.

3.2 Fruit types

3.2.1 Botanical aspects, classification of fruit types

The term 'fruit' is applied to a critical stage in the reproduction of botanical species throughout the plant kingdom: it is the structure that encloses, protects or harbours the seeds until they are ripe, and it often assists in their dispersal. Broadly speaking, fruits can be categorised into two groups according to their physical condition when ripe: dry fruits and succulent or fleshy fruits.

The widest diversity in the manner of seed dispersal is exhibited by the dry fruits. These include the windborne types such as dandelion 'parachutes' or sycamore 'keys'. Mechanical scattering is exhibited by many of the legumes, whose seed pods, when fully ripened and dried out, can split with explosive force to scatter their contents in readiness for a follow-on crop. Another type is made up of those fruits, such as 'cleavers' or 'dock-burrs', that possess small hooks whereby the fruit is caught up in the fur of animals for transportation.

It is the second group, however, that will be the focus of this chapter, the succulent or fleshy fruits, where the seeds are ripened or supported within a soft or fleshy mass containing food materials that may tempt animals to eat the fruit. After being eaten by animals the fruit is digested, but the seeds, protected by a hard shell-like coating, will progress through the alimentary canal of the animal to be passed out in the faeces. This rather primitive method of propagation has been Nature's way almost since the dawn of time, having the advantage of a 'built-in' seedbed growing system rich in nitrogenous compounds and often essential trace elements. The Prunus aves or bird cherry gained its name in recognition of this manner of propagation.

The process of evolution and natural selection ensured that the more successful fruit forms survived in harmony with their surrounding environment.

The appearance of humans on the planet and their own evolution to the status of hunter-gatherer brought a new meaning to the term 'selection', and our ancestors would have taken the first steps in categorising the available fruits according to flavour character.

Many of the commercial fruit varieties popular in the Western world have been developed from specimens whose origins can be traced back to regions east of the Mediterranean, where stone fruits (such as peach, apricot and cherry) and pome fruits (such as apple and pear) grew in fertile surroundings and became part of the staple diet of the inhabitants.

Figures 3.1-3.3 show the structures of various succulent or fleshy fruits.

(a) Remains of stamens

(a) Remains of stamens

Blackcurrant (genus Ribes)
Figure 3.1 The structure of the soft fruits. (a) Currants, e.g. blackcurrant (Ribes); (b) raspberry (Rubus); (c) achenes, e.g. strawberry (Fragaria).

38 CHEMISTRY AND TECHNOLOGY OF SOFT DRINKS AND FRUIT JUICES (a) Lignified ovary wall

(the stone or pit)

38 CHEMISTRY AND TECHNOLOGY OF SOFT DRINKS AND FRUIT JUICES (a) Lignified ovary wall

(the stone or pit)

Stone fruit (e.g. damson) or 'drupe' (genus Prunus)

ovary wall Apple (ripe fruit)

Figure 3.2 (a) A typical stone fruit or drupe, e.g. the damson (Prunus) and (b) structure of a ripe apple (Malus).

ovary wall Apple (ripe fruit)

Figure 3.2 (a) A typical stone fruit or drupe, e.g. the damson (Prunus) and (b) structure of a ripe apple (Malus).

3.2.1.1 The basics of plant reproduction and fruit formation The flower is the reproductive centre of a plant, and the series of changes resulting in the formation of a fruit starts here with the process of pollination, whereby pollen is transferred from anthers to stigma by a series of mechanisms dependent upon the plant species. In essence, these mechanisms can be initiated by an insect visiting a flower and becoming dusted with pollen from the ripe stamens (carrying the pollen-bearing anther) and then visiting another flower where the pollen on its body will adhere to the stigma of the second flower. At this stage the pollen grain, containing the male nucleus, fuses with the stigma, absorbing nutrient and sending out a growing tube that eventually reaches an ovule contained in the plant ovary. The male nucleus passes down the tube during this period and is ideally positioned to fuse with the female nucleus

Juice vesicles

Albedo

Figure 3.3 Component parts of a typical citrus fruit.

Juice vesicles

Flavedo

Consisting of heavily cutinised outer cell walls and covered by a waxy layer. Carries the chromatophores and numerous spherically shaped oil glands.

Albedo

Figure 3.3 Component parts of a typical citrus fruit.

contained in the ovule. From here on rapid changes occur, resulting in the fruit, or receptacle, and seed formation.

3.2.1.2 Respiration climacteric

During their ripening or maturation stages some, but not all, fruit varieties undergo a phase of upsurge in metabolic activity known as the climacteric, a term coined by Kidd and West (1922) to describe the increase in respiration rate and heat evolution as the fruit softens and develops flavour and aroma. Fruits such as apples, pears, bananas and most stone fruit have stored reserves of starch, and during the climacteric these are converted to sugars by starch-degrading enzymes. The carboxylic acid content of the fruit also takes part in the conversion, so that acidity levels are consequently reduced as the maturation takes place.

3.2.2 Harvesting considerations for berry, citrus, pome, stone and exotic fruits

The harvesting of fruit purely for juicing purposes is a relatively minor issue compared with the size of the world market for fresh fruit for direct consumption. Ease of transportation and distribution enables an almost continuous supply of fruit from around the world as harvest time limits are compensated for in the growing areas of both the Northern and Southern hemispheres.

In the Western world, where marketing has become a major influencing factor, fresh fruit for direct consumption is produced to meet very exacting standards. We will have all seen, on the supermarket shelves, high-class displays of, for example, regulation sized Granny Smiths or Red Delicious apples, uniform in shape and appearance, presented at optimum ripeness. The main criteria for juice processing, on the other hand, are more straightforwardly that the fruit should be sound, of good quality, and of correct maturity. In order to achieve these objectives, a highly efficient international quality management system is required to operate throughout the whole procedure of growing, harvesting, ripening, storing and handling of the fruit. Apples and pears can be stored under prescribed temperature and environmental conditions in order to ripen gradually. When they are graded to meet requirements for direct sale, those fruits falling outside size limits may be sent for juicing purposes. Careful handling of the fruit during harvesting is an essential requirement, as also is the post-harvest management. If the fruit is already destined for juicing then it is more usual for mechanically harvested fruit to be transferred direct from the orchard to the processing plant to meet the demands of a tight production programme during the season. (In Europe the season usually runs from the end of June to mid-December).

However, the decision to be taken on exactly what degree of maturity should be reached by the fruit before harvesting is not an easy one, and it is here that the whole operation can encounter success or failure. The soluble solids content of the fruit is a reliable indicator in the case of non-climacteric fruit, whose composition will show little change following harvest and storage under suitable conditions. The suitability for harvesting of citrus fruit and grapes, both non-climacteric fruits, can be assessed successfully, and the ratio of soluble solids to titratable acidity is frequently used in establishing maturity criteria for these fruit. Citrus fruits are harvested direct for commercial sale or juice processing, undergoing washing, grading, and thereafter packaging or juicing. Fruits that undergo climacteric change are more difficult to deal with because the full potential of the fruits will not be known until they are fully ripened, but the important commercial decisions on harvesting cannot be left until then.

Unlike pome and citrus fruits, soft fruits are subject to rapid deterioration when even the slightest 'bruising' takes place, and unless they are harvested for direct use in the press it is best to subject them to rapid freezing and hold them in the temperature range of -18 to -26°C. Two forms of grading are employed for soft fruit. Selected top-quality berries will be individually quick-frozen (IQF) to be used as later whole fruit pieces in jams, yoghurts and culinary uses, whereas fruit for conversion and use as fruit pulp may be cleaned by washing it free from leaves and twigs and 'block-frozen'. Berry fruit intended for juice production will be block-frozen as harvested, complete with any incidental stray foliage. In the case of blackcurrants, redcurrants and the like, this comprises the stalks to which the individual berries are attached; it is known as 'strig' and serves a useful function as a natural filter-aid during the pressing stage to release juice from the fruit.

Although freezing will disrupt the cell structure of soft fruit and render it 'pulpy' upon thawing out, any adverse changes to juice quality will be minimal and flavour and colour can be easily preserved by this treatment

3.3 Fruit types for processing

3.3.1 Pomefruits

Pome fruits include the apple, pear, medlar and quince. The latter two fruits are of little commercial importance in an age in which ease of harvesting and crop yields rule supreme in terms of cost considerations, but they are occasionally seen in speciality outlets. Medlar fruits are brown-skinned and apple-shaped and are best eaten after 'bletting', a process whereby the flesh softens and sweetens during storage. This can take up to 2-3 months from harvest and explains why in the Middle Ages, in Europe, the medlar was a useful fruit to store for winter consumption. Likewise the quince, similar to the pear in shape, although at one time a very popular fruit is now something of a speciality. It is ripe when the fruit turns a bright yellow. It is high in natural pectin and finds its main use in jams and jellies.

The apple and pear are of major commercial importance and are grown in most temperate regions of the world. Argentina, Australia, Bulgaria, Canada, China, France, Germany, Hungary, Italy, Japan, the Netherlands, New Zealand, Poland, South Africa, Spain, the United Kingdom and the United States are among the foremost countries growing pome fruit on a considerable scale for both home and export use. Although local varieties are found to have a following in their own regions, the world markets are dominated by perhaps no more than 20 dessert and culinary varieties, which have been selectively bred for such characteristics as disease resistance, winter hardiness, appearance (colour and shape) and texture together with high average yield. Among these are Bramley's Seedling, Brayburn, Cox's Orange Pippin, Red Delicious, Golden Delicious, Discovery, Granny Smith, Jonathon and Newtown Pippin. The main varieties of pear to be found in the market are the Bartlett or Williams Bon Chrétien, the Comice (Doyenné du Comice) and the Conference.

3.3.2 Citrus fruits

Citrus fruit varieties are grown for commercial use in many parts of the world. Originating in the southern and eastern regions of Asia, China and Cochin China, and the Malay Archipelago, the citron (Citrus medica) is said to have first arrived in Europe during the third century bc, when Alexander the Great conquered western Asia. Later, the orange and lemon were introduced into the Mediterranean region in the days of the Roman Empire, when trade routes from the Red Sea to India became established. Cultivation of citrus fruits has since spread worldwide to all regions where the climate is not too severe during the winter months and suitable soil conditions are available. In the United States, the notable growing areas are in Florida and California and in South America, Brazil has taken over the largest share of the world market for oranges and orange juice products. Morocco, South Africa and parts of Australia have shown increased output during recent years, although within the latter two areas yields are frequently affected by variable weather conditions. China is the largest producer after the United States and Brazil, but over 90% of its output is for the home market.

In the area of citrus juice production, Brazil, California and Florida are the major players, with Spain and Israel being notable producers of specialised concentrates. Israel enjoyed a thriving citrus industry in the 1980s, but has since lost much ground due to strong competition from South America with the emergence of the Brazilian market. In recent years a spate of drought conditions, labour shortages and political instability has done nothing to improve the situation.

The main citrus varieties for juice processing are the orange, lemon, lime and grapefruit.

3.3.2.1 Orange

The most important of all citrus fruits is the sweet orange (C. sinensis), and this is widely grown in those regions of the world suited to citrus. Each region tends to have its own characteristic varieties. Common varieties to be found growing in various parts are Navel, Valencia, Shamouti, Hamlin and Parson Brown. The mandarin orange (C. reticulate) is representative of the 'soft citrus' loose-skinned oranges, 'easy peelers', hitherto of primary importance to the Far East and now popular in other parts, including the United States and Europe. The group includes satsumas, an important crop in Japan, and the clementine, an important cultivar to be found in Mediterranean areas. Other cultivars of note are the tan-gor, a hybrid of mandarin and orange, and the tangelo, a hybrid of mandarin and grapefruit. A third distinctive variety of orange is the bitter orange (C. aurantium), chiefly represented by the Seville orange, which is grown commercially in southern Europe mainly for such products as marmalade. Compared with other citrus crops its yield is small and of little use in the juice market.

An important crop in Italy and some other Mediterranean countries, the lemon is also grown commercially in the United States. The characteristic oval-shaped, yellow fruits, apart from their culinary use, are an important source of juice and flavouring for the soft drinks industry.

3.3.2.3 Grapefruit (C.paradisi)

A large round citrus fruit with a thick yellow skin and somewhat bitter pulp, the grapefruit is generally accepted to be a hybrid between the pummelo and the orange. The pummelo (C. grandis) originated in Asia and is grown in many eastern countries including China, Japan, India, Fiji and Malaysia. It was introduced to the West Indies during the seventeenth century by Captain Shaddock, and hence in that region it is sometimes referred to as a 'Shaddock'. Today the commercially important grapefruit is grown in many parts of the world. Notable producing countries are Argentina, Cuba, Cyprus, the Dominican Republic, Egypt, Honduras, Israel, Mexico, Mozambique, Pakistan, South Africa, Spain, Turkey and the United States. The most predominant cultivar to be seen in the market is the Marsh Seedless, followed by a red, pigmented version, the Star Ruby.

3.3.2.4 Lime (C. aurantifolia)

Limes require warm and humid weather conditions in order to thrive on a commercial scale. India, Egypt, Africa, Mexico and the West Indies are therefore prime growing areas. Mexico and the West Indies together produce a large percentage of the world's lime crop. Relative to the other citrus fruits, limes are a small round fruit; they are green or greenish yellow in colour, and not more than 8-10 cm in diameter, with a sharp, fresh and characteristic flavour.

3.4 General comments on fruit juice processing

The various types of fruit, because of their nature, shape, size, harvesting characteristics and so on, may require specialised treatment during processing. In all instances, however, the operation involves a number of stages: obtaining the fruit supply in a correct state of maturity, expressing the juice in the most efficient manner possible, and then, if required, treating the juice with enzymes (e.g. pectolases and cellulases) for clarification, followed by a suitable filtration stage before concentration and eventual packaging or storage.

In citrus fruits, where the outer skin or epicarp is a composite structure containing certain flavouring substances, it would be detrimental to juice quality if the fruit were subjected to direct pressure as is the case with the fleshy fruits, that is, soft fruits, pome fruits and stone fruits. Stone fruits, before being processed for juice separation, must first be separated from their stones, or pits, in order to facilitate ease of handling and to avoid unwanted notes in the finished product. The pits can be further processed to yield both fixed oils for application in the cosmetics industry and glycosides from which may be sourced other natural flavouring ingredients, such as benzaldehyde, a characteristic of marzipan, almond flavourings and the like.

3.4.1 Processing of 'fleshy'fruits

In the separation of juice from its fruit, the traditional method has been to apply pressure to the mashed, or pulped, fruit in order to force the liquid portion through a cloth or some form of screen. There are several styles of separator available, for both batch and continuous production, and a few of these are referred to in the following.

3.4.1.1 Pack press

The pack press is based on the traditional 'rustic' press widely used in the cider industry. The press comprises a set of frames for containing the fruit. These are loaded, in stages, by placing a loose weave cloth over each rectangular frame and adding an appropriate quantity of the fruit mash from a hopper feed above the assembly. The mash is then smoothed, or trowelled, over the frame by the operator, who then folds the cloth across to cover it over. Another frame is then placed on top and the process repeated until several filled layers are formed. The stack is built up inside a rectangular tray, or bed, which serves as both a collecting device and a platform to be raised by the action of a vertical hydraulic ram in bringing the top of the stack into contact with a fixed frame. In this way pressure is created to express the juice, which runs down into the tray for collection.

Earlier versions of the pack press were made of hardwood. Some still are, but modern pack presses are usually constructed from stainless steel and are frequently designed to accommodate two stacks for improved efficiency. These are assembled in sequence in their respective collecting trays before moving across the hydraulic ram, so that while one pressing operation is under way the next stack is being prepared and charged in readiness to follow on. Thus an almost continuous, albeit labour-intensive, pressing operation can be carried out.

The pack press is ideally suited to a relatively small production output for specialised fruit varieties, but for large-scale production fully mechanised systems are necessary.

3.4.1.2 The horizontal rotary press

Perhaps the most successful of the mechanised systems, to date, has been the horizontal rotary press designed and developed by a Swiss company - Bucher-Guyer AG, of Niederweningen/Zurich. This design carries a horizontal hydraulic piston (HP) operating within the cylindrical hollow body of the press. Between the specially designed endplate and the piston faceplate run a large number of flexible drainage cores, with well defined ribbing along their lengths to act as juice channels. Each line throughout its length is covered with a coarsely woven filter sock. The press will work, self-optimised, to operate in strict sequence with pressing periods appropriate to the fruit in process.

Mash is pumped into the press to partly fill the cylinder space. The piston then moves forward under hydraulic pressure to express the juice by consequently forcing it through the filter sleeves and along the channels in the drainage cores, to be fed through outlets in the specially designed plates for collection in a juice tank. The piston then withdraws whilst a second mash charge is received and the process repeats. During the pressing operation the endplate of the unit rotates with consequent meshing of the filter-lines, so that at the end of a pressing cycle the lines are loosely meshed together within the press-cake, or fruit pomace. As the piston moves back in conjunction with incoming mash, reverse rotation occurs, disentangling the lines and redistributing the pomace amongst the fresh charge of mash. Hence there is no build up of press-cake and maximum efficiency in terms of juice removal. Depending upon the fruit type there will be several charges of mash to feed the pressing sequence until a 'full load' has been achieved.

At the end of a complete pressing sequence the pomace residue is discharged by rotating the whole press body with the piston fully retracted and moving the cylinder flanges away from the endplate to create an opening through which the dry pomace is released for collection; usually along a screw conveyer with a suitable storage hopper. The standard Bucher-Guyer press, the HPX 5005i (similar to the one shown in Figure 3.4), is designed to comfortably accept a 10 tonne loading, although with suitable selection and pre-treatment of the fruit, quantities in excess of this can be handled quite easily. Good quality fruit (e.g. apples) will yield 85-95% by weight of expressed juice (depending on pre-treatment and degree of post-extraction). It is quite feasible therefore for a press of this size to handle in excess of ten tonnes of fruit mash, in stages, leaving around one tonne of pomace to discharge.

3.4.1.3 The use of centrifuges in processing

Although direct pressure has previously been an obvious choice of processing method, in comparatively recent times there has been a move towards employing centrifugal separation of juice from a continuous fruit mash stream. The modern decanter centrifuge can be used in conjunction with a pressing system as a preliminary step to increase throughput efficiency, or, when two units are used, as a complete separation system providing a coarse primary stage, followed by a final clarification stage.

Figure 3.4 Horizontal rotary press: Universal Fruit Press HP5000 (Bucher-Guyer).
Figure 3.5 Clarifying decanter (horizontal scroll centrifuge). Source: Courtesy of Westfalia Separator Ltd.

The decanter is a horizontal scroll centrifuge with a cylindrical-conical solid-wall bowl for the continuous separation of solids out of suspensions (see Figure 3.5). Centrifugation has a particular advantage when producing single-strength cloudy juices for direct consumption since a better definition in terms of particle size distribution can be attained. Typically, with decanter juice, 60% of the particles in suspension are smaller than 1 ^M, whereas this figure is reduced to around 20% for pressed juice. Hence there is greater likelihood in the latter for instability and sedimentation. It should be noted that the major factor in the production of 'naturally cloudy' juices is the rate of processing and that to ensure stability the juicing stage should be followed immediately by pasteurisation in order to deactivate the enzymes naturally present in the fruit.

Decanters are also of use in the production of fruit purée, where the aim is to remove only the undesired particles such as pips, stalk fragments, skin fragments and coarse tissue material, leaving the crushed fruit flesh evenly distributed throughout the juice. By setting the machine parameters accordingly, the undesired components can be selectively removed from the liquid stream output of purée.

Decanters are frequently used in conjunction with disc-stack-type centrifuges in the pre-preparation of clear juices and juice concentrates, where the initial decanter treatment results in a partially clarified juice with a low level of suspended solids. This is followed by a clarification stage using a disc-stack whereby the solids are thrown outwards from the through-flow juice stream into a solids-holding space and automatically discharged therefrom as and when an optimum level of solids is reached (see Figure 3.6).

3.4.2 The use of enzymes in fruit juice processing

Pectin is an essential structural component of fruits, where in combination with hemi-cellulose it binds single cells to form the fruit tissues. Pectins are chains formed almost exclusively of D-galacturonic acid units, partially esterified with methanol. These chains are often referred to as 'polygalacturonic acid' or its synonym 'homogalacturonane'. In the immature fruit the pectins are mainly insoluble, but as the fruit ripens there is a gradual breakdown of some of the pectic substances in the skin and flesh cell walls, resulting in the formation of polysaccharide component materials. The general term 'pectic substances' covers not just pectins but just about everything resulting from the degradation processes involving pectin that take place as the fruit maturates, soluble forms included.

As fruit becomes softer, less acidic and sweeter and heads towards its optimum state of maturity, such changes need to be taken into account by the juice processor. Apples, in particular, are best processed prior to their fully ripened state, as solubilised pectin and softened fruit tissues will seriously affect the

1 Feed

2 Discharge

3 Centripetal pump

4 Sensing liquid clarifying discs

5 Disc stack

6 Separating disc for sensing liquid

7 Solids-holding space

8 Solids ejection ports

9 Annular valve

10 Drain nozzle

11 Storage chamber

12 Nozzle

13 Opening water

14 Piston

15 Closing chamber

16 Closing water

17 Sensing-liquid pump

18 To the control unit

19 Flowmeter

20 Regulating valve

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