Berry juices have long been popular in Europe where the technology has been extensively developed for high value added fruits (Ashurst, 1995). The recognized phytochemical composition of berry fruits is resulting in renewed global interest in and industrial demand for these juices and beverages (Hakkinen, et al., 1999). Economic considerations limit but do not eliminate 100 percent berry juice products. The characteristic dominating flavour and potent colour of these fruits make them ideal for blending. Curiously, the only three commercially important fruits originating in North America are cranberry, blueberry and certain grape species such as Vitis labrusca, rotundafolia and aestavalis.
Berry and berry-like fruit are characterized by relatively soft fleshed, small diameter (5 to 25 mm) pieces (often with complex morphology) that lack a peel or inner core. Seeds are small and easily separated during juicing. However these fruit grow on or close to the ground and are easily contaminated (Figure 14.1). In addition, the soft flesh is more susceptible to physical and insect damage. The small unit size also makes detection of defects and blemished portions difficult to discern and eliminate.
Figure 14.1: Strawberry harvesting on sandy soil.
Cranberries grow in reasonably severe temperate climates and require substantial moisture. In fact peat bogs or fields that can be flooded to protect from frost and facilitate harvest are ideal (Cranberry Institute, 2000). Other Vaccinium species such as the swamp cranberry (Vaccinium oxycoccos), native to Northern Europe, are less well known. They grow up to the Arctic Circle and have health and phytochemical value worth exploring (Konovalchuk and Konovalchuk, 2000).
The model for berry fruit utilization is arguably the North American cranberry (Vaccinium macracarpon). The small, firm, red berry is too high in acid and low in sugar (Brix/acid less than 2.0, pH less than 2.6) to consume directly. However, the strong characteristic flavour and stable red colour make cranberry a popular component of beverage blends (Figure 14.2). In fact, creative blending and marketing of cranberry juice with other fruit juices and non-fruit ingredients have resulted in the current and increasing demand for cranberry beverages. The imputed health value further increases consumer interest in all types of cranberry products as a cure for bladder and urinary tract infections.
Cranberry-based juice beverages.
Other popular blends include apple, grape, cranberry, kiwi and raspberry.
Cranberry, although firmer than most berry fruit, is relatively easy to juice (Nagy, et al., 1993; Somogyi, et al., 1996b). The firm flesh and cool harvest climate permit handling, cleaning and holding procedures that would be quite damaging to other berry fruit. A coarse chopping followed by paddle or screw finisher with ~5mm screening will produce a thick seed and skin containing pulp suitable for juicing. In view of the high value of the juice, extraction procedures are designed for maximum juice yield, consistent with acceptable quality. Macerating enzyme treatment, hot press and water extraction of the press cake optimize yield. The extract can be combined with the press juice or concentrated separately. If not over extracted, the press cake has value in sauces or other co-products, including natural colour extracts.
In view of the popularity of cranberry products, production is expanding rapidly, but the price of concentrate is still much higher than all large volume juices. Whether exploitation of other Vaccinium species will offer processing alternatives remains to be seen. If these or similar species could be grown in regions with low labour and land costs, favourable climate and necessary automation, dramatic industrial growth should follow.
Except in regions where strawberries are grown for processing, the cultivars available and supply of strawberries for puree and juice is usually dependent upon the vagaries of the fresh market. When the fresh market price drops due to competitive pressure from cheaper growing regions, the crop is harvested and either frozen whole (four parts fruit, one part sugar) for bakery, jam, or jelly use or pureed for pulp/juice manufacture.
Furthermore, when the demand for processing raw material drops too far, or the processing/frozen storage pipeline is full (no available capacity), the crop is abandoned in the field. This unfortunate situation often faces juice processors a seasonal peak in fruit that exceeds fresh market demand and factory capacity or processing plant operational difficulties that reduce juice throughput. In either case sound fruit is wasted.
The extremely soft and delicate strawberry is one of the easiest to juice. A rigorous inspection is necessary to remove rotted and mouldy fruit. A gentle chlorinated water spray then eliminates soil. The whole fruit can be fed directly to a paddle or screw pulper fitted with a 0.5 to 10 mm screen to pulp and separate the puree from the cap, leaves and larger seeds; a few small seeds are acceptable in many puree uses. A second pulper with a less than 0.5mm screen effectively removes remaining seeds and produces a smooth, is then filled into 20 to 200 litre containers and frozen at -18ºC. Actually, the puree should be deaerated prior to freezing and frozen in less than 25 litre portion at less than 25ºC for optimum quality retention but this is rarely done. It takes many hours for the centre of a 200 litre barrel of puree or frozen fruit to freeze, ample time for quality deteriorations to commence.
Although frozen and thawed fruit are quite mushy, almost a puree, juice does not separate easily from either fresh or frozen pureed strawberries. The thick seed-containing puree requires a macerating enzyme treatment. About 50 to 100 ppm of enzyme followed by holding at 10 to 20ºC for two to three hours (cold press) or 30 minutes at 60ºC (hot press) produces juice at a 70 to 80 percent yield (puree basis). The cold press provides a better strawberry flavour, while from the hot press yield and colour are higher. Centrifugation followed by filtration produces a clear juice with typical strawberry flavour and colour.
Unfortunately, strawberry anthocyanin pigments are less stable than those in many fruits are. Thus rapid handling and deaeration are recommended. An additional problem is the formation and subsequent precipitation of ellagic acid in the juice. While ellagic acid is a desirable phytochemical in strawberries, in juice it forms unsightly, powdery sediment. This precipitate forms slowly in a brilliantly clear juice, even after microfiltration or sterile filtration and is accelerated by pasteurization (Musingo, et al., 2001). The "brute force" remedy is heating the juice rapidly to ~80ºC, immediately cooling, holding until sedimentation is complete and then polish filtering. If the juice is later subjected to elevated temperature during concentration or pasteurization, quality suffers.
Steam extraction is well suited to strawberry (Figure 6.8). This procedure yields a juice with diminished fresh flavour, but excellent colour and overall stability. Strawberry juice with a ~5 to 8ºBrix and acid level of 0.5 to 1.0 percent is rarely consumed as is, but contributes flavour, ascorbic acid, moderate red colour, (and a good image) to blends (Figure 14.2a. and b.). Strawberry juice can be concentrated, but the colour is somewhat diminished by even the gentlest process.
Chilled strawberry juice cocktail blend.
Contains orange, apple, pineapple and grape.
Figure 14.3b: Pasteurized strawberry-kiwi beverage, 15 percent juice.
Blueberry (Vaccinum species) is another fruit whose popularity is soaring due to its phytochemical properties. Fruit in excess of fresh market needs are much higher priced and less available than strawberry. Hence, the juice is expensive and extraction procedures are designed to maximize yield.
Blueberry juice preparation can be more difficult than other berry juices due to their high level of mucilaginous material. Cold press extracted juice has a light blue colour and delicate flavour, although hot press enzyme treated juice has a deep purple-blue colour and stronger flavour. Blueberry pigments are more stable than strawberry, but less stable than cranberry and blackberry.
Blackberry juice can be extracted either from fresh or frozen fruit. Frozen berries may be cold pressed because freezing plays the part of heating in breaking down the mucilaginous components. This method usually produces a higher yield, allows the plant to operate outside of harvest season, produces a better colour than from fresh cold press juice and nearly as good as from heated fresh berries and seems to have a fresh fruit flavour not present in the heated berry juice.
For hot processing, clean, ripe, wholesome berries are heated and agitated in a steam-jacketed kettle between 60 to 82ºC. When this temperature range is reached, the crushed berries can be pressed in a hydraulic rack and cloth press.
Single strength puree can have a ºBrix from 10.5 to 18; a puree concentrate from 20 to 40ºBrix; a single strength juice 10ºBrix; and a juice concentrate at 45 to 68ºBrix.
Although most berry fruit and the accompanying juice technology originate from the temperate zone, there are related tropical species that can be handled in a similar manner. For example, Mora de Castilla (Rubus glaucusa) tropical blackberry grows well in the Central American highlands. The deep red berry is an important commercial crop and has domestic and export use in canned and frozen form. Mora can produce stable, highly coloured red juice and wine. However, the harvest season does not coincide with cooler temperature, as in the temperate zone. Thus the fruit is more subject to deterioration in the field and spoilage throughout processing steps.
There are a number of berries whose processing steps are similar with the important caveat that many factors discussed in Chapter 5 dictate specific treatments. The juice process of choice for fresh or frozen berries (raspberries, boysenberries, loganberries, young berries and assorted berries) is the hot press method. The fresh ripe berries must be inspected for soundness and cleanliness and cleaned before being put in a steam jacketed kettle and heated to between 60 to 82°C. Berries should be agitated while being heated. As soon as the desired temperature is reached, the berries can be pressed directly or passed through a pulper fitted with screens sized to remove seeds. [Refer to thermal screw discussion in Part 1].
The juice should be deaerated, either in batches (~710mm Hg or higher vacuum for 30 minutes) or by a continuous process (again at a similar vacuum, but almost instantaneously).
Berry juices are preserved best through flash pasteurization. Temperatures of 80ºC or higher for 30 seconds are recommended. Bottles or cans must be filled completely and closed immediately, followed by prompt cooling. Cans must have special enamel lining, since juices high in nitrates or containing SO2 (less used now) easily compromise some can enamels.
Cherries can be processed similar to some berry fruits even though they come from a fruit tree. Cherry cultivars range from extremely sour to sweet low acid with °Brix/A ratios from 7 to 35. Juice processing and utilization will vary accordingly. Fresh or frozen whole cherries can be crushed in a grape crusher or fed directly to a paddle or screw pulper. Machine clearance and speed must be set so that seeds are not broken. Depitting is unnecessary for juice, although it is needed for other cherry products. Heating the whole fruit to about 60ºC with gentle agitation and a macerating enzyme greatly facilitates juice and colour extraction. A cold press produces a more fresh flavoured juice provided that subsequent processing does not adversely affect flavour.
Sour cherry juice is amenable to blends with less acidic juices or as a nectar with added sugar. The clarified juice can be concentrated with good colour retention and, for a 100 percent cherry juice, sour and sweet juices can be blended. Sweetened cherry puree forms the base of syrups and ice cream toppings (Somogyi, et al., 1996b).