CHAPTER 16

VEGETABLE JUICES

Vegetable juice is defined (FAO, 1992) as: "the liquid unfermented but fermentable product or lactic acid fermented product intended for direct consumption obtained from the edible part of one or more sound vegetables and preserved exclusively by physical means. The juice shall be free from skins, seeds and other coarse parts of the vegetables. It may be clear, turbid, or pulpy. It may have been concentrated and reconstituted with water..." "Vegetables for the purpose of the standard are: the parts of edible plants including roots, corms, tubers, stems and shoots, leaves and flowers and legumes." Although the standard applies to edible plants, herbs, botanicals and possibly medical plants may also fit the definition provided that the plant part(s) is edible and not toxic (Figure 16.1).

Figure 16.1: Plant parts as vegetables.
(Kays, 1991)

Vegetable juices are attracting more attention due to the nutritional and phytochemical value of many vegetables. Tomato juice and blends based on tomato have long been popular and account for over 90 percent of the non-fruit juice trade. However, other vegetables are becoming more acceptable. Blurring the distinction between fruits and vegetables are a range of fruit-carrot juice blends (Figure 16.2). Although the major ingredient is carrot, the fruit character (as represented by other juices and acids) predominates. Carrot supplies the health image and contributes to the colour, while the fruit juices provide the flavour and necessary pH reduction. The V-8 juice blend consists of carrot, high fructose corn syrup, pear, apple, pineapple and cherry, with added malic and ascorbic acids, natural flavours, beta-carotene and red 40. (Unless HFCS and flavours count as juice, it's actually V/F-5).

Figure 16.2: Carrot-fruit juice beverages.

Thus the distinction, sweet and acid for fruits and salty for vegetables (Section 2.2) may play a secondary role to the phytochemical composition (and popular image) of vegetable juices and extracts. If the carrot-based juice beverage (albeit only 25 percent total juice) is a market success, other vegetable juice blends will follow. Clever blending and marketing will be needed to provide a fruit character and a vegetable image.

Aside from traditional uses, the major distinction between fruits and vegetables is compositional. Most vegetables are not particularly high in sugars and, tomato excepted, are low in acids, resulting in pH values generally over 4.5. Consequently, the juices are more subject to microbial spoilage and require either pH reduction, thus changing the vegetable character, or more stringent processes to eliminate potentially dangerous microbes. Low acid thermal processing is much more severe, requiring temperatures in excess of 120ºC and process times of minutes in contrast to less than 100ºC and ~ 60 seconds for acid products.

16.1 Tomato juice

Few fruits are easier to juice than tomato or more amenable to a range of juice products and blends. Tomato solids range from 5 to 8ºBrix, somewhat lower than most juices. However, the high pectin content and pulpy nature of the juice provides an adequate taste. Most tomato cultivars have sufficient citric acid to insure a pH below 4.5, although citric acid or lemon juice is often added to compensate for the reduction in acidity that accompanies ripening.

Juice tomatoes can be specific cultivars with high solids and good colour or come from canned whole tomato or tomato piece canning operations. Fresh market fruit are less likely sources, since these fruit are often shipped at the mature green stage too immature for juice and already in distribution channels. Under no circumstances should damaged overripe or immature tomatoes be juiced, as quality will invariably suffer. In view of the delicate nature of ripe fruit and likelihood of machine harvest, thorough washing followed by careful inspection and grading is essential to eliminate damaged and mouldy items. Machine harvest contributes more dirt and field debris that must be removed. Fluming, by which tomatoes are dumped into and transported by water (Figure 16.3) is a dangerous practice that should be replaced by gentle brushing and water spraying, a more sanitary and water-conserving procedure.

Figure 16.3: Tomato fluming operation, circa 1960.

Peeling or coring is unnecessary, but depending upon cultivar and end use, tomatoes receive either a cold or hot break at juicing. A very rapid acting pectin enzyme system, primarily polygalacturonases and pectin methlyesterases, will break down the pectin upon juicing, thus reducing juice viscosity. Consequently whole tomatoes are coarsely chopped and immediately heated to ~80ºC to inactivate these enzymes hot break. The cold break immediate chopping followed by rapid juicing, deaeration and processing is simpler, allows for some pectin enzyme activity, but retains more tomato flavour. The reduced viscosity is desirable for some derived products such as blends and concentrates. A larger mesh finishing screen that allows more insoluble solids in the juice and/or homogenization at ~100Mpa can increase viscosity.

Juicing is accomplished by passing chopped fruit through a disintegrator or a paddle pulper fitted with a 1 to 0.5-mm screen. Skin and seeds are effectively separated with juice yields around 70 percent. At this stage, deaeration is recommended to reduce oxygen exposure, particularly damaging after the hot break. Since most tomato juice is thermally processed, the retention of fresh tomato flavour is not critical, as long as excess cooked flavour is prevented. In fact, fresh unsalted tomato juice is not as acceptable as properly heated juice containing about 0.5 to 0.75 percent added salt.

Prior to thermal processing, the juice pH should be checked and, if necessary, adjusted to below 4.5 with citric acid. Certain cultivars and overripe fruit can exceed pH 4.6 and ascorbic acid levels range from over 30 to less than 10 mg/100g. Rapid processing can retain over 90 percent, whereas poor practices destroy practically all vitamin C. Ascorbic acid is often added in amounts representing the recommended daily intake (RDI) of 70 mg.

Tomato juice was traditionally packed in unlined tin cans or glass jars. The acid etching of the tin plate provided a desirable flavour and minimized pinholing (a danger with enamel lining where weakness in the enamel allows acid to selectively attack the steel plate). Glass jars provide a visually pleasing juice and inertness. Other container options including plastic, laminates, etc. are now available. Thermal processing consists of hot filling containers at ~95ºC, sealing, inverting and holding about 30 seconds followed by rapid cooling. Of course, high pH (near but below 4.6) or highly contaminated juice require a higher temperature hot fill a boiling water bath at 100ºC for 5 to 10 minutes or even a slight pressure process ~105°C for 10 minutes.

Another notable feature of tomato juice is tolerance to heat. Providing that the product is not held at greater than 40ºC for extended time (greater than several hours) the flavour is not damaged. In fact, consumer preference is for thermally processed salted compared to fresh, unsalted juice. However, excessive heating greatly reduces the natural and any added ascorbic acid. Tomato blends well with a variety of other juices (Figure 16.4), spices as well as clam juice.

An appreciable amount of tomatoes are converted to sauce, puree and pulp, which in turn are used as the base for ketchup, salsas and pizza sauce. Depending upon end use the juicing may use a coarser screen and include some tomato pieces. The United States Standards are available (United States Department of Agriculture, 2000b). Most of the grading criteria are descriptive, i.e. colour, flavour, absence of defects, but soluble solids are well defined. The distinction between concentrate, paste, puree and pulp is vague, except for soluble solids (Table 16.1).

Figure 16.4: Tomato vegetable juice blend ingredients.

Tomato products are increasing in popularity due to the discovery that lycopene, the major carotenoid pigment responsible for the red colour in tomatoes, watermelon, pink grapefruit, some papaya cultivars and other fruits, has anti-carcinogenic properties. Curiously, the bioavailability of lycopene is greater in processed/heated tomato products than in fresh items. Unlike B-carotene and other closely related carotenoids, lycopene has no pro-vitamin A activity.

16.2 Carrot

Carrot juice also typifies the current appeal to the health conscious consumer. Carrot has long been a component of tomato blends. Now, unlike tomato, carrot juice is frequently blended in fruit type concoctions where only the colour and natural sweetness carry over. The health image of carrot is strongly promoted while the blending juices, citrus and tropical fruits (Figure 16.1) provide flavour and reduced pH. Pure carrot juice at a ºBrix of 6 to 9 and pH ~5 to 6 requires a retort process at 121ºC. Consequently, the juice has an unappealing cooked carrot flavour and coagulation of carrot solubles occurs.

There is generally an abundant supply of raw carrots from fresh market rejects i.e. roots that are off size or shape, but otherwise high quality (Figure 16.5). Cultivars should be sweet and highly coloured. Topping and peeling are necessary to remove bitter substances associated with those parts. Fresh carrots are difficult to juice and the fresh juice coagulates upon heating. Thus blanching at about 80ºC for several minutes softens the roots and facilitates juicing by comminution and 0.5 to 10 mm screening. Homogenization yields a juice suitable for blending.

Table 16.1: U.S. Soluble solids standards for tomato products.

USDA, 2000b

Figure 16.5: Carrots receiving at a juice plant in ~ 18 MT trailers.

Although only about 25 percent of total carrot beta-carotene is extracted by juicing, retention during processing is reasonable high. Carrot juice may be vacuum concentrated and stored and shipped in the frozen form. This highly cooked carrot-flavoured reconstituted juice is acceptable primarily to those committed to the health value, whereas creative blending produces juices acceptable to a wider market.

16.3 Other juices

While tomato and carrot are the major juices, a number of other vegetables also find uses. The components in V-8 include: tomato, carrot, celery, beet, parsley, lettuce, watercress, spinach as well as salt and spices (Figure 16.4). In truth, the sensory, nutritional, or phytochemical contribution of the minor juices is questionable, but the marketing appeal of 8 juices is clearly successful, as reflected in the market longevity of the product.

As additional valid nutraceutical data support the imputed health value of other vegetables, we can expect a plethora of vegetable juices and blends. For example, recent studies reported that rhubarb juice inhibits browning in apple slices and juice at 10 to 20 percent (Anon. 2000a). The high oxalic acid content (~3 percent in rhubarb) is an effective browning inhibitor. In view of the natural image of rhubarb juice, its use in other browning susceptible foods is logical. However, high levels of oxalic acid can bind essential minerals. Thus, there are downsides to natural products also. To the chemically illiterate, at any level, SO2 is "dangerous", whereas being a "natural" product, rhubarb is "safe", independent of dose. (Consumption of occasional 100 to 200ml portions of 100 percent rhubarb juice would be innocuous; frequent ingestion of this amount or more would not be).

The juice manufacturing practices, although similar in principle, are vegetable-, even cultivar-specific. In general, sound, sanitary pieces free from seeds, stem or inedible/off-flavour components are suitable for juicing. If heating causes coagulation, preheating prior to juicing or subsequent homogenization can provide a stable puree-like product. Salt, spices and creative blending with other juices (fruit and/or vegetable) can result in appealing beverages.

Reduction in pH can be achieved with suitable fruits or fermented vegetable juices. Sauerkraut juice, the excess fluid produced in the manufacture of sauerkraut, is about pH 3.3 and contains appreciable ascorbic acid, ~2 percent salt and typical kraut flavour. In a similar sense lactic fermentation of other vegetables, either singly or in combinations, results in juices possessing distinctive characteristics. Carrot, celery, radish, cucumber, pepper and onion are interesting possibilities. Although shelf stability requires pasteurization, there is undoubtedly a market for freshly fermented juices that maintain the desirable viable microflora associated with probiotics, vegetable analogs to fermented dairy products.

Avocado puree or the Latin American guacamole is fairly removed from beverages, but merits brief mention, since lemon and onion juices contribute to the mix and preservation is an extreme challenge. The high lipids content (10 to 25 percent) and favourable fatty acid and to copherol profile of avocado overcome its calorie-dense reputation. Guacamole is a popular dish, requiring only avocado paste, salt, lemon, onions and additional spices/ingredients, depending upon the creativity of the chef. The major limitation is the susceptibility of avocado to heat. Heating, even to inactivate browning enzymes results in bitter, disagreeable off flavour. Thus, use of browning inhibitors like SO2, ascorbic acid and

rapid freezing are required. Hyperbaric processing is being explored and, conceivably avocado puree could contribute much to a valuable vegetable juice blend where thermal pasteurization can be avoided (Section 8.26).

16.4 Non-fruit/vegetable juices

In the fast paced development of novel juice and juice beverages, there are a number of juices, some traditional in part of the world, others quite novel, that have reached markets in developed countries. While it is too early to evaluate the staying power of these beverages, they represent crops with some potential and merit mention.

Cactus juice (actually iced tea sweetened with prickly pear (agave) nectar) has been noted (Anon. 2000b). Agave is an important crop in Mexico and when fermented and distilled, the source of mescal and tequila spirits. If the supply can be increased, agave as a natural sweetener with unique flavour has intriguing potential for regions where succulent cacti can grow.

Another more common sweetener also has juice possibilities. Sugar cane has long been crushed and the juice sold fresh by street vendors in growing regions. With simple roller crushing, clean, peeled cane produces a light green juice, mostly sucrose, at a pH of ~6 and about 20ºBrix (Figure 16.6). The cane must be clean and well peeled to avoid dark, off-flavoured juice; rapid processing (or the use of SO2) is necessary to prevent browning. When blended with high acid juices and pH adjusted to ~4.2 a very pleasing beverage results. Cane juice blends are amenable to standard processes and have a strong "natural " image. The alternate operation depicted in Figure 16.6 is highly inappropriate for cane juice; the resulting juice is brown, far from sanitary and suitable only for sugar refining.

Figure 16.6: Cane juicer for beverage use and for sugar extraction (trapiche).