CHAPTER 3

FRUIT MORPHOLOGY AND COMPOSITION

3.1 Distinctions

Botanically, fruit is the structure resulting from the growth of a flowering plant. The fleshy component, which is normally the portion eaten, serves to protect and eventually nourish the seed(s) as part of the natural development of the original plant's progeny. The morphology of fruits varies substantially as does the plant tissue consumed (Figure 3.1.).

Figure 3.1: Plant tissue consumed as fruit.
(Coombe, 1976; Kays, 1991)

Fruit types dictate the mechanics and ease of juice extraction (Arthey and Ashurst, 1996). There are about nine categories of fruit/vegetable types (Kader, 1992; Kader and Barrett, Somogyi, et al., 1996a):

• Pome fruits - Apple, pear and quince, are the classic examples,
• Citrus - A range of well and lesser know species make up this category. Orange, grapefruit, tangerine, lemon and lime are in wide use, but other citrus such as pummelo, tangerine and uglifruit, have unique characteristics worth exploring,
• Stone fruits - These have a well-distinguished pit and include peach, plum, apricot, cherry and mango,
• Soft Fruits - Although morphology varies greatly, these fruit have flesh that has a firm structure, but can be easily deformed. Grape, strawberry, pineapple, banana, berry fruits, kiwi, papaya and lychee, are some examples,
• Amorphous fruits - Fruits with little firm flesh structure under the skin such as passion fruit, naranjilla, guava and soursop, fit this category,
• Fruit vegetables - Watermelon, cantaloupe and various melons are fruits by use. Tomato and solanaceous vegetables are also in this category,
• Leafy, flower and stem vegetables - Only rhubarb has fruit-like character and is utilized from among the many vegetables of this type,
• Tree nuts - Although the seed is the desired crop, cashew fruit has value and there are undoubtedly, other fruits in this category where the outer flesh has juice potential. The ripe coffee berry is a remote example,
• Tropical and subtropical - Generally treated as a separate category, although the previous categories cover most species.

There are fruits that do not fit clearly into these definitions, or differ so much according to cultivar and maturity state that they are intermediate. For example, a soft mature mangosteen falls apart upon peeling, whereas a less ripe, firmer one can be peeled and eaten out of the hand.

As with juice definitions, there is some disagreement over what constitutes a fruit. Custom dictates the distinction that a tomato is a fruit, but it is normally regarded and consumed as a vegetable. Similarly, peppers and cucumbers are vegetables; yet watermelon, a member of the cucurbitaceae family along with cucumber, is a vegetable but used as a fruit. Pepper juice is viewed as a strange concoction, although perhaps with some merit (Chapter 16). In contrast, the plant stalk or petiole portion of the rhubarb is consumed as a fruit while the swollen inflorescence of broccoli is the epitome of a vegetable.

A rough, arbitrary, but recognizable distinction is if the juice is sweet and somewhat acid or sweetened with or without added acid for consumption, it is a fruit. If salt and/or spices enhance the juice flavour, it is a vegetable.

For the purpose of this text, most major and some minor horticultural crops capable of yielding a sweet (usually acidic) juice or puree from any portion of the plant customarily consumed and considered a fruit will be covered, along with some major vegetables. Leaf, stem and root extracts are presently gaining greater attention as "teas", herbal medicines or phytochemicals and have relevance in the context of juice by-products and nutraceuticals (Section 3.3)

3.2 Fruit/juice general composition

The structure and functional aspects of fruits dictate their composition. Table 3.1 shows some typical constituents of fruit (and subsequently juices) and the range of values dependent upon fruit, cultivar, cultivation, maturity and other factors to be presented later.

Table 3.1: Fruit edible portion composition ranges* (Fresh weight basis).

*For more specific generalized values see USDA, 2000a.

• The major component is of course, water derived from the extra and intracellular fluid necessary for metabolic processes and maintenance of cell turgor. Water content can range from 97 percent in some wild berries to 70 percent in over ripe grapes and less than 50 percent in fruits drying naturally on the plant;
• The nonaqueous portion contains literally hundreds of identified compounds with a range of natural sugars and/or sugar polymers namely glucose, fructose, starch, cellulose, hemicellulose, pectin, usually representing the majority of solids. These solids are categorized as (1) soluble, which is readily expressed in the juice and (2) insoluble, consisting primarily of the press residue. Other solids such as stem and sometimes skin, core and seeds, are the inedible components, but may have by-product value;
• Other macro components are fruit acids, responsible for the tart taste and low pH of many, but not all fruits. These are water-soluble with specific acids contributing importantly to the flavour and character of fruits and juices. Principal organic acids are citric, tartaric, malic, lactic, acetic and ascorbic and range from trace amounts to over 3 percent. Trace amounts of phenolic acids are also present;
• Protein is generally less than 1 percent, except in oily fruits and all seeds and not readily expressed in juice;
• Next in abundance, are usually either phenolic compounds ranging from anthocyanin or carotenoid pigments to tannins which provide astringency;
• Nutrients such as proteins, lipids, amino acids, vitamins and minerals, are present to varying degrees, as are flavour and aroma substances;
• Lipids deserve special mention. With the exception of olive, avocado, ackee and oil palm fruits, fruit lipid content is very low less than 0.5 percent, except in seeds. The absence of lipids is reflected in the reasonably low caloric value of fruit juices and their important role in fat avoidance diets. Nevertheless, the fatty acid profile of fruit lipids and the accompanying fat-soluble vitamin level are quite good, actually "heart healthy". High lipid fruits are incompatible with juices, although avocado puree merits consideration.

Despite the small amounts of some compounds, they can influence dramatically the appeal, stability, or health value of the fruit. Many intrinsic (fruit-specific) and extrinsic (extraction-dependent) factors influence juice composition, as will be emphasized. Food composition tables provide a reasonable first approximation to fruit and juice make up (USDA, 2000a). Nevertheless, there can be substantial differences in the same species and analytical experience with a given cultivar under controlled production and processing conditions is very important.

3.3 Phytochemicals

This category of compounds, also known as nutraceuticals and functional foods, is increasing in importance and merits special attention. Phytochemicals are defined as:

• "Non-nutritive or nutritive, biologically active compounds present in edible natural foods including fruits, vegetables, grains, nuts, seeds and tea, which prevent or delay the onset or continuation of chronic diseases in humans and animals." (Guhr and LaChance, 1997).
• "Any food ingredient that may provide a health benefit beyond the traditional nutrients it contains" (Hasler, 1998).

Fruits and their juices are an especially good source of phytochemicals (Table 3.2). Close to one thousand different phytochemicals have been found in plants and the
identification and promotion of hitherto unrecognized compounds with real or
imputed health value continues unabated. In fact, the science is in its infancy and additional
phytochemicals, mechanism of action and beneficial effects (toxic properties also), will
become apparent and attributed to traditional and exotic juices.

Given the propensity of some consumers to place the therapeutic value of medicines over foods, one company even incorporates the active ingredients from 17 different fruits, vegetables, grains, fibres and probiotics, into capsules and chewable pills. Their trademark, "Juice PLUS+" acknowledges the phytochemical value of juices. Of course, juices and blends are a less costly and more appetizing alternative, albeit requiring more attention and awareness on the consumer's part.

One unique and fascinating class of phytochemicals is phytoalexins. These compounds are synthesized in plant parts, including fruit, as a response to external stress, i.e. drought, sunburn, temperature extremes, insect damage, pathogen infection, etc. Resveratrol, a
stilbene type compound has been studied extensively as a stress mediated substance with likely chemopreventive effects against cardiovascular disease and cancer. Red grapes, juice and wines have significant amounts of such compounds, believed to be in part responsible for the health benefits of red wine and attributed to their antioxidant properties (Adrian, et al., 2000).

Needless to say, the total composition of fruits and the expressed juice provide a range of pleasing sensory and health attributes that can be optimized by the juice processor, even to the extent of improving upon the raw material.

Table 3.2: Some phytochemicals in fruit and vegetable processing protective properties against cancer and cardiovascular disease*.

* Adapted from Broihier, (1999); Guhr and LaChance, (1997) and expanded.