Review
By-products of plant food processing as a source of functional compounds — recent developments

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Abstract

There is a rapidly growing body of literature covering the role of plant secondary metabolites in food and their potential effects on human health. Furthermore, consumers are increasingly aware of diet related health problems, therefore demanding natural ingredients which are expected to be safe and health-promoting. By-products of plant food processing represent a major disposal problem for the industry concerned, but they are also promising sources of compounds which may be used because of their favourable technological or nutritional properties. The purpose of this review is to highlight the potential of selected by-products as a source of functional compounds.

Introduction

Fruit juices and derived products such as nectars and drinks have experienced growing popularity within the last years. Grapes and apples are the most important fruits in the temperate zone, while oranges, pineapples, bananas, watermelons and mangos are the predominant fruits of tropical and subtropical areas. Among other reasons, the rise in consumption and export of processed fruit juices, pulps and concentrates may be attributed to better transportation and distribution systems, and improved cultivation and processing methods (Askar, 1998). Per capita consumption of juice is highest in Germany, accounting to more than 40 l in 1999 (Horenburg, 2001).

Fruits from the temperate zone are usually characterized by a large edible portion and moderate amounts of waste material such as peels, seeds and stones. In contrast, considerably higher ratios of by-products arise from tropical and subtropical fruit processing. Due to increasing production, disposal represents a growing problem since the plant material is usually prone to microbial spoilage, thus limiting further exploitation. On the other hand, costs of drying, storage and shipment of by-products are economically limiting factors. Therefore, agro-industrial waste is often utilized as feed or as fertilizer. However, demand for feed may be varying and dependent on agricultural yields. The problem of disposing by-products is further aggravated by legal restrictions. Thus, efficient, inexpensive and environmentally sound utilization of these materials is becoming more important especially since profitability and jobs may suffer (Lowe & Buckmaster, 1995).

Epidemiological studies have pointed out that consumption of fruits and vegetables imparts health benefits, e.g. reduced risk of coronary heart disease and stroke, as well as certain types of cancer. Apart from dietary fiber, these health benefits are mainly attributed to organic micronutrients such as carotenoids, polyphenolics, tocopherols, vitamin C, and others. Therefore, a minimum of five servings a day of vegetables and fruits, especially of green and yellow vegetables and citrus fruits, is recommended (Heimendinger & Chapelsky, 1996). Although consumers are increasingly aware of diet related health problems (Gilbert, 1997), a large group of the population lacks a generous intake of fruits and vegetables. Thus, dietary supplements and food fortification may be an alternative route to the consumption of minor plant components that may have health benefits. Since synthetic additives are more and more rejected by consumers, functional ingredients should preferably originate from natural sources. This is particularly valid for phenolic compounds which, in contrast to most carotenoids and vitamins, are not chemically synthesized and need to be extracted from plant material.

The preparation of dietary fiber from by-products has already been summarized (Larrauri, 1999), and residual sources of natural antioxidants were the subject of a recent review (Moure et al., 2001). Therefore, the objective of this review is to highlight the potential of selected by-products of food processing as a source of natural food additives and ingredients with particular reference to investigations on the characterization of low-molecular components.

Section snippets

Apple

The highly variable composition of apple (Malus sp., Rosaceae) pomace and possible strategies of utilization have recently been reviewed (Kennedy et al., 1999a). According to the authors, the ongoing effort indicates that the ideal use has not yet been found. Production of pectin is considered the most reasonable way of utilizing apple pomace both from an economical and from an ecological point of view (Fox et al., 1991, Endreß, 2000). In comparison to citrus pectins, apple pectins are

Tomato

Tomato (Lycopersicon esculentum MILL., Solanaceae) juice is the most important vegetable juice with respect to per capita consumption, followed by carrot juice. About 3–7% of the raw material is lost as waste during tomato juice pressing (Otto & Sulc, 2001). Tomato pomace consists of the dried and crushed skins and seeds of the fruit (Avelino, Avelino, Roseiro, & Collaco, 1997). The seeds account for approximately 10% of the fruit and 60% of the total waste, respectively, and are a source of

By-products of sugar production

Since the world sugar market is currently suffering from very high stocks and low prices, long-term options for the utilization of by-products are urgently required (Anonymous, 2000). Sugar cane (Saccharum officinarum L., Poaceae) and sugar beet (Beta vulgaris L. ssp. vulgaris var. altissima DÖLL, Chenopodiaceae) are the most important crops for the production of sugar. Molasses represents the runoff syrup from the final stage of crystallization. It mainly consists of fermentable carbohydrates

Concluding remarks

Far from claiming completeness, this review discusses the potential of the most important by-products of plant food processing as a source of valuable compounds. The relevance of this topic is illustrated by a number of related reviews that were published during the past 5 years (Sudhaker and Maini, 1995, Anand and Maini, 1997, Larrauri, 1999, Kennedy et al., 1999a, Kennedy et al., 1999b, Shrikhande, 2000, McKee and Latner, 2000, Moure et al., 2001, Das, 2001). For cereal and legume by-products

Future trends

The exploitation of by-products of fruit and vegetable processing as a source of functional compounds and their application in food is a promising field which requires interdisciplinary research of food technologists, food chemists, nutritionists and toxicologists. In the near future, we are challenged to respond to the following research needs: First, food processing technology should be optimized in order to minimize the amounts of waste arising. Second, methods for complete utilization of

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