Use of the whole byproducts 1631 Dietary fibre

The concept of dietary fibre is well established (even though the definition of dietary fibre may still be the subject of debate) and the nutritional benefits (protection against certain types of cancer, regulation of food transit through the digestive system, blood cholesterol lowering) of dietary fibre intake are generally accepted (Lee and Prosky, 1995). Whatever the definition, plant cell walls are the main sources of dietary fibre as they are composed of polysaccharides (mainly cellulose, hemicellulose and pectins) that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine. Our dietary fibre intake comes primarily from cereal products, fruits and vegetables that are consumed in their traditional presentation. However, foods can also be supplemented with dietary fibre. Fibres from cereals (bran essentially), or from fruits or vegetables, are produced and may be added to various foodstuffs in order to increase their dietary fibre content. Indeed, some industries have developed programmes to obtain fibre from agricultural or agro-industrial by-products (Gelroth and Ranhotra, 2001). Sugar beet pulp and leguminous seeds are the main residues from which fibres are currently obtained and marketed, but other pulps (from fruit juice extraction for instance) may also be used. Table 16.1 shows the soluble, insoluble and total dietary fibre content of some typical wastes and by-products from fruit and vegetable processing.

Cell walls in these fruits and vegetables are almost devoid of lignin (content 1-2%), and present the typical composition of primary cell walls

Table 16.1 Soluble (SDF), insoluble (IDF) and total (TDF) dietary fibre content of some fruit and vegetable wastes (values in g/100 g dry product were obtained by the Association of Analytical Communities (AOAC) method)

Origin of the waste

SDF

IDF

TDF

Reference

Orange peel after pectin

21.3

62.9

97.6

Aravantinos-Zafiris et al.

extraction

(1994)

Apple pomace

18.6

69.9

88.5

Renard and Thibault (1991)

Apple pomace after pectin

10.3

78.3

88.6

Renard and Thibault (1991)

extraction

Grape pomace

9.5

68.4

77.9

Valiente et al. (1995)

Beet pulp

12.5

60.3

72.8

Thibault et al. (1994)

Pea hulls

4.1

87.4

91.5

Ralet et al. (1993)

(Thibault et al., 1994). They contain approximately 30% (dry matter basis) cellulose, the remaining polymeric components being polysaccharides (mostly pectins, generally highly methylated), and hemicelluloses, with minor amounts of glycoprotein (extensin). Furthermore, fibres from fruit and vegetable trimmings have an appreciable water-soluble fraction rich in pectins, the amount of which depends on the vegetable, its ripeness, its processing and especially any heat treatments - which can solubilise pectins, mainly by P-eliminative degradations (Thibault et al., 1994). This is a distinctive and potentially important feature of dietary fibre from fruits and vegetables since the nutritional effects of soluble fibres are different from those of insoluble fibres. The capability to bind or hold components (water, ions, small molecules, etc.) and the granularity of the fibres are the two main physicochemical characteristics that can have an impact on the nutritional effects of the fibre as well as on the final product processing. Table 16.2 shows the characteristics of various fibre types in terms of their swelling (measure of bed volume of the fibre in an excess of solvent), water-binding capacity (amount of water bound to the fibre after an external force, e.g. centrifugation, has been applied in given conditions) and cation exchange capacity (number of cations that can bind the fibre by the anionic groups, such as carboxylate in pectins). Fibres arising from fruits and vegetables are characterised by high hydration properties: this property is due to the presence in the fibre of large amounts of hydrophilic pectins. A high waterbinding capacity helps individuals to reduce their calorie intake. This property also has technological interest since a high value helps to retard staling and control ice crystal formation and water migration (Guillon et al., 2000; Gelroth and Ranhotra, 2001).

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