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359

16sugar Beet Fiber: Production, Characteristics, Food Applications, and Physiological Benefits

Marie-Christine Ralet, Fabienne Guillon, Catherine Renard, and Jean-Francois Thibault

CONTENTS

Introduction .........................................................................................................360Fiber Production .................................................................................................. 361Characteristics ..................................................................................................... 361

Sugar Beet Fiber Composition.................................................................. 362Structure of Sugar Beet Fiber Polysaccharides ......................................365

Pectins ..........................................................................................365Hemicelluloses ............................................................................... 369Cellulose .......................................................................................... 369

Sugar Beet Fiber Physicochemical Properties ........................................ 370Hydration Properties ..................................................................... 370Adsorption/Binding of Ions and Organic Molecules .............. 372

Functionality and Food Applications .............................................................. 372Extracted Polysaccharides ........................................................................ 372Whole Sugar Beet Fiber ............................................................................. 373

Ready-to-Eat Breakfast Cereals .................................................... 373Bakery Products ............................................................................. 374Meat Products ................................................................................. 374

Physiological Benefits ......................................................................................... 374Apparent Fermentability or Apparent Digestibility ............................. 374Transit Time and Stool Output ................................................................ 375Minerals Adsorption ................................................................................. 376Glucose Metabolism .................................................................................. 376Lipid Metabolism ....................................................................................... 378Colorectal cancer ........................................................................................ 381Tolerance to Sugar Beet Fiber ................................................................... 382

Safety/Toxicity .....................................................................................................383Conclusion ............................................................................................................383References ............................................................................................................384

© 2009 by Taylor and Francis Group, LLC

360 Fiber Ingredients: Food Applications and Health Benefits

Introduction

Beets originate from the Middle East and have been grown as vegetables or for fodder since antiquity. However, their use as a sugar crop began only in the 18th century. At that time, consumption and production of sugar from sugar cane were very widespread and France had an important place in this trade. The French Revolution drastically modified the sugar world order and con-flicts seriously disrupted shipping transport with the colonies. The Napole-onic Wars at the beginning of the 19th century made worse an already critical situation. In 1807, the British began a blockade of France, preventing the import of cane sugar from the Caribbean. Prices exploded and France had to find an alternative to the production of sugar from sugar cane in the overseas ter-ritories. In 1747, a Prussian chemist, Andreas Sigismund Marggraf, had been successful in recovering crystallized sugar from sugar beet. In France, Benja-min Delessert improved the Marggraf process and opened the first beet sugar factory in 1811. By the end of the wars, over 300 beet sugar mills operated in France and central Europe. The first U.S. beet sugar mill opened in 1838.

Today, sugar beet provides approximately 25% to 30% of the world’s sugar production, which was around 150 million tons in 2005. The European Union (130 million tons for the 2005 –2006 campaign), the United States (25 million tons), and Russia (22 million tons) are the world’s three largest sugar beet producers. The new sugar reform (2006) will probably have only a moderate impact on sugar beet production. Indeed, agricultural surfaces devoted to alternative fuel production will certainly compensate the decrease in those devoted to sugar production.

Sugar beet roots contain ~18% sucrose and ~5% cell wall polysaccharides on a wet weight basis. Beet-sugar producers slice the washed beets and then extract the sugar with hot water in a diffuser. These treatments typically consist of heating at 85°C for approximately 15 min followed by diffusion by water, typically 2 h at ~65°C and pH ~6.5. An alkaline solution (“milk of lime” and carbon dioxide from the lime kiln) then serves to precipitate impurities. After filtration, evaporation concentrates the juice to a content of about 70% solids, and controlled crystallization extracts the sugar. A centri-fuge removes the sugar crystals from the liquid, which gets recycled in the crystallizer stages. When economic constraints prevent the removal of more sugar, the manufacturer discards the remaining liquid known as molasses. Sugar beet pulp is a very abundant by-product (500 kg wet weight per ton of beets). On a wet weight basis (~90% humidity), 120 million tons of beet pulp are produced in the world each year. The wet pulp can be used directly (dry matter ~10%), pressed (dry matter ~27%), or dried (dry matter ~90%).

The pulp is a popular feed for ruminants. However, alternative uses are currently proposed in order to increase the value of the pulp. The extrac-tion of polysaccharides (pectins, arabinans, cellulose) or monomeric compo-nents (arabinose, galacturonic acid, rhamnose, ferulic acid) may be one way of upgrading [1–3]. For example, arabinan may be extracted from the pulp


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sugar Beet Fiber 361

and its potential as fat replacer has been investigated [4]. Another possibility is to find direct uses for the pulp. As this residue consists mainly of cell wall polysaccharides, several if not all sugar companies have studied the use of sugar beet pulp as a high-fiber food ingredient or a dietary fiber.

Fiber Production

Larrauri [5] indicated that the ideal fiber preparation should meet several requirements among which are bland in taste, color, texture, and odor. In that context, beet pulp must be processed before it can be used in food systems because it has a typical unpleasant flavor, may be too colored, and also may contain too high amounts of soil or sand [6]. Essentially physical treatments including cleaning, extraction, sieving, and heating have been described, although some chemical treatments have also been proposed. With special processing, it is possible to produce a dietary fiber, with an off-white color and unobtrusive flavor, suitable for human food. The fibers may be milled to a given particle size from coarse to fine depending on the intended use, or treated with steam in a flaking process.

Several processes have been patented and trade names have been given for such fibers. today, the sole commercial sugar beet fiber is Fibrex® developed by Danisco Sugar A/S (Denmark) and marketed as an ingredient all over the world. Annual Fibrex® production is less than 5000 tons. It includes two steam-drying steps with optimized temperature, pressure, and time, as well as a milling and screening step to remove sand from the end product. Fibrex® is proposed with a variety of particle sizes (from < 32 µm to flake) for easy blending with other ingredients (Figure 16.1).

Characteristics

Dietary fiber in sugar beet comes exclusively from its cell walls, and is devoid of resistant starch or other reserve polysaccharides. Plant cell walls vary enormously in their compositions and physical properties depending on the cell type and plant species (7). Three plant groups are generally defined: dicotyledons (most fruit and vegetables), non-commelinid monocotyledons (mostly alliums), and commelinid monocotyledons (grasses and cereals). Among those groups, two major wall types are typically recognized: pri-mary and secondary, the latter being often lignified (8). The polysaccharide compositions of the wall types in the different plant groups differ widely (Table 16.1). Fiber preparations from fruits or fruit residues and sugar beet pulp (dicotyledons) contain predominantly primary cell walls while cereal


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brans (commelinid monocotyledons) contain both primary and secondary cell walls. This leads to very different cell wall architectures, polysaccharide compositions, and physicochemical properties.

Sugar beet Fiber Composition

Sugar-beet pulp has a high dietary fiber content, typically >75%, and is known for its high soluble fiber content (Table 16.2) (9–11). The AOAC method, because of its lengthy enzyme incubations at pH close to neutral and at high temperature, may however overestimate the amount of fiber actually solubi-lized in the upper parts of the digestive tract. Lignin content of beet fiber is low (< 5%) (12–14). The remainder of the fiber preparations consists of pro-

Table 16.1

The Polysaccharide Compositions of Cell Wall Types in the Different Plant Groups

Wall Type

Plant Group Non-Lignified Primary Lignified Secondary

Dicotyledons and Monocotyledons non-commelinid

Cellulose ~ Pectins > Xyloglucan

Cellulose > Heteroxylans > Glucomannans

Monocotyledons commelinid Heteroxylans (+ mixed β-glucans in Poaceae and other families) > Cellulose >> Pectins and Xyloglucans

Cellulose ~ Heteroxylans >> Glucomannans

Source: Adapted from Harris and Smith, 2006 [8]

FIGuRe 16.1

Fibrex® of different particle sizes. This picture was kindly provided by Danisco Sugar A/S (Denmark).


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teins (< 10%) (13–15); ash (3% to 8%) (13–15); and lipids (< 2%) (15). Some sugar beet pulp fractions may be high in ash (16) arising from contamination by soil particles.

In detailed studies of their composition, beet cell walls, and therefore sugar beet fiber, are characterized by very high pectin content, with about 20% each of galacturonic acid (GalA) and arabinose (Ara) (Table 16.3) (17–20). This amount of pectin and more specifically of Ara is exceptionally high, even in comparison to cell walls from other dicotyledons. Arabinans, which are part of pectins, are still often mistaken for hemicelluloses. Sugar beet fiber also contains approximately 20% of glucose (Glc), mainly of cellulosic origin. In total, sugars account for about 80% of the dry weight, with remarkably low amounts of xylose (Xyl) and mannose (Man). Several non-sugar constituents are also present: methanol, acetic acid, phenolic acids, proteins, lignin, and ash (Table 16.3).

There are little differences in global sugar composition between cell wall material directly isolated from raw beets and sugar beet pulp (Table 16.3). Le Quéré et al. (21) found 4.5% of water-soluble pectin from beet slices alcohol-insoluble solids (AIS) and, surprisingly, still 3.3% from AIS arising from beet pulp after diffusion. Fares et al. (22) also showed that few polysaccharides, mainly of pectic origin, are extractable from sugar beet by water in the sugar factory. This low extraction of pectins could be due to physical limitations to diffusion of the pectic polymers from the cell wall network or to the struc-ture of beet cell walls. Little material is extracted from beet cell walls in mild, non-degradative conditions. Dea and Madden (23) extracted only a total of 5% dry matter from whole beets by successive cold and hot water treatments at pH 3.7. Renard and Thibault (24) and Levigne et al. (19) extracted only 5% to 5.6% of whole beet AIS by buffer or water at pH 4.5 and room temperature. This extracted material is of pectic nature, rich in GalA and Ara.

As pointed out above, the AOAC method leads to higher extraction yields with SDF values around 20%. Compositional analysis reveals that sugar beet SDF is also of pectic nature. Sugar beet IDF still contains large quantities of pectic material and is rich in Glc of cellulosic origin (Table 16.3).

Table 16.2

Dietary Fiber (Total, Insoluble and Soluble, % Dry Weight) of Native and Modified Sugar Beet Fiber Preparations

TDF IDF SDF Ref.

Native sugar beet fiber 87.1 71.7 15.4 (9)Autoclaved at 122°C 78.4 52.5 25.9Autoclaved at 136°C 78.6 48.9 29.7Native sugar beet pulp 76.9 52.1 24.8 (10)Fibrex® 73.0 49.0 24.0 Data provided by the supplier (Danisco)Native sugar beet fiber 70.0 57.8 12.2 (11)H2O2-treated 94.3 61.1 33.2


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364 Fiber Ingredients: Food A

pplications and Health Benefits

Table 16.3

Sugar Composition of Different Sugar Beet Fiber Preparations (% dry weight)

Yield (%) Rha Ara Xyl Man Gal Glc GalA

Total sugars Ref.

Native pulp 100 1.4 19.1 1.6 1.3 4.6 20.6 20.2 68.8 17Acid- and alkali-treated pulp 35 1.0 11.0 3.6 2.7 2.3 54.2 4.9 69.8Native pulp 100 2.4 19.6 1.4 1.3 5.5 21.5 20.6 72.3 18Acid- and alkali-treated pulp 53 2.3 10.0 2.1 2.1 5.7 38.9 16.0 77.1Native fiber 100 1.5 23.6 1.4 1.4 5.4 24.3 23.2 80.8 17Acid- and alkali-treated fiber 46 1.2 5.3 2.5 2.6 4.5 51.0 12.0 79.1AIS from fresh roots 4 2.0 17.2 1.1 1.0 4.5 18.8 20.0 64.6 19Water-extraction at 20°C Residue 82 1.2 19.2 1.4 1.0 4.9 22.2 21.7 71.6 20 Soluble (polymeric) 2 1.2 16.1 tr tr 6.1 1.2 31.6 56.2IDF 60 1.6 24.6 1.6 1.4 6.1 29.6 19.6 84.5 20SDF 13 0.9 7.9 tr 4.2 3.1 0.6 43.4 60.1


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Structure of Sugar beet Fiber Polysaccharides

Sugar beets are mainly composed of parenchymal tissue with thin, supple, and hydrophilic cell walls. Typical primary cell walls of dicotyledonous plants are composed of almost equal amounts of three types of polysaccha-rides: (1) pectin, rich in GalA, Gal, Ara, and Rha; (2) hemicelluloses, typi-cally xyloglucans with minor amounts of (gluco)-mannans; and (3) cellulose. The structure of these cell walls can be summarized as three interlocking networks, namely cellulose/xyloglucans, pectin, and cell wall glycoproteins. Sugar beet cell walls differ from this blueprint in a number of key points, which will be discussed in the following.

Pectins

Most of the data on the structure of the constitutive polysaccharides of sugar beet cell walls and fiber deal with the pectic fraction, as it represents more than 50% of the fiber (Table 16.4) (19, 24–29). Pectin is an extremely com-plex polysaccharide that can be viewed as a multiblock co-biopolymer. The simplest, and the most abundant, of these blocks is homogalacturonan, an unbranched polymer of (1→4)-α-d-GalpA residues that are partly methyl-esterified and sometimes partly acetyl-esterified. A second major block, rhamnogalacturonan I, is mainly composed of a repeating disaccharide unit (→2)-α-l-Rhap-(1→4)-α-d-GalpA-(1→)n decorated with arabinan and (arabino)-galactan side-chains. Assemblies of RG, arabinan, and (arabino)-galactan are often referred to as pectic “hairy” regions in which arabinan and (arabino)-galactan are the “hairs.” A fourth minor block, rhamnogalac-turonan II, is a highly complex molecule made of a short homogalacturonan backbone with four conserved side chains consisting of 12 different mono-saccharides. Sugar beet pectins have distinctive features, notably low aver-age molar mass, high acetic acid contents, and presence of phenolic esters on their side chains. They also contain a high proportion of hairy regions, with very high Ara contents. Oosterveld et al. (29) reported that approximately 70% of the pectin in sugar beet pulp consists of hairy regions.

Backbone

Controlled acid hydrolysis of beet pectins (30) led to isolation of almost pure homogalacturonans. The degree of polymerization of sugar beet homoga-lacturonans is only slightly lower (70–100) than that of citrus or apple homogalacturonans (100–120). The Rha residues are concentrated in rham-nogalacturonans I, where they alternate with the GalA residues (31, 32). Beet pectins, with a Rha:GalA ratio > 1:10 in the cell wall, are particularly rich in Rha (Table 16.2). About 40% of the Rha residues are further substituted at position 4 by neutral sugars, mainly arabinan, side chains. Rhamnogalactur-onan II, a small complex pectic polysaccharide, and its boron-cross-linked dimer, can be isolated from beet after enzymatic digestion (33).


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366 Fiber Ingredients: Food A

pplications and Health Benefits

Table 16.4

Extraction Conditions and Characteristics of Sugar Beet Pectins

Extraction ConditionsYield (%)

GalA Rha (%)

Ara Gal DM DAc FeA (%)

[] (mL/g)

Ref.

single extraction

Buffer pH 4.5 20°C 5.6 51.3 1.3 10.1 5.1 63 32 — 187 24Buffer pH 6.5 80°C 28.9 45.6 1.9 16.4 5.5 52 34 — 70 24CDTA pH 4.5 20°C 7.1 48.4 1.1 8.2 4.6 52 27 — 257 24CDTA pH 6.5 80°C 27.5 48.4 1.6 14.3 4.8 55 35 — 100 24EDTA 2% 85°C 13.6 55.2 1.9 33.7 7.3 — — — — 25HCl pH 3.0 75°C 2.5 44.5 1.1 11.4 3.3 94 39 0.26 454 19HCl pH 1.0 75°C 28.0 29.5 2.8 29.4 6.8 34 37 0.80 342 19HCl pH 3.0 95°C 5.6 44.5 1.4 15.9 2.7 83 36 0.35 351 19HCl pH 1.0 95°C 35.0 45.5 4.1 3.1 8.5 65 28 0.60 304 19NaOH 2% 45°C 19.9 42.4 1.9 8.1 3.7 — — — — 26NaOH 0.05M 4°C — 58.9 2.6 20.1 5.5 16 19 — — 27

sequential extraction scheme

Water 20°C 2.2 54.4 0.9 8.4 6.5 76 31 0.10 259 28NH4 oxalate 1% 20°C 0.5 77.9 0.9 1.9 2.4 60 15 0.04 57HCl 0.05M 85°C 19.9 65.1 2.3 10.0 5.9 62 35 0.48 225NaOH 0.05M 4°C 11.1 54.9 3.2 12.5 8.1 8 4 0.57 181

Water 20°C 6.4 7.6 0.3 7.3 4.0 41 41 0 — 29Autoclave pH 5.2 121°C 17.8 39.9 2.1 26.7 3.9 70 48 0.61 —


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Side Chains

In beet pectins, the side chains are composed of Ara and Gal; other sugars (Xyl, Glc, Man) are present in negligible amounts (19, 28, 34, 35).

Methylation analysis shows a predominant presence of arabinans with a backbone of linked α-(1→5)-Araf residues carrying ramifications pre-dominantly on O-3. Oosterveld et al. (29) used an alkali and a combined autoclave and alkali extraction of sugar beet pulp to extract arabinans. A degree of polymerization of 130 to 170 residues was calculated for those arabinans (36). Methylation analysis and enzymatic degradation using an α-arabinofuranosidase, showed that sugar beet arabinans have a backbone of 60 to 70 residues and that more than 45% to 65% of the Ara residues are present as single unit or oligomeric side group of the arabinan main chain (29, 36).

The Gal residues are mostly present as type I galactans, linear chains of β-(1→4)-linked Galp residues, but the partially methylated derivatives also indicate the presence of type II galactans (29, 34). Sugar beet type I galactans are most likely almost linear and of low degree of polymerization (34).

NMR analysis of the sugar beet pectin supports the evidence of methy-lation analysis with presence of α-(1→5)-linked Araf residues and β-(1→4)-linked Galp residues (37).

Non-Sugar Substituents

In sugar beet, pectin’s backbone carries both methyl esters (on the carboxylic group) and acetyl esters on the secondary alcohols. Sugar beet pectins are not very highly methylated, having a degree of methylation of about 50 to 60 (Table 16.4). The degree of acetylation of the extracted beet pectins is generally 20 to 30 (Table 16.4). Several studies about the exact location of acetyl groups on pectins have been carried out. Comparison of pectic fragments isolated after enzymatic hydrolysis of various tissues from different plant species suggests a high diversity in the degree, distribution among homogalactur-onan and rhamnogalacturonan I, and location of acetyl groups. Keenan et al. (37) presented a 13C NMR study of sugar beet pectin and concluded that both of the available ring positions (O-2 and O-3) of GalA residues can be acetyl esterified. Kouwijzer et al. (38), on the basis of energy calculations, also con-cluded that acetyl groups at both O-2 and O-3 of GalA in the backbone of homogalacturonan and rhamnogalacturonan I are energetically favorable. In sugar beet pectins, around 75% of the acetyl groups appear to be attached to homogalacturonan (39). Only 10% of the GalA residues are present in the rhamnogalacturonan I region (30, 39, 40) so that rhamnogalacturonan I, which carries only 25% of the acetyl groups, is finally very highly acetylated (DAc ~ 60) (39). No methyl esterification was detected on sugar beet rham-nogalacturonan I (39), in agreement with studies on other plant species (41, 42). In sugar beet homogalacturonan, it was shown by mass spectrometry that (a) O-2 and O-3 acetylation are present in roughly similar amounts, (b) 2,3-di-O acetylation is absent, and (c) GalA residues that are at once O-acetyl


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and methyl esterified are rare so that unsubstituted GalA residues are pres-ent in limited amounts (~10%) (39).

Among dicotyledons, in species of the family Amaranthaceae, pectins carry phenolic acids (Table 16.4) (43). These include mainly ferulic acid, which represents about 0.8% of the beet cell walls, and to a lesser extent p-coumaric acid (28). In beet and spinach cell walls, ferulic acid mainly esterifies neutral sugars (Ara and Gal) of pectic side chains (28, 34, 44, 45). More precisely, ferulates are linked for about 50% to 60% to the O-2 position of Ara moieties and for 40% to 50% to the O-6 position of Gal residues (46–48). Structural analysis of longer oligosaccharides (up to DP 8) showed that the feruloyl groups are mainly linked to Ara residues of the core chain of arabinans and to Gal residues of the core chain of type I galactans (47). Recently, minor amounts of ferulic acid linked to O-5 of the Ara residues of the main core of arabinan chains were detected, indicating a potential peripheral location of some ferulic acid on pectic hairy regions (49). Feruloyl esters are not ran-domly distributed among the different pectic polysaccharides in the sugar beet cell wall (50).

Phenolic acids are bifunctional and thus a potential cross-linking element in beet cell walls (51). Indications in favor of that role are the presence of dehydrodimers of ferulic acid in sugar beet pulp (52–57) and the possibility of cross-linking extracted beet pectins in vitro by oxidation of their feruloyl groups (53, 58–62).

Distribution of Pectic Structural Elements

After degradation of partly demethylated sugar beet pectin with polygalac-turonase (39, 40, 63), most of the GalA (~90% of the GalA initially present in pectin) is recovered as oligogalacturonates of low degree of polymeriza-tion arising from homogalacturonans. The remaining GalA is recovered in a high molar mass fraction corresponding to hairy regions and composed mostly of neutral sugars, notably Ara, Gal, and Rha.

Distribution of arabinans and galactans in the hairy regions has been stud-ied by degradation with dilute acids (34) or specific enzymes (64, 65). Diges-tion by a mixture of endo-arabinase and arabinofuranosidase can lead to complete separation of the Ara while the Gal is retained with the rhamnoga-lacturonan I. These results indicate that galactan chains are directly linked to the backbone while arabinans might be connected through an interposed Gal unit or short galactan chain (64, 65).

Extraction and Molar Mass

Sugar beet cell walls contain a very low amount of readily extractable pectin (by water, buffer, or chelating agents at room temperature) even prior to the diffusion step. Though calcium is present in sugar beet in amounts sufficient to neutralize most of the non-methylated GalA (Fares et al., unpublished results), calcium cross-links do not seem to be the main mechanism holding the pectins in the beet cell wall.


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Efficient extraction can be obtained either by heating or by alkaline treat-ments (i.e., demands a degradation of the pectin). Autoclaving as well as heating at pH circa 6.5 (either with buffer, EDTA, or CDTA) leads to degrada-tion of the pectic backbone through β-elimination and therefore to extrac-tion. This causes the presence in the extract of two populations, namely a high molar mass neutral sugars-rich fraction (analogous to the hairy regions obtained after enzymatic degradation) and a lower molar mass fraction, almost exclusively composed of GalA (24, 29, 34, 63).

Hot acid treatments, comparable to those used for industrial extraction of pectins, have been studied using an experimental design (19). The type of acid used (HCl or HNO3) had no effect on the characteristics of extracted pectins. pH was shown to be the main parameter influencing extraction yield. At pH 1, degradation of the arabinan side chains took place. Depend-ing on the extraction conditions used, intrinsic viscosity of the acid-extracted pectins varied from 172 to 493 mL/g and weight-average molar masses from 70 to 355 kDa (19).

Hemicelluloses

Hemicelluloses can be defined as cell wall polysaccharides that have the capacity to bind strongly to cellulose microfibrils by hydrogen bonds (66). The common structural features of hemicelluloses are a main chain with a structural resemblance to cellulose and either short side chains that result in a pipe-cleaner-shaped molecule or a different sugar interpolated in the main chain, both modifications preventing further aggregation (67). In the cell walls of land plants, three classes of polymers correspond to that defini-tion, namely xyloglucans, heteroxylans, and mannans. In the primary cell wall of dicotyledons, the main hemicellulose is usually xyloglucan, which accounts for 15% to 20% of the dry weight of the wall.

Beet cell walls have very low concentrations of the sugars that denote hemicelluloses (i.e., Xyl, Man, non-cellulosic Glc and Fuc; Table 16.3), and their hemicelluloses have been very little studied. Oosterveld (68) isolated from a 4 M NaOH extract from beet a fraction enriched in hemicelluloses, and methylation analysis of this material indicated presence of xyloglucans and mannans. Degradation by a purified endo-glucanase of this fraction allowed identification of xyloglucan oligomers, which confirmed presence, though in very low amounts, of a standard fucogalactoxyloglucan in beet cell walls. Fares et al. (69) identified fucogalactoxyloglucans and xylans in alkali extracts from sugar beet AIS.

Cellulose

Cellulose is the world’s most abundant naturally occurring polymer, rivalled only by chitin. Cellulose is a homopolymer of (1→4)-β-d-Glcp. The β-1,4 configuration results in a rigid and linear structure for cellulose. Cellulose chains exhibit a strong tendency to form intra- and intermolecular hydrogen


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bonds resulting in the formation of microfibrils whose length, width, and crystallinity differ much depending on the cellulose origin. Cellulose arising from primary cell walls are particularly thin (2 to 3 nm width) and of low crystallinity. This has been confirmed by solid-state NMR for beet cellulose (70). Following the initial work of Weibel (71, 72), Weibel and Myers (73), and Dinand et al. (13, 74) purified and evaluated the application potential of sugar beet cellulose.

Sugar beet Fiber Physicochemical Properties

The expression “physicochemical properties” is a generic term, involving structural parameters such as particle size and shape, surface properties and porosity, as well as functional properties such as hydration and cation-exchange properties of cell wall materials (75). For sugar beet fiber, some of those physicochemical properties have been studied in relation to the dietary fiber hypothesis.

Hydration Properties

Hydration capacities partly determine the fate of dietary fiber in the diges-tive tract (fermentation induction) and account for some of their physiological effects (fecal bulking of lowly fermented fiber) (76). Basically, three different parameters were defined (77): (1) swelling, “the volume occupied by a known weight of fiber under the condition used”; (2) water retention capacity (WRC), “the amount of water retained by a known weight of fiber under the condi-tion used”; and (3) water absorption (WA), “the kinetics of water movement under defined conditions.”

Beet fiber, as most of the fibers arising from dicotyledons primary cell walls, exhibits high hydration capacities, in particular compared to fibers from cereal brans. Those hydration properties fluctuate much depending on the fiber preparation and also on the conditions of measurement (Table 16.5) (9, 78–83). The major intrinsic factors affecting hydration properties are particle size and drying conditions. Drying at high temperature results in a decrease of the hydration capacities, as does a decrease in particle size (Table 16.5). Thermal or thermo-mechanical treatments increase the amount of soluble fiber in beet pulp and modify its hydration properties (Table 16.5). In addition, the measured hydration capacities are sensitive to extrinsic fac-tors, such as the ionic strength of the hydrating solution (Table 16.5) and its ion composition. These effects are mostly visible after conversion to the H+ or Na+ form, or after saponification. Beet pulp then appears to behave as a polyelectrolyte resin. The presence of divalent cations results in a decrease in hydration capacities of deesterified beet pulp (78). A number of these effects might be masked in native beet pulp by the presence of a high calcium con-centration. The conditions of hydration also play a role: The presence of shear


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forces in the form of intense stirring can lead to a destructuration of the beet fiber and an increase in apparent WHC (Table 16.5). This sensitivity to the exact method and conditions of measurement explains the variability of the results.

Table 16.5

Hydration Properties of Different Sugar Beet Fiber Preparations

Swelling (mL/g) WA (mL/g) WRC (mL/g) Ref.

Beet Pulp (fiber #) in Water

Native 11.0 — 26.6 7811.5 # — 26.5 # 79

H+-form 25.0 — 22.5 7817.8 — 23.9 80

Na+-form 32.0 — — 8032.6 # — — 79

Beet Pulp in Presence of supporting salts

Native 10.0 — — 78H+-form 13.4 — 16.0

80Na+-form 15.3 — — 80

saponified Beet Pulp in Water

Native 25.0 — 24.8 78H+-form 20.0 — 20.7 78

21.9 — 18.3 81Na+-form 32.4 — — 81

Beet Fiber in Water

Φ 540 µm 21.5 8.5 24.2a 8212.6b

Φ 385 µm 21.4 8.8 22.6a 8212.0b

Φ 205 µm 15.9 7.3 19.2a 82 9.2b

Thermomechanically Treated Pulp

Extruded beet pulp 14.4 (native 19.3) — 28.2 (native 32.9) 83Autoclaved beet pulp at 122°C 20.0 (native 23.0) — 35.0 (native 34.0) 9 at 136°C 21.0 (native 23.0) —– 38.4 (native 34.0) 9

a Long incubation, heavy stirring.b Short incubation, gentle stirring.


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Adsorption/Binding of Ions and Organic Molecules

Sugar beet fibers behave as weak monofunctional cation-exchange resins with a cation-exchange capacity (CEC) of about 0.5 meq/g. This ion-bind-ing capacity is due to the presence of non-methylesterified GalA residues, and the CEC is equal to the concentration of non-methylated GalA residues calculated from independent GalA and methyl groups measurements (79, 80). Beet fibers are devoid of phytic acid, the main ion-binding species in cereal fibers. In spite of the presence of acetyl groups, pectin in sugar beet fiber is able to bind divalent cations, with higher affinities than in solution (18, 80) but with the same selectivity scale: Cu ~ Pb >> Zn ~ Cd > Ni > Ca.

The ability of uronic-acid- and/or phenolic-compounds-rich fibers to inter-act with bile acids in the small intestine has been suggested to explain their hypocholesterolemic effects. Bile acid adsorption to fibers would result in a lower re-absorption, in an increased transport toward the large intestine and, finally, in a higher excretion of bile acids (84). Recent in vitro studies showed that freeze-dried beet, sugar beet pulp, and red sugar beet fiber preparations were able to bind bile acids to a certain extent (~ 10 to 15 μmol bile acid/g of dry matter) (10, 85).

Functionality and Food Applications

extracted Polysaccharides

Pectins from sugar beet do not form gels in the usual conditions (i.e., either with calcium or with high sugar concentrations and acidic conditions) (86, 87). This inability has been ascribed variously to presence of acetyl groups (88), which indeed hinders binding of ions (89), to low molar mass (16, 90) or to excessive amounts of side chains (37). Acetyl groups are the most likely candidates for these weak gelling properties. Several deesterification attempts have been made to improve the gel formation of sugar beet pec-tin: partial deacetylation by mild acid treatments (91), incubation with an enzyme preparation from Aspergillus niger (92), treatment with mixtures of acetyl and methyl esterases from oranges or Aspergillus niger (87, 93), treat-ment with mild acid, alkali, fungus methyl-esterase or plant methyl-esterase (35). All those treatments led to low-ester pectins, which gelled in the pres-ence of Ca2+. However, sugar beet pectin is presently only produced in small amounts for specific applications where it has equal or superior properties compared with apple or citrus pectin. These applications include stabiliza-tion of flavored oil emulsions (94, 95) and stabilization of acidified drinking yogurt (96). As sugar beet pectins may form gels by an oxidative cross-link-ing of ferulic acid (28, 45, 61), the enzymatic gelation of sugar beet pectins in food products was studied (97). Oxidative gelation of sugar beet pectins gives a thermo-irreversible gel that is of great interest for the food industry


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as the product can be heated while maintaining a gel structure. With 2% sugar beet pectin added, a gel was formed in luncheon meat using laccase. The cohesive gel was shown to bind the meat pieces together, thereby mak-ing the product sliceable (97).

Arabinans can be extracted from isolated sugar beet pectins or directly from sugar beet pulp. Alkaline extraction at high temperature (70°C to 98°C) for 15 to 90 min followed by neutralization and ultrafiltration yields a branched arabinan (molar mass of about 50 kDa) containing around 80% of l-Ara (98). Branched arabinan exhibit surface-active properties, which make it suitable for use as an emulsifying agent. Additionally, flavor oil and fragrances may be encapsulated using arabinan (98). However, the arabinan extraction and purification cost is a clear limitation for these uses. Branched arabinan can be linearized using purified α-l-arabinofuranosidase to yield debranched arabinan (98). The debranched arabinan forms an aqueous gel, which has the properties of a fat substitute and may be used in foods (4, 98).

Whole Sugar beet Fiber

Sugar beet fiber is claimed to offer nutritional benefits to consumers as well as manufacturing and functional advantages to food processors. Moisture retention, good texture, and mouthfeel are the main technical properties of the beet fibers (Fibrex®), which are proposed with a variety of particle sizes (from < 32 μm to flake; Figure 16.1) for easy blending with other ingredients. The particle size is important for applications because the ability to bind water may be affected (Table 16.5) (82) and because it may influence the tex-ture of the product and the mouthfeel properties (99). The beet fiber also has the advantage of containing no phytic acid (a substance that may be found in cereal fiber and can tightly bind minerals) and no gluten (6).

Potential applications include cereals, bakery products, pasta, processed meats, soups, and snacks. Fibrex® total volume sales are divided as follows: 55% bakery customers, 30% meat applications, and 15% health. Successful recipes have been proposed for pastries, cakes, biscuits, snack foods, pasta, and meat products. It can be used in breads as a natural improver and to maintain freshness. In biscuits, it increases the fiber content and in meat products, it may provide chewy and juicy character.

Ready-to-Eat Breakfast Cereals

The properties of sugar beet fiber make it a good candidate for fiber enrich-ment in high-fiber ready-to-eat cereals applications (99). It has been incor-porated into extruded ready-to-eat cereals at high quantities (up to 40%) without affecting the mouthfeel, flavor, or color. This property can probably be ascribed to the high water-binding properties of beet fiber. Non-milled versions of the fibers or flaked versions are used in rather high amounts (up to 25%) in muesli products.


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Bakery Products

Fiber-enriched breads have a large commercial success, and diverse fibers can be successfully incorporated into a large variety of bakery products, as a bulking agent and as a dietary fiber source. Cereal bran is generally used to increase the amount of dietary fiber content in breads but this addi-tion influences the color, the taste, as well as the texture/consistency of the product. In comparison with cereal bran, sugar beet fibers are characterized by: (a) low phytate, which is of particular concern to nutritionists because of its possible adverse effects on mineral absorption (100); and (b) better water binding and retention capacity, which is of particular interest for the baking industry (101). Thereby, several research articles deal with the effect of sugar beet fibers onto yield of dough, dough mixing properties, yield of bread, bread volume, and crumb quality (11, 102–105). Up to 15% of flour replace-ment, beet fiber appears to provide beneficial effects on dough textural pro-file, especially for the prominent and suitable decrease in gumminess, and no significant adverse effects on main mechanical, surface, and extensional properties (105). An enrichment with sugar beet fiber decreases bread vol-ume and crumb quality. In that context, less than 10% of flour replacement by sugar beet fiber is recommended (11). Sugar beet fiber is also claimed to prolong the freshness of bread.

Beet fiber can also be used for the production of soft cookies or muffins for which fibers with a high water-binding capacity are required.

Meat Products

Beet fiber (1% to 3%) may be incorporated into meat loaves, patés, meat prod-ucts, and sausages, to give a juicy character even in frozen products, and to improve the consistency or the texture, and as a fat substitute (99, 106–108).

Physiological Benefits

apparent Fermentability or apparent Digestibility

Apparent fermentability and apparent digestibility were investigated in vitro with fecal inoculate (9, 17, 109–113) or in vivo in rats (114, 115) or in pigs (116–118). All indicated a high fermentability or apparent digestibility of sugar beet fiber, in the range of 70% to 90%. GalA and Ara were virtu-ally completely digested; Glc about 85% to 88%; only Xyl, present in small amount, was of low digestibility. It was shown in vitro that all sugars are not fermented at the same rate; Glc disappearance began more slowly than that of GalA and Ara (9, 17, 109, 110, 112). The tridimensional arrangement of the polymers within the cell wall, and thus the access of bacteria or associ-ated enzymes to the polymers, may account for this difference (17). Process-


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ing of fiber, such as autoclaving or chemical extraction followed by drying, influenced its fermentability (9, 17). Harsh drying conditions following pec-tin extraction induce the distortion and shrinking of cells and a noticeable decrease in the total pore volume, especially in the pore volume accessible to bacteria. As a result, fermentability was reduced (17).

The production of short-chain fatty acid (SCFA) was analyzed in vitro (9, 17, 109, 110, 112, 113, 119) or in vivo. In the latter case, the production was deduced either from measurement of SCFA in feces or cecal digesta of animals (115) or from dynamic analysis of porto arterial differences in the concentration of SCFA and of the portal blood flow rate in pigs (120). The data confirmed the high fermentability of sugar beet fiber, especially when compared to other insoluble fibers (from cereal or legumes). Fermentation profiles, expressed as the molar percent of each of the major SCFA—acetic (C2), propionic (C3), and butyric (C4)—was characterized by a high ratio of C2 (60% to 80%) followed by C3 (11% to 23%) and then C4 (9% to 15%). In pigs, a higher level of C2 was observed compared to humans. This might be explained by the fact that the length and the capacity of the large intestine in pigs are approximately 1.5 to 3 times larger than in humans. In vitro, no alterations in the SCFA profile were observed when modulating the chemical composition and physico-chemical properties of sugar beet fiber (17, 110).

Transit Time and Stool Output

The effect of sugar beet fiber on transit time and stool output was evalu-ated in healthy subjects (121), in patients complaining of chronic constipation (122), and in rats (15, 114, 123, 124). Supplementation with sugar beet fiber increased wet fecal mass and number of daily stool. More diverse were the effects on transit time and dry fecal mass.

Sugar beet fiber (33 g/day) in the diet decreased transit time by 25%, as did the wheat-bran-supplemented diet (121). Both increased the number of daily stool and wet fecal mass. Weight of fecal water but not the dry fecal mass changed, while wheat bran increased both dry weight of fecal mass and fecal water. In rats, the sugar beet diet increased the fecal output, as did the other fiber diets (15, 114, 123, 124). Nyman and Asp (114), Johnson et al. (1990) (123), and Harland (15) reported both wet and dry fecal mass increase. In consti-pated patients, a marked decrease in severe and moderate constipation at both the 15th and 30th day of treatment with sugar beet fiber was found, with a significant increase in fecal frequency normalization (122). Moreover, fecal consistency changed from hard and semi-hard stools to soft ones.

The mechanisms by which fiber influences transit time are still not fully understood. Different mechanisms have been suggested, which depend on the physical properties and fermentability of the fiber (125, 126). The fiber may act by increasing the lumen volume, depending on the amount of indi-gestible residue in the colon, the water-retention capacity of the residue, the stimulation of microbial growth, and the production of gas. The fiber can also reduce transit time through modulating colonic motility either by a mechan-


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ical stimulation of mechanoreceptors by the edges of the fiber particle (127), or by a chemical stimulation by the products of fermentation (125, 128), or by the release of compounds trapped by fiber such as biliary acids or fatty acids (126). In the latter case, these products can stimulate not only colon motility but also secretion. Except for stimulation of mechanoreceptors, the different mechanisms mentioned above could contribute to the effect of sugar beet fiber on transit.

The increase in stool output by dietary fiber intake may have several causes (126). It could be related to the amount of excreted residue and its water-binding capacity. The increase of the bacterial mass can also contrib-ute, since bacteria contain 80% water. Finally, the excreted water could be water not absorbed in the colon because of the short transit time or changes in colonic motility. Again, these different mechanisms can all participate in the increase of stool output.

Minerals adsorption

The effect of sugar beet fiber on the absorption of zinc, iron, copper, calcium, and magnesium was investigated in humans (129–131) and rats (15, 132) and led to the same conclusions. Sugar beet fiber has no negative effect on any of the minerals studied. These studies stressed the fact that beet fiber generally has a relatively high mineral content and can therefore contribute to mineral intake.

Glucose Metabolism

The effects of sugar beet fiber on Glc metabolism were investigated with dif-ferent objectives. The effects on fasting plasma Glc and insulin values and on Glc tolerance of sugar beet fiber intake over a period of several weeks (from 3 to 8) were studied in normal (133), normal but with high fasting cholesterol value (134), or non-insulin-dependent diabetes mellitus (NIDDM) subjects (135, 136). These parameters were regarded together with lipid parameters in order to better understand the mechanisms by which daily intake of dietary fiber can decrease the risks of cardiovascular disease. Experiments were also concerned with Glc tolerance (137–140) in healthy volunteers or pigs and focused on acute effects of fiber supplementation.

No clear effect of a long-term sugar beet fiber supplementation on fasting as well as postprandial blood Glc and insulin levels has been demonstrated (Table 16.6). The source, processing, and physical form of the fiber in the diet but also the nature of the meal (amount of fiber, amount of lipids, sources of carbohydrates, etc.), the metabolic status of the subjects, and the duration of the experiment may explain these differences. Similarly, discrepancies in blood Glc and insulin responses in normal subjects to a single meal with added sugar beet fiber are recorded in the literature (Table 16.7).

No clear mechanism explains the effect of sugar beet fiber on postprandial Glc level. It is well known that soluble high molar mass fiber such as oat or guar gum can significantly decrease the postprandial circulating Glc level


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Table 16.6

Chronic and Postprandial Responses of Plasma Insulin and Glucose in Volunteers Given Sugar Beet Fiber Supplements

Intake (g/day/subject) Subjects Duration Results Ref.

20 Healthy 16 days No changes in blood fasting Glc and insulin concentrations.

133

18 Healthy middle-aged with risk ischemic heart disease

3 weeks No effect on fasting plasma Glc and insulin

Effect on postprandial parameters.

134

8 NIDDM 8 weeks Improvement in Glc response to a standardized breakfast.

135

40 NIDDM 8 weeks Blood Glc and insulin fasting or postprandial levels were not significantly affected.

136

Table 16.7

Postprandial Responses of Plasma Insulin and Glucose in Volunteers Given Sugar Beet Fiber Supplements

Intake (g/meal)

Carbohydrate (g/meal) Subjects Results Ref.

20 86 Healthy male human volunteers

No difference in the mean blood and plasma insulin curves at any time between the control and fiber diets.

137

10 100 Healthy male human volunteers

An improved Glc tolerance; no change in insulin level; no decrease in postprandial insulin.

138

7 51 (liquid formula)

Healthy male human volunteers

Lower postprandial blood Glc and serum insulin response compared with formula without fiber.

140

56 653 Pigs No effect on postprandial glycemic and insulinemic values.

139

114 446 Pigs No difference in Glc absorption between sugar beet fiber and wheat bran supplemented diets.

120


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by slowing the gastric emptying and/or influencing the diffusion and mix-ing of the intestinal contents. Sugar beet fiber is only partly soluble and it is unlikely that the soluble fiber fraction can induce a sufficient increase of the viscosity of digesta to delay starch digestion or absorption, especially in the case of a solid meal. Another mechanism suggested is by changing transit time, but, again, results in the literature are discordant. Morgan et al. (138) observed a slightly accelerated liquid gastric emptying with both sugar beet fiber and guar gum supplementation, which was unexpected. Hamberg et al. (141) and Cherbut et al. (121) found, respectively, a decreased and an increased mouth-to-cecum transit time in subjects fed with sugar beet fiber.

lipid Metabolism

Sugar beet fiber, because of its significant content in water-soluble fiber, has been investigated for its effects on lipid metabolism. Studies were carried out in humans either healthy (133, 134, 142) or hypercholesterolemic (143) or with NIDMM (135, 136, 144) and in animals, pigs (145, 146) or rats (123, 147–153). Despite the fact that the dietary pattern (daily intake of dietary fiber, high-fat, low-carbohydrate diet and vice versa) and the duration of the experiments (from two to eight weeks) differed between the studies, most concluded that sugar beet fiber is hypocholesterolemic (Tables 16.8 and 16.9). In humans, it tends to reduce serum total cholesterol, and apo B levels without altering or even slightly increasing the high-density lipoprotein (HDL) cholesterol. Only some studies reported a decrease in serum triglycerides (136, 144, 147, 149).

The mechanisms sustaining such effects are still not clear (154). Dietary fiber may act as hypocholesterolemic resin, which sequesters bile acids and cholesterol, with consequent interruption of the enterohepatic bile acid cycle in the small intestine (intestinal reabsorption of bile salts in humans is 96% to 98% efficient) and loss of cholesterol from increased fecal bile acid excre-tion. This mechanism was clearly demonstrated for viscous fiber such as guar gum and oat gum. In case of sugar beet fiber, most of the studies did not find a significant increase in fecal (124, 142, 149) and ileal (155) excretion of bile acids. These results are in agreement with those from Morgan et al. (156), who did not observe changes in concentrations of circulating postpran-dial bile acids in humans given an acute test meal supplemented with sugar beet fiber (10 g Betafiber per meal), contrary to guar gum or cholestyramine. In vitro data are more controversial. Morgan et al. (156) showed that the insol-uble fraction of sugar beet fiber bound only small quantity of glycocholate and that no bile acids were associated with the soluble fraction. Dongowsky (10) found that cell wall material prepared from sugar beet pulp can be effec-tive in binding bile acids (around 15 µmole/g of alcohol-insoluble material at pH 5). In a study with ileostomists (155) a decrease of 26% of ileal bile acid excretion was noted while cholesterol excretion increased by 52% with the sugar beet fiber diet. The excreted amount of cholesterol corresponded to half of the mean daily intake of cholesterol in this experiment. This pattern is


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Table 16.8

Effect of Sugar Beet Fiber on Lipid Metabolism (Human Studies)

Intake (g/day/subject) Subjects Duration Results Ref

30 Hypercholester-olemic women

2-4 weeks Significant reduction of LDL cholesterol with no change in HDL.

143

8 NIDDM 8 weeks Lower fasting blood Glc; reduction of LDL cholesterol with no change in HDL; lower fasting levels of triglycerides; improvements in Glc response to standardized breakfast.

135

40 NIDDM 8 weeks Decrease of 8% in total cholesterol when compared with the habitual diet, but no decrease compared with the low-fiber diet.

136

18 NIDDM 6 weeks Decrease of 6.2, 10.6, and 6.0% in, respectively, total cholesterol, triglycerides, and Apo B levels.

144

30 Healthy volunteers

3 weeks Decrease of 12 and 15% in total and LDL cholesterol; small changes in HDL; significant decrease in serum triglycerides.

142

20 Healthy volunteers

16 days Decrease of 4.6% in total cholesterol; decrease more marked with subject with a high habitual fat intake.

133

1 Healthy middle-aged volunteers

3 weeks Decrease of 8 and 9.6% in total and LDL cholesterol in subjects in whom fasting plasma cholesterol was above normal; no difference in HDL cholesterol.

134


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different from the pattern generally reported for water-soluble fiber such as oat, guar gums, or pectins. The cholesterol-lowering effect of sugar beet fiber may result from its interference with the lipid absorption through alteration of the digestive processes. The reduced absorption of cholesterol results in a reduced supply to the liver, which, as a second effect, could decrease excre-tion of bile acids, as they are synthesized from cholesterol in the liver (155).

Table 16.9

Effect of Sugar Beet Fiber on Lipid Metabolism (Animal Studies)

Level of Incorporation

(g/kg diet) Animals Duration Results Ref.

100 g/kg semi- synthetic diet

Rats 28 days Significant reduction of serum cholesterol, but less than that of guar gum.

123

300 g/kg fructose base diet

Rats 3 weeks Decrease in plasma triglyceride and cholesterol concentration in the postprandial as well as the post-absorptive period.

147

100 g/kg semi-synthetic diet

Rats 28 days Depress of the liver triglyceride level in concert with decreased liver lipogenesis; no change in liver cholesterol; animal less fat.

149

150 g/kg cholesterol free diet

Rats 14 days Lower circulating cholesterol, hepatic cholesterol, and circulating triacylglycerol; no change in total hepatic lipid concentrations and hepatic adipose tissue lipogenesis; reduced expression of hepatic lipoprotein A-1gene.

150

100 g/kg 25% casein diet

Rats 28 days Lower plasma total cholesterol; lower HDL cholesterol.

148

120 g/kg semi-synthetic diet

Weaning piglets

4 weeks No change in serum cholesterol and HDL cholesterol concentrations; lower fasting triacylglycerol due to reduction in VLDL synthesis.

145

100 g/kg semi- synthetic diet ±0.3% cholesterol

Rats 40 days Lower plasma total cholesterol, LDL and triglycerides; decrease in HDL phospholipids and total phospholipids in cholesterol group.

Diet free of cholesterol, no effect on measured parameters.

153

100-220 g/kg diet

Growing pigs

Fattening period

Gradual increase in fiber content caused a linear decrease in total cholesterol and cholesterol fractions in blood serums; decrease in adipose tissue cholesterol.

146


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The influence of sugar beet fiber on lipid absorption may account at least for the acute postprandial effect of dietary fiber on lipemia, but the mechanisms involved have not been explored. Moreover, the extent to which the repeti-tion of the single meal effect can lead to a new metabolic steady state in the long run remains to be further investigated. In rats fed with sugar beet fiber, hypocholesterolemia was accompanied by a reduction in hepatic cholesterol and in circulating triacylglycerol and bile acids, with no increase in bile acid fecal excretion (149). The authors pointed to another possible mechanism involving disruption of the bile acid circulation, possibly via changes in the rate of absorption patterns of triacylglycerol and its subsequent handling by circulating lipoproteins.

Other mechanisms of action of dietary fiber have been suggested. Modifica-tion in hormonal status, especially insulin, could influence lipoprotein lipase activity, cholesterol, and bile acid synthesis and very low-density lipoprotein (VLDL) secretion. Only few groups (133–136) have investigated the effects of sugar beet fiber on both gastrointestinal hormones and cholesterol. Most of the authors reported no significant changes in the fasting levels of insulin.

It has been suggested that the hypocholesterolemic effect of dietary fiber might also be mediated through the fermentation products, which can modify the activity of regulatory enzymes involved in hepatic cholesterol synthesis. A study in rats (148) has demonstrated that an intact cecum and colon is necessary for the fiber to be effective. One of the SCFA, propionate, has been shown in pigs and rats to significantly lower plasma and liver cho-lesterol concentrations and to inhibit cholesterol synthesis in isolated rat hepatocytes. However, no such effect has been reported in humans, and the role of propionate in reducing low-density lipoprotein (LDL) cholesterol levels is controversial. Hara et al. (151) showed that plasma cholesterol level decreased following ingestion of SCFA mixture simulating cecal fermen-tation products of sugar beet fiber. They further investigated mechanisms involved in the cholesterol-lowering effects of SCFA by feeding rats either with SCFA or sugar beet diet (152). They concluded that SCFA can decrease the hepatic cholesterol synthesis rate, which probably contributes to the low-ering of plasma cholesterol level, as observed in rats fed with sugar beet fiber. It seems therefore likely that the cholesterol-lowering effect of sugar beet fiber is not dependent on increased fecal bile acid and is affected by a number of factors rather than a single mechanism.

Colorectal cancer

The effect of sugar beet fiber on experimentally induced colorectal cancer was mainly studied in rats (157–162). Results have been equivocal. In three studies, beet fiber reduced the incidence of precancerous lesions, aberrant crypt foci (159, 161, 162). In contrast, Thorup et al. (157, 158) reported no pro-tective effect of sugar beet fiber at any stage of the colorectal carcinogenesis process.


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Although the contribution of dietary fiber to cancer protection is not very clear, several mechanisms by which they can be protective have been sug-gested. Sugar beet fiber may reduce the risk for colon carcinogenesis through enhancement of defecation and dilution of carcinogens. They can exert a pro-tective role through decreasing the concentration of fecal bile acid. Through acidification of colonic content via fermentation, sugar beet fiber can prevent the conversion of primary bile acids to secondary bile acids, lithocholic acid and deoxycholic acid, which are considered promoters of colon cancer. Gal-laher et al. (124) showed that sugar beet fiber slightly increased the total bile acid daily excretion but the fecal bile acid concentration was much lower than with the fiber-free basal diet. This concentration was even lower than with oat or rye bran diets. When compared to other sources of fiber, sugar beet fiber produced the lowest concentration of lithocholic acid.

Some fibers can prevent oxidative damage to important molecules such as DNA, membrane lipids, and proteins. The mechanisms may include quenching free radical, chelating transition metal, or stimulating antioxida-tive enzyme systems. Antioxidant properties of sugar beet fiber were inves-tigated in pigs (lipid peroxidase) (153) and in rats (liver antioxidant enzymes and serum enzymes) (163). Both studies concluded that sugar beet fiber has no protective role against oxidation.

Sugar beet fiber significantly increased the concentration of many organic acids, especially acetate and propionate as well as butyrate. Ishizuka and Kasai (159) suggested that butyrate produced by sugar beet fiber fermenta-tion may account for the decrease of aberrant crypt foci in 1,2 dimethylhy-drazine induced aberrant crypt foci rats. Butyrate has diverse and apparent paradoxical effect on cellular proliferation, apoptosis, and differentiation. It is the primary energy source for colonic epithelium, and in an environment deficient in alternative substrate, it will paradoxically promote cell prolif-eration and growth and inhibit cell death. There is also some evidence that, delivered in adequate amount in the appropriate site, butyrate will protect against early colorectal carcinogenesis process (164). The mucosal epithelium has a characteristic immune system and intraepithelial lymphocytes play a role in the initial immune action against exogenous antigens. The immune response to a tumor is thought to be an early event leading to its destruction before it becomes clinically apparent (165). Ingestion of sugar beet fiber in luminal content was shown to promote an accumulation of CD8+ intraepi-thelial lymphocytes that participate in the elimination of abnormal epithelial cells after initiation (162). Thus, the protective effect of sugar beet fiber on col-orectal carcinogenesis may be related to its capacity to stimulate the immune surveillance in the colorectal mucosa. SCFA are candidates as the mediators of this property (166).

Tolerance to Sugar beet Fiber

In human studies, the daily intake varied greatly, from 7 to 40 g per subject. Generally, the fiber intake was gradually increased, in particular when large


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doses were concerned. The form under which fiber was ingested also dif-fered: it can be included in foods (prepared dishes, bread, biscuits, chocolate bars), pressed in tablets, or mixed as a powder in water. Generally toler-ance was good. Only afew studies reported cases of discomfort, abdominal cramping, and bloating or trouble with flatulence or borborygmi. This gener-ally occurred with the largest doses. One study (144) mentioned that subjects (five of seven) found the bread and biscuits supplemented with sugar beet fiber less palatable than normal products, which led to a reduction in compli-ance during the last two of six weeks of sugar beet fiber supplementation.

Three studies (133, 143, 144) reported an increase in energy and mean daily fat intakes during the period of sugar beet fiber supplementation. In these studies, fiber was incorporated into bread and it was suspected that the increase arose from an increased use of high-fat spread. However, no changes in subject body weight were noticed.

In a subacute feeding study of male rats, sugar beet fiber at levels up to 10% was well tolerated by the animal (167). There were no reductions in food consumption and no reductions in body weight.

Safety/Toxicity

Potential toxic effects of sugar beet fiber supplementation have not been extensively investigated (124, 167). Dongowski et al. (167) showed in rats that the enrichment of the diet with a sugar beet fiber preparation up to a level of 10% for four weeks did not substantially influence urinary, hematological, and serum parameters indicative of a toxic effect.

Conclusion

On a wet weight basis (~ 90% humidity), 120 million tons of beet pulp are produced in the world each year and many laboratories are involved in find-ing new end uses to beet fiber. Beet fiber has thereby been extensively stud-ied and has been used as a standard fiber in many functional and nutritional studies. Beet fiber has a high natural concentration of dietary fibers (~ 70%) with a particularly high soluble fiber content (~ one-third) due to its high pectin content. It exhibits a high water-holding capacity, which provides a broad application area.


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1995. 3. Micard, V., Renard, C.M.G.C. and Thibault, J.-F., Enzymic saccharification of

sugar beet pulp, Enzyme Microbial Technol., 19, 162, 1996. 4. Cooper, J.M. et al., Preparation, physical properties and potential foods applica-

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