ARTICLE IN PRESS

0733-5210/$ - se

doi:10.1016/j.jc

Abbreviations

oligosaccharides

G 0, storage (or

time (h); HDL-c

cholesterol; Mw

UDP-Glc, uridi

mg); Z, apparenshear’ specific v�CorrespondE-mail addr

Journal of Cereal Science 46 (2007) 101–118

www.elsevier.com/locate/jcs

Molecular aspects of cereal b-glucan functionality: Physical properties,technological applications and physiological effects

A. Lazaridou�, C.G. Biliaderis

Laboratory of Food Chemistry & Biochemistry, Department of Food Science and Technology, School of Agriculture, Aristotle University,

P.O. Box 256, Thessaloniki 541 24, Greece

Received 2 February 2007; received in revised form 7 May 2007; accepted 8 May 2007

Abstract

Cereals b-glucans are linear homopolysaccharides of consecutively linked (1-4)-b-D-glucosyl residues (i.e. oligomeric cellulose

segments) that are separated by single (1-3)-linkages. b-Glucans display all the functional properties of viscous and gel forming food

hydrocolloids combined with all the physiological properties of dietary fibres. This review focuses on the relationships between the

molecular–structural characteristics of b-glucans and their physicochemical properties in aqueous dispersions and in food systems as well

as their physiological functions in the gastro-intestinal tract. The physical properties of b-glucans, such as solubility and rheological

behaviour in the solution and gel states, are controlled by their molecular features, such as their distribution of cellulosic oligomers, their

linkage pattern and their molecular weight as well as by temperature and concentration. The technological and nutritional functionality

of b-glucans is often related to their rheological behaviour. Incorporation of b-glucans into various products (bread, muffins, pasta,

noodles, salad dressings, beverages, soups, reduced-fat dairy and meat products) showed that attributes, such as breadmaking

performance, water binding and emulsion stabilising capacity, thickening ability, texture, and appearance appear to be related to the

concentration, molecular weight and structure of the polysaccharide. The health benefits of b-glucans, such as reducing blood serum

cholesterol and regulating blood glucose levels, are also correlated with the amount and molecular weight of the solubilised b-glucans inthe gastro-intestinal tract.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Cereal b-glucans; Structure; Molecular weight; Function; Rheology; Gels; Viscoelasticity; Applications; Physiological effects

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

2. Structural features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

3. Physical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

3.1. Solubility—solution behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

3.2. Aggregation phenomena—gelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

e front matter r 2007 Elsevier Ltd. All rights reserved.

s.2007.05.003

: DP, degree of polymerisation; DP 3, 3-O-b-cellobiosyl-D-glucose; DP 4, 3-O-b-cellotriosyl-D-glucose; DPX5, cellodextrin-like

with more than three consecutive 4-O-linked glucose residues; DSC, differential scanning calorimetry; FI, fluidity index; f, frequency (Hz);

elastic) modulus (Pa); G 00, loss modulus (Pa); GI, glycemic index; Glcp, D-glucopyranosyl; G 0max, pseudo-plateau G 0 value (Pa); Gt, gelation

holesterol, high-density-lipoprotein-cholesterol; IE, elasticity increment (h�1); IL-1b, interleukin 1b; LDL-cholesterol, low-density-lipoprotein-

, molecular weight; o/w, oil-in-water emulsion; PBGR, peak blood glucose rise; TBG, total b-glucan content; tand, loss tangent ( ¼ G 00/G 0);

ne diphosphoglucose; _g, shear rate (s�1); _g1=2, shear rate at which Z ¼ Z0/2; DG, peak blood glucose level; DH, apparent melting enthalpy (mJ/

t viscosity (Pa s); Z*, complex viscosity (Pa s); [Z], intrinsic or limiting viscosity (dl/g); Z0, viscosity at the Newtonian zone (Pa s); (Zsp)0, ‘zeroiscosity

ing author. Tel.: +302310 991716; fax: +30 2310 991797.

ess: athlazar@agro.auth.gr (A. Lazaridou).

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118102

3.2.1. Dynamic rheometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

3.2.2. Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

3.2.3. Uniaxial compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

4. Functional properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4.1. Technological aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4.2. Physiological responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.2.1. Effect of viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.2.2. Effects of the food matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

4.2.3. Fate of b-glucan in the gastro-intestinal tract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

4.2.4. Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

1. Introduction

Over the last two decades the acceptance of b-glucans asfunctional, bioactive ingredients has increased the popu-larity and consumption of cereal-based foods as well as ofmany other foods fortified with cell wall-enriched grainfractions, b-glucan concentrates and isolates. In thisrespect, fractions rich in b-glucan have been obtainedfrom cereal grains by dry milling (pin or roller), sieving,and air classification processes (Izydorczyk et al., 2003;Knuckles and Chiu, 1995; Vasanthan and Bhatty, 1995) orwet milling, sieving, and solvent-extraction protocols usingdifferent solvent systems (Beer et al., 1996; Bhatty, 1995;Cavallero et al., 2002; Jaskari et al., 1995; Oste Trianta-fyllou, 2000; Wood et al., 1989). These approaches result inconcentrates or isolates containing approximately 8–30%b-glucans for the former and up to 95% for the latter. Theappearance of such ingredients in the market has recentlystimulated the research and development of many novelfood products rich in cereal b-glucans. From a nutritionaland a functional viewpoint, such foods fit into thedescription of ‘functional foods’ as they provide some ofthe normal quality attributes of a food, such as mouthfeeland texture, as well as conferring specific health benefits.

b-Glucans are major components of starchy endospermand aleurone cell walls of commercially important cereals,such as oat, barley, rye and wheat. The localisation ofb-glucans in cereal grains influence the isolation andpurification procedures, which aim to produce fractions/preparations enriched in b-glucans. In oat and barley,b-glucans are located throughout the starchy endospermwhereas in wheat the highest concentration is in thesubaleurone layer with little in the rest of the starchyendosperm (Izydorczyk and Biliaderis, 2000).

b-Glucans from cereals are linear homopolysaccharidesof D-glucopyranosyl residues (Glcp) linked via a mixture ofb-(1-3) and b-(1-4) linkages, with blocks of consecutive(1-4)-linked residues (i.e. oligomeric cellulose segments)separated by single (1-3)-linkages (Fig. 1). Although mostof the cellulose segments are trimers and tetramers, longercellodextrin units are also present in the polymeric chains(Dais and Perlin, 1982; Izydorczyk et al., 1998a; Varumand Smidsrod, 1988; Wood et al., 1991a, 1994a; Wood-

ward et al., 1983a, 1988). Cereal b-glucans exhibitconsiderable diversity in their structures, including theratio of tri- to tetramers, the amount of longer cellulosicoligomers and the ratio of b-(1-4):b-(1-3) linkages(Izydorczyk and Biliaderis, 2000). These structural featuresappear to be important determinants of their physicalproperties, such as water solubility, viscosity, and gelationproperties (Bohm and Kulicke, 1999a, b; Cui et al., 2000;Doublier and Wood, 1995; Irakli et al., 2004; Izydorczyket al., 1998a–c; Lazaridou and Biliaderis, 2004; Lazaridouet al., 2003, 2004; Skendi et al., 2003; Storsley et al., 2003;Tosh et al., 2004a, b; Vaikousi et al., 2004), as well as oftheir physiological action in the gastro-intestinal tract(Wood, 2002).The water-soluble fibre seems to improve blood glucose

regulation and reduce serum cholesterol levels in diabeticand hypercholesterolemic subjects, respectively. Suchbeneficial health effects have been attributed to thesolubility of b-glucans in water and their capacity to formhighly viscous solutions (Kahlon et al., 1993; Wood et al.,1994b). In 1997, the US Food and Drug Administration(FDA) approved a health claim for the use of oat-basedfoods for lowering the risk of heart disease and passed aunique ruling that allowed oat bran to be registered as thefirst cholesterol-reducing food at a dosage of 3 g b-glucanper day, with a recommendation of 0.75 g of b-glucan perserving (Anonymous, 1997). Recently, a similar healthclaim for the barley b-glucan has also been approved(Anonymous, 2005). However, cereal b-glucans exhibitonly partial solubility in water. Therefore, in additionto the physiological benefits of the soluble dietary fibre,their action also includes the physiological effects asso-ciated with the consumption of insoluble fibre, such as anincrease of faecal bulk and on ability to relieve constipa-tion. The aforementioned dry milling techniques can pro-vide fractions with b-glucan content up to 30%, includingboth soluble and insoluble material. Such fractions arebelieved to offer the combined beneficial physiologicaleffects of soluble and insoluble dietary fibre when incorpo-rated into food systems. On the other hand, increasedb-glucan solubility is observed for concentrates or isolatesobtained by wet milling processes involving aqueous ex-tractions, hydrolytic enzyme treatments (amylases, proteases),

ARTICLE IN PRESS

Cellotriose unit

Cellotetraose unit

Long cellulosic unit (n≥3)

….4G1→4G1→3G1→4G1→4G1→

3G1→4G1 →4G1→4G1→3G1→[4G1]n→4G1 →

3G1→4G1….

Lichenase

4G1→4G1

3G1

4G1→4G1→4G1 →→3G1

[4G1]n→4G1→3G1

DP 3 DP≥5DP 4

Fig. 1. Generalised structure of cereal b-glucans and their debranching with lichenase; dotted arrows indicate the lichenase hydrolysis sites on the

polysaccharide chain. G: b-D-glucopyranosyl unit; DP3: 3-O-b-cellobiosyl-D-glucose; DP4: 3-O-b-cellotriosyl-D-glucose; DPX5: cellodextrin-like

oligosaccharides containing more than three consecutive 4-O-linked glucose residues.

A. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 103

sieving/centrifugation and precipitation of the polysacchar-ide components with alcohol solution.

The potential use of b-glucans as hydrocolloids in thefood industry is based mainly on their rheologicalcharacteristics, i.e. their gelling capacity and ability toincrease the viscosity of aqueous solutions. Thus, b-glucanscan be utilised as thickening agents to modify the textureand appearance of food formulations or may be used asfat mimetics in the development of calorie-reduced foods.b-Glucan-rich fractions from cereals or purified b-glucanshave in fact been successfully incorporated into productssuch as breakfast cereals, pasta, noodles and baked goods(bread, muffins), as well as dairy and meat products(Anonymous, 2003; Cavallero et al., 2002; Dexter et al.,2004; Duss and Nyberg, 2004; Hatcher et al., 2005; Hudsonet al., 1992; Inglett, 1990; Izydorczyk et al., 2005; Knuckleset al., 1997a; Marconi et al., 2000; Morin et al., 2002;Newman et al., 1990, 1998; Oste Triantafyllou, 2002;Volikakis et al., 2004).

The main objective of this review is to address recentresearch findings on structure—function relationships ofcereal b-glucans in the context of their technological andphysiological functionality. It also considers the prospectsfor further research on these topics.

2. Structural features

The structural characteristics of cereal b-glucans ofdifferent botanical origin, including molecular weight,distribution of cellulose oligomers and ratios of tri- totetramers and of b-(1-4) to b-(1-3) linkages in thepolysaccharide chain, are summarised in Table 1.

Despite the structural similarity of b-glucans fromdifferent genera of cereals, as suggested from methylationanalysis and their almost identical NMR spectra, oats,barley and wheat b-glucans are in fact structurally distinct,as shown by quantitative HPLC analysis of lichenase-released oligosaccharides (Cui et al., 2000; Dais and Perlin1982; Lazaridou et al., 2004; Wood et al., 1991a). The

enzyme lichenase, a (1-3)(1-4)-b-D-glucan-4-glucanohy-drolase (EC 3.2.1.73), specifically cleaves the (1-4)-glycosidic linkages of the three-substituted glucose residuesin b-glucans, yielding oligomers with different degrees ofpolymerisation (DP) (Fig. 1). The major hydrolysisproducts for the cereal b-glucans are 3-O-b-cellobiosyl-D-glucose (DP 3) and 3-O-b-cellotriosyl-D-glucose (DP 4), butcellodextrin-like oligosaccharides are also released insmaller amounts (�5–10%) from the polymer regionscontaining more than three consecutive 4-O-linked glucoseresidues. The DP of the long cellulose-like fragments hasbeen found to vary between 5 and 20, with DP5, 6, and 9being the most abundant (Izydorczyk et al., 1998a, b;Lazaridou et al., 2004; Wood et al., 1994a). Furthermore,cellotriosyl and cellotetraosyl segments are distributed in arather random pattern in the polymer (Buliga et al., 1986;Staudte et al., 1983).The oligosaccharide distribution within the same genera

of cereals is generally similar with major differences onlyoccurring between b-glucans of different botanical origin(Cui et al., 2000; Lazaridou et al., 2004; Wood et al.,1991a). The relative amount of trisaccharide (DP3) in theb-glucans decreases from wheat (67–72%), to barley(52–69%) and oats (53–61%), whereas the relative amountof tetrasaccharide (DP4) follows the opposite trend, i.e.increases from wheat (21–24%), to barley (25–33%) andoats (34–41%) (Table 1). However, the combined contentsof tri- and tetrasaccharides are similar among b-glucansfrom different cereal genera, resulting in similar totalamount of cellulose-like oligomers with DPX5. Thedifferences in the proportions of tri- and tetrasaccharidesobserved among different b-glucans from various sourcesare also reflected in the molar ratio of cellotriose tocellotetraose units (DP3:DP4), following the order ofwheat (3.0–4.5), barley (1.8–3.5), rye (1.9–3.0) and oats(1.5–2.3). This ratio is considered to be a fingerprint ofthe structure of cereal b-glucans. Literature data indicatethat there also are some differences in the ratio ofDP3:DP4 within the same genera, which could be

ARTICLE IN PRESS

Table 1

Molecular-structural features of cereal b-glucans

Source DP3a DP4a DPX5a Molar ratio

DP3/DP4

(1-4)/(1-3) Molecular

weight (10�3)

References

Oat – – – – 2.3–2.6 Dais and Perlin, (1982)

55.0–58.1 34.2–36.0 7.7–8.9 2.1–2.3 2.4 360–3100 Doublier and Wood (1995)

and Wood et al. (1991a–c)

– – – – – 1500 Autio et al. (1992)

– – – – – 1100–1500 Malkki et al. (1992)

– – – 1.5–2.3 – – Miller and Fulcher (1995)

– – – – 2.5 – Westerlund et al. (1993)

– – – – – 600–840 Jaskari et al. (1995)

– – – – – 1200–2500 Beer et al. (1997a, b)

57.6 34.1 8.2 1.7 – – Izydorczyk et al. (1998b)

– – – – – 120–2400 Zhang et al. (1998)

58.3 33.5 8.1 2.2 – 1160 Cui et al. (2000)

53.4–53.8 40.4–41.4 – 1.7–1.8 – 1100–1600 Johansson et al. (2000)

– – – – 2.4 214–257 Roubroeks et al. (2000a,

2001)

56.7 34.6 8.7 2.2 – 611–1700 Wang et al. (2002, 2003)

55.6–55.9 33.6–34.4 7.1–7.5 1.6–1.7 2.4 – Colleoni-Sirghie et al.

(2003a)

54.2–60.9 33.8–36.7 3.6–9.7 2.0–2.3 2.4–2.8 65–250 Lazaridou et al. (2003,

2004)

54.6–56.8 35.3–36.3 7.7–9.2 2.0–2.1 2.3–2.6 180–850 Skendi et al. (2003)

– – – – – 2060–2300 Aman et al. (2004)

Barley – – – – 1.9–2.3 Balance and Manners

(1978)

– – – – 2.3–2.6 Dais and Perlin (1982)

56–61 28–32 6–13 2.3–2.9 2.2–2.6 150–290 Woodward et al. (1983b,

1988)

62.1 29.4 8.4 2.8–3.4 2.4 1700–2700 Wood et al. (1991a–c) and

Wood (1994)

59.2–64.9 25.3–30.4 9.4–10.2 2.6–3.4 2.4 80–150 Saulnier et al. (1994)

– – – – 2.4 – Henriksson et al. (1995)

– – – – – 1300–1500 Beer et al. (1997a)

~200–600 Gomez et al. (1997a)

– – – – – 570–2340 Knuckles et al. (1997b)

56.8–61.6 26.1–32.3 10.6–11.2 1.8–2.4 – – Izydorczyk et al. (1998a, c)

– – – – – 31–560 Morgan and Ofman (1998)

– – – – – 100–375 Bohm and Kulicke (1999a)

63.7 28.5 7.8 3.3 – – Cui et al. (2000)

51.8–61.9 28.1–32.1 6.3–12.5 2.3–2.8 – 708 Jiang and Vasanthan

(2000)

66.0 25.7 8.2 3.4 – 693 Wang et al. (2003)

61.5–64.3 27.9–30.1 7.8–8.6 2.7–3.0 – – Wood et al. (2003)

59.4–64.3 24.8–31.0 8.2–17.5 2.5–3.2 1.9–2.2 – Storsley et al. (2003)

57.7–62.4 29.4–32.9 7.7–9.5 2.3–2.8 2.2–2.7 1320–450 Irakli et al. (2004)

62.0–63.3 27.5–29.2 8.8–9.1 2.8–3.0 – 213 Lazaridou et al. (2004)

62.0–69.3 26.2–29.1 4.5–8.9 2.8–3.5 2.1–2.8 250 Vaikousi et al. (2004)

Rye – – – 2.7–3.0 – 1100 Wood et al. (1991a–c)

– – – 1.9–2.3 2.3 21 Roubroeks et al. (2000b)

Wheat – – – 3.0–3.8 – Wood et al. (1991a)

72.3 21.0 6.7 4.5 – 267–487 Cui et al. (2000) and Li

et al. (2006)

67.1 24.2 8.7 3.7 – 209 Lazaridou et al. (2004)

aHydrolysis products of cereal b-glucans by lichenase: DP3 is 3-O-b-cellobiosyl-D-glucose, DP4 is 3-O-b-cellotriosyl-D-glucose and DPX5 is

cellodextrin-like oligosaccharides containing more than three consecutive 4-O-linked glucose residues.

A. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118104

attributed to genotypic and environmental factors (Jiangand Vasanthan, 2000; Miller et al., 1993; Storsley et al.,2003; Wood et al., 2003). b-Glucans in waxy barley

varieties appear to have a higher DP3:DP4 ratio comparedto those from non-waxy cultivars. Moreover, the ratios oftri- to tetrasaccharides in b-glucans from oats and barley

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 105

aleurone tissues are higher than those from starchyendosperm tissues (Izydorczyk et al., 2003; Wood et al.,1994a). According to a current model for the biosynthesisof mixed linkage (1-3),(1-4)-b-glucans proposed byBuckeridge et al. (2004), the (1-3),(1-4)-b-glucansynthase makes cellobiosyl and even-numbered cellodex-trin units, and a distinct glycosyl tranferase adds a thirdglycosyl residue to complete the cellotriosyl and higherodd-numbered units. Buckeridge et al. (1999, 2001)determined the b-glucan synthase activity in vitro andsuggested that the cellodextrin unit distribution is altereddrastically depending on the uridine diphosphoglucose(UDP-Glc) concentration. At suboptimal UDP-Glc con-centrations the synthesis of longer cellodextrin units inb-glucan is favoured, particularly cellotetraosyl units,whereas as saturation with UDP-Glc is approached,cellotriose units become the predominant oligomericspecies added along the polysaccharide chain via single(1-3)-b-linkages.

The calculated ratios of the two types of linkages, (1-4)to (1-3), in the native cereal b-glucan structures, based onNMR data and methylation analysis, were found to bewithin the range of 1.9–2.8 (Table 1). Furthermore,molecular weight values for b-glucans in the range of65–3100� 103, 31–2700� 103, 21–1100� 103, and 209–487� 103 have been reported for oats, barley, rye, andwheat, respectively. The apparent discrepancies in themolecular weight estimates of cereal b-glucans mayoriginate from varietal and environmental (growth) fac-tors, as well as from differences in the methods used forextraction (solvent and temperature affect the solubilisa-tion) and purification, aggregation phenomena (dependenton the structural features and solvent quality) anddepolymerisation events (by endogenous or microbialb-glucanases from contaminating microorganisms) occur-ring during the extraction step. Also, the reportedmolecular weights are dependent on the analytical meth-odology used for determination of molecular size (includ-ing detector and the standards used) (Beer et al., 1996,1997a, b; Colleoni-Sirghie et al., 2003b; Cui et al., 1999;Gomez et al., 1997a; Izydorczyk et al., 1998a, c, 2003;Jaskari et al., 1995; Knuckles et al., 1997b; Malkki et al.,1992; Morgan and Ofman, 1998; Skendi et al., 2003;Storsley et al., 2003; Wang et al., 2002; Wood et al., 1989,1991c; Zhang et al., 1998).

3. Physical properties

3.1. Solubility—solution behaviour

The chemical features of cereal b-glucans are reflected bytheir solubility in water and their extended, flexible chainconformation (Woodward et al., 1983b). The cellulose-likesegments of cereal b-glucans might contribute to thestiffness of the molecules in solution (Varum andSmidsrod, 1988); b-glucans containing blocks of adjacentb-(1-4) linkages may exhibit a tendency for interchain

aggregation (and hence lower solubility) via strong hydro-gen bonds along the cellodextrin portions. The b-(1-3)linkages break up the regularity of the b-(1-4) linkagesequence, making the molecule more soluble and flexible(Buliga et al., 1986). It has been suggested that the irregularspacing of (1-3) linked b-glucosyl residues in the b-glucanchain is responsible for the non-ordered overall conforma-tion of the polysaccharide and hence the chains are unableto align closely over extended regions, keeping thepolysaccharide in solution (Woodward et al., 1988). Onthe other hand, it has been reported that helical segmentsmade up of at least three consecutive cellotriosyl residuesmay constitute a conformationally stable motif (ordereddomain) in mixed linked (1-3),(1-4) b-glucans (Tvar-oska et al., 1983) and that the b-(1-3) linkages could beinvolved in the ordered conformation of barley b-glucans(Morgan et al., 1999). It is possible, therefore, that a highercontent of consecutive cellotriosyl units might impose someconformational regularity on the b-glucan chain, andconsequently a higher degree of organisation of thesepolymers in solution (i.e. lower solubility) (Izydorczyket al., 1998c, 2003). In accordance with these two aggrega-tion mechanisms, it has also been suggested that higheramounts of cellotriosyl fragments and a higher ratio of(1-4):(1-3) linkages, might explain the solubility differ-ences between cereal b-glucan fractions obtained by usingdifferent aqueous or alkali extraction conditions and/ordifferent concentrations of ammonium sulphate (Cui et al.,2000; Izydorczyk et al., 1998a–c; Storsley et al., 2003;Woodward et al., 1988).Dynamic and steady shear rheological tests of freshly

prepared (or heat-treated before measurement) solutions ofcereal b-glucans reveal a behaviour typical of non-interacting disordered polysaccharides with chain entan-glements in the concentrated state. Thus, for aqueousdispersions of b-glucan a Newtonian region at low shearrates and a shear-thinning flow at high shear rates areobserved (Fig. 2a). Moreover, under dynamic rheologicalmeasurements the loss modulus, G00, is larger than thestorage (or elastic) modulus, G0 at lower frequencies, whilethe behaviour approaches that of solid-like materials athigher frequencies, with G0 being greater than G00. Asexpected, with increasing molecular weight (Fig. 2a) and/orconcentration of polysaccharide, there is an increase inviscosity as well as in the shear thinning and viscoelasticproperties of aqueous dispersions of b-glucans (Irakli et al.,2004; Lazaridou et al., 2003, 2004; Skendi et al., 2003;Vaikousi et al., 2004).The reported intrinsic or limiting viscosity ([Z]) values for

cereal b-glucans vary between 0.28 and 9.6 dl/g, dependinglargely on the molecular weights of the isolated poly-saccharides. It has been suggested that b-glucans have anextended random coil conformation. Consequently, thedouble logarithmic plots of ‘zero shear’ specific viscosity,(Zsp)0, vs. c[Z] for several b-glucan preparations differing infine structure and molecular size superimpose closely, oftenfalling into three linear parts (Bohm and Kulicke 1999a;

ARTICLE IN PRESS

0.0001

0.001

0.01

0.1

1

0.001 0.01 0.1 1 10 100 100010000

η/η 0 200 2.1

200 3.0

200 3.7

100 2.1

100 2.8

Mw x 10-3 DP3/DP4

0.01

0.1

1

10

0.01 1 100 10000

η (P

a s

)

250

210

180

140

110

70

γ (1/s)⋅ γ γ 1/2

⋅ ⋅

Mw x 10-3

Fig. 2. Molecular weight (70–250� 103Mw) effects on apparent viscosity (Z) of barley b-glucan aqueous dispersions (3%w/v, 20 1C and DP3:DP4 ratio

2.8) (adapted from Vaikousi et al., 2004) (a) and generalised shear rate dependence of viscosity for cereal b-glucans (8%w/v, 20 1C); the solid line

represents the fit of the experimental data to Eq. (1) (adapted from Lazaridou et al., 2004) (b).

A. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118106

Burkus and Temelli 2003; Colleoni-Sirghie et al., 2003b;Doublier and Wood 1995; Grimm et al., 1995; Gomezet al., 1997b; Irakli et al., 2004; Izydorczyk et al., 1998a, b;Lazaridou et al., 2003, 2004; Roubroeks et al., 2000a, b;Skendi et al., 2003; Vaikousi et al., 2004; Varum andSmidsrod 1988; Wang et al., 2003; Woodward et al., 1983a,1988).

The flow curves of aqueous solutions of b-glucans havebeen described by the power law model and are largelydepended on molecular size, concentration and tempera-ture. Moreover, the effects of temperature on the viscosityof b-glucan solutions have been modelled by the Arrheniusequation (Autio et al., 1988; Colleoni-Sirghie et al., 2003b;Papageorgiou et al., 2005; Wikstrom et al., 1994). Theshear-thinning curves for aqueous solutions of b-glucanswere found to follow a common master curve using atransformation of viscosity values vs. shear rate (Fig. 2b).This general form of shear-thinning behaviour is typical ofdisordered polysaccharides irrespective of primary struc-ture, molecular weight, temperature, solvent environment,and concentration, and can be matched with goodprecision by the simple relationship (Morris, 1990):

Z ¼ Z0=½1þ ð_g=_g1=2Þ0:76�, (1)

where Z is viscosity, Z0 is viscosity at the Newtonian zone, _gis shear rate, and _g1=2 is shear rate at which Z ¼ Z0/2.Indeed, the experimental data from the generalised shear-thinning profile of cereal b-glucan solutions at certainconcentrations (8, 10%w/v) and temperature (20 1C) fittedwell (r2 ¼ 0.99) to this equation regardless of differences inthe molecular weight and fine structure of the b-glucanisolates (Lazaridou et al., 2004; Vaikousi et al., 2004). Theapplication of the Cox–Merz rule (i.e. the relationshipbetween rheological responses under destructive and non-destructive deformation) has also been tested for cerealb-glucan preparations, differing in molecular weight and

in the concentration of dissolved polysaccharide. This ruleis generally observed for random coil polymer chainsinteracting solely by physical entanglements and no largedepartures from the empirical Cox–Merz correlation werefound for cereal b-glucan samples, except in the case ofhigh polysaccharide concentrations and samples of highmolecular weight (Autio, 1988; Bohm and Kulicke 1999a;Lazaridou et al., 2003, 2004; Skendi et al., 2003).

3.2. Aggregation phenomena—gelation

Although fresh solutions of cereal b-glucans fallrheologically into the category of random coil-typepolysaccharides, their rheological properties may changedepending on their molecular characteristics (size, struc-ture), storage time (i.e. waiting time before analysis) andthermal history (Bohm and Kulicke 1999a, Gomez et al.,1997c; Lazaridou et al., 2003, 2004; Tosh et al., 2003;Vaikousi et al., 2004). Cereal b-glucans with certainstructural features may display time-dependent rheologicalbehaviour, often revealed in thixotropic loop experiments,implying a time-dependent formation of intermolecularnetworks. Thus, with increasing storage time solutions ofb-glucans show unusual shear-thinning flow behaviour atlow shear rates. This behaviour becomes more pronouncedwith increasing polysaccharide concentration and storagetime prior to the rheological testing, as well for b-glucanswith low molecular weight and high proportions ofcellotriosyl units in the polymeric chains (Lazaridouet al., 2003, 2004; Vaikousi et al., 2004).In addition to exhibiting increased solution viscosity on

storage, b-glucans have also been shown to gel undercertain conditions. Thus, cereal b-glucan hydrogels withdifferent molecular characteristics and properties havebeen obtained under isothermal (5–45 1C, 4–12%w/vpolymer concentration) conditions (Bohm and Kulicke1999b; Irakli et al., 2004; Lazaridou et al., 2003, 2004; Li

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 107

et al., 2006; Papageorgiou et al., 2005; Skendi et al., 2003;Tosh et al., 2003, 2004a; Vaikousi et al., 2004), as well asafter repeated freezing and thawing cycles of relativelydilute (1–4%w/v) polysaccharide solutions due to aggrega-tion of the polymeric chains upon freeze-concentration(Lazaridou and Biliaderis 2004; Vaikousi and Biliaderis,2005). The later process is called cryogelation or cryos-tructurisation and the gels formed cryogels. Cerealb-glucan hydrogels formed at temperatures above 0 1Cand cryogels belong to the category of physically cross-linked gels whose three-dimensional structure is stabilisedmainly by multiple inter- and intrachain hydrogen bonds inthe junction zones of the polymeric network. The structureand properties of b-glucan hydrogels have been exploredby small strain dynamic rheometry, differential scanningcalorimetry (DSC) and large deformation mechanical tests.

3.2.1. Dynamic rheometry

The gelation capacity of aqueous dispersions of cerealb-glucans differing in molecular size and fine structure hasbeen monitored isothermally by dynamic rheometry atdifferent temperatures above 0 1C and polymer concentra-tions (Irakli et al., 2004; Lazaridou et al., 2003, 2004; Liet al., 2006; Skendi et al., 2003; Vaikousi et al., 2004). Afteran induction period, G0 and G00 increase with time and theaqueous dispersions begin to adopt gel-like properties(G04G00), as shown in Fig. 3. At the end of the gel curingprocess, the behaviour becomes typical of an elastic gelnetwork and the G0 attains a pseudo-plateau value G0max.The time where G0 crosses G00 is often considered as thegelation time (Gt) and the maximum slope of the G0 curve isreported as an index of the gelation rate; the latter, knownas ‘elasticity increment, IE’, can be calculated asIE ¼ (dlogG0/dt)max (Bohm and Kulicke 1999b). The Gt

0.1

1

10

100

1000

10000

0 20 40 60

Ti

G´,

G´´´´

(P

a)

G

G

G

IE

0.01

0.1

1

10

100

1000

0.1 1 10 100

f (Hz)

G´,

G´´´´

(P

a)

0.1

1

10

η∗ (P

a s

)

G´´´´

G´

η∗

Fig. 3. Sol–gel phase transition of an aqueous b-glucan dispersion (10%w/v) a

frequency 1.0Hz); the respective mechanical spectra before (left) and after (rig

decreases and the IE values increase with decreasingmolecular weight in cereal b-glucans with similar distribu-tions of cellulose-like fragments (Fig. 4a), possibly due tothe higher mobility of the shorter chains that enhancesdiffusion and lateral interchain associations (Bohm andKulicke 1999b; Doublier and Wood 1995; Irakli et al.,2004; Lazaridou et al., 2003, 2004; Li et al., 2006; Skendiet al., 2003; Tosh et al., 2004b; Vaikousi et al., 2004).Among cereal b-glucans of equivalent molecular weight,the gelation time decreases and the gelation rate increasesin the order of oats, barley, and wheat b-glucans, reflectingthe order of the molar ratio of DP3:DP4 units (Fig. 4b)(Bohm and Kulicke 1999b; Cui et al., 2000; Lazaridou etal., 2004; Tosh et al., 2004a). Two different gelation modelshave been proposed in the literature for mixed-linkage (1-3),(1-4) b-glucans. One involves the side-by-side interactionsof cellulose-like segments of more than three contiguousb-(1-4)-linked glucosyl units (Fincher and Stone 1986)and the other the association of chain segments withconsecutive cellotriosyl units linked via b-(1-3) bonds(Bohm and Kulicke 1999b). Overall, the observed differ-ences in gelation propensity seem to be attributed mostly tothe differences in the DP3:DP4 ratio between cerealb-glucans; the likelihood of ordered repeating segmentsof the cellotriosyl units being increased with an increase inthe DP3:DP4 ratio. Furthermore, a decrease in molecularweight and an increase in the DP3:DP4 ratio in theb-glucan chains resulted in gels of increased G0max anddecreased values of loss tangent, tan d ( ¼ G00/G0), (Lazar-idou et al., 2004; Tosh et al., 2004a). The gelation rate alsoincreases with increasing concentration and increasing gelcuring temperature, reaching a maximum at �25–35 1C.At higher temperatures the IE values decrease (Lazaridouet al., 2003, 2004; Skendi et al., 2003; Vaikousi et al., 2004).

80 100 120 140

me (h)

´

´´´´

G′max

t

= (dlogG´/dt)max

1

10

100

1000

0.01 0.1 1 10 100f (Hz)

G´,

G´´´´

(P

a)

1

10

100

1000

η∗ (P

a s

)

G´´´´

G´

η∗

s probed by dynamic rheological testing (gel curing at 25 1C, strain 0.1%,

ht) gelation are also shown.

ARTICLE IN PRESS

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 6 12

Number of cycles

tan

δ

3 9

65 2.1

110 2.1

140 2.1

200 2.1

200 3.0

200 3.7

Mw x 10-3 DP3/DP4

Fig. 5. Cereal b-glucan molecular weight and structure effects on tan drecorded at 5 1C, 1.06Hz, and 0.1% strain of 3% (w/v) cryogels obtained

after repeated freeze–thaw cycles (adapted from Lazaridou and Biliaderis,

2004).

0

10

20

30

40

50

60

0 50 100 150 200 250 300

Mw x10-3

I E (

h-1

)

0

0.5

1

1.5

2

2.5

Gt (h

)

IE

Gt

0

20

40

60

80

100

120

140

160

180

2 2.5 3.5 4

DP3/DP4

I E (

h-1

)

0

0.05

0.1

0.15

0.2

0.25

Gt (h

)

IE

Gt

3

Fig. 4. Molecular weight (a) and structure (b) dependence of elasticity increment (IE) and gelation time (Gt) for cereal b-glucans cured at 25 1C and 8%w/v

concentration (frequency 1Hz, strain 0.1%); the b-glucan preparations of (a) have a common DP3:DP4 ratio, 2.8, and those of (b) have a common

molecular weight, 210� 103 (adapted from Lazaridou et al., 2004 and Vaikousi et al., 2004).

A. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118108

Furthermore, the dependence of the storage modulus(apparent G0max value) on b-glucan concentration followspower law relationships; G0 varying as C7.2–7.5 (Lazaridouet al., 2003). For cereal b-glucan cryogels, the G0 valuesobtained from their mechanical spectra at 5 1C increase andtan d decrease with increasing initial polysaccharide con-centration, number of freeze–thaw cycles and trisaccharideunits (DP3), as well as with decreasing molecular weight ofthe polysaccharide (Fig. 5) (Lazaridou and Biliaderis,2004).

Food formulation also has an impact on the gelationability and the rheological properties of cereal b-glucangels (Irakli et al., 2004; Lazaridou and Biliaderis, 2007;Lazaridou et al., 2007; Vaikousi and Biliaderis, 2005). Theincorporation of various sugars (glucose, fructose, sucrose,xylose, and ribose) at a concentration of 30% (w/v) toaqueous dispersions of barley b-glucan (6%w/v) led to anincrease of the gelation time upon curing at roomtemperature (Irakli et al., 2004). Similarly, the addition ofseveral sugars (sucrose, glucose, fructose, and xylose) ata concentration range of 5–30% (w/w) to aqueous dis-persions of b-glucan (3%w/w) without or with skimmedmilk (12%w/w), submitted to repeated cycles of free-zing–thawing, induced a significant delay in the transitionto a gel state. The resultant cryogels were weaker(Lazaridou and Biliaderis, 2007; Lazaridou et al., 2007;Vaikousi and Biliaderis, 2005) and the inhibitory effectsof xylose and fructose were stronger than those of glucoseand sucrose. On the other hand, sorbitol appears topromote network formation (Lazaridou and Biliaderis,2007; Lazaridou et al., 2007). However, whereas theeffects of polyols were significant when added to prepara-tions of high molecular weight b-glucans (210� 103), theygenerally decreased with decreasing molecular weight andseemed to be independent on the type of polyol forlow molecular weight b-glucan preparations (e.g. 140 and70� 103).

3.2.2. Calorimetry

The DSC thermal scans of cereal b-glucan hydrogelsformed by curing at temperatures above 0 1C (Fig. 6a) andby freeze–thaw cycling exhibited rather broad endothermicgel-sol transitions at 55–80 1C (Lazaridou and Biliaderis,2004; Lazaridou et al., 2003, 2004; Skendi et al., 2003;Vaikousi et al., 2004). Similar to the rheological responses,the DSC kinetic data showed that the rate of endothermdevelopment (short range molecular order) during gelcuring at room temperature (Fig. 6b) (Lazaridou et al.,

ARTICLE IN PRESS

30 40 50 60 70 80 90 100

Temperature (°C)

En

do

therm

ic H

eat

Flo

w

ΔH: 5.1 mJ/mg

0.2 mWatts

ΔH: 2.9 mJ/mg

ΔH: 2.1 mJ/mg

curing: 72h

curing: 47h

curing: 21h

ΔH: 4.1 mJ/mg

Mwx10-3 DP3/DP4

curing: 40h

35 2.3

65 2.3

110 2.3

110 2.8

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Time (h)

ΔH (

mJ/m

g)

35 2.3

65 2.3

200 2.1

200 3.0

200 3.7

Mw x 10-3 DP3/DP4

Fig. 6. Cereal b-glucan molecular weight and structure effects on differential scanning calorimetry (DSC) thermal curves of gels cured for specified time

periods (a), and on apparent melting enthalpy (DH) kinetic data of gel structures (b); heating rate 5 1C/min and curing at 25 1C with 10%w/v

polysaccharide concentration (adapted from Lazaridou et al., 2003, 2004).

A. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 109

2003, 2004), as well as the apparent melting enthalpy values(plateau DH) of the gels increase with decreasing molecularsize and with increasing DP3:DP4 ratio (Fig. 6) (Lazaridouet al., 2003, 2004; Li et al., 2006; Vaikousi et al., 2004). TheDH values of the cryogels also increase with increasingDP3:DP4 ratio and with decreasing molecular weight ofcereal b-glucans (Lazaridou and Biliaderis, 2004). Further-more, the melting temperatures of the gel networks studied,as measured by dynamic rheometry and DSC, increasedwith the molecular weight and the amount of cellotriosylunits in the polysaccharide chains (Lazaridou and Bilia-deris, 2004; Lazaridou et al., 2003, 2004; Li et al., 2006;Vaikousi et al., 2004). It appears that although a slowergelation process is observed for high molecular weightb-glucans, the gel network structure consists of structuralelements (microaggregates) with better organisation and/or involves interchain associations over longer chainsegments.

It is also notable that cryogels obtained after repeatedfreeze–thaw cycles, at low initial polysaccharide concentra-tions (3%w/v) (Lazaridou and Biliaderis, 2004), areanalogous with the gel networks prepared at roomtemperature and using much higher polysaccharide con-centrations (10%w/v) (Lazaridou et al., 2004). In fact, theDSC thermal scans of these two systems show that theapparent melting enthalpy (DH) values are similar for bothtypes of gels whereas comparison of the broadness of theendotherm peak reveals that cryogelation generally yieldsa more cooperative gel-sol transition. Similarly, after

curing 3% (w/v) b-glucan dispersions at 5 1C for 135 days,which is long enough for G0 to reach a ‘pseudo-plateau’value, the strength (G0 value) of the resultant gel is lowerthan that of cryogels obtained from dispersions atequivalent initial polysaccharide concentration after 14freeze–thaw cycles (Lazaridou et al., 2007). The reinforce-ment of b-glucan network structures induced by cryos-tructurisation is attributed to the increased concentration(cryoconcentration) of the polysaccharide in the unfrozenbulk phase resulting in the promotion of associativeinteractions among the polysaccharide chains (physicalcross-linking).

3.2.3. Uniaxial compression

Variation in the mechanical properties of cereal b-glucangels has been also revealed by large deformation compres-sion tests. For gel samples cured at room temperature, anincrease in strength and a decrease in brittleness werefound with increasing concentration, molecular weight andDP3:DP4 ratio of the polysaccharide (Lazaridou et al.,2003, 2004; Vaikousi et al., 2004). Instead, cereal b-glucanswith high molecular weight and low amounts of DP3 unitsformed strong cryogels when tested under large deforma-tion protocols; these findings were attributed to differencesin the nature of the network microstructure betweensamples formed under different gel curing conditions(Lazaridou and Biliaderis, 2004). Microscopic images ofcereal b-glucan gels aged for seven days at 5 1C revealedthat the microstructure was not homogenous and there

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118110

seemed to be a coarsening of the gel network structure asthe b-(1-3)-linked cellotriosyl unit content increased(Tosh et al., 2004a).

4. Functional properties

4.1. Technological aspects

The incorporation of fractions enriched with oats andbarley b-glucans into cereal-based foods has been widelystudied. The addition of b-glucan concentrates at highlevels (�10–30%) in yeast-leavened breads leads to highwater retention, as well as to darker coloured end-productsand undesirable effects on crumb texture and loaf volume(Bhatty, 1986; Cavallero et al., 2002; Dexter et al., 2004;Izydorczyk and Dexter, 2004; Knuckles et al., 1997a;Krishnan et al., 1987; Newman et al., 1998). However,b-glucan rich fractions from oats and barley may result inincreased loaf volumes when used at certain concentrationsin wheat flour, probably by increasing the viscosity of thedough (Delcour et al., 1991). It is considered that b-glucansmay play a role in improvement of bread crumb structureby stabilising air cells in the dough and preventing theircoalescence (Wang et al., 1998). Relationships have alsobeen established between b-glucan concentration and thepasting characteristics of oat flours. The increase in thepasting peak viscosity which occurred with increasingb-glucan content of oat cultivars was explained by anincrease in the water binding capacity of the flours(Colleoni-Sirghie et al., 2004; Zhou et al., 2000). Inaddition to the polysaccharide concentration, the molecu-lar weight (Mw) also seems to affect the breadmakingperformance of b-glucans, as shown in a recent investiga-tion (Skendi et al., 2006) in which two isolates of barleyb-glucan with different molecular weights were incorpo-rated into breads made from two types of wheat flourdiffering in their breadmaking quality. The addition ofb-glucans with high molecular weight resulted in higherloaf volumes than the incorporation of those with lowmolecular weight. The highest loaf volume was obtainedfor the poor breadmaking quality flour with 0.6%w/wb-glucan, with the incorporation of the high molecularweight preparation giving quality exceeding that of thegood breadmaking quality wheat cultivar. Furthermore,incorporation of b-glucans up to 5% into baked goods,such as muffins, did not seem to affect the overallacceptability of the products (Hudson et al., 1992; New-man et al., 1990, 1998). The addition of barley fractionsenriched in b-glucans in semolina or wheat flours at up to�20%, resulted in pasta or noodles with acceptable sensoryproperties and cooking quality despite their decreasedbrightness and yellowness and the increased redness and‘speckiness’ of the product (Dexter et al., 2004; Hatcheret al., 2005; Izydorczyk et al., 2005; Knuckles et al., 1997a;Marconi et al., 2000).

The aforementioned development of weak gel networkstructures by cereal b-glucans with certain structural

characteristics and under certain conditions is a desirableattribute in water-continuous low-fat spreads. In thiscontext, cereal b-glucans exhibit the potential to be usedas fat replacers due to their high viscosity and water-binding and their foaming and emulsion stabilising capa-bilities (Burkus and Temelli, 2000; Kontogiorgos et al.,2004). Kontogiorgos et al. (2004) investigated the effects ofpure barley and oat b-glucans on the rheological andcreaming behaviour of concentrated egg-yolk-stabilisedmodel emulsions (salad dressing model). The high mole-cular weight b-glucans (apparent Mw�110� 103) stabilisedthe oil-in-water (o/w) emulsions, mainly by increasing theviscosity of the continuous phase, while the low molecularweight b-glucans (apparent peak Mw�40� 103) influencedemulsion stability through network formation in thecontinuous phase. Moreover, it was found that incorpora-tion of barley b-glucan gum (76.2% purity) into reduced-fat breakfast sausages at a level that provides 0.3–0.7%b-glucan in the product gave improved water binding,without having any significant effects on product texture orflavor at a level of 0.3% (Morin et al., 2002). A commercialconcentrate of oat b-glucans (22.2% b-glucan content) hasbeen also incorporated into low-fat white-brined cheesefrom bovine milk (70% fat reduction) at two levels, 0.7%and 1.4% (w/w), resulting in a product with increased yield,greater proteolysis and higher levels of short chain fattyacids (lactic, acetic, and butyric) as well as with improvedtexture compared to its low-fat (b-glucan-free) counter-part. However, the colour, flavour and overall impressionscores were significantly inferior to those of a typical white-brined cheese product, particularly for the cheese pro-duct made with the high level of b-glucan (Volikakiset al., 2004). Recently, non-dairy ready-to-use, lactose-free,milk substitutes (oat milk beverage, yogurt, ice cream,oat-based cream, whipped cream, and buttermilk) havebeen introduced into the market. These products containcereal b-glucans as bioactive ingredients as well as actingas stabilisers and texturisers (Anonymous, 2003; OsteTriantafyllou, 2002).The molecular weight of b-glucans does not only affect the

viscosity of foods but also has a significant impact on theperceived (sensory) thickness of products as shown forbeverage and soup food prototypes containing 0.25–2%(w/w) b-glucans (Lyly et al., 2003, 2004). These appear to besignificant correlations between textural characteristics andmeasured viscosity as well as a moderately strong relationshipbetween flavor attributes and the viscosity of soups. From atechnological point of view, b-glucans may be useful asalternative thickening agents in soups. It may also be possibleto prepare beverages and soups containing a physiologicallyeffective amount of b-glucan from low molecular weightb-glucan preparations, as they are easier to process than theirhigh molecular weight counterparts. However, the relation-ship between the physiological functionality and molecularweight of b-glucans has to be borne in mind.The potential use of b-glucans as biodegradable edible

food packaging material has recently been suggested

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 111

(Skendi et al., 2003). The mechanical properties of castb-glucan films were affected by the amount of plasticiser(water, polyol) used and the molecular weight of thepolysaccharide. Water as well as sorbitol, added as a co-plasticiser at a 15% (w/w) level, improved the extensibility,but decreased the mechanical strength of the b-glucanfilms. Furthermore, high molecular weight b-glucans gavestronger and more extensible films than low molecularweight b-glucan preparations.

4.2. Physiological responses

4.2.1. Effect of viscosity

The physiological functions of dietary fibre are oftenattributed to their physico-chemical properties: waterholding capacity, swelling, diffusion-suppressing ability(through viscosity enhancement and gel-formation), bind-ing properties, and susceptibility or resistance to bacterialdegradation and fermentation (Dikeman and Fahey, 2006;Schneeman, 2001). The mechanisms by which a solublefibre, such as b-glucan, exerts hypocholesterolemic andhypoglycemic effects are still debated but the mostcommon hypothesis is based on increased lumen viscosity(Battilana et al., 2001; Bell et al., 1999; Bourdon et al.,1999; Dikeman and Fahey, 2006; Wood, 2002). It has beensuggested that cereal b-glucans decrease the absorption andreabsorption of cholesterol, bile acids, and their metabo-lites by increasing the viscosity of the gastro-intestinal tractcontents as well as delaying gastric emptying and theintestinal absorption of nutrients, such as digestiblecarbohydrates, and thereby reducing post-prandial hyper-glycemia and insulin secretion. The latter have healthbenefits for those with type-2 diabetes, and are alsoassociated with reduced risk of developing the diseaseand insulin insensitivity. Carbohydrate and lipid metabo-lism are closely interrelated and insulin has also beenreported to increase hepatic cholesterol synthesis. There-fore, if fibre decreases carbohydrate absorption and insulinsecretion, it may also contribute indirectly to the hypocho-lesterolemic effects (Bell et al., 1999; Bourdon et al., 1999;Dikeman and Fahey, 2006; Wood, 2002). Furthermore, theincreased viscosity caused by soluble dietary fibre influ-ences fat emulsification by increasing emulsion droplet sizewhich may impair fat absorption (Pasquier et al. 1996).However, Battilana et al. (2001) found that the adminis-tration of frequent meals with or without b-glucansresulted in similar carbohydrate and lipid metabolism,whereas ingestion of a single meal containing b-glucanlowered post-prandial glucose concentrations. This mightsuggest that the beneficial action of b-glucans is mainly dueto delayed and reduced carbohydrate absorption from thegut and does not result from the effects of metabolitesproduced by fermentation of b-glucans in the colon. It isalso likely that b-glucans do not only decrease the post-prandial glucose response due to high viscosity in thegastro-intestinal tract, but also reduce starch digestion bya-amylase. Symons and Brennan (2004) found that

inclusion of 5% of a b-glucan-rich fraction in breadresulted in a significant decrease in the release rate ofreducing sugars following the in vitro digestion of breadwith pepsin and a-amylase. Similarly, Izydorczyk et al.(2005) showed that replacement of 25% wheat flour innoodles with a fibre-rich fraction from barley significantlydecreased the in vitro release of glucose compared to 100%wheat noodles. In both cases, it was speculated thatb-glucans decrease the accessibility of starch-degradingenzymes to their starch substrate by forming a gel matrixor by limiting the water available for starch hydration andthus, for starch gelatinisation.A study with ileostomy subjects showed that bile acid

excretion increased by a mean of 450% in a diet with oatbran bread, compared with a similar diet in which 70% ofthe b-glucan was degraded by b-glucanase. This indicatesthat the high molecular weight b-glucan mediates increasedexcretion of bile acids: whole micelles or bile acids maybecome entrapped or encapsulated in the highly viscous b-glucan matrix, and then excreted from the small bowel orthe viscous environment to reduce the formation of mixedmicelles in the small intestine and/or decrease thereabsorption of bile acids in the terminal ileum (Liaet al., 1995). Furthermore, a dependence of the hypocho-lesterolemic action on extractability, viscosity, pseudo-plastic flow behaviour, hydrodynamic properties, andmolecular weight of b-glucans was found in rats fed ondiets based on various oat bran concentrates, supporting amechanism based on high lumen viscosity (Malkki et al.,1992). A difference in the mechanism of physiologicaleffects has also been observed between cereal b-glucansdiffering in their molecular features. Wilson et al. (2004)demonstrated that the cholesterol-lowering activity ofbarley b-glucan in hamsters occured with both low(175 kDa) and high molecular weight (1000 kDa) prepara-tions. However, although cholesterol accumulation did notdiffer between the molecular weight preparations, theesterification of cholesterol was affected. Thus, a signifi-cantly lower accumulation of cholesterol ester was ob-served in the hamsters fed the lower molecular weightbarley b-glucan: tissue cholesterol ester concentration isviewed as the hallmark in fatty streak formation associatedwith atherogenic progression. Several other animal studiesof diets supplemented with cereal fibre or cereal fractionsshowed that the content of b-glucans and particularly thatof soluble b-glucans, the molecular weight of the b-glucansand endogenous b-glucanase activity, all of which affectextract viscosity, appeared to be strong predictors of theserum cholesterol response (Kahlon et al., 1993; Newmanand Newman, 1991; Newman et al., 1992). In a meta-analysis report, Brown et al. (1999) identified 25 clinicalstudies involving a total of 1600 healthy, hyperlipidemicand diabetic individuals in the age range of 26–61. Dailydoses of between 2 and 10 g of oat soluble fibre gavesignificant mean reductions in the total (�0.04mmol/L/g offibre) and low-density-lipoprotein (LDL, �0.037mmol/L/gof fibre) cholesterol levels. It is generally considered that

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118112

the cholesterol-lowering effect is larger in subjects withhigher cholesterol levels, whereas the dose–response effectis not always clear. Overall, the findings from many humanstudies reveal a substantial variation in the responsesbetween the different trials and types of subjects studied. Inaddition to variation in the design (intervention) of theclinical trial (i.e. number and type of subjects involved,daily intake and number of servings, length of treatment,etc.), variation is also observed in the physiologicalresponses of individual. Also, in some cases, inconsistentresults may be related to factors that influence the viscositydevelopment by b-glucans, which is in turn determined bythe molecular weight and extent of solubilisation of thepolysaccharide from the food matrix.

According to Anderson (1990) and Malkki and Virtanen(2001), soluble fibre can affect satiety and promote weightloss for a number of reasons including a slower rate of mealintake, a delay in gastric emptying, moderate responses ofplasma glucose levels and insulin secretion by the pancreas(insulin stimulates hunger), elevation of cholecystokinin(a gut hormone correlated with prolonged satiety) andproduction of gas and short-chain fatty acids by fermenta-tion of the fibre in the colon.

It has recently been shown that a gelling b-glucan frombarley (Glucagel) can cause a number of effects on themammalian immune system, including upregulation of therelease of the cytokine interleukin 1b (IL-1b) (indicatingenhanced monocyte differentiation), changes to woundhealing processes and enhancement of gut mucin secretion(Porter et al., 2006). Furthermore, the in vitro secretion ofIL-1b, which is one of the early responses of the immunesystem to infection, increased with increasing dose andmolecular weight of the b-glucan.

4.2.2. Effects of the food matrix

The viscosity effects of b-glucans are often modulated byformulation and food processing and storage protocols.The dose–response of oat gum on plasma glucose andinsulin levels of healthy humans consuming oat gum in adrink (in the range of 1.8–14.5 g after an oral glucose loadof 50 g) has been studied (Wood et al., 1994b). Increasingthe dose of oat gum successively reduced the plasmaglucose and insulin responses relative to a control drinkwithout gum, reaching a plateau dose–response at anintake of �6 g of b-glucan (the maximum effective dose). Ina recent study by Cavallero et al. (2002), a linear decreasein the glycemic index of non-diabetic humans was foundwith increasing consumption of barley b-glucan in bread,the GI (glycemic index) being related to the total b-glucancontent (TBG) by the equation:

GI ¼ 91:27� 3:68TBG ðr2 ¼ 0:96Þ. (2)

It should also be noted that the dose–response for a solidmeal might differ from that for a liquid (Wood et al.,1994b). The food matrix or food processing or both couldhave adverse effects on the hypocholesterolemic propertiesof b-glucan. A clinical study (Kerckhoffs et al., 2003) with

mildly hypercholesterolemic subjects showed that con-sumption of an b-glucan-enriched preparation from oatbran with orange juice was more effective in lowering totaland LDL-cholesterol concentrations and the ratio of totalto high-density-lipoprotein (HDL)-cholesterol than whenthe same preparation was administered in bread andcookies. Some studies have failed to show a physiologicalaction of b-glucan preparations despite the daily b-glucanintake being much higher than the amount recommendedby the FDA. This fact has been attributed to the reductionof polysaccharide molecular weight occurring during theisolation stage and/or as a result of food processing (Keoghet al., 2003). Commercial oat foods including porridgemade of rolled oats, and extruded and breakfast cerealproducts all contain high molecular weight b-glucans(600–2930� 103), crisp bread (950� 103) and yogurt-likeproducts (830� 103) contain moderately degradedb-glucans while bread loaves, fried pancakes, pancakebatter, and fermented oat soup contained extensivelydegraded b-glucans (190–630� 103) (Aman et al., 2004;Wood et al., 1991c). During baking of yeast-leavened oat-and barley-containing breads, the b-glucan is partiallydegraded (from 1200–2300� 103 to 240–1920� 103), prob-ably due to the action of b-glucanases in the flour mixtures.This degradation is reduced with decreasing mixing andfermentation time and with increasing particle size of thebran (Aman et al., 2004; Andersson et al., 2004; Sundberget al., 1996; Trogh et al., 2004). Furthermore, during thetypical pasteurisation conditions used for an acidic liquidproduct such as fruit and tomato juice, a reduction ofviscosity resulting from the inclusion of barley b-glucanisolates is observed, probably as a consequence of acidhydrolysis (Vaikousi and Biliaderis, 2005). The effects ofacid hydrolysis were dependent on pH, temperature andtime, and were more pronounced for a high molecularweight isolate (molecular weight 250� 103) than for a lowmolecular weight isolate (140� 103), indicating thatdifferences in the flow behaviour of liquid productscontaining b-glucans of different molecular size may occurunder different processing and formulation protocols.Furthermore, food processing and storage can alter thesolubility and thereby, the availability of b-glucans (Beeret al., 1997b; Brummer et al., 2006). Food processing canincrease the physiological activity of b-glucans by increas-ing availability (cooking, extrusion) and although themolecular size of the polymer may be partly reduced (dueto enzymic hydrolysis, milling, stirring, and pumping)(Izydorczyk et al., 2000; Robertson et al., 1997), theb-glucan can be still effective in improving plasmacholesterol, glucose and insulin responses (Sundberget al., 1995; Van Der Sluijs et al., 1999; Yokoyama et al.,1998). Animal studies have shown that the molecularweight of b-glucans can be reduced by an order ofmagnitude (e.g. from 1000 to 200 kDa) and the prepara-tions are still able to reduce plasma cholesterol levels(Yokoyama et al., 2002; Wilson et al., 2004). However, arecent clinical study showed that there were no appreciable

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118 113

effects on the concentrations of blood lipids, insulin orglucose in moderate hypercholesterolemic humans on con-sumption of yeast-leavened oat breads with high (797 kDa)or low (217 kDa) molecular weight b-glucans (Frank et al.,2004). It is probable that these responses are due to thesolid nature of the food matrix which can influence thesolubility and extractability of the added polysaccharide.

Wood (2002) has suggested that the net result of foodprocessing might be to improve the bioavailability orbioactivity of the b-glucan, and that this is related to theproduct of (C�Mw); C being the concentration and Mw

the molecular weight. Human studies have shown inverselinear relationships between log(viscosity) and post-pran-dial glucose and insulin responses after consuming glucosesolutions containing oat gum in the dose range of 1.3–10.5 g pure b-glucan (Wood et al., 1994b). The viscosity ofaqueous oat gum solutions which improved post-prandialglucose and insulin responses when consumed by healthyhumans was within the range of 20–8000mPa s (at 30 s�1),with little additional effect being achieved above 5000mPa s.Regression analysis of their experimental data suggestedthat even at viscosities as low as 10mPa s, an average12–13% reduction in peak plasma glucose could beobserved. In addition, a relationship between the changein peak blood glucose levels (DG), the polysaccharideconcentration (c) and the weight average molecular weight(Mw) of the b-glucan has been found (Wood et al., 2000):

DG ¼ 7:93� 0:68 log10ðcÞ � 1:01 log10ðMwÞðr2 ¼ 0:88Þ.

(3)

However, real foods may differ greatly from the drinkmodel described above. Since the viscosity developed in thegastro-intestinal tract seems to be the most critical variablefor the physiological effects of b-glucans, it is important toalso take into account the modification of the fibre matrixduring its transit in the small intestine when assessing thedietary response to a fibre source. There is also someevidence of changes in the solubility and molecular weightof b-glucan during digestion. Beer et al. (1997b) extractedb-glucan from oat bran and from oat bran muffins using anin vitro system that included a sequential treatment withamylase and pepsin to crudely mimic the human digestionprocess. The original brans used, prior to mixing with otheringredients and cooking, showed about 25–30% b-glucanextractability and a molecular weight of 1–2� 106, whilecooking of the oat bran muffins resulted in reduction of themolecular weight to 600–950� 103, but increased thepercentage of b-glucan solubilised by about threefold (to55–85%), depending on the recipe followed. During frozenstorage, the extractable b-glucan was reduced to 27–40% inall muffins, but no change in the peak molecular weightof b-glucans was detected; the reduced solubility of theb-glucan possibly reflects changes in the molecular organi-sation (chain aggregation) during frozen storage. In arecent study, Brummer et al. (2006) successfully used this invitro assay to predict the effectiveness of a solid food in

attenuating the glycemic response; this was achieved inmuffins subjected to freeze–thaw cycles to modify thesolubility of the polysaccharide. The freeze–thaw treatmentreduced the solubility from 30% to 40% in fresh muffins to10%, whereas the molecular weights of the b-glucans weresimilar for both fresh and frozen products. Blood glucoseresponse was determined in healthy subjects over 2 hand the peak blood glucose rise (PBGR) was signifi-cantly reduced after consuming fresh muffins compared tothose submitted to four freeze–thaw cycles (1.84 vs.2.31mmol/L), i.e. fresh muffins reduced the area underthe glucose response curve twice as much as muffinssubmitted to the freeze–thaw process. Furthermore, inmuffins containing b-glucans of similar molecular weight,there was a significant inverse linear relationship betweenlog(concentration) of solubilised b-glucan and PBGR.Similarly, Ostman et al. (2006) reported a useful tool forprediction of the glycemic responses to bread productsenriched with b-glucans. These researchers found a highcorrelation (r ¼ 0.98) between the GI of normal humansubjects after consumption of b-glucan-enriched breadsand the fluidity index (FI) that was estimated in the in vitroenzymatic digests of bread products using a Bostwickconsistometer. This relationship was described by thefollowing equation:

GI ¼ 50:8þ 0:441FI: (4)

4.2.3. Fate of b-glucan in the gastro-intestinal tract

Animal studies indicate a decrease of b-glucan molecularweight during digestion. Thus, Wood and co-workersdemonstrated a significant depolymerisation of oatb-glucan during digestion in the small intestine of ratsand chicks (Wood, 1994; Wood et al., 1991a); the maxi-mum b-glucan concentration was found in the ileal section,where most of the starch has been absorbed. The molecularweight of oat b-glucan extracted from the intestinal contentof rats was lower relative to that in the intact diet; a 10-foldlower molecular weight of b-glucan was present in thesmall intestine than in the stomach, whereas there werefurther decreases in the caecum and faeces (Wood et al.,1991c). Dongowski et al. (2002) found that b-glucan washighly fermentable in the rat caecum and was not found infaeces. The molecular weight of b-glucan from the stomachand intestines of hamsters was about 100,000, lower thanthat of fibre subjected to processing treatments (Yokoyamaet al., 1998, 2002). Even in pigs, which are usually con-sidered good models for humans, depolymerisation ofb-glucans to a molecular weight of about 100,000 occurredduring passage through the stomach and the upper jejunum(Johansen et al., 1993). This depolymerisation was attri-buted to enzymic activity of microbes present in the gut,but their action does not lead to a loss in b-glucan from thedigesta before it reaches the lower small intestine; trypsinhad no effect on the viscosity of b-glucan solutions. On theother hand, Robertson et al. (1997) showed that endogen-ous proteases from the small intestine of ileostomate

ARTICLE IN PRESSA. Lazaridou, C.G. Biliaderis / Journal of Cereal Science 46 (2007) 101–118114

humans were capable of enhancing the extractability of thebarley b-glucan measured in vitro to a level similar to thatfound in ileal effluent from the patients fed an acute barleybased-diet. The extractability measured in vivo was signi-ficantly higher than that measured in the original barleybut without protease treatment. Increased extractabilitycan be accompanied by a reduction in the molecular weightof the b-glucan in the upper gut, 10% of the total b-glucanrecovered had a molecular weight of o12,000. Similarly,Sundberg et al. (1996) observed some depolymerisation ofb-glucans during the transit through the upper gastro-intestinal tract of ileostomates consuming breads ofdifferent cereals. Thus, ‘unextractable’ b-glucans have thepotential to behave as a source of soluble fibre because ofthe modification during transit in the upper gut (Robertsonet al., 1997). It is also notable that the decrease in b-glucanmolecular weight was significant only in breads containinghigh molecular weight b-glucans, e.g. the 1.3� 106 peakmolecular weight (Mwp) of b-glucan included in a oatbread was found in excreta of ileostomic bags to be�0.7� 106. In contrast, this reduction was limited for abarley b-glucan of relative low molecular weight in theconsumed bread, i.e. the Mwp from 285� 103 was reducedto 262� 103 after passage through the small intestine(Sundberg et al., 1996).

4.2.4. Future perspectives

The ability of soluble fibre to increase intestinal transittime has been also attributed to the gel-formation proper-ties of these materials (Anderson and Chen, 1986);however, soluble fibre such as oat fibre may increase faecalbulk and alleviate constipation (Malkki and Virtanen,2001). The gel network that is formed could also act as amolecular sieve in the small intestine; large molecules couldpass rapidly through the hydrated network, but smallermolecules would be trapped in the various pores forvariable lengths of time slowing down the digestion(retarded diffusion and contact between gastro-intestinalenzymes and their substrates) and absorption of variousnutrients. However, the possible relationships between thegelling properties of cereal b-glucans and their physiologi-cal functions have not yet been examined. To our knowl-edge, there are no reports in the literature addressing thepossible relationships between the structural features of b-glucans and their physiological effects. The latter can bemodulated by interchain aggregation phenomena whichare controlled by the ratio of cellotriosyl/cellotetraosylunits, the sequence of cellotriosyl residues along the chain,and the presence and distribution of long cellulose-likefragments. Further clinical studies must be properlydesigned with well-characterised materials to provideevidence for such structure–function relationships.

5. Conclusions

The physical and physiological properties of b-glucansare of commercial and nutritional importance. Recent

studies show that the molecular and structural features ofcereal b-glucans are important determinants of theirphysical properties, such as water solubility, dispersibility,viscosity, and gelation properties, and thereby affect thefunctionality of these polysaccharides from a technologicalviewpoint when added to food systems. The physiologicalfunctions of cereal b-glucans also seem to be associatedwith viscosity enhancement properties, and this may beaffected by the amount and molecular weight of solubilisedb-glucan in the gastro-intestinal tract. However, thepossible relationships between the gelling properties ofb-glucans and their physiological effects need to be furtherstudied.

Acknowledgements

This work has been carried out with the financial supportfrom the Greek Ministry of Development, GeneralSecretariat for Research & Technology, InternationalCooperation in Industrial Research and DevelopmentProgram DSEBPRO-2005, # 05 DSEBPRO-100, 2006-2008.

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