INTRODUCTION

The ß-(13),(14)-D-glucans (ß-glucans) of barley and oat endosperm cell walls have been suggested to have an important influence on human nutrition and health. Foremost, has been a reported hypocholesterolemic effect^1-3. As such, beta-glucans would seem a prime candidate as a cereal nutraceutical. Neutraceuticals are defined as any substance that is a food or part of a food that provides medical and/or health benefits⁴.

Prior to incorporation of cereal beta-glucans into food products, it is essential that their influence on processing parameters and product quality be investigated. Barley flour has been satisfactorily used in baked products^{5, 6}. Although the incorporation of a beta-glucan-rich fraction into white pan bread has been reported⁷, information on the use and effects of extracted beta-glucans in baked products is limited. The objective of this study was to investigate the effects of beta-glucans extracted from a hull-less barley on wheat starch and flour functionality.

MATERIALS AND METHODS

Samples: The hull-less barley variety, Wanubet, (1989, Martin, ND) was used throughout this study. Hard red spring wheat flour was used for baking and dough rheology studies. Wheat starch and carboxymethylcellulose (CMC) were purchased from Sigma Chemical Company (St. Louis, MO).

Preparation of -Glucan: Barley beta-glucans were extracted with sodium carbonate⁸. Crude -glucan was dissolved in water (90C), and any undissolved material was removed by centrifugation. The supernatant was adjusted to 50% with respect to iso-propanol. The precipitate was allowed to settle and was collected by centrifugation. This fraction, which was used in all additional studies, contained 85.0% -glucan and 1.9% protein. The remainder of this preparation was probably predominately arabinoxylan. Structural and rheological features of a highly purified beta-glucan from the same source were previously described⁹.

Pasting Properties: Starch suspensions (5.5% and 8.0% db) were prepared by shaking wheat starch with distilled water (460 ml), and starch/-glucan suspensions were prepared by dispersing wheat starch (8% db) into an aqueous beta-glucan solution (0.45% db)(460 ml). Starch (8% db)/CMC (0.45% db) suspensions and gum solutions (CMC and beta -glucan, 0.45% db) also were evaluated. Pasting properties were analyzed with a Brabender Amylograph (C.W. Brabender Instruments, Inc., South Hackensack, N.J.). The suspensions were heated to 30^oC, and then to 96^oC at a rate of 1.5^oC/min. Following 20 min at 96^oC, the suspensions were cooled to 50^oC at a rate of 1.5^oC/min.

Dough Rheology and Bread Properties: The absorption and mixing characteristics were measured with the Brabender farinograph, using the AACC¹⁰ constant flour weight procedure. A portion of the flour (1%) was replaced with wheat starch for the control. Beta-glucan was evaluated by replacing 1% of the flour. Test baking was conducted with 100 g loaves using a straight-dough method with a 2-hr fermentation time, a 55- min proof time, and a 25-min bake at 221C. The baking formula based on flour weight (14% moisture basis) was: flour, 100 g; compressed bakers yeast, 3.0%; sugar, 5.0%; salt, 2.0%; shortening, 3.0%; potassium bromate, 20 ppm; and -amylase, 15 SKB units. Baking absorption was estimated by farinograph absorption, and mixing times were determined by the feel and appearance of the dough. Loaf volume was measured 20 min after baking.

Bread characteristics were scored by a trained judge. Bread was sliced 4 hr after baking and stored at 25^oC in sealed plastic bags to prevent moisture loss. Bread firmness¹⁰ was evaluated on a Universal Instron Testing Instrument Model 1000 (Instron Corp., Canton, Mass.) at 0, 1, 2, and 4 days after baking with compression force value used to measure the bread firmness¹¹. Three slices were used for each loaf, and three positions on each slice were tested.

RESULTS AND DISCUSSION

Pasting Properties of Starch/-Glucan Suspensions: The addition of crude beta-glucan or CMC to starch water suspensions may increase the force exerted on the starch granules in the shear field during heating. The sensitivity to swelling of wheat starch during the early stage of heating was increased by heating the wheat starch suspension (8% w/v) in the presence of CMC (0.45% w/v). Miller et al.¹² suggested that an increase in viscosity of starch-water suspensions after the starch granule swelling was due to the released soluble amylose and inter-molecular entanglement through hydrogen bonds and van der Waals forces. The addition of CMC may contribute to network formation through intermolecular interaction; thus, enhancing the viscosity effects. As was observed for CMC, beta -glucan hastened the onset of initial paste viscosity and significantly increased the pasting viscosity of wheat starch (Table I). The combined viscosity of starch and beta-glucan was greater than the sum of the individual viscosities. This observation suggests synergism between wheat starch and -glucan. Synergism between beta-glucan and methylcellulose has been previously reported⁹. Beta-glucan appeared to function in a similar manner as CMC but exhibited greater interaction with starch. Beta-glucan, as well as CMC, also may compete with starch for water. This increases the concentration of soluble starch in the remaining free water; thus, sharply increasing viscosity.
 
 

Table 1. Effect of -Glucan on the Pasting Properties of Wheat Starch Suspensions^a.

	Transition	Viscosity (BU)
Suspensionsb	temp (oC)	96o C	20-min. hold	Cool to 50oC
5.5% starch	87 a	27 a	24 a	98 a
8.0% starch	75 b	141 b	224 b	508 b
8.0% starch/0.45% CMC	60 c	252 c	486 c	879 c
8.0% starch/0.45% -glucans	57 d	501 d	720 d	1171 d

 
 
 

^a Mean of duplicate determinations. Means for each column with the same letter do not differ significantly (P<0.05).

^b Dry weight basis.

Since a normal peak viscosity was not obtained, the 96C and 20 min hold viscosities were used to compare the differences in pasting viscosity. The 96C viscosity of beta-glucan/starch blends was much greater than that of the CMC/starch blends. Differences in viscosities following 20 min of heating suggests that beta-glucan interacts with starch to a greater extent than CMC. The setback pattern of the starch/beta-glucan suspension showed a rapid rise in viscosity and had a similar rate of setback to the starch/CMC suspension. It was concluded that beta-glucan had great synergistic thickening effect with wheat starch.

Effect of -Glucan on Mixing Characteristics of Dough: The control (99% flour and 1% starch) had a mixing time of 8 min and 58.6% absorption. When a dry mixture of 99% flour and 1% beta-glucan preparation was tested with the farinograph at the same absorption level, the dough consistency increased from 500 to 700 BU, and the mixing time was considerably shortened. The beta-glucan absorbed more water; thus, requiring an increased farinograph water absorption. With adjustment of the water absorption to the 500 BU line; however, the beta-glucan increased dough development time from 8 min to 9.5 min (Table II). The water-binding capacity of the beta -glucan appeared to be similar to that reported for water-soluble pentosans¹³. It should be noted that the crude beta-glucan preparation utilized in this study may have contained a small portion (10%) pentosan (arabinoxylan).

Table 2. Effect of -Glucan on Farinogram Properties^a.

	Control	1% -glucan
Absorption (%) at 500 BU	58.6 a	63.8 b
Dough Development Time (min)	8.0 a	9.5 b
Arrival Time (min)	4.0 a	5.0 b
Departure Time (min)	19.0 a	18.0 a
Stability (min)	15.0 a	13.0 a
Mechanical Tolerance Index (BU)	23.5 a	20.0 a

 

^a Mean of duplicate determinations. Means for each row with the same letter do not differ significantly (P<0.05).

Dough stability and mechanical tolerance index (MTI) also were affected somewhat when beta-glucan was added and compared at an equal consistency level of 500 BU (Table II). Compared to the control doughs, the stability of beta-glucan containing doughs decreased, while MTI slightly decreased. Similar to barley beta-glucan, barley meal replacement of white flour increased the farinograph absorption, arrival time, and dough development time¹⁴. Since the beta-glucan fraction used in this study contains only a small amount (1.9%) of protein, the effect of the protein on the mixing characteristics was probably negligible. Therefore, the barley -glucan may exert its largest effect on mixing characteristics through increased water absorption.

Effect of -Glucan on Bread Properties: The addition of 1% -glucan preparation to the baking formula increased absorption. The mixing time of the doughs also increased with the incorporation of the beta-glucan. Despite the increased water absorption, beta-glucan addition produced stiffer doughs than those of the control after mixing. Addition of the beta -glucan also appeared to slightly increase loaf volume (Table III), perhaps because of improved gas retention. This effect however, was not significant (P<0.05).

Various gums, used as gluten substitutes, have been used to produce acceptable breads from starches and certain non-wheat flours^15-17. Likewise, incorporation of -Glucan (1%) into wheat flour may slow the rate of carbon dioxide diffusion in the dough by providing appropriate dough consistency. The synergistic increase in viscosity with starch during gelatinization, as shown by the amylograph, may support the hypothesis of improved gas retention. However, direct interaction of -glucan with gluten proteins has not been demonstrated. Beta-glucan slightly improved bread grain and texture, cells were more elongated cells, and the crumb had a silkier texture. Addition of beta-glucan did not affect crumb color.

Table 3. Effect of -Glucan on Baking Properties^a .

	Control	1% -glucan
Baking absorption (%)	57.0 a	62.3 b
Mixing Time (min)	2.8 a	3.3 a
Specific Loaf Volume (g/cc)	6.3 a	6.4 a

 
 
 

^a Values represent the mean of five loaves. Means for each row with the same letter do not differ significantly (P > 0.05).

Effect on Bread Firming: As all other variables, except the level of water absorption were constant, the differences in bread firmness between the control and the beta -glucan-containing breads can probably be attributed to the addition of beta-glucan or to the increased absorption. When compared to the control breads, the breads containing beta-glucan consistently gave higher values in compression force measurements. This indicated that beta -glucan, despite the increased absorption, yielded breads with firmer crumb, probably by creating higher dough consistency. The firmness of control and beta-glucan containing breads increased with storage time at similar rates. The incorporation of 1% beta-glucan to bread did not affect bread firming during the 4-day storage period (P<0.05) when breads where compared at the same storage times. Starch retrogradation is the most important factor involved in bread firming, and the effect of other flour and bread components are minor¹³. Beta-glucan gels, however, also may retrograde during aging⁹.

CONCLUSIONS

Beta-glucan was determined to have a pronounced thickening effect with wheat starch. The synergistic effect of beta-glucan with starch could result in many food applications, where starch is a primary ingredient. Beta-glucan could be incorporated into baked products as an alternative to cellulose gums. Results of this study suggest that beta-glucans, utilized at low levels, may have some improving effects on bread quality. While somewhat similar in composition to cellulose derivatives, cereal -glucans can be considered a natural product and are associated with nutritional benefits.

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