Evaluation of Starch Degradation and Textural Characteristics of Dietary Fiber Enriched Biscuits

Abstract

The effects of dietary fiber inclusion on biscuit texture, cooking properties, and sugar release after in vitro degradation were investigated. Inulin, β-glucan enriched fraction (BGEF), potato fiber, and a resistant starch were used. Effects of the dietary fibers on the pasting properties of flour based mixtures were investigated using a rapid visco analyzer. Results showed a reduction in visco-pasting properties (peak viscosity and final viscosity) of the flour–dietary fiber systems with increasing fiber content. Addition of dietary fiber into the biscuits affected biscuit shrinkage and height development during cooking, as well as generally reducing the resistance of biscuits to snapping during texture analysis tests. Sugar release during in vitro degradation studies showed a highly significant decline when comparing dietary fiber enriched biscuits to the control samples. This would have a beneficial effect in potentially reducing the glycaemic index and subsequent glycaemic loading of such foods.

Keywords:

• Biscuits
• Diabetes
• Dietary fiber
• Glycaemic index
• Inulin
• Starch gelatinization
• Rapid Visco Analyzer

Introduction

Consumer attention has recently focused on low-calorie food products, with emphasis being placed on healthy eating and increased fiber within a balanced diet. From the food industry's perspective, understanding of the function of these raw ingredients is critical in order to manipulate the quality of the final product.1 2 3 4

The determination of beneficial implications of dietary fiber in a wide variety of food products has resulted in fiber being regarded as a “functional” ingredient. The physiological responses to fiber consumption have been well documented in relation to reduced risks of chronic diseases.5 6 7 8 Dietary fiber, especially soluble, viscous fibers, such as β-glucans are generally recognized as having important positive health benefits such as regulation of serum cholesterol and low density lipoprotein (LDL) cholesterol, prevention of constipation, production of short-chain fatty acids, reduction of the risk of diabetes and intestinal cancer and stimulation of beneficial gut microflora.6 9 10 11 12 13 As dietary fiber is not digestible by enzymes in the small intestine any effects on metabolism are mediated through physical-chemical changes to food as it passes through the gastrointestinal tract. These changes arise from affects on water-holding capacity, food bulking, viscosity developments, ability to absorb or bind bile acids and fermentibility by microbes in the gut.6 7

Biscuits may be regarded as relatively simple cereal based food products. Basic biscuit ingredients include flour, sugar, fat, and water. In terms of quality, biscuits tend to be influenced highly by the physico-chemical properties of their doughs,2 4 which are in turn influenced by protein composition of the flour, fat, and water levels and baking parameters.14 15 Starch is another important structural element in dough and biscuits, with granules occupying space within the overall structure. Biochemical and physio-chemical reactions that occur during baking (including water evaporation, protein denaturation, and Mailliard reactions) may be related to the gelatinization properties of starch. As such, many studies have focused on starch degradation and on components that effect gelatinization of starch.16 17 18 19 20

The rate and extent of the degradation of starch are also reflected in the magnitude and the duration of the glycaemic response.21 22 23 Nutritional classification based on in vitro evaluation of the rate and extent of starch digestion is useful in predicting the likely glycaemic response to foods.24 However, starch degradability in the gut is also influenced by a number of other factors including, the extent to which the food is chewed, the amount of pancreatic amylase available and transit time through the small intestine, nature of the carbohydrate, food form, food processing, fiber, and anti-nutrient content.23 25 26

One of the major problems involved in incorporating dietary fibers into a food system is the negative it has on the functional properties of other components within food. In meeting the expectations of consumers in terms of overall quality of color, texture, and mouthfeel, fiber can only be added to food if the product being manufactured has good sensory characteristics, irrespective of the nutritional benefits.27

There has been limited research aimed at investigating how dietary fiber additions affect biscuit quality, characteristics, texture, and starch digestibility. The aim of the present study investigates the possible mechanisms involved in the modulation of postprandial glycaemia, after the consumption of biscuits with the inclusion of different types of dietary fiber.

Materials and Methods

Starch Pasting Properties

Pasting properties of the samples were examined using a Rapid Visco Analyzer (RVA, Newport Scientific Pty. Ltd., Warriewood, NSW, Australia) interfaced with a computer equipped with Thermocline and Thermoview software (Newport Scientific). For all pasting experiments purified wheat starch was used as the prime starch (product number 9005-25-8, VWR International–BDH, Loughborough, UK). Prime starch (3.5 g) was added to 25 mL of distilled water in an aluminum canister. Fibers replaced prime starch at the following proportions (w/w): 2.5, 5, and 10%. Fiber used included an RS2 resistant starch (National Starch & Chemical Ltd., Manchester, UK); inulin (Frutafit HD, Calleva Ltd., Reading, UK); potato fiber (Potex, Lyckeby Stärkelsen Food & Fiber Ltd., Kristinstad, Sweden); beta-glucan enriched flour (Nutrim-OB, Futureceutical Ltd., Momence, USA). Total run time was 13 min using methodology described by El-Khayat et al.28

Biscuit Preparation

Biscuit formulation is detailed in Table 1 using the biscuit flour golden dawn plain (Allied Mills, Tewkesbury, UK). For preparation of biscuits, flour, fiber, fat, and baking powder were placed in an electric cake mixer (Kithchenaid, Europa Inc., Brussels, Belgium) and mixed for 1 min (speed 4). Water was then added and mixed for a further 2 min (speed 4). Dough was allowed to rest at room temperature for 15 min before being sheeted to a thickness of 2 mm using aluminum guides, and by rolling with a rolling pin. Biscuits were shaped with a cutter (52 mm diam) and baked on and aluminum tray at 180°C for 14 min. Biscuits were cooled for 30 min to room temperature, and analyzed for fracture properties.

Table 1 Formulation of biscuits

Sample	Flour (g)	Fiber (g)	Other ingredients
Control	225	—
2.5% Fiber addition	219.4	5.6	50 g Vegetable fat
5% Fiber addition	213.7	11.3	100 mL Water
10% Fiber addition	202.5	22.5	8 g Baking powder

Evaluation of Biscuits Physical Characteristics

The diameter (mm) and thickness (mm) of 10 biscuits prior to, and after, baking were evaluated using a micrometer.

Fracturability Characteristics

The fracture properties of biscuits were analyzed using a texture analyser (TA-TX2, Stable Micro Systems, Godalming, Surrey, UK) calibrated for a load cell of 25 kg. Fracturability was determined by the bend or “snap” test, using the 3-point bend rig (settings: pretest speed, 2 mm/s; test speed, 0.5 mm/s; posttest speed, 10 mm/s, distance, 5 mm; trigger type, auto 20 g; rate for data acquisition; 400 points per second). At least 10 biscuits were tested for each batch and the measurements were taken in the central part of the biscuit. Each biscuit was compressed once and peak force was recorded.

Starch Content

Total starch was determined in the cooked biscuit using a Megazyme starch determination kit (Megazyme International, Wicklow, Ireland) according to the Approved Methods 55-10, 44-15, 56-81B.29 Dry matter was determined according to Approved Method 76-21.29

In Vitro Digestibility of Starch

Samples (4 g) were mixed with 19.5 mL of sodium phosphate buffer (pH 6.9), 1.5 mL 8 M HCl (pH 1.5) and 5 mL porcine pepsin (115 Au/mL, product number 9001-75-6 VWR International–BDH, Loughborough, UK) and allowed to digest for 30 min at 37°C. After digestion, the pH of the samples were corrected with 1.5 mL of 10% NaOH to 6.9 and 22.5 mL of phosphate buffer and 1 mL of porcine α-amylase (110 Au/mL, product number 9000-90-2 VWR International–BDH, Loughborough, UK) was added. The mixtures were placed into dialysis bags made of 15 cm strips from dialysis tube (TWT-400-070 M, 24/32′′, 19.0 mm, Medicell International Ltd., Reading, UK). Dialysis bags were placed into 450 mL phosphate enzyme/buffer solution and held at 37°C. Every 30 min, from 0 to 120 min, duplicate 1 mL samples were taken and 0.5 mL samples were taken at 150, 180, and 270 min for analysis of reducing sugar released (RSR). Dialysate was analyzed for total dilysable sugars by the 3,5-dinitrosalicylic acid method.30 A sample blank (sample + deactivated enzyme) and a maltose blank (sample, deactivated enzyme, and 5 mL 20% maltose solution) were analyzed. Total reducing sugars released are expressed in terms of milligram of sugars per gram of available starch as of the formula of Brighenti et al.31 A control solution containing 1g pure maltose dissolved in 50 mL phosphate buffer in the absence of sample. Digestibility determinations were conducted in triplicate.

Statistical Analysis

The data collected from all experiments were calculated as mean ± SEM and all determinations were made at least in triplicate. Analysis of variance, followed by Tukey's test was achieved using Minitab software.

Results and Discussion

Flour Pasting Properties

The starch pasting properties of flour and dietary fiber mixes, as measured by the RVA, are shown in Table 2. Pasting parameters generated by the RVA provide a relative measure of starch gelatinization, swelling, and gelling ability. As starch is heated in the presence of water, granules swell, and absorb moisture leading to disruption of hydrogen bonds, and leaching of amylase from the starch granules. As the mixture is subsequently cooled, re-association between starch molecules occurs to form a gel network.

Table 2 Pasting properties of flour and dietary fiber mixes as measured by the rapid visco analyzer (RVA)

Peak viscosity (CU)	Final viscosity (CU)	Setback
Control	0	131.80^a	117.00^a	74.20^a
RS	2.5	129.56^a	117.06^a	73.75^a
	5	118.69^ab	108.53^abc	69.14^abcd
	10	96.72^cd	95.80^bd	64.70^bde
Inulin	2.5	127.75^a	118.50^a	74.19^a
	5	107.03^bc	104.14^bc	68.22^abd
	10	84.53^d	91.03^de	62.06^de
Potato fiber	2.5	141.17^e	130.47^abc	79.92^ab
	5	130.69^abc	127.81^abc	81.39^a
	10	149.92^e	156.50^g	96.86^f
BGEF	2.5	119.86^ab	101.75^bc	66.58^bd
	5	111.11^abc	85.17^eh	58.64^dg
	10	114.78^abc	78.42^h	51.22^g

Values are means of at least three determinations. Within columns means with the same subscript are not significantly different (P < 0.05).

Incorporation of fiber influenced flour pasting properties. Peak viscosity showed significant differences between potato fiber and beta-glucan enriched flour (BGEF). Final viscosity and setback displayed significant differences between BGEF against Inulin, Potato fiber, and RS (Table 2).

Eliasson32 observed that changes in rheological properties during gelatinization of starch depended on the presence of swollen starch granules as a dispersed amylose-amylopectin matrix, and the interaction between both constituents. These physico-chemical properties are affected by fiber inclusion. Low inclusion levels of the resistant starch, showed similar pasting properties to that of the control. However, as the concentration of RS increased, the visco-properties of the flour paste reduced (Table 2) this is likely to be due to a reduction in gelatinisable starch resulting from the replacement of available prime starch with RS starch.

Addition of Inulin also showed a decrease in peak and final viscosity development associated with increasing levels of inclusion, possibly due to the replacement of starch as observed in the RS sample. Alternatively, the soluble fiber may be inhibiting starch gelatinization and hence pasting. The inclusion of potato fiber into the flour sample resulted in an increase in visco-properties of the pastes. This may be due to the high water-holding capacity of the fiber and a tendency to form a networked gel structure. The addition of BGEF to flour did not significantly alter the peak viscosity of the material but did reduce the final viscosity of the paste. This may be due to the relatively high β-glucan content in the fiber (12%). The increased levels of soluble cell wall polysaccharides in the flour paste may give the paste a more elastic characteristic.

Biscuit Physico-Chemical Characteristics

The mechanical properties of biscuits are important when evaluating the quality attributes from the point of view of consumer acceptance. Amongst other things, the mechanical characteristics of biscuits depend on the properties of its matrix. Biscuit dough is representative of complex systems containing relatively low amounts of water, and a matrix in which gas bubbles of various size and shape are embedded within a starch, protein, and lipid paste.1 14 Results obtained from biscuit physical characteristics and textural analysis are presented in Table 3.

Table 3 Some physical characteristics of biscuits containing different dietary fiber

Shrinkage (cm)	Height (cm)	Firmness (N)
Control	0	3.78 ± 0.23^ab	3.85 ± 0.40^abc	10.81 ± 2.62^a
RS	2.5	3.97 ± 0.18^ab	4.28 ± 0.17^ab	8.58 ± 0.92^a
	5	4.01 ± 0.17^ab	4.29 ± 0.22^ab	8.86 ± 2.01^a
	10	4.03 ± 0.28^ab	4.37 ± 0.33^ab	9.31 ± 1.80^a
Inulin	2.5	3.93 ± 0.08^a	4.22 ± 0.40^a	8.75 ± 1.84^a
	5	3.54 ± 0.39^abc	3.58 ± 0.26^ac	8.96 ± 1.96^a
	10	2.68 ± 0.14^d	3.82 ± 0.40^ac	8.20 ± 1.83^a
Potato fiber	2.5	3.85 ± 0.12^b	4.80 ± 0.10^d	10.36 ± 1.32^a
	5	3.80 ± 0.20^ab	4.12 ± 0.03^ab	11.09 ± 1.46^a
	10	3.76 ± 0.04^ab	4.17 ± 0.18^ab	11.29 ± 1.66^a
BGEF	2.5	3.27 ± 0.24^c	4.80 ± 0.16^d	10.43 ± 2.46^a
	5	2.88 ± 0.04^e	5.52 ± 0.21	8.55 ± 1.67^a
	10	2.90 ± 0.12^e	4.60 ± 0.11^d	8.49 ± 1.59^a

Although when comparing the fiber enriched biscuits against the control, biscuit diameter appeared to decrease after baking (Table 3), this effect was nonsignificant (excluding the BGEF sample). The nonsignificant reduction in biscuit shrinkage observed in the RS, inulin, and potato fiber enriched samples indicates that the fibers may be acting as stabilizers to the biscuit dough mixture, enabling the reformed biscuit dough to retain its diameter during baking. Variations in biscuit height were also nonsignificant (excluding the BGEF sample).

Texture is also an important factor in the consumer acceptance of baked products, with research focusing on the use of carbohydrates and proteins as fat replacers.19 33 34 Table 3 illustrates that there was a general (nonsignificant) trend of fiber additions reducing biscuit firmness compared with the control (with the notable exception of Potato fiber). The fact that the fiber enriched biscuits did not exhibit significant differences in texture compared against the control sample indicates that, fiber addition to biscuits of up to 10% may be used without having any negative effect on the fracturability characteristics of the biscuit.

In Vitro Starch Degradation

The rate and extent of starch degradation is related to the reducing sugar release during digesta and hence the glycaemic response of an individual.35 In biscuits, starch is entrapped within a food matrix comprising fully and partially gelatinized starch granules in a protein matrix. Brighenti et al.31 suggested that dietary fiber inclusion may reduce the glycaemic response of an individual by reducing the accessibility of α-amylase to starch within a food matrix. Further research by Brennan et al.36 and Huth et al.37 indicated that the digestion of starch, and hence sugar release from foods, may be delayed due to dietary fibers adhering to starch granules and possibly increasing digesta viscosity.

Figure 1 illustrates the rate of reducing sugar release during in vitro digestion. Fiber addition significantly reduced the extent and rate of sugar release during digestion. These results are consistent with other studies on cereal based food products. Figure 1 illustrates that the inclusion of inulin, potato fiber, and resistant starch have similar effects in affecting the release of reducing sugar, and hence the degradation of starch from biscuits. In all three cases the reduction in reducing sugar release (compared against the control) is greater than what would be expected from a straight replacement factor. As such this clearly illustrates the role some forms of dietary fibers have in inhibiting starch degradation. The rate of sugar release observed during the digestion of biscuits with added BGEF could be explained by the fact that the BGEF itself contained 60% starch (manufacturer specifications). However, even this dietary fiber ingredient significantly reduced sugar release, possibly due to the β-glucan in the BGEF regulating starch degradation.

Figure 1. Reducing sugar release (RSR) over a 270 min (expressed as mg of RSR/g available starch) digestion of biscuits with added (a) inulin, (b) BGEF, (c) potato fiber, (d) resistant starch.

Surprisingly there appeared to be a slight increase in sugar release with increasing fiber concentration. This is apparently contrary to other studies, which has indicated a general decline in sugar release with increasing fiber inclusion.38 39 As such the results suggest that there may be an optimum level of fiber addition with regards to regulating starch degradation in biscuits. Further studies need to be conducted to evaluate this possibility.

Conclusion

Results from this study demonstrate that the potential benefits of dietary fiber inclusion in bread and pasta food products can be transferred to biscuit items, in that the inclusion of dietary fibers into biscuits can significantly decrease the amount of sugars released during digestion. This reduction in sugar release appears to be independent of fiber concentration, indicating that even small levels of fiber addition may prove useful in regulating sugar. The use of such ingredients could therefore be used in the regulation of starch degradation and hence the glycaemic response of biscuits without significantly affecting biscuit texture.

Acknowledgments

The authors wish to thank Miss Carmen Tudorica for her help in the preparation of this document.