Comparison of mixing and pasting behaviors of commercially available wheat and gluten-free flours

K.A.F. Gajo (1,2 )* and J.M.C. Dang (1)
(1) Perten Instruments of Australia Pty. Ltd., Unit 13, 2 Eden Park Drive, Macquarie Park NSW 2113, Australia
(2) Food Science and Nutrition, School of Chemical Engineering, Faculty of Engineering, University of New South Wales, Sydney NSW 2052, Australia *Corresponding author: kgajo@perten.com

Abstract

There has been a general increase in the demands for gluten-free (GF) products due to the recent heightened awareness of food intolerances and allergies. A commercially available GF product may employ lipids, carbohydrates and/or other gluten substitutes to achieve a final product that resembles the textural properties of its gluten-containing counterpart. Processing behavior, taste and texture are all important considerations for a manufacturer of GF products. Instrument producers face the challenge to develop a standardized method that can assess the processing suitability of GF ingredients, with the aim of optimizing processing conditions to produce fit-for-purpose GF products. The objectives of this study were to compare the mixing and starch pasting properties of commercially available regular wheat flours (WFs) and GF flours. Four GF flours and four regular (gluten-containing) flours with different ingredients and composition were analyzed using the Perten Instruments micro-doughLAB (mdL, mixing qualities) and Rapid Visco Analyser (RVA, pasting properties). Mixing qualities of regular and GF flours were differentiated on the mdL, with most GF flours exhibiting lower water absorption (WA), earlier dough development (DDT) and shorter stability than regular flours when a control bulking agent was used. RVA results showed that the flours varied in pasting behavior. Expectedly, the GF flours exhibited higher viscosities than regular flours due to their higher starch contents. Owing to differences in ingredients, all flours were differentiated by the RVA and mdL. Future studies will focus on using other bulking agents, assessing nutritional and baking qualities of the various flours, and investigating other quality parameters of different GF flours in order to widen the scientific understanding of GF systems and their properties.

Introduction

Gluten and gluten-free (GF) products are current hot topics in the food industry. GF products are costly to produce, and their sensory qualities are often inferior to their gluten-containing counterparts. A GF flour, when mixed with water, is unable to develop into a traditional dough due to the absence of gluten, forming instead a “batter” that can consist of carbohydrates, lipids and other gluten substitutes. Despite the amount of research on the topic, commercially available GF products are still quite limited. Instrument manufacturers face the challenge of adapting traditional dough quality testing methods to include testing of GF systems so that their properties can be compared. Therefore, analyzing and understanding pasting and mixing properties of GF systems are essential in developing better quality GF products. The Rapid Visco Analyser (RVA) is a cooking and stirring viscometer that analyzes the pasting properties of different samples. The micro-doughLAB (mdL) is a small-scale dough mixer that can analyze the quality and processing characteristics of flours. This project aims to compare the mixing and processing qualities of regular and GF flours.

Material and Methods

Samples and treatments

Four GF flours and four regular WFs were purchased from various local supermarkets (Sydney, Australia), and designated as shown in Table 1. Each sample was thoroughly mixed and stored in separate airtight containers in a cool (18°C), dry environment until analysis.

Sample analyses

The moisture content of each sample was determined by AACCI Method 44-15.02 (AACC International, 1999). Mixing quality was analyzed on an mdL (Perten Instruments of Australia, Macquarie Park, Australia), using 4.00 ± 0.01 g sample (14% moisture basis), according to the method shown in Table 2. For comparison purposes, the mixing characteristics of all flours were analyzed with added wheat gluten (14% flour weight). Pasting properties were analyzed on an RVA 4500 (Perten Instruments of Australia, Macquarie Park, Australia), using 3.50 ± 0.01 g sample (14% moisture basis) and 25.0 ± 0.1 mL distilled water, according to AACCI Method 76-21.01 (AACC International, 1999 ). Sample and water weights were corrected for sample moisture content in mdL and RVA analyses. End of test time was adjusted according to the sample’s mixing properties, to ensure that stability and softening data were obtained.

Table 1: WFs and GF flours used in mixing tests.


aHPMC = hydroxypropyl methylcellulose, CMC = sodium carboxymethylcellulose.

Table 2: micro-doughLAB standard 120 rpm method used for studying the mixing characteristics of WFs and GF flours.

Results and Discussion

Rapid Visco Analyser

Within a sample, it is mainly the starch and its interaction with other components in the system that is measured by the RVA. Starch is the main contributor to viscosity development in a typical pasting curve. In the absence of gluten, a GF product relies on carbohydrates and/or other bulking agents in its formulation to achieve the textural properties of its gluten-containing counterpart. The thickening power of the bulking agent(s) when heated determines the quality of the end product (Swinkels, 1985). The GF samples exhibited typical RVA starch pasting curves and were readily differentiated by the RVA (Figure 1). The higher starch content of GF samples contributed to their high pasting viscosities compared to WF samples (Figure 1, Table 3). The GF samples had complex formulations, containing various thickeners and at different concentrations (Table 1), and interactions between components within the systems influenced the individual swelling power of each thickener. 


Figure 1: Rapid Visco Analyser (RVA) pasting curves of GF flours and regular WFs.

Table 3: Pasting properties of regular WFs and GF flours.a


a All viscosity values (peak, hold, breakdown, final, setback) measured in centipoise (cP); peak time measured in minutes (min); breakdown = peak viscosity – hold viscosity; setback = final viscosity – hold viscosity; PT = pasting temperature (°C); figures are the means of triplicate and duplicate analyses for WFs and GF flours, respectively.

GF1 showed a typical pasting curve for legume flours, with a flat (broad) peak, small breakdown and low setback (Figure 1). With its main ingredient of garbanzo bean (chickpea) flour (Table 1), GF1 exhibited the lowest pasting temperature, earliest peak time and lowest overall viscosities among the GF samples (Figure 1, Table 3). Chickpea flours (or flours from food legumes) generally exhibit lower viscosities than cereal flours due to their lower carbohydrate and higher protein and lipid contents (Wood, 2004). The lowered viscosity of GF1 compared to the other GF samples may also have been a result of interactions between the starches, lipids and proteins within the sample. GF1 had higher overall viscosities than the WFs (Figure 1, Table 3) due to the presence of potato starch and tapioca flour.

GF2 exhibited the highest viscosity among the samples (Figure 1, Table 3) due to its high starch content and the presence of four hydrocolloids (Table 1), including guar and xanthan gums, which are known to have synergistic effects on viscosity (Young, 2007). Tapioca is known for its clear, long stringy texture when gelatinized, which can trap air bubbles within the starch matrix, while its elastic nature is characterized by its poor ability to withstand excessive shear stress (Thomas & Atwell, 1999). The trapped air bubbles and/or prolonged heating of the sample may have caused gel destabilization, resulting in an unstable curve for GF2 (Figure 1).

Even though GF3 and GF4 have the same ingredients (Table 1), the sensitivity of the RVA repeatedly differentiated the two samples (Figure 1, Table 3). The slight differences in the RVA curves may be attributed to differences in flour particle size, and concentrations and qualities of the various components.

The GF samples (GF2, GF3 and GF4) containing guar gum had higher final viscosities than those samples without guar (GF1 and all WFs). These observations agree with Young (2007), who reported that guar gum in a 1% aqueous concentration had the highest viscosity during the cooling stage in comparison to locust bean gum and xanthan gum.

The WF samples exhibited typical pasting curves for WFs (Figure 1, Table 3). There was less discrimination among the WF samples since they have essentially the same makeup (WF of varying starch content). The wholemeal FLOUR2 had the lowest overall viscosity among WFs due to its lower starch content (~64%) compared to white flours (~71-73%) (FSANZ, 2013, Gómez, et al., 2009). FLOUR3 is a strong, high-protein flour intended for bread and pizza doughs. In comparison to other WFs, the added wheat gluten in its formulation may have contributed to its overall high viscosities by strengthening the starch-protein matrix.

The RVA was able to differentiate between and within the GF and WF sample sets, with less discrimination among the WF samples due to the similarities in starch components. The GF samples had higher viscosities than WF samples (Figure 1) due to the high starch and/or gum contents in GF flours and poorer swelling power of wheat starch in WFs.

micro-doughLAB

A bakery dough (especially for bread) is mixed to develop the gluten, forming a protein-starch matrix where trapped air bubbles can expand during proofing and baking. These bubbles are held in place as the gelatinized starch sets after baking, and the final product volume is achieved. In the absence of gluten, starch is the main component of GF flours. The creation of volume in baked goods relies on the ability of starch to swell when cooked at higher temperatures, as demonstrated by the RVA. Mixing tests are performed at 30°C to exploit the mixing quality of gluten in WFs. However, at this low test temperature, the mixing tests do not exploit the pasting properties of starch in GF flours.

Without gluten, the GF “doughs” were adhesive and had poor mixing properties. The “mixing” curves showed a high initial torque as water hydrated the flour components, followed by collapse of torque as energy was imparted on the undeveloped “dough” (Figure 2) and gave unstable “mixing” curves. In order to compare GF and WF samples, the same amount of the chosen control bulking agent (wheat gluten) was added to each sample during mixing tests. Although it seems counterintuitive for this study, wheat gluten was chosen as the bulking agent since it is a widely studied protein with known behavior, and is sufficient for validating the feasibility of this experiment. Addition of gluten to the GF flour resulted in a more typical mixing curve with an initial hydration peak followed by a development peak (Figure 2). 


Figure 2: Effect of adding a bulking agent (wheat gluten) on the mixing curve of GF dough.


Figure 3: micro-doughLAB (mdL) mixing curves of regular WFs and GF flours, with added bulking agent (wheat gluten) and optimized to 130 mNm.

Table 4: Mixing properties of regular WFs and GF flours with and without the addition of bulking agent (wheat gluten), tested on the micro-doughLAB (mdL).a


a WA = water absorption; DDT = dough development time.

Mixing curves of GF flours supplemented with gluten showed similarities to a typical mixing curve of WFs (Figure 3). The mixing curve of GF1 (supplemented) was most similar to those of the WFs, and had the longest stability among supplemented GF samples (Table 4), due to its protein-rich garbanzo bean flour component (Table 1). The other supplemented GF samples had earlier development times and shorter stabilities than the supplemented WFs. In other words, the GF samples, with the exception of GF1, had poorer mixing abilities than WFs. The results indicate that, if volume is a main quality consideration for a GF product, then including a non-gluten protein in the formulation as a main component may be essential in achieving and maintaining final product volume.

The mixing curves of WFs were harder to differentiate in comparison to GF flours. This observation was not surprising since WF often contains wheat only. Addition of vital wheat gluten to WFs increased their WA, stability and DDT (Table 4).

Conclusions

The mixing and pasting characteristics of regular WFs and GF flours were compared using pasting (RVA) and mixing (mdL) methods. Both methods differentiated the WF and GF flour sets. By exploiting the main component (starch) of GF formulations, the RVA provided better discrimination of samples among each of the WF and GF flour sets than the mdL. The RVA would therefore be the method of choice for determining the suitability of GF flours for particular purposes (e.g. breadmaking). It should be noted that GF formulations are complex systems, and the quality of their final products are affected by processing conditions (e.g. mixing and baking). This research will be extended to include other control bulking agents, testing with full formulation and testing the quality of the resultant baked products with the Bread Volume Meter.

References

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