Ultrasonic investigation of the effects of composition on the volume fraction of bubbles and changes in their relative sizes in non-yeasted gluten-starch blend doughs
Introduction
There is a significant relationship between dough aeration during mixing and the cellular structure of the baked bread (Campbell et al., 2001, Campbell et al., 1998). It has been shown that dough aeration is influenced by mixer type (Peighambardoust et al., 2010, Whitworth and Alava, 1999), mixing headspace pressure (Chin et al., 2004, Elmehdi et al., 2004), mixing time (Campbell et al., 1998, Mehta et al., 2009), water content (Chin et al., 2005, Peighambardoust et al., 2010) and various other dough ingredients (Chin et al., 2005, Mehta et al., 2009). Resolving how dough properties are affected by changes in ingredients and mixing process parameters is not a trivial task (Koksel and Scanlon, 2012), so that understanding the mechanisms governing the changes in dough aeration is a longstanding research challenge (Baker and Mize, 1941).
Working with model gluten-starch (G-S) blend doughs enables the role of gluten and starch in dough systems to be probed in a simple way (Uthayakumaran and Lukow, 2003, Watanabe et al., 2002, Yang et al., 2011). The complexity of interactions of protein and starch with other constituents (e.g., pentosans, damaged starch, endogenous lipids and enzymes) is minimized (Petrofsky and Hoseney, 1995, Uthayakumaran and Lukow, 2003), while the use of gluten from one source eliminates variations that arise from proteins of different characteristics. Moreover, non-yeasted G-S blend doughs are relatively stable systems that do not allow bubbles to cream out so that changes in the concentration and sizes of bubbles can be studied as a function of time. Despite the simplifications afforded by G-S blends, it is still experimentally very challenging to study bubbles and their evolution since all doughs lack optical transparency, bubbles change rapidly and they are very fragile (Bellido et al., 2006, Shimiya and Nakamura, 1997, Strybulevych et al., 2012).
Investigations of bubble size distributions (BSDs) in dough have been conducted with several methods, including light microscopy (Carlson and Bohlin, 1978), conventional bench-top X-ray microtomography (Bellido et al., 2006), synchrotron X-ray microtomography (Koksel et al., 2016, Turbin-Orger et al., 2012), magnetic resonance imaging (De Guio et al., 2009), and confocal laser scanning microscopy (Upadhyay et al., 2012). Low-intensity ultrasound has also been used to characterize dough aeration (Elmehdi et al., 2005, Elmehdi et al., 2004), because its rapid and non-destructive nature makes it well suited for studying these optically opaque systems (Koksel et al., 2014, Létang et al., 2001, Ross et al., 2004, Scanlon et al., 2008, Strybulevych et al., 2012). Of particular interest in determination of bubble sizes in dough, a broad band of appropriate frequencies can be used to ascertain bubble sizes from the measured ultrasonic parameters, i.e., from the phase velocity and attenuation coefficient (Leroy et al., 2008, Scanlon and Page, 2015). Precise ultrasonic determinations of the bubble size distribution (BSD) in bread dough is still being established (Leroy et al., 2008, Scanlon and Page, 2015), but changes in the distribution are readily accessible from changes in the bubbles’ acoustic signature (Koksel et al., 2014, Strybulevych et al., 2012).
To better understand how the various components of the dough matrix interact to alter dough aeration properties, the first objective of this study was to use the bubbles’ acoustic signature to investigate how changes in the volume fraction of starch granules and the hydration of the gluten affect the amount of gas occluded into the dough during mixing. The second objective was to investigate the rate of relative change in the BSD in these different “dough” systems based on time-dependent changes in the acoustic signature.
Section snippets
Sample preparation
Dough ingredient specifications (Koksel and Scanlon, 2012) and sample preparation for ultrasonic measurements (Koksel et al., 2014) are in accordance with previous descriptions. Gluten-starch (G-S) blend doughs of varying composition were prepared by addition of saline solution (3.2% w/w) at 90, 95 and 100% (total G-S blend weight basis). Neither yeast nor leavening agents were used in the G-S blend dough formulation. Therefore changes in bubbles will arise only from disproportionation (
Effects of gluten, starch and water on dough density
The effects of composition on dough density, dough matrix density and air volume fraction are shown in Table 2. At a given water content, dough density decreased as gluten content increased, which resulted in a greater air volume fraction since the calculated dough matrix density decreased only slowly with increasing gluten. This result accords with the results of Koksel and Scanlon (2012), who reported that when doughs are mixed for a fixed period of time, dough density decreases as gluten
Conclusion
An ultrasonic transmission technique has been used for monitoring the relative changes in bubble sizes and their time evolution as a function of gluten, starch and water content in non-yeasted G-S blend doughs. Frequency-dependent peaks in attenuation coefficient and phase velocity, characteristic of a low frequency bubble resonance, were seen for all G-S blends. Based on interpretation of ultrasonic results, gluten content affected the mean bubble radius entrained into the G-S blend doughs
Acknowledgements
The authors acknowledge financial support from the Discovery grants program of the Natural Sciences and Engineering Research Council of Canada and Archer Daniels Midland Agri-Industries Company for supplying the starch and gluten. Financial support for a University of Manitoba Graduate Fellowship for F. K. is also gratefully appreciated.
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