Interaction of bubbles with sugar in bread

Interaction of bubbles with sugar in bread 

L. Trinh and P. J. Martin 

University of Manchester, Manchester, UK (P.Martin@manchester.ac.uk) 

ABSTRACT 

An investigation was conducted to assess the interaction of sugar with bubbles in bread dough. Sweetened 

bread is a desirable product. However, large quantities of sugar in the dough formulation result in sticky 

dough, causing processing difficulties. An understanding of how sugar interacts with bubbles in bread dough 

would ease the development and production of sweetened bread products. 

Batches of bread dough were made up with both strong and weak flours and different quantities of 

sugar, up to 15% of the flour weight. Doughs were mixed in a high speed Tweedy mixer and the extent of 

dough aeration investigated through calculations of dough density using the double cup method. On 

increasing the sugar content the dough density increases. Pressure step change experiments were conducted 

during mixing with doughs containing 0% and 15% sugar. Through calculating the density using the double 

cup method and inputting this data into a population balance model of aeration, entrainment and 

disentrainment rates were calculated. Results showed that doughs containing 15% sugar had a lower steady 

state voidage than doughs containing 0% sugar. Doughs containing 15% sugar had both lower air 

entrainment rates and higher air disentrainment coefficients than doughs containing 0% sugar. 

Keywords: Bread; Dough; Sugar; Aeration; Bubbles 

INTRODUCTION 

Bread is a worldwide staple food available in many shapes and variations. Sweetened bread products in the 

UK such as fruit loaves and Chelsea buns contain sugar, and are further sweetened with dried fruit, sugar 

glazes or both. This reflects the desire to further sweeten these products. Bakers use the term, “bakers 

percentage”. This is the percentage of an ingredient in relation to the quantity of flour present. The addition 

of more than 10% sugar in the bread formulation poses a challenge to bakers [1], due to the presence of sugar 

resulting in sticky dough, making the dough difficult to handle and causing it to stick to processing 

equipment. Dough’s bubble structure is also affected by the presence of sugar, which affects the quality of 

the bread produced. In African and Asian countries, bread containing as much as 30% sugar exists [2]. It is 

produced using strains of yeast tolerant to high sugar concentrations. However, the processing equipment is 

likely to differ to that used in the UK. 

Ample research has been carried out on the presence of bran, salt and surfactants within bread 

dough, but a literature search through scientific databases found very little research has been carried out on 

the presence of sugar in the bread dough formulation. The aim of this research is to investigate how sugar 

changes the behaviour of a basic dough (dough made from flour, water, yeast, and salt) made up in a high 

speed mechanical mixer and how it affects the bread made from this dough. This work shows the differences 

in entrainment and disentrainment rates in doughs containing 0% sugar and 15% sugar, and illustrates the 

voidage differences in doughs mixed at different pressures and containing different quantities of sugar. Bread 

dough consists of gas bubbles held in a gluten-starch matrix. The voidage is known as the proportion of gas 

present in the bread dough. 

The high speed mechanical mixing used is known as the Chorleywood Bread Process (CBP) and 

achieves dough development in less than five minutes. When dough is developed, it becomes a viscoelastic 

material capable of retaining expanding gas bubbles. The CBP is the main breadmaking process in the UK as 

well as Australia, New Zealand, South Africa and a number of other countries worldwide, thus research 

focused on this method of producing high sugar doughs may be beneficial to a number of countries. 

The bubbles in bread, from the number of bubbles to their size, distribution and orientation, 

determine the quality of bread. A key quality attribute bakers strive for is a fine bubble distribution. All 

stages of breadmaking involve manipulation of gas bubbles. Mixing folds the dough over itself, trapping air 

between layers of dough and creating the initial bubbles. Moulding subdivides these bubbles, increasing the 

number of bubbles present. During proving these bubbles are inflated with carbon dioxide produced by yeast.

Finally, during baking, the bubbles embedded in the protein starch matrix ruptures and gas escapes from 

these bubbles. As the heat causes the structure of the bread to set, the dough becomes interconnected with 

bubbles. Adapting the CBP, through first understanding how sugar affects each of these stages may be the 

key to producing high sugar breads. 

MATERIALS & METHODS 

The Effect of Sugar on Dough Density after Mixing 

Doughs were made up in a Tweedy 1 mixer, operated at a speed of 720rpm. Doughs were mixed under 

compressed air for three minutes at absolute pressures of 0.25 bara, 0.50 bara, 1.00 bara, 1.50 bara, 2.00 bara 

and 2.50 bara, whilst maintaining the flow rate of compressed air at 10-15L/min when mixing above 

atmospheric pressure. Doughs were made up from both strong and weak flour (3663, High Wycombe, UK). 

The weak flour had a protein content of 10.1% and the strong had a protein content of 11.0%, as stated on the 

packaging. Tap water was used, and its temperature controlled using ice to achieve a final dough temperature 

of 30±2°C. Caster sugar (Tate and Lyle, London, UK) was used due to its fineness. Table salt and fresh yeast 

were purchased from a local supermarket (Sainsburys, London, UK). The dough formulations made up are 

listed in Table 1. Six samples were taken from each dough mix and the density of each measured using 

Campbell et al’s double cup technique [3]. This involves using the weight of the dough in air and the weight 

of the dough in silicon oil (Basildon Chemicals, Oxon, UK) to calculate the density of the dough, as detailed 

in (1). Silicon oil was used as it is a low hazard, non-wetting and non-dissolving liquid. 

Table 1. Dough formulation. The first number for the water quantity is that used in the strong flour dough formulation 

and the second number is the quantity of water used in the weak flour dough formulation. 

Sugar Content (%) Sugar (g) Flour (g) Yeast (g) Salt (g) Water (g) 

0.0 0 400 10 7.2 232 / 225 

1.5 6 400 10 7.2 232 / 225 

5.0 20 400 10 7.2 232 / 225 

10.0 40 400 10 7.2 232 / 225 

15.0 60 400 10 7.2 232 / 225 

mair 

d 

 

m m 

air 

Where ρ d is the dough density; m air and m oil the weight of the dough in air and oil; and ρ oil is the density of 

the oil. 

Pressure Step Change during Mixing 

Both pressure step increase and pressure step decrease experiments were conducted in a Tweedy 1 mixer. In 

the pressure step increase experiment, the pressure was changed from 1.0 to 2.0 bara using compressed air. In 

the pressure step decrease experiment the pressure was changed from 1.0 bara to 0.5 bara. For both 

experiments the doughs were mixed for 230 seconds, with the step change at 110 seconds. Numerous mixing 

sessions were carried out using separate dough mixes of the 0% and 15% sugar strong and weak flour dough 

formulations in Table 1. The times in seconds in which mixing was stopped to take density measurements 

were 110, 115, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230. The density of the dough was 

taken using the double cup method detailed above. By inputting the values for dough density into a 

population balance model of aeration developed by Martin et al [4], the entrainment and disentrainment rates 

were obtained. 

oil 

 

oil 

(1)

Density of Dough removed from Mixer 

RESULTS & DISCUSSION 

The Effect of Sugar on Dough Density after Mixing 

Figure 1 shows the density of strong flour doughs containing different quantities of sugar mixed at different 

pressures. Throughout all the doughs, as the pressure increases, the density of the dough once removed from 

the mixer decreases. This is due to the increase in pressure causing more air molecules to enter the mixer 

headspace and thus the dough, once the pressure is released the bubbles expand. The doughs containing 

10.0% and 15.0% sugar also consistently have a lower voidage. There is some overlap in results between the 

0.0%, 1.5% and 5.0% sugar doughs, where a less dense dough has been obtained in doughs containing 1.5% 

and 5.0% sugar than 0.0% sugar, although this effect is within the bounds of experimental error. It is known 

that bakers sometimes add 1.0-2.0% sugar to the dough formulation to favour the growth of yeast. The results 

in Figure 1 may suggest that up to 5.0% sugar has the beneficial effect of also lowering the dough density. A 

sticky dough making entrainment of air during mixing less efficient may be a cause of denser doughs in the 

10.0% and 15.0% sugar doughs. Sugar also successfully competes with starch and proteins for water. Water 

is required for the hydration of starch and protein which make up bubble walls, thus the presence of less 

available water could interfere with this. The gas free dough density of each dough can be obtained by 

extrapolation to an absolute pressure of 0.0 bara. In Figure 1, extension of the linear fit line would show a 

general increase in the gas free dough density with an increase in sugar. Experimental repeats with the weak 

flour dough illustrated the same trend. 

1.30 

1.25 

1.20 

1.15 

1.10 

0.0% sugar 





1.05 

0.0 0.5 1.0 1.5 2.0 2.5 

Figure 1. Change in density at different pressures with different sugar content strong flour doughs. A linear fit line has 

been applied to each set of points. The error bars indicate the standard deviation of the six measured samples. 

Pressure Step Change during Mixing 

Mixer Headspace Absolute Pressure (Bara) 

Figures 2 shows the results of the pressure step increase experiment for two strong flour doughs. It shows a 

sudden decrease in in-situ voidage on increasing the pressure. The voidage decreases because the pressure 

increase compresses the bubbles and reduces their volume. As mixing continues the voidage increases as 

more air is entrained than disentrained. The levelling off and reaching of steady state observed beyond 200 

seconds, is due to the air entrainment rate becoming balanced with the disentrainment rate, which increases 

with voidage. The result for the 0% sugar dough at 230 seconds is likely to be an anomaly, since it appears 

as though the voidage was levelling off. The dough containing no sugar consistently has a higher voidage 

than the dough containing 15% sugar which corresponds to the results displayed in Figure 1. Experimental 

repeats with the weak flour dough illustrated the same trend.

Voidage of Dough Removed 

from Mixer 

Voidage of Dough removed 

from Mixer 

0.09 

0.08 

0.07 

0.06 

0.05 

0.04 

0.03 

0.02 

0.01 

0 

90 110 130 150 170 190 210 230 250 

Time (seconds) 

0% sugar 

15% 

sugar 

Figure 2. Pressure step change experiment for strong flour doughs with an increase in headspace pressure from 1 bara to 

2 bara occurring at 110 seconds. Error bars indicate the standard deviation of six measured samples. 

Figure 3 shows the results of the pressure step decrease experiment for two strong flour doughs. The opposite 

trend to that shown in Figure 2 is displayed. It shows a sudden increase in voidage on decreasing the 

pressure. The cause to the voidage increase is the pressure decrease pulling the bubbles outwards and 

resulting in the bubbles occupying a larger volume. However, in addition to their expansion, a number of 

bubbles will also rupture. It can take up to 15 seconds for the mixer to reach a pressure of 0.5 bara. As the air 

is sucked out of the mixer and a partial vacuum is obtained, the entrainment of air becomes difficult. New 

bubbles cannot be formed through mixing under a vacuum [5]. Consistent with Figures 1 and 2 the dough 

containing no sugar consistently has a greater voidage than the dough containing 15% sugar. Experimental 

repeats with the weak flour dough illustrated the same trend. 

0.12 

0.1 

0.08 

0.06 

0.04 

0.02 



0 

90 110 130 150 170 190 210 230 250 

Time (seconds) 

Figure 3. Pressure step decrease experiment with the decrease in pressure from one bara to half bara occurring at 110 

seconds. 

Tables 2 and 3 show the entrainment rate and disentrainment coefficient for the pressure step change 

experiments. In the pressure step increase experiment, the ratio of entrainment rate to disentrainment 

coefficient is approximately 1.6 times greater in the doughs containing no sugar than the dough containing 

15% sugar, implying greater aeration of doughs containing no sugar, in agreement with the results in Figure 

1. In the pressure step decrease experiment, the entrainment rate to disentrainment coefficient is 

approximately 4.5 times greater in the no sugar strong flour dough than 15% sugar strong flour dough and 

approximately 5.1 times greater in the no sugar weak flour dough than 15% sugar weak flour dough. Taking

into account the voidage of the dough before and immediately after the step change, the proportion the 

voidage changed was calculated. Although the quantity of the voidage changes varies between doughs 

containing no sugar and 15% sugar, the proportion to the voidage before the step change of which the 

voidage changes is similar. Since the same proportion of air is entrained or disentrained, this suggests that the 

difference in dough density is based on the steady state density of each dough, which as illustrated in Figure 

1 is lower in the dough containing more sugar. As the internal structure of the dough has not been observed at 

this key stage, it is not known if the voidage decrease observed during the pressure step increase experiments 

is due to more bubbles rupturing or shrinking. Conversely, it is not known if the voidage increase observed 

during the pressure step decrease is due to the bubbles expanding or more bubbles forming. It is unlikely to 

be due to the formation of new bubbles since mixing is necessary in the formation of new bubbles. Sugar 

weakens the gluten structure [1], thus making rupture of bubbles more likely. It may be that for the doughs 

containing 15% sugar, bubble rupture at the surface leading to gas disentrainment is more likely than in 

doughs containing no sugar, especially at this point in mixing when the gluten has not had sufficient time to 

develop. 

The entrainment and disentrainment rates for the two flours show similar rates throughout the 

mixing process. The largest difference between the entrainment and disentrainment ratio in the two flours is 

in the 0% sugar dough with a pressure step decrease. However, when examining the proportional change in 

voidage before and after the step change, they both change by a factor of 1.89. This illustrates that sugar is 

responsible for the changes in the entrainment and disentrainment rates. 

Table 2. Entrainment rate, disentrainment coefficients and voidage changes for different strong flour doughs following 

pressure step changes during mixing. PSI stands for pressure step increase and PSD stands for pressure step decrease. 

Mix 

Entrainment, Disentrainment, v/k Voidage change at 

v (rev -1 ) 

k (rev -1 ) 

step change 

PSI, 0% sugar 8.05x10 -5 0.897x10 -3 0.0898 ± 0.00149 Decrease 0.0259 

± 1.23x10 -6 ± 2.78x10 -5 (x 0.513) 


± 1.04x10 -6 ± 3.80x10 -5 (x 0.511) 

PSD, 0% sugar 7.00x10 -5 1.15x10 -3 0.0669 ± 0.00379 Increase 0.0489 

± 2.20x10 -6 ± 2.73x10 -5 (x 1.89) 


± 9.27x10 -7 ± 3.09x10 -5 (x 1.93) 

Table 3. Entrainment rate, disentrainment coefficients and voidage changes for different weak flour doughs following 

pressure step changes during mixing. PSI stands for pressure step increase and PSD stands for pressure step decrease. 

Mix 

Entrainment, Disentrainment, v/k Voidage change at 

v (rev -1 ) 

k (rev -1 ) 

step change 


± 1.06x10 -6 ± 2.54x10 -5 (x 0.516) 


± 1.40x10 -6 ± 4.05x10 -5 (x 0.511) 


± 4.08x10 -5 ± 7.90x10 -5 (x 1.89) 


± 7.71x10 -7 ± 3.03x10 -5 (x 1.92) 

CONCLUSION 

This study has illustrated some key ways in which dough sugar content affects the aeration processes during 

bread dough mixing. Less dense dough was obtained following mixing with more sugar indicating a change 

in the rates of gas entrainment and disentrainment during mixing. Although results show the addition of more 

sugar generally increases the density of dough, it is not known if this increase in density is due to a decrease 

in the number of gas bubbles or a decrease in the size of these bubbles. Results of the step change 

experiments show less entrainment of air and suggest a weakening of the bubble structure so that doughs 

containing more sugar cannot retain as large a quantity of air. If this were the case, fewer bubbles would be 

present due to the likely rupture of a larger proportion of bubbles later on in the breadmaking process. A 

method employing x-ray imaging is currently being developed to investigate the internal structure of bread 

dough.

REFERENCES 

[1] McGee H. 2004. McGee on Food and Cooking, Hodder & Stoughton, London, 525. 

[2] Cauvain S. P., Young L. S., 2007. Technology of Breadmaking, Springer Science & Business Media, New York, 87. 

[3] Campbell G. M., Herrero-Sanchez R., Payo-Rodriguez R., Merchan M. L., 2001. Measurement of Dynamic dough 

Density, and the Effect of Surfactants and Flour Type on Aeration during Mixing and Gas Retention during Proving. 

Cereal Chemistry, (78), 272-277. 

[4] Martin P.J., Chin N. L., Campbell G. M. 2004. Aeration During Bread Dough Mixing II. A Population Balance Model 

of Aeration. Food and Bioproducts Processing, 82(C4), 268-281. 

[5] Baker J. C., Mize M. D., 1941. The Origin of the Gas Cell in Bread Dough. Cereal Chemistry, (18), 19-34.

Interaction of bubbles with sugar in bread

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