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Alessandro Angioloni · Santina Romani · Gian Gaetano Pinnavaia · Marco Dalla Rosa

Characteristics of bread making doughs: influence of sourdough fermentation on the fundamental rheological properties

Received: 5 May 2005 / Revised: 14 June 2005 / Accepted: 14 June 2005 / Published online: 31 August 2005 C Springer-Verlag 2005

Abstract The proofing process is an essential step in the bread making technology in providing a link between the bubble structure created in the mixer and the final baked loaf structure. This research was carried out to test the effect of two different types of fermentation on bread making dough properties. Fundamental rheological tests were used to evaluate and compare doughs prepared by using compressed yeast and only natural yeast. Dynamic oscillation tests were performed using a controlled stress­strain rheometer; the phase angle () and the value of the storage modulus (G ) were measured for all samples. Significant differences were found between the doughs which were made using commercial compressed yeast (baker's yeast (BY)) and those prepared using sourdough (SD). In particular SD was less elastic, less firm and easily extensible having higher phase angle and lower G . Keywords Sourdough . Yeast . Proofing . Bread making . Fundamental rheology Introduction Sourdough (SD) fermentation is one of the oldest biotechnological processes used in food production and indeed it was the only bread leavening method used before the discovery of yeast in beer production [1]. The acidification process effected by the use of SD remains a necessary prerequisite for the preparation of rye bread [2]; actually it is also used to prepare some special kind of bakery products, in Italy it is often utilized to produce several types of regional breads. Despite this long tradition, various details about the SD influence on the dough and bread structure

A. Angioloni ( ) · S. Romani · G. G. Pinnavaia · M. D. Rosa Alma Mater Studiorum Universit` di Bologna, Campus Scienze a degli Alimenti, piazza Goidanich, 60-47023 Cesena, (FC) Italy e-mail: [email protected] Tel.: +39-0547-636120 Fax: +39-0547-382348

have not yet been fully understood [2­4]. The SD application on the wheat bread process make several documented effects including leavening [5], acidification [6], improvement of aroma [7], delayed firmness and staling [8­10] and microbiological stability [11­13]. It has also been reported that the use of the SD can have positive nutritional implications by increasing mineral bioavailability [14, 15] and lowering the glycemic response to the baked goods [16]. The rheological characteristics of fermented dough are determined by many factors. At the beginning of the mixing process, physical actions such as hydration take place, the gluten network is formed when proteins and starch granules absorb water. The influence of SD on dough rheology has been shown by several empirical rheological techniques such as farinograf, extensigraf and rheofermentometer [17, 18]. However, in order to achieve a comprehensive understanding, a variety of effects have to be taken into account The structural effects of SD in wheat-based system may first be due to the direct influence of low pH on structureforming dough components such as gluten, starch, arabinoxylans etc. [8]. It has been moreover noted that when SD is added, there are changes in the fundamental rheological properties of wheat dough, making it soft, less elastic and therefore easily extensible [17]. The impact of such changes in dough rheology must be considered in order to choose an appropriate proof time and to obtain good quality bread. The dough must contain a large volume of gas and also gas retention in reserve for oven rise [19]. The proofing is really important in the bread making process in providing a link between the bubble structure created in the mixer and the final baked loaf structure [17]. The objective of this research was to study and compare the fundamental rheological properties of two different bread making doughs supplied by a bakery and realised by different kind of fermentation: the first one was realised with baker's yeast (BY, Saccharomyces cerevisiae), the second with only natural yeast (SD). Many analogous studies have been carried out on fermented doughs obtained in laboratory in small scale [20­22], in which the fermentation process was made using at the same time both SD and baker's yeast (BY). In these cases it is difficult to evaluate the effect of each kind

Table 1 flour Flour Salt Sugar Yeastc Sourdough

a b

Bread dough recipes. Quantities (g) based on 1000 g of BYa 1000 16 10 20 ­ SDb 800 18 5 ­ 400

Dough obtained with baker's yeast Dough obtained with sourdough c Commercial baker's yeast. Water addition for each formula was based on % farinograph water absorption Table 2 flours Chemical composition and rheological properties of the Flour A Proteina (%) Gluten (%) Asha (%) Fata (%) Moisture (%) Alveograph W (× 10-4 J) P (height × 1.1) Length, mm P/L

a

Flour B 12.22±0.05 11.13±0.12 0.54±0.01 1.88±0.30 14.23±0.06 215.7±5.07 62.7±1.10 112.67±2.52 0.55±0.02

14.91±0.03 13.93±0.06 0.57±0.01 1.75±0.21 13.8±0.17 363.17±6.58 78.10±1.10 140.67±3.51 0.56±0.03

6 D-73760 Ostfildern Germany-Europe), using parallelplate geometry (25 mm plate diameter and 2 mm plate gap). The upper serrated 25 mm plate was lowered until the thickness of sample was adjusted to 2 mm and the excess was trimmed of. The exposed surface was covered with a thin layer of mineral oil to prevent moisture loss during testing. A strain sweep test was used to identify the linear viscoelastic region; on the basis of this data, a target strain of 0.03%, which was within this linear region, was chosen for measurement. A frequency sweep test, ranging from 0.1 to 10 Hz, was used to study the rheological dough changes during fermentation. Fermentation tests were recorded for three fermentation batches of each dough type; the samples for the measurements were taken in two different steps of the bread making process. The first test was performed taking dough after the molding step (T0), the second one taking dough at the end of the last proof (T1). The fermentation times, after the molding step, were 2 and 4 h for the BY dough and the SD respectively. Statistical analysis Analysis of variance was performed on data adopting LSD (Fisher test) procedure, to test the statistical significance of the differences between means at a p0.05 significance level. Data were processed using Statistic for Windows r (Statsoft, Tulusa, UK) package. Results and discussion pH level development Initial pH (T0) levels for BY and SD were 5.4 and 4.8, respectively; the lower value for SD is due to the presence of certain amount of acid in this kind of dough [25]. After proofing (T1), pH levels dropped to 5.2 for the dough fermented with BY and to 4.5 where SD was added. pH decrease affects the solubility of certain protein fractions [26]. The optimum pH value for carbohydrate-degrading enzymes activity, such as amylase, pentosanase or cellulase, vary widely (between 3.6 and 5.6), depending on wheat variety and germination status; acidification is used to inhibit -amylase activity. Proteinases and peptidases in flour are active at pH levels ranging between 7 and 9, depending on substrate [27]. The lowest pH level reached in SD could reduce amylolytic activity, while the higher pH level reached in BY may allow to achieve further starch degradation. Rheological changes during fermentation The effects of SD on the dough viscoelastic properties were assessed by dynamic oscillatory measurements. Figures 1 and 2 show the obtained results for these measurements. All doughs showed the same behaviour for phase angle () and storage modulus (G ) over the range (from 0.1 to 10 Hz) of frequencies measured. Both BY fermented doughs, BY

Corrected to 14% moisture content. Values represent mean of three replicates ± standard deviation

of fermentation on the real bread making dough rheological properties. Materials and methods All the samples, flours and doughs, were supplied by Mulino Briganti (RA-Italy); the bread dough recipes utilized are shown in the Table 1. Two different types of commercial baker's wheat flours and two different kind of bread making doughs were used; flour A was used to prepare and revive the SD, flour B was utilized in the bread making process for both type of fermentation. Potable water was used in the dough making processes and for rheological tests. Flour analysis (moisture44-19 , ash08-01 , proteins46-10 , gluten38-12 and Alveograph characteristics54-30A ), were determined following AACC methods [23], fats using Soxhlet method. Chemical composition and rheological properties of used flours are reported in Table 2. pH were measured potentiometrically in a suspension of 10 g of dough and 100 ml of distilled water [24]. Rheological measurements Dynamic oscillation tests were performed on a controlled stress­strain rheometer (MCR 300, product by Physica/Anton Paar; Messtechnik GmbH Helmut-Hirth-Strasse,

27 26

25 24 23 22 SD (T0) SD (T1) BY (T0) BY (T1)

Fig. 1 Phase angle (), at 1 Hz of frequency, for wheat dough formulation: BY (baker's yeast) and SD (sourdough). T0 and T1 are the two different times of control. Mean value±standard deviation of three replicates

80

frequencies. The increase in phase angle and decrease in G were more evident in SD, this fact is due to the presence of SD that develop into less elastic and simultaneously less firm dough at the low rate of strain applied (0.03%); some Authors [17, 28] obtained similar results. Differences obtained between BY and SD dough rheological properties during and at the end of fermentation have been influenced by different structural components such as gluten and by occurred changes in the absolute pH value of the dough system [29]. At the low pH values there is a sizeable positive net charge and proteins are more soluble. An increase in protein solubility promotes intramolecular electrostatic repulsion among gluten proteins and makes protein groups more reactive (denaturation). However, strong intermolecular electrostatic repulsive forces prevent the formation of new bonds [28]. The results of the present study are in agreement with this hypotheses. Increased softness of the gluten might be a consequence of repulsion on the molecular level, causing a weaker structure. Conclusions

Phase angle, (º)

60

40

20

BY(T0) SD (T0)

BY(T1) SD (T1)

0

0.1

1

Frequency (Hz)

10

Fig. 2 Value of the storage modulus (G ) as a function of frequency for wheat dough formulation: BY (baker's yeast) and SD (sourdough). T0 and T1 are the two different times of control. Mean value of three replicates

at T0 and T1, showed (Fig. 1) a lower phase angle () than the samples proofed with SD. The use of SD did increase the phase angle values and significant difference (p0.05) were found between SD (T0) and SD (T1). The phase angle ranges from 0 (ideally elastic material, Hookean solid) to 90 (ideally viscous material, Newtonian liquid). For all viscoelastic materials, the phase angle is between 0 and 90 ; the lower the values, the more elastic the material. Therefore, the use of SD reduced the dough elasticity in contrast to BY fermentation, were no significant difference in the elasticity value were found. The value of the storage modulus (G ) increased with increasing frequency and the shape of the curves was similar for all doughs tested (Fig. 2). These data show that the BY (T0) doughs had the highest storage modulus values over the whole frequency range, higher values of storage modulus indicated that the dough was firmer. The lower storage modulus found in SD show that the addition of SD reduce the dough elastic component. The values of BY and SD (T0) were significantly (p0.05) greater than those obtained from BY and SD (T1) over all

In this work the influence of SD fermentation on dough rheological properties was studied; significant differences were found between the doughs obtained using commercial compressed yeast and those which were prepared utilizing SD. Dough viscoelastic properties changed entirely with proofing; types of fermentation and pH levels had an effect on final rheological characteristics. The results of fundamental rheological tests show that the incorporation of SD material provoked changes that were different from those seen in BY fermentation. Such changes may be attributed to a number of intrinsically related factors, including variations in the rate or amount of acid produced. The obtained data show that the addition of SD lead into a less elastic and firm dough; it may be hypothesized that physicochemical changes in the protein network induced by the addition of SD may have been responsible for the greater expansion upon proving due to the more extensible nature of the dough.

Acknowledgments The authors gratefully acknowledge Mulino Briganti (Ra-Italy) for his assistance in the experimental work.

G' (KPa)

References

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