A new micro-baking method for determination of crumb firmness properties in fresh bread and bread made from frozen dough / Entwicklung eines Mikrobackversuches zur Evaluierung der Krumeneigenschaften von frischen Broten und Broten aus vorgegarten Tiefkühlteiglingen

Three different wheat cultivars have been used in this study: bread making cv. Midas (A), waxy cvs. Waxydie (B) and Waximum (C). The seed samples were provided by Saatzucht Donau (Probstdorf, Austria), Dieckmann Seeds (Rinteln, Germany) and BOKU Plant Breeding Division (Tulln, Austria), respectively. Samples were milled on a laboratory roller mill (E8, Haubelt Laborgeräte GmbH, Berlin, Germany) and flour was sieved with 180 µm sieves. Flours were stored for two weeks at 4°C in paper bags, before baking experiments were carried out. Salt (iodized) and dry yeast (saf-instant, Lesaffre Group, France) were obtained locally.

A schematic overview of the bread making process is shown in Figure 1. For baking pans, unused Rapid Visco Analyzer (RVA) sample canisters (h = 67.7 mm, d = 38 mm, wall thickness 0.45 mm) were applied. Usually, these aluminum canisters are exclusively used for viscosity measurements by the RVA device (Perten Instruments, Hägersten, Sweden). On the bottom of each pan, two holes (d = 3 mm) were drilled for easier depanning of breads after baking (shown in Figure 2). The baking formula included 100 g flour (14% moisture basis), 2% salt, 2% dry yeast and 63 ml water. All dry ingredients were equilibrated at room temperature before use. The water amount was kept constant for all flours in this experiment, as changes would have had substantial effects on crumb firmness of bread (Yi et al., 2009). All ingredients were mixed in a Flourgraph E6 (Haubelt Laborgeräte GmbH, Berlin, Germany) for 5 minutes. Water was pre-warmed to 30°C and added at the beginning of the mixing. The mixing bowl of Flourgraph E6 was tempered to 30°C, whereby a final dough temperature between 29 and 30°C was reached (and the dough temperature was monitored). The dough was removed by hand, rounded and placed into a fermentation cabinet (G66W, Manz Backtechnik GmbH, Creglingen, Germany). After proofing for 30 min at 30°C and 85% RH (relative humidity), the dough was divided into 15±0.5 g portions. Each piece was rounded by hand for 10 s and placed into an RVA canister, which was greased carefully with baking spray (Boeson-Trennwachs, CSM Deutschland GmbH, Bingen, Germany) prior to use.

One part of such prepared dough pieces underwent a second proofing step of 45 min (30°C/85% RH) for fresh bread production, followed by the baking process. For the production of frozen dough pieces, another part of the dough pieces was submitted to a reduced second proofing step of 30 min (30°C/85% RH) followed by freezing in a blast freezer (IF101L, Sagi S.p.a., Ascoli Piceno, Italy) at an ambient temperature of −36°C. The freezing duration of 20 min was experimentally determined before, as a core temperature of −15°C was reached by these settings. Then, the dough pieces were taken out of the cans, packed in airtight plastic bags, sealed and stored for 1 week at −18°C. After that, the frozen storage doughs were placed into baking pans again, and thawed in the fermentation chamber for 30 min (30°C/85% RH). Fresh and frozen dough were baked under same conditions. First the baking oven (60/3 W, Manz Backtechnik GmbH, Creglingen, Germany) was pre-heated to 230°C, top and bottom heat. Five bread pans were placed in the oven, evenly distributed inside the oven. Subsequently, the top temperature was reduced to 200°C and bottom temperature to 180°C; the breads were baked for 20 min. After baking, the breads were cooled at room temperature within the pans for 10 min; then, the breads were removed from the pans and were cooled for further 50 min in a controlled atmosphere (20°C/50% RH). The baking trials were done in duplicate, resulting in 10 fresh breads and 10 breads from frozen dough for each flour. Specific loaf volume and crust color was measured for all breads. Bread crumb properties were determined from 6 breads on the day of baking. The other four breads were used for determination of firming kinetics. For the storage study, breads were packed into plastic bags and stored at 20°C. After 24, 48, 72 and 96 hours of storage, one bread per sample was analyzed on each day.

Mean values, standard deviations and coefficients of variation were calculated using Microsoft Office Excel 2016 (Microsoft, Redmond, USA). One-way ANOVA was performed by using SPSS 21 for Windows (SPSS Inc., Chicago, IL, USA) to analyze the significance of flour type on bread properties. To determine the individual differences between groups, the Tukey test was performed at p < 0.05. Bread staling parameters (k and n) were fitted to the Avrami equation by using the website www.mycurvefit.com (accessed 16^th December 2016): (F∞<!-- 8 -->−<!-- - -->Ft)(F∞<!-- 8 -->−<!-- - -->F0)=e−<!-- - -->k∗<!-- * -->tn$$ \frac{(F_\infty\,-\,F_t)}{(F_\infty\,-\,F_0)}=e^{-k*t^n} $$

where F_∞ and F₀ were the measured crumb firmness at the beginning and final stage of bread staling and F_t corresponds to firmness at time t (Cornford et al., 1964).

Specific loaf volumes obtained by the micro-baking procedure varied between 330 and 425 cm³/100 g for fresh bread and from 236 to 331 cm³/100 g for the frozen dough procedure. The highest volume for fresh bread was achieved by flour C, while for frozen dough bread, flour B reached the highest volume. Specific volumes of waxy wheat flours (B and C) were not significantly different or even higher as compared to the volume of the standard bread wheat (A). These results are likely to be related to a higher loaf expansion of waxy wheat breads during baking (Blake et al., 2015).

Guenzel (1981) argued that the baking process in micro-baking is hardly comparable with standard baking tests, because a very low oven rise occurs, and additionally, the dough can shore up more at the walls of the baking pans than in standard sizes. This could be critical in this baking procedure, as the geometry of the RVA canisters supports this disadvantage. For instance, flours B and C show surprisingly high loaf volume for waxy wheat. Figure 3 demonstrates that breads obtained from these flours have a rather flat top in comparison to the standard bread wheat (A). This occurred due to the weak structure of the waxy wheat doughs, which allows a high oven rise, but during baking, the dough partly collapses again. This effect would be higher when the bread is baked without pans or in standardized pans with relatively larger base area and lower height, as they are used in the ICC standard method 131, for example. However, within the used micro-pans, the shearing of the dough at the walls is higher, thus the collapse of the dough was probably diminished. Therefore, the specific loaf volume could be higher in micro-pans than in standard sized baking experiments.

The coefficients of variation were similar for each flour and bread making procedure. Values were comparable to data shown by Kieffer et al. (1998) for the micro-Rapid-Mix-Test. Other studies (Sedlácek and Horćićka, 2011; Thanhaeuser et al., 2014) reported extremely low (<0.5%) coefficients of variation for micro-baking tests. However, in this study, significant differences among wheat flours could be detected.

In Figure 3, it is visualized that height measurements of micro-baking breads would only roughly describe the volume, as the bread volume increase does not occur evenly. For example, a comparison between fresh breads produced by flour C to frozen dough breads by flour A reveals on one hand a “rectangular” shape, flat top and flat bottom (flour A), and on the other hand a rather “circular” shape, round top (higher rise in the middle part of the bread) and sometimes also round bottom (flour C).

The texture properties of the bread crumb were measured by a compression test which describes the viscous and the elastic deformation (represented by F_REL). Coefficients of variation for maximum firmness (F_max ) ranged between 7.5 and 21.3% and were higher for the frozen dough procedure. For F_REL all coefficients of variation were lower than 7%. Significant differences between flours and bread making procedure were found for both parameters. Maximum firmness (F_max) of breads made from waxy wheat flours (B, C) were significantly lower than from breads made with standard bread wheat (A), which is a typical quality characteristic of waxy wheat flours (Bhattacharya et al., 2002). Moreover, there was no difference between the firmness of fresh and frozen dough bread made by waxy wheats. Standard bread wheat (A) showed substantial increase in firmness with the frozen dough procedure, following the results of numerous frozen dough studies (Rosell and Gómez, 2007).

Results of relative elasticity (F_REL) showed diverse effects, flour A and C showed an increase after the freezing procedure, whereas flour B F_REL did not significantly change. Crumb elasticity does not only affect the mouthfeel, but also the cutting ability and therefore, it is an important quality parameter in industrial bread production (Wassermann, 1973).

To observe the repeatability of the baking procedure, color is also a noteworthy parameter. For the measurements expressed as CIELAB parameters, only the top of the bread, which was not in direct contact with the baking pan surface, was used. The color of the sidewalls can be seen on exemplary breads in Figure 3. For the parameter lightness (L*), the coefficient of variation was 1.4 and 3.8%, for redness (a*) 4.0 to 8.8% and for yellowness (b*) 0.5 to 3.4%. Bread made from frozen dough had a significantly lower L*-value than fresh bread with the same flour, this resulted in a darker color, as shown in Figure 3. For all color parameters, significant differences between flours and bread making procedure were identified.

The increase in F_max of bread crumb, due to retrogradation during 5 days of controlled storage is shown in Figure 4. For both procedures, irrespective of the storage duration, highest firmness was achieved with the standard bread flour (A). Waxy wheat breads (B, C) showed similar behavior and only very little increase of firmness in the first 48 hours.

The kinetics of bread crumb firming can be described by measuring crumb firmness after different storage periods; this mechanism is following the model of Avrami equation (Armero and Collar, 1998). The Avrami parameters determined by curve fitting are presented in Table 2. The parameter k is the firming rate and defines the initial stage of firming. The Avrami exponent n indicates the nucleation type and describes the behavioral approach to reach the final state of staling (Amigo et al., 2016). Both parameters revealed big differences between waxy wheat and standard bread wheat. This behavior was expected, as slower retrogradation for breads with the addition of waxy wheat has been shown by Bhattacharya et al. (2002) previously.

Table 2

Staling kinetic parameters according to the Avrami equation

Tabelle 2. Kinetische Parameter des Altbackenwerdens laut Avrami-Gleichung