- Email: [email protected]
Bread staling
23 Bread staling P. Rayas-Duarte, Oklahoma State University, USA and S. Mulvaney, Cornell University, USA
Abstract: Bread staling is as old as bread itself. Much has been learned about the causes of bread staling, but loss of bread quality in storage is still a problem. There are several theories related to bread staling, but the complex interactions between bread components make it difficult to pin down exact cause-and-effect relationships. Several additives are used to interfere with the crumb firming and loss of crust crispness. Newer techniques look directly at moisture mobility using electrical impedance or nuclear magnetic resonance (NMR). These instruments provide useful information on the migration of water between components. This chapter will summarize newer research related to staling of bread and its molecular basis. Key words: bread staling, moisture mobility, water migration, crumb firmness, glass transition temperature, NMR.
23.1
Introduction
After almost 160 years of study, bread staling continues to firm our daily bread. Due to its dynamic non-equilibrium state, changes in the physical and organoleptic characteristics of bread start right after coming out from the oven during the cooling period and storage. Despite significant advances in the knowledge of factors that contribute to or accelerate staling and those contributing to ‘control’ it or slow it down, the bread-staling puzzle has not been solved completely. It is an important problem to the baking industry, implicating significant economical losses. From the research point of view, it is still a relevant issue that deserves a new comprehensive approach to its elucidation. Traditional definitions of bread staling begin by stating its complexity due to the number of components that participate in bread formulations as well as the different processing variables that affect the staling phenomenon. Even the nature of the main components in bread (proteins and carbohydrates), which are complex
© Woodhead Publishing Limited, 2012
Bread staling
581
matrices in their own right, prevents simplification within the context of bread staling. The definition of bread staling also includes the time and temperature dependent events that lead to the deterioration of its physical properties. Specifically, they lead to the loss of resilience of the crumb, decrease of crispness of the crust, loss of volatile flavor compounds and soluble starch content. Thus, bread staling includes a series of interrelated events. It is not possible to single out a particular event without underestimating related causes/effects in this dynamic system. In this chapter, a summary of selected relevant ingredients and processes known to influence bread staling will be presented. We will also offer a point of view for the systematic advancement of the understanding of bread staling molecular reactions involving kinetic studies, specifically of the reaction rates of important chemical and physical changes that occur in bread during storage at and above its glass transition. In the pursuit to find the connection between the microscopic and macroscopic properties of bread during staling, suggestions from complex material (such as foams) can be used (Weaire et al., 2007). Weaire and collaborators suggested multidisciplinary collaboration with material scientists actively advancing in chemistry, physics, mathematics and engineering disciplines. Such collaborations will bring expertise with models, simulations and advances in methodology that can improve our understanding of bread staling.
23.2
Breadcrumb structure
The multi-component breadcrumb has been considered a resilient deformable open-celled solid foam; a natural sponge is also an open celled solid foam, though its interconnected fibrils are more cylindrical and thinner than those of breadcrumb (see Figure 23.1 in Weaire and Hutzler, 1999). The air filled structure (alveoli) of the crumb is formed by inter-dispersed polymer rich gels and an interconnected network of films (lamellae) rich in low molecular weight compounds. Molecular mobility affects the rate at which substances diffuse across the semisolid (gel) structures to liquid film (lamellae) and interstitial spaces or to neighboring gel structures. The number of bubble contacts (connectivity) of a solid foam material (bread) influences its physical properties such as thermal or electrical conductance, as these properties are highly correlated to connectivity (Hindermann-Bischoff and Ehrburger-Dolle, 2001; Weaire and Hutzler, 1999).
23.3
Bread stability
The stability of food systems depends on the reaction rates (chemical and physical changes) of the different components (Roos and Lievonen, 2002). Among the critical parameters used for determining bread stability (staling) is the glass transition temperature (Tg) and its range (Slade and Levine, 2003). Tg is a property of amorphous materials and is measured in the laboratory as the transition from a rubbery, viscous amorphous solid, to a brittle, glassy amorphous solid of a
© Woodhead Publishing Limited, 2012
582
Breadmaking
glass with solid-like mechanical properties. At the molecular level, the measurement of breadcrumb glass transition describes the temperature at which the polymer chains (such as proteins and starch) have lower mobility compared to the temperature at which the polymers will undergo a phase transition from a crystalline or semi-crystalline phase to a solid amorphous phase (Slade and Levine, 2003). The glass transition temperature (Tg) in polymers is described as the temperature at which the Gibbs free energy is such that the activation energy of the cooperative process (translational and segmental transposition) of 50 or so adjacent chain segments is exceeded. This allows molecular chains to slide past each other in the direction of the stress (Cowie, 1991). As an example, the local segmental transposition of six carbon atoms (crankshaft motion) is reported to involve an activation energy of about 25 kJ mol−1 (Cowie, 1991). Tg can be measured by a number of methodologies such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectic analysis (DEA), thermomechanical analysis (TMA) and different spectroscopic methods (Roos and Lievonen, 2002). The broad range of Tg (onset and end, about 20°C) of white bread has been suggested to reflect its complexity and heterogeneity (Buera et al., 1998; Le Meste et al., 1992). For example, the polymers in bread are in different physical state; gluten is completely amorphous and starch is in a partially crystalline state (Le Meste et al., 1992). Due to its non-equilibrium state, bread will tend to form a more ordered structure or crystalline state, which is more stable. Thus, microdomains of ‘subphases’ are found in the bread system. These bread ‘subphases’ will have different diffusivity properties, which will affect the reaction rates of different changes that are time and temperature dependent. To advance in our understanding of bread staling kinetic studies are needed in which a systematical description of the reaction rates of each of the main factors that affect bread staling have been documented. Specifically, each of the ingredients (flour, water, sugar, salt, yeast, additives) and processing parameters such as temperature and time should be included in the study of their effect on state and phase transition, diffusion, molecular mobility (α- and β-relaxations), and reaction rates. Several authors have pointed to the lack of studies demonstrating the relationship of Tg and reaction rates in different food systems, such as low and intermediate moisture foods (Roos and Lievonen, 2002). The comparison of diffusion-controlled and well stirred systems over the same temperature range would illustrate the magnitude of change of the effect of glass transition on the reaction rates that are of interest in bread staling. Another series of experiments would need to be planned to document the reaction rates of bread formulated to cover a wide range of Tg (Roos and Lievonen, 2002). Similar experiments are needed to record the effect of Tg on the reaction rates of bread systems containing the additives reported to control or delay as well as promote staling. Such experiments will assist in developing a kinetic description of bread staling in relation to the complex factors that contribute to the deterioration of bread quality. For example Baik and Chinachoti (2000) showed that the glass transition temperature of fresh bread was about 0°C, while the appearance of a second peak in tan delta appeared after storing bread with and without the crust for seven days (Fig. 23.1). © Woodhead Publishing Limited, 2012
Bread staling
583
Fig. 23.1 Dynamic mechanical analysis (DMA) thermograms for breadcrumb during storage at 25°C. E′ = storage modulus, E″= loss modulus, tan delta = E″/E′ (Baik and Chinachoti, 2000).
Tian et al. (2009) investigated the role that the addition of β-cyclodextrin had on the x-ray diffraction patterns of fresh and stored crust and crumb (Fig. 23.2 and 23.3). They concluded that β-cyclodextrin had a significant impact on the staling of crust and crumb. It significantly lowered the rate of retrogradation of breadcrumb and affected the x-ray diffraction patterns suggesting new complexes with amylose and lipid. 23.3.1 Molecular mobility Bread average moisture content (about 38%) and storage temperature of bread (about 25°C) are favorable to a number of reactions that can cause loss of bread
© Woodhead Publishing Limited, 2012
584
Breadmaking
Fig. 23.2 Effect of 1.5% beta-cyclodextrin on x-ray diffraction pattern of fresh crust (a) and stored crust (b) in the absence (control) or presence of β-cyclodextrin (Tian et al., 2009).
quality. The physical state of the polymers (glassy solid or rubbery liquid) present in bread is assumed to affect its diffusion properties, meaning how rapidly the molecules move through the bread matrix (Roos and Lievonen, 2002). The diffusion properties are expected to affect the reaction rates of the major
© Woodhead Publishing Limited, 2012
Bread staling
585
Fig. 23.3 Effect of 1.5% β-cyclodextrin on x-ray diffraction pattern of fresh crust (a) and stored crust (b) in the absence (control) or presence of β-cyclodextrin (Curti et al., 2009).
macroscopic changes reported in bread staling although there are no experimental data reported. Water mobility studies of polymers are successfully studied by measuring the mobility of protons using nuclear magnetic resonance and magnetic resonance imaging methodologies and a summary of their application to bread staling is
© Woodhead Publishing Limited, 2012
586
Breadmaking
Fig. 23.4 (a) −1H T2 relaxation times distributions of breadcrumb during storage; (b) −1H T2 relative abundance in each proton population during storage (Curti et al., 2011).
available in the literature (Ruan and Chen, 2001; Curti et al., 2011). An example of T2 relaxation times distribution of breadcrumb in storage is shown in Fig. 23.4 (Curti et al., 2011). The mobility of protons reveals the equilibrium state of the material as they coexist in mobile (liquid phase) and immobile (solid phase) regions in food
© Woodhead Publishing Limited, 2012
Bread staling
587
systems (Ruan and Chen, 2001). Solid-state magnetic resonance methodologies provide structural and dynamic information of polymers and are suitable for providing insight into the mechanisms of self-assembly, which are of much relevance in modern polymer science (Brown and Spiess, 2001). For example, 1H solid-state NMR has provided valuable structural information on partially ordered samples gained on account of the marked sensitivity of the 1H chemical shift to hydrogen-bonding and aromatic π–π effects (Brown and Spiess, 2001). It can also probe dynamic events, such as the making and breaking of hydrogen bonds as well as guest mobility in a supramolecular complex (Brown and Spiess, 2001). Advances in high-resolution 1H solid-state NMR methods have opened up the possibility of routinely directly probing hydrogen-bonding and aromatic π–π interactions that will translate in molecular mobility in bread staling (Sereno et al., 2007). Studies of changes in water mobility using transverse relaxation time (T2) of protons (1H nuclear magnetic resonance (NMR) Relaxometry) revealed two distinctive regions of T2 associated with multiple domains of water in fresh white breadcrumb and gluten-starch gels (Wang et al., 2004). Bread crumb results revealed a less mobile water region (170–220 μs T2) and a more mobile region (3–6 ms T2) which were associated with multiple domains of water (Wang et al., 2004). Multiple 1H T2 populations in baked products that underwent major changes over storage, resulting in a reduced mobility (shorter 1H T2 relaxation time) in stored products have been reported (Ruan et al., 1996; Chen et al., 1997; Engelsen et al., 2001). Wang et al. (2004) also reported that the water mobility (peak T2 values) in breadcrumb were similar for bread prepared with protein content in the range of 11.2, 13, and 14.2 g of protein/100 g of flour (levels obtained by adding dried gluten) but significant differences were found for breads prepared with moisture content within the range of 180, 220, and 260 g of water/345 g of flour. Slow water mobility can be interpreted as an environment of higher viscosity with more polymer–water interaction. The authors suggested that during gelatinization starch granules absorbed water (presumably coming from the hydrated gluten) and the mobility of water associated with starch decreased significantly. The latter phenomenon can be associated with the formation of crystallites of amylopectin, which has 36 water molecules associated with the unit cell of the B-type structure (Sarko and Wu, 1978; Gray and Bemiller, 2003) or with the amorphous phase (Kim-Shin et al., 1991). Other methodologies such as dielectric techniques of thermally stimulated depolarization currents and dielectric relaxation spectroscopy have been used to study the kinetics of slow molecular mobility in the amorphous state of condensed complex systems, and have potential to be applied to kinetic studies of bread staling (Moura Ramos et al., 2006). The slow molecular mobility in amorphous solids includes the main relaxation (α) and the secondary relaxations (β, corresponding to intramolecular reorientations) (Moura Ramos et al., 2006). This is of particular interest since bread is found mostly in an amorphous state. Dielectric and mechanical analysis studies of commercial bread, wheat starch and vital wheat gluten have shown significant changes in the molecular mobility of
© Woodhead Publishing Limited, 2012
588
Breadmaking
their components as measured in changes in the glass transition temperature Tg (Huang et al., 1996). 23.3.2 Crumb firmness Crumb firmness of bread products is a function of time, temperature and formulation. It has been related to changes in starch components such as reorganization (recrystallization) of amylopectin and changes in amorphous domains of the gelatinized starch, as well as the molecular associations with components such as protein, starch and lipids. All these molecular changes are related to the organoleptic and mechanical properties (stiffness) of the bread structures. The molecular basis for these reorganizations has been reviewed extensively in the literature but up to date there is no agreement on the cause and effect of the molecular processes involved (Gray and Bemiller, 2003; Chinachoti and Vodovotz, 2001; Cauvain and Young, 2000). Among the mechanism of staling proposed is the explanation of two independent processes. The first one occurs during the first two days of storage and is governed by organizational changes of the starch polymers, and the second is associated with the water migration from the gluten to the starch and from the crumb to the crust (Garimella Purna et al., 2011; Hug-Iten et al., 2003). This can cause a loss of crispness in the crust. However, the permeability of the crust to water vapor can mitigate this effect (Hirte et al., 2010). But this will not stop the loss of moisture from the crumb itself. The change in water mobility and distribution causes changes in diffusion properties in the crumb and crust, and the perception of the crumb is that it has become stiff and dry (Lodi et al., 2007; Baik and Chinachoti, 2000; Cauvain and Young, 2000). In general, the loss of textural quality in fresh bread after cooling and over time is due to the retrogradation/recrystallization of starch, especially the short amylopectin side chains (Goesaert et al., 2009). This in turn leads to a firmer, less resilient crumb and a softer crust due to moisture migration to the crust. These authors describe the main ingredients in bread such as flour: starch, gluten proteins, non-starch polysaccharides and lipids, and how amylases act as antistaling agents. Slade and Levine (1991) have used a food polymer approach to understanding staling based on starch and gluten networks in bread. Conclusions were stated in Goesaert et al. (2009) as follows. A conventional bacterial alphaamylase can weaken the starch network in bread, but does not prevent amylopectin recrystallization. The latter process leads to sequestering water in amylopectin crystallites, thus reducing the available plasticizing water to the gluten network and increased firmness. On the other hand, a maltogenic alpha-amylase limits amylopectin recrystallization, with the result that plasticizing water remains available to the gluten network resulting in a softer more resilient crumb. Gray and Bemiller (2003) came to similar conclusions. Their analysis of the literature supports the idea that gluten serves as a reservoir of water for transfer to starch, which in turn retrogrades. They point out that amylopectin retrogradation alone does not explain bread staling, but one must also consider gluten–starch
© Woodhead Publishing Limited, 2012
Bread staling
589
interactions. Slade and Levine relate degree of recrystallization to the glass transition temperature (Tg) (Slade and Levine, 1991). Tg was said to be around room temperature for freshly baked bread and increasing to about 60°C for the fully retrograded (staled) system. Baik and Chinachoti (2000) looked at moisture distribution and phase transitions for white bread, with and without crust, stored at 25°C in hermetically sealed containers. The crumb moisture content decreased significantly more for the sample with crust than the one without crust. The freezable water was also reduced in the sample with crust. Storing the samples beyond seven days resulted in greater firmness and increased amylopectin retrogradation in the sample stored with crust. This was attributed to moisture migration from crumb to crust. Electrical impedance spectroscopy (EIS) was used to observe the effects of staling on the physicochemical properties of bread (Bhatt and Nagaraju, 2009) (Fig. 23.5 and 23.6). The authors found that the glass transition temperature of the crust occurred after 96 hours of storage at room temperature for moisture contents greater than 17%. EIS results were validated via DSC experiments. Recrystallization in the crumb followed by EIS with time was also validated via DSC. Thus, EIS provides for rapid nondestructive measurement of electrical properties of bread at different zones from the crumb to the crust.
Fig. 23.5 Variation in capacitance at the crust with moisture content (Bhatt and Nagaraju, 2009).
© Woodhead Publishing Limited, 2012
590
Breadmaking
Fig. 23.6 Variation in resistance at the crumb with time (Bhatt and Nagaraju, 2009).
23.3.3 Transport processes in breads Based on the previous discussions some ideas will be further explored from the point of view of transport matter. The moisture migration or moisture transport in a solid material of medium moisture like bread is affected by several phenomena including the diffusion of water through the structure and permeability. It is assumed that diffusion of different compounds plays an important role in the deterioration of quality during bread storage, for example loss of moisture and flavor compounds. In an oversimplified version, in solid materials and more specifically in composite materials in quasi-equilibrium, if concentration gradients exist, there is a trend to even out such concentrations by diffusion, generally as a function of time (Walstra, 2003). In the case of moisture, it will diffuse from a higher moisture region to a drier region. The rate of change is limited by the diffusion coefficients, which are inversely proportional to particle or molecule radius and viscosity of the media (Walstra, 2003). The diffusion in compound systems is model-dependent, meaning it depends on the structure of the matrix. For simplicity’s sake, let us consider bread as a composite solid matrix, interspersed with a continuous liquid phase and a discontinuous gas (air) phase. Liquids flowing through the matrix of a composite solid encounter pressure gradients proportional to the material constant called permeability. In an oversimplified description, permeability is about proportional to pore diameter squared and pore volume fraction (surface fraction of the pores in a cross section of the material) (Walstra, 2003).
© Woodhead Publishing Limited, 2012
Bread staling
591
Permeability is not an easy property to predict. Some relations have been proposed based on the volume fraction of the matrix material and the specific surface area of the matrix (in m2 per m3 of the whole material). Some models were developed for powders and have limited validity. Examples of approximate magnitudes of permeability (B) for ground coffee, renneted milk gel, and polysaccharide gel are 10−8, 10−12, and 10−17 per m2, respectively. So, the two examples of gels have relatively lower permeability than ground coffee and hold water tenaciously (Walstra, 2003). In summary, studies are needed on the effective diffusion coefficients and permeability of breadcrumb and crust and model systems with single and binary ingredients used in bread to enhance our understanding of the staling phenomena. This would have several approaches such as comparison of the effective diffusion coefficient of water in various matrices (bread, starch, amylose, amylopectin, gluten, etc.) as a function of their mass fraction of water. There is also a need to explore the relationship of these material properties to firmness evaluations used to assess staling. To our knowledge, there are no experimental measurements on the effect of the thickness of the cells or other measure of porosity of breadcrumb to staling. Cell thickness and crumb porosity being a macroscopic scale approach that would reflect the properties previously discussed which could be considered a microscopic level. Common sense suggests that porosity of the crumb is one important attribute that will affect moisture diffusion and therefore moisture loss. Let us assume this oversimplified scenario, given the same formulation (effect of ingredients, specifically water in the system) and process (up to a certain point since the difference in porosity is generally attributed to processing or ingredients differences or a combination of these two), we could speculate (with no rigorous proof nor physical models) that a more dynamic system with faster moisture migration is found in a finer crumb structure. But more careful arguments may be instructive. For example, thickness and solubility of diffusional species in the material (in the assumed example of moisture migration is water diffusing through multicomponent layers or media) control the diffusional resistance of a thin sheet. It is expected that very thin sheets will tend to cause negligible resistance to most solutes (Walstra, 2003). One can assume that this could also be an approximation to thin sheets of cell walls and water.
23.4 Anti-staling agents A number of anti-staling agents, alone or in combination due to their synergistic effects, are used in the bread in industry. Due to their economic importance in extending the shelf life of baked goods, anti-staling agents are actively developed by ingredient companies. Among the most common anti-staling agents are surface-active components or emulsifiers and enzymes, some of them covered in this book. In this section, we will limit our scope to emulsifiers only. The reader can find comprehensive treatises including the chemistry and applications of
© Woodhead Publishing Limited, 2012
592
Breadmaking
emulsifiers in breadmaking in the literature (Knighthly, 1996; Whitehurst, 2004; Williams and Pullen, 2007; Popper, 2006). Most likely emulsifiers interact with several components in dough. They act by stabilizing the dough interfaces via decreasing surface tension and increasing/ decreasing molecular interactions. In summary, the major properties of emulsifiers in bread baking are surface activity, gluten interaction, starch complexation and foam formation (Popper, 2006). These properties are governed by different specific functions. For example, the surface activity of emulsifiers causes a reduction of surface tension at oil/water interfaces (thus stabilizing the lamella of bubbles), improvement of fat dispersion allowing reduction of fat in the formulation, as well as an improvement in water absorption. It is assumed that emulsifiers form complexes with gluten, similar to the native polar lipids found in flour, thus increasing inter- and intra protein chain interactions. Emulsifiers could also act as plasticizers, increasing the slippage of layers of proteins when passing over each other (Popper, 2006). It is expected that emulsifiers will attach their polar end to the protein and project their hydrophobic portion to the outside, preventing adhesion to the polar starch molecules. The result will be increased dough elasticity and loaf volume. Gelatinization and recrystallization of starch is mitigated in the presence of emulsifiers by forming inclusion complexes (α-helix of amylose and side branch chains of amylopectin) with the hydrophobic ends. The overall effect is an improvement of crumb softness and texture. The stabilization of the foam structure in dough is improved with the addition of emulsifiers, which results in improved gas retention in the dough (Whitehurst, 2004; Popper, 2006). A list of the most common emulsifiers used in baked products is included in Table 23.1, adapted from (Zobel and Kulp, 1996). It includes a summary of the effect of the selected emulsifiers in strengthening and fermentation of the dough and softening of the crumb. References from the Code of Federal Regulations and online links to their E numbers used in the European Union are also included.
23.5
Future trends
We offer a point of view of the need for systematic studies of the effect of antistaling additives on Tg and rate of diffusivity to obtain relationship maps of specific macro- and micro-molecular changes. Diffusion in food systems has been associated with the glass transition and viscosity. The relationship between a specific parameter and a specific physical change (i.e. crystallinity) needs to be determined. For example, trehalose increases Tg of bread and is an effective antistaling agent (Zhou et al., 2007). Since different studies are conducted with different bread formulas and processing, it would be of interest to compare the kinetic studies and relationship maps on Tg and rate of diffusivity on future studies comparing trehalose with other anti-staling agents. There is also a need for quick and relatively inexpensive methods (perhaps dielectric methodologies) that can reliably measure Tg shifts as a function of all the main components in bread
© Woodhead Publishing Limited, 2012
Table 23.1
Surfactants commonly used in baked products and their effect
© Woodhead Publishing Limited, 2012
Product
Strengthening Softening
Fermentation* Reference**
E Number
Calcium steroyl-2lactylate
Excellent
Good
0.5%1
21 CFR, 172.844
Sodium steroyl-2lactylate
Excellent
Very good
0.5%1
21 CFR, 172.846
DATEM
Excellent
Fair
No limit2
21 CFR,184.1101
Mono- and diglycerides of fatty acids Succinylated monoglycerides
No
Excellent
No limit2
21 CFR, 184.1505
Good
Good
0.5%1
21 CFR, 172.830
Polysorbate 60
Fair
Very good
0.5%1
21 CFR, 172.836
Ethoxylated monoand diglycerides
Very good
Poor
0.5%1
21 CFR, 172.834
Sucrose fatty-acid esters
Very good
Very good
No limit2
21 CFR, 172.859
E482 http://www.ukfoodguide.net/e482.htm Accessed 16 May 2011 E481 http://www.ukfoodguide.net/e481.htm Accessed 16 May 2011 E472e for Mono- and diacetyl tartaric acid esters of mono- and diglycerides of fatty acids http://www.ukfoodguide.net/e427e.htm Accessed 16 May 2011 E471 http://www.ukfoodguide.net/e471.htm Accessed 16 May 2011 E472g http://en.wikipedia.org/wiki/E_number Accessed 16 May 2011 E436 http://www.ukfoodguide.net/e436.htm Accessed 16 May 2011 E488 http://en.wikipedia.org/wiki/E_number Accessed 16 May 2011 E473 http://www.ukfoodguide.net/e473.htm Accessed 16 May 2011
Source: Adapted from Zobel and Kulp (1996) with permission. Notes: * 1Gives limits for standardized products; 2no limits (only GMP compliance) for nonstandardized products. ** References given are from Code of Federal Regulations, Section 21, Washington, D.C.: Office of the Federal Register, National Archives and Records, revised 16 May 2011.
594
Breadmaking
and follow macro- and micro-molecular changes that conduce to bread staling (through the assistance of some software prediction models). However, there continues to be a need for direct measurement of the texture of bread (breadcrumb and crust) using rheological methods. The long-term objective is to use a combination of physicochemical and rheological methods for product development/QC purposes. Also proposed is the continued adaptation of methods to baked products and which would measure directly or indirectly (1) the shift of organized/ordered to disordered state, such as evaluation of Tg and molecular mobility, (2) the effect of ingredients used in the formulation of bread, including additives, and (3) the reaction rates of individual measurable variables associated with staling. A more precise understanding is needed of subtle changes in the physical state of the bread that affect the mobility of the polymers and thus reaction rates during storage.
23.6
References
BAIK, M.-Y.
and CHINACHOTI, P. 2000. Moisture redistribution and phase transitions during bread staling. Cereal Chemistry, 77, 484–8. BHATT, C. and NAGARAJU, J. 2009. Studies on electrical properties of wheat bread as a function of moisture content during storage. Sensing and Instrumentation for Food Quality and Safety, 4, 61–6. BROWN, S. P. and SPIESS, H. W. 2001. Advanced solid-state NMR methods for the elucidation of structure and dynamics of molecular, macromolecular, and supramolecular systems. Chemical Reviews, 101, 4125–56. BUERA, M. P., JOUPPILA, K., ROOS, Y. H. and CHIRIFE, J. 1998. Differential scanning calorimetry glass transition temperatures of white bread and mold growth in the putative glassy state. Cereal Chemistry, 75, 64–9. CAUVAIN, S. and YOUNG, L. 2000. Effects of water upon textural properties and their changes during storage. In: Cauvain, S. and Young, L. (eds) Bakery Food Manufacture and Quality: Water Control and Effects. Oxford: Blackwell Science Ltd. CHEN, P. L., LONG, Z., RUAN, R. and LABUZA, T. P. 1997. Nuclear magnetic resonance studies of water mobility in bread during storage. Food Science and Technology – LebensmittelWissenschaft und -Technologie, 30, 178–83. CHINACHOTI, P. and VODOVOTZ, Y. 2001. Bread Staling. Boca Raton, FL: CRC Press LLC. COWIE, J. M. G. 1991. The amorphous phase. In: Cowie, J. M. G. (ed.) Polymers: Chemistry and Physics of Modern Materials, 2nd edn. Boca Raton, FL: CRC Press. CURTI, E., BUBICI, S., CARINI, E., BARONI, S. and VITTADINI, E. 2011. Water molecular dynamics during bread staling by Nuclear Magnetic Resonance. LWT – Food Science and Technology, 44, 854–9. ENGELSEN, S. B., JENSEN, M. K., PEDERSEN, H. T., NORGAARD, L. and MUNCK, L. 2001. NMRbaking and multivariate prediction of instrumental texture parameters in bread. Journal of Cereal Science, 33, 59–69. GARIMELLA PURNA, S. K., MILLER, R. A., SEIB, P. A., GRAYBOSCH, R. A. and SHI, Y.-C. 2011. Volume, texture, and molecular mechanism behind the collapse of bread made with different levels of hard waxy wheat flours. Journal of Cereal Science, 54, 37–43. GOESAERT, H., SLADE, L., LEVINE, H. and DELCOUR, J. A. 2009. Amylases and bread firming – an integrated view. Journal of Cereal Science, 50, 345–52. GRAY, J. and BEMILLER, J. 2003. Bread staling: Molecular basis and control. Comprehensive Reviews in Food Science and Food Safety, 2, 1–21.
© Woodhead Publishing Limited, 2012
Bread staling HINDERMANN-BISCHOFF, M.
595
and EHRBURGER-DOLLE, F. 2001. Electrical conductivity of carbon black-polyethylene composites: Experimental evidence of the change of cluster connectivity in the PTC effect. Carbon, 39, 375–82. HIRTE, A., HAMER, R. J., MEINDERS, M. B. J. and PRIMO-MARTIN, C. 2010. Permeability of crust is key to crispness retention. Journal of Cereal Science, 52, 129–35. HUANG, V. T., HAYNES, L., LEVINE, H. and SLADE, L. 1996. Glass transitions in starch, gluten and bread as measured with dielectric spectroscopy and TMA methods. Journal of Thermal Analysis and Calorimetry, 47, 1289–98. HUG-ITEN, S., ESCHER, F. and CONDE-PETIT, B. 2003. Staling of bread: Role of amylose and amylopectin and influence of starch-degrading enzymes. Cereal Chemistry, 80, 654–61. KIM-SHIN, M. S., MARI, F., RAO, P. A., STENGLE, T. R. and CHINACHOTI, P. 1991. Oxygen – 17 nuclear magnetic resonance studies of water mobility during bread staling. Journal of Agricultural and Food Chemistry, 39, 1915–20. KNIGHTHLY, W. H. 1996. Surfactants In: Hebeda, R. E. and Zobel, H. F. (eds) Baked Goods Freshness. New York: Marcel Dekker, Inc. LE MESTE, M., HUANG, V. T., PANAMA, J., ANDERSON, G. and LENTZ, R. 1992. Glass transition of bread. Cereal Foods World, 37, 264–7. LODI, A., ABDULJALIL, A. M. and VODOVOTZ, Y. 2007. Characterization of water distribution in bread during storage using magnetic resonance imaging. Magnetic Resonance Imaging, 25, 1449–58. MOURA RAMOS, J. J., CORREIA, N. T. and DIOGO, H. P. 2006. TSDC as a tool to study slow molecular mobility in condensed complex systems. Journal of Non-Crystalline Solids, 352, 4753–7. POPPER, L. 2006. Flour treatment. In: Popper, L., Shafer, W. and Freund, W. (eds) Future of Flour. A Compendium of Flour Improvement. Dumme: AgriMedia GmbH. ROOS, Y. M. and LIEVONEN, S. M. 2002. State transitions and reaction rates in concentrated food systems. In: Welti-Chanes, J., Barbosa-Canovas, G. V. and Aguilera, J. M. (eds) Engineering and Food for the 21st Century. Boca Raton, FL: CRC Press. RUAN, R. R. and CHEN, P. L. 2001. Nuclear magnetic resonance techniques. In: Chinachoti, P. and Vodovotz, Y. (eds) Bread Staling. Boca Raton, FL: CRC Press LLC. RUAN, R., ALMAER, S., HUANG, V. T., PERKINS, P., CHEN, P. and FULCHER, R. G. 1996. Relationship between firming and water mobility in starch-based food systems during storage. Cereal Chemistry, 73, 328–32. SARKO, A. and WU, H. C. H. 1978. The crystal structures of A-, B- and C-polymorphs of amylose and starch. Starch – Stärke, 30, 73–8. SERENO, N. M., HILL, S. E., MITCHELL, J. R., SCHARF, U. and FARHAT, I. A. 2007. Probing water migration and mobility during the aging of bread. In: I. A. Farhat, Belton, P. S. and Webb, G. A. (eds) Magnetic Resonance in Food Science: From Molecules to Man. London, UK: RSC Publishing. SLADE, L. and LEVINE, H. 1991. Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety. Crit Rev Food Sci Nutri, 30, 115–360. SLADE, L. and LEVINE, H. 2003. Food polymer science approach to studies on freshness and shelf-life of foods. In: Cadwallader, K. R. and Weenen, H. (eds) Freshness and ShelfLife of Foods. Washington, DC: ACS. TIAN, Y Q., LI, Y., JIN, Z Y., XU, X M., WANG, J. P. et al. 2009. β-Cyclodextrin (β-CD): A new approach in bread staling. Thermochimica Acta, 489, 22–6. WALSTRA, P. 2003. Physical chemistry of foods. New York: Marcel Dekker, Inc. WANG, X., CHOI, S.-G. and KERR, W. L. 2004. Water dynamics in white bread and starch gels as affected by water and gluten content. Lebensmittel-Wissenschaft und -Technologie, 37, 377–84. WEAIRE, D. L. and HUTZLER, S. (eds) 1999. The Physics of Foams, Oxford: Clarendon Press, Oxford University Press.
© Woodhead Publishing Limited, 2012
596
Breadmaking
WEAIRE, D., LANLGOIS, V., SAADATFAR, M. and HUTZLER, S. 2007. Foam as granular matter. In:
Aste, T., DI Matteo, T. and Tordesillas, A. (eds) Granular and Complex Materials, World Scientific Lecture Notes in Complex Systems. Singapore: World Scientific Publishing Co. Pte. Ltd. WHITEHURST, R. J. 2004. Emulsifiers in Food Technology, Oxford Backwell Publishing Ltd. WILLIAMS, T. and PULLEN, G. 2007. Functional Ingredients. In: Cauvain, S. P. and Young, L. S. (eds) Technology of Breadmaking. 2nd ed. New York: Springer Business & Multimedia. ZHOU, J.-C., PENG, Y.-F. and XU, N. 2007. Effect of trehalose on fresh bread and bread staling. Cereal Foods World, 52, 313–6. ZOBEL, H. F. and KULP, K. 1996. The staling mechanism. In: Hebeda, R. E. and Zobel, H. F. (eds) Baked Goods Freshness. New York: Marcel Dekker, Inc.
© Woodhead Publishing Limited, 2012
Abstract: Bread staling is as old as bread itself. Much has been learned about the causes of bread staling, but loss of bread quality in storage is still a problem. There are several theories related to bread staling, but the complex interactions between bread components make it difficult to pin down exact cause-and-effect relationships. Several additives are used to interfere with the crumb firming and loss of crust crispness. Newer techniques look directly at moisture mobility using electrical impedance or nuclear magnetic resonance (NMR). These instruments provide useful information on the migration of water between components. This chapter will summarize newer research related to staling of bread and its molecular basis. Key words: bread staling, moisture mobility, water migration, crumb firmness, glass transition temperature, NMR.
23.1
Introduction
After almost 160 years of study, bread staling continues to firm our daily bread. Due to its dynamic non-equilibrium state, changes in the physical and organoleptic characteristics of bread start right after coming out from the oven during the cooling period and storage. Despite significant advances in the knowledge of factors that contribute to or accelerate staling and those contributing to ‘control’ it or slow it down, the bread-staling puzzle has not been solved completely. It is an important problem to the baking industry, implicating significant economical losses. From the research point of view, it is still a relevant issue that deserves a new comprehensive approach to its elucidation. Traditional definitions of bread staling begin by stating its complexity due to the number of components that participate in bread formulations as well as the different processing variables that affect the staling phenomenon. Even the nature of the main components in bread (proteins and carbohydrates), which are complex
© Woodhead Publishing Limited, 2012
Bread staling
581
matrices in their own right, prevents simplification within the context of bread staling. The definition of bread staling also includes the time and temperature dependent events that lead to the deterioration of its physical properties. Specifically, they lead to the loss of resilience of the crumb, decrease of crispness of the crust, loss of volatile flavor compounds and soluble starch content. Thus, bread staling includes a series of interrelated events. It is not possible to single out a particular event without underestimating related causes/effects in this dynamic system. In this chapter, a summary of selected relevant ingredients and processes known to influence bread staling will be presented. We will also offer a point of view for the systematic advancement of the understanding of bread staling molecular reactions involving kinetic studies, specifically of the reaction rates of important chemical and physical changes that occur in bread during storage at and above its glass transition. In the pursuit to find the connection between the microscopic and macroscopic properties of bread during staling, suggestions from complex material (such as foams) can be used (Weaire et al., 2007). Weaire and collaborators suggested multidisciplinary collaboration with material scientists actively advancing in chemistry, physics, mathematics and engineering disciplines. Such collaborations will bring expertise with models, simulations and advances in methodology that can improve our understanding of bread staling.
23.2
Breadcrumb structure
The multi-component breadcrumb has been considered a resilient deformable open-celled solid foam; a natural sponge is also an open celled solid foam, though its interconnected fibrils are more cylindrical and thinner than those of breadcrumb (see Figure 23.1 in Weaire and Hutzler, 1999). The air filled structure (alveoli) of the crumb is formed by inter-dispersed polymer rich gels and an interconnected network of films (lamellae) rich in low molecular weight compounds. Molecular mobility affects the rate at which substances diffuse across the semisolid (gel) structures to liquid film (lamellae) and interstitial spaces or to neighboring gel structures. The number of bubble contacts (connectivity) of a solid foam material (bread) influences its physical properties such as thermal or electrical conductance, as these properties are highly correlated to connectivity (Hindermann-Bischoff and Ehrburger-Dolle, 2001; Weaire and Hutzler, 1999).
23.3
Bread stability
The stability of food systems depends on the reaction rates (chemical and physical changes) of the different components (Roos and Lievonen, 2002). Among the critical parameters used for determining bread stability (staling) is the glass transition temperature (Tg) and its range (Slade and Levine, 2003). Tg is a property of amorphous materials and is measured in the laboratory as the transition from a rubbery, viscous amorphous solid, to a brittle, glassy amorphous solid of a
© Woodhead Publishing Limited, 2012
582
Breadmaking
glass with solid-like mechanical properties. At the molecular level, the measurement of breadcrumb glass transition describes the temperature at which the polymer chains (such as proteins and starch) have lower mobility compared to the temperature at which the polymers will undergo a phase transition from a crystalline or semi-crystalline phase to a solid amorphous phase (Slade and Levine, 2003). The glass transition temperature (Tg) in polymers is described as the temperature at which the Gibbs free energy is such that the activation energy of the cooperative process (translational and segmental transposition) of 50 or so adjacent chain segments is exceeded. This allows molecular chains to slide past each other in the direction of the stress (Cowie, 1991). As an example, the local segmental transposition of six carbon atoms (crankshaft motion) is reported to involve an activation energy of about 25 kJ mol−1 (Cowie, 1991). Tg can be measured by a number of methodologies such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectic analysis (DEA), thermomechanical analysis (TMA) and different spectroscopic methods (Roos and Lievonen, 2002). The broad range of Tg (onset and end, about 20°C) of white bread has been suggested to reflect its complexity and heterogeneity (Buera et al., 1998; Le Meste et al., 1992). For example, the polymers in bread are in different physical state; gluten is completely amorphous and starch is in a partially crystalline state (Le Meste et al., 1992). Due to its non-equilibrium state, bread will tend to form a more ordered structure or crystalline state, which is more stable. Thus, microdomains of ‘subphases’ are found in the bread system. These bread ‘subphases’ will have different diffusivity properties, which will affect the reaction rates of different changes that are time and temperature dependent. To advance in our understanding of bread staling kinetic studies are needed in which a systematical description of the reaction rates of each of the main factors that affect bread staling have been documented. Specifically, each of the ingredients (flour, water, sugar, salt, yeast, additives) and processing parameters such as temperature and time should be included in the study of their effect on state and phase transition, diffusion, molecular mobility (α- and β-relaxations), and reaction rates. Several authors have pointed to the lack of studies demonstrating the relationship of Tg and reaction rates in different food systems, such as low and intermediate moisture foods (Roos and Lievonen, 2002). The comparison of diffusion-controlled and well stirred systems over the same temperature range would illustrate the magnitude of change of the effect of glass transition on the reaction rates that are of interest in bread staling. Another series of experiments would need to be planned to document the reaction rates of bread formulated to cover a wide range of Tg (Roos and Lievonen, 2002). Similar experiments are needed to record the effect of Tg on the reaction rates of bread systems containing the additives reported to control or delay as well as promote staling. Such experiments will assist in developing a kinetic description of bread staling in relation to the complex factors that contribute to the deterioration of bread quality. For example Baik and Chinachoti (2000) showed that the glass transition temperature of fresh bread was about 0°C, while the appearance of a second peak in tan delta appeared after storing bread with and without the crust for seven days (Fig. 23.1). © Woodhead Publishing Limited, 2012
Bread staling
583
Fig. 23.1 Dynamic mechanical analysis (DMA) thermograms for breadcrumb during storage at 25°C. E′ = storage modulus, E″= loss modulus, tan delta = E″/E′ (Baik and Chinachoti, 2000).
Tian et al. (2009) investigated the role that the addition of β-cyclodextrin had on the x-ray diffraction patterns of fresh and stored crust and crumb (Fig. 23.2 and 23.3). They concluded that β-cyclodextrin had a significant impact on the staling of crust and crumb. It significantly lowered the rate of retrogradation of breadcrumb and affected the x-ray diffraction patterns suggesting new complexes with amylose and lipid. 23.3.1 Molecular mobility Bread average moisture content (about 38%) and storage temperature of bread (about 25°C) are favorable to a number of reactions that can cause loss of bread
© Woodhead Publishing Limited, 2012
584
Breadmaking
Fig. 23.2 Effect of 1.5% beta-cyclodextrin on x-ray diffraction pattern of fresh crust (a) and stored crust (b) in the absence (control) or presence of β-cyclodextrin (Tian et al., 2009).
quality. The physical state of the polymers (glassy solid or rubbery liquid) present in bread is assumed to affect its diffusion properties, meaning how rapidly the molecules move through the bread matrix (Roos and Lievonen, 2002). The diffusion properties are expected to affect the reaction rates of the major
© Woodhead Publishing Limited, 2012
Bread staling
585
Fig. 23.3 Effect of 1.5% β-cyclodextrin on x-ray diffraction pattern of fresh crust (a) and stored crust (b) in the absence (control) or presence of β-cyclodextrin (Curti et al., 2009).
macroscopic changes reported in bread staling although there are no experimental data reported. Water mobility studies of polymers are successfully studied by measuring the mobility of protons using nuclear magnetic resonance and magnetic resonance imaging methodologies and a summary of their application to bread staling is
© Woodhead Publishing Limited, 2012
586
Breadmaking
Fig. 23.4 (a) −1H T2 relaxation times distributions of breadcrumb during storage; (b) −1H T2 relative abundance in each proton population during storage (Curti et al., 2011).
available in the literature (Ruan and Chen, 2001; Curti et al., 2011). An example of T2 relaxation times distribution of breadcrumb in storage is shown in Fig. 23.4 (Curti et al., 2011). The mobility of protons reveals the equilibrium state of the material as they coexist in mobile (liquid phase) and immobile (solid phase) regions in food
© Woodhead Publishing Limited, 2012
Bread staling
587
systems (Ruan and Chen, 2001). Solid-state magnetic resonance methodologies provide structural and dynamic information of polymers and are suitable for providing insight into the mechanisms of self-assembly, which are of much relevance in modern polymer science (Brown and Spiess, 2001). For example, 1H solid-state NMR has provided valuable structural information on partially ordered samples gained on account of the marked sensitivity of the 1H chemical shift to hydrogen-bonding and aromatic π–π effects (Brown and Spiess, 2001). It can also probe dynamic events, such as the making and breaking of hydrogen bonds as well as guest mobility in a supramolecular complex (Brown and Spiess, 2001). Advances in high-resolution 1H solid-state NMR methods have opened up the possibility of routinely directly probing hydrogen-bonding and aromatic π–π interactions that will translate in molecular mobility in bread staling (Sereno et al., 2007). Studies of changes in water mobility using transverse relaxation time (T2) of protons (1H nuclear magnetic resonance (NMR) Relaxometry) revealed two distinctive regions of T2 associated with multiple domains of water in fresh white breadcrumb and gluten-starch gels (Wang et al., 2004). Bread crumb results revealed a less mobile water region (170–220 μs T2) and a more mobile region (3–6 ms T2) which were associated with multiple domains of water (Wang et al., 2004). Multiple 1H T2 populations in baked products that underwent major changes over storage, resulting in a reduced mobility (shorter 1H T2 relaxation time) in stored products have been reported (Ruan et al., 1996; Chen et al., 1997; Engelsen et al., 2001). Wang et al. (2004) also reported that the water mobility (peak T2 values) in breadcrumb were similar for bread prepared with protein content in the range of 11.2, 13, and 14.2 g of protein/100 g of flour (levels obtained by adding dried gluten) but significant differences were found for breads prepared with moisture content within the range of 180, 220, and 260 g of water/345 g of flour. Slow water mobility can be interpreted as an environment of higher viscosity with more polymer–water interaction. The authors suggested that during gelatinization starch granules absorbed water (presumably coming from the hydrated gluten) and the mobility of water associated with starch decreased significantly. The latter phenomenon can be associated with the formation of crystallites of amylopectin, which has 36 water molecules associated with the unit cell of the B-type structure (Sarko and Wu, 1978; Gray and Bemiller, 2003) or with the amorphous phase (Kim-Shin et al., 1991). Other methodologies such as dielectric techniques of thermally stimulated depolarization currents and dielectric relaxation spectroscopy have been used to study the kinetics of slow molecular mobility in the amorphous state of condensed complex systems, and have potential to be applied to kinetic studies of bread staling (Moura Ramos et al., 2006). The slow molecular mobility in amorphous solids includes the main relaxation (α) and the secondary relaxations (β, corresponding to intramolecular reorientations) (Moura Ramos et al., 2006). This is of particular interest since bread is found mostly in an amorphous state. Dielectric and mechanical analysis studies of commercial bread, wheat starch and vital wheat gluten have shown significant changes in the molecular mobility of
© Woodhead Publishing Limited, 2012
588
Breadmaking
their components as measured in changes in the glass transition temperature Tg (Huang et al., 1996). 23.3.2 Crumb firmness Crumb firmness of bread products is a function of time, temperature and formulation. It has been related to changes in starch components such as reorganization (recrystallization) of amylopectin and changes in amorphous domains of the gelatinized starch, as well as the molecular associations with components such as protein, starch and lipids. All these molecular changes are related to the organoleptic and mechanical properties (stiffness) of the bread structures. The molecular basis for these reorganizations has been reviewed extensively in the literature but up to date there is no agreement on the cause and effect of the molecular processes involved (Gray and Bemiller, 2003; Chinachoti and Vodovotz, 2001; Cauvain and Young, 2000). Among the mechanism of staling proposed is the explanation of two independent processes. The first one occurs during the first two days of storage and is governed by organizational changes of the starch polymers, and the second is associated with the water migration from the gluten to the starch and from the crumb to the crust (Garimella Purna et al., 2011; Hug-Iten et al., 2003). This can cause a loss of crispness in the crust. However, the permeability of the crust to water vapor can mitigate this effect (Hirte et al., 2010). But this will not stop the loss of moisture from the crumb itself. The change in water mobility and distribution causes changes in diffusion properties in the crumb and crust, and the perception of the crumb is that it has become stiff and dry (Lodi et al., 2007; Baik and Chinachoti, 2000; Cauvain and Young, 2000). In general, the loss of textural quality in fresh bread after cooling and over time is due to the retrogradation/recrystallization of starch, especially the short amylopectin side chains (Goesaert et al., 2009). This in turn leads to a firmer, less resilient crumb and a softer crust due to moisture migration to the crust. These authors describe the main ingredients in bread such as flour: starch, gluten proteins, non-starch polysaccharides and lipids, and how amylases act as antistaling agents. Slade and Levine (1991) have used a food polymer approach to understanding staling based on starch and gluten networks in bread. Conclusions were stated in Goesaert et al. (2009) as follows. A conventional bacterial alphaamylase can weaken the starch network in bread, but does not prevent amylopectin recrystallization. The latter process leads to sequestering water in amylopectin crystallites, thus reducing the available plasticizing water to the gluten network and increased firmness. On the other hand, a maltogenic alpha-amylase limits amylopectin recrystallization, with the result that plasticizing water remains available to the gluten network resulting in a softer more resilient crumb. Gray and Bemiller (2003) came to similar conclusions. Their analysis of the literature supports the idea that gluten serves as a reservoir of water for transfer to starch, which in turn retrogrades. They point out that amylopectin retrogradation alone does not explain bread staling, but one must also consider gluten–starch
© Woodhead Publishing Limited, 2012
Bread staling
589
interactions. Slade and Levine relate degree of recrystallization to the glass transition temperature (Tg) (Slade and Levine, 1991). Tg was said to be around room temperature for freshly baked bread and increasing to about 60°C for the fully retrograded (staled) system. Baik and Chinachoti (2000) looked at moisture distribution and phase transitions for white bread, with and without crust, stored at 25°C in hermetically sealed containers. The crumb moisture content decreased significantly more for the sample with crust than the one without crust. The freezable water was also reduced in the sample with crust. Storing the samples beyond seven days resulted in greater firmness and increased amylopectin retrogradation in the sample stored with crust. This was attributed to moisture migration from crumb to crust. Electrical impedance spectroscopy (EIS) was used to observe the effects of staling on the physicochemical properties of bread (Bhatt and Nagaraju, 2009) (Fig. 23.5 and 23.6). The authors found that the glass transition temperature of the crust occurred after 96 hours of storage at room temperature for moisture contents greater than 17%. EIS results were validated via DSC experiments. Recrystallization in the crumb followed by EIS with time was also validated via DSC. Thus, EIS provides for rapid nondestructive measurement of electrical properties of bread at different zones from the crumb to the crust.
Fig. 23.5 Variation in capacitance at the crust with moisture content (Bhatt and Nagaraju, 2009).
© Woodhead Publishing Limited, 2012
590
Breadmaking
Fig. 23.6 Variation in resistance at the crumb with time (Bhatt and Nagaraju, 2009).
23.3.3 Transport processes in breads Based on the previous discussions some ideas will be further explored from the point of view of transport matter. The moisture migration or moisture transport in a solid material of medium moisture like bread is affected by several phenomena including the diffusion of water through the structure and permeability. It is assumed that diffusion of different compounds plays an important role in the deterioration of quality during bread storage, for example loss of moisture and flavor compounds. In an oversimplified version, in solid materials and more specifically in composite materials in quasi-equilibrium, if concentration gradients exist, there is a trend to even out such concentrations by diffusion, generally as a function of time (Walstra, 2003). In the case of moisture, it will diffuse from a higher moisture region to a drier region. The rate of change is limited by the diffusion coefficients, which are inversely proportional to particle or molecule radius and viscosity of the media (Walstra, 2003). The diffusion in compound systems is model-dependent, meaning it depends on the structure of the matrix. For simplicity’s sake, let us consider bread as a composite solid matrix, interspersed with a continuous liquid phase and a discontinuous gas (air) phase. Liquids flowing through the matrix of a composite solid encounter pressure gradients proportional to the material constant called permeability. In an oversimplified description, permeability is about proportional to pore diameter squared and pore volume fraction (surface fraction of the pores in a cross section of the material) (Walstra, 2003).
© Woodhead Publishing Limited, 2012
Bread staling
591
Permeability is not an easy property to predict. Some relations have been proposed based on the volume fraction of the matrix material and the specific surface area of the matrix (in m2 per m3 of the whole material). Some models were developed for powders and have limited validity. Examples of approximate magnitudes of permeability (B) for ground coffee, renneted milk gel, and polysaccharide gel are 10−8, 10−12, and 10−17 per m2, respectively. So, the two examples of gels have relatively lower permeability than ground coffee and hold water tenaciously (Walstra, 2003). In summary, studies are needed on the effective diffusion coefficients and permeability of breadcrumb and crust and model systems with single and binary ingredients used in bread to enhance our understanding of the staling phenomena. This would have several approaches such as comparison of the effective diffusion coefficient of water in various matrices (bread, starch, amylose, amylopectin, gluten, etc.) as a function of their mass fraction of water. There is also a need to explore the relationship of these material properties to firmness evaluations used to assess staling. To our knowledge, there are no experimental measurements on the effect of the thickness of the cells or other measure of porosity of breadcrumb to staling. Cell thickness and crumb porosity being a macroscopic scale approach that would reflect the properties previously discussed which could be considered a microscopic level. Common sense suggests that porosity of the crumb is one important attribute that will affect moisture diffusion and therefore moisture loss. Let us assume this oversimplified scenario, given the same formulation (effect of ingredients, specifically water in the system) and process (up to a certain point since the difference in porosity is generally attributed to processing or ingredients differences or a combination of these two), we could speculate (with no rigorous proof nor physical models) that a more dynamic system with faster moisture migration is found in a finer crumb structure. But more careful arguments may be instructive. For example, thickness and solubility of diffusional species in the material (in the assumed example of moisture migration is water diffusing through multicomponent layers or media) control the diffusional resistance of a thin sheet. It is expected that very thin sheets will tend to cause negligible resistance to most solutes (Walstra, 2003). One can assume that this could also be an approximation to thin sheets of cell walls and water.
23.4 Anti-staling agents A number of anti-staling agents, alone or in combination due to their synergistic effects, are used in the bread in industry. Due to their economic importance in extending the shelf life of baked goods, anti-staling agents are actively developed by ingredient companies. Among the most common anti-staling agents are surface-active components or emulsifiers and enzymes, some of them covered in this book. In this section, we will limit our scope to emulsifiers only. The reader can find comprehensive treatises including the chemistry and applications of
© Woodhead Publishing Limited, 2012
592
Breadmaking
emulsifiers in breadmaking in the literature (Knighthly, 1996; Whitehurst, 2004; Williams and Pullen, 2007; Popper, 2006). Most likely emulsifiers interact with several components in dough. They act by stabilizing the dough interfaces via decreasing surface tension and increasing/ decreasing molecular interactions. In summary, the major properties of emulsifiers in bread baking are surface activity, gluten interaction, starch complexation and foam formation (Popper, 2006). These properties are governed by different specific functions. For example, the surface activity of emulsifiers causes a reduction of surface tension at oil/water interfaces (thus stabilizing the lamella of bubbles), improvement of fat dispersion allowing reduction of fat in the formulation, as well as an improvement in water absorption. It is assumed that emulsifiers form complexes with gluten, similar to the native polar lipids found in flour, thus increasing inter- and intra protein chain interactions. Emulsifiers could also act as plasticizers, increasing the slippage of layers of proteins when passing over each other (Popper, 2006). It is expected that emulsifiers will attach their polar end to the protein and project their hydrophobic portion to the outside, preventing adhesion to the polar starch molecules. The result will be increased dough elasticity and loaf volume. Gelatinization and recrystallization of starch is mitigated in the presence of emulsifiers by forming inclusion complexes (α-helix of amylose and side branch chains of amylopectin) with the hydrophobic ends. The overall effect is an improvement of crumb softness and texture. The stabilization of the foam structure in dough is improved with the addition of emulsifiers, which results in improved gas retention in the dough (Whitehurst, 2004; Popper, 2006). A list of the most common emulsifiers used in baked products is included in Table 23.1, adapted from (Zobel and Kulp, 1996). It includes a summary of the effect of the selected emulsifiers in strengthening and fermentation of the dough and softening of the crumb. References from the Code of Federal Regulations and online links to their E numbers used in the European Union are also included.
23.5
Future trends
We offer a point of view of the need for systematic studies of the effect of antistaling additives on Tg and rate of diffusivity to obtain relationship maps of specific macro- and micro-molecular changes. Diffusion in food systems has been associated with the glass transition and viscosity. The relationship between a specific parameter and a specific physical change (i.e. crystallinity) needs to be determined. For example, trehalose increases Tg of bread and is an effective antistaling agent (Zhou et al., 2007). Since different studies are conducted with different bread formulas and processing, it would be of interest to compare the kinetic studies and relationship maps on Tg and rate of diffusivity on future studies comparing trehalose with other anti-staling agents. There is also a need for quick and relatively inexpensive methods (perhaps dielectric methodologies) that can reliably measure Tg shifts as a function of all the main components in bread
© Woodhead Publishing Limited, 2012
Table 23.1
Surfactants commonly used in baked products and their effect
© Woodhead Publishing Limited, 2012
Product
Strengthening Softening
Fermentation* Reference**
E Number
Calcium steroyl-2lactylate
Excellent
Good
0.5%1
21 CFR, 172.844
Sodium steroyl-2lactylate
Excellent
Very good
0.5%1
21 CFR, 172.846
DATEM
Excellent
Fair
No limit2
21 CFR,184.1101
Mono- and diglycerides of fatty acids Succinylated monoglycerides
No
Excellent
No limit2
21 CFR, 184.1505
Good
Good
0.5%1
21 CFR, 172.830
Polysorbate 60
Fair
Very good
0.5%1
21 CFR, 172.836
Ethoxylated monoand diglycerides
Very good
Poor
0.5%1
21 CFR, 172.834
Sucrose fatty-acid esters
Very good
Very good
No limit2
21 CFR, 172.859
E482 http://www.ukfoodguide.net/e482.htm Accessed 16 May 2011 E481 http://www.ukfoodguide.net/e481.htm Accessed 16 May 2011 E472e for Mono- and diacetyl tartaric acid esters of mono- and diglycerides of fatty acids http://www.ukfoodguide.net/e427e.htm Accessed 16 May 2011 E471 http://www.ukfoodguide.net/e471.htm Accessed 16 May 2011 E472g http://en.wikipedia.org/wiki/E_number Accessed 16 May 2011 E436 http://www.ukfoodguide.net/e436.htm Accessed 16 May 2011 E488 http://en.wikipedia.org/wiki/E_number Accessed 16 May 2011 E473 http://www.ukfoodguide.net/e473.htm Accessed 16 May 2011
Source: Adapted from Zobel and Kulp (1996) with permission. Notes: * 1Gives limits for standardized products; 2no limits (only GMP compliance) for nonstandardized products. ** References given are from Code of Federal Regulations, Section 21, Washington, D.C.: Office of the Federal Register, National Archives and Records, revised 16 May 2011.
594
Breadmaking
and follow macro- and micro-molecular changes that conduce to bread staling (through the assistance of some software prediction models). However, there continues to be a need for direct measurement of the texture of bread (breadcrumb and crust) using rheological methods. The long-term objective is to use a combination of physicochemical and rheological methods for product development/QC purposes. Also proposed is the continued adaptation of methods to baked products and which would measure directly or indirectly (1) the shift of organized/ordered to disordered state, such as evaluation of Tg and molecular mobility, (2) the effect of ingredients used in the formulation of bread, including additives, and (3) the reaction rates of individual measurable variables associated with staling. A more precise understanding is needed of subtle changes in the physical state of the bread that affect the mobility of the polymers and thus reaction rates during storage.
23.6
References
BAIK, M.-Y.
and CHINACHOTI, P. 2000. Moisture redistribution and phase transitions during bread staling. Cereal Chemistry, 77, 484–8. BHATT, C. and NAGARAJU, J. 2009. Studies on electrical properties of wheat bread as a function of moisture content during storage. Sensing and Instrumentation for Food Quality and Safety, 4, 61–6. BROWN, S. P. and SPIESS, H. W. 2001. Advanced solid-state NMR methods for the elucidation of structure and dynamics of molecular, macromolecular, and supramolecular systems. Chemical Reviews, 101, 4125–56. BUERA, M. P., JOUPPILA, K., ROOS, Y. H. and CHIRIFE, J. 1998. Differential scanning calorimetry glass transition temperatures of white bread and mold growth in the putative glassy state. Cereal Chemistry, 75, 64–9. CAUVAIN, S. and YOUNG, L. 2000. Effects of water upon textural properties and their changes during storage. In: Cauvain, S. and Young, L. (eds) Bakery Food Manufacture and Quality: Water Control and Effects. Oxford: Blackwell Science Ltd. CHEN, P. L., LONG, Z., RUAN, R. and LABUZA, T. P. 1997. Nuclear magnetic resonance studies of water mobility in bread during storage. Food Science and Technology – LebensmittelWissenschaft und -Technologie, 30, 178–83. CHINACHOTI, P. and VODOVOTZ, Y. 2001. Bread Staling. Boca Raton, FL: CRC Press LLC. COWIE, J. M. G. 1991. The amorphous phase. In: Cowie, J. M. G. (ed.) Polymers: Chemistry and Physics of Modern Materials, 2nd edn. Boca Raton, FL: CRC Press. CURTI, E., BUBICI, S., CARINI, E., BARONI, S. and VITTADINI, E. 2011. Water molecular dynamics during bread staling by Nuclear Magnetic Resonance. LWT – Food Science and Technology, 44, 854–9. ENGELSEN, S. B., JENSEN, M. K., PEDERSEN, H. T., NORGAARD, L. and MUNCK, L. 2001. NMRbaking and multivariate prediction of instrumental texture parameters in bread. Journal of Cereal Science, 33, 59–69. GARIMELLA PURNA, S. K., MILLER, R. A., SEIB, P. A., GRAYBOSCH, R. A. and SHI, Y.-C. 2011. Volume, texture, and molecular mechanism behind the collapse of bread made with different levels of hard waxy wheat flours. Journal of Cereal Science, 54, 37–43. GOESAERT, H., SLADE, L., LEVINE, H. and DELCOUR, J. A. 2009. Amylases and bread firming – an integrated view. Journal of Cereal Science, 50, 345–52. GRAY, J. and BEMILLER, J. 2003. Bread staling: Molecular basis and control. Comprehensive Reviews in Food Science and Food Safety, 2, 1–21.
© Woodhead Publishing Limited, 2012
Bread staling HINDERMANN-BISCHOFF, M.
595
and EHRBURGER-DOLLE, F. 2001. Electrical conductivity of carbon black-polyethylene composites: Experimental evidence of the change of cluster connectivity in the PTC effect. Carbon, 39, 375–82. HIRTE, A., HAMER, R. J., MEINDERS, M. B. J. and PRIMO-MARTIN, C. 2010. Permeability of crust is key to crispness retention. Journal of Cereal Science, 52, 129–35. HUANG, V. T., HAYNES, L., LEVINE, H. and SLADE, L. 1996. Glass transitions in starch, gluten and bread as measured with dielectric spectroscopy and TMA methods. Journal of Thermal Analysis and Calorimetry, 47, 1289–98. HUG-ITEN, S., ESCHER, F. and CONDE-PETIT, B. 2003. Staling of bread: Role of amylose and amylopectin and influence of starch-degrading enzymes. Cereal Chemistry, 80, 654–61. KIM-SHIN, M. S., MARI, F., RAO, P. A., STENGLE, T. R. and CHINACHOTI, P. 1991. Oxygen – 17 nuclear magnetic resonance studies of water mobility during bread staling. Journal of Agricultural and Food Chemistry, 39, 1915–20. KNIGHTHLY, W. H. 1996. Surfactants In: Hebeda, R. E. and Zobel, H. F. (eds) Baked Goods Freshness. New York: Marcel Dekker, Inc. LE MESTE, M., HUANG, V. T., PANAMA, J., ANDERSON, G. and LENTZ, R. 1992. Glass transition of bread. Cereal Foods World, 37, 264–7. LODI, A., ABDULJALIL, A. M. and VODOVOTZ, Y. 2007. Characterization of water distribution in bread during storage using magnetic resonance imaging. Magnetic Resonance Imaging, 25, 1449–58. MOURA RAMOS, J. J., CORREIA, N. T. and DIOGO, H. P. 2006. TSDC as a tool to study slow molecular mobility in condensed complex systems. Journal of Non-Crystalline Solids, 352, 4753–7. POPPER, L. 2006. Flour treatment. In: Popper, L., Shafer, W. and Freund, W. (eds) Future of Flour. A Compendium of Flour Improvement. Dumme: AgriMedia GmbH. ROOS, Y. M. and LIEVONEN, S. M. 2002. State transitions and reaction rates in concentrated food systems. In: Welti-Chanes, J., Barbosa-Canovas, G. V. and Aguilera, J. M. (eds) Engineering and Food for the 21st Century. Boca Raton, FL: CRC Press. RUAN, R. R. and CHEN, P. L. 2001. Nuclear magnetic resonance techniques. In: Chinachoti, P. and Vodovotz, Y. (eds) Bread Staling. Boca Raton, FL: CRC Press LLC. RUAN, R., ALMAER, S., HUANG, V. T., PERKINS, P., CHEN, P. and FULCHER, R. G. 1996. Relationship between firming and water mobility in starch-based food systems during storage. Cereal Chemistry, 73, 328–32. SARKO, A. and WU, H. C. H. 1978. The crystal structures of A-, B- and C-polymorphs of amylose and starch. Starch – Stärke, 30, 73–8. SERENO, N. M., HILL, S. E., MITCHELL, J. R., SCHARF, U. and FARHAT, I. A. 2007. Probing water migration and mobility during the aging of bread. In: I. A. Farhat, Belton, P. S. and Webb, G. A. (eds) Magnetic Resonance in Food Science: From Molecules to Man. London, UK: RSC Publishing. SLADE, L. and LEVINE, H. 1991. Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety. Crit Rev Food Sci Nutri, 30, 115–360. SLADE, L. and LEVINE, H. 2003. Food polymer science approach to studies on freshness and shelf-life of foods. In: Cadwallader, K. R. and Weenen, H. (eds) Freshness and ShelfLife of Foods. Washington, DC: ACS. TIAN, Y Q., LI, Y., JIN, Z Y., XU, X M., WANG, J. P. et al. 2009. β-Cyclodextrin (β-CD): A new approach in bread staling. Thermochimica Acta, 489, 22–6. WALSTRA, P. 2003. Physical chemistry of foods. New York: Marcel Dekker, Inc. WANG, X., CHOI, S.-G. and KERR, W. L. 2004. Water dynamics in white bread and starch gels as affected by water and gluten content. Lebensmittel-Wissenschaft und -Technologie, 37, 377–84. WEAIRE, D. L. and HUTZLER, S. (eds) 1999. The Physics of Foams, Oxford: Clarendon Press, Oxford University Press.
© Woodhead Publishing Limited, 2012
596
Breadmaking
WEAIRE, D., LANLGOIS, V., SAADATFAR, M. and HUTZLER, S. 2007. Foam as granular matter. In:
Aste, T., DI Matteo, T. and Tordesillas, A. (eds) Granular and Complex Materials, World Scientific Lecture Notes in Complex Systems. Singapore: World Scientific Publishing Co. Pte. Ltd. WHITEHURST, R. J. 2004. Emulsifiers in Food Technology, Oxford Backwell Publishing Ltd. WILLIAMS, T. and PULLEN, G. 2007. Functional Ingredients. In: Cauvain, S. P. and Young, L. S. (eds) Technology of Breadmaking. 2nd ed. New York: Springer Business & Multimedia. ZHOU, J.-C., PENG, Y.-F. and XU, N. 2007. Effect of trehalose on fresh bread and bread staling. Cereal Foods World, 52, 313–6. ZOBEL, H. F. and KULP, K. 1996. The staling mechanism. In: Hebeda, R. E. and Zobel, H. F. (eds) Baked Goods Freshness. New York: Marcel Dekker, Inc.
© Woodhead Publishing Limited, 2012