Baking: Chapter 19 - PDF

19 Baking Tiphaine Lucas IRSTEA Food Process Engineering Research Unit, Rennes Cedex, France Introduction 336 Structural changes in starch and protein Heat and water transport during baking 336 during baking and subsequent M...

19 Baking Tiphaine Lucas IRSTEA Food Process Engineering Research Unit, Rennes Cedex, France Introduction 336 Structural changes in starch and protein Heat and water transport during baking 336 during baking and subsequent Modes of heat transfer 336 changes in dough/crumb Modes of heat transport into mechanical properties 345 the dough 337 Basic mechanisms of starch Measurement of temperature gelatinization 345 during baking 338 Factors affecting starch gelatinization Spatial distribution of water and extent of starch gelatinization in bakery products 338 in bakery products 345 Measurements of water content Protein coagulation 346 in bakery products 339 Impact on dough viscosity during Cell expansion and squeezing, crumb baking 347 formation, and overall collapse 339 Impact on mechanical properties Growth from gas nuclei 339 of crumb 347 Mechanisms favoring volume Crust formation 348 increase in cell 340 Dehydration and its effect on Mechanisms limiting volume reactions in and mechanical increase in cell 340 properties of the crust 348 Starch gelatinization 340 Role of steam injection on the crust Ruptures in cell walls 341 formation 349 Setting of the crust and its effect Cell expansion in the crust; of cell collapse or coalescence 343 effect on permeability of crust Collapse of products 344 to gases 350 Total water loss 344 Factors affecting the crispiness Measurements of water content of bakery products, and in bakery products 344 bread crust 350 Spatial distribution of water content The end point of baking 351 in different bakery products 344 References 352 Bakery Products Science and Technology, Second Edition. Edited by W. Zhou, Y. H. Hui, I. De Leyn, M. A. Pagani, C. M. Rosell, J. D. Selman, and N. Therdthai. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. 336 CH 19 BAKING Introduction Oven-rise (the change of dough dimensions during baking, look at a video at http://www. During baking, heat and mass transfer takes weekendbakery.com/posts/baking-bread- the- place in the dough simultaneously and interde- movie/) together with dough/crumb transition pendently, and involves four major changes: and crispiness are the most outstanding macro- scopic features of baking, with relevance to 1. Gases are vaporized as temperature texture. Reactions are another essential feature, increases; the gas cell increases in volume related to taste (production of flavour), digestibil- provided that the cell wall retains gases and ity (starch gelatinization), and safety (production is deformable. of acrylamides). 2. Starch gelatinizes upon the increase of tem- These different changes do not occur sequen- perature to an extent depending on the local tially but do overlap. Roughly in the first stage of availability of water, and proteins coagulate. baking, cell expansion and water loss begin; one These changes limit dough extensibility. It of these can, however, be delayed during this may limit the cell expansion described in (1), stage. During stage 2, both the total height of and hence be a local driving force for pres- the product and the rate of water loss reach sure build-up and possibly fracturing of the their maximum. Color develops at the surface cell wall (3). during the last stage 3, concomitantly to a possi- 3. The initial structure with closed gas cells ble collapse of the porous structure and a separated by dough walls is transformed decrease in the rate of water loss. These changes into a porous structure with interconnected are detailed in the following sections. pores, known as the dough-crumb transition. Dough membranes rupture when they can no longer withstand the gas pressure, and Heat and water transport changes in (2) are a possible cause. In the- ory, ruptured walls limit the cell expansion during baking described in (1); gas molecules are Temperature dominates product quality because exchanged between adjacent open cells, it affects enzymatic reaction, volume expansion, and finally transported out of the dough. starch gelatinization, protein denaturation, non- Pressure forces in such regions may then enzymatic browning reactions, and water migra- decrease, a possible explanation for the col- tion, as will be detailed later. lapse of cells locally. Time-course changes in temperature and its 4. Under the action of high temperatures at distribution through the dough during baking are boundaries, water vaporizes in the oven determined by multiple factors, among them: the atmosphere. Depending on the product size of the product (biscuit versus bread loaf); thickness, but also on baking conditions, this the ratio between top, side and bottom heat flows; supports the formation of a dry, hard crust in the expansion of cells which reduces thermal con- dessert or bread dough, and may also lead to ductivity, but likewise favors the transport of water a complete drying in biscuits and cookies. vapor evaporated at the surface layers to the core, This is accompanied by specific chemical where it condenses and releases energy. reactions (in particular Maillard reactions), responsible for the browning of the crust and the development of flavour. Early setting Modes of heat transfer of the crust restricts total volume and modi- Heat is transferred through the combination of fies the balance in pressure forces between conduction, convection and radiation. Radiation the inner and outer dough and hence cell comes from the heating elements (burner flames) expansion (1) locally. and all hot metal parts (walls) in the oven. It is  HEAT AND WATER TRANSPORT DURING BAKING 337 responsive to changes in the absorptive capacity Under the dough surface, where there is still of the dough and there is a tendency for color liquid water, water evaporates to saturate the changes upon heating to accelerate after the first partial water vapor pressure in the pores. The browning has occurred (Therdthai and Zhou water vapor pressure reaches a maximum at a 2003). Contribution of radiation to heat transfer location from which the vapor can diffuse in at the top surface can be estimated by h-monitor two directions, towards the dough surface and and was found to be much higher in direct ovens, towards the core. The driving force for the trans- 70–80% (Baik and others 1999, 2000a) than in port of water vapor from this location to the sur- indirect ovens, 45–60% (Fahloul and others face has been detailed earlier. On the other hand, 1995). The contributions of the different heat the temperature gradient induces a water vapor sources were calculated through modeling for pressure gradient between this location and the tunnel type multizone industrial ovens; for bis- core, which produces a diffusive flow of vapor in cuit, heat transferred by conduction through the the same direction. Because the water migrates band was minimal (10%), radiant and convective counter to the thermal gradient, it is accompa- heat being of the same order (Fahloul and others nied by condensation, release of its latent heat 1995). Conduction represents more than half of and an increase in liquid water at the core. This is the total heat transferred to the product during why this mechanism is also called transport by tin baking, possibly reaching two-thirds in certain “evaporation-condensation”. This mechanism conditions (Baik and others 2000a). Conduction takes place as long as a temperature gradient mode is also predominant for products deposited remains (and water remains!) and the core tem- on the hearth of the oven, like Indian flat bread perature has not reached 100 °C. Such transport (chapatti) (Gupta 2001) or some French breads is promoted by the opening of the porous struc- (Sommier and others 2005). ture which occurs during baking; nevertheless it also proceeds in a cellular structure (closed cells) Modes of heat transport with four distinct phases (De Vries and others into the dough 1989): As soon as the product is introduced in the oven, 1. evaporation proceeds on the warmer side of heat transfer results in superficial water evapora- the gas cell, tion and heating up of the product. In the oven, 2. water migrates through the gas phase, where the temperature is high, the partial water induced by the vapor concentration gradient vapor pressure is far from saturation. Hence, inside the cell, water vapor in the superficial layers of the prod- 3. water condenses on the colder side of the gas uct diffuses into the oven atmosphere. As the dif- cell, fusive flow of liquid water from the core is less 4. liquid water is transported by diffusion than the flow of evaporated water at the surface, through the dough wall to the warmer side of the product dries at the surface, leading to a the next gas cell, where the series of processes very low water activity, and the water activity starts again. Thorvaldsson and Skjöldebrand decreases rapidly with water contents below (1998) suggested, however, that as the con- 0.15 kg/kg. Temperature near the product surface densed water is likely to be absorbed by does not exhibit a marked plateau at 100 °C (the starch gelatinization, transport of water by temperature of water at the pressure close to that this mechanism to and through the ungelati- of the atmosphere, and water activity close to nized closed region is unlikely to occur. that of pure water). Above 100 °C, the tempera- ture rises in thermodynamic equilibrium The transfer of water vapor with vaporization with the locally decreasing water content and near the surface and condensation at the core rises towards that of the oven temperature. contributes to heat transfer, and was proposed by 338 CH 19 BAKING Sluimer (cited by De Vries and others 1989) to and others (2005) reported higher heating rates explain rapid heat transport during baking. De at the core for small bread loaves, of 15–17 °C/min. Vries and others (1989) recorded the tempera- Higher heating rates can also be generated by ture in the centre of gas-free and fermented steam condensation onto the dough/batter sur- doughs during baking with the same external face subsequently to its injection into the oven. dimensions, and showed that heat transport in Sommier and others (2005) and Chevallier and fermented dough was much faster than in gas- others (2002) reported heating rates of 25 °C/min free dough. If conduction was the only mecha- for bread (baguette) and 37–150 °C/min for short nism, heat transport in a gas-free dough should dough. In a tunnel type multizone industrial oven, have been faster than in fermented dough pieces heating rates in the range of 18–50 °C/min were because of cells acting as isolating elements. Such reported for cakes (Baik and others 2000a). results were explained by the contribution of the evaporation-condensation-diffusion mechanism Spatial distribution of water to heat transport. The location where the partial water vapor in bakery products pressure is at maximum progressively moves Whatever the type of bakery product it is impor- inside the product, making the dry area grow tant to realize that moisture gradients exist in the thicker. Progressively, the superficial dry layer product at the end of the baking process. Such constitutes a major barrier to heat transport distribution in water content through the prod- inside the product, hence temperature and water uct governs textural properties such as the con- content gradients at the core progressively trast between the softness of crumb and the vanish. Heat transport is no longer intensified crispiness of the crust in high-moisture bakery by the diffusion of water vapor to the core. In products. It governs more generally the storage high-moisture products, temperature at the properties of the product such as the loss in crisp- core stabilizes at 100 °C; this plateau lasts as iness in high-moisture products or the checking long as its water content remains similar to that of biscuits (Ahmad and others 2001; Saleem and of the dough. others 2003). Heat transport towards the centre during baking induces water transport and there- Measurement of temperature fore a change in the water distribution (refer to the heat transport subsections). Water also plays during baking an important role in the major structural changes Reproducible measurements of the temperature that take place during bread baking, at the micro- inside dough during baking are difficult owing to scopic (for example, water vaporization in the the oven-rise and to the difficulty of accurately cell expansion) and molecular scale (for exam- locating the thermocouples. Temperature gradi- ple, starch gelatinization). ents are considerably increased in thin dough The initial water content typically ranges from pieces (about 15–50 °C in 2 mm thick biscuits), 11 to 30% for biscuit and crackers (Wade 1988), amplifying the inaccuracy in positioning the ther- 32–38% for cake batter (Baik and others 2000b) mocouple. Thermal conduction along the ther- and 38–48% for bread dough. mocouple wire is an additional source of bias; The final moisture of a freshly baked biscuit is however, this can be overcome by using optic fib- usually within the range of 1–5% depending on ers of less accuracy. Nevertheless, some general the type of product (Wade 1988). Water distribu- trends can be drawn for the core. tion is very difficult to measure in the thickness Heating rate in bread is close to 3–6 °C/min of a biscuit; it can be however assessed by simula- in the absence of steam injection (Therdthai tion or experimentally along the diameter of a and Zhou 2003; Sommier and others 2005; Patel disk-shaped biscuit, with differences up to 1.7% and others 2005; Wagner and others 2007). Patel (Ahmad and others 2001). The final water  CELL EXPANSION AND SQUEEZING, CRUMB FORMATION, AND OVERALL COLLAPSE 339 content in cakes varies between 17 and 24% (Hayakawa and Hwang 1981), 1.4 × 10–3 to (Baik and others 2000b). 9.4 × 10–4 kg kg/kg for yellow and chocolate cakes As bread loaves are larger pieces, reports of of various sizes (Baik and others 2000b), final water content are scarce for the loaf as a 2.5 × 10–4 kg kg/kg for sponge cake (Lostie and whole, with most data concerning the distribu- others 2002) and 2.8 × 10–5 to 2.4 × 10–4 kg kg/kg tion of moisture through the loaf. Total water (Hasatani and others 1992) and 2–5 × 10–4 kg kg/kg loss can be between 10 and 20%; it is enhanced (Sommier and others 2005) for bread. by higher baking temperatures (Hasatani and others 1992). The lowest water content at the end of baking is obviously encountered in the Measurements of water content bread crust, being 2–15% depending on the per- in bakery products meability of crust (Hirte and others 2010) or the In practical terms, although the overall water loss oven temperature (Ahrne and others 2007). is easy to assess, the determination of water con- Bread crumb located beneath the crust exhibits tent profiles in crumb during baking requires the intermediate water contents, from 2– to 12% setting up of a method for accurate cutting of the lower than that of dough (Thorvaldsson and loaf into samples while still hot and deformable, Skjöldebrand 1998) or from 3–9% lower than and the minimization of water losses by evapora- that of dough (Wagner and others 2007). The tion from cut surfaces. Water content is also sen- crumb at the core retained its initial moisture sitive to the time elapsed from the end of baking content or even increased (Wagner and others and the relative humidity in the atmosphere of 2007). When sampling took place just after storage; these important factors are rarely removal of bread loaves from the oven, an reported and make experimental results almost increase in water content by 1 to 3% was reported impossible to compare between studies. at the core (De Vries and others 1989; Zanoni and Peri 1993; Wagner and others 2007). In addi- tion to the conventional, destructive method of sampling combined with oven-drying for water Cell expansion and squeezing, content determination, a continuous method crumb formation, and overall based on near infrared reflectance (NIR) was proposed by Thorvaldsson &and Skjöldebrand collapse (1998). Light was sent into the sample through an During baking, total volume may increase by optic fibre and the intensity of the reflected light 16–100% (volume unit per 100 volume units of was measured. The technique is known to be the initial dough) for bread (Zanoni and Peri sensitive to the structure and temperature of 1993; Sommier and others 2005; Wagner and the sample. If dependence on temperature was others 2008), 50–80% for sponge cake (Lostie incorporated into the calibration, structural and others 2002) and 100–300% for biscuit dependence made quantitative measurements (Wade 1988; Chevallier and others 2002). in the sample difficult during the oven-rise. Thorvaldsson &and Skjöldebrand (1998) showed an increase in water content in the core immedi- Growth from gas nuclei ately after the local temperature had reached The presence of gas nuclei is a prerequisite for 70 ± 5 °C °C (i.e. after cell expansion was more or inflation (Baker and Mize 1937). The aeration less complete), which reached +8% at the end of of dough or batter is achieved during mixing baking (35 min at 225 °C °C) relative to that of or whipping for sponge cake. For bread, dough dough initially. porosity after mixing is in the range 3–15% Magnitudes of the overall drying rate were depending on the mixer type and mixer speed 1.8 × 10–3 kg kg water/kg dry matter for cookies (Cauvain and others 1999). The inflation of cells 340 CH 19 BAKING may be started before baking by fermentation, macroscopic scale (for example, pores connected while sheeting – involved in the production of to the exterior of the dough or setting of the outer cream crackers for instance – largely eliminates layers of dough). Few studies have encompassed the gas produced from previous steps of process- all these different scales in attempting to provide ing within the dough (Wade 1988). a clear explanation for controversial data and theories in the literature. In this context, the rela- tive contribution of the different mechanisms to Mechanisms favoring volume the restrictions of expansion will not be discussed increase in cell further as more multiscale research is required. Thermal expansion of gases initially occluded in the bubble is responsible for 10–15% of cell infla- Starch gelatinization Starch gelatinization and/ tion. Further inflation is hence supported by the or protein aggregation contribute to the stiffen- vaporization of dough components at the cell– ing of dough (Figure 19.1) and reduce the capac- dough interface; vaporization is hastened by tem- ity of cell walls to elongate and the gas cell to perature increase during baking. The solubility of increase in volume. Some evidence has been carbon dioxide accumulated during fermenta- obtained when studying the early interruption of tion decreases during baking and the dissolved the baking process, the most demonstrative case gases are vaporized. The vapor pressure of water being the growth of columnar cells in crumpets increases rapidly upon heating; the same applies where heat transfers unidirectionally (Pyle 2005). to ethanol accumulated during fermentation, As a consequence, the onset temperature of although this gas has received little attention in these reactions (in the vicinity of 60 °C for starch past studies. It is important to understand that gelatinization and 70 °C for protein aggregation) water vapor is the predominant gas for cell infla- is often related to a build-up in gas pressure and tion in specific bakery products like sponge cake the restriction of, and ultimately the cessation of, (Lostie and others 2002) or non-yeasted puff cell growth. This molecular mechanism would pastries. Finally, most biscuits, cookies, and cakes favor the influence of gases with low tempera- are chemically aerated, the most commonly used tures of vaporization on cell expansion, high- reagents being sodium bicarbonate, alone or with lighting a contrasting effect of temperature rise an acidulant, and ammonium bicarbonate. With which favors gas production on the one hand but sodium bicarbonate, for instance, the hydrogen limits dough extension on the other hand. ion reacts with the bicarbonate ion to produce However, pressure measurements recently per- water and carbon dioxide, the latter supporting formed at different locations in bread dough with cell inflation. The decrease in H+ concentration is miniaturized sensors showed no temperature- accompanied by an increase in pH in the batter. dependence of the pressure build-up during This is slightly counterbalanced at the end of baking (Grenier and others 2010); this result baking by the decrease in overall water content questions the assumed relationship between (Baik and others 2000b). dough stiffening and pressure build-up, but could be explained if ruptures in the cell walls occur simultaneously or earlier (see later). In addition, Mechanisms limiting volume neither of these explanations seems to apply to biscuit or cookie baking. Chevallier and others increase in cell (2002) observed that the cessation of the oven- Mechanisms resisting or limiting the expansion rise in biscuit, which occurred in the temperature are multiscale, from molecular scale (for exam- range of protein aggregation, was usually ple, stiffening subsequent to starch gelatiniza- followed by collapse, hence questioning the rigid tion), microscopic scale (for example, ruptures in character of the dough matrix produced by cell walls and opening of the porous structure) to protein aggregation.  CELL EXPANSION AND SQUEEZING, CRUMB FORMATION, AND OVERALL COLLAPSE 341 106 1 2 3 5°C/min-tensile-44% water (Vanin and others 2010) 15°C/min-tensile-44% water (Vanin and others 2010) 105 Viscosity (Pa.s) 4°C/min-squeezing-32% water (Lassoued-Qualdi 2005) 2.5°C/min-tensile-45% water (Dresse and others 1988) 3°C/min-squeezing-?% water 104 (Bloksma 1985) 6°C/min-squeezing-?% water (Bloksma 1985) 9°C/min-squeezing-?% water (Bloksma 1985) 103 20 40 60 80 100 120 Temperature (°C) Figure 19.1 Wheat dough viscosity as a function of temperature, a compilation of literature data, where ‘squeezing’ means the use of the testing mode employing biaxial deformation (lubricated squeezing mode), and ‘tensile’ means the testing mode employing uniaxial deformation. Refer to the text for more details about stages 1, 2, 3. For color detail, please see color plate section. Ruptures in cell walls Ruptures in cell walls and protein coagulation (70–90 °C in the presence the formation of an open porous structure are of sugar). For optimal oven-rise, Eliasson and evidenced by observation of a product section Larsson (1993) also recommended adjusting the with the naked eye or with microscopy (Lostie thermal gradient inside the bread dough to the and others 2002), as well as by the uniformity strength of the gluten film forming the interface in pressure as reported at the end of baking or of cells. during post-chilling (Grenier and others 2010). Rupture is more generally determined by the Nevertheless, closed pores would still remain at extent and rate of deformation in cell walls; resist- the end of baking, occupying a still uncertain vol- ance to rupture can hence be modulated by dough ume fraction, of 1–20% (Datta and others 2007). formulation (high protein content in the flour, Rupture in cell walls is a prerequisite for limiting low water content in dough, addition of emulsi- cell shrinkage upon cooling and for preserving fiers) and/or the mastering of the mixing step in the oven-rise. Drastic shrinkage of bread loaves terms of under- or over-kneading (Dobraszczyk formulated with tapioca starch was related to the and Salmanowicz 2008). Consistently, dynamic absence of ruptures in cell walls (Kusunose and monitoring by MRI showed that for low interfer- others 1999). ence of the crust setting, the expansion of bread Pressure gradients between neighboring cells dough ceased locally at temperatures lower than tend to promote rupture. In the literature, rup- the onset temperature of starch gelatinization, a ture in cell walls is very often assumed to be due result which was associated with early ruptures in to the pressure build-up subsequent to dough cell walls (Wagner and others 2008). In this study, stiffening. Mizukoshi and others (1980) working an influx of gas higher than the outflux was sup- on biscuit dough heated in a liquid paraffin bath posed to maintain the open porous structure observed the formation of bubbles on the dough and avoid its collapse until its setting by starch surface and the massive emission of gas towards gelatinization. the exterior of the bath as soon as the internal The net balance between influx and outfluxes temperature of the sample exceeded that of is crucial for evaluating the impact of the pore  (a) Region 4 Region 3 Region 2 Region 1 100 110 Total thickness 100 Region 1 Region 2 90 90 Region 3 Thickness variation (%) 80 Region 4 70 Large bubble 80 Area (mm2) in region 4 60 50 70 40 30 60 20 10 50 0 –10 40 0 2 4 6 8 10 12 14 16 18 20 Time (min) (b) 110 Total thickness 100 Region 1 90 Region 2 Thickness variation (%) Region 3 80 Region 4 70 60 50 40 30 20 10 0 –10 0 2 4 6 8 10 12 14 16 18 20 Time (min) Figure 19.2 Changes in local porosity as monitored by MRI during bread pan-baking (Wagner and others 2008. Reproduced with permission of Elsevier). Dough regions are separated by one or two rows of oil microcapsules of which a high MRI signal (bright spots) permit the tracking of dough displacement subsequent to bubble inflation. Heat is transferred from bottom and top faces only. Associated MRI images are displayed at the top of each sub-Figure 19.for selected baking times; high signal intensity is related either to oil microcapsules (bright spots) or to dense regions of crumb especially present during the second part of baking. The tracking of microcapsules in case b was stopped when crumb structure became too dense  CELL EXPANSION AND SQUEEZING, CRUMB FORMATION, AND OVERALL COLLAPSE 343 in their vicinity (see last image). (a) case of late crust setting (oven temperature 130 °C): the different regions stop expanding all together; final porosity is unevenly distributed because of the limited mass transfer at the bottom (next to the baking tin); standard deviation from three repetitions was less than 3% in thickness variation (b) case of early crust setting (oven temperature 180 °C): inflation at the core (region 2) occurs after the cessation of the overall expansion and proceeds at the expense of the other regions which get squeezed. For color detail, please see color plate section. opening on cell expansion: for tin baking, mass pushing of the inner part of the dough will have exchange between the bottom region and the no squeezing effect; if a cell wall is still partly exterior is limited by the tin walls and also the flexible, it will be at least partly squeezed. The dough core structure becomes porous later, limit- relationship between dough stiffening and rup- ing exchange of the bottom region with the top ture is hence crucial; the greater the separation region and the oven; the bottom is hence likely to between the two events, the less the degree of retain its gases despite ruptures in cell walls and, squeezing (Jefferson and others 2007). Since as a result, the porosity will expand proportion- these events are often associated with threshold ally more than in other regions, by a factor of two values of temperature, and the dough is heated to four (Wagner and others 2008). from the outside mainly by conduction, the outer crumb (beneath the crust) is more prone to Setting of the crust and its effect of cell collapse or squeezing. Zhang and others (2007) developed coalescence Early crust setting is an additional, a device with a fabric lid placed above the top macroscopic determinant of cell expansion. There is surface of a bread pan and which made it pos- minimal interference by the crust with local cell sible to stop the oven-rise at different heights. inflation if crust setting occurs (just) after the Using MRI they showed that the later the oven- dough at the core has expanded (Wagner and oth- rise was stopped, the deeper the location of the ers 2008) (Figure 19.2a). The reverse is, however, squeezed crumb. Consistently with the above more frequently encountered in realistic baking mechanistic sequence, this result was explained conditions (Figure 19.2b). The intensity of the crust by the isotherm of wall rupturing moving deeper interference depends on the dynamics of dough into the product as more time was allowed for stiffening at the product surfaces and heat trans- heat transport into the dough. A wider tempera- port to the dough core, supporting cell inflation. ture gradient would be expected to develop in Crust setting results in a constant total volume; larger cells, which are more prone to squeezing if the crust sets early, it is accompanied by the (Jefferson and others 2007). The growth of a building of pressure at the core, resulting from large cell followed by its squeezing upon the the vaporization of gases under limited increase expansion of the dough core is shown in in cell volume (Grenier and others 2010). The Figure 19.2a; a cell formed in the bread dough at pressure build-up particularly concerns the the end of fermentation might no longer exist unbaked region with closed pores. The force in the final baked structure of the loaf. Because exerted by the core on other dough regions of these force balances developing in the dough, becomes higher as baking progresses; thus, the changes in local porosity are not monotonic core inflates at the expense of other dough during baking, and phases of compression and regions in which gas gets compressed, resulting in inflation occur alternately. Thus, the outer layers a reduction in volume as visualized by MRI (see of bread may be 2–3 times denser than the initial Figure 19.2b), and/or an increase in pressure dough (Jefferson and others 2007). (Grenier and others 2010). The amount by which The increase in pressure generated by the crust a cell collapses is determined by the extent to setting also imposes additional stress on the cell which the cell wall is set when it ruptures walls, possibly leading to bubble coalescence in (Jefferson and others 2007). If the cell wall is the still unbaked part of the product and finally to completely set when it fractures, the outward coarse texture of the crumb. Hayman and others 344 CH 19 BAKING (1998) mimicked the crust setting by applying to permeability (Chevallier and others 2002); (ii) the top of the bread pan various levels of over- the late shrinkage of the crust subsequent to pressure and at different baking times. They found cross-linking reactions such as extensive protein that an over-pressure of 0.3 kPa applied when the coagulation and Maillard browning reactions temperature at the dough core was between 60 (Sommier and others 2005). and 70 °C accentuated the coalescence between neighboring cells, as shown by the more open- grain crumb structure of the final bread slices. When pressure was continuously monitored Total water loss inside bread dough during baking, sudden falls in Whatever the type of bakery product, it is impor- pressure were associated with bubble coalescence, tant to realize that moisture gradients exist in the which was frequently observed at the dough core product at the end of the baking process. The and less frequently elsewhere (Grenier and oth- distribution in water content through the product ers 2010). These pressure drops started at 40 °C governs thesection). Water also plays an important and stopped at 70 °C; dough stiffening that pro- role in the major structural changes that take place ceeds beyond this temperature may render the during bread baking, at the microscopic (e.g. water coalescence more difficult. Pressure drops were vaporization in the cell expansion) and molecular also more frequent when higher pressure gradi- scale (for example, starch gelatinization). ents were developed such as for short-proved doughs (Grenier and others 2010). The increase in pressure generated by the crust Measurements of water content setting may also lead to its own rupture, allowing in bakery products prolongation of the oven-rise, but also contribut- In practical terms, although the overall water loss ing to strong spatial heterogeneity in crumb is easy to assess, the determination of water con- expansion as monitored by MRI for bread tent profiles in crumb during baking requires a (Wagner and others 2008) or X-rays for cake method to be set up for the accurate cutting of (Whitworth 2008). loaf samples while still hot and deformable, and Finally, pressure build-up in bread is accompa- the minimization of water losses by evaporation nied by a release of CO2 (Zhang and others from cut surfaces. Water content is also sensitive 2007). The net balance of the two mechanisms on to the time elapsed from the end of baking and cell expansion has not been assessed to date; the relative humidity of the storage atmosphere; despite the release of CO2, pressure build-up these important factors are rarely reported and might be favorable to cell expansion. On the make experimental results between studies almost other hand, CO2 release may be desired since it is impossible to compare. assumed to be a vector of aroma compounds (Eliasson and Larsson 1993). Spatial distribution of water Collapse of products The collapse of products is content in different bakery often reported in the literature, and to significant extents. For example: −13% (Hayman and others products 1998) and −30% (Zanoni and Peri 1993) for The initial water content typically ranges from bread, −20 to −25% for sponge cake (Lostie and 11 to 30% for biscuit and crackers (Wade 1988), others 2002), and −100 to −150% for biscuit 32–38% for cake batter (Baik and others 2000b), (Chevallier and others 2002). Nevertheless, this and 38–48% for bread dough. phenomenon has received little attention so far. The final moisture of a freshly baked biscuit is Two main hypotheses have been proposed to usually within the range 1–5%, depending on the explain it: (i) the insufficient rigidity of the matrix type of product (Wade 1988). Water distribution is to overcome the effect of gas loss or increase in very difficult to measure in the thickness of a  STRUCTURAL CHANGES IN STARCH AND PROTEIN DURING BAKING 345 biscuit; it can, however, beBread crumb located by the starch granules and the increase in volume beneath the crust exhibits an intermediate water of these starch granules; (ii) hydration-facilitated content, reported as being 2–12% lower than that melting whereby the crystalline structure of the of dough which was at 42% (Thorvaldsson and granules is irreversibly lost, and (iii) disruption of Skjöldebrand 1998) or 3–9% lower than that of starch granules owing to further hydration of the dough which was at 49% (Wagner and others disordered polymer chains, followed by the partial 2007) or at 3% itself (Marston and Wannan 1976). dispersion of amylose and amylopectin. Amylose The crumb at the core retained its initial moisture association increases as the concentration of solu- content or it even increased (Wagner and others ble amylose increases and when the temperature 2007). When sampling took place just after removal is less than the melting temperature of amylose of bread loaves from the oven, an increase in water crystallites (Morris 1990). Granular swelling content by 1–3% was reported at the core (De together with exudation of amylose from gran- Vries and others 1989; Zanoni and Peri 1993; ules, are responsible for the increase in viscosity. Wagner and others 2007). In addition to the Starch gelatinization is an endothermic pro- conventional, destructive method of sampling cess, and the number of endothermic transitions combined with oven-drying for water content (one to four) depends on dough composition, determination, a continuous method based on near mainly water content (Figure 19.3). The first and infrared reflectance (NIR) was proposed by second endothermic transitions reflect the disor- Thorvaldsson and Skjöldebrand (1998). Light was ganization (melting) of starch crystallites; third sent into the sample through an optic fiberThor- and fourth endothermic transitions (endotherms) valdsson and Skjöldebrand (1998) showed an are attributed to the dissociation of amylose–lipid increase in water content in the core immediately complexes that formed in the previous process after the local temperature had reached 70 ± 5 °C steps. Attainment of the first two endotherms is (that is, after cell expansion was more or less com- accompanied by the loss of birefringence, which plete), which reached +8% at the end of baking is characteristic of native starch (Burt and Russell (35 min at 225 °C) relative to that of dough initially. 1983). The extent of starch gelatinization during Magnitudes of the overall drying rate were baking can be estimated by the quantity of heat 1.8 × 10–3 kg water/kg dry matter for cookies uptake as assessed during calorimetric analysis (Hayakawa and Hwang 1981), 1.4 × 10–3 to relative to a reference; the reaction follows first- 9.4 × 10–4 kg/kg for yellow and chocolate cakes of order kinetics (Marcotte and others 2004). various sizes (Baik and others 2000b), 2.5 × 10–4 kg/kg for sponge cake (Lostie and others 2002), and 2.8 × 10–5 to 2.4 × 10–4 kg/kg (Hasatani Factors affecting starch and others 1992) and 2–5 × 10–4 kg/kg (Sommier gelatinization and extent of starch and others 2005) for bread. gelatinization in bakery products In the presence of sodium chloride or sucrose Structural changes in starch at low concentrations, gelatinization occurs at and protein during baking and slightly higher temperatures, and is less complete as evidenced by lower enthalpies of melting subsequent changes in dough/ (Biliaderis 1991; Ghiasi and others 1983). Marcotte crumb mechanical properties and others (2004) reported a peak gelatinization Basic mechanisms of starch temperature in the presence of cake ingredients in the range 94–97 °C. Fats and emulsifiers also gen- gelatinization erally delay gelatinization (Richardson and others Starch gelatinization comprises a series of 2003). In the temperature range 60–70 °C, a high processes on a molecular scale, which overlap in level of activity of α-amylase in doughs causes time: (i) swelling, that is the absorption of water enzymatic breakdown of gelatinized starch, which 346 CH 19 BAKING 200 Toven = 100°C (Vanin and others 2012) Toven = 180°C (Wagner and others 2007) 180 Toven = 205°C (Zanoni & Peri 1993) Toven = 235°C (Marston & Wannan 1976) 160 Toven = 250°C (Vanin and others 2012) 140 Starch gelatinization and melting Temperature (°C) 120 3rd endotherm 100 Tg 80 (gluten and bread) 2nd endotherm 60 1st endotherm 40 20 - 10 20 30 40 50 Water content (% wet basis) Figure 19.3 Heat-moisture pathways of crust (filled symbols) and crumb (open symbols) from the literature, as represented in the state diagram of starch and gluten, and where Toven is the oven temperature. in turn attenuates the increase in viscosity nor- leaching, phase separation from amylopectin and mally associated with gelatinization. As a result of network formation, possibly accounting for the the limiting amount of water and/or the presence differences in gel properties and baked product of co-solutes, the starch is only partly gelatinized texture (Patel and others 2005). in many baked products. The degree of gelatiniza- tion is low in dry products, such as biscuits, where a considerable degree of birefringence can still Protein coagulation be observed after baking (Wade 1988). Up to 70% Heating above 60 °C also gradually modifies the of the starch granules gelatinize at the center of a protein network, with the polymerization of bread loaf (Biliaderis 1991); some starch granules glutenins and the changes from gluten gel to lose their birefringence but still appear intact, coagel. In dough, water does not migrate rapidly; whereas other are disrupted. The partially swollen thus, every starch granule gelatinizes by using the granules usually become stretched into elongated water in or near the granule. As there is insuffi- forms enabling gas cells to expand. cient water in the dough to gelatinize completely, The swelling of starch granules as observed by some of the water will transfer from the protein light microscopy is also affected by the heating membranes to the starch as the baking proceeds. rate. A higher heating rate increases the tempera- This is facilitated by protein denaturation upon ture range over which the granules increase in size; heating, a reaction which is accompanied by it also results in incomplete folding of granules water release. However, some of the water will (Patel and Seetharaman 2006). These differences remain unavailable for starch gelatinization potentially result in altered patterns of amylose (Ghiasi and others 1983). Therefore starch  STRUCTURAL CHANGES IN STARCH AND PROTEIN DURING BAKING 347 gelatinization and protein coagulation both in all studies, because either the range of affect the diffusion of water by absorbing and temperature was limited in the upper values releasing water, respectively. (Bloksma 1985) or the water content in the sample started to decrease above 95 °C Impact on dough viscosity (Vanin and others 2010). Two possible mech- anisms governing phase 3 were proposed during baking (Lassoued-Oualdi 2005; Vanin and others Data for viscosity in dough at constant water 2010): (i) the gas nuclei originally present content as reported in past studies (Bloksma in the dough, and those formed at the mixing 1985; Dreese and others 1988; Lassoued-Oualdi stage, might expand upon heating, or (ii) the 2005; Vanin and others 2010) are compared in gelatinized starch grains become much softer Figure 19.1. Note that these studies differed in and disrupt; both hypotheses affect measure- the heating rates used, and two different testing ment and lead to a reduction in the modulus. modes were used: biaxial deformation (lubri- During the testing of different hydrothermal cated squeezing mode) or uniaxial deformation pathways under rheological measurements, (tensile mode). Nevertheless, the same amplitude however, the passage of the second endo- of variations in dough viscosity was observed, therm (associated with the melting of the with three distinct phases at around the same remaining crystallites) was directly linked transition temperatures of 55, 75 and 100 °C. to the onset of viscosity decrease (Vanin and others 2012), a conclusion which supports the 1. Firstly, the logarithm of viscosity slightly latter hypothesis. decreased until the dough temperature reached around 55–60 °C (phase 1, It must be emphasized here that the uniaxial Figure 19.1). Phase 1 was not observed in all tensile mode gave results in coherence with lubri- studies, especially when the values to be cated squeezing measurements leading to biaxial measured were below the sensitivity thresh- extension of the dough, as expected around the old of the measuring apparatus (Lassoued- cell. These tests were performed at high exten- Oualdi 2005; Vanin and others 2010). This sion rates compared to the ones involved during decrease in viscosity in dough is common to baking, however, being between 1 and 2 × 10–3 s−1 all materials in which no transformation (Bloksma 1985), and this point introduces some takes place. uncertainty in the application of these results to 2. Between 55 and 75 °C, an almost linear oven-rise. increase in the logarithmic viscosity was observed (phase 2, Figure 19.1). This phase Impact on mechanical was attributed to the starch gelatinization (in particular the swelling) and/or to protein properties of crumb coagulation; their relative contribution Gelation mainly proceeds during the cooling remains unclear (Dreese and others 1988; phase, resulting in a three-dimensional network Lassoued-Oualdi 2005). The temperature at where the swollen, yet deformable, granules are which the viscosity is minimal becomes embedded into a continuous matrix of entangled higher as the dough is heated more rapidly amylose molecules. Such a gel is metastable and (Bloksma 1985); this is explained by the finite undergoes transformation during storage com- rate of starch structure changes governing prising further chain aggregation and recrystalli- the increase in viscosity. zation (retrogradation). 3. Between about 75 and 100 °C, the logarith- The heating rate during baking and possibly mic viscosity decreased almost linearly the post-baking cooling rate, are known to affect (phase 3, Figure 19.1). This was not observed the process of retrogradation (Biliaderis 1991). 348 CH 19 BAKING Le-Bail and others (2009) studied different heat- weight loss, is an essential part of crust formation ing rates during bread baking (8, 10 and 13 °C/min and, therefore, manipulating the oven conditions obtained with oven temperatures of 180, 200, and to reduce water loss will unavoidably change the 220 °C) and evaluated the staling rate of bread characteristics of the crust. crumb through the changes in the Young’s modu- The extent of starch gelatinization is governed lus and by the melting enthalpy of amylopectin. by water content and temperature history they concluded that breads baked at 8 °C/min (Fukuoka and others 2002). The fast evaporation exhibited a staling rate almost twice as slow as the of water at the dough surface due to high tem- others. Patel and others (2005) also tested differ- perature impedes the full gelatinization of starch ent heating rates in crumb (6–17 °C/min) by vary- in the crust (Figure 19.3 and Figure 19.4). Using ing the heating mode (convective, impingement, DSC, Primo-Martin and others (2007) estimated or combined) as well as the dough size (small that the fraction of starch crystallites in the crust versus large) using an even oven temperature. was between 32 and 43% depending on the bak- Consistent with Le-Bail and others (2009), they ing conditions; these crystallites melted at 70 °C, reported lower enthalpies of amylopectin recrys- suggesting that during baking the less stable crys- tallization and lower incidences of bread firmness tals in the crust had gelatinized (melting point of when breads were exposed to slower heating rates 50–60 °C). and longer times above the melting temperature The decrease in water content, rather than par- of native starch. This was related to the extent of tial starch gelatinization, is mainly responsible amylose leaching and the enhanced associations for the increase in viscosity in the surface layers of amylose inside and outside the granules, as evi- during baking of bread dough (for example, denced by lower levels of soluble amylose. Vanin and others 2010, 2012). Similarly, protect- ing dough samples with paraffin oil to prevent dehydration allowed continuous expansion until Crust formation 100 °C, whereas with dehydration, expansion ceased well before a temperature of 100 °C was Because of exposure to high temperature, crust reached (Le Meste and others 1992). This differs from crumb in many respects: it is colored, increase in viscosity may govern the cessation of but it is also dry, dense, and with elongated or the oven-rise if the viscosity increase occurs early small cells; all these aspects confer more cohe- during baking (see previous section about cell sion and hardness to the crust material (Vanin expansion). and others 2009). Rigidity, together with thickness, is a crucial feature of the crust as it helps to maintain the Dehydration and its effect whole alveolar structure. Many observations are on reactions in and mechanical consistent with this conclusion; for example: (i) when crust is thin and still flexible such as in properties of the crust part-baked bread loaves, collapse of the loaves as Water content and thickness of the crust depend a whole is often observed during cooling together on many factors relating to water evaporation; with the formation of side riddles; (ii) the lower- among them the initial porosity of the dough, ing of pressure measured in loaves during post- duration of baking, intensity of drying at the baking cooling is associated with a reduced dough surface, and the temperature at which capability of the product boundaries to deform cells collapse (Jefferson and others 2006). The (Grenier and others 2010). In fact, the condensa- crust is dense despite its lower water content tion of vapor upon cooling first occurs in the because the cells are few, small or elongated. peripheral layers of dough. In this latter study, When the crust–crumb ratio is expressed in terms it was not immediately counterbalanced by a of its mass, it is high, between 1 and 2 (Le-Bail decrease in pore volume locally, meaning that the and others 2011). Note that water loss, and hence peripheral layers were not very deformable;  CRUST FORMATION 349 (a) (b) Crust 0.3 mm (c) Crumb Figure 19.4 Microstructure of bread crust and crumb analyzed by (a) 3D confocal microscopy (Primo-Martin and others 2007. Reproduced with permission of Elsevier.) and (b, c) by SEM at 50× magnification (Datta and others 2007. Reproduced with permission of Elsevier). Crust at the top of image (a) is composed of a continuous protein network (stained in red) and a discontinuous starch network (stained in green), with a preserved granular shape. When moving from the outer crust towards the crumb an increased extent of starch gelatinization is observed and only remnants of granules are recognized. SEM samples (b) crust, (c) crumb were baked with microwave heating combined to with infrared. neither was it immediately counterbalanced by methional, (E)-2-nonenal, methyl-propanal, and an enrichment in gases from the surroundings, 2- and 3-methylbutanal for bread crust – are also especially the air from the outside, meaning that produced, mainly by the Maillard reaction the peripheral layers were not very permeable to (Poinot and others 2008). The transfer of these gases (more details are given later). compounds towards the oven atmosphere or the Low water activity (optimal between 0.4 and crumb during baking has been recently studied 0.8) and high temperatures (commonly above (Onishi and others 2011). 105 °C as mentioned earlier) accelerate the Maillard reaction in the crust. Highly reactive compounds Role of steam injection are initially produced which then polymerize, yielding brown products. Browning is linearly on the crust formation correlated to water loss (for example, Wahlby Steam injection delays the evaporation of water at and Skjoldebrand 2002). Finally, some of the the dough surfaces by condensation of water from desired flavors and aromas of bread – among the oven atmosphere onto the dough surface. which are 2-acetyl-1-pyrroline, 4-hydroxy-2, Condensation proceeds as long as the crust tem- 5-dimethyl-3(2H)-furanone, 2,3-butanedione, perature is below the dew point temperature of the 350 CH 19 BAKING oven atmosphere, which in practice occurs during and others (2012) reported higher porosity in the the first minute of baking under classical operating superficial layers of bread dough when starch conditions. The rate of increase in crust tempera- granules remained intact. Moreover, the surface ture will then level out as it approaches the dew layers cannot deform fast enough to accommo- point temperature, marking the point where con- date the core expansion (see section about cell densation changes to evaporation (Wiggins 1998). expansion). This leads to squeezing, elongation of When baked in a dry atmosphere, bakery cells located near the surface, as observed dynam- products have a dull surface appearance and a ically during baking (Whitworth 2008). rather harsh color. It is commonly believed that It is well known that the permeability of a water condensation also permits full starch gelat- porous medium is affected by the pore fraction inization at the surface, favoring the formation of and structure. Permeability to gases developed in a glossy crust. However, most microscopic stud- the surface layers during baking is low; as a result, ies did not find evidence of starch gelatinization it limits water migration during baking (Wählby at the surface of bakery products. High levels and Skjöldebrand 2002; Hirte and others 2010; of injected steam might be required for this Baik and Chinachoti 2000) and favors pressure (Altamirano-Fortoul and others 2012). Another build-up (Grenier and others 2010) with possible alternative mechanism, valid for biscuit or cookie impacts on the local development of the alveolar dough, is the leaching out of sugars at the surface structure (see previous section on cell expansion). by condensed water droplets which later form a Although receiving a renewed interest, gas per- glassy coating with gloss (Wade 1988). meability in bakery products has been the object It is important to understand, however, that of very few studies so far, and beyond this basic the intensification of heat transfer at the dough need for understanding, searching for ways to surface by increasing the amount of steam control the setting up of this property during injected into the first part of the travelling oven, baking would also be of industrial relevance (see is known to reduce the total thickness of thin more details later). Permeability of bread crumb bakery products like semi-sweet biscuits and was directly related to porosity (0.1–0.6), what- cream crackers (Wade 1988). ever the water (15–50% wb) and fat (2–8%) contents (Goedeken 1993). Chaunier and others Cell expansion in the crust; (2008) reported differences by a factor of almost effect on permeability 10 between bread and brioche crumbs exhibiting the same porosity; this was explained by the size of crust to gases of connections (holes) in the walls separating Because of the proximity of the boundary with cells, large-sized for bread and tiny for brioche. the oven air, gases easily escape the superficial From these studies, the permeability of typical dough layers and the vapor pressure remains crumb (> 80% of porosity) was estimated at close to the atmospheric pressure (also see pres- around 3 × 10–10 m2, and of typical crust (40% of sure profiles calculated by Zhang and Datta porosity) around 10–12 m2. Hirte and others (2012) 2006). Additionally, the loss of extensibility of cell estimated the permeability in the crust of crispy walls would be expected to be more rapid, mainly rolls to water vapor as being 5 × 10–9 g/(m s Pa). because of the great reduction in water content. Thus, one explanation of the smaller size of cells Factors affecting the crispiness in the crust than in the crumb (Figure 19.4) is that the forces favoring expansion are not sufficient of bakery products, and bread crust for cell expansion. The full mechanism underly- In cookies, crispiness develops with the setting of ing cell expansion in crust warrants in-depth a sponge structure (open pores) at the final stage study, however; for instance, Altamirano-Fortoul of baking (Piazza and Masi 1997); the mechanisms  THE END POINT OF BAKING 351 governing the opening of pores have been high-moisture products (Wade 1988; Ahrne and described in a previous section. others 2007), while producing an unbaked core in Several studies also found a relation of crispi- bread (Ahrne and others 2007) and paler inner ness to water content or water activity (Roudaut crumb in biscuit (Wade 1988). High heat transfer and others 2002; Primo-Martin and others 2006). at the surfaces also hastens the setting of the At low water contents, gelatinized starch and outer layers and limits the extent of the oven-rise gluten matrices are in a glassy state making cell because the gases do not have enough time to walls more prone to fracture (Stokes and Donald complete their potential for expanding cells 2000). Crispiness of biscuits, cookies, and crack- (Wade 1988; Wagner and others 2008). Long ers can be preserved easily by packaging which baking durations tend, however, to decrease keeps their water activity constant. However, crumb softness (Cauvain and Young 2001). maintaining a low water activity in bread crust is The hottest temperatures (300–350 °C) are used not possible because water migrates from the wet for cracker products containing little or no added crumb to the dry crust, causing a loss of crispiness sugars. Semi-sweet biscuits and sugar-containing within a few hours (Hirte and others 2010). crackers are baked at lower temperatures (250 °C) In an interesting study, lower water contents in and the short and soft doughs at about 200 °C the crust at the end of baking were obtained by using (Wade 1988). Bread doughs are baked between 200 flour with lower protein content, or by hydrolyzing and 275 °C (Zanoni and Peri 1993; Sommier and proteins in the dough surface layers by the applica- others 2005; Ahrne and others 2007), but at much tion of a protease onto the dough surface at the end lower temperatures (160 °C) for part-baking pur- of proving (Primo-Martin and others 2006); in turn, poses. Oven temperatures during cake baking are these lower water contents at the end of baking per- in the range 150–200 °C (Baik and others 2000a). mitted longer retention of crispiness. The time taken to achieve a satisfactory baked Increasing the crust permeability to water vapor product depends to a considerable extent upon would also provide an effective way to increase the properties (especially thickness) of the product crispiness retention during storage by up to 1 day itself. A thin biscuit such as a soda cracker can be (Hirte and others 2010), with only a small impact baked satisfactorily in a time of about 2 min whereas on the amount of water lost by the inner crumb a thicker product such as large short bread or wire- (Hirte and others 2010) or on crumb dryness cut cookie may take 10 min or more. Microwave (Hirte and others 2012); this remains valid for heating could be combined with conventional permeability values lower than 8 × 10–9 g/(m s Pa). baking to homogenize the final water content rather than reduce the baking time (Ahmad and others 2001). Baking durations for bread loaves range from 10 to 30 min; baking time is reduced to The end point of baking 7–10 min for crispy rolls with the shortest baking The completion of the baking process is in gen- times being achieved with the highest oven tem- eral judged by two properties of the product: its perature of 220 °C in a deck oven (Le-Bail and oth- color and its moisture for low-moisture products, ers 2010) or by using combined microwaves and and for high-moisture products, its color and the airjet impingement (Datta and others 2007). Baking completion of starch gelatinization at the core times of 10–15 min are required for 200 g dough (often associated with a temperature exceeding depending on the oven temperature in the range 98 °C). Satisfactory baking conditions optimize 200–260 °C (Ahrne and others 2007); 30 min for the thermal gradient through the product. 350 g dough with oven temperatures of 200 °C Attempts to reduce baking times by increasing (Zanoni and Peri 1993) or 235–275 °C (Sommier oven temperatures increases the darkness of the and others 2005); and 35 min for 600 g dough at outer layers, and affects the crust–crumb ratio of 225 °C (Thorvaldsson and Sköldebrand 1998). 352 CH 19 BAKING Chaunier L, Chrusciel L, Delisee C, Della Valle G, References Malvestio J. 2008. Permeability and Expanded Ahmad SS, Morgan MT, Okos MR. 2001. Effects of Structure of Baked Products Crumbs. Food Biophys microwave on the drying, checking and mechanical 3(4):344–51. strength of baked biscuits. J Food Eng 50(2):63–75. Chevallier S, Della Valle G, Colonna P, Broyart B, Ahrne L, Andersson CG, Floberg P, Rosen J, Lingnert H. Trystram G. 2002. Structural and chemical modifica- 2007. 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J Food Eng 80(4):1302–11. 20 Packaging and Shelf-life Prediction of Bakery Products Virginia Giannou, Dimitra Lebesi and Constantina Tzia Laboratory of Food Chemistry and Technology, School of Chemical Engineering, National Technical University of Athens, Athens, Greece Introduction 355 Bakery products with low Classification of bakery products 356 moisture content 362 Characteristics of bakery products 357 Bakery products with intermediate Bread 357 or high moisture content 364 Cakes 357 The shelf-life of bakery products 365 Biscuits and cookies 357 Factors affecting the shelf-life Crackers 357 of bakery products 365 Storage – quality characteristics Storage temperature 366 and deterioration during storage 358 Product reformulation 366 Bakery products with low Other shelf-life extension techniques 367 moisture content 358 Shelf-life estimation and prediction 367 Bakery products with intermediate Conclusions 369 or high moisture content 358 References 370 Packaging 359 Introduction appealing due to their nutritional, sensory, and textural characteristics. Bakery products have been a basic part of the A wide variety of bakery products is pro- human diet for centuries with bread being duced, such as cakes, crackers, and cookies. the staple product consumed daily by most of the Common characteristics of all bakery products population. Bakery products are popular and are that they contain cereal flour as their basic Bakery Products Science and Technology, Second Edition. Edited by W. Zhou, Y. H. Hui, I. De Leyn, M. A. Pagani, C. M. Rosell, J. D. Selman, and N. Therdthai. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd. 356 CH 20 PACKAGING AND SHELF-LIFE PREDICTION OF BAKERY PRODUCTS ingredient, and require baking as part of their pies, pastries, sandwiches, cream cakes, pizza manufacture. and quiche); The quality features of bakery products start to leavened with yeast (most commonly Sacch- deteriorate immediately after baking, however, aromyces cerevisiae) or chemical agents leading to staling, loss of moisture and flavor, and a (baking powders made up most commonly of limited shelf-life. During recent decades, due to con- baking soda, acid salts, and inert fillers), or sumers’ needs and demands as well as changes in the unleavened (without fermentation or gas- way food products are manufactured, distributed, producing agents) products; and retailed, a great variety of bakery products have high acid (pH < 4.6), low acid (pH > 4.6 and been developed together with alternative or novel pH < 7), and nonacid or alkaline bakery methods for their production and preservation. products (pH > 7). For these reasons, research has been focused on the development of advanced packaging and pres- For the scope of this chapter, bakery products are ervation technologies and techniques for bakery classified according to Smith and Simpson (1995) products. Inherent overall product quality, as well based on their moisture content as: as environmental storage conditions, influence the characteristics of bakery products and hence affect low moisture (aw < 0.6) products (cookies and their acceptance. Thus packaging materials, meth- crackers); ods, and conditions have been investigated for dif- intermediate moisture (aw > 0.6 and aw < 0.85) ferent bakery products, as well as the methodology products (soft cookies, doughnuts and danish for the estimation and prediction of their shelf-life pastries); and (Matz 1989; Kulp and others 1995; Laaksonen and high moisture (aw > 0.85) bakery products Roos 2000; Bhattacharya and others 2003). (bread, cakes, pies). Classification of bakery Low moisture products are usually fragile and highly hygroscopic and are characterized by their products crisp or crunchy texture. Under proper packaging Bakery products comprise an enormous variety (in and storage conditions, they can be preserved for size, shape, color, texture, taste, and flavor) of goods a long period. Intermediate and high moisture which can be either classified according to their for- bakery products exhibit a pliable or creamy tex- mula (mainly sugar content or leavening type), pH ture and a shorter, and in some cases very limited, level and moisture content, or water activity (aw). shelf-life. The shelf-life of low and intermediate Therefore they can be categorized (Table 20.1) into: moisture bakery products is usually limited by physical (moisture gain or loss and staling) and unsweetened (bread, rolls, crackers, pizza chemical (rancidity) factors; while microbiologi- base, crumpets, bagels), sweet goods (cakes, cal spoilage by bacteria, yeast, and molds is the biscuits, cookies, muffins, pancakes, dough- main concern in high moisture products (Smith nuts, waffles), and filled goods (fruit and meat and Simpson 1995; Smith and others 2004). Table 20.1 Classification categories for bakery products Product type Leavening type pH Moisture aw Unsweetened Unleavened High acid pH < 4.6 Low moisture content aw < 0.6 Sweet Yeast raised Low acid pH = 4.6–7 Intermediate moisture content aw = 0.60–0.85 Filled Saccharomyces cerevisiae Non acid pH > 7 High moisture content aw > 0.85 Chemically leavened Baking powders  CHARACTERISTICS OF BAKERY PRODUCTS 357 Characteristics of bakery density have a major impact on cake eating quality. Lower moisture contents yield firm, products dry-eating products, while more dense cake The main characteristics of the most important products tend towards pastry eating characteris- bakery products are summarized in the following tics (Pyler 1988; Cauvain and Young 2006; Hui sections. 2006; Lai and Lin 2006a). Bread Biscuits and cookies Bread is characterized by a dry thin crust Biscuits are small baked products of many shapes (moisture content 12–17%), that encloses a soft, and sizes made principally from flour, sugar, and sponge-like cellular structure (moisture content fat. After baking they may be coated with choco- 35–40%). The basic ingredients required for the late, sandwiched with a fat-based filling or have production of bread are flour, yeast, salt, and other pleasantly flavored additions. They can be water. Other ingredients may also be used such divided into two groups. Those with very low mois- as fat, sugar, milk or milk substitutes, oxidizing ture content and those containing, for example, agents, enzymes, dietary fibers, protein products, caramel, jam or fruit paste, that have higher mois- surfactants, hydrocolloids, and/or preservatives. ture content. Most biscuits fall into the first group Each of these ingredients may serve specific and typically have a moisture content of < 4%. functions in the production of bread, the most When packaged in moisture proof containers, they important of which are easy dough handling, have a long shelf-life, perhaps 6 months or more. improved texture or sensory characteristics, Biscuit dough can be either hard or soft and the higher nutritional value, and longer shelf-life difference is determined by the amount of water (Autio and Laurikainen 1997; Cauvain and required to make the dough. Hard doughs have Young 2006)