Bakery Products Science and Technology (Hui/Bakery Products Science and Technology) || Baking
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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.
Introduction 336Heat and water transport during baking 336
Modes of heat transfer 336Modes of heat transport into
the dough 337Measurement of temperature
during baking 338Spatial distribution of water
in bakery products 338Measurements of water content
in bakery products 339Cell expansion and squeezing, crumb
formation, and overall collapse 339Growth from gas nuclei 339Mechanisms favoring volume
increase in cell 340Mechanisms limiting volume
increase in cell 340Starch gelatinization 340Ruptures in cell walls 341Setting of the crust and its effect
of cell collapse or coalescence 343Collapse of products 344
Total water loss 344Measurements of water content
inbakery products 344Spatial distribution of water content
in different bakery products 344
Structural changes in starch and protein during baking and subsequent changes in dough/crumb mechanical properties 345Basic mechanisms of starch
gelatinization 345Factors affecting starch gelatinization
and extent of starch gelatinization in bakery products 345
Protein coagulation 346Impact on dough viscosity during
baking 347Impact on mechanical properties
of crumb 347Crust formation 348
Dehydration and its effect on reactions in and mechanical properties of the crust 348
Role of steam injection on the crust formation 349
Cell expansion in the crust; effect on permeability of crust to gases 350
Factors affecting the crispiness of bakeryproducts, and bread crust 350
The end point of baking 351References 352
BakingTiphaine LucasIRSTEA Food Process Engineering Research Unit, Rennes Cedex, France
336 CH 19 BAKING
IntroductionDuring baking, heat and mass transfer takes place in the dough simultaneously and interde-pendently, and involves four major changes:
1. Gases are vaporized as temperature increases; the gas cell increases in volume provided that the cell wall retains gases and is deformable.
2. Starch gelatinizes upon the increase of tem-perature to an extent depending on the local availability of water, and proteins coagulate. These changes limit dough extensibility. It may limit the cell expansion described in (1), and hence be a local driving force for pres-sure build-up and possibly fracturing of the cell wall (3).
3. The initial structure with closed gas cells separated by dough walls is transformed into a porous structure with interconnected pores, known as the dough-crumb transition. Dough membranes rupture when they can no longer withstand the gas pressure, and changes in (2) are a possible cause. In the-ory, ruptured walls limit the cell expansion described in (1); gas molecules are exchanged between adjacent open cells, and finally transported out of the dough. Pressure forces in such regions may then decrease, a possible explanation for the col-lapse of cells locally.
4. Under the action of high temperatures at boundaries, water vaporizes in the oven atmosphere. Depending on the product thickness, but also on baking conditions, this supports the formation of a dry, hard crust in dessert or bread dough, and may also lead to a complete drying in biscuits and cookies. This is accompanied by specific chemical reactions (in particular Maillard reactions), responsible for the browning of the crust and the development of flavour. Early setting ofthe crust restricts total volume and modi-fies the balance in pressure forces between the inner and outer dough and hence cell expansion (1) locally.
Oven-rise (the change of dough dimensions during baking, look at a video at http://www.weekendbakery.com/posts/baking-bread- the-movie/) together with dough/crumb transition and crispiness are the most outstanding macro-scopic features of baking, with relevance to texture. Reactions are another essential feature, related totaste (production of flavour), digestibil-ity (starch gelatinization), and safety (production of acrylamides).
These different changes do not occur sequen-tially but do overlap. Roughly in the first stage of baking, cell expansion and water loss begin; one of these can, however, be delayed during this stage. During stage 2, both the total height of the product and the rate of water loss reach their maximum. Color develops at the surface during the last stage 3, concomitantly to a possi-ble collapse of the porous structure and a decrease in the rate of water loss. These changes are detailed in the following sections.
Heat and water transport during bakingTemperature dominates product quality because it affects enzymatic reaction, volume expansion, starch gelatinization, protein denaturation, non-enzymatic browning reactions, and water migra-tion, as will be detailed later.
Time-course changes in temperature and its distribution through the dough during baking are determined by multiple factors, among them: the size of the product (biscuit versus bread loaf); theratio between top, side and bottom heat flows; the expansion of cells which reduces thermal con-ductivity, but likewise favors the transport of water vapor evaporated at the surface layers to the core, where it condenses and releases energy.
Modes of heat transferHeat is transferred through the combination of conduction, convection and radiation. Radiation comes from the heating elements (burner flames) 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 of the dough and there is a tendency for color changes upon heating to accelerate after the first browning has occurred (Therdthai and Zhou 2003). Contribution of radiation to heat transfer at the top surface can be estimated by h-monitor and was found to be much higher in direct ovens, 7080% (Baik and others 1999, 2000a) than in indirect ovens, 4560% (Fahloul and others 1995). The contributions of the different heat sources were calculated through modeling for tunnel type multizone industrial ovens; for bis-cuit, heat transferred by conduction through the band was minimal (10%), radiant and convective heat being of the same order (Fahloul and others 1995). Conduction represents more than half of the total heat transferred to the product during tin baking, possibly reaching two-thirds in certain conditions (Baik and others 2000a). Conduction mode is also predominant for products deposited on the hearth of the oven, like Indian flat bread (chapatti) (Gupta 2001) or some French breads (Sommier and others 2005).
Modes of heat transport into the doughAs soon as the product is introduced in the oven, heat transfer results in superficial water evapora-tion and heating up of the product. In the oven, where the temperature is high, the partial water vapor pressure is far from saturation. Hence, water vapor in the superficial layers of the prod-uct diffuses into the oven atmosphere. As the dif-fusive flow of liquid water from the core is less than the flow of evaporated water at the surface, the product dries at the surface, leading to a very low water activity, and the water activity decreases rapidly with water contents below 0.15 kg/kg. Temperature near the product surface does not exhibit a marked plateau at 100 C (the temperature of water at the pressure close to that of the atmosphere, and water activity close to that of pure water). Above 100 C, the tempera-ture rises in thermodynamic equilibrium with the locally decreasing water content and rises towards that of the oven temperature.
Under the dough surface, where there is still liquid water, water evaporates to saturate the partial water vapor pressure in the pores. The water vapor pressure reaches a maximum at a location from which the vapor can diffuse in two directions, towards the dough surface and towards the core. The driving force for the trans-port of water vapor from this location to the sur-face has been detailed earlier. On the other hand, the temperature gradient induces a water vapor pressure gradient between this location and the core, which produces a diffusive flow of vapor in the same direction. Because the water migrates counter to the thermal gradient, it is accompa-nied by condensation, release of its latent heat and an increase in liquid water at the core. This is why this mechanism is also called transport by evaporation-condensation. This mechanism takes place as long as a temperature gradient remains (and water remains!) and the core tem-perature has not reached 100 C. Such transport is promoted by the opening of the porous struc-ture which occurs during baking; nevertheless it also proceeds in a cellular structure (closed cells) with four distinct phases (De Vries and others 1989):
1. evaporation proceeds on the warmer side of the gas cell,
2. water migrates through the gas phase, induced by the vapor concentration gradient inside the cell,
3. water condenses on the colder side of the gas cell,
4. liquid water is transported by diffusion through the dough wall to the warmer side of the next gas cell, where the series of processes starts again. Thorvaldsson and Skjldebrand (1998) suggested, however, that as the con-densed water is likely to be absorbed by starch gelatinization, transport of water by this mechanism to and through the ungelati-nized closed region is unlikely to occur.
The transfer of water vapor with vaporization near the surface and condensation at the core contributes to heat transfer, and was proposed by
338 CH 19 BAKING
Sluimer (cited by De Vries and others 1989)