Journal of Cereal Science 36 (2002) 167–176 doi:10.1006/jcrs.2001.0445, available online http://www.idealibrary.com on Cellular Structure of Bread Crumb and its Influence on Mechanical Properties M. C. Zghal, M. G. Scanlon* and H. D. Sapirstein Department of Food Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 Received 18 September 2001 ABSTRACT Quantification of structure is a desirable goal if the mechanical properties of cellular solids such as bread crumb are to be predicted. This study determined the influence of cellular structure on the mechanical properties of bread crumb. Variation in structural and mechanical properties was obtained by varying flour type, water content and dough processing conditions. Cellular structure was characterised by crumb relative density and various structural parameters determined by digital image analysis. Mechanical properties were determined by tensile testing. Young’s modulus and failure stress were successfully fitted (0·55ΖR 2 Ζ0·94) to the relative density power law model proposed by Gibson and Ashby. However, the fitting exponents between mechanical properties and relative density were lower than their theoretical values. By incorporating effects of strain hardening in the crumb solids, and of missing cell walls (due to gas cell coalescence), with the theoretical effect of relative density, estimates could be obtained for strain hardening exponents (p) of doughs of different baking strength (1·5<p<6·5). These values concurred with p values from other researchers’ biaxial extension experiments. Estimates of Young’s modulus and failure stress of the crumb solids ranged from 280–440 kN m -2 , and from 14–22 kN m -2 , respectively, when the revised Gibson and Ashby model was used.  2002 Elsevier Science Ltd Keywords: bread, structure, mechanical properties, relative density, tensile testing, image analysis. mechanical behaviour of bread crumb. This is INTRODUCTION particularly true since bread, like many cellular Bread crumb is a composite two-phase solid 1 of materials, is produced through random physical high porosity, which has been classified as a cellular processes 8 , so that its structure is generally hetero- solid 2 . Gibson and Ashby 3 showed that relative geneous 9 . This heterogeneity includes non-peri- density, defined as the density of the cellular ma- odic ordering of cells, variation in the properties terial divided by the density of the solid material, of the cell walls, and occasional imperfections due is the main structural characteristic affecting the to the absence of cell walls that would normally elastic properties and mechanical strength of syn- separate adjacent cells. Because structural vari- thetic cellular materials. Relationships between ation and the presence of defects have significant relative density and mechanical properties have effects on mechanical properties 9–11 , such hetero- also been successfully applied to processed geneity should be accounted for when predicting materials of cereal origin, such as extruded maize 4 , mechanical behaviour. For trabecular bone, 23– sponge cake 5 , starch bread 2 and starch foams 6,7 . 65% of the variation in elastic properties was In addition to relative density, other structural explained by power-law functions of density, but parameters are also expected to influence the this was increased to 66–94% when a measure of cellular anisotropy was added to density 12 . A useful tool for quantifying the contribution of these ad- ditional structural features is image analysis 13 . For * Corresponding author. E-mail: scanlon@cc.umanitoba.ca 0733–5210/02/050167+10 $35.00/0  2002 Elsevier Science Ltd