Using ultrasound to investigate the cellular structure of bread crumb H.M. Elmehdi a,b,1 , J.H. Page a , M.G. Scanlon b, * a Department of Physics and Astronomy, University of Manitoba, Winnipeg, Man., Canada R3T 2N2 b Department of Food Science, University of Manitoba, Winnipeg, Man., Canada R3T 2N2 Received 10 June 2002; revised 11 November 2002; accepted 13 December 2002 Abstract In this paper, ultrasonic techniques were used to study how the mechanical properties of bread crumb are affected by changing the size, concentration and shape of the crumb cells. Since the gas cells determine the structural integrity of the bread crumb, these effects can form the basis of methods for predicting loaf quality, and hence are of considerable importance to food and cereal scientists. Freeze-dried bread crumb samples were prepared using red spring wheat flour. The density (r) of crumb was varied from 100 to 300 kg/m 3 by proving the dough for various times. Both the ultrasonic velocity and the amplitude of 54 kHz longitudinal waves increased with r, with the velocity varying approximately as r 0.5 . The signal amplitude was found to increase linearly with density. To investigate the effects of anisotropy in the cell structure of bread crumb, freshly baked samples were compressed uniaxially, thereby transforming the shape of the cells from approximately spherical to ellipsoidal. Ultrasonic measurements were taken in the directions parallel and perpendicular to the applied stress. The density dependence of the velocity in the compressed samples was opposite to that in the non-compressed samples, with velocity decreasing with increasing compression, more prominently along the direction parallel to the stress. Signal amplitude showed a slight increase. These results demonstrate that the mechanical properties of the compressed and non-compressed samples are different. The sensitivity of ultrasonic waves to changes in the size and shape of crumb cells demonstrates the potential for using ultrasound as a tool for characterizing the mechanical and structural properties of bread crumb, and hence for measuring some of the determining factors of bread quality. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Bread; Structure; Ultrasound; Cell anisotropy; Mechanical properties 1. Introduction From a structural point of view, bread crumb is a porous material consisting of gas cells and crumb cell walls. Often the crumb wall phase is referred to as the matrix (Scanlon and Zghal, 2001; Keetels et al., 1996). It is this material that contributes to the bread’s mechanical strength and structural architecture. The crumb cell walls consist of partly gelatinized starch in a heterogeneous protein-based matrix (Eliasson and Larsson, 1993). The final structure in the bread crumb (and in many baked goods) depends to a large extent on creating and controlling the development of gas bubbles in the dough matrix, and then retaining them as distinct bubbles until the bread is baked (and thus the matrix fixed) (Campbell et al., 1998; Cauvain, 1999). The crumb structure contributes to the texture (or grain quality), mechanical strength and perceived product freshness of the bread as well as to its visual appearance. Each of these product attributes varies with the numbers, sizes and uniformity of distribution of these gas bubbles, which may be determined and controlled in the breadmaking process as early as the mixing stage (Cauvain, 1999). In general, the gas bubbles have a relatively small size; for example, at the end of mixing their mean diameter is about 75 mm, but they expand to reach a mean diameter as large as a few millimetres (Shimiya and Nakamura, 1997). In the creation of fine- celled white bread, such as sandwich loaves, it is generally accepted by consumers that small holes that are uniformly distributed throughout the crumb are required and that large holes or irregular cell distributions are undesirable (Cau- vain, 1999). For a good product, the crumb cell wall, or matrix, is required to be as thin as possible, yet it must be resilient enough to recover from modest deformation, such as squeezing and pressing—the two most common ways by which consumers assess product freshness (Ponte and 0733-5210/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0733-5210(03)00002-X Journal of Cereal Science 38 (2003) 33–42 www.elsevier.com/locate/jnlabr/yjcrs 1 Present address: Agriculture and Agri-Food Canada, Cereal Research Centre, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2N9. * Corresponding author. Tel.: þ 1-204-474-6480; fax: þ1-204-474-7630. E-mail address: scanlon@cc.umanitoba.ca (M.G. Scanlon).