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首页> 外文学位 >An ultrasonic investigation of the effect of voids on the mechanical properties of bread dough and the role of gas cells in determining the cellular structure of freeze-dried breadcrumb.
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An ultrasonic investigation of the effect of voids on the mechanical properties of bread dough and the role of gas cells in determining the cellular structure of freeze-dried breadcrumb.

机译：超声研究空隙对面包面团机械性能的影响以及气室在确定冻干面包屑的细胞结构中的作用。

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摘要

This thesis is an analysis of voids in the breadmaking process, more specifically the effect of gas cells entrapped in the dough during mixing, their expansion during fermentation, and their relationship to the breadcrumb structure in the final product. This is important to food scientists because the voids ultimately influence the structural integrity of bread and hence its quality. Understanding how voids affect the viscoelastic properties of dough is also a challenging problem in soft condensed matter physics. Longitudinal ultrasonic velocity and attenuation measurements, performed at 54 kHz, investigated changes in the mechanical properties of dough and bread as void concentration was varied.; In the first part of the thesis, the effect of voids on the properties of unyeasted dough at the end of mixing was investigated. As &phis; is increased, the attenuation coefficient increased linearly with &phis;; hence the change in attenuation is proportional to the number of voids, allowing the combined effects of scattering and absorption by single voids to be directly determined. By contrast, the ultrasonic velocity decreased dramatically with increasing &phis; in the range 0.0 12 &phis; 0.03, while at higher &phis;, the velocity decrease was less rapid. An effective medium model successfully modeled the viscoelastic behavior of the dough at all void fraction values, provided that the shear modulus of the matrix was permitted to vary. The same ultrasonic technique was also used to monitor the increase in gas cell size due to CO 2 production during fermentation of yeasted dough. A large decrease in velocity and an increase in the attenuation coefficient were observed as the gas cells grew. In addition, at early fermentation times, a substantial contribution to the velocity decrease arises from a reduction in the shear modulus of the dough matrix. This occurs because the pH drops as CO2 molecules dissolve in the matrix and intermolecular interactions are weakened due to protein chain charge repulsion effects.; In the second part of the thesis, freeze-dried breadcrumb structure was investigated. To change the size of the air cells, the dough was proofed for various times. Ultrasonic velocity and amplitude decrease with increasing &phis;. The experimental data were found to be in reasonable agreement with theoretical models for the elasticity of isotropic cellular foams and tortuosity. The effects of anisotropy in breadcrumb structure were studied by compressing samples uniaxially, thereby transforming the shape of the air cells from approximately spherical to elongated ellipsoids. Ultrasonic measurements were taken in the directions parallel and perpendicular to the strain. These results indicated that the path by which sound propagates is critical. The data were interpreted using the same two theoretical models, taking into account anisotropy effects. The tortuosity model was able to interpret the void fraction dependence of the velocity along the two orthogonal directions, thus giving a way of relating changes in ultrasonic velocity to changes in breadcrumb structure.; This thesis demonstrates the potential for using ultrasound as a non-destructive, cheap and accurate tool for studying the effect of voids (and their expansion) on dough properties. These ultrasonic techniques can also be used to investigate the effect of air cells on the structural integrity of breadcrumb and hence be a useful tool for quantitatively assessing bread quality.
机译：本论文对面包制作过程中的空隙进行了分析，更具体地说是分析了混合过程中滞留在面团中的气室的作用，发酵过程中气室的膨胀以及它们与最终产品中面包屑结构的关系。这对食品科学家很重要，因为空隙最终会影响面包的结构完整性，进而影响面包的质量。了解空隙如何影响面团的粘弹性质也是软质凝聚态物理中的一个难题。纵向超声波速度和衰减测量是在54 kHz下进行的，研究了随着空隙浓度的变化面团和面包的机械性能的变化。在论文的第一部分，研究了混合结束后空隙对未发酵面团特性的影响。作为＆phis;增加，衰减系数随φ线性增加。因此，衰减的变化与空隙的数量成正比，从而可以直接确定单个空隙对散射和吸收的综合影响。相反，超声速度随着φ的增加而急剧下降。在0.0 12 <＆phis;范围内<0.03，而在φ较高时，速度下降的速度较慢。一个有效的介质模型可以成功地模拟面团在所有空隙率值下的粘弹性行为，前提是允许基质的剪切模量发生变化。同样的超声技术也被用来监测发酵面团发酵过程中由于产生CO 2 而引起的气室尺寸的增加。随着气室的增长，观察到速度大大降低，衰减系数增加。此外，在发酵初期，面团基质的剪切模量降低是速度降低的重要原因。之所以发生这种情况，是因为随着CO 2 分子溶解在基质中，pH下降，并且由于蛋白质链电荷排斥作用，分子间的相互作用减弱。在论文的第二部分，研究了冷冻干燥的面包屑结构。为了改变气室的大小，面团经过了多次发酵。超声波速度和振幅随φ的增加而减小。实验数据与各向同性多孔泡沫的弹性和曲折性的理论模型基本吻合。研究了各向异性对面包屑结构的影响，方法是单轴压缩样品，从而将气泡的形状从近似球形转变为细长的椭圆形。在平行于和垂直于应变的方向上进行超声测量。这些结果表明声音传播的路径至关重要。考虑到各向异性效应，使用相同的两个理论模型解释了数据。曲折模型能够解释沿两个正交方向的速度对空隙率的依赖性，从而提供了一种将超声波速度的变化与面包屑结构的变化联系起来的方法。本论文证明了使用超声波作为研究空隙（及其膨胀）对面团特性影响的无损，便宜且准确工具的潜力。这些超声技术还可以用于研究气泡对面包屑结构完整性的影响，因此是定量评估面包质量的有用工具。

著录项

  • 作者

    Elmehdi, Hussein Mohamed.;

  • 作者单位

    The University of Manitoba (Canada).;

  • 授予单位 The University of Manitoba (Canada).;
  • 学科 Physics Acoustics.; Agriculture Food Science and Technology.
  • 学位 Ph.D.
  • 年度 2001
  • 页码 196 p.
  • 总页数 196
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 声学;农产品收获、加工及贮藏;
  • 关键词

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