Characterization of Cereals and Flours_Bài gởi qua mail admin của HUONGKIM

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Characterization of Cereals and Flours_Bài gởi qua mail admin của HUONGKIM

Bài gửi  chilinhkgcc on Wed Apr 07, 2010 8:55 pm

Cereal-based foods comprise a substantial portion of the world’s food supply, despite regional, economical, and habitual differences in consumption. In the human diet, cereals are considered excellent sources of fiber and nutrients (e.g.,starches, proteins, vitamins, and minerals). In many developing countries, cereals provide as much as 75% of human dietary energy. In 1992, the U.S. Department of Agriculture emphasized the importance of cereal-based foods in the human diet by introducing the Food Guide Pyramid. This graphical guideline organizes foods into five groups and recommends daily consumption of 6–11 servings of
bread, cereals, rice, and pasta (two to three times more than the number of servings for other food groups), thereby stressing the relative significance of the grains group. As economical and abundant raw materials, cereals have long been used for the production of a wide range of food and nonfood products, including breads, cookies, pastas, breakfast cereals, snack foods, malted cereals, pharmaceuticals, and adhesives.
The improvement and development of cereal products and processes require an understanding of the impact of processing and storage conditions on the physical properties and structure of pre- and postprocessed materials. In this book, we focus on techniques used to characterize the influence on the physical properties of cereal flours of several cereal processing technologies, including Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved. baking, pasta extrusion, and high-temperature extrusion, as well as cookie and cracker production. This text facilitates viewing the impact of various cereal processing technologies on cereal flours from three complementary perspectives: characterization of thermal, mechanical, and structural properties. Establishing quantitative relationships among the various physical observables and between the physical properties and the sensory attributes of end products should provide a rapid and objective means for assessing the quality of food materials, with the overall goal of improving this quality. To this end, a fourth perspective is also included: namely, sensory end-product attributes of significance to the consumer.
In several chapters, in fact, correlations between sensory attributes and physical properties are reported. Cereal processing consists basically of mixing cereal flours with water, followed by heating to various temperatures, cooling, and storing. Consequently, for the purpose of improving processing, it would be most useful if one could predict the physical properties of pre- and postprocessed cereal flours when subjected
to varied processing and storage conditions. Part I of this book, which includes Chapters 1 through 5, focuses on discussions of thermal analysis techniques to assess the impact of various cereal processing conditions on the physical properties of cereal flours in high-temperature extrusion, cookie manufacturing, and baking. Chapters 1, 2, and 3 describe thermally induced transitions (glass, melting, gelatinization) in cereal flours as a function of conditions relevant to cereal processing technologies. Chapter 4 addresses the influence of moisture on the processing conditions and the physical properties of the product. The final chapter of Part I (Chapter 5) focuses on the utilization of a database created from the studies described in the previous chapters to establish state diagrams that define the state of the cereal flour prior to, during, and after processing. This chapter also describes the application of such state diagrams to map the path of processes, to assess the impact of processing conditions, and, ultimately, to design processing conditions that achieve desired end-product attributes. Part II includes Chapters 6 through 10 and focuses on the characterization
of mechanical properties of cereal flours, prior to, during, and after processing. Chapter 6 reports on the assessment of the stability of cereal flours in terms of caking or loss of flowability as a result of moisture sorption or exposure to elevated temperatures during storage. Chapter 7 covers the rheological characteristics of cereal flours during processing and their relation to end-product physical properties such as expansion of extrudates. Chapters 8 and 9 describe the mechanical properties of postprocessed cereal flours as a function of processing conditions, additives, and postprocessing storage conditions in relation to pasta drying, textural attributes, and shelf life of extruded products. Mechanical properties of biopolymers change as their physical state is altered during processing or storage. Chapter 10 focuses on the application of this information in product and process development.
The third and final part includes studies exploring the microscopic determinants of macroscopic properties. These studies employ techniques such as light and electron microscopy and nuclear magnetic resonance (NMR) spectroscopy. Chapters in this part focus on the development of correlations between the microscopic structural features of pre- and postprocessed food biopolymers and their macroscopic physical properties. Chapter 11 describes how image analysis techniques can be used to evaluate macrostructures created in expanded extrudates as a function of formulation and processing conditions. The cell structure and cell size distribution in these products are responsible for the characteristic crispy texture of cereal products. Macroscopic observables do not reveal whether an observed order–disorder transition reflects a change in the overall structure or whether the transition is specific for local structural domains. As with all food materials, ompositional and microstructural heterogeneity are intrinsic characteristics of pre- and postprocessed cereal flours. Consequently, it is most useful to characterize chemical and structural composition at a microscopic level. Chapter 12 focuses on the use of microscopy as a tool to gain such information about structural organization, as well as the distribution of various domains within proteins, starches, and other
components in pre- and postprocessed cereal flours. Chapters 13 and 14 focus on probing the relationships between structure, dynamics, and function using NMR and phosphorescence spectroscopy. Due to the noninvasive character of NMR and the richness of its information content, its use to study pre- and postprocessed cereal biopolymers has increased in the past decade. Such NMR studies range from structural characterization of starch granules to observations of changes in water mobility in staling bread. Phosphorescence spectroscopy is a promising emerging technique for studying the molecular dynamics of the glassy state in which the mobility is very limited. Chapter 15 is devoted to two converging lines of starch research with implications for the cereal processing industry. Chemical studies link the molecular characterization of starch granules and starch-bound proteins to the properties of starch-based products. Biochemical and genetic studies provide information on starch modification and biosynthesis with the ultimate objective being to enhance the starch yield and quality. All of the chapters in this book are designed
1. To develop a fundamental understanding of the influence of processing on cereal flours by creating a database via systematic studies of the physical properties of pre- and postprocessed cereal flours
2. To demonstrate how this knowledge can be used as a predictive tool for evaluating the performance of cereal flour during processing, and, ultimately, for adjusting, in a rational fashion, the formulation of raw
materials and processing parameters so as to achieve desired endproduct attributes This book bridges the gap between basic knowledge and application. We be- lieve it will prove to be a comprehensive and valuable teaching text and reference book for students and practicing scientists, in both academia and industry.
Go¨nu¨l Kaletunc¸
Kenneth J. Breslauer

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