Shahidi Biochemistry of Foods

3. Auflage 2012
ISBN: 978-0-08-091809-9
Verlag: Elsevier Reference Monographs
Format: PDF
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 584 Seiten

This bestselling reference bridges the gap between the introductory and highly specialized books dealing with aspects of food biochemistry for undergraduate and graduate students, researchers, and professionals in the fi elds of food science, horticulture, animal science, dairy science and cereal chemistry. Now fully revised and updated, with contributing authors from around the world, the third edition of Biochemistry of Foods once again presents the most current science available. The first section addresses the biochemical changes involved in the development of raw foods such as cereals, legumes, fruits and vegetables, milk, and eggs. Section II reviews the processing of foods such as brewing, cheese and yogurt, oilseed processing as well as the role of non-enzymatic browning. Section III on spoilage includes a comprehensive review of enzymatic browning, lipid oxidation and milk off-flavors. The final section covers the new and rapidly expanding area of rDNA technologies. This book provides transitional coverage that moves the reader from concept to application.
Features new chapters on rDNA technologies, legumes, eggs, oilseed processing and fat modification, and lipid oxidationOffers expanded and updated material throughout, including valuable illustrationsEdited and authored by award-winning scientists

Autoren/Hrsg.

Shahidi, Fereidoon

Leseproben

Chapter 2
Fruits and Vegetables
N.A. Michael Eskin* and Ernst Hoehn† *Department of Human Nutritional Sciences, Faculty of Human Ecology, University of Manitoba, Winnipeg, Manitoba, Canada †Swiss Federal Research Station, Switzerland Chapter Outline
I. Introduction II. Respiration A. Fruits B. Vegetables 1. Control of the Climacteric Rise 2. Enzymatic Control III. Initiation of Ripening IV. Biosynthesis of Ethylene A. Methionine as Precursor of Ethylene 1. The Yang Cycle, Recycling of Methionine 2. Methionine and Ethylene Biosynthesis B. Regulation of Ethylene in Ripening Fruits 1. ACC Synthase and ACC Oxidase 2. Cyanide 3. Organic Acids 4. Lipid Peroxidation: Lipoxygenase 5. Galactose V. Color Changes A. Chlorophyll Changes during Ripening B. Chlorophyll Biosynthesis 1. Phase 1: Glutamate to Chlorophyll a 2. Phase 2: The Chlorophyll Cycle C. Regulation of Chlorophyll Biosynthesis D. Mechanism of Chlorophyll Degradation 1. Chlorophyll Degradation: Processing and Storage E. Carotenoids 1. Carotenoid Changes during Ripening 2. Carotenoid Degradation: Processing and Storage F. Anthocyanins 1. Biosynthesis of Anthocyanins 2. Anthocyanins: Effect of Processing VI. Texture A. Cell Wall 1. Cell Wall Constituents 2. Cell Wall Degradation VII. Flavor A. Aroma 1. Aldehydes, Alcohols, and Esters B. Taste 1. Starch–Sugar Conversion 2. Organic Acids VIII. Storage A. Cold Storage B. Controlled Atmosphere Storage C. New Developments in Postharvest Storage References I Introduction
Characteristics of fruits and vegetables such as flavor, color, size, shape, and absence of external defects ultimately determine their acceptance by consumers. The development of these characteristics is the result of many chemical and biochemical changes that occur following harvesting and storage. Since harvesting fruits and vegetables at their correct stage of maturity is critical for the development of a highly acceptable product for the fresh market, or for processing, it is important to understand more fully what changes are taking place. This chapter will highlight those changes occurring within fruits and vegetables during the postharvest period. It is during this period that fruits and vegetables show a gradual reduction in quality concurrent with transpiration and respiration, as well as with other biochemical and physiological changes. Ultimately the plant material deteriorates because of the undesirable enzyme activity and spoilage microorganisms. The growth and maturation of fruits and vegetables are dependent on photosynthesis and absorption of water and minerals by the parent plant. Once detached, however, they are independent units in which respiratory processes play a major role. This chapter will focus on those changes in postharvest fruits and vegetables that affect quality. II Respiration
Respiration is the fundamental process whereby living organisms carry out the exothermic conversion of potential energy into kinetic energy. In higher plants the major storage products are sucrose and starch. These are completely oxidized in the presence of oxygen to carbon dioxide and water, with the production of adenosine triphosphate (ATP): The latter is the form in which energy is stored within the cell. The contribution of proteins and lipids to plant respiration is difficult to assess but can occur via the formation of acetyl-coenzyme A (CoA). In the absence of oxygen, anaerobic respiration occurs, resulting in only a partial degradation of carbohydrates and a lower ATP production. The metabolic pathways involved in the respiration of plant tissue result in the conversion of starch or sucrose to glucose-6-P. The latter is then oxidized by glycolysis (Embden–Meyerhoff pathway) or the pentose phosphate pathway to triose phosphate, which enters the tricarboxylic acid cycle by way of pyruvate (Scheme 2.1) (ap Rees, 1977). Finally, in a third stage, oxidative phosphorylation converts NADH and FADH2 into chemical energy in the form of ATP (Browse et al., 2006). SCHEME 2.1 Glycolytic and pentose phosphate pathways. The contribution of these two major pathways of carbohydrate oxidation to plant respiration remains unresolved. Difficulties were encountered with the experimental techniques used in assessing the relative roles of these pathways based on the production of 14CO2 or labeled intermediates from labeled hexoses (ap Rees, 1980). Evidence shows that both pathways exist in plant tissues (ap Rees, 1974) and that they change considerably during plant development (ap Rees, 1977). Current evidence supports the glycolytic pathway as the predominant one operating, while the maximum contribution of the pentose phosphate pathway may not exceed 30% of the total (ap Rees, 1980). The relative importance of these pathways probably depends on the particular plant, the organ, and the state of maturity. Respiration rates of fruits and vegetables are affected by many environmental factors. In cases where this leads to negative effects on plant tissue it is defined as stress. During the storage of fruits and vegetables (Section VIII) effects of low temperatures, reduction in oxygen (O2) concentration and increase in carbon dioxide (CO2) concentration in the storage atmosphere are utilized to extend the storage life of produce. However, maintaining an adequate energy status is required to prevent browning or senescence of harvested fruits and vegetables (Saquet et al., 2000, 2003a; Xuan et al., 2005; Song et al., 2006; Jiang et al., 2007). It is well established that lowering of O2 concentrations during controlled atmosphere storage reduces respiration rates and energy supply and that severe limitations of O2 induce fermentative (anaerobic) respiration and metabolism in stored produce (Scheme 2.1). The net yield of ATP during anaerobic respiration is only 2 moles of ATP of hexose sugar compared with 36 moles of ATP per mole of hexose in aerobic respiration. Hence, energy status may be insufficient and provoke storage disorders (Jiang et al., 2007). A Fruits
A large number of fruits exhibit a sudden sharp rise in respiratory activity following harvesting, referred to as the climacteric rise in respiration. This phenomenon was first noted by Kidd and West (1922, 1930a) as an upsurge in carbon dioxide gas at the end of the maturation phase of apples. Since then there have been numerous reports on this phenomenon in a wide range of fruits. The appropriateness of the term climacteric was questioned by Rhodes (1970), who suggested that it should be all inclusive and describe the ‘whole of the control phase in the life of fruit triggered by ethylene and the concomitant changes occurring’. McGlasson et al. (1978), however, suggested that respiratory climacteric was the more appropriate term to describe this gaseous phenomenon. Biale and Young (1981) nevertheless still preferred the more inclusive description in which climacteric defined those physical, chemical, physiological, and metabolic changes associated with the increased rate of respiration covering the transition phase from growth and maturation to the final stages of senescence. Essentially, climacteric defines the last stages of the fruit at the cellular level, which determine the quality of the fruit that is shipped to the consumer. Biale (1960a, b) tentatively classified fruits as either climacteric or non-climacteric according to their respiratory rates. A later review by Biale and Young (1981), however, suggested a more extensive list of fruits from both groups as shown in Table 2.1. Eventually fruits such as cantaloupe, honeydew melon, and figs were included, all of which are considered climacteric (Lyons et al., 1962; Pratt and Groeschel, 1968; Marei and Crane, 1971). A few rare fruits were also added, namely, breadfruit (Biale and Barcus, 1970), guavas, and mammee apples (Akamine and Goo, 1978, 1979a, b; Saltveit, 2004). TABLE 2.1 Respiratory Activity of Selected...