Food Carbohydrate Chemistry E-Book

Ronald E. Wrolstad

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Institute of Food Technologists Series
• Sprache: Englisch

Seitenzahl: 246

Veröffentlichungsjahr: 2011

Beschreibung

Not since "Sugar Chemistry" by Shallenberger and Birch (1975) has a text clearly presented and applied basic carbohydrate chemistry to the quality attributes and functional properties of foods. Now in Food Carbohydrate Chemistry, author Wrolstad emphasizes the application of carbohydrate chemistry to understanding the chemistry, physical and functional properties of food carbohydrates. Structure and nomenclature of sugars and sugar derivatives are covered, focusing on those derivatives that exist naturally in foods or are used as food additives. Chemical reactions emphasize those that have an impact on food quality and occur under processing and storage conditions. Coverage includes: how chemical and physical properties of sugars and polysaccharides affect the functional properties of foods; taste properties and non-enzymic browning reactions; the nutritional roles of carbohydrates from a food chemist's perspective; basic principles, advantages, and limitations of selected carbohydrate analytical methods. An appendix includes descriptions of proven laboratory exercises and demonstrations. Applications are emphasized, and anecdotal examples and case studies are presented. Laboratory units, homework exercises, and lecture demonstrations are included in the appendix. In addition to a complete list of cited references, a listing of key references is included with brief annotations describing their important features. Students and professionals alike will benefit from this latest addition to the IFT Press book series. In Food Carbohydrate Chemistry, upper undergraduate and graduate students will find a clear explanation of how basic principles of carbohydrate chemistry can account for and predict functional properties such as sweetness, browning potential, and solubility properties. Professionals working in product development and technical sales will value Food Carbohydrate Chemistry as a needed resource to help them understand the functionality of carbohydrate ingredients. And persons in research and quality assurance will rely upon Food Carbohydrate Chemistry for understanding the principles of carbohydrate analytical methods and the physical and chemical properties of sugars and polysaccharides.

Leseprobe

Contents

Cover

Series One

Title Page

Copyright

Series Two

Dedication

Contributors

Acknowledgments

Introduction

Chapter 1: Classifying, Identifying, Naming, and Drawing Sugars and Sugar Derivatives

Structure and Nomenclature of Monosaccharides

Different Ways of Depicting Sugar Structures

Classifying Sugars by Compound Class—Hemiacetals, Hemiketals, Acetals, and Ketals

Structure and Nomenclature of Disacchaarides

Structure and Optical Activity

A Systematic Procedure for Determining Conformation (C-1 or 1-C), Chiral Family (D or L), and Anomeric Form (α or β) of Sugar Pyranoid Ring Structures

Structure and Nomenclature of Sugar Derivatives with Relevance to Food Chemistry

Vocabulary

Chapter 2: Sugar Composition of Foods

Introduction

Sugar Content of Foods

Composition of Sweeteners

Sugar Composition of Fruits and Fruit Juices

Vocabulary

Chapter 3: Reactions of Sugars

Introduction

Mutarotation

Oxidation of Sugars

Glycoside Formation

Acid Catalyzed Sugar Reactions

Alkaline-Catalyzed Sugar Reactions

Summary

Vocabulary

Chapter 4: Browning Reactions

Introduction

Key Reactions in Maillard Browning

An Alternate Free-Radical Mechanism for Nonenzymatic Browning

Measurement of Maillard Browning

Control of Maillard Browning

Other Browning Reactions

Vocabulary

Chapter 5: Functional Properties of Sugars

Introduction

Taste Properties of Sugars

The Shallenberger–Acree Theory for Sweetness Perception

Sugar Solubility

Crystallinity of Sugars

Hygroscopicity

Humectancy

Viscosity

Freezing Point Depression and Boiling Point Elevation

Osmotic Effects

Vocabulary

Chapter 6: Analytical Methods

Introduction

Physical Methods

Colorimetric Methods

Chromatographic Methods

Enzymic Methods

Carbon Stable-Isotopic Ratio Analysis (SIRA)

Chapter 7: Starch in Foods

Introduction

Sources of Starch

Molecular Structure of Starch

Starch Granules

Gelatinization and Pasting: The Cooking of Starch

Retrogradation and Gelation: The Cooling of Cooked Starch

Monitoring Starch Transitions

Starch Hydrolytic Enzymes

Modified Starches

Resistant Starch

Concluding Remarks

Vocabulary

Chapter 8: Plant Cell Wall Polysaccharides

Introduction: Why Plant Cell Walls are Important

Cellulose

Hemicelluloses

Pectic Polysaccharides

Interactions between Polysaccharides and Cellulose

The Plant Cell Wall Structure

Vocabulary

Chapter 9: Nutritional Roles of Carbohydrates

Introduction

The Digestive Process: From the Bucchal Cavity through the Small Intestine

The Large Intestine and the Digestive Process

Carbohydrate Nutrition and Human Health

Vocabulary

Appendices

Unit 1: Laboratory/Homework Exercise—Building Molecular Models of Sugar Molecules

Expected Outcomes

Materials

Assignment

Unit 2: Homework Exercise—Recognizing Hemiacetal, Hemiketal, Acetal, and Ketal Functional Groups

Answers to Exercise

Unit 3: Laboratory/Homework Exercise—Specification of Conformation (C-1 or 1-C), Chiral Family (D or L), and Anomeric Form (α or β) of Sugar Pyranoid Ring Structures

Supplementary materials

Prologue

Determination of Chair Conformation

Determination of Chiral Family

Determination of Anomeric Form

Exercise: Calculating and Specifying Multiple Chirality of Sugar Pyranoid Ring Structures

Answers to Exercise

Unit 4: Demonstration of the Existence of Plane-Polarized Light and the Ability of Sugar Solutions to Rotate Plane-Polarized Light

Materials

Demonstration

Unit 5.: Laboratory Exercise—Sugar Polarimetry

Expected Outcomes

Materials

Experimental

Unit 6: Laboratory Exercise or Lecture Demonstration—The Fehling's Test for Reducing Sugars

Expected Outcomes

Materials

Background Information

Experimental

Unit 7: Laboratory Exercise—Student-Designed Maillard Browning Experiments

Expected Outcomes

Prologue

Materials

Equipment

Hypotheses

Unit 8: Laboratory Exercise or Lecture Demonstration—Microscopic Examination of Starch

Expected Outcomes

Materials and Equipment

Experimental

Anecdote

Anecdote

Unit 9: Names and Structures of Oligosaccharidesa

Index

Food Science and Technology

The IFT Press series reflects the mission of the Institute of Food Technologists—to advance the science of food contributing to healthier people everywhere. Developed in partnership with Wiley-Blackwell, IFTPress books serve as leading-edge handbooks for industrial application and reference and as essential texts for academic programs. Crafted through rigorous peer review and meticulous research, IFT Press publications represent the latest, most signifi-cant resources available to food scientists and related agriculture professionals worldwide. Founded in 1939, the Institute of Food Technologists is a nonprofit scientific society with 22,000 individual members working in food science, food technology, and related professions in industry, academia, and government. IFT serves as a conduit for multidisciplinary science thought leadership, championing the use of sound science across the food value chain through knowledge sharing, education, and advocacy.

IFT Press Advisory Group

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IFT Press Editorial Board

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Library of Congress Cataloging-in-Publication Data

Wrolstad, Ronald E., 1939– Food carbohydrate chemistry / Ronald E. Wrolstad. – 1st ed. p. cm. – (Institute of food technologists series ; 48) Includes bibliographical references and index. ISBN 978-0-8138-2665-3 (hardback) 1. Carbohydrates. I. Title. QD321.W88 2012 547′.78–dc23 2011036449

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Titles in the IFT Press series

Accelerating New Food Product Design and Development (Jacqueline H. Beckley, Elizabeth J. Topp, M. Michele Foley, J.C. Huang, and Witoon Prinyawiwatkul)Advances in Dairy Ingredients (Geoffrey W. Smithers and Mary Ann Augustin)Bioactive Proteins and Peptides as Functional Foods and Nutraceuticals (Yoshinori Mine, Eunice Li–Chan, and Bo Jiang)Biofilms in the Food Environment (Hans P. Blaschek, Hua H. Wang, and Meredith E. Agle)Calorimetry in Food Processing: Analysis and Design of Food Systems (Gönül Kaletunç)Coffee: Emerging Health Effects and Disease Prevention (YiFang Chu)Food Carbohydrate Chemistry (Ronald E. Wrolstad)Food Ingredients for the Global Market (Yao–Wen Huang and Claire L. Kruger)Food Irradiation Research and Technology (Christopher H. Sommers and Xuetong Fan)Foodborne Pathogens in the Food Processing Environment: Sources, Detection and Control (Sadhana Ravishankar, Vijay K. Juneja, and Divya Jaroni)High Pressure Processing of Foods (Christopher J. Doona and Florence E. Feeherry)Hydrocolloids in Food Processing (Thomas R. Laaman)Improving Import Food Safety (Wayne C. Ellefson, Lorna Zach, and Darryl Sullivan)Innovative Food Processing Technologies: Advances in Multiphysics Simulation (Kai Knoerzer, Pablo Juliano, Peter Roupas, and Cornelis Versteeg)Microbial Safety of Fresh Produce (Xuetong Fan, Brendan A. Niemira, Christopher J. Doona, Florence E. Feeherry, and Robert B. Gravani)Microbiology and Technology of Fermented Foods (Robert W. Hutkins)Multiphysics Simulation of Emerging Food Processing Technologies (Kai Knoerzer, Pablo Juliano, Peter Roupas and Cornelis Versteeg)Multivariate and Probabilistic Analyses of Sensory Science Problems (Jean–François Meullenet, Rui Xiong, and Christopher J. FindlayNanoscience and Nanotechnology in Food Systems (Hongda Chen)Natural Food Flavors and Colorants (Mathew Attokaran)Nondestructive Testing of Food Quality (Joseph Irudayaraj and Christoph Reh)Nondigestible Carbohydrates and Digestive Health (Teresa M. Paeschke and William R. Aimutis)Nonthermal Processing Technologies for Food (Howard Q. Zhang, Gustavo V. Barbosa– Cánovas, V.M. Balasubramaniam, C. Patrick Dunne, Daniel F. Farkas, and James T.C. Yuan)Nutraceuticals, Glycemic Health and Type 2 Diabetes (Vijai K. Pasupuleti and James W. Anderson)Organic Meat Production and Processing (Steven C. Ricke, Michael G. Johnson, and Corliss A. O’Bryan)Packaging for Nonthermal Processing of Food (Jung H. Han)Preharvest and Postharvest Food Safety: Contemporary Issues and Future Directions (Ross C. Beier, Suresh D. Pillai, and Timothy D. Phillips, Editors; Richard L. Ziprin, Associate Editor)Processing and Nutrition of Fats and Oils (Ernesto M. Hernandez and Afaf Kamal–Eldin)Processing Organic Foods for the Global Market (Gwendolyn V. Wyard, Anne Plotto, Jessica Walden, and Kathryn Schuett)Regulation of Functional Foods and Nutraceuticals: A Global Perspective (Clare M. Hasler)Resistant Starch: Sources, Applications and Health Benefits (Yong–Cheng Shi and Clodualdo Maningat)Sensory and Consumer Research in Food Product Design and Development (Howard R. Moskowitz, Jacqueline H. Beckley, and Anna V.A. Resurreccion)Sustainability in the Food Industry (Cheryl J. Baldwin)Thermal Processing of Foods: Control and Automation (K.P. Sandeep)Trait–Modified Oils in Foods (Frank T. Orthoefer and Gary R. List)Water Activity in Foods: Fundamentals and Applications (Gustavo V. Barbosa–Cánovas, Anthony J. Fontana Jr., Shelly J. Schmidt, and Theodore P. Labuza)Whey Processing, Functionality and Health Benefits (Charles I. Onwulata and Peter J. Huth)

This book is dedicated to two special mentors, one being my Major Professor at the University of California, Davis, Dr. Walter G. Jennings. His concern for students and his enthusiasm for research and teaching continue to inspire. The second is the late Robert S. Shallenberger with whom I was fortunate to work while on sabbatical leave at Cornell University in 1979–1980. His influence on this book should be evident on nearly every page. I would also like to dedicate the book to the many undergraduate and graduate students, who through their suggestions, understanding, and misunderstanding helped me to revise, discard, and improve lecture presentations, homework assignments, demonstrations, and laboratory exercises. All of those items were a platform for this book.

Contributors

Chapter 7

Andrew S. Ross Department of Crop and Soil Science/Department of Food Science and Technology Oregon State University Corvallis, Oregon

Chapter 8

Bronwen G. Smith and Laurence D. Melton Food Science Programme The University of Auckland Auckland, New Zealand

Acknowledgments

A sincere thanks to Andrew Ross, who authored Chapter , and to Laurence Melton and Bronwen Smith for Chapter . Thanks also to Dan Smith for his insightful reviewing and to Carole Jubert, who came to the rescue of this novice in ChemDraw™ and prepared the chemical structures and figures.

Introduction

Carbohydrates are major components of foods, accounting for more than 90% of the dry matter of fruits and vegetables and providing for 70–80% of human caloric intake worldwide (BeMiller and Huber 2008). Thus, from a quantitative perspective alone, carbohydrates warrant the attention of food chemists. From the standpoint of food quality, carbohydrates are multifunctional. Sugars are the major source, as well as our reference point, for sweetness. Although carbohydrates are described as being odorless, the volatile reaction products from the Maillard reaction, Strecker degradation, and carmelization reactions can provide desirable, undesirable, or neutral flavor compounds. And, although carbohydrates are colorless, sugars participate in Maillard and carmelization reactions to produce desirable and undesirable brown colors. Cellulose, hemicellulose, pectin, and starch are the structural components of plants that are largely responsible for the textural characteristics of fruits and vegetables. Starch and starch derivatives and various hydrocolloids isolated from plants, seaweed, and microbial sources are used as thickeners, gelling agents, bodying agents, and stabilizers in foods. When it comes to nutrition, a sizable portion of the lay public view carbohydrates in a bad light. Carbohydrates are often blamed for health issues such as obesity, diabetes, and dental caries. It should be realized that carbohydrates are, or should be, the principal source of energy in our diet. After all, we evolved as a species to efficiently use carbohydrates that can be converted to glucose for our body's fuel. Good nutrition is based on the consumption of the appropriate carbohydrates in the right amounts in balance with other nutrients. It is widely accepted that consumption of various forms of complex carbohydrate can reduce the risk of diabetes, coronary heart disease, diverticulitus, and colon cancer. For peak athletic performance, the advice of professional nutritionists will emphasize consumption of the appropriate carbohydrates, in the appropriate amounts, at the appropriate time. Although the percentage of carbohydrates contributing to caloric intake in the United States is highly variable, the average is considerably less than 70%. Dietary recommendations call for increased consumption of fruits and vegetables and a greater proportion of complex carbohydrate (Walker and Reamy 2009; WHO 2010).

The major thrust of this book is to apply basic carbohydrate chemistry to the quality attributes and functional properties of foods. Structure and nomenclature of sugars and sugar derivatives is covered but limited to those compounds that exist naturally in foods or are used as food additives and food ingredients. Review and presentation of fundamental carbohydrate chemistry is minimized, with the assumption that readers have taken general organic chemistry and general biochemistry and have ready access to those books for reference. Chemical reactions focus on those that have an impact on food quality and occur under processing and storage conditions. How chemical and physical properties of sugars and polysaccharides affect the functional properties of foods is emphasized. Taste properties and nonenzymic browning reactions are covered. The nutritional roles of carbohydrates are covered from a food chemist's perspective. One chapter describes selected carbohydrate analytical methods, emphasizing the basic principles of the methods and their advantages and limitations. There is an extensive appendix that includes some laboratory and classroom exercises and lecture demonstrations.

References

BeMiller JM, Huber KC. 2008. Carbohydrates. In: Damodaran S, Parkin KL, Fennema OR, editors. Fennema's Food Chemistry, 4th ed. Boca Raton, FL: CRC Press, Taylor & Francis, pp. 83–154.

Walker C, Reamy BV. 2009. Diets for cardiovascular disease prevention: what is the evidence? Am Fam Physician 79:571–8. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19378874. Accessed September 2, 2010.

WHO 2010. Global strategy on diet, physical activity and health. Available from: http://www.who.int/dietphysicalactivity/diet/en/index.html. Accessed September 2, 2010.

1

Classifying, Identifying, Naming, and Drawing Sugars and Sugar Derivatives

Structure and Nomenclature of Monosaccharides

Aldoses and Ketoses

Configurations of Aldose Sugars

D- vs. L-Sugars

Different Ways of Depicting Sugar Structures

Fischer, Haworth, Mills, and Conformational Structures

Classifying Sugars by Compound Class—Hemiacetals, Hemiketals, Acetals, and Ketals

Structure and Nomenclature of Disacchaarides

Structure and Optical Activity

A Systematic Procedure for Determining Conformation (C-1 or 1-C), Chiral Family (D or L), and Anomeric Form (α or β) of Sugar Pyranoid Ring Structures

Structure and Nomenclature of Sugar Derivatives with Relevance to Food Chemistry

Glycols (Alditols)

Glyconic, Glycuronic, and Glycaric Acids

Deoxy Sugars

Amino Sugars and Glycosyl Amines

Glycosides

Sugar Ethers and Sugar Esters

Vocabulary

References

Structure and Nomenclature of Monosaccharides

Sugars are polyhydroxycarbonyls that occur in single or multiple units as monosaccharides, disaccharides, trisaccharides, tetrasaccharides, or oligosacharides (typically three to ten sugar units). Monosaccharides (also known as simple sugars) exist as aldoses or ketoses, with glucose and fructose being the most common examples. Glycose is a generic term for sugars. Sugars are also classified according to the number of carbon atoms in the molecule (e.g., trioses, tetroses, pentoses, hexoses, heptoses, etc.).

Figure 1.1 Structure and nomenclature of glucose, fructose, and arabinose.

Aldoses and Ketoses

Aldoses contain an aldehyde functional group at carbon-1 (C-1), whereas ketoses contain a carbonyl group that is almost always located at carbon-2 (C-2). C-1 for aldoses and C-2 for ketoses are the reactive centers for these molecules and are known as the anomeric carbon atoms. Figure 1.1 shows the structure for D-glucose, D-fructose, and, in addition, D-arabinose. Sugars have common or trivial names with historical origins from chemistry, medicine, and industry. There is also a systematic procedure for naming sugars (some examples are shown in Table 1.1). Glucose is also commonly known as dextrose. In systematic nomenclature, its suffix is hexose, indicating a 6-carbon aldose sugar, and the prefix is gluco-, which shows the orientation of the hydroxyl groups around carbons 2–5. The symbol D refers to the orientation of the hydroxyl group on C-5, the highest numbered asymmetric carbon atom, also known as the reference carbon atom. Since fructose (also known as levulose) has just three asymmetric carbon atoms, its configurational prefix is the same as that for the pentose sugar arabinose. Thus, the systematic name for glucose is D-gluco-hexose and fructose is D-arabino-hexulose.

Table 1.1 Trivial and Systematic Names of Selected Sugars

Trivial (or Common)SystematicaD-ErythroseD-erythro-tetroseD-ThreoseD-threo-tetroseD-ArabinoseD-arabino-pentoseD-LyxoseD-lyxo-pentoseD-RiboseD-ribo-pentoseD-XyloseD-xylo-pentoseD-AlloseD-allo-hexoseD-AltroseD-altro-hexoseD-GalactoseD-galacto-hexoseD-GlucoseD-gluco-hexoseD-GuloseD-gulo-hexoseD-IdoseD-ido-hexoseD-MannoseD-manno-hexoseD-TaloseD-talo-hexoseaIn the systematic name, the configurational prefix is italicized, and the stem name indicates the number of carbon atoms in the molecule.

Configurations of Aldose Sugars

Figure 1.2 shows all possible configurations around the asymmetric carbon atoms for the triose, tetrose, pentose, and hexose D-aldose sugars. Diastereoisomers are molecular isomers that differ in configuration about one or more asymmetric carbon atoms; there are eight hexose diastereoisomers. Epimer is yet another term in sugar chemistry that refers to diastereoisomers that differ in configuration about only one asymmetric carbon atom (e.g., D-galactose is the 4-epimer of D-glucose). The term has historical significance because the melting point of dinitrophenylhydrazone derivatives was a classical procedure used in identifying sugars. The 2-epimers (e.g., glucose and mannose, allose and altrose, etc.) gave identical dinitrophenylhydrazone derivatives.

Figure 1.2 Structures of the D-aldoses containing from 3 to 6 carbon atoms as depicted by the Rosanoff shorthand convention.

Figure 1.3 D- and L-glucose as depicted in the Fischer and conformational projections.

D- vs. L-Sugars

L-sugars are the mirror images of D-sugars. Figure 1.3 depicts the structures of D- and L-glucose in the Fischer and conformational projections. (Note: When drawing an L-sugar, the orientation of the hydroxyl groups on every asymmetric carbon atom need to be reversed; a frequent error is to only change the orientation on the reference carbon atom.) D- and L-glucose are enantiomers, nonsuperimposable stereoisosmeric molecules that are mirror images. L-sugars occur rarely in nature. A pair of enantiomers are identical in chemical reactivity, and they have the same taste properties. They are handled differently in biological systems, however. Although humans absorb L-sugars, L-sugars are not metabolized and thus have no caloric value.

Figure 1.4 Furanose and pyranose structures of β-D-glucose and β-D-fructose.

Figure 1.5 β-D-glucopyranose as depicted by the Haworth, Mills, and conformational conventions.

Figure 1.6 Basic chair and boat conformations for β-D-glucopyranose.

Different Ways of Depicting Sugar Structures

Fischer, Haworth, Mills, and Conformational Structures

It is important to realize that pentose and hexose sugars exist as ring forms, and only a small amount will occur in the acyclic form. Thus, the Fischer projections shown in Figure 1.2 are in no way representative of actual structural form. The Fischer projections are useful in that they provide a code for the orientation of hydrogen and hydroxyl substituents on adjacent carbon atoms. Sugars exist as both six-membered (pyranose) and five-membered (furanose) ring structures (Figure 1.4). The furanose ring is relatively planar, whereas the pyranose ring is "puckered," existing in chair and boat forms. Figure 1.5 illustrates β-D-glucopyranose by the Haworth, Mills, and conformational conventions. The Haworth representation is troublesome because it shows the pyranose ring as being planar. The Mills structure takes an aerial view with hydroxyl groups depicted as being either "upward" or "downward." The conformational representation shows perspective where the ring oxygen is remote, equatorial substitutents are drawn at an angle, and axial substituents are vertical. The conformational representation is highly preferred and will be used predominately throughout this book.

There are two stable chair forms and six stable boat forms for pyranose sugars; the forms for β-D-glucopyranose are shown and named in Figure 1.6. Sugar conformation is best visualized and demonstrated using molecular models. Relatively inexpensive and stereochemically accurate kits are available, and practice with them is highly recommended. Showing three-dimensional structure on the two-dimensional printed page has obvious limitations. Given the conformational structure of a pyranose sugar, one should be able to make the correct molecular model; and, vice versa, given a molecular model, one should be able to draw the correct conformational structure. An exercise is included in the Appendix for making molecular models of sugar molecules with molecular model kits.

Classifying Sugars by Compound Class—Hemiacetals, Hemiketals, Acetals, and Ketals

Aldehydes and ketones will condense with alcohols to form hemiacetals and hemiketals. In excess alcohol and dehydrating conditions, further condensation will take place to form acetals and ketals. A generalized reaction of an aldehyde with alcohol is shown in Figure 1.7, and the generalized structures of hemiacetals, hemiketals, acetals, and ketals are shown below.

Structure 1.1 Hemiacetal, Hemiketal, Acetal, Ketal

Pyranose and furanose ring forms of sugars are hemiacetals or hemiketals and are formed from intramolecular condensation of the carbonyl group with a hydroxyl substituent. When this reaction occurs, two products are formed, as the hydroxyl group on the anomeric carbon has two possible orientations, as illustrated by α- and β-D-glucopyranose. They have different chemical properties, with the β-form being more stable because of its equatorial disposition. They are known as anomers, diastereoisomers that differ in configuration only at C-1 for aldoses and C-2 for ketoses.

Figure 1.7 Reaction of an aldehyde with an alcohol to form a hemiacetal and an acetal.

Being able to recognize hemiacetal, hemiketal, acetal, and ketal functional groups is critical to carbohydrate chemistry. The chemical reactivity and functionality of different sugars is directly related to the presence of those functional groups. Figure 1.8 illustrates the location of the hemiacetal and hemiketal functional groups for β-D-glucopyranose and β-D-fructopyranose. To correctly recognize these functional groups, one should proceed as follows:

Figure 1.8 Location of the hemiacetal and hemiketal functional groups in β-D-glucopyranose and β-D-fructopyranose.

Locate the anomeric carbon atom. This will be C-1 for aldose sugars and C-2 for ketose sugars.The anomeric carbon atom will be to one side of the ring oxygen.The carbon on the other side of the ring oxygen may or may not be the reference carbon atom. The reference carbon atom is C-5 for hexoses and C-4 for pentose sugars. Once the anomeric carbon atom has been located, it helps to number all carbon atoms.

An exercise for identifying hemiacetal, hemiketal, acetal, and ketal functional groups in sugars is included in the Appendix.

Structure and Nomenclature of Disacchaarides

Disaccharides are either reducing or nonreducing, and a reducing disaccharide can be identified by the presence of a hemiacetal or hemiketal functional group. Disaccharides are formed from the reaction of an anomeric carbon atom with the hydroxyl group of another sugar. This acetal or ketal linkage is also referred to as a glycosidic linkage. The hydroxyl group reacting with the anomeric carbon may be an alcohol functional group of another sugar, or it could be the hydroxyl substituent located on the anomeric carbon. When the condensation is between two anomeric carbons, the compound will not contain a hemiacetal or hemiketal functional group, and it will be a nonreducing sugar. Reducing disaccharides are systematically named by having the nonreducing sugar moiety be a substituting group on the reducing sugar. The nature of the glycosidic linkage, whether α or β, the number of the carbon atom where the sugar is substituted, and the ring size all need to be indicated. Importantly, reducing disaccharides have an -ose suffix. For example, the systematic name for lactose is 4-O-β-D-galactopyranosyl-D-glucopyranose. Reducing disaccharides will have both α and β forms, the designation being for the orientation of the anomeric hydroxyl, not the glycosidic linkage. If the glycosidic linkage is changed from β to α, a different sugar is formed (e.g., the sugar will no longer be lactose). The structures along with systematic and trivial names for disaccharides that are important in foods are shown in Figure 1.9. Lactose is also known as milk sugar and has nutritional relevance because of lactose intolerance (inability to digest lactose) that is prevalent in some human populations.

Figure 1.9 Structures of five disaccharides of particular importance in foods: sucrose (β-D-fructofuranosyl-α-D-glucopyranoside or α-D-glucopyranosyl-β-D-fructofuranoside), lactose (4-O-β-D-galactopyranosyl-D-glucopyranose), maltose (4-O-α-D-glucopyranosyl-D-glucopyranose), cellobiose (4-O-β-D-glucopyranosyl-D-glucopyranose), and trehalose (α-D-glucopyranosyl-α-D-glucopyranoside).

Sucrose is a nonreducing sugar; hence, both anomeric carbons participate in the glycosidic linkage. There is no need to indicate which carbons are involved in systematic nomenclature, because it is known that both C-1 of glucose and C-2 of fructose are involved. Either sugar can be the substituting sugar, and the nature of both glycosidic linkages needs to be given. The suffix for nonreducing sugars is -ide. The correct name for sucrose is either α-D-glucopyranosyl-β-D-fructofuranoside or β-D-fructofuranosyl-α-D-glucopyranoside. The relevance of sucrose to food technology, nutrition, and world trade is monumental. Maltose (4-O-α-D-glucopyranosyl-D-glucopyranose) is the building unit of starch, whereas cellobiose (4-O-β-D-glucopyranosyl-D-glucopyranose) is the building unit for cellulose. Trehalose (α-D-glucopyranosyl-α-D-glucopyranoside) consists of two glucose molecules linked head-to-head and is nonreducing. It occurs in mushrooms and insects and is metabolized by humans. (This gives credence to the hypothesis that insects played an important role in man's diet from an evolutionary perspective.) The nonreducing and moderate sweetness properties of trehalose make possible some innovative food applications. The structures and names for an extensive list of known disaccharides are given in Unit 9 of the Appendix.

Table 1.2 Specific Rotation [α]D20 of Selected Pairs of D- and L-Sugars at Equilibrium

Structure and Optical Activity

Enantiomers were previously defined as nonsuperimposable stereoisomeric molecules that are mirror images. This is best demonstrated with molecular models by making a model of β-D-glucopyranose, for example, and then also creating its mirror image, β-L-glucopyranose. The two molecules can be visualized as mirror images by placing them head-to-head or, alternatively, placing one below the other. They are nonsuperimposable, meaning that they cannot be stacked with C-1 being over C-1, C-2 over C-2, etc. They are asymmetric, they have "handedness," and they are chiral. They have the property of being able to rotate plane-polarized light in equal amounts but in opposite directions. (Optical activity is the ability to rotate plane-polarized light.) The specific rotations of several D- and L-sugars are shown in Table 1.2. Note that some D-sugars (e.g., D-glucose and D-xylose) are dextrorotary, having the ability to rotate plane-polarized light to the "right" or in a clockwise direction, whereas other D-sugars (e.g., D-arabinose, D-ribose, D-gulose, and D-fructose) are levorotary, having the property of rotating plane-polarized light to the "left" or in a counter-clockwise direction. Thus, being a D- or L-sugar does not predict whether a sugar is dextrorotary or levorotary. The designation D and L should not be confused with d and l, which denotes having optical activity that is dextrorotary or levorotary. (The trivial names dextrose and levulose for glucose and fructose, respectively, have their origin from D-glucose being dextrorotary and D-fructose being levorotary.) The appendix contains instructions for demonstrating the existence of polarized light and the ability for sugar solutions to rotate it using plastic polarizing material, a light source such as a flashlight, and sugar solutions.

Anecdotal information

D- and L-glucose have identical physical and chemical properties (i.e., the same solubility, reactivity, and taste properties). L-Sugars are not used by humans, however, and are noncaloric. In 1981, Bioospherics Inc. of Beltsville, MD received a patent for use of several L-sugars as low-calorie sweeteners in foods (Levin and Zehner 1991). Their petition to the FDA for its use, however, was denied. The medical and nutritional communities supported the FDA's action, as the wisdom of having considerable quantities of reactive L-sugars circulating in the bloodstream from consumption of L-sugars was questionable.

Specific rotation is a physical property of sugars that is useful in identifying sugars and measuring their concentration. Measurements are taken with a polarimeter that has a source of plane-polarized light, and a prism that enables measurement of the degree that a sugar solution in a cell through which the light is being transmitted will be rotated. The equation for calculating specific rotation is shown below. α is the observed rotation in degrees, c is the concentration of the sugar solution in g/mL, l is the path length of the cell in dm, and D is the wavelength for the D line of Na, 589.3 nm.

Table 1.3 Specific Rotation [α]D20 of Selected Sugarsa

Sugar[α]D20 (Initial)[α]D20 (Final)α-D-Glucose+112°+52°β-D-Glucose+19°+52°α-D-Galactose+144°+80°β-D-Galactose+52°+80°α-D-Mannose+34°+15°β-D-Mannose−17°+15°β-D-Fructopyranose−133°−92°β-D-Fructofuranose−21°−92°α-D-Xylose+92°+19°β-D-Xylose−20°−19°α-D-Lactose+90°+55°β-D-Lactose+35°+55°α,α-Trehalose+197°+197°Sucrose+66.5°+66.5°Raffinose+124°+124°Stachyose+148°+148°Invert Sugar−20°α-L-Glucose−96°−52°Levoglucosan−67°−67°aSource: Shallenberger and Birch 1975; Shallenberger 1982.

(1.2)

Table 1.3