Handbook of Plant Food Phytochemicals E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1284

Veröffentlichungsjahr: 2013

Beschreibung

Handbook of Plant Food Phytochemicals

Phytochemicals are plant-derived chemicals which may bestow health benefits when consumed, whether medicinally or as part of a balanced diet. Given that plant foods are a major component of most diets worldwide, it is unsurprising that these foods represent the greatest source of phytochemicals for most people. Yet it is only relatively recently that due recognition has been given to the importance of phytochemicals in maintaining our health. New evidence for the role of specific plant food phytochemicals in protecting against the onset of diseases such as cancers and heart disease is continually being put forward. The increasing awareness of consumers of the link between diet and health has exponentially increased the number of scientific studies into the biological effects of these substances.

The Handbook of Plant Food Phytochemicals provides a comprehensive overview of the occurrence, significance and factors affecting phytochemicals in plant foods. A key objective of the book is to critically evaluate these aspects. Evaluation of the evidence for and against the quantifiable health benefits being imparted is expressed in terms of the reduction in the risk of disease conferred through the consumption of foods that are rich in phytochemicals.

With world-leading editors and contributors, the Handbook of Plant Food Phytochemicals is an invaluable, cutting-edge resource for food scientists, nutritionists and plant biochemists. It covers the processing techniques aimed at the production of phytochemical-rich foods which can have a role in disease prevention, making it ideal for both the food industry and those who are researching the health benefits of particular foods. Lecturers and advanced students will find it a helpful and readable guide to a constantly expanding subject area.

Leseprobe

Contents

Contributor list

1 Plant food phytochemicals

1.1 Importance of phytochemicals

1.2 Book objective

1.3 Book structure

Part I Chemistry and Health

2 Chemistry and classification of phytochemicals

2.1 Introduction

2.2 Classification of phytochemicals

2.3 Chemical properties of phytochemicals

2.4 Biochemical pathways of important phytochemicals

3 Phytochemicals and health

3.1 Introduction

3.2 Bioavailability of phytochemicals

3.3 Phytochemicals and their health-promoting effects

3.4 General conclusions

4 Pharmacology of phytochemicals

4.1 Introduction

4.2 Medicinal properties of phytochemicals

4.3 Phytochemicals and disease prevention

4.4 Phytochemicals and cardiovascular disease

4.5 Phytochemicals and cancer

4.6 Summary and conclusions

Part II Sources of Phytochemicals

5 Fruit and vegetables

5.1 Introduction

5.2 Polyphenols

5.3 Carotenoids

5.4 Glucosinolates

5.5 Glycoalkaloids

5.6 Polyacetylenes

5.7 Sesquiterpene lactones

5.8 Coumarins

5.9 Terpenoids

5.10 Betalains

5.11 Vitamin E or tocols content in fruit and vegetables

5.12 Conclusions

6 Food grains

6.1 Introduction

6.2 Phytochemicals in cereal grains

6.3 Phytochemicals in legume grains

6.4 Stability of phytochemicals during processing

6.5 Food applications and impact on health

6.6 Cereal-based functional foods

6.7 Legume-based functional foods

7 Plantation crops and tree nuts: composition, phytochemicals and health benefi ts

7.1 Introduction

7.2 Composition

7.3 Phytochemicals content

7.4 Health benefits

8 Food processing by-products

8.1 Introduction

8.2 Phytochemicals from food by-products

8.3 By-products from fruit and vegetables

8.4 Tuber crops and cereals

8.5 Extraction of bioactive compounds from plant food by-products

8.6 Future trends

Part III Impact of Processing on Phytochemicals

9 On farm and fresh produce management

9.1 Introduction

9.2 Pre-harvest factors affecting phytochemical content

9.3 Harvest and post-harvest management practices

9.4 Future prospects

10 Minimal processing of leafy vegetables

10.1 Introduction

10.2 Minimally processed products

10.3 Cutting and shredding

10.4 Wounding physiology

10.5 Browning in lettuce leaves

10.6 Refrigerated storage

10.7 Modified atmosphere storage

10.8 Conclusions

11 Thermal processing

11.1 Introduction

11.2 Blanching

11.3 Sous vide processing

11.4 Pasteurisation

11.5 Sterilisation

11.6 Frying

11.7 Conclusion

12 Effect of novel thermal processing on phytochemicals

12.1 Introduction

12.2 An overview of different processing methods for fruits and vegetables

12.3 Novel thermal processing methods

12.4 Effect of novel processing methods on phytochemicals

12.5 Challenges and prospects/future outlook

12.6 Conclusion

13 Non thermal processing

13.1 Introduction

13.2 Irradiation

13.3 High pressure processing

13.4 Pulsed electric field

13.5 Ozone processing

13.6 Ultrasound processing

13.7 Supercritical carbon dioxide

13.8 Conclusions

Part IV Stability of Phytochemicals

14 Stability of phytochemicals during grain processing

14.1 Introduction

14.2 Germination

14.3 Milling

14.4 Fermentation

14.5 Baking

14.6 Roasting

14.7 Extrusion cooking

14.8 Parboiling

14.9 Conclusions

15 Factors affecting phytochemical stability

15.1 Introduction

15.2 Effect of pH

15.3 Concentration

15.4 Processing

15.5 Enzymes

15.6 Structure

15.7 Copigments

15.8 Matrix

15.9 Storage conditions

15.10 Conclusion

16 Stability of phytochemicals at the point of sale

16.1 Introduction

16.2 Stability of phytochemicals during storage

16.3 Food application and stability of phytochemicals

16.4 Edible coatings for enhancement of phytochemical stability

16.5 Modified atmosphere storage for enhanced phytochemical stability

16.6 Bioactive packaging and micro encapsulation for enhanced phytochemical stability

16.7 Conclusions

Part V Analysis and Application

17 Conventional extraction techniques for phytochemicals

17.1 Introduction

17.2 Theory and principles of extraction

17.3 Examples of conventional techniques

17.4 Conclusion

18 Novel extraction techniques for phytochemicals

18.1 Introduction

18.2 Pressurised solvents

18.3 Enzyme assisted extraction

18.4 Non-thermal processing assisted extraction

18.5 Challenges and future of novel extraction techniques

19 Analytical techniques for phytochemicals

19.1 Introduction

19.2 Sample preparation

19.3 Non-chromatographic spectrophotometric methods

19.4 Chromatographic methods

20 Antioxidant a ctivity of phytoche micals

20.1 Introduction

20.2 Measurement of antioxidant activity

20.3 Concluding remarks

21 Industrial applications of phytochemicals

21.1 Introduction

21.2 Phytochemicals as food additives

21.3 Stabilisation of fats, frying oils and fried products

21.4 Stabilisation and development of other food products

21.5 Nutracetical applications

21.6 Miscellaneous industrial applications

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Index

This edition first published 2013 © 2013 by John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Handbook of plant food phytochemicals : sources, stability and extraction / edited by Brijesh Tiwari, Nigel Brunton, Charles S. Brennan. p. cm.Includes bibliographical references and index.

ISBN 978-1-4443-3810-2 (hardback : alk. paper) – ISBN 978-1-118-46467-0 (epdf) – ISBN 978-1-118-46469-4 (emobi) – ISBN 978-1-118-46468-7 (epub) – ISBN 978-1-118-46471-7 (obook) 1. Phytochemicals. 2. Plants–Composition. 3. Food–Composition. 4. Food industry and trade. I. Tiwari, Brijesh K. II. Brunton, Nigel. III. Brennan, Charles S.QK861.H34 2012580–dc23

2012024779

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

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Cover image credits, left to right: © iStockphoto.com/KevinDyer; © iStockphoto.com/aluxum; © iStockphoto.com/FotografiaBasicaCover design by Meaden Creative

Contributor list

Editors

B.K. TiwariFood and Consumer Technology, Manchester Metropolitan University, Manchester, UKNigel P. BruntonSchool of Agriculture and Food Science, University College Dublin, Dublin, IrelandCharles S. BrennanFaculty of Agriculture and Life Sciences, Lincoln University, Lincoln, Canterbury, New Zealand

Contributors

El-Sayed M. Abdel-AalGuelph Food Research Centre,Agriculture and Agri-Food Canada,Guelph, Ontario, CanadaLaura Alvarez-JubeteSchool of Food Science and Environmental Health,Dublin Institute of Technology,Dublin, IrelandAnil Kumar AnalFood Engineering and Bioprocess Technology,Asian Institute of Technology,Klongluang, ThailandFazilah AriffinFood Biopolymer Research Group,Food Technology Division,School of Industrial Technology,Universiti Sains Malaysia,Penang, MalaysiaRajeev BhatFood Biopolymer Research Group,Food Technology Division,School of Industrial Technology,University Sains Malaysia,Penang, MalaysiaPeter Bongers*Structured Materials and Process Science,Unilever Research and Development Vlaardingen,The NetherlandsCharles S. BrennanFaculty of Agriculture and Life Sciences,Lincoln University, Lincoln,Canterbury, New ZealandNigel P. BruntonSchool of Agriculture and Food Science,University College Dublin, Dublin, IrelandRocio Campos-VegaKellogg Company Km,Campo Militar,Querétaro, MéxicoHelena Soares CostaNational Institute of Health Dr Ricardo Jorge, Food and Nutrition Department, Lisbon, PortugalPJ CullenSchool of Food Science and Environmental Health,Dublin Institute of Technology,Dublin, IrelandEnda CumminsUCD School of BiosystemsEngineering,Agriculture and Food Science Centre,Dublin, IrelandTamer GamelGuelph Food Research Centre,Agriculture and Agri-Food Canada,Guelph, Ontario, CanadaNiamh HarbourneFood and Nutritional Sciences,University of Reading,Reading, UKXiangjiu HeSchool of Pharmaceutical Sciences,Wuhan University,Wuhan, Hubei, ChinaJean Christophe JacquierSchool of Agriculture and Food ScienceUniversity College Dublin,Dublin, IrelandIan T. JohnsonInstitute of Food Research,Norwich Research Park, Colney,Norwich, UKRod JonesDepartment of Primary Industries,Victoria, AustraliaAlias A. KarimFood Biopolymer Research Group,Food Technology Division,School of Industrial Technology,Universiti Sains Malaysia,Penang, MalaysiaAmritpal KaurDepartment of Food Science and Technology,Guru Nanak Dev University,Amritsar, IndiaBhupinder KaurFood Biopolymer Research Group,Food Technology Division,School of Industrial Technology,Universiti Sains Malaysia,Penang, MalaysiaHongyan LiGuelph Food Research Centre,Agriculture and Agri-Food Canada,Guelph, Ontario, CanadaEunice MareteSchool of Agriculture and Food ScienceUniversity College Dublin,Dublin, IrelandJosé M. MatésDepartment of Molecular Biology and Biochemistry,Faculty of Sciences, Campus de Teatinos,University of Málaga, SpainPradeep Singh NegiHuman Resource Development,Central Food Technological Research Institute (CSIR),Mysore, IndiaColm P. O’DonnellUCD School of Biosystems Engineering,University College DublinBelfield, Dublin, IrelandB. Dave OomahPacific Agri-Food Research Centre,Agriculture and Agri-Food Canada, Summerland,British Columbia, CanadaDolores O’RiordanSchool of Agriculture and Food ScienceUniversity College Dublin,Dublin, IrelandAnkit PatrasDepartment of Food Science,University of Guelph, GuelphOntario, CanadaSanaa RagaeeDepartment of Food Science,University of Guelph, Guelph,Ontario, CanadaKim ReillyHorticulture Development Unit,Teagasc, Kinsealy Research Centre,Dublin, IrelandAna Sanches-SilvaNational Institute of Health Dr Ricardo Jorge, Food and Nutrition Department, Lisbon, PortugalKoushik SeethramanDepartment of Food Science,University of Guelph,Guelph, Ontario, CanadaNarpinder SinghDepartment of Food Science and Technology,Guru Nanak Dev University,Amritsar, IndiaB.K. TiwariFood and Consumer Technology,Manchester Metropolitan University,Manchester, UKUma TiwariUCD School of Biosystems Engineering,Agriculture and Food Science Centre,Dublin, IrelandBruce TomkinsDepartment of Primary Industries,Victoria, Ferntree Gully,DC, AustraliaOlivera TrifunovicStructured Materials and Process Science,Unilever Research and Development Vlaardingen,The NetherlandsRong TsaoGuelph Food Research Centre,Agriculture and Agri-Food Canada,Guelph, Ontario, CanadaJuan ValverdeTeagasc Food Research Centre Ashtown, Dublin, IrelandHilde H. WijngaardDutch Separation Technology Institute,Amersfoort, The NetherlandsJun YangFrito-Lay North America R&D,PepsiCo Inc.,Plano, TX, USAYvonne V. YuanSchool of Nutrition,Ryerson University,Toronto, Ontario, CanadaDongjun ZhaoDepartment of Food Science,Cornell University, Ithaca, NY, USA

1Plant food phytochemicals

B.K. Tiwari, Nigel P. Brunton and Charles S. Brennan

1.1 Importance of phytochemicals

Type the word ‘phytochemical’ into any online search engine and it will return literally thousands of hits. This is a reflection of the role plant derived chemicals have played in medicine and other areas since humans have looked to nature to provide cures for various ailments and diseases. While it is often stated, it is worth repeating that the evolution of modern medicine derived from applying scientific principles to herbalism and to this day plants derived compounds provide the skeletons for constructing molecules with the abilities to cure many diseases. In recent times applications of phytochemicals have extended into other areas especially nutraceuticals and functional foods. The focus here is not on ­curing existing conditions but delaying the onset of new ones and it is not surprising to note that plant foods and plant derived components make up the vast majority of compounds with European Food Safety Authority validated Article 13.1 health claims. Whilst there has been a renewed interest in the use of medicinal plants to treat diseases in recent times and the use of phytochemicals as pharmaceuticals is covered in the present book, this is not the core theme of the book. Given that plant foods are still a major component of most diets worldwide the greatest significance of phytochemicals derives from their role in human diets and health. In fact it is only in relatively recent times that due recognition has been given to the importance of phytochemicals in maintaining health. This has driven a huge volume of work on the subject ranging from unravelling mechanisms of biological significance to discovery and stability studies.

An overview of the health benefits of phytochemicals is essential as many phytochemicals have been reported to illicit both positive and negative biological effects. In recent times some evidence for the role of specific plant food phytochemicals in protecting against the onset of diseases such as cancers and heart diseases has been put forward. Most researchers in this field will however agree that in most cases more evidence is needed to prove the case for the ability of phytochemicals to delay the onset of these diseases. Nevertheless the increasing awareness of consumers of the link between diet and health has exponentially increased the number of scientific studies into the biological effects of these substances.

1.2 Book objective

The overarching objective, therefore, of the Handbook of Plant Food Phytochemicals is to provide a bird’s eye view of the occurrence, significance and factors affecting phytochemicals in plant foods. A key of objective of the handbook is to critically evaluate some of these with a ­particular emphasis on evidence for or against quantifiable beneficial health effects being imparted via a reduction in disease risk through the consumption of foods rich in phytochemicals.

1.3 Book structure

The book is divided into five parts. Part I deals with the health benefits and chemistry of phytochemicals, Part II summarises phytochemicals in various food types, Parts III and IV deal with a variety of factors that can affect phytochemical content and stability and Part V deals with a range of analytical techniques and applications of phytochemicals. The subject of the biological activity of phytochemicals is approached both from a disease risk reduction perspective in Chapter 3 and from a more traditional pharmacological viewpoint in Chapter 4. Together these chapters are intended to give the reader a sound basis for understanding the biological significance of these substances and to contextualise their roles either as a medicinal plant or as a nutraceutical/functional food. Key to understanding both the stability and biological role of phytochemicals is a sound knowledge of their chemistry and biochemical origin. This often neglected topic is covered in detail here along with an overview of the classification of these compounds. This reflects the ambition of the book to serve as a reference text for students in the field and is intended to provide a basis for understanding some of the complex subjects covered in earlier chapters.

The chemical diversity and number of plant food phytochemicals with reported abilities to protect against diseases numbers in the many thousands. Therefore, to cover all these substances in detail would be impossible. However, myself and my fellow editors felt that providing readers with a reference manuscript for plant food phytochemicals and a basic understanding of the types of phytochemicals in plant foods was essential. Part II of the handbook covers this subject matter by giving an overview of the phytochemicals present in four food categories – fruit and vegetables, food grains, natural products and tree nuts and food processing by-products. Fruits and vegetables are perhaps the best recognised source of phytochemicals and this is reflected in the depth and volume of literature on this food type. Chapter 5 summarises information on major phytochemicals groups in fruits and vegetables as well as some of the more obscure and recently emerged groups. From a consumption perspective food grains form a huge proportion of most diets worldwide – however, due recognition of grains as sources of phytochemicals has only emerged relatively recently. Chapter 6 summarises the phytochemical composition of both cereals and legumes and underlines the importance of this food group as a source of phytochemicals in human diets. Early humans were of course hunter gatherers and nuts would have been important of their diets. It is therefore perhaps not surprising that tree nuts and other natural products have been shown to contain a range of phytochemicals with the potential to deliver benefits beyond basic nutrition. The importance of tree nuts as sources of these compounds is hence covered in detail in Chapter 7 along with related food types such as plantation products. Whilst a core objective of the handbook is to cover the breadth of subject matter in phytochemicals from plant foods this is not merely an academic exercise. Phytochemicals have real commercial uses and this is given due recognition in Chapters 7 and 8 where an overview of the application of phytochemicals derived from foods grains and trees is given. In fact throughout the handbook authors provide detailed information and examples of real applications of plant food derived phytochemicals with a view to underlining the commercial importance of these compounds. Food processing by-products do not of course constitute a food group – however, they have become hugely important sources of phytochemicals in recent times and Chapter 8 is dedicated to revealing the potential of food processing ­by-products as sources of phytochemicals with real commercial potential. Recovering value from by-products is of course hugely significant to food processors as they seek to maximise the value of a resource that hitherto was considered a waste. This also reflects the drive to identify more sustainable food processing practices and increasing pressures from regulators to reduce waste.

As with most other foods, plant foods are often not consumed in their native form. Therefore, investigators have long been interested in developing an understanding of how processing effects phytochemical composition with view to maximising their potential health promoting properties. Today’s consumers are demanding foods that are healthy, convenient and appetising. The drive for healthy foods has fuelled interest in the effect of processing on the level of components responsible for imparting this benefit, especially phytochemicals. Therefore, much work has been devoted to assessing the effect of processing and storage on levels of potentially important phytochemicals in foods. In addition, a number of novel thermal and non-thermal technologies designed to achieve microbial safety, while minimising the effects on its nutritional and quality attributes, have recently become available. Minimising changes in phytochemicals during processing is a considerable challenge for food processors and technologists. Thus, there is a requirement for detailed industrially relevant information concerning phytochemicals and their application in food products. In addition, industrial adoption of novel processing techniques is in its infancy. Applications of new and innovative technologies and resulting effects on those food products either individually or in combination are always of great interest to academic, industrial, nutrition and health professionals. Part III gives an oversight as to how processing affects phytochemicals in plant foods. This is an area that has received huge attention recently and this has reflected the number of chapters dedicated to it in the handbook. This part of the handbook also summarises and evaluates an area that is often neglected when in the phytochemicals arena but can have profound impact on final phytochemical content, namely on farm and fresh produce management. Given the investment and scale of research required to carry out replicated field trials elucidating the impact of pre-harvest factors, such as fertiliser application, light, temperature, biotic and abiotic stress, this area has perhaps been the most challenging of any of the ‘farm to fork’ factors involved in determining the phytochemical content of plant foods. Indeed assessing the relative effects of intensive and organic farming practices is a highly controversial area but one that consumers appear to take an active interest in given the premium demand for organically produced plant foods. Post-harvest management pertains to the period between harvesting of the plant food and its arrival at the processing plant. This covers many operations including mechanical harvesting, storage and transport. Unsurprisingly many of these operations constitute a stress to the still respiring plant food and thus can activate or deactivate pathways leading to the synthesis of phytochemicals. Ready to eat fruit and vegetables are a relatively recent phenomenon on supermarket shelves. Their emergence is a reflection of consumers’ busy lifestyles and the need to provide healthy and convenient solutions for time poor customers who desire a healthy diet. Products of this nature are often referred to as minimally processed and are subjected to a variety of operations ranging from peeling and cutting to washing. Unlike plant foods, which have been subjected to heat processing, minimally processed products remain viable, albeit in many cases in a wounded state. Therefore a wide variety of responses to minimal processing have been reported and these are summarised and evaluated in Chapter 10, with a particular emphasis on salad mixes. A huge spectrum of full processing techniques is available to food processors nowadays. These range from severe (canning) to mild (sous-vide processing) to non-thermal examples such as high pressure processing, ultrasound and irradiation. Not surprisingly these can have a range of effects on phytochemical content and Chapters 11, 12 and 13 summarise the work done to date on these processes. Grains and pulses undergo a distinctly different processing route to other plant foods involving germination, milling, fermentation and finally baking. Therefore we have dedicated a standalone chapter to food grains, which reviews reports on the grain ­processing techniques on the content of phytochemicals. Finally, in tune with the farm to fork approach adopted by the handbook, the last chapter in Part III reviews the stability of foods containing phytochemicals during storage after processing. Like most chemical constituents the nature of the matrix they are contained in has a profound effect on their stability. Therefore, in Chapter 15 the stability of phytochemicals with different properties such as low moisture contents, ethnic foods and of course traditional foods is reviewed.

The final part of the book deals with perhaps the first question a researcher must ask him/herself when entering the field namely how do we extract these compounds and how do we measure them. The chapter on extraction is particularly relevant as this is an important ­consideration not only when analysing these compounds but also when preparing to include them as an ingredient in another food. Phytochemical analysis techniques are advancing at an exponential rate and therefore a chapter reviewing the state of the art in this discipline was one of the first we put on paper when deciding on the content of the book. Finally, the reason we have dedicated a book to the subject of phytochemicals in plant foods is because they have very real applications in industry and everyday life. The final chapter of the handbook drives this point home by providing real examples of industrial uses for phytochemicals ranging from maintaining stability in oxidatively labile foods to enhancing the health promoting properties of others. To conclude we hope you find the proceeding chapters to be informative, clear, concise and that they provide a clear thinking perspective on a subject matter that has benefitted mankind from many perspectives and will no doubt continue to do so into the future.

Part I

Chemistry and Health

2 Chemistry and classification of phytochemicals

Rocio Campos-Vega1 and B. Dave Oomah2

1 Kellogg Company Km. Querétaro, Qro. México2 Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Summerland,British Columbia, Canada

2.1 Introduction

The word ‘biodiversity’ is on nearly everyone’s lips these days, but ‘chemodiversity’ is just as much a characteristic of life on Earth as biodiversity. Living organisms produce several thousands of different structures of low-molecular-weight organic compounds. Many of these have no apparent function in the basic processes of growth and development, and have been historically referred to as natural products or secondary metabolites. The ­importance of natural products in medicine, agriculture and industry has led to numerous studies on the synthesis, biosynthesis and biological activities of these substances. Yet we still know ­comparatively little about their actual roles in nature.

Clearly such research has been stimulated by scientific curiosity in the substances and mechanisms involved in the protective effects of fruits and vegetables. Dietary ­phytonutrients appear to lower the risk of cancer and cardiovascular disease. Studies on the mechanisms of chemoprotection have focused on the biological activity of plant-based phenols and ­polyphenols, flavonoids, isoflavones, terpenes, and glucosinolates. However, most, if not all, of these bioactive compounds are bitter, acrid, or astringent and therefore aversive to the consumer. Some have long been viewed as plant-based toxins. The analysis of ­phytochemicals is complicated due to the wide variation even within the same group of compounds, and the metabolic degradation or transformation that may occur during crushing or processing of plants (e.g. for Allium and Brassica compounds), thus increasing the complexity of the ­mixture. Many phytochemical analyses require mass spectroscopy and therefore are ­time-consuming and expensive. Furthermore, some compounds tend to bind to macromolecules, making quantitative extraction difficult. Furthermore, many plant food ­phytochemicals that are poorly absorbed by humans usually undergo metabolism and rapid excretion. It is clear from in vitro and animal data that the actions of some phytochemicals are likely to be achieved only at doses much higher than those present in edible plant foods. Thus, extraction or synthesis of the active ingredient is essential if they are to be of prophylactic or ­therapeutic value in human subjects.

Figure 2.1 Classification of phytochemicals.

2.2 Classification of phytochemicals

Many phytochemicals have a range of different biochemical and physiological effects, ­isoflavonoids, for example have antioxidant and anti-oestrogenic activities. These ­activities may require different plasma or tissue concentrations for optimum effects. A diagram ­illustrating the classification of the phytochemicals covered in this chapter is shown in Figure 2.1.

In addition, plants contain mixtures of phytochemicals (Table 2.1), with considerable opportunity for interaction (Rowland et al., 1999). Plant secondary metabolites are an ­enormously variable group of phytochemicals in terms of their number, structural heterogeneity, and distribution.

A summary of the main groups of bioactive chemicals in edible plants, their sources, and their biological activities is presented in Table 2.2 (Rowland et al., 1999).

2.2.1 Terpenes

The term terpenes originates from turpentine (balsamum terebinthinae). Turpentine, the ­so-called “resin of pine trees”, is the viscous pleasantly smelling balsam that flows upon cutting or carving the bark and the new wood of several pine tree species (Pinaceae). Turpentine contains the “resin acids” and some hydrocarbons, which were originally referred to as terpenes. Traditionally, all natural compounds built up from isoprene subunits and, for the most part, originating from plants are denoted as terpenes (Breitmaier, 2006).

All living organisms manufacture terpenes for certain essential physiological ­functions and therefore have the potential to produce terpene natural products. Given the many ways in which the basic C5 units can be combined together and the different selection pressures under which organisms have evolved, it is not surprising to observe the enormous number and diversity of structures elaborated (Gershenzon and Dudareva, 2007). Terpenes (also known as terpenoids or isoprenoids) are the largest group of natural products comprising ­approximately 36 000 terpene structures (Buckingham, 2007), but very few have been investigated from a functional perspective (Figure 2.2).

Table 2.1 Phytochemical content of some edible plants (modified from Caragay, 1992; Rowland et al., 1999)

Table 2.2 Sources and biological activities of phytochemicals (adapted from Rowland et al., 1999)

Figure 2.2 Examples of terpenes with established functions in nature (adapted from Gershenzon and Dudareva, 2007).

The classification of terpenoids is based on the number of isoprenoid units present in their structure. The largest categories consist of compounds with two (monoterpenes), three (­sesquiterpenes), four (diterpenes), five (sesterterpenes), six (triterpenes), and eight (tetraterpenes) isoprenoid units (see Figure 2.3) (Ashour et al., 2010).

Terpenoids have well-established roles in almost all basic plant processes, including growth, development, reproduction, and defence (Wink and van Wyk, 2008). Gibberellins, a large group of diterpene plant hormones involved in the control of seed germination, stem elongation, and flower induction (Thomas et al., 2005) are among the best-known lower (C5–C20) terpenes. Another terpenoid hormone, abscisic acid (ABA), is not ­properly ­considered a lower terpenoid, since it is formed from the oxidative cleavage of a C40 ­carotenoid precursor (Schwartz et al., 1997).

Figure 2.3 Parent hydrocarbons of terpenes (isoprenoids) (modified from Breitmaier, 2006).

2.2.2 Polyphenols

Polyphenols, secondary plant metabolites are the most abundant antioxidants in human diets. These compounds are designed with an aromatic ring carrying one or more hydroxyl moieties. Several classes can be considered according to the number of phenol rings and to the structural elements that bind these rings. In this context, two main groups of polyphenols, termed flavonoids and nonflavonoids, have been traditionally adopted. As seen in Figures 2.4 and 2.5, the flavonoid group comprises compounds with a C6-C3-C6 structure: flavanones, flavones, dihydroflavonols, flavonols, flavan-3-ols, anthocyanidins, ­isoflavones, and proanthocyanidins. The nonflavonoids group is classified according to the number of carbons and comprises the following subgroups: simple phenols, benzoic acids, ­hydrolyzable tannins, acetophenones and phenylacetic acids, cinnamic acids, coumarins, ­benzophenones, xanthones, stilbenes, chalcones, lignans, and secoiridoids (Andrés-Lacueva et al ., 2010).

Figure 2.4 Chemical structures of the main classes of polyphenols (adapted from Scalbert and Williamson, 2000).

Figure 2.5 Chemical structures of some representative flavonoids (adapted from Tapas et al., 2008).

2.2.3 Carotenoids

Carotenoids are fat-soluble natural pigments with antioxidant properties (Krinsky and Yeum, 2003), with various other additional physiological functions, such as immunostimulation (McGraw and Ardia, 2003). The more than 600 known carotenoids are generally classified as xanthophylls (containing oxygen) or carotenes (purely hydrocarbons with no oxygen). Carotenoids in general absorb blue light and serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and protect chlorophyll from ­photodamage (Armstrong and Hearst, 1996). In humans, four carotenoids ( α -, β -, and γ -carotene, and β -cryptoxanthin) have vitamin A activity (i.e. can be converted to retinal), and these and other carotenoids can also act as antioxidants (Figure 2.6). In the eye, certain other carotenoids (lutein and zeaxanthin) apparently act directly to absorb damaging blue and near-ultraviolet light, in order to protect the macula lutea. People consuming diets rich in carotenoids from natural foods, such as fruits and vegetables, are healthier and have lower mortality from a number of chronic illnesses (Diplock et al., 1998).

2.2.4 Glucosinolates

Glucosinolates (GLS), a group of plant thioglucosides found among several vegetables (Larsen, 1981), are a class of organic compounds containing sulfur and nitrogen and are derived from glucose and an amino acid (Anastas and Warner, 1998). Over 100 different GLS have been characterized since the first crystalline glucosinolate, sinalbin, was isolated from the seeds of white mustard in 1831. GLS occur mainly in the order Capparales, ­principally in the Cruciferae, Resedaceae, and Capparidaceae families, although their ­presence in other families has also been reported (Larsen, 1981). Some economically ­important GLS ­containing plants are white mustard, brown mustard, radish, horse radish, cress, kohlrabi, cabbages (red, white, and savoy), brussel sprouts, cauliflower, broccoli, kale, turnip, swede, and rapeseed (Fenwick et al., 1989).

Figure 2.6 Some examples of carotenoids ((a) adapted from Sliwka et al., 2010; (b) adapted from Yahia et al., 2010)).

GLS hydrolysis and metabolic products have proven chemoprotective properties against chemical carcinogens by blocking the initiation of tumours in various tissues, for example, liver, colon, mammary gland, and pancreas. They exhibit their effect by inducing Phase I and II enzymes, inhibiting the enzyme activation, modifying the steroid hormone metabolism and protecting against oxidative damage. GLS facilitate detoxificiation of carcinogens by ­inducing Phase I and Phase II enzymes. Some enzymes of Phase I reaction that activate the carcinogens, are selectively inhibited by glucosinolate metabolites (Das et al., 2000).

2.2.5 Dietary fiber (non starch polysaccharides)

Dietary fiber is the edible parts or analogous carbohydrates resistant to digestion and ­absorption in the small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant ­substances. Dietary fibers promote beneficial physiological effects including laxation, and/ or blood cholesterol attenuation, and/or blood glucose attenuation (AACC, 2001) (Table 2.3). Dietary fibers are polymers of monosaccharides joined through glycosidic linkages and are defined and classified in terms of the following structural considerations: (a) identity of the monosaccharides present; (b) monosaccharide ring forms (six-membered pyranose or five-membered furanose); (c) positions of the glycosidic linkages; (d) configurations (a or b) of the glycosidic linkages; (e) sequence of monosaccharide residues in the chain, and (f) presence or absence of non-carbohydrate substituents. Monosaccharides commonly ­present in cereal cell walls are: (a) hexoses – D-glucose, D-galactose, D-mannose; (b) ­pentoses – L-arabinose, D-xylose; and (c) acidic sugars – D-galacturonic acid, D-glucuronic acid and its 4-O-methyl ether (Choct, 1997).

Table 2.3 Constituents of dietary fiber according to the definition of the American Association of Cereal Chemists (adapted from Jones, 2000)

Non starch polysaccharides (NSP) and resistant

    Cellulose    Hemicellulose    Arabinoxylans    Arabinogalactans    Polyfructose    Inulin    Oligofructans    Galacto-oligosaccharides    Gums    Mucilages    Pectins

Analogous carbohydrates

    Indigestible dextrins    Resistant maltodextrins (from maize and others sources)    Resistant potato dextrins    Synthesized carbohydrate compounds    Polydextrose    Methyl cellulose    Hydroxypropylmethyl cellulose    Indigestible (“resistant”) starches

Lignin substances associated with the NSP and lignin complex in

    Plants    Waxes    Phytate    Cutin    Saponins    Suberin    Tannins

According to Cummings (1997), the health benefits of DF do not provide a distinct ­disease-related characteristic that can be exclusively associated with it. Constipation comes closest to fulfilling such a criterion and it is clear that some functional and physiological effects have been demonstrated with some specific fibers: (a) faecal bulking or stool output (ispaghula, xanthan gum, and wheat bran); (b) lowering of postprandial blood glucose response (highly viscous guar gum or β -glucans); (c) lowering of plasma (LDL-) cholesterol (highly viscous guar gum, β -glucans or oat bran, pectins, psyllium). Other effects have not yet been demonstrated in human subjects, such as colonic health effects related to ­fermentation products, although a substantial body of evidence is available from in vitro or animal models (Champ et al., 2003 and references therein).

2.2.6 Lectins

Lectins (from lectus, the past participle of legere, to select or choose) are defined as ­carbohydrate binding proteins other than enzymes or antibodies and exist in most living organisms, ranging from viruses and bacteria to plants and animals. Some examples are given in Table 2.4. Their involvement in diverse biological processes in many species, such as clearance of glycoproteins from the circulatory system, adhesion of infectious agents to host cells, recruitment of leukocytes to inflammatory sites, cell interactions in the immune system, in malignancy and metastasis, has been shown (Ambrosi et al., 2005 and references therein).

Table 2.4 Examples of lectins, the families to which they belong and their glycan ligand specificities (modified from Ambrosi et al., 2005 and references therein)

Lectin name

Family

Glycan ligands

Plant lectins

Concanavalin A (Con A; jack bean)

Leguminosae

Man/Glc

Wheat germ agglutinin (WGA; wheat)

Gramineae

(GlcNAc)1–3, Neu5Ac

Ricin (castor bean)

Euphorbiaceae

Gal

Phaseolus vulgaris

(PHA; French bean)

Leguminosae

Unknown

Peanut agglutinin (PNA; peanut)

Leguminosae

Gal, Galb3GalNAca (T-antigen)

Soybean agglutinin (SBA; soybean)

Leguminosae

Gal/GalNAc

Pisum sativum

(PSA; pea)

Leguminosae

Man/Glc

Lens culinaris

(LCA; lentil)

Leguminosae

Man/Glc

Galanthus nivalus

(GNA; snowdrop)

Amaryllidaceae

Man

Dolichos bifloris

(DBA; horse gram)

Leguminosae

GalNAca3GalNAc, GalNAc

Solanum tuberosum

(STA; potato)

Solanaceae

(GlcNAc)n

2.2.7 Other phytochemicals

2.2.7.1 Alkaloids

The term “alkaloid” was coined by the German pharmacist Carl Friedrich Wilhelm Meissner in 1819 to refer to plant natural products (the only organic compounds known at that time) showing basic properties similar to those of the inorganic alkalis (Friedrich and Von, 1998) The ending “-oid” (from the Greek eidv, appear) is still used today to suggest similarity of structure or activity, as is evident in names of more modern vintage such as terpenoid, ­peptoid, or vanilloid (Hesse, 2002).

Among the secondary metabolites that are produced by plants, alkaloids figure as a very prominent class of defense compounds. Over 21 000 alkaloids have been identified, which thus constitute the largest group among the nitrogen-containing secondary ­metabolites (besides 700 nonprotein amino acids, 100 amines, 60 cyanogenic glycosides, 100 ­glucosinolates, and 150 alkylamides) (Roberts and Wink, 1998; Wink, 1993). An alkaloid never occurs alone; alkaloids are usually present as a mixture of a few major and several minor alkaloids of a particular biosynthetic unit, which differ in functional groups (Wink, 2005).

2.2.7.2 Polyacetylenes

Polyacetylenes are examples of bioactive secondary metabolites that were previously ­considered undesirable in plant foods due to their toxicity (Czepa and Hofmann, 2004) (Figure 2.7). However, a low daily intake of these “toxins” may be an important factor in the search for an explanation of the beneficial effects of fruit and vegetables on human health. For example, polyacetylenes isolated from carrots have been found to be highly cytotoxic against numerous cancer cell lines. Over 1400 different polyacetylenes and related compounds have been isolated from higher plants.

Figure 2.7 Polyacetylenes structure (a) Falcarinol (FaOH), (b) Falcarindiol (FaDOH), (c) Falcarindiol 3-acetate (FaDOAc).

Aliphatic C17-polyacetylenes of the falcarinol type such as falcarinol and falcarindiol (Figure 2.7), are widely distributed in the Apiaceae and Araliaceae (Bohlmann et al., 1973; Hansen and Boll, 1986), and consequently nearly all polyacetylenes found in the utilized/edible parts of food plants of the Apiaceae, such as carrot, celeriac, parsnip, and parsley are of the falcarinol-type. Falcarinol, a polyacetylene with anti-cancer properties, is commonly found in the Apiaceae, Araliaceae, and Asteraceae plant families (Zidorn et al., 2005). Other polyacetylenes had been reported from other plants like Centella asiatica, Bidens pilosa (Cytopiloyne), Panax quinquefolium L. (American ginseng), and Dendranthema zawadskii(Dendrazawaynes A and B), among others.

2.2.7.3 Allium compounds

Early investigators identified volatile odour principles in garlic oils – however, these compounds were only generated during tissue damage and preparation. Indeed, the vegetative tissues of Allium species are usually odour-free, and it is this observation that led to the hypothesis that the generation of volatile compounds from Allium species arose from non-volatile precursor substances. It was in the laboratory of Stroll and Seebrook in 1948 that the first stable precursor compound, (+)-S-allyl-L-cysteine sulfoxide (ACSO), commonly known as alliin, was identified; it makes garlic unique sulfur-containing molecules among vegetables (Stoll and Seebeck, 1947). Alliin is the parental sulfur compound that is responsible for the majority of the odorous volatiles produced from crushed or cut garlic. Three additional sulfoxides present in the tissues of onions were later identified in the laboratory of Virtanen and Matikkala, these being (+)-S-methyl-L-cysteine sulfoxide (methiin; MCSO), (+)-S-propyl-L-cysteine sulfoxide (propiin; PCSO), and (+)-S-trans-1-propenyl-L-cysteine sulfoxide or isoalliin (TPCSO). Isoalliin is the major sulfoxide present within intact onion tissues and is the source of the A. cepa lachrymatory factor (Virtanen and Matikkala, 1959). With regards to chemical distribution, (+)-S-methyl-L-cysteine sulfoxide is by far the most ubiquitous, being found in varying amounts in the intact tissues of A. sativum, A. cepa, A. porrum, and A. ursinum L (Table 2.5).

Table 2.5S-Alk(en)yl cysteine in Allium spp (modified from Rose et al., 2005)

Common name

Chemical name

Chemical structure

Methiin

S

-Methyl-L-cysteine sulfoxide

Aliin

S

-Allyl-L.cysteine sulfoxide

Propiin

S

-Propyl-L-cysteine sulfoxide

Isoalliin

S

-Propenyl-L-cysteine sulfoxide

Ethiin

S

-Ethyl-L-cysteine sulfoxide

Butiin

S

-n-Butyl-L-cysteine sulfoxide

Upon hydrolysis and oxidation, oil-soluble allyl compounds, which normally account for 0.2–0.5% of garlic extracts, such as diallyl sulfide (DAS), 5 diallyl disulfide (DADS), diallyl trisulfide (DATS), and other allyl polysulfides (2), are generated. Alternatively, it can be slowly converted into watersoluble allyl compounds, such as S-allyl-cysteine and S-allylmercaptocysteine (SAMC) (Filomeni et al., 2008 and references there in).

2.2.7.4 Chlorophyll

Chlorophyll (also chlorophyl) is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words chloros (“green”) and phyllon (“leaf”). Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light. Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion. However, it is a poor absorber of green and near-green portions of the spectrum; hence the green color of chlorophyll-containing tissues (Speer, 1997). Chlorophyll was first isolated by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817 (Pelletier and Caventou, 1951).

In pepper, unripe fruit colors can vary from ivory, green, or yellow. The green color derives from accumulation of chlorophyll in the chloroplast while ivory indicates chlorophyll ­degradation as the fruit ripens (Wang et al., 2005). The persistent presence of chlorophyll in fruit ripening to accumulate other pigments like carotenoids or anthocyanins produces brown or black mature fruit colors. Chlorophyll in black pepper fruit is 14-fold higher ­compared to violet fruit (Lightbourn et al., 2008).

Figure 2.8 Capsaicin.

2.2.7.5 Betalains

The name “betalain” comes from the Latin name of the common beet (Beta vulgaris), from which betalains were first extracted. The deep red color of beets, bougainvillea, amaranth, and many cacti results from the presence of pigments. Betalains are a class of red and ­yellow indole-derived pigments found in plants of the Caryophyllales, where they replace ­anthocyanin pigments, as well as some higher order fungi (Strack and Schliemann, 2003).

There are two categories of betalains: Betacyanins include the reddish to violet betalain pigments and betaxanthins that are those betalain pigments that appear yellow to orange. Among the betaxanthins present in plants include vulgaxanthin, miraxanthin and portulaxanthin, and indicaxanthin (Salisbury et al., 1991).

The few edible known sources of betalains are red and yellow beetroot (Beta vulgaris L. ssp. vulgaris), coloured Swiss chard (Beta vulgaris L. ssp. cicla), grain or leafy amaranth (Amaranthus sp.), and cactus fruits, such as those of Opuntia and Hylocereus genera (Azeredo, 2009 and references there in).

2.2.7.6 Capsaicinoids

The nitrogenous compounds produced in pepper fruit, which cause a burning sensation, are called capsaicinoids. Capsaicinoids are purported to have antimicrobial effects for food preservation (Billing and Sherman, 1998), and their most medically relevant use is as an analgesic (Winter et al., 1995). Capsaicinoids have been used successfully to treat a wide range of painful conditions including arthritis, cluster headaches, and neuropathic pain. The analgesic action of the capsaicinoids has been described as dose dependent, and specific for polymodal nociceptors. The gene for the capsaicinoid receptor has been cloned (TRPV1) and the receptor transduces multiple pain-producing stimuli (Caterina et al., 1997; Tominaga et al., 1998). Capsaicin (trans-8-N-vamllyl-6-nonenamide) is an acrid, volatile alkaloid responsible for hotness in peppers (Figure 2.8).

2.3 Chemical properties of phytochemicals

2.3.1 Terpenes

The basic structure of terpenes follows a general principle: 2-Methylbutane residues, less precisely but usually also referred to as isoprene units, (C 5 ) n, build up the carbon skeleton of terpenes; this is the isoprene rule 1 formulated by Ruzicka (1953) (Figures 2.2 and 2.3). The isopropyl part of 2-methylbutane is defined as the head , and the ethyl residue as the tail (Breitmaier, 2006). In nature, terpenes occur predominantly as hydrocarbons, alcohols and their glycosides, ethers, aldehydes, ketones, carboxylic acids, and esters (Breitmaier, 2006).

Several important groups of plant compounds, including cytokinins, chlorophylls, and the quinone-based electron carriers (the plastoquinones and ubiquinones), have terpenoid side chains attached to a non-terpenoid nucleus. These side chains facilitate anchoring to or movement within membranes. In plants, prenylated proteins may be involved in the control of the cell cycle (Qian et al., 1996; Crowell, 2000), nutrient allocation (Zhou et al., 1997), and abscisic acid signal transduction (Clark et al., 2001).

The most abundant hydrocarbon emitted by plants is the hemiterpene (C5) isoprene, 2-methyl-1,3-butadiene. Emitted from many taxa, especially woody species, isoprene has a major impact on the redox balance of the atmosphere, affecting ozone, carbon monoxide, and methane levels (Lerdau et al., 1997). The release of isoprene from plants is strongly influenced by light and temperature, with the greatest release rates typically occurring under conditions of high light and high temperature (Lichtenthaler, 2007). Although the direct function of isoprene in plants themselves has been a mystery for many years, there are now indications that it may serve to prevent cellular damage at high temperatures, perhaps by reacting with free radicals to stabilize membrane components (Sasaki et al., 2007).

2.3.2 Polyphenols

Simple phenols (C6), the simplest group, are formed with an aromatic ring substituted by an alcohol in one or more positions as they may have some substituent groups, such as ­alcoholic chains, in their structure (Andrés-Lacueva et al., 2010). Phenolic acids (C6<-C1) with the same structure as simple phenols are hydroxylated derivatives of benzoic and ­cinnamic acids (Herrmann, 1989; Shahidi and Naczk, 1995). They act as cell wall support materials (Wallace and Fry, 1994) and as colourful attractants for birds and insects helping seed dispersal and pollination (Harborne, 1994). Hydrolyzable tannins are mainly glucose esters of gallic acid. Two types are known: the gallotannins, which yield only gallic acid upon hydrolysis, and the ellagitannins, which produce ellagic acid as the common ­degradation product (see Figure 2.4) (Andrés-Lacueva et al., 2010).

Acetophenones are aromatic ketones, and phenylacetic acids have a chain of acetic acid linked to benzene. Both have a C6-C2 structure. Hydroxycinnamic acids are included in the phenylpropanoid group (C6-C3). They are formed with an aromatic ring and a three-carbon chain. There are four basic structures: the coumaric, caffeic, ferulic, and sinapic acids. In nature, they are usually associated with other compounds such as chlorogenic acid, which is the link between caffeic and quinic acids (Andrés-Lacueva et al., 2010). Coumarins belong to the benzopyrone group of compounds, all of which consist of a benzene ring joined to a pyrone. They may also be found in nature, in combination with sugars, as glycosides. They can be categorized as simple furanocoumarins, pyranocoumarins, and coumarins substituted in the pyrone ring (Murray et al., 1982). Benzophenones and xanthones have the C6-C1-C6 structure. The basic structure of benzophenone is a diphenyl ketone, and that of xanthone is a 10-oxy-10 H-9- oxaanthracene. More than 500 xanthones are currently known to exist in nature, and approximately 50 of them are found in the mangosteen with prenyl substituents (Andrés-Lacueva et al., 2010). Stilbenes have a 1,2-diphenylethylene as their basic structure (C6-C2-C6). Resveratrol, the most widely known compound, contains three hydroxyl groups in the basic structure and is called 3,4,5-trihydroxystilbene. Stilbenes are present in plants as cis or trans isomers. Trans forms can be isomerized to cis forms by UV radiation (Lamuela-Raventós et al., 1994). Lignans in the strict sense are phenylpropanoid dimers linked by a C-C bond between carbons 8 and 8 ‘ prime’ in the side chain; they can be divided into several subgroups, depending on other linkages and substitution patterns introduced into the original hydroxycinnamyl alcohol dimmer. More than 55 plant families contain lignans, mainly gymnosperms and dicotyledonous angiosperms (Dewick, 1989).

Flavonoids constitute one of the most ubiquitous groups of all plant phenolics. So far, over 8000 varieties of flavonoids have been identified (De Groot and Raven, 1998). In plants, flavonoids are usually glycosylated mainly with glucose or rhamnose, but they can also be linked with galactose, arabinose, xylose, glucuronic acid, or other sugars (Vallejo et al., 2004). All flavonoids contain 15 carbon atoms in their basic nucleus: two six-­membered rings linked with a three-carbon unit, which may or may not be parts of a third ring (Middleton, 1984). The rings are labeled A, B, and C (see Figure 2.5). The ­individual carbon atoms are based on a numbering system that uses ordinary numerals for the A and C and “primed” numerals for B-ring (1). Primed modified numbering system is not used for ­chalcones (2) and the isoflavones derivatives (6): the pterocarpans and the rotenoids. The different ways to close this ring associated with the different oxidation degrees of ring A provide the various classes of flavonoids. The six-membered ring ­condensed with the benzene ring is either a α -pyrone (flavones (1) flavonols (3) or its dihydroderivative (flavanones (4) and flavan-3-ols (5)). The position of the benzenoid substituent divides the flavonoids into two classes: flavonoids (1) (2-position) and isoflavonoids (6) (3-position). Most ­flavonoids occur naturally associated with sugar in ­conjugated form and, within any one class, may be characterized as monoglycosidic, diglycosidic, etc. The glycosidic linkage is normally located at position 3 or 7 and the carbohydrate unit can be L-rhamnose, D-glucose, glucorhamnose, galactose, or arabinose (Tapas et al., 2008 and references therein).

2.3.3 Carotenoids

Carotenoids consist of 40 carbon atoms (tetraterpenes) with conjugated double bonds. They consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position and the remaining nonterminal methyl groups are in a 1,5-position relationship. They can be acyclic or cyclic (mono- or bi-, alicyclic or aryl) (see Figure 2.6) (Yahia and Ornelas-Paz, 2010).

Carotenoids are often used in visual displays through deposition in skin or feathers. Given these multiple uses that all require substantial amounts of carotenoids for normal functioning, carotenoids have been suggested to be in limited supply for reproduction, health related functions, or the expression of sexual coloration. For example, it has been suggested that carotenoids may limit vital functions, such as scavenging of free radicals, eliminating ­peroxides, and enhancing immune function (production of lymphocytes, enhancement of phagocytic ability of neutrophils and macrophages, production of tumor immunity), in which they have been shown to be involved (Møller et al., 2000).

In nature, carotenoids exist as only two varieties: (1) unelaborated hydrocarbons, or (2) with functional groups, these are always attached via oxygen to the carotenoid skeleton. Carotenoids with heteroatoms other than oxygen have not yet been discovered in nature, but have been synthesized (Pfander, 1976). Hydrocarbon carotenoids generally form colored monomolecular solutions in nonpolar organic solvents, whereas and typically remain ­colorless in water. At extremely low carotenoid concentration, water unexpectedly exhibits an orange tint (the highly unsaturated polyene chain acting as a hydrophilic component). Strangely enough, the first carotenoid aggregates in water were obtained from β , β -carotene (von Euler et al., 1931). Its well-known hydrophobicity did not prevent other studies with β , β -carotene, and lycopene, an acyclic carotenoid hydrocarbon (Song and Moore, 1974; Bystritskaya and Karpukhin, 1975; Mortensen et al., 1997; Lindig and Rodgers, 1981). The many natural carotenols and carotenones (zeaxanthin, lutein, violoxanthin, astaxanthin) are undoubtedly more suited for aggregation studies in water (Mori, 2001; Zsila et al., 2001; Billsten et al., 2005; Köpsel et al., 2005).

The overwhelming majority of the ~750 known naturally occurring carotenoids are hydrophobic (Britton et al., 2004). It is therefore a striking paradox that the most utilized carotenoid since antiquity is extremely water-soluble: crocin has no saturation point in water. Crocin illustrates the typical surfactant structure, the hydrophobic polyene chain linked to two hydrophilic sugars; it is surface active, and the molecules associate to small oligomers at high concentration. The surface and aggregation properties of crocin have only recently been determined (Nalum-Naess et al., 2006). Meanwhile, other natural sugar carotenoids have been isolated and characterized, however, the low occurrence and abundance of these “red sugar derivatives” prevents practical applications (Dembitsky, 2005). Another group of naturally occurring carotenoids – sulfates – are considerably less hydrophilic; the first characterized compound was bastaxanthin sulphate (Hertzberg et al., 1983). A proposed application of carotenoid sulfates as feed/flesh colorants for cultured fish requires the additional help of an organic solvent for good outcomes (Yokyoyama and Shizusato, 1997). The “strange” appearance of the first recorded carotenoid sulfate visible spectrum in water was not immediately recognized as a sign of H-aggregation (Hertzberg and Liaaen-Jensen, 1985). The aggregation of a carotenoid sulfate was later observed as a negative outcome (Oliveros et al., 1994). Norbixin is the other carotenoid utilized since ancient times; it is reported to be water-soluble up to 5%. Recent measurements could not confirm solubility; only negligible dispersibility was observed (Breukers et al., 2009).

In the modern age, in addition to crocin and norbixin, several carotenoids have become extremely important commercially. These include, in particular, astaxanthin (fish, swine, and poultry feed, and recently human nutritional supplements); lutein and zeaxanthin (animal feed and poultry egg production, human nutritional supplements); and lycopene (human nutritional supplements). The inherent lipophilicity of these compounds has limited their potential applications as hydrophilic additives without significant formulation efforts; in the diet, the lipid content of the meal increases the absorption of these nutrients, however, ­parenteral administration to potentially effective therapeutic levels requires separate ­formulation that is sometimes ineffective or toxic (Lockwood et al., 2003).

2.3.4 Glucosinolates

Glucosinolates are amino acid-derived secondary plant metabolites found exclusively in cruciferous plants. The majority of cultivated plants that contain glucosinolates belong to the family of Brassicaceae such as brussel sprouts, cabbage, broccoli, and cauliflower. These are the major source of glucosinolates in the human diet – about 120 different glucosinolates have been characterized. Glucosinolates and their breakdown products are of particular interest because of their nutritive and antinutritional properties, their potential adverse effects on health, their anticarcinogenic properties, and finally the characteristic flavour and odour they give to many vegetables (Verkerk and Dekker, 2008).

The majority of cultivated plants that contain glucosinolates belong to the family of Brassicaceae. Mustard seed, used as a seasoning, is derived from B. nigra, B. juncea (L.) Coss, and B. hirta species. Vegetable crops include cabbage, cauliflower, broccoli, brussel sprouts, and turnip of the B. oleracea L., B. rapa L., B. campestris L., and B. napus L. ­species. Kale of the B. oleracea species is used for forage, pasture, and silage. Brassica ­vegetables such as brussel sprouts, cabbage, broccoli, and cauliflower are the major source of ­glucosinolates in the human diet. They are frequently consumed by humans from Western and Eastern cultures (McNaughton and Marks, 2003). In the Netherlands, the average ­consumption of these vegetables is more than 36 g Brassica per person per day (Godeschalk, 1987). The typical flavor of Brassica vegetables is largely due to glucosinolate-derived ­volatiles. The versatility of these compounds is also demonstrated by the fact that ­glucosinolates are quite toxic to some insects and therefore could be included as one of many ­natural pesticides. However, a small number of insects, such as the cabbage aphids, use glucosinolates to locate their favorite plants as feed and to find a suitable environment to deposit their eggs (Barker et al., 2006). Furthermore, glucosinolates show antifungal and antibacterial properties (Fahey et al., 2001).

Table 2.6 Glucosinolates commonly found in Brassicca vegetables (adapted from Verkerk and Dekker, 2008)

Trivial name

Chemical name (side chain R)

Aliphatic glucosinolates

Glucoiberin

3-Methylsulfinylpropyl

Progoitrin

2-Hydroxy-3-butenyl

Sinigrin

2-Propenyl

Gluconapoleiferin

2-Hydroxy-4-pentenyl

Glucoraphanin

4-Methylsulfinylbutyl

Glucoalyssin

5-Methylsulfinylpentyl

Glucocapparin

Methyl

Glucobrassicanapin

4-Pentenyl

Glucocheirolin

3-Methylsulfonylpropyl

Glucoiberverin

3-Methylthiopropyl

Gluconapin

3-Butenyl

Indole glucosinolates

4-Hydroxyglucobrassicin

4-Hydroxy-3-indolylmethyl

Glucobrassicin

3-Indolylmethyl

4-Methoxyglucobrassicin

4-Methoxy-3-indolylmethyl

Neoglucobrassicin

1-Methoxy-3-indolylmethyl

Aromatic glucosinolates

Glucosinalbin

p

-Hydroxybenzyl

Glucotropaeolin

Benzyl

Gluconasturtiin

2-Phenethyl

Only a limited number of glucosinolates have been investigated thoroughly although there are about 120 different ones currently characterized. A considerable amount of data on levels of total and individual glucosinolates are now available. The levels of total glucosinolates in plants may depend on variety, cultivation conditions, climate, and agronomic practice, while the levels within a particular plant vary between the parts of the plant. Generally the same glucosinolates occur in a particular sub-species regardless of genetic origin, and in most species only between one and four glucosinolates are found in relatively high concentrations (Table 2.6). Glucosinolates are chemically stable and biologically ­inactive when separated within sub-cellular compartments throughout the plant. However, tissue damage caused by pests, harvesting, food processing, or chewing initiates contact with the endogenous enzyme myrosinase in the presence of water leading to hydrolysis releasing a broad range of biologically active products such as isothiocyanates (ITCs), organic cyanides, oxazolidinethiones, and ionic thiocyanate.

Glucosinolate breakdown products exert a variety of toxic and antinutritional effects in higher animals amongst which the adverse effects on thyroid metabolism are the most ­thoroughly studied (Tripathi and Mishra, 2007). Tiedink et al. (1990, 1991) investigated the role of indole compounds and glucosinolates in the formation of N-nitroso compounds in vegetables. These studies revealed that the indole compounds present in Brassica vegetables can be nitrosated and thereby become mutagenic. However, the nitrosated products are ­stable only in the presence of large amounts of free nitrite.

2.3.5 Dietary fiber (non starch polysaccharides)

Polysaccharides are widespread biopolymers, which quantitatively represent the most important group of nutrients in botanical feed. Carbohydrates constitute a diverse nutrient category ranging from sugars easily digested by monogastric animals in the small ­intestine to dietary fiber fermented by microbes in the large intestine.

The structure of the plant cell wall influences the physical and chemical properties of the individual NSP and these vary considerably between different polymers and different molecular weights of the same polymer (Choct, 1997). Another factor that differentiates the physical properties among polysaccharides is the way the monomer units of polysaccha rides are linked together (Moms, 1992). Different sugars linked together in the same way often give polysaccharides with very similar physical properties.

On the other hand, despite being built up from the same monomer units, polysaccharides can have different physical properties when the monomer units are linked together in ­different ways. The physiological effects of NSP on digestion and absorption of nutrients in human and monogastric animals have been attributed to its physicochemical properties. The main physicochemical properties of NSP that are of nutritional significance include: (a) hydration; (b) viscosity; (c) cation exchange capacity; and (d) organic compound absorptive properties. The hydration properties of NSP influence its water holding and binding ­capacity (Bach Knudsen, 2001). These depend on the physicochemical structure of the ­molecule and its ability to incorporate water within the molecular matrix. The viscosity properties of the NSP depend on its molecular weight or size (linear or branched), ionically charged groups, the surrounding structures, and the concentration of NSP (Smits and Annison, 1996). The cation exchange capacity is formed because the three-dimensional structure of the NSP molecule allows a chelation of ions to occur. The organic compound absorptive properties of NSP are due to its capacity to bind small molecules by both ­hydrophobic and hydrophilic bond interactions.

2.3.6 Lectins

Although it seems apparent now that Weir Mitchell had already observed lectin activity in rattle snake venom before (Kilpatrick, 2002) it wasn’t until at least six years later, when Stillmark reported the dramatic action of ricin on red blood cells and then Helin followed it up by a similar report on abrin, that agglutinins caught the attention of the medical ­community. Reports of hemagglutinins from various sources were quick to follow. Besides plants, agglutinins were discovered in fungi, bacteria, viruses, invertebrates, and vertebrates. Although this early period established, beyond any doubt, the proteinaceous nature of ­lectins and their cell-agglutination and precipitation capabilities, lectin research thereafter was beset with problems and difficulties for the next quarter of a century. Studies, by Sugishita, Jonsson, Boyd, and Renkonen, provided the proverbial “shot in the arm” for research on lectins by identifying lectins as cell-recognition molecules that could have ­practical applications (Kocoureck, 1986). Reports of blood-group specificity, mitogenicity, and tumor cell-binding of lectins followed almost immediately.

The number of known properties and possible applications of lectins grew rapidly. Concanavalin A (Con A), a lectin from jack beans, became the first lectin to be crystallized and then extensively characterized by Sumner and Howell (1936) who also showed for the first time that sucrose could inhibit its agglutination activity. Two other major discoveries set the tone of the research that was to follow. Funatsu and his collaborators isolated the first ­non-toxic lectin from Ricinus communis , shattering the prevalent notion at that time that ­lectins were necessarily toxic proteins (Ishiguro et al ., 1964). Secondly, it was shown that several of these lectins, such as that from soybean, were glycoproteins (Lis and Sharon, 1973).

On the other hand, the effect of plant lectins on different cell types had already set the agenda for early research on them, leading to an extensive search for lectins in plant extracts and identification of a large number of lectins with practical applications. Such an objective did not require identification of the biological function of the protein per se . Indeed, in ­several cases where biological functions have been hypothesized or proven, the effect of the plant lectins on microbial or animal cells has provided clues to their putative function in vivo . Research on the endogenous roles of plant lectins has therefore been a late starter, although some progress has been made in this direction. Despite this, interest in studying plant lectins has been sustained, owing to the fact that their natural abundance makes their applications in a large number of areas much more feasible (Komath et al ., 2006 and ­references therein).