Some 700 carotenoids, in great structural diversity, occur widely distributed in nature (e.g. in plants, animals, algae and microorganisms), being responsible for their yellow to red colour. Carotenoids have important, well-established functions, from their essential role in photosynthesis as light-harvesting agents and for photoprotection in plants to vitamin A activity in humans. A large body of epidemiologic evidence from the past 30 years suggests that consumption of carotenoid-rich foods is associated with health. In this way, substantial research efforts have been undertaken to unveil the biochemical and physiological mechanisms behind some of the health-promoting functions of such compounds related to, for instance, reducing the risk of developing certain types of cancers and age-related macular disease, as well as being beneficial for cognitive function, among other benefits.
While all of the photosynthetic organisms (i.e. vegetables, fruits, algae, bacteria, archaea and fungi) synthesise carotenoids, animals are not able to do so; thus, they must obtain these compounds from their diet. Although the six carotenoids accounting for 90–95% of the total carotenoids in human plasma are found almost exclusively free, the 70–100 carotenoids present in foods are frequently associated with other molecules, like sugars, proteins or fatty acids. Such associations strongly influence their properties, both physical–chemical and biological.
The occurrence of long-chain acyl esters of carotenoids—known by their short name ‘carotenoid esters’—in nature has been known for a long time. However, research on carotenoid esters, especially in foods, only became intense in recent years, driven by the development of new analytical hyphenated techniques. As a result, a large volume of information related to these compounds has been published in recent years, mostly in research or review papers. Those facts were the motivation for the organisation of the first book dedicated to carotenoid esters. The topics of the book, organised in different parts, aim to provide a state of the art, to review the current knowledge related to various aspects of carotenoid esters and to encourage research into this renewed area.
Basic, but essential, information is provided in Part I: Physical and Chemical Properties of Carotenoids. The nomenclature and structural aspects of isoprenoids, carotenoids, fatty acids and, finally, carotenoid esters, with references to some physical–chemical properties and their importance at different levels, are addressed in Chapter 1. Chapter 2 provides an overview of the full array of carotenoid deposition forms in plants, focusing on the occurrence of exceptional tubular and presumably liquid-crystalline form in carotenoid ester-rich plants. Considering that few commercial standards of carotenoid esters are available, an overview on the three to four well-established (trans)esterification methods used for the chemical synthesis of carotenoid esters is given in Chapter 3.
The mechanisms, genes and enzymes involved in carotenoid biosynthesis, especially related to their accumulation in fruits and metabolisation by marine organisms, along with the occurrence of carotenoid esters in foods from plant and animal origins are the topics of Part II: Carotenoid Biosynthesis and Occurrence of Carotenoid Esters. Chapter 4 gives a general overview of carotenoid biosynthesis by integrating results from all kinds of investigations of reaction mechanisms, genes and enzymes to summarise the current state of knowledge. Chapter 5 focuses on the common mechanisms governing carotenoid biosynthesis and accumulation in fleshy fruits—with an emphasis on those with significant accumulation of esterified carotenoids—underlying both biochemical and molecular mechanisms that modulate carotenoid changes during fruit development and ripening. Focusing on another kingdom, Chapter 6 presents the occurrence and metabolism of carotenoid esters in marine organisms, considering that carotenoids from the primary food chains, phytoplankton and other algae may be structurally modified by metabolism within the animal. Finally, a compilation of relevant data on the occurrence of xanthophyll esters in foods from plant and animal origins with the aim of contributing to our understanding of this important natural process is presented in Chapter 7.
The current knowledge and advances in the analytical techniques applied for analysis of carotenoid esters are discussed in Part III: Analytical Methods for Determination of Carotenoid Esters. The methods for carotenoid extraction, including classical methods and those employing new technologies, as well as the use of green solvents and different procedures for extract clean-up, are presented in Chapter 8. In Chapter 9, analytical methods for the separation of carotenoid esters is overviewed, including one- and two-dimensional chromatographic approaches using either liquid or supercritical fluids as mobile phases. Identification based on the practical information provided by ultraviolet–visible spectrum characteristics and mass spectrum fragmentation patterns of carotenoids, in both free and esterified forms, obtained by liquid chromatography connected to diode array and atmospheric pressure chemical ionisation–mass spectrometry detectors is given in Chapter 10. To close this part, an overview of the current strategies used for food sampling, as well as for the quantification of carotenoid esters and the validation of methods used in the quantitative determination of such compounds, is provided in Chapter 11.
Besides composition, the amounts of carotenoids released from foods that are able to become bioaccessible and available for absorption in the intestine and be further metabolised should be considered along with the estimation of dietary carotenoid intake by different populations. These issues are the focuses of Chapters 12–14 in Part IV: Dietary Intake, Digestion, Absorption and Metabolism of Carotenoids and Their Esters. Chapter 12 estimates the dietary intake of the major carotenoids found in the human diet (i.e. high-percentage consumption as ester forms) and discusses dietary methods for their assessment. Also, the adequacy of carotenoid consumption is discussed in relation to their contribution to vitamin A intake and regarding the effects of individual carotenoids. Chapter 13 provides a comprehensive review of the digestion, absorption and metabolism of dietary carotenoid esters and discusses the possibility of endogenous acylation of xanthophylls in animal and human tissues. The current knowledge about the application of in vitro digestion protocols to measure the micellarisation of carotenoids from foods, along with the effects of food structure, processing features and physiological issues, among others, are discussed in Chapter 14.
A short review of the research highlights related to carotenoid esters, needs for further research and perspectives and the importance of interdisciplinary studies are discussed in Part V: Final Remarks (Chapter 15).
I believe that this book provides useful information for both academia and industry as carotenoids are versatile compounds that are eliciting increasing interest in different disciplines, such as chemistry, plant science, agriculture, food science and technology, nutrition and health, among others. I also hope this book will promote research on all of the aspects related to carotenoid esters in foods and encourage new researchers to join this exciting area.
Finally, I would like to thank the Royal Society of Chemistry for the invitation to edit this first book on carotenoid esters in foods, and also to thank all of the authors who contributed their scientific research knowledge and time to make this book possible.
Adriana Z. Mercadante