Food Flavours: Biology and Chemistry

FOOD FLAVOURS

Biology and Chemistry

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FOOD FLAVOURS

Biology and Chemistry

CAROLYN FISHER* and THOMAS R. SCOTT

Departments of Animal and Food Sciences and Psychology

University of Delaware

Newark DE 19716, USA

*Present address: McCormick and Company Inc.,

202 Wight Avenue, Hunt Valley, MD 21031, USA

ISBN 0-85404-538-4

EPUB ISBN 9781788018371

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

© The Royal Society of Chemistry 1997

All rights reserved.

Apart from any fair dealing for the purposes of research or private study, or criticism or review as permitted under the terms of the UK Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page.

Published by The Royal Society of Chemistry, Thomas Graham House,

Science Park, Milton Road, Cambridge CB4 4WF, UK

Typeset by Vision Typesetting Manchester

Printed by Athenaeum Press Ltd, Gateshead, Tyne and Wear, UK

Preface

This book is designed for students of food flavours, and for those in related fields who seek an integrated overview of this area. Most books on flavours are edited contributions of individual scientists, each addressing a specialized aspect of the discipline. Here, we offer the joint perspectives of a flavour biochemist (CLF) and a neuroscientist (TRS), permitting a full sweep from synthetic chemistry through the chemical senses to the hedonic reactions of human tasters.

The purpose is to offer a deeper understanding of the chemical products generated in the food industry, and the biological and psychological reactions to them. By this means, our goal is to help position others to achieve a fuller understanding of current flavours and to discover new compounds to serve the flavour industry.

This book is presented in five chapters. The first offers an introduction to the complexities flavour scientists encounter. The second provides the chemical background on flavour compounds. In the third chapter, we describe the anatomy and physiology of the chemical sensory systems: olfaction and taste. In the fourth, we detail the sensory and the instrumental analyses of flavourants. Finally, we offer five complex flavour problems, and the methods by which students may be directed toward their solutions.

Contents

Acknowledgements

Chapter 1

Introduction – Problems in Flavour Research

Definition of Flavour

Classification of Food Flavours

Chemical Compounds Responsible for Flavour

Difficulties of Flavour Chemistry Research

Objectives of Flavour Chemistry Research

References

Chapter 2

Flavour Compounds

Chemical Compound Classes and their Flavour Responses

Flavour Development during Biogenesis

Flavour Development during Food Processing

The Use of Biotechnology to Develop Flavours

Conclusion

References

Chapter 3

The Chemical Senses

Introduction

Anatomy of the Chemical Senses

Neural Development of the Chemical Senses

Receptor Mechanisms

Neural Coding

The Control of Eating

References

Chapter 4

Flavour Analysis

Subjective versus Objective

Psychophysics and Sensory Evaluation

Instrumental Analysis

Sample Handling and Artefacts

Data Handling

References

Chapter 5

Teaching Flavour Concepts

Problem Based Learning

Tongue and Nose

Onion

Beverage

Maillard Reaction

Thio-stench

References

Bibliography

Glossary

Subject Index

Acknowledgements

We would like to thank our spouses and families for their encouragement during the writing. Special mention should be given to Dr Fisher’s son, Dan Fisher, who helped by scanning the figures into the computer and manipulating them.

We would like to acknowledge the help of those who gave their time to read the manuscript and provide very valuable comments: Dr Chi-Tang Ho, Dr Herbert Stone, Dr Ann C. Noble, Mr Paul Todd, and Ms Leora Hatchwell. Special mention must be given to Dr Chi-Tang Ho, who so graciously allowed Dr Fisher to peruse his flavour chemistry lecture notes during the formative stages of this book.

Dr Fisher would also like thank her students of the spring semesters of 1995 and 1996 for reading the manuscript drafts as they evolved and working through the problems and/or case studies that are included in Chapter 5.

We dedicate this book to Leonard Phillip Fisher and Bonnie Kime Scott.

Chapter 1

Introduction – Problems in Flavour Research

The inspiration for this chapter comes from the first chapter in Flavour Research: Recent Advances, by Flath et al.,¹ which has the same name.

DEFINITION OF FLAVOUR

What is flavour? There are two main definitions of flavour which depend upon the viewpoint of the definer. Flavour can refer to a biological perception, such that it is the sensation produced by a material taken in the mouth, or flavour can refer to an attribute of the material being perceived. The attribute is the aggregate of the characteristics of the material that produces the sensation of flavour. Flavour is perceived principally by the aroma receptors in the nose and taste receptors in the mouth. However, flavour descriptors, such as hot, pungent and biting, are also given to sensations received by the general pain, tactile, and temperature receptors in the mouth, nose and eyes.

Whether flavour refers to the chemicals responsible for the stimulation or the biological receptor stimulation itself, is immaterial to the consumer of foods. Consumers consider flavour one of the three main sensory properties decisive in their selection, acceptance, and ingestion of a particular food. The other two sensory properties are appearance and texture (Figure 1.1). We are all familiar with the basic five senses: sight (eyes), taste (tongue), odour (nose), hearing (ears) and touch (fingers, mouthfeel). The sense of touch, giving mouthfeel, can be broken down into three sensations: pressure, trigeminal and kinaesthesis. Pressure represents the feeling when force is applied over the surface of the food, trigeminal refers to a pain sensation and kinaesthesis denotes feedback from masticatory muscles during chewing.

Figure 1.1 Relationship of the five senses with sensory properties.

CLASSIFICATION OF FOOD FLAVOURS

Flavours can be classified by the general sensations that one feels when eating different foods (Figure 1.2). As observed in Figure 1.1, flavour comes from three different sensations: taste, trigeminal and aroma (odour). It is generally agreed that taste sensations are divided into four major categories: saltiness, sweetness, sourness and bitterness. However, some Japanese scientists also include a fifth category called umami (savoury) that can be represented by the flavour of glutamate. Trigeminal sensations give us the descriptors of astringency, pungency and cooling. Both taste and trigeminal sensations occur upon contact with food in the mouth, as most substances which produce these flavours are non-volatile, polar, and water-soluble. For aroma sensations to occur, an aromatic compound must be sufficiently volatile to allow detection at a distance. The physical interaction between the volatile compound and the receptor site occurs in the nasal passages. Those molecules that reach the olfactory receptors, either via the nasal passage or oral passageway, trigger the odorous sensations.

However, food flavourants are usually classified by the food sources in which they are normally detected (Table 1.1) because more than one flavour sensation is usually triggered by a food flavourant. Given a specific flavourant, the food industry wants to know what type of image the average consumer will envision when he or she encounters it. For example, celery flavourant (from an extract of celery seed) used in a soup is bitter with a floral aroma, but to an average consumer this flavourant just elicits the thought of celery soup.

Figure 1.2 Correlation of flavour types with sensation descriptors.

The problem with using food sources to classify flavours is that flavours may vary with the history of the food source. For example, fresh cabbage has a quite different aroma than cooked cabbage and sauerkraut is a vastly different olfactory and gustatory experience! Thus classifying flavours by food source is somewhat arbitrary, with the processing method frequently denoted in the descriptive name of the flavour.

Fruit Flavours

The tastes of fruit are a blend of the sweetness due to sugars (such as glucose, fructose and sucrose) and the sourness of organic acids (such as citric and malic). However, it is the aromas of the different volatile components of fruits that allow us to distinguish among them. When one’s sense of smell is eliminated (temporarily having a stuffy nose from a cold), it is extremely difficult to distinguish between onions and apples.

Table 1.1 Classification of food flavours

A typical fruit may have well over a hundred different volatile components, but in total, these compose only a few parts per million of the entire fruit.

Fruit aromas vary widely. Citrus, such as grapefruit, orange, lemon and lime, are rich in terpenoids whereas most non-citrus fruits, such as apple, raspberry, cranberry and banana, are characterized by esters and aldehydes.

Vegetable Flavours

Most cultivated vegetables have a milder flavour than the corresponding wild species. Over the years of plant cultivation, the milder varieties, that were high yielding and disease resistant, were chosen for propagation unless the plant was also used to ‘spice’ up other foods. Many vegetable flavours are only released from the raw vegetable when they are chopped or cooked, because the aroma compounds are tied up as glycosides (celery, lettuce) or glucosinolates (cabbage, radish), which makes them non-volatile. When the glycoside or glucosinolate linkage is broken via either enzymatic cleavage or heat, then the aroma compounds are released.

The ‘green’ flavour of many vegetables (peas, pepper, beans, asparagus, carrot, lettuce) comes from alkylalkoxypyrazines. Other alkylalkoxy-pyrazines are responsible for earthy aromas (potato). Phthalides give the bitter flavour to celery.

When vegetables (or fruits) are dried, many of the original flavour volatiles are removed with the water. Heat is usually used to speed the drying process (unless freeze-dried) and many of the flavour compounds change with elevated temperatures and air oxidation. New flavours can be developed from non-volatile precursors (see Maillard reaction, carotenoid degradation, lipid oxidation).

Spice Flavours

Some vegetables, such as onion and garlic, can also be considered spices. The onion is classified as lachrymatory as the initial flavour compound released upon enzymatic cleavage will bring tears to the eyes. Luckily it is short-lived and reacts to form other more appreciated flavour compounds. This lachrymatory compound is not formed in garlic.

Aromatic spices are the dried fruits and aromatic herbs are the dried leaves of plants. Volatile compounds give the characteristic aromas to the spices: eugenol (cloves), cinnamaldehyde (cinnamon) and menthol (mint). Some of these volatile substances, such as eugenol and cinnamaldehyde, also produce a slight pungent sensation via the trigeminal nerves.

The hot spices include chilli or red pepper, black pepper and ginger. All have aromatic characters, but the pungent sensation in the mouth is overwhelming. Garlic, nutmeg and cinnamon are also sometimes considered hot spices; however, here the trigeminal sensation occurs mainly in the nose.

In food processing, spices are often used in the form of essential oils or oleoresins. Essential oils are prepared by steam distillation of the dried ground spices and contain the volatile flavour compounds. Oleoresins are the solvent extracts of the spices and contain both the volatile essential oil as well as non-volatile resinous material and are more characteristic of the original ground spice.

Beverage Flavours

Beverage flavours can be divided into three types: unfermented, fermented and compounded. Unfermented beverages include milk and fruit and vegetable juices. Coffee might fall under this classification as it is not fermented, but because the beans are roasted to develop the flavour, it also can be considered an empyreumatic flavour.

Tea is usually classified as a fermented flavour. However, this is a misnomer. Fermentation refers to microbial growth (e.g. yeast), but the formation of flavour (and colour as well) during ‘fermentation’ in tea manufacturing is related predominantly to the oxidation of the phenolic compounds by enzymes found in the fresh tea leaves.

Alcoholic beverages use microbes to process the beverage and the chemical transformations that occur during fermentation generate flavours. However, the primary distinguishing flavours between beer and wine develop via non-fermentative processes. The bitter flavour of beer comes from hops that are transformed during the boiling of the wort before fermentation begins. Many wine flavours develop from interactions among fermentation products, flavonoid and the wooden containers during the long ageing process after fermentation has stopped.

Compounded beverage flavours can be found in the soft drinks and cordials of today that have been completely blended by flavourists. Here, the flavourist has been creative in the combination of natural and/or artificial flavours to make beverages that excite the palate.

Meat Flavours

Meats are cooked, dried, or even smoked to develop their flavours. The application of heat produces complex reactions between amino acids (often sulfur containing) and sugars (containing a carbonyl), that are given a singular name of Maillard reaction and are discussed in detail later. How long the meat is cooked, whether a dry method (broiling) or wet method (stewing) is used, and the temperatures obtained during cooking can alter the compounds formed and change the flavours dramatically.

Besides the cooking methods giving different flavour reactions, each animal contains a unique ratio of amino acids, fatty acids and sugars and thus generates its own flavours. In beef, lamb and pork, the lipids contain mostly saturated fatty acids that do not break down as quickly as do unsaturated fatty acids. However, in fish and fowl, there are many unsaturated lipids that generate flavours and small reactive molecules which interact with the amino acid/sugar reaction products to produce even more complex flavours. Also, because of these unsaturated lipids, rancid flavours develop more quickly in fish and fowl than in beef.

Fat Flavours

As suggested above, unsaturation in fats leads to oxidative cleavage and the formation of both desirable and undesirable flavours. The development