356 BOOK REVIEWS

FLAVOR SCIENCE-SENSIBLE PRINCIPLES A N D TECH- NIQUES, edited by Terry E. Acree and Roy Teranishi, American Chemical Society, Washington, DC, 1993. No of pages: xvi + 252, price US$ 79.95. ISBN 0-8412-2516-8.

Although this book is based on a Flavor Research Workshop held 25-26 August 1990 in Washington, DC, USA, i t is not one of the well-known ACS Sym- posium Series but rather an ACS Professional Refer- ence Book-the first I have seen. This suggests an agreed platform for further work rather than the state-of-the-art reports of the Symposium Series. In the first chapter, Terry Acree discusses ‘Bioassays for flavor’- a flavour-active chemical must react with a (human) receptor to get a flavour response. To quan- tify this response the concepts of flavour units, dose- response curves and iso-intensity scores are intro- duced. The bulk of the chapter, however, is con- cerned with gas chromatography-olfactometry. In particular, in Aroma Extract Dilution Analysis (AEDA, Grosch) and ‘Charm’ analysis (Acree), serial dilutions are made and examined by GC-0 and the greatest dilution at which the odour is still detected is measured. The difference between the two methods is that, whereas AEDA just determines the maximum dilution value, ‘Charm’ analysis deter- mines the dilution value, using computer software, over the whole time the compound elutes. No doubt using these or similar techniques the impact com- pounds in most foodstuffs and fragrances will be determined over the next decade.

The next three chapters deal with taste. H. T. Lawless and C. B. Lee discuss ‘Common chemical sense in food flavor’. They divide oral chemorecep- tion (taste) into oral chemical irritation- pungency, astringency and cooling compounds - and gustation - sweet, sour, salty, bitter (and umani?) and discuss the compounds responsible for trigeminal or chem- aesthetic effects and their analysis. Among the sen- sory methods, variations of Scoville’s method for the measurement of pungency are discussed, but there is no agreed method for astringency; indeed, some un- trained observers confuse astringency and bitterness. Instrumental methods of analysis for capsaicinoids, the pungent principles of pepper and ginger, and tan- nins are reviewed together with attempts to correlate the results with sensory analysis. Psychophysical char- acterization of these chemaesthetic flavours empha- sizes the time element in perception. For example, the pungency of capsaicin may take 3-6 minutes to decay and strong doses may lead to desensitization. A. van der Heijden reviews ‘Sweet and bitter tastes’ which are perceived by the interaction of tastants with re- ceptors on different papillae, mainly on the tongue. The current hypothesis of sweet taste transduction is given. The main classes of sweeteners are listed, together with a discussion as to how they fit Shallen-

berger’s AH-B receptor and the need to propose a third binding site. Structure-activity relationships, in- hibition and cross-adaptation are also discussed. In contrast to sweeteners, where sucrose is a natural standard of sweetness, there is no agreed standard bitter substance. The various classes of bitter com- pounds are listed in Table VI where, somewhat sur- prisingly, anti-isohumulones are given as representa- tives of hop compounds. The occurrence of the anti- isohumulones in beer has not been confirmed where cis- and trans-isohumulones are the accepted hop bit- ter compounds. Structure-activity relationships with bitter compounds are limited to amino acids and pep- tides. Much less is known about bitter receptors than sweet receptors but the balance of evidence suggests they are not the same site. Knowledge of sweetness receptors may be helped by a knowledge of ‘Sweet- ness antagonists’ reviewed by M. G. Lindley. Follow- ing the discovery of gymnemic acid, the inhibition of sweetness by phenylpropanoic acids, phenoxypropanoic acids, benzyloxypropanoic acids, sulphates and sulpho- nates has been studied. So far these studies suggest that they act as competitive antagonists at a single receptor -but more work is necessary.

The remaining six chapters deal with aroma. Roy Teranishi and Saima Kint discuss ‘Sample preparation’. In many cases flavours may be present at less than

so isolation for identification may be difficult. The isolation of the flavour extract may be by distilla- tion, extraction, simultaneous distillation-extraction, cold trapping or adsorption. Further resolution of this extract may be by distillation or liquid chromatography, but usually high resolution gas chromatography is em- ployed. Even then, for example, repeated analyses on megabore capillary columns may be necessary to get sufficient sample for say NMR analysis. In the longest chapter in the book (56 pages) Cynthia J. Mussinan reviews ‘Instrumental analysis in thc flavor industry’, starting with gas chromatography where ten different types of detector are compared. This is followed by an overview of mass spectrometry, hyphenated tech- niques, such as GC-MS, LC-MS, MS-MS, infrared spectroscopy, and, in particular, FTIR and NMR spectroscopy. In multidimensional GC cuts from the eluate from one column are fed on to a second column. Often this second column has a chiral phase so that the enantiomeric composition of the component may be determined. Most natural products occur in one enan- tiomeric form so adulteration by the racemic compound can be readily detected. Other tests for naturalness involve isotopic analysis. Both the US Federal Register and EEC regulations state that products obtained by enzymic action and fermentation are considered natu- ral. Accordingly ‘Biotechnology: challenge for the flavor industry’ by Peter Winterhalter and Peter Schreier reviews the production of flavours by plant cells, microorganisms and isolated enzymes. The de novo synthesis of flavour compounds by plant cell or

BOOK REVIEWS 357

tissue culture only proceeds, if at all, in very low yields but biotransformation of added precursors is more promising. Fermentations and microbial transforma- tions are more successful, especially with fungi where the de novo biosynthesis of lactones is useful. Enzymes are principally used for esterolytic and dehydrogenase reactions but glycosidases are used to release bound forms of flavour compounds discussed more fully in Chapter 9. In Chapter 8 Ron G. Buttery discusses ‘Quantitative and sensory aspects of flavor of tomato and other vegetables and fruits’. Thirty years ago, long before the advent of AEDA and ‘Charm’ analysis, Buttery and his co-workers introduced the concept of odour units and used them to highlight the important impact flavour compounds in tomatoes and other fruits. They showed that the volatiles in a tomato homogenate changed very rapidly and it was necessary to deactivate the enzymes present, with calcium chloride, to stabilize the (Z)-hex-3-enal content (at 50000 x the threshold level). Fifteen other compounds were found to have positive log odour units and thus to influence the tomato flavour, including 2-isobutyl-thiazole and 1- nitro-2-phenylethane. Tomato paste has a different pat- tern of volatiles: (Z)-hex-3-enal is almost completely lost, but new compounds are released from glycosides. In Chapter 9 Patrick J. Williams reviews ‘Hydrolytic flavor release in fruit and wines through hydrolysis of nonvolatile precursors’. Most of the precursors that have been studied are glycosidic derivatives and en- zymic hydrolysis may give either free volatiles or polyhydroxylated derivatives which liberate free vola- tiles with acid. Aglycones may be aliphatic, monoter- pene, sesquiterpene, or norisoprenoid derivatives, and in many cases the P-glucopyranosyl unit is substituted with another sugar. The individual glucoconjugates are

often isolated to study their structure and behaviour with different enzymes and the knowledge obtained used to enhance the flavour of processed foods. In the last chapter Richard Scarpellino and Robert J. Soukup review ‘Key flavors from heat reactions of food ingredients’. They set out very clearly the products of the thermal breakdown of sugars by Maillard brown- ing reactions to give inter alia furfural, 5- hydroxymethylfurfural, furaneol, cyclotene, maltol, isomaltol, nor-furaneol and maple lactones. Thermal breakdown of amino acids by Strecker’s reaction, using the a-dicarbonyls formed from sugars by the Maillard reaction, gives rise to flavourful aldehydes - isovaler- aldehyde, phenylacetaldehyde and methional. Self- condensation of a-aminocarbonyls, from Strecker and Amadori reactions, leads to pyrazines which are potent flavourants. The last group of compounds discussed are those which give rise to bread and cracker flavours and include acetylpyrazine, 2-acetyl-l,4,5,6tetrahydro- pyridine, 2-acetyl-2-thiazoline, 2-acet yl- 1 -pyrroline, and 2-(methyldithio)methylfuran.

Thus, the book Flavor Science- Sensible Principles and Techniques is a welcome guide to the isolation and identification of compounds which influence flavour. The emphasis is on techniques and it does not attempt to list the major flavourants found in different food- stuffs. Nevertheless, the examples given cover a wide range and the index (15 pages) is good; so it is easy to locate them, where the references lead to further infor- mation. I think that the book well fulfils its aim as a professional reference book which can be recom- mended to new and established flavour chemists.

ROGER STEVENS Threlkeld