news and views fore chose simply to tabulate the percentage common spice combinations bring together circuits of my salsa- and vindaloo-loving of all bacteria tested towards which a given complementary antimicrobials. family are a legacy of unspicy northern spice or its extract has consistently shown The strong biological slant of Billing and Europe, and they suggest not. at least some inhibitory activity, regardless of Sherman’s hygienic hypothesis will also help Harold McGee is at 14 Chemin Las Crozes, 31450 dose. Four spices are said to be inhibitory sharpen the thinking of anyone who is Pompertuzat, France. He is author of On Food and towards all bacteria tested — garlic, onions, intrigued by the complexities of human spice Cooking: The Science and Lore of the Kitchen, and allspice and oregano — and a total of 15 use and its persistence. The past few decades The Curious Cook (Scribner’s/Allen & Unwin). inhibited 75% of bacteria, or more. have brought Paul Rozin’s rich behavioural e-mail: [email protected] This simple ranking of antimicrobial analysis of flavourings and the challenges of 1. Billing, J. & Sherman, P. W. Q. Rev. Biol. 73, 1–38 (1998). 4 potencies disguises large differences in the omnivory , fascinating studies of the cere- 2. Thimmayamma, B. V. S., Rau, P. & Radhaiah, G. Ind. J. Nutr.8 5 6 Dietet. 20, 153–162 (1983). extent to which different spices have been monial and mythopoetic , social and eco- 3. Davidson, P. M. in Food Microbiology (eds Doyle, M. P., 7 tested. An appendix reveals that in the most nomic aspects of European spice habits — Beuchat, L. R. & Montville, T. J.) 520–556 (Am. Soc. inhibitory group, garlic was tested against 29 and a continuing boom in per capita spice Microbiol., Washington DC, 1997). 4. Rozin, P. in Flavor: Its Chemical, Behavioral, and Commercial bacteria, but allspice against only five, and consumption in the United States. Is the last Aspects (ed. Apt, C. M.) 101–127 (Westview, Boulder, CO, onion against four. Chillies were successful a manifestation of new selection pressures 1978). against just four of five bacteria. A review of exerted by Escherichia coli, salmonella and 5. Detienne, M. The Gardens of Adonis, transl. Lloyd, J. (1972; reprint Princeton Univ. Press, 1994). chemical food preservation concludes that campylobacter? Of an immigrant popula- 6. Schivelbusch, W. Tastes of Paradise (Pantheon, New York, 1992). of the 15 spices that Billing and Sherman tion reverting to type? The chemosensory 7. Laurioux, B. Food Foodways 1, 43–76 (1985). classify as ‘highly inhibitory’ and find signifi- cantly associated with warm climates, seven, Mantle mineralogy including chillies and allspice, have shown “limited or no activity” towards food Olivine emerges from isolation spoilage microbes3. Even for the apparently Craig R. Bina inhibitory spices (cinnamon and cloves, gar- lic, and the Mediterranean herbs), the prob- lem remains that most evaluations have been ear a depth of 410 km, the speeds at magnesium and iron between olivine and done in vitro rather than in victu. The few which seismic waves travel in Earth’s other mantle minerals is an important factor spices to have been tested in foods show far Nmantle rise sharply, an observation in resolving these apparent discrepancies. less activity than they do in microbiological long attributed to high-pressure transfor- Such discrepancies are significant media, probably because food particles mation of the mineral olivine to wadsleyite. because they constrain the extent to which sequester the inhibitory compounds3. The However, seismological studies of this mantle discontinuities arise from phase active sulphur compounds in garlic and ‘410-km discontinuity’ reveal wave-speed transitions in minerals, such as those in onions are formed in disrupted tissue by increases that are smaller and more abrupt olivine, rather than changes in bulk chemi- enzymatic activity, which ordinary cooking than expected from extrapolation of the cal composition. The mineralogy and chem- procedures limit (but which the Indian laboratory behaviour of these minerals to istry of Earth’s mantle, especially that part practice of grating or grinding probably mantle conditions. On page 702 of this known as the ‘transition zone’ between the maximizes). And little is known about the issue1, Irifune and Isshiki show that the seismic discontinuities at depths of 410 and stability of the inhibitory compounds to hitherto largely neglected exchange of 660 km, govern the dynamics of convective cooking temperatures. Volume fraction (%) In short, we don’t yet have enough infor- Figure 1 Equilibrium mineral proportions mation to say whether spices have significant 0 20 40 60 80 100 0 100 and compositions as functions of mantle antimicrobial effects in ordinary cooking. depth, for (Mg0.89Fe0.11)2SiO4 olivine in On balance, it looks as though a simpler and isolation (Fo89) and for a model opx Fo89 (pyrolite) mantle composition. Mineral more reliable defence against foodborne dis- Pyrolite ease is to heat food well and often, as Indians proportions are as volume fraction, have long done with highly perishable and 200 shown by bold white lines; compositions unspiced liquid milk. are in terms of Mg/[Mg+Fe], and are If Billing and Sherman are unable to shown by colours and by dotted contours make a convincing case for the hygienic at 1% intervals. With increasing depth, hypothesis, their labours are to be valued for a a olivine (a) transforms to wadsleyite (b) reviving an excellent question and demon- near 410 km, and orthopyroxene (opx) strating the need for in victu research initia- 300 cpx gradually dissolves into clinopyroxene (cpx) which in turn dissolves into garnet tives. After testing culinarily appropriate Depth (km) spice doses in a model food, one would want (gt). In the pyrolite mantle composition, to contaminate genuine, pre-refrigeration all phases grow more Mg-rich with versions of, say, chicken in a sauce — Chinese increasing depth. (Figure based on data red-cooked, Indian curried, Italian caccia- from Irifune and Isshiki1.) tore, French coq au vin, American pot pie, 400 Mexican mole — and follow pathogen growth in each, which will surely be affected by ingredients such as soy sauce, vinegar and b b wine. Given the worldwide popularity and gt yet limited antibacterial activity of black pep- per, Billing and Sherman’s speculative ratio- nale — that pepper acts as a ‘bioavailability enhancer’ to sensitize bacteria to other spices 75Mg/[Mg+Fe] (%) 95 — needs testing, as does the possibility that 650 NATURE | VOL 392 | 16 APRIL 1998 Nature © Macmillan Publishers Ltd 1998 news and views flow of heat and matter in the interior2. This Irifune and Isshiki1 have shown such transition to higher pressures (as can be seen flow, in turn, drives the tectonic evolution artificial separation to be misleading by in Irifune and Isshiki’s Fig. 3 on page 704), so of the surface, including the occurrence of experimentally demonstrating Mg–Fe ex- the onset of the transition in pyrolite is post- volcanism and seismicity. change between a, b, garnet and clino- poned to greater depths relative to that in Olivine, a solid solution of forsterite pyroxene. We have known5 for some time Fo89. On the other side of the phase change, (Mg2SiO4) and fayalite (Fe2SiO4), has the that the proportions of these minerals vary b does not partition Fe into garnet as strong- approximate composition (Mg0.89 Fe0.11)2SiO4 with depth, as shown by the bold white lines ly as does a, so b is not initially Mg-enriched, (called forsterite-89 or Fo89) in samples from in Fig. 1, but Irifune and Isshiki have now and the completion of the a–b transition the shallow mantle. It transforms from the measured the accompanying variations in is not postponed relative to Fo89. The net olivine (a) phase to wadsleyite (b), changing the compositions of coexisting minerals, effect of postponing onset but not co8m- again to ringwoodite (g) at greater depths shown by the colours. In isolated Fo89 pletion is to concentrate the transition into and eventually breaking down to a mixture of olivine, mineral compositions must remain a narrower range of depths, decreasing the silicate perovskite and magnesiowüstite. In constant, except in the narrow zone of a–b depth-extent of the wave-speed increase by standard compositional models of the upper coexistence. In pyrolite, however, all phases some 6 km relative to that expected for Fo89 mantle such as the pyrolite3 model, in which a grow increasingly Mg-rich with depth. (>10 km), in better agreement with seismic6 mix of different minerals is taken to account Given that bulk composition is fixed, this estimates (<5 km). for the rocks and melts erupted at the surface entails an increasing abundance of the most Furthermore, mineral wave-speeds from the interior, olivine accounts for only Fe-rich phase, garnet. Indeed, gradual dis- themselves also vary with composition. about 60% of the mantle by volume. Other solution of the less Fe-rich pyroxenes into Wave-speed increases across the a–b transi- minerals concerned include orthopyroxene, the garnet both raises the proportion of tion in isolated Fo89 can be measured in clinopyroxene and garnet, which gradually garnet and dilutes the garnet to lower Fe the laboratory, extrapolated to mantle dissolve each into the next with increasing levels. However, given constant Mg–Fe conditions, and scaled for a mantle model depth. Despite a pioneering study nearly 20 partitioning ratios (incidentally confirmed which is about 60% olivine by volume. years ago, in which Akaogi and Akimoto4 by Irifune and Isshiki), such dilution drives Although subject to remaining uncertain- pointed out that knowledge of element parti- Fe-depletion (that is, Mg-enrichment) of ties7 in the pressure- and temperature- tioning amongst these minerals is ‘indispen- a to maintain equilibrium levels of Fe in dependence of mineral wave-speeds, some sable’ for understanding the mantle, to this garnet.