EFFECT OF MICROWAVE HEATING AND CONVENTIONAL ROASTING ON VOLATILE COMPOUNDS IN OREGANO LEAVES, CORIANDER AND CUMIN SEEDS.

Processing of spices using microwaves is a newer dimension. This alternative methodology is preferred due to the convenience and ease of handling. In the present study, oregano leaves (Origanum vulgare L.), coriander (Coriandrum sativum L.) and cumin seeds (Cuminum cyminum L.) were subjected to conventional roasting and microwave heating to study their effects on the volatile components of each spice. Hydrodistilled oil of each spice was subjected to GC and GC- Ms analysis. A comparison study was  done between the two methods of roasting and with raw sample of each spice. Twenty three volatile components were identified in essential oil of oregano leaves, 17 components in essential oil of coriander seeds and 16 components in essential oil of cumin seeds. In general, the effect of heating on the three spices caused reduction in their essential oil concentrations compared with the  raw samples. This reduction was due to the formation of lipid degradation products, hexanal and (E)-2-hexenal in all cases. However the sum of their total percentages in microwave treated samples were always smaller than that in conventional roasted samples, which  revealed that the  microwave heating is more preferable. Moreover, the higher total percentages of thymol and carvacrol (the two major isomeric monoterpene phenols) in microwave treated sample (25.82%) of oregano leaves than that  in conventional roasting sample (20.44%) make the microwave heating sample of higher quality, since these two monoterpene phenols are responsible for the potent odorant compounds and used as the quality index for this herb. Also, the higher percentage of linalool in microwave heated coriander seeds (57.26%) than that in conventional roasted coriander seeds (55.53%) make the microwave method more preferable due to the fact that linalool increases quality and add fruity and minty aroma to this spice. On the other hand, the total concentration of monoterpene aldehydes in microwave heated cumin are higher than that in conventional roasting sample (35.68% and 34.17%, respectively). These aldehydes (cuminaldehyde; ρ-mentha-1,3-dien-7-al; ρ-mentha-1,4-dien-7-al and myrtenal) contribute to the pleasant characteristic flavour of cumin seeds. Besides, microwave heated sample showed a decrease in concentrations of  γ-terpinene, p-cymene and β- pinene in comparison with those in conventional roasted sample, these compounds thought to reduce the quality of this spice. Therefore microwave treatment was more retentive of the flavour impact compounds than conventional roasting treatment and considered the best choice as an alternative- heating medium for processing.