Effect of Microwaves on Essential Oils of Coriander and Cumin Seeds and on their Antioxidant and Antimicrobial Activities

Abstract

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 oil and on both their antioxidant and antimicrobial activities. HD 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. Seventeen and sixteen components were identified in essential oils of coriander and cumin seeds respectively. HD oil of raw coriander consisted mainly of linalool (72.7 %) followed by λ-terpinene (8.8 %), α-pinene (5.5 %), camphor (3.7 %), limonene (2.3 %), geranyl acetate (1.9 %) and p-cymene (1.5 %). However HD Oil of raw cumin consisted mainly of cuminaldehyde (26.5 %) followed by λ-terpinene (20.9 %), β-pinene (20.9 %), p-mentha-1,4-dien-7al (12.3 %) p-cymene (7.7%) and p-mentha-1,3-dien-7-al (4.0 %) . In general, the effect of heating on the two spices caused reduction in their essential oil concentrations compared with the raw samples, due to the formation of lipid degradation products, hexanal and (E)-2-hexanal in two treatments. 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 percentage of linalool in microwave heated coriander seeds (57.3 %) than that in conventional roasted coriander seeds (55.5 %) makes 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 monoterpenes aldehydes in microwave heated cumin are higher than that in conventional roasting sample (35.7 % and 34.2 %), respectively). These aldehydes contribute to the pleasant characteristic flavour of cumin seeds. Therefore microwave treatment was more retentive of the flavour impact compounds than the conventional roasting treatment and considered the best choice as an alternative-heating medium for processing. Microwaved treated samples showed a remarkable antioxidant activity against DPPH and β-carotene assays, due to the higher concneitation of oxygenated manoterpenes than that in conventional treated samples also for the remarkable percentages of α-pinene, β-pinene, p-cymene and λ-terpinene which may be contribute to the antioxidant activities of both spices, however still lower than that of synthetic antioxidant TBHQ. Both microwave conventionally roasted oils exhibit similar antimicrobial effect against microorganisms under investigation but were higher effect than that of raw oils.