Natural Food Resource Valorization by Microwave Technology: Purslane Stabilization by Dielectric Heating.

The application of microwave-assisted drying is a promising technique due to the features of process sustainability that are usable for responsible productions. It is largely applied for the stabilization of food products, especially in the agro-food sector. In this study, the weed Portulaca oleracea L. (purslane), with its richness in antioxidant components in addition to its recognized pharmacological properties, has been considered due to its potential to be a natural, well-accepted future food. Attention was focused on the role of the heat and mass transfer rates involved in the drying processes on the nutritional profile of the dried products. For this purpose, different drying protocols (convective, microwave irradiation, microwave-vacuum irradiation) were applied to different parts of purslane herb (apical, twigs, entire structures) and chemical characterizations were performed by a GC/MS analysis of the extracts of the dried products. The results show that microwave treatments can assure a better preservation of fatty acids such as SFAs, MUFAs, and PUFAs (which constitute over 90% of the total components in the apical part, 65% in twigs, and 85% in microwave-vacuum-dried entire purslane samples) and phytosterols (their highest preservation was found in microwave-dried twigs) compared with convective treatments. The chemical composition variability as well as treatment times depend on the drying rates (in microwave treatments, the times are on a minute scale and the rates are up to three orders of magnitude greater than convective ones), which in turn depend on the heating transport phenomena. This variability can lead towards products that are diversified by properties that transform a weed into a valorized food source.

Impact of drying methods on the yield and chemistry of Origanum vulgare L. essential oil.

Oregano (Origanum vulgare L.) is mainly cultivated, both as fresh and dried herb, for several purposes, such as ailments, drugs, and spices. To evaluate the influence of some drying methods on the chemical composition of the essential oil of oregano, its aerial parts were dehydrated by convective drying techniques (shade, static oven), microwave-assisted heating (three different treatments) and osmotic treatment. The oils were analyzed by GC-FID and GC-MS. The highest essential oil yield was achieved from microwave and shade drying methods. In total, 39 components were found, with carvacrol (ranging from 56.2 to 81.4%) being the main constituent; other compounds present in lower amounts were p-cymene (1.6-17.7%), γ-terpinene (0.8-14.2%), α-pinene (0.1-2.1%), thymol methyl ether (0.4-1.8%) and thimoquinone (0.5-3.5%). The essential oil yields varied among the different treatments as well as the relative compositions. The percentages of p-cymene, γ-terpinene and α-pinene decreased significantly in the dried sample compared with the fresh sample; on the other hand, carvacrol, isoborneol and linalool increased significantly in the dried materials. The choice of the drying method for obtaining the essential oil therefore appears crucial not only in relation to the higher yield but also and above all in reference to the percentage presence of components that can direct the essential oil toward an appropriate use.