Effect of Microwave Oven and Warm-Air Drying on the Microflora and Volatile Oil Profile of Culinary Herbs

A number of culinary herb species Allium schoenoprasum L., Anethum graveolens L., Anthriscus cerefolium (L.) Hoffm., Artemisia dracunculus L., Coriandrum sativum L., Levisticum officinale Koch. Mentha spicata L., Origanum majorana L., Petroselinum crispum (Mill.) Nym. ex A. W. Hill, Salvia officinalis L., Satureja hortensis L. and Thymus vulgaris L. were dried by warm-air and microwave ovens. The microbiological quality of the raw and dried material was determined for total bacterial count, Staphylococcus aureus, Bacillus cereus and coliforms. The volatile oil content of seven plant species was determined by gas chromatography following drying at temperatures from 40–100°C, revealing that at temperatures >60°C, most of the volatile constituents were lost. Exposure of herbs to microwaves was evaluated as a method of both drying and reducing the microbial load present on the plants. The microflora was reduced by two to three logarithmic cycles, but the effect on the volatile oil profile was profound. In addition to losses of oil constituents, novel peaks were detected on the GC trace.