The effects of different drying methods on drying speed, essential oil and microbial load in Peppermint

Today special attention has been paid to the microbial quality of medicinal plants.One of the operations in order to maintain the quality of plants is drying which may also affect the quantity and quality of the essential oil. In order to evaluate drying process effect in Peppermint,the experiment was conducted in randomized complete block design with 4 treatments and 3 replications. The treatments included:1-Cotrol,2-Shade drying,3-Oven drying at 45oc,4-Microwave drying at 450W power. Samples were analyzed for their drying rate, essential oil content and microbial load(mesophilic aerobic bacteria, total coliforms, yeasts and moulds, and E. coli)according to the standard guideline. Results indicated that the treatments significantly affected traits(p < 0.01). Accordingly the fastest method was using the microwave. The maximum essential oil content was obtained from the shade dried samples(3.7%V/W)and the minimum one was obtained from the microwave dried samples(0.23%V/W). Microbial contamination in mesophilic aerobic bacteria and yeasts and moulds was at the lowest level in microwave dried samples (4.47 logCFU/g and 3.42 logCFU/g respectively); and total coliforms contamination was the lowest in samples which were dried in shade (3.36 log CFU/g). The maximum microbial contamination was observed in fresh samples. The existence of E. coli was not proved in any of the treatments. According to these results we can conclude that drying in microwave oven could reduce microbial load under standard levels, however it reduced essential oil content; whereas oil content could be maintained in higher levels in shade method, but this methods could not decrease microbial load.