The effect of different drying methods on the quantity and quality of peppermint essential oil with the help of electronic nose and gas chromatography-mass spectrometry

The use of plant-derived compounds is common in medicine and preventive health care, while the scope of use of some substances is steadily increasing. The mint family, with more than 200 genera and 3000 species, is very important economically and medicinally. The mint genus contains 25 to 30 species that grow in different temperate regions of Asia, Europe, Australia and South Africa. There is a great diversity in terms of chemical composition among the species of the mint genus. Peppermint essential oil (Mentha spicata L.) is rich in carvone, which produces the special aroma of mint. The yield of essential oil of Sentha spicata is lower than that of Mentha piperita. Carvone is the main component of Mentha spicata and Mentha longlifolia, while Carvone is absent in Mentha piperita, Mentha aquatic, Mentha arvensis and Mentha pulegium. Peppermint essential oil and extract are used in the pharmaceutical, cosmetic and food industries all over the world. Mentha spicata essential oil and leaves have therapeutic uses and its general properties are analgesic, tonic, stomach tonic, antitussive, anticonvulsant, astringent, analgesic and sedative. Peppermint oil has been used since ancient times for medicinal purposes, mostly to treat headaches, colds and neuralgia. It can also relieve skin irritations and digestive problems and has antispasmodic effects. Although, there is mixed information about the chemical composition of Mentha spicata essential oil, many studies have confirmed carone and limonene as its main components. Carvone is responsible for the smell of peppermint essential oil. The high price of carvone in the market has pushed breeders to improve mint varieties with high carvone. Different chemotypes are characterized by specific odors and biological activities, which indicate different applications in the aromatic and pharmaceutical industries. For example, Europeans enjoy the scent of Carvone. The use of medicinal plants in the food and pharmaceutical industries depends on the amount of biologically active substances and their chemical composition. Changes in the concentration of volatile compounds of mint during drying also depend on several factors, including drying conditions (temperature, air speed), humidity, variety and age of the plant, climate, soil and harvesting method. The drying process and storage conditions of the dried plant can have an adverse effect on the medicinal properties of the essential oil. Drying is one of the efficient methods to preserve agricultural products and maintain food quality. Drying, as an important food preservation technique, is used in the food industry. Drying is required to reduce the water activity of the product to suppress the growth of microorganisms and inhibit chemical reactions to increase the shelf life of the product at room temperature. In addition, drying lightens shipping weight and reduces storage space. Conventional drying methods include hot air drying (HAD), vacuum drying (VD), vacuum freeze drying (VFD), and microwave-hot air alternating drying (MW-HAD). HAD is the most common method that dries food in an oven with a constant flow of hot air. As an optimal approach for drying raw vegetable food, this method has easy operation and low cost, but it requires a long drying time and has low energy consumption.

After the drying process, the essential oil was extracted from the dried product, and for this purpose, a Clonger machine was used using the water distillation method. Distillation with water is a method of extracting essential oils. This method is cheap because it mostly uses water as a solvent. Qualitative GC-MS analysis of the extracted essential oils was performed using an HP 6890 gas chromatograph coupled to an HP 5973 mass-selective detector (Agilent Technologies, Foster City, CA, USA) operating at 70 eV mode. The electronic nose consists of three parts: (1) a sample transport system (2) a detection system consisting of a set of gas sensors with partial characteristics and (3) an odor data processing system. The e-nose instrument can detect the presence of VOCs in various molecular structures with high accuracy and reliability regardless of more or less odor. Samples were analyzed using a portable e-nose, which consists of a multiple gas sensor array, a signal acquisition unit, and pattern recognition software. Essential oil samples (1 mL) were placed in a 10 mL sealed glass vial and equilibrated at 40 °C for 30 min under stirring. Clean ambient air was used as the carrier gas to transport the volatiles in the headspace of the sealed glass vials to the temperature and humidity controlled sensing chamber. The conductivity change in the sensor array is expressed by the normalized response of the sensor. Each measurement cycle lasted 100 seconds, which allows the sensor to reach a steady state, and the data collection interval using a computer was 1 second. Between measurement cycles, the sensor was purged for 200 s with purge gas filtered through activated charcoal to return the sensor signal to baseline. 15 measurements were made for each sample of peppermint essential oil. Data obtained from GC-MS analysis were first processed by in-house MSD Chemstation and structural identification was performed through NIST 2014 library research along withretention index (RI) validation. The dataset consists of pre-processed signals from 9 MOS gas sensors obtained in the e-nose during 120 measurements corresponding to 8 independent samples evaluated with 15 repetitions. The performance of e-nose for evaluating peppermint essential oil samples was evaluated using three supervised statistical methods, namely QDA, MDA and ANN.

Drying is the most suitable method used to preserve the natural products of plants. Choosing a special drying method is one of the important costs in the production and commercialization of medicinal plants. This study determined the effect of different drying methods on the quantity and quality of peppermint essential oil. The results showed that the highest yield of essential oil was in the HAD1A drying method and the lowest yield was related to the sun drying method. Also, the obtained compounds of the essential oil were determined by the GC-MS method, and in the HAD drying method, 18 compounds were determined, and the content of some of them decreased significantly with the increase of the drying temperature. In the dried samples, the main components were Carvone (64.30-7.45%), Limonene (24.21- 6.59%) and Carveol (18.34-1.92%). Also, the aroma characteristics of mint essential oil were evaluated with the help of an E-nose. Three classification algorithms QDA, MDA and ANN were used, and the highest percentage of classification related to QDA and MDA methods was 100%, and the accuracy of the ANN method was also 0.967%. The findings of this study provide a theoretical basis for the development of hot air thin layer drying process for medicinal plants and improving their sensory quality and related products. The future perspective is to continuously improve the in situ drying technique for medicinal plants and develop a suitable monitor system to control the sensory quality of the final products based on the findings of the current study.