Elsevier

Industrial Crops and Products

Volume 42, March 2013, Pages 613-619
Industrial Crops and Products

Evaluation of six drying treatments with respect to essential oil yield, composition and color characteristics of Thymys daenensis subsp. daenensis. Celak leaves

https://doi.org/10.1016/j.indcrop.2012.06.012Get rights and content

Abstract

Thymus daenensis subsp. daenensis. Celak is known as an endemic and valuable spice plant. The effect of six different drying treatments (sun, shade, oven 50 °C, oven 70 °C, microwave and freeze–drying) on the essential oil yield, composition and color characteristics of leaves was assessed. The essential oils from fresh and dried samples were isolated by hydrodistillation in a Clevenger apparatus and analyzed using gas chromatography–mass spectrometry (GC–MS). The highest essential oil yields were obtained by freeze–drying (1.7%) followed by oven 50 °C (1.46%), sun drying (1.42%), oven 70 °C (1.01%), shade drying (0.91%) and microwave (0.89%). Results showed that air drying at ambient temperature increased significantly the essential oil yield. Higher temperatures (microwave and oven 70 °C) lead to increase in thymol/carvacrol and β-caryophyllene content. In this study, Lightness (L*), greenness (a*), and yellowness (b*) of dried leaves were also compared with fresh one. Oven drying at higher temperature resulted in a considerable decrease in the color quality of the leaves. Air drying and oven 50 °C and 70 °C had the highest yellowness in comparison to other treatments. In spite of relatively low essential oil yield in Microwave drying, this method had many advantages such as shortening of drying time, high color quality and increased major compounds of thyme leaves.

Highlights

► The effects of six drying treatments on the essential oil content, composition and color characteristics of Thymus daenensis subsp. daenensis were studied. ► Oven drying at higher temperatures resulted in a considerable decrease in color quality and essential oil content. ► Concentration of major compounds such as thymol/carvacrol and β-Caryophyllene were increased in the case of microwave and oven 70 °C. ► Microwave drying showed the appreciable characteristics such as shortening of drying time, high color quality and high thymol and carvacrol content.

Introduction

Thyme (Thymus L.) belongs to the Lamiaceae family and consists of about 215 species spread worldwide. The Mediterranean region is considered as the center of origin of that genus (Mozaffarian, 1998, Morales, 2002). It is a perennial dwarf shrub used as a medicinal and spice plant due to its volatile constituents. Fourteen Thymus species have been reported in Flora Iranica, four of which have been known to be endemic (Rechinger, 1982). Thymus species are commonly used as herbal tea, flavoring agents (condiment and spice) and medicinal plants (Morales, 2002) Thymus daenensis. subsp. daenensis Celak is an endemic to Iran subspecies which grows at high altitudes in Zagros mountains range (Ghasemi Pirbalouti et al., 2011, Rahimmalek et al., 2009). Its fresh leave and flowers can be used as aroma additive in food, pharmaceuticals and cosmetics (Nickavar et al., 2005). Infusion and decoction of aerial parts of Thymus species are used as tonic, carminative, digestive, antispasmodic, anti-inflammatory, antitussive, expectorant and for the treatment of colds in Iranian traditional medicine (Nickavar et al., 2005, Zargari, 1990). Recent studies have showed that Thymus species have strong antibacterial, antifungal, antiviral, antiparasitic, spasmolytic and antioxidant activities (Ghasemi Pirbalouti et al., 2011, Morales, 2002).

The demand for high-quality dried food products is permanently increasing all over the world. The main purpose of drying is to extend product shelf life, minimize packaging requirements and reduce shipping weights (Hamrouni-Sellami et al., 2011a). Drying process increases the shelf life by slowing microorganisms growth and preventing certain biochemical reactions that might alter the organoleptic characteristics (Díaz-Maroto et al., 2003, Hamrouni-Sellami et al., 2011b). Furthermore, artificial drying has been one of the most important needs of the pharmaceutical industries, which have to use dried plants for industrial production.

Volatile constituents are the most sensitive components in the process of food drying. The effect of drying on yield and composition of essential oil in many aromatic plants has been studied by various researchers (Dong et al., 2011, Doymaz, 2011, Salehi Shanjani et al., 2010, Sefidkon et al., 2006, Hamrouni-Sellami et al., 2011b, Tarhan et al., 2011, Yousif et al., 1999). The loss of volatile constituents in herbs and spices depends mainly on drying parameters and biological characteristics of the plants (Venskutonis, 1997). Moreover, the new concern for spice plants is the final color of dried plant in the ultimate product which influences the marketing. The latter aspect is not considered in most drying researches and there are limited reports for color characteristics of dried spices (Arslan and Ozcan, 2008, Yousif et al., 1999).

Thyme is a valuable spice and drying is a useful method to increase its shelf life in food technology (Doymaz, 2011). There are few reports on different drying methods of thyme. On the other hands, most of researches were done on Thymus vulgaris (Venskutonis et al., 1996, Venskutonis, 1997, Doymaz, 2011). There is only one report about the effect of heat treatment on chemical composition and antioxidant property of T. daenensis essential oil (Alavi et al., 2010). Alavi et al. (2010) used three thermal treatments including 80, 120 and 180 °C on T. daenensis shoots. The assessment of various treatments can introduce the best methods in terms of essential oil composition, yield and color quality. However, there are no reports for color changes of thyme during drying. Furthermore, different drying methods such as sun, microwave, shade, oven 50 °C, oven 70 °C and freeze drying might have variable effects on essential oil composition of different thyme species.

The aims of this study were: (1) to assess the effects of six drying methods (sun, microwave, shade, oven 50 °C, oven 70 °C and freeze–drying) on the essential oil components and yield in T. daenensis subsp. daenensis and (2) to determine the best drying method in respect to conserving the main essential oil compounds of the thyme plants such as thymol and carvacrol and to achieve the best final color of dried product.

Section snippets

Materials and methods

Leaves of T. daenensis subsp. daenensis were collected in May 2011 from natural habitat in Afoos, Isfahan province, Iran (32°59′8N and 50°24′43E) at altitude of about 1900 m above sea level. The identity of the plant was confirmed by V. Mozaffarian (Research Institute of Forests and Rangelands, Tehran) using Flora Iranica (Rechinger, 1982). In order to preserve their original quality, leaves were stored in a refrigerator at 4 °C for 2 h before drying experiments. The initial samples were divided

Effect of drying methods on the essential oil and moisture content

The effect of drying methods on the essential oil (EO) yield of thyme leaves is illustrated in Fig. 1. Results showed that the essential oil yield of thyme was influenced by different drying methods. Generally, drying the plant material before distillation resulted in both increased and reduced essential oil yield depending on time of drying and temperature. (Hamrouni-Sellami et al., 2011a). Freeze drying showed the highest oil yield (1.7%) whereas those dried by microwave and shade drying were

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