Comparison of Microwave-Assisted Hydrodistillation and Hydrodistillation Methods for the Fruit Essential Oils of Foeniculum vulgare

Journal of Essential Oil Research: JEOR, Sep/Oct 2007 by Kosar, Müberra, Özek, Temel, Kürkçüoglu, Mine, Baser, K Hüsnü Can

Abstract

Microwave-assisted hydrodistillation (MWHD) and hydrodistillation (HD) were carried out for the analysis of volatile components in whole and ground fruits of Foeniculum vulgare Miller (fennel). Fruits were distilled using a microwave oven modified to fit a Clevenger-type apparatus. The effect of microwave energy on the yield and composition of the essential oil was investigated against the classical hydrodistillation. All the essential oils were analyzed by GC-FID and GC/MS. (E)-anethole was found as the main compound in the oils of both whole and ground materials (82.2-86.8%) using die two methods. Methyl chavicol (4.0-4.9%) and limonene (2.2-4.9%) were also found in fennel oils obtained by HD and MWHD. The amounts of identified components in the oils obtained from whole and ground fennel fruits were not affected significantly by microwave energy except for limonene.

Key Word Index

Foeniculum vulgare, Apiaceae, fennel, microwave-assisted hydrodistillation, hydrodistillation, essential oil composition, (E)-anethole.

Introduction

The fruits oiFoeniculum vulgare Miller (Fennel) (Apiaceae) are used as a spice in Europe, Northern Africa, West, Central and South Asia, as well as Turkey. Fennel fruits are used as appetizers, digestive, sedative and colic (1) and added in breads, fishes, salads and cheeses as a spice (2). Two varieties of fennel are known and reported in the European Pharmacopoeia. Foeniculum vulgare Miller subsp. vulgare var. vulgare known as bitter fennel contains (E)-anetiiole (60%) and fenchone (15%) and F. vulgare Miller subsp. vulgare var. dulce (Miller) Thellung known as sweet fennel contains (E)-anethole (80%) as die main component. The oil of fennel fruits is used as an ingredient of cosmetic and pharmaceutical products for its balsamic, cardiotonic, digestive, lactagogue and tonic properties. The composition of die oil of fennel has been reported. (E)-Anediole, fenchone (present in bitter varieties), estragóle (metìiyl chavicol), limonene, a- pinene, and a- phellandrene were found as the main components (2-5). The amount of methyl chavicol and the ratio of (E)-anethole and methyl chavicol vary between the two types. The carcinogenic effect of methyl chavicol has been reported. Scientific committee on food of European Commission reported its opinions on methyl chavicol in 2001 (6). In this report, chemical characterization, exposure assessment, metabolism, and toxicity of mediyl chavicol were given. According to this report, limited widi respect to the standard long-term bio-assays, have shown diat mediyl chavicol is a weak inducer of hepatocarcinogenicity in mice treated orally, by i.p. or s.c. injection. The induction of liver tumours seems to depend on formation of l'-hydroxymetabolites. Metabolic studies indicate that in the high dose range of carcinogenicity studies (150-600 mg/kg bw) the production of G-hydroxy-methyl chavicol, expressed as percentage of die dose, is about 5-10 times higher dian that at lower doses (0.05-50 mg/kg bw). l'-Hydroxyestragole has been found also in the urine of men dosed with 100 µg estragole/day for six months. Mediyl chavicol occurs naturally in a variety of foods including tarragon (60-75%), sweet basil (20-43%), sweet fennel (5-20%), green anise (1%), and star anise (5-6%). There are 28 food categories identified by industry to which mediyl chavicol can be added. For these food categories, a concentration of 10 mg methyl chavicol/kg food was assumed for food in general and a concentration of 50 mg/kg for food containing herbs and spices. For alcoholic beverages, canned fish and fats and oils, die following concentrations were applied as specified by die Council of Europe (2001): alcoholic beverages 100 mg/kg; canned fish 50 mg/kg; fats and oils 250 mg/kg (6).

Fennel oils are obtained by distillation of dried fruits . There are several reports on their composition (2-5). Conventional hydrodistillation (HD) using modified Clevenger-type apparatus is widely used to obtain the essential oil from plants for scientific investigations and for oil yield determinations. It involves distillation of die plant material in water for 3 h. Microwave-assisted mediods have been used increasingly in die last few years especially for extraction (7-12). Microwave-assisted hydrodistillation (MWHD) metiiod is amore recent technique used to recover volatile components (13-17). In this method, plant material placed in a Clevenger-type apparatus is heated inside a microwave oven for a short period of time to extract the essential oil. Heat is produced by microwave energy. The sample reaches its boiling point very rapidly, leading to a very short extraction or distillation time (7-17). Widi the microwave distillation technique it is possible to achieve distillation with the indigenous water of the fresh plant material.

The aim of diis study was to provide a rigorous comparison between microwave-assisted hydrodistillation and classical hydrodistiUation techniques to obtain essential oil from fennel fruits. Both whole and ground fruits were evaluated from the point of view for the effect of microwave energy on oil composition.