Biological Significance of Spicy Essential Oils

Sumitra Maurya; Ajeet Kumar Kushwaha; Gurdip Singh

Abstract

Spices are used as food additives since ancient times， as flavouring agents but also as natural food preservatives. Spice essential oils are complex mixtures of volatile substances， ordinarily terpenes， sesquiterpenes and oxygenated derivatives. They have been largely employed for their properties already observed in nature i.e.， for their antibacterial， antifungal and insecticidal properties. At present， approximately 3000 essential oils are known， 300 of which are commercially important especially for pharmaceutical， agronomic， food， sainitary， cosmetic and perfume industries. It is important to develop a better understanding of their mode of action for new applications in human health， agriculture and environment. Some of them constitute effective alternatives or complements to synthetic compounds of chemical industry.

Key words： Spice essential oil； Antibacterial； Antifungal； Insecticidal； Food additives

INTRODUCTION

Spices， which are important part of human diets， have been used for thousands of years to enhance the flavour， colour and aroma of food. In addition to boosting flavour， herbs and spices are also known for their preservative（Neilsen & Rios， 2000）， antioxidative （Shobana & Naidu， 2000）， antimicrobial and various other medicinal values （Wood et al.， 2001）， which forms one of the oldest sciences. Scientific experiments since 19th century have documented the antioxidative and antimicrobial properties of spices and their components. Essential oils of spices are volatile， natural complex compounds， characterized by a strong odour and are found in aromatic plant as secondary metabolites for which aromatic plants are used in the pharmaceutical， food and fragrance industries. They comprise as several hundred constituents， especially hydrocarbon （terpenes and sesquiterpenes） and oxygenated compound （aldehydes， ketones， alcohols， acids， phenols， oxides， lactones， acetate， ethers and esters）（Anitescu et al.， 1997）.

The extraction of essential oil components using solvents at high pressure， or supercritical fluids， has received much attention in the past several years， especially in food， pharmaceutical and cosmetic industries， because it presents an alternative to conventional processes such as organic extraction and steam distillation （Eikani et al.， 1999）. The extraction product can vary in quality， quantity and in composition according to climate， soil composition， plant organ， age and vegetative cycle stage （Masotti et al.， 2003； Angioni et al.， 2006）. A lot of data of spice essential oils are reported in the literature but they are not systematical and concise. Therefore， this review is written in keeping in view to provide a better understanding of methods of extraction of spices essential oils and their mode of biological action for new applications in human health， agriculture and environment. Some of them constitute effective alternatives or complements to synthetic compounds of chemical industry.

1. BOTANICAL ASPECTS

Essential oils may be found throughout the plant cellular tissue or in special cells， glands or ducts located in several parts of the plants， i.e.， in the leaves， barks and roots， flowers， fruits and seeds， sometimes confined to special structures and in others not localized. The type of structure of the secretory tissues is one of the characteristics of the botanical family. With few exceptions （e.g.， clove）， the essential oil is present at a low percentage and constituents only a small fraction of the total plant weight. The quantity and composition of the essential oil varies not only with the type of plant but also in particular， with the conditions prevailing during its growth i.e. climate， soil， altitude etc. The function of essential oils in the living plant tissue is far being completely understood. Odours of flowers， for instance， may be directly associated with insect attraction or repulsion and so influences pollination and some to act as a form of protection against parasites and others have such a repulsive odour which affords the plant protection from animal depredation （Heath， 1978）. Plant produces volatile oils because of the flowing biological activity （Hay& Waterman， 1993）.

a. Attractants： Volatile oils associated with flowers can play a significant role in attracting pollinators.

b. Feeding deterrents： Mono and sesquiterpenes have both been widely implicated in the defence of plants against herbivores （e.g.， Leaf cutter ants）. Some simple acyclic sesquiterpenes appear to act as insect juvenile hormones； for example， juvabione， which occurs in basil （Ocimum basilicum）， has the ability to arrest the development of some insects， leading imperfect adult forms or the complete failure to produce adults. Some components of volatile oils， notably alcohols such as linalool and α-terpineol are able to reduce digestive efficiency in ruminants by their antibacterial activity.

c. Monoterpenes as natural products： When oils glands are fractured， the monoterpenes flow rapidly over the leaf surface carrying with them the less-volatile germacrone. They then evaporate to leave the germacrone more widely distributed.

2. CHEMICAL ASPECTS

3. METHODS OF EXTRACTION OF ESSENTIAL OILS

3.2 Solvent Free Microwave Extraction （SFME）

Recently， an original method for extracting natural products by using microwave energy has been developed（Chen & Spiro， 1994； Pare & Belanger， 1997； Lucchesi et al.， 2004）. One of the advantages of the SFME is rapidity and high yield. Based on relatively simple principle， this method involves placing spice material in a microwave reactor without any added solvent or water. The internal heating of the in situ water within the plant material distends it and makes the glands. This process thus free essential oils， which is evaporated by the in situ water of the plant by azeotropic distillation. The vapours then pass through a condenser outside the microwave cavity， where it is condensed. The distillate is collected continuously in the receiving flask. The excess of water is refuxed through the extraction vessel. The essential oil is collected directly and dried without any added solvent extraction. SFME is based on the combination of microwave heating and dry distillation， and is performed at atmospheric pressure. The isolation and concentrations of essential oils are performed in a single stage. The obtained solution can be analysed directly without any preliminary clean-up or solvent exchange steps by GC-MS. SFME extraction can be continued until no more essential oil is extracted. However， extraction time of SFME must be lower than that of hydrodistillation to be interesting in terms of time and energy savings. The irradiation power determines the rate of evaporation of water or the azeotropic mixture（water and essential oil） during SFME. The greater the rate of evaporation， the greater the yield the quantity of essential oil extracted. Solvent free microwave extraction has been used to obtain essential oils from different raw materials. The SFME is neither a modified microwave assisted extraction （MAE） which uses organic solvents， or a modified hydro-distillation which uses a large quantity of water （Chemat， 2003； 2004）.

The volatile oils are a complex class of natural substances. Generally， they contain a wide variety of components， although in some instances one or a few constituents make up the major portion of the oil. These aromatic materials are evaluated on the basis of their organoleptic properties， particularly their odour and flavour. In this regards， the roles of physico-chemical constants， introduced by Otto Wallach （Wallach， 1959； Teesserie， 1964； Wallach， 1961） are still considered important. These properties （Guenther， 1948； Teranishi， 1967） included boiling point， refractive index， density， optical rotation， solubility， acid value， saponification value， hydroxyl value etc. These methods proved to be of great value in the essential industry because of their simplicity， rapidity and reproducibility. However， these routine methods are insufficiently selective when it is desired to determine， the actual concentration of particular constituents in the essential oils. The techniques used to analyse the essential oils is summarized under two heads.

a. Separation techniques

b. Identification techniques

4.1 Separation Techniques

Resolution of different constituents of essential oils and their quantitative estimation can be carried out by following methods：

a. Fractional distillation

b. Chemical methods

c. Chromatography

4.1.1 Fractional Distillation

It is the classical methods for separating the components of the volatile oils and is the backbone of essential oil industry. Through it， the complete separation of all the components is not readily achieved because many terpenoids and other classes of compounds have a small range of boiling points. This method has not been standardized with regard to distillation equipment or fractionation schedule. Fractional distillation is always carried out under reduced pressure （commonly 20-25mm of mercury）， since some polymerization occurs when the volatile oils are distillation at atmospheric pressure （Mirov， 1946）. The elevated temperatures during fractionation make the products more difficult to purify because of degradation or loss of odour. The detailed procedure is well expressed in literature.

Calcium chloride form addition compounds with some alcohols and may permit a partial separation of alcohols （e.g.， geraniol）. Other examples of crystalline adduct formations are the formation of addition products of carvone with hydrogen chloride， cineole with resorcinol and cinnamaldehyde with sodium bisulphite（Kirchner et al.， 1951）. The unsaturated hydrocarbons（terpenes） of volatile oils may be separated by the formation of bromo derivative followed by refluxing with Zn-dust. Thus limonene was separated by making its tetrabromoderivative （God leveski & Wagner， 1898）. For this purpose the essential oil containing limonene as major fraction was dissolved in equal part of amyl alcohols and ether， and then the solution was added drop by drop to ice-cold solution of bromine in ether. The solid tetrabromolimonene was separated by filtration and then converted to limonene by reducing with zinc in alcohol.