Application of Essential oils:
An alternative method for controlling post harvest losses
By: Madhu Prakash Srivastava and Neeta Sharma
Problem and need
Food security, in a populous country like India is the most pressing national issue. There never has been enough food for the health of all the people and so, the food production must be greatly increased. Besides enhancement of yield, the post-harvest losses need to be controlled. The redemption of such losses is the most feasible option for meeting the ever increasing demand for food in the future.
Fresh produce can become infected before or after harvest by pathogens widespread in the air, soil and water. It has been estimated that out of 100,000 species of fungi, less than 10% are plant pathogens and more than 100 species of fungi are responsible for the majority of post-harvest diseases. The agricultural produce before it is transported from fields and reaches the consumers, it undergoes various processing procedures, which improve their storability. The techniques and procedures followed to reduce loses after the crop harvest are referred to as post harvest technology.
Fungicides are chosen as most easily available and simply applicable option by the farmers. Synthetic fungicides such as Imazalil, thiabendazole, pyrimethanil prochloraz and guazatine are generally used on packlines as the first line of defense against post harvest pathogens. In recent past many problems have popped up relating to indiscriminate use of fungicides as post-harvest chemicals. The severity of damage caused by direct application of fungicides to agro-products is much higher than those associated by their use as foliar sprays or soil mixes.
The chemical pesticides have been tested for their ill effects such as reproductive toxicity and carcinogenesis in mammals. High doses of these agents have been proved to be fatal to animals and fruits. The direct exposures of humans to the fungicides have caused grave health concern due to residual toxicity and long degradation periods and other side-effects on humans. In recent years public perspective towards the use of chemicals as pesticides has shown drastic shift due to hazards such as pest resurgence, environmental as well as soil pollution. Ultimately, biomagnification of pesticides in the food chain may occur through accumulation of these xenobiotic substances in organisms Dip solutions such as Imazalil, prepared in large tanks of at least 1500 l volume, are maintained for several days before the residue is disposed. Toxic waste disposal is a costly exercise and hazardous waste poses serious environmental problems.
Resistance in populations of post-harvest pathogen to commonly used fungicides has posed a serious problem. For example; many synthetic fungicides are currently used to control blue mould rot of citrus fruit. However, the reported acquired resistance by Penicillium sp. to fungicides used on citrus has become a matter of grave concern in recent years. In view of serous side-effects of synthetic fungicides, alternative strategies need to be developed for reducing losses due to post harvest decay. These strategies include natural, non chemical and organic amendments that are perceived as safe by the public, and pose negligible risk to human health and environment.
At present biocontrol by the application of microbial antagonists (fungi, bacteria, and yeasts), natural plant-based antimicrobial substances (volatile aromatic compounds, acetic acid, jasmonates, glucosinolates, essential oils, plant extracts and propolis), antimicrobial substances from soil (fusapyron and deoxyfusapyrone) and natural animal-based antimicrobial substances like chitosan have emerged as the latest non-conventional technologies and as a new hope in the present scenario.
Over the past 20 years the use of antagonistic microorganisms has emerged as an effective non-conventional bio-control strategy to combat major post harvest decay of perishables. Bio-control offers a viable option, however, they are inconsistent, show great variability in their efficacy and confer only a protective effect that diminishes with time. The limitations are inherent in most biological control agents and their successful application will depend on integration with low toxicity compounds or chemicals generally recognized as safe.
The use of non-selective fungicides (sodium carbonate, sodium bicarbonate, active chlorine and sorbic acid) and physical treatments such as heat therapy, low temperature storage, hot water treatments and radiation can significantly lower the disease pressure on harvested commodities. Harvesting and handling techniques that minimize injury to the commodity, along with storage conditions that are optimum for maintaining host resistance will also aid in suppressing disease development after harvest. However, none of these treatments are consequently effective, and many of these cause damage to the commodities.
The alternative methods are also not completely reliable; the drawbacks as well as the risk associated with these strategies bring to focus, other control methods, particularly those which are eco-friendly.
Eco-friendly and biodegradable approaches
Various plants have an important historical tradition in healing or are particularly valued for their medicinal, savory and aromatic quality. They are considered as medicinal plants if they are collected only for their medicinal or aromatic properties Essential oils are aromatic, volatile extracts from components of such plants (leaves, flowers, fruits, bark, roots, rhizomes, and wood) that are usually obtained by technologically simple processes of maceration and water solution, steam or hydro-distillation. The extracts are used as flovours, fragnaces and for medicinal purposes or health care purposes. Nevertheless, the term volatile is preferred because it refers to the most component of the oils, which are stored in extracellular space in the epidermis or mesophyll, have low boiling points and can be recovered from the plant tissues by steam or hydro-distillation.
Production of essential oils by plants is believed to be predominently a defense mechanism against pathogens and pests and indeed, essential oils have been shown to possess antimicrobial and antifungicidal properties Essential oils and their components are gaining increasing attention because of their relatively safe status, their wide acceptance by consumers, and their exploitation for potential multi-purpose functional use. Essential oils are made up of many different volatile compounds and the chemical composition of the oil quite often varies between species. It is difficult to correlate the fungitoxic activity to single compounds or classes of compounds. It seems that the antifungal and antimicrobial effects are the result of many compounds acting synergistically. Thus, there would be negligible chance of development of resistant races of fungi after application of essential oils to fruit and vegetables. So essential oils are among the most promising groups of natural compounds that are used to develop safer antifungal agents.
Chemical composition of essential oils
Modern research has shown that medicinal plants act through a relatively small number of constituents called active principles. In a number of cases, tannins have a more extensive action than the isolated active principles. Essential oils are volatile, natural, complex compounds characterized by a strong odour and are synthesized by aromatic plants as secondary metabolites. They are liquid, volatile, limpid and rarely coloured, lipid soluble and soluble in organic solvents with a generally lower density than that of water. They can be synthesized by all plant organs, i.e. buds, flowers, leaves, stems, twigs, seeds, fruits, roots, wood or bark, and are stored in secretory cells, cavities, canals, epidermic cells or glandular trichomes.
The general categories of plant products are as follows:
The lipids including the simple and functionalized hydrocarbons, as well as terpenes;
Aromatic compounds, including phenols:
Amines, amino acids and proteins;
Nucleosides, nucleotides and nucleic acids
There is no reason to doubt that the antifungal and antimicrobial activity of some volatile oils components in vitro exhibits the same antibiotic activity in nature. The extent of recent interest in antimicrobial activity is shown by the wide range of organisms which have been tested against volatile oils, including filamentous fungi, yeasts and plant viruses The major constituents of many of these oils are phenolic compounds (terpenoids and phenylpropanoids) like thymol, carvacrol or eugenol, of which antimicrobial and antioxidant activities are well documented
Essential oils are very complex natural mixtures which can contain about 20–60 components at quite different concentrations. They are characterized by two or three major components at fairly high concentrations (20–70%) compared to other components present in trace amounts. For example, carvacrol (30%) and thymol (27%) are the major components of the Origanum compactum essential oil, linalol (68%) of the Coriandrum sativum essential oil, a- and b-thuyone (57%) and camphor (24%) of the Artemisia herba-alba essential oil, 1,8-cineole (50%) of the Cinnamomum camphora essential oil, a-phellandrene (36%) and limonene (31%) of leaf and carvone (58%) and limonene (37%) of seed Anethum graveolens essential oil, menthol (59%) and menthone (19%) of Mentha piperita essential oil. Generally, these major components determine the biological properties of the essential oils. The components include two groups of distinct biosynthetical origin. The main group is composed of terpenes and terpenoids and the other is composed of aromatic and aliphatic constituents, all characterized by low molecular weight.
Extraction of Essential Oils
Essential oils are extracted from various aromatic plants generally localized in temperate to warm countries like Mediterranean and tropical countries where they represent an important part of the traditional pharmacopoeia.
They are usually obtained by steam or hydro-distillation first developed in the middle Ages by Arabs. There are several methods for extracting essential oils. These may include use of liquid carbon dioxide or microwaves, and mainly low or high pressure distillation employing boiling water or hot steam.
Role in post-harvest
Essential oils possess a wide spectrum of different impressive qualities including antiphlogistic, spasmolytic, antinociceptive and antioxidant activity. Moreover, they exert immunomodulant, psychotropic, acaricide and expectorant effects. Due to their multifunctionality, EOs find a huge application area in medicine and aromatherapy.
The general antifungal activity of essential oils is well documented and there have been some studies on the effects of essential oils on post-harvest pathogens. These essential oils are thought to play a role in plant defense mechanisms against phytopathogenic micro-organisms. Most of the essential oils have been reported to inhibit post-harvest fungi in in vitro conditions and the reported effect of eighteen essential oils against post harvest fruit pathogens. Thyme oil proved to be the best inhibitor against all of the pathogens tested, like Lasodiplodia theobromae; Colletotrichum gloeosporioides; Alternaria citrii; Penicillium digitatum; B. cinerea. The biological activity attributed to the action of oils in in vitro conditions could be the cytotoxicity of the oil components, completely squashed and severely collapsed hyphae are recorded in case of Alternaria alternata, the conidiation was deeply hampered and ultimately at higher concentration no conidia were formed.
In vitro efficacy of citrus oil against A. niger was tested by a few researchers. Citrus sinensis essential oil caused complete growth inhibition of A. niger on agar plates. Higher concentration of oil was found to be lethal. The oil showed fungistatic activity at low concentration. The essential oil significantly reduced the growth of A. niger in a dosage -response manner. Citrus sinensis is a result of attack of oil on the cell wall and retraction of cytoplasm in the hyphae and ultimately death of the mycelium. Essential oil is found to be inhibitory for A. flavus as well as A. parasiticus, the two aflatoxin producing fungi, the activity of oil is reported to be affecting aflatoxin production.
The small amount of oil applied to the pathogens gave a promising result to use them in vivo to manage the storage fungi. The incorporation of essential oils into fruit coatings, primarily applied to retain moisture, has gained popularity. The advantage of using coatings amended with essential oils, rather than vapour, is that there is a closer contact between the essential oils and fruit surfaces, allowing exposure of each fruit to similar concentrations of inhibitor over a longer period. Amiri et al. (2008) applied different formulations amended with eugenol oil (Eugenia caryophylata) to two apple cultivars and successfully reduced the disease incidence after cold storage. Trans (isomerized) jojoba oil was applied by Ahmed et al. as a coating for ‘Valencia’ oranges. They effectively maintained fruit quality for up to 60 days using concentrations of 20–30% of (trans)-jojoba oil
Regnier et al., (2008) successfully achieved pathogen control, without any observed physiological breakdown, by applying commercial coatings amended with L. scaberrima essential oil to mango fruit. Further, general observation was that the mango fruits with amended coatings exhibited no shriveling or browning even after ten days of storage. Other alternatives such as the inclusion of sachets impregnated with essential oils such as thymol oil during 35 days packaging reduced the development of moulds of table grapes.
E. Bosquez-Molina et al., (2010) reported for the first time the use of essential oils of thyme and Mexican lime in reducing C. gloeosporioides and R. stolonifer infection of papaya fruit. In addition, another positive effect of thyme and Mexican lime is that they do not have the strong flavor that characterizes other essential oils. Coating application extends the storage life of horticultural commodities since they cover the fresh produce by providing physical barriers to reduce loss of water vapor and aroma volatiles and delay the side effects of respiration. Some studies have concluded that whole essential oils have greater antimicrobial activity than the major components mixed, which suggests that the minor components are critical to the activity and may have a synergistic effect or potentiating influence.
Although the antimicrobial activity of essential oils is documented, the normal amounts added to foods for flavor is not sufficient to completely inhibit microbial growth. The antimicrobial activity varies widely, depending on the type of essential oils, test medium, and microorganism. For these reasons, essential oils should not be considered as a primary preservative method. However, the addition of essential oils can be expected to aid in preserving foods stored at refrigeration temperatures, at which the multiplication of microorganisms is slow.
A partial listing of the summary of the antimicrobial effectiveness of essential oils is as follows.
Microorganisms differ in their resistance to a given essential oil.
A given microorganism differs in its resistance to various essential oils.
Bacteria are more resistant than fungi.
The effect on spores may be different than that on vegetative cells.
Gram-negative bacteria are more resistant than gram-positive bacteria.
The effect of essential oils may be inhibitory or germicidal.
Essential oils harbor microbial contaminants.
Essential oils may serve as substrates for microbial growth and toxin production.
Amounts of essential oils added to foods are generally too low to prevent spoilage by microorganisms.
Active components of essential oils at low concentrations may interact synergistically with other factors (NaCl, acids and preservatives) to increase preservative effect.
Nutrients present in essential oils may stimulate growth and/or biochemical activities of microorganisms.
Application of essential oil is a very attractive method for controlling post harvest diseases. Thus, food product safety and shelf life depend, in some measure, on the type, quantity, and character of essential oils added to the products.
Mycology & Plant Pathology Division, Department of Botany, University of Lucknow, Lucknow-226 007. India