Preservation of food in a sound and safe condition has long been and has remained on going challenge for humans. Micro organisms are associated in many ways with the food we eat. These microorganisms will be able to influence the quality and availability of food. Naturally occurring food such as fruits and vegetables normally contain microorganisms on their surface and may be contaminated by variety of microorganisms from food handling, equipment and other sources such as water, air, dust, and soil. But it is also well established that intrinsic factors in food and extrinsic factors of storage environment decides the type of microorganisms that dominate the micro flora.
Food may also carry pathogenic microorganisms as a result transmits diseases. Other microorganisms if allowed to grow in certain food products produce toxic substances that results in food poisoning when the food is ingested. Food borne illness caused by microbial contamination has been a serious issue in recent years. The major concern is the control of microorganisms to increase the shelf life of food products. Drying, salting and fermentation were the traditional methods of preservation. In recent times the preservation of foods by the antimicrobials of plant products against the growth of microorganisms is being studied. Food spoilage by microorganisms can be prevented potentially by the use of plant extracts that possess antimicrobial activity as food additives especially those that preserve foods and enhance food quality. This method of prevention is termed as "Allelopathy" which is real term means reputed bane-full effect of living things by some toxins i.e., Antimicrobial property (Beuchat and Golden, 1989).
The control of survival and proliferation of food associated pathogens has been the major concern in now-a-days. The incorporation of generally recognized, safe, natural compounds as additives to common food systems is highly beneficial to human health. Many higher plants produce organic compounds which possess antimicrobial activities. Plants possessing antimicrobial principles in human diet that include the Allium species are A.sativum (garlic), A.cepa (Onion) and A.porrum (leek). Many food borne pathogens were reported to be sensitive to extracts from garlic and onion. Other plant extracts especially used as spices and herbs in foods are known to possess antimicrobial activity. The active ingredients identified include allicin, thymol, cinnamic aldehyde, eugenol, and vanillin. Phenols, suphur compounds, flavanoids, alkaloids, sterols etc. are the major group of compounds identified in natural plant products having the antimicrobial property.
Essential oils are the organic compounds which are odoriferous and highly volatile in nature and obtained from the plant part or plant tissues. Essential oils have been used in gastric, discomfort, flatulent, colic and to stimulant the appetite. In recent years essential oils have not only use in perfumery but also in the pharmaceutical industry due to their antiseptic, carminative, simulative, expectorant, diuretic and irritant properties.
Incorporation of generally recognized safe natural compounds as additives to common food system is highly beneficial to human health. Many higher plants produce organic compounds which possess antimicrobial activities, include onion, garlic, and leek. Many other plants extract especially those used as herbs and spices are also known to possess antimicrobial activity. The active ingredients include allicin, thymol cinnamic aldehyde, eugenol and essential oils.
Mode of action: The antibacterial and antifungal activity of essential oils act by causing lysis of cell, reducing protein synthesis and leucine incorporation and reducing growth and development of mycelial and conidia.
Causing lysis of cells: Carvacrol a phenolic monoterpenoid that is found primarily in the essential oils of oregano (Origanum vulgare) and thymol is a phenolic monoterpenoid that is found in the essential oils of thyme (Thymus vulgaris) disintigrated the outer membrane associated material from the cells to the external medium. The intracellular and extracellular ATP levels of cells treated with eugenol, cinnamaldehyde and carvacrol inhibit the ATPase activity of bacterial cells. The decreased intercellular ATP pool of E.coli and also increased the extra cellular ATP indicating disruptive action on the cytoplasmic membrane.
The structure of the gram-positive bacteria cell wall allows hydrophobic molecules to easily penetrate the cells and act on both the cell wall and within the cytoplasm. Phenolic compounds, which are also present in the essential oils, generally show antimicrobial activity against gram-positive bacteria
Neutral recimic antifungal alcohols of 8, 0, 4-neolignan type (found in virola surinamensis, V.carnata, V.paronis, V.elongata and Myristrica fragrance) were inhibit the activity towards the fungal cell wall. Although these compounds were found to be inhibitory of the enzymes, these compounds do not act via the inhibition of glucon synthesis.
Reducing protein synthesis and leucine incorporation: Mode of action of verbascoside an anti microbial agent extracted from Buddleja cordata acts against Streptococcus aureus. Verbascoside induced lethal effect on S.aureus by affecting protein synthesis and inhibiting leucine incorporation, since leucine is an important metabolite in protein synthesis.
Reducing growth and development of mycelia and conidia: The dichloro-methane extracts of Tabebuia chrysantha, Oroxylum indicum, Fernando adenophylla and Jacaranda filicifera exhibited antifungal activity against dermatophytes and wood rot fungi. Activity has been attributed to the presence of lapachol and beta-lapachone present in these plant extracts. These extracts inhibited the development of mycelium and conidia in the filamentous fungi (Guddewar et al., 1999)
Table 1: Showing Antimicrobial property of plant extracts from different sources against various food organisms.
| Plant sources | Antimicrobial compounds | Organism inhibited |
| Carum | Carvone, limonene | Aspergillus parasiticus |
| Chilli | Capsanthin, Capsacin | Aspergillus flavus |
| Cinnamon | Cinnamic aldehyde | Salmonella listeria |
| Clove | Eugenol (4-allyl-2-methoxyphenol) | Salmonella listeria, E.coli, Staphylococcus spp. |
| Cymbopogan tree | Essential oil | Aspergillus niger , Alternaria, Trichoderma |
| Garlic | Ajoene, sulphur compounds | Bacillus, Streptococcus, E.coli, Pseudomonas |
| Ginger | Zingiberene | Lactobacillus acidophillus, B.cereus, Saccharomyces cervisiae , Aspergillus niger and Mycoderma spp. |
| Shredded carrot | Dodecanoic acid, Methyl esters of Pentadecanoic acid | Staphylococcus aureus |
| Wax gourd peel | Phenols | Salmonella typhimurium, Pseudomonas aeruginosa, Proteus vulgaris, Serratia liquefaciens, Cronobacter muytjens, Shigella boydii and Serratia marcescens |
Antimicrobials from spice extracts
Clove (Syzygium aromaticum):
Clove is known to be growth potential inhibitor of food borne bacteria and fungi due to eugenol the major component of clove oil. Essential oils active against organisms like E.coli, Enterobacter sakavaki and Klebsiella pneumonia. Eugenol inhibit the production of alpha-amylase, protease and subtilism by Bacillus subtilis and also prevent the production of extra cellular enzymes. Gram positive bacteria were more sensitive to antimicrobial compounds in spices than gram negative bacteria. At a concentration of 150ppm it prevented germination of spores and had greater inhibitory effect on vegetative growth and germicidal effect against various bacteria and Candida albicans. Clove extract posses broad range of antifungal activity against different fungi without exhibiting any phytotoxicity on the host.
Garlic (Allium sativum):
Garlic which has worldwide use as a spice in food and folk medicine with antibacterial and antifungal activities. Inhibition was presumed to be due to Ajoene, a Sulfur- containing compound present in garlic bulbs though active inhibitory principle is allicin (diallyl disulphide and diallyl trisulphide). Growth of gram positive bacteria such as B.cereus, B.subtilis, Mycobacterium smegmatis and Streptomyces griseus were inhibited at 4-5 micro grams of ajoene per milli litre.
Cinnamon (Cinnamomum verum):
Cinnamaldehyde an aromatic aldehyde is known to be the effective inhibitory compound present in cinnamon extract. Cinnamon oil had the antimicrobialactivity on germination of spores and vegetative growth of Clostridium botulinum 67B in broth media. Inhibit the Aflatoxin B production.
Ginger (Zingiber officinale): The essential oil extracted from ginger showing varied degrees of inhibition againstLactobacillus acidophillus, B.cereus, Saccharomyces cervisiae, Aspergillus niger and Mycoderma spp. ( Indu and Nirmala, 2010)
Chilli (Capsicum annum):
Capsanthin and capsaicin, the coloring and pungent principles of red chilli influenced the growth and aflatoxin producing potentials of Aspergillus flavus
Black Pepper (Piper nigrum):
Alcoholic extract of black pepper was quite active in inhibiting the Clostridium botulinium in culture media
Carum (Carumcarvi):
Carum also known as meridian fennel or Persian cumin, the fruits are usually used whole, have a pungent, anise like flavor and aroma that comes from essential oils, mostly carvone and limonene. The essential oil is used in liqueurs, mouthwashes, toothpastes, soaps, and perfumes. Carum essential oils inhibited the growth of Aspergillus parasiticus and yeasts and Gram positive and Gram negative antibacterial activity of essential oils against bacterial pathogens of mushrooms.
Antimicrobials from vegetables
Vegetables which form the important nutritious food of human diet have a very short shelf life. Microbial spoilage is more common in case of vegetables but due to the presence of antimicrobial compounds there attack by food spoilage microorganisms and the incidence of health hazardous microorganism’s growth is prevented to a certain extent.
Carrot (Daucus carota):
The antimicrobial effect of purified ethanolic extracts of peeled and shredded carrots against a range of food-borne microorganisms. 6-mthoxymellein as a common phytoalexin produced by carrot roots which inhibit the growth of several fungi, yeasts and bacteria.
Cabbage (Brassicaoleraceavar. capitata): Saurkraut, the fermentation product of cabbage contains sulfur compounds namely S-methyl-1-cysteine sulfoxie, sinigrin, dimethyl disulphide dimethyl trisulphide, methyl methanethiosulfinate and allylisothiocyanate. Cabbage extract showed antimicrobial activity against gram negative bacteria. Against fungi, only Aspergillus sp. was inhibited and to others it was less effective.
There is lot of evidence for the antifungal, antibacterial and antinematicidal properties of essential oils, they can be used for the control of both animal and plant diseases. These can become the safest control as they are non toxic to humans and animals, biodegradable and non-pollutants. They provide less phytotoxic and more systematic fungicides. There is a need for knowing the field application of these essential oils for controlling plant diseases. From the above, it can be seen that essential oils are quite effective against many fungi, bacteria and nematodes. Since, they are ecofriendly and they do not leave any residual effects, they hold a great future for their role as biopesticides.
References:
1. Beuchat, L.R and Golden, D.A. (1989). Antimicrobials occurring naturally in foods. Food Tech. 43:134-142.
2. Guddewar, M., Naik, S.N and Prasad, D. (1999). Evaluation of fungicidal activity of certain essential oils against Fusarium oxysporium Sclecht. Indian Perfum. 43(1):26-28.
3. Indu Sasidharana and Nirmala Menon A. (2010). Comparative chemical composition and antimicrobial activity fresh & Dry ginger oils (Zingiber officinale). International Journal of Current Pharmaceutical Research. 2(4): 40-43.
About Author / Additional Info:
Scientist (Horticulture), Germplasm Exchange Unit, Indian Council of Agricultural Research (ICAR)-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi-110012.