Role of essential oils as feed additive
aSikhamoni Haloi, M.V.Sc
bChandra Prakash Dixit, M.V.Sc
aDepartment of Animal Nutrition, College of Veterinary Science, Assam Agricultural University, Khanapara, Guwahati 781022, India
bDepartment of Animal Reproduction, Gynaecology and Obstetrics, College of Veterinary Science, Assam Agricultural University, Khanapara, Guwahati 781022, India
Introduction:
Essential oils (EOs) are aromatic oily liquids extracted from plant parts such as seeds, leaves, twigs, flower, roots, bark, wood, fruits etc. They are steam volatile substances which are used since ages for their strong aroma and bioactivity and traditionally used to add flavour and fragrances in cosmetics and foods. EOs and its different compounds possess antibacterial, antifungal and pesticidal properties and recently have been known for its natural preservatives and additives action. The term essential comes from the theory of ‘quinta essentia’ suggested by Paracelsus Von Hohenheim in the 16th century. Essential oils are produced by secondary metabolism of plants and its bioactivity depends on their complex composition of the volatile molecules. It also differs with plant species, harvesting type and plant chemotype and their activities in animals are affected by their processing, storage and status of the gastrointestinal tract of the animals. It is found that more than 17000 plant species of about 60 plant families including Alliaceae, Apiaceae, Asteraceae, Lamiaceae, Poaceae, Myrtaceae and Rutaceae own the properties of essential oils which play a major role in protecting the plant against external agents like plant pathogen, herbivores and insects.
EOs contains various compounds like terpenes, terpenoids, phenylpropanes and phenols which impart the unique aromatic and bioactive properties in a range of plants. Terpenoids are one of the primary components of essential oil and synthesised from plant glandular tissues, particularly glandular trichomes. The two main precursors of terpenoids, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP) are formed by two independent reaction chains in a plant cell. The acetate-mevalonate pathway of cytoplasm with the condensation of acetyl-CoA leads to the formation of sesquiterpenoids while plastidial methylerythritol phosphate (MEP) pathway uses pyruvate and glyceraldehyde-3-phosphate to synthesise isoprene, monoterpenes, and diterpenes. Apart from terpenoids, the other aromatic compounds of essential oils are synthesized by a separate shikimate pathway.
Technique for extraction of essential oils:
Essential oils are generally extracted from different parts of aromatic plants like leaves, seeds, roots, and flowers through the hydro or steam distillation. Cold pressing is also used for isolation of essential oils from different parts of the plant. Supercritical fluid extraction (SCFE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), solvent-free microwave extraction (SFME), and microwave hydro diffusion and gravity (MHG) are some of the recent techniques for extraction of the volatile fraction of plant.
Antimicrobial activity of essential oils:
The essential oils are commonly used as flavouring and aromatic agent but it also has antimicrobial property. Their effectiveness depends upon the pH, concentration of the bioactive compound, type and population of affected microorganism. EOs substances such as thymol and carvacrol have an antimicrobial activity which is related to the lipophilic nature of the active compounds which cause disruption of bacterial cell membrane and mitochondria, inhibits the energy metabolism and membrane-bound electron flow that disintegrates the proton pump and drains out the ATP pool. These substances enter the phospholipid bilayer of microorganism and get situated between the fatty acid chain which makes the cell membrane permeable and increases the membrane fluidity as a result of expansion and destabilization of the membrane thereby increases the passive permeability which hinders the survivability of microorganism. The partition coefficient of EOs can also affect the rate of diffusion through the bacterial cell membrane. The antimicrobial activity of EOs are potent against both gram-positive and gram-negative organism but they are found to be more effective against gram-positive than gram-negative food spoiling microorganism as gram-negative microorganism has an outer layer surrounding the cell wall that prevents the entry of hydrophobic compound through lipopolysaccharide layer. EOs from plant compound, such as eugenol and coriander, clove, oregano and thyme are widely used as food preservatives due to their antimicrobial activity. Mint oils are used as preservatives in dairy products.
Anti-parasitic activity of essential oils:
Essential oil has been used recently in the poultry industry as coccidiostat due to prevailing resistance of anticoccidial drugs. Plant species like Dichroa febrifuga and Sophora flavescens are rich in alkaloids that act against coccidiosis. It has been found that essential oils from Carvacrol, thymol and oregano have effective anticoccidial action against Eimeria tenella, Eimeria acervulina and mixed Eimeria spp. These components are hydrophobic in nature and exert toxic effect on the upper layer of mature enterocytes of the intestinal mucosa. EOs from various herbs and spices are effective against Eimeria tenella due to its strong antioxidant property which prevents the lipid peroxidation in the gut. Essential oils from varieties of plant species have been used as anti parasitic drug such as Garlic (Allium sativum), mint (Mentha spp.) and onion (Allium cepa) are used against gastrointestinal parasites. Extract from tobacco plant (Nicotiana tabacum) is used in skin scabies treatment. The plants EOs can also be used as a possible alternative to control parasitic load in the animal. Some plant secondary metabolites have antinutritional properties. Feeding of such metabolites to the animal may cause a reduction in feed intake, toxicity and death. The consumption of condensed tannin leads to reduced feed intake, reduced digestibility and impaired rumen metabolism. Therefore, it is important to eliminate the negative effects of EOs on animal health and performance.
Antioxidant activity of Essential oils:
Essential oils can act as a natural antioxidant which provides health benefits against oxidative stress in animals. They are also used in the food industry to prevent oxidation that causes food spoliation. Antioxidant activity of EOs may be related to their ability scavenge reactive oxygen species. Phenolic compounds are found to be more potent as anti oxidant than vitamins E, C and carotenoids. EOs derived from rosemary (Rosmarinus Officinalis), lavender (Lavandula spp.), eucalyptus (Eucalyptus oblique), clove (Syzygium aromaticum), chamomile (Anthemis nobilis), ginger (Zingiber officinale),), peppermint (Mentha piperita) and thyme (Thymus vulgaris) are found to be effective against oxidative damage.
Immuno-stimulating activity of essential oils:
The immunostimulatory actions of various EOs like garlic and oregano have been observed in animals. Inclusion of garlic in chicken diets increases the antibody production against Salmonella enteritidis, Pasteurella multocida and Leptospira Pomona. It has been observed that incorporation garlic in the diet of broiler increases antibody titre against new castle disease and infectious bursal disease virus. Supplementation of garlic in bird increases the lymphocyte proliferation and WBC production which leads to an increase in weight of spleen and thymus. The immune-stimulating activity of garlic may be related to its ability to enhance phagocytosis of peritoneal macrophages. They increase the production of interleukins, interferon (INF-γ) and tumour necrosis factor (TNF-α), enhance the secretary metabolism of macrophages and increase the antigen-presenting cells and antioxidant activity.
Anti-inflammatory activity of essential oils:
The anti-inflammatory effects of EOs were elucidated in number of studies. Essential oils are rich in the phenolic compound which has strong anti-inflammatory action. Extracts of curcuma, red pepper, black pepper, cumin, cloves, cinnamon, mint and ginger showed to have significant anti-inflammatory effects. Anti-inflammatory effect of EOs can be attributable to its ability to scavenge reactive oxygen species which decreases the oxidative damage in tissue and thereby reduces inflammation. Terpenoids and flavonoids are major substances of EOs that possess anti-inflammatory activities. They suppress the metabolism of inflammatory prostaglandins. Flavonol glycosides and flavonoid aglycons exert anti-inflammatory activity in the animal by modulating arachidonic acid metabolism through inhibition of cyclo-oxygenase and lipooxygenase activity.
Effects of essential oils in pig and poultry:
Essential oils derived from different species of plants are used in pig and poultry to promote growth and production. Their effects are variable due to differences in animals’ gastrointestinal tract anatomy, function and have a positive effect on the gastrointestinal tract. They increase the palatability of feed, stimulate secretion of digestive fluids, improve intestinal morphology and reduce inflammation. Increased palatability stimulates the appetite and thereby increases the feed intake. Thus they improve the overall growth of animal. Effects of EOs are variable among species. Pigs are sensitive to odour. They do not prefer ginger or oregano in the feed as they have a strong aroma. Improved nutrient absorption has been observed in pig and poultry fed on EOs at the rate of 24mg/kg of complete feed and 1-10g/kg of feed, respectively. EOs increases the absorptive surface area and height of villi in the small intestine of pigs and poultry and also has a positive impact on bile synthesis, secretion of saliva, bile and mucus and activity of the enzyme. The antibiotic activity of EOs reduces the effects of pathogenic bacteria and promotes the growth of probiotic microflora in pig and poultry. Moreover, EOs act as an immunomodulating agent and heat stress reliever in pig and poultry. The antioxidant properties of EOs prevent spoilage of egg and meat by reducing lipid oxidation.
Effects of essential oils in Ruminants:
During the last few years, the concern over the use of antibiotic in ruminants has increased due to emerging antibiotic resistance and its transmission to human. The scientist is exploring the alternatives of antibiotic to improve feed efficiency and animal productivity in ruminants. Essential oils can be used as alternatives to antibiotics due to their antimicrobial properties and its consumption at the rate of 0.2 to 2g/kg of feed improves nitrogen and energy utilization in ruminants. They reduce deamination of amino acids (AAs) and production of ammonia nitrogen in rumen and can also inhibit the process of ruminal fermentation thereby decrease the total volatile fatty acid production. The active component of EOs selectively inhibits the ruminal methanogenesis which reduces the production of methane. These responses are evident with high doses of EOs. The active components of EOs favourably alter rumen fermentation and improve feed efficiency and promote health. The uses of novel EOs in ruminant nutrition are advantageous to animal production and the environment.
Conclusions:
Essential oils and their compounds are efficient as antimicrobial, antioxidant, immuno-stimulating and anti-inflammatory agents. These compounds can be considered as safe as a feed additive. Essential oils and their compounds can be used as natural non-antibiotic growth promoters in an animal where problems of resistance or unwanted side-effects of chemical substances, such as antioxidants or antimicrobials arise.
