The basic raw material for mint oil is leaves of a plant Mentha arvensis . Mentha is widely cultivated in India and its leaves are used in making a sauces/chatni in most households.
Mint oil is obtained by steam distillation of Mentha arvensis leaves. The oil is used for treating certain stomach disorders like indigestion, gas problem, acidity, etc. It is the main ingredient of ayurvedic medicines like Daburs ‘Pudin Hara’.
The oil is a natural source of menthol, which is the main ingredient of cough drops and ointments like Vicks Vaporub, etc.
The mint crop gives maximum oil content when it has just reached the flowering stage, after which the oil content begins to decline, but in cases where there is delayed flowering, as at Delhi, yellowing of the lower leaves is an indication for cutting in time. 2-3 cutting are done during the season. Freshly cut herb is left in the field for 2-4 hours during the sunny weather. This partially dried herb is further dried in small bundles by hanging over wires in shade till it is reduced to 1/3 or ¼ of its original we ight, taking care that the leaves do not get crisp. The crop should not be heaped for drying in the sun as it is reported to result in reduction of oil by about 20-25%.
Japanese mint can be cultivated both in tropical and sub-tropical areas. The mean temperature between 20-400 C during major part of the growing period and rainfall between 100-110 cm. (light showers at planting stage and ample sunshine at the time of harvesting) is ideal for its cultivation.
Well drained loam or sandy loam soils rich in organic matter having pH between 6 and 8.2 are ideally suited for its cultivation. It can also be cultivated on both red and black soil. In case of acidic soil having pH less than 5.5, liming is recommended.
Varieties are: MAS-1, Hybrid-77,Shivalik(selection from Chinese cultivar), EC-41911(selection from Russian germplasm),Gomti,Himalaya,Kosi,Saksham, Kushal.
Mint can be propagated vegetatively through stolons and runners. By and large, most area under the crop is propagated by planting live juicy 8 to 10 cm. long stolons (underground stems) during early spring season. The seed rate used is 400-450 kg. of stolons per ha. and the spacing varies from 40 to 60 cm., depending upon soil fertility and the kind of the intercultural implements used. In northern India, planting of Japanese mint is suitable from first week of February to second week of March.
Ten irrigations are given during summer season at intervals of 10-12 days whereas another 4-6 for autumn crop harvested in late October. In order to obtain luxuriant growth, sufficient fertilizers and water must be applied to mint crop. A minimum water of about 100 mm is required to obtain good crop yield. Water logging during rainy season should be avoided by providing adequate drainage. In case of heavy soils and the soils prone to water logging, it is preferable to cultivate mint on ridges. The frequency of irrigation can also be reduced by 25% through the application of leaf mulches @ 5 t/ha.
The recommended dose for chemical fertilizers is Nitrogen 120 kg, phosphorus 60 kg and potassium 40 kg per ha. The entire quantity of P and K along with one-fifth of N is mixed with the soil at the time of planting, the remaining four-fifth of N is given as top-dressing twice for each harvest in available split doses. About 20 tonnes of well-rotten FYM, 150 kg DAP and 100 kg MOP per hectare are applied at the time of planting. Subsequently, half of N in the form of calcium ammonium nitrate or urea is applied in 2 split doses at 30 and 60 days after planting and similar quantities for ratoon crop at 25 days and 45 days of the harvest.
By and large, 4 to 14 weeks after planting is crucial period for weed control. The crop requires intensive weeding and this is the most expensive cultural operation which contributes to a higher yield of the crop. Weeding with hand or mechanical hoes within the first six weeks of planting does control weeds. This process can be repeated once and rarely twice at an interval of about two to three weeks, after the first weeding. Since weeding and hoeing accounts for 30% of the cost of cultivation, use of wheel hoes either driven by hand or bullock drawn helps in reducing cost on interculture. Several pre-and post application of weedicides are recommended but these weedicides cannot control monocot weeds after the rainy season. Therefore best method is to combine manual, mechanical and chemical methods. Some of the effective herbicides includes Oxyflurofen (0.5 kg a.i. / ha), Pendimethalin (0.75 a.i. / ha), Simazine and Atrazine (1 kg a.i. / ha). The best procedure is to first apply a weedicide followed by manual or mechanical weeding at 8 to 10 weeks when mulching should also be applied.
The rotation of mint crop with other food crops is found to be a good way of controlling weeds. Continuous cropping of any of the mints is not advisable.The best rotaion is Mint : Rice and Mint : Potatoes and Mint : Vegetables : Peas etc. depending upon cropping system followed in the region.
The crop planted through stolons in January and February is harvested twice i.e. in June and October months. The first crop is harvested after 100-120 days of growth and the second harvest in about 80-90 days following the first harvest. The fresh herbage at harvesting stage contains 0.5 to 0.68% of oil and is ready for distillation after wilting for 6-10 hrs. The wilted crop is cut 10cm. above the ground by means of a sickle on bright sunny days, since harvesting on cloudy or rainy days decrease the menthol content in the oil.
The recovery of oil from the herb is 0.5-0.8%. Oil is obtained through steam distillation. The oil is of golden yellow colour, containing not less than 75% menthol. The duration of steam distillation is 2-2.5 hours for complete recovery of the oil. About 80% of the oil is received in the receiver in about one hour’s time. The oil that is received later is richer in menthol.
The fresh or semi dried herbage is placed in a tank and treated with passing steam under pressure. The steam that comes out of the tank is then passed through a condenser. The condenser receiving the steam, carrying the oil extracted from the herbage in the tank is kept constantly cool by circulating cold-water over/around it. The condensed oil and water mixture is collected in a receiver. Since the water and oil have different densities, oil floats on the surface of the water in the receiver. The oil is skimmed off and collected.
The oil that is skimmed off must be cleaned of traces of water that it may carry. For this purpose, a separator funnel is used. Treating with anhydrous sodium sulphate and decanting removes any remnant moisture in the oil. The whole process is highly critical. Steam rectification process may be applied in case the colour of the oil changes due to rusting.