Pepper-oil.—The fruits of Piper nigrum (see Spices—Pepper), when distilled with water, give a thin, colourless oil, of hot, peppery odour and flavour, sp. gr. 0.864, boiling at 167*°-170° F.).
Other Piper-oils are described under Cubebs and Matico (pp. 1420, 1424).
Peppermint-oil.—Peppermint (Mentha piperita) yields an essential oil second to none in commercial importance. The plant is found in several parts of England and the Continent, and is extensively cultivated in England, France, Germany, and N. America. It prefers good garden soil, and abundance of moisture, yielding a better oil in a temperate climate than in an arid one. The ground is well tilled some 8-10 in. deep, and the planting takes place in April-May, according to the season. The plants send out a number of runners, which take root at short intervals. These are cut off in spring when about 1 in. high, and are set out in plantations at about 1 ft. apart each way. if the soil is not very humid, or the weather wet, watering will be compulsory. Until July-August, the period of the first harvest, one or two weedings are necessary. After the cropping, runners start in all directions, take root, and cover the ground, sometimes even affording stems for a second gathering. Between August and November, one or two more weedings are required ; and towards winter, the plants are covered with a light bed of straw sprinkled with mould or dung. In the second year, the plants have completely occupied the soil ; they grow with great vigour, and attain a height of over 2 ft. Two weedings generally suffice during the second year ; one crop is cut in July-August, and a second of less importance in the autumn, the usual precautions being taken before winter seta in. In the 3rd and 4th years, the growth relaxes, and the ground has become completely filled with an inextricable mass of runners and roots. The locality is then changed, and the soil is thoroughly well ploughed up ; some few gardens, however, will last 5 years. The weight of the crop, and the yield of oil from the plants, vary exceedingly with the seasons. The 6-years' average on a French plantation of 31 aces of (119i sq. yd.), including plants from 1 to 4 years old, was 145 kilo. of fresh plants per are all round, or say 115 cwt. from an acre.
The herb, when cut, is almost universally allowed to dry on the ground before distillation. This needs the greatest degree of care, for though an incipient fermentation may, and probably does, increase the quantity and improve the quality of the oil, any excess would result. in total destruction. The advantage of sun-drying the herb before distillation has been proved by experi ments on a large scale, the product being 7 per cent. greater, and of superior fragrance. The main reason for this appears to be that, in the fresh plant, the oil-cells are so strongly protected that it is difficult to rupture them and secure their contents, while the prolonged boiling to which the herb is subjected with this object tends to destroy the natural fragrance of the oil. Some extra cost is
entailed for labour in drying ; but as it is a common custom for small cultivators to hire the use of a still at so small a charge, irrespective of the weight, an advantage is gained in having the herb dry, as more can then be distilled at once. The best moment for distilling is when the flowers are blowing, the quality of the oil being then superior, though the quantity is perhaps greater some what later. The flowering lasts about a month, and the still-accommodation should suffice to complete operations within that time. Usually the entire herb is distilled, for though the leaves and tops afford the most and beat oil, the exclusion of the stems reduces the product in quantity, and entails additional labour.
The stills used in Europe are most commonly heated by fire ; but in America, preference seems to be given to steam-heating the herb in wooden stills. A good form of still for fire-beat is such as is used for distilling brandy from grape-marc in France, built into masonry, and protected from direct action of the flue, with a copper strainer fixed about 3-4 in. from the bottom for the support of the herb. The distillation is conducted at the lowest possible temperature, and the products are cooled and separated in the ordinary manner, as described on p. 1457. In England, the water that comes over with the oil is mostly allowed to run away, and none of it is used for a second charge. In France, preference is given to the water of former charges. The advantage of old water is that, being already saturated with essential oil, it will not abstract more from the fresh plant ; but it is impossible to obtain oil of the highest quality with old waters. The duration of the operation depends upon the firing, and averages about 2; hours. The spent herb is taken out, dried, and used as cattle-food in America ; here it seems to be disregarded. The water remaining in the still after each operation becomes very foul ; the still must therefore be washed out every 2-3 days. The yield of oil is extremely variable. In England, it is usually reckoned to average 8-12 lb. from an acre, or 2i-31 lb. from a ton of dried herb, say per cent.; but some growers pretend to get 6 lb. from a ton, or per cent. The results obtained by M. L. Rose from his plantations in France were as follows :—In the 6 consecutive years 1856-61, 26,639 kilo. of the plant in flower, weighed within 24 hours of cutting, and distilled entire, gave a mean of 1 kilo. of oil from each 609 kilo. of plant, the maximum quantity required for 1 kilo. having been 638 kilo., and the minimum 548 kilo ; the mean would be 21 lb. of oil from 1342 lb. of almost fresh herb. The average in America is said to be 7 lb. from au acre of plant, but Stearns makes it exceed the English yield.