Hop acids have a mild antibiotic effect against Gram-positive bacteria which favors the exclusive activity of brewing yeast in the fermentation of beer. The flavor imparted by hops varies greatly by variety and use; hops boiled with the beer ("bittering hops") produce a bitterness while hops added to beer later impart some degree of "hop flavor" (final 10 minutes of boil) and "hop aroma" (final 3 minutes, or less of boil ) and a lesser degree of bitterness. Adding hops after the boil, a process known as "dry hopping," adds very little bitterness. The degree of bitterness imparted by hops depends on the degree to which otherwise insoluble alpha acids (AAs) are isomerized during the boil. Unboiled hops are only mildly bitter. The bitterness impact of a given amount of hops is specified in International Bitterness Units.
Flavors and aromas are described appreciatively using terms including: grassy, floral, citrusy, and spicy. Most of the common Pilsener-style beers have fairly low hop influence, while beers made by homebrewers known as "hop-heads" can be dramatically hoppy.
The first documented instance of hop cultivation is 736 A.D. in the Hallertau region of present-day Germany but the first mention of the use of hops in brewing is in 1079 A.D. Hops were introduced to British beers in the early 1500's and hop cultivation was begun in the United States in 1629.
The hop is planted on various soils, and chiefly in valleys. Hops are generally of the best quality from strong clay land. The crop, however, is there very precarious. Those on peat are much more productive, but are liable to be affected by mold in some seasons, which reduces their value considerably. The best plantations are on a deep, loamy soil, where the produce of the latter and the quality of the former are sometimes obtained. Those which are grown on sandy and gravelly lands are seldom remarkable for either great produce or superior quality.
The plant is extremely liable to disasters from its first putting up in the spring until the time of picking the crop, which is in September. Snails or slugs, ants and flies, are formidable enemies in the first instance. Frosts are inimical to its growth, and the vines are frequently blighted even after they have reached the top of the poles. Small green flies and other insects which make their appearance in the months of May and June, when the wind is about northeast, often greatly injure them, and they are subject to take damage by high winds from the southwest. The best situation for a plantation, therefore, is a southern aspect, well shaded on three sides either by hills or planting, which is supposed to be the chief protection that can be given them.
In the winter time provide the soil and manure for the hop-ground against the following spring. If the dung be rotten, mix it with two or three parts of common earth, and let it incorporate together till there is occasion to make use of it in making the hop-hills; but if it be new dung, then let it be mixed as before till the spring in the next year, for new dung is very injurious to hops. Hops require to be planted in a situation so open that the air may freely pass round and between them to dry up and dissipate the moisture, which often destroys the middle of large plantations, while the outsides remain unhurt.
The hills should be eight or nine feet asunder. If the ground be intended to be ploughed with horses between the hills, it will be best to plant them in squares, chequerwise; but if the ground is so small that it may be done with the breast-plough or spade, the holes should be ranged in a quincunx form. Which way soever is made use of, a stake should be stuck down at each of the places where the hills are to be made.
Be very particular in the choice of the plants as to kind, for if the hop-garden be planted with a mixture of several sorts of hops that ripen at several times, it will cause much trouble and great detriment.
The two best sorts are the white and the gray bind; the latter is a large, square hop, more hardy, bears more abundantly, but ripens later than the former. There is another sort of the white bind, which ripens a week or ten days before the common, but this is a tenderer and a less plentiful bearer, though it has this advantage, that it comes first to market. If there be a sort of hop you value, and would wish to increase, the superfluous binds may be laid down when the hops are tied, cutting off the tops and burying them in the hill, or when the hops are dressed all the cuttings may be saved, for almost every part will grow and become a good set the next spring.
English planters approve the months of October and March. The most usual time of procuring the cuttings is in March, when the hops are out and dressed. As to the manner of planting the sets, there should be five good sets planted in every hill, one in the middle, and the rest round about, sloping. Let them be pressed close with the hand and covered with fine earth; a stick should be placed on each side of the hill to secure it.