Information provided by Hopunion
The use of hops for beer production has been documented back to 736 AD in south central Europe. They were introduced into the United States in 1629 by the colonists. Today virtually all commercially grown hops are found in the Pacific Northwest states of Washington, Oregon, and Idaho.
Hops are perennial plants that can be male or female. Male plants are used in breeding and have no use in the brewing of beer. The female plant on the other hand produces a hop cone which contains the chemical properties that are used in the brewing process. This material is described as hop lupulin.
There are basically two types of hops. The aroma hops are typified by low alpha acids, higher levels of beta acids, and an oil profile associated with good aroma. These hops would generally be used as a finishing or conditioning hop. Bitter hops have a much higher level of alpha acids than beta acids. These are generally used in the boiling process to extract bitterness. There are some varieties considered dual-purpose, such as Perle,Cluster and Northern Brewer that can be used in both parts of the process.
There are a number of ways to use hops in the brewing process. Whole hops are the natural hop cones that have been dried and baled. It can be argued that this form is the most inconsistent, bulky, poorest storage, and inefficient way to brew of all product forms. Still a number of the world's brewers use the whole hop claiming they prefer the all natural product.
Hop pellets are basically whole hops that have been ground through a hammer mill and then pressed together through a pellet die. The ground hops are kept together as a compressed pellet by the hops natural resins. No additives have been put into a standard type 90 pellet. This product is then put into a vacuum foil package. The major advantages are less storage space, better consistency, and enhanced utilization. The disadvantage is that the crushing of the cones changes the behavior of the hops to some extent that can result in different beer flavor.
Many breweries around the world use a liquefied form of hops called hop extract. Basically this process removes the resins from the vegetative hop matter. There are many types of this product. Advantages are consistency, less storage space required, minimal deterioration, and greater utilization. A disadvantage is that the hop has been changed in character.
Some other products are hop essential oils and essences. As the beer industry changes, many of these later products are gaining predominance.