Hops are traditionally added to the beer at the brewing stage as a natural preservative and to give that bitter taste which has become a characterisic of the beer flavour. Modern varieties have been developed to increase the amount of alpha acid in the plant, and therefore, the degree of bitterness, enabling brewers either to use less hops or to produce a stronger flavour. Nowadays, the dried catkins are compressed into pellets at a factory in Nelson, making them easier to handle and transport; in some countries further processing into hop oil is carried out.