What's That Taste?

The hop (Humulus lupulus), it imparts the bitterness, the zest, tartness, sharpness, or my favourite word - piquancy to the beer. The compounds that are responsible for those lovely adjectives are called Alpha acids, and more specifically Lupulins. There are also Beta acids, I'll deal with those later.
Alpha Acids
The bitterness level of a particular hop variety is measured in percent alpha acid by weight. For instance the Saaz Hop has 3.2 - 4.5% Alpha acid. The higher the percentage, the more bitter the hop - simple. As hops age, the alpha acid oxidizes and the bittering potential diminishes. Alpha acid content is the accepted method in the brewing industry for assessing the quality hops the world over, as a result Alpha acids play a decisive role in determining prices and quantities bought in the hop trade today.
Hop varieties get their properties or active ingredients from tiny yellowish glands tucked at the base of the cones' petals within the scales of hop. The gland is covered by a waxy skin within which lies all of the hops unique bittering (alpha acids) and aromatic (oil) compounds. These glands are referred to as Lupulin. The quality of the Hop depends largely on the amount of lupulin they contain.
To compare bitterness of Czech beer, a Pilsner Urquell is about 45 IBU and a Budvar is about 35 IBU, whereas a StaroBrno has a very low IBU about 15.
Beta acids
do not isomerise, (does not react) during the boil of wort and have a negligible effect on beer flavour. Instead they contribute to beer's bitter aroma, and high beta acid hop varieties are often added at the end of the wort boil for aroma only. Beta acids can however quickly oxidize in the finished product and oxidized beta acids form sulfur compounds such as dimethyl sulfide (DMS), that can give beer off-flavours of rotten vegetables, eggs and or cooked corn. One Czech beer that seems to have higher levels of Beta acids is Krusovice.
International Bittering Units (IBUs)
A measure of the bitterness of a beer in parts per million(ppm), or milligrams per liter(mg/l) of alpha acids.
A measure of the bitterness of a beer in parts per million(ppm), or milligrams per liter(mg/l) of alpha acids.
IBU= (ounces of hops x %alpha acid of hop x % utilization) gallons of wort x 1.34
Percent utilization varies because of wort gravity, boiling time, wort volume and other factors. Home brewers get about 25% utilization for a full one-hour boil, about 15% for a 30-minute boil and 5% for a 15 minute boil. As an example, 1 ounce of 6% alpha acid hops in 5 gallons of wort boiled for one hour would produce a beer with 22 IBUs:
IBU = (1*6*25)/(5*1.34) IBUs
Hops also have antibiotic effects and contribute to bacteria free beer.
In a large Czech beer there is about 1.2 to 1.8 grams of hop.