Hops
Karl J. Siebert
Introduction
Of the four main ingredients of beer (malt, hops, yeast and water), hops
can, but don't necessarily, contribute two aspects of beer flavor, the
characteristic sharp bitterness and hop aroma. Brewers often think of
hops as either "bitter hops" or "aroma hops". Bitter hops contribute
mainly alpha acid (which leads to bitterness); their essential oil
content is not considered very significant. Aroma hops contribute a
significant amount of desirable essential oils and typically lower
amounts of alpha acid.
Hops
The only significant commercial use of hops is in brewing. Hops are
climbing bines (not vines) that are typically grown on strings leading to wires 14 feet
above ground. Hops can grow in a number of climates, but the aspect of primary
interest to the brewer, alpha acid production, is triggered by a shift
in day length from about 16 to 14 hours. This only occurs at latitudes
far removed (either North or South) from the Equator. Hops are grown
commercially in southern Australia (especially Tasmania), Germany and
nearby parts of Europe, China, Japan and the Pacific Northwest of the
US. The alpha acid and also the essential oils are produced in the tiny
yellow lupulin glands inside the hop cones. The cones are green and of
very low density.
Hop Processing
The hops spiral around strings
attached to the trellis. At harvest the strings are cut and together
with the attached hop bines taken to the processing plant. There the hop
cones are separated from the other matter. In the traditional method the
cones are immediately dried in a kiln and then tightly packed into 75-80
kg burlap bales. This form of hop material is referred to as 'raw' or
'leaf' hops. Today most hops are processed into hop pellets or hop
extract.
Hop Pellets
The dried hop cones are ground and
then extruded through a pellet press to make a much denser and more
compact product.
Hop Extract
Ground hops are extracted with a
solvent (typically liquid CO2). This results in material that
can readily be chemically modified (isomerized to produce isoalpha acid
and/ or reduced to prevent development of 'skunky' flavor).
Hop Use in the Brewery
Typical hop use in the
brewery is to add the hop material to the kettle in two or three
increments relatively late in the kettle boil. The slightly bitter
alpha acid is extracted into the wort and undergoes a ring rearrangement
to form the very bitter isoalpha acid. Much of the essential oil is
extracted as well. Some of the oil components distil out of the kettle,
but the more oxidized compounds remain and survive into the beer where
they may contribute the "kettle hop" aroma.
Dry Hopping
The practice of dry hopping is
rarely used by major brewers. Micro and pub breweries use it in some ale
styles. Traditionally leaf hops were placed in a burlap bag that was put
into the storage tank. This enabled direct extraction of essential oils
into the beer with relatively little opportunity for subsequent loss.
The character of the resulting hop aroma is different from kettle
hopping and is referred to as "dry hop". Today this is sometimes
duplicated by direct addition of extracted hop oil.
Changes During the Brewing Process
The alpha
acid is converted into isoalpha acid. Practically no alpha acid survives
into the finished beer. The essential oils are extracted in the kettle.
The low boiling compounds are removed by distillation. The higher
boilers remain. The more non-polar compounds have low solubility in wort
and beer and are lost in foam, by adsorption onto yeast or vessel walls.
The more polar compounds survive. Some of these are modified during
fermentation by reduction or esterification.
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FS430:
Understanding Wine and Beer