Hops: Oils/Acids, Varieties, and the Struggle

Hops are one of the four major ingredients in beer. They provide a complexity through bitterness, aroma, and taste. During the brewing process, they are added during the boil for a length of time dependent on their role in the beer. As the time the hop is exposed to the heat of the boil increases, the flavor components will go from aroma to flavor to bitterness. This is due to the fact that different chemicals are created or driven off depending on the boil time.

Oils and Acids

They glory of hops lies in their oils and acid and each variety contains a unique chemical package. The acids are typically described with two different quantities: %alpha- and %beta- acids. Alpha acids are largely responsible for the bitter flavor in beer, while beta acids contribute to the bitterness to a lesser degree. It is interesting to note that hops high in alpha acids posses strong anti-bacerial properties that promote exclusively brewer's yeast to drive the fermentation (as opposed to countless bacteria).

Alpha acids refer to a class of chemicals that posses a carbon hexagon with branch groups containing ketones and alcohols. Under enough heat, these chemicals can be isomerized. That is, the atoms contained within the molecule will change in relation to one another. Isomerization does not refer to a chemical reaction where atoms are switched between molecules, only a restructuring of the molecule. Heat is required to isomerize the compounds, which is obtained in during the boil. As the hops boil a longer period of time, a greater proportion of the alpha acids are converted to iso-alpha acids, resulting in a more bitter beer. Bittering hops are boiled for at least 45 minutes for most beers. Hops varieties used for bittering will have high levels of alpha acids, up to 20% alpha acid by weight, while those for flavor and aroma can be less than 5% by weight.

Beta-acids differ from alpha-acids in that they cannot be isomerized. However, a chemical reaction with oxygen will yield a compound that is perceived as bitter. Iso-alpha acids will degrade with time, making the oxidation of beta-acids play an important role in lagers and aged beers. The ratio of alpha acids:beta acids changes depending on the variety, but there is almost always more alpha acids than beta acids. The alpha:beta ratio ranges between 1:1 to 2:1.

The majority of the weight in flavor/aroma hops comes from essential oils as opposed to the acids. These oils, which vary in concentration among different varieties, are highly volatile and are driven off during a long boil. Brewers will add these hops either: 1)in the mash which doesn't get above 170degF,  2) in the last few minutes of the boil ( <5 minutes), or 3) into a fermentor in a process called 'dry hopping'. By minimizing the amount of time the hop tissue is exposed to high heat, the volatile compounds are maximized.

Varieties

Noble Hops - Europe. Much like wine, the specific climate and soil conditions have a huge effect on the chemical profile of hops. There are four types of noble hops: tettnang, saaz, haullertau, spalt. These hop varieties are named after the region in which they are grown. Each variety is named after the town or region in which it is cultivated, in the same way burgundy wine is named after the region in France. Geographically speaking, saaz is from the Czech Republic, while the others are from Germany. Noble hops have low levels of alpha- and beta- acids (<5%) and an alpha:beta ratio ≤1. They are renowned for their consistent bitterness with age, and are the most common hop variety in most German/Czech lager and pilsners.

Sorachi Ace - Japan. Sapporo brewing company in Japan developed this hop from Saaz and Brewer's Gold. Their intention was to create a hop that would be similar in flavor to the Czech Saaz, but would possess the intense bittering power of the high alpha acid Brewer's Gold. While it does possess a high alpha acid content, the hop is still viable as a late addition hop due to it's unique lemon/spice flavor and aroma. It was first cultivated in the 1970s in Japan, but the hop wasn't available in the US until 2006. During the hop shortage in 2008, many breweries and home brewers began to experiment with it, with some having commercial success (Brooklyn Sorachi Ace saison).

There are dozens of other hop varieties grown though the US, Canada, and the UK. Each region has its own unique varieties breed for bittering, aroma, or flavor.

The Struggle

Roy Farms Hops

Hops are a pain in the ass to grow and harvest. The picture above shows a typical hop farm. The hops grow on bines which require a trellis for support. As the hops grow up the trellis, it increases the area of sunlight they are exposed to, which in turn increases the total yield of the plant. The trellises are constructed by hand, and are created whenever a new crop is planted. Each spring the vine starts from scratch, and will grow to the top of the trellis by the end of June. Each plant has to be trained by hand to grow up the trellis and not across the ground.

The recent craft explosion has put a strain on the hop growers around the world, particularly the United States. Hops can only be grown in certain regions, and the only areas suitable in the united states are in the northwest (Washington, but some in Idaho and Oregon). While the recent craft beer explosion provides a new market for hop growers, the constant thirst for something new creates demand for new hop varieties. Maintaining a hop crop is hard enough, but to switch between varieties can lead to financial ruin. Hop plants do not produce a crop their first year, so farmers will typically sign contracts with hop distributors guaranteeing that their crop will be purchased for several years regardless of demand. It is for this reason that hops are so damn expensive.

Preparing the Malt

Before the brewing process begins, the barley has to be partially germinated in a process called modification. The modification metabolizes the embryonic nutrients to produce a nugget of starch, which is in turn converted into simple sugars in the mash. After the barley is modified, it may be roasted to add complexity to the malt profile. While kilning produces a more complex flavor, it destroys the enzymes responsible for converting the starches into sugars. A beer will often get the enzymatic power from lowly/un-roasted barley, and will use kilned malts to add flavor complexity.

Anatomy of barley kernel  (Palmer, How to Brew)

The modification process was likely discovered by accident. Barley kernels which are soaked in water and then kept moist for several days will begin to germinate. The figure to the right shows the inside of a barley kernel a few days into the modification process. Endosperm contains globules of starch contained within tough cell walls. As the kernel germinates, the acrospire metabolizes the cell walls of the endosperm as it beings to engulf the volume of the kernel. If the acrospire takes up less than 75% of the volume, it is considered under-modified; if it beings to sprout, it is over-modified.

As the acrospire metabolizes the endosperm, it liberates the starches which will eventually become the sugars that the yeast turn into alcohol. The modification process changes the physical as well as the chemical properties of the kernel. Prior to modification, the kernel is hard as a rock due to the cell wall infrastructure. As the kernel is modified and the cell walls metabolized, the kernel becomes more friable and won't break your tooth during a sensory test.

The modification process requires the kernel to be saturated; up to 50% water by weight. This water content must be reduced to below 5% by slowly increasing the temperature to 100degF. After the malted barley has a low enough water content, it is then kilned to halt enzymatic activity as well as impart a complexity of sweetness. Pale and pilsner malt are kilned and low temperatures for a brief duration, while chocolate and Black Patent malt are essentially broiled for hours. There is a spectrum of malts between the two classes that is kilned at intermediate temperatures and duration. The kilning process was mechanized during the Industrial Revolution by Daniel Wheeler's invention of the drum roaster.

The kilning process provides heats that destroys some of the proteins required to convert starches into sugars. For this reason, you cannot make a beer strictly out of darkly roasted malts; you must have some pale/pilsner malt to provide the catalysts involved in saccrification. The proteins in highly roasted malts are broken down and often react with sugars in a Maillard Reaction

The Maillard Reaction is what gives steak and bread their color and flavor. It occurs between 140 and 160 degrees Fahrenheit, when sugars form compounds with amino acids. This reaction "stores" the sugar as yeast are unable to metabolize it. This reaction is responsible for the sweetness of Samuel Adam's Boston Lager. 

The Reinheitsgebot forced brewers to be creative with the four ingredients allowed. This lead to the spectrum of malts as well as the variety of different styles that exist today. The modification and kilning process have become so intricate, that they are typically preformed by maltsters and not your average brewery. While the brewer does not have to prepare their own malt, they are still responsible for converting the starches stored in the malted barley into fermentable sugars in a process known as mashing.