Hopalicious — Let’s get a little science-y about hops
Hold onto your hats, as Sarah “Salty” Otton takes us deep into the world of hop science. By Sarah Otton
This article is taken from:
Froth Craft Beer Magazine by Froth
What are they?
I assume I am writing this article for individuals who love or at least appreciate the glory and splendour of hops. But, how much do you really know? This article is written in two parts, the first is an introduction to hops and the second is where I get “science-y”. You may need a couple of ice-cold problem-solvers for the second part. Humulus lupulus is the scientific name for hops and they are a member of the Cannabaceae family. Yes, that word looks familiar, the closest botanical relative to marijuana is hops.
Hops grow on BINES not VINES which grow very, very tall (even taller than me). Hops are grown in several regions around the world, and thrive between the latitudes of 35° and 55°. The main producers are Germany and the USA. (In Australia they are mainly grown in northern Victoria and Tasmania.) According to the ever-unreliable Wiki, North Korea grew 2000 tonnes in 2012. I wonder what kind of beer powers Kim Jong-un?
The hop products in our beer are derived from the flowers (aka cones) of the unfertilised female flowers. The yellow at the base of the petals near the stem are the lupulin glands. These are the tasty bits containing hop resins, polyphenols, essential oils and other delicious things. Nerd alert! Hop resins are divided into hard and soft. Soft resins contain the hop acids, both alpha-acids (α-acids) also known as humulones, and beta acids (β-acids) also known as lupulones. I don’t know a whole lot about β-acids, my research focused on α-acids.
What I do know is that as a general rule they are undesirable as they don’t isomerize in the boil (I’ll explain this in the second part) and they contribute bitterness as a result of oxidation. Some hops are revered for their ability to provide aroma, such as the four Noble hops: Hallertau, Tettnang, Spalt and Saaz. Bittering hops, on the other hand, are full of α-acids waiting to be isomerized (I’ll explain this a bit later). So, why do we add hops to beer? Lots of reasons. Hops are pretty amazing and accomplish heaps of stuff other than contributing to the bitterness and aroma of beer.
Hops are an antibacterial and inhibit the growth of gram-positive bacteria such as Staphylococcus, Streptococcus, Listeria and Clostridium. Hops also contribute to foam stability, so your frothy stays frothy! Generally, hops are added to the “boil” phase of brewing. The bittering hops are added first so they can isomerize (explanation is coming, I promise), and aroma hops are added towards the end of the boil. Pre-isomerized hop extract can be added when moving beer from vessel to vessel, and beer can be dry-hopped, although this is for aroma.
Now, if you’ll allow me, I am going to digress into India Pale Ales (IPAs). In the 18th-century a sailor’s beer ration was one gallon or 4.5ish litres every day (again risky Wiki fact, if you’re a 300-year-old sailor and can correct me, please do). When sailing to cooler climates the beer kept reasonably well. However, as the British started to sail into warmer climates, it became infected with lactobacillus (causing sourness) and lacked carbonation. Beer was just not important for the sailor’s morale, but back then if the sailors were crook, the B-vitamins in beer actually made them healthier. And no one wanted to give them rum because we all know rum makes you crazy.
Lots of experimentation took place, including a beer concentrate which James Cook took on a voyage, but nothing worked. The beer available to the navy was sour and flat and sailors were sad. No one wants sad sailors. Then along came George Hodgson of Bow Brewery in East London. George created a new pale ale. More hops were added in the boil and dry hops were added to the casks going to India. George also added more malt to the boil, increasing the fermentable sugars which allowed the yeast to survive longer, thereby increasing carbonation.
And the IPA was born — George, what a bloody legend!
Let’s get Science-y!
So, to recap, we have a hop resin. Within this hop resin we have α-acids and β-acids. This section is going to focus primarily on α-acids, which give beer its bitterness. α-acids have five analogues, or five constituents. There are three primary α-acids: cohumulone, humulone, adhumulone, and two secondary α-acids: posthumulone and prehumulone.
Let’s focus on the primaries. The α-acids as they exist in the lupulin glands have very poor solubility and minimal bitterness. I hear you ask, “How do they become bitter? How do they become soluble?” Well, when your hops are boiled they isomerize. Isomerization is when a molecule changes — it has the exact same number and type of atoms, it just changes its shape
Now would be a good time to get an ice-cold problem-solver from the fridge. We’re about to get deep. In isomerizing, the three primary α-acids — cohumulone, humulone, adhumulone — transform into isocohumulone, iso-humulone, isoadhumulone. Each of these have a cis- and trans- configuration. Isohumulone is said to impart a soft bitterness and iso-cohumulone is believed to impart a harsher bitterness.
It is this isomerisation that enables the formation of the desirable characteristics including foam stability, antimicrobial properties, increased solubility and — maybe most importantly –bitterness. Iso- α-acid bitterness can be detected at levels as low as six parts per million in water. All iso-α-acids are sensitive to light and will rapidly undergo photo-degredation (alteration as a result of light exposure).
This is commonly referred to as ‘light strike’ and results in a ‘skunky’ aroma and flavour due to the formation of a volatile compound detectable at 2–20 parts per trillion. Beer in dark bottles, now you know why! Brown being best and green coming in second and cans let in no light so they’re better than bottles (it’s a well-known fact I love tinnies). So, what about beer in clear bottles? In the 1970s breweries started to research how they could sell a beer in clear bottles without it becoming skunky.
“No one wanted to give them rum because we all know rum makes you crazy.”
Scientists (obviously doing God’s work) discovered that you could “reduce” iso-α-acids, i.e. eliminating the double bond present in the iso-α-acid carbon side chain. There are three major reduced iso-α-acids but, I’ll just talk about tetrahydro-iso-α-acids as they are most stable and most commonly used. In figure 4 the cis- and transform of tetrahydro-iso-α-acids are shown (sorry if you have no idea what I am talking about, but stay with me). If you compare it to figure 3 there are no double bonds in the left-most carbon chain. All reduced iso-α-acids are made in laboratories.
Tetrahydro-iso-α-acids are interesting as not only do they improve light stability but also foam stability as a result from increased hydrophobicity as a result of the removal of the double bond. Interestingly, according to the Reinheitsgebot (German purity law), reduced iso-α-acids are not permitted in beer, despite being derived from hops. But who pays attention to the Reinheitsgebot these days? (apart from Germans).
Now I am sure everyone has seen or heard beer discussed in terms of IBUs or International Bitterness Units. The Australian Tax Office defines a beer (for tax purposes) as a beverage that contains hops, or extracts of hops, with an IBU reading of no less than four. IBUs are measured using a spectrometer. A sample of beer is put in the spectrometer and UV light is shone into it. Some of this light is absorbed which gives a reading, this is then multiplied by 50 and hey, presto, you’ve got an IBU reading.
However, an IBU reading does not distinguish between the constituents of hops and can be affected by a cloudy beer or anything else that also absorbs at the same wavelength. High-performance liquid chromatography is slowly getting adopted as the preferred method of bitterness or iso-α-acid concentration measurement. It extracts all the hop acids and is immune to interference from other compounds present. It is however extremely expensive.
So, as a result, we can thank tetrahydroiso- α-acids for providing clear bottle beers with the IBU measurement they need to be classed a beer by the ATO. I really enjoyed writing this article and introducing you to a bit of hop chemistry.
I hope I didn’t get to science-y. I am more than happy to explain stuff. So, if you see me, buy me a beer and ask.
