1. Where Hop Bitterness Comes From: Lupulin in Hop Flowers
Inside the female flowers of the hop plant, there is a yellow, powdery tissue called lupulin. According to an announcement by Kyoto University, hop bitterness components accumulate specifically in this lupulin [1]. The main representative of these components is alpha acid (humulone).
Does adding this powder to beer make it bitter immediately? Kyoto University’s announcement writes simply that alpha acid dissolves in beer to provide bitterness [1]. However, when reading alongside general brewing explanations, the main cause of beer bitterness is said to be "iso-alpha acids," which are created when alpha acids change during boiling [2]. This article follows that explanation, noting that the university text may have simplified the story.
So, what exactly is this change, and how much of it actually happens? We will look at this in the next section.
2. How Boiling Changes Alpha Acids Into Iso-Alpha Acids
Alpha acids change during boiling because heat alters their shape. This process is called isomerization, and the resulting substance is iso-alpha acid [2].
What is surprising is how little actually changes. In one experiment, isomerization progressed the most after 100 minutes of boiling, with a maximum of 18.1% of the alpha acid changing. It was reported that boiling for longer than this caused both iso-alpha acids and essential oils (which provide aroma) to decrease [3].
However, if conditions change, the numbers change too. Another study found that bitter acid relatives were produced most frequently when kept at 80°C for more than 100 minutes, which aligns with old brewing rules of thumb [4]. Because laboratory conditions are not the same as actual wort boiling conditions, it is dangerous to memorize 18.1% as a standard value.
How do people compare this bitterness? The next section explains the unit used.
Note: The amount of bitterness produced depends on boiling time, temperature, and ingredient conditions. There is no single fixed number.
3. How Beer Bitterness Is Measured in IBU
The unit for measuring beer bitterness is IBU (International Bitterness Units). Originally, it was defined as a calculated value: the amount of alpha acids from hop resin multiplied by 5/7 (a historical correction factor). As a rough guide, 1 IBU corresponds to about 1 mg/L (1 ppm) of dissolved iso-alpha acid [5].
References note that American mass-produced lagers are around 5–10 IBU, while American IPAs are around 55–70 IBU [5]. This means bitterness can differ by more than five times depending on the beer type. However, remember that 1 IBU equals 1 mg/L is only a guideline; actual measured values may differ slightly.
Beyond taste, do these bitterness components have other roles?
4. How Bitter Components in Hops Help Stop Bacteria
There is a quote attributed to Hildegard in the 12th century, stating that hops prevent the spoilage of drinks to some extent. However, this quote comes from secondary sources, and the original text has not been verified [6].
In current brewing explanations, iso-alpha acids are said to suppress the growth of gram-positive bacteria (such as lactic acid bacteria) that spoil beer. One study introduced an example where the growth of a specific bacterium, *Lactobacillus buchneri*, was completely stopped at 10 ppm. This result applies only to that specific bacterium and condition [6]. On the other hand, these acids do not work against gram-negative bacteria [6].
The explanation is not that "hops kill bacteria," but that they suppress the rate of bacterial growth. The component used to create bitterness is also involved in making beer less prone to spoilage.
However, these components have a weakness: light.
5. Why Beer Left in the Sun Smells Skunky
When light hits beer, riboflavin (vitamin B2) and other substances in the beer are excited by the light. This energy causes isohumulone to break down. As a result, a substance called 3-methyl-2-buten-1-thiol (MBT) is created, producing a skunky "lightstruck" smell [7].
The surprising part is the amount needed. It is reported that as little as 1 ng/L (one-billionth of a gram per liter) is enough to make the beer unpleasant to drink [7]. Only a tiny amount is needed in one liter.
Iso-alpha acids change into another substance under light, causing the smell. This shows that iso-alpha acids can change under light.
6. How You Can Investigate Hops and Beer Further
What you can do today: Compare beer bottles and cans at a store or at home. Colors and materials vary. After reading about how light changes the smell, you might wonder if the container color has a reason. This article’s sources do not explain the reason for container colors, so talk to an adult or research it yourself. Beer is an adult drink. Children should not drink it.
If you want to learn more: Reading the Kyoto University announcement shows where hop bitterness components accumulate. Reading the IBU explanation reveals how bitterness numbers are created.