Lesson 2 - Wort Boiling Theory

Beer foam is bubbles of gas surrounded by beer. Amphiphilic substances in beer help to prevent the gas bubbles from collapsing. The most important substances in the beer in this regard are proteins.

Two proteins have been identified as being particularly good at stabilising beer foam. These are protein Z and lipid transfer protein one (LTP1). Protein Z was the first protein to be identified as foam positive. It is a major protein found in beer accounting for 10-25% of the non-dialysable protein. It has a molecular weight of around 45 kDa. In malting and/or during the boil, protein Z is glycated (has carbohydrate attached to it). This is thought to be through the Maillard reaction. The glycosylated protein Z is much more effective at promoting foam than the non-glycosylated form.

LTP1 in barley unsurprisingly transfers lipids and is thought to be involved in protecting the barley from microbial pathogens. In its undenatured form LTP1 is not particularly foam positive. However, when it is denatured, or unfolded, during wort boiling, it becomes more hydrophobic which means it is able to associate with the gas in the foam bubble more easily. As a result, it holds beer in the foam and supports the gas bubble.

In wort boiling you can have too much of a good thing when it comes to foam stability. Experiments where wort was boiled under pressure at 103°C found that the resultant beer had poor foam stability. Another study found that a 102°C boil produced poorer foam stability than a 96°C boil where the level of lipids in the beer were higher. This suggests that the proteins’ ability to prevent lipids reducing foam stability was reduced when boiled at a higher temperature.

Use of antifoam during boiling

While we are on the subject of foam, it is a good opportunity to discuss the use of antifoams. Every time foam is produced in the brewing process, there is a risk that the compounds that support foam are lost in the foam that is formed. Some brewers feel that foaming in the kettle reduces beer foam stability. Therefore, we try to keep it to a minimum. Foaming in the kettle also leads to the loss of hop compounds and increases the risk of wort overflowing the vessel. This reduces yield and presents a hazard in the brewhouse. To prevent foam, some brewers add antifoam to the kettle. There is a range of products available with some based on hop hard resins, lipids, and waxes obtained by solvent extraction of hops or fatty acids from malt. The most popular antifoam is dimethylpolysiloxane which is added at a rate of a few mg/L to the kettle before the start of the boil.

Summary

By way of a brief summary of the lesson, here’s a table detailing some of the changes that can occur during wort boiling. It is not meant to be exhaustive.