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Understanding the wort boiling process

Understanding the wort boiling process

Usually, homebrewers don’t think about boiling, and aside from the need to add hops, nothing seems to happen. But, the boiling of wort is a key link in the winemaking process. The chemical reaction that occurs during this process has an important impact on the flavor of the final beer. Wort is a mixture of substances formed during mashing and filtering. Hops are the main ingredient added during the boiling process of wort, and a small amount of calcium sulfate and zinc are also added. Interestingly, because of the complex chemical reactions that occur during the boiling process, the composition of the wort varies greatly before and after boiling.
Understanding the wort boiling process
Ingredients of wort

Ingredients of wort

The important component of wort is water, and other non-main components include carbohydrates, nitrogen-containing compounds, and other compounds.


The carbohydrates in wort are divided into fermentable carbohydrates and non-fermentable carbohydrates. Under the action of enzymes, most starches can be converted into monosaccharides and dextrins in the mashing process.

Nitrogen compounds

Nitrogen-containing compounds account for about 4% of the wort solids, and the types are complex and diverse, the most important being amino acids, proteins, and nucleic acids. To amino acids, wort also contains complex nitrogen compounds called nucleic acids. Nucleic acids are mainly composed of purines, pyrimidines, carbohydrates, and phosphoric acid. These components are the same as the nucleic acids that make up the genetic code in all living cells.

Other compounds

Also to carbohydrates and nitrogen-containing compounds, wort solids also contain many other ingredients. For example, polyphenols, vitamins, oils, and minerals, etc.

The purpose and function of boiling wort

Below we will introduce the purpose and function of wort boiling. Wort boiling has many important functions. From a chemical point of view, the boiling of wort is the most complicated part of the whole winemaking process.

Decrease enzyme activity

The most important effect of boiling wort is to reduce the activity of residual enzymes after the mashing process. Boiling can end the mashing process that converts starch into sugars while stabilizing the fermentable sugar components in the wort. So, reducing the enzyme activity can maintain the required sugar/dextrin ratio in the wort. This ratio has been clearly defined during the saccharification process, and it is a necessary condition for obtaining the RDF target value.

Extract hop resin

One of the main functions of boiling wort is to extract the required hop resin from hops.

Isomerized hop composition

Adding hops in the early stage of boiling in the boiling pot can maximize the leaching of hops and ensure full isomerization. So, adding hops in the later stage helps to maintain the subtle smell and aroma of hops produced by the hop oil. This is because hop oil has high volatility and is easy to “evaporate” during the long boiling process.

Wort concentrate

Wort concentrate is another important function of the boiling process. During the mashing and filtration process, the addition of more water can maximize the extraction and recovery of the extract. The boiling process fully evaporates this part of the water to get the desired (target) wort Brix. The main factors that determine the final sugar content are the initial sugar content and the boiling time. The longer the boiling time, the higher the final sugar content.
The relationship between the amount of water added during mashing and filtration and the boiling time of wort is an important factor in determining the best boiling time in beer recipes. Due to differences in equipment, the actual boiling time varies from brewery to brewery.

Sterilize wort

Sterilization of wort is another important function of the process of boiling wort. Generally speaking, the bacteria in the wort will be killed during the boiling process. Fifteen minutes of boiling time is enough to kill almost all types of bacteria.
The impact of hops on bacteria, hops have natural bactericidal properties, which can inhibit the growth of bacteria to a least. These characteristics are especially important before pasteurization and refrigeration technology are available. In modern brewing methods, boiling wort is still the main sterilization method.

Add liquid excipients

Generally speaking, auxiliary materials such as rice and corn are usually added in solid form at the time of feeding. But, for certain beer recipes, it is selected to add liquid auxiliary materials to the boiling pot during the boiling of the wort. Many other breweries use a high proportion of liquid excipients.

Evaporate unwanted volatiles

Another effect of the process of boiling wort is to evaporate some unwanted volatile compounds. These compounds include:
  • A: Sulfur-containing compounds, such as dimethyl sulfide (DMS).
  • B: Vegetable odor from green malt and malting process.

Protein flocculation precipitation

Wort contains a lot of protein compounds, some of which can affect the taste and stability of beer, and are thus undesirable ingredients. But, the properties of these proteins are not fully known to us. Boiling can cause changes (denaturation) of some protein components, reducing solubility, and causing the protein to precipitate as flocs. These flocs are called hot coagulum or boiling coagulum. The hot coagulum is formed during the boiling of the wort and is subsequently removed in the whirling sedimentation tank.
It should be noted that the boiling process is not to remove all proteins, and wort without any protein is not what we want. There is a delicate balance between proper protein removal and excessive removal. Some proteins are beneficial to maintain good foam and form the desired mouthfeel characteristics to a certain extent.

Protein and polyphenol polymerization

Also to proteins, polyphenols are another type of compound. For winemaking, polyphenols and tannins are considered synonymous. Polyphenols have both positive and negative effects on beer. Some polyphenols are what we expect because they are conducive to the color, bitterness, and mellowness of beer. Some polyphenols also act as antioxidants. But, because its reaction with protein can cause the beer to be cold and cloudy, we generally do not expect the presence of these polyphenols. Polyphenols can also cause the beer to produce a harsh bitter taste.

Color formation

The process of boiling wort has an important influence on the formation of wort color (intensification of color). The color formation is caused by the complicated Maillard reaction. It is named after a French chemist and is sometimes called the “brown reaction”. This reaction usually occurs between certain amino acids and sugars to produce a product called melanoid. Melanoid is the name of a type of melanin.
In practice, malt drying will provide about one-third of the wort color, while the remaining two-thirds come from the Maillard reaction. The color of the wort is directly proportional to the boiling time. The longer the boiling time, the darker the color formed; conversely, the shorter the boiling time, the lighter wort will be produced.

Flavor formation

The Maillard reaction that causes color formation also affects the flavor of the beer. The odor of melanoidin produced by this reaction depends on which amino acid compound reacts with sugar. For example, when reacting with glucose, ammonia will produce a bitter taste, glycine will produce a burnt aroma of bread, and valine will produce a malt taste. Leucine produces the aroma of fresh bread, while phenyl arsine produces the taste of withered roses. Also produced are aldehydes as intermediate products of the Maillard reaction. They have a very strong flavor, but most of them are converted into melanoidin, which is “evaporated” or reduced during the fermentation stage. Any aldehydes remaining in the brewing process will speed up the aging reaction of beer. The fats and oils from malt or auxiliary materials can also be decomposed into strong-tasting aldehydes during the boiling process.

Lower PH

The actual pH of the wort is mainly determined by the malt ingredients. The boiling process can often lower the pH slightly. Adding gypsum to the boiling pot can also produce a similar effect. The calcium in gypsum can lower the pH by reacting with phosphate. Gypsum is the mineral name for calcium sulfate. By adding calcium sulfate, the calcium reacts with the phosphate in the malt to lower the PH value of the wort. Calcium sulfate can also promote the precipitation of oxalate in malt. The typical wort has a pH close to 6.0, and the final wort has a pH of 5.5. Low PH can increase protein flocculation and precipitation, thereby improving the precipitation effect of coagulum. As mentioned earlier, a lower pH value will also produce “excellent” hop bitterness characteristics.
In short, the chemical reaction that occurs wort boiling has an important impact on the characteristics of the final product, so the process of wort boiling is very important.
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