Boil Chemistry 

You have just spent much time and effort mashing with careful planned precision to produce malt rich wort.

The starch has been converted to sugar and the sugar is ready for the yeast. So why boil?


1)   Sterilize the wort

2)   Stop the enzymatic activity

3)   Drive off DMS

4)   Decrease pH level

5)   Isomerization

6)   Hop Utilization

7)   Color or maillard reaction

8)   Increase gravity

9)   Coagulate proteins (clear beer)

Sterilize – Wort can contain contaminates that could compete with yeast and ruin a beer. The boil ensures a more stable product by killing off any organisms that made it through the mash.

Enzymatic activity – At this point you want to end all such activity. Heating to 180+ will denature all remaining enzymes.

Drive off DMS– DMS (Dimethyl Sulfide) is a byproduct of mashing and fermentation, so it is present to some degree in all beers. It has the aroma of cooked or creamed corn. It can be perceived at very low thresholds. The pre-cursor for DMS is produced in the malt kilning process and the heating of wort in both the mash and boil will produce DMS.  Luckily DMS is quite volatile and a vigorous boil will drive it off. DMS is going to be present in higher quantities in lighter kilned malts, and less so as the SRM of the malt increases. The lightest kilned malt is Pilsner, and this malt will contain the most DMS pre-cursor. Boil pilsner based beers for longer time periods and never cover your boil!

Decrease pH – pH will under normal circumstances drop in the boil. During boiling the pH drops by about 0.1 – 0.2 pH units from 5.3 – 5.5 pH to about 5.2 – 5.3 pH. This has many affects on the beer overall. It is critical to most of the other processes happening during the boil.

Isomerization – the conversion of alpha acids to iso-alpha acids. Basically it is a rearranging of the molecular structure of the acid.

How does isomerization occur? Heat and time.

Not all alpha acids will be converted to iso-alpha acids, but those that do are more soluble and will contribute more of a bitter flavor.

Hops that are added at the start of a 60-minute boil will undergo the most conversion. Thereby adding a high amount of bittering to a beer. At the same time the long boil destroys the compounds that produce aroma and flavor.

Hops added late in the boil will convert very little to none of their alpha acids. Thereby these will contribute large amounts of the VOCs for aroma and flavor, and very little bitterness.

Hop Utilization: Before hop alpha acids can be transformed into their iso- forms, which are more soluble, they need to be dissolved into the boiling wort. Boil pH can vary the solubility of hop acids.  It has been shown that this solubility increases with pH which is why the bitterness extraction from hops is greater at a higher boil pH.

Notes: Bitterness extracted at higher boil pH can be perceived as being harsher than that which is extracted at lower pH levels


Maillard Reaction: Maillard reactions, or non-enzymatic browning, start with a reaction between an amino acid and a reducing sugar. The final products are melanoidins which are responsible for the darkening of both, malt during kilning and wort during boiling. The rate of Maillard reactions is affected by wort concentration, temperature and pH. The higher pH the faster the rate of Maillard reactions will be.


The same wort boiled for 15 min at pH 5.5 (left) and pH 6.5 (right). The increased color of the higher pH wort is remarkable. It is a result of stronger Maillard reactions at higher pH.

Increase gravity – Boil off of water increase the concentration of sugar, raising the worts gravity.Protein Coagulation – This is a fairly complicated subject by itself.  Simply we want to see large flakes of protein in our hot and cold breaks. In addition to the chains of proteins sticking together, tannins from the husks and hop material, which are present in wort, will also attach themselves. This will help us with chill haze, beer clarity, and possibly taste. Protein with bond more easily at pH of 5.2


Wort clarity and break formation before, at the beginning, during, and at the end of the boil. The starting boil pH was 5.35 and the cast out wort pH was 5.30. It can be seen that the wort 10 min before boiling is still clear. At the beginning of the boil protein have coagulated to form a haze. Shortly after that, they started to form flocks (hot break) which cleared up the wort. At the end of the 60 min boil the flocks are larger while the wort is brilliantly clear

A visual approach to checking pH in the boil

Fortunately, there is an easy way to check on your boil pH and adjust it — all without requiring a pH meter or anything you don’t likely already have in your brewery.

Below a pH of 5, the coagulation of proteins decreases. (Hot break, the light-colored flakes that form in your kettle during the boil, is coagulated protein.) Above a pH of 5, the amount of protein coagulated is constant, but the break material looks different at different pH levels. At a pH of 5.2, hot break is seen as relatively large, fluffy flakes floating in otherwise clear wort. In some cases, these flakes may even clump together into larger “globs” of break material. The farther you move away from a pH of 5.2 (in either direction), the smaller hot break particles become. A few tenths of pH point away and the wort may simply look cloudy. So, you can use the presence or absence of big, fluffy hot break flakes as an indication of whether or not you are in the right boil pH range. The malts you use also effect the appearance of the hot break. Their protein levels and amount of protein modification achieved in malting also play a role.

The same chemical reactions, primarily between calcium and phosphates, that help a brewer establish the correct mash pH can also help him or her establish the proper boil pH. If your boil is cloudy, but large flakes of break material are not present, the problem is likely that your boil pH is too high. Adding a small amount of calcium will fix this problem most of the time. Plus, you will be able to tell if it worked by the appearance of large, fluffy flakes in your kettle.

So here’s how to hit the right boil pH in most ordinary brewing circumstances. If, after the first 15 minutes of boiling your wort, you think your boil pH is too high, add approximately 50 ppm of calcium ions to your kettle. For 5 gallons (19 L) of wort, adding 1 tsp of calcium chloride (CaCl2) or calcium sulfate (CaSO4) will yield about 50 ppm. After another 10 minutes, check your wort again — if the break material still seems like little specks as opposed to bigger flakes, add another 50 ppm of calcium.

Most of the time, you’ll see your hot break improve after the first addition. If two additions of calcium don’t work, adding more calcium is probably not going to solve the problem.