Why do we have to aerate the wort?

Let’s admit, that brewers are the most inconsistent people in the world! They pass the whole year trying to avoid oxygen, and at the end of wort production what do they do? Put tons of O2 on it! What a crazy people! But let’s investigate the reasons in this article! Please subscribe to this newsletter to read a new article every week!

Everything Because of Sterols!

The short answer to the title question is sterols! They provide flexibility to cell membranes and improve the ability to transport compounds, not to mention the boost of stress resistance. We can say that they regulate the yeast's growth because in budding, the mother shares the sterols with the daughter, meaning that when the sterol levels reach a lower value it will not have more cell division.

Let’s investigate the following image showing the pathway of creation of sterol involving mevalonic acid (a cell lipid):

We can see in the simplified pathway that in the early stages, it requires energy in the form of ATP. That’s the reason we need to pitch yeasts with good levels of glycogen on our wort! If we store the yeast longer than 48 h the glycogen will be depleted, and the sterol will not be formed. Read more about glycogen and trehalose here.

To convert squalene into lanosterol and further into ergosterol, the main sterol in the yeast, it is necessary to use oxygen, and that’s the reason we must aerate the wort! Without oxygen, these steps of the pathway will not occur, leading to yeast with no ergosterol, and consequently, no growth.

Many people may think the yeast would respire (with the presence of oxygen), but that's not true due to the Crabtree effect inhibiting respiration when the sugar content is higher than 0.5 °P (yes, it is the case of wort).

How to aerate the wort?

The aeration is usually done after the cooling system, in the transfer line to the cellar. Craft breweries normally use a sintered metal candle, which is a porous metal part that creates microbubbles, increasing the surface area and making easier solubilization. ?This technology has a disadvantage from a cleaning point of view, where the breweries must sterile it by chemical immersion, which is not the most efficient way to clean something. This system requires a static mixer or at least several curves on the piping downstream to generate a whirlwind and incorporate the oxygen.

Commercial breweries prefer using venturi systems. This device uses the principle of Venturi, where the diameter of the wort pipe is reduced (increasing the velocity of the liquid), creating a low-pressure zone in the piping and ‘sucking’ the air to the liquid pipe. After passing through the orifice, the liquid with air enters an expansion chamber, where it can decelerate, and the air is incorporated. This technology does not require any further mixing system. The CIP (clean-in-place) can be performed in all system during the piping cleaning.

How much Oxygen is needed?

This answer depends on the yeast strain and the original gravity of the wort; however, we can say that 8 to 10 ppm of pure oxygen is enough for normal fermentation. When using sterile air, it must be considered that the air has only 21% oxygen, so the volume of air will be greater.

High-gravity beer used to demand more oxygen; nonetheless, there is a limit to how much gas can be solubilized in the liquid (you can read the article about carbonatation here, it follows the same principle), in this case, it can be performed a second aeration 12 or 18 hours after the first, this one in the fermentation tank.

Under aerated wort will produce low ergosterol, the growth will be compromised and the fermentation will be slow, producing several off-flavors in the process (more esters). If we have a slightly overaerated wort, we will have no injuries but with high among of oxygen it will lead to unnecessary compounds, making the fermentation inefficient and with low ester production.

How about Olive Oil?

It is possible to ‘hack’ this system. The oxygen is used to perform a pathway we described earlier, but what if we provide sterols in the wort, yeast will not require oxygen anymore, right?

Right!

Adding Unsaturated fatty acids (UFAs), such as palmitoleic acid and oleic acid (present in olive oil), the yeasts use other pathways to reach the ergosterol, demanding less oxygen. The studies point out that a great part of the method to add UFAs leads to an ester reduction, because both, lipids and esters, use acetyl-CoA, and changing the synthesis of one will affect the other. However, the differences are not noted in the sensories panel.

The replacement of aeration by UFA addition has strong potential because breweries are concerned about the oxidation power of aeration, and how it helps to spoil the beer earlier, but yet no comercial brewery adopted this practice industrially!

Thank you for reading! Comment on your opinions about adding olive oil to beer and like and share it!