Here's a great deal of information about foeder sours and the process that Petrus uses:

"Why use such large oak foeders?
Because we don’t want the taste of the oak, as in cask aging. The wood is simply a special container, a kind of breeding ground for the various micro-organisms that convert the remaining sugars from the main fermentation into acids, higher alcohol content and esters. We also need the wood as a permeable wall: air slowly penetrates the wood and enters the beer, creating a controlled oxidation. This process proceeds very slowly – it takes 2 years to result in a refreshingly sour beer with a very fruity aroma.

During the 24 months of ageing, the microbiological flora in the wood reacts with the beer to produce its slightly sour flavor and distinctive aroma.

Here's a video on how Foeders are made!  https://youtu.be/BzFfMdurYBg

Petrus is all but an average beer. Petrus is a real sour, with roots that date from ages ago, brewed in Belgium according to the finest brewing tradition. Despite its age, it is young at heart!

It is the sour beer reference. Though not for sour people, but for people who don’t care about trends and hypes. People who trust their senses, rely on their own good taste and are eager to explore new boundaries.

Petrus is a slow, unpredictable and wild beer that takes lots of patience. No wonder it is a beer for people who understand the urge to slow down. This beer, aged for 2 years in oak foeders, should be enjoyed sip by sip. Indeed, a real sour... takes time.

Petrus is intriguing, enigmatic and therefore impossible to copy. Only the master brewer fully grasps the complex brewing processes. He rules the mysterious microbiology in the oak foeders. That truly is science with a touch of magic. So why not accept that mystery, cherish the secret and embrace the experiment?

Do you dare to take that experiment to the next level?

Simply savoring Petrus is already quite a rewarding experience, but do you dare to start your own blending experiment with the Petrus sour beers? How sour do you want your Petrus blend? Try and taste until you reach your personal level of sour beer perfection.