Lesson 6 - Wort Boiling Final SAQ

***How can the wort boiling process and technology be optimised to improve colloidal and foam stability?

Response:

Proteins and polypeptides are central to this question. They are the major component in haze and in the liquid surrounding and supporting beer foam bubbles. For colloidal stability we need to remove them and for foam stability we need to activate specific types.

Colloidal stability

Colloidal stability is how unlikely beer is to form haze. Haze forms principally due to the binding of proteins and polypeptides with polyphenols. Some proteins and polypeptides are more problematic than others. The most potent haze forming proteins are above 10 kD. We do not have the option of selectively removing proteins/polypeptides so in the boil we need to remove as many of the problematic ones as possible. The mechanism for removing them is to coagulate them, making them insoluble and enabling them to form a complex with polyphenols (increasing the particle size and hence the rate of precipitation from the wort). To do this we need to apply heat and give them the opportunity to interact. We give them this opportunity by ensuring the wort is moving vigorously and also by maximising vapour bubble formation. Vapour bubbles are particularly important because parts of proteins/polypeptides are hydrophobic, so they concentrate in the interface between the vapour and the wort. Most modern boiling systems are designed to maximise vapour bubble formation. However, the two systems that are said to do it best are the Briggs symphony external wort boiler with its long heating tubes and maximised nucleate boiling and low pressure dynamic boiling with it extensive vapour bubble formation. Both systems are designed to prevent excess thermal load. The high heating surface area in the external wort boiler means that ΔT can be low while in low pressure dynamic boiling the boiling phases are divided between thermal coagulation and vapour bubble formation. Minimising shear forces is also vital as these reduce trub particle size and hence its rate of sedimentation. Wort boiling systems operating on forced convection, some older designs of external wort boilers where the route of the wort is torturous and using a plate heat exchanger as an external wort boiler should all be avoided for this reason.

Process factors that encourage trub formation include a long boil and high wort temperatures. Both of these are undesirable if we want beer that tastes good and stays tasting good. Excess thermal load can also reduce foam stability which we are also trying to maximise in this question! However, we could increase temperature and time to a level below one that causes a problem by adjusting the process and testing the beer. A process factor that we can optimise without compromising the beer is pH. A pH of around 5.2 is close to the isoelectric point of the problematic proteins so will help increase trub formation and protein, polypeptide and polyphenol removal.

We can also use process aids to help improve beer colloidal stability. Kettle finings, silica sols and gallotannins can help precipitate proteins and polyphenols and PVPP can be added to remove polyphenols. The rate of use of each of these should be optimised regularly to ensure that they are effective.

Whatever changes we make to the process or equipment, we should also regularly analyse the other elements of the beer like flavour, to ensure that the changes we make are not negatively affecting them.

Foam stability

Interestingly some of the changes that favour colloidal stability, improve foam stability while others reduce it. Trub formation is beneficial because lipids that destroy foam stick to the trub particles and can be removed in wort clarification. Not all proteins and polypeptides are the same and many haze forming proteins and polypeptides may not assist with foam stability. Despite this complex scenario, we do know that very long and very hot boils that strip out the vast majority of proteins and polypeptides can reduce foam stability. However, we do need to ensure wort is hot enough and the vigour of the boil is sufficient to activate foam positive proteins like protein Z and LTP1. Too much heat can cause them to lose their effectiveness. A wort boiling system that removes enough trub without excessive thermal load strikes the balance between maximising foam and colloidal stability. Suppliers of all the modern wort boiling systems claim that theirs do just that and provide impressive trial data that suggest they are telling the truth.

These systems also usually feature facilities that minimise foam formation during the boil. This is important because many feel that foaming in the process leads to weaker foam in the beer. We could use antifoam in the kettle to further suppress foaming during the boil.

As with process changes made to maximise colloidal stability, we need to measure process parameters and analyse the wort and beer to ensure the changes have been successful. And of course that they have not affected other aspects of beer quality.