Our three basic fining aids; copper finings, auxiliary finings and isinglass finings are all charge-based processes. All molecules in solution or colloidal suspension possess an electrostatic charge. The amount of charge is specific to the substance in question and is highly dependant upon the pH of the solution it finds itself in, but we will come to pH effects later. An electrostatic charge behaves just like a magnet, except that a magnet has two poles, whereas a molecule has just one pole or a charge of just one polarity, either a positive charge or a negative charge (or a neutral charge). Protein, for example, has a positive charge whereas yeast has a negative charge. Going back to our magnet analogy, we know that opposite poles on a magnet attract each other, whereas like poles repel. The same is true of an electrostatic charge. Molecules of the same charge repel one another and force each other apart, whereas two molecules of opposite charge will attract each other and coalesce. Protein molecules have a positive charge and when in suspension these molecules repel one another which spaces them away from each other, and they cannot easily come together to coalesce into larger particles heavy enough to drop out of solution. Yeast behaves in the same way, except that it has a negative charge. Finings have the opposite charge to the stuff we are trying to remove. The finings then attract the yeast or protein to itself, causing larger, heavier flocs which drop out of suspension faster. Isinglass finings, for example, has a strong positive charge. When isinglass is added to beer its strong positive charge causes its molecules to space themselves out like a web and attract negatively charged yeast cells to it. This then drop down though the beer, attracting more yeast cells to it, which are pulled out of suspension with the isinglass.

Isoelectric Points

Substances that cause turbidity problems in beer: yeast and complexed proteins, are not normally in solution as such, but in suspension or colloidal dispersion. These substances are held in suspension by the electrical charge that they carry. Because similar substances carry the same electrical charge they repel one another and push themselves apart in the solution or in suspension, dispersing themselves more or less equidistantly throughout the medium. The magnitude of the charge on the substance is highly dependant upon pH. Alterations in the pH will change the charge on these molecules. At one end of the pH scale a substance may have a strong negative charge, and at the opposite end of the pH scale the same substance may have a strong positive charge. It therefore follows that between the two pH extremes there must be a point where the net charge is zero. This point, where the charge is zero, is known as the isoelectric point in boffinspeak. Different substances have different charges and different isoelectric points. With a net charge of zero the molecules no longer push themselves apart, or hold themselves in suspension, so, if they are heavy enough, they can precipitate out of suspension under the influence of gravity. If they are of too small to be greatly influenced by gravity, they are free to move together and coalesce to form larger particles that then become heavy enough to drop out of suspension quickly.

Those proteins that are responsible for head formation and retention have an isoelectric point of about pH 5.9, whereas those proteins that cause problems with haze and stability have an isoelectric point of about pH 4.9. Because the pH scale is logarithmic, the difference between pH 5.9 and pH 4.9 is a factor of ten, a much bigger difference than would be expected by taking the figures at face value. It therefore follows that if the pH of the wort during the boil is around the pH 5.0 mark, this will aid the precipitation of haze-forming proteins, but without excessively precipitating useful components, such as head-forming proteins. Yeast behaves the same way, with an isoelectric point of about pH 4.0. Irish moss copper finings added to the wort at the end of the boil work best at pH 5.2, but will still work well over the range pH 5.0 - 5.4. The pH of beer gradually falls throughout the brewing process, starting off at around pH 5.3 during the mash, and ending up below pH 4 at the end of fermentation. Therefore the pH of beer falls gradually during the brewing process, passing through all the appropriate isoelectric points, coincidentally and conveniently at appropriate times. This means that even without finings, protein or yeast in suspension or colloidal dispersion will, at sometime during the life-cycle of beer production, experience zero-charge conditions and can more easily drop out of suspension. With finings added the same things apply. Adding a lot of positively-charged isinglass to a beer will attract negatively-charged yeast to it, but the isinglass can still end up with a net positive charge, holding the whole lot in suspension if the flocs are not heavy enough to overcome the charge. However, isinglass is added when the pH of the beer is close to the isoelectric point of isinglass, so its net charge should be close to zero, facilitating its precipitation. The same applies to other fining aids. It gets a bit more complicated because two molecules of, say, isinglass can have a net zero charge with respect to each other, but can have a strong positive or negative charge with respect to molecules of a different substance. But that is too complicated for me.