We’ve got as far as filtering. I thought we’d already done that. Didn’t it come before carbonation? Or am I getting confused?
Let’s start with some interesting points about beer clarity and filtering.
“Filtering of Bottled Beers
Experience has revealed the fact that the filter plays a more important part in the subsequent beers than was at first understood. As a result, several improvements have been made in modern filters, and they are still being further developed. The whole idea of the filter is centralized in the production of a brilliant beer which will remain bright for an unlimited time after bring bottled. Up to the present we know of no plant which will invariably produce such a beer, and we rather doubt if one will ever be made. So much depends upon the actual brewing of the beer and the colloidal balance of its constituents. We feel justified in saying that the brilliancy of beer depends first upon the quality of the barley, then upon its correct and careful malting, subsequently upon proper mash tun treatment, and finally upon satisfactory fermentation. When beer has been brewed under these conditions and is given time to mature it should drop bright spontaneously. In such a case the filter merely becomes a polisher. It is impossible, however, always to guarantee that beer has been produced under such favourable conditions, and the bottler has to rely to a far greater extent upon his filter.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, page 341.
That’s an interesting admission: no method produced a beer that would stay bright indefinitely. I wonder if that’s still the case? If you filtered and pasteurised enough, wouldn’t that be the case? Despite having used isinglasss finings for centuries, British brewers still had a thing about beer spontaneously dropping bright. They reckoned that, given time, a beer brewed properly should become clear. Unfined does not equal murky, despite what some modern charlatans might claim.
So if you’d brewed your beer well, the filter was just adding the final polish. If not, then hopefully it was going to save your bacon.
Now something about filter design:
“Considerable strides have been made in filter design in the past ten years or so. At one time most beer filters used filter pulp, which was supplied in thick cakes by the makers and had to be broken up, soaked and then continually agitated with hot water to give an even mass, then pumped through perforated frames, the pulp being retained by the gauze to form a filter. The breaking up and disintegration of the pulp, and the filling of the frames and their even pressure to form a satisfactory filter bed was a matter of considerable trouble and variations in brilliancy of the filtered beer were difficult to avoid. After use the pulp had to be washed, redispersed and sterlized before using again. The mixture of 3% to 5% of asbestos fibre with the pulp (the cakes as bought were usually treated in this way) was found to assist in giving better packing. Comparatively few bottlers now use pulp filters of this kind. Modern beer filters fall into two main categories. Those winch use prepared sheets of compressed filter material, known as plate or sheet filters, and those which use a bed of kieselguhr (infusorial earth), known as kieselguhr or earth filters.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, pages 341 - 342.
That pulp sounds like a pain the arse. And the presence of asbestos isn’t making me like it any more. It seems like these are still the two main methods of filtering.
“Different mechanisms of filtration can be used:
Sieving or surface filtration in which the particles are trapped in pores in the filter medium and retained in a layer. Filtration quality improves with time but the volume flow decreases continuously."Brewing Science and Practice" by Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes and Roger Stevens, 2000, page 575.
Depth filtration in which a separation medium, e.g., kieselguhr is used on a support and which causes the particles in the beer to take a very elongated route through a large surface area. The particles are retained by mechanical sieving because of size and will gradually block the pores in the medium and so reduce flow rate and the particles can also be retained by adsorption as a result of electrical charge effects.”
Fascinating that nothing has changed much in this area in the last 60 years.
Next we’ll look at the filters themselves in more detail.