Kegged beer is a product which has been chilled and filtered, usually carbonated then packaged into pressurised metal containers which have a spear or extraction device of some description to aid dispense. In this way it is a different beverage to cask beer although often starts out brewed in the same way. The ability of the brewer to keg enables him to exploit different sales channels compared to cask where his keg beer can tolerate longer shelf life requirements, dispense points with no cellar cooling or sporadic turnover. Typical shelf life for a keg beer for the UK market is 12 weeks whereas for export, anything over 9 months is required.
Raw materials, brewhouse and fermentation processes are the same as for cask beer. It is in the preparation of the beer for packaging that the major changes occur. The first step is in the maturation. Beer destined for kegging is usually chilled to as low a temperature as the brewer can get it. The lower the better as this precipitates chill haze so it helps to have used carrageenan in the brewhouse too. A beer kept at -2.0˚C can be filtered after 48hrs, -1.0˚C would be 4 days, but if a beer can only be kept at 4 or 5˚C, it may be better to keep it for several weeks.
The second step is a filtration stage. This is to remove all yeast and as much protein and other material that would otherwise promote the formation of haze and off-flavour. Filtration can be through cellulose sheets or cartridge filters of different porosity or by using a kieselghur (diatomaceous earth) filter. All these techniques achieve the same thing; the production of a star bright beer which maintains its chemical and physical stability for the length of shelf life required by the customer. In practice beer is pumped from the cold tank through the filter and collected in a bright beer tank. At all stages it is important to keep air, in particular oxygen from contacting the beer. Oxygen readily reacts with residual proteins and hop resins in the beer to form compounds that eventually lead to oxidised flavours and haze which detract from the flavour. Therefor the rough beer tank often has a top pressure of CO2 gas acting as a blanket as the tank empties and likewise the receiving bright beer tank has a CO2 atmosphere to do the same as it fills.
Keg beer has a higher carbonation than its cousin in cask and this is achieved by carbonating the beer. The most efficient way is to do this inline as the beer exits the filter and fills the bright beer tank. However it can be done by carbonating the bright beer tank directly although this risks stripping out hop aroma and causing floaters through collapsed fob. A typical keg beer carbonation could range between 1.4 to volumes (cask beers are rarely greater than 1.0vol).
Once in the bright beer tank, keg beer is packaged into kegs as soon as possible. The product although stabilised by filtration, is not sterile so most brewers fill their kegs after first flash pasteurising. Pasteurisation is the name given to heat treatment of a liquid to render it microbiologically safe. The amount of pasteurisation given is measured in pasteurisation units, PU’s, defined as the amount of heat delivered to kill microbes in a unit of time. 1 PU is the amount of microbial death achieved at 60˚C for 60 seconds. The scale is logarithmic and can be found from tables but keg beers are generally given around 20 PU’s equivalent to holding beer at 72˚C for a 15 – 20 seconds. The whole packaging process, including pasteurisation, is done under pressure to prevent the loss (fobbing) of CO2. The pasteuriser feeds the keg racker, usually via a buffer tank to cope with variable flows seen during keg filling. Kegs are washed on a specially designed washer as they need to be de-ullaged, cleaned, sterilised, rinsed and back-pressured prior to filling with the pasteurised beer.
Due to possible damage to the beer at the elevated temperatures of pasteurisation, beer can be sterile filtered instead. This usually involves passing the beer through a series of Filter Cartridges, the final cartridge being 0.45 micron.
After filling, kegs are check-weighed to comply with trading standards legislation, labelled and a tamper-proof keg cap placed over the filling/dispense point to protect the contents.
This is the basic process of kegging beer. It is obvious that a different process technique and different equipment is required to keg beer. The underlying principle is to bring a greater stability to the beer to deliver a longer shelf life.
Some further considerations about this process are given below:
- Even longer shelf life can be achieved through chemical stabilisation in cold tank prior to filtration by using silica gels, PVPP or enzymes according to the raw materials used and shelf life desired by the brewer.
- There are many different types of keg fitting to found in pubs, and some of the one trip kegs have their own fitting. Your market needs to be investigated when considering keg and fitting type.
- The prevention of oxygen contact throughout the whole process is imperative to achieve the shelf life stated by the brewer. Oxidised beer is really unpleasant to drink and indicates a fault in the above process. It is difficult to prevent oxygen ingress but permitted anti-oxidants are available to minimise the inevitable contact, e.g. ascorbic acid, sulphur dioxide (max. permitted in beer before declaration is 20ppm).
- It can be seen that much capital equipment is required to install the correct processing equipment. However keg beer can be made employing sterile filtration technology which precludes the need to pasteurise with its risk on flavour change and energy requirement. Sterile filtering is made immediately after the rough filtering step and produces a microbially stable beer that can be carbonated and kegged as described.
- The introduction of another product stream to the brewery requires more management, knowledge and expertise. It is worth considering purchasing additional quality control equipment like CO2 and O2 analysers especially to assist with the control of these important parameters.