The universal popularity of alcoholic beverages has had the result that all governments have viewed them as a convenient and lucrative method of raising revenue. In addition, the effect of ethanol on human behaviour has had the consequence that the production and sale of such beverages has usually been subject to stringent regulation. In many countries and at various times in history, the supply of crucial raw materials has been controlled by the state as a means of regulating consumption by controlling production. Occasionally this method has also been used to maintain a state monopoly.

With regard to brewing, the weight of legislation has influenced the materials which are used, the conduct of the process as a whole and fermentation in particular. With respect to raw materials, there may be restrictions on what may be used. Thus, as described in Section 1.1, the German beer purity laws allow only malt, water, yeast and hops to be used in brewing. This precludes the use of alternative sources of fermentable sugar and many so-called 'process aids', such as fining agents to assist with beer clarification after fermentation is complete. Even where the legislation allows greater latitude with respect to raw materials, the precise system of raising excise revenue influences the composition of the wort, in particular the sugar concentration. Thus, because of the limits of accuracy and reproducibility of analytical methods it is usual to assess liability to excise payment in discrete bands of alcohol concentration in the beer. The width of these bands varies from country to country but the result is that worts have to be prepared that contain fermentable sugar at a concentration that will produce a desired alcohol concentration. Of course beers at the top and bottom of any particular alcohol concentration excise band will attract the same excise payment and therefore it is incumbent on the prudent brewer to steer an appropriate course between legality and profitability.

The modern practice of high-gravity brewing (see Section 2.5), where a concentrated wort is fermented, followed by dilution to a desired alcohol concentration, places less onus on the need to achieve an accurate initial wort sugar concentration. However, at the other end of the process after fermentation is complete, a means of precise blending of concentrated beer and water is needed.

The liability for excise payment is usually gauged by self-assessment. Since brewing is still predominantly a batch process, this system has produced a requirement for keeping scrupulous records. Of equal importance, as alluded to already, it has produced a need for accurate measurement of volume, weight and concentration. Whilst many methods may be used in a brewery to measure, for example, ethanol concentration, only those which have been given government approval may be employed for the purposes of assessing excise duty payments.

In the majority of countries, excise payments are now based on the alcohol con tent of the beer, termed end-product duty systems. This was not always the case and in some countries, for example, Belgium and the United Kingdom, excise was levied on the basis of sugar usage, expressed in terms of the specific gravity of the wort. Such systems arose since in historical times when the precision with which some measurements could be made left much to be desired it was simpler to police a levy based on quantities of raw materials used. In order to manage this system it was necessary to develop the concept of original gravity. For reasons of microbiological safety, it is not generally considered good practice to store uninoculated (unpitched) wort for any length of time. This, coupled with the possibility of further sugar additions being made after the primary fermentation had started made it necessary to have a method of computing initial specific gravity from samples taken during or at the end of fermentation. These methods allowed correction for the relative proportions of wort sugars converted to ethanol and that used for new yeast biomass formation.

In the United Kingdom, this measure, the original gravity, was based on data gathered by measurements made in ten British breweries during the years 1909-10 and published as the 'mean brewery table' by Thorpe and Brown (1914). In essence, a representative sample of beer of known volume at a controlled temperature is taken and distilled. The residue and distillate are both diluted to the same initial volume and the specific gravity of each is measured. The difference in degrees of specific gravity between the two, termed the spirit indication, is calculated. This figure is used in the mean brewery table to obtain a value for the proportion of the specific gravity of the original wort that must have been used in fermentation. This added to the specific gravity of the residue gives the original gravity of the wort.

The data used to formulate the mean brewery tables were obtained from British ale fermentations. In the case of bottom-fermented lager the proportions of sugar used for yeast biomass formation are altered and in continental Europe an alternative system, the Balling or Plato tables, are used, as in the following relationship:

where p is the original gravity in degrees Plato; A is the alcohol content in % by weight; n is the residual gravity.

Wort-based excise assessment has fallen out of favour and, for example, in the United Kingdom a system of end-product duty payment was introduced in 1993. Thus, the wort-based system was unwieldy and the method of arriving at a figure for original gravity was subject to serious inaccuracy. This worked in favour of more efficient breweries since it proved possible by careful control of fermentation to convert a greater proportion of the fermentable extract into ethanol than that assumed in the excise calculation. Hence, there was a significant but legal underpayment of duty and consequent loss to the Exchequer. The development of routine methods allowing precise measurement of ethanol concentration made the introduction of end product assessment inevitable and a lucrative loophole has been closed.

Brewing Yeast and Fermentation Chris Boulton, David Quain Copyright © 2001 by Blackwell Publishing Ltd

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