As a certain quantity of alcohol is evolved in the ale by the fermentation, it is obvious that the last column is not quite accurate. The real quantity of saccharine matter in each of these also must be greater than what is indicated in that column, because the effect of the saccharine matter, in increasing the specific gravity of the ale, is counteracted by the alcohol, which tends to diminish that specific gravity. By casting the eye over the preceding table, it will be seen that the attenuation does not follow the ratio of the strength. It was greatest of all in the third, and least in the first brewing. These brewings being the same with those given in the fourih chapter, in order to illustrate the quantity of yeast used in fermenting, the reader, by comparing the two tables together, will be able to form some conclusions respecting the effect of different quanti" ties of yeast, and different temperatures, upon the attenuation of strong ale.
Porter is much weaker than strong ale. The average specific gravity of porter wort, according to Shannon, as deduced by the saccharometer, is 1064Ô, which indicates 60 lbs. per barrel of saccharine extract. Hence the reason why it is so much less glutinous and adhesive than strong ale. The fermentation which porter undergoes is, we believe, much less than that of ale ; but we have no very accurate information on the subject. According to the experiments of Mr Brande, brown stout, which is the strongest porter made in London, contains 6 8 per cent, by measure, of alcohol of the specific gravity 0-825. If he had given us the specific gravity of this porter before distillation, it would have enabled us to determine in some measure the error in the attenuation, as indicated by the saccharometer.
The porter brewers in London use three kinds of malt; namely, pale malt, amber malt, and brown malt. * These three are mashed separately, and the worts from each are afterwards mixed together in the same fermenting vessel. In some breweries, mi in that of Barclay and Perkins in the Borough, there are three separate mash-tuns. In other breweries, the custom is to mash one kind of malt the first day, another kind the second day, and a third kind the third day. The first day's wort is put into the fer-menting-vessel, and mixed with yeast; and the other two worts are added to it successively as they are formed. Hence it is very difficult to determine with accuracy the strength of the worts in the London breweries* It could only be done by knowing the quantity of wort from each mash, and its specific gravity when let into the fermenting-vessel. We have had an opportunity of determining the strength of the porter, wort in all the principal breweries in London. The average specific gravity of brown-stout wort is 1-0624. The wort of the best common porter is of the specific gravity 1-0535, that of the worts of the weakest is as low as 1-0374. The average specific gravity deduced from twenty brewings was 1-0500. Such wort contains about 46-4 lbs. per barrel of saccharine matter. Judging from the taste of some of the worts, quassia seems to be employed in considerable quantity by some of the brewers, and much more sparingly, if at all, by others. The fermentation of porter is carried on with considerable rapidity, so that it is over in two or three days. The specific gravity of the porter is usually brought down to 1-013 or 1-017. The specific gravity of the best brown stout, after standing some months in the bottle, is 1-0106. The proportion of pale and brown malt used in the different houses varies. One of the best brewers in London uses nearly two parts pale malt to one part brown.
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