It was not recognized that many of the diseases of fermented liquids are occasioned by foreign yeasts; moreover, this process, as was shown later by Hansen, favours the development of foreign yeasts at the expense of the good yeast.
About this time Hansen, who had long been engaged in researches on the biology of the fungi of fermentation, demonstrated that yeast free from bacteria could nevertheless occasion diseases in beer.
Hansen set himself the task of studying the properties of the varieties of yeast, and to do this he had to cultivate each variety in a pure state.
Having found that some of the commonest diseases of beer, such as yeast turbidity and the objectionable changes in flavour, were caused not by bacteria but by certain species of yeast, and, further, that different species of good brewery yeast would produce beers of different character, Hansen argued that the pitching yeast should consist only of a single species - namely, that best suited to the brewery in question.
Pure Cultivation of Yeasts.-The methods which were first adopted by Hansen for obtaining pure cultures of yeast were similar in principle to one devised by J.
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