2. The cream to be ripened must first be pasteurised, in order to destroy the developing organisms already in it, and thus be prepared for the addition of the pure culture. 3. The addition of the developing starter to the pasteurised cream and the holding of the cream at such a temperature as will readily induce the best development of flavour. 4. The propagation of the starter from day to day. A fresh lot of pasteurised milk should be inoculated daily with some of the pure culture of the previous day, not the ripening cream containing the culture. In this way the purity of the starter is maintained for a considerable length of time. Those starters are best which grow rapidly at a comparatively low temperature (60-75° F.), which produce a good flavour, and which increase the keeping qualities of the butter. Now, whilst it is true that the practice of using pure cultures in this way is becoming more general, very few species have been isolated which fulfil all the desirable qualities above mentioned. In America starters are preferred which yield a "high" flavour, whereas in Danish butter a mild aroma is commoner. In England as yet very little has been done, and that on an experimental scale rather than a commercial one.[67] In 1891 it appears that only 4 per cent. of the butter exhibited at the Danish butter exhibitions was made from pasteurised cream plus a culture starter; but in 1895, 86 per cent. of the butter was so made. Moreover, such butter obtained the prizes awarded for first-class butter with preferable flavour. Different cultures will, of course, yield different flavoured butter. If we desire, say, a Danish butter, then some species like "Hansen's Danish Starter" would be added; if we desire an American butter, we should use a species like that known as "Conn's Bacillus, No. 41." But whilst these are two common types, they are not the only suitable and effective starters. On certain farms in England there are equally good cultures, which, placed under favourable temperatures in new cream, would immediately commence active ripening.