Lactic Ferments, Limited, make use of a different principle to maintain the temperature steady during the lactifying period. Their apparatus (Fig. 47) consists
Fig. 47 of a water vessel mounted on a stand. The milk vessels (tumblers) are placed in the water, and the temperature maintained at the proper figure by a small night light burning underneath. It is recommended that the milk should be placed in an earthenware jar or jug, which is stood in a pot of water kept boiling for an hour. After cooling add from three to six previously crushed tabloids of culture, and stir well with a glass rod which has been sterilised in boiling water. The milk is then transferred to three tumblers, which it should fill, and these are put into the water vessel, the water in which should be at about 100° F., and the night light started.
Messrs. Allen & Hanbury, Limited, also make use of the night light to maintain the proper temperature during incubation in their "Sauerin" apparatus (Fig. 48).
No water, however, is placed in the metal container. The procedure is the same as that already described, and both tablet and liquid pure cultures are supplied. For children it is recommended that the incubation should occupy from three to four hours only, in other cases eight to ten hours. Grated nutmeg, ground cinnamon, or other flavouring, and cream may be used with the soured milk.
The night light is also employed in the "Veronelle" apparatus of Messrs. Clay, Paget and Company, Limited (Fig. 49).
The containing vessel is of tin or aluminium, and has two stands, the high one for hot and the low one for cold weather, as in the latter case greater heat is needed to maintain the incubating temperature. The milk is placed in an earthenware jar and is sterilised by placing it in a saucepan of water and boiling it; continuing the boiling for half an hour. It is allowed to cool to about 98° F., and placed in the incubator, culture added, and the lamp lighted, the cover of the incubator being kept on. The period of incubation is given as six hours. To prepare the next day's supply a tablespoonful of the soured milk is retained and used instead of the culture. This may be continued for fourteen days, when a fresh start with culture is necessary. The soured milk will keep for thirty-six hours. Capacity, one and one half and two pints; also a large size for family use.
For the preparation of soured milk on a small scale, one of the various forms of vacuum flasks now on the market may be used with satisfactory results. A little cold water must be poured into the flasks, and warm water added, until, by means of three to four changes, boiling water can be safely poured in without cracking the flask.
This boiling water must be allowed to remain in for about twenty minutes, and then replaced by freshly boiled milk that has been cooled, so that its temperature in the flask is about 105° F. The culture of lactic organisms should then be added, the opening of the flask plugged with clean cotton-wool, and the cap screwed on.