This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
If brewer's yeast cannot be had, baker's stock yeast might be employed, but when neither of the above can be obtained a very good yeast can be made by taking the dry yeast cake - such as are kept for sale by nearly all the grocers in the country. Crumble it up fine into a small cup of Indian meal or flour. If meal is not handy, stir in milk-warm water (not hot water, as this kills the yeast) until the meal or flour is about as thick as butter; set this in a warm place until it ferments and works up lively, which will take from six to twelve hours, and sometimes longer. In place of clear warm water, as above mentioned, use, as a substitute, warm water in which a small handful of hops has been steeped. Some use a few boiled potatoes, mashed up fine, in place of meal or flour with the steeped hops. This is what is called hop or potato yeast. Malt is very nice in making yeast. Soak a pint of malt with a few hops in scalding water (not boiling water), for three or four hours, strain this water off and mix with meal, flour or potatoes, as before directed; and it will make, if properly worked, a prime yeast.
The following is also a very good formula for making yeast. Take two pounds of ground malt and about two ounces of hops; add to this one gallon of scalding water, of about 170° F soak the malt and hops in this for about six hours, then strain off the liquid, and add two or three boiled potatoes mashed fine; then put in, in fine crumbs, two or three dry yeast cakes; have the mixture just milk-warm when the yeast cakes are added; keep this in a warm place until it ferments. It will then be ready for use. If you cannot get malt, make a thin porridge of mashed potatoes, adding a little sugar and the water from some steeped hops, and proceed in other respects as directed for the use of malt, but never use any yeast when it has become sour.
If yeast or yeast cake of any kind is not obtainable in the locality where the bottler is, mix up a little meal in warm water, and let it set, and in time it will ferment itself. What is wanted is to get, when it is possible, some of the yeast plant or germ cells, and transplant them into new material, that they may grow and bring forth a hundred fold. The raising of yeast is like raising wheat or barley or any other plant, and requires about the same care and attention.