The nature of "dunder" may be described as follows: When the fermented mash from which the rum is to be made is placed in the still, it contains practically all of the yeast cells, living and dead, which aided in or were produced during fermentation. The warming of the fermented material in the still produces a rapid extraction of the soluble materials from the yeast cells. These soluble materials, as is well known, are of the nature of diastases or enzymes. This whole mass, after the removal of the alcohol by distillation, is naturally concentrated and the extracts are in a more usable form than they were before. This material, consisting of numerous mineral matters and other substances, as well as the remains of the yeast cells, forms an excellent food for the nourishment of the new yeast cells of the succeeding fermentation. Naturally, the "dunder" itself does not afford any alcohol, but it stimulates the growth of the yeast, so as to produce a larger yield and of a finer character, provided the quantity of "dunder" employed is not too great.

It must not be forgotten that not only does this residue of the fermentation of the rum contain valuable qualities, but it also has some disadvantages. The "dunder" is very apt to be infected with bacteria, some of which are not killed during the process of distillation. Especially is this true of the bacteria which are produced from spores. Inasmuch as the constituents of this residue are an excellent food for yeasts, they also become likewise a most excellent food for bacteria.

The development of bacteria may interfere very seriously with the succeeding fermentations and introduce elements of activity which tend, or may tend, to produce a product of an inferior quality. Hence, as in the case of using sour mash, attention must be paid to the process of fermentation, in order that no undesirable strain of bacterial or yeast life may be produced.