Brown. — Some Studies on Yeast. 203 
from separate flasks maintained under exactly similar conditions, but in most 
cases the samples for examination were derived from one flask of several 
litres capacity, so arranged that uniform samples could be forced out by 
pressure of CO^ 1 * 
Occasions sometimes arise when it is inconvenient or even impossible 
to determine the progress of cell-reproduction by direct counting, and we 
have to fall back on an indirect method which gives equally good results. 
As I have fully described this method elsewhere (see Journ. Institute 
of Brewing, 1909, p. 232), I need only refer to it briefly. It is based on the 
fact that freshly cultivated yeast when seeded in a malt-wort maintains 
a constancy in both the average dimensions of its constituent cells and in 
their nitrogen content. 
Let us assume that we have determined by the Kjeldahl method the 
percentage of nitrogen in a sample of washed and pressed yeast which we 
are about to use for seeding a wort, and that we have also determined the 
number of cells per unit volume of 40V0 cm., which are given by mixing 
a definite weight of this same yeast with water, and making up the volume 
to 100 c.c. From these data we can calculate the actual amount of yeast- 
nitrogen in 100 c.c. if the distribution of the cells corresponded to exactly 
one cell per unit volume of cm * This value I have termed the ‘nitrogen 
coefficient ’ of the yeast, and since the ‘ nitrogen coefficient * under the 
experimental conditions of growth in malt-wort remains constant, the cell- 
reproduction which occurs between any two intervals of time is a measure 
of the actual amount of nitrogen assimilated from the liquid ; and con- 
versely, if the loss of nitrogen per 100 c.c. has been determined for the same 
period by two successive nitrogen determinations in the malt-worts filtered 
from the yeast, this nitrogen loss can be used as a measure of cell-re- 
production. As an example, let us suppose that a particular sample of 
pressed yeast contains in its moist condition 1-98 per cent, of nitrogen, and 
that 1 grm. of this yeast, corresponding to 0-0198 grm. of nitrogen when 
mixed with water up to 100 c.c., gives a cell-count in the haemocytometer of 
10*39 ce ^ s P er unit volume. Then the 4 nitrogen coefficient ’ of the yeast (that 
is to say, the total nitrogen in the 100 c.c. if the distribution of the cells were 
exactly one cell per unit volume) would be represented by ~~~~ — 0*00191. 
This value can now be used as a measure of the cell-increase between any 
two intervals of time, if the loss of nitrogen in the filtered liquid has been 
determined : we then have 
i, . Loss of nitrogen per 100 c.c. 
cell-increase per unit volume = aT : 7 • 
Nitrogen coefficient of yeast 
1 Certain obvious precautions have to be taken in such cases to ensure that a sample drawn off 
in this way is truly representative of the bulk. 
