By A. PERCY SMITH, F.I.C., F.C.S., Rugby.
The method usually adopted for estimating the peptonizing power of
pepsina porci consists in dissolving 1 to 2 grains in 8 to 12 ounces
of water, to which 40 to 60 minims of hydrochloric acid has been
added. 500 to 1,000 grains of hard-boiled white of egg, granulated by
rubbing through a wire sieve, is immersed in the liquid, and the whole
kept at 98° to 130° F. for four hours, when the undissolved albumen is
filtered off through muslin, and, after partial drying, is weighed to
ascertain the amount dissolved. The variable numbers above quoted
embrace various formulæ recommended by different experimenters.
This method of analysis is excessively crude and untrustworthy. When
hard-boiled white of egg is kept in warm water, it absorbs a
considerable quantity of that menstruum, as much as several units per
cent.; consequently, on weighing the residual albumen, you may find
that the weight is greater instead of less than that with which you
started, the gain in weight due to absorbed water more than
counterbalancing the loss obtaining through solution, as has happened
with indifferent samples of pepsin. Then who shall say when, by simple
air drying, the albumen has regained its former condition? The
enormous quantity of albumen is foreign to the usual habits of the
scientific analyst, and involves an enormous waste of time in
manipulation.
One trial of this method was enough for me. The first modification I
adopted consisted in substituting for the large quantity of granulated
albumen a single half of the white of an egg in one piece. I likewise
arranged a check experiment in which the pepsin was omitted, other
conditions remaining unaltered. At the end of four hours the residual
pieces of albumen were placed on blotting paper to remove superfluous
moisture, and weighed. The gain in weight of the albumen in the check
experiment, due to absorbed water, was calculated into percentage, and
the same deducted from the weights of the other portions which had
been subjected to the action of various pepsins. This, although an
improvement upon the old method, proved likewise unreliable, because
the water absorbed was not equal in each experiment. The albumen which
was immersed in acidulated water only quickly dried, superficially,
when placed on blotting paper, whereas that which had been acted on by
pepsin was rendered glutinous and incapable of being dried in this
manner. In fact, one sample weighed considerably more than it did at
starting, even after deducting the allowance for water absorbed.
I next tried much smaller pieces of albumen, about 1 c.c., in hope
that complete solution might ensue, and a time value be obtained. I
soon found, however, that the solubility does not depend upon the
mass, but upon the surface exposed.
Finally I discarded altogether the use of fresh white of egg, and had
recourse to dry powdered albumen, prepared by drying in a steam oven
and levigation in a mortar. With this I succeeded in getting accurate
comparisons between the digestive powers of various pepsins. Albumen
in this form dissolves with rapidity, owing to its state of fine
division. Any remaining undissolved can be filtered off on a
counterpoised filter paper, and heated in a water oven until
absolutely dry. It is, however, unnecessary to do this when two
samples only are compared against each other, nor is it essential to
know the actual weight of albumen employed, provided it be the same in
each experiment. This is insured by placing some on the naked pan of
the balance (there is no objection to so doing, as it is a dry gritty
powder, and does not adhere to the metal), and counterpoising by a
similar addition to the other pan.
Let the albumen fall on the center of the filtered liquid, avoiding,
if possible, contact with the glass of the beaker. It soon sinks, and
after the lapse of some time, a simple inspection will show which is
dissolving with the greater rapidity. Agitation assists solution.
Therefore take the two beakers, one in each hand, and rotate the
contents equally. When one sample has dissolved all the albumen, it is
manifestly superior to the other which has failed to do so in the
given time. If many samples have to be compared, it will be necessary
to start with known quantities of albumen, and weigh the undissolved
residues in the manner above indicated.
An objection may possibly be raised to this modified method, viz.,
that albumen as ingested is not in the form of a dry powder, and that
we ought to copy as nearly as possible the conditions existing in the
stomach. To this I would reply that it does not matter in the least,
to us, as analysts, what are the conditions which obtain in the
stomach; since there is no absolute test for pepsin, we can only
compare one sample against another, and that which dissolves the most
albumen in the shortest time is taken to be the best.
Another imperfect method of analysis is that employed in the
examination of malt extracts for diastase, in which a certain weight
of extract ought to dissolve a certain weight of starch in ten
minutes, when if it does so dissolve it, the extract is a good one; if
not, it is to be condemned. The more correct way is to ascertain the
reducing power on Fehling's solution, before and after digestion with
an _excess_ of starch, and I intend to say a few words upon this
subject on a future occasion, when I have ascertained the maximum
amount of diastase existing in the best samples of malt.--_The
Analyst._
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