The preservative in general use in ketchup is sodium benzoate.
Salicylic acid is used, but only to a limited extent. The amount
of sodium benzoate used, according to the labels, varies from
one-sixteenth to one-tenth of 1 per cent; but on some labels the amount
is not stated. Experiments were made to determine the amount necessary
to check the spoilage of ketchup.
Two organisms, a mold and a yeast, were selected on which to make the
tests. The mold was the ordinary blue mold, Penicillium, which was
present in many of the brands of ketchup and is found commonly on acid
foods. It was selected on account of its prevalence and resistive
power. The yeast was obtained from ketchup and was also a vigorous
grower, forming a thick, wrinkled film on various media. Any effect on
the growth of the yeast could be seen readily in its manner of forming
the film.
Portions of tomato gelatin to which 0.1, 0.5, 1, and 2 per cent,
respectively, of sodium benzoate were added, were first inoculated with
the mold. There was no development in those containing 1 and 2 per
cent; a retarded development resulted in that containing 0.5 per cent,
and the growth when 0.1 per cent was used was nearly normal, showing
very little difference from that in the gelatin without sodium benzoate.
Ketchup was next tried as a medium, but the amount of benzoate was
reduced to one-sixteenth, one-twelfth, and one-tenth of 1 per cent,
as it was thought that some of the other constituents of the ketchup
were antiseptic to a slight degree. The growth in the ketchup was
irregular, though the benzoate checked development in all. Equal
amounts of benzoate were used in tomato bouillon, with practically the
same results as in the ketchup. The development was checked in all,
and in some plates one-sixteenth of 1 per cent seemed to be fully as
efficacious as one-tenth of 1 per cent. When the mold was examined
under the microscope, the filaments were found to be much swollen and
distorted in shape, and filled with a coarsely granular protoplasm,
containing much fat, as indicated by the blackening with osmic acid.
The culture containing the mold which gave the least development seemed
to show the least distortion and swelling of the filaments.
The results indicated that in using sodium benzoate as a preservative
there is uncertainty as to results, even when using the maximum amount
allowed--one-tenth of 1 per cent. They also indicated that this
preservative had an injurious effect on the living matter of the mold.
(See Pl. II; compare with normal growth, Pl. I.)