516 FOOD PRESERVATION 
pressed down till a film of the product about 0.1 mm. thick is obtained. 
After some experience this can be done fairly well. A film much thicker 
than this is too dense to be examined successfully, while a much thinner 
film necessitates pressing the liquids out, which gives a very uneven- 
appearing preparation. When a satisfactory mount has been obtained, 
it is placed under the microscope and examined. The power used is 
about 90 diameters, and such that the area of substance actually exam- 
ined in each field of view 1s approximately 1.5 sq. mm. 
A ficld is examined for the presence or absence of mold filaments, 
the result noted, and the slide moved so as to bring an entirely new 
field into view. This is repeated till approximately 50 fields have 
been examined, and the percentage of fields showing molds present 
is then calculated. Our experience has demonstrated that for home- 
made ketchups this is practically zero, and with some manufactured 
ketchup it is as low as from 2 to 5 per cent, while for carelessly made 
products it may be 100 per cent; that is, every field would show the 
presence of mold. Investigations under factory conditions clearly 
indicate that with only reasonable care the proportion of fields having 
molds can be kept below 25 per cent. A specimen in which 60 per cent 
of the fields have molds is in more than twice as bad a condition as one 
containing 380 per cent. 
After the percentage reaches 30 to 40 per cent it will be found that 
some of the fields frequently have more than one filament or clump of 
mold, and the number of such fragments might be counted, but in this 
laboratory this usually is not done. A Thoma-Zeiss counting cell with 
a center disk of 0.75 in. instead of 0.25 in., as usually furnished, would 
give a regular depth of liquid and would be more exact than the method 
described, but this must be specially manufactured, not being listed 
in any of the catalogues of microscopic supplies, and the method as given 
is sufficiently accurate for the purpose. When the number of fragments 
of mold per cubic centimeter is estimated, it has been found to range 
from virtually zero to over 20,000. There is no excuse for a manufac- 
turer allowing such conditions to prevail that his ketchup shows more 
than 2000 per cubic centimeter, while some manufacturers by careful 
handling hold it down to 150. 
Estimation of Yeasts and Spores. Though the spores referred to 
are those coming from molds and correspond to seeds in more highly 
developed plants, it is frequently very difficult to differentiate some of 
them with certainty from some yeasts without making cultures, which is 
obviously impossible in a product that has been sterilized by heat. 
For this reason the yeasts and spores have been reported together, and 
