EXAMINATION OF TOMATO PRODUCTS 517 
if there seemed to be a larger percentage of the latter, mention was made 
of that fact. 
To make a count 10 c.c. of the product is thoroughly mixed with 
20 c.c. of water, and, after being allowed to rest for a moment to permit 
the very coarsest particles to settle out, a small drop is placed on the 
central disk of the Thoma-Zeiss counting cell and then covered with 
a glass. Care must be exercised to have the slide perfectly clean, so 
that, when the cover glass is put in place, a series of Newton’s rings * 
results from the perfect contact of the glass surfaces; and furthermore, 
the drop should be of such size as not to overrun the moat around the 
central disk and creep in underneath the cover glass, thus interfering 
with the contact. 
With the magnification of 180, it has been the practice in this 
laboratory to count the number of yeasts and spores on one-half of 
the ruled squares on the disk. With the dilution used this calculates 
back to a volume equal to one-sixtieth of a cubic millimeter in the 
original sample, and reports are made on that basis rather than on 
the number in a cubic centimeter, because the former number is more 
readily grasped by the mind and affords a simpler notation. To 
obtain the numbers per cubic centimeter the count made is simply 
multiphed by 60,000. 
It has been found in practice that the number of yeasts and spores 
varies, for one-sixtieth of a cubic millimeter, from practically none in 
homemade and first-class commercial ketchups up to 100 or 200, and 
in one sample the number was as high as 1200. Laboratory experiments 
show that, when the number of yeasts in raw pulp reaches from 30 to 35 
in one-sixtieth of a cubic millimeter the spoilage may frequently be 
detectable by an expert by odor or taste, and from experiments made 
under proper factory conditions, it seems perfectly feasible to keep the 
number in commercial ketchups below 25. 
Estimation of Bacteria. The bacteria are estimated from the same 
mounted sample as that used for the yeasts and spores. A power of 
about 500, obtained by using a high-power ocular, is employed in this 
case, and because of the greater number present a smaller area is counted 
over. Usually the number in several areas, each consisting of five of the 
small-sized squares, is counted and the number of organisms per cubic 
centimeter is calculated by multiplying the average number in these 
areas by 2,400,000. Thus far it has proved impracticable to count the 
micrococci present, as they are likely to be confused with other bodies 
* These are rainbow-colored rings produced at the point of contact when polished 
plates of glass are pressed against each other. 
