l8o MEDICAL BACTERIOLOGY 



onstrated that for homemade 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 be- 

 low 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 30 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 inch in- 

 stead of 0.25 inch, 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 manu- 

 facturer 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 differ- 

 entiate some of them with certainty from some yeasts without making cultures, 

 which is obviously impossible in a product that has been sterilized by heat. 

 I or this reason the yeasts and spores have been reported together, and if there 

 seemed to be a larger percentage of the latter, mentioned was made of that fact. 



To make a count 10 cc. of the product is thoroughly mixed with 20 cc. of 

 water, and after being allowed to rest for a moment to permit the very coarsest 

 particles to settle out, a small drop is place 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 %o 

 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 mo- 



* These are rainbow-colored rings produced at the point of contact when polished plates of 

 glass are pressed against each other. 



