TOMATO KETCHUP UNDER THE MICROSCOPE. O 



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 1 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 

 multiplied 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 1,200. Labora- 

 tory experiments show that, when the number of yeasts in raw pulp 

 reaches from 30 to 35 in one-sixtieth of a cubic millimeter the spoil- 

 age 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 micro- 

 cocci present, as they are likely to be confused with other bodies fre- 

 quently present in such products, such as particles of clay, etc. A 

 comparison of this method with the ordinary cultural methods on 

 samples in which the organisms had not been killed has almost in- 

 variably shown that the one used gives too low instead of too high 

 results. In some cases it was found to give not more than one-third 

 of the entire number present. The estimates of the laboratory on 

 this point may, therefore, be considered very conservative. 



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

 of glass are pressed against each other. 



