14 



BULLETIN 563, U. S. DEPARTMENT OF AGRICULTURE. 



Table VII. — Comparison of bacterial counts obtained from the same samples of ice 

 cream but with different dilutions — Continued. 



Lot. 



Sam- 

 ple 

 No. 



Dilu- 

 tion. 



Number of 



colonies on 



duplicate 



plates. 



Average 



count per 



c. c. 



Varia- 

 tion be- 

 tween 

 lowest 



and 

 highest 

 counts. 



Dilution. 



Num- 

 ber of 

 colonies 

 on du- 

 plicate 

 plates. 



Avcerage 



count per 



c. c. 



Varia- 

 tion be- 

 tween 

 lowest 



and 

 highest 

 counts. 





D ... 



1 

 2 

 3 



4 

 5 

 6 

 7 

 8 

 9 



T60»&0» 



321 



270 

 386 

 278 

 367 

 354 

 431 

 470 

 434 



310 



305 

 372 

 305 

 356 

 357 

 466 



31,500,000 

 28,750,000 

 37,900,000 

 29, 150, 000 

 36,150,000 

 35,550,000 

 44,850,000 

 47,000,000 

 43,400,000 



Per ct. 

 63.47 



T7060.17TO 



22 

 35 

 24 

 33 

 35 

 35 

 43 

 52 

 55 



31 

 36 

 37 

 25 

 34 



27' 

 39 

 45 



26,500,000 

 35,500,000 

 30,500,000 

 29,000,000 

 34,500,000 

 35,000,000 

 35,000,000 

 45,500,000 

 50,000,000 



Perct. 

 88.69 





. E.... 



1 

 2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 



Ti&inr 



219 

 214 

 222 

 192 

 212 

 197 

 212 

 231 

 218 



186 

 190 



194 



200 

 208 

 205 

 224 



202,500 

 214, 000 

 206,000 

 192,000 

 203,000 

 198, 500 

 210,000 

 218, 000 

 221,000 



15.10 



TG,T5TTG 



24 

 25 

 23 

 13 

 26 

 36 

 20 

 31 

 36 



20 



24' 

 22 

 16 



"24' 

 25 

 22 



220,000 

 250,000 

 235,000 

 175,000 

 210, 000 

 360,000 

 220, 000 

 280,000 

 290,000 



105. 71 





F..„. 



1 



2 

 3 

 4 

 5 

 6 

 7 

 8 



T00.&06 



310 

 286 

 330 

 293 

 284 

 328 

 286 

 275 



298 



288 

 288 

 292 

 27S 

 285 



30,400,000 

 28, 600, 000 

 33,000,000 

 29, 050, 000 

 28,600,000 

 31,000,000 

 28,200,000 

 28,000,000 



17.85 



17060.751$ 



19 

 27 

 21 

 29 

 31 

 36 

 35 

 27 



13 

 29 

 22 

 22 

 28 

 31 

 39 

 32 



16,000,000 

 28,000,000 

 21,500,000 

 25,500,000 

 29,500,000 

 33,500,000 

 37,000,000 

 29,500,000 



131. 25 





G.... 



1 



2 

 3 

 4 

 5 

 6 

 7 

 8 

 9 



! 



144 

 128 

 179 

 200 

 235 

 152 

 130 

 270 

 222 



178 

 136 

 161 



190 



133 



16, 100, 000 

 13,200,000 

 17,000,000 

 20, 000, 000 

 21,250,000 

 15, 200, 000 

 13,150,000 

 27,000,000 

 22,200,000 



105. 32 



T7000.000 



22 

 17 



7 

 30 



6 

 15 

 15 

 38 

 33 



20 

 47 

 14 

 14 



1 

 15 



9 



'45" 



21,000,000 

 32,000,000 

 10,500,000 

 22,000,000 

 3,500,000 

 15, 000, 000 

 12,000,000 

 38,000,000 

 39, 000, 000 



1,014.28 









VARIATION BETWEEN DUPLICATE COUNTS FROM SAME SAMPLE AND 



SAME DILUTION. 



As stated, we do not intend to say just what variation should be 

 allowed between duplicate counts or a series of samples of ice cream 

 from the same lot. Table VIII, however, shows some colony counts 

 obtained by making a series of from five to eight plates from the 

 same dilution. In five different samples a variation of from 7 to 

 26.6 per cent was found. Among the duplicate plates in the exami- 

 nation of other samples of ice cream a variation as high as 41 per 

 cent was found. From figures given by other investigators of the 

 results of bacterial counts of ice cream, variations in counts between 

 duplicate plates as high as 88 per cent have been observed. It is 

 obvious that the factor of variation among plates from the same 

 dilution of ice cream must be taken into consideration when inter- 

 preting the results of the bacterial analysis of duplicate samples or 

 a series of samples from a given mass of ice cream. To this factor 

 of variation must be also added that caused by the difficulty of 

 removing exactly equal quantities of different samples of ice cream 

 in order to make the first dilution. 



