14 
BULLETIN 1460, U. S. DEPARTMENT OF AGRICULTURE 
Table 8. — Influence of time, method, and sample on protein test results — Con. 
GUNNING METHOD 
Sample No. 
Time of digestion— minutes 
15 
30 
45 
60 
75 
90 
105 
120 
High heat: 
12004 
Per cent 
12.37 
14.88 
11.00 
11.86 
14.59 
10.89 
0) 
0) 
C 1 ) 
Per cent 
12.74 
15.10 
11.34 
12.71 
15.10 
11.29 
0) 
0) 
0) 
Per cent 
12.84 
15.32 
11.40 
12.71 
15.22 
11.34 
12.48 
14.65 
11.23 
Per cent 
12.82 
15.36 
11.39 
12.88 
15.38 
11.40 
12.54 
15.03 
11.06 
Per cent 
12.83 
15.38 
11.39 
12.82 
15.38 
11.40 
12.48 
15.16 
11.17 
Per cent 
12.87 
15.36 
11.44 
12.86 
15.38 
11.44 
12.48 
15.16 
11.23 
Per cent 
12.81 
15.38 
11.41 
12.85 
15.38 
11.43 
12.71 
15.22 
11.28 
Per cent 
12.85 
12006. __ 
15.38 
12008 
11.40 
Medium heat: 
12004.. 
12.88 
12006 
15.38 
12008 
11.42 
Low heat: 
12004 
12.77 
12006. . 
15.22 
12008 
11.34 
At medium heat, the Gunning method becomes more satisfactory 
and is in line with the other two methods. The digestion time with 
the Kansas City Protein Referee Board method was reduced 20 per 
cent by raising the heat from low to medium, whereas the time was 
reduced 50 per cent when the Gunning method was used. By using 
high heat it was found that all three methods were on a par. It was 
further brought out, when using high heat with a 1-gram sample and 
20 cubic centimeters of acid, that no longer than 45 minutes is neces- 
sary to oxidize the sample completely. 
A summary of the investigations in this section shows that protein 
tests results will vary, depending upon the length of time of diges- 
tion, and that this period of digestion depends upon the intensity of 
the heat. At high heat, by all three methods, complete oxidation can 
be obtained in 45 minutes; at medium heat, in 60 minutes. At low 
heat, the Kjeldahl method was the only one giving correct results 
in less than an hour. With the Kansas City Protein Eeferee Board 
method 75 minutes were necessary. The Gunning method was not 
satisfactory at this intensity of heat. 
To check further the times recorded above, 19 samples representing 
the five commercial classes of wheat were digested for two hours. 
The same samples were also digested for 45 minutes at high heat and 
for 60 minutes at medium heat. As it is believed that low heat will 
never find place in commercial protein testing, it was not tried out. 
In the aggregate, with this intensity of heat the results are unsatis- 
factory. The oxidation is not regular and duplicative results are not 
the rule. The data from the check series, proving that 45 minutes 
is sufficient to oxidize a 1-gram sample at high heat and that 60 
minutes is sufficient to do the same at medium heat, are given in 
Table 9. 
