16 BULLETIN 1041, U. S. DEPARTMENT OF AGRICULTURE. 
been found necessary even to remove considerable tissue in advance, 
of the fungous growth. 
There is another type of discoloration met with in the canning of 
sweet potatoes which is due to the influence of heat. At very high 
temperatures, even in well-exhausted cans, the potatoes turn reddish 
brown in color ; but it is more marked if oxygen is present. The con- 
dition is due to carmelization of the sugars and to changes in other 
constituents of the product. It is also accompanied by an alteration 
of flavor. Discoloration of this type is easily avoided by carefully 
adjusting the time and temperature of processing. 
HEAT PENETRATION AND STERILIZATION. 
The rate of temperature changes in sweet potatoes during canning 
and the influence which initial temperatures have upon it have been 
fully considered by the writers in United States Department of 
Agriculture Bulletins Nos. 956 and 1022. Extended discussion of it 
here, therefore, is unnecessary. Sweet potatoes have a heavy con- 
sistency, and the penetration of heat into a can of such material is 
very slow. If the potatoes are introduced into the can after they 
have cooled considerably a much longer time is required for process- 
ing and the danger of spoilage is increased. For the same reason 
short heating in the exhaust box is ineffective in producing a proper 
vacuum. No attempt was made in these studies to determine the 
processing temperatures and time periods necessary to effect com- 
plete sterilization. Though the statement of T\ r einzirl (20) that the 
sweet potato offers an unusual test of sterilization would lead one 
to think that it is very difficult to can successfully, three years of 
observation and study of sweet-potato canning lead the writers to 
the conclusion that this product is very easily preserved. Rigorous 
processing, such as is demanded by some of our standard food prod- 
ucts, seems not to be essential to satisfactory results with sweet pota- 
toes. Packing the potatoes into the cans at a temperature not below 
70° C. and processing at 116° C. for a length of time sufficient to 
bring the material at the center of the can somewhat above 100° C. 
has given uniformly good results in the work here recorded. 
Differences in the rate of heat penetration in the " dry " firm 
varieties and in those which are soft and " moist " have been found 
too slight to be of any practical significance. Modifications in the 
processing due to any varietal differences in potatoes, therefore, ap- 
pear unnecessary. 
CONSISTENCY. 
The behavior of different varieties of sweet potatoes on cooking is 
extremely variable. Some remain quite dry and mealy and are firm 
