16 BULLETIlSr 1041, U. S. DEPARTMEiSTT 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 heavj?^ con- 

 sistency, and the penetration of Jieat 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 m these studies to determine the 

 processing temperatures and time periods necessary to effect com- 

 plete sterilization. Though the statement of Weinzirl {W) 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 easilj^ preserved. Eigorous 

 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 



