6 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 



in at first by conduction and later by convection. Any formulas 

 which take into consideration such factors as these must be very com- 

 plex, indeed, and their application would be difficult and of doubtful 

 value. This investigator may be able to overcome some of these 

 difficulties in further work. 



METHODS AND APPARATUS. 



As has been pointed out, the earlier work upon the time-tempera- 

 ture relations in foods during canning made use of maximum ther- 

 mometers, which were sealed into the material in the cans. While 

 the information obtained in this way is valuable so far as it goes, for 

 practical purposes and for the carrying out of careful scientific inves- 

 tigations the use of the maximum thermometer is out of the ques- 

 tion. In the first place, for one experiment, which may require from 

 one to many hours to complete, only one temperature reading can be 

 obtained. In an experiment of this sort nothing is known of the 

 exact length of time required for the material at the center of the 

 can to reach the recorded temperature or of the length of time the 

 temperature may have remained at that point. Furthermore, it is 

 necessary to carry out mam 7 tests in order to record even a partial 

 story of the time-temperature relations in a single can of material. 

 To make studies of this kind of the most value, it is important to 

 know not only what is the highest point reached, but also something 

 of the rate of rise in temperature before that point is reached, and 

 especially for how long it remains at or above the pasteurizing or 

 sterilizing temperature during the processing. 



To overcome the disadvantages of the maximum thermometer, 

 thermocouples have been used in more recent investigations. These 

 enable the worker to record the entire story of the temperature 

 changes in any part of the can if desired, and when properly stand- 

 ardized they are highly accurate. The principal drawback in their 

 use is the complexity of the equipment, which requires considerable 

 technical skill to operate properly and the fact that the equipment 

 is not available for many who would care to carry on studies in this 

 field. Furthermore, thermocouples must be confined primarily to 

 laboratory investigations, as the} 7 are unsuited to practical routine 

 work. 



With these facts in mind an endeavor was made to devise an ap- 

 paratus which would be inexpensive to install, simple and easy to use, 

 and at the same time sufficiently accurate for the determination of 

 temperature changes under various conditions of processing. A 

 standard method of determining temperature is by the use of the mer- 

 cury thermometer, and it should be acceptable for this work, pro- 

 vided it is suitably constructed, properly calibrated, held securely, 



