6 CIRCULAR 



FACTORS INFLUENCING PRESSURE-TEST READINGS 

 Variations in Methods of Making Tests 



Various factors that might affect the pressure-test reading have been 

 studied. Magness and Taylor (48) determined that if the pressure 

 is applied very rapidly the results may be about 0.5 of a pound higher 

 than if it is applied slowly. This would represent an extreme differ- 

 ence, as the rate of application varied more than is likely to occur in 

 practice. The rate of application of pressure, therefore, is not likely 

 to be an important source of error. 



The tip of the plunger as furnished with the testers is uniformly 

 slightly rounded. Magness 2 compared plungers having a flat tip, a 

 hemispherically rounded tip, and a tip intermediately round. The 

 average pressure test of four varieties was 13.5 pounds for the rounded 

 tip, 13.7 for the slightly rounded one, and 13.8 for the flat one. These 

 extreme differences in shape did not cause significantly different 

 results. However, Ryall 3 compared a rounded-tip plunger originally 

 furnished with the testers with a plunger having the less rounded tip 

 of the later models and found that pressure tests with the rounded 

 tip were sometimes as much as a pound less than with the slightly 

 rounded tip. Apparently differences in the shape of the tip may cause 

 appreciable error, and the shape should therefore be kept uniform. 



Pressure testers should be calibrated occasionally to insure that 

 they are reading correctly. This can be conveniently done by placing 

 the plunger of the tester against the platform of an accurate set of 

 scales and pressing down until the scale registers a given amount and 

 checking this against the pressure-test reading. The calibration 

 should be made at various points on the scale of the pressure tester, 

 as the correction may vary for the different points. If the tester is 

 held upright on the scales in calibrating, the weight of the plunger 

 rod, theoretically, should be deducted from the scale reading. Prac- 

 tically, this is not important and is not done in the original calibration 

 of the instrument. If an instrument is found to read incorrectly, 

 the readings should be corrected or the instrument should be returned 

 to the factory for recalibration. 



Maturity and Ripeness of Fruit 



As the fruit becomes more mature on the tree or ripens after harvest, 

 there is a gradual softening of the flesh. Softening of the fruit after 

 harvest has been associated with the hydrolysis of the insoluble cell- 

 wall cementing material, protopectin, to soluble pectin in peaches 

 (1, 8, 53), apples (12, 13, 25), and pears (22, 24, 41). As the fruit 

 matures on the tree it is likely that the increase in the size of the cells 

 and possibly the stretching and thinning of the cell walls are import- 

 ant factors influencing the softening of the fruit. 



Magness et al. (46) observed that apples grown in districts with 

 long growing seasons were generally softer when picked than those 

 grown in districts of short growing seasons. This was attributed in 

 part to greater maturity at time of picking for apples grown with 



2 Unpublished data by J. R. Magness. 



3 Unpublished data by A. L. Ryall. 



