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HOW MUCH DOES A PRESSURE-TEST TELL YOU? 



William J. Bramlage 

 Department of Plant and Soil Sciences 



Crispness is one of the most important attributes of apple quality, 

 and consequently measurement of fruit firmness is an important index of 

 quality. Firmness is most commonly measured with a Magness -Taylor pres- 

 sure tester, an instrument first described in 1925, yet one which is 

 widely used even today. 



Because individuals use the pressure tester differently, the read- 

 ings obtained by different persons often do not agree. However, if an 

 individual is careful to apply the pressure tester in a consistent manner, 

 his own readings taken on different fruit samples are usually rather com- 

 parable. I have found that if the fruit is tested against a solid sur- 

 face (for example, a table top) and the plunger is inserted slowly, quite 

 comparable results are obtained. 



But how much does a pressure test tell you? Presumably, it is an 

 index of fruit texture. During storage tests in the 1965-66 season, 

 samples of Mcintosh apples were pressure-tested and then were offered to 

 a panel of 10-14 individuals to evaluate for texture under controlled 

 conditions. These judges had to evaluate 6 different samples against a 

 standard fruit sample, and evaluations were made at 15 different time 

 intervals. In 27 of these comparisons, significant texture differences 

 were detected by the panel, and in 23 of the 27 instances, significant 

 differences had been measured with a pressure tester. In 7 other compar- 

 isons, samples with significant pressure differences were not detected 

 as having significant texture differences by the panel. Because the over- 

 all test was a large one involving many samples, an average pressure dif- 

 ference of only 0.2 lbs. between samples (20 fruits per sample) was sig- 

 nificant statistically. That such small differences in firmness could 

 be so indicative of detectable texture differences is indeed remarkable. 

 The conclusion is inescapable that pressure tests were a reliable index 

 of fruit texture . 



Pressure tests are very commonly used as a maturity index to deter- 

 mine time of harvest, and also as a means of predicting storage life or 

 shelf life after storage. These usages are based on the fact that as 

 fruits ripen, the layer of pectin between individual cells breaks down, 

 causing the cells to separate to some degree and therefore to soften. 

 As a maturity index, pressure tests are valuable if used along with 

 other indices, but they certainly are not definitive guides to be used 

 by themselves. As a means of predicting storage or shelf life, pressure 

 test may be very misleading. This was clearly shown by our storage tests 

 of the past 2 seasons. 



In 1964, we harvested Mcintosh at 2 stages of maturity and the pres- 

 sure tests averaged 16.2 and 14.8 lbs., respectively. In 1965, fruits 

 of approximately comparable maturity averaged 15.9 and 14.4 lbs. at har- 

 vest. Thus, if firmness were a reliable predictive index, the fruits 

 should have responded similarly in both years to similar storage condi- 



