A MECHANICAL THUMB FOR MEASURING 

 FIRMNESS OF FRUITS 



H. A. Schomer, K. L. Olsen, and J. N. Yeatman 

 Horticultural Crops Branch 

 Agricultural Marketing Service 



Scientists of the Agricultural Marketing Service have devised a "mechan- 

 ical thumb" with which fruit inspectors can measure objectively the firmness 

 of the flesh of fruits, without cutting or discarding the samples. It is used as 

 an attachment on the Magness -Taylor pressure tester by substituting it for 

 the regular plunger at the end of the tester shaft (fig. 1). 



The "thumb" consists of a plunger l/2-inch in diameter, with a slightly 

 convex end, encased in a close-fitting but free-sliding sleeve (fig. 2, p. 4). 

 The plunger does not penetrate the fruit flesh, but, as it is pressed against 

 the fruit, the sleeve is forced back as the plunger indents the fruit to a pre- 

 set distance. At this point, an electrical contact is made which lights a small 

 attached flashlight, and the firmness is read on the Magness -Taylor tester. 

 These testers are available with scales reading either to 10 pounds or 30 

 pounds pressure, depending on the sensitivity desired and the product being 

 tested. A tester reading to 30 pounds is used for apples. 



Material components: 



Suggested material components shown in the scale drawing (fig. 2) are as 

 follows, or as otherwise indicated in the drawing: 



1. Plunger and housing of stainless steel or aluminum. 



2. Sleeve of brass or synthetic material. 



3. Spring wire of .032-inch diameter, 5 revolutions, steel. 



4. All components easily accessible for purposes of maintenance and 

 ease of calibration. 



Specific requirements: 



1. The plunger head is a segment of a 1-inch sphere. The actual height 

 it extends above the sliding sleeve is not critical. It could actually be 



Washington, D. C 



2 



May 1963 



