Measuring Fruit Firmness with a Penetrometer
Apple firmness is used worldwide as a measure of ripeness and "condition" of the fruit. The most widely used instrument for firmness measurement is the Magness-Taylor pressure tester (devised in 1925), although the Effegi tester (developed recently in Italy) has met some acceptance due to its compact size and convenience. Tests comparing the Magness-Taylor with the Effegi indicate that readings of the two instruments are quite comparable, and I shall assume that what is said in this article about use of a Magness-Taylor is equally true about use of an Effegi tester.
With its worldwide and longstanding use, and the importance of its measurements, one may assume that the Magness-Taylor is used in a standard way. However, readings are often grossly variable among users of the instrument. In one test in Geneva, New York, it was found that professional users of a Magness-Taylor varied as much as 3 to 4 pounds in the readings they obtained on the same lots of apples! Following an informal discussion at a meeting in December, 1975, where it was evident that use of pressure testers differed widely, 10 Northeastern post-harvest horticulturists agreed to gather data on factors that can influence pressure test determinations, in hopes of standardizing a technique. The results of this collaborative effort, coordinated by Dr. G. D. Blanpied of Cornell University, are summarized here.
The Pressure Tester
The instrument itself may be a cause of erroneous readings. First, there are two sizes of plunger "heads" that might be used. For apples, the larger one, with a diameter of 7/16-inch, is always used; the smaller, 5/16-inch head, is for use on pears, which are much harder than apples until nearly ripe. A second problem is that the instrument may not be calibrated. Calibration is relatively simple and should be checked regularly. To calibrate, place the plunger on an accurate scale and press down slowly until the scale registers a weight that occurs on the pressure tester scale. Check this weight against the recorded reading on the pressure tester. Several different points on the scale should be tested in this manner.
The user should consciously and carefully choose the fruits that will be tested, knowing the factors that may influence the readings.
If you are testing in the orchard, it is likely that fruit from outside of the tree will test firmer than those toward the inside of the tree.
Fruit size is a very important factor. In general, the larger the fruit, the softer it will be. Sometimes a 1/4 inch difference in diameter can make a one or two pound difference in the pressure test. Following years of careful record-keeping, Dr. George Mattus suggests that you not vary more than 1/4 inch in diameter among the fruit you test. Obviously some kind of sizing device is therefore necessary in choosing a sample. Further, you should test a size that is representative of the majority of the crop, and specify the size you are testing. You cannot accurately compare firmness of lots of fruit if you sample 3-inch fruit in one lot and 2 1/4 inch fruit in the other.
The temperature of the fruit can have a small but sometimes significant influence on pressure tests. Firmness tends to be slightly less when apples are warm than when they are cold. This is not nearly as important a point as is the size of the fruit, but for maximum accuracy, the user should be consistent about testing either warm fruits or cold fruits.
The number of fruit to sample is an important consideration.
- For harvest measure:
- 10 apples per orchard
- 2-3 tests per apple
- one set of apples
[Editor: Use the same typical trees in a block each week.]
- For storage measure:
- 20 apples per grower lot, 2-3 tests per apple
- 10 apples per grower lot, set to ripen 10 days at 65°, 95% R. H., 2-3 tests per apple
- For harvest measure:
Making the Test
Having calibrated the pressure tester and carefully chosen a sample, how should you test the fruits? First, you should recognize that each fruit is not of uniform firmness. Generally, the blush side is firmer than the green side. This difference may be as much as one pound of pressure. Therefore, either consistently test the blush side, knowing it is firmer, or the green side, knowing it is softer, or else test both the blush and the green sides and average the readings. Since the skin badly distorts a pressure test on an apple, it must be removed from the area to be tested. The depth of the cut removing this skin influences the reading: the deeper the cut, the higher the reading. Dr. Robert Hardenburg suggests use of a potato peeler (stainless steel to avoid rusting) for quick, shallow, consistent cuts. These cuts should be made at a point half way between the stem and calyx ends of the fruit.
Never test a bruised area. For testing, the fruit should be placed on a hard surface (e.g., table top) rather than being hand held. The plunger should be inserted to the line inscribed on the plunger. Testing only to the "yield point" of the fruit tissue (i.e., when it "gives") produces an erroneously low reading, and going beyond the line gives a high reading.
However, the most critical feature of testing is the speed of applying the force. The faster you apply the pressure, the higher will be the reading. The proper speed is about two seconds, and to regulate your speed, it is suggested that you say to yourself, "1,001, 1,002" as you insert the plunger into the fruit. This may sound childish, but it is extremely critical as can be seen simply by applying force at different speeds during calibration. The user needs to frequently check himself during testing to make sure he is testing at the proper speed. Applying pressure too fast is probably the greatest source of false readings by users of the pressure tester.
Reading the Scale
Having tested the fruit, how do you read the scale? Some read it to the nearest whole pound, others to the nearest 1/2 pound and some may even read to the nearest 1/10 pound. It seems clear that reading to the nearest 1/2 pound is sufficient, and if your sample size is reasonably large, the nearest one pound is satisfactory. Again, my preference is to the nearest 1/2 pound.
Sources of Error
With an accurate instrument, careful sampling and precise testing, you should obtain a quite accurate firmness measurement of the fruits. But this accurate measurement still may not truly represent the "condition" of the apple. Some sources of error are as follows:
Nitroqen (N) level of the fruit: Increasing the N level in apples may reduce firmness of apples more than it affects post harvest "condition" of them if the apples were at the threshold of N-deficiency before treatment. Thus, you may misjudge "condition" by comparing lots of widely varying N levels.
Water Core: The more water core in a fruit, the firmer it may pressure test, even though increasing water core indicates increasing fruit maturity. Pressure tests may indicate very little about "condition" of water-cored apples.
Water loss: If apples are losing water rapidly, they may "soften" due to loss of turgor (i.e., wilting). This softening does not represent what is usually regarded as "loss of condition"
There are probably other complicating factors, also, but these examples illustrate the importance of observing the fruit you are testing, recognizing symptoms of complicating conditions, and being careful about how you interpret the results of pressure tests.
Need for Uniformity
With the importance of firmness in the acceptability of apples, and the ease of using pressure testers, these instruments seem certain to remain as key determinants of apple quality in the foreseeable future. Yet, it is shocking to see how erratically these devices are used. At present, a term like "10-pound McIntosh" may actually mean little to anyone but the person who tested the fruit. These same apples may test 12 pounds to another person and 8 pounds to a third person. Yet, McIntosh apples truly testing 12 pounds of pressure have grossly different potential than ones truly testing 8 pounds. If we are going to use firmness as a meaningful guide to apple quality, we all need to reexamine our testing procedures and do our utmost to standardize them so that our determinations can become more comparable and our interpretation can be more accurate. Here is a problem that can be overcome with good judgment and little or no expense. Your results are only as good as your sample, consistent technique and a properly functioning instrument.
Use of this chart will allow you to standardize firmness readings to box size 100 fruit. This is necessary since fruit larger than size 100 are normally less firm, while smaller fruit are firmer. By combining firmness readings with diameter, week to week or orchard to orchard comparisons become more meaningful. A convenient method of measuring circumference is with a Cranston Machinery Co. gauge.
Adapted from: FRUIT NOTES, University of Massachusetts, Vol. 42, No. 2, March /April, 1977. Collaborators were: G. D. Blanpied, Cornell University; D. H. Dewey, Michigan State University; R. E. Hardenburg and A. Watada, USA, Beltsville, MD; M. Ingle, West Virginia University; R. LaBelle and L. Massey, Geneva NY; G. Mattus, Virginia Polytechnic Institute and State University, Blacksburg; W. Stiles, University of Maine; and W. J. Bramlage
William J. Bramlage
Department of Plant and Soil Sciences, University of Massachusetts
Post Harvest Pomology Newsletter, August 1983, Vol 1, No. 3.