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Examination of Operational Aspects of Fruit Pressure Tests
Examination of Operational Aspects of Fruit Pressure Tests! Peter W. Voisey Engineering Research Service Research Branch, Agriculture Canada Ottawa, Ontario KIA OC6 lContribution No. 671 from Engineering Research Service
Abstract The Ballauf fruit pressure tester and a compact Italian instrument were compared in testing apples. It was found that the p~imary' source of differences between the mstrument readmgs was systematic calibration errors. The different size, shape and spring rate of the two instruments did not affect the rate of increase of force applied by the average operator. Females, however, used nearly twice the rate of males. It appeared advantageous to calibrate the instruments dynamically since this simulated the operating conditions of the instruments.
Resume Pour mesurer la texture des pommes, on a fait la comparaison entre Ie texturometre Ii pression Ballauf et un instrument italien compact. II a ete trouve que la principale source de differences entre les lectures instrumentales etait des erreurs systematiques de calibration. Les differences dans la taille, la forme et la vitesse du ressort des deux instruments a ete sans effet sur III vitesse d'augmentation de la force appliquee par l'operateur moyen. Cette vitesse a ete presque deux fois plus elevee avec les femelles qu'avec les males. II est apparu avantageux de faire la calibration dynamique des instruments afin de simuler les conditions des instruments en operatIOn.
Introduction The puncture test is widely used for measuring fruit and vegetable texture (Bourne, 1976) and many instruments have been developed for this purpose (Voisey and DeMan, 1976). An early application was in the determination of fruit maturity for field and grading station applications. This led to the use of spring scales to register the maximum force required to push a cylindrical probe into the fruit, and instruments of this type, commonly called fruit pressure testers, are established in the industry. Operationally the inherent simplicity of the puncture test makes it an ideal method but it is difficult to analyze and establish a theoretical base for interpreting the readings in fundamental terms (e.g. Timbers et al., 1965; Mohsenin et al., 1970; Anon, 1974; Barber, 1974) by classical analysis of the indentation problem. A simplified approach was to analyze the puncture force in terms of shear and compression coefficients that describe the behaviour of the test material (Bourne, 1966, 1975; DeMan, 1969; Su and Humphries, 1972; Peleg and Brito, 1975). A complete analysis of fruit pressure tester operation was reported by Yang and Mohsenin (1974). These analyses were investigated using a powered mechanism that forced the punch into the sample at a constant speed. A number of such mechanized devices have been used for laboratory investigations (Voisey, 1971, 1976), but the hand held fruit pressure tester is still the most widely applied unit in the field because of its simplicity, portability and low cost. Pressure Can. Ins!. Food Sci. Technol. J. Vol. 10, No.4, October 1977
testers have been designed for specific crops such as corn (Culpepper and Magoon, 1924), cherries (Brekke and Sandomire, 1961) and are to be found in most segments of the food industry (e.g. dairy products, Kruisheer and Herder, 1938). The fruit pressure test appears to have originated at the State College of Washington in 1917 where a marble was imbedded in wax and supported by a spring scale (Morris, 1925). Apples were pressed on to the ball and the indentation force noted. In fact the standard radius for fruit pressure tester tips was apparently established by the diameter of this original marble (1.588 cm). The same procedure was extended to pears by Lewis et al. (1919) and Murneek (1921). The efficacy of the method led to the development of hand held instruments using a linear spring scale with the puncture probe mounted directly on the scale plunger to form a self-contained unit (Magness and Taylor, 1925; Blake, 1928) for testing apples, pears and peaches. The unit of Magness and Taylor (1925) included a mechanism to indicate when the spherical tip penetrated the fruit a preselected distance. This device was improved by Schomer and Olsen (1962) and called the "mechanical thumb" as it sim1.!lated the sensory "thumb test". There are two types of pressure testers used, those that indent the surface of the fruit (thumb test) and those that penetrate the fruit, the latter being the most commonly used. The two methods have been compared by a number of workers (e.g. Mattus, 1965). A major difference between the methods is the distance the probe penetrates the fruit. This is small in the case of the mechanical thumb and normally considered to be a surface phenomina (indentation) whereas in the penetration test the probe enters the fruit flesh a much larger (but preselected) distance. Bourne (1965, 1969) pointed out that the "mechanical thumb" which measures the indentation force, Le. when the flesh in the region of the probe collapses, provides an approximate index of the bioyield force (Mohsenin et al., 1963) but that this does not bear any relationship to firmness according to accepted definitions (the force to attain a given deformation). Nevertheless, fruit pressure testers measuring either indentation or penetration force are popularly considered to measure fruit firmness. Bourne (1965) and others also found that the penetration force was reduced by removal of the fruit skin. Sophistica'ted portable instruments have been devised to indicate both force and deformation during penetration of the fruit (e.g. Texprobe, Agricultural Specialty Co., 284
Beltsville, Maryland) and can also be found in applications to meat (Haughey and Marer, 1971) and soil testing (Prather et al., 1970). However, such equipment has not been widely adopted. The general penetration technique is to measure the force required to cause a pressure tester tip to penetrate the fruit flesh a preselected distance (0.79 cm Bourne, 1974) and a number of instruments are made specifically for this purpose (Bourne, 1976). Obviously any spring force gauge can be adapted to the test. It has been recognized that there is a need to standardize the methods used to apply the instrument (Haller, 1941). A number of techniques have been tried, including mounting a spring force gauge on a lever giving the operator a mechanical advantage (Claypool et al., 1966) so that force can be applied more evenly during the test. Operator effects have been observed (Nichols, 1960) but Bourne (1974) found, that for apples and pears, the results from a pressure tester were highly correlated with results from a power driven probe providing the operator used proper techniques. Breene et al. (1974) found with cucumbers that manually obtained readings were less than those obtained with a machine providing the driving force. Since a power drive for the instrument detracts from its simplicity, a key point is the technique used by the operator as opposed to a machine where the rate of applied deformation is constant and the forces are measured under reproducible conditions. Bourne (1974) recommended that the fruit should be rested against a vertical surface and the force applied to the instrument evenly and steadily since it is a maximum indicating device, and theoretically the readings are affected by the rate of force application. A steadily increasing force was best achieved by resting the instrument on the hip. This can be readily accomplished with the standard pressure testers (e.g. D. Ballauf Manufacturing Co., Washington, D.C.) because they are long, slender instruments. A different version made in Italy (Effe-gi Alfonsine, Italy) is extremely compact and must be operated differently because it fits in the palm of the hand. This raises the question of the suitability of the Italian instrument for achieving a constant rate of force application as recommended by Bourne (1974). The manufacturer recommends using the instrument vertically pushing down on the fruit held in the hand with the hand supported on a horizontal surface. In view of the forces involved this could be a painful procedure. The two instruments (Ballauf and Italian) were compared by Abbott et al. (1976) who found that they were not entirely interchangeable even though the probes and indicated force ranges are essentially the same. They concluded that the difference was caused by the different size and shape of the two instruments and 'the fact that the spring rates are different because of spacer limitations. The purpose of the work here was to test the hypothesis that the differences in size, shape and spring rate of the two instruments affects the operator's control of force application and to compare mechanical performance aspects in relation to the use of hand operated instruments applying a constant rate of force increase as opposed to mechanized units applying a constant rate of deformation.
Materials and Methods The experiments were performed with three pressure
285
testers (Table 1) but only one standard tip was used and transferred between instruments to eliminate possible vari_ ations from this source. The tip was 1.115 cm (0.439 in.) in diameter. Table I. Instruments used for the test. Instrument No.
Range of force reading* 12kg
2
IOlb
3
30lb
Manufacturer Effe.gi C. Garibaldi 102 480 II Alfonsine (Ra) Italy. D. BallaufManufacturing Co. Inc., 619 H Street, N.W. Washington, D.C. 2001, U.S.A. D. Ballauf
*English units are used for the Ballauf instruments as these are the graduations on the instruments.
The instruments were suspended on the crosshead of an Instron Testing Machine (Model TM-M, Instron Canada Ltd., Burlington, Ontario) so that their springs could be deformed at a constant rate thus controlling the rate of increase offorce (Figure lA, B). The probe rested on a 500 kg load cell (Model FLIU, Strainsert Ltd., Bryn Mawr, Pa.). The deflection of the load cell under the test forces was negligible. The load cell was connected to the Instron recorder and calibrated with weights so that force was indicated over the range 0-15 kg within ± 0.1 % ofreading. Static calibration of the pressure testers (i.e. comparison of the graduations with force applied by the springs in the instruments) was performed by manually lowering the crosshead until a preselected reading was obtained on the pressure tester and the force applied at the tip then noted from the recorder chart. This was repeated at fixed increments of instrument reading up to the maximum and then in the reverse direction. This test was repeated with the "hold at maximum indicator", (a sliding sleeve on the Ballauf models and a friction clutch on the Italian unit) operative and inoperative. Dynamic calibrations, with and without the hold at maximum indicators operative, were obtained by lowering and raising the crosshead over an appropriate stroke at 0.5 cm/min and recording force against time. In order to stimulate the operating conditions recommended by Bourne (1974), the Instron was tilted to the horizontal position so that all experimental readings were obtained with the testers operating along a horizontal axis. Red Delicious apples were tested with the same installation by supporting the apple on the load cell (Figure 1C, D) and using a crosshead speed of 5.0 cm/min (the maxImum permIssIble to prevent SIgnal attenuation by recorder response). The apples were supported on the load cell by a ring 35 mm inside the 42 mm outside diameter with a 45 0 chamfered edge. This distributed the supporting force over the apple and prevented damage at future puncture sites. The Instron crosshead compressed the springs of the fruit pressure testers at an approximately constant rate, that depended only on the penetration of the probe into the fruit, to achieve a controlled rate of force increase. The fruit were punctured in the equatorial area (midway between the stem and blossom ends). Each apple was first punctured with the Ballauf 30 Ib instrument, roJ. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
Fig. I
A. The Ballauf pressure tester and B. the Italian instrument installed in the Instron for static calibration; C and D. the same in-
stallation was used for puncture testing apples. Note for the experiment the machine was placed in the horizontal position.
tated 90° and a portion of skin removed and the test repeated. The apple was rotated 90° and tested with the Italian instrument and finally rotated a further 90° for a similar test with the skin removed. This procedure was performed on two occasions as the maximum number of fruits that could be conveniently tested per day was 25. The puncture force indicated by the instrument and the maximum force indicated on the recorder chart were noted for each penetration test. Theoretically, these are readings of the same force. It was not possible to test the 10 lb Ballauf instrument (No.2, Table 1) as it had insufficient range. Operator performance was evaluated by installing the 500 kg load cell in a fixture so that the compression surface attached to the cell was vertical. This provided a reaction surface for the apple at a convenient height (Figure 2A, B) so that the instrument or arm could be pressed into the hip and leaning of the body could be used to apply a steadily increasing force (Bourne, 1974). The 30 lb Ballauf and Italian instruments were used in the same manner except that for the Italian unit, the arm as opposed to the instrument was rested on the hip. The load cell was connected to a recorder (Figure 2C) to record force against time. The load cell was enclosed and the recording apparatus placed so that the equipment could not be observed by the operators during the test. The test was executed by 40 operators (31 male and 9 female) who were inexperienced in fruit pressure tester operation. Each operator was given a brief training session with the two instruments testing apples for practice. The following information was given: "These are two instruments used to test apple texture and maturity and we suspect that one of them does not work. We wish to test the instruments not your skill. Rest the apple against the vertical surface (i.e. the load cell) and increase the force that you apply at a steady rate. We will record what happens.". Each operator used each instrument to puncture one apple twice, the puncture points being diametrically opposed. The puncture force indicated on the pressure tester and the maximum force recorded on the chart were noted for each penetration. The average rate of rise of force during each test was estimated from the records using a straight edge to determine the slope of the forcetime curves. The mean puncture force for each instrument on each apple was calculated.
Table 2. Static calibration readings - instrument No. I Italian. Hold at maximum indicator
287
Applied force
-
Increasing (g)
Error'
(g)
(%)
Decreasing (g)
Off
2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
1890 2865 3810 4875 5865 6765 7845 8745 9600 10650 11805
5.50 4.50 4.75 2.50 2.25 3.36 1.94 2.83 4.00 3.18 1.63
1800 2760 3810 4770 5730 6675 7620 8530 9465 10395 11805
10.00 8.00 4.75 4.60 4.50 4.64 4.75 5.22 5.35 5.50 1.63
On
2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
1845 2850 3780 4770 5715 6630 7695 8685 9345 10500 11640
7.75 5.00 5.50 4.60 4.75 5.29 3.81 3.50 6.55 4.55 3.00
1650 2670 3720 4650 5580 6720 7650 8400 9750 10245 11640
17.50 11.00 7.00 7.00 7.00 4.00 4.38 6.67 2.50 6.86 3.00
Error (%)
a. Error = Instrument reading - Applied force x 100% Instrument readmg
taken in this direction, this was unimportant. The manufacturer provides shims that can be placed between the sring and housing to adjust the static readings by altering t e spring length at zero applied force. Theoretically it is possible to obtain a correct static reading at one point on the scale by this modification (ideally the average reading for the fruit being tested), however, this was not attempted. A similar arrangement is used in the Ballauf instruments. Table 3. Static calibration readings - instrument No.2 Ballauf 10 lb. Hold at maximum Instrument indicator reading
Applied force Increasing (g)
Error' (%)
Decreasing (g)
Error'
(lb) Off
I 2 3 4 5 6 7 8 9 10
495 930 1385 1815 2250 2720 3150 3550 4000 4500
-9.13 -2.51 -1.78 -0.03 0.79 -0.27 0.79 2.17 2.08 0.79
450 890 1340 1780 2270 2690 3125 3590 4030 4500
0.79 1.90 1.53 1.90 -0.09 1.16 1.58 1.07 1.28 0.79
On
I 2 3 4 5 6 7 8 9 10
495 995 1420 1860 2300 2770 3160 3650 4040 4550
-9.13 -9.68 -4.35 -2.51 -1.41 -1.78 0.48 -0.58 1.04 -0.31
450 895 1350 1775 2225 2670 3100 3550 4015 4550
0.79 1.34 0.75 2.17 1.90 1.90 2.37 2.17 1.65 -0.31
Results and Observations Static calibrations indicated that there were errors in calibration of the three instruments tested. It should be noted that these include the errors inherent in taking the readings from the pressure testers. Judgement \yas needed due to the low resolution of the graduated scales. Also, the results are only typical for the instruments tested. Different results would be expected with other units of the same models due to the variation between instruments as with all mechanical force indicating apparatus (Voisey, 1971). Since all the readings were taken by one operator, the errors are comparable between instruments within the experiment. The Italian instrument had the largest reading errors (Table 2) as the applied force was increasing and ranged up to 7.75%. The errors were generally larger when the hold at maximum indicator was operative. The errors were larger for decreasing force; however, since readings are not
Instrument reading
(%)
a. Error = Instrument reading - Applied force x 100% calculated in g. Instrument reading J. ln5t. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
Table 4. Static calibration readings - instrument NO.3 Ballauf 30 lb. Hold at maximum Instrument indicator reading
Off
On
Applied force
(lb)
Increasing (g)
Error' (%)
Decreasing (g)
5 10 15 20 25 5 10 15 20 25
2250 4540 6600 9660 11055 2295 4530 6870 9015 11250
0.8 -0.1 3.0 -6.5 2.5 -1.2 0.1 -1.0 0.6 0.8
2265 4530 6585 8700 11055 2295 4620 6465 8580 11250
Error' (%) 0.1 0.1 3.2 4.1 2.5
-1.2 -1.9 5.0 4.8 0.8
a. Error = Instrumement reading - Applied force x 100% calculated Instrument reading
Fig. 2.
Recording the force applied to the apples during puncture tests. A. Ballauf 30 Ib model; B. Italian instrument; C. a test in progress showing the recording apparatus.
The hold at maximum indicators did not increase the reading errors of the 10 and 30 lb Ballauf units (Table 3 and 4). The 10 lb instrument (Table 3) had larger calibration errors than the 30 lb (Table 4) reflecting the larger influence of friction in the more sensitive instrument. With both Ballauf instruments the errors were approximately the same over most of the scale in the increasing and decreasing force directions. The observed errors for the 30 lb Ballauf instrument were, on the average, the smallest of the 3 instruments tested (Table 4). Dynamic calibration curves could be repeated exactly indicating that the same readings would be obtained for each operation under dynamic conditions. The Italian unit required the application of 1.2 kg to start moving the probe relative to the instrument body and commerce compressing the spring (Figure 3). This "breakout" force repCan. Inst. Food Sci. Techno!. J. Vo!. 10. No.4. October 1977
in g.
resented the forces necessary to overcome pretension in the spring and friction within the mechanism. There was no evidence of a breakout force with the 10 lb Ballauf unit (Figure 4) but the 30 lb instrument required up to 0.3 kg (Figure 5). The higher breakout force of the Italian model is possibly because the spring is operated in compression, thus, any misalignment in the mechanism introduces friction, whereas in the Ballauf instruments the spring is in tension and the mechanism is self-aligning. The friction forces introduced by the different hold at maximum indicators were also not the same. There was hysterisis in the relationships between applied force and spring deflection in the increasing and decreasing force directions, and the relationships were slightly non-linear in both the increasing and decreasing directions for the three instruments. Again since readings are not taken in the decreasing direction the greater errors in this directioILare not important. The hold at maximum indicator did not affect the dynamic spring rate (force/unit deflection) of the Ballauf units but did have an effect in the Italian instrument (Table 5). The Italian instrument had the greatest nonlinearity in the force-deformation relationships and the 30 lb Ballauf the least, whereas with respect to the hysterisis between the increasing and decreasing force curves, the situation was reversed (Table 5).
288
12.0 INSTRUMENT N~ 1
10.5 ( ITALIAN)
9.0 MAXIMUM OFF
MAXIMUM ON
7.5
Cl
~
I.&J U ~
6.0
0
u..
4.5
3.0 1.5 O+--t'---+----t--+--+---+----t---t 0.25 0.5 0.75 1.0 1.25 1.5 1.75
o
o
0.25
SPRING DEFLECTION Fig. 3.
0.5
0.75
1.0
1.25
1.5
1.75
mm
Dynamic calibration of Italian instrument.
5.0 INSTRUMENT N°.2
(Ballauf 101b)
4.0
30
'" ....
~
U
Q:
o
lL
20
1.0
0+----i--+-+----if----+--+----i'---+-+----1--+_+----1
o
10
20
30
40
50
60
70
80
90
100
110
120
130
0
10
20
30
40
50
60
70
80
90
100
110
120
130
SPRING DEFLECTION mm
Fig. 4.
289
Dynamic calibration of 10 lb Ballauf instrument. J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
12.0 INSTRUMENT
N~
3
10.5 (Bailout 30 Ibl 9.0
s'" 7.5
'~" 60
...o
4.5
30
1.5
O+--+--t-----l--+--+---f--+--t---+--+--+---f---i
o
10
20
30
40
50
60
70
80
90
100
110
120
0
10
20
30
40
50
60
70
80
90
100
110
120
SPRING DEFLECTION mm
Fig. 5.
Dynamic calibration of 30 Ib Ballauf instrument.
Table 5. Summary of dynamic calibration data. Instrument No.
I
2 3
Maximum non-linearitya (%) Maximum Maximum on' of!" 1.9 1.5 0.5
1.9 1.0 0.1
Maximum hysterisis' (%) Maximum Maximum on' of!" 5.6 6.5
5.6 5.0
10.6
10.6
Breakout force (B)' (kg) Maximum Maximum on' of!" 1.20 0.00 0.23
a. During the period of increasing force. b. The device for indicating maximum force was either on or deactivated. c. These factors establish the relationship between spring deflection and force: F = RD tion mm, R the spring rate kg/mm, C is the breakout force kg and F the force kg.
The dynamic calibration data for increasing force (which simulates actual operating conditions) showed that the relationship between force (F) and instrument reading (N) was of the form F = NR + B where R is the spring rate and B the breakout friction (Table 5). Only in the case of the 10 lb Ballauf instrument was B negligible. It should be noted that the spring rate R is fixed but the constant B depends on both friction and spring pretension, the latter being adjustable by adding or subtracting shims. These results suggest that the best means of calibration is the dynamic method as an accurate relationship is easily established under conditions similar to those used in testing fruit. Typical records of force applied to the apples against crosshead movement (and, therefore, time), obtained when the instruments were powered by the Instron, show how the force on the probe increased at an approximately constant rate with the Ballauf 30 lb tester and to a less controlled extent with the Italian unit (Figure 6). Deviations in the curves suggested bioyield points (i.e. initial rupture of the flesh), however, it was assumed that these readings Can. lnst. Food Sci. Technol. J. Vol. 10, No.4, October 1977
1.20 0.00 0.30
+
Average spring rate (RY (kg/mm) Maximum Maximum on' of!" 6.0940 0.0368 0.1034
6.1540 0.0364 0.1034
C where D is the spring deforma-
were affected by the "breakout" forces in the instruments, thus preventing a comparison between the two instruments. For example, the indications of bioyield points (Y, Figure 6) were quite different for the two instruments. Thus, only maximum force was read off the charts and assumed to coincide with the abrupt penetration of the fruit by the probe. The differences between the instrument and chart readings of the maximum force were much less for the Italian instrument than the Ballauf 30 lb unit (Table 6) but in both cases errors in the readings were mainly attributable to errors in the factory calibration of the instruments and are thus systematic errors that can be corrected. The variation within groups of apples was similar for both the chart and instrument readings suggesting a similarity of performance in resolving differences between apples for the mechanical and electronic force indicating devices. Generally the instrument readings were higher than the chart readings,. the converse of the observation of Breene et al. (1974) with cucumbers. As observed by Bourne (1965) removal of the apple skin reduced the forces. Corre-
290
7.5
INSTRUMENT W I
\
SKIN ON
(Italian)
6.0
SKIN OFF
4.5 3.0
y
1.5 0
o
I
0.25
0.5 0.75
1.25
1.0
1.5
I
0.5 0.75
1.75
1.25
1.0
1.5
1.75
Ol
~
W
u
a:::
9.0
0
l.L.
N~
INSTRUMENT
7.5
3
(Bailout 301b) SKIN ON
6.0 SKIN OFF
4.5 3.0 1.5 0
o
2
3
5
4
670
I
2
3
CROSSHEAD MOVEMENT Fig. 6.
4
5
7
6
8
9
10
em
Typical records of force applied to apples by the instruments when powered by the Instron. Y indicates points that possibly indicate initial yielding of the apple flesh.
Table 6. Summary of results for apple tests with instruments operated by the Instron. 3 Instrument No. Instrument reading (kg)' Apples 1-25 26-50 I-50
Mean C.V. (%) Mean C.V.(%) Mean C.V. (%)
Chart reading (kg)
Difference b (%)
Instrument reading (kg)
Chart reading (kg)
Difference b
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
6.7 2\.4 6.5 16.1 6.6 18.9
4.9 15.2 5.1 9.9 5.0 12.8
7.2 14.7 6.8 15.2 7.0 15.1
5.6 17.1 5.3 9.1 5.4 14.0
-6.9
-12.5
3.9
- 7.4
5.2 20.5 5.1 12.9 5.2 17.0
-1.3
-5.7
7.6 12.9 7.5 9.7 7.5 11.3
0.0
- 3.8
5.2 14.3 5.3 10.4 5.3 12.4
-1.3
-4.4
7.5 14.2 7.4 9.1 7.5 I\.9
0.0
\.9
a. Converted to kg from the original readings in lb. b. Difference = Instrument reading - Chart reading x 100% Chart readmg
291
J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4, October 1977
table 7. Correlation among readings from apples when the instruments were powered by the Instron. Instrument No.
3 I
Instrument Vs. chart reading Skin on Skin off 0.76 0.94
Skin on Vs. Skin off readings Instrument Chart 0.40 0.33
0.29 0.80
-0.01 0.51
lation between the instrument and chart readings (Table 7) were, with one exception, high. However, the level of correlation suggested that the instrument readings did not predict the force precisely (i.e. using the chart readings as the standard for comparison). This again can be partly attributed to the calibration errors. The correlation between the readings with and without the skin were not high suggesting that they are indices of different characteristics (e.g. skin toughness and flesh toughness, Bourne, 1976). The characteristic manner of force application by the operators in testing apples covered a range including: a) a smooth linear increase with time up to a peak followed by a second increase to the maximum (Figure 7A, D) suggesting that the operator sensed that yielding was commencing and momentarily relaxed the applied force; b) a smooth increase at a fast rate and then a sudden change in rate prior to the maximum also suggesting that the operator was preparing for the sudden penetration by the plunger (Figure 7B, E) and c) a smooth increase up to the maximum (Figure 7C, F). The latter characteristic was observed in the majority of cases (68%), the remainder being about equally divided between a and b. Similar characteristics were observed with both instruments with about equal frequency. During operation, it was observed that the operators generally prepared themselves for the sudden probe penetration particularly with the Ballauf 30 Ib instrument. The energy stored in the spring caused a large jerk and the operator was concerned with being splashed with juice (in spite of a laboratory coat being provided). This was also observed by Claypool et al. (1966). The Italian instrument had an advantage in this respect as its spring was deformed only 1I59th of the distance of the Ballauf unit so that the shock was less severe. The lack of roundness of the apples, a characteristic of the Delicious variety, caused occasional difficulties in aligning the tester, apple and compression surface.
mean rate achieved 'by the groups of operators were virtually identical with the two instruments. The only significant difference P > 0.05) observed with both instruments was between male and female operators. The females increased the force at almost twice the rate of the males. This was observed to arise because the females found the operating forces to be high so they pushed the instruments energetically and rapidly to overcome the resistance. This suggests that the operators' physique may have some influence on the technique used. Also, it was observed subjectively that both the operators' physique and personality had some influence on the manner in which the instruments were operated in a similar manner to that observed when people squeezed foods to test firmness (Voisey and Crete, 1973). The maximum force readings obtained for the two instruments from the recorder charts were, on the average, almost identical (Table 9) suggesting that the instruments did not affect the applied forces. The readings obtained directly from the Ballauf instrument, however, were consistently lower than those of the Italian unit (Table 9). Again, this was attributed to systematic errors of instrument calibration. In all other respects the data from the two instruments were comparable.
Conclusions The hypothesis that the different physical characteristics (size, shape and spring rate) of the Italian and Ballauf fruit pressure testers affect the operator's control of the rate of force application is not correct. On the average operators achieve the same rates with both instruments, however, females used a higher rate than males. The sources of errors within and differences between readings from the two instruments are most likely differences between operators and systematic calibration errors. The latter can be corrected. The obtained results indicate that a dynamic calibration is useful since an equation (F = NR + B) can be quickly obtained under conditions that simulate actual operation to accurately determine the relationship between the force applied and the instrument readings. The results obtained with untrained operators suggest that trained operators should achieve satisfactory control of the rate of force increase. It may be worthwhile to use the described force recording apparatus as a training aid. The operator could learn to apply a uniformly increasing
Table 8. Summary of results showing average rates of loading achieved with two instruments by 40 operators. Operators
Loading rate (kg/sec) Instrument No. I 1st test 2nd test
31 Males Mean 2.18 C.V. (%) 49.3 9 Females Mean 4.12 C.V. (%) 35.0 40 Males and females pooled Mean 2.62 C.V. (%) 53.8
Mean
1st test
Instrument No.3 2nd test
Mean
2.32 48.7
2.25 48.8
2.15 46.0
2.41 50.1
2.28 47.3
3.91 42.6
4.01 35.9
3.53 30.2
3.64 36.4
3.60 31.3
2.68 52.9
2.70 51.4
2.46 46.8
2.69 49.2
2.58 47.0
The results indicated that the average rate of increase in force varied widely between operators (Table 8) but the Can. Ins!. Food Sci. Technol. J. Vol. 10, No.4, October 1977
force by observing how the force applied actually increased to develop a sense of feel for the correct rate. 292
12.0
INSTRUMENT NO.1 ( Italian)
10.5
9.0
7.5
6.0
A
4.5
B
C
3.0
1.5
0 C'
0
~
2
4
6
8
I
I
I
I
2
4
6
8
I 10
0
2
4
8
6
w U 0::
0
u.
10.5
INSTRUMENT N°.3 (Ballauf 301b)
9.0
7.5
6.0
4.5
D
3.0
F
E
1.5
0 0
2
4
6
I 0
2
I 4
I
I
6
8
I 10
I 12
I
2
I 4
I
6
TIME SECONDS Fig. 7.
293
Typical records of force applied by operators to punch test apples. J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4, October 1977
Table 9. Summary of readings obtained during manual operation of two instruments showing maximum force from the instruments and recorder chart. operators
Maximum force reading from the recorder chart (kg) Instrument No. I Instrument No.3 1st test 2nd test Mean 1st test 2nd test Mean
31 Males Mean 10.2 C.V.(%) 15.0 9 Females Mean 10.9 C.V. (%) 17.7 40 Males and females pooled Mean 10.3 C.V.(%) 15.7
Maximum force readings from the instruments (kg)" Instrument No. I Instrument NO.3 1st test 2nd test Mean 1st test 2nd test Mean
10.4 15.3
10.2 14.1
10.1 13.8
10.1 13.4
10.1 12.2
9.0 11.6
9.4 11.3
9.2 10.5
8.8 10.8
8.9 15.0
8.8 11.5
11.2 1l.6
11.0 13.7
11.0 13.3
10.9 13.4
10.9 12.5
10.3 12.3
10.4 9.8
10.3 8.9
9.1 12.0
8.9 6.7
9.0 9.0
10.5 14.7
10.4 14.1
10.3 14.0
10.3 13.5
10.3 12.5
9.3 13.0
9.6 1l.8
9.4 1l.2
8.8 11.0
8.9 13.5
8.9 10.9
"Converted from readings in Ib for purposes of comparison.
It would be worthwhile to develop a damper to prevent the rapid return of the puncture probe at penetration and eliminate the shock that this gives to the operator. The stiffer spring of the Italian fruit pressure likely makes it more difficult than the Ballauf unit to adjust the calibration by adding shims to change the initial pretension of the spring. The same shim thickness would have 59 times the effect on the breakout force of the Italian unit compared to the 30 Ib Ballauf instruments. The problems of calibrating and operating the fruit pressure testers suggest that a better solution would be to use an electronic force gage with strain gages to detect force and an electronic circuit to display the maximum force. Such a device was recently introduced (AccuForce gage Ametek, Hunter Spring Division, Hatfield, Pennsylvania 19440) as a hand held unit with a built in digital readout. However, such devices are not suitable for hand operation because they are extremely sensitive to operator technique (Voisey, 1976).
References Abbott, J. A., Watada, A. E. and Massie, D. R. 1976. Effe-gi, Magness Taylor and Instron fruit pressure testing devices for apples, peaches and nectarines. 1. Amer. Soc. Hort. Sci. 10 I:698. Anon. 1974. Compression test of food materials of convex shape. Agr. Eng. Yearbook pp. 386-389. Barber, J. R. 1974. Determining the contact area in elastic-indentation problems. J. Strain Analysis 9:230. Blake, M. A. 1928. A device for determining the texture of peach fruits for shipping and marketing. New Jersey Agr. Exp. Station, New Brunswick, N.J., Circular No. 212. pp. 2-8. Bourne, M. C. 1965. Studies on punch testing of apples. Food Techno!. 19: 1l3. Bourne. M. C. 1965. How skin affects apple pressure tests. Farm Res. 31: 10. Bourne, M. C. 1966. Measure of shear and compression components of puncture tests. 1. Food Sci. 31 :282-291. Bourne. M. C. 1969. Two kinds of firmness in apples. Food Techno!. 23:59-60. Bourne, M. C. 1974. Comparison of results from the use of the Magness-Taylor pressure tip in hand and machine.operation. J. Texture Studies 5:105. Bourne, M. C. 1975. Method for obtaining compression and shear coefficients of foods using cylindrical punches. J. Texture Studies 5:459. Bourne, M. C. 1976. Texture of fruits and vegetables. In Rheology and Texture in Food Quality. Eds. J. M. DeMan, P. W. Voisey, V. F. Rasper and D. W. Stanley. AVI Pub. Co., Westport, Conn., p. 296.
Can. Ins\. Food Sci. Technol. J. Vol. 10, No.4, October 1977
Breene, W. M., leon, I. J. and Bernard, S. N. 1974. Observation on texture measurement of raw cucumbers with the fruit pressure tester. J. Texture Studies 5:317. Brekke, J. E. and Sandomire, M. M. 1961. A simple, objective method of determining firmness of brined cherries. Food Techno!. 15:335. Claypool, L. L., Fridley, R. B. and Johns, R. 1966. Precision in a pressure tester. Western Fruit Grower 20: 18. Culpepper, C. W., and Magoon. C. A. 1924. Studies upon the relative merits of sweet corn varieties for canning purposes and the relation of maturity of com to the quality of the canned product. J. Agr. Res. 28:403. DeMan, 1. M. 1969. Food texture measurements with the penetration method. 1. Texture Studies I: 114. Haller, M. H. 1941. Fruit pressure tester and their practical applications. U.S. Dept. of Agr. Circular No. 627: I. Hau~hey, D. P. and Marer. 1. M. 1971. The softening of frozen meat: Criteria for transportation in insulated containers without refrigeration. J. Food Technol. 6: 119. Kruisheer, C. I. and Den Herder, P. C. 1938. Research concerning the consistency of butter (in Dutch). Chemical Weekly-J. Dutch Chern. Assoc. & Assoc. Chern. Ind. Netherlands 35:719. Lewis, C. I., Murneek. A. E. and Catc, C. C. 1919. Pear harvesting and storage investigation in Rogue River Valley (2nd Report). Oreg. Agr. Exp. Sta. Bull. 162. pp. 1-39. Magness, J. R. and Taylor, G. F. 1925. An improved type of pressure tes:er for the determination of fruit maturity. U.S. Dept. of Agr. Circular 350:1·7. Mattus, G. E. 1965. Mechanical thumb tests of apple firmness. Proc. Amer. Soc. Hort. Sci. 87: 100. Mohsenin, N., Goelich, H. and Tukey, L. D. 1963. Mechanical behavior of apple fruit, as related to bruising. Amer. Soc. Hort. Sci. 81 :67. Mohsenin, N. N., Morrow, C. T. and Yang, Y. M. 1970. On the spherical indenter as a means for determining the "firmness" and "hardness" of food materials. Proc. Fifth Int. Congo on Rheology 2:647. Morris, O. M. 1925. Studies in apple storage. Washington State College, Agr. Expt. Sta. Bull. 193:5-44. Murneek, A. E. 1921. A new test for maturity of the pear. Oreg. Agr. Exp. Sta. Bull. 186: 1-28. Nichols, R. C. 1960. Some observations on the use of fruit pressure testers. Quar. Bull. Michigan Agr. Expt. Sta. 43:312. Peleg, M. and Brito, L. G. 1975. Estimation of the components of a penetration force of some tropical fruits. J. Food Sci. 40: 1030. Prather, O. C.. Hendrick, J. G. and Schafer, R. L. 1970. An electronic hand.operated recording penetrometer. Trans. Amer. Soc. Agr. Engrs. 13:385-386, 390. Schomer, H. A. and Olsen, -K. L. 1962. Mechanical thumb for determining firmness of apples. Proc. Amer. Soc. Hort. Sci. 81:61. Su, C. S. and Humphries. E. G. 1972. Rupture properties of cucumber skin. Pickle Pack. Sci. 2:3. Timbers, G. E., Staley, L. M. and Watson, E. L. 1965. Determining modulus of elasticity in agricultural products by loaded plungers. Agr. Eng. 46:274. Voisey, P. W. 1971. Modernization of texture instrumentation. J. Texture Studies.2:129. Voisey, P. W. and Crete, R. 1973. A technique for establishing instrumental conditions for meas· uring food firmness to simulate consumer evaluations. 1. Texture Studies 4:371. VOlsey, P. W. 1976. Engineering assessment and critique of instruments used for meat tenderness evaluation J. Texture Studies 7: II. Voisey, P. W. and DeMan, J. M. 1976. Applications of instruments for measuring food texture. In Rheology and Texture in Food Quality. Eds. J. M. DeMan, P. W. Voisey, V. F. Rasper and D. W. Stanley. AVI Publishing Co., Westport, Conn., pp. 142-243. Yang, Y. M. and Mohsenm, N. 174. Analysis of the mechanics of the fruit pressure tester. 1. Texture Studies 5:213. Received March 31, 1977
294
Abstract The Ballauf fruit pressure tester and a compact Italian instrument were compared in testing apples. It was found that the p~imary' source of differences between the mstrument readmgs was systematic calibration errors. The different size, shape and spring rate of the two instruments did not affect the rate of increase of force applied by the average operator. Females, however, used nearly twice the rate of males. It appeared advantageous to calibrate the instruments dynamically since this simulated the operating conditions of the instruments.
Resume Pour mesurer la texture des pommes, on a fait la comparaison entre Ie texturometre Ii pression Ballauf et un instrument italien compact. II a ete trouve que la principale source de differences entre les lectures instrumentales etait des erreurs systematiques de calibration. Les differences dans la taille, la forme et la vitesse du ressort des deux instruments a ete sans effet sur III vitesse d'augmentation de la force appliquee par l'operateur moyen. Cette vitesse a ete presque deux fois plus elevee avec les femelles qu'avec les males. II est apparu avantageux de faire la calibration dynamique des instruments afin de simuler les conditions des instruments en operatIOn.
Introduction The puncture test is widely used for measuring fruit and vegetable texture (Bourne, 1976) and many instruments have been developed for this purpose (Voisey and DeMan, 1976). An early application was in the determination of fruit maturity for field and grading station applications. This led to the use of spring scales to register the maximum force required to push a cylindrical probe into the fruit, and instruments of this type, commonly called fruit pressure testers, are established in the industry. Operationally the inherent simplicity of the puncture test makes it an ideal method but it is difficult to analyze and establish a theoretical base for interpreting the readings in fundamental terms (e.g. Timbers et al., 1965; Mohsenin et al., 1970; Anon, 1974; Barber, 1974) by classical analysis of the indentation problem. A simplified approach was to analyze the puncture force in terms of shear and compression coefficients that describe the behaviour of the test material (Bourne, 1966, 1975; DeMan, 1969; Su and Humphries, 1972; Peleg and Brito, 1975). A complete analysis of fruit pressure tester operation was reported by Yang and Mohsenin (1974). These analyses were investigated using a powered mechanism that forced the punch into the sample at a constant speed. A number of such mechanized devices have been used for laboratory investigations (Voisey, 1971, 1976), but the hand held fruit pressure tester is still the most widely applied unit in the field because of its simplicity, portability and low cost. Pressure Can. Ins!. Food Sci. Technol. J. Vol. 10, No.4, October 1977
testers have been designed for specific crops such as corn (Culpepper and Magoon, 1924), cherries (Brekke and Sandomire, 1961) and are to be found in most segments of the food industry (e.g. dairy products, Kruisheer and Herder, 1938). The fruit pressure test appears to have originated at the State College of Washington in 1917 where a marble was imbedded in wax and supported by a spring scale (Morris, 1925). Apples were pressed on to the ball and the indentation force noted. In fact the standard radius for fruit pressure tester tips was apparently established by the diameter of this original marble (1.588 cm). The same procedure was extended to pears by Lewis et al. (1919) and Murneek (1921). The efficacy of the method led to the development of hand held instruments using a linear spring scale with the puncture probe mounted directly on the scale plunger to form a self-contained unit (Magness and Taylor, 1925; Blake, 1928) for testing apples, pears and peaches. The unit of Magness and Taylor (1925) included a mechanism to indicate when the spherical tip penetrated the fruit a preselected distance. This device was improved by Schomer and Olsen (1962) and called the "mechanical thumb" as it sim1.!lated the sensory "thumb test". There are two types of pressure testers used, those that indent the surface of the fruit (thumb test) and those that penetrate the fruit, the latter being the most commonly used. The two methods have been compared by a number of workers (e.g. Mattus, 1965). A major difference between the methods is the distance the probe penetrates the fruit. This is small in the case of the mechanical thumb and normally considered to be a surface phenomina (indentation) whereas in the penetration test the probe enters the fruit flesh a much larger (but preselected) distance. Bourne (1965, 1969) pointed out that the "mechanical thumb" which measures the indentation force, Le. when the flesh in the region of the probe collapses, provides an approximate index of the bioyield force (Mohsenin et al., 1963) but that this does not bear any relationship to firmness according to accepted definitions (the force to attain a given deformation). Nevertheless, fruit pressure testers measuring either indentation or penetration force are popularly considered to measure fruit firmness. Bourne (1965) and others also found that the penetration force was reduced by removal of the fruit skin. Sophistica'ted portable instruments have been devised to indicate both force and deformation during penetration of the fruit (e.g. Texprobe, Agricultural Specialty Co., 284
Beltsville, Maryland) and can also be found in applications to meat (Haughey and Marer, 1971) and soil testing (Prather et al., 1970). However, such equipment has not been widely adopted. The general penetration technique is to measure the force required to cause a pressure tester tip to penetrate the fruit flesh a preselected distance (0.79 cm Bourne, 1974) and a number of instruments are made specifically for this purpose (Bourne, 1976). Obviously any spring force gauge can be adapted to the test. It has been recognized that there is a need to standardize the methods used to apply the instrument (Haller, 1941). A number of techniques have been tried, including mounting a spring force gauge on a lever giving the operator a mechanical advantage (Claypool et al., 1966) so that force can be applied more evenly during the test. Operator effects have been observed (Nichols, 1960) but Bourne (1974) found, that for apples and pears, the results from a pressure tester were highly correlated with results from a power driven probe providing the operator used proper techniques. Breene et al. (1974) found with cucumbers that manually obtained readings were less than those obtained with a machine providing the driving force. Since a power drive for the instrument detracts from its simplicity, a key point is the technique used by the operator as opposed to a machine where the rate of applied deformation is constant and the forces are measured under reproducible conditions. Bourne (1974) recommended that the fruit should be rested against a vertical surface and the force applied to the instrument evenly and steadily since it is a maximum indicating device, and theoretically the readings are affected by the rate of force application. A steadily increasing force was best achieved by resting the instrument on the hip. This can be readily accomplished with the standard pressure testers (e.g. D. Ballauf Manufacturing Co., Washington, D.C.) because they are long, slender instruments. A different version made in Italy (Effe-gi Alfonsine, Italy) is extremely compact and must be operated differently because it fits in the palm of the hand. This raises the question of the suitability of the Italian instrument for achieving a constant rate of force application as recommended by Bourne (1974). The manufacturer recommends using the instrument vertically pushing down on the fruit held in the hand with the hand supported on a horizontal surface. In view of the forces involved this could be a painful procedure. The two instruments (Ballauf and Italian) were compared by Abbott et al. (1976) who found that they were not entirely interchangeable even though the probes and indicated force ranges are essentially the same. They concluded that the difference was caused by the different size and shape of the two instruments and 'the fact that the spring rates are different because of spacer limitations. The purpose of the work here was to test the hypothesis that the differences in size, shape and spring rate of the two instruments affects the operator's control of force application and to compare mechanical performance aspects in relation to the use of hand operated instruments applying a constant rate of force increase as opposed to mechanized units applying a constant rate of deformation.
Materials and Methods The experiments were performed with three pressure
285
testers (Table 1) but only one standard tip was used and transferred between instruments to eliminate possible vari_ ations from this source. The tip was 1.115 cm (0.439 in.) in diameter. Table I. Instruments used for the test. Instrument No.
Range of force reading* 12kg
2
IOlb
3
30lb
Manufacturer Effe.gi C. Garibaldi 102 480 II Alfonsine (Ra) Italy. D. BallaufManufacturing Co. Inc., 619 H Street, N.W. Washington, D.C. 2001, U.S.A. D. Ballauf
*English units are used for the Ballauf instruments as these are the graduations on the instruments.
The instruments were suspended on the crosshead of an Instron Testing Machine (Model TM-M, Instron Canada Ltd., Burlington, Ontario) so that their springs could be deformed at a constant rate thus controlling the rate of increase offorce (Figure lA, B). The probe rested on a 500 kg load cell (Model FLIU, Strainsert Ltd., Bryn Mawr, Pa.). The deflection of the load cell under the test forces was negligible. The load cell was connected to the Instron recorder and calibrated with weights so that force was indicated over the range 0-15 kg within ± 0.1 % ofreading. Static calibration of the pressure testers (i.e. comparison of the graduations with force applied by the springs in the instruments) was performed by manually lowering the crosshead until a preselected reading was obtained on the pressure tester and the force applied at the tip then noted from the recorder chart. This was repeated at fixed increments of instrument reading up to the maximum and then in the reverse direction. This test was repeated with the "hold at maximum indicator", (a sliding sleeve on the Ballauf models and a friction clutch on the Italian unit) operative and inoperative. Dynamic calibrations, with and without the hold at maximum indicators operative, were obtained by lowering and raising the crosshead over an appropriate stroke at 0.5 cm/min and recording force against time. In order to stimulate the operating conditions recommended by Bourne (1974), the Instron was tilted to the horizontal position so that all experimental readings were obtained with the testers operating along a horizontal axis. Red Delicious apples were tested with the same installation by supporting the apple on the load cell (Figure 1C, D) and using a crosshead speed of 5.0 cm/min (the maxImum permIssIble to prevent SIgnal attenuation by recorder response). The apples were supported on the load cell by a ring 35 mm inside the 42 mm outside diameter with a 45 0 chamfered edge. This distributed the supporting force over the apple and prevented damage at future puncture sites. The Instron crosshead compressed the springs of the fruit pressure testers at an approximately constant rate, that depended only on the penetration of the probe into the fruit, to achieve a controlled rate of force increase. The fruit were punctured in the equatorial area (midway between the stem and blossom ends). Each apple was first punctured with the Ballauf 30 Ib instrument, roJ. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
Fig. I
A. The Ballauf pressure tester and B. the Italian instrument installed in the Instron for static calibration; C and D. the same in-
stallation was used for puncture testing apples. Note for the experiment the machine was placed in the horizontal position.
tated 90° and a portion of skin removed and the test repeated. The apple was rotated 90° and tested with the Italian instrument and finally rotated a further 90° for a similar test with the skin removed. This procedure was performed on two occasions as the maximum number of fruits that could be conveniently tested per day was 25. The puncture force indicated by the instrument and the maximum force indicated on the recorder chart were noted for each penetration test. Theoretically, these are readings of the same force. It was not possible to test the 10 lb Ballauf instrument (No.2, Table 1) as it had insufficient range. Operator performance was evaluated by installing the 500 kg load cell in a fixture so that the compression surface attached to the cell was vertical. This provided a reaction surface for the apple at a convenient height (Figure 2A, B) so that the instrument or arm could be pressed into the hip and leaning of the body could be used to apply a steadily increasing force (Bourne, 1974). The 30 lb Ballauf and Italian instruments were used in the same manner except that for the Italian unit, the arm as opposed to the instrument was rested on the hip. The load cell was connected to a recorder (Figure 2C) to record force against time. The load cell was enclosed and the recording apparatus placed so that the equipment could not be observed by the operators during the test. The test was executed by 40 operators (31 male and 9 female) who were inexperienced in fruit pressure tester operation. Each operator was given a brief training session with the two instruments testing apples for practice. The following information was given: "These are two instruments used to test apple texture and maturity and we suspect that one of them does not work. We wish to test the instruments not your skill. Rest the apple against the vertical surface (i.e. the load cell) and increase the force that you apply at a steady rate. We will record what happens.". Each operator used each instrument to puncture one apple twice, the puncture points being diametrically opposed. The puncture force indicated on the pressure tester and the maximum force recorded on the chart were noted for each penetration. The average rate of rise of force during each test was estimated from the records using a straight edge to determine the slope of the forcetime curves. The mean puncture force for each instrument on each apple was calculated.
Table 2. Static calibration readings - instrument No. I Italian. Hold at maximum indicator
287
Applied force
-
Increasing (g)
Error'
(g)
(%)
Decreasing (g)
Off
2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
1890 2865 3810 4875 5865 6765 7845 8745 9600 10650 11805
5.50 4.50 4.75 2.50 2.25 3.36 1.94 2.83 4.00 3.18 1.63
1800 2760 3810 4770 5730 6675 7620 8530 9465 10395 11805
10.00 8.00 4.75 4.60 4.50 4.64 4.75 5.22 5.35 5.50 1.63
On
2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
1845 2850 3780 4770 5715 6630 7695 8685 9345 10500 11640
7.75 5.00 5.50 4.60 4.75 5.29 3.81 3.50 6.55 4.55 3.00
1650 2670 3720 4650 5580 6720 7650 8400 9750 10245 11640
17.50 11.00 7.00 7.00 7.00 4.00 4.38 6.67 2.50 6.86 3.00
Error (%)
a. Error = Instrument reading - Applied force x 100% Instrument readmg
taken in this direction, this was unimportant. The manufacturer provides shims that can be placed between the sring and housing to adjust the static readings by altering t e spring length at zero applied force. Theoretically it is possible to obtain a correct static reading at one point on the scale by this modification (ideally the average reading for the fruit being tested), however, this was not attempted. A similar arrangement is used in the Ballauf instruments. Table 3. Static calibration readings - instrument No.2 Ballauf 10 lb. Hold at maximum Instrument indicator reading
Applied force Increasing (g)
Error' (%)
Decreasing (g)
Error'
(lb) Off
I 2 3 4 5 6 7 8 9 10
495 930 1385 1815 2250 2720 3150 3550 4000 4500
-9.13 -2.51 -1.78 -0.03 0.79 -0.27 0.79 2.17 2.08 0.79
450 890 1340 1780 2270 2690 3125 3590 4030 4500
0.79 1.90 1.53 1.90 -0.09 1.16 1.58 1.07 1.28 0.79
On
I 2 3 4 5 6 7 8 9 10
495 995 1420 1860 2300 2770 3160 3650 4040 4550
-9.13 -9.68 -4.35 -2.51 -1.41 -1.78 0.48 -0.58 1.04 -0.31
450 895 1350 1775 2225 2670 3100 3550 4015 4550
0.79 1.34 0.75 2.17 1.90 1.90 2.37 2.17 1.65 -0.31
Results and Observations Static calibrations indicated that there were errors in calibration of the three instruments tested. It should be noted that these include the errors inherent in taking the readings from the pressure testers. Judgement \yas needed due to the low resolution of the graduated scales. Also, the results are only typical for the instruments tested. Different results would be expected with other units of the same models due to the variation between instruments as with all mechanical force indicating apparatus (Voisey, 1971). Since all the readings were taken by one operator, the errors are comparable between instruments within the experiment. The Italian instrument had the largest reading errors (Table 2) as the applied force was increasing and ranged up to 7.75%. The errors were generally larger when the hold at maximum indicator was operative. The errors were larger for decreasing force; however, since readings are not
Instrument reading
(%)
a. Error = Instrument reading - Applied force x 100% calculated in g. Instrument reading J. ln5t. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
Table 4. Static calibration readings - instrument NO.3 Ballauf 30 lb. Hold at maximum Instrument indicator reading
Off
On
Applied force
(lb)
Increasing (g)
Error' (%)
Decreasing (g)
5 10 15 20 25 5 10 15 20 25
2250 4540 6600 9660 11055 2295 4530 6870 9015 11250
0.8 -0.1 3.0 -6.5 2.5 -1.2 0.1 -1.0 0.6 0.8
2265 4530 6585 8700 11055 2295 4620 6465 8580 11250
Error' (%) 0.1 0.1 3.2 4.1 2.5
-1.2 -1.9 5.0 4.8 0.8
a. Error = Instrumement reading - Applied force x 100% calculated Instrument reading
Fig. 2.
Recording the force applied to the apples during puncture tests. A. Ballauf 30 Ib model; B. Italian instrument; C. a test in progress showing the recording apparatus.
The hold at maximum indicators did not increase the reading errors of the 10 and 30 lb Ballauf units (Table 3 and 4). The 10 lb instrument (Table 3) had larger calibration errors than the 30 lb (Table 4) reflecting the larger influence of friction in the more sensitive instrument. With both Ballauf instruments the errors were approximately the same over most of the scale in the increasing and decreasing force directions. The observed errors for the 30 lb Ballauf instrument were, on the average, the smallest of the 3 instruments tested (Table 4). Dynamic calibration curves could be repeated exactly indicating that the same readings would be obtained for each operation under dynamic conditions. The Italian unit required the application of 1.2 kg to start moving the probe relative to the instrument body and commerce compressing the spring (Figure 3). This "breakout" force repCan. Inst. Food Sci. Techno!. J. Vo!. 10. No.4. October 1977
in g.
resented the forces necessary to overcome pretension in the spring and friction within the mechanism. There was no evidence of a breakout force with the 10 lb Ballauf unit (Figure 4) but the 30 lb instrument required up to 0.3 kg (Figure 5). The higher breakout force of the Italian model is possibly because the spring is operated in compression, thus, any misalignment in the mechanism introduces friction, whereas in the Ballauf instruments the spring is in tension and the mechanism is self-aligning. The friction forces introduced by the different hold at maximum indicators were also not the same. There was hysterisis in the relationships between applied force and spring deflection in the increasing and decreasing force directions, and the relationships were slightly non-linear in both the increasing and decreasing directions for the three instruments. Again since readings are not taken in the decreasing direction the greater errors in this directioILare not important. The hold at maximum indicator did not affect the dynamic spring rate (force/unit deflection) of the Ballauf units but did have an effect in the Italian instrument (Table 5). The Italian instrument had the greatest nonlinearity in the force-deformation relationships and the 30 lb Ballauf the least, whereas with respect to the hysterisis between the increasing and decreasing force curves, the situation was reversed (Table 5).
288
12.0 INSTRUMENT N~ 1
10.5 ( ITALIAN)
9.0 MAXIMUM OFF
MAXIMUM ON
7.5
Cl
~
I.&J U ~
6.0
0
u..
4.5
3.0 1.5 O+--t'---+----t--+--+---+----t---t 0.25 0.5 0.75 1.0 1.25 1.5 1.75
o
o
0.25
SPRING DEFLECTION Fig. 3.
0.5
0.75
1.0
1.25
1.5
1.75
mm
Dynamic calibration of Italian instrument.
5.0 INSTRUMENT N°.2
(Ballauf 101b)
4.0
30
'" ....
~
U
Q:
o
lL
20
1.0
0+----i--+-+----if----+--+----i'---+-+----1--+_+----1
o
10
20
30
40
50
60
70
80
90
100
110
120
130
0
10
20
30
40
50
60
70
80
90
100
110
120
130
SPRING DEFLECTION mm
Fig. 4.
289
Dynamic calibration of 10 lb Ballauf instrument. J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4. October 1977
12.0 INSTRUMENT
N~
3
10.5 (Bailout 30 Ibl 9.0
s'" 7.5
'~" 60
...o
4.5
30
1.5
O+--+--t-----l--+--+---f--+--t---+--+--+---f---i
o
10
20
30
40
50
60
70
80
90
100
110
120
0
10
20
30
40
50
60
70
80
90
100
110
120
SPRING DEFLECTION mm
Fig. 5.
Dynamic calibration of 30 Ib Ballauf instrument.
Table 5. Summary of dynamic calibration data. Instrument No.
I
2 3
Maximum non-linearitya (%) Maximum Maximum on' of!" 1.9 1.5 0.5
1.9 1.0 0.1
Maximum hysterisis' (%) Maximum Maximum on' of!" 5.6 6.5
5.6 5.0
10.6
10.6
Breakout force (B)' (kg) Maximum Maximum on' of!" 1.20 0.00 0.23
a. During the period of increasing force. b. The device for indicating maximum force was either on or deactivated. c. These factors establish the relationship between spring deflection and force: F = RD tion mm, R the spring rate kg/mm, C is the breakout force kg and F the force kg.
The dynamic calibration data for increasing force (which simulates actual operating conditions) showed that the relationship between force (F) and instrument reading (N) was of the form F = NR + B where R is the spring rate and B the breakout friction (Table 5). Only in the case of the 10 lb Ballauf instrument was B negligible. It should be noted that the spring rate R is fixed but the constant B depends on both friction and spring pretension, the latter being adjustable by adding or subtracting shims. These results suggest that the best means of calibration is the dynamic method as an accurate relationship is easily established under conditions similar to those used in testing fruit. Typical records of force applied to the apples against crosshead movement (and, therefore, time), obtained when the instruments were powered by the Instron, show how the force on the probe increased at an approximately constant rate with the Ballauf 30 lb tester and to a less controlled extent with the Italian unit (Figure 6). Deviations in the curves suggested bioyield points (i.e. initial rupture of the flesh), however, it was assumed that these readings Can. lnst. Food Sci. Technol. J. Vol. 10, No.4, October 1977
1.20 0.00 0.30
+
Average spring rate (RY (kg/mm) Maximum Maximum on' of!" 6.0940 0.0368 0.1034
6.1540 0.0364 0.1034
C where D is the spring deforma-
were affected by the "breakout" forces in the instruments, thus preventing a comparison between the two instruments. For example, the indications of bioyield points (Y, Figure 6) were quite different for the two instruments. Thus, only maximum force was read off the charts and assumed to coincide with the abrupt penetration of the fruit by the probe. The differences between the instrument and chart readings of the maximum force were much less for the Italian instrument than the Ballauf 30 lb unit (Table 6) but in both cases errors in the readings were mainly attributable to errors in the factory calibration of the instruments and are thus systematic errors that can be corrected. The variation within groups of apples was similar for both the chart and instrument readings suggesting a similarity of performance in resolving differences between apples for the mechanical and electronic force indicating devices. Generally the instrument readings were higher than the chart readings,. the converse of the observation of Breene et al. (1974) with cucumbers. As observed by Bourne (1965) removal of the apple skin reduced the forces. Corre-
290
7.5
INSTRUMENT W I
\
SKIN ON
(Italian)
6.0
SKIN OFF
4.5 3.0
y
1.5 0
o
I
0.25
0.5 0.75
1.25
1.0
1.5
I
0.5 0.75
1.75
1.25
1.0
1.5
1.75
Ol
~
W
u
a:::
9.0
0
l.L.
N~
INSTRUMENT
7.5
3
(Bailout 301b) SKIN ON
6.0 SKIN OFF
4.5 3.0 1.5 0
o
2
3
5
4
670
I
2
3
CROSSHEAD MOVEMENT Fig. 6.
4
5
7
6
8
9
10
em
Typical records of force applied to apples by the instruments when powered by the Instron. Y indicates points that possibly indicate initial yielding of the apple flesh.
Table 6. Summary of results for apple tests with instruments operated by the Instron. 3 Instrument No. Instrument reading (kg)' Apples 1-25 26-50 I-50
Mean C.V. (%) Mean C.V.(%) Mean C.V. (%)
Chart reading (kg)
Difference b (%)
Instrument reading (kg)
Chart reading (kg)
Difference b
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
Skin on
Skin off
6.7 2\.4 6.5 16.1 6.6 18.9
4.9 15.2 5.1 9.9 5.0 12.8
7.2 14.7 6.8 15.2 7.0 15.1
5.6 17.1 5.3 9.1 5.4 14.0
-6.9
-12.5
3.9
- 7.4
5.2 20.5 5.1 12.9 5.2 17.0
-1.3
-5.7
7.6 12.9 7.5 9.7 7.5 11.3
0.0
- 3.8
5.2 14.3 5.3 10.4 5.3 12.4
-1.3
-4.4
7.5 14.2 7.4 9.1 7.5 I\.9
0.0
\.9
a. Converted to kg from the original readings in lb. b. Difference = Instrument reading - Chart reading x 100% Chart readmg
291
J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4, October 1977
table 7. Correlation among readings from apples when the instruments were powered by the Instron. Instrument No.
3 I
Instrument Vs. chart reading Skin on Skin off 0.76 0.94
Skin on Vs. Skin off readings Instrument Chart 0.40 0.33
0.29 0.80
-0.01 0.51
lation between the instrument and chart readings (Table 7) were, with one exception, high. However, the level of correlation suggested that the instrument readings did not predict the force precisely (i.e. using the chart readings as the standard for comparison). This again can be partly attributed to the calibration errors. The correlation between the readings with and without the skin were not high suggesting that they are indices of different characteristics (e.g. skin toughness and flesh toughness, Bourne, 1976). The characteristic manner of force application by the operators in testing apples covered a range including: a) a smooth linear increase with time up to a peak followed by a second increase to the maximum (Figure 7A, D) suggesting that the operator sensed that yielding was commencing and momentarily relaxed the applied force; b) a smooth increase at a fast rate and then a sudden change in rate prior to the maximum also suggesting that the operator was preparing for the sudden penetration by the plunger (Figure 7B, E) and c) a smooth increase up to the maximum (Figure 7C, F). The latter characteristic was observed in the majority of cases (68%), the remainder being about equally divided between a and b. Similar characteristics were observed with both instruments with about equal frequency. During operation, it was observed that the operators generally prepared themselves for the sudden probe penetration particularly with the Ballauf 30 Ib instrument. The energy stored in the spring caused a large jerk and the operator was concerned with being splashed with juice (in spite of a laboratory coat being provided). This was also observed by Claypool et al. (1966). The Italian instrument had an advantage in this respect as its spring was deformed only 1I59th of the distance of the Ballauf unit so that the shock was less severe. The lack of roundness of the apples, a characteristic of the Delicious variety, caused occasional difficulties in aligning the tester, apple and compression surface.
mean rate achieved 'by the groups of operators were virtually identical with the two instruments. The only significant difference P > 0.05) observed with both instruments was between male and female operators. The females increased the force at almost twice the rate of the males. This was observed to arise because the females found the operating forces to be high so they pushed the instruments energetically and rapidly to overcome the resistance. This suggests that the operators' physique may have some influence on the technique used. Also, it was observed subjectively that both the operators' physique and personality had some influence on the manner in which the instruments were operated in a similar manner to that observed when people squeezed foods to test firmness (Voisey and Crete, 1973). The maximum force readings obtained for the two instruments from the recorder charts were, on the average, almost identical (Table 9) suggesting that the instruments did not affect the applied forces. The readings obtained directly from the Ballauf instrument, however, were consistently lower than those of the Italian unit (Table 9). Again, this was attributed to systematic errors of instrument calibration. In all other respects the data from the two instruments were comparable.
Conclusions The hypothesis that the different physical characteristics (size, shape and spring rate) of the Italian and Ballauf fruit pressure testers affect the operator's control of the rate of force application is not correct. On the average operators achieve the same rates with both instruments, however, females used a higher rate than males. The sources of errors within and differences between readings from the two instruments are most likely differences between operators and systematic calibration errors. The latter can be corrected. The obtained results indicate that a dynamic calibration is useful since an equation (F = NR + B) can be quickly obtained under conditions that simulate actual operation to accurately determine the relationship between the force applied and the instrument readings. The results obtained with untrained operators suggest that trained operators should achieve satisfactory control of the rate of force increase. It may be worthwhile to use the described force recording apparatus as a training aid. The operator could learn to apply a uniformly increasing
Table 8. Summary of results showing average rates of loading achieved with two instruments by 40 operators. Operators
Loading rate (kg/sec) Instrument No. I 1st test 2nd test
31 Males Mean 2.18 C.V. (%) 49.3 9 Females Mean 4.12 C.V. (%) 35.0 40 Males and females pooled Mean 2.62 C.V. (%) 53.8
Mean
1st test
Instrument No.3 2nd test
Mean
2.32 48.7
2.25 48.8
2.15 46.0
2.41 50.1
2.28 47.3
3.91 42.6
4.01 35.9
3.53 30.2
3.64 36.4
3.60 31.3
2.68 52.9
2.70 51.4
2.46 46.8
2.69 49.2
2.58 47.0
The results indicated that the average rate of increase in force varied widely between operators (Table 8) but the Can. Ins!. Food Sci. Technol. J. Vol. 10, No.4, October 1977
force by observing how the force applied actually increased to develop a sense of feel for the correct rate. 292
12.0
INSTRUMENT NO.1 ( Italian)
10.5
9.0
7.5
6.0
A
4.5
B
C
3.0
1.5
0 C'
0
~
2
4
6
8
I
I
I
I
2
4
6
8
I 10
0
2
4
8
6
w U 0::
0
u.
10.5
INSTRUMENT N°.3 (Ballauf 301b)
9.0
7.5
6.0
4.5
D
3.0
F
E
1.5
0 0
2
4
6
I 0
2
I 4
I
I
6
8
I 10
I 12
I
2
I 4
I
6
TIME SECONDS Fig. 7.
293
Typical records of force applied by operators to punch test apples. J. Inst. Can. Sci. Technol. Aliment. Vol. 10. No.4, October 1977
Table 9. Summary of readings obtained during manual operation of two instruments showing maximum force from the instruments and recorder chart. operators
Maximum force reading from the recorder chart (kg) Instrument No. I Instrument No.3 1st test 2nd test Mean 1st test 2nd test Mean
31 Males Mean 10.2 C.V.(%) 15.0 9 Females Mean 10.9 C.V. (%) 17.7 40 Males and females pooled Mean 10.3 C.V.(%) 15.7
Maximum force readings from the instruments (kg)" Instrument No. I Instrument NO.3 1st test 2nd test Mean 1st test 2nd test Mean
10.4 15.3
10.2 14.1
10.1 13.8
10.1 13.4
10.1 12.2
9.0 11.6
9.4 11.3
9.2 10.5
8.8 10.8
8.9 15.0
8.8 11.5
11.2 1l.6
11.0 13.7
11.0 13.3
10.9 13.4
10.9 12.5
10.3 12.3
10.4 9.8
10.3 8.9
9.1 12.0
8.9 6.7
9.0 9.0
10.5 14.7
10.4 14.1
10.3 14.0
10.3 13.5
10.3 12.5
9.3 13.0
9.6 1l.8
9.4 1l.2
8.8 11.0
8.9 13.5
8.9 10.9
"Converted from readings in Ib for purposes of comparison.
It would be worthwhile to develop a damper to prevent the rapid return of the puncture probe at penetration and eliminate the shock that this gives to the operator. The stiffer spring of the Italian fruit pressure likely makes it more difficult than the Ballauf unit to adjust the calibration by adding shims to change the initial pretension of the spring. The same shim thickness would have 59 times the effect on the breakout force of the Italian unit compared to the 30 Ib Ballauf instruments. The problems of calibrating and operating the fruit pressure testers suggest that a better solution would be to use an electronic force gage with strain gages to detect force and an electronic circuit to display the maximum force. Such a device was recently introduced (AccuForce gage Ametek, Hunter Spring Division, Hatfield, Pennsylvania 19440) as a hand held unit with a built in digital readout. However, such devices are not suitable for hand operation because they are extremely sensitive to operator technique (Voisey, 1976).
References Abbott, J. A., Watada, A. E. and Massie, D. R. 1976. Effe-gi, Magness Taylor and Instron fruit pressure testing devices for apples, peaches and nectarines. 1. Amer. Soc. Hort. Sci. 10 I:698. Anon. 1974. Compression test of food materials of convex shape. Agr. Eng. Yearbook pp. 386-389. Barber, J. R. 1974. Determining the contact area in elastic-indentation problems. J. Strain Analysis 9:230. Blake, M. A. 1928. A device for determining the texture of peach fruits for shipping and marketing. New Jersey Agr. Exp. Station, New Brunswick, N.J., Circular No. 212. pp. 2-8. Bourne, M. C. 1965. Studies on punch testing of apples. Food Techno!. 19: 1l3. Bourne. M. C. 1965. How skin affects apple pressure tests. Farm Res. 31: 10. Bourne, M. C. 1966. Measure of shear and compression components of puncture tests. 1. Food Sci. 31 :282-291. Bourne. M. C. 1969. Two kinds of firmness in apples. Food Techno!. 23:59-60. Bourne, M. C. 1974. Comparison of results from the use of the Magness-Taylor pressure tip in hand and machine.operation. J. Texture Studies 5:105. Bourne, M. C. 1975. Method for obtaining compression and shear coefficients of foods using cylindrical punches. J. Texture Studies 5:459. Bourne, M. C. 1976. Texture of fruits and vegetables. In Rheology and Texture in Food Quality. Eds. J. M. DeMan, P. W. Voisey, V. F. Rasper and D. W. Stanley. AVI Pub. Co., Westport, Conn., p. 296.
Can. Ins\. Food Sci. Technol. J. Vol. 10, No.4, October 1977
Breene, W. M., leon, I. J. and Bernard, S. N. 1974. Observation on texture measurement of raw cucumbers with the fruit pressure tester. J. Texture Studies 5:317. Brekke, J. E. and Sandomire, M. M. 1961. A simple, objective method of determining firmness of brined cherries. Food Techno!. 15:335. Claypool, L. L., Fridley, R. B. and Johns, R. 1966. Precision in a pressure tester. Western Fruit Grower 20: 18. Culpepper, C. W., and Magoon. C. A. 1924. Studies upon the relative merits of sweet corn varieties for canning purposes and the relation of maturity of com to the quality of the canned product. J. Agr. Res. 28:403. DeMan, 1. M. 1969. Food texture measurements with the penetration method. 1. Texture Studies I: 114. Haller, M. H. 1941. Fruit pressure tester and their practical applications. U.S. Dept. of Agr. Circular No. 627: I. Hau~hey, D. P. and Marer. 1. M. 1971. The softening of frozen meat: Criteria for transportation in insulated containers without refrigeration. J. Food Technol. 6: 119. Kruisheer, C. I. and Den Herder, P. C. 1938. Research concerning the consistency of butter (in Dutch). Chemical Weekly-J. Dutch Chern. Assoc. & Assoc. Chern. Ind. Netherlands 35:719. Lewis, C. I., Murneek. A. E. and Catc, C. C. 1919. Pear harvesting and storage investigation in Rogue River Valley (2nd Report). Oreg. Agr. Exp. Sta. Bull. 162. pp. 1-39. Magness, J. R. and Taylor, G. F. 1925. An improved type of pressure tes:er for the determination of fruit maturity. U.S. Dept. of Agr. Circular 350:1·7. Mattus, G. E. 1965. Mechanical thumb tests of apple firmness. Proc. Amer. Soc. Hort. Sci. 87: 100. Mohsenin, N., Goelich, H. and Tukey, L. D. 1963. Mechanical behavior of apple fruit, as related to bruising. Amer. Soc. Hort. Sci. 81 :67. Mohsenin, N. N., Morrow, C. T. and Yang, Y. M. 1970. On the spherical indenter as a means for determining the "firmness" and "hardness" of food materials. Proc. Fifth Int. Congo on Rheology 2:647. Morris, O. M. 1925. Studies in apple storage. Washington State College, Agr. Expt. Sta. Bull. 193:5-44. Murneek, A. E. 1921. A new test for maturity of the pear. Oreg. Agr. Exp. Sta. Bull. 186: 1-28. Nichols, R. C. 1960. Some observations on the use of fruit pressure testers. Quar. Bull. Michigan Agr. Expt. Sta. 43:312. Peleg, M. and Brito, L. G. 1975. Estimation of the components of a penetration force of some tropical fruits. J. Food Sci. 40: 1030. Prather, O. C.. Hendrick, J. G. and Schafer, R. L. 1970. An electronic hand.operated recording penetrometer. Trans. Amer. Soc. Agr. Engrs. 13:385-386, 390. Schomer, H. A. and Olsen, -K. L. 1962. Mechanical thumb for determining firmness of apples. Proc. Amer. Soc. Hort. Sci. 81:61. Su, C. S. and Humphries. E. G. 1972. Rupture properties of cucumber skin. Pickle Pack. Sci. 2:3. Timbers, G. E., Staley, L. M. and Watson, E. L. 1965. Determining modulus of elasticity in agricultural products by loaded plungers. Agr. Eng. 46:274. Voisey, P. W. 1971. Modernization of texture instrumentation. J. Texture Studies.2:129. Voisey, P. W. and Crete, R. 1973. A technique for establishing instrumental conditions for meas· uring food firmness to simulate consumer evaluations. 1. Texture Studies 4:371. VOlsey, P. W. 1976. Engineering assessment and critique of instruments used for meat tenderness evaluation J. Texture Studies 7: II. Voisey, P. W. and DeMan, J. M. 1976. Applications of instruments for measuring food texture. In Rheology and Texture in Food Quality. Eds. J. M. DeMan, P. W. Voisey, V. F. Rasper and D. W. Stanley. AVI Publishing Co., Westport, Conn., pp. 142-243. Yang, Y. M. and Mohsenm, N. 174. Analysis of the mechanics of the fruit pressure tester. 1. Texture Studies 5:213. Received March 31, 1977
294