(N)
(3.9)
(3.10)
(3.11 )
At maximum deformation, crt = rmax and F = F max. Substituting equation (3.1 2) into (3.8), the maximum flesh tensile stress (r max) is obtained as:
=
Procedure
3W t X L x
y
x sin�
B x n 3
(pa)
(3.13)
Before each experiment, the equipment was calibtated by testing five specimens in a preliminary trial and recording the voltage output and the equ ivalent distances along the vertical axis (force) and horizontal axis (deformation) on the output of the chart recorder. During this trial test, suitable jaw size to accommodate the length of specimens was selected. Additional weights were added when testing 'hard' fruit. These sizes were recorded and used for the calculation of the tensile properties.
For all experiments, a l O-mm rectangular cork borer was used to obtain test samples from fruit. Each fruit was sampled on two opposite sides along the vertical axis. During testing, the core sample was placed on the platform jaw and the cam was moved slowly and steadily in the anti-clockwise direction until the sample broke.
After testing, the maximum deformation (x) and the angle at fai lure (�) were evaluated from the force-deformation curve produced by the chart recorder. These results were then used to calculate the flesh tensile strain and stress from equations (3. 1 1 ) and (3.1 3), respectively. Young' s modulus of elasticity (E) was calculated as the ratio of stress to strain.
3.4.6 Skin Bursting Stress
A new technique was devised for testing the skin bursting stress of fruit. The equipment comprised an Effegi penetrometer fitted with a 7 .97 mm head and an electric drill press fitted
with an 8 m m drill bit. This technique involved removing the flesh from the test site using the drill and 'bursting' the skin with a penetrometer probe in a way that mimics the splitting of the skin due to excessive internal pressure. In addition to the obvious advantage of testing the skin in the intact condition, this technique also obviates the traditional problems of specimen preparation and handling which have continued to raise questions on existing test methods. This technique was found to be simple and quick, and it gave fairly consistent results during preliminary testing. By this technique, it was also possible to test fruit with different amounts of flesh attached to the skin.
General Procedure
First, the fruit sample was sectioned into two equal halves along the vertical axis. Each half was placed on a flat surface and an 8mm diameter drill bit connected to an electric motor was used to make a hole through the fruit flesh by lowering the drill bit onto the cut surface. The vertical displacement of the drill was pre-set to leave a gap between the drill tip and the flat surface on which the fruit skin made contact. After making the hole, the sample was then placed on a non-hard surface with a hole bigger than that in the fruit and the probe of a penetrometer was passed through the hole in the fruit to burst the skin. The scale reading
COp)
of the penetrometer was recorded. Figure 3.4 shows the photograph of apple sections tested for skin bursting stress.The skin bursting stress
CSbs)
is related to the probe diameter and skin thickness by:where :
=
0bf =
COp
X g) = bursting force, N IT = pId
p
= diameter of penetrometer probe, mtsk = thickness of skin, m
3.4.7 Fruit-stem Adhesion Force
A technique was developed to measure the force required to detach the stem from already harvested fruit. The system was made up of penetrometer and a device to gri p the stem using a drill chuck. Figure 3 .5 shows the arrangement of the system called a "stem-puller" . Fruit were tested as fol lows : the stem was i nserted i n to a chuck and securely gripped by l ocking the chuck; the penetrometer was zeroed, and the frui t was held by both hands and pull ed vertically downwards until the stem was detached. The penetrometer readi ng (kg) at this point was recorded.
It was important that the pul l i ng was done slowly and steadily until detachment occurred i n order t o minimise the effect o f loading rate o n the results. Also, care was taken t o observe accurately the penetrometer readin g at the time of stem detachment as this varied when the chain oscillated about its original position.
The fruit-stem adhesion force (aar) was determined from the rel ationship :
=
(N)
(3.15)where :
= mass of fruit, kg
= penetrometer reading, kg
3.5 Analysis of Data
Experimental data were analyzed for variance, means, and standard errors using the Statistical Analysis Systems (SAS) programmes (SAS/STA T User' Guide, 1 988). Prior to any analysis
of variance, data were tested for normality and homogeneity of variance. Where necessary, appropriate transformations of the original data were performed before statistical analysis and bac k transformed for presentation (Little, 1 985). Means were compared using the Duncan' s Multiple Range Test (Duncan, 1 955). For tests involving only two treatments, the means were compared using a standard t-test (Cochran and Cox, 1 957).
Graphs were plotted using Cgle (Version 3.2 ) graphics packages (Pugmire, 1 992). Where applicable, regression and correlation analyses were carried out using the method of Steel and Torrie ( 1 9 80) and the SAS package (SAS/STAT User' Guide, 1 988).