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Sunday, February 17, 2019

WSU-TFREC/Postharvest Information Network/Apple Bruising Research Update: Packingline Impact Evaluations

Apple Bruising Research Update: Packingline Impact Evaluations


The amount of impact bruising that occurs to apples depends upon the number and severity of impacts that the apples experience, and upon the bruise susceptibility of the fruit. Another article (click here to read article J3I3A) discusses apple bruise susceptibility. This article discusses number and severity of impacts in packing lines and why they occur.

Figure 1 shows the equipment involved in I.S. (Instrumented Sphere, Zapp et al. 1990) and video evaluation of handling equipment. The impact evaluation procedure is as follows:

  1. Synchronize the I.S. and video camera clocks so that the time on the videotape matches the time in the I.S. data.
  2. Pass the I.S. three times through each transfer point in the handling equipment while tracking it with the video camera.
  3. Dump data from the I.S. to the computer, analyze it, and print it out.
  4. Mark large impacts.
  5. Play back the videotape to the time of each large impact to see where and how it occurred.

Figure 1

What the I.S. Tells Us

The I.S. tells us both the severity of the impact (in g's) and the amount of cushioning involved (as velocity change, dV). Figure 2 is a graph of the impacts measured in one packing line, plotted with g's on the horizontal axis and dV on the vertical axis. Impacts farther to the right are the more severe; those higher up are more cushioned. When we add impact surface type reference lines to the graph (the steel, and 1/8" and 1/4" Poron™ lines), we can tell what kind of impacts they were (how much cushioning). The bottom sloping line is for impacts on steel; the top curved line is for impacts on 1/4"Poron™ or equivalent, and the middle line is for 1/8" Poron™.

Adding 10% and 100% bruise threshold lines for the cultivars to be handled to the graph (Figure 3) tells us whether the impacts would bruise the apples and how much. (The threshold data are from Schulte et al. 1992, at Michigan State University. Thresholds for both Red and Golden Delicious are shown.) The left line is for 10% Red Delicious damage and the next is for 10% Golden Delicious damage. Any impacts falling on the line will bruise 10% of that cultivar; impacts farther to the right of the line will damage more and more fruit, until at the 100% lines, all of the fruit will be bruised. The fact that the Red Delicious line is to the left of the line for Golden Delicious means that Reds are more bruise susceptible than Goldens. This trend agrees with our data.

The shaded bruise zone is defined by the cushioning and threshold curves. Any impacts falling in the shaded area will cause damage to Reds and Goldens; and impacts farther to the right within the shaded area will bruise a higher percentage of the fruit.

Figure 3 shows only selected impacts at locations that either may need attention or are of interest because they are less of a problem than they appear to be (e.g., the overrun drop). The legend indicates the location in the packing line of each of the impacts. The weigh sizer output (circles) showed possible occasional problems. One, a 78 g impact on "steel" at the 100% probable damage level, occurred where the I.S. apparently rolled past the deflector belts and hit the steel side of the cross conveyor under the weigh sizer. This situation may or may not represent a real problem for apples, because they roll less easily than does the I.S.

The drop into the weigh-cup showed only one recorded impact (the +), and that one was not in the bruise zone. Though the drop into the weigh cup looks and sometimes sounds severe, it is generally less damaging than it appears. However, the tub delivery plate (a padded ramp) impacts (triangles) showed possible damage at the 10% or more level and may need more cushioning material.

When the I.S. rolled over against the steel side of the finger belt conveyor (diamond symbol in Fig. 3; photo, Fig. 4), the impact was in the bruise zone. These impacts could easily be fixed by padding the sides of that conveyor with 1/8" Poron™ or equivalent. Surprisingly, the singulator overrun drop impacts (the squares) were well cushioned and not in the bruise zone at all. We would expect bruise at that location only when fruit hits fruit.

Impacts at the Fancy corner drop (a special component in this particular packing house) have apparently been a problem in this packing line and unfortunately still seem to be (the X's). Some of these impacts may have been the I.S. colliding with apples, but apples colliding with apples can cause bruising. This transfer involves the possibility of apples rolling into other apples with enough velocity to damage both fruit involved in some cases.

Figure 4

Other Packingline Results

Brush Transfer Height Experiment

Figure 5 shows two positions for the brushes that transfer fruit into a dryer. Lowering the brushes only 3/4" reduced impact acceleration by 12 g's or 27%. However, as Figure 6 shows, many of the low brush impacts are still in the bruise zone. This transfer is often a problem area. An alternative design, using a padded ramp and keeping the transfer fully loaded with fruit, can eliminate the problem.

Figures 5 and 6

Other Experimental Results

The following is a summary of results of other packingline experiments that we conducted:

  1. Small bruises (less than 1/4 inch in diameter) appear to occur mostly in the rotating brush conveyers used for washing, polishing and waxing, while larger bruises (greater than 1/4 inch diameter) occurred progressively through the entire packingline.
  2. Reducing brush speeds reduced the severity of the small impacts that cause small bruises from the rotating brushes. (The small bruises are probably caused by apple hitting apple.)
  3. Reducing brush speeds also reduced the number of impacts per second; however, if the fruit flow rate is also reduced, the total impacts per fruit may still be the same. Thus, for bruise susceptible cultivars, it would appear that the best method of operation is to reduce brush speed while maintaining fairly fast fruit flow.
  4. Changing weigh sizer speed in a limited range of 246 to 290 cup rods/min. had no significant effect on damage potential.

In general, bruising impacts occur where fruit is allowed to roll or drop too far and collide with insufficiently cushioned surfaces or with other fruit. Severity of impacts can be reduced by minimizing the height of elevation changes and/or by dissipating the drop energy with baffles and by keeping the transfers full of fruit. The number of impacts can be reduced by reducing the number of drops and transfers in the packingline. Conditioning the fruit to reduce its bruise susceptibility is another way to eliminate bruise. Both temperature and moisture are important factors (see article on p. 10 on effects of moisture, temperature and cultivar), but condition is still not well understood. We hope to do more research on both impact reduction and fruit conditioning.


Schulte, N. L., G. K. Brown and E. J. Timm. 1992. Apple Impact Damage Thresholds. Applied Engineering in Agriculture 8(l):55-60.

Zapp, H. R., S. H. Ehlert, G. K. Brown, P. R. Armstrong and S. S. Sober. 1990. Advanced Instrumentation Sphere (IS) for Impact Measurements. Transactions of the ASAE 33(3):955-960.


This research was funded by the Washington State IMPACT Center at Washington State University.

Gary M. Hyde and Weihua Zhang

Biological Systems Engineering Department
Washington State University, Pullman, WA 99164-6120

Tree Fruit Postharvest Journal 3(3):12-15
August 1992

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