Bruising-Impacts, Why Apples Bruise, and What You Can Do to Minimize Bruising
Impacts and Impact Severity
This article describes impacts, what determines their severity, the determination of the severity of impact that will begin to bruise fruit, methods of reducing impact severity in handling systems, and the principles involved in conditioning fruit to reduce its impact sensitivity. The general principles involved in reducing bruising are to reduce the number and severity of impacts by adding cushioning, reduce effective drop heights, eliminate unnecessary drops, and very slightly dehydrate the fruit to improve its bruise threshold.
This article discusses impacts, which cause much of the bruise damage in fruits and vegetables. While fruit (apples, pears, and other)1 at the bottom of a bin may get crushed or bruised under some conditions, the up-to-4-lb static force on fruit at the bottom of a 2-foot-deep bin is usually not sufficient to cause bruising. What creates impacts and determines their severity? Quite simply, impacts result from either dropping or throwing fruit against other fruit or other surfaces. Thus, if there were no change in elevation in a handling system, there would be few if any impacts and little bruising.
Impact severity relates to the old adage, "the bigger they are, the harder they fall." The energy (E) involved in fruit impact depends on:
- The mass of the individual fruit (m).
- How far it falls (h).
- Earth's gravity (g).
The relationship is E = m*h*g. If the impact energy absorbed by an apple stresses its tissue beyond its bruise threshold, a bruise will result (more about bruise thresholds below).
Cushioning reduces impact severity by absorbing some of the impact energy, thus reducing the amount that dissipates in the fruit. The cushion also spreads the remaining impact force over a greater area of the fruit, further reducing tissue stress (force per unit area) and the likelihood of bruising. Thus, cushioning reduces the effective severity of an impact two ways: by absorbing energy and by spreading the load.
Why Fruits Bruise
Inherent strength exists in fruit and vegetable tissue as in other materials such as a rubber band, a steel bolt, or a piece of glass. A major difference is that plant tissue is visco-elastic rather than primarily elastic like the latter materials. This means that fruits and vegetables act differently, depending upon how fast you apply loads to them. Under slow loading, they act more rubbery and tough; under fast loading, they are more brittle. Dropping potato tubers, for example, from a low bruising height may cause one type of bruising (black spot discoloration with no cracking), while dropping the same tubers from a much higher height causes a very different type of bruising (cracks and shattering with no discoloration; Mathew and Hyde, 1997).
Fruit and vegetable tissue behavior and strength may also be influenced by temperature more than other materials, and definitely by hydration level (turgor). Colder and/or more turgid fruit and vegetable tissue is usually more brittle and more sensitive to impacts, i.e., more easily bruised (but there are exceptions).
The impact sensitivity of fruits and vegetables has two components:
- Bruise threshold is the
drop height at which bruising begins to occur, i.e., when
the tissue strength is first exceeded. Its inverse is the
percent of individual fruit bruised at a given drop
- Bruise resistance is the amount of energy absorbed in bruising a specimen divided by the resulting bruise volume. Its inverse is approximately the bruise size resulting from a given bruising drop height for a given size fruit.
Factors Affecting Impact
Zhang (1994) found that temperature had little effect on either bruise threshold or resistance for Red and Golden Delicious apples, but that hydration level (turgor) had a large effect. Bruise threshold improved (doubled) from 9 mm to 18 mm (3/8 to 3/4") when the fruit were slightly dehydrated to an additional weight loss of about 2% (Fig. 1). However, the bruise resistance of the same fruit decreased with turgor (Fig. 2). Since the ideal would be to have high threshold and high resistance, there exists a yet undefined optimal hydration level that would give the best compromise among bruise threshold, bruise resistance, and weight loss.
What You Can Do to Minimize Bruising
Impact Evaluation--The Instrumented Sphere
The severity of impacts in handling equipment can be assessed using a device called the instrumented sphere (I.S.) developed by agricultural and electrical engineers at Michigan State University and the USDA (Zapp et al., 1990). The I.S. is a survey tool that records time and acceleration of impacts encountered as it moves through handling equipment along with the fruit. The data are reported as peak acceleration in Gs and velocity change (dV) in meters per second (m/s). Tracking the I.S. through the handling system with a video camera and synchronizing the I.S. and camera clocks adds the location and appearance of each impact to the data base. Thus, the I.S. characterizes the severity of the impact and the video tape shows how and where it occurred. All that remains is to determine the level of impact severity that will bruise the fruit (provided in the I.S. software and discussed below) and then to fix the problem areas by modifying the equipment, pre-conditioning the fruit to reduce its impact sensitivity, or both.
Bruise Thresholds and the
To make the I.S. data meaningful, we need to know the bruise threshold for the commodity to be handled. Michigan data (Schulte et al., 1992) show that, for Red and Golden Delicious apples of 200 gram (0.44 lb) size 1 day after harvest, about 10% of the fruit were bruised when dropped onto steel from a height of 2 mm (5/64"). Their data show also that this bruising threshold impact is equivalent to I.S. peak acceleration of 21.7 Gs and velocity change of 0.22 m/s. A graph (Fig. 3) of I.S. Gs vs. velocity change using reference curves for cushioning and the Michigan bruise thresholds defines the "bruise zone." Any impact falling within that zone will likely bruise fruit, with the likelihood increasing as the impact moves toward the right in the bruise zone.
Reducing impact severity in handling equipment is a matter of reducing the number and severity of impacts. Reducing the severity requires moving bruise zone impacts toward the left (Fig. 3) and out of the bruise zone. That reduction in severity can be achieved by reducing drop height, adding cushioning, or both as the arrows in the lower right of Figure 3 indicate.
Example solution to an impact problem. For example, consider the diamond-shaped impact labeled "finger belt side" in Figure 3. The video tape (Fig. 4) shows that impact to be against a sheet steel side rail, with a peak acceleration of 65 Gs and velocity change of 0.93 m/s. This was a hard surface impact as indicated by the nearness of the impact symbol to the steel line in Figure 3. The fact that the impact did not fall on the steel line is because that line represents impacts on a steel anvil. Since the side rail is thin material that can flex, it was less rigid and provided a small amount of cushioning.
To fix the side rail (Fig. 4) so that it would not bruise apples, it is necessary to move the impact out of the bruise zone to the left in Figure 3. Notice that there are two cushioning reference curves in Figure 3, one for 1/8" and one for 1/4" Poron. Figure 3 tells us that adding a layer of 1/8" Poron to the side rail of Figure 4 would move the diamond-shaped impact symbol to the left beyond the 1/8" Poron line, completely out of the bruise zone, and solve the bruise problem at the side rail.
In general, to reduce impact hazards in handling equipment, do the following:
- Remove hard support under belts where
fruit drops onto them.
- Add retarders to
- Match flows so commodity mass prevents
rolling on ramps. Fruit flowing en masse prevents
individuals from rolling and hitting each other.
- Minimize elevation change at each transfer in the handling system.
Zhang's threshold data (Fig. 1 above) show thresholds for apples, and that it is possible to improve (double) the bruise threshold by slight dehydration of the fruit. Those bruise thresholds varied from 8 to 16 mm for Red Delicious for fruit purchased in October, held in cold storage, and tested in January. The thresholds for the Michigan experiments (Schulte et al., 1992) were 2 mm at one day after harvest, implying much more turgid fruit. Clearly, hydration conditioning of apples can reduce bruising problems in handling. Highly turgid Golden Delicious coming out of CA storage in February often need such conditioning. However, research is needed to compare the bruise thresholds of that fruit to those of the above fruit and to find efficient methods of achieving the hydration conditioning.
Mathew, R. and G. M. Hyde. 1997. Potato impact damage thresholds. Trans. of the ASAE, in press.
Schulte, N. L., G. K. Brown and E. J. Timm. 1992. Apple impact damage thresholds. Applied Engineering 8(1):55-60.
Zapp, H. R., S. H. Ehlert, G. K. Brown, P. Armstrong and S. Sober. 1990. Advanced Instrumented Sphere (Is) for impact measurements. Trans. of the ASAE 33(3):955-960.
Zhang, W. 1994. Apple impact bruise analysis. Ph.D. Dissertation, Program in Engineering Science, Washington State University.
1 This article is written with apples in mind, but the same principles apply to other fruit and even potatoes and onions. Each crop has different impact sensitivities.
G. M. Hyde
Biological Systems Engineering Department
Washington State University, Pullman, WA 99164-6120
Tree Fruit Postharvest Journal 8(4):9-12