Optimizing Temperature and Moisture to Minimize Fruit Bruising
We now have the analytical tools to measure the impact sensitivity (bruise threshold and bruise resistance) of individual apples. We can also measure the relative hydration level reliably at the turgid end of the scale where apples are handled and marketed. Experiments to date show that, while temperature has only a small influence, slight dehydration of the fruit (1% to 2% weight loss) can greatly improve (double) bruise threshold allowable drop height. However, that slight dehydration may mean that, for a given bruising impact, more turgid fruit will have larger bruises (poorer bruise resistance). More research is needed to determine the best compromise between weight loss, bruise threshold, and bruise resistance and to determine efficient conditioning procedures to achieve that optimal hydration level.
Fruit Impact Sensitivity Components
Impact sensitivity has two components: bruise threshold and bruise resistance.
- Bruise threshold is the drop height at which bruising begins to occur. Its inverse is the percent of individual fruit bruised at a given drop height.
- 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.
Bruise threshold and bruise resistance seem, at this point, to be somewhat independent of each other and may be influenced in differing ways by changes in hydration level and other factors.
Factors that Influence Impact Sensitivity
The factors that influence impact sensitivity probably include the following, with the importance of each factor varying depending upon the species:
- Hydration level
- Production practices
The influences of production practices probably include not only the amount of water and nutrient stresses, but the patterns of such stresses during the growing season.
Impact Test Procedures
In order to measure not only the impact sensitivity of the commodities, but also their condition, we developed instrumentation and three test procedures: paired increasing-height multiple-impacting (PIHMI), constant-height multiple impacting (CHMI), and dynamic axial compression (DAC).
- PIHMI measures bruise threshold for individual apples
- CHMI measures bruise resistance and bruise volume for a given drop height for individual specimens
- DAC measures relative hydration level (turgor), toughness, and failure stress and strain for tissue samples.
Effects of Hydration Level (Turgor)
Zhang (1994) conducted experiments using three temperatures and three hydration levels. The hydration levels were determined by measuring fruit weight loss (Table 1). Reducing hydration level dramatically improved bruise threshold for both Red and Golden Delicious apples (Figure 1). Bruise threshold doubled in going from high to low hydration level, an increase in weight loss of only 2%. Note that Reds had a lower bruise threshold than Goldens; this is consistent with results from our research and research done elsewhere (Schulte et al. 1992)
On the other hand, lower hydration level resulted in poorer bruise resistance (Figure 2). In other words, reducing hydration level meant that the apples would have to be dropped from a higher height to bruise them, but if that bruising height were reached, the bruises would be larger.
Effects of Temperature
Temperature had no significant effect on either bruise threshold or bruise resistance for either Reds or Goldens (Figures 3 and 4) (Zhang 1994). These results seem to indicate that to reduce bruising of Goldens coming out of CA storage in February, warming is not as important as reducing hydration.
Fruit Conditioning Trade-Offs
From this work it appears that there is a compromise between hydration level (weight loss), bruise threshold, and bruise resistance. Slight dehydration of the fruit will reduce the likelihood of bruising by improving bruise threshold; but when bruises occur, they may be larger than for slightly more turgid fruit. Thus, managers must try to find the best compromise among bruising, bruise size, and shrink loss. Still to be developed is an efficient method of hydration level conditioning for bruise control, possibly one that avoids unnecessary warming of fruit that won't be waxed.
Schulte, N. L., G. K. Brown, and E. J. Timm. 1992. Apple Impact Damage Thresholds. Applied Engineering in Agriculture 8(1):55-60.
Zhang, Weihua. 1994. Apple Impact Bruise Analysis. Ph. D. Dissertation, Program in Engineering Science, Washington State University.
G. M. Hyde, A. L. Baritelle; R. W. Bajema, and W. Zhang
Biological Systems Engineering Dept.,
Washington State University, Pullman, WA 99164-6120
13th Annual Postharvest Conference