Apple Size and Length of Storage Affects Firmness
The apple packinghouse industry has long known that two quality factors in apples, size and firmness, are related. The exact nature of that relationship, and the ability of the industry standard firmness measurement, the Magness-Taylor Firmness Test (MT) to measure firmness independent of apple size in not known. In the 1950s a Magness-Taylor correction table (Table 1) for firmness based on apple size was circulated among the Washington State apple industry. This table attempted to eliminate the differences in firmness between apples of different sizes. For example, it suggests that a box size 64 apple would be 1 pound less firm than the equivalent apple at box size 100. The table also suggested that the steps between box sizes was even -- that firmness changed by 0.25 lb per box size. Larger apples were 0.25 lb less firm that those of size 100, and apples smaller than size 100 lost 0.25 lb per box size.
Recent study of apple tissue physiology has led to some understanding into reasons that large apples are less firm than smaller apples. The number of cells in an apple is fairly constant (about 3 million) regardless of apple size, leading to the conclusion that large apples have large cells. Many apple cells are tubular shaped, their length is about twice their diameter, with the major axis orientated perpendicular to and radial from the core line. Apple tissue also has a number of intercellular air spaces. Large apples are likely to have larger air spaces. Figures 1A, 1B and 1C illustrate cell shape, orientation and shape of air spaces in apple tissue.
Both larger cells and air spaces provide more stress to cell walls, resulting in less firm apple tissue that fractures under smaller loads than small apple tissue. Figure 2 demonstrates the two modes of tissue failure thought to occur in apple tissue. Compression of the tissue places stress on the cell walls and on the pectin bonds holding the cells together. If the cell walls fail first, the tissue fails rapidly as the cascading increase in cell wall stress ruptures the remaining cells. If the pectin bonds fail, the detached cells will move, but will not rupture until the intercellular spaces are filled.
Larger cells and intercellular spaces will place additional stress on cell walls and cell bonds. It is not known if the increase stress on cell walls and pectin bonds alter human perceptions of firmness and/or the force required to insert the MT plunger into a large apple. The need for a Magness-Taylor correction to reconcile human perceived firmness with MT firmness may be because humans sense one failure mode while the MT senses another, and the failure modes change differently with apple size. A third type of measure of apple firmness might provide additional insight to tissue failure in large apples.
As part of a project to evaluate nondestructive apple firmness sensors, we obtained data from over 10,000 Red Delicious apples collected over a range of sizes, growing locations over three years. In addition to measuring MT values measurements from various prototype firmness sensors, we measured the failure stress (crushing force) in tissue from these apples. The crushing force is a third measure of apple firmness.
Figures 1b and 1c
The objectives of this study were to determine
- If larger apples are less firm than smaller apples
- If the difference in firmness between large and small apples changes as the apple ages
- If the Magness-Taylor correction table (Table 1) is appropriate for Red Delicious apples.
Materials and Procedures
Red Delicious apples from the Okanogan, Wenatchee and Yakima growing regions were collected from the 1990, 1991 and 1992 crops. From each region, we collected blush and stripe varieties in box sizes ranging from 64 to 125. Each box collected was a unique combination of crop year, growing region, stripe or blush, box size and replicate.
Three samples of 20 apples were removed from each box. The first sample was tested after warming to room temperature for 24 hours. The second sample of 20 apples was held at room temperature for two weeks (3 weeks for the 1992 crop), and then tested. The third sample of 20 apples was held at room temperature for an additional 2 weeks (6 weeks for the 1992 crop) before testing.
After the appropriate aging period, two MT measurements were made, both 90 degrees from the sunny side of the apple. A MT head was fitted to an Instron Universal Testing machine, along with a force transducer to measure the force required to penetrate the apple. The average of the two MT values were recorded.
After making MT measurements on the apples, cylindrical tissue samples (15 mm length x 10 mm dia.) were removed and crushed to failure between parallel plates closing at a rate of 25 mm/min. In 1990 and 1991 the cylinder was removed at a point 180 degrees from the sunny side of the apple. In 1992, the cylinder was removed from a undisturbed tissue near the MT site. Force and deformation measurements were recorded as the cylinder was crushed. The peak force required to crush the cylinder (Crush Force) was used in this study.
Results and Discussion
Apple size did have an effect on firmness measured by both the MT value and crush force (Figures 3-4, 5-6 and 7-8). The data for the graphs in these figures include all apples of a particular box size which included apples that had aged at room temperature. At the 95% significance level, both the MT value and the crush force data indicated that all box sizes were significantly different from other sizes with the exception of size 72 and 80 apples. These results were consistent across the three years of the study. Of particular interest is that the MT value and crush force both decreased in similar trends as box size increased.
Leaving apple samples at room temperature had the desired effect of softening the apples, as indicated in Figures 9 and 10. Although storage at room temperature is not an industry practice, the ambient temperatures allowed the apple samples to rapidly senesce to a lower range of firmness values. Large apples tended to lose more firmness than small apples. Figures 11 and 12 show the decrease in firmness with length of time stored at room temperature (LTSRT) by box size for the 1991 crop. The 1990 and 1992 crops have similar trends. For each crop, at a 95% significance level, the LTSRT, box size and the interaction between LTSRT and box size all had a significant effect on apple firmness measured by MT values and by crush force. That longer LTSRT and larger box size lower the firmness in an apple is not surprising. The interaction of LTSRT and box size is more interesting.
Why do large apples soften more quickly than small apples? Two inferences about the effect of this interaction on firmness come to mind. One is that there is a physiological reason that large apples soften more quickly due to the growth of the fruit. The other is that there is a physical reason due to the geometric differences between large and small apples. The data in this study does not provide much insight into the cause of the interaction between LTSRT and box size.
Figures 2 and 3
Figure 4 and 5
Figures 7 and 8
Figures 9 and 10
Figures 11 and 12
From our analysis of apple data from three growing seasons, including blush and stripe cultivars of sizes from 125 to 64 grown in three regions of Washington State, we have reached the following answers from the questions stated in the objectives.
- At harvest, large apples will tend to be softer than small apples. This trend was observed in two different measures of apple firmness.
- During senescence, large apples will lose more firmness than small apples. This observation is also confirmed by both MT values and crush force.
- From our analysis of the data presented, we did not see a justification for adjusting the MT values based on apple size.
If the difference in firmness between large and small apples is the subject of further research, we recommend that effort be placed into determining if the dominant failure mode of apple tissue changes with apple size. Identifying the failure modes, and the conditions which cause apple tissue to fail via these modes, may lead to more insight in the management of apple storages.
Funding for the evaluation of nondestructive apple firmness sensors project, which provided the data for this paper, was provided by the Washington State Tree Fruit Research Commission, the IMPACT Center, the Washington Technology Center, USDA and Washington State University. Their combined support is gratefully acknowledged.
Marvin Pitts (1), Steve Drake(2) and Ralph Cavalieri (1)
(1)Department of Biological Systems Engineering. L.J. Smith 213 Washington State University, Pullman, WA 99164-6120 (2)USDA, ARS 1104 Western Ave, Wenatchee, WA 98801
13th Annual Postharvest Conference