Softening at the onset of grape ripening alters fruit rheological properties and decreases splitting resistance
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Applying principles of shell theory, we found that grape berries rapidly change their behavior from thick-walled spheres to pressurized thin-walled spheres and become susceptible to splitting during berry softening.
Knowledge of the rheological properties of the skin of berry fruits is needed to make decisions concerning berry splitting prevention. However, how these properties and splitting resistance respond to varietal differences and developmental changes is poorly understood. In a customized injection test, pressurized water was injected into the berries of four grape varieties until they split. In a compression test, the deformation of berries in response to berry softening or dehydration was measured. Shell theory was applied to estimate how the internal pressure translates to tensile stress on the skin. The results suggested that berry softening at the onset of ripening drastically alters berry rheological properties; berries rapidly changed from brittle to ductile materials. The skin became the major stress-bearing structure during berry softening and became vulnerable to tensile stress, which was associated with a rapid decline in splitting resistance. The rate of decline and the absolute extent of the skin’s ability to bear stress varied by variety. Dehydration of overripe or water-stressed berries did not alter the skin properties but reduced the risk of berry splitting. These results indicate that the vulnerability to berry splitting is closely related to developmentally regulated changes in fruit rheological properties and water relations.
KeywordsFracture mechanics Fruit ripening Shell theory Tensile stress Water balance
Strain at splitting
Offset yield point
Offset yield strength at 0.2% strain
Critical shell tension
Injected water volume
This work was supported by the USDA National Institute of Food and Agriculture, Hatch project 1000186, the Chateau Ste. Michelle Distinguished Professorship, and the Graduate School of Washington State University. We thank Lynn Mills and Alan Kawakami for technical assistance.
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