Articles
FRUIT CRACKING MECHANISMS IN SWEET CHERRIES (PRUNUS AVIUM L.) – A REVIEW
The phenomenon has been paid attention to by researchers during many decades.
In spite of this, the understanding of the basic mechanisms involved in cherry fruit cracking is not clear, and the subject is still a matter of discussion.
Cracks in sweet cherry fruits are shallow or deep, oblong wounds in the fruit flesh.
They are occurring all over the fruit surface, although characteristic patterns are often described.
Fractures in the fruit cuticle are commonly occurring as well, but are not described as fruit cracks.
The prevailing theories have been based on surface water penetrating the fruit surface causing a volume increase of the fruit that itself causes cracking.
The driving force for the water diffusion is differences in osmotic potential between the surface water and the fruit juice.
The many results obtained in experiments based on this model have often lacked consistency when being compared, or have even been directly contradictory.
Studies on other crops, e. g. grapes, tomatoes, lemons and apples, indicate that water uptake to the fruit from the root system, causing internal turgor pressure buildup, play an important role in cracking mechanisms.
According to a model based on this, the turgor pressure is a driving force in the cracking mechanisms, while the role of surface water covering the fruit is limited to causing destruction of the bearing structures of the fruit surface, i. e. epidermal layers and the cuticle.
Such mechanisms were suggested by some of the first researchers studying cherry cracking as well, but were opposed and have been brought to attention only recently.
The model is now well documented both theoretically, by electron microscopy documentation and by experiments in which the forces acting from inside the fruit and their distribution on the fruit surface have been measured.
Results from experiments carried out at Ullensvang Research Centre support this model as well; by differentiating the moisture content of the soil, cherry fruits developed cuticular fractures differently, evidently caused by the soil moisture differences.
According to this model, the properties and function of the fruit cuticle, including the phase where it meets the outer epidermal cell layer, are crucial to the cracking mechanisms.
Similarly, the properties and function of the water conducting (xylem and phloem) tissue both in the tree, the fruit pedicel and the fruit itself will influence cherry fruit cracking.