The effect of surface water on the frequency of microcracks in the cuticular membrane (CM) of exocarp segments (ES) of developing sweet cherry fruit (Prunus avium L.) was studied. Strain of CM and ES on the fruit surface was preserved by mounting a stainless steel washer on the fruit surface in the cheek region using an ethyl-cyanacrylate adhesive. ES were excised by tangentially cutting underneath the washer. Frequency of microcracks in the CM of ES was determined following infiltration for 10 minutes with a 0.1% acridine orange solution by fluorescence microscopy before and after exposure to deionized water (generally 48 hours). Exposing the surface of ES of mature `Burlat' sweet cherry fruit to water resulted in a rapid increase in microcracks in the CM that approached an asymptote at about 30 microcracks/cm2 within 24 hours. There was no change in microcracks in the CM when the surface of the ES remained dry. Incubating ES in polyethylene glycol solution that was isotonic to fruit juice extracted from the same batch of fruit resulted in a greater increase in frequency of microcracks as compared to incubation in deionized water. The water-induced increase in microcracks was closely related to strain of the CM across different developmental stages within a cultivar [between 45 and 94 days after full bloom (DAFB); r 2 = 0.96, P ≤ 0.001, n = 9] or across different cultivars at maturity (r 2 = 0.92, P ≤ 0.0022, n = 6). Incubating ES of developing fruit in enzyme solution containing pectinase and cellulase such that the outer surface remained dry resulted in complete rupture and failure of the ES. Time to rupture and percentage of ruptured ES were closely related to the strain of the CM (r 2 = 0.92, P ≤ 0.001, n = 9 and r 2 = 0.68, P ≤ 0.0063, n = 9, respectively). Removal of epicuticular wax had no effect on frequency of water-induced microcracks. Also, temperature had no effect on frequency of water-induced microcracks, but frequency of microcracks increased exponentially when exposing the outer surface of ES to relative humidities above 75%. At 100% humidity the increase in frequency of microcracks did not differ from that induced by liquid water. Local wetting the surface of intact fruit in the pedicel cavity or stylar end region resulted in formation of macroscopically visible cracks despite of a net water loss of fruit. Uniaxiale tensile tests using dry and fully hydrated CM strips isolated from mature `Sam' sweet cherry fruit established that hydration increased fracture strain, but decreased fracture stress and moduli of elasticity. Our data demonstrate that exposure of the fruit surface to liquid water or high concentrations of water vapor resulted in formation of microcracks in the CM.
Moritz Knoche and Stefanie Peschel
Martin Brüggenwirth and Moritz Knoche
Rain cracking of sweet cherry fruit (Prunus avium L.) is said to occur when the volume increase associated with water uptake, extends the fruit skin beyond its upper mechanical limits. Biaxial tensile tests recorded fracture strains (εfracture) in the range 0.17 to 0.22 mm2·mm−2 (equivalent to 17% to 22%). In these tests, an excised skin segment is pressurized from its inner surface and the resulting two-dimensional strain is quantified. In contrast, the skins of fruit incubated in water in classical immersion assays are fractured at εfracture values in the range 0.003 to 0.01 mm2·mm−2 (equivalent to 0.3% to 1%)—these values are one to two orders of magnitude lower than those recorded in the biaxial tensile tests. The markedly lower time to fracture (tfracture) in the biaxial tensile test may account for this discrepancy. The objective of our study was to quantify the effect of tfracture on the mechanical properties of excised fruit skins. The tfracture was varied by changing the rate of increase in pressure (prate) and hence, the rate of strain (εrate) in biaxial tensile tests. A longer tfracture resulted in a lower pressure at fracture (pfracture) and a lower εfracture indicating weaker skins. However, a 5-fold difference in εfracture remained between the biaxial tensile test of excised fruit skin and an immersion assay with intact fruit. Also, the percentage of epidermal cells fracturing along their anticlinal cell walls differed. It was highest in the immersion assay (94.1% ± 0.6%) followed by the long tfracture (75.3% ± 4.7%) and the short tfracture (57.3% ± 5.5%) in the biaxial tensile test. This indicates that the effect of water uptake on cracking extends beyond a mere increase in fruit skin strain resulting from a fruit volume increase. Instead, the much lower εfracture in the immersion assay indicates a much weaker skin—some other unidentified factor(s) are at work.
Grace Q. Chen, Louisa Vang, and Jiann-Tsyh Lin
Harrington, J.F. 1972 Seed biology Academic Press New York, NY Harry-O'kuru, R.E. Carriere, C.J. Wing, R.E. 1999 Rheology of modified Lesquerella gum Ind. Crops Prod. 10 11 20
Moritz Knoche, Bishnu P. Khanal, and Matej Stopar
-treated control fruit at 75 and 138 DAFB. Thus, the decrease in formation of microcracks caused by GA 4+7 cannot be explained by effects on CM rheology. Hydration decreased fracture force and moduli of the hydrated cuticle, but fracture strains remained constant
Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda, and Kazuyuki Abe
varieties Food Structure 11 339 349 Lin, T.-T. Pitt, R.E. 1986 Rheology of apple and potato tissue as affected by cell turgor pressure J. Texture Stud. 17 291 313 Motomura, Y. Takahashi
Jaroslav Ďurkovič, František Kačík, Miroslava Mamoňová, Monika Kardošová, Roman Longauer, and Jana Krajňáková
.J. Doherty, W.O.S. 2013 Thermophysical properties and rheology of PHB/lignin blends Ind. Crops Prod. 50 270 275 Nelson, M.L. O’Connor, R.T. 1964a Relation of certain infrared bands to cellulose crystallinity and crystal lattice type. Part I. Spectra of
Kareen Stanich, Margaret Cliff, and Cheryl Hampson
. 2007 An analysis of elastic and plastic fruit growth of mango in response to various assimilate supplies Tree Physiol. 27 219 230 Lin, T.-T. Pitt, R.E. 1986 Rheology of apple and potato tissue as affected by cell turgor pressure J. Texture Stud. 17 291