Cherry cracking would, at first blush, seems to be a simple concept. When sweet cherries have more water than their skin can handle, the pressure increases and the skin bursts – almost like a tire of a car. But, not so fast, says a team of German researchers who wants to understand why cherry fruit burst following rains.
“There are many reasons why a cherry could crack. The important thing is to find the right cause of cracking. Cracking is way more complicated than everybody thinks it is. The phenomenon of cracking is still poorly understood,” says Moritz Knoche, a horticulture professor and agricultural plant scientist with Leibniz University in Hannover, Germany.
Knoche, along with Martin Brüggenwirth, a PhD student at Leibniz University, assessed different cherry varieties to see what susceptibility the cultivar has to cherry cracking.
Knoche and Brüggenwirth studied the differences between ‘Regina,’ a variety less susceptible to cracking, and ‘Burlat,’ a more vulnerable variety. What they learned is that cherry cracking is the end result of several “events” — not just one.
Knoche says cherries grow inconsistently between the cuticle and fruit from color change onward – much like a balloon.
“A constant amount of cuticle is distributed over an enlarging surface during stage III (from color change onwards). This means that the sweet cherry fruit cuticle is strained,” he says. “Comparison with other fruit crops reveals that the amount of strain is extreme in cherries — the worst case of all data that I am aware of.”
What exacerbates this ballooning effect is how thin the cuticle of a sweet cherry is — approximately 1 micrometer, or 1,000 times smaller than a millimeter. This, Knoche says, is very, very thin — typically cuticles are thicker.
“Usually fruit cuticles are thicker than those of leaves. Also, the cuticle of a cherry is tremendously strained,” he says. “When peeled off the fruit — we have procedures that can do this very gently — it decreases in area by up to 100% and more. This is like wearing clothing 10 sizes too small.”
Knoche says this thin cuticle cracks on a microscopic scale when exposed to water vapor (high relative humidity) or liquid. This cracking is undetectable by the naked eye. It creates a flaw in what should be a protective barrier, and negatively impacts fruit quality. In addition, cherries contain fairly high concentrations of malic acid.
“When this acid leaks out of the cells into the cell wall space, cells die, their cell walls swell and become mechanically weak,” Knoche says. They’re also learning how the xylem in the fruit changes over the span of a season. This, in turn, may help growers to assess whether irrigation strategies may be helpful in preventing cracking.
To simulate cherry cracking in the lab, Knoche and Brüggenwirth use an elastometer. Knoche says although his research team did not invent the device, they modified procedures to simulate the proper amount of fruit strain. This device subjects isolated fruit skin to biaxial tension, which Knoche says is key. Biaxial tension causes the skin to bulge — much like excess water would cause cherry skin to bulge.
The team measured the stiffness of the skin, the pressure that caused the skin to rupture, and the strain of this rupture.
“The elastometer is capable of detecting differences between (extreme) varieties that markedly differ in susceptibility,” Knoche says. “We also demonstrate that the cell wall properties and the amount of cell wall per unit fruit surface area differed between the two varieties. So this may be a factor contributing to differences in susceptibility.”
Knoche and Brüggenwirth learned the cellular layers form the mechanical backbone of the sweet cherry skin. The cuticle is only responsible for the skin’s permeability and microscopic cracks in the cuticle increase the permeability.
Knoche says this research is promising as researchers learn more about cherry cracking.
“Our research is far from over,” he says.
So far they have not seen close relationships between cuticle thickness or cuticle permeability in different varieties and their cracking susceptibility. However, Knoche says recent research shows some promising leads emerging on physical cell wall characteristics that are critical in determining the mechanical properties of cherries’ skin.
“Unless the exact mechanism in sweet cherry fruit cracking is clarified, the search for a cure of the problem will not be successful,” he says. “Similar to fixing the tube of a bicycle tire, you need to find the hole first before you can apply the patch.”