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Three German researchers have shed significant light on the process of fruit cracking in cherries. While researchers have known for many years the presence of too much water — either within cherries or cherry trees — results in cracking, little was known
about the exact mechanism causing it until a few months ago.

Moritz Knoche, Max Ossenbrink and Andreas Winkler of the Institute for Horticultural Production Systems at Leibniz-University, Hannover, believe they have identified a crucial link in the chain reaction leading to cracking.

The culprit is malic acid, a naturally occurring compound imparting tartness to such fruit and vegetables as apples, rhubarb, grapes and cherries.

What led them to this conclusion?

“We observe a surprising and dramatic increase in cracking when sweet cherry fruit are brought into direct contact with the expressed juice of sweet cherries,” they wrote in an article published in the July 2015 issue of the Journal of the American Society for Horticultural Science.

Sweet cherry juice caused rapid fruit cracking when water intake was limited.

Cracking also occurred when cherries were placed in an artificial juice made of the majority of compounds normally passing through its cell walls as the fruit matures.

Proof came when cracking occurred at a very high rate when cherries were placed in a solution made only of malic acid.
Trail of evidence

In 2014, Knoche found what he called “a surprisingly low turgor in Stage III sweet cherries.” Turgor refers to the amount of pressure pushing against the plasma membrane in plant cells. Stage III is the final ripening period before harvest.

Knoche, the article’s lead author, likened cell turgor in immature sweet cherries to that of immature grapes, another fruit prone to rain cracking.

Research has shown low turgor in grapes results from a balance of pressure created by the flow of water in the spaces between grape cells (called apoplastic flow) versus the flow that passes through cell walls to adjacent cells (symplastic flow).

Other research has revealed that as fruit matures, cell wall strength weakens. In later maturation stages, it virtually disappears, and the two flows combine to cause outward pressure on the fruit’s skin, leading to cracking.

Though well documented in grapes, this process was not yet proven to occur in cherries. Knoche thinks the similarity between the two fruits’ structures, as well as the occurrence of cracking in both species, means it very likely does.

Some of Knoche’s research has already shown excessive moisture uptake by sweet cherries causes very small cracks in cell walls.

He has also observed significant tissue crushing around the pit in many cherry varieties at maturity.

Knoche and his team think these findings point to significant cell structure degradation when cherries reach maturity. So, he wondered, what are the consequences of leakage that occurs when sweet cherry cell walls can’t contain cell moisture?

Trial setup

The scientists randomly selected Leibnitz University-grown Sam and Adriana sweet cherries as well as Bings procured from a local market. They selected fruit based on uniformity of size and color and an absence of visual defects.

To assure there was no water uptake, they cut the fruits’ stems about one-fifth of an inch above the fruit and sealed them with a rubber coating.

They then put the cherries into cold storage overnight at 2 degrees Celsius (35.6 degrees Fahrenheit), removed them from storage the following day and allowed them to reach room temperature before proceeding.

Initially, they set about determining which of the materials that are known to pass through cherry cell walls as they mature appeared when cracking occurred. Using cherries in water as controls, they compared cracking and water uptake rates of cherries in real cherry juice and an artificial cherry juice made of 98 percent of the compounds known to pass through cherry cell walls.

Next, they did the same thing with cherries in solutions comprising the individual components of the artificial juice and water controls.

It was within this set of experiments that Knoche’s team identified malic acid as the compound with which fruit was markedly more susceptible to cracking.

The team observed that fruit cracking was most severe at low pH levels and decreased as the pH increased and when malic acid was present in treatment solution in higher concentrations.

Suspected mechanism

Scientists have known for years that exposing cells to too much water causes them to burst.

If cherry cell walls tend to be weaker when fruit is closer to maturity, the presence of too much water would certainly cause some cells to burst. This, in turn, releases malic acid into the space between the cells, weakening even more cell walls and creating a domino effect within the fruit, which ultimately results in cracking.

Perhaps now, with a greater understanding of how cherry cracking occurs, it will be easier to create solutions to significantly decrease its incidence. •

– by Dave Weinstock