Research on citrus fruit provides insight into causes of tartness

While citrus fruits are generally known for their tartness and acidity, it was not known why this was until UCR researchers recently discovered the genetic cause for the flavor profile of fruits like lemons and limes.

A team of researchers including Dr. Mikeal Roose, a professor of genetics in the department of botany and plant sciences, and staff research associate Claire Federici, published their research this past February in the scientific journal Nature Communications. By testing the flesh of different varieties of citrus fruits, they found that certain genes, namely CitPH1 and CitPH5, were expressed more prominently in the genetic makeup of the tarter varieties of the fruits than in the sweeter, less acidic varieties.

Our perception of flavor is influenced by the balance between sugars and acid, mainly citric acid,” said Dr. Roose in an email to the Highlander. “As citrus fruit ripen, the acidity typically decreases more rapidly than sugars increase so the amount of acid that accumulates and how quickly it decreases are major determinants of fruit maturity.”

It turns out that hydrogen ions play an important role in the development of acidity in these fruits. The level of these hydrogen ions found in the pulp of tart fruits compared to sweeter ones suggest that they are responsible for the low pH levels (acidity) that account for the sour flavor. The mechanisms through which these hydrogen ion levels occurred in plants remained a mystery to scientists until the UCR researchers observed the genetic processes of petunias. These observations revealed that the genes CitPH1 and CitPH5 played a larger role than just aiding in the coloration of plants, which was the previous understanding of their function.

This research could have a positive impact on both citrus producers as well as consumers. According to Dr. Roose, “We need to have citrus varieties that ripen at different times of the year. This benefits consumers in having fresh fruit available that are closer to peak maturity. It also benefits producers and workers by extending the citrus harvest season.” Dr. Roose further elaborated on the potential benefits of this research, stating that “understanding the mechanism that leads to acid accumulation will help us develop varieties with excellent flavor at different times.”

Beyond the capacity to grow sweeter tasting citrus fruit varieties, the researchers hope that their work can be built upon. Dr. Roose said that “We hope that an eventual benefit will be development of new citrus varieties with better taste that encourage consumption of more citrus fruit. We hope to couple this with fruit having enhanced levels of health-beneficial chemicals.”

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