There is something interesting about the sharp and face-puckering sour taste of all citrus fruits. It doesn’t matter which is your favorite type because all of them have something to offer, including oodles of vitamins and essential nutrients. These juicy and succulent fruits can be squeezed to make juice or eaten whole by simply peeling the skin. Citrus fruits can also be used to prepare a variety of dishes such as salads, marmalade, jams, pies, and others. Some of the known citrus fruits are orange, grapefruit, clementine, lemon, pomelo, tangelo, and kumquat.

Ever wondered why these fruits are sour in taste? Well, scientists have now found out the science behind the sour taste of these fruits. A team of researchers from the University of California, Riverside, discovered that the presence of a single chemical element called hydrogen in the fruit pulp is a big factor responsible for the sourness and sweetness of fruits. It was found that citrus fruits have a greater amount of hydrogen ions, which give the fruits a lower pH and a tangy taste. On the other hand, the pulp of sweet fruits contains fewer hydrogen ions and tastes less acidic. The study was published in the open access scientific journal, Nature Communications.

Ronald Koes, the lead author of the study and his team went to find out how some citrus fruits yield more acidic juice than others. This interest arose from a previously published a study which revealed that higher acidity in purple petunia flowers led to more petal pigmentation. Curious about the Faris variety of lemon tree, which grows sweet and sour fruits as well as white and purple-tinged flowers, the scientists consulted UCR plant scientists, Mikeal Roose and Claire Federici. By leveraging the university’s huge Citrus Variety Collection, which stores more than 1,000 living citrus and related fruit varieties, Roose and Federici opted for the Faris lemon and 20 other citrus fruits with taste profiles varying from wincingly sour to sugary sweet for the study.

The Koes’ team analyzed the gene expressions related to those controlling acidity in petunias and recognized two citrus genes namely, CitPH1 and CitPH5. The identified genes are more pronounced in sour varieties and less prominent in sweet-tasting varieties. The CitPH1 and CitPH5 genes encode transporter proteins that push hydrogen ions into the vacuole, a large space inside juice cells, thereby increasing the level of their acidity.

The scientists then began focusing on genes that control the levels of CitPH1 and CitPH5 in juice cells. While down-regulation of CitPH1 and CitPH5 in sweeter tasting varieties occurred numerous times in several varieties, they discovered that mutations in genes for a number of transcription factors (proteins that are responsible for turning certain genes on and off) caused the reduced content of CitPH1 and CitPH5, leading to a sweeter taste.

According to Roose, the discoveries could assist breeders to create better-tasting citrus fruits. He also said that breeding varieties with severe mutations in the transcription factors such as those in the “acidless” citrus would be overkill, producing sugary citrus fruits. He said that plant scientists should focus on targeting mutations that have a less dramatic effect on the production and activity of transporter proteins.