Why is there acid in lemons?

The sour taste of lemons and other citrus fruits is primarily due to the presence of citric acid. Citric acid is a colorless, crystalline organic acid with a sour taste and a chemical formula of C6H8O7. It is the most prevalent acid found in citrus fruits and is also present in other fruits and vegetables, including oranges, grapefruits, limes, pineapples, strawberries, and tomatoes.

1. Biochemical Pathways

In citrus fruits, including lemons, citric acid is produced through a series of biochemical processes that occur in the fruit during its development. These processes involve various enzymatic reactions and the metabolism of carbohydrates, particularly glucose and sucrose.

2. Krebs Cycle Intermediates

During the breakdown of carbohydrates, several intermediate compounds are formed, which eventually enter the citric acid cycle, also known as the Krebs cycle. The citric acid cycle is a central metabolic pathway in living organisms that generates energy (ATP) and various precursors for cellular synthesis.

3. Citrate Accumulation

As the citric acid cycle operates in the cells of citrus fruits, there is a build-up and accumulation of citrate, which is a salt or ester of citric acid. Citrate is further converted into citric acid through the enzyme aconitase.

4. Vacuolar Storage

Citric acid and citrate are then transported and accumulated in the vacuoles of the cells, particularly in the juice sacs and pulp of the fruit. Vacuoles act as storage compartments for various compounds and contribute to the overall flavor, acidity, and nutrient content of the fruit.

5. Sour Taste

The high concentration of citric acid in lemons gives them their distinctive sour taste. The acidity of lemons is what makes them useful in various culinary applications, including adding a tangy flavor to dishes, preserving food, and making beverages such as lemonade.