MCQ Detailed View

The sour taste of lemons is primarily due to citric acid, a naturally occurring organic acid found in citrus fruits. Citric acid is a weak acid that contributes not only to the flavor but also to the preservation and chemical properties of fruits. In lemons, citric acid can make up to 5–8% of the fruit’s juice, which is why the taste is distinctly sour compared to other fruits.

Citric acid belongs to a class of compounds called carboxylic acids, which are characterized by the presence of one or more carboxyl groups (-COOH). These acids release hydrogen ions (H⁺) when dissolved in water, which is responsible for the sour or acidic taste detected by our taste buds. The high concentration of citric acid in lemons triggers sour taste receptors on the tongue, giving the fruit its signature tangy flavor.

Other acids like tartaric acid and acetic acid are present in some fruits or foods but are not the main contributor to lemon’s sourness. Tartaric acid is common in grapes, while acetic acid is the main acid in vinegar. Hydrochloric acid is a strong mineral acid and is not naturally present in fruits.

Beyond taste, citric acid plays important roles in plant metabolism. It is a key intermediate in the citric acid cycle (Krebs cycle), which is fundamental for energy production in plant cells. It also helps regulate the pH of fruit juice, enhancing preservation and preventing microbial growth.

Understanding why lemons are sour gives insight into plant chemistry, organic acids, and how fruits develop distinct flavors. This knowledge is part of everyday biology and helps students grasp the chemical nature of food and its interactions with human senses.