Question

Many people use lemon when eating fish. This custom is a carryover from the days when it was difficult to preserve fish, and the lemon acted to diminish the unpleasant odor (if not the decomposition). Lemons contain 5-8\% citric acid, and this acidity contributes to their sour taste. Explain why lemon juice is effective at reducing the odor of fish.

Short answer

Lemon juice neutralizes the basic amines in fish, forming non-volatile salts and reducing the odor.

Step-by-step solution

Understand the Fish Odor

Fish odor is primarily caused by amines, such as trimethylamine, which are basic compounds produced during the decomposition of fish proteins.

Know the Nature of Citric Acid

Lemon juice contains citric acid, which has a pH level of 2-3, making it a weak acid. This means it can neutralize basic compounds.

Neutralization Reaction

When lemon juice is added to fish, the citric acid in the lemon reacts with the amines, neutralizing them and forming non-volatile salts which do not have a strong odor.

Impact on Odor

The neutralization reaction reduces or eliminates the fishy odor because the amines that cause the smell are converted into salts that are not fragrant.

Key concepts

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Citric acid, found abundantly in lemon juice, is an important player in various chemical processes, including odor neutralization. It is a weak organic acid, giving lemons their characteristic sour taste. Citric acid has a pH level typically ranging from 2 to 3, which is quite acidic.
This acidity allows citric acid to participate effectively in neutralization reactions. When you squeeze lemon juice onto fish, the citric acid can neutralize the basic compounds responsible for the fishy odor. This transformation is not only fascinating but also practical in everyday life.

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A neutralization reaction occurs when an acid and a base react to form water and a salt. In the context of fish odor, the bases are usually amines such as trimethylamine, which are produced during the decomposition of fish proteins.
When lemon juice, which contains citric acid, is added to fish, a neutralization reaction takes place. The citric acid reacts with the amines (basic compounds), reducing their basicity and forming non-volatile salts. These salts do not have a strong odor, thus diminishing the fishy smell.

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Amines are organic compounds and derivatives of ammonia. They are responsible for the fishy odor of decomposed fish due to the breakdown of proteins. Examples include trimethylamine, which is commonly associated with the characteristic fishy smell.
During the process of decomposition, proteins in the fish break down into simpler compounds, including amines. These amines are basic in nature and react strongly with acids. When neutralized by citric acid, these basic compounds form salts that are less volatile and odorous.

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The pH level measures the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 being neutral. Values below 7 are considered acidic, and those above 7 are basic. Citric acid in lemon juice typically has a pH level between 2 and 3, making it quite acidic.
The acidity of citric acid is effective in neutralizing basic compounds like amines. When these bases encounter the acidic environment, their basic nature is neutralized, which significantly reduces the unpleasant odor.

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The decomposition of proteins is a natural process where proteins break down into smaller compounds, including amines. This process usually results in the release of gases and compounds that can have strong, unpleasant odors.
In fish, this decomposition starts to occur soon after the fish dies. Amines such as trimethylamine are formed, which are responsible for the fishy smell. By applying lemon juice, the citric acid can interact with these amines, neutralizing their effects and thus reducing the odor.