Determination of Acidic Substances in Food

1、 The concept of acidity

1. Total acidity

Total acidity refers to the total amount of acidic components in food, including the concentration of dissociated acid and undissociated acid. Due to the use of standard alkaline solution for titration, total acidity, also known as titratable acidity, is generally expressed by the mass fraction of the main representative acids in the sample (such as citric acid, malic acid, tartaric acid, etc.).

2. Effective acidity

Effective acidity refers to the concentration of free hydrogen ions in the sample, which is the concentration of H+in the measured solution (more accurately, it should be activity). It reflects the concentration of dissociated acid and is commonly expressed as pH.

3. Volatile acidity

Volatile acidity refers to the volatile organic acids in food, such as low carbon straight chain fatty acids such as acetic acid, formic acid, and butyric acid.

4. Milk acidity

Milk has two types of acidity, namely external acidity and true acidity.

External acidity, also known as intrinsic acidity (latent acidity), refers to the acidity inherent in freshly squeezed milk, which is caused by factors such as phosphoric acid, casein, albumin, citric acid, and CO2. The external acidity accounts for 0.15% to 0.18% (calculated as lactic acid) in sour milk.

True acidity, also known as fermentation acidity, refers to the portion of acidity that increases during the placement of milk due to the fermentation of lactose by lactic acid bacteria. If the acidity of milk exceeds 0.15% to 0.20%, it indicates the presence of lactic acid. Therefore, it is customary to call milk with an acidity below 0.20% fresh milk. If it reaches 0.3%, it will have a sour taste, and if it reaches 0.6%, it will solidify.

The acidity of milk is generally expressed as the volume (mL) of 0.1000mol/L NaOH consumed to titrate 100mL of milk sample, known as T, which is the titration acidity, abbreviated as acidity. It can also be expressed as the mass fraction of lactic acid, using the same method as calculating total acidity. Lactic acid is used to represent the acidity of milk.

2、 Types and functions of acidic substances in food

The sour components in food are mainly organic and inorganic acids. Most organic acids have a refreshing sour taste, while inorganic acids generally have an unpleasant bitter taste, so people often do not use inorganic acids as food acidifiers.

Organic acids in food exist in both free and acidic salt states. Common organic acids include citric acid, malic acid, tartaric acid, oxalic acid, succinic acid, lactic acid, and acetic acid. Some of these organic acids are natural ingredients inherent in food, such as citric acid and malic acid in fruits, and lactic acid in fish and meat; Some are artificially added in food processing, such as citric acid in formulated beverages; Some are also produced during the production, processing, and storage processes, such as lactic acid in yogurt.

The role of acidic substances in food is reflected in multiple aspects. One is that it affects the odor and taste of food, such as organic acids with strong fruit aromas that can stimulate appetite. Secondly, it affects the color of food. The color tone of pigments contained in fruits and vegetables is closely related to their acidity. For example, reducing pH during fruit processing can inhibit enzymatic browning of fruits, thereby maintaining their natural color. Thirdly, it affects the stability of food, for example, organic acids can prevent the oxidation of vitamin C and improve its stability.