What is citric acid? You are probably familiar with the fact that citric acid is naturally found in citrus fruits like oranges, lemons, grapefruits, and limes, among others. Citric acid is neither a vitamin nor a mineral. It does not have dietary importance and should not be confused with vitamin C or ascorbic acid, which is also found in citrus fruits.
Citric acid is classified as a weak organic acid because it partially dissociates, or separates, when dissolved in a solvent, particularly water. The strength of an acid is measured not necessarily in terms of pH but on its dissociation in a solvent.
Its concentration in citrus fruits varies depending on the species. One indicator of its concentration in fruits is the level or intensity of sour taste. For instance, lemon is very sour because it has a citric acid concentration of about 5-6% with a pH level of around 2.2.
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Is Citric Acid Produced on an Industrial Scale?
Approximately two million tons of citric acid are mass-produced annually. It is mainly used in the food industry as a flavouring and chelating agent. The process of chelation – bonding ions and molecules into metal ions – is useful in the food industry in terms of nutritional additives.
Citric acid along with its derivatives are also very useful in the pharmaceutical industry because of its antioxidant properties. It is used to maintain the stability of active ingredients in some medicines. It is also used as an anticoagulant by chelating the calcium in the blood.
Citric Acid Formula
Just like other acids, citric acid is defined by its chemical formula, its average pH level for a given concentration, and its dissociation constant. The molecular formula of citric acid can be written as:
C6H8O7
It has six atoms of carbon, eight atoms of hydrogen, and seven atoms of oxygen with a total molar mass of 192.12 g/mol. Citric acid is classified as an alpha-hydroxy acid with a three-carbon skeleton. It has a hydroxyl group (OH) and three carboxylic acid groups (COOH). Structurally, three of its oxygen atoms are double-bonded with carbon atoms while four of its oxygen atoms are bonded with hydrogen atoms.
What is Citric Acid Used for?
Citric acid, whether in its natural form or in its synthetic form, has several practical uses, particularly in:
- Food and drink: Citric acid is used both as a flavouring and a preservative in a wide range of food and beverages, especially soft drinks and sweet treats. It is also used as an emulsifying agent for ice cream and sold in powder form ‘sour salt’. It is used as an alternative to vinegar or lemon juice in many culinary recipes.
- Cleaning as a chelating agent: You can use the natural citric acid from citrus fruits, particularly from lemon juice, as a cleaning agent for ceramic tiles. In fact, many bathroom and kitchen cleaning solutions have this acid as one of their active ingredients. It is excellent for removing metallic stains because it acts as a chelating agent, binding metallic molecules into a soluble form. It can be used to remove limescale, and its chelating effect makes it useful in treating hard water.
- Cosmetics and dietary supplements: Many cosmetic products such as creams, gels, and liquids use citric acid as an acidulant. The acid helps preserve the stability of enzymes in many cosmetic products. It is also used in dietary supplements as a processing aid.
Is Citric Acid a Strong Acid?
Citric acid is classified as a weak acid because the hydroxyl and the carboxylic acid groups only partially dissociate when dissolved in water. The strength of an acid is computed as a disassociation constant, which is the ratio between the dissociated ions and the non-dissociated molecules.
Ka = [H+] [A–]/HA
When dissolved in water, citric acid dissociates in three steps with corresponding dissociation constants:
C6H8O7(aq)+H2O(l)⇌C6H7O−7(aq)+H+3O(aq) (1)
C6H7O−7(aq)+H2O(l)⇌C6H6O2−7(aq)+H+3O(aq) (2)
C6H6O2−7(aq)+H2O(l)⇌C6H5O3−7(aq)+H+3O(aq) (3)
For each of the three steps, the disassociation constant also varies. The specific constants computed for each step are the following:
Step (1): Ka1=7.5 x 10−4
Step (2): Ka2=1.7 x 10−5
Step (3): Ka3=4.0 x 10−7
As you can see, the dissociation constant becomes weaker as the acid solution reaches its final equilibrium. This makes it a very useful component in a buffer solution. It is useful for a pH range of 2.1–7.4.
Is Citric Acid Natural?
Citric acid is actually an intermediate byproduct in the biochemistry of the citric acid cycle, which occurs in all aerobic organisms. Aerobic organisms are those that need oxygen to release energy from food. Hence, they undergo aerobic respiration.
Technically, plants, including citrus fruits, do not need molecular oxygen to “breathe” as animals do. The process may seem reversed because plants take in carbon dioxide and release oxygen as a byproduct.
However, it is not as simple as it seems because plants also use oxygen to release energy from the food (usually in the form of sugar) that they make. This stage is called the citric acid cycle, also known as the Krebs cycle.
In the Krebs cycle, stored energy in food is released through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The acetyl-CoA is then converted into ATP and carbon dioxide. Along the way, citric acid is produced as an intermediate biochemical byproduct.
Is Citric Acid in Food Harmful?
Whether naturally produced or synthetically produced, citric acid is safe as a food additive as it is naturally a component of citrus fruits. Its wide range of use as a food preservative, flavouring, and chelating agent makes it a safe and necessary ingredient of many culinary recipes.
Understanding what citric acid means to the food and beverage industry will make you appreciate the industrial-level use of the acid in food processing. Some food products would have a very short shelf life without it. However, in powder form, this substance has a slight fire hazard risk. You should be careful in the kitchen when using powder citric acid. Do not expose it to direct flame as it may burst.
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