Lactic Acid

1.0What is Lactic Acid?

Lactic acid is an organic acid widely found in nature, playing a crucial role in biological processes and industrial applications. It is chemically classified as an alpha-hydroxy acid (AHA) because it has a hydroxyl group (−OH) attached to the carbon atom adjacent to the carboxyl group (−COOH).

This compound is naturally produced in the human body during intense exercise and is also the key ingredient responsible for the sour taste in fermented foods like yogurt and sourdough bread.

• IUPAC Name: 2-Hydroxypropanoic acid
• Common Name: Milk acid or Lactic acid
• Chemical Formula: C₃​H₆​O₃​

2.0Chemical Structure and Formula

The lactic acid molecule consists of a three-carbon backbone. Its structure makes it a chiral molecule, meaning it exists in two optically active forms (enantiomers): L-(+)-Lactic acid (found in muscles) and D-(-)-Lactic acid (produced by certain bacteria).

Structural Analysis

• Functional Groups: It contains two functional groups—a carboxyl group (−COOH) and a hydroxyl group (−OH).
• Asymmetry: The central carbon atom (C-2) is chiral because it is bonded to four different groups: a hydrogen atom, a methyl group (−CH₃​), a hydroxyl group, and a carboxyl group.

Structural Formula Representation:

3.0Production of Lactic Acid

Lactic acid is primarily produced through two methods: natural fermentation and synthetic preparation.

1. Natural Fermentation (Biological Method)

This is the most common commercial method. Bacteria such as Lactobacillus ferment carbohydrates (sugars) like glucose, sucrose, or lactose under anaerobic conditions (absence of oxygen).

General Reaction:

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\stackrel{\text{ Bacteria }}{\to }2{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3+\text{ Energy (ATP) }$

This process is vital in the food industry for making yogurt, cheese, and pickles.

2. In Human Muscles (Anaerobic Respiration)

During strenuous physical activity, when the body demands energy faster than oxygen can be supplied, muscle cells switch to anaerobic respiration. Glycogen is broken down into glucose, which is then converted into lactic acid to release energy.

Mechanism:

$\text{ Glucose }\text{ → }\text{ Pyruvate }\stackrel{\text{ Lactate Dehydrogenase }}{\to }\text{ Lactic Acid }$

3. Synthetic Method

Industrially, it can be synthesized by the hydrolysis of lactonitrile, which is obtained by reacting acetaldehyde with hydrogen cyanide.

4.0Physical Properties

• State: Pure lactic acid is a white, crystalline solid. However, it is most commonly available as a colorless to yellowish syrupy liquid (aqueous solution).
• Solubility: It is highly soluble in water and ethanol due to its ability to form hydrogen bonds.
• Melting Point: Pure crystals melt at 53∘C (127∘F), while the commercial liquid form varies based on concentration.
• Acidity: It is a weak acid with a pKa of approximately 3.86, meaning it partially dissociates in water.

5.0Types of Lactic Acid

There are two optical isomers of lactic acid:

1. L-lactic acid (Levo) – Produced naturally in animals and humans during metabolism.
2. D-lactic acid (Dextro) – Found mainly in some bacterial fermentation processes.

Both forms are chemically similar but differ in molecular orientation and biological behavior.

6.0Uses of Lactic Acid

Lactic acid is a versatile chemical used in skincare, food processing, and pharmaceuticals.

1. Skincare and Cosmetics (Lactic Acid for Skin)

Lactic acid is a popular Alpha Hydroxy Acid (AHA) used in serums, lotions, and chemical peels.

• Exfoliation: It gently removes dead skin cells from the surface, promoting a smoother complexion and brighter skin tone.
• Hydration: Unlike other acids, it is a humectant, meaning it attracts moisture to the skin, making it ideal for dry and sensitive skin types.
• Anti-aging: It stimulates collagen production, helping to reduce fine lines and wrinkles.

2. Food Industry

• Preservative: Its acidic nature lowers the pH of food, preventing the growth of spoilage bacteria (e.g., in pickles and olives).
• Flavoring Agent: It provides the characteristic tangy or sour taste in yoghurt, cheese, sourdough bread, and sour beers.
• Curdling Agent: It is used to coagulate milk proteins (casein) during cheese production.

3. Industrial and Medical Applications

• Biodegradable Plastics: It is polymerized to form Polylactic Acid (PLA), a sustainable, biodegradable plastic used in packaging and 3D printing.
• Pharmaceuticals: It is used in intravenous fluids (Lactated Ringer's solution) to treat blood loss and restore fluid balance.

4. Lactic Acid in Food Industry

• Lactic acid is widely used in the food industry as a preservative, flavoring agent, and acidity regulator. 
• It helps in fermentation processes like yogurt and kimchi making. Its antimicrobial properties prevent food spoilage, enhancing shelf life naturally.

5. Lactic Acid in Healthcare

• In medicine and skincare, lactic acid acts as an exfoliant, promoting new cell growth. 
• It’s also used in intravenous fluids to treat dehydration and metabolic acidosis. 

7.0Lactic Acid in Muscles: Myth vs. Reality

For decades, lactic acid was blamed for Delayed Onset Muscle Soreness (DOMS)—the pain felt days after a workout. However, modern science has corrected this view.

• The Fuel Source: Lactic acid (specifically lactate) is not a waste product but a fuel source. The liver recycles it back into glucose through a process called the Cori Cycle.
• The Burn: The "burning" sensation during exercise is caused by the accumulation of hydrogen ions (H+), which increases acidity, not the lactate itself.
• Recovery: The body clears lactic acid from the muscles within an hour after exercise. It does not stay in the muscles to cause soreness days later

8.0Lactic Acid Bacteria

Lactic acid bacteria are beneficial microorganisms that convert sugars into lactic acid.

Examples

• Lactobacillus
• Streptococcus
• Leuconostoc

Importance

• Improve digestion
• Enhance food preservation
• Used as probiotics

9.0Importance of Lactic Acid in Daily Life

Lactic Acid in Food Industry: Lactic acid is widely used in the food industry as a preservative, flavoring agent, and acidity regulator. It helps in fermentation processes like yogurt and kimchi making. Its antimicrobial properties prevent food spoilage, enhancing shelf life naturally.

Lactic Acid in Healthcare: In medicine and skincare, lactic acid acts as an exfoliant, promoting new cell growth. It’s also used in intravenous fluids to treat dehydration and metabolic acidosis.