Kojic acid is a by-product of the fermentation process of certain types of fungi. It's a popular ingredient in skincare products, particularly those designed to lighten skin and reduce the appearance of dark spots.

At a molecular level, kojic acid works by inhibiting the function of tyrosinase, the enzyme responsible for the production of melanin. Melanin is the pigment that gives skin, hair, and eyes their color. By inhibiting tyrosinase, kojic acid can prevent and reduce hyperpigmentation and dark spots.

Serums containing kojic acid are particularly effective because serums are designed to deliver a high concentration of active ingredients to the skin. They're typically lightweight and easily absorbed, allowing the kojic acid to penetrate deeply into the skin where it can exert its effects.

However, it's important to note that while kojic acid is effective, it can also be irritating to some people, particularly those with sensitive skin. It's always a good idea to do a patch test before using a new product containing kojic acid. And, as with any skincare product, it's important to use it as directed to avoid potential side effects.

In terms of potency, a concentration of 1-4% kojic acid is generally considered safe and effective for over-the-counter products. Higher concentrations may be used in professional treatments under the supervision of a dermatologist.

It's also worth noting that kojic acid can degrade and lose its effectiveness when exposed to air and light. Therefore, it's important to look for products that are packaged in opaque, air-tight containers to ensure the stability and effectiveness of the kojic acid.

In conclusion, kojic acid is a potent ingredient for reducing dark spots and hyperpigmentation. However, as with any active ingredient, it's important to use it responsibly and under the guidance of a skincare professional.