Which of the following is often used in the formation of emulsions?

Lecithin is a phospholipid that plays a crucial role in the formation of emulsions due to its amphiphilic nature, which means it has both hydrophilic (water-attracting) and hydrophobic (water-repelling) properties. This characteristic enables lecithin to stabilize mixtures of oil and water by reducing surface tension at the interface between the two phases. In emulsions, lecithin molecules align themselves at the oil-water interface, with the hydrophilic heads attracting water and the hydrophobic tails interacting with oil, thereby helping to keep the emulsion stable.

Other substances listed, while they have their own unique functions in food systems, do not serve as effective emulsifiers. For instance, cholesterol can influence membrane fluidity and stability in biological systems but does not specifically stabilize emulsions. Casein is a protein found in milk and can contribute to emulsion stability to some extent, but it is more commonly recognized for its role in the formation of foams and gels rather than as a primary emulsifier. Rennin, an enzyme used to curdle milk, is not used in emulsion formation. Therefore, lecithin stands out as the key ingredient most explicitly associated with the stabilization of emulsions.