Synthesis and Commercial Preparation of Food Emulsifiers

Food emulsifiers, more correctly referred to as surfactants, are molecules, which contain a nonpolar, and one or more polar regions. In general, nonpolar groups are aliphatic, alicyclic, or aromatic hydrocarbons. Polar functional groups contain heteroatoms such as oxygen, nitrogen, and sulfur. As shown in Fig. 2.1, the polar functionality makes the emulsifier anionic, cationic, amphoteric, or nonionic. Anionic surfactants contain a negative charge on the bulky molecule, associated with a small positive counterion. Cationics have a positively charged molecule with a negative counterion. Amphoteric surfactants contain both positive and negative charges on the same molecule. A nonionic surfactant contains no formal positive or negative charge, but a polar heteroatom produces a dipole with an electron dense and electron-depleted region. Many food products use emulsifying agents present in the foods themselves. For example, casein and egg yolk proteins are excellent emulsifiers. Alanine, phenylalanine, leucine and isoleucine contain nonpolar aliphatic and aromatic side chains. Amino acids, such as arginine, lysine and tryptophane, contain amino groups, which promote cationic character to the protein. Aspartic and glutamic acids possess side chains with carboxyl groups, which contribute to anionic character. The nature, number and location of the polar amino acids determine the isoelectric point of a protein; e.g., the pH at which the protein is uncharged. In food systems where the pH is above the isoelectric point, the protein will behave as an anionic emulsifiers, while at pH values below their isoelectric point, it will become cationic. One complicating factor in using emulsifiers is that their charge makes them vulnerable to interactions with other charged species, such as calcium ions and some gums. In addition, proteins may denature under some processing conditions, such as high temperature and shear forces. Phospholipids from egg and soy have found many applications in food products. Structurally, these molecules contain two fatty acids esterified to glycerol and a phosphatidyl group esterified to a terminal −OH group on the glycerol. Phosphatidylcholine (PC), phosphatidylethanolamine (PE), Phosphatidylinositol (PI), and phosphatidylserine (PS) are the predominant polar functional groups.