Emulsifiers make possible to form and/or maintain homogenous mixtures of two or more immiscible phases such as oil and water in food preparations.
An emulsion is a system that contains two immiscible phases: the dispersed phase (present in drops) and the continuous phase (the matrix wherein the drops are dispersed).
Abbreviations are frequently used to describe the type of emulsion:
- • O / W (oil in water) such as milk, mayonnaise, salad dressings, ice cream, or
- • W / O (water in oil) such as butter or margarine
Emulsions are normally unstable in food preparations. Stabilization of the mixture can be achieved either by physical processes—for example, by high-pressure homogenization (a consolidated practice for milk and cream) or by adding emulsifiers with surfactant properties.
Emulsifiers are molecules composed of a nonpolar group (aliphatic groups, alicyclic, or aromatic) and a polar group (composed of atoms such as oxygen, nitrogen, and sulfur). According to the type of the polar group, they can be distinguished as:
- • Anionic emulsifiers contain a polar group with negative charge—for example, soaps, amine soaps, sulfates of higher alcohols, sulfur
- • Cationic emulsifiers contain a polar group with positive charge, composed up almost entirely of quaternary ammonium salts.
- • Amphoteric emulsifiers contain polar groups with either negative or positive charge. They behave as anionic or cationic, depending on the pH of the solution in which are contained—for example, lecithin.
- • Nonionic emulsifiers are molecules that do not have charged groups. The hydrophilic part is composed of neutral molecules soluble in water. They have a wide use thanks to their insensitivity to changes of pH and to the presence of electrolytes. In this group are included the nonionic emulsifiers:
- - lipophilic—alcohol ethoxylates (mono and diglycerides of fatty acids), sorbitan esters (SPAN®)
- - hydrophilic—esters of polyethylene glycols (PEG), polysorbates (TWEEN)
- • Other surfactants cannot be divided according to their polar group are blocks of copolymers, surfactants derived from carbohydrates (alkyl polyglucosides).
Based on the chemical structure of emulsifiers, it is possible to create:
an emulsion oil + water
a foam gas + liquid
a suspension liquid + solid
an aerosol liquid/solid + gas
Emulsifiers added to food interact with the following:
- • Fats: for example, reducing the viscosity in chocolate and increasing the aeration of the cream
- • Starch: for example, delaying the hardening of the bread
- • Gluten: for example, improving the quality of flour baking and giving better structure and volume to baked goods
Emulsifiers allow freezing with a more homogeneous structure in the industrial ice cream that thaws more gradually. In addition to the emulsifying properties, they can also have other properties such as foaming, anti-foaming, wetting, and serving as bacteriostatic agents.
Choosing the right emulsifier for any food is not trivial, because it must protect texture and keep the appearance impeccable over a long shelf-life. The method applied by Griffin (ATLAS POWDER COMPANY) in 1949 can help in this choice. Griffin laid down the method for the calculation of HLB (Hydrophilic / Lipophilic Balance). The HLB values can predict the behavior of an emulsifier, so they could be used as W/O emulsifiers, wetting agents, O/W emulsifiers, detergents, or solubilizing.
Clearly, these numbers are not precise, but combined with the experience of the food technologist they can be a useful help to narrow the field of possible emulsifiers used.
Emulsifiers are used to replace part of the fat with water, like in margarines and reduced-fat butter.
An example for this application is represented by the products denominated SIMPLESSE® (introduced on the market by 1988 da CP Kelco). These products are obtained with the process of microparticulation, or reduction in particle size of diameter of 0.1 to 2 pm, of noble proteins such as casein and whey proteins and soy. These products have the same characteristics of spreadability as butter has, but they have low calorie content. SIMPLESSE has been affirmed as GRAS by the FDA for use in several products. The main limitation, which is the limitation for most of the currently approved fat substitutes, is that it cannot be used for heated product preparations, and particularly not for frying.
Another example of fat replacers are alkyl glycoside fatty acid esters. They are nonionic, nontoxic, odorless, and biodegradable compounds with emulsification properties. Alkyl glycoside fatty acid esters could be used to replace fat (from 5%-95%) in such items as frying oils and Italian salad dressings (Curtice-Burns et al., 1989).