Food Lipids: Chemistry, Nutrition, and Biotechnology

may be composed of surface-active components of a variety of types, including
small-molecule surfactants, phospholipids, polysaccharides, and proteins. Some of
the ingredients in food emulsions are not located exclusively in one phase but are
distributed between the oil, water, and interfacial phases according to their partition
coefficients. Despite having low concentrations, many of the minor components present
in an emulsion can have a pronounced influence on its bulk physicochemical
properties. Food emulsions may consist of oil droplets dispersed in an aqueous phase
(e.g., mayonnaise, milk, cream, soups), or water droplets dispersed in an oil phase
(e.g., margarine, butter, spreads). The droplets and/or the continuous phase may be
fluid, gelled, crystalline, or glassy. The size of the droplets may vary from less than
a micrometer to a few hundred micrometers, and the droplets themselves may be
more or less polydisperse.
To complicate matters further, the properties of food emulsions are constantly
changing with time because of the action of various chemical, physical, and biological
processes. In addition, during their processing, storage, transport, and handling,
food emulsions are subjected to variations in their temperature (e.g., via sterilization,
cooking, chilling, freezing) and to various mechanical forces (e.g., stirring, mixing,
whipping, flow through pipes, centrifugation high pressure) that alter their physicochemical
properties. Despite the compositional, structural, and dynamic complexity
of food emulsions, considerable progress has been made in understanding the major
factors that determine their bulk physicochemical properties.
III. LIPID-BASED EMULSIFIERS
A. Molecular Characteristics
The most important types of lipid-based emulsifier used in the food industry are
small-molecule surfactants (e.g., Tweens, Spans, and salts of fatty acids) and phospholipids
(e.g., lecithin). The principal role of lipid-based emulsifiers in food emulsions
is to enhance the formation and stability of the product; however, they may
also alter the bulk physicochemical properties by interacting with proteins or polysaccharides,
or by modifying the structure of fat crystals [9]. All lipid-based emulsifiers
are amphiphilic molecules that have a hydrophilic ‘‘head’’ group with a high
affinity for water and lipophilic ‘‘tail’’ group with a high affinity for oil [8,10,11].
These emulsifiers can be represented by the formula RX, where X represents the
hydrophilic head and R the lipophilic tail. Lipid-based emulsifiers differ with respect
to type of head group and tail group. The head group may be anionic, cationic,
zwitterionic, or nonionic. The lipid-based emulsifiers used in the food industry are
mainly nonionic (e.g., monoacylglycerols, sucrose esters, Tweens, and Spans), anionic
(e.g., fatty acids), or zwitterionic (e.g., lecithin). The tail group usually consists
of one or more hydrocarbon chains, having between 10 and 20 carbon atoms per
chain. The chains may be saturated or unsaturated, linear or branched, aliphatic and/
or aromatic. Most lipid-based emulsifiers used in foods have either one or two linear
aliphatic chains, which may be saturated or unsaturated. Each type of emulsifier has
unique functional properties that depend on its chemical structure.
Lipid-based emulsifiers aggregate spontaneously in solution to form a variety
of thermodynamically stable structures known as association colloids (e.g., micelles,
bilayers, vesicles, reversed micelles) (Fig. 2). These structural types are adopted
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.