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Multilayer emulsions stabilized by vegetable proteins and polysaccharides
César Burgos-Díaz
1*
, Traudy Wandersleben
2
, Ana M. Marqués
3
, Mónica Rubilar
1,4
1
Agriaquaculture Nutritional Genomic Center, CGNA, Technology and Processes Unit, Temuco, Chile.
2
Agriaquaculture Nutritional Genomic Center, CGNA, Genomic and Bioinformatic Unit, Temuco, Chile
3
Laboratory of Microbiology, Faculty of Pharmacy, University of Barcelona. Barcelona, Spain.
4
Departament of Chemical Engineering, Scientific and Technological Bioresource Nucleus, BIOREN, Universidad de La
Frontera, Temuco, Chile
*
Corresponding author: Tel.: +56 45 2744232; Fax: +56 45 2732402.
E-mail address: cesar.burgos@cgna.cl (C. Burgos-Díaz)
Abstract
There is great interest in the food, cosmetic and pharmaceutical industry in the use of
proteins and polysaccharides as natural hydrocolloids to create novel emulsion systems
with improved stability and functionality. For example, the electrostatic interaction between
proteins and polysaccharides may be used to form oil-in-water (O/W) emulsions with
multilayered interfacial membranes around oil droplets or multilayer emulsions. This type
of emulsions have been developed using the layer-by-layer (LbL) technique, which consists
of direct adsorption of an oppositely charged polyelectrolyte layer (e.g. polysaccharides) on
a primary layer of ionic emulsifiers (e.g. proteins). The polymeric structure and electrical
charge of proteins make them a special class of compounds very suitable for its utilization
in the LbL technique. In recent years, the utilization of proteins as emulsifiers in food and
pharmaceutical industry has been turning towards plants as a preferred alternative to
animal-based sources. This article reviews the current understanding of the utilization of
different vegetable proteins as emulsifier in order to stabilize O/W multilayer emulsions
systems. Additionally, it highlights some potential applications of the multilayer emulsion
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technology in the industry, for improving the stability of emulsions to environmental
stresses and for developing controlled or triggered release systems.
Key words: multilayer emulsions, vegetable proteins, polysaccharides, oil-in-water, layer-
by-layer
1. Introduction
Current technological development in industry demands new emulsifiers or methodologies
to obtain stable emulsions, thus creating novel products or improving its shelf-live
associated with an adequate delivery activity, efficiency and yield of active principles. Oil-
in-water emulsions (O/W) are widely used in the cleaning, cosmetics, pharmaceutical and
food industries for encapsulating different bioactive compounds and increasing their
solubility and stability. Homogenization of oily and aqueous phases is achieved in the
presence of one or more emulsifiers. The emulsifier is adsorbed to the surface of just
formed droplets reducing the interfacial tension and facilitating droplet disruption.
In recent years, proteins from animal and vegetables sources, have been considered as
natural emulsifiers in different industrial processes [1]. Their biodegradability,
compatibility and excellent characteristics are remarkable properties of these natural
polymers. Moreover, industries in their search for protein ingredients have been turning
towards plants as a preferred alternative to animal-based sources, e.g. in vegetarian diets,
due to increased consumer concerns over the safety of animal-derived products [2*].
Many types of proteins can be used as emulsifiers due to their amphiphilic character,
polymeric structure, and electrical charge characteristics [3]. The amphiphilic character
means that they can be adsorbed to the droplet surfaces during homogenization. Despite
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their good features, protein-stabilized emulsions are highly sensitive to environmental
stresses such as pH, ionic strength and temperature, affecting encapsulated compounds
[4**]. In this regards, it has been shown that these emulsions are particularly sensitive to
pH and ionic strength. They tend to flocculate at pH values close to the isoelectric point of
the adsorbed proteins and when the ionic strength exceeds a particular level, because the
electrostatic repulsion between the droplets is then no longer sufficiently strong to
overcome the various attractive interactions. This instability may limit their application in
some commercial products [5]. As a way to overcome this disadvantage, the strategy is the
incorporation of additional polysaccharide coating layers that stabilize the O/W emulsions
by means of electrostatic interaction with the protein layer. Along these lines, Guzey and
McClements [4**] convey that, one strategy to improve the physical stability of O/W
emulsions to environmental stresses is to form multilayer emulsions. Multilayer emulsions
consist of various interfacial layers of proteins (emulsifier) and/or polysaccharides around
oil droplets which are deposited using the LbL electrostatic technique. LbL has been shown
to have potential applications in foods, including controlled/triggered release, stabilization
of emulsions, processing and storage conditions. The production of stable multilayered
emulsions using this technique depends on biopolymer properties (e.g., charge density,
molecular weight and conformation), emulsifier layer thickness and bulk physicochemical
conditions [1]. Additionally, multilayer emulsions have the potential to decrease lipid
oxidation rates due to their ability to alter both the emulsion droplet charge and thickness of
the interfacial region [6]. The electrical properties of the first layer of a multilayer emulsion
are determined by the emulsifier, and they can therefore be controlled by selecting different
types of emulsifiers [4**].
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Several studies have shown that vegetable proteins can be used to prepare multilayer
emulsions using the LbL technique [1, 3, 7]. The use of vegetable proteins as emulsifier
reflects the present “green” trend in the pharmaceutical, cosmetics and food industries. In
food applications, vegetable proteins are known to be less allergenic compared to animal
derived proteins [2*]. This review presents the recent works dealing with the use of
vegetable proteins in the formation of multilayer emulsions systems. The influence of the
combination of different biopolymers, proteins, and polysaccharides on stability of O/W
multilayer emulsions, and the industrial applications in several industrial processes will be
particularly discussed. Besides, the review is focused on the utilization of LbL technique in
the formation on multilayer emulsions. The information collected to prepare this revision
follows the description of main published experimental studies in the last ten years.
2. Vegetable proteins as emulsifier
Proteins are commonly used in the food and pharmaceutical industry as emulsifiers in the
stabilization of O/W emulsions. These natural polymers present several advantages:
biocompatibility, biodegradability, good amphiphilic and functional properties such as
water solubility, emulsifying and foaming capacity. Proteins are amphiphilic compounds,
and therefore, they are able to adsorb strongly at the oil-water interface, favoring emulsion
formation [8]. The amount adsorbed and the conformation adopted at the oil-water interface
will depend very much on the protein amino acid composition since adsorption occurs
through hydrophobic groups present within their structure [9].
Nowadays, the utilization of vegetable proteins as emulsifiers has increased even
surpassing the use of proteins from animal sources. In food applications, for example,
vegetable proteins are known to be less allergenic compared to animal derived proteins.
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Vegetable proteins consist of several fractions: the major fraction is glutenin, followed by
globulin fraction, albumin and prolamin [2*].
Among vegetable proteins used as emulsifiers, we can mainly find soy protein isolate, pea
protein isolate, lupin protein isolate, broad beans and cereal proteins (such as wheat
proteins) [10, 11]. These types of proteins have been used to facilitate the formation,
improve the stability, and provide specific physicochemical properties to emulsions [2*].
Many vegetable proteins are surface-active molecules that can be used as emulsifiers
because of their ability to facilitate the formation, improve the stability and produce
desirable physicochemical properties in O/W emulsions [12*].
Despite their functional properties and benefits, emulsions stabilized by proteins are highly
sensitive to environmental stresses such as pH, ionic strength and temperature [13]. For
example, at pH values close to the isoelectric point of protein and/or high salt concentration
in the emulsion, the electrostatic repulsion of the protein adsorption layers decreases and
therefore, coalescence and flocculation happen [13]. Besides, when emulsion is subjected to
heat treatment, for pasteurization or sterilization purposes, flocculation happens because of
the protein denaturation which holds the droplets together [14]. For this reason, several
strategies have been developed to improve the stability of protein-stabilized emulsions to
droplet flocculation induced by pH or ionic strength effects. For example, (1) the
incorporation of multivalent counterions, such as Ca
2+
, Fe
2+
or Fe
3+
, to emulsions systems;
(2) the addition of ionic surfactants to protein-stabilized emulsion to change the pH
dependence of the ζ-potential of the droplets, thereby changing the range of pH values so
that the emulsion is stable to flocculation [12*]; and, (3) the addition of electrically charged
biopolymers to the surface of oppositely charged droplets to a protein-stabilized emulsion
in order to increase its physical stability to environmental stresses [15*]. The LbL
