The functional properties of biopolymer particles are ultimately determined by the type, concentration, arrangement, and interactions of the biopolymer molecules used to assemble them. In addition, the type of preparation method that can be used to fabricate a biopolymer particle depends on the specific characteristics of the biopolymer molecules involved (Table 1). For example, electrostatic-driven assembly depends on the biopolymers having particular electrical characteristics under the environmental conditions used. The 2 major classes of food-grade biopolymers that are used to fabricate functional biopolymer particles are proteins and polysaccharides. In this section, we provide a brief overview of the molecular and physicochemical attributes of some of the most widely used biopolymers, restricting ourselves to a discussion of proteins (Table 2) and ionic polysaccharides (Table 3) for the purpose of this review.
Proteins are polypeptide chains consisting of 20 common amino acids linked together through peptide bonds. Amino acids are distinguished by their side groups, notably amines, carboxylates, hydroxyls, phenolics, and sulfhydryls (Damodaran 1996). The type, number, and sequence of amino acids along the polypeptide chain determine the molecular weight, conformation, electrical charge, hydrophobicity, physical interactions, and chemical reactivity of proteins. Detailed information on protein structure and conformation has been given by Damodaran (1997). Some basic characteristics of proteins (Table 1) are discussed, as well as specific food protein properties (Table 2).
In globular proteins, the polypeptide chain is usually folded into a compact spheroid conformation with most of the nonpolar amino acids located in the interior and the polar amino acids located at the exterior (Seno and Trovato 2007). The major driving force for this configuration is the hydrophobic effect (see below), yet other interactions (such as van der Waals, hydrogen bonding, disulfide bonds, and electrostatic forces) also play a role. Disulfide bonds, specifically, play a key role in maintaining the durable internal structures of certain globular proteins, such as β-lactoglobulin, bovine serum albumin, and soy glycinin (Wolf 1993; Hoffman and van Mill 1997; Fox 2003). Loss in this globular conformation is termed denaturation, which is further discussed, below.
Electrical charge on proteins may be both positive and negative, contributing to electrostatic interactions (attractive or repulsive) among differing environments. Protonated amino side groups (-NH3+) contribute positive charges below pH 10, whereas deprotonated carboxylate side groups (-CO2−) contribute negative charges above pH 2. The pH value where positive and negative charge contributions are equal is usually referred to as the isoelectric point (pI); when the pH = pI, a protein has zero net-charge. Some common food protein pI values are listed in Table 2.
Protein hydrophobicity is divided into 2 functional categories: surface hydrophobicity and internal hydrophobicity. Surface hydrophobicity plays a key role in determining the functional attributes of a bulk protein, such as interactions with nonpolar materials (for example, fatty acids and flavors), nonpolar surfaces (oil or air), and other proteins (in the formation of aggregates) (Damodaran 1996). Internal hydrophobicity results from globular conformations and is relatively ineffective in bulk protein interactions.
Proteins interact with other components through a variety of physical or chemical interactions. Physical interactions (for example, electrostatic or hydrophobic forces) play a role in the interchange with solvents, cosolvents, surfactants, phospholipids, polysaccharides, sugars, and minerals. Chemical reactions among proteins include disulfide interchanges, dehydration, phenolic oxidation, Maillard, and transglutaminase reactions. These reactions have been summarized in a review by Gerrard (2002). Disulfide interactions play a stronger role in high temperature protein aggregation (Monahan and others 1995; Hoffmann and van Mill 1999), specifically at high pH (where the cysteine residue is partially deprotonated) (Sava and others 2005). The remaining reactions are discussed in more detail in the section titled “Cross-Linking of Biopolymer Particles.”
Protein denaturation and aggregation. In nature, globular proteins have certain biological functions (including enzyme activity, signaling, transport, and molecular recognition), which require a specific 3-dimensional conformation usually referred to as the native state. Globular proteins in food may lose their native states during the extraction, isolation, and purification of functional food ingredients (such as milk, egg, or soy protein concentrates or isolates) or after incorporation into a food product due to changes in their environment, such as pH, ionic strength, solvent type, temperature, adsorption to interfaces, high pressure, dehydration, or chemical treatments (Damodaran 1996). Understanding the specific factors and mechanisms of protein aggregation is helpful in the rational formation of protein structures, such as spheroids, filaments (Akkermans and others 2007; Jung and others 2008), and nano-tubes (Graveland-Bikker and others 2009).
In principle, denaturation may be either reversible or irreversible, but it is typically irreversible for the globular proteins commonly used in foods. When a globular protein becomes denatured its physical and chemical interactions change appreciably through exposure of nonpolar and sulfur-containing groups that were originally present within the compact interior of the globular protein. Consequently, denatured proteins have a greater tendency to aggregate, irreversibly, with each other through hydrophobic bonding and disulfide bond formation.
Globular proteins experience a number of physicochemical phenomena during thermal denaturation and aggregation, which is often characterized by the model protein β-lactoglobulin (Sawyer 2003). At ambient temperatures globular proteins may self-associate into various types of quaternary structures (for example, dimers or octamers), depending on solution conditions (such as pH, ionic strength, and solvent type). Upon heating, these quaternary structures dissociate (Galani and Apenten 2000), followed by partial unfolding of the secondary and tertiary structures of the protein (Qi and others 1997; Bryant and McClements 1998). Hydrophobic groups are then exposed to the surrounding aqueous phase (Lametti and others 1996; Relkin 1998), leading to protein self-association or binding of nonpolar components (Carrotta and others 2001). Hydrophobically driven aggregation is especially prevalent near the isoelectric point or at high ionic strengths, where the electrostatic repulsion between protein molecules is low (Hoffmann and van Mill 1999). At sufficiently high pH values, disulfide interchanges may also occur (Relkin and others 1998; Sava and others 2005) leading to the formation of irreversibly denatured structures (Creamer and others 2004).
The temperature at which a given protein denatures is termed as the thermal denaturation temperature (Tm). This temperature depends on its unique molecular characteristics, as well as solution conditions (such as pH, ionic strength, solvent composition, and the presence of surface-active substances) (Ragone 2004). Thermal denaturation tends to be less favorable (higher Tm) at pH values close to the isoelectric point, which has been attributed to increased hydrogen bonding within secondary structures, increased electrostatic attraction between oppositely charged regions, and reduced electrostatic repulsion between similarly charged regions along the polypeptide chain (Kella and Kinsella 1988; Boye and others 1996; Relkin and others 1998; Baeza and Pilosof 2002).
Polysaccharides are polymers of monosaccharides that possess varying molecular weights, conformations, branching, electrical characteristics, flexibility, and hydrophobocity (Table 1) (Rinaudo 2008). Compared to protein, the monomer composition of polysaccharides is more uniform (Table 3), granting particular physicochemical and functional properties (for example, solubility, binding properties, viscosity enhancement, gelation, and surface activity). However, polysaccharides within 1 batch often possess variation in monosaccharide arrangement, molecular weight, and electrical charge, making initial characterization a necessary step in all molecular-level research (Autio 2006). The following section is far from exhaustive; a thorough discussion on polysaccharide structure and relevant analytical methods can be found in books (Eliasson 2006; Stephen and others 2006) and reviews (Muralikrishna and Rao 2007; Rinaudo 2008).
Molecular conformations of polysaccharides are limited to random coil or helical structures. Linear structures are favored as glycosidic bond rotation and chain flexibility are restricted. Chain flexibility in ionic polysaccharides is especially restricted due to intra-chain charge repulsions. Helical polysaccharides (such as carrageenan and agar) are constructured from hydrogen bonds and undergo conformational changes with heating. Helix-to-coil transition occurs at a specific temperature (Tm), whereupon the polysaccharide may reassociate into intermolecular junctions. These junctions are responsible for many polysaccharide gels.
Polysaccharide charge may be anionic, cationic, or nonionic depending on the nature of their functional groups and varies with pH depending on the pKa value of the ionizable side-groups. Common ionized groups on polysaccharides include carboxylates (-CO2−, pKa about 2.5 to 4.5), sulfates (-SO4−, pKa < 0), and amines (-NH3+, pKa about 9.4). Polysaccharides will generally only have 1 ionized group; for example, pectins contain carboxylates (Ridley and others 2001), carrageenans contain sulfates (Piculell 1995; Whistler and BeMiller 1997), and chitosan contains amines (Kurita 2006; Rinaudo 2006).
The ability of polysaccharides to self-associate and form gels has important implications for biopolymer particle formation. In an excellent review by Williams (2007), gelling biopolymers are categorized by their gel formation mechanism: cooling (for example, carrageenan); heating (methyl cellulose); ion addition (alginate); and retrogradation (amylopectin). Physical interactions in polysaccharides are largely based on hydrogen bonding and, among ionic species, electrostatic interactions (often via divalent ions). These play a role in the formation of intermolecular junctions and solubilization. Nonpolar-like interactions are also possible among helical, uncharged polysaccharides (for example, maltodextrin) and nonpolar residues (such as fatty acids) (Siswoyo and Morita 2003). True nonpolar properties among polysaccharides generally result from conjugation with amphiphilic protein fractions (for example, gum arabic or pectins) (Gaspar and others 2001; Funami and others 2007; Yapo and others 2007; Kirby and others 2008; Mahendran and others 2008; Nakauma and others 2008). Other physicochemical properties may be attained in polysaccharides through chemical modification, such as changes in molecular weight (Coffey and others 2006; Gidley and Reid 2006; Wurzburg 2006) or the addition of specific functional groups (for example, phosphorylation) (Whistler and BeMiller 1997).