Food Proteins and Their Applications

Srinivasan Damodaran

• Publisher: CRC Press
• Year: 2017
• Print ISBN:
  9781498783057
• eBook ISBN:
  9781351447522
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Reviews the physiochemical properties of the main food proteins and explores the interdependency between the structure-function relationship of specific protein classes and the processing technologies applied to given foods. The book offers solutions to current problems related to the complexity of food composition, preparation and storage, and includes such topics as foams, emulsions, gelation by macromolecules, hydrolysis, microparticles/fat replacers, protein-based edible films, and extraction procedures.

1

Food Proteins: An Overview

Srinivasan Damodaran

University of Wisconsin—Madison Madison, Wisconsin

I Introduction

Proteins play several important roles in biological and food systems. Some of these include biocatalysts (enzymes), structural components of cells and organs (e.g., collagen, keratin, elastin, etc.), contractile proteins (actin, myosin, tubulin), hormones (insulin, growth factor, etc.), transport proteins (serum albumin, transferrin, hemoglobin), metal chelation (phosvitin, ferritin), antibodies (immunoglobulins), protective proteins (toxins and allergens), and storage proteins (seed proteins, casein micelles, egg albumen) as nitrogen and energy source for embryos.

Proteins are highly complex polymers, and their functional diversity mainly arises from their chemical make-up. For instance, while other biopolymers, such as polysaccharides and nucleic acids, are made up of one or a few monomers, proteins and polypeptides are made up of combinations of 20 different amino acids. In some proteins, some of the amino acid residues are enzymatically modified by the cytoplasmic enzymes. Examples of such modifications include glycosylation (ovalbumin, κ-casein, lectins, and vicilin-type proteins of legumes) and phosphorylation (α- and β-caseins, phosvitin, kinases, phosphorylases). In addition, unlike the ether and phosphodiester bonds that link the monomers in polysaccharides and nucleic acids, respectively, the substituted amide linkage in proteins is a partial double bond; this adds to the structural complexity of protein polymers. The various types of noncovalent interactions between the amino acid constituents and the specific properties of the amide linkage impart a multitude of spatial structural forms to proteins with diverse biological functions. Literally innumerable numbers of proteins with distinct structures and functions can be synthesized by varying the amino acid composition and sequence.

The functional properties of proteins in foods are related to their structural and other physicochemical characteristics. A fundamental understanding of the physical, chemical, and functional properties of proteins and the changes these properties undergo during processing is essential if the performance of proteins in foods is to be improved and if underutilized proteins, such as plant proteins and whey proteins, are to be increasingly used in traditional and processed food products.

Several of the chapters in this book deal with the physicochemical bases of protein functionality and the structure-function relationships of specific food proteins. In this chapter, a general overview of protein structure will be presented and its influence on the physical and chemical properties and functional properties of proteins will be discussed.

II Protein Structure

Proteins are polymers made up of 19 different α-amino acids and one imino acid linked via amide bonds, also known as peptide bonds. The constituent amino acids differ only in the chemical nature of the side-chain group at the α-carbon atom. The physicochemical properties, such as charge, solubility, and chemical reactivity, of the amino acids (hence proteins) are dependent on the chemical nature of the side-chain group. Amino acids with aliphatic (Ala, Ile, Leu, Met, Pro, and Val) and aromatic (Phe, Trp, and Tyr) side chains are nonpolar; they exhibit limited solubility in water. Amino acids with charged (Arg, Lys, His, Glu, and Asp) and uncharged (Ser, Thr, Asn, Gln, and Cys) side chains are quite soluble in water. Proline is the only imino acid present in proteins. The net charge of a protein at any pH is determined by the relative numbers of basic (Arg, Lys, and His) and acidic (Glu and Asp) amino acid residues in the protein.

One of the major factors influencing the properties, such as conformational stability, solubility, surface activity, fat binding, etc., of proteins is the overall hydrophobicity of the constituent amino acid residues. Hydrophobicity is generally defined as the excess free energy of a solute in water compared to that in an organic solvent. Thus, the hydrophobicities of amino acids can be determined by measuring their solubilities in water and in a reference organic solvent, such as ethanol [1], and using the relation:

ΔGt(Et→W)=−RT ln(SAA,EtSAA, W)⁢(1)

where ΔG_t(Et→W) is the transfer free energy of the amino acid from ethanol to water, S_AA,Et and S_{AA, W} are the solubilities of the amino acid in ethanol and water, respectively, T is the temperature, and R is the gas constant.

Because an amino acid can be considered as a derivative of glycine, and since ΔG_t(Et→W) is an additive function, the transfer free energy of an amino acid can be considered to be sum of the transfer free energies of glycine and the side chain, i.e.,

ΔGt,side chain=ΔGt,AA−ΔGt,Gly⁢(2)

The hydrophobicities of amino acid side chains obtained in this manner are listed in Table 1. Amino acid side chains with large positive ΔG_t values are hydrophobic; they prefer to be in an organic phase or in the protein interior rather than in an aqueous environment. Amino acid side chains with negative ΔG_t values, especially the charged amino acid side chains, prefer to be on the protein exterior exposed to the aqueous environment.

The functional behaviors of biologically important proteins and food proteins are dependent on their structures. Four levels of structural hierarchy, namely, primary, secondary, tertiary, and quaternary structures, exist in proteins.

Table 1 Hydrophobicity of Amino Acid Side Chains at 25°C

Source: Ref. 1.

A Primary Structure

The primary structure of a protein denotes the linear sequence in which the constituent amino acids are linked via peptide bonds. The chain length and the amino acid sequence of the polypeptide determines its ultimate three-dimensional structure in solution.

One of the structural features that distinguishes proteins from other biopolymers, such as polysaccharides and nucleic acids, is the partial double-bond character of the peptide bond, resulting from its resonance structure:

Because of this resonance structure, the rotation of the peptide bond is restricted to a maximum of about 6°, the six atoms of the peptide unit becomes planar, protonation of the peptide N—H group becomes impossible, the oxygen and hydrogen atoms of C=O and N—H groups acquire partial negative and positive charges, respectively, and the peptide unit can exist in either cis or trans configuration. Almost all peptide bonds in proteins exist in the trans configuration, because it is more stable than the cis configuration. Because of the partial charges of the C=O and N—H groups, interchain and intrachain hydrogen bonding between these groups is possible under appropriate conditions.

Because the peptide bonds constitute one third of all covalent bonds of the polypeptide backbone, the restriction on their rotational freedom drastically reduces the flexibility of the polypeptide chain. Among the covalent bonds of the peptide backbone, only the N—C_α and the C_α—C bonds have rotational freedoms, and these are termed ϕ (phi) and ψ (psi) dihedral angles, respectively. However, steric hindrance arising from bulk side chains attached to the α-carbon atom restricts rotational freedoms of the ϕ and ψ angles as well. Because of these constraints, a majority of proteins do not exist in flexible random coil conformations in solution. On the contrary, most proteins assume a highly compact ordered structure because of steric factors and other noncovalent interactions among the amino acid residues. The selection of the 20 different amino acids and the choice of peptide (amide) bond as the primary linkage appear to be a deliberate design by nature to precisely control the flexibility and/or rigidity of polypeptides so that they may be used to perform several biological functions.

B Secondary Structure

The secondary structure relates to periodic structures in polypeptides and proteins, in which the consecutive amino acid residues of the segment assume the same set of ϕ and ψ angles. The most commonly found secondary structures in proteins are the α-helix and the β-sheet. The α-helix is characterized by a pitch of 5.4 Å involving 3.6 amino acid residues. It is stabilized by intrachain hydrogen bonding between the i^th N—H group and the C=O group of the i-4^th residue.

Recent studies indicate that the instruction for α-helix formation is coded as a binary code (related to the arrangement of polar and nonpolar residues) in the amino acid sequence [2]. Peptide segments with repeating heptet sequences of [—P—N—P—P—N—N—P—], where P and N are polar and nonpolar residues, respectively, readily form α-helices. In this binary code, the precise identities of the polar and nonpolar residues are irrelevant. Slight variations in the binary code are tolerated, provided other interactions in the protein are favorable for α-helix formation. A good example of this is tropomyosin, which exists entirely in a coiled-coil α-helical rod form. In this protein, the repeating heptet sequence is [—N—P—P—N—P—P—P—]; despite this variation, tropomyosin exists in a α-helix form because of other favorable interactions in the coiled-coil rod [3].

The α-helical structures in protein are predominantly amphiphilic, that is, one half of the helical surface is hydrophilic and the other half is hydrophobic. In fact, if an α-helix is made out of the heptet sequence mentioned above, one would find that all the hydrophobic amino acid residues would lay on one side of the helix surface and all the hydrophilic residues would lay on the other side. Generally, in native proteins, the hydrophobic surface of the α-helix faces the interior of the protein and is engaged in hydrophobic interactions with other nonpolar groups in the interior. Such interactions generally contribute to the stability of the folded form of the protein.

The β-sheet structure is an extended structure in which the C=O and N—H groups are oriented perpendicular to the direction of the backbone. In this configuration, hydrogen bonding can occur only between sheets. Depending on the directions of the sheets, two types of β-pleated sheet structure, namely, parallel and antiparallel β-sheets, can form. The binary code in the amino acid sequence that specifies β-sheet formation is [—N—P—N—P—N—P—N—. . . .]. In other words, segments containing alternating polar and nonpolar amino acid residues exhibit a high probability of forming β-sheets. Generally, β-type proteins are more hydrophobic than the α-type proteins. They exhibit high denaturation temperatures. Examples are β-lactoglobulin (51% β-sheet) and soy US globulin (64% β-sheet), which have thermal denaturation temperatures of 75.6 and 84.5°C, respectively. In contrast, bovine serum albumin, which is an α-type protein, has a thermal denaturation temperature of only about 64°C [4,5]. When protein solutions are heated, more often than not, α-helix is converted to β-sheet [4], but conversion of β-sheet to α-helix has not been reported.

Polypeptide segments in which the consecutive amino acid residues assume random combinations of ϕ and ψ angles have disordered or aperiodic structures. Proteins containing high levels of proline residues usually assume a random structure. This is because of their pyrrolidine ring structure, in which the ϕ angle is fixed at 70°C, and their inability to form hydrogen bonds. A good example is casein: α- and β-caseins consist of about 8.5 and 17% proline residues, respectively. The uniform distribution of these residues in its sequence hinders with formation of α-helical and β-sheet structures, and these proteins exist predominantly in disordered states. About one third of the residues in collagen are either proline or hydroxyproline, which exists in a helical form in which three polypeptide chains are entwined to form a triple helix; the triple helix is stabilized by interchain hydrogen bonds. The geometry of this triple helix is different from that of the α-helix. Other food proteins that contain a large amount of proline residues include cereal proteins, such as gliadins and glutenins. Sin...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• 1. Food Proteins: An Overview
• 2. Thermal Denaturation and Coagulation of Proteins
• 3. Protein-Stabilized Foams and Emulsions
• 4. Protein Gelation
• 5. Protein-Lipid and Protein-Flavor Interactions
• 6. Protein-Polysaccharide Interactions
• 7. Structure-Function Relationships of Caseins
• 8. Structure-Function Relationships of Whey Proteins
• 9. Structure-Function Relationships of Soy Proteins
• 10. Structure-Function Relationships of Wheat Proteins
• 11. Structure and Functionality of Egg Proteins
• 12. Structure-Function Relationships of Muscle Proteins
• 13. Enzyme and Chemical Modification of Proteins
• 14. Genetic Engineering of Food Proteins
• 15. Functionality of Protein Hydrolysates
• 16. High-Pressure Effects on Proteins
• 17. Protein and Protein-Polysaccharide Microparticles
• 18. Edible Protein Films and Coatings
• 19. Effects of Processing and Storage on the Nutritional Value of Food Proteins
• 20. Edible Protein Films and Coatings
• 21. Chemical and Physical Methods for the Characterization of Proteins
• 22. Applications of Immunochemistry for Protein Structure Control
• Index