Proteins in food processing

Proteins in food processing

Proteins in food processing

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Proteins in food processing

Edited by

R Y Yada

Published by Woodhead Publishing Limited

Abington Hall, Abington

First published 2004, Woodhead Publishing Limited and CRC Press LLC

ß 2004, Woodhead Publishing Limited

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1 Introduction

R Y Yada, University of Guelph, Canada

2 Properties of proteins in food systems: an introduction

E C Y Li-Chan, The University of British Columbia, Canada

2.1 Introduction

2.2 Chemical and physical properties of food proteins

2.3 Factors affecting properties of proteins in food systems2.4 Structure and function of proteins: classification andrelationships

P F Fox and A L Kelly, University College, Cork, Ireland

3.1 Introduction: the caseins

3.2 Heterogeneity of the caseins

3.3 Molecular properties of the caseins

3.4 The caseins as food constituents and ingredients

3.5 The casein micelle: introduction

3.6 Properties and stabilisation mechanisms of casein micelles

Contents

3.7 Structure models of the casein micelle

3.8 Stability of casein micelles

4.1 Introduction: whey proteins as food ingredients

4.2 Analytical methods for determining protein content

4.3 Structure of whey proteins

4.4 Improving functionality of whey proteins in foods: physicalprocesses and enzymatic modification

4.5 Sources of further information and advice

7 Proteins from oil-producing plants

S D Arntfield, University of Manitoba, Canada

7.1 Introduction

7.2 Oilseed protein characteristics

7.3 Factors limiting protein utilization

7.4 Extraction and isolation of proteins

7.5 Functional properties of proteins

7.6 Improving functionality of oilseed protein

8.4 Gluten: formation, properties and modification

8.5 Processing and modification of cereal proteins in cerealproducts

8.6 Future trends

8.7 References

9 Seaweed proteins

J Fleurence, University of Nantes, France

9.1 Introduction: seaweed and protein content of seaweed9.2 Composition of seaweed proteins

9.3 Algal protein digestibility

9.4 Uses of algal proteins in food

9.5 Future trends

9.6 Sources of further information and advice

9.7 References

Part II Analysing and modifying proteins

10 Testing protein functionality

R K Owusu-Apenten, Pennsylvania State University, USA

10.1 Introduction

10.2 Protein structure: sample characteristics and commercialproteins

10.3 Testing functionality

10.4 Model foods: foaming

10.5 Model foods: emulsification and gelation

10.6 Conclusions and future trends

10.7 Sources of further information and advice

10.8 Acknowledgement

10.9 References

11 Modelling protein behaviour

S Nakai, University of British Columbia, Canada

11.1 Introduction

11.2 Computational methodology

11.3 Computer-aided sequence-based functional prediction11.4 Future trends

11.5 Further information and advice

11.6 Conclusion

11.7 Acknowledgement

11.8 References

12 Factors affecting enzyme activity in foods

J R Whitaker, University of California, USA

12.1 Introduction

12.2 Types of enzymes and post-harvest food quality

12.3 Parameters affecting enzyme activity

12.4 Future trends

12.5 Sources of further information and advice

12.6 References

13 Detecting proteins with allergenic potential

R Krska, E Welzig and S Baumgartner, IFA-Tulln, Austria

13.1 Introduction

13.2 Methods of analysing allergenic proteins

13.3 Methods of detecting food allergens

13.4 Developing new rapid tests: dip-sticks and biosensors13.5 Future trends

13.6 Sources of further information and advice

13.7 References

14 The extraction and purification of proteins: an introduction

R E Aluko, University of Manitoba, Canada

14.1 Introduction

14.2 Factors affecting extraction

14.3 Extraction and fractionation methods

A Kato, Yamaguchi University, Japan

15.1 Introduction

15.2 Lysozyme-polysaccharide conjugates

15.3 Constructing polymannosyl lysozyme using genetic

engineering15.4 Improving functional properties of lysozymes

15.5 Acknowledgement

15.6 References

16 Modifying seeds to produce proteins

A M Nuutila and A Ritala, VTT Biotechnology, Finland

16.1 Introduction

16.2 Methods of seed modification

16.3 Application and use of modified seeds for protein production16.4 Future trends

16.5 Sources of further information and advice

16.6 References

17 Processing approaches to reducing allergenicity in proteins

E N C Mills, J Moreno, A Sancho and J A Jenkins,

Institute of Food Research, UK and H J Wichers,

Wageningen UR, The Netherlands

17.1 Introduction: food allergens

17.2 Protein allergens of animal origin

17.3 Protein allergens of plant origin

17.4 General properties of protein allergens: abundance,

structural stability and epitopes

17.5 Factors affecting protein allergenicity in raw foods

17.6 Reducing protein allergenicity during food processing17.7 Reducing protein allergenicity using enzymatic processing17.8 Future trends: low allergen proteins

17.9 Acknowledgements

17.10 References

Part III Applications

18 Using proteins as additives in foods: an introduction

H Luyten, J Vereijken and M Buecking, Wageningen UR,

The Netherlands

18.1 Introduction

18.2 Rheological properties of proteins

18.3 Surfactant properties of proteins

18.4 Protein-flavour relationships

18.5 Protein structure and techno-functionality

18.6 References

19 Edible films and coatings from proteins

A Gennadios, Cardinal Health, Inc., USA

20.2 Food proteins and their gels

20.3 Mechanisms of protein gel formation

S Barnes, T Sanderson, H McCorkle, L Wilson, M Kirk and

H Kim, University of Alabama at Birmingham, USA

21.1 Introduction

21.2 Protein separation techniques

21.3 Using mass spectrometry to identify and characterizeproteins

21.4 The impact of food processing on soy protein

21.5 Conclusion

21.6 Acknowledgements

21.7 References

22 Texturized soy protein as an ingredient

M N Riaz, Texas A & M University, USA

22.1 Introduction: texturized vegetable protein

22.2 Texturized vegetable protein: raw material characteristics22.3 Soy based raw materials used for extrusion texturization22.4 Wheat and other raw materials used for extrusion

texturization22.5 Effect of additives on texturized vegetable protein

22.6 Types of texturized vegetable protein

22.7 Principles and methodology of extrusion technology22.8 Processing texturized soy protein: extrusion vs

extrusion-expelling22.9 Economic viability of an extrusion processing system forproducing texturized soy chunks: an example

22.10 Uses of texturized soy protein

22.11 References

23 Health-related functional value of dairy proteins and peptides

D J Walsh and R J FitzGerald, University of Limerick, Ireland

23.1 Introduction

23.2 Types of milk protein

23.3 General nutritional role of milk proteins

23.4 Milk protein-derived bioactive peptides

23.5 Mineral-binding properties of milk peptides

23.6 Hypotensive properties of milk proteins

23.7 Multifunctional properties of milk-derived peptides

23.8 Future trends

23.9 Acknowledgement

23.10 References

24 The use of immobilized enzymes to improve functionality

H E Swaisgood, North Carolina State University, USA

25 Impact of proteins on food colour

J C Acton and P L Dawson, Clemson University, USA

25.1 Introduction: colour as a functional property of proteins25.2 Role of proteins in food colour

25.3 Improving protein functionality in controlling colour25.4 Methods of maintaining colour quality

25.5 Future trends

25.6 Sources of further information and advice

25.7 References

The University of British Columbia

Faculty of Agricultural Sciences

Food Science Building

University College, CorkIreland

Tel: 353 21 490 2362Fax: 353 21 427 0001E-mail: pff@ucc.ie

Chapter 4

Dr A KilaraArun Kilara Worldwide

1020 Lee Road, Suite 200Northbrook

Illinois 60062-3818USA

Tel/fax: 847 412 1806E-mail: kilara@ix.netcom.comContributor contact details

Dr M N Vaghela

Group Manager ± Ice cream

Nestle R & D Center

Tel: 1 (204) 474 9866Fax: 1 (204) 474 7630E-mail: susan_arntfield@umanitoba.ca

Chapter 8

Dr N GuerrieriDepartment of Agrifood MolecularScience

University of MilanVia Celoria 2

20133 MilanItalyTel: +39 (0) 25031 6800/23Fax: +39 (0) 25031 6801E-mail: nicoletta.guerrieri@unimi.it

Chapter 9Professor J FleurenceFaculty of SciencesMarine Biology LaboratoryUniversity of Nantes

BP 92208

44 322 Nantes Cedex 3France

Tel: 33 2 51 12 56 60Fax: 33 2 51 12 56 68E-mail:

joel.fleurence@isomer.univ-nantes.fr

Konrad Lorenzstr 20

A ± 3430 TullnAustria

Tel: +43 2272 66280 401Fax: +43 2272 66280 403E-mail: rudolf.krska@boku.ac.at

Chapter 14

Dr R E AlukoUniversity of ManitobaDepartment of Foods and Nutrition400A Human Ecology BuildingWinnipeg MB R3T 2N2Canada

Tel: (204) 474-9555Fax: (204) 474-7592E-mail: alukor@ms.umanitoba.ca

Chapter 15

Dr A KatoDepartment of Biological ChemistryFaculty of Agricultural ScienceYamaguchi University

JapanTel: 083 933 5852Fax: 083 933 5820E-mail: akiokato@yamaguchi-u.ac.jp

Dr A Sancho and Dr J A Jenkins

Food Materials Science

Institute of Food Research

Norwich Research Park

Agrotechnology & Food Innovations

Programme Leader Food and Health

Agrotechnology & Food Innovations(A&F)

PO Box 17

6700 AA WageningenThe NetherlandsTel: +31 (0) 317 475120Fax: +31 (0) 317 475347E-mail: hannemieke.luyten@wur.nl

Chapter 19

Dr A GennadiosCardinal Health, Inc

Oral Technologies Business Unit

14 Schoolhouse RoadSomerset NJ 08873USA

Tel: 732 537 6366Fax: 732 537 6480E-mail: Aris.gennadios@cardinal.com

Chapter 20

Dr J M AguileraDepartment of Chemical andBioprocess EngineeringUniversidad CatoÂlica de ChileSantiago

ChileTel: (562) 686 4256Fax: (562) 686 5803E-mail: jmaguile@ing.puc.cl

Dr B Rademacher

Institute of Food Process Engineering

Technical University of Munich

Room 452 McCallum Building

University of Alabama at Birmingham

AL 35294USATel: 205 975 0832Fax: 205 934 6944E-mail: Landon.Wilson@ccc.uab.eduMarion.Kirk@ccc.uab.edu

Chapter 22

Dr M RiazFood Protein R&D CenterTexas A & M UniversityCollege Station

TX 77843 2476USA

Tel: 979 845 2774Fax: 979 458 0019E-mail: mnriaz@tamu.edu

IrelandTel: +353 61 202 598Fax: +353 61 331 490E-mail: dick.fitzgerald@ul.ie

Chapter 24

Professor H E Swaisgood

Department of Food Science

North Carolina State University

Clemson UniversityA203J Poole HallClemson

SC 29634-0316USA

Tel: 864 656 1138Fax: 864 656 0331E-mail: pdawson@clemson.edu

Through their provision of amino acids, proteins are essential to human growth,but they also have a range of structural and functional properties which have aprofound impact on food quality Proteins in food processing reviews thegrowing body of research on understanding protein structure and developingproteins as multi-functional ingredients for the food industry.

Chapter 2describes what we know about the common chemical and physicalproperties of proteins and the range of factors that influence how theseproperties are expressed in particular food systems It provides a context for Part

I which discusses the diverse sources of proteins, whether from milk, meat orplants Individual chapters review the structure and properties of these groups ofproteins and ways of improving their functionality as food ingredients.Part II builds on Part I by summarising the range of recent research onanalysing and modifying proteins A first group of chapters reviews ways oftesting and modelling protein behaviour, understanding enzyme activity anddetecting allergenic proteins They are followed by chapters reviewing the range

of techniques for extracting, purifying and modifying proteins The bookconcludes by analysing the many applications of proteins as ingredients, fromtheir use as edible films to their role in modifying textural properties andimproving the nutritional quality of food

The financial support from the Natural Sciences and Engineering ResearchCouncil of Canada is gratefully acknowledged

1

Introduction

R Y Yada, University of Guelph, Canada

2.1 Introduction

The word `protein' is defined as

any of a group of complex organic compounds, consisting essentially ofcombinations of amino acids in peptide linkages, that contain carbon,hydrogen, oxygen, nitrogen, and usually, sulfur Widely distributed inplants and animals, proteins are the principal constituent of the

protoplasm of all cells and are essential to life (`Protein' is derivedfrom a Greek word meaning `first' or `primary,' because of the

fundamental role of proteins in sustaining life.) (Morris, 1992)

Proteins play a fundamental role not only in sustaining life, but also in foodsderived from plants and animals Foods vary in their protein content (Table 2.1),and even more so in the properties of those proteins In addition to theircontribution to the nutritional properties of foods through provision of aminoacids that are essential to human growth and maintenance, proteins impart thestructural basis for various functional properties of foods

The objective of this chapter is to provide an introduction to the chemical andphysical properties of food proteins that form the basis for their structural andfunctional properties However, food scientists wishing to study proteins in foodsystems must be cognizant of the complexity of such systems in terms ofcomposition and spatial organization Food systems are usually heterogeneouswith respect to (a) protein composition (foods usually do not contain a singleprotein entity, but multiple proteins); (b) other constituents (most foods containnot only water and other proteins, but also lipids, carbohydrates as majorcomponents, and various other minor components such as salt, sugars,

micronutrients, minerals, phenolic compounds, flavour compounds, etc.); and(c) structural or spatial organization (proteins exist in foods as tissue systems,gels, coagula, films, emulsions, foams, etc., and not usually as the dilutesolutions or crystalline forms that are typically investigated in model systems).Furthermore, significant changes in the properties of the proteins are induced byenvironmental factors and processing conditions that are typical of food systems.Lluch et al (2001) have written an excellent chapter describing thecomplexity of food protein structures The diversity of the structural role ofproteins in various food raw materials is illustrated by comparing proteinstructures in the muscle tissues of meat, fish and squid, the protein bodies ofplant tissues such as cereals, legumes, oilseeds and shell (nut) fruits, and thecasein micelle structure of bovine milk Interactions of proteins with othercomponents are exemplified in protein-starch interactions observed duringdough processing and baking, protein-hydrocolloid interactions in dairy

Table 2.1 Total protein contents of the edible portion of some foods and beveragesa

products, protein-fat interactions in comminuted meat emulsions, mayonnaiseand cheese, protein-water as well as protein-protein matrix interactions in fishsurimi gels, yogurt and cheese (Lluch et al., 2001).

With this complexity in mind, in addition to describing the basic chemicaland physical properties of proteins and their amino acid building blocks, thischapter provides an overview of the factors that can influence the properties ofproteins in food systems, and suggests approaches that may be useful toelucidate the structure±function relationships of food proteins

2.2 Chemical and physical properties of food proteins

2.2.1 Amino acids commonly found in proteins

It is commonly recognized that 20 amino acids form the building blocks of mostproteins, being linked by peptide (amide) bonds formed between -amino and-carboxylic acid groups of neighbouring amino acids in the polypeptidesequence Nineteen of these 20 amino acids have the general structure of H2N-

CH (R)-CO2H, differing only in R, which is referred to as the side chain, whilethe 20th amino acid is in fact an `imino' acid, in which the side chain is bonded

to the nitrogen atom With the exception of the amino acid glycine, in which theside chain is a hydrogen atom, the -carbon atom exhibits chirality Typically,only the L-form of the amino acids is found in proteins, being incorporatedthrough the transcription and translation machinery of the cell The D-enantiomers of amino acids are present in some peptides

Table 2.2 shows the three-letter and single letter abbreviations as well assome key properties of the 20 amino acids The reader is referred to Creighton(1993) and Branden and Tooze (1999) for illustrations depicting the structure ofthe side chains of the 20 amino acids Similar information can also be viewed atnumerous internet sites, such as those maintained by the Institut fuÈr MolekulareBiotechnologie (2003a), and the Birbeck College (University of London) School

of Crystallography (1996) As shown in Table 2.2, the 20 amino acids can beclassified according to their side chain type: acidic (Asp, Glu), basic (Arg, His,Lys), aliphatic (Ala, Ile, Leu, Val), aromatic (Phe, Tyr, Trp), polar (Ser, Thr),thiol-containing (Cys, Met), amide (Asn, Gln) In addition, as noted above, twoamino acids are unique in being achiral (Gly) or an imino rather than amino acid(Pro)

It is interesting to note that the two amino acid residues occurring at greatestfrequency in proteins possess aliphatic side chains (9.0 and 8.3% for Leu andAla, respectively), while Gly is the third most frequently occurring amino acid at7.2% (Creighton, 1993) With the exception of His, more than 80 or 90% of thebasic and acidic amino acid residues in proteins usually locate such that they areprimarily exposed to the solvent (Institut fuÈr Molekulare Biotechnologie, 2003a;Bordo and Argos, 1991) Similarly, amino acid residues with polar side chains(Ser, Thr, Asn, Gln) as well as Pro are also primarily accessible to the solvent.Conversely, with the exception of Tyr, which contains an aromatic phenolic

Table 2.2 Some properties of the 20 amino acid residues commonly found in proteins

areac chain burial >30AÊ2<10AÊ2(AÊ2) (kcal/mol)

a Mass of the amino acid (from NIST Chemistry WebBook, 2001) minus the mass (18.00) of a water molecule.

b From Creighton (1993).

c From Institut fuÈr Molekulare Biotechnologie (2003a) and Karplus (1997); aliphatic and aromatic surface areas are reported separately for aromatic amino acids; percentages of each residue with solvent exposed area >30AÊ 2 or <10AÊ 2 were calculated based on 55 proteins in the Brookhaven database using solvent accessibility data

group, less than 50% of the aliphatic and aromatic groups have solvent exposedareas greater than 30AÊ Nevertheless, only 40±50% of aliphatic and aromaticresidues would be considered to be `buried', with solvent exposed areas of lessthan 10AÊ These observations indicate that while charged residues are almostalways located near the surface or solvent-accessible regions of proteinmolecules, the converse cannot be assumed for nonpolar aliphatic or aromaticresidues, probably due to insufficient capacity in the interior of the molecule.Thus, both charged and hydrophobic groups reside at the surface or solvent-accessible regions of protein molecules, whereas charged groups are found muchless frequently in the buried interior of protein molecules In fact, it has beenreported that approximately 58% of the average solvent accessible surface or

`exterior' of monomeric proteins is nonpolar or hydrophobic, while 29% and13% of the surface may be considered polar and charged, respectively (Lesk,2001)

Table 2.2shows that 54% of Cys residues are `buried' with solvent-exposedarea <10AÊ, although the estimated hydrophobic effect of Cys side chain burial is0.0 kcal/mol The highly reactive thiol groups of Cys residues may interact withother thiol-containing residues to undergo sulfhydryl-disulfide interchangereactions or oxidation to disulfide groups Internal disulfide bonds frequentlyplay an important role in the stability of the three-dimensional structure ofglobular proteins, while disulfide bonds between Cys residues on the surface ofmolecules may be responsible for the association of subunits or the formation ofaggregates from denatured molecules

Similarly, as mentioned previously, the percentage of buried His residues ishigher than that observed for the other basic amino acid residues The pKaof Hisresidues lies near neutrality, and the ionization state of imidazoyl groups hasbeen implicated in important biological or catalytic functions of His residues,particularly those located in the interior of protein molecules, which may berelated to the unusual ionization properties that can result from the influence ofenvironment in the folded protein molecule

2.2.2 Other naturally occurring amino acids

While most of this chapter will be focused on food proteins composed of the 20amino acids listed in Table 2.2, it is important to acknowledge the presence ofother naturally occurring amino acids, as these can confer distinctive andinteresting properties to some food systems Over 300 naturally occurring aminoacids have been reported, and the reader is encouraged to consult Mooz (1989)and the references cited therein for a listing of these amino acids and theirproperties Some of these amino acids exist as free amino acids, while othershave been found in peptides or proteins

Some examples of the unusual amino acids that have been reported fromfood sources include O-phosphoserine in casein, 4-hydroxyproline in gelatin,4-hydroxy-4-methyl-proline, 4-methylproline and pipecolic acid in apples,citrulline in watermelon, 1-aminocyclopropane-1-carboxylic acid in pears and

apples, 2-alanyl-3-isoxazolin-5-one in pea seedlings, sulfoxide in cabbage, ,-dihydroxynorleucine in bovine tendon, -N-methyl-lysine in calf thymus histone, S-(2-carboxypropyl)-cysteine, S-allylcysteineand other sulfur derivatives in onions, S-methylmethionine in asparagus, S-methylcysteine in Phaseolus vulgaris and hercynin (histidine betaine) inmushrooms.

S-methylcysteine-Other amino acids may be found as a result of processing, such as furosine N-(2-furoyl-methyl)-lysine) and pyridosine (-(1,4-dihydro-y-methyl-3-hydroxy-4-oxo-1-pyridyl)-lysine) in heated milk, or N--(2-amino-2-carboxyethyl)-lysine in alkali-treated protein In addition, , -unsaturatedamino acids stabilized by peptide bond formation are present in natural products.Examples include dehydroalanine and -methyldehydroalanine in the peptidesnisin and subtilin (Fasman, 1989)

(-Incorporation of amino acids that are not coded by mRNA into peptides orpeptidomimetic compounds, has generated much interest due to the increaseddiversity in physicochemical properties with potential pharmacological interest,

as well as to the possibility for reduced sensitivity of such peptides tobiodegradation by peptidases (Sandberg et al., 1998) Recent research reportshave also appeared on methods for genetic encoding of additional amino acids,beyond the 20 amino acids commonly occurring in living organisms Forexample, Mehl et al (2003) reported the generation of a completely autonomousbacterium Escherichia coli with a 21 amino acid genetic code The bacteriumdemonstrated the capacity to synthesize the additional amino acid p-aminophenylalanine from simple carbon sources and to incorporate it intoproteins with fidelity rivaling the common 20 amino acids The authorsconcluded that their pioneering research could open the door to allowinvestigations into the evolutionary consequences of adding new amino acids

to the genetic repertoire, and to generate proteins with novel or enhancedbiological functions

2.2.3 Levels of structural organization

Four levels of hierarchical organization are used to describe protein structure orarchitecture The primary structure of a protein refers to its peptide bond linkedsequence of amino acids, described from the N-terminus to the C-terminus Theprimary structure also includes other covalently bonded structures, such as thelocation of disulfide bridges and the sites of posttranslational modifications ofside chains (e.g methylation, glycosylation, phosphorylation) The enormouspotential for diversity of proteins arises from the fact that theoretically, each site

in the primary sequence could be occupied by one of the 20 amino acids Thus,for example, excluding posttranslationally modified residues and unusual aminoacids, there would be 20100unique sequences of proteins containing 100 aminoacids In fact, only a small percentage of the potential sequences have actuallybeen found to exist in nature As described later, the native structure of mostproteins possess only marginal stability conferred by specific intramolecular

interactions in the folded state Furthermore, the planar nature of the atomsaround the peptide bond and the bulky side chains of some of the amino acidresidues impose restrictions on the flexibility of the polypeptide chains, and thusthe primary structure dictates the final three-dimensional structure of a proteinmolecule.

The secondary structure describes the regular local conformations of thepolypeptide backbone, which are determined by the planarity of the peptidebond, hydrogen bonding between the C=O acceptor and N-H donor groups ofpeptide bonds, and the possible rotation around N-Cand C-C bonds Periodicstructures, such as the -helix or -sheet structures, are characterized byrecurring values of the dihedral phi () and psi ( ) angles, generating auniformity of backbone conformation (Ludescher, 1996; Lesk, 2001) Images ofsome of these periodic secondary structures can be viewed at the IMB JenaImage Library (Institut fuÈr Molekulare Biotechnologie, 2003b) In contrast,aperiodic structures such as reverse ( ) turns or loops involve regular backboneconformations, but without a repeating sequence of dihedral angles Manyvariants of -turns have been described, including -hairpins that link thestrands of an antiparallel -sheet Reverse turns are commonly found on thesurface of proteins, providing purely structural roles in some cases, andfunctional residues accessible to the solvent in other cases

In the most commonly found helical structure, the right handed -helix, with3.6 residues per turn, the characteristic  and angles are approximately ÿ60oand ÿ50o, respectively Intrachain hydrogen bonding occurs between the C=Ogroup at position i with the NH group at position i+4, resulting in a dipolemoment along the helical axis, with a positive pole at the N-terminus andnegative pole at the C-terminus The side chains of the residues point away fromthe surface of the helix, and many -helices possess hydrophilic andhydrophobic faces (Lesk, 2001) The  and angles are approximately ÿ70oand ÿ20oor less, respectively, for the more tightly packed 310helix with an i+3hydrogen bonding pattern (Ludescher, 1996; Institut fuÈr MolekularBiotechnologie, 2003b) The polyproline II conformation found in collagenand gelatin is also an example of a periodic secondary structure, but is anextended, left-handed helical structure with 3.3 residues per turn, and  and angles of ÿ80oand +150o, respectively Unlike the other helical structures, thepolyproline II structure is not stabilized by intra chain hydrogen bonds, but byspecific conformational restraints resulting from the many proline andhydroxyproline residues that are characteristic of the collagen molecule(Ludescher, 1996)

The individual -strands of a -pleated sheet have a helical structure arisingfrom the recurring  and angles of 120oand +140o, respectively, while thefully extended polypeptide chain has both  and angles at 180o Inter-chainhydrogen bonding occurs between two or more ... structuralproteins, contractile proteins, storage or nutrient proteins, regulatory proteins, defense proteins, etc Alternatively, proteins can be viewed in terms of theirfunctional role in food systems... or basic proteins, sulfur-containing proteins, proline-rich proteins) , shape (e.g globular or fibrous), secondary structure propensity(e.g proteins with predominantly -sheet or predominantly -helical... classification of proteins such as albumins, globulins, glutelinsand prolamins, is an example of applying a functional attribute (solubility) todistinguish food proteins (Regenstein and Regenstein, 1984;