Methods of Testing Protein Functionality

Protein functionality is big business in the food industry, both academic and commercial. This reflects the fascination of protein chemistry and the commercial value of proteins in giving texture and substance to foods. My first encounter with the subject came about through an interest in assessing the extent of thermal damage in spray-dried proteins. A change in functional properties seemed a useful guide to the thermal damage. I found the literature replete with methods (and theory) and so did what everybody else seemed to do - I devised my own methods which were slight variations on the general theme. However, being an analyst at heart I thought there must be a systematic approach to the various tests available. This book is an attempt to assess the practice of protein functionality testing, keeping theory to a minimum, followed by a suggested 'standard' method or protocol to follow. Each chapter is devoted to a single functional property and the reader will see that attempts to 'standardise' them have had varied success, perhaps reflecting the complexity of the mechanisms at work. As mentioned above theory has been kept to a minimum but hopefully it supports the practical methodology described. I hope that the book stimulates debate on the subject; it is certainly not the last word! G. H. Contributors Dr G. E. Arteaga Centro de Investigacion en Alimentation y Desarrollo, Hermosillo, Sonora AP 1735, Mexico Dr. S.

1 Basic concepts.- 1.1 Introduction.- 1.2 Sources of functional proteins.- 1.3 Factors affecting protein functional properties.- 1.3.1 Intrinsic factors.- 1.3.2 Processing treatments.- 1.3.3 Application conditions.- 1.4 Conclusions.- References.- 2 Solubility.- 2.1 Introduction.- 2.1.1 Solubility definition and theory.- 2.1.2 Factors that influence protein solubility.- 2.1.3 Definition of terms used for protein solubility.- 2.2 Review of methods for measuring solubility.- 2.2.1 Protein solubilization.- 2.2.2 Separating the undissolved protein (clarification).- 2.2.3 Methods for measuring protein concentration.- 2.3 Protocols for methods.- 2.3.1 Protein dispersion and solubilization.- 2.3.2 Clarification and phase separation.- 2.3.3 Determination of protein concentration.- Acknowledgement.- References.- 3 Viscosity.- 3.1 Introduction.- 3.2 What is viscosity?.- 3.2.1 Factors which affect viscosity.- 3.2.2 Non-linear viscosity.- 3.2.3 Viscosity of suspensions.- 3.3 Viscometers for measuring shear viscosity.- 3.3.1 Rotational viscometers.- 3.3.2 Other techniques.- 3.4 Protocols for measuring viscosity.- 3.4.1 Shear viscosity.- 3.5 Summary.- References.- List of manufacturers.- 4 Gelation.- 4.1 Introduction.- 4.2 Scientific basis of protein gelation.- 4.2.1 Definition of gelation and types of gels.- 4.2.2 Conformational change of proteins.- 4.2.3 Chemical forces involved in gelation phenomena.- 4.2.4 Rheological aspects.- 4.3 Methods of testing gel properties of proteins.- 4.3.1 Determination of heating temperature.- 4.3.2 Detection and analysis of the early stage of gelation.- 4.3.3 Detection of the sol-gel transition (determination of gelling points).- 4.3.4 Effects of pH and ionic strength.- 4.3.5 Spectroscopic analysis of protein structural change through the gelation process.- 4.3.6 Contribution of chemical bonding to the gelation process.- 4.3.7 Rheological properties of gels.- 4.3.8 Microscopic observation.- 4.4 Protocol for testing the gelling ability of proteins.- 4.4.1 Information on samples.- 4.4.2 Solubilization or dispersion.- 4.4.3 Determination of the denaturation temperature.- 4.4.4 Determination of the gellingpoint.- 4.4.5 The effects of pH, ionic strength and other conditions.- 4.4.6 Evaluation of gel properties.- Acknowledgement.- References.- 5 Foam formation and stability.- 5.1 Scientific background.- 5.1.1 Introduction.- 5.1.2 Principles of protein foam formation.- 5.1.3 Principles of protein foam stability.- 5.1.4 Improving protein foam production.- 5.2 Review.- 5.2.1 Methods of foam generation.- 5.2.2 Methods of protein foam measurement.- 5.2.3 Analysis.- 5.3 Protocols.- 5.3.1 Pure proteins.- 5.3.2 Crude protein extracts.- 5.3.3 Real systems.- References.- 6 Emulsions.- 6.1 Introduction.- 6.2 Emulsion definition.- 6.3 Forces involved with emulsions.- 6.3.1 Attractive forces.- 6.3.2 Repulsive forces.- 6.3.3 Steric forces.- 6.3.4 Depletion forces.- 6.4 Emulsion stability.- 6.4.1 Creaming and drainage.- 6.4.2 Flocculation.- 6.4.3 Coalescence.- 6.4.4 Phase inversion.- 6.5 Stabilizers and emulsifiers.- 6.5.1 Interfacial properties.- 6.5.2 Types of protein.- 6.5.3 Protein modification.- 6.5.4 Mixed biopolymers.- 6.6 Forming emulsions.- 6.6.1 Procedure for the laboratory scale production of an emulsion.- 6.7 Methods of measuring emulsification properties.- 6.7.1 Measurement of droplet size.- 6.7.2 Emulsifying activity (EA).- 6.7.3 Emulsion capacity (EC).- 6.7.4 Oil phase volume (OPV).- 6.7.5 Emulsion stability (ES).- 6.8 Other useful parameters when assessing emulsion properties.- 6.8.1 Surface hydrophobicity.- 6.8.2 Viscosity.- 6.9 Conclusions.- References.- 7 Determining water and fat holding.- 7.1 Introduction.- 7.2 Scientific basis.- 7.3 Waterholding.- 7.3.1 Press method.- 7.3.2 High-speed centrifugation.- 7.3.3 Low-speed centrifugation.- 7.3.4 Capillary suction methods.- 7.3.5 Absorption/rehydration methods.- 7.3.6 Optical methods.- 7.3.7 Microscopy.- 7.3.8 Special instruments (NMR, DSC, X-ray).- 7.4 Fat holding.- 7.4.1 Review of methods.- 7.4.2 Cooking/processing.- 7.4.3 High-speed centrifugation.- 7.4.4 Low-speed centrifugation.- 7.4.5 Emulsification test.- 7.4.6 Solvent extraction.- 7.4.7 Optical methods.- 7.4.8 Microscopy.- 7.4.9 Special instruments.- 7.5 Development of a test protocol.- 7.6 Conclusions.- References.- 8 Measurement of surface hydrophobicity.- 8.1 Introduction.- 8.1.1 Definition of hydrophobicity.- 8.1.2 Protein structure and surface hydrophobicity.- 8.1.3 Quantitative structure-activity relationships.- 8.2 Measurement of surface hydrophobicity.- 8.2.1 Hydrophobicity scales for proteins.- 8.2.2 Empirical scales.- 8.2.3 Comparison of relative hydrophobicity values of proteins.- 8.2.4 Prospect of measurement of protein surface hydrophobicity: exposure of hydrophobic side chains.- 8.3 Experimental protocols.- 8.3.1 Probe spectrofluorometry.- 8.3.2 Hydrophobic-interaction chromatography.- 8.3.3 Contact angle determination.- 8.3.4 SDS binding capacity.- 8.3.5 Triglyceride binding capacity.- 8.3.6 Hydrophobic partition.- References.