Heat is the major way in which physical modifications are carried out with proteins. For example, the industry is replete with many soy protein products that have been modified by heating to improve foaming, emulsification, water-binding, and gelation properties (38). Heat causes proteins to partially denature through changes in second- ary and primary structure. In theory, this should increase the functionality of the protein by improving its ability to unfold at interfaces and form films around air bubbles or lipid droplets. In particular, the gelling ability of several proteins, such as those from dairy whey, has been sub- stantially improved through heat treatment (4). Many of the procedures used to manufacture heated protein ingre- dients for use in formulated foods are proprietary. It is known, however, that the procedures used to maximize the functionality of a protein source for one application are usually not the same for another. In other words, heat- ing techniques used to enhance foaming or gelling are not the same as for emulsification (4).
A specific type of modification of plant proteins using heat has widespread application. Texturized Vegetable Protein (TVP) is commonly used in the processed food and food service industry to bind water and fat. Most TVP products are made from soy and are manufactured under very specific environmental conditions of temperature, pH, ionic strength, and pressure. TVP is produced using extrusion techniques where the proteins are subjected to heat, pressure, and shear forces simultaneously. The pro- teins are extensively denatured and insolublized; however, the resulting matrix is almost sponge-like in its ability to bind other liquids, such as excess fat and moisture, in food systems.
One interesting application of physical alteration of proteins is the manufacture of several fat substitutes through the manipulation of their physical characteristics. Using a process generically called “microparticulation,”
proteins from milk and/or eggs are denatured and refolded into smaller, denser particles that can act as lubricants during the chewing and swallowing of foods. These mate- rials have the mouthfeel of lipids, but are actually a blend of proteins and water. Simplesse (registered trade name by the NutraSweet Company) is an example. Since these ingredients are proteins, they cannot be used in products that will see extremes of environment, such as heat or pH, since they would most likely denature under such condi- tions. Their use is probably limited to products that will only see low temperature heating processes, such as pas- teurization.
IV. SUMMARY
The functional properties of proteins are defined as their physical or chemical properties that affect foods during their preparation, processing, storage, and consumption. They contribute greatly to the quality and acceptability of a wide range of natural and processed/prepared foods in the food supply. Proteins are considered by many experts to be the most multifunctional components of foods in that they can play many different roles in foods. Natural products made from meat, poultry, dairy, eggs, cereal grains, and legumes, as well as the majority of formulated foods developed and marketed through the food industry, all rely on the functionality of proteins for their accept- ability and quality.
The functional properties include solubility, water- holding ability or capacity, gelation, emulsification, and foaming. Solubility is paramount for the successful use of proteins in most food systems. To function in a food the protein must be able to migrate throughout the aqueous phase to seek interfaces (foaming and emulsification), hold water, or form extensive three-dimensional networks (gelation). The solubility of a protein is determined by its amino acid sequence and is greatly influenced by envi- ronmental factors such as solvent polarity, pH, tempera- ture, and concentration of dissolved salts.
Food gels are diverse structures primarily composed of immobilized water held within a cross-linked protein matrix. Both thermally reversible (gelatin) and thermally irreversible (frankfurters) gels are important food prod- ucts. The ability of a protein to gel is primarily evaluated by measuring the texture or strength of the gel using a tor- sion test.
Water binding by proteins is influenced by tempera- ture, concentration of protein, concentration of salt, degree of protein denaturation, and the presence of other compounds, such as sugars or alcohols. Two main meth- ods, water absorption and expressible moisture, are com- monly used to measure the water binding of proteins.
Emulsification and foaming are cousins in the protein functionality family. They both rely on similar properties of the protein in the food system, such as solubility and
ease of denaturation. Proteins must be able to migrate to the interface formed with the second phase of oil in emul- sions and air in foams. Proteins must be able to form cohesive, multilayered, flexible films around droplets of lipid or air. The main model tests involve measuring the overall ability of a protein to emulsify or foam, as well as the stability of the emulsion or foam created.
The functional properties of proteins are often meas- ured using model systems that can vary widely from researcher to researcher. To make the results of our research valuable, we must strive to use protocols that are widely accepted and used by our colleagues. In many cases, model systems do not mimic the conditions seen in real food systems. We tend to measure, and correctly so, the fundamental properties that are related to the desired functional property seen in the food.
The modification of proteins by chemical, physical, and enzymatic methods has increased the utilization of less conventional protein sources and decreased the costs of manufacture for a number of food products. Most methods attempt to increase the solubility of proteins. Although chemical methods are effective, there are ques- tions regarding safety and nutritional losses. Enzymatic and physical methods have been adapted for use in the protein ingredients industry. Heating remains the most widely used modification technique and has been very successful in increasing the functionality of a wide range of proteins, particularly those from soybeans and whey. Texturization produces very useful protein ingredients which have the ability to adsorb excess moisture and fat in foods. Lastly, the ability of proteins to be compressed allows them to be used as fat substitutes in foods.
REFERENCES
1. Smith, D.M. and Culbertson, J.D. 2000. Proteins: Functional Properties, Ch. 9. In Food Chemistry: Principles and Applications, G.L. Christen and J.S. Smith (Eds.). pp. 131–148. STS. West Sacramento, CA. 2. Smith, D.M. 2003. Measurement of Functional Properties: Overview of Protein Functionality Testing. Unit B5.1.1–B5.1.9. In Current Protocols in Food Analytical Chemistry, Vol. 1, E.W. Harkins (Ed.). pp. Unit B5.1.1. John Wiley & Sons, Inc., New York. 3. Schwenke, K. D. 1997. Enzyme and chemical modifica-
tion of proteins. Ch 13. In Food Proteins and Their Applications, S. Damodaran and A. Paraf (Eds.). pp. 393–423. Marcel Dekker, Inc., New York.
4. Beuschel, B.C., Culbertson, J.D., Partridge, J.A. and Smith, D.M. 1992. Gelation and emulsification proper- ties of partially insolubilized whey protein concentrates. J. Food Sci. 57:605–609.
5. Hung, T.Y. and Smith, D.M. 1993. Dynamic rheological properties and microstructure of partially insolubilized whey protein concentrate and chicken breast salt soluble protein gels. J. Agric. Food Chem. 41:1372–1378.
6. Kneifel, W. Paquin, P, Albert, T. and Richard, J.P. 1991. Water-holding capacity of proteins with special regard to milk proteins and methodological aspects—A review. J. Dairy Sci. 74:2027–2041.
7. AOCS. 1999. Official Methods and Recommended Practices, 5th ed. American Oil Chemists Society, Champaign, Ill.
8. AACC. 2000. Approved Methods, 10th ed. American Association of Cereal Chemists, St. Paul, MN. 9. Clark, A. H. 1992. Gels and Gelling. Ch. 5. In Protein
Functionality in Foods, H.G. Schwartzberg and R.W. Hartel (Eds.). pp. 263–305. Marcel Dekker, Inc., New York. 10. Doi, E. 1993. Gels and gelling of globular proteins.
Trends Food Sci. Technol. 4:1–5.
11. Foegeding, E.A. and Hamann, D.D. 1992. Physico- chemical aspects of muscle tissue behavior. Ch 8, in Physical Chemistry of Foods. H.G. Schwartzberg and R.W. Hartel (Eds.). pp. 423–441. Marcel Dekker, Inc., New York.
12. Gel Consultants, Inc. 1991. Torsion Gelometer Manual, Gel Consultants, Inc., Raleigh, NC.
13. Vittayanont, M., Steffe, J.F., Flegler, S.L. and Smith, D.M. 2003. Gelation of chicken pectoralis major myosin and heat-denatured beta-lactoglobulin. J. Agric. Food Chem. 51:760–765.
14. Lee, C.M., Filipi, I.Y., Xiong, Y., Smith, D.M., Regenstein, J., Damodaran, S., Ma, C.Y. and Haque, Z.U. 1997. Standardized failure compression test of pro- tein gels from a collaborative study. J. Food Sci. 62:1163–1166.
15. Richardson, R.I. and Jones, J.M. 1987. The effects of salt concentration and pH upon water binding, water- holding and protein extractability of turkey meat. Int. J. Food Sci. Technol. 22:683–692.
16. Honikel, K.O. 1987. How to measure the water binding capacity of meat: Recommendation of standardized methods. In Evaluation and Control of Meat Quality in Pigs, P.V. Tarrant, G. Eikelenboom and G. Monin (Eds.). pp. 129–142. Martinus Nijhoff. Dordrecht, the Netherlands.
17. Krocher, P.N. and Foegeding, E.A. 1993.
Microcentifuge-based method for measuring water- holding of protein gels. J. Food Sci. 58:1040–1046. 18. Cameron, D.R., Weber, M.E., Idziak, E.S., Neufeld, R.J.
and Cooper, D.G. 1991. Determination of interfacial areas in emulsions using turbidimetric and droplet size data; Correction of the formula for emulsifying activity index. J. Agric. Food Chem. 39:655–659.
19. Dickinson, E. 1994. Protein stabilized emulsions. J. Food Engineering 8:59–74.
20. Halling, P.J. 1981. Protein stabilized foams and emul- sions. Crit. Rev. Food Sci. Nutr. 15:155–203.
21. Nakai, S., Li-Chan, E. and Arteaga, G.E. 1996. Measurement of surface hydrophobicity. In Methods of Testing Protein Functionality, G.M. Hall (Ed.). pp. 226–259. Blackie Academic and Professional. New York. 22. Damordaran, S. 1997. Protein-stabilized foams and emulsions. Ch. 3. In Food Proteins and Their Applications, S. Damordaran and A. Paraf (Eds.). pp. 57–110. Marcel Dekker Inc., New York.
23. Wang, J.C. and Kinsella, J.E. 1976. Functional proper- ties of novel proteins: Alfalfa leaf protein. J. Food Sci. 41:286–292.
24. Smith, D.M. 2003. Measurement of Functional Properties: Overview of Protein Functionality Testing. Unit B5.1.1–B5.1.9. In Current Protocols in Food Analytical Chemistry, Vol. 1, E.W. Harkins (Ed.). pp. B5.1.6. John Wiley & Sons, Inc., New York.
25. Pearce, K.N. and Kinsella, J.E. 1978. Emulsifying prop- erties of proteins: Evaluation of a turbidimetric tech- nique. J. Agric. Food Chem. 26:716–723.
26. Weiss, Jochen. 2002. Emulsion Stability Determination. Unit D3.4.1–D3.4.17. In Current Protocols in Food Analytical Chemistry, Vol. 1, E.W. Harkins (Ed.). pp. D3.4.4. John Wiley & Sons, Inc., New York.
27. Fligner, K.L., Fligner, M.A. and Mangino, M.E. 1991. Accelerated tests for predicting long-term creaming sta- bility of infant formula emulsion systems. Food Hydrocolloids 5:269–280.
28. McClements, D.J. 1999. Food Emulsions: Principles, Practice and Techniques. CRC Press, Boca Raton, FL. 29. Wilde, P.J. and Clark, D. C. 1996. Foam formation and
stability. Ch. 5. In Methods of Testing Protein Functionality, G.M. Hall (Ed.). pp. 110–152. Blackie Academic and Professional, New York.
30. Dickenson, E. 1994. Protein stabilized emulsions. J. Food Engineering. pp. 59–74.
31. Phillips, L.G., German, J.G., O’Neil, T.E., Foegeding, E.A., Harwalker, V.R., Kilara, A., Lewis, B.A.,
Mangino, M.E., Morr, C.V., Regenstein, J.M., Smith, D.M. and Kinsella, J.E. 1990. Standardized procedure for measuring foaming properties of three proteins: A collaborative study. J. Food Sci. 55:1441–1444. 32. Phillips, L.G., Haque, Z. and Kinsella, J.E. 1987. A
method for the measurement of foam formation and sta- bility. J. Food Sci. 52:1074–1077.
33. Schwenke, K.D. 1997. Enzyme and chemical modifica- tion of proteins. Ch. 13, In Food Proteins and Their Applications, S. Damoraran and A. Paraf (Eds.). pp. 393–423. Marcel Dekker, Inc., New York.
34. Singh, H. 1991. Modification of food proteins by covalent cross-linking. Trends in Food Sci. Tech. 2:196–200.
35. Vojdani, F. and Whitaker, J.R. 1994. Chemical and enzymatic modification of proteins for improved func- tionality. Ch. 9. In Protein Functionality in Food Systems, N.V. Hettiarachchy and G.R. Ziegler (Eds.). pp. 261–309. Marcel Dekker, Inc., New York.
36. Kato, A., Wada, T., Kobayashi, K., Seguro, K. and Motoki, M. 1991. Ovomucin-food protein conjugates prepared through the transglutaminase reaction. Agric. Biol. Chem. 55:1027–1031.
37. Hamada, J.S. 1992. Effects of heat and proteolysis on deamidation of food proteins using peptidoglutaminase. J. Agric. Food Chem. 40:719–823.
38. Hill, S.E. 1996. Emulsions. Ch. 6. In Methods of Testing Protein Functionality, G.M. Hall (Ed.). pp. 153–185. Blackie Academic and Professional, New York.
I. INTRODUCTION
Although the intake of excess fat may result in significant health problems, it is important to note that fats are a crit- ical part of a proper diet. The human body can produce most fatty acids but certain “essential” fatty acids (e.g.,
linoleic and linolenic acid) are typically derived from lipid-containing foods since the body cannot produce them. In addition, fats in foods are sources of vitamins A, D, E, and K. There is considerable evidence that thera- peutic components that can improve human health are endogenous to food fats. Fats also contribute unique
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