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Enhancing Wet Cellulose Adhesion with Proteins Xin Li and Robert Pelton* McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering, JHE-136, McMaster University, Hamilton, Ontario, Canada L8S 4L7
Twenty proteins were compared as potential paper wet strengthening additives by measuring the peel force required to delaminate wet, regenerated cellulose films laminated with a thin (3 mg/m2) protein layer. Wet adhesion results ranged over nearly an order of magnitude, reflecting the importance of protein composition. The proteins with the highest contents of lysine and arginine gave the strongest adhesion with secondary contributions from hydroxyl and phenolic amino acid residues. Wet adhesion was performed with TEMPO oxidized cellulose and with laminates that were cured at high temperatures (120 °C), suggesting that protein grafting to the cellulose and protein cross-linking was important for good wet strength. Although none of the protein laminates was as strong as polyvinylamine or a commercial PAE resin used in the paper industry, this paper suggests that increasing the primary amine (amino group) content as well as optimizing heat-induced bond formation may someday lead to a protein-based paper wet strength resin. Introduction Wet paper is usually weak because fiber-fiber joints swell and lose adhesive contact. Although moisture sensitivity is advantageous for recycling and biodegradation, there are many applications where paper must have temporary or permanent wet strength. Examples include towels, tissue, and packaging. Surface hydrophobization with sizing agents will protect paper from casual water contact; however, to have strong wet paper requires that the fiber-fiber bonds be reinforced with wet strength resins. Conventional wet strength resin technology and mechanisms have been described in a number of recent reviews.1,2 In most cases, the key element of wet strength resins appears to be the ability to cross-link and to graft upon the fiber surfaces to minimize fiber joint swelling. Because nature provides many spectacular examples of high wet strength, including our own bodies, we believe that it should be possible to engineer proteins and/or other biomacromolecules to achieve improved wet strength in paper products. Advantages over existing technologies might include the ability to use enzyme enhanced repulping and the absence of organo chlorine, formaldehyde, and other undesirable synthetic chemicals potentially released into water or air.3,4 The paper industry has long used proteins as surface sizing and coating agents.5-8 Many wet proteins are naturally sticky, and several proteins have been used as wood adhesives for indoor and outdoor applications.9-12 However, based on these disclosures, it is difficult to form general conclusions about the types of protein structures likely to be effective wet strengthening agents. In a previous publication, we reported an investigation of simple synthetic polypeptides as potential wet strengthening agents.13 For this, we developed a new laboratory method for evaluating wet strengthening agents including biomacromolecules. Our approach was to wet laminate two regenerated cellulose films using * To whom correspondence should be addressed. Phone: (905) 525-9140 ext 27045. Fax: (905) 528-5114. E-mail:
[email protected].
a very thin layer of the target macromolecule as adhesives for the two cellulose plies. The laminate was dried, rewetted, and the 90° delamination peel force was measured for the water saturated laminate. This approach offers a number of advantages over conventional handsheet testing including that (1) the quantity of polymer in the failure plane can be carefully controlled; (2) there are no artifacts associated with polymer induced fiber flocculation; (3) polymers that do not spontaneously adsorb onto aqueous wood pulp fibers can be evaluated; and (4) the method is relatively rapid. Using this methodology, we made the following conclusions linking polypeptide structure to adhesion. Polylysine, a synthetic polypeptide with pendant primary amine groups, gave wet strength if the laminate was first equilibrated at 50% relative humidity, 23 °C (Tappi test guideline T402), whereas never-dried laminates where very weak. Thus, the polypeptide showed promise as a wet strength agent but is unlikely to improve wet-web strength, a more difficult target. Interestingly, the incorporation of tyrosine, an amino acid with pendant phenolic groups, increased wet adhesion, which was counter intuitive. Nevertheless, none of the polylysines was as effective as polyvinylamine, an analogous synthetic polymer.14,15 In this paper, we present results of a survey of 20 proteins and additional synthetic polypeptides. Our goal is to identify those protein characteristics that are most clearly are associated with wet adhesion. Experimental Procedures Protein, Peptides, and Polymers. Twenty proteins and peptides were purchased from Sigma, and their properties are summarized in Table 1. The proteins were dispersed in 20 mM phosphate buffer (pH 7.2) and stored at 4 °C. Carboxymethyl cellulose (250 000 Da and degree of substitution 1.2, Aldrich) was used as supplied. Spectra/Por 2 regenerated cellulose dialysis membranes (product no. 132682, Spectrum Laboratories, Inc.) were provided as cylindrical membranes that were cut into pieces of 2 cm × 6 cm (top ply) or 3 cm × 6 cm (base ply) and soaked in deionized water (MilliQ) for 30 min to remove water-soluble impurities. After soak-
10.1021/ie050635c CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005
Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7399 Table 1. Proteins, Polypeptides, and Polymers Evaluated as Adhesives for Wet Cellulose sample
designation
Albumin Albumin Albumin Albumin Casein Fibrinogen Gelatin Gelatin Gliadin Globulins Hemoglobin Lysozyme Peptone soy protein wheat gluten Zein Histone Insulin Tyrosinase Protamine poly-L-arginine poly (Arg, Pro, Thr) 6:3:1 poly-L-asparagine poly (Glu, Thr) 1:1 poly-L-lysine poly (Lys, Ser) 3:1 poly-L-glutamic acid
AE AG AS BSA CA FG GA GB GD GG HG LY PM SP WG ZN HT IS TY PT PR PRFT6 PN PET PK PKS PE
molecular wt (kDa) 33.8
comments from chicken egg white, grade II from goat, fraction V from sheep serum, fraction V from bovine serum, fraction V from New Zealand bovine milk factor 1, type IV type A, from porcine skin type B, from bovine skin from wheat, crude from goat, Cohn fraction II and III from bovine blood from chicken egg white from meat type 1, not roasted from wheat, crude from corn from calf thymus, type VIII-S from bovine pancreas from mushroom grade IV, from salmon
45.0 66 ∼30 47-63.5 10-80 81 50 14.3 N/A N/A 19.4 27.8 14.7 60.7 10-30 15.0 14.6, 27.4, 57.8, 152, and 353 31.0 112
ing, the membranes were rinsed thoroughly with deionized water. The washing process was carried out 3 times. The inside surfaces of the tubular membranes were used for the adhesion measurements. For some experiments, the cellulose membrane was oxidized with TEMPO using the method described by Iosagai and coworkers.16,13 In a typical experiment, 21 pairs of cut cellulose membrane strips were added into 1190 mL of a solution containing 40.6 mg of 2,2,6,6-tetramethyl-1piperidinyloxy radical (TEMPO), 405.7 mg of sodium bromide, and 1.6 mL of 0.0039 M sodium hypochlorite solution. The oxidizing reaction was carried out at room temperature for 30 min, and the pH was maintained at 10.5 by 1 M NaOH addition. Preparation of Cellulose Film Laminates. Strips of regenerated cellulose films were laminated with proteins by the following procedure. A wet base membrane was carefully placed on the surface of a polished stainless steel plate (Tappi standard drying plate), and a 10 mm wide, 54 µm thick ribbon of PTFE tape (Masters Thread Seal Tape, G. F. Thompson Co. Ltd. Canada) was placed across one end of the base strip. A drop of 15 µL of protein solution (0.222 mg/mL) was placed in the center of the base membrane, and the top membrane was carefully laid over the drop in an effort to produce a uniform distribution of protein between the membranes while minimizing loss of protein solution. For most experiments, the film-protein-film specimen and base plate were covered with two blotting papers and dried at room temperature for 30 min. Then, the film-protein-film specimen was removed from the steel plate, covered with two new blotting papers, and dried at room temperature for another 30 min. The room temperature dried specimens were further pressed at 2.2 MPa and 120 °C for 10 min. After pressing, the specimens were conditioned at 23 °C and 50% relative humidity for 24 h. Wet Peel Strength. Peeling tests were performed using an Instron model 4411 universal testing system (Instron Corporation, Canton, MA) with the lower jaw replaced with a freely rotating 14 cm diameter aluminum peeling wheel running on SKF-6,8-2RS1 radial bearings,
which has a 40 mm wide flat outer surface. On the basis of a Paprican design, peeling from a sample attached to the surface of the peel wheel gives a 90° peeling geometry.17 In a typical experiment, a laminated film was soaked in 20 mM phosphate buffer (pH 7.2) for 30 min at room temperature. The wet laminates were taken out of buffer and then gently pressed between blotting paper to remove excess buffer. The base laminate was fixed to the peel wheel with 3M Medical Double Coated Tape (polyethylene Model No: 1522), a 4 cm wide, water-resistant double-sided adhesive tape. The edge of the top membrane was separated from the Teflon tape and clamped in the upper jaw of the Instron. The peeling force was measured with a 50 N load cell and the elongation rate was 20 mm/min. In most cases, three or four samples were tested for each experimental condition. The results are expressed as N/m obtained by dividing the steadystate peel force by the sample width. Polymer Films. Heat treatment may cause covalent cross-linking of protein layers. To test for this possibility, thin films of model polymers were heat-treated, and swelling/dissolution behaviors of the dried films were used as indicators of covalent cross-linking. Three types of films were cast: one from CMC; one from poly-Llysine (PK); and one from a mixture of CMC and 2 wt % PK. Two g of CMC was dissolved in 100 mL of deionized water and diluted with 100 mL of pure formic acid, giving a 1% (w/v) CMC solution. A total of 0.1 g of PK was dissolved with 10 mL of deionized water, and the final concentration of PK was 1% (w/v). For the one component films, 3.6 mL of CMC or PK solution was evaporated in a 35 mm Falcon Polystyrene Tissue Culture Dish, giving dried films approximately 47 µm thick. For the mixed film, 0.78 mL of PK solution was mixed with 2.82 mL of CMC solution. After drying at room temperature, some films were hot pressed (2.2 MPa, 120 °C) for 10 min. All films were conditioned at 23 °C and 50% relative humidity for 24 h before testing. The solubility of the films was determined by immersion in deionized water, 20 mM phosphate buffer
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Figure 1. Effect of amino residues on wet peel strength. Polypeptide was coated with an aqueous solution at pH 7.2 for PE and PK and at pH 4.0 for PN, PR, and PRFT6. The protein coverage was 3.33 mg/m2. The laminates were dried at 120 °C for 10 min and rewetted at the same pH used to apply the protein.
Figure 2. Influence of hydroxyl bearing comonomers on the wet adhesion with polyglutamic acid and polyglycine. The polypeptide coverage was 3.33 mg/m2.
(pH7.2), NaOH (pH 12.5), or 50% formic acid solution at room temperature. After 30 min, the residual films were rinsed with same solvent and then with deionized water. After washing, the films were dried (4 h, 105 °C) and weighed. Results Model Polypeptides. In a preliminary investigation, we reported the adhesion characteristics of cellulose films laminated with simple synthetic polypeptides.13 The main conclusions were that polylysine-rich structures gave the highest wet adhesion; adhesion was much
higher with oxidized cellulose membranes; and there appeared to be an advantage to having some tyrosine tryptophan or phenylalanine. However, this preliminary study only involved laminates slowly dried at room temperature as compared to the high (120 °C) temperature drying used in this investigation. Figure 1 compares five synthetic polypeptides; polylysine (PK) and polyarginine (PR) gave the highest adhesion, whereas carboxylated polypeptides gave very low adhesion. The terpolymer PRFT6 gave intermediate adhesion. Note that a commercial PAE resin known for strong permanent wet strength (3.33 mg/m2 Kymene from Hercules;
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Figure 3. Effect of poly-L-lysine molecular weight on wet peel strength. Polylysine coverage was 3.33 mg/m2. Testing specimens were rewetted in 20 mM phosphate buffer (pH 7.2) for 30 min.
oxidized cellulose membranes; hot pressed at 120 °C, 20 000 lbs, for 10 min) gave a wet peel strength of 96 N/m in our experiments. Heating the laminates provides an opportunity for covalent bond formation such as the condensation of alcohols with carboxylic acids. Figure 2 shows the effect of including hydroxyl containing amino acids. For PE and PK, adding hydroxyl moieties did not influence wet adhesion. The cellulose substrate employed in this work was a porous dialysis membrane with a molecular weight cutoff of 12-14 kDa. Thus, we would expect polypeptides and proteins, larger than the cutoff, to remain on the exterior surfaces, whereas smaller macromolecules could spread throughout the film structure, giving lower adhesion. Furthermore, the cohesive strength of low molecular weight polymers increases with molecular size reflecting the role of entanglements. Figure 3 shows the influence of a polylysine molecular weight on laminate adhesion. Adhesion increased with molecular weight up to 150 kDa, above which it was approximately constant. One of the potential mechanisms by which proteins influence wet strength is via the formation of amide bonds between lysine and surface carboxyl groups on cellulose. To determine if this was possible under the heated drying conditions used in this work, films of CMC, polylysine, and their mixtures were cast, heated, and immersed in various solvents; the results are summarized in Table 2. The one-component films where destroyed by the solvents irrespective of the drying temperature. By contrast, the heated mixed films were only swollen by the solvents, indicating the formation of amide cross-links between lysine and carboxyl moieties. As expected, high temperature was required for covalent bond formation; the unheated mixed films dissolved in the solvents. Proteins. The influence of goat globulin on the wet delamination peel strength of oxidized and untreated cellulose membranes was measured as a function of
Figure 4. Effect of dosage of goat globulin on the wet peel strength of laminates based on oxidized and nonoxidized cellulose membranes. The laminates were heated to 120 °C and soaked for 30 min in 20 mM phosphate buffer (pH 7.2) before testing.
Figure 5. Effect of curing time on wet peel strength of oxidized cellulose films treated with HT and SP. The protein concentrations were 1.66 mg/m2, and the laminates were pressed using 2.2 mPa at 120 °C.
protein coverage, and the results are summarized in Figure 4. The protein contents are expressed as mass per area of the joint. Up to 10 mg/m2 oxidized cellulose membranes gave substantially higher adhesion, whereas the trend was reversed at higher protein coverage. In an effort to minimize protein use, most measurements were made at 15 mg/m2 for nonoxidized membranes and only 3.33 mg/m2 for laminates based on oxidized cellulose films. These extremely low quantities of protein are in the same order of magnitude as those obtained by equilibrium adsorption from solution.18 Furthermore, such low protein contents ensure that the protein/ cellulose interface must be involved in the failure because the protein films would be too thin to exhibit exclusively cohesive failure. The laminates were cured in a press at 120 °C. The influence of drying time on wet laminate adhesion is shown in Figure 5 for two proteins and for a protein-
Table 2. Mass Fraction of Films of Carboxymethyl Cellulose (CMC), Polylysine, and CMC-Polylysine Mixtures Remaining after 30 min Solvent Immersion at Room Temperature mass fraction of films (%) solvents drying temp. °C 50% formic acid DI water 20 mM phosphate buffer, pH 7.2 NaOH, pH 12.5
CMC 120 0 0 0 0
poly-L-lysine 23 0 0 0 0
120 0 0 0 0
CMC with poly-L-lysine 23 0 0 0 0
120 100 100 100 100
23 0 4.8 51 6.4
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Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 Table 3. Amino Acid Content of Protein (Mol Fractions) aspartic acid glutamic acid serine threonine tyrosine arginine lysine histidine
Figure 6. Effect of protein type (see Table 1) on wet peel strength. GD, SP, WG, and ZN were dissolved in aqueous sodium hydroxide at pH 12.5, whereas the other proteins were dissolved in 20 mM phosphate buffer at pH 7.2 for preparation of the laminates. The laminates were rewetted with water at the same pH and ionic strength used to apply the protein.
free control experiment. Low drying times gave little adhesion, presumably because there was insufficient time or temperature for bond formation. Adhesion also decreased at long times, perhaps because of protein degradation. The optimum time was protein dependent, and for the remaining work, the hot pressing time was fixed at 10 min. Note, the adhesion for the control was very low and did not improve with extended hot pressing time. Twenty proteins, spanning a broad range of composition and properties, were compared as potential wet strength enhancing agents for paper. The protein des-
AG
BSA
0.0684 0.1789 0.0682 0.0522 0.0263 0.0204 0.0549 0.0209
0.0675 0.0954 0.0531 0.0555 0.0344 0.0430 0.0987 0.0265
GB
GG
HT
SP
0.0327 0.0684 0.0665 0.0494 0.1806 0.0043 0.1619 0.0051 0.0682 0.0682 0.0522 0.0457 0.0263 0.0283 0.0578 0.0204 0.1902 0.0542 0.0506 0.0549 0.0832 0.0584 0.0073 0.0209 0.0465 0.0273
ignations and molecular weights are given in Table 1, and the wet peel strengths are summarized in Figure 6 for laminates based on oxidized and nonoxidized cellulose. The delamination force for the control samples was 2.2 and 5.5 N/m for nonoxidized and oxidized cellulose films with standard deviations less than 7% of the mean for triplicate measurements. The fact that TEMPO oxidation increases cellulose wet adhesion has been recently reported in both the patent19,20 and the paper technology literature,21 and the explanation is that surface aldehydes induced by oxidation can react with cellulose to give hemiacetal bonds that help prevent the cellulose-cellulose joint swelling in water. Some proteins were particularly ineffective. Peptone gave lower adhesion than the control for both types of membranes, showing that it was acting as a debonding agent. In general, peptone contains about 52.% (wt/wt) hydrophobic amino acids, and its molecular weight is lower than 10 kDa22sboth of these factors will contribute to low adhesion. Casein (CA) and gelatin (GA) also gave lower adhesion than the control for nonoxidized cellulose; however, these proteins gave nearly twice the adhesion as compared to the control for oxidized membranes, suggesting covalent bond formation with aldehydes on the oxidized cellulose surface. The six proteins with the strongest adhesion to the oxidized cellulose membrane were HT > SP ≈ BSA > GB > GG > AS. The adhesion values were less for nonoxidized membranes despite the fact that the nonoxidized laminates had 3 times more protein than the laminates made from oxidized membranes. The relative order also changed slightly, giving HT > SP > BSA ≈ AG ≈ GG >AS > GB for the nonoxidized membranes.
Figure 7. Influence of protein amine and hydroxyl content on wet adhesion.
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The amino acid compositions of these proteins are summarized in Table 3.23-26 The adhesion results were fitted to the amine and hydroxyl contents of the proteins yielding the following empirical fit:
PF ) 27.66A - 23.38H + 228.46AH + 8.19 where PF is the dimensionless peel force obtained by dividing the peel force by N/m, A is the mole fraction of arginine + lysine + histidine in the protein, and H is the mole fraction of serine and threonine. The surface described by this model is plotted in Figure 7 together with data. Clearly, the amine content is a dominate factor. The extrapolated empirical fit suggests that there may be some advantage in having high contents of both amine and serine + threonine. Discussion Regenerated cellulose films and wood fibers are hydrophilic materials that spontaneously swell in water. Virtually all commercial paper wet strength resins contain reactive functional groups capable of crosslinking the resin and grafting onto surface groups on the fibers and fines.1 The result is a covalently bonded network that may swell in water but requires significant work to rupture. We believe covalent bond formation across the fiber joint is a universal requirement for paper wet strength. The types of potential covalent bonds in this work are now considered. The dominant type of surface group in the regenerated cellulose films is hydroxyls that are rather unreactive. However, at least one end of every cellulose chain contains a reactive hemiacetal group. Furthermore, many of our measurements were made with TEMPO oxidized cellulose films, which contain both reactive aldehydes and carboxyl groups. The cellulose supported aldehyde groups will form imines with lysine moieties under mild drying conditions, giving some wet strength without heating.13 Carboxyl groups on the cellulose will also react with lysine to give amides; however, this reaction requires high temperatures to remove water and drive the condensation reaction. Both our model film studies (Table 2) and the temperature dependence of protein adhesion (Figure 5) suggest that the drying conditions used in this work were indeed sufficient to promote amide bond formation. This is consistent with many studies in the food processing literature that have reported protein carbohydrate amide coupling with heat treatment.27-31 Proteins in vivo display specific secondary and tertiary structures that contribute to mechanical properties. In this work, the proteins were denatured to generate simple polymer solutions. Exploiting protein secondary and tertiary structure is a future objective of our research. From a technological perspective, none of the off-theshelf proteins evaluated in this study was as effective as commercial PAE wet strength resins. For example, the highest adhesion we observed for proteins or polypeptides was 74 N/m, whereas a commercial PAE resin gave a wet peel strength of 96 N/m in our experiments. Thus, protein based materials for commercial application will have to become more effective. Approaches to developing such materials might involve protein engineering or chemical modification.
Conclusions (1) A variety of proteins increases the peel strength of rewetted, laminated cellulose films. In general, the highest adhesion values were obtained with proteins with the highest content of lysine and arginine. Hydroxyls containing serine, threonine, and tyrosine also were associated with strong adhesion, although these seemed less important than the amine moieties. (2) Covalent bonds within in the protein layer and possibly bonded to the cellulose surfaces were formed by heating. On the basis of model studies with CMCPK cast films, amide formation between amine and carboxyl groups is possible under the hot pressing conditions. Excess heating lowers the wet strength, indicating a concurrent degradative process. (3) The uniform application of proteins on the cellulose film requires that they be applied as homogeneous solutions that, in most cases, means that the proteins were denatured. Although proteins such as soy protein gave good wet adhesion, the need to dissolve them at pH 12.5 limits their usefulness in paper technology applications. (4) Although none of the protein laminates was as strong as polyvinylamine or a commercial PAE resin used in the paper industry, this work suggests that increasing the primary amine (amino group) content as well as optimizing heating induced bond formation may someday lead to a protein-based paper wet strength resin. Acknowledgment We acknowledge Buckman Laboratories Canada for supporting this research. Also, we thank Xianhua Feng for providing a method for preparing the polymer films and Lou DiFlavio for the mathematical model. Literature Cited (1) Espy, H. H. The Mechanism of Wet-Strength Development in Paper: A Review. Tappi J. 1995, 78 (4), 90. (2) Linhart, F. The Practical Application of Wet-Strength Resins. In Applications of Wet-End Paper Chemistry; Au, C. O., Thorn, I., Eds.; Blackie Academic and Professional: Bishop Briggs, Glasgow, 1995; p 102. (3) Henderson, J. H. Volatile Emissions from the Curing of Phenolic Resins. Tappi J. 1979, 62 (8), 93. (4) Chan, L. L.; Lau, P. W. K. Urea-Formaldehyde and Melamine-Formaldehyde resins. In Wet-Strength Resins and Their Application; Chan, L. L., Ed.; Tappi Press: Atlanta, GA, 1994; p 1. (5) Quack, R.; Gehrden, G. ProteinssInnovative Base Materials for Adhesives and Coatings. Paintindia 2002, 52 (7), 39. (6) Kimpimaki, T.; Lindstro¨m, M.; Nurmi, K. Surface Size Composition. PCT Int. Appl. WO 2001004416, 2001. (7) Krinski, T. L.; Hou, K. C. Soy Protein Thickener. European Patent Application EP1054103 A1, 2000 (8) Dupont, A. L. Study of the degradation of gelatin in paper upon aging using aqueous size-exclusion chromatography. J. Chromatogr. A 2002, 950, 113. (9) Brandis, R. L. Animal Glue. In Handbook of Adhesives, 3rd ed.; Skeist, I., Ed.; Van Nostrand Reinhold: New York, 1990; p 123. (10) Bye, C. N. Casein and Mixed Protein Adhesives. In Handbook of Adhesives, 3rd ed.; Skeist, I., Ed.; Van Nostrand Reinhold: New York, 1990; p 135. (11) Liu, Y.; Li, K. Chemical Modification of Soy Protein for Wood Adhesives. Macromol. Rapid Commun. 2002, 23 (13), 739. (12) Zhong, Z.; Sun, X. S.; Wang, D.; Ratto, J. A. Wet Strength and Water Resistance of Modified Soy Protein Adhesives and Effects of Drying Treatment. J. Polym. Environ. 2003, 11 (4), 137.
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(13) Kurosu, K.; Pelton, R.; Simple Lysine-Containing Polypeptide and Polyvinylamine Adhesives for Wet Cellulose. J. Pulp Paper Sci. 2004, 30 (8), 228. (14) Pelton, R.; Hong, J. Some properties of Newsprint Impregnated with Polyvinylamine. Tappi J. 2002, 1 (10), 21. (15) DiFlavio, J. L.; Bertoia, R.; Pelton, R.; Leduc, M. The Mechanism of Polyvinylamine Wet-Strengthening; Transactions of the 13th Fundamental Research Symposium: Cambridge, September 2005; p 1293. (16) Kitaoka, T.; lsogai, A.; Onabe, F. Chemical modification of pulp fibers by TEMPO-mediated oxidation. Nordic Pulp Paper Res. J. 1999, 14, 279. (17) Skowronski, J.; Bichard, W. Fiber-to-Fiber Bonds in Paper. Part I. Measurement of Bond Strength and Specific Bond Strength. J. Pulp Paper Sci. 1987, 13 (5), J165. (18) Nylander, T. Interfacial Behavior of Proteins in Relation to Solution Association and Aggregation. In Encyclopedia of Surface and Colloid Science 2745; Hubbard, A., Ed.; Marcel Dekker: New York. (19) Jaschinski, T.; Gunnars, S. Oxidized Polymeric Carbohydrates and Products Made Thereof. U.S. Patent 6,635,755 B1, 2003. (20) Cimecioglu, A. L.; Thomaides, J. S. Polysaccharide Aldehydes Prepared by Oxidation Method and Used as Strength Additives in Papermaking. U.S. Patent Application 2003/0209336 Al, 2003. (21) Saito, T.; Isogai, A. A Novel Method to Improve Wet Strength of Paper. Tappi J. 2005, 4 (3), 3. (22) da Cruz, S. H.; Cilli, E. M.; Ernandes, J. R. Structural Complexity of the Nitrogen Source and Influence on Yeast Growth and Fermentation. J. Inst. Brew. 2002, 108 (1), 54. (23) Posati, L. P.; Orr, M. L. Composition of Foods. Dairy and Egg Products. USDA-ARS, Consumer and Food Economics Institute, Agricultural Handbook: Washington, DC, 1976; No. 8-1, p 77.
(24) Stein, W. H.; Moore, S. Amino acid Composition of _-Lactoglobulin and Bovine Serum Albumin. J. Biol. Chem. 1948, 178, 79. (25) Calvery, H. O. Methods of Analysis and Reactions of the Amino Acids and Proteins. In The Chemistry of the Amino acids and Proteins; Schmidt, C. L. A., Ed.; Charles C. Thomas: Springfield, IL, 1945; p 183. (26) Wolf, W. J.; Cowan, J. C. Soybeans as a Food Source; CRC Press: Cleveland, OH, 1975. (27) Oste, R. E.; Brandon, D. L.; Bates, A. H.; Friedman, M. Effect of Maillard Browning Reaction of the Kunitz Soybean Trypsin Inhibitor on its Interaction with Monoclonal Antibodies. J. Agric. Food Chem. 1990, 38 (1), 258. (28) Hurrell, R.; Carpenter, K. J. Digestibility and Lysine Values of Proteins Heated With Formaldehyde or Glucose. J. Agric. Food Chem. 1978, 26 (4), 976. (29) Kato, Y.; Watanabe, K.; Nakamura, R.; Sato, Y. Effect of Preheat treatment on Tryptic Hydrolysis of Maillard-reacted Ovalbumin. J. Agric. Food Chem. 1983, 31 (2), 437. (30) Hurrell, R. F.; Carpenter, K. J. Mechanism of Heat Damage in Proteins. 7. The Significance of Lysine-Containing Isopeptides and Lanthionine in Heated Proteins. Br. J. Nutr. 1976, 35, 383. (31) Bjarnason, J.; Carpenter, K. J. Mechanism of Heat Damage in Proteins. 2. Chemical Changes in Pure Proteins. Br. J. Nutr. 1970, 24, 313.
Received for review May 31, 2005 Revised manuscript received July 21, 2005 Accepted July 26, 2005 IE050635C
