A soy protein-based hydrogel capable of imbibing a large amount of water
was synthesized by first
modifying the lysyl residues of soy protein isolate (SPI) with
ethylenediaminetetraacetic dianhydride
(EDTAD) followed by cross-linking with glutaraldehyde.
Modification of SPI with EDTAD at pH 9
resulted in polymerization involving intramolecular cross-linking of
soy 11S and 7S globulin, which
decreased the number of carboxyl moieties introduced per lysyl residue
and impaired the swellability
of the hydrogel. Pretreatment of SPI involving heating for 30 min
at 65 °C and pH 12.0 followed
by modification with EDTAD at 25 °C/pH 12 did not result in
polymerization and, under these
conditions, about 3.3 carboxyl moieties/lysyl residue were incorporated
into the protein. The hydrogel
prepared under these conditions was able to absorb more than 105 g of
water/g of dry gel. The
structural changes in SPI caused by the modification and its relevance
to swelling properties of the
gel are discussed.
Keywords: Chemical modification; protein; hydrogel; soy protein
The novel use of fish protein (FP) in the manufacture of a superabsorbent hydrogel was investigated. Following the introduction of a large number of hydrophilic groups into fish protein by modification with ethylenediaminetetraacetic dianhydride (EDTAD), proteins were cross-linked by sulfhydryl-disulfide interchange reaction between the endogenous sulfhydryl groups and disulfide bonds to produce an insoluble gel structure. The water uptake of 76% EDTAD-modified FP hydrogel was 540 g water/g dry gel at 214 × g, and it depended on pH and ionic strength of the solution. The structural changes in proteins consequent to modification were also investigated. EDTAD-FP hydrogels had increased rigidity after glutaraldehyde treatment but displayed diminished water uptake characteristics.The total world fishery catch was about one million metric tons in 1993 (1), of which approximately 30% can be considered underutilized and unexploited by-catch (2). Typically, most of these low-cost fish resources are used in the manufacture of fish meal. Besides the underutilized trash fish, the waste generated from fish processing operations contains 27-49% protein on a dry weight basis and constitutes another valuable resource (3). Since the 1940s, a variety of chemical modifications have been employed to modify the functional properties (water absorption, emulsifying activity, emulsifying capacity, gelation and aeration capacity) of fish protein (FP) concentrate (4-6). However, these chemically modified protein products have never been introduced in the market owing to safety concerns. In the present study, a new application for FP as a superabsorbent hydrogel for nonfood uses has been investigated. The process to synthesize FP hydrogel and the swelling properties of the gel are described.
EXPERIMENTAL PROCEDURESMaterials. Ethylenediaminetetraacetic dianhydride (EDTAD) and N-acetylhomocysteine thiolactone (NAHCTL) were obtained from Aldrich Chemical Co. (Milwaukee, WI). Picrylsulfonic acid (TNBS), 25% glutaraldehyde solution, and 5,5′-dithio-bis(2-nitrobenzoic acid) were from Sigma Chemical Co. (St. Louis, MO). All other chemicals were of analytical grade. Heat-sealable, water-wettable paper was purchased from Bolmet Inc. (Dayville, CT).Preparation of crude FP. The fish used in this study was walleye pike (Stizostedion vitreum), obtained from a Wisconsin fish farm. The fish were filleted immediately upon arrival, and whole muscle was chopped and blended in chilled deionized water at a meat-to-water ratio of 1:10. The pH of the fish suspension was adjusted to 12 by the addition of 2.5 N NaOH solution, and the suspension was mixed for 30 min to totally solubilize the fish protein (FP) from muscle cells. The suspension was filtered through a 0.5-mm sieve to remove insoluble material. The filtrate was dialyzed against water (the molecular weight cut-off of the membrane was 6,000-8,000 Daltons) and lyophilized.Protein determination. The protein content of the lyophilized sample was determined by subtracting the lipid and moisture content of ...
The nitrate and nitrite contents of four kinds of vegetables (spinach, crown daisy, organic Chinese spinach and organic non-heading Chinese cabbage) in Taiwan were determined during storage at both refrigerated (5 +/- 1 degrees C) and ambient temperatures (22 +/- 1 degrees C) for 7 days. During storage at ambient temperature, nitrate levels in the vegetables dropped significantly from the third day while nitrite levels increased dramatically from the fourth day of storage. However, refrigerated storage did not lead to changes in nitrate and nitrite levels in the vegetables over 7 days.
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