| Abstract | Optimization of the process parameters for gelatin extraction from lizardfish (Saurida spp.) scales for maximum gelatin yield, gel strength, and viscosity were undertaken. The physiochemical properties of gelatin extracted from lizardfish scales were evaluated and microbial transglutaminase (MTGase) was evaluated for its ability to improve gelatin properties. The characteristics and the properties of gelatin films were also determined.
The effects of alkali pretreatment and hot-water extraction on the extract yield of gelatin from lizardfish scales were investigated using response surface methodology (RSM) with a 4-factor, 5-level central composite design (CCD) to ascertain the optimum gelatin extraction conditions. The effects of concentration of NaOH (0.1-0.9%, X1), treatment time (1-5 hr, X2), extraction temperature (70-90oC, X3) and extraction time (1-5 hr, X4) were determined. The responses included extraction yield (%), gel strength (g) at 910oC and viscosity (cP) at 25oC. The normality distribution of experimental data was determined and adequately fitted to a 2nd order model with multiple regression coefficients (R2) of 0.951 for extraction yield (Y1), 0.949 for gel strength (Y2), and 0.935 for viscosity (Y3). The three main effects impacting gelatin extraction from the lizardfish scales were the concentration of NaOH (%), extraction temperature (°C) and extraction time (hr). The optimization of the multiple responses was developed using desirability functions with the responses to be maximized. The results showed that the optimum conditions for the best values of each of the three responses occurred with a concentration of NaOH of 0.51%, a treatment time of 3.10 hr, an extraction temperature of 78.5oC and an extraction time of 3.02 hr. The predicted responses were a 10.7% extraction yield, 240 g gel strength and 5.61 cP viscosity. The experimental values were 10.6±0.82% extraction yield, 252±1.2 g gel strength and 7.50±0.28 cP viscosity.
The physicochemical properties of the lizardfish scales gelatin were characterized and the results indicated high protein and low ash content. The color value of lizardfish scales gelatin powder evaluated as L, a, and b values were 75.1±0.11, 1.96±0.12, and 11.8±0.35, respectively. This was significantly lower (P≤0.05) in whiteness and yellowish than commercial bovine gelatin power. Texture profile analysis (TPA) with compression was carried out at 30% deformation. The gel characteristic of lizardfish scales gelatin was not significantly different (P>0.05) in terms of springiness and cohesiveness but was significantly different (P≤0.05) in terms of hardness, gumminess, and chewiness from the gel from bovine gelatin.
The amino acid profile of lizardfish scales gelatin showed that the degree of hydroxylation for proline and lysine were 19.2% and 5.12%, respectively. The lizardfish scales gelatin was found to contain 20.4% imino acids (proline and hydroxyproline) and glycine was found to be predominant at 17.3% of the total amino acids. The relative percentage of amino acids that were acidic, basic, hydrophobic and uncharged polar group were 11.3, 12.3, 47.3, and 19.3%, respectively. Furthermore, the essential and nonessential amino acids components were 36.4 and 63.5%, respectively. The gelatins from lizardfish scales had molecular weight peptides in the range of 35-220 kDa. The main protein components were shown to be in the molecular weight range of 50-105 kDa. The βcomponent was seen in lizardfish scales gelatin but not in bovine gelatin and the α1 and α2chains were found as a major component in both gelatins.
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The gel microstructure was observed using scanning electron microscopy (SEM). The gels of gelatin from lizardfish scales showed slightly looser strands than the gels of bovine gelatin.
The addition of MTGase generally increased the gel strength of lizardfish scales gelatin gel (P<0.05) with an obvious increase in gel strength with the addition of MTGase up to 0.5% (w/v). TPA compression test with lizardfish scales gelatin gel with and without MTGase showed that the addition MTGase significantly increased the hardness (P≤0.05) with the maximum hardness being observed with 0.5% MTGase. Springiness was not significantly different (P>0.05) at all concentration of MTGase. Cohesiveness was significantly different (P≤0.05) between 0.2% and 0.6% MTGase but was not significantly different at 0, 0.1, 0.3, 0.4 and 0.5% MTGase. The lizardfish scales gelatin gels with and without MTGase had a gumminess of 440-633 g and a chewiness of 429-637 g. The addition of MTGase to gels of lizardfish scales gelatin influenced the β- and α-components with continuously decreased band intensity of these components with increasing concentrations of MTGase. The gel microstructures with various concentration of MTGase showed that there were denser strands in the gels with enzyme compared with the looser stands in non-enzyme treated gel samples. The greatest amount of denser aggregates with negligible voids in the gel network structure was observed with 0.5% MTGase.
Films cast from lizardfish scales gelatin with and without 0.5%MTGase and bovine gelatin films were produced for the determination of film characteristics. All gelatin films were transparent and flexible. The films from lizardfish scales gelatin with and without 0.5% MTGase were slightly yellowish in color and transparent. The bovine gelatin films were clearer and more transparent. The color of the films were measured using L, a, and b values and the total color difference (ΔE) was also calculated. The L value of bovine gelatin film had the highest value (P≤0.05) whereas lizardfish scales gelatin films with and without 0.5% MTGase were not significantly different (P>0.05) for L, a, and b values and ΔE. The addition of MTGase into lizardfish scales gelatin decreased the L value with concomitant increases in the a and b values, indicating more yellowness and greenness in the films. The mechanical properties of the film samples included tensile strength (TS) and elongation at break (E). The films of bovine gelatin had the highest TS (P≤0.05) but E was not significantly different (P>0.05) from lizardfish scales gelatin films. Therefore, the films of lizardfish scales gelatin with 0.5% MTGase had a higher TS than the films without MTGase (P≤0.05). The water vapor permeability (WVP) of films from lizardfish scales gelatin with and without 0.5% MTGase and bovine gelatin films were bovine gelatin films were 21.0±0.17, 26.3±0.79, and 25.8±0.09 (g.mm/m2.d.kPa), respectively, while the oxygen transmission rate (OTR) of all three types of films were lower than 50 (cc O2/m2.d).
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