الملخص الإنجليزي
The present work dealt with the physic-chemical and thermal properties of commercial bovine and porcine gelatins and of shaari (Lethrinus microdon) fish skin gelatin extracted under different conditions (temperature and acetic acid concentration). In addition an innovative method that measures thermal relaxation characteristics was developed.
The state diagram of bovine gelatin was developed by measuring the freezing curve, glass transition, unfolding, solids-melting lines, and ultimate maximum freeze-concentration conditions. The freezing point decreased with the increase of solids; whereas glass transition, unfolding, and solids-melting decreased with the decrease of solids up to solids' content 0.84 kg/kg gelatin and then remained constant. The freezing point, glass transition and solids-melting were modeled by Chen's model based on the Clausius-Clapeyron equation, modified Gordon-Taylor model, and Flory's equation, respectively. The ultimate maximum-freeze-concentration conditions were found as (Tm')w equal to -11.9°C and (T8'"')# equal to -14.9°C, and the characteristic solids content, X, as 0.80 kg/kg sample (i.e. un-freezable water, X=0.20), respectively. Similarly the value of Tom (i.e. intersection of vertical line passing thru Tm' and glass transition line) was estimated as 34°C.
A new innovative method to measure the thermal-relaxation by maintaining isothermal condition in a Differential Scanning Calorimetry (DSC) was developed. The DSC thermal-relaxation characteristics of gelatin and date flesh were found to be related with the structural- and mechanical-glass transition measured by conventional DSC linear-heating and thermal-mechanical analysis (ie Differential Thermal
chanical Analysis, DMTA). Initial slope of the thermal relaxation curve (i.e., heat flow versus relaxation time) was determined and it was plotted as a function of relaxation temperature (i.e. isothermal condition). The initial slope-temperature graph showed three thermal relaxation characteristics: limited mobile, freely mobile and restricted mobile regions. The region 1 showed limited molecular mobility with increasing temperature, while region 2 showed free mobility with increasing temperature sensitivity. The intersection of the regions 1 and 2 was related to the
structural-glass transition measured by conventional linear-heating in a DSC. The region 3 showed restricted mobility due to enhanced interactions of the phases at higher temperature. This region 3 was related to the entangled flow region measured by thermal-mechanical analysis in a DMTA.
TIPI ICO Gelatin from shaari skin was extracted as a function of temperature and concentration of acetic acid. The concentration and temperature of extracted solutions were varied from 0.01 to 1.0 N and 4 to 80°C, respectively. Gelatin extraction yield increased with the increase of acetic acid concentration and temperature. Gelatin extracted from shaari skin using 0.1 N acid solutions and temperatures of 50 and 80°C gave highest protein content comparable to that of commercial bovine and porcine gelatins. Shaari gelatin contained higher amount of the amino acids alanine, glycine, and threonine; and lower contents of hydroxyproline, isoleucine and proline than bovine and porcine gelatins. The L values of bovine and porcine gelatins were significantly higher (p<0.05) (i.e. less dark) than those of shaari gelatins extracted under experimental conditions used in this study, In general, gelatin extracted from shaari gelatin showed lower onset of glass transition temperature than mammalian gelatins. For shaari skin gelatin, the onset of glass transition temperature decreased with the increase of extraction temperature up to 50°C and then remained nearly constant. The decrease in glass transition was more pronounced for gelatin extracted at 0.01 N compared to the 0.1 and 1.0 N samples. Unfolding temperature decreased exponentially with the increase of extraction temperature. The unfolding temperature shifted to lower temperature, and the decrease was more pronounced in the case of higher (1.0 N) concentrated samples. The extraction concentration and temperature did not show significant effect on the onset solids-melting temperature. Fourier Transform Infrared Analysis (FTIR) was used to characterize secondary structure of gelatins extracted from shaari skin and commercial bovine and porcine gelatin. The secondary structure of gelatin extracted using mild treatment (concentration: 0.01 N and temperature: 4°C) was observed completely different from other extracted gelatin. This study generated detailed thermal and molecular characteristics of gelatin from different sources.
