Mechanical Properties of Gelatin Gels; Effect of Molecular Weight and Molecular Weight Distribution
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The goal of the gelatin manufacturer is to partial hydrolyze the covalent cross-linkages that organize the collagen molecules into a quarter staggered arrangement found in connective tissue, to minimize the hydrolysis of the peptide bonds and to obtain the appropriate molecular weigh distribution (MWD) for a specific application. The mechanical properties of the resulting gelatin are known to be influenced by the tissues and species from which it is produced as well as the pretreatment and extraction procedures. Cold water fish gelatins from saithe (Pollachius virens) skins were in this work extracted at different temperatures, by different acetic acid concentrations and in different extraction times. Size-exclusion chromatography coupled to multi-angle laser light scattering (SEC-MALLS), small-strain oscillatory measurements and optical rotation measurements were used to characterize the resulting gelatins according to their molecular weight, mechanical and structural properties. Saithe gelatins with high weight average molecular weight (Mw) and excellent gel forming properties were produced during the extractions made at low temperature while highly hydrolyzed gelatins with reduced mechanical properties were extracted under harsher conditions. Gelatins from cold water fish species, acid porcine skin gelatins (type A) and lime bovine bone gelatins (type B) were compared on the basis of low deformation mechanical properties, Bloom value, Mw, MWD and polydispersity index (PI). The Bloom value and dynamic storage modulus (G') for all types of gelatin increased with increasing Mw. Cold water fish gelatins and mammalian gelatins exhibit comparable Bloom values at different Mw and different PI. The Bloom values for gelatins extracted at room temperature (22 C) from haddock (Melanogrammus aeglefinus), saithe and cod (Gadus morhua) were determined to be 200, 150 and 100 g from a linear correlation between G' and Bloom. Acid porcine skin gelatins and lime bovine bone gelatins with similar Mw exhibited different G' and Bloom values, most likely due to varying MWD and PI. Removing low molecular weight (LMW) molecules from a gelatin sample improved the mechanical properties whereas adding LMW gelatin molecules had the opposite effect on the mechanical properties of the resulting gel. Based on principal component analysis (PCA) and partial least squares regression (PLSR), two linear relationships between the mechanical properties and the MWD were suggested. Both the G' value for cold water fish gelatin and the Bloom value for mammalian gelatin were positively correlated with their corresponding fractions of β - chains and high molecular weight (HMW) molecules and negatively correlated with the fractions of LMW molecules. In contrast to the Bloom value for mammalian gelatin, the G' value for cold water fish gelatin was negatively correlated with the fractions of α - chains.