Gas chromatographic evaluation of amino acid epimerisation in the course of gelatin manufacturing and processing
Using capillary gas chromatography on chiral stationary phases total hydrolysates of gelatins were investigated for their amounts of D-amino acids (D-AAs). Gelatins of different quality and origin were analysed including gelatin-containing final products like candies and meat products. In gelatins manufactured by stepwise extraction (first to fifth extract) with water of increasing temperature (ca. 55–95°C) amounts of 2.7–4.8% of D-aspartic acid (D-Asp) were determined in extracts obtained at ≤ 85°C. The amounts D-Asp were as high as 28.1% in fifth extracts obtained at 95°C (data corrected for 4.3% of D-Asp determined in native collagen as a result of acid-induced epimerisation of L-Asp during total hydrolysis). Epimerisation of aspartyl or asparaginyl residues of proteins in general on heating in water is assumed to proceed via formation of aspartyl succinimide, tautomerisation and release of D-Asp on total hydrolysis. The other AA residues of gelatin showed only a slight increase of epimerisation with increasing thermal treatment. Furthermore, amounts of 3–4% of cis-4-D-hydroxyproline, formed from native trans-4-L-hydroxyproline in collagen on hydrolysis, were detected in total hydrolysates of all gelatins.
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