Studies on proteins and amino acids exposed to supercritical carbon dioxide extraction conditions

Abstract

Lysozyme was treated with humid supercritical carbon dioxide and nitrogen (300 bar, 80°C and room temperature, 6 and 2h). No alterations could be demonstrated by amino acid analysis and assays of TNBS-reactive lysine. Independent of the gas used, digestion experiments and SDS-PAGE indicated an unfolding, a partial oligomerization and some fragmentation of the protein molecules in samples treated at 80°C. Such alterations are caused by heating proteins in the presence of water as shown elsewhere. Furthermore, l-glutamic acid, l-glutamine, l-methionine, l-leucine, l-alanine, β-alanine and l-lysine were treated with humid supercritical carbon dioxide (300 bar, 80°C, 6h). Automated amino acid analysis demonstrated a loss of 15–23% only with glutamine and a loss of 10% with glutamine exposed to nitrogen under the same reaction conditions, while glutamine treated at room temperature remained unaltered. This loss was caused by conversion of glutamine to 2-pyrrolidinone 5-carboxylic acid, identified by ion-exchange and thin-layer chromatography and hydrolysis to glutamic acid. Protein alterations to the extent observed here, as well as the formation of pyrrolidinone carboxylic acid, do not negatively influence food quality under the reaction conditions used in supercritical carbon dioxide extraction of foods.