Antioxidant and biocide behaviour of lignin fractions from apple tree pruning residues

Different lignin fractions were obtained from apple tree pruning residues following different isolation processes (autohydrolysis, organosolv with ethanol and acetic acid, and soda processes) and acid hydrolysis purification to obtain the acid insoluble lignin fractions. All the obtained lignin samples were characterised for chemical composition, chemical structure (by ATR-IR, NMR and Pyr-GC/MS analyses), molecular weight distribution (by GPC), total phenolic content, antioxidant power and antimicrobial activity towards a typical food and environment contaminant (Aspergillus niger) and a food processing yeast (Saccharomyces cerevisiae). The results showed different chemical composition and structure for the lignin samples, in particular for the soda lignin characterised by a higher hemicellulose and inorganic matter content. The acid hydrolysis was effective in removing hemicellulose but partially modified the lignin structure. The total phenolic content was the lowest for the soda lignin, the highest for the autohydrolysis lignin and was generally reduced after acid hydrolysis. However, the specific antioxidant capacity (in relation to the phenolic content) was higher for the organosolv lignin samples and was not reduced by acid hydrolysis (with exception for the soda lignin). Addition of lignin samples in the culturing media of A. niger, delayed the growth and brought to colonies with different morphological aspects. On the contrary, the lignin samples showed a clear inhibitory effect on S. cerevisiae growth.