TABLE OF CONTENTS Tannin properties, including radical scavenging effects and protein-binding ability, largely depend on their structure and particularly on the nature and number of constitutive units. Formal identification of proanthocyanidins can be achieved by bidimensional NMR techniques.70 The same approach enables to identify tannin-like derivatives.45 ,49 ,61 ,71 However, this is restricted to pure compounds, which become increasingly difficult to isolate as their degree of polymerization (DP) increases, owing to the larger number of possible isomers, smaller amounts of each individual compound, and poorer resolution of the chromatographic profiles. Therefore, specific methods have been developed to characterize proanthocyanidins polymers in rather crude plant extracts and fractions.
Immediate processing of the fresh material is preferable but storage in the frozen or freeze-dried form is possible. Comparison of conventional freeze drying of frozen material, freeze drying without prefreezing and vacuum drying procedures on leaves concluded that only heat drying methods resulted in significant proanthocyanidin losses.72 Lyophilisation of fresh leaves has also led to slight enhancement of condensed tannin extraction (usually less than 10%) in other studies.73 Care must be taken to avoid enzymatic oxidation ( e.g. immediate sample stabilization in liquid nitrogen or organic solvent, use of antioxidants such as sulfur dioxide or ascorbic acid). Spraying of the fresh material with aqueous formic acid (3% v/v) during peeling and grinding enabled the prevention of enzymatic oxidation of apple tissues.74
Extraction of proanthocyanidins is performed by grinding or homogenizing the ground material in...
