Six types of chitosan molecules were prepared from crab shellsas previously described (Limpanavech et al., 2008). Polymer (P)and Oligomer (O) forms of chitosan, each with a degree ofdeacetylation of 70, 80 and 90% (designated P70, P80 and P90; andO70, O80 and O90, respectively), were solubilised in 1N acetic acid.Before application, the chitosan solution was adjusted to pH 5 withNaOH