Antimicrobial and rheological properties of chitosan as affected by extracting conditions and humidity exposure

Abstract

The antimicrobial activity of differently processed chitosans, of varying molecular weights (Mwv = 5600–690 kDa) and deacetylation degrees (DD = 77–86 %), was tested against 26 microorganisms. Chitosan solutions and films were prepared by solubilizing chitosan with lactic acid without adding plasticizers. Films with different water activity (aw) were prepared by varying either drying time or relative humidity during film conditioning. In dilute solution (1 g/100 g, w/w), chitosan inhibited the growth of Gram-positive and Gram-negative bacteria, of which the most sensitive was Debaromyces hansenii. High antimicrobial activity was found in chitosans with different molecular weights and similar deacetylation degree (ChQ4: 1600 kDa, 82% DD; ChR2: 2200 kDa, 81% DD; ChR4: 830 kDa, 83% DD). The films obtained from these chitosans were more effective when they were physically similar to coatings as a result of the increased aw, allowing diffusion of the active amino groups; moreover, this activity was further augmented at the more acidic pH. The superior rheological properties of the ChQ4 chitosan (1600 kDa, 82% DD) would also confer high mechanical resistance, which makes it a versatile option for the development of antimicrobial coatings to be used for a wide range of food applications.