The antimicrobial properties of chitosan, a deacetylated derivative of chitin, were assessed in a synthetic medium and in a blueberry and red grape juice against six heat resistant moulds (HRM) associated with food spoilage, in order to evaluate its use as a potential food preservative of natural origin. In both synthetic medium and natural substrate, the Minimum Inhibitory Concentration (MIC) of chitosan proved to vary in a wide range, regardless to both fungal genus and medium considered. Although Aspergillus neoglaber, Talaromyces bacillisporus and Aspergillus niger proved most resistant to chitosan (MIC > 5000 mg/L) on both media, a marked difference was detected for other strains tested. On HRM inoculated in MEB, the MIC was equal to 100 mg/L for Byssochlamys fulva or Monascus floridanus, and to 1000 mg/L for Talaromyces macrosporus. On HRM inoculated in blueberry and red grape juice, the MIC was equal to 2500 mg/L for Byssochlamys fulva or Monascus floridanus, and to 5000 mg/L for Talaromyces macrosporus.If combined with thermal treatment (80°C for 5 min) at a concentration (500 mg/L) that did not prove effective in inhibiting fungal spores, chitosan did not affect spore survival of microorganisms tested, thus allowing the spoilage of tested fruit juice after 5 days at 30°C. Also heat resistance parameters of a selected heat-resistant strain, T. bacillisporus, were not significantly affected by supplementation of blueberry and red grape juice with 500 mg/L of chitosan, D values ranging between 47.6 and 71.4 minutes at 82°C; 13.3 and 23.3 minutes at 85°C; 3.6 and 5.9 minutes at 88°C; 0.9 and 1.8 minutes at 91°C (z=5.2°C to 5.5°C). Since chitosan did not prove effective alone or combined with thermal treatment either to inactivate heat resistant microorganisms tested or to modify heat-resistance parameters of a heat resistant mould such as T. bacillisporus, its use cannot be considered as an interesting strategy to apply milder thermal treatments and to achieve stability of acid products. aim of this study was: (i) to assess the inhibiting and inactivating effect of chitosan on heat resistant Fungi, representing a matter of concern for manufacturers of pasteurized fruit products, and to test the synergistic effect of chitosan and thermal treatments on (ii) survival and (iii) heatresistance parameters of selected strains, to evaluate its possible use as a technological aid during the pasteurization of fruit-based products. Inhibiting and Inactivating Effect of Chitosan on Heat Resistant Moulds Materials and Methods MicroorganismsThis study was carried out on different microbial strains: