Insect chitosan as a natural antimicrobial against vegetative cells of Bacillus cereus in a cooked rice matrix.

This study investigates the antimicrobial activity of insect chitosan against vegetative cells of Bacillus cereus in a rice matrix. Sample culture solutions were prepared with different concentrations of insect chitosan (150, 180, 220 and 250 µg/mL) and tested at three temperatures (30 °C, 20 °C and 10 °C), which simulate different storage temperature scenarios of precooked rice. The results indicate that insect chitosan has antimicrobial activity that depends on temperature and chitosan concentration. For the assays with chitosan at 10 °C, all concentrations were bactericidal during the study time, reaching a maximum inactivation of 6 log cycles for 250 µg/mL. At 20 °C and at 30 °C a bacteriostatic activity was observed for concentrations of 150 µg/mL and 180 µg/mL. Results also showed that concentrations of 220 µg/mL and 250 µg/mL were bactericidal for all the temperatures tested during the storage time. When rice is cooked and not stored at an appropriate temperature, below 10 °C, the consumer's health is at risk. In these cases, insect chitosan could be a good additional control measure to control B. cereus growth and toxin formation in cooked rice.

Joint effect of temperature and insect chitosan on the heat resistance of Bacillus cereus spores in rice derivatives.

The heat resistance of Bacillus cereus spores inoculated in a rice substrate supplemented with insect chitosan as an alternative antimicrobial was studied. Two concentrations of insect chitosan were considered in order to assess the role of the insect chitosan concentration during the heat process. Results of the study indicated that the DT values were higher in the substrate without chitosan than in the substrate containing chitosan thus indicating a greater heat resistance to heat treatment of the microorganism inoculated in the substrate without chitosan. This behaviour was also evidenced in the survival curves. There were no great differences between either of the insect chitosan concentrations tested regarding the DT values. The z values were 9.8°C on rice substrate and8.9°C on rice substrate supplemented with insect chitosan at 150 µg/mL and 10.7°C on rice substrate supplemented with 250 µg/mL of insect chitosan. The chitosan concentration appears to affect the z value of the microorganism. Our results indicate that the combination of heat with insect chitosan as an antimicrobial on foodstuffs subjected to cooking is feasible and can improve the safety of rice derivatives.