Bárbara Ramos

Influence of pH, type of acid and recovery media on the thermal inactivation of Listeria innocua

Acidification of foods with organic acids, either by fermentation or by intentional addition, is an important and common mechanism for controlling foodborne pathogens in a diversity of food products. The objective of this work was to study thermal inactivation of Listeria innocua, an acid tolerant microorganism, at 52.5, 60.0 and 65.0 degrees C, at different pH values (4.5, 6.0 and 7.5), using three types of acid (lactic, acetic and hydrochloric) and three different plating media (Tryptic Soy Agar with 0.6% yeast extract-TSAYE; TSAYE plus 5% NaCl-TSAYE+5%NaCl; and Palcam Agar with selective supplement-Palcam Agar), according to a 3(4) factorial experimental design. Survival data experimentally obtained were fitted with a Gompertz-inspired model and kinetic parameters (shoulder, maximum inactivation rate-k(max), and tail) were estimated for all conditions considered. The influence of temperature, pH, type of acid and enumeration media on kinetic parameters was assessed. Results showed that, with the exception of the type of acid, all the remaining factors and their combinations significantly affected the shoulder period and k(max). In relation to tail, temperature and recovery media were the affectable factors. It was concluded that the survival of this bacteria is higher when combining low temperature with neutral pH, and when TSAYE is the enumeration medium. Bigelow-inspired models were successfully developed and describe accurately the temperature and pH effects on the kinetic parameters.

Antilisterial active compound from lactic acid bacteria present on fresh iceberg lettuce

Pediococcus pentosaceus DT016, a bacteriocin producing strain, was isolated from fresh lettuce. A protein with antilisterial activity (bacDT016), between 11 to 17 kDa, was identified and characterized as the bioactive substance from the LAB culture. Highest bacteriocin production was noted after 15 h of growth. Antibacterial activity was affected by some enzymes and detergents, as well as by temperatures equal or above 80 oC. DT016 strain contains an 1110 bp DNA fragment with homology to pediocin AcH/PA-1.