Behavior of listeria monocytogenes in avocado pulp and processed guacamole

The potential ability of Listeria monocytogenes to grow or survive in avocado pulp (AP) and processed guacamole (PG) stored at 22, 4 to 7, and -18degreesC was studied. Both products were obtained from a factory in Michoacan, Mexico. PG consisted of AP mixed, with dehydrated vegetables, antioxidants, and preservatives. Populations of L. monocytogenes in AP stored at 22degreesC increased from 2 to 6 and 9 log CFU/g after 24 and 48 h, respectively. At 4 to 7degreesC, the growth rate of L. monocytogenes in AP was greatly decreased; generation time was 8.2 h, in contrast with 1.35 h observed at 22degreesC. L. monocytogenes populations did not increase in PG either at 22degreesC for 48 h or at 4 to 7degreesC for 15 days. The bacteriostatic effect in PG may have resulted from the presence of added substances, especially citric acid and disodium dihydrogen pyrophosphate. Aerobic plate counts and coliforms. increased in AP and PG stored at ambient temperature and under refrigeration. However, these increments did not affect the growth of the pathogen. L. monocytogenes (50,000 most probable number [MPN]/g) survived at least 58 weeks in both products stored frozen at -18degreesC; the final population was 335 MPN/g in AP and 23 MPN/g in PG. Although the composition of avocado fruit differs significantly (high content of lipids and scarcity of simple carbohydrates) from that typical of most fruits, these results underline AP as a potential vehicle of human listeriosis and indicate that freezing should not be used as the sole mechanism to control this pathogen.