Assessment of the potential for Listeria monocytogenes survival and growth during alfalfa sprout production and use of ionizing radiation as a potential intervention treatment.
Alfalfa seeds (Australian, nondormant, nonscarified) were treated with 20,000 ppm active chlorine, sprouted in canning jars for 5 days, and packaged and stored at 5 degrees C for up to 9 days. Seeds or sprouts were inoculated with a three-strain cocktail of Listeria monocytogenes at one of three points during the process-day 0 (before 24-h aqueous seed soak), day 1 (after 24-h aqueous seed soak), or day 5 (after sprouting, before prepackaging 10 ppm chlorine rinse)--or control (no inoculum), and the ability of the inoculum to survive and grow was evaluated. Total bacterial numbers on uninoculated seeds increased dramatically during the first 24-h the seeds were soaked, from 3.5 to ca. 8.0 log CFU/g, and remained at this level during refrigerated storage. When the seeds were inoculated with a cocktail of L. monocytogenes (log 5 CFU/10 ml) on day 0 or 1, the population of the pathogen increased dramatically, to within 1 to 2 logs of the total, and remained high during refrigerated storage. When sprouted seeds were inoculated with L. monocytogenes later in the process (day 5), the inoculum survived but did not grow more than ca. 1 log CFU/g, regardless of whether the inoculation level in each jar was low (10(3)) or high (10(5)). Irradiation of sprouts with beta radiation at 3.3 or 5.3 kGy, but not 1.5 kGy, was effective at eliminating L. monocytogenes from inoculated sprouts (6 log CFU/g) without causing noticeable changes in appearance or odor. In summary, L. monocytogenes can grow on sprouts during production, can survive on refrigerated sprouts, and may be eliminated on sprouts with beta radiation.