Pilot-scale continuous ultrasonic cleaning equipment reduces Listeria monocytogenes levels on conveyor belts.

Ultrasonic cleaning of a conveyor belt was studied by building a pilot-scale conveyor with an ultrasonic cleaning bath. A piece of the stainless steel conveyor belt was contaminated with meat-based soil and Listeria monocytogenes strains (V1, V3, and B9) and incubated for 72 h to allow bacteria to attach to the conveyor belt surfaces. The effect of ultrasound with a potassium hydroxide-based cleaning detergent was determined by using the cleaning bath at 45 and 50 degrees C for 30 s with and without ultrasound. The detachment of L. monocytogenes from the conveyor belt caused by the ultrasonic treatment was significantly greater at 45 degrees C (independent samples t test, P < 0.001) and at 50 degrees C (independent samples t test, P = 0.04) than without ultrasound. Ultrasonic cleaning efficiency was tested with different cleaning durations (10, 15, 20, and 30 s) and temperatures (30, 45, and 50 degrees C). The differences in the log reduction between cleaning treatments were analyzed by analysis of variance with Tamhane's T2 posthoc test using SPSS (Chicago, IL). The lengthening of the treatment time from 10 to 30 s did not significantly increase the detachment of L. monocytogenes (ANOVA 0.633). At 30 degrees C and at the longest time tested (30 s), the treatment reduced L. monocytogenes counts by only 2.68 log units. However, an increase in temperature from 30 to 50 degrees C improved the effect of the ultrasonic treatment significantly (P < 0.01). Ultrasonic cleaning for 10 s at 50 degrees C reduced L. monocytogenes counts by more than 5 log units. These results indicate that ultrasonic cleaning of a conveyor belt is effective even with short treatment times.

Survival of Listeria monocytogenes strains in a dry sausage model.

The survival of five inoculated Listeria monocytogenes strains (DCS 31, DCS 184, AT3E, HT4E, and HR5E) was studied in dry fermented sausages prepared using two different starter cultures (starter A and B) with or without a protective Lactobacillus plantarum DDEN 2205 strain. L. monocytogenes was detected throughout ripening in every sausage sample in which the L. plantarum DDEN 2205 strain had not been used. The use of either starter A, with a high concentration of protective culture, or starter B, with a low concentration of protective culture, resulted in L. monocytogenes-negative sausages after 17 days of ripening. Differential survival was noted among the L. monocytogenes strains during fermentation. Strains AT3E and DCS 31 survived in sausages with protective cultures more often than did the other strains, whereas HT4E and HR5E were inhibited during ripening by all starter and protective cultures used. Protective cultures such as L. plantarum may be used as part of a hurdle strategy in dry sausage processing, but variations in susceptibility of different L. monocytogenes strains can create problems if other hurdles are not included.

An 8-year surveillance of the diversity and persistence of Listeria monocytogenes in a chilled food processing plant analyzed by amplified fragment length polymorphism.

Contamination routes of Listeria monocytogenes were examined in a chilled food processing plant that produced ready-to-eat and ready-to-reheat meals during an 8-year period by amplified fragment length polymorphism (AFLP) analysis. A total of 319 L. monocytogenes isolates were recovered from raw materials (n = 18), the environment (n = 77), equipment (n = 193), and products (n = 31), and 18 different AFLP types were identified, five of which were repeatedly found to be persistent types. The three compartments (I to III) of the plant showed markedly different contamination statuses. Compartment I, which produced cooked meals, was heavily contaminated with three persistent AFLP types. AFLP type A1 dominated, and it comprised 93% of the isolates of the compartment. Compartment II, which produced uncooked chilled food, was contaminated with four persistent and five nonpersistent AFLP types. The equipment of compartment III, which produced cooked ready-to-reheat meals, was free of contamination. In compartments that produced cooked meals, the cleaning routines, product types, and lack of compartmentalization seemed to predispose production lines to persistent contamination. The most contaminated lines harbored L. monocytogenes in coolers, conveyors, and packing machines. Good compartmentalization limited the flow of L. monocytogenes into the postheat-treatment area and prevented the undesired movement of equipment and personnel, thus protecting the production lines from contamination. In compartment II, grated cheese was shown to cause product contamination. Therefore, special attention should be paid to continuous quality control of raw ingredients when uncooked ready-to-eat foods are produced. In compartment II, reconstruction of the production line resulted in reduced prevalence rates of L. monocytogenes and elimination of two persistent AFLP types.

Ultrasonic cleaning of conveyor belt materials using Listeria monocytogenes as a model organism.

Persistent Listeria monocytogenes contamination of food industry equipment is a difficult problem to solve. Ultrasonic cleaning offers new possibilities for cleaning conveyors and other equipment that are not easy to clean. Ultrasonic cleaning was tested on three conveyor belt materials: polypropylene, acetal, and stainless steel (cold-rolled, AISI 304). Cleaning efficiency was tested at two temperatures (30 and 45 degrees C) and two cleaning times (30 and 60 s) with two cleaning detergents (KOH, and NaOH combined with KOH). Conveyor belt materials were soiled with milk-based soil and L. monocytogenes strains V1, V3, and B9, and then incubated for 72 h to attach bacteria to surfaces. Ultrasonic cleaning treatments reduced L. monocytogenes counts on stainless steel 4.61 to 5.90 log units; on acetal, 3.37 to 5.55 log units; and on polypropylene, 2.31 to 4.40 log units. The logarithmic reduction differences were statistically analyzed by analysis of variance using Statistical Package for the Social Sciences software. The logarithmic reduction was significantly greater in stainless steel than in plastic materials (P < 0.001 for polypropylene, P = 0.023 for acetal). Higher temperatures enhanced the cleaning efficiency in tested materials. No significant difference occurred between cleaning times. The logarithmic reduction was significantly higher (P = 0.013) in cleaning treatments with potassium hydroxide detergent. In this study, ultrasonic cleaning was efficient for cleaning conveyor belt materials.