Application of Superheated Steam-Based Technology for Inactivation of Foodborne Pathogens

Abstract

Superheated steam (SHS) is steam which is given additional heat to raise its temperature above the saturation temperature at a constant pressure. SHS has been known as a safe, non-polluting technology with low energy consumption and proven to be one of the most effective methods for the drying of biological or non-biological products, including foods. However, the inactivation of foodborne pathogens by SHS has rarely been studied. This study researched the effects of SHS treatment for inactivating foodborne pathogens including Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes by comparing and evaluating the effectiveness of SS and SHS. Bacteria can attach to solid surfaces of food processing facilities and form slimy, slippery biofilms consisting of hydrated extracellular polymeric substances. Adhesion of bacteria to food processing facility surfaces leads to potential hygienic problems in the food processing industry. Biofilms were formed on PVC and stainless steel coupons by using a mixture of three strains each of three foodborne pathogens at 25°C. After biofilm development, PVC and stainless steel coupons were treated with saturated steam (SS) at 100°C and SHS at 125, 150, 175, and 200°C for 5, 10, 20, and 30 s on both sides. The viable cell numbers of biofilms were significantly (P < 0.05) reduced as SHS temperature and exposure time increased. For all biofilm cells, SHS treatment resulted in an additional log reduction compared to SS treatments. After exposure to 200°C steam for 30 s or 10 s on PVC or stainless steel, respectively, the numbers of biofilm cells were reduced to below the detection limit (1.48 log CFU/coupon). SHS treatment effectively reduced populations of biofilm cells and reduced disinfection time compared to SS treatments. Effectiveness of SHS on the inactivation of foodborne pathogens on agriculture produce including almonds, in-shell pistachios, cherry tomatoes, oranges, radish seeds, and alfalfa seeds and on quality by measuring color, texture, ascorbic acid contents, antioxidant capacity, and germination rate were evaluated. Exposure of almonds and pistachios to SHS for 15 or 30 s at 200˚C reduced all tested pathogens to below the detection limit (0.3 log CFU/g) without causing significant changes in color values or texture parameters (P > 0.05). For both almonds and pistachios, acid and peroxide values following SS and SHS treatment for up to 15 s and 30 s, respectively, were within the acceptable range. Exposure to SHS for 3 or 20 s at 200˚C reduced all tested pathogens on cherry tomatoes and oranges, respectively, to below the detection limit (1 and 1.7 log CFU/g, respectively) without causing significant changes in color values or texture parameters, ascorbic acid contents, and antioxidant capacity (P > 0.05). SHS treatment caused to an additional 0.79?2.05 and 0.78?1.77 log reductions of the three pathogens on radish seeds and alfalfa seeds treated continuous and intermittent (1 s heating followed by cooling at 25˚C for 2 min) steam treatment, respectively, compared to SS treatments. A continuous steam treatment for 3 and 2 s resulted in a considerably drop in percent germination compared to the water control for radish seeds and alfalfa seeds, respectively. However, 10 times intermittent SHS treatment at 200°C did not decrease germination rate of radish seeds and alfalfa seeds under the 90%. Simulation using computational fluid dynamics (CFD) was studied to evaluate the inactivation of foodborne pathogens on food samples by SHS treatment. COMSOL multi-physics software to predict temperature distribution and concentration of the live bacteria on an orange were used. The governing equations for continuity, compressible fluid flow, and energy are solved numerically together with bacteria concentration, using a finite element method. Arrhenius equation was used to describe bacteria deactivation kinetics. The simulations have provided flow pattern, live bacteria concentration, and temperature profiles from different periods of heating. The simulated results show the slowest heating and little effect zones, which are correlated to the concentration of the live bacteria. The simulations also show bacteria were eliminated during SHS treatment at 200°C for 20 s. Portable superheated steam generator for field application was developed and the ability of inactivation of foodborne pathogens biofilm cells on stainless steel evaluated. The populations of viable biofilm cells on stainless steel coupons were reduced below the detection limit when subjected to SHS treatment at 160°C for 30 s. Healthy cells and heat-injured cells on stainless steel coupons following SS or SHS heating were compared. There were no significant (P > 0.05) differences between the levels of cells enumerated on the appropriate selective agar (SMAC, XLD, and OAB) versus the agar for resuscitation (SPRAB, OV-XLD, and OV-OAB) during the whole SHS treatment time. Also, the results have revealed that the Weibull model, which had been mostly used for describing inactivation of the bacterial cells by heat treatment, could be successfully used to describe foodborne pathogens biofilm cells on stainless steel inactivation by SHS. This study demonstrated that SHS treatment effectively reduced populations of biofilm cells on materials and foodborne pathogens on agricultural produce compared to SS treatments. And inactivation of bacteria on food during SHS treatment using CFD and development of portable SHS generator can be used for application to feeding facilities. SHS treatment has potential as an excellent intervention for controlling foodborne pathogens and enhancing safety in the food industry.