Food Safety / Innovation
Engineering R&D

Moving dry food sterilization ahead full-steam

A new pasteurization and sterilization approach for dry goods relies on saturated steam to purge pathogenic germs

May 13, 2013
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Equipment to decontaminate naturally occurring microorganisms in low-moisture food products uses a variety of treatment technologies, but until now, the machinery has been generally designed as a batch-style or continuous system, depending on the volume of material being processed. 
 
Steristep is a new saturated steam approach claiming to be the first continuous pasteurization and sterilization system to process product lots as small as 11 lbs. up to throughputs of 40 tons. The new system has no requirement regarding product size or type, and can treat medicinal plants, seeds, nuts, roots, spices, dried fruits or dehydrated vegetables in whole product, flake or powder (up to 200 mesh) form on the same equipment.
 
Steristep uses a unique direct heat, vibration and saturated steam approach for pasteurization or sterilization that provides a range of advantages compared to treatment technologies such as ETo (ethylene oxide), irradiation, microwave, ozone, UV or other steam machines, including a gentle cooling step, short cleaning/changeover times and a lower electrical cost. 
 
The process equipment was developed jointly by two French engineering companies: FCD SAS of Beauchastel, France, which received a patent for its DS steam sterilization system technology in July 2012, and Natprocess SAS of Lyon, which is responsible for worldwide sales of the DS technology and a special cooling technology under the Steristep name, according to Nicolas Braun, managing director at Natprocess. 
 
Braun joined Natprocess in August 2012 and is helping the company commercialize its expanding product line to include new equipment for cleaning, conveying and dosing, mixing and processing, cooling, sorting, packing and quality control. He holds a degree in business and administration from Groupe École Supérieure de Commerce de Troyes.
 
FE: How is Steristep different from competing systems?
Braun: The process comprises a decontamination unit and a separate cooling unit that are isolated from each other. A bypass section at the end of the decontamination unit diverts sufficiently treated product to the cooling unit and undertreated material away from the process. There is unique technology employed in both the decontamination and cooling sections. 
 
The decontamination section consists of an electrically heated, step-type vibrating conveyor that thermally heats and transports product. The stainless steel decontamination chamber heats the product from four sides, including the plate surface, the side walls and top lid. The temperature of each surface is independently controlled.
 
The vibrating plate is a core component in the technology; it consists of a flat section that ends in a vertical drop down to another heated vibrating plate section. The vibration transports the material from one end of the plate to the other. The vertical drop is important as it helps to homogenize the treatment, gently tumbling and mixing the product. With a flat vibrating plate, the product surface contacting the plate may see different heat than the surface facing the top of the chamber. The steps ensure all sides experience a similar heat treatment.
 
Steristep’s use of simple electrical resistances powered by conventional three-phase electricity lowers energy consumption while providing high heating surface for fast rise of product temperature.
 
FE: What is the role of heat and the steam in product treatment?
Braun: Both play a role in product decontamination. We operate the heating surfaces at temperatures ranging from 110°C to 140°C [230°F to 284°F]. We group the 12 vibrating steps into four separate temperature zones, with three steps per zone. Temperature sensors along the vibrating plate ensure tight control. We increase the temperature as the product moves through the decontamination section, but control it so that we do not burn or cook the product. Heating the product kills pathogens.
 
Steam is essential to sterilize and carry away the contaminants. We install four steam injection nozzles at the bottom of each step [48 nozzles in the system] where we introduce clean, filtered vaporized water at a temperature between 100°C and 120°C [212°F to 248°F]. 
 
The saturated steam provides a minimal amount of condensation on the product which helps decontaminate the product by increasing its heat conductivity. The small air space inside the decontamination system is saturated with hot air and steam. 
 
Operators of the system can tailor the decontamination process to the product by inputting temperature set points through each vibrating plate section and controlling the activation and volume of the steam injection nozzles. Operators can choose to introduce steam from every injection nozzle at every step or not, depending on whether the product calls for it. A recipe function is integrated into the PLC so any operator can use the equipment without being an expert on steam sterilization.
 
FE: How efficient is Steristep in terms of usage and productivity?
Braun: The difference between pasteurization and sterilization is just a difference of processing parameters. For example, we are working on a project for treating almonds and pistachios. These products only require pasteurization, and this means a shorter treatment time than sterilization; approximately two to three minutes at 85°C [185°F] is enough to kill Salmonella or E. coli. Sterilization is a longer treatment, approximately five to six minutes. In both cases, Steristep delivers a shorter residence time which helps preserve the organoleptic properties of the product.
 
The design of the decontamination system also provides a significant ease-of-cleaning benefit. The injection nozzle can spray cleaning solution that cascades over and down each step of the stainless steel system. The treatment unit can be entirely cleaned and ready for the next product in a maximum of two hours, whereas competing equipment may take one or two shifts to changeover. Steristep is simple in design, accessible and easily cleanable. There are no valves, PTFE or jacketed systems to clean. We wanted a simple design that minimized maintenance and downtime. The only major change is the fan filter once a year.
 
Another advantage of Steristep is that the drying step is eliminated, replaced by a special cooling step. A bypass section at the end of decontamination evacuates the steam through a top vent, while product falls through a bypass valve. Good, treated product falls gently onto a flat cooling system conveyor, where filtered, cold, dry air is injected at the input point of the conveyor to avoid condensation of any remaining steam on the product. The conveyor features a closed loop cold water circuit that reduces product temperature to room temperature, approximately 20°C to 30°C [68°F to 86°F]. 
 
The combination of the dried cool air and the cooled conveyor arrests bacterial development. The inoculated end product can be packaged into standard net bags. No drying is required since moisture doesn’t increase during treatment. 
 
FE: What level of germ reduction have you achieved?
Braun: A pilot plant in Valence, France tested dozens of materials over several months with success achieving up to 5 logs reduction of the total plate count [TPC]. E. coli, Salmonella, and the reduction of yeast, molds and coliforms were inactivated, with minimum degradation of product properties. 
 
Natprocess treated raw mushrooms with the Steristep process reducing TPC (or total anaerobic plate count) from 750,000 to 1,000; trimming Enterobacteriaceae from 41,000 to nil; paring yeast from 120,000 to nil; and reducing mold spores from 600 to nil.
 
In another test using raw parsley, Steristep slashed TPC from 1.6 million to 1,000; cut Enterobacteriaceae from 200 to less than 10; and reduced molds from 280 to less than 10. 
 
Steristep has successfully tested most spices in powder form and has treated shrimp-, chili-, cinnamon- and paprika-powder.

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