The ohmic heater assembly can be visualized in the context of a complete product sterilization or cooking process. A typical line diagram of such a plant is shown in Figure 3. In common with other types of continuous systems, careful design of the ancillary process equipment is necessary, and once heated, the product must be cooled by more conventional means such as scraped surface or tubular heat exchangers. The latter is generally preferred for particulate processing to maximize the greatest advantage of ohmic heating, that of a minimal structural damage to the particulates.
Presterilization of the ohmic heater assembly, holding tube and coolers, is carried out by recirculation of a solution of sodium sulfate at a concentration that approximates the electrical conductivity of the food material that will subsequently be processed. Sterilization temperatures are achieved through the passage of electrical current and backpressure is controlled using a backpressure valve. The aseptic storage reservoir, interface catch tank, and connecting pipework to the filler are sterilized by traditional steam methods.
The use of sterilizing solution of similar product electrical conductivity minimizes adjustment of electrical power during subsequent change to product, thus ensuring a smooth and efficient changeover period with little temperature fluctuation.
Once the plant has been sterilized, the recycled solution is cooled using a heat exchanger in the recirculation line. When steady-state conditions are reached, sterilizing solution is run to drain and product introduced into the hopper of a positive displacement feed pump. Typically, this might be an auger-fed mono or rotary or a Marlen reciprocating piston pump. Product is usually prepared in pre-mix vessels, which can incorporate preheating, or blanching operations.
Backpressure during the changeover period is controlled by regulation of top pressure in a catch tank using sterile compressed air or nitrogen. This tank serves to collect the sodium sulfate/product interface. Once the interface has been collected, product is diverted to the main aseptic storage vessel where top pressure is similarly used to control backpressure in the system.
Backpressure is maintained at a constant 1 bar when sterilizing high-acid food products at temperatures of 90 to 95°C. A 4-bar backpressure is used for low-acid food products where sterilization temperatures of 120 to 140°C are necessary. Safety features are incorporated to ensure power is automatically switched off should there be any loss of pressure.
The use of pressurized storage vessels has proven to be an extremely effective method of controlling backpressure in pilot-plant heaters where throughputs are typically less than 750 kg/h. In larger systems operating in excess of 2 t/h, aseptic positive displacement pump downstream of the cooler can be used as an alternative depending on the composition of the food product. This alternative overcomes the requirement for two aseptic storage vessels in order for the system to continuously feed product to the filling machine.
Product is progressively heated to the required sterilization temperature as it rises through the ohmic heater assembly. It then enters an air-insulated holding tube before being cooled in a series of tubular heat exchangers. More rapid cooling can be achieved with some products in which the final particulate level will be less than 40%. This system involves a combination of ohmic and traditional heat treatment and takes advantage of the capability of the ohmic heater in being able to process products containing up to 80% particulates. During preparation, the product is formulated into a high concentration particulate stream and a separate liquid stream. The liquid stream is conventionally sterilized and cooled in a plate or tubular heat exchanger system before being injected into the particulate stream leaving the holding tube of the ohmic heater. Such operation has the advantage that it reduces the capital and operating cost for a given throughput. It also allows the electrical conductivity of the carrier fluid in the ohmic heater to be more closely matched to that of
Holding Q tubes _
Ohmic heater y
Catch tank i-c
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