In high-temperature short-time (HTST) pasteurization, also known as Flash pasteurization, the product is heated to the minimum temperature and held continuously at or above that temperature for at least the minimum time required. This thermal treatment extends the shelf life of product by eliminating vegetative and pathogenic microorganisms. For HTST temperature ranges between 72°C- 76°C for 15 to 20 seconds. It targets resistant Pathogenic bacterial and their spores e.g., Coxiella burnetii.
Need of pasteurization is not limited to make product contamination free or to increase the shelf life. As high temperature is used during processing it alters some of the characteristics of end product that can be due to enzymatic changes or denaturation of protein. Products that can be completely pasteurized are beer, canned food, milk and milk product, eggs, juices, low alcoholic beverages, syrups, vinegar, water, wines, nuts, etc. It is important to handle the pasteurized product in refrigerated condition after pasteurization to prevent the recontamination of product.
Temperature and time for process is set on the basis of product and targeted microorganism of the product. Effectiveness of pasteurized product can be detected by various means. Heat treatment in milk causes denaturation of alkaline phosphatase and in liquid eggs the alpha amylase. These can be used as indicatives for checking the effectiveness of the process.
HTST pasteurization: Working
Stepwise flow of process in HTST starts with product entering the balance tank and is drawn under reduced pressure to the regenerator section. In the regenerator section, the product is pre-warmed by hot product flowing through regenerator plates. The product is then drawn through the timing pump to the heating section. The now-hot product flows through the holding tube. The product contacts the indicating thermometer and the recording thermometer. If it has not reached the minimum required temperature, it is returned back to the constant level tank via the diversion port. If the product is at or above the minimum required temperature, it passes through the regenerator plates (on the pasteurized side) and then to the cooling section. The product exits the cooling section and rises to an elevation of at least 12 inches above any raw product. Finally, the product passes to a storage tank for packaging.
HTST pasteurizer: Components
- Constant Level Tank/ Balance Tank: The constant level tank must have a sanitary design, and the overflow level must be at least 1 inch below the lowest level of raw milk in the regenerator. It provides a continuous supply of product to the HTST unit and it is also a return storage for sub-legal milk from the flow diversion valve.
- Regenerator: The regenerator section pre-warms cold raw milk by the heat given up by hot pasteurized milk flowing in a counter-current direction and separates pasteurized and raw product with stainless steel plates.
- Timing Pump: The timing pump controls the flow rate within the HTST system. It is located after the raw regenerator and before the holding tube. It draws raw milk through the raw regenerator and pushes it forward. The pasteurized product is always under greater pressure than the raw. The timing pump must be set so the maximum delivery rate is equal to or less than the calculated maximum flow rate. This ensures that the desired minimum holding time is obtained.
Common types of timing pumps include:
- Gear-driven positive displacement pump: two rotors revolve within an oval case to carry the fluid around the periphery of the pump body.
- Piston-type pump, such as a homogenizer. The homogenizer is equipped with a recirculation loop, so that product does not continue to the holding tube if the timing pump is not operating.
- Magnetic flow meter-based pump. This pump uses a centrifugal pump in conjunction with product flow-controlling methods.
- Heat exchange systems: There are two types of thermal exchange systems, both types use indirect heating methods, which involve transferring heat from the heat medium through a partition into the product. They all function to heat the product, hold product at the required time and temperature, and Cool the product.
- Plate heat exchanger (PHE): The PHE uses metal plates to transfer heat from pasteurized product to raw product. These plates are pressed with surface patterns to create and increase turbulence in the product stream and enhance the heat transfer. Molded gaskets around the plate edges and ports prevent leakage and intermixing of fluid.
- Tubular heat exchanger (THE): Unlike PHEs, THEs have no contact points in the product channel. THEs may have either a double-tube or triple-tube design. However, from the standpoint of heat transfer, the THE is less efficient than the PHE. The tubes within a THE have spiral or corrugated surfaces to increase turbulence and heat exchange.
- Holding Tube: The holding tube ensures that the product is at sterilization temperature for the proper time, and that temperature variation does not exceed 1 °F. The holding tube has a minimum upward slope of 0.25 inch per running foot to ensure uniform product flow and preclude air entrapment and are less than 7 inches in diameter. The length of time the product spends in the holding tube is determined by the pumping rate of the timing pump, the length of the holding tube, and the product surface friction.
- Indicating Thermometer: It shows the accurate, “official” temperature of the product. The indicating thermometer is located at the end of the holding tube and as near as practical to the recording thermometer having an accuracy rate within ±0.5 °F.
- Recorder Controller: The recording thermometer must be located within 18 inches of the upstream form the flow diversion device. The recording controller/ safety thermal limit record (STLR) may be one of the following two types:
- Capillary: It is an older design with tube filled with a volatile liquid that boils at fairly low temperatures, including in the pasteurization ranges. As it boils, it builds pressure in the sealed tube, which causes a flat coiled tube called the Bourdon coil to unwind and collapse. A significant disadvantage of this system is that as the distance from the sensor bulb to the coil increases, the speed of the response decreases.
- Electronic: Electronic designs may either have analog controls or microprocessor controls. In either case, these STLRs meet the same operational functions as the capillary design. A disadvantage for this design is that some of these units have shown susceptibility to outside radio interference.
- Flow Diversion Device (FDD): The purpose of the flow diversion device is to safely and accurately control and separate raw and pasteurized product flow. In order to do this, the flow diversion device controls pump and other valves and allows product flow forward only when minimum temperature is met. Flow diversion devices may be one of the following two types:
- Single stem: It consists of a three-way valve that automatically controls the direction of product flow. It is air activated for the open (forward) flow position, and spring activated for the closed (divert or fail-safe) position.
- Dual stem: It is made up of two three-way valves operating in tandem and multiple controls. This device will switch to the fail-safe divert position in the event of loss of adequate temperature, electronic power, or air pressure.
- Cooler Section: In the cooler section, the product is chilled down to a present temperature (below 45°F) by glycol or ice water solutions. When the product exits the cooler section, it rises to an elevation at least 12 inches above any raw product. Finally, the product passes to a storage tank or vat to await packaging
- Vacuum breakers: Vacuum breakers are atmospheric devices that help maintain proper pressure relationships in milk-to-milk regenerator sections. It prevents a negative pressure between the flow diversion device and any downstream flow promoting device. These are installed after the pasteurized milk regenerator.
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