VAT Pasteurization

VAT Pasteurization

One of the most effective methods of pasteurization is heating the product in a vessel in a process known as vat pasteurization, also referred to as batch or low-temperature long-time pasteurization. The product is heated up to a temperature of 62-64˚C and held at this temperature for about 30 minutes and then quickly cooled. Sudden cooling of product causes thermal shock to microorganism. Vat pasteurization provides the shelf life of 2-3 weeks in refrigerated condition.

Vat pasteurization is used primarily in the dairy industry as a means of preparing milk in the processing of cheese, yogurt, and other common food products. In vat pasteurization, the product is heated in a jacketed stainless steel vat which has been fitted with pipes to deliver water and steam to the jacket liner and a thermometer to monitor and record product temperatures. A method for agitation is also there to assure uniformity in temperature distribution.

Vat Pasteurization –Working

The product components (e.g., Dairy products, liquid sugar and sweeteners, water, stabilizers, vitamins) are added to the vat through the inlet line, with the outlet valve in the closed position. Steam or hot water inside the vat jacket heats the product while agitators continuously move the product in the vat. The product is held in the vat for the required time and thermometers monitor and record the temperature throughout the process. The pasteurized product then leaves the vat via the outlet valve.

Vat Pasteurization –Components & Design Parameter     

In general, vat pasteurization equipment is constructed to prevent or reduce contamination. The design of the equipment should meet the following criteria:

1. Smooth, non-absorbent surfaces
2. Self-draining and easily cleanable
3. Use of radiuses to avoid corners
4. Proper slope to ensure self-drainage
5. Raised edges to prevent potential contaminant from flowing back into equipment
6. Overlapping drip deflectors on inserted implements

1. Inlet Line: The inlet line is designed to run the product down the side of the vat, which reduces the development of foam during the pasteurization process. The line should be disconnected from the vat after filling to prevent contamination of product during or after pasteurization.
2. Covers: Covers of vat pasteurizers are constructed to prevent the entrance of surface contamination or foreign material and cover edges must be designed with overlapping “shoe box” style lids. This design prevents contaminates from entering when the cover is raised, and drains away splashes and drips to the outside of the vat pasteurizer.
3. Jacket: The vat pasteurizer jacket is a double-walled covering. In the space between the walls circulating water, steam, or heating coils of water and steam heat the product in the vat. The product should be heated in as short a time as practicable, and in no case should exceed 4 hours.

4. Agitators: An agitator is designed to keep milk or milk product moving at all times by running constantly during the pasteurization cycle. This results in uniform product and temperature throughout the vat. The most efficient agitators push the product down and sweep it against the heat exchange surface on the sides and bottom of the vat. Agitator shafts must be fitted with effective drip deflection shields to prevent contamination of the product. They must also be easily cleanable.
5. Indicating Thermometers: The indicating thermometer is the “official” thermometer. It occupies an inlet port in the vat. They are made of mercury in glass or are electronic resistance thermal devices (RTDs). They must be long enough to reach into the product zone during the pasteurization cycle. However, the probe should not reach the bottom of the vat.
6. Air Space Thermometers: Air space thermometers measure the temperature of the air space above the product in the vat. These thermometers are shorter probes than indicator thermometers. The air space thermometer bulb must be positioned 2 to 3.5 inches from the bottom of the cover, and at least 1 inch from the top surface of the product during pasteurization

7. Recording Thermometers: The recording thermometer provides a record of the pasteurization cycle and heat treatment, including the holding time.

8. Air Space Heaters: An air space heater, typically an electric boiler with steam traps and filters to produce culinary steam (i.e., steam that has been properly filtered for use in food processing), may be necessary to maintain these minimum air space temperatures. The air space above the product in the vat must be maintained at least 5 °F higher than the minimum pasteurization temperature for the product being pasteurized.

9. Valves: The inlet and outlet valves of the vat pasteurizer are normally constructed of solid stainless steel to permit adequate heat transfer and to prevent accumulation of unpasteurized product when the valves are in closed position. Non stainless steel plugs (e.g. rubber or plastic) must not be used as any milk that may have made its way around the sides of the plug will not be adequately heated.

Valve bodies or valve plugs include leak detection grooves, which are designed to prevent leakage of raw milk past the valve body. These grooves are curved or placed at such an angle to allow proper draining, and must remain fully open and free from obstruction during pasteurization.

Vat Pasteurizer- Holding Period

It should be remembered that vats are operated so that every particle of product is heated for at least 30 minutes and no more than 4 hours at or above the required minimum temperature for that product. To ensure that these requirements are reached, it is important that the operator maintain accurate charts and take careful note of cooling periods, preheating, filling time, and emptying time. It has to be ensured that the sensors of both the recording and indicating thermometers are fully covered by the product during pasteurization. In the event of any mechanical failure of any kind (e.g., a lifted cover or an agitator malfunction), the holding period must be restarted.

VAT pasteurization is suitable for small capacity milk handlers who don’t have milk capacity to go with continuous type of pasteurization. Sensory impact on the product is also negligible as they are processed at comparatively low temperature thus avoiding chances of development of any flavour or nutritional change. On other hand it protects the natural enzymes and nutritional components of the product from degradation due to rapid cooling of product after heating it at low temperature over longer period of time.