Description Of The Aseptic Processing System

Although the equipment for aseptic processing systems varies, all systems have certain common features:

1. A pumpable product

2. A means to control and document the flow rate of product through the system

3. A method of heating the product to sterilizing temperatures

4. A method of holding product at an elevated temperature for a time sufficient for sterilization

5. A method of cooling product to filling temperature

6. A means to sterilize the system before production and to maintain sterility during production

7. Adequate safeguards to protect sterility and prevent nonsterile product from reaching the packaging equipment

Preproduction Sterilization

Producing a commercially sterile product cannot be assured unless the processing system and filler have been adequately sterilized before starting production. It is important that the system be thoroughly cleaned before sterilization; otherwise the process may not be effective. Some systems, or portions thereof, use saturated steam for sterilization. However, for most systems, equipment sterilization is accomplished by circulation of hot water through the system for a sufficient length of time to render it commercially sterile. When water is used, it is heated in the

Aseptic Processing Chamber
Figure 1. Simplified diagram of an aseptic processing system.

product heater and then pumped through all downstream piping and equipment up to (and generally past) the filler valve on the packaging unit. All product contact surfaces downstream from the product heater must be maintained at or above a specified temperature by continuously circulating the hot water for a required period of time.

Surge tanks are generally sterilized with saturated steam rather than hot water owing to their large capacity. Although sterilization of surge tanks may occur separately, it is usually conducted simultaneously with hot water sterilization of the other equipment.

To control aseptic system sterilization properly, it is necessary that a thermometer or thermocouple be located at the coldest point(s) in the system to ensure that the proper temperature is maintained throughout. Thus, the temperature-measuring device is generally located at the most distant point from the heat exchangers. Timing of the sterilization cycle begins when the proper temperature is obtained at this remote location. If this temperature should fall below the minimum, the cycle should be restarted after the sterilization temperature is reestablished.

Flow Control

Sterilization time or residence time, as indicated in the scheduled process, is directly related to the rate of flow of the fastest-moving particle through the system. The fastest-moving particle is a function of the flow characteristics of the food. Consequently, a process must be designed to ensure that product flows through the system at a uniform and constant rate so that the fastest-moving particle of food receives at least the minimum amount of heat for the minimum time specified by the scheduled process. This constant flow rate is generally achieved with a pump, called a timing or metering pump.

Timing pumps may be variable speed or fixed rate. The pumping rate of the fixed-rate pump cannot be changed without dismantling the pump. Variable-speed pumps are designed to provide flexibility and allow for easy rate changes. When a variable-speed pump is used, it must be protected against unauthorized changes in the pump speed that could affect the rate of product flow through the system.

Product Heating

A product heater brings the product to sterilizing temperature. There are two major categories of product heaters in aseptic food processing: direct and indirect.

Direct heating, as the name implies, involves direct contact between the heating medium (steam) and the product. Direct heating systems can be one of two types: steam injection and steam infusion.

Steam injection introduces steam into the product in an injection chamber as product is pumped through the chamber (Figure 2). Steam infusion introduces product through a steam-filled infusion chamber (Fig. 3). These systems are currently limited to homogeneous, low-viscosity products.

Direct heating has the advantage of very rapid heating, which minimizes organoleptic changes in the product. The problems of fouling or burn-on of product in the system may also be reduced in direct heating systems compared with indirect systems.

There are also some disadvantages. The addition of water (from the condensation of steam in the product) in-

Product inlet {

Product inlet {

Steam inlet

Heated product outlet

Steam inlet

Heated product outlet

I 1 Steam 3 Product

Product and condensed steam mixture

Figure 2. Steam injection.

Figure 2. Steam injection.

Product in

Product in mms

Figure 3. Steam infusion.

creases product volume. Because this change in volume increases product flow rate through the hold tube, it must be accounted for when establishing the scheduled process. Depending on the product being produced, water that was added as steam may need to be removed. Water removal is discussed under product cooling. Steam used for direct heating must be of culinary quality and must be free of noncondensable gases. Thus, strict controls on boiler feed water additives must be followed.

The other major category of product heaters is indirect heating units. Indirect heating units have a physical separation between the product and the heating medium. There are three major types of indirect heating units: plate, tubular, and swept-surface heat exchangers.

Plate heat exchangers are used for homogeneous liquids of relatively low viscosity. The plates serve as both a barrier and a heat-transfer surface with product on one side and the heating medium on the other. Each plate is gasketed, and a series of plates are held together in a press. The number of plates can be adjusted to meet specific needs.

Tubular heat exchangers employ either two or three concentric tubes instead of plates as heat-transfer surfaces. Product flows through the inner tube of the doubletube style and through the middle tube of the triple-tube style, with the heating medium in the other tube(s) flowing in the opposite direction to the product. In a shell and tube heat exchanger (considered a type of tubular exchanger), the tube is coiled inside a shell. Product flows through the tube while the heating medium flows in the opposite direction through the shell. As with plate heat exchangers, tubular heat exchangers are used for homogeneous products of low viscosity.

Scraped-surface heat exchangers are normally used for processing more viscous products (Fig. 4). The scraped-surface heat exchanger consists of a mutator shaft with scraper blades concentrically located within a jacketed, insulated heat exchange tube. The rotating blades continuously scrape the product off the wall. This scraping reduces buildup of product and burn-on. The heating medium flowing on the opposite side of the wall is circulating water or steam.

Some systems incorporate the use of product-to-product heat exchangers. These devices are either plate or tubular heat exchangers with product flowing on both sides of the plates or through both sets of tubes. This process allows the heat from the hot, sterile product to be transferred to the cool incoming, nonsterile product. Energy and cost savings can be significant by recycling the heat from sterile product.

When a product-to-product regenerator is used, the regenerator must be designed, operated, and controlled so that the pressure of the sterilized product in the regenerator is at least 1 psi greater than the pressure of any non-sterilized product in the regenerator. This pressure differential helps ensure that any leakage in the regenerator will be from sterilized product into nonsterilized product.

Hold Tube

Once the product has been brought to sterilizing temperature in the heater, it flows to a hold tube. The time required for the fastest product particle to flow through the hold tube is referred to as the residence time. The residence time must be equivalent to or greater than the time necessary at a specific temperature to sterilize the product and is specified in the scheduled process. Hold-tube volume, which is determined by hold-tube diameter and length, combined with the flow rate and flow characteristics of the product, determines the actual residence time of the product in the hold tube. Because the hold tube is essential for ensuring that the product is held at sterilization temperatures for the proper time, certain precautions must be followed:

1. The hold tube must have an upward slope in the direction of product flow at least 0.25 in./ft to assist in eliminating air pockets and prevent self-draining.

Media inlet

Media inlet

Product !|L outlet

Media outlet

Product !|L outlet

Media outlet

I_I Heat-transfer media

I Product

Figure 4. Scraped-surface heat exchanger.

2. If the hold tube can be taken apart, care should be taken that all parts are replaced and that no parts are removed or interchanged to make the tube shorter or different in diameter. Such accidental alterations could shorten the time the product remains in the tube.

3. If the hold tube can be taken apart, care should be exercised when reassembling to ensure that the gaskets do not protrude into the inner surface. The tube interior should be smooth and easily cleanable.

4. There must be no condensate drip on the tube, and the tube should not be subjected to drafts or cold air, which could affect the product temperature in the hold tube.

5. Heat must not be applied at any point along the hold tube.

6. The product in the hold tube must be maintained under a pressure sufficiently above the vapor pressure of the product at the process temperature to prevent flashing or boiling because flashing can decrease the product residence time in the hold tube. The prevention of flashing is usually accomplished by use of a back-pressure device.

The temperature of the food in the hold tube must be monitored at the inlet and outlet of the tube. The temperature at the inlet of the tube is monitored with a temperature recorder-controller sensor that must be located at the final heater outlet and must be capable of maintaining process temperature in the hold tube. A mercury-in-glass thermometer or other acceptable temperature-measuring device (such as an accurate thermocouple recorder) must be installed in the product sterilizer hold tube outlet between the hold tube and the cooler inlet. An automatic recording thermometer sensor must also be located in the product at the hold tube outlet between the hold tube and cooler to indicate the product temperature. The temperature-sensing device chart graduations must not exceed 2°F (1°C) within a range of 10°F (6°C) of the required product sterilization temperature.

Product Cooling

Product flows from the hold tube into a product cooler that reduces product temperature before filling. In systems that use indirect heating, the cooler will be a heat exchanger that may be heating raw product while cooling sterile product. Those systems that use direct heating will typically include a flash chamber or vacuum chamber. The hot product is exposed to a reduced pressure atmosphere within the chamber, resulting in product boiling or flashing. The product temperature is lowered, and a portion or all of the water that was added to the product during heating is removed by evaporation. On discharge from the flash chamber, product may be further cooled in some type of heat exchanger.

Maintaining Sterility

After the product leaves the hold tube, it is sterile and subject to contamination if microorganisms are permitted to enter the system. One of the simplest and best ways to prevent contamination is to keep the sterile product flowing and pressurized. A back-pressure device is used to prevent product from boiling or flashing and maintains the entire product system under elevated pressure.

Effective barriers against microorganisms must be provided at all potential contamination points, such as rotating or reciprocating shafts and the stems of aseptic valves. Steam seals at these locations can provide an effective barrier, but they must be monitored visually to ensure proper functioning. If other types of barriers are used, there must be a means provided to permit the operator to monitor the proper functioning of the barrier.

Aseptic Surge Tanks

Aseptic surge tanks have been used in aseptic systems to hold sterile product before packaging. These vessels, which range in capacity from about 100 gal to several thousand gallons, provide flexibility, especially for systems in which the flow rate of a product sterilization system is not compatible with the filling rate of a given packaging unit. If the valving that connects a surge tank to the rest of the system is designed to allow maximum flexibility, the packaging and processing functions can be carried out independently, with the surge tank acting as a buffer between the two systems. A disadvantage of the surge tank is that all sterile product is held together, and if there is a contamination problem, all product is lost. A sterile air or other sterile gas supply system is needed to maintain a protec tive positive pressure within the tank and to displace the contents. This positive pressure must be monitored and controlled to protect the tank from contamination.

Automatic Flow Diversion

An automatic flow-diversion device may be used in an aseptic processing system to prevent the possibility of potentially unsterile product from reaching the sterile packaging equipment. The flow-diversion device must be designed so that it can be sterilized and operated reliably. Past experience has shown that flow-diversion valves of the gravity drain type should not be used in aseptic systems owing to the possibility of recontamination of sterile product. Because the design and operation of a flow-diversion system is critical, it should be done in accordance with the recommendations of an aseptic processing authority.

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