When wandering the grocery store aisles, it’s easy to forget that for every colorful package that sits on the shelves, a myriad of processes go into preparing the foods inside. Whether it’s cereal, breakfast pastries, cookies or canned peaches, everything must be either steamed, dehydrated, pulverized, pasteurized or otherwise treated to fit into the cans, boxes and cellophane packages.
For every one of those processes, valves are employed in many ways, especially in the management of steam, the single most critical component of food processing. With the very safety of the food supply at risk, regulation of the quality and quantity of steam is a challenging job for valves, actuators and control systems.
Because safety is involved, many regulations and standards are in place around the world. Yet there are few specific guidelines and none that are accepted worldwide to manage the quality and purity of the steam that comes into direct contact with the food or processing that food. In the U.S., there are basically three mentions in current regulations: Accepted Practices for a Method of Producing Culinary Steam, the FDA Code of Federal Regulations and a National Organic Standards Board review.
GRADES OF STEAM
Several grades of steam are used in food processing and each has its own level of contamination risk. Each also presents its own challenges for the equipment in process control.
Industrial or plant steam is the lowest grade of steam. It is the starting point for all steam used in food and beverage processing, but it’s the steam that doesn’t come into direct contract with the food or drink product. In other words, it’s used in heat exchangers or used for hot water generation, in boiling pans and other areas. Softened water, reverse osmosis-treated or de-alkalized water is generally used for plant steam. This well-treated water is easy on valve materials and causes few corrosion issues. The typical valves used in these systems are standard-issue for the steam and power industry. They include gate, globe and check valves. Some applications may also allow the use of quarter-turn valves as long as their design contains no pockets where fluid or debris could collect.
Even though plant steam does not come into direct contact with the food or beverages, it still needs to be at the correct pressures, and it must remain clean, dry and free from other gases and air. This is achieved by the use of plant steam control systems containing pressure control valves, filters and steam traps.
While no hard and fast rules about plant steam materials exist, some food and beverage manufacturers stipulate any steam products, including pressure regulation valves, that supply steam either directly or indirectly to a beverage or food product must be constructed of a material sufficiently inert to preclude contamination of the food. For this reason, austenitic stainless steels such as 304ss, 316ss or 316Lss are often used.
FILTERED OR CULINARY STEAM
The next level of steam is filtered or culinary steam. Culinary steam is used in applications used to sanitize the processing system. These are called “Clean in Place” (CIP) procedures, and they are employed to ensure the proper level of hygiene in pipes, valves, fittings and related components in the food processing systems themselves. Air and steam filters are often integrated into an overall system for the CIP process with appropriate valves, drains, pumps, pressure gauges, pressure relief valves and associated controls.
There also are portable products designed specifically for CIP. Similar to a power washer for a car or sidewalk, these systems use super-heated steam with power nozzles to sanitize and eliminate hazardous food-borne bacteria. Dry vapor-steam is the most efficient cleaning solution for food and beverage facilities because most pathogens can be eliminated at 160°F (71°C). Many of these systems produce constant hot dry steam ranging from 212-240°F (100-116°C).
Moisture reduction control systems consisting of steam traps and filters are employed to keep the steam as pure as possible. When additional moisture is required in the fluid stream (for example, when chocolate or heavy grease are involved), moisture control systems and valves are used.
The highest grade of steam is clean steam, and it is typically raised from purified water in a dedicated clean steam generator. This is the area in which the foods or beverages are in direct contact with the steam. To create clean steam, a secondary generator with a controlled feed water system is used. Clean steam requires the use of stainless-steel pipework and components that eliminate the potential for corrosion of steam traps, valves and pipeline equipment.
As in other sanitary process piping systems, a paramount concern in food and beverage processing is the need for ease of cleaning. For this reason, the inside of all piping components, including valves, must be very smooth and free from crevices or pockets that might entrap fluid or debris or be difficult to remove via standard cleaning processes. This limits the use of many as-cast components because of their inherent surface roughness. It also requires electro-polishing or other processes to ensure that the bore of the fluid stream is free from potential areas of fluid entrapment.
Standards and specifications for sanitary process lines are detailed in documents created by the 3-A Standards organization (Figure 1). This is a standards organization whose membership is made up of industry professionals as well as governmental agencies such as the Food and Drug Administration.
Most food-contacting valves are made of 316 stainless steel for corrosion resistance (or 316L if the valve has welded end fittings), although other alloys are used for certain applications. An important difference between general application valves and those made for food service is that the food-related valves tend to be forged or machined out of bar stock, rather than cast. This is because cast valves are more likely to have pores, while forgings are more dense with no hidden cavities.
WHERE CLEAN IS VITAL
Typical segments of the food processing area where the steam might come into contact with the food are:
Retorts are vessels in which jars, bottles or cans are heated to cook and sterilize food products (Figure 2). Precise temperature control is crucial here so that the fragile containers in the retort do not rupture. Yet these containers must be thoroughly heated to ensure no bacteria survive.
Bubble-tight steam shutoff is essential to prevent overheating and to keep steam from entering the retort between cycles. A globe-style control valve with 3–15 psi signal controls the flow of steam sent to the equipment to sterilize canned or bottled products.
There also are times when two valves are used in this application. In this case, the valves operate at different pressures. An example is when the system requires fast start-up and then tight temperature control. The first valve would complete the entire span within a 3–9 psi range. Once the retort is up to temperature, that valve begins to throttle back and a 9–15 psi valve will open and complete its span within that range. Once the retort is up to temperature, the 9–15 psi valve will throttle back and perhaps shut entirely, leaving the 3–9 psi valve operating to handle the flow.
FLASH STEAM PEELING
Another area where steam comes into direct contact is in flash steam peeling systems (Figure 3). In these systems, vegetables such as potatoes, carrots and other root crops are fed into a peeler in batches. High-pressure steam is introduced, which causes rapid heating of the surface layer of the food. When that pressure is then instantly released, the resulting steam forms under the skin of the vegetable, causing it to flash off.
Valves required here would be those for maintaining the steam pressure and temperature from the boiler as well as valves to open and close when needed to dump waste or debris. Sliding gate or diaphragm control valves are often used in this process.
Another process for removing skin is blanching. Blanching equipment is used to take the skin off vegetables such as carrots, green beans and peas by scalding. Blanchers also heat the vegetables to de-activate natural enzymes that cause spoilage.
In steam blanching, the product is transported by a chain or conveyor belt through a chamber where steam is directly injected at about 212°F (100°C). Usually temperature in the headspace (the unfilled space above the container contents) is measured and the flow rate of steam is controlled.
Forced convection blanchers are made of nested chambers, which allow recirculating steam with a fan that interconnects both chambers. The fan forces the flow of steam through a packed bed of product conveyed by a mesh belt. Another technology is individual quick blanching (IQB), which was developed to minimize product treatment involving lack of uniformity. In IQB, a single layer of product is conveyed through the steam chamber and each ‘‘individual’’ piece of product immediately enters in contact with the steam.
Generally, in blanching processes, a pressure relief valve with a pressure regulator is used to ensure a constant low-pressure flow of steam, while a control valve throttles the flow based on a temperature setpoint.
When processing poultry, jacketed tanks are heated with steam to cook large quantities of chickens. Good control and fast response are needed to adjust to load changes. Ideally, valves used here would be self-regulated. One common option for this application is a temperature-regulated sliding gate control valve.
Evaporation is used to concentrate food products such as orange juice, fruit roll-ups and similar products.
The efficiency of the evaporation process can be increased by using thermo-compressors that recycle the vapor as a heating medium. For example, the process vapor can preheat incoming feed liquor. In these cases, each effect operates at a progressively lower pressure.
Programmable logic controllers are normally used to adjust pressures, temperatures and flowrates. Condensate contamination detection may be required to ensure the condensate can be returned to the boiler feed tank.
Typical final control elements used for this process often include globe-bodied control valves.
DIRECT STEAM INJECTION
Direct steam heating can be used for heating products if the steam is of culinary quality and the product can be diluted. This heating may be combined with other methods such as jacketed vessels, but steam injection is efficient, inexpensive and provides needed agitation.
The steam may be injected using sparge pipes for low-pressure steam or injector nozzles for higher pressures. In-pipeline injection also is increasingly common.
Some methods of drying food require production of hot air. Because many food plants already have steam on site, steam is often used to heat the air using air heater batteries.
Conditioning of process hot air may be required because the air used to supply equipment such as fluidized bed dryers or belt or tunnel dryers may come into direct contact with the food product. Valving in these processes also must be clean and is subject to the 3-A specifications. Specially manufactured gate, globe and check valves with electro-polished interior surfaces are often used here.
While many of the valves used in food and beverage processing are in steam lines, these certainly are not the only processes that use valves. For example, cryogenic (or flash freezing) can be done dockside or on huge fish processing ships. Valves used in this process must be especially robust to handle the cryogenic temperatures. Butterfly valves, often with stainless-steel bodies, are frequently used to control this flow, and the valve seats are generally metal because it is more durable in these harsh conditions.
Homogenization of everything from milk to peanut butter is also an area in which valves play an essential role. In this case, poppet valves are used for moderately abrasive products such as vegetable oils, and ball valves are used with abrasive products such as peanut butter and pigments.
What all of this shows is that modern food processing could not occur without a full range of valves, actuators and controls, and as processing methods improve, so does the equipment that makes those methods possible.
It’s not something the average person thinks about when they sit down to a bowl of ice cream, but the food and beverage industry, like many valve industries, can only get bigger.