January 01, 1996
by Teresa Acklin
By taking various precautions, feed millers can reduce the risk of contaminated product.
By Jan L. Vahl
In practice, it is possible to produce feed that is extremely free of salmonella, although absolute non-contamination cannot be guaranteed.
If constant pressure is exercised on all salmonella sources in our food chain, we can produce bacterially safer food in the long run. Everyone who works in the chain, including equipment and process designers, should be aware of salmonella and informed of the preventive measures required.
Salmonella is a family of enterobacteria that grows depending on its surroundings. Even though bacteria do not multiply under certain conditions, they are merely “sleeping” and can multiply again if favorable conditions return.
In general, bacteria multiply favorably under moist conditions at moderate temperatures of 10° to 50°C. Bacteria do not multiply at temperatures of less than -20°C or in dry materials under 13% to 14% moisture.
A feed miller can use several methods to kill enterobacteria. The most important include heating to a specific temperature for a certain time; pressurizing at a certain temperature; or adding bacteria-killing chemicals, often organic acids.
Once feed is produced, it is important to keep it salmonella free. This means recontamination must be prevented. In feed pelleted at 80°C, two to 50 enterobacteria per gram are present. But the level can rise to 1,000 to 10,000 per gram if the surviving bacteria start to multiply under favorable conditions.
By purchasing good, non-contaminated raw materials, the possibility of feed contamination can be lessened. Meatmeal and fishmeal can be purchased “salmonella free,” but in practice, meals often contain small numbers of salmonella, as do all other raw materials. This means that a pasteurization step in the mill is necessary.
A relationship exists between feed temperature and the time the feed has been at that temperature. At 80°C, two minutes are necessary to kill salmonella; at more than 100°C, several seconds are sufficient to obtain the same result.
It is very important to assure that all feed receives the same temperature treatment for the same length of time. Equipment for heating up feed (conditioners, ripeners) must be designed so that variances in the residence time of feed in the heater are low. Capacity adjustments and good maintenance can help minimize variances in residence time. Temperature can be determined by measuring the feed after heat treatment. If the temperature is too low, other measures should be taken, such as recirculating the feed.
It is not sufficient to read the temperature only periodically. Heating equipment should include automatic temperature monitors and an alarm to warn when the temperature is too low.
Expanding processes, during which temperatures reach a minimum of 100°C, are reliable in killing salmonella. The pelleting process also is a good method to make feed salmonella free, but only if pelleting can be done at a minimum of 80°C.
Unfortunately, it is not always possible to guarantee pelleting at that temperature. Start/stop equipment problems can cause temperature variations, or the condition of raw materials may preclude pelleting at elevated temperatures.
Because the aim of pelleting includes killing salmonella, every part of the mill beyond the pelleting section and all other feed and materials in the subsequent production flow must be kept salmonella free. If mash is produced, it should be stored and loaded in a separate area of the mill. Heating mash with steam or applying infrared radiation will kill salmonella, and pelleting is not necessary. If steam heat is used, the mash subsequently must be dried.
A number of chemicals can slow down the multiplication of bacteria or kill bacteria, and they can be added in the main mixer. Chemical solids, frequently salts of organic acids, are added in quantities of 0.2% to 2%. Precautions must be taken, as they can irritate eyes and skin.
Another method is the addition of liquid organic acids. In sufficiently high concentration, these acids kill all salmonella after a period of time.
Unfortunately, this period can range from several hours to up to several days. Another drawback is that the liquid acids, when used repeatedly in large quantities, can damage concrete, as well as steel equipment, which should be replaced by st-ainless steel equipment.
Feed should come in contact with as few free surfaces as possible after sterilization. For this reason, countercurrent coolers are better then belt coolers; pneumatic conveyors are better than elevators; and revolving distributors are better than screw conveyors with slides. Avoid dead ends.
Coolers and other equipment used after the sterilization step should be designed to avoid the collection of large amounts of feed in dead angles. Evidence exists that the number of enterobacteria increases in the cooler, sometimes in an alarming way. This is caused by contaminated material mixing in with the just-sterilized material, for example, feed falling from the cover of a belt cooler.
It is also important to avoid condensation. Feed after pelleting is moist and warm, even steaming. This steam will condensate on cold surfaces. Feed dust then clings to the wet surfaces, and a dirty layer of moist material develops.
This layer provides ideal conditions for bacteria to multiply. If one salmonella bacterium escapes from the pellet mill and finds its way to this moist layer, a nice source of infection is formed. It is important to assure that conditions after pelleting do not foster the multiplication of the few bacteria that may remain.
Condensation may be prevented by insulating all places where condensation can occur. These places include the warm parts of the cooler, the exhaust pipes of the cooling air, the cyclone, the front cover of the pellet mill and the feeder funnel of the pellet mill can cake.
If the outdoor air is colder than the mill interior, condensation on some equipment can occur. Examples include elevator heads, over the roof.
Water also can condensate on silo walls and roofs. Steel silos without insulation are especially vulnerable to condensation, particularly the underside of the silo roof. Heating of the silo deck is the only prevention, and sometimes this can be done with waste heat.
In concrete silos, sweating is often a problem. Concrete is porous, and rain or other moisture can pass through the silo wall. The solution is coating the silo exterior with a non-permeable paint. This action also can help prevent concrete corrosion by acid rain.
Locations where condensation can be expected should be ventilated. Common aspiration of coupled elevator heads is one example. Although coupled heads are undesirable in bulk grain storage because of the dangers of explosion, the explosion risk for pelleted products is much less so that common aspiration is possible.
Preheating installations with warm dry air also will prevent condensation. Check critical places with a thermometer. Heating of other equipment and locations where troublesome condensation occurs is possible. This can be done electrically, with resistance cable, or by making the equipment double walled and heating with hot water or steam.
Care should be taken to prevent “cold bridges,” which are large pieces of steel exposed to potential condensation. Cold bridges can be prevented in construction through appropriate civil and mechanical engineering. In existing structures, cold bridges should be insulated.
If residues are permitted to remain in storage or equipment, heating or rot can occur, creating an ideal place for bacterial growth. Storage installations designed to minimize or avoid residues must be used to discourage feed contamination.
Reinforcing strips should be on the installation's exterior. Internal construction components, such as boltheads, nuts or interrupted welds, should be avoided.
Spouts and outlets should be designed in a way that assures complete and thorough product flow. Dead ends or dead angles, such as those created by rectangular elevator feet, should not be permitted.
Avoid flanges if possible. The crevices are sources for collecting residues, and flanges can become cold bridges. If flanges must be used, choose placement carefully, and if necessary, apply putty to close crevices.
Storage and other mill facilities must be constructed so that they can be cleaned internally. Sweepings must be easy to remove. Interior spaces should be easily accessible, with sufficient space for cleaning.
Contaminated equipment must be sterilized by one of the following methods:
dry heating to 100°C using hot air;
“gassing” with decontaminants such as formaldehyde. Formaldehyde is a controversial product, but for the present, it is a good solution if used properly. Companies specializing in decontamination also provide these services;
applying dry powders, often organic chlorine products or salts of organic acids. These powders often work well only in moist surroundings, and decontamination time can be lengthy.
Thorough cleaning and removal of unsound feed is often sufficient, provided the equipment stays dry. The remaining bacteria cannot multiply and are flushed out of the equipment.
Because the interiors of equipment used in feed processing should be clean, smooth and noncorroding surfaces are preferred. This characteristic applies to equipment ranging from coolers, ducts and cyclones to silos.
Stainless steel or internal coatings are options. Internal coatings may consist of plastic, such as glass-fiber reinforced polyester, or heat-cured or epoxy finishes. But heat-cured or epoxy coatings cannot be welded without damaging the protective coating, which can require costly repair or replacement.
Because equipment may require decontamination, it must be resistant to either chemicals or high temperatures. Gaskets should be made of ethylene-propylene rubber or teflon. Do not use alloys containing copper or zinc, as these corrode easily in feed mills. Aluminium is often a good material.
Galvanized steel is rough, and in the long run not better than normal steel. Concrete also is rough and difficult to keep clean. Prefabricated silos with “Z-type” walls permit residues to accumulate, and bolted, corrugated steel silos have the same problem. Prefabricated silos constructed of plastic can be a solution.
Other precautions include recirculating suspect feed before the decontamination step, in general before pelleting, and assuring that air used for the coolers is taken from a clean area. Never sweep feed in finished product silos.
Personnel should not be a source of contamination. Clean sanitary facilities must be available for all staff, who should be encouraged to practice good hygiene, including hand washing and drying with hot air or throw-away towels. Medical check ups or testing for salmonella carriers also may be considered.
Transport of feed requires care to avoid contamination. Condensation can occur in bulk trucks, and the inner topside of tanks often becomes caked. These cakes must be removed regularly at least twice a year. Decontamination can be accomplished by blowing a decontaminant, often calcium propionate, into the tank while the truck is in a clean, dry area.
Mice, rats and birds can be carriers of salmonella. This means that close attention should be paid to pest abatement.
Preventive measures should be taken to assure pests cannot enter the finished product. Seal openings, and keep compartments closed. A good abatement program using a professional service is often the best and cheapest way to keep pests and birds under control.
Jan L. Vahl is with Hendrix Voeders, Boxmeer, the Netherlands. This article is based on his presentation at the VICTAM '95 Feed and Food Industries Show in Utrecht, the Netherlands, sponsored by VICTAM International.