Dr. Frank Jones—Professor Emeritus, University of Arkansas
Alexander the Great is thought to have died in 323 BC of a Salmonella infection and a similar fate is thought to have met Great Britain's Prince Albert in 1861. Obviously, salmonellosis is an ancient disease caused by an organism that has probably always existed. That organism (Salmonella) continues to present challenges in the production of foods for man and animals.
Why is Salmonella so difficult to control? Perhaps there are two characteristics that make Salmonella such a challenge: persistence and adaptability. Salmonella can persist from months to years in a wide range of materials. As a result, Salmonella may be found nearly anywhere one wants to look. Once presented with favorable conditions, some organisms gradually begin to grow. However, Salmonella is capable of quickly adapting and growing in a wide range of conditions. This means that while high numbers of Salmonella (≥ 10,000 cfu/100g) have been found in feed, even the small numbers usually found in feed (≤ 10 cfu/100 g) present a potential threat. Thus, Salmonella contamination in feed is an issue that must be addressed.
First of all, it should be clearly understood that effective programs to control of Salmonella are MUCH more than simply the application of heat or chemicals to kill the microbe. Both heat and chemical inhibitors destroy a limited number of Salmonella cells and if cells remain after the application control method(s), feeds will (obviously) be contaminated. Therefore, it is important to keep the number of Salmonella cells present in the feed as low as possible. How do we keep Salmonella numbers low? By continuing attention to detail and sustained efforts to prevent contamination from entering the facility and work to reduce microbial multiplication within the plant. Overlooking either of these areas increases the likelihood that Salmonella will be found in the final product.
Although pelleting and extrusion are effective at Salmonella control, the processes are most effective when applied correctly. Effective control of microbes (including Salmonella) in feeds or pet foods using pelleting or extrusion involves: heat penetration, moisture, temperature, time and protection from recontamination.
The application of heat is most effective when all particles within the batch are heated to the target temperature for an adequate amount of time. Coarse particles will require more time for heat penetration than will fine particles. Therefore, grinding procedures can directly influence the effectiveness of pelleting or extrusion at microbial control. In most cases, feeds should be ground ≤ 700 microns prior to pelleting.
Moist heat is generally more effective at killing microbes (including Salmonella) than is dry heat. While the amount of moisture added to feed by steam conditioning is highly dependent on the formula involved, the goal should be ≥ 15% total moisture in the conditioned mash. Yet, too much water can be introduced by poor quality steam and the added moisture can encourage microbial growth in finished feeds. Consequently, it is important to ensure that high quality steam (i.e. ≥ ¥80%) is used in the pelleting or extrusion process.
The temperature and time required to kill Salmonella are inversely related. For example, the same number of Salmonella cells may be destroyed in feeds heated to 180°F for 40 seconds as in feeds heated to 170°F for 160 seconds. It is also important to remember Salmonella is not evenly distributed in feeds. Thus, small portions of feed may contain high numbers of Salmonella and harsher conditions will be required to destroy these higher numbers. Significant destruction of Salmonella in feeds generally starts at about 160°F and target temperatures of 180 to 185°F are generally recommended. While new equipment designs can allow manufacturers to keep feeds at these temperatures for ≥ 6 min., a minimum of 30 seconds should be maintained. However, even if pelleting or extrusion destroys 100% of the Salmonella cells present, recontamination of feeds following heat treatment can occur with relative ease.
As conditioned mash is pressed through dies, contamination can quickly be re-introduced via dust. This means the area around the pellet mill or extruder must be kept sanitary and dust free. Preventing dust from entering the cooler or drier is also crucial. In addition, it is important to ensure that coolers or driers produce a minimum of condensation since Salmonella can grow in these locations and contaminate feeds.
Chemicals offer some protection from recontamination and may, in some situations (e.g. mash feeds), be the primary means of controlling Salmonella in feeds. Yet, the heat from pelleting or extrusion has been shown to enhance the effectiveness of chemical inhibitors. Still, it is important to understand that, like pelleting or extrusion, chemical inhibitors are most effective when applied correctly.
To be effective, any compound added to feed must be evenly distributed, which obviously requires the use of equipment capable of delivering a homogeneous mix. Each particle must come in contact with the inhibitor for maximum effectiveness and each feed particle is potentially contaminated, thus a uniform distribution is especially important when using chemical inhibitors. A fine mist should be used to spray liquid products on feeds and dry products should contain very small particles so that as many particles of feed as possible are contacted for maximum effectiveness. It is also important to prevent the escape of dust and vapors from feed following the application of inhibitors since these materials can be irritating or toxic to feed manufacturing personnel.
Chemicals used to control Salmonella in feeds have primarily consisted of blends of organic acids (mainly formic and propionic acids) or formaldehyde. Although organic acids have been preferred as mold inhibitors, are naturally occurring, and are easily digested by almost all animals; inclusion rates of about 1% are required for significant kill of Salmonella. The use of such high levels of organic acids may be costly, corrosive to milling or feeding equipment, affect feed palatability, and interfere with utilization of vitamins. Consequently, organic acids are generally considered less effective at killing Salmonella than products containing formaldehyde. It should also be noted that formaldehyde is the only chemical approved by the Food and Drug Administration for the control of Salmonella in feeds.
Formaldehyde is effective against a wide range of microbes (including Salmonella) and it has long been used for preservation of feed ingredients in ruminant production. One reason for the effectiveness of formaldehyde is due to the fact that it forms vapors. When used in closed systems, formaldehyde vapors can effectively kill a wide variety of pathogens. Formaldehyde has also been applied to carriers such as grains, wheat feeds or woodchips to decontaminate the inaccessible interior surfaces of equipment. Yet the long-term effectiveness of formaldehyde may be limited when it is used in open systems or bins. Consequently, many commercial formaldehyde-based products also contain acids such as propionic acid or other antimicrobial compounds to reduce evaporation. Formaldehyde has also been used to treat certain feed ingredients that have been considered high risk (e.g. animal protein products) prior to shipment. Such treatments assist in protecting the ingredient from contamination during transit and provide some residual activity in feeds. However, formaldehyde use is not without risks.
Formaldehyde has been identified as a potential human carcinogen and workers may be exposed through the inhalation of vapors generated during the application process or as a result of dust from the treated feed. In addition, inhaled vapors may irritate the eyes, cause headaches, produce a burning sensation in the throat, result in difficulty breathing and trigger or aggravate asthma symptoms in feed mill personnel. While it should be noted that 90-95% of the formaldehyde applied to feed almost immediately binds to feed particles, quantities sufficient to cause human symptoms to remain. Consequently, it is imperative that workers be protected from exposure.
Effective Salmonella control programs require continuing attention to detail and sustained effort in three broad areas:
Killing Salmonella in feeds usually involves the application of heat through steam conditioned pelleting or extrusion and/or the addition of chemical inhibitors. Controlling Salmonella via steam-conditioned pelleting or extrusion requires:
Heat enhances the effectiveness of chemical inhibitors. Chemical inhibitors must be evenly distributed within the feed for maximum effectiveness. Chemical inhibitors are primarily made up of organic acids (mainly formic and propionic acids) or formaldehyde.