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 remain. Consequently, it is
imperative that workers be protected from exposure.
Summary
Effective Salmonella control programs require continuing attention to
detail and sustained effort in three broad areas:
➥ Preventing contamination from entering the facility,
➥ Reducing microbial multiplication within the plant and
➥ Killing the pathogen.
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:
➥ Grinding feed ≤ 700 microns,
➥ Total moisture in the conditioned mash of ≥15% ,
➥ Use of high quality steam (i.e. ≥80%),
➥ Temperatures of 180-185˚F for a minimum of 30 seconds and
➥ Protecting feeds from recontamination.
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.
