Controlling Salmonella

by Emily Buckley
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The link between the microbiological safety of food and the consequent health of the consumer is firmly established. The subsequent connection made is that the feed eaten by the broiler, layer or pig also needs to be microbiologically safe in order to help insure that the meat and eggs eaten by people are free of potentially pathogenic bacteria. The European Commission published a ‘White Paper’ on food safety in 2000 that stated that the safety of food of animal origin begins with safe animal feed and that feed manufacturers, farmers and food operators have the primary responsibility for food safety.

Where there is a commitment to the production of ‘clean feed,’ the mill’s approach needs to be comprehensive and integrated. The incorporation of a HACCP plan (Hazard Analysis of Critical Control Points) into a feedmill’s quality system should aim for Salmonella control and should include not only corrective actions, but also aim to prevent Salmonella contamination of feed through strategic and routinely implemented actions. An approved supplier system and the monitoring of incoming feed ingredients, specific points along the feed production process, the mill environment, finished feed and feed transport vehicles are all relevant to an integrated control system. This article will describe some of the procedures that can be considered by feedmill management.Raw material receival and storage

Due to the uneven distribution of Salmonella bacteria throughout a mass of feed ingredient or finished feed and difficulties with the logistical effectiveness of sampling from a large mass of material, it may be advisable to measure total enterobacteriaceae as an indication of microbiological status, and/or to assume that all raw materials are a potential source of Salmonella contamination.

When Salmonella is not found in a sample, it is certainly more appropriate to state ‘not detected’ rather than ‘negative.’

While every raw material delivery should be physically inspected, samples of specified raw materials should be sent to an approved laboratory for Salmonella testing according to a prescribed schedule. This schedule will depend on:

• the type of raw material, e.g. the emphasis is generally on protein meals;

• the frequency of deliveries of certain raw materials, e.g. every delivery may not need to be tested when there is more than one delivery per month;

• the feed tonnage emphasis of the mill with regard to critical or ‘sensitive’ feed types;

• the supplier’s history, e.g. a new supplier or suppliers with previous positive Salmonella test results.

While it would be desirable to have the more ‘sensitive’ raw materials tested for Salmonella or declared ‘not detected’ prior to their acceptance into the mill, this is not always possible. The next preference would be to place the delivery in a quarantine area at the mill and not use it in feed production until the laboratory result has been received but this also is not always practical. Hence, raw materials might be used in feed production prior to receiving the laboratory result on Salmonella status. Apart from notifying the suppliers, and asking them to review their quality system, corrective actions upon receipt and use of a Salmonella positive raw material should include:

• isolating and spraying the remaining raw material with a suitable liquid Salmonella inhibitor;

• spraying a suitable liquid inhibitor or powder fogging with a dry inhibitor (solid state disinfection) any equipment that had been used to transport/convey the contaminated raw material (A portable powder fogger is shown in Figure 1.);

• solid state disinfection by fogging the emptied bins that held the contaminated raw material with a suitable dry Salmonella inhibitor;

• liquid spraying or solid state disinfection of areas used to hold the raw material if it was bagged or stored in loose bulk form;

• flushing the entire intake and milling system with an appropriate quantity (around 500 kilograms) of grain or grain byproduct mixed with a high concentration (5% to 10%) of a suit able dry Salmonella inhibitor. The intake system can also be flushed on a weekly basis to help prevent Salmonella growth.

Additional actions aimed at maintaining a ‘clean’ storage and milling system include a regular silo cleaning and sanitation program, including powder fogging with a dry Salmonella inhibitor.

An alternative is to routinely treat high-risk raw materials with an appropriate Salmonella inhibitor upon receival into the mill. Storage systems should be maintained in order to keep raw materials dry and the dust level in work areas should be well controlled.

The results of a silo hygiene trial using a dry Salmonella inhibitor applied with a portable dry powder fogger are shown in Table 1. Swabs were taken at four points inside a silo, and total enterobacteriaceae was measured before fogging and then 24 and 48 hours after fogging (Adams, 1999).

The use of organic acid-based products applied either as liquids (spraying) or powders (fogging) along the feedmill-ing process serves as both corrective and preventative measures, constituting a part of the integrated approach to minimizing the risk of producing Salmonella-contaminated feed. Solid state disinfection via fogging with dry powders is effective in enclosed areas and should also be used where the introduction of moisture is to be avoided and where the drying of wet surfaces is considered to be undesirably slow.

In-process sampling

In order to identify ‘hot spots’ and deploy necessary corrective and preventative actions, monitoring the feedmilling process is advised. (See table 2 for survey data.) Samples should be taken according to a prescribed schedule. In-process sampling sites should include the meal intake pit, the inside top surface of the mixer and the inside surfaces of the pellet press door, cooler, bulk out-loading bins and bag packing bins. Material can be scraped from the site with several scrapings combined and placed in a sterile container.

Mash feed and pelleted feed

The aim is to manufacture feed to insure no risk of Salmonella contamination during processing. Finished feeds selected for testing should represent the more critical and ‘sensitive’ feed types. An appropriate dry or liquid Salmonella inhibitor should be included in the more sensitive mash feeds as a preventative measure.

Conditioning temperature and retention times should be monitored for pelleted feeds to ensure they meet the minimum standard set by the feedmill. While these standards may be quite difficult to arrive at, they should be determined by the feedmill for the formulations of concern.

If bypassing the cooler is not possible during start-up of the pellet press, then a Salmonella inhibitor should be included in the first three batches of feed produced and/or until the minimum conditioning temperature is achieved.

The press should be cleaned on a weekly basis and any build-up of wet material removed from inside the door and chutes – the inside surfaces of the door can then be sprayed or powder fogged with a Salmonella inhibitor. The weekly flush material referred to above should also pass through the press and cooler. A Salmonella inhibitor may also be included in pelleted/crumbled feed in order to maintain its improved microbiological status beyond the feedmill.

No raw materials should be stored near the cooler air intake, and the area around the cooler should be kept clean and free of dust. The inside surfaces of the cooler should be kept free of moist feed build-up and should be powder fogged on a weekly basis with a dry Salmonella inhibitor.

The standards used along with the cleaning and sanitation program should be reviewed when Salmonella positive samples are reported. The frequency of cleaning and sanitation using Salmonella inhibitory products may need to be increased.

Application equipment

Solid state disinfection requires special applicators to distribute the dry powdered disinfectant products over the surfaces of interest. Two special applicators for quick and appropriate distribution of solid state disinfection products have been developed. These are suitable for all kinds of feed production machinery, storage silos, transportation systems, conditioners, coolers and vehicles. These applicators offer a fast and efficient way to disinfect inaccessible places within the mill system and also larger areas, such as grain silos.

Figure 1 shows a portable powder duster that is suitable for vehicles, production machinery, conveying lines and small silos. Figure 2 shows a mobile silo duster for larger production and storage areas as well as floor surfaces and larger capacity silos. Figure 3 illustrates effective liquid spraying arrangements suited to treating raw materials and finished feed.

Conveyance to bins and feed delivery

Feed must remain free of Salmonella during transfer from the mill to storage. Equipment that allows build-up of feed and moisture penetration should be regularly inspected and cleaned. Samples should be submitted for Salmonella detection as per the prescribed schedule.

Elevator boots should be cleaned out regularly and liberally dusted with a dry Salmonella inhibitor. An outloading/ bagging sanitation program should be in place involving powder fogging with an inhibitor. The weekly Salmonella inhibitor flush should be allowed to pass right through to bulk outloading and bag packing bins.

The finished feed storage area should be kept clean and sanitized. Feed delivery trucks should be dedicated to feed transportation and not used for transporting raw materials.

A truck hygiene program should exist and could include solid state disinfection of emptied trucks with a dry Salmonella inhibitor.

Contamination of feed after it leaves the feedmill presents yet another challenge. This requires not only good hygiene practices throughout the feed production process, but also with feed delivery systems and subsequent storage conditions on-farm. Contamination from rodents, birds or the environment is likely to be local in distribution and not homogeneously distributed through the material.

Table 1. Control of total enterobacteriaceae (cfu/swab) by solid state disinfection with Sal CURB® Dry applied at 1kg/10 tonnes capacity

Swabbing point

Before fogging

+24 hours

+48 hours

Flange of outlet side

> 10,000

< 10

< 10

Flat side of hopper wall

> 10,000

< 10

< 10

Welded joint of hopper wall

> 10,000

< 10

< 10

Underside of wall panels

> 10,000

> 10,000

< 10

Table 2. Salmonella detection in UK feedmills


Davies & Wray, 1997

Shrimpton, 1989

# of samples

% +ve

% +ve

Intake pits & augers



Ingredient bins & augers












Pellet press








Finished product bin




Outloading gantry



Warehouse & bagging out area



On farm