Salmonella Control in Feed02 March 2016
Regulations to control Salmonella in the animal industry are one of the major challenges.
Salmonella is found everywhere and can survive, even at low moisture levels, for a long time.
Animal feed is generally considered to be an important factor to control Salmonella. Due to the complexity of Salmonella control, several measures need to be taken in parallel in feed production: prevention of Salmonella contamination, avoid growth of Salmonella and killing of Salmonella.
In many cases a combination of heat treatment and chemical treatment is used to kill bacteria. Chemical treatment normally consists of a blend of acids.
A novel approach is to combine organic acids with specific essential oils. Results have shown that such a combination is more effective in killing Salmonella.
Infections due to Salmonella species are one of the most common public health problems. Although many researchers have tried to tackle the problem, Salmonella continues to cause significant human morbidity and even mortality and consequently high economic losses.
For this reason, there is an increasing concern by consumers about Salmonella contaminations in their food.
Foods of animal origin are still one of the major routes of infection (Zhao et al., 2009). Within the EU this has resulted in legislation to control Salmonella in the animal production by setting clear standards. The industry is putting a lot of effort in their organisations to meet these standards.
Salmonella normally lives in the intestinal tracts of humans and animals. However, outside humans and animals Salmonella is widely found in nature (Winfield and Groisman, 2003). The main season is that Salmonella is able to survive, but not multiply, for long periods on material that contain very low levels of moisture.
Consequently, Salmonella may enter anywhere in the animal production chain. Therefore, good biosecurity measures are crucial in a Salmonella control program. Moreover, to control Salmonella challenges in the animal production chain, producers need to make use of all possible measures to control Salmonella.
Importance of salmonella in feed
It is generally accepted that Salmonella present in animal feed is a significant source of infection in animals (Jones, 2011; Li et al., 2011). The presence of Salmonella in many type of ingredients such as grains, oilseed meals, and fish meal has been documented for many years (Cox et al., 1983, Veldman et al., 1995). Salmonella is particularly frequently determined in protein-rich ingredients (Wierop and Häggblom, 2010).
In Table 1, results of a surveillance of the presence of salmonella in animal feeds and feed ingredients are given, between 2002 and 2009 (Li et al., 2011). Data presented in this table are obtained from the Feed Contaminants Programme (2002 – 2009), while in the period of 2007 – 2009 samples were also used from the Salmonella Assignment programme.
Salmonella data from ingredients and animal feeds show a high variation between studies (Jones and Richardson, 2004). Factors such as the sampling method, the method used for detection of Salmonella and the number of samples taken strongly influence the outcome.
Therefore, it is very difficult to compare data from different studies.
For example, it is important to take samples aseptically (Jones, 2008). Jones and Richardson (2004) stated that to test the prevalence of Salmonella in diets several hundreds of samples need to be taken. To analyse if a batch of a certain ingredient is Salmonella free, at least 30 samples need to be analysed individually (Mitchell and McChesney, 1991).
Table 1. Prevalence of Salmonella in different categories of animal feeds. (1 - Source: Li et al., 2011; 2 - Number of samples positive/number of samples tested; a,b - Rows with unlike superscripts differ significantly (p ≤ 0.05).
Control of Salmonella in feed
Feed manufacturing is generally considered to be a primary source of Salmonella contamination. As stated before, Salmonella is very difficult to control and every possible tool needs to be used to in the control program.
Therefore, in general three different strategies are used in combination to eliminate Salmonella presence in animal feeds.
First of all actions need to be taken to prevent contamination of ingredients and/or feeds with Salmonella. Secondly, measures should be taken to prevent growth of bacteria. The third action should focus on killing pathogens (Jones, 2008).
It is considered that the feed ingredients, arriving at the feed mill, are the predominant source for contamination (Radcliff, 2006). Radcliff stated that a further increase in contamination often happens during the processing steps.
Some ingredients have more than 10 processing steps before it arrives at the feed mill. Therefore, a good selection of suppliers which are able to provide a specification that their products are Salmonella negative is essential.
At arrival, goods should be inspected by the people of the feed mill. After approval it should be avoided that the ingredients get contaminated within the feed mill.
Dust has been considered as the most important risk factor in the mill (Butcher and Miles, 1995). Therefore, dust control is one of the most important measures in the Salmonella control program and requires continuous attention.
Moreover, it has been observed that fat/oil can protect Salmonella (Morita et al., 2006). Therefore, besides dust prevention it should also be avoided that fat or oil contaminates the environment.
It is generally known that all types of animals can be carrier of Salmonella. Consequently, measures should be taken to get the feed mill environment free of rodents and birds. Even the presence of insects should be avoided as much as possible.
Besides all above measures to prevent Salmonella, people who work in the plant are an important factor in spreading Salmonella. Special clothing and shoes can help to reduce the risk. In addition, it is good to segment the feed mill in so called “dirty” and “clean” areas and avoid the movements of personnel between the two areas as much as possible.
Within the whole hygiene protocol of the feed mill, working with clean trucks that transport the final feed to the customers is off course important. The trucks that deliver the final feed should be sanitised, but also the trucks that deliver the feed ingredients should follow a strict hygiene protocol.
The most important criterion for growth of bacteria is moisture. In general, the environment of the feed mill does not contain enough moisture to allow bacteria to grow. However, there are some sources of moisture that are very difficult to avoid such as condensation or high humidity.
Also during a good manufacturing process moisture is used sometimes as a “kill” measure, e.g. in the conditioner to increase temperature. This moisture can enable Salmonella to survive and grow, particularly when Salmonella is present in niches in the spots somewhere in the production system (Israelsen et al., 1996). Finding those spots is an important task of the mill personnel.
A good sampling method and risk assessment can help to identify those growth niches. Areas where Salmonella spots are most frequently found are the ingredients receiving area, poorly designed milling facilities and in coolers.
Killing of Salmonella
In principle there are two measures to kill bacteria in the feed mill process: heat treatment, particularly via pelleting, and chemical treatment. Normally these measures are taken alone or in combination. The effectiveness of these actions is strongly affected by various other actions, as described previously, in the process, and sometimes they even act synergistically (Tabib et al., 1984).
The conditioner is the most essential part in the heat treatment process. In the conditioner steam is added to increase the temperature of the mash.
The combination of moisture level, the temperature achieved and the time that the mash stays at this temperature are the main factors that determine the killing potential of the heat treatment.
Although time and moisture level are important it has been estimated that a temperature of 80–85°C needs to be reached to achieve significant Salmonella destruction (Veldman et al., 1995).
As well as pelleting, other processing techniques such as expanding or extrusion are used to improve the killing potency as these technologies, creating temperatures of above 100°C. Dietary factors such as composition and particle size play an important role in the ability of the heat treatment to kill Salmonella.
Chemical treatments to kill Salmonella are generally done via the addition of acids to the diet. Formic and propionic acids are the most common used acids and numerous tests have proven their ability to kill Salmonella in animal diets (Garland, 1994; Carter, 2006). The efficacy of acids varies a lot and depends on numerous factors such as diet composition, moisture level in the diet, physical form of the diet, inclusion level of the acid blend, the composition of the acid blend, the chemical form of the acid product (e.g. pure acids or salts of acids).
Formaldehyde is included in several acids blends to improve the efficacy of the product. It is generally accepted that formaldehyde is one of the most effective products to kill Salmonella (Duncan and Adams, 1972).
The main difference between the effects of acids and formaldehyde is that formaldehyde kills Salmonella immediately, while the effect of acids takes some time. Usage of formaldehyde is nowadays more restricted because it has been demonstrated that it has carcinogenic properties (Sheldon and Brake, 1991). Moreover, the product is very volatile and workers need to protect themselves as the product is very aggressive.
More recently, a novel development is to use essential oils to kill bacteria in diets. Many essential oils have been shown to have antibacterial properties against Salmonella (Burt, 2004).
An important limitation for the usage of essential oils is that they are very volatile. A good protection of these components is essential. Our lab has carried out several studies to investigate the possibility to combine essential oil components with acids to study the potential to replace formaldehyde containing products.
Results of such a test are shown in figure 1. These results show that when the acid blend was combined with a blend of essential oils, Salmonella levels were reduced at a much faster rate. This indicates that a combination of acids with essential oils might be a good alternative for a formaldehyde product.
Figure 1. The antibacterial effect of an acid blend alone, or in combination with essential oils, against Salmonella contaminated in an animal diet.
It has been demonstrated for a long time that there is a clear link between animal feed and the presence of Salmonella in animals and even in humans.
Because Salmonella can be found anywhere and is able to survive easily for a long time, thorough Salmonella control strategies need to be implemented.
At the feed mill, three different strategies should be implemented: preventing multiplication, avoid growth and killing bacteria.
A novel strategy is a chemical treatment of the animal diet with a combination of acids and essential oils. This combination has shown to have similar potential to kill Salmonella as formaldehyde containing products, without the negative aspects of formaldehyde.
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