Contaminated feed represents one of the first introduction pathways for Salmonella and other Enterobacteriaceae into the food value chain. One potential measure to reduce this risk is targeted heat treatment of the feedstuff. But for the implementation of heat treatment as a validated “kill-step” systematic studies are still lacking.
Dr. Edyta MARGAS
Food and Feed Safety Leader
The following article is the first one of the series which describes step by step, how to validate a heat treatment of feed to turn it into a trusted kill step for Salmonella as one member of the family Enterobacteriaceae (and any other pathogenic bacteria). The series will include background information on the topic and selection of most appropriate Salmonella strains, a series of studies in lab scale and pilot scale.
Numerous outbreaks of human salmonellosis can be traced back to contaminated animal feed (reviewed in Crump et al. 2002). This demonstrates that animal feed – even if it is far away of the end product – represents one of the first introduction pathways for pathogens into the food value chain. Effective preventative measures which are currently applied to control Salmonella in animal feed are the avoidance of the introduction, presence and multiplication in the production lines. These can be done by e.g. raw material control, well-designed equipment according to hygienic design guidelines as well as regular inspection and cleaning of critical spots. As nevertheless contamination of raw material cannot be completely prevented, a “kill-step” is essential to ensure safe feed. The two most common options to reduce Salmonella contaminations are chemicals and heat treatment. Due to risks for workers handling chemicals, fast corrosion on the equipment and stricter regulations, increasing emphasis is currently set on thermal treatment. The questions are: What has to be regarded to efficiently reduce Salmonella load by heat treatment and how does the process have to be designed to ensure no Salmonella is detected in feed?
Heat inactivation of bacteria is directly influenced by three parameters: temperature, moisture and retention time. The inactivation effect increases with an increase of each parameter. Therefore, to inactivate bacteria by heat treatment, a finely tuned interplay between all three parameters is needed. Currently, however, only time and temperature are taken to consideration in most evaluations when designing the heat treatment.
During feed processing, the main effect regarding microbial inactivation is expected to happen during conditioning and retentioning. The additional heating-up during pelleting can be assessed as negligible due to the short residence time in the die. For conditioning, superheated steam is added to the feed material introducing heat and moisture into the system. The feed material is then mixed and transported further into the retentioner where the product temperature is maintained for a certain time to ensure a homogeneous heat and moisture distribution. Process conditions vary between 60-90°C, 30-240 seconds retention time and 12-19% moisture (equivalent of water activity of 0.7-0.9) representing the relevant parameters for thermal inactivation of Salmonella. If the initial product moisture is very low, for example in countries in the MEA region, water has to be added to ensure.
The main difficulty of heat inactivation in feed is the low moisture content. It is known that in low moisture conditions the heat resistance of Salmonella is significantly increased. To achieve the same inactivation effect in low moisture conditions higher temperatures and / or longer retention times are needed to achieve the same effect compared to higher moisture. This is especially challenging in countries where the initial product moisture is very low what requires separate water addition. A further challenge for animal feed is the broad variation of recipes. Feed components like fat, minerals and protein influence the inactivation of the bacteria making it difficult to define generally applicable inactivation parameters for feed production.
Despite the challenges for thermal treatment of feed there is a huge potential for its application as existing infrastructure and process technology to control Salmonella can be used. Furthermore, it is not a new topic, also not in feed production. Numerous scientific studies are published evaluating the effect of heat treatment on Salmonella reduction in feed.
In most published studies, the effect of heat treatment was assessed in the industrial process by taking samples throughout the production process from raw material intake to load-out. This is a practicable but no reliable approach. The main issue is that due to the inhomogeneous distribution of contaminations no conclusions about the effect of processing can be drawn as the knowledge about the real initial contamination of the sample is lacking. A further variable falsifying the results is the missing knowledge about the heat resistance of the natural microflora which can vary over a broad spectrum. Furthermore, detailed information about the processing conditions are often missing. Therefore, controlled studies with knowledge about the initial contamination and the behavior of the used strains are needed.
A few studies with this approach can also be found. Liu et al. (1969), for example, determined the inactivation kinetics of S. Senftenberg 775W in meat and bone meal in lab and pilot scale at varying moisture levels. They concluded that processing at 15% moisture and 87°C (190 F) would be indicated to reduce Salmonella by 5 log, whereas no information about the retention time is provided. Himathongkham et al. (1996) evaluated inactivation kinetics of three different Salmonella strains in turkey grower feed in lab trials as well as in field tests. The different strains showed significant differences under the same process conditions demonstrating the need to specify the observed strain. Based on the evaluated data they concluded that 93°C for 90 seconds at 15% moisture are needed to achieve a 4 log reduction to ensure safe feed for young chicks.
Numerous values about “the right” heat treatment parameters can be found. But even when scientific literature is analyzed, the provided information are not sufficient to design a verified “kill-step”. Feed processors currently use heat processing conditions which were historically established, however, the rationale for choosing these parameters is often missing. To really ensure a reliable “kill-step” by thermal processing, systematic validation studies are needed including determination of laboratory-, pilot- and industrial-scale inactivation kinetics in different matrices. These studies, together with verification trials at each feed processor facility, would help to choose best processing conditions for specific recipes and particular equipment to significantly reduce Salmonella in feed. With such a detailed knowledge, a combination of processing conditions could be chosen which not only ensures safe feed, but also highest product quality.
The results of a first laboratory study evaluating these inactivation kinetics under controlled conditions done by Bühler AG will be presented in the following article.