Managing Salmonella Risks from Surface Coatings

Disclaimer: This blog article was written in Europe but has applicability to the risks that the global pet food industry faces.

Salmonella contamination is a core topic in terms of food safety within the pet food sector. There is a high level of concern when discussing the ingredients that are directly applied to the surface of the diets such as fats. Recent publications revealed that bulk fat may support the survival and growth of pathogens, adding it to the list of potential dangers.¹

Understanding this pathogen challenge that persists in pet food is critical to implementing solutions that help manage the food safety risks associated with Salmonella.

Salmonella: A Persistent Problem

Going through the data of the RASFF (Rapid Alert System for Food and Feed) since 2020, Salmonella is for three consecutive years at the first place of the hazards.² It is also to be noticed that the number of alerts is increasing year after year; from 10 in 2020, it went to 15 in 2022 and 2023, making Salmonella account for around 42% of the overall alerts notified by the RASFF.

Salmonella is not only an actual threat, but also the increasing number of cases shows that we need to think more proactively on how those contaminations occur and how to avoid or manage them with additional strategies.

Until now, high-moisture foods were seen as the primary risk for microbial contamination. In food safety discussions, water activity (a_w) is a key factor to monitor, as it indicates the amount of available water that can support the growth of mold, yeast, and bacteria.³ Gram-negative bacteria need an a_w greater than 0.93 to grow, a threshold that is also significant for Salmonella.⁴

Historically considered a minimal risk in terms of food safety, the low-moisture food/ingredients have come back on the radar recently. Their water activity is below the level known to support the growth of most microorganisms. But all those matrices remain susceptible to microbiological contamination and therefore need to be included in the risk evaluation.⁵

Emerging Concerns in Pet Food

In the pet food sector, recently, the focus has begun to be on the fats. These are used in pet food diets as a source of high-quality energy and flavor. Since they are typically applied to the diet surface after thermal processing steps, ensuring the microbial safety of the fats is paramount. It can be a source of microbial contamination in pet food.^6,7

When assessing contamination risk, the quality of the fat is crucial. Higher levels of impurities and moisture increase the risk. Purifying fat is essential, but some rendering processes can introduce microbial recontamination. To mitigate this, some rendering companies use heat treatment before loading the bulk fat. However, this doesn’t eliminate the possibility of contamination in two key steps:

1. During bulk loading: If the bulk contains unwashed residues with water, contamination can occur. Although bulk cleaning usually follows strict procedures, exceptions exist.
2. During unloading at the pet food manufacturer’s site: Reception tanks can be contaminated by residues, water, or external factors like dust and human contact.

Wet cleaning in this dry processing environment is increasing the possibility of contamination and should be avoided if possible.⁸

Pulling that together, we begin to understand the complexity of this threat and the difficulty in solving it. Additionally, application of some acidifiers to bulk fat have been reported to potentially help in Salmonella reduction but may also increase lipid oxidation in fat and eventually cause the appearance of brown or black deposits.^9,10 They are therefore only a partial answer to Salmonella contamination.

Finally, it is good to keep in mind, while trying to avoid Salmonella contamination, that there are also regulations and market constraints. We tend to limit the use of some acids in pet food. The pet food manufacturers are also seeking for “clean” label, easy to read and natural claim.

A Solution to Protect the Rendered Fat Against Salmonella

Avoiding Salmonella, keeping the oxidation under control, not adding “stress” on the production process, following the guidelines in term of organic acid use and finally, if possible, still answering to the clean label/natural claim, those are the constraints that face the renderers and petfood manufacturers.

Among the solutions to prevent Salmonella, many organic acids have demonstrated positive effects. This makes them a promising option for treating fats. Additionally, they can be naturally sourced, addressing another important requirement.

Organic acids exist in two forms: dissociated and undissociated. Only the undissociated form is effective against bacteria. The more acidic the solution, the higher the concentration of the undissociated form. The pKa of an organic acid is the pH at which both forms are present in equal amounts. Lower pH levels increase the presence of the undissociated form, enhancing the acid’s effectiveness.

To address the initial Salmonella issue, a solution should include an acidifier along with organic acid to adjust the pH of the aqueous portion of the fat. However, organic acids can accelerate product oxidation, so adding an antioxidant is necessary to tackle both microbial growth and oxidation.

An optimized blend of natural acidifier and antioxidant offers dual benefits: protection against Salmonella risk and control of oxidation. One major challenge is the distinct lipid and aqueous phases in the fat, requiring an emulsion system for precise delivery in both phases.

Kemin managed to combine these elements and has found a potential solution to prevent Salmonella in fat at destination of the pet food. This one now needs to be tested.

Measuring The Efficacy¹¹

To confirm the efficacy of the blend, untreated fat and fat with varying levels of treatment have been contaminated by dry and liquid forms of inoculum. We usually consider only the dry forms as they are the ones that may occur the most in the environment. But, in the case of the fat, the liquid form is also relevant as Salmonella may be present in residual waters. Contamination can come from the two forms.

The inoculated fats were held at two temperatures: 57°C and 32°C. The first one being a typical fat storage temperature and the second aiming to simulate a low storage temperature or the transport conditions. Samples were analyzed at time 0 and then hourly for 7 hours, except for the study with liquid inoculum held at 57°C in which testing intervals were done every 10 minutes for 65 minutes. 

The application of organic acid treatment led to a rapid reduction of Salmonella in fats. For the liquid inoculum, Salmonella levels dropped to 0 after 30 minutes in untreated fat. When treated with 3000 ppm, Salmonella was reduced in less than 15 minutes, and with 5000 ppm, it was reduced immediately at T0. In the case of the dry inoculum, controlling Salmonella was more challenging. In untreated fat, the Salmonella count decreased by 2 LOG after 7 hours. With a 3000ppm treatment, this reduction was achieved in just 3 hours, and the count continued to decrease over the 7-hour duration of the experiment. A similar but faster reaction was observed with a 5000ppm treatment. This suggests that the treatment effectively protects fats from microbial contamination or re-contamination.

Overall, the Kemin treatment, which combines organic acid and antioxidants in an emulsion, effectively reduces Salmonella counts in fats, providing a tool to manage pathogen hazards. Applying this treatment early, either at the rendering factory or upon receipt at the pet food manufacturer, supports mitigation of contamination risk to the final pet food by adding a necessary barrier, especially since fats are applied directly to the kibbles. Additionally, the use of antioxidants helps prevent fat oxidation, which can significantly impact the palatability of pet food. This approach may offer a natural solution to minimize potential for new Salmonella outbreaks.

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References

1 Sarah Finn, et al, (2013), Mechanisms of survival, responses and sources of Salmonella in low-moisture environments, Frontiers in Microbiology, Vol4(331)pp1-15

2 RASFF

3 https://www.cscscientific.com/csc-scientific-blog/moisture-content-and-water-activity-what-are-they

4 D’Aoust, J.-Y., Maurer, J., and Bailey, J. S. (1997). “Salmonella species,” in Food Microbiology: Fundamentals and Frontiers, eds M. P. Doyle, L. R. Beuchat, and T. J. Montville (Washington, DC: American Society for Microbiology), 135–137.

5 Beuchat, L. R. et al. 2013. Low-Water Activity Foods: Increased Concern as Vehicles of Foodborne Pathogens. Journal of Food Protection Vol. 76, No. 1, pp. 150-172.

6 Beuchat, L. R. et al. 2013. Low-Water Activity Foods: Increased Concern as Vehicles of Foodborne Pathogens. Journal of Food Protection Vol. 76, No. 1, pp. 150-172.

7 Lambertini, E. et al. 2016. Modeling the Long-Term Kinetics of Salmonella Survival on Dry Pet Food. Food Microbiology Vol. 58, pp. 1-6.

8 Sarah Finn, et al, (2013), Mechanisms of survival, responses and sources of Salmonella in low-moisture environments, Frontiers in Microbiology, Vol4(331)pp1-15

9 Dhakal, J., C. G. Aldrich, and C. Knueven. 2019. Assessing the Efficacy of Sodium Bisulfate and Organic Acid Treatments for Control of Salmonella Typhimurium in Rendered Chicken Fat Applied to Pet Foods. Journal of Food Protection Vol. 82, pp. 1864-1869.

10 Kiehl, R. C, et al. 2018. Influence of Storage Temperature, Moisture Content, and Physical Impurities on the Distribution and Survival of Salmonella enterica in Poultry Fat intended for Pet Food Use. Journal of Food Protection Vol. 81, No. 8, pp. 1364-1372.

11 Kemin internal data INFO-22-2047

12 Kemin internal data INFO-22-2047

13 Kemin internal data INFO-22-2047