I think that you are underestimating the risks. Cultivating Food Pathogens can be risky because of the risks associated with cross contamination. It involves many steps, from pre-enrichment to enrichment, testing and sterilizing the resulting incubated samples. I believe that the greatest risk comes from personnel mobility. Contamination can happen through spillage on the floor, on equipment and contaminated space. Sooner or later employees can carry a cell or two in or on their shoes.
When you incubate food samples you tend to incubate and concentrate the target organisms at around 10^3 and 10^4 CFU/ml (sometimes higher) to get a decent amplification (Malorny, et al., 2004). I know I am citing an old article and that current detection levels may be lower, but current incubation protocols lead to as many organisms. Lab spills happen all the time and technicians can carry the unwanted pathogens into many common areas of the facility.
Even if you decide to test in-house you need to become accredited on a continuous basis for your results to be officially recognized. The accreditation process takes time, money and effort to put in place.
When you add all related costs associated with testing and the potential risk of contamination it may not seem like a good idea, but everyone knows their own capabilities. If you decide to test in-house, ensure that the testing facility is not in the same location as the plant and that you follow proper decontamination steps for people visiting the testing building and the facility.
I hope it helps!
Malorny, B., Paccassoni, E., Fach, P., Bunge, C., Martin, A., & Helmuth, R. (2004). Diagnostic real-time PCR for detection of Salmonella in food. Applied and environmental microbiology, 70(12), 7046–7052. https://doi.org/10.1....7046-7052.2004