To get all math about it:
Risk = Probability + Seriousness - (Preventative Actions + Contingent Actions)
In the context of food safety for fresh produce, it's easy to forget this. So many resources spell out the things we can do to mitigate risk - from washing your hands and keeping food off the floor to using sanitized pallets and requiring workers to bathe daily - without providing any context about the probability of contamination.
Prevention in food safety comes down to keeping the poop off of the food. Preventative actions vary in their effectiveness. For my money, good hand washing - thorough scrubbing in running potable water with soap and drying with a single-use towel afterwards - provides the single biggest risk reduction. Everything else (except not dumping raw manure on your vegetables) pales in consideration.
Operating under the assumption that our produce is contaminated, we take contingent actions to keep the bacteria from spreading or growing. Washing in running water, sanitizing wash water, and cooling produce to slow enzymatic activity all reduce risk.
Of course, the seriousness of a food safety outbreak is high - E. coli 0157:H7 and Salmonella enterica can kill people. But the probability is relatively low - in the 2006 spinach - E. coli 0157:H7 outbreak, only three people died, and a couple hundred were sickened, despite over 250 billion servings of fresh bagged salad greens having been sold in the United States that year.
Effective food safety plans leverage preventative actions that are relatively straightforward and common sense to reduce the risk of a contamination incident, and back them up with contingent actions that reduce the risk of a single contamination incident spreading or multiplying.
When you are considering a food safety plan for your farm, focus on those preventative and contingent actions that yield a high return. A series of one-percent reductions in risk will add up, but you get far more bang for your buck by focusing first on those actions that yield big results.