The researchers outlined that one of the reasons people may believe peanut butter is low-risk for salmonella contamination is because it has a low moisture content (via Foods). Following roasting, peanuts have a moisture content of only about 3%. Furthermore, the final product is calculated to have a water activity (aw) measurement of approximately 0.35, which seems low-risk when you consider that salmonella requires about 0.99 water activity to grow.
Yet this doesn’t make peanut butter immune to potential bacterial contamination. Rather, peanuts’ low moisture content may be one factor that supports potential salmonella contamination in peanut butter. This is because bacteria can become heat-resistant in foods that are low in moisture, but simultaneously high in protein, sugar, and/or fat (like in peanut butter). Past cases of salmonella outbreaks have been traced to contaminated water used in processing facilities. Michael Doyle, director of the Center for Food Safety at the University of Georgia in Griffin, told Scientific American that exposure to animal fecal matter is one way in which the water may become contaminated.