Known as black bread mold, R. stolonifer is a fungus commonly found in contaminated food products such as meat, preserved, and/or baked goods. Potassium sorbate is a common food preservative used to prevent fungi, mold, and mycotoxin growth by damaging the cell membrane or altering proteins in the cell. Based on background research on potassium sorbate growth prevention, it was hypothesized that a 25% concentration of potassium sorbate will most efficiently prevent R. stolonifer growth compared to lower concentrations. R. stolonifer was grown on potato dextrose agar with eight levels of potassium sorbate concentrations. During a two week period, fungal growth was observed, photographed, and the height and width of each sample were recorded. At the end of two weeks, the fungi were stained using lactophenol cotton blue dye and observed under a microscope. Qualitative observation at the cellular level showed healthy R. stolonifer solely in the control group (see Figure 1). When comparing the fungi samples across the 5% and 30% concentration groups, large areas of dead mass dominated their cellular makeup. At the end of two weeks, the 1% concentration sample group had an observed increase of 1.383 cm2 in total area, a significant decrease compared to the control group. Between the 5% and 30% concentration sample groups, there were minor changes in total area with no change exceeding 0.125 cm2. Between the 1% and 5% experimental groups, there was a significant decrease in fungal area growth, and a similarly large decrease between the 10% and 15% concentration fungi samples. Thus, it can be concluded that it is unnecessary to exceed a 1% concentration of potassium sorbate to prevent fungal growth.