© 2020 Elsevier Ltd We establish the total amino acids (AA) concentration in wash water as an alternative indicator of free chlorine (FC) levels, and develop a model to predict FC concentration based on modeling the reaction kinetics of chlorine and amino acids. Using single wash of iceberg lettuce, green cabbage, and carrots, we report the first in situ apparent reaction rate β between FC and amino acids in the range of 15.3 – 16.6 M−1 s−1 and an amplification factor γ in the range of 11.52–11.94 for these produce. We also report strong linear correlations between AA levels and produce-to-water ratio (R2 = 0.87), and between chemical oxygen demand (COD) and AA concentrations (R2 = 0.87). The values of the parameters γ and β of the model were validated in continuous wash experiments of chopped iceberg lettuce, and predicted the FC (R2 = 0.96) and AA (R2 = 0.92) levels very well.