Inhibition of Enzymatic Browning of Fresh-Cut Potato by Immersion in Citric Acid is Not Solely Due to pH Reduction of the Solution

In order to elucidate the mechanisms of citric acid in fresh-cut potato browning inhibition, various concentrations of immersion solutions were prepared using either citric or sulfuric acid for pH ≤ 4 and sodium hydroxide for pH > 7. Dipping potato slices in water (control, pH = 5.84) allowed severe browning to develop during 6 days of storage at 5C. Citric acid at concentrations >0.5% inhibited browning on the cut surfaces of fresh-cut potato without affecting the antioxidant content or PPO activity, and induced the highest increase of tissue pH during storage. Dipping in 1 or 2% citric acid (pH 2.42 and 2.24) reduced all color changes equally, resulting in the lowest discoloration scores. Acidification of the immersion solution using H2SO4 also reduced discoloration, as NaOH solutions with pH > 7, but neither was as effective as citric acid. Further reducing the pH of 0.5% citric acid from 2.59 to 2.24 with H2SO4 also did not improve browning inhibition. Practical Application Acidification or alkalization of the immersion solution using inorganic compounds reduces browning of potato slices, although not as efficiently as citric acid, by reducing PPO activity. Citric acid reduces browning of potato slices, without affecting PPO activity, either by bonding with phenolic substrates or PPO enzyme to form complexes, or by generating the formation of other compounds, as implied by the pH increase of the surface tissue. No correlation between the total phenolic content, antioxidant capacity, and discoloration score were observed. This research shows that the effectiveness of citric acid in reducing browning in fresh-cut plant tissues is not predominately due to acidification, and that the effectiveness of citric acid as an antibrowning agent cannot be duplicated by simple pH adjustment of the immersion solution—either below or above the PPO optimum range.