Citric Acid Reduces Browning of Fresh-Cut Potato by Means Other Than as An Acidulant

Enzymatic discoloration in fresh-cut potato during handling develops due to polyphenoloxidase (PPO) activity. The optimum PPO activity pH is 5.25-5.75, with limited activity below 4 and above 7. Therefore; in order to study the solution pH effect on browning, potato slices were dipped in organic (citric acid; CA) and inorganic (HNO3, H2SO4 or NaOH) solutions at various pH levels (2.24-11.58) and browning on slices was evaluated after 6 days storage at 5oC. The most severe browning occurred after dipping in pH 5.7-5.8 solutions as indicated by 12-14% ΔL* and 9-10 ΔE, where ΔΕ=(ΔL*2+Δa*2+Δb*2)1/2. Browning was reduced in proportion to the concentration of CA from 0.5 to 2%, even when the pH was held constant at the lowest level (2.24). Color changes of potato slices previously dipped in alkaline solutions of pH 7.9-11.6, remained at the same high levels as the control (10.3-12.5% ΔL* and 8-9 ΔE), indicating that there was no inhibitory effect of the high pH solutions on browning development. In contrast, a pH 2.3 HNO3 solution (0.0353%) significantly reduced %ΔL* and ΔE, similarly to 2% CA, whereas HNO3 solutions of pH 3 or 4 proved insufficient to control browning. Dipping potato slices in pH 2.4-4 H2SO4 solutions (0.04021-0.00098%) also reduced browning, although not as well as 2% CA or pH 2.3 HNO3 solutions. Visual evaluation based on the per cent slice discolored area revealed that dipping in a ≥1% CA solution resulted in the lowest discoloration score (DS=2.6-3.2 out of 6), not significantly different from dipping in a pH 2.3 HNO3 solution (DS=3.4), but lower than all the other treatments (DS=3.6-4.3). From the above, it appears that browning inhibition by CA is due to a mechanism other than acidification – possibly its ability to act as a copper chelating agent in the active site of PPO. It is possible that HNO3 and H2SO4 similarly reduce browning by chelating copper rather than by reducing the solution pH.