Oxalic Acid in Commercial Pectins Inhibits Browning of Raw Apple Juice

Various commercial pectin preparations were tested for their ability to prevent enzymatic browning of fresh, raw, Granny Smith apple juice. Six of eight commercial pectin samples inhibited browning to some degree, and two of the pectin preparations completely prevented fresh, unrefrigerated, apple juice from browning for 24 h when added to juice at a concentration of 0.5% (w/v). Fractionation and testing of one pectin preparation showed that the antibrowning activity was not due to pectin, per se, but to the presence of low levels of oxalic acid (<0.5%) in the pectin preparations. When oxalic acid was removed from the commercial pectins, antibrowning activity was lost or greatly reduced. Pure oxalic acid was also found to inhibit browning reactions in fresh juice. Pure oxalic acid is not an approved food additive, but it does occur naturally in fruits and vegetables such as rhubarb and spinach.