| Millions of dollars in crop losses occur yearly as a result of enzymatic browning. This oxidative browning reaction caused by polyphenol oxidase (PPO) results in deterioration of food quality and limits production of specialty products like sliced fruits. Prevention of unfavorable browning would have enormous economic benefits to the food industry and consumers; thus, studies were conducted to characterize two natural browning inhibitors: oxalic acid and inhibitor(s) isolated from common house fly (Musca domestica L.).; The mode of inhibition for oxalic acid on PPO was investigated. The extent of inhibition was influenced by oxalic acid concentration and also by pH changes. Inhibition of PPO by oxalic acid was due to its binding with copper to form an inactive complex, and the inhibition was characterized as noncompetitive. Oxalic acid was a more potent inhibitor of PPO compared with other structurally related acids. The relative antibrowning potency of oxalic acid on fresh-cut surfaces of bananas and apples was investigated using a machine-vision system. Degree of browning inhibition was evaluated based on oxalic acid concentration and storage time. Browning inhibition was most prominent on banana and apple slices treated with oxalic acid solutions at concentrations of 60 and 5 mM, respectively. Combining oxalic acid with ascorbic acid synergistically enhanced browning inhibition for apple slices, but was not effective for bananas. Apple browning was suppressed with a solution containing 0.5% ascorbic acid and oxalic acid in the range of 1.5 to 5 mM. Average residual oxalic acid levels in the tissues for an effective antibrowning activity were measured.; Additionally, a new type of inhibitor(s) was discovered from house fly extracts. The inhibitor(s) isolated by buffer extraction, heat treatment, and dialysis from house fly pupae inhibited the activity of apple PPO up to 90% at pH values above 5.0. Inhibitor(s) was further characterized by employing heat, freezing and thawing, irradiation, pH adjustment, and ultrafiltration studies. Potential PPO inhibitor(s) was stable to heating at 100°C for 1 h, repeated freezing and thawing, and irradiation. Inhibitor(s) was most stable near pH 5.0 and least stable at alkaline pH. The PPO inhibition profile of the house fly during metamorphosis was also evaluated. |
