Bikolano-led study suggests electric field may alter discoloration in fruits, vegetables

A recent study at the Food Engineering Laboratory of the Ohio State University (OSU) showed that the enzyme causing browning in plant and animal tissues can be activated or inactivated at certain frequency and holding time under moderate electric field.

In agriculture, browning of produce is often regarded as a sign of quality deterioration. This is observable in banana, apple, mango, eggplant, cabbage and other crops. As such, the control of such discoloration can save the product from significant loss of market value. There are exceptions of course, where browning is desirable, as in the case of tea and raisin processing. Whatever the desired result, moderate electric field treatment appears promising in either enhancing or preventing discoloration.

“The discoloration in biological materials is generally due to three different causes: enzymatic, Maillard and caramelization reactions,” says Engr. Jerry James M. de la Torre, principal investigator in the research Effect of Frequency on Polyphenoloxidase Activity During Moderate Electric Field (MEF) Treatment.

“Here, we focused on enzymatic browning by isolating the enzyme and applying 10 V/cm, 60, 600 and 6000 Hz at varying holding times (up to 15 min) and different isothermal conditions (40, 50, 60°C). Later, we added the substrate L-dihydroxyphenylalanine to see if the enzyme is still active or not,” he adds.

Results showed that MEF stimulated higher enzyme activity at 60 and 6000 Hz 40°C 10 min and 6000 Hz 40°C 15 min and at all frequencies at 60°C 15 min. Reduced activity was noted at all frequencies but at different conditions in the first 10 min of 60°C treatments: 60 Hz 5min as well as 600 and 6000 Hz both at 10 min. Overall, the data suggest that MEF activation is likely to occur at higher frequency and at longer holding periods.

Enzymatic browning occurs when an active polyphenoloxidase reacts with phenolic substrates in the presence of oxygen to produce compounds that automatically form melanin pigments. Both the enzyme polyphenoloxidase and phenolic substrates are commonly existent in plant and animal tissues.

To arrest browning, the conventional treatments are either thermal or application of chemical inhibitors. Heat treatments are energy intensive and damaging to other nutrients and sensory attributes of the food. Inhibitors like sulfites are effective but allergic reactions in some consumers are among the big stumbling blocks. Electric field treatment is promising because of rapid and uniform stimulation and lethality of target objects in food. Its efficacy is dependent on the conductivity of the target substance, dipole moment, isoelectric point and molecular aggregation.

“Our findings have relevant implications not only in agriculture but also in medical and industrial fields. The control of enzymatic browning is a key area in skin cancer treatments in humans and animals. Enzyme activation is also very important in improving efficiency in enzyme catalyzed reactions such as those in fermentation and biofuel processing,” according to Engr. de la Torre. “The response of polyphenoloxidase also implies that electric field may also be applied to other structurally similar biological compounds,” he said.

Engr. de la Torre, science research specialist II at the Bureau of Postharvest Research and Extension (BPRE), conducted the study as part of his MSFood Engineering program through a Fulbright scholarship. He’s a Bikolano agricultural engineer from Camarines Sur State Agricultural College (CSSAC) who placed 5th in the 1998 licensure examinations.