Infrared radiation inhibits browning of dried bananas.

The browning of fruits during the drying process impacts product quality and is a major concern. The enzyme polyphenol oxidase (PPO) appears to be the main cause of browning in bananas.

A drying process that utilizes infrared radiation could be harnessed to minimize enzymatic browning and, in turn, eliminate the need for further treating fruit to prevent it from turning brown. USDA-ARS researchers and colleagues at the University of California examined the quality characteristics of bananas that were dried using infrared radiation and natural convection. The results were compared to what occurred after drying the product using conventional hot air and forced convection. Infrared radiation gave the product a lighter color and completely inactivated PPO.

Cavendish bananas that were 8 millimeters thick were dried by researchers using infrared and hot air dryers at 60 C, 70 C and 80 C. The investigators measured any changes in residual PPO activity, color, moisture content and water activity after four hours of drying. Freshly cut bananas were used as a control.

Total inactivation of PPO was achieved with infrared radiation at all temperatures. However, only a 3% reduction of PPO was reached using hot air at 60 C. L-values were reduced to 72%, 73% and 61% in the infrared radiation-dried bananas and to 60%, 65% and 71% in hot air at 60 C, 70 C and 80 C, respectively.

Infrared radiation drying reduced water activities to 0.72, 0.65 and 0.52. With hot air, they declined to 0.69, 0.55 and 0.45 at 60 C, 70 C and 80 C, respectively. After 4 hours of drying, the moisture content in the hot air-dried bananas declined to 19%, 12% and 8%. The moisture content fell to 27%, 21% and 12% when using infrared radiation at 60 C, 70 C and 80 C, respectively.

Nevertheless, drying rates were higher when infrared radiation was used than when using hot air, at least during the first 20 minutes of drying. Drying the product with infrared radiation and natural convection did not shorten the drying time over the conventional hot air drying process when the product's temperature remained constant.

Further information. Zhongli Pan, Processed Foods Research, USDA-ARS Western Regional Research Center, Room 1111, 800 Buchanan St., Albany, CA 94710; phone: 510-559-5861; fax: 510-559-5851; email: zpan@pw.usda.gov.