Intermittent heating aids in blanching, dehydrating produce.

Infrared (IR) heating controlled by intermittent heating is able to simultaneously dry-blanch and dehydrate apple slices while at the same time giving them desirable quality. In order to better understand the performance of intermittent heating for simultaneous dry-blanching and dehydration applications, scientists at the USDA-ARS and colleagues at the University of California examined the relationship between process variables and product quality.

Their goal was to study the impact of simultaneous dry-blanching and dehydration, used with an intermittent heating mode, on apple processing characteristics and product quality. It turns out that intermittent IR heating can be used to simultaneously blanch and dehydrate fruits and vegetables since it satisfactorily inactivates enzymes in apples and removes a desirable amount of moisture while preserving the surface color of apples.

The researchers used an IR dry-blancher-dehydrator equipped with two catalytic IR emitters powered by natural gas. Apple slices were heated from both their top and bottom sides. A constant product surface temperature was maintained by controlling the natural gas supply to the emitters.

The investigators used three processing parameters: surface temperatures of 70 C, 75 C and 80 C; processing times of up to 40 minutes; and sample thicknesses of 5 mm, 9 mm and 13 mm. They measured certain processing characteristics, including: moisture removal, drying and heating rates, product quality, the residual enzyme activity of polyphenol oxidase (PPO) and peroxidase (POD), and changes in surface color.

Their results indicated that a higher surface temperature of 80 C led to a higher center temperature. This also caused a more rapid inactivation of enzymes and less change in surface color than did a lower surface temperature of 70 C. After 10 minutes, all of the processing conditions led to more than a 90% inactivation of PPO, but the residual POD activity was still high, ranging from about 4% to 23%. Compared to continuous heating, intermittent heating took a longer time to reach the same level of enzyme inactivation but never caused the samples to burn.

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