JOURNAL OF FOOD PROCESS ENGINEERING

The spinach puree was heated from 30C to prescribed temperatures (60, 70, 80 or 90C) ohmically and conventionally. Ohmic heating was performed by application of four different voltage gradients in the range of 10-40 V/cm, while conventional heating was conducted at constant temperature in a water bath. Although ohmic heating time required from 30 to 70C was similar for water heating (129 +/- 7 s) and ohmic heating by 20 V/cm (127 +/- 2 s), faster heating (51 +/- 0.5 s) could be obtained as the voltage gradient for ohmic heating increased to 30 V/cm. Ohmic heating caused browning more than conventional water heating for the same temperature range. The effect of voltage gradient applied was not found statistically significant on chlorophyll (total, a and b), beta-carotene and color values (L, a, b, Lb/a, Lba, hue angle) (P < 0.05). On the other hand, temperature (60, 70, 80 or 90C) at constant voltage gradient (30 V/cm) affected color values of spinach puree (P < 0.05), whereas holding time (0 and 600 s) at constant temperature increased chlorophyll-a and beta-carotene contents (P < 0.05). This increase could be caused by the possible enhancing effect of ohmic heating on beta-carotene biosynthesis and formation of chlorophyll derivatives. It was concluded that ohmic heating could be applied to vegetable purees, resulting in high retention of color attributes and beta-carotene. PRACTICAL APPLICATIONS The present study suggests that ohmic heating could be used to heat the spinach puree faster than conventional heating by adjusting voltage gradient. Moreover, this treatment resulted in high retention of color attributes. In this study, detailed information was given about the effects of ohmic heating parameters such as temperature, voltage gradient and holding time on chlorophyll, beta-carotene and color values of spinach puree. There are no published data in the open literature about effects of ohmic heating on beta-carotene and chlorophyll, to the authors' knowledge. These data could be used in designing of ohmic heating units for functional foods