Volatile Organic Compounds, Total Phenolic Content, Color, and Heating Uniformity of Lemon Peel as Affected by Rotational Speed of Turntable During Microwave Drying

Abstract

In this study, the effects of rotational speed of microwave turntable (0, 6.5, 9.5, and 12.5 rpm) on surface temperature distribution of lemon peels during drying and, on some quality attributes (color, water activi-ty, total phenolic content (TPC), and volatile organic compounds) of dried lemon peel powders were investigated. The quality analyses were also performed in freeze-dried peels. During microwave drying with rotation, speed of turntable affected the surface temperature values and TPC depending on the power level but did not exert a clear effect on the homogeneity of the surface temperature distribution and the color parameters. The percentage amount of major monoterpene hydrocarbons (limonene, γ-terpinene, myrcene, p-cymene, and α-p-dimethylstyrene) detected in fresh lemon peel decreased after microwave and freeze-drying. At 600W, formation of furan compounds (furfural, furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-acetyl furan, 5-methyl furfural, and HMF) were identified. Microwave drying without rotation (0 rpm) caused uneven heating which led to the production of peel powders with unacceptable dark color especially at 450 and 600W. Drying at 600W-0 rpm gave the darkest color, the highest amount of TPC (1730.7 mg GAE/100g d.b.) and furan compounds. Therefore, microwave drying at low power levels with rotation function preserved the quality of lemon peel powder better.