Heat and moisture transport behaviour and quality of chopped garlic undergoing different drying methods

• FIR combined with HP drying provided a shorter drying time compared with HA or HP drying.
• Drying by HA and HP at 50 °C could retain allicin in the dried chopped garlic.
• The allicin retention was decreased when FIR was combined with HP, due to an increase in the internal product temperature.
• The volatile oil content was rather insensitive to heat.
• The colour of the garlic powder was found to be darker when the FIR power level increased.

A far-infrared radiation-assisted heat pump (HP-FIR), which is a novel drying method, was used for drying chopped garlic and compared with conventional hot-air (HA) drying. The drying characteristics and the qualities of the dried chopped garlic in terms of the allicin content, volatile oil content and colour were investigated. HA and heat pump (HP) drying were carried out at a temperature of 50 °C. The far-infrared radiation (FIR) power levels used in combination with HP drying were 250, 350 and 450 W. A coupled heat and mass transfer model was developed to estimate the effective moisture diffusivity (Deff) and to predict the evolution of the moisture content and internal temperature of chopped garlic. The experimental results showed that an increase in the FIR power resulted in a faster decrease in the moisture content and a shorter drying time. The amounts of allicin in samples dried by HP-FIR at different FIR powers were not significantly different; their percentage losses were about 20% higher than those obtained from HA or HP drying. However, the volatile oil contents in all dried samples were similar to those of fresh samples. HP drying provided garlic powder with the brightest colour as manifested by the highest values of L and the hue angle. The values of L and the hue angle had a decreasing trend when the FIR power increased. In addition to quality issues, the developed model could estimate the Deff of chopped garlic rather well. The Deff value was in the range 0.41–1.43 × 10−10 m2/s at the internal product temperature range from 50 to 62 °C. The predicted moisture content and internal temperature of chopped garlic agreed quite well with the experimental data.