due to the effect of lipid oxidation during the preservation process. To avoid oily products, hot air drying (HA) is proposed as an alternative method. However,this method requires a long drying time with low energy efficiency and also deteriorates the product quality in terms of nutrients, colour, flavour, shrinkage and texture (Bai-Ngew et al., 2011). Product quality can be better maintained by freeze drying and vacuum drying. However, freeze–drying requires high investment and operating costs and thus is only suitable for high value products such as herbs.