Microwave Drying - A promising alternative for the herb processing industry

Abstract

Herbs just like any other biological product are susceptible to rotting and degradation which can render the product unsuitable for consumption, or at least make them visually unappealing for consumers. Fresh and dried herbs are widely used as flavouring agents in many different food products on account of their powerful aromatic odour. The introduction of a microwave drying technique could offer a promising alternative for the herb processing industry. Hence the efficacy of microwave drying of herbs viz., mint, coriander, dill and parsley leaves with respect to drying characteristics such as drying time, moisture loss, drying constant, rehydration ratio and water activity and the product quality in terms of colour and chlorophyll content at selected levels of microwave power density (10, 30, 50, 70 and 90 Wg-1) was investigated and compared with convection air drying (45, 60 and 75°C). An increase in air temperature from 45 to 75°C resulted in 77 to 90 per cent reduction in drying time. The microwave drying technique was more efficient than conventional hot air drying and resulted in savings to an extent of about 95-98 percent of drying time. The single exponential model used to describe the drying kinetics of leaves gave an excellent fit for all the data points with higher coefficient of determinations. The value of the drying constant increased with the increased microwave output power signifying faster drying of the product. The microwave dried leaves exhibited less shrinkage and thus had better rehydration characteristics. The dried leaves were safe and stable with respect to microbial growth, chemical/biochemical reaction rates, and physical properties based on water activity values. In general, compared to hot air dying, microwave drying can be effectively used for drying herbs (mint, dill, coriander and parsley leaves) owing to improved drying kinetics (sharp reduction of drying time, increased drying rate) and better quality attributes (higher rehydration ratio, ensured economic viability and microbiological safety, retention of colour and chlorophyll content).