Kinetics and Mathematical Modeling of Microwave Drying of Sri Lankan Black Pepper (Piper nigrum)
Abstract
Drying characteristics of black pepper (Piper nigrum) was investigated in a microwave drying system. Drying experiments were carried out at three different microwave power levels, 180, 360 and 540 Wand the moisture content was measured at different time intervals. Experimental results were fitted to seven thin layer drying models; Newton, Page, Henderson and Pabis, Logarithmic, Midilli et. al., Weibull and Kaleta et. al.. Statistical indicators; Coefficient of determination (R2), Root mean square error (RMSE) and reduced chi square values (2) showed Midilli et. al., Weibull and Kaleta et. al. models give better fit to the experimental values. Drying rate constants and the equation constants were compared and analyzed. Similarities of the drying models were observed and discussed with respect to the equation parameters. The absence of the constant drying rate period in drying curves show the drying of black pepper lies totally in the falling rate period where the drying rate is controlled by the moisture diffusion. Maximum drying rates observed were 0.02, 0.05 and 0.08 kg moisture/kg of dry material/ min at 180, 360 and 540 W power levels. Results revealed drying rate constant and the effective moisture diffusivity values increases with the microwave power level. Drying rate constants were 0.03, 0.09 and 0.16 min-1 and the effective moisture diffusivity values were 2.43 x 10-10, 4.87 x 10-10, 1.42 x 10-9 m2/sfor power levels of 180, 360 and 540 W respectively. The Activation energy of black pepper calculated based on the Arrhenius equation is 86.7 W/g.
Keywords
Download Options
Introduction
Black pepper (BP) (Piper nigrum) is the most widely used spice in the world. BP is also used in perfume and pharmaceutical industries. The crop is grown in many tropical countries including Sri Lanka. The world production of pepper in 2017 is approximately 472,500 MTand is gradually growing. Sri Lanka produces around 3% of the world pepper production [1]. The mature peppercorns are green in colour and become red when ripe (Fig 1). This mixture of green and red peppercorns is dried after harvesting to reduce the moisture content to avoid fungal growth and wastage. The moisture content of the pepper is around 77% when harvested and reduced up to 11% (wet basis) before storage. Solar drying is the widely used method in Sri Lanka for drying of black pepper. However, improper drying specially during rainy season causes postharvest losses. National Committee on Postharvest Technology & Value Addition in Sri Lanka has identified BP as a high priority crop and further development of postharvest drying technologies is recommended as amain thrust research area [2]. Food and grain drying have been paid much attention by researchers. Solar drying, hot air drying, fluidized bed or spouted bed drying are the widely used techniques for food drying [ 3-8]. Application of Microwave technology has been investigated over the last few years for food and grain drying [9-13]. However, despite the wide use of BP as a spice in the world, the previous studies on drying of black pepper is limited. Solar tunnel dryers and fluidized bed or spouted bed dryers have been tested for black pepper drying [14-16].
Chacko et. al. [16] and Magda et. al. [13] have found that microwave drying is suitable for BPdrying and have observed improvement in aroma despite minor losses of some volatiles. Drying kinetics is important in determining the drying time required and hence the dryer size.
Therefore, the objective of this work is to study drying kinetics of black pepper in microwave drying and determine moisture diffusivity and activation energy of the material.
Conclusion
Microwave drying kinetics of black pepper was studied and the experimental data was fitted to seven thin layer drying models. The Midilli et. al., Weibull and Kaleta et. al. models showed statistically better fit compared to other models. Similarities of the model parameters were observed and the significance of the parameters was analyzed. The drying rate constants obtained by Midilli et. al.model were 0.03, 0.09 and 0.16 min-1 for power levels of 180, 360 and 540 W respectively and the other models also showed values in the similar range. The Effective moisture diffusivity increased with the microwave power level and values were in the range 2.43x 10-10-1.42 x 10-9 m2/s. The drying rate constants and the effective diffusivity values were in the same order of magnitude as the values obtained by other workers for similar products. The Activation energy of black pepper calculated based on the Arrhenius equation is 86.7 W/g. Drying of BPunder low microwave power level is recommended to preserve volatile matter in the product. The time required to dry peppercorns from 75% MC to 11% MCwet basis under 180 W power level is 88 minutes compared to several days under sun drying.
