THIN LAYER DRYING KINETICS, CHARACTERIZATION AND EMPIRICAL MODELING OF GINGER RHIZOMES

Mahesh Ganesapillai1, Prithvi Simha1,* and Ashita Gugalia1 1 Mass Transfer Laboratory, Chemical Engineering Division, School of Mechanical and Building

Sciences,

VIT University, Vellore-632014, India

* Corresponding Author: Prithvi Simha, Chemical Engineering Division, VIT University, Vellore Email: prithvisimha092@gmail.com prithvisimha@mespom.eu

Keywords: Ginger rhizome; Microwave drying; Modeling; Food preservation; Shelf life

Extended abstract:

Background: Ginger, the underground stem or rhizome of the plant Zingiber officinale Roscoe (Musales: Zingiberaceae), has attracted widespread attention for its medicinal properties. Though it is native to tropical Asia, it is now grown as a commercial crop in Latin America and Africa. Because of lack of suitable storage and transport facility, ginger, which is a semi-perishable crop, is wasted by respiration and microbial spoilage. It is sold loose, bunched, packaged fresh in bags, canned, or frozen. Drying is the one of the storage methods, which has the capability of extending the consumption period of ginger while maintaining its vitamin content. Drying processes not only inhibit microbial growth but also several biological and chemical degradation reactions. Dehydration by open-air sun drying, one of the oldest preservation practices for storage of agricultural products is still a common method practiced in most countries of the world with adequate solar radiation. In all the methods, the heat transfers from the surface to the bulk. Prolonged exposure to elevated temperatures results in shrinkage of cells, loss of rehydration ability, case hardening and substantial degradation in the quality of the desired product causing reduction in color, odor, flavor and nutrients. Microwave heating at specific experimental conditions is aimed at the restoration of

nutritional and medicinal properties of the raw material. In contrast with conventional heating mechanisms, where energy is first converted to heat then transferred along temperature gradients from the surface to the core of the material, microwave (MW) induce heat at the molecular level by direct conversion of the electromagnetic energy into heat. Its properties such as uniform energy, high thermal conductivity to the inner sides of the material, space utilization, sanitation, energy savings, precise process control, fast startup and shutdown conditions has led to its rapidly growing market. The objectives of this study were: (1) To describe microwave drying characteristics of ginger and discuss the influence of sample thickness, microwave output power and sample load on drying characteristics; (2) To calculate effective moisture diffusivity with respect to sample thickness, load and microwave output power; (3) To study the applicability of several thin layer models and to develop a new suitable model from the basic equation; (4) To study the effect of microwave power on extraction of oleoresins.

Results: Ginger rhizomes of different thickness, microwave output power and sample load were dehydrated in a modified microwave oven, and effective moisture diffusivity was calculated. According to the drying rate curves for microwave drying of thin ginger slices, acceleration of drying process was observed with increase in microwave output power from 100 to 300 W when initial moisture content of samples were kept almost same. Unlike other investigators, only falling period was observed under all test conditions. By working at 300 W instead of 100 W, the drying time could be shortened by 3.1-fold for ginger slices. Moreover, Drying times were seen to be directly proportional to sample thickness. It was observed that with increase in thickness from 0.001 to 0.003 m, drying time increased from 10 to 15.6 mins. The moisture content versus drying time curves for microwave drying of ginger were plotted. Higher the material load, the longer was the drying time as shown in the figure given below. The results confirm that maximum drying rate can be achieved by microwave drying of ginger rhizomes with samples of 0.001 m thick at microwave output power of 300 W for a sample load of 25 g, giving a final moisture content of 31.53% (d.b.). The results also showed that the effective moisture diffusivity increased with the increase in microwave output power and decreased with the increase in sample amounts and thickness. The effective moisture diffusivity values ranged from 20.24 x10-12to 9.87 x 10-12m2/s for 0.001-m-thick samples at power of 300 W for 40 g and 25 g of sample loads, respectively. Although the use of MW drying technology is well

established in society, and are used in domestic and industrial processes, the developments of these driers in commercial large scale are limited.

The drying behavior of a number of food products has recently been a subject of interest for various investigators. The drying kinetics of materials may be described completely using their transport properties together with those of the dried medium. In MW drying, the drying constant combines all the transport properties and may be defined by the thin layer equation. Although, it describes the drying phenomena in a unified way, regardless of the controlling mechanism, it has been used to estimate the drying time of several products and to generalize drying curves. Thin-layer equations can contribute to the understanding of the heat and mass transfer phenomena and computer simulations for the design of new processes and the improvement of existing commercial drying operations. In the development of thin layer models for food products, generally the moisture content of the material at any time is measured and correlated to the drying parameters. Many investigators have successfully used different thin layer equations available in the literature to explain drying of a wide variety of products. Hence, there is a need to investigate the microwave drying performance of ginger and develop a fitting model to study the drying kinetics to preserve without compromising the quality of natural flavor and aroma. The moisture content data obtained at different microwave output power, sample thickness and load were converted into MR expression, and then curve fitting computations with drying time were performed for all eight models. Among the various basic models considered, diffusion equation was best fit to predict the drying behavior for the test conditions, yielding the highest R2 of

0.9983. Based on the interpretations, the constants and coefficients were rewritten in Arrhenius and logarithmic expressions for microwave power. Validation of the derived mathematical models was carried out according to statistical parameters which are root mean square error, reduced chi-square mean bias error and t-test values. The effect of microwave drying on extraction of oleoresins with hexane as the solvent was in the range of 2.534.55%. Integrity of texture and functional groups were enhanced as revealed by scanning electron microscopy and Fourier transform infrared spectroscopy. Mathematical models applied in this research will be helpful to analyze, simulate as well as to design and scale up the microwave drying process for industrial and commercial applications. Furthermore, the product may have an increased shelf life, finished product quality; may reduced energy cost in comparison with the conventionally dried rhizomes; and may serve as ingredient for medicinal, pharmaceutical and cosmetics industry for the extraction of essential oils such as oleoresins.