Modeling sorption behavior and process kinetics of lemon balm (Melissa officinalis L.) for optimization of drying with regard to quality and energy requirement

Publikations-Art

Dissertation

Autoren

Argyropoulos, D

Erscheinungsjahr

2015

Veröffentlicht in

Schriftenreihe des Lehrstuhls für Agrartechnik in den Tropen und Subtropen der Universität Hohenheim

Herausgeber

Prof. Dr. Joachim Müller

Verlag

Shaker Verlag , Aachen

Band/Volume

10/2015/

ISBN / ISSN / eISSN

978-3-8440-4110-1

Schlagworte

Lemon balm, Melisse

Drying not only represents a standard postharvest technology for preservation of medicinal plants but also a fundamental processing step for the development of herbal medicinal products. Lemon balm (Melissa officinalis L.) is among the most important herbs in Germany. The objective of this study was to find the optimum drying temperature of M. officinalis in terms of drug quality and energy requirement. Therefore, drying experiments with a range of air temperatures were carried out in order to assess the quality of leaves with regard to color, rosmarinic acid as well as essential oil content and composition. Besides moisture content, another important material property related to drying and storage is water activity. Modern techniques were used in this work for the determination of moisture sorption isotherms at temperatures typically found in handling of medicinal plants. The equilibrium moisture contents of leaves and stems were separately established at 25, 35 and 45 °C over a stepwise increase of relative humidity ranging from 0 to 90% by dynamic vapor sorption. The modified Oswin equation was the best model to describe the experimental sorption isotherms of both plant organs. Predicting microbial stability at 25 °C, the leaves and stems should be stored at a maximum moisture content of 11 and 10% wet basis, respectively. Further analysis of the sorption data revealed that the isosteric heat of adsorption of leaves was greater than that of stems. The evolution of essential oil content was investigated in a high precision laboratory dryer at different drying temperatures varied from 30 to 90 °C, constant specific humidity of 10 g kg-1 dry air and uniform airflow of 0.2 m s-1. The essential oil yield was determined by hydrodistillation and the essential oil constituents were analyzed by GC/FID. The process was described by a first-order reaction kinetics model in which the temperature dependence of the rate constant was modeled by the Arrhenius-type relationship. Essential oil loss was observed primarily in the initial phase of drying and was proportional to air temperature. Drying at 30 and 45 °C resulted in 16% and 23% losses in essential oil, respectively, whereas drying at increased temperatures caused significantly higher losses, for instance 65% at 60 °C. Distinct changes in the major essential oil components were found to occur at 60 °C: neral, geranial and citronellal decreased, while citronellol showed an increasing tendency. Scanning electron images indicated that in addition to volatility of the oil constituents at increased temperatures, the losses in essential oil yields can also be attributed to the structural modifications caused by drying. The effect of drying temperature on CIELAB color changes was also examined. Total hydroxycinnamic acid derivatives were quantified by the photometric method and expressed as rosmarinic acid equivalents. Kinetic modelling was very useful in describing both redness and yellowness during hot-air drying. Color deterioration started immediately in the initial phase of drying, when moisture content was still high in the material. The rate of color degradation was found to increase as temperature increased. The rosmarinic acid equivalents in the leaves decreased with increasing drying temperature and this decrease was assumed to be responsible for browning, especially at higher temperatures. Based on the results, optimal drying air temperature was found to be 30 °C, because the least quality deterioration in terms of color, rosmarinic acid, essential oil content and composition was documented. However, the choice of drying temperature remains a decision of the operator, implying a trade-off between drying time and quality. Due to the temperature sensitivity of the herb, an attempt was made to investigate the impact of alternative drying methods on moisture sorption behavior and quality of dried leaves. An electric hygrometer was used to generate equilibrium relative humidity data in the head space from a common set of partially dried samples treated by convective-, vacuum- and freeze drying. The sorption data was modelled by the modified Halsey equation, while the isosteric heat of sorption was directly calculated by the model in which the Clausius-Clapeyron equation had been incorporated. The drying method was found to influence the moisture sorption properties of the material to some extent, for example the freeze dried leaves were more hygroscopic as compared to the vacuum- and air-dried ones within the storage-relevant level of water activity. Sample cross sections displayed a less dense cell structure with indications of open pores between the lower and upper epidermal layers in the freeze-dried leaves. The isosteric heat of sorption increased with reducing moisture content and temperature. Sorption heat was greater for the freeze dried samples within the monolayer and polymolecular regions of the sorption mechanism, which meant that more energy was required to remove water vapor by freeze drying. Over drying should be prevented as an additional energy requirement is generated at low moisture contents. With respect to quality, freeze drying caused minimal essential oil losses, preserving large amounts of the key aroma compounds, namely geranial and neral. Vacuum drying caused more pronounced essential oil losses and significant reduction in citrals. The interrelation between essential oil losses and structural alterations was also supported by scanning electron images. Finally, freeze drying and vacuum drying enhanced the conservation of rosmarinic acid, presumably due to the induced structural rearrangement at reduced operating pressure.