INFLUENCE OF DRYING ON THE VOLATILE COMPOUNDS OF SPEARMINT (Mentha spicata L.)
Tamás Antal, Benedek Kerekes, László Sikolya Department of Vehicle and Agricultural Engineering, College of Nyíregyháza Kótaji Str. 9-11., Nyíregyháza, H-4400, Hungary Tel: +36 (42) 599-400, E-mail:
[email protected],
[email protected],
[email protected] Abstract: Spearmint was dried using a freeze drier and conventional hot air drier. The effect of the drying method on the content of volatiles and dehydration kinetics of the herb was evaluated. The volatile constituents of herb were isolated by extraction and analysed by gas chromatography. A total of 9 compounds were identified in spearmint. It was found that drying time and essential oil content were strongly influenced by chamber pressure in freeze drier. Freeze-drying at high pressure was the method that produced the best results. Freeze-drying at low pressure and hot-air drying caused great losses of these compounds. Four different mathematical models were fitted to the drying data. Keywords: spearmint, freeze-drying, hot-air drying, volatile compounds, pressure INTRODUCTION Volatile aroma compounds are the most sensitive components in the process of drying. The effect of drying on the composition of essential oil of various aromatic plants, fruits and vegetables has been the subjects of numerous studies, which show that the changes in the concentrations of the volatile compounds during drying depend on several factors, such as the drying method and drying conditions (temperature, air velocity, relative humidity) (Venskutonis, 1996; Yousif et al., 2000, Diaz-Maroto et al., 2003, Kaya and Aydin, 2009; de Torres et al., 2010). Drying of plant material can be achieved by several processes, including hot-air and freeze-drying. Hotair drying can cause thermal damage and can severely modify the physical and chemical characteristics of the products. Although freezedrying can be used to avoid damage caused by heat, producing a product with superior physical and chemical qualities, it is considered a costly and time consuming process (Ratti, 2001). Mints are regarded as one of the most important spices throughout the world. Mint leaves in botany are the common name for members of the Labiatae, a large family of chiefly annual or perennial herbs. The essential oils of mints are widely used as flavourings in the food, spicing, tea infusions, cosmetic and pharmaceutical industries (Özbek and Dadali, 2007). This study has examined the influence of two different drying methods (freeze-drying and hot-air drying) on the volatile compounds of spearmint. The effect of the pressure (high and low) of the freezedrying procedure was also investigated. To best of our knowledge, no study has been reported on freeze-
drying of spearmint and its effect on the retention of volatiles and its effect on the drying kinetics. MATERIAL AND METHODS Raw material Fresh spearmint (Mentha spicata L.) leaves were purchased from a local grower in Nyíregyháza, Hungary. Harvest was done in the second half of September 2010, when spearmint just before flowering. Only leaves were used for analyses. Fresh leaves were stored in the refrigerator at 5±2°C before isolation of volatile compounds; the rest of material was put for drying immediately after sorting. Drying equipments and drying procedure The sample was placed on the drying tray in a thin single layer. The sample weight was kept constant at 50g for all runs. The moisture loss was recorded at 1 hour intervals during drying. Drying tests were replicated three times to obtain a reasonable average. The dried samples were powdered prior to extraction of volatiles. Convective drying was done by using a conventional hot-air dryer. The fresh herb was dried in a laboratory cylindrical dryer (LP-302 type, Hungary). Convective method was operated at 43°C (air temperature) with a relative humidity of 11-48%. During the experiments, the temperature changes and relative humidity are measured using nickel-chromenickel thermocouple and humidity meter (Testo, Germany). During the drying experiments, the velocity of air-flow (U=0,5 m/s) were measured with Testo 4510 Vane Probe Anemometer. Moisture loss of mint leaves were recorded during drying for
determination of drying curves by digital balance (JKH-500, Taiwan). Freeze-drying was carried out in Armfield FT33 laboratory drying equipment. The product was dried for 14 h period at 150-250 Pa and 12 h period at 1030 Pa with the heating plate maintained at 17°C. Condenser temperature was kept at -50 to -55°C. For an exact monitoring of the drying process a mass balance instrument was designed to supplement this freeze-drier (EMALOG, Hungary). The initial and final moisture content of products is determined using an infrared moisture analyzer (PRECISA HA 60, Switzerland). Analysis of volatiles About 50 g of the fresh and 10 g each of dried leaves of the plant were separately subjected to extraction. Gas Chromatography (GC) analysis of the essential oil was performed by Perkin-Elmer Clarus 500 model Auto System GC with flame ionization detector (FID). Analytical standards of the flavour principles were obtained from Sigma-Aldrich. Quantitative data were obtained electronically from FID area percent data without the use of correction factors. The result was an average of three determinations. Mathematical modelling of drying data Drying curves were fitted with four thin-layer drying models, namely, the third-degree polynomial, sigmoid, Page and exponential models (Table 1). Parameters of sigmoid and exponential model have a physical meaning. The parameter a is the asymptotic value of water content during drying. That value can be a bit lower than zero and is thus not reached during drying. The parameter b is a theoretical interval of moisture content values. Parameter c corresponds to the time coordinate of the inflexion point in the drying curve (only at sigmoid). That point can thus be treated as a critical point (K). Thus the point K constitutes an extremum in the drying rate function. The d parameter is the time constant of the drying process. The greater the d the longer is the drying process (Figiel, 2009). Table 1. Mathematical models applied to the drying curves Methods
Model name Polynomial
Freezedrying
Sigmoid
Hot-air
Page Exponential
Model M = at 3 + bt 2 + ct + d b
M =a+
1+ e
M = a ⋅e
t −c d
− b⋅t c −
t
M = a + b ⋅e d M: moisture content (kg water/kg dry matter), t: drying time (h).
Parameters of third-degree polynomial: a, b, c, d are coefficients of the third-degree polynomial. The values of those parameters depend on the characteristics of the material: the variety, the freezing speed, the ripeness and the tendency to lose water. The sample moisture content M was calculated on a dry basis (db) according to Equation (1): M=
Wt − Wk Wk
(1)
Statistical analysis Data were subjected to one-way analysis of variance (ANOVA) using appropriate software (PASW Statistics 18). When it is required, the differences between methods were analyzed by Tukey’s test (significance level p
