H. Bahmanpour; S. M. Sajadiye; M. J. Sheikhdavoodi; M. Zolfaghari
Abstract
Introduction Mint (Mentha spicata L.) cbelongs to the Lamiaceae family, is an herbaceous, perennial, aromatic and medicinal plant that cultivated for its essential oils and spices. Since the essential oil is extracted from dried plant, choosing the appropriate drying method is essential for gaining high ...
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Introduction Mint (Mentha spicata L.) cbelongs to the Lamiaceae family, is an herbaceous, perennial, aromatic and medicinal plant that cultivated for its essential oils and spices. Since the essential oil is extracted from dried plant, choosing the appropriate drying method is essential for gaining high quality essential oil.Vacuum drying technology is an alternative to conventional drying methods and reported by many authors as an efficient method for improving the drying quality especially color characteristics. On the other side, solar dryers are also useful for saving time and energy. In this study the effect of two method of dryings including vacuum-infrared versus solar at three different conventional temperatures (30, 40 and 50°C) on mint plant is evaluated while factorial experiment with randomized complete block is applied. Drying time as well as color characteristics areconsidered for evaluation of each method of drying. Materials and Methods Factorial experiment with randomized complete block was applied in order to evaluate the effect of drying methods (vacuum-infrared versus solar) and temperature (30, 40 and 50°C) on drying time and color characteristics of mint. The initially moisture content of mint leaves measured according to the standard ASABE S358.2 during 24 hours inside an oven at 104 °C. Drying the samples continued until the moisture content (which real time measured) reached to 10% wet basis. The components of a vacuum dryer consisted of a cylindrical vacuum chamber (0.335 m3) and a vacuum pump (piston version). The temperature of the chamber was controlled using three infrared bulbs using on-off controller. Temperature and weight of the products registered real time using a data acquisition system. The components of a solar dryer were consisting of a solar collector and a temperature control system which was turning the exhaust fan on and off in order to maintain the specific temperature. A date acquisition system was applied to register and monitoring product weight real time. For imaging of dried samples, a semi-professional digital cameras Fujifilm Fine Pix HS55model Barzvlvshn 921000 pixel was applied. Dry samples were used to determine the RGB color model that consists of three whole red (Red), green (Green) and blue (blue) light intensity 0 to 255 (in this case, zero for black and 255 for white pixels) Finally, the average of RGB changes color index were calculated as the mean change color of samples during the drying. Results and Discussion The results showed that drying time of solar dryer is more than vacuum-infrared (averaged: 201 versus 153 minutes). For two methods of drying, increasing temperature, made reduction in drying time. The maximum drying time registered 237 minutes for solar method which was set to 30°C and minimum drying time was registered 112 minutes relating to vacuum –infrared which was set to 50°C. Color evaluation showed that the effect of drying method on the changes of colour index (before and after drying) is reasonable. Vacuumed-infrared dryer case with 8.75% color change was showed to be much efficient than solar dryer with 11.96% change. Analysis of variance was performed due to the drying temperature index mint color changes and results showed the reasonable difference. The highest and lowest color change related to the temperature of 50°C (11.767%) and 30°C (9.197%) respectively. Conclusion Drying method as well as applying temperature showed rescannable effects on daring time and color quality of mint. The vacuum-infrared method reduces drying time for all temperature treatments considered in this study. Beside this, using vacuum-infrared showed minimum changes on color characteristic and can be say more efficient in aspect of color quality especially at its lowest applicable temperature (30°C). Increasing temperature causes the samples to be more darken for both drying methods. This phenomena may be related to replacement of magnesium by hydrogen inside the chlorophyll and then causing the chlorophyll to be destroyed.
N. Hafezi; M. J. Sheikhdavoodi; S. M. Sajadiye; M. E. Khorasani
Abstract
Introduction
Potato (Solanumtuberosum L.) is one of the unique and most potential crops having high productivity, supplementing major food requirement in the world. Drying is generally carried out for two main reasons, one to reduce the water activity which eventually increases the shelf life of food ...
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Introduction
Potato (Solanumtuberosum L.) is one of the unique and most potential crops having high productivity, supplementing major food requirement in the world. Drying is generally carried out for two main reasons, one to reduce the water activity which eventually increases the shelf life of food and second to reduce the weight and bulk of food for cheaper transport and storage. The quality evaluation of the dried product was carried out on the basis of response variables such as rehydration ratio, shrinkage percentage, color and the overall acceptability. Drying is the most energy intensive process in food industry. Therefore, new drying techniques and dryers must be designed and studied to minimize the energy cost in drying process. Considering the fact that the highest energy consumption in agriculture is associated with drying operations, different drying methods can be evaluated to determine and compare the energy requirements for drying a particular product. Thermal drying operations are found in almost all industrial sectors and are known, according to various estimates, to consume 10-25% of the national industrial energy in the developed world. Infrared radiation drying has the unique characteristics of energy transfer mechanism. Kantrong et al. (2012) were studied the drying characteristics and quality of shiitake mushroom undergoing microwave-vacuum combined with infrared drying. Motevali et al. (2011) were evaluated energy consumption for drying of mushroom slices using various drying methods including hot air, microwave, vacuum, infrared, microwave-vacuum and hot air-infrared. The objectives of this research were to experimental study of drying kinetics considering quality characteristics including the rehydration and color distribution of potato slices in a vacuum- infrared dryer and also assessment of specific energy consumption and thermal utilization efficiency of potato slices during drying process.
Materials and Methods
A laboratory scale vacuum-infrared dryer, developed at the Agricultural Machinery and Mechanization Engineering Laboratory of Shahid Chamran University of Ahvaz has been used. The dryer consists of a stainless steel drying chamber; a laboratory type piston vacuum pump, which was used to maintain vacuum in the drying chamber; an infrared lamp with power of 250 W which was used to supply thermal radiation to a drying product; and a control system for the infrared radiator.
Sample Preparation
Fresh potatoes were purchased from a local market in Hamadan province. Potatoes were peeled, washed, and cut into sliced with thickness of 1, 2 and 3 mm by a manual slicer. Drying experiments of potato slices were performed in a vacuum chamber with absolute pressure levels of 20, 80, 140 and 760 mmHg; and radiation intensity of infrared lamp was 0.2, 0.3 and 0.4 W cm-2. The mass change of the sample during drying was detected continuously using an electronic weight scale (Lutron, GM- 1500P, Taiwan) with the accuracy of ±0.05 g.
Evaluation of rehydration capacity of dried potato slices
The rehydration tests measured the gain in weight of dehydrated samples (~5 g), dehydrated samples were rehydrated in 200 cc of distilled water at 100°C for 3 minutes.
Evaluation of color
The color of potatoes was measured on five slices selected randomly, and was described by three coordinates in the RGB color space using computer vision.
Evaluation of specific energy consumption
Energy consumption of dying process came from the electrical energy consumed by the operation of the vacuum pump and the infrared lamp. Specific energy consumption was defined as the energy required for removing a unit mass of water in drying the potato slice.
Evaluation of thermal utilization efficiency
Thermal utilization efficiency is defined as the latent heat of vaporization of moisture of sample to the amount of energy required to evaporate moisture from free water. The latent heat of vaporization of water at the evaporating temperature of 100°C was taken as 2257 kJkg-1.
Results and Discussion
The results of the evaluation of rehydration capacity of potato slices during drying process are shown in Table 1. Statistical analysis (ANOVA, post-hoc Duncan) showed that thickness at probability level of 1% had statistically significant influence on rehydration capacity values of dried potato slices. Moisture of dried slice of potato compared to its fresh was obtained nearly 80% in boiling water (at temperature 100°C) for 3 min. The most color changes of slice after drying was related to green color. According to Table 2 and statistical analysis results showed that factor of thickness was not statistically significant on specific energy. The effect of absolute pressure (p<0.05) and radiation intensity (p<0.01) parameters also interaction of absolute pressure and radiation intensity (p<0.05) had statistically significant influence on specific energy of dried potato slices. According to Table 3 and statistical analysis the factor of absolute pressure had statistically significant at probability level of 5% on thermal utilization efficiency. Also the effect of interaction of absolute pressure and radiation intensity had statistically significant at probability level of 5% on thermal utilization efficiency of dried potato slices. The drying efficiency of potato slices varied between 2.13% to 31.01%.
Conclusions
Dried potato slices at a thickness of 1 mm put in boiling water for three minutes; showed the most amount of water absorption ratio that it was able to absorb the value of 86% more than the initial moisture. The lowest rate of color change before and after the drying process is related to the thickness of the thinnest sliced potatoes. Comparison of energy consumption showed that the radiation intensity of 0.4 W cm-2, absolute pressure level of 80 mmHg and slice thickness of 1 mm had shorter drying time in experimental conditions.
