تاثیر دما و روش خشک کردن بر مدت زمان خشک شدن و کیفیت رنگ گیاه نعناع

نعناع گیاهی علفی، معطر و چندساله است که برای استخراج اسانس و اهداف دارویی و ادویه‌ای کشت می‌شود. از آن‌جایی‌که استخراج اسانس از پیکره خشک گیاهی صورت می‌گیرد، انتخاب روش مناسب جهت خشک کردن نعناع برای استحصال اسانس با کیفیت بالا ضروری است. در تحقیق حاضر اثر دو روش خشک کردن (خشک کن خورشیدی و خلائی مادون قرمز) در سه دمای مختلف (30، 40 و 50 درجه سلسیوس) در قالب آزمون فاکتوریل بر پایه طرح کاملاً تصادفی در گیاه نعناع مورد ارزیابی قرار گرفت. پارامترهای مورد بررسی شامل: مدت زمان خشک شدن (دقیقه) و شاخص کیفیت رنگ (%) اندازه گیری شد. نتایج بررسی نشان داد که خشک کن خلائی با میانگین مدت زمان 153 دقیقه در مقایسه با خشک کن خورشیدی 201 دقیقه، زمان کمتری صرف کرده است. همچنین با افزایش دما در هر دو نوع خشک کن روند خشک شدن محصول افزایش می یابد، به‌طوری‌که بیشترین زمان خشک شدن به مدت 237 دقیقه و مربوط به دمای 30 درجه سلسیوس در خشک کن خورشیدی و کمترین زمان خشک شدن به مدت 112 و مربوط به دمای 50 درجه سلسیوس در خشک کن خلائی بود. نتایج مربوط به شاخص تغییرات رنگ نشان داد که بهترین نمونه از نظر حفظ رنگ نعناع، مربوط به نمونه خشک شده در خشک کن خلائی است. خشک کن خلائی با %8/75 تغییرات رنگ کمتری در مقایسه با خشک کن خورشیدی %11/96 نشان داد. نتایج حاصل از بررسی اثر دما بر شاخص تغییرات رنگ نشان داد که کمترین و بیشترین میزان تغییر رنگ به‌ترتیب مربوط به نمونه خشک‌شده در دمای 30 و 50 درجه سلسیوس می‌باشد.

The Effect of Temperature and Drying Method on Drying Time and Color Quality of Mint

<strong >Introduction </strong > 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 vacuuminfrared 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. <strong >Materials and Methods </strong > Factorial experiment with randomized complete block was applied in order to evaluate the effect of drying methods (vacuuminfrared 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 onoff 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 semiprofessional 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. <strong >Results and Discussion </strong > The results showed that drying time of solar dryer is more than vacuuminfrared (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. Vacuumedinfrared 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. <strong >Conclusions </strong > Drying method as well as applying temperature showed rescannable effects on daring time and color quality of mint. The vacuuminfrared method reduces drying time for all temperature treatments considered in this study. Beside this, using vacuuminfrared 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.