تاثیر متغیرهای خشک کردن جت برخوردی بر سینتیک خشک شدن و برخی مشخصه‌های کیفی برگه‌های توت‌فرنگی

توت‌فرنگی به‌دلیل داشتن آنتی‌اکسیدان‌های طبیعی و ریز مغذی‌ها نقش مهمی را در سلامت انسان ایفا می‌کند. تقاضا برای افزایش زمان نگهداری، کاهش سرعت فرآیند‌های میکروبی، کاهش وزن و حجم محصولات و در نهایت صادرات، خشک کردن این محصول ارزشمند را حائز اهمیت می‌نماید. یکی از روش‌های جدید خشک کردن استفاده از هوای جت برخوردی است که با کاهش زمان خشک شدن باعث افزایش کیفیت محصول نهایی از جمله تغییر رنگ پایین و ظرفیت آبگیری بالا می‌گردد. در این تحقیق از خشک‌کن جت برخوردی با قابلیت تنظیم دما و سرعت هوا و نسبت فاصله نازل تا سطح محصول به قطر نازل استفاده گردید. آزمایش‌های خشک کردن برگه‌ها به‌صورت فاکتوریل در قالب طرح کاملاً تصادفی با دو متغیر شامل دمای هوا در سه سطح 45، 55 و 65 درجه سلسیوس و سرعت هوای خروجی از نازل‌ها در سه سطح 6، 9 و 12 متر بر ثانیه و طرح یک متغیره برای بررسی تاثیر نسبت فاصله نازل تا سطح محصول به قطر نازل در سطوح 4، 5، 6، 7 و 8 انجام شد. نتایج نشان داد که متغیرهای دما و سرعت هوا بیشترین تاثیر و نسبت فاصله نازل تا سطح محصول به قطر نازل کمترین تاثیر را بر مدت زمان خشک شدن داشتند. همچنین تاثیر دمای هوا بر نرخ خشک شدن بیشتر از سرعت هوا بود و با افزایش نسبت فاصله نازل تا سطح محصول به قطر نازل، سرعت خشک شدن برگه‌ها کمتر شد. از دیگر نتایج تحقیق افزایش ضریب انتشار موثر همراه با افزایش دما و سرعت هوا است. مقدار ضریب نفوذ موثر از 10-^10×1.62 تا 10-^10×3.24 متر مربع بر ثانیه تغییر کرد و انرژی فعال‌سازی 12.8 تا 16.7 کیلوژول بر مول تعیین شد. نتایج مدل‌سازی ریاضی نشان داد که مدل وانگ و سینگ به دلیل داشتن کمترین ریشه میانگین مربعات خطا (rmse=0.02) و بیشترین ضریب تبیین (0.996= r^2) مدل مناسبی برای پیش‌بینی رطوبت خشک کردن می‌باشد. همچنین مشاهده شد که افزایش دما و سرعت هوای خروجی باعث کاهش نسبت آبگیری مجدد نمونه‌های خشک شده توت‌فرنگی شده و افزایش h/d افزایش آن را به دنبال داشته است. تحلیل آزمایش‌های تغییر رنگ نمونه‌ها نیز نشان داد که افزایش دما و سرعت هوا میزان تغییر رنگ را کاهش و بالا رفتن نسبت h/d مقدار آن را افزایش داده است.

Effects of Air Impingement Jet Drying on Drying Kinetics and Some Quality Attributes of Strawberry Slices

IntroductionStrawberry plays an important role in human health because of its micronutrients and natural antioxidant content. Increasing storage time and decreasing microbial processes, weight and volume, and eventually facilitating export, has bolded the need for drying this product. The most common drying method is sun drying. This technique requires large areas and lengthens the time to complete the process which is undesirable economically. Furthermore, the final product may be contaminated by dust and insects, and the exposure to solar radiation results in color deterioration. In order to improve the quality, traditional sun drying techniques can be replaced by a more rapid and efficient drying method such as hotair drying. In recent years air impingement technology has got more attention in the field fruit slices drying due to high heat and mass transfer, decreasing drying time and increasing product quality. The objectives of this study were to investigate the effects of drying conditions on the drying kinetics and quality characteristics including the rehydration ratio of the strawberry slices in an air impingement jet dryer.Materials and MethodsAn air jet impingement dryer with controllable temperature, air velocity, and the relative nozzletoproduct distance (H/D) was used in this study. The experiments were conducted under different temperatures (45, 55, and 65°C), air velocities (6, 9, and 12 m s1) and H/D ratios (4, 5, 6, 7, and 8). The initial moisture content, effective moisture diffusivity, activation energy, and rehydration ratio were evaluated.Results and DiscussionThe effects of drying temperature and air velocity on the moisture ratio and the drying rate are shown in Figs 2 and 3. As it can be seen, the moisture ratio of strawberry slices decreased with the increase of drying time. The analysis of variance for drying time indicated that increasing drying temperature and air velocity could reduce the drying time. In addition, the effect of drying temperature on drying time was more significant than that of the air velocity. It is clear that the drying rate decreased with moisture content. There was a rapid decrease in drying rate during the initial period and slow decrease at the later stages of the drying process. It is also found that the drying process generally took place in the falling rate period. It is observed that the moisture ratio decreased as H/D ratio fall. The response of drying time was affected significantly (p lt; 0.05) by H/D ratio. The effective moisture diffusivity increased with increasing drying temperature and air velocity. Based on the results reported in this study, the Wang and Singh model with the lowest Root Mean Square Error (RMSE=0.02) and the highest Coefficient of determination (R2=0.996) provided the best fit to describe the experimental drying data of strawberry slices. The statistical analysis shows that drying temperature and air velocity have significant (P lt;0.01) effect on the rehydration ratio (RR) of slices, while the interaction effect was not significant. The means comparison shows that the RR of dried slices decreased as drying temperature and air velocity rose. H/D ratio significantly (1%) affected rehydration ratio. The means comparisons shows that the rehydration ratio increased when H/D value varied from 4 to 8. Also, the results of color change represented that color change of dried samples decreased with increase of temperature and air velocity and increased with increase of the H/D ratio.Conclusionsa) Increasing drying temperature and air velocity dropped the drying time. In addition, the effect of drying temperature on drying time was more significant than that of the air velocity.b) A constant rate period was not observed in drying of strawberry slices and the whole process of strawberry slices was carried out in the falling rate period.c) The moisture ratio decreased as H/D ratio dropped, which in turn resulted in saving drying times.d) The Wang and Singh model was found to be the best model to describe the drying kinetics of strawberry slices.e) The effective moisture diffusivity of strawberry slices ranged from 1.62×1010 to 3.24×1010 m2 s1.f) The values of activation energy of strawberry slices were found to be 12.88, 15.055 and 16.746 kJ mol1 for air velocities of 6, 9 and 12 m s1, respectively.g) The rehydration ratio of dried slices dropped as the drying temperature and air velocity rose and increased with increase of the H/D ratio.h) The color change of dried samples decreased with the increase of temperature and air velocity and increased with the increase of the H/D ratio.