اثر فرآیند تغلیظ بر خصوصیات رئولوژیکی و فیزیکوشیمیایی آبلیمو

در عملیات صنعتی، محصول تحت تاثیر دامنه‌ای از نرخ‌های برشی قرار می‌گیرد، بنابراین، شناخت تغییرات در رئولوژی با تغییرات دما، فرکانس و نرخ‌های برشی برای طراحی تجهیزات عملیات مورد نیاز است. از آبلیمو در صنعت غذا به‌طور گسترده‌ای استفاده می‌شود. اطلاعات رئولوژیکی و فیزیکوشیمیایی می‌تواند در افزایش مقیاس و بهینه‌سازی فرآیند، حمل و نقل و نگهداری کنسانتره لیمو مهم باشد. هدف از این مطالعه بررسی رئولوژی و ویژگی‌های فیزیکوشیمیایی کنسانتره آبلیمو تولید شده با روش تغلیظ تحت خلا می‌باشد. فرآیند تغلیظ نمونه آبلیمو تا غلظت‌های 25، 35، 45، 55 و 65 درجه بریکس انجام گردید. میزان مواد جامد محلول، ph، اسیدیته و رنگ، رفتار جریانی و ویسکوالاستیسیته تابع کرنش، فرکانس و دما بررسی شد. مقایسه میانگین‌ها با روش تفاوت حداقل معنی‌دار صورت گرفت. با افزایش بریکس،ph کاهش و اسیدیته افزایش یافت. افزایش غلظت موجب کاهش پارامتر *l و *b و افزایش در پارامتر *a شده است. نمونه کنترل و نمونه‌ها با بریکس 25 و 35 رفتار نیوتونی اما نمونه‌ها با بریکس 45، 55 و 65 رفتار رقیق‌شونده با برش داشتند و برازش مناسبی با مدل کراس نشان دادند. این سه نمونه رفتار تیکسوتروپ داشته و با افزایش بریکس، وابستگی به زمان بیشتر شد. در کل دامنه کرنش و فرکانس مورد مطالعه، مقادیر مدول ویسکوز در کلیه نمونه‌ها بیشتر از مقادیر الاستیک بوده است و هیچ برخوردی میان دو مدول مشاهده نشد. در کل دامنه دمایی مورد مطالعه، ویژگی ویسکوز برجسته‌تر از ویژگی الاستیک بود. مدول کمپلکس و ویسکوزیته کمپلکس به‌ترتیب برای نمونه 45 ، 55 و 65 افزایش یافت. نمونه کنسانتره آبلیمو یک ماده رقیق وتیکسوتروپ بوده، ژلی تشکیل نداد و رفتار نمونه‌ها مایع ویسکوالاستیک بود. با افزایش بریکس، اگرچه، مقاومت به نرخ های برشی بالا افزایش می یابد اما توانایی در بازیافت ساختار تخریب شده در اثر حرارت کاهش می یابد.

Effect of concentration on the rheological and physicochemical properties of lemon juice

Introduction: In industrial operations, a product is submitted to a range of shear rates. The knowledge of the changes in rheology with temperature, frequency and shear rate is needed to design the equipment of operations. The lemon juice is used extensively in the food industry. Information about rheological and physicochemical properties is important for scale up and optimization of processes such as transport and storage of concentrated lemon in commercial applications. The aim of this study was toevaluate the rheological and physicochemical properties of the concentrated lemon produced using vacuum evaporator.Materials and Methods: Lemon juice was concentrated to 25, 35, 45, 55 and 65 Brix by a rotary evaporator under vacuum at 65°C. The content of soluble solids in terms of brix using refractometer, pH, acidity in terms of citric acid, color in terms of a* , b* and L*, flow behavior and viscoelasticity as function of strain, frequency and temperature using rheometer were investigated. Mean comparison carried out using the least significant difference.Results &Discussion: As brix increased, pH reduced and acidity increased. An increase in the concentration resulted in a decrease in L* and b* parameters and an increase in a* parameter. Enzymatic browning in this study could be ignored due to the sensitivity of enzymes to temperatures above 50°C. Millard 's nonenzymatic browning (formation of hydroxyl methyl furfural and its polymerization) and the degradation of pigments might be a major contributor to color variation, although a decrease in moisture content and an increase in soluble solids are other reasons for darkening of color. Based on coefficient of determination (R2) and root mean square error (RMSE), we could state that the Newtonian and Bingham models had the best fit with laboratory data of the stressstrain obtained for the control sample with the 7Brix, and the samples with 25 and 35 Brix. Newtonian and Bingham viscosity increased as brix increased (p <0.05). However, the observed yield stress in these samples is very low and can be indicative of their viscous behavior. HerschelBulkley model, despite it had the high R2, had a higher RMSE than the cross model. In addition, the yield stress for the sample with 65 brix was a negative number, which is meaningless rheologically. Therefore, the HerschelBulkley model is not a suitable model. Therefore, the cross model with high R2, low RMSE and concept parameters was superior model for samples with 45, 55 and 65 brix. Using cross model, viscosity values were obtained at infinite and zero shear rates. As brix increased, viscosity values increased at zero shear rates. The viscosity values at infinite shear rates were very small in 65 brix. The increase in viscosity at infinite and zero shear rates with increasing of brix can be attributed to an increase in pectin concentration and also the presence of sugar. Increasing in viscosity at low shear rates in the cross model can be due to interactions between small particles as a result of van der waals forces or electrostatic forces between negatively charged pectins and positively charged proteins. At high shear rates, hydrodynamic forces are effective on the rheological fluid. The area of the hysteresis ring was very small in samples with low brix. Higher brixes (55 and 65) indicated higher hysteresis. Increasing in the hysteresis indicates an increase in the time dependence. Therefore, samples showed higher thixotropic behavior in higher brixs. Thixotropic behavior may be due to the internal structure of juice that is formed by physical interactions between soluble solids, sugars and proteins. In strain sweep, with increasing of the brix, the values of both viscous and elastic modules increased (p <0.05). In frequency and strain sweep, viscose modulus was greater than the elastic modulus and was not observed any crossover between them. This behavior is an indicative of dilute material. In acidic state, hydrolysis of polysaccharides is a barrier in formation of a network due to molecular fluidity in the matrix. In fact, due to the low concentration of pectin and sucrose in the low brixes, the gel is not formed, but with the increase of the brix, which increases the concentration of pectin and sucrose, the ability in formation of gel increased. However, no gel was formed in any of the samples. The dependence of the viscous modulus on the frequency in the range of 161 0 Hz to 117 Hz, in three samples with 45, 55 and 65 brix, fit well with the power law model. Model parameters showed the dependence of the viscous modulus on the frequency increased with increasing of brix, which can be attributed to an increase in the concentration of pectin. In total amplitude of temperature, viscose modulus was greater than the elastic modulus. Complex modulus and complex viscosity increased in samples as brix increased. Reducing of viscose and elastic modules and complex viscosity with increasing in temperature indicates that the material 's resistance to flow decreases with increasing temperature. As the brix increased, the area of the hysteresis loop between the structural destruction and the formation of the structure increased. That is, the ability of the structure return decreased with increasing in brix during decreasing of temperature.