تاثیر پیش حرارت‌دهی امواج مادون قرمز و میدان الکتریکی پالسی بر خصوصیات فیزیکوشیمیایی آب گوجه‌فرنگی

امروزه، میدان الکتریکی پالسی (pef) به‌عنوان روشی نوین جهت سالم سازی و افزایش مدت زمان ماندگاری آب میوه‌ها مورد استفاده قرار می گیرد. آنزیم پکتین متیل استراز (pme) به دلیل دارا بودن مقاومت حرارتی بالا به‌عنوان شاخص کفایت فرآوری آب‌میوه‌ها با اسیدیته بالا، مانند آب گوجه‌فرنگی، شناخته می شود. با توجه به ساختار پروتئینی این آنزیم، افزایش دمای اولیه آب میوه قبل از فرآوری pef، به غیرفعال سازی بیش تر آن و در نتیجه افزایش کارایی این روش کمک می کند. روش نوین حرارت دهی سریع مواد غذایی با استفاده از امواج مادون قرمز (ir) در کاهش میزان افت مواد مغذی موثر شناخته می شود. از این رو، در این پژوهش به تاثیر همزمان دو فرایند حرارتی (ir) و غیرحرارتی (pef) بر برخی از ویژگی های فیزیکوشیمیایی آب گوجه‌فرنگی پرداخته شد. برای این منظور، ابتدا آب گوجه‌فرنگی با استفاده از امواج ir تا دماهای 40، 45 و 50 درجه سانتی گراد حرارت‌دهی شد. سپس، نمونه تحت فرایند pef مداوم (73/22، 27/27، 82/31 و 36/36 کیلوولت بر سانتی متر به مدت 3520 میکروثانیه) قرار گرفت. میزان تغییرات اسید اسکوربیک، آنزیم پکتین متیل استراز (pme)، کدورت و رنگ آب گوجه‌ فرآوری شده به‌ترتیب با استفاده از روش های یدومتری، کیم بال، اسپکتوفوتومتری و پردازش تصویر تعیین گردید. نتایج نشان داد که پیش حرارت دهی آب میوه در کنار وقوع پدیده حرارت دهی اهمیک طی فرایند pef، به دلیل افزایش بسیار سریع دمای آب گوجه‌فرنگی موجب کاهش بیش تر میزان فعالیت آنزیم و افزایش کدورت محصول شد. همچنین، مشاهده شد که با ترکیب pef و حرارت دهی ملایم و سریع محصول می توان موجب کاهش میزان افت محتوای اسید اسکوربیک و جلوگیری از بروز تغییرات نامطلوب رنگ نمونه شد.

The effect of infrared preheating and pulsed electric field on physicochemical properties of tomato juice

Introduction: Tomato consumption is recommended due to its brilliant color, amazing taste, high antioxidants and vitamins content. Pectin methylesterase (PME) is known as a pasteurization index in tomato juice. This enzyme plays important roles in cloud stability, color, viscosity and organoleptic properties of the juice. PME induces the cloud loss in the juice due to the formation of the insoluble calcium pectate; its inactivation is therefore needed to maintain the juice stability. Degradation of ascorbic acid (AA) in the juice is considered as a major challenge during thermal food processing procedures. Furthermore, during its degradation non enzymatic browning occurs, affecting the taste and color. Novel juice processing methods such as infrared (IR) heating and pulsed electric field (PEF) reduce the adverse effects of the conventional thermal method. Quick IR heating produces a juice having higher nutritional value and better quality. During PEF processing, the juice is subjected to high voltage electric pulses for a few microseconds to inactivate target microorganisms and enzymes. Considering the protein structure of the PME, PEF processing at a higher temperature can be effective in this enzyme inactivation. A quick preheating of the juice using IR heating was therefore applied in this study. Furthermore, the ohmic heating, occurring during PEF treatment, was not prevented. The synergistic effect of thermal treatment and a nonthermal one (continuous PEF system) on some physicochemical properties of tomato juice were investigated.; ;Materials and methods: Tomato juice, with an initial temperature of 30℃, was firstly preheated using a continuous IR heating system. During the IR treatment, a temperature controller was applied to set the outlet temperature of the juice to 40, 45 and 50℃. The preheated juice was then passed through a continuous PEF system. The process chamber of the PEF system consists of two parallel stainless steel electrodes with 0.55 mm distance. The length and width of the exposed electrode surface were 10 mm and 0.5 mm, respectively. A squarewave bipolar pulse with a width of 1 ms was selected to perform the treatment within the range of 22.73 to 36.36 kV/cm at 32 Hz. The outlet temperature of the juice was recorded using a data acquisition system during 3.52 s PEF treatment. The treated juice was cooled in an icewater bath to minimize the effect of cooling period on PME inactivation. AA content, PME activity, cloud value and color of the juice were measured using iodine titration, Kimball, spectroscopy and image processing, respectively.; ;Results and discussion: Results showed that higher PME inactivation was observed in the juices treated at higher IR preheating temperature in combination to higher PEF intensity, resulting in higher cloud stability of the juice. While preheating of the juice decreased the AA content, PEF treatment has no significant effect reflecting the heat sensitivity of this vitamin. Furthermore, color aspects of the juice were more affected by heating treatment in comparison to the PEF processing. Browning index (BI) increased as the AA content decreased. By measuring the total color difference (TCD) it was shown that the PEF had no impact on color properties of the juice. Therefore, the synergistic effect between the IR heating and PEF treatment was also effective in producing the juice having the high nutritional value and better appearance.