The effect of ultrasound processing on free radical formation and product quality characteristics in sour cherry juice

This thesis aims to investigate the potential of the ultrasound (US) process in generating free radicals, the optimization of the ultrasound process in processing sour cherry juice produced from concentrate, its potential use compared to conventional thermal processing, and the integration of image processing technology in determining the quality characteristics of fruit juice, using sour cherry juice produced from concentrate and a water environment as a model. Acoustic cavitation generated by ultrasound process in a liquid medium can cause the formation of radicals, including OH• and H• radicals, which can initiate radical chain reactions. Therefore, experiments were conducted using both water and sour cherry juice produced from concentrate as model environments, with glutathione (GSH), which has antioxidant properties, and terephthalic acid (TFA), which gains fluorescent properties through oxidation reactions, used as indicator agents to develop an indirect method for observing free radical formation during the ultrasonication process. Additionally, the ultrasonication process was optimized for its use in sour cherry juice produced from concentrate, and the quality characteristics of the juice produced under these optimal conditions were evaluated. Image processing technology was also utilized to determine color parameters, which are among the physical quality characteristics. Free radical formation was primarily studied in a water environment using GSH, and the oxidation level of GSH was measured by HPLC in samples taken at specific intervals during a 30-minute sonication process, applied with two different probe diameters (13 mm and 19 mm) under 60% and 80% amplitude conditions. It was observed that as the ultrasonication time increased, GSH reacted with the generated free radicals, leading to a decrease in GSH concentration and an increase in oxidized glutathione (GSSG) concentration. At this stage, since the GSSG level generated with the 19 mm probe was approximately 5 times higher than that with the 13 mm probe, indicating a higher level of free radical formation, the 19 mm probe was selected to represent more challenging conditions, and the subsequent stages continued with the 19 mm probe. The ultrasound process conditions for preserving bioactive compounds in sour cherry juice produced from concentrate were optimized using a 19 mm probe and based on a central composite experimental design. Amplitude (60%-100%) and duration (2-15 minutes) were used as independent variables, while total monomeric anthocyanin (TMA) and total phenolic content (TFM) were used as response variables. According to the experimental design, the optimal ultrasound conditions that maximized TMA (R²=0.8427) and TFM (R²=0.8887) content were determined to be 67% amplitude and 7 minutes of processing time. Under these optimal conditions, both free radical formation and product quality characteristics were examined. The free radical generation potential of the sonication process at optimal amplitude was measured using an indirect method, taking advantage of TFA's ability to acquire fluorescent properties through oxidation. In this context, the ultrasound process was applied to an aqueous TFA solution (80 mM) at 67% amplitude for 30 minutes, followed by measurements using a fluorescence spectrophotometer. In samples taken over time, it was observed that as the sonication duration increased, the fluorescence intensity of TFA also increased, which was associated with oxidation reactions occurring due to the free radicals generated during sonication. In addition to the aqueous solution, TFA and GSH were added to a sour cherry juice environment in the same manner, and the model fruit juice environment was subjected to sonication at 67% amplitude for 30 minutes, during which fluorescence properties were examined; however, no significant differences were observed in the sour cherry juice. Furthermore, in the LC-MS scan of the fruit juice with added GSH, no oxidized glutathione (GSSG) was detected. The lack of a significant oxidation process for the TFA and GSH indicators in sour cherry juice was attributed to the juice's more complex structure compared to water and its high antioxidant content, which likely caused the free radicals to rapidly interact with these components rather than with the indicators. In the final part, the effects of the ultrasound (US) process under optimal conditions (19 mm probe, 67% amplitude, 7 minutes) on the physicochemical quality characteristics and bioactive compound contents of sour cherry juice were examined and compared to conventional thermal processing (90°C, 1 minute). No significant differences (p>0.05) were observed between the samples in terms of pH, soluble solids content, and titratable acidity. The total monomeric anthocyanin (TMA) content in unprocessed sour cherry juice (control) was 158.93 mg/L, while it significantly decreased with thermal processing (154.39 mg/L) compared to the US process (160.57 mg/L) (p<0.05). The total phenolic content (TFM) in the control juice was 417.89 mg/100 mL, and it significantly decreased with both US (384.86 mg/100 mL) and thermal processing (379.65 mg/100 mL), while antioxidant activity remained stable regardless of the treatment. In addition to colorimetric measurements for determining color properties, an innovative technology, image processing using MATLAB, was employed to measure the color values of sour cherry juice, and its applicability to food quality analysis was evaluated. Both methods revealed that the L* value remained the same across all samples, while the a* and b* values were preserved during the ultrasound process but showed significant differences compared to the control after thermal processing (p<0.05). Although the conclusions from the colorimetric method and image processing method were consistent, it was observed that differences could arise between the values determined by the two methods when comparing the color parameters measured colorimetrically to those predicted by image processing. The obtained data demonstrate that, when the ultrasound process is specifically optimized for the product, it allows the processing of sour cherry juice by preserving its anthocyanin content and antioxidant capacity, while minimizing the negative effects of thermal processing on quality characteristics. Although the ultrasound process raises some concerns regarding the formation of free radicals, the strong antioxidants present in sour cherry juice neutralize these radicals, preventing significant losses in phenolic content compared to conventional thermal pasteurization. Additionally, promising results have been obtained regarding the integration of image processing technology—which is increasingly used in areas such as classification, defect detection, microbial analysis, and color analysis, and holds great potential for the food industry—into the fruit juice industry, where its use is still limited, for the evaluation of food quality.