## 4 Discussion
### Total Polyphenol Content (TPC) and Antioxidant Capacity  
Antioxidants in juice, such as polyphenols, are crucial in neutralizing free radicals such as DPPH\(\bullet\) by donating a hydrogen atom. This reaction leads to the formation of a reduced form of DPPH\(\bullet\), causing a change in the initial coloration of the solution [64]. The observed decrease in absorbance is directly proportional to the content of the oxidized form of DPPH\(\bullet\) remaining in the solution. Hence, the higher the concentration of antioxidants in the juice, the faster the reduction of cationic radical's DPPH\(\bullet\) over time [65]. Chlorogenic acid, epicatechin, and phloridzin are typical polyphenic compounds found in apple juice [66]. Many studies have observed that as the content of polyphenic ingredients decreases, the antioxidant activity decreases [67; 68; 69]. On the other hand, the increase in polyphenol content and the associated increase in antioxidant activity induced by microwave radiation on fruit tissue may lead to tissue damage and better extraction of components into juice [52]. Research conducted by various scientists has shown that both conventional heating and microwaving significantly impact enzyme activity and antioxidant content in juices. Our research study emphasized the effect of temperatures at low pH, which can accelerate the hydrolysis of higher phenolic compounds into smaller molecules. Temperature also had an impact during the studies by [PERSON] et al., 2021 [70]. A study by [PERSON] et al., 2021 compared conventional and microwave heating in the temperature range from 50 to 90 \({}^{\circ}\)C [70]. The studies adopted three kinetic models to reduce polyphenol oxidase (PPO) activity in buffer and coconut water. The results underscored the significant impact of temperature, time, and heating methods on polyphenol oxidase activity. The study indicates that the Weibull model is suitable for describing the inactivation of the PPO enzyme. It was chosen to simulate PPO inactivation curves and compare traditional heating with microwave heating. The predicted inactivation of PPO using focused microwave heating was higher than that of conventional heating at temperatures above 70 \({}^{\circ}\)C.  
A study by [PERSON] et al., 2017 showed that applying MFP to pomelo juice positively affected the concentration of phenolic compounds, surpassing the effects achieved through TP [71]. The total phenolic content of fresh pomelo juice was observed to be 710 mg GAE/L. After processing, it decreased to 690.5 mg GAE/L and 705.3 mg GAE/L for conventional and microwave pasteurization. These results confirm the potential use of the microwave method to improve the concentration of bioactive compounds in pomelo juice. Similar effects were observed in the case of apple-peach and apple-chokeberry juices. Compared to the fresh sample, the TPC in these juices significantly increased by 15% after MFP and TP processing. The research conducted by [PERSON] et al., 2019 [72] aimed to examine the impact of phenol content under twelve processing conditions using microwave heating and conventional heating. The research findings indicated that changes in phenol content in pasteurized samples were independent of the processing technology. However, it was noted that the content of polyphenols increased during thermal processing, resulting in better extraction from the juice tissues. Moreover, apple juices treated by microwave pasteurization retained their nutritional values better than those pasteurized using conventional heating, with a similar storage time. The conclusions from the study conducted by [PERSON] et al., 2019 [72] indicate that microwave processing technology has promise in thermal preservation. These results may indicate shorter heating time and can help maintain product quality. Proper storage conditions depend on enzymes' activity and antioxidants' stability. Temperature control is crucial for maintaining fruit juices' quality and nutritional value, such as apple-peach or apple-chokeberry juice.