## 1 Introduction  
Browning is a common occurrence during the preparation and storage of fruits and vegetables such as apples, potatoes, bananas, and avocado. It occurs when food undergoes chemical reactions that turn it brown. There are two types of browning: enzymatic browning and non-enzymatic browning. Enzymatic browning is believed to cause more than 50% loss during pre-harvest and post-harvest processing of fruits and vegetables [1]. The enzyme that is responsible for food browning is polyphenol oxidase (PPO). Browning occurs when PPO oxidises phenolic substances, resulting in food darkening [2]. PPO activity is a major problem in the production and marketing of food items, particularly fruit juices. When browning occurs, the appearance, and the quality of fruit juices are altered. Many researchers have investigated how to inhibit undesirable browning to increase their shelf life [3]. However, in certain foods such as coffee, cocoa beans, and tea, browning is needed to produce the desired colour and flavour [4]. This is also the case for dry fruits such as raisins. It is critical in food processing to use physical (thermal and non-thermal processing) or chemical agents (Such as citric or ascorbic acid) to avoid or slow down enzymatic browning. Thermal or non-thermal processing can be used to inactivate polyphenol oxidase. The PPO enzyme in fruit (juice) is inactivated during thermal processing by heating the juice to a temperature of 70 \({}^{\circ}\)C to 90 \({}^{\circ}\)C. Thermal treatment of fruits and vegetables is a classic method for destroying bacteria and inactivating enzymes. For a long time, thermal food preservation methods have dominated the food processing sector [5]. Thermal treatment, on the other hand, can result in negative changes in food attributes such as loss of colour, texture, and flavour, as well as nutrient quality degradation [6; 7]. Heat treatment of fruits can cause heat-sensitive components including vitamins to be destroyed [1]. Industry and researchers have developed non-thermal food preservation techniques in response to consumer demand for safe and nutritious juices. Some of the technologies include high pressure processing (HPP), high pressure carbon dioxide (HPCD), ultrasound, and irradiation [8; 9; 10; 11]. The latter can be used alone or in combination with heat to enhance bacterial and enzyme inactivation. The adverse effects of thermal treatment on a variety of fruits have been mitigated using these strategies [6]. Food will be less impacted by the moderate heat/no heat employed in these processes, allowing for the retention of nutritional, physical, taste, and flavour properties identical to those of fresh foods [12; 13; 14; 15]. These non-thermal processing methods have the potential to help with product development in order to address the rising demand for high-value, complex, and diverse food items. The approach for thermal treatment, high pressure processing, and ultrasonic technologies for the inactivation of PPO enzymes are discussed in this study. The purpose of this review is to observe the resistance of polyphenol oxidase (PPO) enzymes in different fruits and fruit cultivars to thermal treatment, high pressure processing, and ultrasound in terms of inactivation and estimation of energy requirements. The review will include discussions on fruits cultivars that achieved at least 80% PPO inactivation after the specified treatments using the kinetic PPO inactivation models. In addition, the estimation of energy requirements for different processing technologies is thoroughly discussed. In this review, at least 80% inactivation of PPO was regarded to be reasonable to control or slow down the enzymatic browning in fruit products, based on the studies of [PERSON] et al [16; 17] that showed no regeneration of PPO activity during storage after thermal, HPP, PEF and ultrasound processing, when <20% residual activity (or >80% enzyme inactivation) was obtained with these processes.