## 1 Introduction  
As stated by previous studies, food by-products are approximately one-third of the total food production, and this amount is increasing annually. These wastes can give rise to the generation of a significant quantity of greenhouse gases, causing many problems such as climate change [1]. Nonetheless, food by-products are still regarded as principal sources for the extraction of food bioactive compounds [2; 3; 4; 5; 6]. Bioactive compounds are found numerously in nature and are extensively used in the food industry [7]. The recovery of these valuable compounds decreases the financial and environmental impact caused by food wastes. Thus, the challenge for the contemporary time is likely to find processing methods that are able to reuse by-products in industries and recover the nutrients still contained in the food wastes [8; 9].  
Polyphenols are the most pervasive bioactive compounds found in agroindustry by-products [10; 11], various fruits [12; 13], seeds [14], cereals [15], nuts [16], vegetables [17], tea [18; 19], coffee [20], etc. These compounds are a group of secondary metabolites derived from phenylalanine, and they own one or more phenolic rings with one or several bound hydroxyl groups. Polyphenols act as natural antioxidants, enhancing the nutritional value of food by retarding the oxidation of lipids [8; 21; 22]. To date, many studies have classified polyphenols as nutritional sources possessing a highly beneficial impact on the prevention of many diseases, including diabetes, obesity, atherosclerosis, hyperlipidemia, hypertension, Alzheimer's, and thrombosis [21; 23]. Therefore, it is crucial to recover these compounds from herbal food wastes and use them also in the production of functional foods and ingredients.  
However, there is a large challenge regarding the efficient extraction of polyphenols from food by-products, since conventional extraction methods consume plenty of time and require a relatively large amount of solvent and energy. Indeed, extract quality, extraction rate, final cost, extraction yield, consumer, and environmental protection are some determining factors in choosing the more useful technology for recovering polyphenolic compounds [24]. Another challenge is to prevent climate change caused by the emission of heat from different industries [25]. Hence, there is a growing demand for novel extraction methods with less solvent consumption, shorter time, and higher extraction yield [7]. Recently, many sustainable technologies such as microwave [26], ultrasound [27], cold plasma [28], pulsed electric field [29], pressurized liquid [30], and ohmic technology [31] have been utilized in the pre-treatment and extraction process of polyphenols. In these techniques, the temperature is not the decisive factor, and thus they require lower energy and have a lower effect on the loss of sensitive compounds, resulting in better recovery of bioactive compounds [32].  
Therefore, it is critical to study the efficiency of mild technologies in the extraction of polyphenols from wastes produced in the food industry and agricultural processes. In addition, the control of polyphenol oxidase during the extraction of polyphenolic compounds is another crucial issue for maintaining their nutritional attributes. Accordingly, this paper seeks to review up-to-date information regarding the valorization of food by-products as rich sources of polyphenols in the extraction process using novel technologies that could be useful also for avoiding the impact of enzymatic browning on the polyphenolic compounds recovered.