## 1 Introduction  
Citrus fruits (Rutaceae family) are fruits that are widely produced and consumed worldwide. Citrus fruits (orange, grapefruit, lemon, tangerine, etc.) contain large amounts of phytochemical and bioactive compounds as well as pectin [1]. Large amounts of citrus waste are generated in the food sector. This situation creates a negative burden for the environment. In citrus juice operations, approximately 50% of the whole fruit is waste, and 50-55% of this waste consists of peels. Pectin is found in nature in cell walls, intercellular spaces, and middle lamella of soil plants. Commercially, pectin is generally obtained from three main residues: sugar beet pulp, citrus peel, and apple pulp. Pectin-rich citrus peels (approximately 28% for oranges, 23% for grapefruit, and 19% for tangerines on a dry weight basis) are highly preferred in pectin production, and studies of pectin extracted from citrus peels have concluded that pectin contains rhamnodastoman I (RG I) and a small fraction of rhamnodastoman II (RG-II) [2; 3; 4]. [PERSON] and [PERSON] [5] stated that the highest pectin yield was achieved from GP compared to other citrus peels. GP pectin is valuable due to its high RG-I content. GP pectin is widely preferred as a thickening, stabilizing, gelling, and emulsifying agent in varying industries. In terms of health, citrus pectin helps to regulate the gut microbiota, treat intestinal inflammation, and help the liver [5; 6; 7; 8]. Pectin's structure and amount are of critical importance in plants and vary depending on the region where the plants are located, the degree of ripening, and the storage and processing stages. The main components of pectic substances are D-galacturonic acid (GaLA) and are attached at the \(\alpha\)-1,4 position. The GalA content of pectin in citrus fruits varies between 58.5 and 85.4%. The linear structure of the poly-(GalA) chain of pectin is interrupted by 8-1,2 linked L-rhamnose units, forming the RG-I region of pectin, which contains covalently linked sugars, the most important of which are D-galactose, L-arabinose, and L-rhamnose. The minor components of pectin are galacturonans such as rhamn-ligand and xylogalacturonan (XGA). RG-II consists of four different glycosidic complexes covalently cross-linked to at least eight units of the \(\alpha\)-1,4 linked GalA backbone. These glycosidic complexes consist of several sugars. Xylogalacturonan is much less complex; its structure consists of a homogalacturonan substituted with xylose. Methyl groups esterify part of the carboxyl group at carbon 6 in the continuous poly-(GalA) chain; this indicates a change in the degree of methyl esterification (DE or DM); that is, DE gives the percentage of the ratio of esterified galacturonic acid units to the total galacturonic acid in pectin's structure [9; 10]. Pectin with a 50% and above DE is known as high-methoxy pectin (HMP), while below 50%, it is known as low-methoxy pectin (LMP). Generally, the esterification degree of commercial HMP is between 60 and 75%, while this rate is between 20 and 40% in LMP. The degree of esterification for various citrus fruits varies between 6.77 and 85.7%. These two different forms of pectin have different applications. For instance, HMP is able to form gels when heated in an acidic solution with a high sugar concentration (55-75%). On the other hand, LMP is able to form gels over an extensive pH range [9; 11]. The extraction technique of pectin has a significant impact on pectin's characteristics and yield. The conventional extraction (CE) method is based on the extraction of pectin with a hot diluted solution of strong mineral acids (such as sulfuric, nitric, phosphoric, acetic, or hydrochloric acid) followed by precipitation using ethanol. Some new technologies have been adopted in pectin extraction to overcome disadvantages such as long processing time, excessive energy consumption, the use of harmful chemicals, and the degradation of the pectin.