## 4 Chemical Aspects of Ulvan from _Ulva_ sp.
### Extraction  
In general, there are two types of extraction: acid and enzyme. The quantity and quality of extracted ulvan are influenced by the extraction procedure and the selection of the solvent used for extraction. The extraction procedure has several parameters such as pH, temperature, and mechanical movements that determine the amount of obtained extract and metal content [32]. Acid extraction causes almost complete desulfation of isolated ulvan while purifying enzymatic methods maintain a significant SO3 substituent level [33]. Several _Ulva_ sp. has been studied for their extraction methods in the last five years. For instance, to increase the purity of the ulvan, _U. ohnoi_ is extracted at 90 \({}^{\circ}\)C with a pH of 2.2 [34]. To increase the purity of the ulvan from _U. ohnoi_, salt removal can be done by the ultrafiltration method [35]. In addition, ulvan from _U. fasciata_ is extracted twice using distilled water and then doing the oxidation process for 72 h [36]. To improve the quantity of ulvan, extraction of ulvan from _U. pertusa_ is carried out with hot water followed by alcohol precipitation [37]. The latest method is extraction with the one freeze-dried method [14]. In the extraction of ulvan from _U. prolifera_ by enzyme method, to catalyze the initial depolymerization step of ulvan, ulvan lyase can be obtained from fermentation of _Catenovulum_ sp. [38]. The choice of extraction method generally depends on the physicochemical properties of ulvan, which can interact with components of the cell wall when it comes in contact with a solvent [39; 40]. The many active components contained in ulvan that can be obtained through the extraction process are polysaccharides (starch, cellulose, xyloglucuan, and glucoronan); other components such as protein, lipids, minerals, and secondary metabolites can be obtained by co-extraction. This review specifically describes ulvan extraction from _U. lactuca_ that provides a high yield, high selectivity, and low degradation [2]. Extraction with pure distilled water is the most widely used method [41; 42]. This procedure provides ulvan that is glucuronic acid, glucose, arabinose, and xylose, with an average yield of 25-40% [22; 33; 43]. Another extraction method involves adding 0.05-1% hydrochloric acid (pH 1.5 to 2), with an average total yield of 15-45% [10; 22; 33]. Enzyme extraction utilizes onozula, pectinase macrozyme, \(\alpha\)-amylase, and proteinase K, which are suspended in the buffer. The yield in enzyme extraction is 15-47% [10; 33; 44]. This yield has the potential to be improved because there has been no optimization. In addition to the three widely used methods, other methods include multilevel extraction, using water followed by NaCO3 and NaOH solvents, with a 3.14-6.50% yield. There is also a sequential extraction method that involves acidic solutions and ammonium oxalate with an average ulvan yield [45]. Various methods can be employed to extract ulvan depending on the active component that will be analyzed. Variations in concentration, pH, and temperature are important parameters to achieve the desired results. One study compared hot water, NaCO3, and NaOH and reported that the highest yield is obtained from hot water extraction [46]. The combination of extraction temperature, solvent pH, and extraction duration influences the yield and quality of extraction (e.g., purity and molecular integrity). The higher extraction temperatures allow the greater ulvan dissolution and the lower pH allows the higher ulvan selectivity. In addition, an increase in extraction duration will increase the yield of ulvan. However, critical care must be taken to protect the integrity of the ulvan structure. For example, stability may be decreased at high temperature, low pH, and longer extraction time. The time duration and the lower pH cause a significant depolymerization of ulvan.