# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 2 Starch Sources and Chemical Composition  
Several starch sources have been reported in the literature, primarily derived from fruits, stems, cereals, tubers, and roots. Starch is commonly isolated using the wet milling method, which involves soaking procedures at varying temperatures and the use of chemical agents such as alkalis or acids [13]. For example, in the extraction of starch from beans, a sodium sulfite solution is used during the soaking and blending stages. This facilitates water penetration into the starch granules while also preventing microbial growth. The resulting mixture is then filtered, and the suspension is decanted. This process is repeated, followed by resuspension in distilled water and centrifugation to obtain purified starch [19]. Similar methods are used to extract starch from quinoa [20], amaranth [21], and peas [22], typically involving an NaOH solution ranging from 0.1% to 0.3%. This alkaline treatment reduces the protein content through repeated filtration over approximately 15 days. In the case of lentils, starch extraction also involves an alkaline treatment; however, prior to this step, lentil flour is treated with an aqueous solution at pH 8 to solubilize proteins and facilitate starch isolation [23]. A different methodological modification was reported for extracting starch from sweet potato and cassava [13], where an NaHSO\({}_{3}\) solution is used to inhibit the oxidation of metabolites such as tyrosine and dihydroxyphenylalanine. This prevents the formation of melanin, which imparts a red-brown coloration [13]. Table 1 presents various starch sources and their chemical composition, including the percentages of starch, protein, ash, lipids, and fibers. Many sources have high starch concentrations (>90%), such as cassava, banana, taro, yam, chestnut, unripe fruits, and loquat seeds. Thus, these starch sources, particularly cassava and banana, are optimal for  
producing SHs on a large scale, as they show high starch content and are produced in large quantities around the world. Conversely, some sources, such as corn and mango seeds, have been reported to contain low starch concentrations (<25%). According to the data, the AM concentration is not directly correlated with the total starch concentration. For example, longan contains 85.7% starch but has a higher AM concentration (26.4%) than cassava (16.71%). This suggests that the AM concentration varies depending on the source, ranging from 1.25% to 45.69%. The concentrations of AM and AP influence the crystallinity of starch granules. These granules consist of a semi-crystalline structure composed of crystalline lamellae (closely packed glucan chains) and amorphous lamellae (non-ordered branching glucan chains) [24]. The organization of AM and AP forms rings or shells known as growth rings, which are constructed by alternating amorphous and semi-crystalline regions. These structures can be observed using a light microscope or a scanning electron microscope after treating the sample with a dilute acid solution [11]. Granule sizes are classified into three types based on their dimensions: A-type (>15 \(\upmu\)m), B-type (5-15 \(\upmu\)m), and C-type (<5 \(\upmu\)m) [25]. Typically, all starch sources exhibit a wide granule size distribution within the same sample, containing different proportions of starch granule types. However, exceptions exist, such as cassava, potato, yam, loquat seed, and unripe fruits, where the granule size is predominantly classified as type A. This type is typically associated with a high AM content and low crystallinity [26; 27]. The AM content increases as seeds mature and starch granules grow in size. For example, in corn endosperm, the AM content rises from 9.2% at 12 days after pollination to 24.2% at 30 days after pollination [28]. Starch granules are formed through a complex packing process, with their chemical composition--including lipids, proteins, and moisture--varying depending on growth conditions and the source.