Isolation, Characterization, Modification, and Applications of PMS: Comparison
Please note this is a comparison between Version 1 by Manop Suphantharika and Version 2 by Camila Xu.

Proso millet starch (PMS) as an unconventional and underutilized millet starch is becoming increasingly popular worldwide due to its health-promoting properties. PMS can be isolated from proso millet grains by acidic, alkaline, or enzymatic extraction. PMS exhibits typical A-type polymorphic diffraction patterns and shows polygonal and spherical granular structures with a granule size of 0.3–17 µm. PMS is modified by chemical, physical, and biological methods. The native and modified PMS are analyzed for swelling power, solubility, pasting properties, thermal properties, retrogradation, freeze–thaw stability, and in vitro digestibility.

  • proso millet starch
  • extraction
  • characterization
  • modification
  • in vitro digestibility
  • application


1. Introduction

Global production of cereal grains has reached record levels. Cereal grains play an important role in the human diet as a primary source of energy. The Food and Agriculture Organization (FAO) [1] reported in 2020 that the global production of cereal grains in 2019 reached a record high of 2715 million tons. At the same time, the global community is facing climatic changes, pollution, water scarcity, rising food costs, population growth, and other socioeconomic issues. These negative aspects can affect regional agricultural progress and limit grain production, leading to high food prices and serious food security concerns worldwide [2]. Moreover, smallholder farmers facing these conditions become economically vulnerable due to their limited resources and have difficulty maintaining their yields and profitability [3]. As a consequence of unfavorable global phenomena and their adverse impacts that constrain agricultural production, there is an urgent need among experts in nutrition and technology to identify a suitable cereal crop that could serve as a viable food source to address these challenges [4]. Under these circumstances, millets may be a nutritious option to supplement the nutritional needs of a growing world population in an uncertain global environment [5].
Millet belongs to the Poaceae family and is cultivated in subtropical and tropical regions of marginal drylands. Over 10,000 years ago, prior to the widespread consumption of wheat, maize, barley, and rice, this food item served as a staple for the people of that era. Currently, the most commonly cultivated species include proso millet (Panicum miliaceum), pearl millet (Pennisetum glaucum), and finger millet (Setaria italica) [6]. Millet is abundant in proteins, fats, carbohydrates, fiber, minerals, vitamins, and phenolic compounds [7]. Nutritionally, millet contains proteins (6–19%), carbohydrates (60–70%), fats (1.5–5%), minerals (2–4%), dietary fiber (12–20%), and various phytochemicals [8]. In addition, millet is gluten-free. This is desirable for people with celiac disease, and because of millet’s blood sugar-lowering properties, it is also effective in treating type II diabetes [8].
Proso millet (Panicum miliaceum L.) is also known as common millet, hog millet, Russian millet, and broomcorn millet in certain areas [9]. Proso millet is characterized by its adaptability to unfavorable environmental conditions (such as salt, drought, temperature, and pH). It also has a short lifecycle (about 12 weeks) and is grown in slightly acidic, saline, sandy, and low-fertility soils with limited nitrogen and carbon dioxide [10][11][12][10,11,12]. Proso millet contains carbohydrates (70–74%), proteins (9.4–9.9%), ash (1.2–3.8%), and fats (1.2–3.8%), along with a variety of phytochemicals and vital minerals [13].
Starch is a major constituent of millet and is divided into two types, namely, amylose and amylopectin. Based on the amylose content, millets are classified into nonwaxy (high amylose content) and waxy (low amylose content) [14]. Yang et al. [15] measured the range of starch content in nonwaxy (high amylose content) proso millet as 59–77% and for waxy millet as 55–69%. Starch serves as a crucial energy source for humans and is extensively utilized in the food and food-related industries. It is a renewable, biodegradable, economical, and natural material used to modify the textural properties of various foods. It can be modified into thickeners, stabilizers, and sweeteners, and can also serve as a water-retention agent [9].
A number of researchers conducted an analysis comparing various types of millet starches, but, unfortunately, they did not provide a thorough study of proso millet starch (PMS) [6][16][17][6,16,17]. According to Banger et al. [18], a comprehensive account of PMS, including its physiochemical and functional properties, modification, and applications, was presented. However, it was noted that more detailed information on isolation, digestibility, and recent advances in its applications is lacking.

2. Isolation, Yield, and Composition of Proso Millet Starch

The starch granules within proso millet grains exhibit strong binding affinity to the surrounding protein matrix. Various methods and chemical reagents are used to extract starch and solubilize the proteins in the grain [19]. Generally, starch extraction methods consist of three phases, i.e., fragmentation, cell disruption, and purification or separation [20]. Millet starch is usually isolated by the wet milling method. The grain or flour is soaked in an aqueous solution (water, alkali, or acid) for a certain time, depending on its chemical properties and composition [16]. The particular method of starch extraction (e.g., acidic, alkaline, or enzymatic) has a significant effect on starch yield. Starch isolation methods vary widely and depend on the inherent starch content of the grain and the initial soaking conditions (neutral, alkaline, or acidic) [21]. The procedure for isolating proso millet starch (PMS) is depicted in Figure 1.
Figure 1.
Isolation of proso millet starch.
In the alkaline steeping method, the grains are soaked with 0.3% sodium hydroxide (NaOH) solution for 24 h at 4 °C. The soaked grains are then ground to a wet slurry using a mill and then sieved through a 100-mesh sieve. After this, the samples are centrifugated at 3000 rpm for 15 min, the supernatant is removed, and the remaining contents are resuspended in water. This washing step is repeated for a total of 3 cycles; the slurry is then neutralized with hydrochloric acid (HCl). After washing/neutralization, the starch cake is dehydrated at 40 °C for 48 h [22]. In the acid steeping method of PMS extraction, the grains are soaked with 0.15% sulfur dioxide (SO2) solution for 48 h at 52 °C. The soaked grains are then crushed with a blender, sieved through a 40-mesh sieve, and washed with water. The residual material is then crushed using a mortar and pestle and filtered through a 200-mesh sieve and then through a 270-mesh sieve. The residue is again washed with water, filtered through a Buchner funnel with No. 2 Whatman filter paper, centrifuged, and dried overnight at 45 °C [6]. To extract starch from proso, pearl, kodo, foxtail, little, and barnyard millets, a practically neutral solution (pH 6.5) containing a minute amount of either sodium azide (0.01%) or mercury (II) chloride (0.01%) is used to prevent bacterial growth and inhibit amylase activity [6]. However, it should be noted that the use of sodium azide and/or mercury (II) chloride can cause serious health problems if ingested. A small amount of sulfur dioxide (0.5 g/L) and lactic acid (0.15 g/L) are added to isolate starch from proso millet in the acid steeping procedure. Similarly, the addition of a small amount of NaOH (0.1%) and sodium borate buffer comprising SDS (0.5%) and Na2S2O5 (0.5%) are used to isolate starch from these treatments. The use of these isolating solutions can significantly impact the chemical composition and characteristics of the extracted starch. In a comparative study between acid and alkaline steeping, acid steeping has a higher residual protein content (4.3%) than alkaline steeping (0.7%) [23]. The amount of millet starch obtained and the resulting chemical compositions differ significantly in the studies presented in Table 1. Millet contains about the same amount of starch as other cereal grains. The millet starch usually contains 20–30% amylose and 70–80% amylopectin. The presence of impurities in millet starch grains has significant implications for achieving some desired functional objectives [16]. For example, millet starch contains mainly nonpolar and polar lipids. The majority (89%) of the overall lipid content is attributed to polar phospholipids, whereas the rest primarily comprises nonpolar triglycerides [24]. These lipids can combine with the amylose component of starch to form complexes, which can lead to a decrease in the starch’s swelling capacity and flowability. This is caused by the lipids’ hydrophobic bonds and cohesive nature [16].
Table 1.
Starch yield and chemical composition of proso millet starch.

3. Morphology and Crystallinity of Proso Millet Starch

The size of starch granules in millet varies depending on the plant species. Despite being generally spherical and polygonal in shape (as indicated in Table 2), the dimensions of these granules range from 0.3–17 µm. The polygonal shapes are also larger and have more indentations than the spherical shapes [25], and the morphology of the starch is strongly influenced by its treatment and/or biomodification [30]. In addition, differences in particle size of PMS obtained from proso millet grown in different regions can be due to local environmental aspects. An increase in altitude and reduced mean temperature can lead to bigger granules [26]. Additionally, the morphology of starch is influenced by the arrangement of starch granules inside the endosperm of the grain [31]. Cavities are dispersed randomly throughout the entire outer layer of the starch granules due to surface pores and protein bodies. These pores are connected to the central cavity of the granules, enabling specific molecules from the external environment to penetrate the granules [16]. From a starch modification perspective, this phenomenon is helpful. These pores allow OH ions or water to enter the granules, destroying the amylose-containing amorphous region. Consequently, the restrictive qualities of amylose are reduced, leading to enhanced starch swelling and hydration properties [32].
Table 2.
Proso millet starch’s native and modified morphological properties.
29] found that the solubility of PMS was higher than other millets such as foxtail, barnyard, hybrid barnyard, and pearl millets, but lower than finger millet. However, all millet starches exhibited lower SP and solubility patterns in the temperature range of 60–90 °C than other commonly used starch sources (e.g., wheat and potato), suggesting stronger swelling resistance and binding strength within the starch granules [21]. It is thought that the interaction between starch and water molecules upon heating is the cause of the increased solubility and swelling power, and that the starch exposes additional groups that become associated with water molecules [41].

4.2. Pasting Properties

In the majority of cases, rheological evaluation of starch was carried out using both the Rapid Visco Analyzer (RVA) and the Brabender Visco-Amylograph (BVA), and the findings are presented in Table 3. This technique involves heating starch with a substantial quantity of water under continuous shear. The viscosity changes at a given temperature cycle are recorded. Pasting is affected by several parameters, including starch structure, water content, temperature program, and shear rate, which are closely monitored. The amount of starch used in the studies that was examined ranged from 6 to 10% [6]. Three sections can be identified in a typical pasting curve, each representing a specific phase of starch granule transformation during the pasting process [9]. The first phase involves the gradual absorption of water by the starch granules, causing them to expand; the second phase involves the leaching of the amylose component; and the final phase involves the loss of structural integrity of the expanded starch granules, causing them to disintegrate into fragments [42]. The pasting properties and attributes of starch paste are subject to the influence of several factors, including the concentration of starch, its composition in terms of amylose content and amylose-to-amylopectin ratio, and cooking and cooling temperatures, as well as the presence of solutes such as pH, lipids, and sugars. For instance, waxy starch has a greater tendency to absorb water and expand quickly, enabling it to attain its maximum pasting temperature in a shorter duration as compared to starches with a higher amylose content [43]. Yang et al. [14] reported that the peak viscosity (PV), trough viscosity (TV), and breakdown viscosity (BD) of waxy proso millet starch were greater, while the setback viscosity (SB) and pasting temperature (PT) were lower compared to nonwaxy millet starch. The study conducted on proso millet starch revealed that amylose content had a strong negative correlation with PV, TV, and BD, but a substantial positive correlation with SB and PT. A lower SB indicates better stability, and a lower BD indicates high shear resistance. Waxy proso millet starch demonstrates superior stability, making it a desirable choice for frozen food and thickening applications. On the other hand, nonwaxy proso millet starch exhibits higher temperature stability and improved shear resistance, indicating its potential suitability for medicinal resources [14].
Table 3.
Pasting properties of proso millet starch.
6]. Table 4 presents the thermal properties of PMS. The characteristics of gelatinization in starch vary not only between different species of millet, but also among various genotypes within the same species [16]. Various factors, including the granule size and the ratio of amylose to amylopectin, have an impact on the gelatinization properties of diverse types of millet starch. Moreover, these differences are also observed between different varieties of the same plant species. The gelatinization temperature of waxy and low-amylose starches takes a longer time to reach compared to nonwaxy, high-amylose starches [45]. Gelatinization temperatures are also important in the selection of specific starch properties for various food applications [21]. Thermal properties of PMS observed by Yang et al. (2019) [14] in both nonwaxy and waxy starch are as follows: To (64.6–71.1 °C); Tp (70.5–77.9 °C); Tc (77.4–82.3 °C); and ΔH (9.6–10.8 J/g). A higher gelatinization temperature indicates a perfect crystal structure of starch, while a higher enthalpy indicates that the gelatinization of starch requires more energy [46].
Table 4.
Thermal properties of proso millet starch determined by differential scanning calorimetry (DSC).
Millet starches are semi-crystalline and are similar to other starches that contain both crystalline and amorphous regions. Millet exhibits typical A-type polymorphic diffraction patterns [16]. The relative crystallinity observed for native starch is 35.7% with a diffraction peak at 2θ values of 15.3–23.1° for a single peak and about 17°–8° for a double peak [36]. Sun et al. [22] observed that the native starch of the proso millet exhibited A-type X-ray diffraction patterns with 2θ of 15°, 17°, 18°, and 23.5°, confirming a previous report by Kim et al. [34], which also confirmed an A-type diffraction pattern for PMS. In a 2019 study, the relative crystallinity of PMS was measured to range from 37.6% to 38.4% [14]. The differences in the degree of relative crystallinity can be attributed to a variety of factors, including the biological origin, plant variety, composition of amylose and amylopectin, conditions during cultivation, and maturity stage of the parent plant at the time of harvest [37]. Impurities present in the starch, such as other millet constituents, result in a shift of the peaks and a decline in intensity. This is because the occurrence of impurities alone increases the size of the amorphous region compared to the crystalline portion [38]. These general differences in starch granules all affect the degree of crystallinity, and due to the absence of amylose, this occurs without affecting the granular size [39]. A stable crystalline structure for starch is formed by long amylopectin chains, in contrast to the less stable shorter amylopectin chains, which are easily broken down by high temperatures [24]. Food processing techniques, such as milling, frequently cause damage to the physical structure of starch. The crystalline amylopectin is transformed into amorphous amylopectin during these processes, and the resulting material develops low-molecular-weight fractions. These changes in crystallinity affect food functionality [16].

4. Physiological and Functional Properties

4.1. Swelling Power and Solubility

With an appropriate quantity of water, the starch is subjected to heating, causing the granules to absorb moisture and undergo swelling. In this process, the components of the starch granule are leached out and largely dissolved in the form of amylose. Eventually, the swollen starch granules break down and disintegrate when they continue to be exposed to high temperatures. This activity is influenced by several factors, including the physical associations of the chemical components in the granules, the molecular structure of amylose and amylopectin, the intrinsic phosphorus groups, and the restricting entanglement of the lipid–amylose complex [40]. Starch granules undergo swelling when exposed to temperatures between 50–90 °C in the presence of water. Studies have shown that the swelling power (SP) of millet is lower compared to that of rye, potato, and wheat. This indicates that millet starch has greater resistance towards swelling due to its relatively strong binding force between the granules [16]. Much research has been conducted on SP of PMS, and some representative results are presented below. Singh and Adedeji [28] studied the SP of PMS at different temperatures (70–90 °C) and recorded the percentage range of their size changes, i.e., native starch (4.69–24.99%), acid-modified starch (4.94–21.26%), and hydrothermally modified starch (5.29–10.37%). At 95 °C, Xiao et al. [41] studied the SP of native PMS (13.77 g/g) and PMS with proanthocyanidins (14.15–19.83 g/g). Wu et al. [29] reported that the SP of PMS in their research was greater than other millet varieties, such as foxtail, barnyard, and finger millet, as well as a hybrid of barnyard and pearl millet. Li et al. [33] studied the SP of PMS (2–35%) at 50–90 °C and found that after ultra-high pressure, the treated starch showed lower SP than native starch. The following solubility of PMS at different temperatures (70–90 °C) was observed for native starch (2.62–34.88%), acid-modified starch (18.97–86.17%), and hydrothermally modified starch (1.71–12.45%) [28]. The solubility of acid-modified starch was higher than that of native starch, which is due to the fact that increasing temperature causes structural weakening and depolarization of starch granules in the former [28]. Li et al. [33] observed the solubility of PMS in a temperature range of 50–90 °C and found that the ultra-high-pressure-treated starch exhibited lower solubility than the native starch at a higher temperature. At 95 °C, Xiao et al. [41] investigated the solubility of native PMS (5.32%) and found a higher solubility of PMS with proanthocyanidins (8.64–16.35%). Wu et al. [
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