Kidney bean (Phaseolus vulgaris) starch: A review

The mini review focuses on the morphology, pasting, rheological and in vitro digestibility of kidney bean starch. In legumes seeds, starch is the most abundant carbohydrate reserve in plants and have been ascribed medicinal and cultural as well as nutritional roles. The major carbohydrate of kidney bean seeds is starch, which accounts for 25–45% of the dry matter. Lower swelling and high solubility of kidney bean starches indicate their higher functional properties than cereal starches. High amount of resistant starch (RS) and slow digestible starch (SDS) and low amount of rapidly digestible starch (RDS) present in kidney bean starches provide their potentiality as a good source of RS. Starch is a macro‐constituent of many foods and its properties and interactions with other constituents, particularly water and lipids, are of interest to the food industry and for human nutrition as starch properties may greatly determine the product quality.

carbohydrates (50%-60%) and a good source of vitamins, minerals, poly-unsaturated fatty acids (Rehman, Salariya, & Zafar, 2001;Reyes-Moreno & Paredes-Lopez, 1993), and appreciable amount of folate and fiber (Shi, Xue, Kakuda, Ilic, & Kim, 2007). The major kidney bean seed storage polysaccharide is starch accounting 25%-45% (Su, Lu, & Chang, 1997;Yoshida et al., 2003). Starch is the main storage carbohydrate of plants and contributes 50%-70% of the energy in the human diet, providing a direct source of glucose. It is often used as an additive ingredient in food products such as sauces, soups, confectionery, sugar syrups, ice cream, snack foods, meat products, baby foods, and fat replacers (Copeland, Blazek, Salman, & Tang, 2009). As the demand for convenience foods increases, the use of starch and its by-products increases rapidly. Among different types of starches, we maize, wheat, and potato are widely used in a diverse range of applications. Finding alternative to these commercial starch sources may offer additional substitutes for meeting the rising demand in the starch industry (Ngobese et al., 2018). Legume starch exhibits better gel characteristics and resistant starch (RS) contents when compared with cereal and tuber starches. This mini-review will provide an update on chemical composition, morphology, thermal properties, and in vitro digestibility of kidney bean starch with a view to providing suggestions for needed research to improve the implementation of kidney bean starch in the food and nonfood industries.
Because of presence of low protein content in kidney bean starches, it may be used for manufacturing high glucose syrups Amylose plays an important role in the pasting and gel texture properties of starch during cooking (Srichuwong, Sunarti, Mishima, Isono, & Hisamatsu, 2005). Bajaj, Singh, Kaur, and Inouchi (2018) and Du, Jiang, Ai, and Jane (2014) reported the amylose content of 49.73% and 32.4% for kidney bean starches.

| MOLECULAR STRUCTURE
Amylose and amylopectin are the main α-glucan components of starch. Amylose is small, linear, or slightly branched molecules and located primarily in amorphous regions, whereas amylopectin is large, highly branched molecules and interactions of the outer branches of the polymer creating crystalline regions (Donald, 2001;Jane, Xu, Radosavljevic, & Seib, 1992;Yoshida et al., 2003).

| SWELLING POWER AND SOLUBILITY
Swelling and solubility indices provide an evidence of the magnitude of interaction between starch chains within the amorphous and crystalline domains. Swelling power and solubility defines as a function of temperature, and it increased with the rise in temperature. Kidney bean starch possess single stage restricted swelling and low solubility behavior (Hoover & Sosulski, 1985) which, is indicative of the existence of strong bonding forces within the starch granule (Lineback & Ke, 1975). Hoover and Sosulski (1985) observed an increase in swelling and solubility between 70 and 80 C, thereafter the increases were gradual. They concluded that as the temperature increases, intragranular binding forces become weak, which facilitate less restricted swelling. Gani et al. (2016) reported swelling power and solubility of 1.62-2.13 (g/g) and 2.11%-2.285% at 60 C for starches from different kidney beans cultivars. They reported the highest values of swelling power and solubility for kidney bean starches at 90 C over temperature range of 60 C-90 C. The swelling index and solubility of 10.4-11.6 g/g and 14.7%-17.9% has been reported in starch from kidney beans (Gani, (Cheetham & Tao, 1998). Compared with A-and B-type starches, the C-type starch is complex and contains both A-and B-type allomorphs. A-type characteristics from cereal starches, Btype found in tubers and C-type X-ray pattern, an intermediate between A and B type, present in legumes is reported (Hoover & Sosulski, 1985). Crystalline structure of kidney bean starch was observed to be of C-type (mixture of A-and B-type) (Hoover & Sosulski, 1985;Singh, Belton, & Georget, 2009). Typical X-ray pattern of kidney bean starch is shown in Figure 1. F I G U R E 1 A typical X-ray diffraction pattern of kidney bean starch (Sharma et al., 2015) F I G U R E 2 Scanning electron micrographs of kidney bean starch (Du et al., 2014) T

| MORPHOLOGICAL CHARACTERISTICS
The size and shape of starch granules are important in relation to their technological properties, including the viscosity of their pastes (Wojeicchowski, Siqueira, Lacerda, Schnitzler, & Demiate, 2018). The scanning electron microscopy (SEM) micrographs of kidney bean starch granules showed round, elliptical, irregular, and oval shapes with smooth surfaces (Wani et al., 2010) with some indentations or hollows at one end . Scanning electron micrographs of kidney bean starch are shown in Figure 2. Wang and Ratnayake (2014) reported no evidence of starch damage or extraneous matter of kidney bean starches when examined under SEM.
Starch granules of kidney beans were reported to have 6-32 μm width range and 8-42 μm length range (Wani et al., 2010;Hoover and Ratnayake, 2002). Kidney bean starches exhibited bimodal granule size distribution with peaks less than 10 and greater than 10 μm. Bajaj et al. (2018) reported that the major proportion in kidney bean is occupied by granules that are greater than 10 μm (94.76%) followed by size less than 10 μm (5.13%).

| PASTING CHARACTERISTICS
Pasting properties provide relevant information about the cooking behavior of starches during heating and cooling cycles. Brabender visco-amylograh (BVA), rheometer, and rapid visco analyzer (RVA) are commonly used to examine the pasting characteristics of starches.
Pasting profile of kidney bean starches is presented in Table 1. Sharma, Singh, Virdi, and Rana (2015) reported pasting temperature (PT), peak viscosity (PV), final viscosity (FV), breakdown viscosity (BV), and setback viscosity (SV) ranged from 75.4 C to 83 C; 2,534 to 5,703 cP; 4,250 to 7,251 cP; 213 to 2,388 cP and 2,156 to 3,936 cP, respectively for 17 landraces of kidney bean starches. Pasting properties established that kidney bean starches had high PV in comparison with commercial corn and potato starches, making them good thickening agent (Wani et al., 2010). Kidney bean starch is reported to have high PT indicates the higher resistance of starch granules towards the swelling, which may be attributed to the higher amylose content and the tighter granular structure than other legumes starches (mung bean and lima bean; Chang et al., 2018).

| THERMAL CHARACTERISTICS
Differential scanning calorimetry (DSC) provides quantitative measurements of heat flow associated with gelatinization, where the endothermic peaks are indicative of melting (Rupollo et al., 2011).
Thermal characteristics of kidney bean starches are shown in Table 2. For starches, the onset, peak, and conclusion temperatures of gelatinization (To, Tp, and Tc, respectively) are determined by DSC and have been reported to be influenced by the molecular architecture of the crystalline region, which corresponds to the distribution of amylopectin short chains (DP6-11; Noda et al. 1998).
This phenomenon has been ascribed to the water-mediated melting of starch crystallites, initiated by the stripping of starch chains in the swollen amorphous regions of the granule (Biliaderis, Maurice, & Vose, 1980). Singh et al. (2008) reported To, Tp, and Tc of 68.3 C, 73.4 C, 79.1 C for kidney bean starch. Cook and Gidley (1992) explained that the enthalpy of gelatinization (ΔHgel) reflects primarily disrupting double helices than long-range disruption of crystallinity. Wani et al. (2010)

ACKNOWLEDGMENTS
The authors acknowledge Department of Food Science & Technology, Chaudhary Devi Lal University, Sirsa for providing necessary infrastructure for this research work. No funds were received from any agency for this work.

CONFLICT OF INTEREST
The authors declare no conflict of interests.

AUTHOR CONTRIBUTIONS
Sneh Punia conceptualized the study, conducted the literature of review, and prepared the review draft. All authors contributed to the critical review and editing of the manuscript.

DATA AVAILABILITY STATEMENT
Due to technical limitations, the full dataset is unable to be published at this time. However, it is available upon request from the authors.

ETHICS STATEMENT
This article does not contain any studies with human and animal subjects.