**4. Discussion**

Starchy foods or ingredients are digested by amylolytic α-amylases and α-glucosidase enzymes, including maltase-glucoamylase and sucrase-isomaltase, at the brush border of the small intestine [24]. Thereafter, the absorbable glucose is transported into the bloodstream through the glucose transporter in the small intestine. The high rate of digestion and absorption of these foods contributes to a rise in postprandial glucose, related to health consequences. The physio-chemical properties of carbohydrate foods are normally investigated by measurement of the rate and extent of glucose release after enzymatic digestion under controlled conditions [25]. Our findings demonstrated that a higher amount of maltose and glucose released from flours was observed after in vitro digestion. When the CTE was mixed with the flours, the release of maltose and glucose was significantly decreased. These findings sugges<sup>t</sup> that CTE has a potential to reduce the release of maltose and glucose from flours, leading to a delay in the rate of starch digestibility. In agreemen<sup>t</sup> with another study, CTE inhibited the pancreatic α-amylase and intestinal α-glucosidase related to its phytochemical compounds [26]. It has been revealed that the phytochemical compounds in CTE are delphinidin-3, 5-glucoside, delphinidin-3-glucoside, malvidin-3-glucoside, delphinidin-based ternatins (ternatins A1–A3, B1–B4, C1–C5, and D1–D3), kaempferol, quercetin-3-*O*-(2-rhamnosyl) rutinoside, and rutin [18]. In particular, rutin and kaempferol could inhibit the pancreatic α-amylase and intestinal α-glucosidase activity [27,28]. Moreover, the natural delphinidin and malvidin compounds have shown a competitive inhibiting effect against the intestinal α-glucosidase [29,30]. A study conducted by Pods ˛edek et al. [31]

observed that the degree of the inhibitory effect on the carbohydrate digestive enzymes is positively correlated with the concentration of anthocyanins. We sugges<sup>t</sup> that the phytochemical compounds in CTE may contribute to delaying the hydrolysis of starch by inhibiting the carbohydrate digestive enzymes, including pancreatic α-amylase and intestinal α-glucosidase. Additionally, Zhu et al. [32] also explained other mechanisms of polyphenols on delayed starch digestibility. It found that polyphenol could interact with the starch chains to form the complex, resulting in the alteration of enzyme susceptibility. This evidence was supported from the higher content of undigested starch after in vitro digestion of starch and anthocyanins in blue maize [33]. The interaction between polyphenols and starch was due to the non-covalent bonding and/or the hydrogen binding formation [34]. Further studies are needed to define the hypothesized mechanisms specific to the interaction between the anthocyanins in the CTE and flours, and the type of enzyme inhibition.

For nutritional purposes, the starch in food is generally classified into three categories, including rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) [35]. In terms of RDS, starch is readily and completely digested in the small intestine associated with more rise postprandial plasma glucose within first 20 min. SDS is a complete digestion of starch in the small intestine with slow rate [5]. Furthermore, RS is defined as the dietary starch that resists digestion in the small intestine. According to a previous study [36], undigested starch contributes to the resistant starch, which is due to the inhibitory activity of α-amylase by antinutrients (e.g., polyphenols). The various amount of starch fractions in the different types of flour depends on their physical and chemical characteristics [37]. The observed results indicate that all flours had a higher level of RDS with a concomitant lower content of undigested starch. The gelatinization is one of the factors affecting starch hydrolysis during the cooking process [38]. Thus, the gelatinization of the starch granule induces an increase in the RDS response with the release of the glucose molecule. The results also showed that the flour with a high RDS content produced a significantly higher level of HI and pGI, whereas the higher SDS content with lower RDS reduced the levels of HI and pGI. A previous study supported our findings, indicating that the pGI value was correlated with the parameters of the digestible starch fractions, including RDS and SDS. In particular, SDS is found to be the main contributing factor to the GI [39]. It has been reported that an intake of diet containing a high RDS level could induce a rapid hyperglycemic response and a subsequent glucose-induced insulin secretion from pancreatic β-cells [5]. In contrast, undigested starch (RS) in human diets provides functional properties and applications for delaying postprandial glucose [40] and improving postprandial insulin [41]. Our findings demonstrated that the addition of CTE into glutinous rice, wheat, corn, and cassava flour causes the reduction of RDS and SDS, in relation to the increased content of undigested starch. Moreover, the undigested starch of wheat and cassava flours significantly and strongly correlated with the concentration of CTE. For traditional use, glutinous rice flour, ground from glutinous rice or sticky rice, is usually used as the ingredient for desserts, sweets, rice cakes, and puffed rice in Asia and Southeast Asia [42]. Basically, glutinous rice has been classified as high GI because of its high amylopectin content and rate of digestion [43]. The starch digestibility of glutinous rice produces more rapidly and is more complete than other high-amylose rice varieties. Chan et al. [44] found an increased glycemic response and GI values in Caucasian and Asian populations after the consumption of glutinous rice, which was similar to a previous study of Ranawana et al. [45]. In the current study, the mixture of CTE into glutinous rice flour can reduce the pGI value, suggesting that CTE suppresses the digestive process of glutinous rice flour to absorbable monosaccharides. A combination of glutinous rice flour and CTE might have opportunities for flour applications to reduce the GI of the food products.

Several studies have reported that the plant-based diets containing polyphenols alter the glycemic index of various foods. The current study found that the addition of CTE caused the rate of carbohydrate digestion and pGI of wheat bread to slow down. Our results are in agreemen<sup>t</sup> with Reshmi et al. [9], who reported the in vitro glycemic impact of bread fortified with pomelo fruit. Because of the action of phenolic compounds and the flavonoids in pomelo, the bread fortification with pomelo caused a lower level of digestible starch with a concomitant increased level of undigested starch. Lemlioglu-Austin et al. [46] also found that the incorporation of phenolic-rich sorghum bran extract into porridges contributes to slow starch digestion with a reduced GI and increased undigested starch. In addition, thermal processing also affects starch digestibility by the alteration of its granular structure [47]. The cooking process with heating and excess water induces the gelatinization of the starch granule, increasing in starch digestibility. The crumb portions of baked bread increased the starch digestibility when compared with the crust portion, because the starches in the crust portion are not completely gelatinization after baking. The fortification of green tea polyphenol in baked bread reduced in glucose release for both the crumb and crust after in vitro digestion [48] and reduced the rapidly digestible starch of white bread samples [49]. During baking, the interaction between gelatinized starch granules and the gluten network occurs in crumb, causing a loss of kinetic energy and a subsequent increase in firmness [50]. Previous evidence revealed that polyphenols can form a complex with bread ingredients including protein and polysaccharides [50]. The formation of polyphenols and polysaccharides or protein as enzymes clearly indicates a reduced in vitro digestibility [34]. Moreover, polyphenols affect the breadmaking quality by altering the flour protein properties [51]. The interaction of polyphenols and gluten proteins in wheat bread is associated with a reduction in protein cross-linking, resulting in decreased bread volume. Our study showed that the addition of CTE caused the bread volume to and air pocket in wheat bread to reduce. These findings are consistent with a study of Pathak et al., who reported that the addition of mango fruit peel powder could decrease the volume and height of loaf, whereas it increased the density of the loaf with less visible air pockets of bread, owning to the compact crumb structure [52]. It could be explained that the gluten network was not completely formed, leading to the ineffectiveness to hold air during fermentation, which caused the decreased loaf volume [53]. According to our findings, the addition of CTE into various types of flours successfully altered the parameters of starch digestibility and consequently decreased the level of HI and pGI. Further studies are warranted to elucidate whether the consumption of bread incorporated with CTE delays postprandial glucose in humans.
