Flavonoids in plants can slow down starch hydrolysis and contribute to the formation of resistant starch. In this study, non-glutinous rice flour, joshin-ko, was heated in water or in an aqueous extract of sorghum grains, and then fractionated into small and large particles. In the small particles, pancreatin-induced starch hydrolysis was faster in joshin-ko heated in the sorghum extract than that heated in water. The faster hydrolysis was attributed to the removal of resistant starch, which was present in the small particles of joshin-ko, by forming complexes with sorghum components. In the large particles, the hydrolysis of starch was slower in joshin-ko heated in the sorghum extract than in water; the slowdown of amylose hydrolysis was greater than that of amylopectin hydrolysis. In addition, the heating of joshin-ko in the sorghum extract resulted in the increase in the content of resistant starch and the formation of poorly hydrolysable proteins combined with sorghum flavonoids such as 3-deoxyanthocyanidins in the large particles. It is postulated that the above resistant starch might be formed through the following processes: (i) the surrounding of the protein/flavonoid complexes by gelatinized starch, amylose in which is interacting with flavonoids in the protein/flavonoid complexes, during retrogradation, and (ii) the hydrolysis of the surrounding starch and proteins not formed complexes with flavonoid, leaving amylose-rich resistant starch, amylose in which is interacting with flavonoids in the protein/flavonoid complexes.

Sorghum grains contained taxifolin (dihydroquercetin) and taxifolin 7-O-glucoside (T7G). Quercetin and its 7-O-glucoside (Q7G) were formed by heating sorghum grains in distilled water. The formation of the above components was also observed by heating an extract of the grains prepared using 0.1 M sodium phosphate with 0.15 M NaCl (pH 7.0). It was shown that quercetin and Q7G could be formed from taxifolin and T7G, respectively, which were isolated from sorghum grains, by heating around pH 7. The formation of quercetin was accompanied by the formation of two taxifolin isomers, namely, taxifolin 2,3-cis isomer and alphitonin. Two isomers were also formed from T7G accompanying the formation of Q7G, and the characteristics of the isomers were similar to the taxifolin isomers. Taking the mechanism of the isomer formation into account, it is discussed that quercetin and Q7G might be formed by the oxidation of chalcones, which were formed via quinone methides of taxifolin and T7G, respectively.

Taxifolin (dihydroquercetin), which has various pharmacological functions, is contained in edible plants. Some taxifolin-containing foodstuffs such as adzuki bean and sorghum seeds are cooked by themselves and with other starch-containing ingredients. In this study, non-glutinous rice flour (joshin-ko) and potato starch were heated with taxifolin. The heating resulted in the slowdown of pancreatin-induced hydrolysis of suspendable starch in joshin-ko and soluble starch in potato starch. The products of taxifolin formed by the heating such as quercetin were combined with starch during the heating and/or retrogradation, which was converted into the suspendable starch in joshin-ko and the soluble starch in the potato. Taking the difference in protein content and amylose chain length between joshin-ko and potato starch into account, the slowdown is discussed to be due to the binding of the reaction products of taxifolin to proteins in suspendable starch in joshin-ko and to soluble amylose in potato starch.

Black soybean (BSB), which contains cyanidin-3- O -glucoside (C3G) and procyanidins, is cooked with rice in Japan. The color of the cooked rice is purplish red due to the binding of C3G and reddish oxidation products of procyanidins. These components can slowdown pancreatin-induced hydrolysis of amylose more significantly than the hydrolysis of amylopectin, and can react with nitrous acid in the stomach. This manuscript deals with the effects of nitrous acid on pancreatin-induced hydrolysis of amylose heated with BSB extract. The hydrolysis of amylose heated with BSB extract was slow, and the slowdown was due to the binding of C3G/its degradation products and degradation products of procyanidins. The amylose hydrolysis was slowed down further by treating with nitrite under gastric conditions. The further slowdown was discussed to be due to the binding of the products, which were formed by the reaction of procyanidins with nitrous acid, to amylose. In the products, dinitroprocyanidins were included. In this way, the digestibility of amylose heated with BSB extract can be slowed down further by reacting with nitrous acid in the stomach.

Polyphenols in plant can interact with amylose and amylopectin in different ways affecting their hydrolysis by α-amylase. Pancreatin liberated starch from non-glutinous rice flour heated with and without an aqueous extract of sorghum seeds, and hydrolyzed the liberated starch. The hydrolysis of the liberated starch was slowed down by the sorghum extract. Then, the liberated starch was fractionated into soluble starch and suspendable starch. In the soluble starch, amylose hydrolysis was slowed down more significantly than amylopectin hydrolysis, and in the suspendable starch, the hydrolysis of amylopectin was slowed down efficiently by the sorghum extract. It is discussed that (i) the slowdown in the former might be due to the binding of sorghum components including procyanidins to amylose, and that (ii) the slowdown in the latter might be due to the complex formation between amylopectin and shorter amylose combined with the sorghum components. The contribution of amylose to the slowdown was supported by the result that the sorghum extract inhibited the starch hydrolysis only slightly in glutinous rice flour, the starch of which was almost composed of amylopectin. It was proposed a possible mechanism of the slowdown of amylopectin hydrolysis in suspendable starch by shorter amylose combined with the sorghum components.

Black soybean (BSB), which contains cyanidin 3-O-glucoside (C3G) and procyanidins, is an ordinary ingredient in Japan. The BSB extracts slowed down pancreatin-induced reducing sugar formation and starch hydrolysis more significantly in non-glutinous rice flour (joshin-ko) than glutinous rice flour (mochi-ko), suggesting that BSB components inhibited amylose digestion more effectively than amylopectin digestion. In the components, C3G, a procyanidin dimer (proB2), and procyanidin oligomers were included. C3G could cooperate with proB2 for the inhibition. Procyanidin oligomers inhibited the amylose hydrolysis significantly but slightly the reducing sugar formation in joshin-ko, and inhibited the both reactions slightly in mochi-ko, suggesting that the oligomers could effectively inhibit amylose digestion. C3G could also cooperate with the procyanidin oligomers to inhibit amylose digestion in joshin-ko. The above results suggest that the cooperation of C3G and procyanidins might also be included in the BSB extract-dependent inhibition of amylose digestion in joshin-ko.

Cyanidin 3-O-glucoside (C3G), which has various health-promoting functions, is contained in black soybean (BSB). In Japan and Korea, BSB is cooked with rice and the cooked rice appears purplish in colour. In this study, BSB was cooked with glutinous rice, non-glutinous rice, and high-amylose rice. The amount of C3G detected in high-amylose rice was greater than that detected in glutinous rice, suggesting that C3G combined more efficiently with amylose than with amylopectin. Pancreatin induced the liberation of starch/C3G complexes from the purplish cooked rice, and rate of the liberation was in the following order; glutinous rice < non-glutinous rice < high-amylose rice. The amylose/C3G complexes liberated from high-amylose rice was hydrolysed slowly, while the amylopectin/C3G complexes liberated from glutinous rice were hydrolysed into smaller amylopectin/C3G complexes that were difficult to further hydrolysis. Thus, C3G may be useful for preparing foods whose starch hydrolysis is slow.

Procyanidins are contained in various foods, and their effects on starch hydrolysis have been reported. In Japan, black soybeans, which contain a trimeric procyanidin, procyanidin C1 (proC1), are cooked with rice and used to prepare dumplings. In this study, the effects of proC1 on the pancreatin-induced formation of reducing sugars and starch hydrolysis were studied using potato starch and corn starch. ProC1 inhibited both reactions; the inhibition was greater in potato starch than corn starch when added to heated potato starch and corn starch. When heated with proC1, its inhibitory effects decreased, especially in potato starch, suggesting the important role of proC1 itself for the inhibition of potato starch hydrolysis. ProC1 also inhibited the hydrolysis when added to heated, longer amylose (average molecular weight: 31,200), and the inhibition decreased when heated with the amylose. On the other hand, proC1 could not inhibit the hydrolysis when added to heated, shorter amylose (average molecular weight: 4500), but could when heated with the amylose, suggesting the important role of the degradation products of proC1 for the inhibition. We discuss the mechanism of the proC1-dependent inhibition of amylose hydrolysis, taking the molecular weight into account.

Adzuki bean is cooked with rice in Japan, and the color of rice becomes pale red by the cooking. In previous papers, we showed that pancreatin-induced starch liberation was slower in red rice prepared from non-glutinous rice. In this study, it was shown that pancreatin-induced amylose liberation from high-amylose rice was divided into two phases independent of cooking with or without adzuki bean; the first phase was amylose liberation from starch leached from rice during the cooking and the second phase was the liberation of amylose from starch in rice grains. The slower amylose liberation from rice cooked with adzuki bean was due to the binding of cyanidin-producible components derived from procyanidins to rice. The binding was confirmed by treating rice with a 1-butanol/HCl/Fe(III) system. The digestion of amylose liberated from high-amylose rice cooked with adzuki bean was also slower, and the liberated amylose could produce cyanidin by the 1-butanol/HCl/Fe(III)-treatment. Furthermore, it was shown (i) that the heating of rice flour with reagent procyanidin B2 resulted in the binding of the oxidation products to the flour, and (ii) that the binding resulted in the slower digestion of amylose in the rice flour. It was concluded that the slower liberation and digestion in high-amylose rice cooked with adzuki bean were due to the binding of cyanidin-producible components, which were derived from adzuki bean procyanidins, to amylose during the cooking.

In Japan, adzuki bean is cooked with rice. During the cooking, the colour of rice becomes pale red. It is postulated that the red pigment is produced from procyanidins and that the ingestion of red rice causes the production of nitric oxide (•NO) in the stomach by reacting with salivary nitrite. The increase in colour intensity accompanied the decrease in the amounts of procyanidins, suggesting the conversion of procyanidins into the red pigment during the cooking. In addition, the red pigment combined with rice strongly. The red-coloured rice produced •NO by reacting with nitrite in artificial gastric juice, and the amounts were dependent on the contents of procyanidins and the equivalents. It is suggested that although adzuki procyanidins were oxidised during cooking with rice, procyanidins and the equivalents bound to rice still have the ability to produce bioactive •NO in the stomach using nitrite in mixed whole saliva.

Adzuki bean is often cooked with non-glutinous rice in Japan, and the dish is called adzuki-meshi. By the cooking, flavonoids in adzuki bean are transferred to rice, and the color of the rice becomes pale red. However, it has not been reported on starch digestion of the rice of adzuki-meshi. The purpose of this study is to elucidate that the transferred flavonoids, especially procyanidins could slow down the digestion of rice starch. The principal results obtained are (1) that pancreatin-induced starch digestion, which was observed as the liberation of reducing sugars and starch fragments from the rice, was slower in the pale red rice of adzuki-meshi than the rice cooked without adzuki bean, (2) that the starch fragments liberated from the rice of adzuki-meshi were digested slowly, and (3) that procyanidins and the oxidation products, which were not extracted by methanol, were present in the pale red rice. From the results, it was concluded that adzuki bean procyanidins and the oxidation products, which bound to rice starch during cooking, could contribute to slow down the starch digestion.

Adzuki beans are used to prepare foods with glutinous and non-glutinous rice in Japan, and adzuki bean pigments are able to color rice starch a purplish red. This study deals with the adzuki bean extract-dependent suppression of starch digestion of non-glutinous rice flour (joshinko in Japanese), which was gelatinized in boiling water and then cooled to 37 °C. Accompanying the treatment of joshinko with pancreatin, amylose and amylopectin were released from the joshinko particles, and the released amylose and amylopectin were further digested. The adzuki extract suppressed the release and digestion by binding to amylose and amylopectin, which were present in the particles and at the surfaces of the particles. Fatty acids and flavonoids in the adzuki extract contributed to the suppression. In addition, the starch digestion in the joshinko particles appeared to be suppressed if the amylose/fatty acid complexes and amylose/flavonoid and amylopectin/flavonoid complexes, which are poor substrates of α-amylase, surrounded the particles. It is discussed that the suppression was due to the prevention of α-amylase access to the particles.

Starch is digested to glucose in the intestine and absorbed into the body. If the increased blood concentrations of sugar after meals decrease slowly or are maintained for a long time, various adverse effects are induced. Therefore, it is important to decrease the rate of the digestibility of starch in the intestine in the patients of hyperglycemia. One of the ways to effect a decrease is the inhibition of α-amylase secreted from the pancreas. Flavonoids are a group of compounds that can inhibit this enzyme's activity, and many investigators have studied the flavonoid-dependent inhibition of this enzyme and presented mechanisms for the inhibition of its activity. Starch containing foods, however, cooked or ingested with flavonoid containing foods are mixed with saliva and gastric juice in the stomach. Thus, flavonoids in the foods can interact with starch and can react with nitrous acid derived from the oral cavity in the stomach before being transported to the intestine. This review mainly deals with: (i) the inhibition of α-amylase activity by flavonoids suggesting the mechanisms of the inhibition, (ii) suppression of starch digestion by flavonoids by forming starch-flavonoid complexes by hydrophobic interactions, and (iii) formation of starch not easily digested by α-amylase by the formation of covalent bonds between flavonoids and starch during cooking and in the stomach. In addition, the cooperation of flavonoids with fatty acids are discussed taking their binding to amylose into account.

Flavonoids of adzuki bean bind to starch when the beans are cooked with rice. The purpose of this study is to show that adzuki flavonoids can suppress pancreatin-induced digestion of cooked rice starch. The diethyl ether extract of water boiled with adzuki bean inhibited starch digestion, and quercetin and a cyanidin-catechin conjugate (vignacyanidin) but not taxifolin in the extract contributed to the inhibition. The order of their inhibitory effects (taxifolin < quercetin < vignacyanidin) suggested that the effects increased with an increase in their hydrophobicity. The diethyl ether extract also inhibited the starch digestion of cooked rice preincubated in artificial gastric juice, and the level of inhibition was decreased by nitrite. The decrease was due to nitrite-induced consumption of quercetin and vignacyanidin. Taking these results into account, we discuss mechanisms of quercetin- and vignacyanidin-dependent inhibition of starch digestion and the possibility of the decrease in their inhibitory effects by nitrite in the stomach.

Foods are mixed with saliva in the oral cavity and swallowed. While staying in the stomach, saliva is contentiously provided to mix with the ingested foods. Because a salivary component of nitrite is protonated to produce active nitrous acid at acidic pH, the redox reactions of nitrous acid with phenolic compounds in foods become possible in the stomach. In the reactions, nitrous acid is reduced to nitric oxide (•NO), producing various products from phenolic compounds. In the products, stable hydroxybezoyl benzofuranone derivatives, which are produced from quercetin and its 7-O-glucoside, are included. Caffeic acid, chlorogenic acid, and rutin are oxidized to quinones and the quinones can react with thiocyanic acid derived from saliva, producing stable oxathiolone derivatives. 6,8-Dinitrosocatechis are produced from catechins by the redox reaction, and the dinitrocatechins are oxidized further by nitrous acid producing the quinones, which can make charge transfer complexes with the dinitrosocatechin and can react with thiocyanic acid producing the stable thiocyanate conjugates. In this way, various products can be produced by the reactions of salivary nitrite with dietary phenolic compounds, and reactive and toxic quinones formed by the reactions are postulated to be removed in the stomach by thiocyanic acid derived from saliva.

Non-glutinous rice boiled with adzuki bean (adzuki-meshi) is often prepared in Japan, and the colour of rice in adzuki-meshi is purplish red, suggesting the transfer of adzuki bean components to rice. In fact, red-coloured rice of adzuki-meshi contained adzuki bean flavonoids such as (+)-catechin, (−)-epicatechin, taxifolin, quercetin 7-O-glucoside, rutin, quercetin, and cyanidin-catechin conjugates (vignacyanidins). A salivary component nitrite reacted with the above flavonoids in adzuki-meshi under the conditions simulating the stomach. The reaction resulted in the decrease in the antioxidative components of adzuki-meshi, and the decrease was accompanied by the production of functional compounds like nitric oxide (•NO), 6,8-dinitrosocatechin, 6,8-dinitrosoepicatechin, and 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone from nitrous acid, (+)-catechin, (−)-epicatechin, and quercetin, respectively. In this manuscript, functions of adzuki-meshi are discussed considering the reactions of the flavonoids with nitrite in the stomach.

By the ingestion of fresh apple fruit, it is masticated squeezing apple juice into the oral cavity and the juice is mixed with saliva. The mixture of saliva and apple juice is swallowed into the stomach where the pH is around 2. This paper deals with the reactions of polyphenols in the juice obtained by mastication of apple fruit with salivary nitrite under acidic conditions. The concentrations of catechins, procyanidins, and chlorogenic acid in the apple juice were approximately 55, 55, and 170μM, respectively, and the polyphenols were oxidized by salivary nitrite under conditions of the stomach. Rates of the oxidation increased in order chlorogenic acid<catechins<procyanidins. The oxidation of catechins and procyanidins resulted in the formation of the nitroso compounds, and the oxidation of chlorogenic acid resulted in the formation of the thiocyanate conjugate. The production of dinitroso compounds is proposed to be due to the addition of nitric oxide (NO) to radicals of catechins and procyanidins. As the mechanism of thiocyanate conjugate formation, reaction of o-quinone of chlorogenic acid with a salivary component thiocyanate is proposed, and the formation of the thiocyanate conjugate is discussed from the point of detoxification of chlorogenic acid quinone.

Abstract Catechins are ingested as food components and supplements. It is known that catechins are transformed to dinitrosocatechins by nitrite under acidic conditions, suggesting the possibility of their formation in the stomach because saliva contains nitrite. This paper deals with nitrite-induced transformation of (+)-catechin in methanol extracts of adzuki bean into 6,8-dinitrosocatechin in acidified saliva (pH ≈ 1.9). As the mechanism of its formation, addition of nitric oxide (NO) to (+)-catechin semiquinone radical, both of which were produced in nitrous acid/(+)-catechin systems, was proposed. The dinitrosocatechin was oxidized to the quinone by nitrous acid, and the quinone reacted with a salivary component thiocyanate producing 6′-thiocyanato-6,8-dinitrosocatechin. Since quinones are toxic, we propose a function of thiocyanate as a scavenger of the o-quinone formed from dinitrosocatechins in the stomach.

When foods that contain catechins and quercetin glycosides are ingested, quercetin glycosides are hydrolyzed to quercetin during mastication by hydrolytic enzymes derived from oral bacteria and the generated quercetin aglycone is mixed with catechins in saliva. The present study deals with the interactions between (+)-catechin and quercetin during their reactions with nitrous acid under the conditions simulating the gastric lumen. Nitrous acid reacted with (+)-catechin producing 6,8-dinitrosocatechin, and quercetin partially suppressed the dinitrosocatechin formation. Nitric oxide, which was produced by not only (+)-catechin/nitrous acid but also quercetin/nitrous acid systems, was used to produce 6,8-dinitrosocatechin. Furthermore, 6,8-dinitrosocatechin was oxidized by nitrous acid to the quinone form. The quinone formation was significantly suppressed by quercetin. Quercetin-dependent suppression of the above reactions accompanied the oxidation of quercetin, which was observed with the formation of 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. Taking the above results into account, we proposed a possible mechanism of 6,8-dinitrosocatechin formation and discuss the importance of quercetin to prevent the quinone formation from 6,8-dinitrosocatechin in the gastric lumen, taking the interactions between quercetin and catechins into account.

Foods of plant origin contain flavonoids. In the adzuki bean, (+)-catechin, quercetin 3-O-rutinoside (rutin), and quercetin 7-O-β-D-glucopyranoside (Q7G) are the major flavonoids. During mastication of foods prepared from the adzuki bean, the flavonoids are mixed with saliva and swallowed into the stomach. Here we investigated the interactions between Q7G and (+)-catechin at pH 2, which may proceed in the stomach after the ingestion of foods prepared from the adzuki bean. Q7G reacted with nitrous acid producing nitric oxide (˙NO) and a glucoside of 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. (+)-Catechin reacted with nitrous acid producing ˙NO and 6,8-dinitrosocatechin. The production of the dinitrosocatechin was partly suppressed by Q7G, and the suppression resulted in the enhancement of Q7G oxidation. 6,8-Dinitrosocatechin reacted further with nitrous acid generating the o-quinone, and the quinone formation was effectively suppressed by Q7G. In the flavonoids investigated, the suppressive effect decreased in the order Q7G≈quercetin>kaempferol>quercetin 4'-O-glucoside>rutin. Essentially the same results were obtained when (-)-epicatechin was used instead of (+)-catechin. The results indicate that nitrous acid-induced formation of 6,8-dinitrosocatechins and the o-quinones can be suppressed by flavonols in the stomach, and that both a hydroxyl group at C3 and ortho-hydroxyl groups in the B-ring are required for efficient suppression.