**1. Introduction**

Carbohydrates are one of three basic macronutrients that produce energy for our body. Nowadays, edible flour enriched with carbohydrates has been made from several parts of plants, including roots, seeds, and germs [1]. Flour is a common ingredient used for foods and desserts, according to different cooking purposes. The excessive and chronic consumption of flour markedly increases the postprandial blood glucose level and leads to excess visceral fat, which increases both insulin resistance and inflammation, and predisposes one to diabetes, hypertension, and cardiovascular diseases [2]. It has been shown that various types of flour contribute to a different rate and degree of starch hydrolysis, resulting in varying degrees of postprandial blood glucose rise [3]. Normally, the glycemic index (GI) is the measure of the immediate effect on the postprandial glucose level after food consumption, by comparing the percentage of incremental glucose area under the curve (iAUC) of a test food with reference to a standard food. However, the in vivo measurement of GI requires the recruitment of human subjects under ethical committee approval, financial supports, and it is time consuming, and all of these reasons led to widespread acceptance of in vitro starch digestibility studies [4].

The predicted glycemic index (pGI) is a common technique used to measure the rate of carbohydrate hydrolysis in foods [4]. It has been found that in vitro methods used to classify foods based on their digestion characteristics are similar to the in vivo situation [5]. There is a positive correlation between the in vitro and in vivo glycemic response [4]. In the focus of nutritional aspects, carbohydrate foods with a low pGI value (<55) can be considered as beneficial foodstuff for human health, in terms of the prevention and treatment of the metabolic syndromes, diabetes, and cardiovascular diseases [6,7]. Replacing or mixing flours with other ingredients such as fruits and vegetables is one of the alternative approaches to reduce the pGI in carbohydrate foods [8]. For example, pomelo containing polyphenols incorporated into bread could lower the predicted glycemic index probably by inhibiting the activity of the carbohydrate hydrolyzing enzymes [9]. It is becoming clear that plant-based ingredients containing polyphenols delay the action of carbohydrate digestive enzymes and thereby reduce the absorption rate of glucose [10,11]. Previously, polyphenols from the extracts of pomegranate, cranberry, grape, and cocoa could bind to the digestive enzymes ( α-amylase and glucoamylase), resulting in the inhibition of starch hydrolysis [12]. Therefore, the addition of plant-based ingredients may be capable of reducing the glycemic index during starch hydrolysis.

*Clitoria ternatea* L., commonly known as Butterfly pea, is a plant species belonging to the Fabaceae family. This plant is widely distributed in tropical zones such as Asia, the Caribbean, and Central and South America. In traditional Ayurvedic medicine, *Clitoria ternatea* L. has been used for treating stress and depression and enhancing memory [13]. There have been many pharmacological activities reported for this plant, such as antidiabetic [14], antipyretic [15], anti-inflammatory [16], and antimicrobial activity [17]. *Clitoria ternatea* has been reported to contain rutin, kaempferol, delphinidin, and related glycosides [18]. Our recent reports demonstrated that an aqueous extract of CTE inhibited the activity of carbohydrate digestive enzymes such as intestinal α-glucosidase and pancreatic α-amylase [19]. In addition to the biological pigment, the flower of *Clitoria ternatea* has been used as a colorant in various foods, beverages, and desserts in Asia. This colorant flower is regularly mixed with rice, bread, cookies, flours, and other traditional foods and desserts with a variety of ratios. For example, some traditional Thai desserts are made using cassava flour or glutinous rice flour mixed with butterfly pea juice at various concentrations to color and are then steamed until cooked. Moreover, sticky rice noodles are made by the mixture of rice flour with CTE juice. Although the pancreatic α-amylase and α-glucosidase inhibitory activity of CTE is well-documented, studies regarding its effect of pancreatic α-amylase action and in vitro starch digestibility using various types of flour have not been taken. Particularly, the potential food application of CTE in flour-based products remains unknown. Therefore, the aim of the present study was to investigate the effect of *Clitoria ternatea* L. flower extract on the activity of pancreatic α-amylase, in vitro starch hydrolysis, and predicted glycemic index of potato, cassava, rice, corn, wheat, and glutinous rice flour. The application in bread prepared from wheat flour and CTE was also determined.

#### **2. Materials and Methods**

#### *2.1. Chemicals and Reagents*

Commercial flours including potato, rice, glutinous rice, wheat, corn, and cassava flours were purchased from a supermarket. Porcine pancreatic α-amylase Type VI-B (catalogue number: A3176) and 3,5-dinitrosalicylic acid were purchased from the Sigma-Aldrich Chemical Co., Ltd. (St. Louis, MO, USA). Amyloglucosidase was obtained from Roche Diagnostics (Indianapolis, IN, USA). The glucose oxidase-peroxidase (GOPOD) kit was purchased from HUMAN GmbH (Wiesbaden, Germany).
