1. Introduction
Diabetes and obesity have been the primary health problems worldwide. The International Diabetes Federation recorded that 537 million adults, or more than 10% of the world population, suffer from diabetic mellitus type 2 [
1]. This number is predicted to increase to 783 million in 2045. The epidemic’s prevalence demands a higher level of responsibility from many stakeholders worldwide to find solutions, or at least slow down its rate. A high glycemic index (GI) diet with poor fiber and other essential nutrients may lead to a high risk of diabetes [
2]. In addition, stress-oxidative and low physical activities could speed up the path to diabetes [
3]. Many efforts have been made to slow down those processes. One of them is by developing low-GI foods that are easy to serve, have the desired nutritional value, and are preferred by the community.
Purple sweet potato (
Ipomoea batatas L.) is a tropical root that is easily cultivated and contains a significant amount of fiber (2.3–3.9%) with a low-medium GI (54–68) and strong antioxidant activity (IC
50 65.35 ppm) that might be beneficial in preventing diabetes and other non-communicable diseases [
3,
4,
5,
6]. Among other sweet potatoes, the deep purple sweet potato has the highest anthocyanin and antioxidant capacity [
5]. However, the short shelf life and complicated processing method hinder the broad utilization of this tuber. To improve its shelf life and acceptability, purple sweet potato might be processed into flour and extruded products by using a high-temperature, short-time technique. However, this high-temperature process might transform this commodity from a low-GI food into a high-GI food and lower its antioxidant capacity [
7].
Extruded foods have been a topic of discussion in the food industry for many years and continue to be a popular and convenient food choice for many consumers. Some concerns have been raised regarding the nutritional quality and safety of extruded foods [
8,
9]. Extruded foods are made by processing a mixture of ingredients, such as grains and legumes, under high pressure and temperature to create a dough-like material that is then pushed through a die to give it a specific shape. One concern with extruded foods is that the high heat and pressure used during processing can cause a loss of nutrients, especially vitamins and bioactive compounds [
8,
9]. Additionally, some studies have suggested that certain compounds formed during the extrusion process may have negative health effects. Some alternative approaches for handling this issue are low-temperature extrusion and legume substitution in the mixture [
10,
11].
Substitution of legumes has been acknowledged to offer various benefits, i.e., protein enrichment, texture improvement, satiety improvement, and positive alteration of the glycemic profile, lipidemic profile, and gut microbiota [
12,
13]. Legumes contain significant amounts of protein, fiber, and various secondary compounds, like phenolic compounds, that might provide various health benefits [
14]. Indonesia preserves abundant biodiversity, including legume diversity [
15]. The present research utilized kidney bean (
Phaseolus vulgaris L.) as a main ingredient substitution in developing extruded purple sweet potato (EPSP). This legume was selected since it has the lowest GI value (26), with a significant amount of fiber (5.29%) and protein (24.79%) [
16]. Furthermore, it is adaptable to being cultivated at high temperatures with low water intake [
17].
This study aimed to evaluate the effect of kidney bean flour (KBF) substitution in the EPSP on nutritional quality, including macronutrients, GI function, and antioxidant capacity. Other important parameters that might influence the GI value were also observed in this research, i.e., polyphenol content and resistant starch [
18]. This study is expected to formulate an extruded, ready-to-eat food containing significant fiber and having a low GI made from purple sweet potatoes and kidney beans. The final purpose of this research is to develop a nutritious product that is well-suited for promoting and maintaining the nutritional status and health of the community.
2. Materials and Methods
This research has obtained permission from the Commission on Research Ethics Involving Human Subjects, IPB University Number: 661/IT3.KEPMSM-IPB/SK/2022.
2.1. EPSP Processing and Nutrient Analysis
The EPSP was made from purple sweet potato flour (PSPF) and kidney bean flour (KBF) with three formulas, i.e., 100% PSPF (F0, Formula 0), 70% PSPF and 30% KBF (F1), and 60% PSPF with 40% KBF (F2). These PSPF and KBF combinations made up 76%
w/
w of the total formula. The PSPF and KBF were produced through the following processing stages: cleaning, steaming (100 °C, 20 min), double drum drying (70–120 °C), and cabinet drying (60 °C, 2 h). The additional ingredients (%
w/
w) included rice flour (12.46), cornstarch (1.6), powdered milk (0.9), palm oil (1.8), water (7.2), and emulsifier (0.046) (
Table 1). All these ingredients were mixed and then processed using a twin screw extruder at a temperature of 60 °C, an auger speed of 40 Hz, and a screw speed of 40 Hz. The resulting extrusion product is shown in
Figure 1. The proximate analysis was performed to measure the contents of water, ash, protein, fat, and carbohydrates with the methods of oven-gravimetric, dry-ashing, Kjeldahl, soxhlet, and by-difference, respectively. Dietary fiber content was also measured using the enzymatic gravimetry method.
2.2. Polyphenol Content Analysis
The polyphenol analysis was performed based on Jayanegara et al. [
19]. A total of 0.2 g of sample was added to 10 mL of methanol (50%) and put in an ultrasonic water bath for 20 min at room temperature. Then it was centrifuged for 10 min at 3000×
g at 4 °C. Then, a 0.2 mL aliquot of sample extract was added with 1.25 mL Folin-Ciocalteu reagent and 6.25 mL sodium carbonate, and distilled water was added so that the volume reached 10 mL, was vortexed, and was recorded at 725 nm. Non-tannin phenol was analyzed using 0.1 g of Polyvinylpolypyrrolidone (PVPP) and inserted into a centrifuge tube to separate the content of tannin and non-tannin. 1.0 mL of distilled water and 1.0 mL of sample extract were added, then vortexed and stored at 4 °C for 15 min. After that, it was vortexed again and centrifuged (3000×
g for 10 min). The supernatant containing only non-tannin phenol was collected. Then 0.4 mL aliquots were taken and added along with 1.25 mL Folin-Ciocalteu reagent and 6.25 mL sodium carbonate. Then, distilled water was added so that the volume reached 10 mL. Total phenol and total tannin were calibrated against gallic acid solution as standards, and values were expressed as mg gallic acid equivalents (GAE).
2.3. Resistant Starch Analysis
Resistant starch analysis was performed based on the AOAC 2002.02 method and the AACC 32–40 method available from the Megazyme International Protocol [
20]. A total of 0.1 g of sample was mixed with 3.5 mL of sodium maleate buffer (pH 6.0) in a 37 °C water bath for 5 min. Then, 0.5 mL of pancreatic amylase enzyme was added to each sample tube. They were placed in a shaking water bath horizontally at 37 °C for 4 h (200 strokes/min), added with 4 mL of 95% ethanol, and then vortexed and centrifuged (4000 rpm, 10 min). The pellet was added to 2 mL of ethanol (50%
v/
v) and vortexed. Then, 6 mL of ethanol was added (50%
v/
v), shaken, and centrifuged again (1500 rpm, 10 min). Pellets were added to 2 mL of 1.7 M NaOH and shaken for 20 min. After that, 8 mL of sodium acetate buffer (1 M, pH 3.8) was added to each tube and shaken again. Then, 0.1 mL of AMG solution was added immediately and incubated for 30 min. Take 1.5 mL of the aliquot, then centrifuge (4000 rpm, 5 min); 0.1 mL of the supernatant was transferred to a glass test tube, added with 3 mL of GOPOD reagent, and incubated (50 °C, 20 min). The absorbance was read at 510 nm.
2.4. Glycemic Index Testing Procedure
The GI testing procedure is referred to as the ISO 26642 [
21] method. The ISO 26642 mentions that the GI should be determined based on the average glucose response of a minimum of 10 healthy subjects [
21,
22]. The subjects in this study were recruited using a purposive sampling method. The inclusion criteria for the subjects were that they were male or female in the age range of 18–30 years, had a normal Body Mass Index (BMI) of 18.5–24.9 kg/m
2, had no allergies or food intolerances, had a normal fasting blood sugar value (<110 mg/dL), had a healthy condition as stated by a doctor, engaged in light to moderate physical activity, and were willing to have their blood glucose levels measured. Exclusion criteria for the subjects included having a history of diabetes mellitus, being pregnant or breastfeeding, experiencing digestive disorders, consuming alcohol, undergoing medication, and smoking [
21].
The GI value in this study was assessed in vivo with 13 participating subjects, including seven males (53.85%) and six females (46.15%) (
Table 2). The nutritional status category [
23] showed that all subjects were in the normal nutritional status category (BMI = 21.13 ± 1.40 kg/m
2). Statistical tests showed that the data on height, weight, and BMI were not significantly different (
p > 0.05). So, it can be said that the subjects involved in this study were homogenous.
The reference food was 25 g of D(+)-Glucose anhydrous for biochemistry (Merck) that was dissolved in 250 mL of water, which was repeated twice [
21,
22]. The test foods were three formulas of extruded purple sweet potato and kidney bean, i.e., F0 (100% PSP flour), F1 (70% PSP flour and 30% kidney bean flour), and F2 (60% PSP flour and 40% kidney bean flour). Based on ISO 26642 [
21] about the determination of the glycaemic index (GI), in Section 2.2 titled “Carbohydrate Portion”, it is stated that the GI values could be determined by the weighed portion of food containing either 50 g of glycaemic carbohydrate or, if the portion size is unreasonably large, 25 g of glycaemic carbohydrate may be used. The evaluated samples have a very low specific gravity, so they have a high density (volume per weight). This high volume per weight made the 50 g glycemic carbohydrate equal to 82 g of product (about two bowls), which is unreasonable to be consumed within 12 min. Therefore, the study used 25 g of available carbohydrate instead of 50 g of available carbohydrate. The extruded products that must be consumed by the subjects were 33, 37.7, and 41 g for F0, F1, and F2, respectively. The test food was served in a plastic bowl with 250 mL of water.
The blood glucose was measured by taking a sample of the subject’s blood using the finger-prick capillary blood sample method with the Accu-Check Performance Glucometer. The subjects underwent complete fasting (except for water) for approximately 10 h (22:00 p.m. to 8.00 a.m. the next day). Before they were given intervention food, their blood samples were taken at minute 0 to determine fasting blood glucose levels. The subjects were required to finish the food provided for 12 min [
21,
24]. Blood samples were taken at 15, 30, 45, 60, 90, and 120 min. The time interval for giving the test food is five days.
Data on blood glucose levels at each time was plotted into a graph of time (x) and blood glucose (y). The GI value was calculated by comparing the area under the curve between the test and reference foods. The method used to calculate the Incremental Area Under the Curve (IAUC) was the trapezoid rule [
21,
22]. The method measured the area above the baseline by ignoring the area under the curve. The glycemic index values of the subjects were then averaged to obtain the food’s GI value.
2.5. GI for Mixed Food and Glycemic Load Assessment
Suggestions for serving the extruded were adjusted to match the suggestion for serving commercial cereal products in general, which is as much as 35 g and served with 125 g milk. The value of a meal’s Glycemic Index was calculated mathematically using the weighted average GI values for each individual ingredient based on its contribution to the overall available carbohydrate content [
24]. The glycemic index (GI) is a system that ranks foods based on how they affect blood sugar levels. The GI values are classified into three categories: high GI (≥70), medium GI (55–70), and low GI (≤55). Another metric used to measure the impact of a food on blood sugar levels is the glycemic load (GL), which takes into account both the GI of the food and the amount of carbohydrates in a typical serving. The GL categories are defined as follows: high GL (>20), medium GL (11–19), and low GL (<10) [
21]. The formula for GL is [GI of food × carbohydrate content (g) in one serving size] × 100.
2.6. Antioxidant Capacity Analysis
The capacity of the antioxidant products was analyzed by the 2,2 diphenyl-1-picrylhydrazyl (DPPH) method, which is a free radical compound. The principle of this method is to measure the concentration of the sample needed to counteract the DPPH free radicals. The DPPH method consisted of several stages: the extraction of the sample, preparation of standard vitamin C solutions, preparation of DPPH solutions, and analysis of the antioxidant. The antioxidant capacity was expressed as the IC50 value and the AEAC (Ascorbic Acid Equivalent Antioxidant Capacity). An analysis of antioxidant capacity was carried out on PSP flour and selected formulas.
2.7. Statistical Analysis
The data were analyzed using the Analysis of Variance test (ANOVA) followed by Duncan’s test at alpha 0.05. The data on antioxidant capacity was statistically tested using a t-test at alpha 0.05. The relationship analysis among the variables was performed using the Pearson correlation test. All those statistical analyses have been performed using IBM SPSS statistical software version 23.
4. Conclusions
In conclusion, the substitution of 40% of legumes in the extruded purple sweet potato has proven to be a promising approach for enhancing protein and fiber contents of the final product by up to 13.33% and 16.31%, respectively. Furthermore, this supplementation has resulted in a desirable glycemic index profile, making the extruded tuber a potentially valuable food source with high fiber and protein contents that may be suitable for consumption by both normal and pre-diabetic individuals. However, to fully unlock the potential health benefits of purple sweet potatoes, innovative flour and extrusion processing methods are necessary to preserve the native antioxidant capacity of the plant. By doing so, it may be possible to create a new generation of extruded food products that offer enhanced nutritional value and promote better health outcomes for consumers.