Original papers
Increasing resistant starch of processed Thai rice using citric acid, extra virgin coconut oil and herbs
2024 Volume 30 Issue 1 Pages 37-46
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2024 Volume 30 Issue 1 Pages 37-46
This study aimed to increase resistant starch (RS) content in cooked KDML 105 rice. Various factors were investigated, such as rice type, cultivation area, oil type, rice cooker, and Thai herbs. This study found that the choice of oil and the incorporation of specific Thai herbs had a significant impact on the RS content of cooked KDML 105 rice. Using extra virgin coconut oil increased RS content, while certain Thai herbs, particularly Pandan leaves juice and butterfly pea flower, contributed to both increased RS content. Sao Hai rice had the highest RS content (4.31 ± 0.30 %). Saline soil areas did not significantly affect RS content in KDML 105 rice. Using extra virgin coconut oil and pressure rice cookers increased RS content significantly (1.40 ± 0.23 %). Incorporating four Thai herbs improved RS content. Notably, soaking rice in 2 % citric acid, mixing with extra virgin coconut oil, Thai herb and pressure cooking significantly increased RS content.
Enhancing resistant starch in rice-based functional foods was provided with added health benefits. Recognized by esteemed institutions such as the American Association of Cereal Chemists (AACC) and the Food and Nutrition Board of the Institute of Medicine-National Academy of Sciences, resistant starch falls under the definition of dietary fiber. The notable advantage of resistant starch is its ability to bypass digestion and be fermented in the large intestine, resulting in the production of short-chain fatty acids. This physiological process is associated with numerous benefits, including a reduced glycemic index, decreased cholesterol absorption, and potential prevention of colon cancer (Tan et al., 2021). Scientists have explored various techniques to modify the functional properties of resistant starch, such as the use of low or no-calorie sugars, formation of starch-lipid complexes, and specific processing methods like heat moisture treatment and extrusion (Hung et al., 2016; Kumar et al., 2018; Kim et al., 2019; Cervantes-Ramirez et al., 2020).
Resistant starch occurs naturally in starchy foods, and its content can be influenced by factors such as the amylose: amylopectin ratio, compactness, and crystallinity of the food product. Here, RS content was evaluated on Glutinous rice RD 6 (low amylose), KDML 105 (medium amylose), and Sao Hai rice (high amylose) (Hung et al., 2016). Furthermore, cooking methods such as electric and pressure cooking techniques are compared. Temperature, post-cooking storage, and interactions with lipids and other carbohydrates can impact the digestibility of starch (Birkett and Brown, 2008). Heat and chemical interventions can influence the texture, water holding capacity, processing stability, and nutritional functionality of food products (Bello-Perez and Paredes-Lopez, 2018). Retrogradation involve the heating of starch followed by cooling to rearrange the crystal structure of amylose, contributes to the formation of resistant starch with a robust crystalline structure, thus rendering it resistant to enzymatic degradation (Asp, 1992). Our experiments, chemical modifications as 2 % citric acid can also induce cross–linking within the starch structure, resulting in the formation of resistant starch may be through the creation of new bonds.
Furthermore, KDML 105 rice has proven to be a suitable cultivar for arid areas due to its ability to thrive with minimal water requirements, resistance to acidic and saline soils, and positive response to organic fertilizers (Prasertsuk and Wijitkosum, 2021). Researchers have explored different approaches to increase the resistant starch content in KDML 105 cooked rice, including retrogradation and chemical modification to obtain amylose-lipid starch complexes.
Due to coconut oil composition of saturated fatty acids, which are efficiently metabolized by the body and cold–pressed coconut oil contains medium–chain saturated fatty acids that have been shown to enhance colonic efficiency and stimulate excretion (den Besten et al., 2013). Here, the present study presents the incorporation of extra virgin coconut oil and citric acid into the rice mixture, thereby enabling potential alterations in the properties of starch. Two cooking techniques–electric and pressure cooking–are employed to cook rice permitting potential modifications in starch properties. The cooling and chilling steps were applied to ensure form amylose-lipid starch complexes. In this study, they were kept in −20 °C refrigerators until analysis. For consumer, they can bring RS rice to warm it by microwave for eaten. KDML 105 rice cultivation locations had an impact on cooked rice quality. Pasting and thermal properties of gelatinization and retrogradation parameters could explain the quality of KDML 105 rice obtained from different cultivated locations. The eating quality of KDML 105 rice could be differentiated by its planting location (Pitiphunpong and Suwannaporn, 2009). Our challenge is to determine the cooking method, cooling and chilling steps retains RS content while considering the influence of cooking additives.
Furthermore, the incorporation of herbs in food preparation provides not only flavor and natural color but also bioactive compounds with antioxidant properties and essential vitamins. Asian pigeon wings, for example, have demonstrated the ability to reduce starch digestion when added to bread (Chusak et al., 2018). Pandan leaves have been associated with anti-inflammatory properties (Londonkar et al., 2010), and Noni fruit consumption has shown promising effects in reducing obesity and ocular abnormalities (Zhang et al., 2019), attributed to the presence of lignin, pinoresinol, quercetin, kaempferol, scopoletin, isoscopoletin, and vanillin (Shengmin et al., 2001).
As mentioned above, this study compared the RS content of three types of rice which has different amylose content (low, medium and high of glutinous rice RD 6, KDML 105 and Sao Hai rice, respectively) and grown in two cultivation areas using three types of oil, two cooking methods, and four herbs (Pandan leaf, butterfly pea flower, Gac fruit and Noni leaf). During retrogradation, as the starch granules recrystallize, amylose molecules become more ordered and compact. This can enhance the interaction between amylose and lipids, facilitating the formation of amylose-lipid complexes (Bello-Perez and Paredes-Lopez, 2018). Our challenge aims to provide valuable insights for creating healthier and more nutritious rice-based meals. This research has the potential to reshape dietary choices and contribute to improved overall health and well-being.
Plant materials Three different types of rice, (Oryza sativa L.), most of indica varieties such as KDML 105, Sao Hai, and glutinous rice RD 6 were purchased from a Bangkok market, while KDML 105 from a saline soil area was sourced from Wapeeprathum Market, Maha Sarakham Province, Thailand (Fig 1). The herb was obtained from Wapeeprathum Market, Maha Sarakham Province, Thailand where it was grown.
Shows a saline soil area (indicated by 
) in Wapeeprathum district, Maha Sarakham province and a non-saline soil area (indicated by 
) in Thailand.
Preparation of cooked rice Rice was cooked by electric cooker (Smart Home, Model: MV-017, PEOPLE'S REPUBLIC OF CHINA) at 100 °C. Cooked rice products were prepared from two-hundred grams of rice mixed with 250 mL of water. The cooking time was about 15 min.
Cooked rice products were prepared using a pressure cooker (HOMEMATE, Model: HOM-12LC62, PEOPLE'S REPUBLIC OF CHINA). Two-hundred grams of rice were soaked in 250 mL of reverse osmosis water under pressure (13 psi or 90 kPa) at 121 °C for 15 min.
Cooked rice preparation for RS analysis Three types of rice (200 g of each) which has different amylose content (low, medium and high of glutinous rice RD 6, KDML 105 and Sao Hai rice, respectively) were soaked in 250 mL of reverse osmosis water in pressure rice cookers, and then cooked under pressure (13 psi or 90 kPa) at 121°C for 15 min. Then, transfer the cooked rice to a refrigerator and store it at 4°C overnight, store the rice samples at -20°C until RS analysis.
Weigh 200 g of Sao Hai rice or KDML 105 for each cooking method.
Electric Cooker Method: Each rice type combined 5 mL of extra virgin coconut oil and 2.5 mL of citric acid with the 200 g of rice. Mix thoroughly to ensure even distribution. Let the rice mixture sit at room temperature for 30 min to allow for any potential modifications in starch properties. Add 250 mL of reverse osmosis water to each rice mixture. Transfer the rice mixture to electric cooker or pressure cooker. Set the electric rice cooker to its standard cooking cycle for rice. For the pressure cooker, ensure that it is securely sealed and set it to cook at 13 psi (90 kPa) at 121 °C for 15 min.
Allow the cooked rice from both methods to cool at room temperature for 15 minutes. Then, transfer the cooked rice to a refrigerator and chilled at 4 °C overnight, stored at −20 °C until analysis.
Two-hundred grams of rice was mixed with 5 mL cooking coconut oil or extra virgin coconut oil in ratio 1:1 (2.5 mLcooking coconut oil: 2.5 mL extra virgin coconut oil) or 5 mL extra virgin coconut oil and then left at room temperature for 30 min after which it was soaked in 250 ml of reverse osmosis water in pressure rice cookers and then cooked under pressure (13 psi or 90 kPa) at 121 °C for 15 min. Then, transfer the cooked rice to a refrigerator and store it at 4 °C overnight, store the rice samples at −20 °C until RS analysis.
Two-hundred grams of Sao Hai rice or KDML 105 rice from normal or saline soil areas is mixed with 5 mL of extra virgin coconut oil for uniform coating, and another serving as a control group cooked without the addition of extra virgin coconut oil. Then added 250 mL of reverse osmosis water used for each rice sample. After cooking, the rice is cooled at room temperature for 15 minutes and then chilled at 4 °C overnight and then stored at −20 °C until analysis.
Two-hundred grams of KDML 105 rice from normal or saline soil areas was mixed with 5 mL of 2 % citric acid and another serving as a control group cooked without the addition of 2 % citric acid and left at room temperature for 30 min and then soaked in 250 ml of reverse osmosis water in pressure cookers followed by cooking under pressure (13 psi or 90 kPa) at 121 °C for 15 min. Then, transfer the cooked rice to a refrigerator and store it at 4 °C overnight, store the rice samples at −20 °C until RS analysis.
One-hundred grams of four Thai herbs (Pandan leaf, butterfly pea flower, Gac fruit and Noni leaf), was mixed individually with 250 ml water before grinding with a blender until thoroughly homogenous. The filtered with a double layer of cheesecloth on a colander was used to retain only the juice. After that, 250 mL of juice was cooked with 200 g rice using a pressure rice cooker.
All experiments, once the rice was cooked in the electric or pressure cookers, it was cooled at room temperature for 15 min, then chilled at 4 °C overnight and kept in −20 °C until analysis.
Prepared cooked rice for RS analysis An approximately 50 g sample of grain or lyophilized food product was ground in a grinding mill to pass a 1.0 mm sieve, transferred to a wide-mouthed plastic jar and mixed well by shaking and inversion. Industrial starch preparations are usually supplied as a fine powder, so grinding was not required. Fresh samples were minced in a hand operated or electric meat mincer to pass a ~ 4.5 mm screen. Moisture content of dry samples was determined by the AOAC Method 925.10 (15) and of fresh samples by lyophilization followed by oven drying according to AOAC Method 925.10.
RS analysis The RS content of all samples was estimated using a Megazyme kit (Megazyme, K-RSTAR 08/11, Wicklow, Ireland) with slight modifications. A rice flour sample (100 mg) was digested with 4 mL of pancreatic α-amylase (10 mg/mL) containing amyloglucosidase (300 U/mL) in a 50 mL conical flask at 37 °C with continuous shaking for 16 h, followed by addition of 4 mL absolute ethanol (Merck, Germany) with vigorous stirring. The contents were then centrifuged at 3 000 rpm for 10 min. The pellet was suspended in 2 mL 50 % ethanol with vigorous stirring followed by addition of 6 mL 50 % ethanol and vigorous stirring. It was then centrifuged at 3 000 rpm for 10 min. The supernatant was decanted and the process was repeated with the residue. The supernatant was decanted and excess liquid was drained out by inverting the tubes on absorbent paper. The pellet was suspended in 2 mL KOH (2 M) (MP Biomedicals Santa Ana, CA, OSA) in a test tube and stirred in an ice bath for 20 min. Then, 8 mL of 1.2 M sodium acetate buffer was added while stirring, immediately followed by the addition of 0.1 mL of amyloglucosidase (3 300 U/mL). After shaking, the tubes were incubated at 50 °C for 30 min followed by centrifugation at 3 000 rpm for 10 min. Aliquots of supernatant measuring 0.1 mL were transferred to glass tubes in duplicates, mixed well with 3 mL of GOPOD reagent, and incubated at 50 °C for 20 min. The absorbance was measured at 510 nm against a reagent blank. The RS percentage was calculated using the formula provided by the manufacturer. The experiment was conducted with three replicates. The data was expressed on a dry weight.
Statistical analysis One-way analysis of variance (ANOVA) was performed using SPSS to determine significant differences between treatment groups using Tukey's honest significance test (Tukey's HSD) at p < 0.05.
Comparison of RS contents of cooked rice types The results showed that RS contents of cooked Sao Hai, KDML 105 from saline soil, KDML 105 from normal soil, and glutinous rice RD 6 were different. The RS content of Sao Hai rice was the highest (4.31 ± 0.30 °%) because the amylose content was high ranging from 24–27 % whereas KDML 105 has medium amylose content (13–17 %) (Isnaini et al., 2019) and low amylose content (3–4 %) of glutinous rice RD 6 (Udomrati et al., 2022). Glutinous rice RD 6 had the lowest RS content (0.22 ± 0.026 %), whereas KDML 105 rice planted in saline soil had higher RS values (0.75 ± 0.09 %) than rice grown in normal soil (0.65 ± 0.07 %) (Fig 2), but not significantly different. The RS contents of freshly steamed japonica, indica, and waxy rice were 0.7 %, 6.6 %, and 1.3 %, respectively were reported by Reed et al., 2013. It showing that RS content depended on rice varieties.
Comparison of RS contents of cooked rice types: KDML 105, KDML 105 from saline soil, Sao Hai, and glutinous rice RD6.
Values with different letters are significantly different at p < 0.05.
From Fig. 2, the results showed significant differences in RS content among these rice varieties. Sao Hai rice had the highest RS content at 4.31 %, which was attributed to its high amylose content. KDML 105 had medium amylose content and showed RS values of 0.75 % in saline soil and 0.65 % in normal soil. Glutinous rice RD 6 had the lowest RS content at 0.22 % due to its low amylose content. Sao Hai rice, known for its low glycemic properties, offers significant health benefits due to its slow digestibility, primarily attributed to its high amylose content. Research has consistently shown that rice varieties with higher amylose content tend to exhibit higher levels of RS (Tuaño et al., 2021). This phenomenon is attributed to the faster retrogradation process in starches with high amylose/amylopectin ratios compared to those containing predominantly amylopectin. Furthermore, high amylose starches with branched polymers have been found to yield elevated levels of resistant starch (Asp, 1992).
Resistant starch contents of KDML 105 and Sao Hai rice cooked by electric and pressure cookers This study demonstrated that pressure cooking resulted in higher RS content in Sao Hai rice. Additionally, no significant difference in RS content was observed when comparing the use of extra virgin coconut oil with or without citric acid in the cooking process, using various rice cookers (Table 1). However, it was noted that Sao Hai rice cooked with extra virgin coconut oil and 2 % citric acid in the pressure cooker exhibited the higher RS content (6.93 ± 0.68 %) than those in electric cooker (4.31 ± 0.30 %) significantly. This outcome suggests that the addition of citric acid positively influenced RS formation (Isnaini et al., 2019). Previous research supports these findings, as it has been shown that rice soaked in citric acid affects starch characteristics and enhances RS formation. Specifically, citric acid facilitates the opening of a small pore located above the hilum of the rice seed, allowing lipids to easily penetrate and form amylose-lipid complexes within the cooked rice, leading to RS production (Hung et al., 2016; Krishnan et al., 2020).
| Cooker type | Rice type | % RS |
|---|---|---|
| Electric cooker | KDML 105 rice and extra virgin coconut oil | 0.99 ± 0.10c |
| KDML 105 rice, extra virgin coconut oil and 2 % citric acid | 0.88 ± 0.15c | |
| Sao Hai rice and extra virgin coconut oil | 4.36 ± 0.37b | |
| Sao Hai rice, extra virgin coconut oil and 2 % citric acid | 4.31 ± 0.30b | |
| Pressure cooker | KDML 105 rice and extra virgin coconut oil | 0.98 ± 0.16c |
| KDML 105 rice, extra virgin coconut oil and 2 % citric acid | 1.22 ± 0.02c | |
| Sao Hai rice and extra virgin coconut oil | 4.68 ± 0.32b | |
| Sao Hai rice, extra virgin coconut oil and 2 % citric acid | 6.93 ± 0.68a |
*Mean ± standard deviation. Values with different superscripts are significantly different at p < 0.05.
The results showed that pressure cooking increased the RS content, especially in Sao Hai rice which was high in amylose. Pressure cooking resulted in higher RS content in Sao Hai rice. When comparing the use of extra virgin coconut oil with or without citric acid in the cooking process using different rice cookers, no significant difference in RS content was observed. However, when citric acid was added (2%) and the rice was cooked in a pressure cooker, the RS content significantly increased to 6.93 % compared to 4.31 % in an electric cooker. The addition of citric acid was noted to positively influence RS formation, likely by facilitating the formation of amylose-lipid complexes in the cooked rice (Hung et al., 2016; Krishnan et al., 2020)
Higher heat in the pressure cooker broke down the amylose and amylopectin as demonstated in cooked rice in pressure cooker shows the result of higher RS content comparing between Sao Hai rice, extra virgin coconut oil and 2 % citric acid cooked in pressure cooker in this study. For rice with high amylose content found the higher RS content. This due to the increase in opportunity of forming amylose-lipid complex was occurred rather than low amylose content. This finding was agreed with the report from Kim et al. (2019) that high amylose content provided high RS content in rice.
Numerous studies have investigated the impact of different processing methods and conditions on RS content in various starch sources such as Sievert and Würsch (1993) found that gelatinization at temperatures ranging from 73.4 to 77.1 °C led to starch with high RS percentages. These findings were consistent with our own research, which demonstrated that the heating-cooling treatment, particularly in a pressure cooker, resulted in high RS rice. Interestingly, for starches with low and medium amylose levels, gelatinization temperatures above 100 °C were found to improve RS levels (Liu et al., 2014). It is important to note that starches derived from different types of rice and cooking methods exhibit varying gelatinization temperatures. From 2 experiments above (rice varieties and cooking methods), Sao Hai rice had the highest RS content due to its high amylose content, while glutinous rice RD 6 had the lowest RS content due to its low amylose content. KDML 105 exhibited medium RS content. Pressure cooking was found to increase RS content in Sao Hai rice compared to electric cooking. This suggests that rice varieties and cooking methods can influence RS formation.
Comparison of RS content of cooked rice types among KDML 105 from saline soil area with three types of oil The choice of oil also plays a role in determining RS content. Our results demonstrated that extra virgin coconut oil had the potential to significantly increase RS content (1.40 ± 0.23 %) (Fig. 3). The specific mechanisms underlying this observation warrant further investigation.
Comparison of RS content of cooked rice types among KDML 105 from saline soil area with three types of oil.
Values with different letters are significantly different at p < 0.05.
In a study by Krishnan et al. (2020), it was observed that when rice was mixed with virgin coconut oil before cooking, it resulted in a higher RS content compared to when mixed with regular coconut oil. This finding aligns with the present study, where the use of extra virgin coconut oil increased the RS content in KDML 105 rice (1.40 ± 0.23 %). The study by Krishnan et al. (2020) also reported that rice bran oil, coconut oil, and extra virgin coconut oil yielded RS contents of 0.83 %, 0.84 %, and 0.91 %, respectively, in white rice. Additionally, when brown rice was mixed with soybean oil, the RS content reached 2.99 % (Kim et al., 2019). These findings suggest that oils can contribute to the modification of starch in rice, increasing its RS content.
Comparison of RS contents of cooked rice types: KDML 105 from saline soil, KDML 105, and Sao Hai with and without extra virgin coconut oil Furthermore, the results indicated that extra virgin coconut oil increased RS content in all rice types. Among them, Sao Hai rice exhibited the highest RS content (4.68 ± 0.32%) due to its high amylose content. The small pore above the hilum at one end of the rice seed closes when Sao Hai rice is cooked with extra virgin coconut oil, making it more resistant to enzymatic digestion (Oupathumpanont and Wisansakkul, 2021). It is worth noting that Sao Hai rice is not widely consumed due to its firm texture after cooking (Jadwong and Therdthai, 2018). Therefore, it was intriguing to observe that KDML 105 rice had an increased RS content, particularly when cultivated in saline soil areas. The RS content of KDML 105 from saline soil areas (1.40 ± 0.08 %) was significantly higher than that from nonsaline areas (1.20 ± 0.02 %) (Fig. 4).
Comparison of RS contents of cooked rice types: KDML 105 from saline soil, KDML 105, and Sao Hai with and without extra virgin coconut oil.
Values with different letters are significantly different at p < 0.05.
From Fig. 4, the RS content of KDML 105 rice, particularly when cultivated in saline soil, showed a significant increase compared to rice grown in normal soil when cooked with extra virgin coconut oil (1.40 ± 0.08 %). The increase in RS content could be due to the stress conditions of saline soil, which may alter the rice's starch structure, making it more resistant to digestion (Kong-ngern et al., 2011). Various types of oils demonstrated different RS contents, with higher RS values attributed to collective effects such as starch retrogradation, amylose-lipid complex formation, restricted starch swelling/gelatinization, non-starch dietary fiber, and chemical cross-linking (Kim et al., 2019; Krishnan et al., 2020). Among the oils, extra virgin coconut oil exhibited superior complex capacity, leading to increased RS content and reduced glycemic response across all rice types (Kumar et al., 2018). These findings suggest that as RS content increases, the glycemic response decreases. Similar findings were reported by Srikaeo and Sangkhiaw (2014), who observed a decreased glycemic response when rice was cooked with different cooking oils.
Comparison of RS contents of cooked rice types: KDML 105 from saline soil, and KDML 105 with and without 2 % citric acid The addition of citric acid had an impact on the physicochemical properties of starch, including the RS content (Kim et al., 2019). This could be attributed to the ability of citric acid to modify the hydroxyl groups present in starch (Bao et al., 2022). This finding aligns with previous reports indicating that the RS content of rice starches can be enhanced through the use of acids (Zhao and Lin, 2009; Hung et al., 2016). Our study revealed that adding 2 % citric acid increased the resistant starch content in KDML 105 rice significantly after being cooked in a pressure cooker. This effect was observed for rice from both saline (1.19 ± 0.09%) and nonsaline soil regions (1.03 ± 0.03 %), compared to the initial levels of 0.65 ± 0.07 % and 0.75 ± 0.09 %, respectively (Fig 5). For increasing resistant starch content, citric acid needed to add in different types of starch (Zhao and Lin, 2009).
Comparison of RS contents of cooked rice types: KDML 105 from saline soil, and KDML 105 with and without 2% citric acid.
Values with different letters are significantly different (p < 0.05).
The addition of citric acid modifies the hydroxyl groups on the starch molecule, resulting in changes in the physicochemical properties, including the RS content, as observed in the study by Kim et al. (2019). This finding aligns with the findings of Pham et al. (2016), who reported an improvement in the RS content of rice starches with the addition of acid.
During the acidification process using citric acid, citric anhydride substitutes the hydroxyl glucans present in the starch chains, creating modified structures that resist digestion by amylolytic enzymes. It is important to optimize the thermal-acid treatment of starch for RS production, as excessive treatment intensity can potentially lead to the destruction of the resistant starch structure, as highlighted by Hung et al. (2016). A study by Liu et al. (2014) demonstrated that heat and citric acid treatment of normal rice starch resulted in a significant increase in RS yield, reaching 35.62 %.
Comparison of RS contents of cooked KDML 105 with four Thai herb juices Additionally, the inclusion of four Thai herbs in the rice preparation contributed to an increase in RS content, as depicted in Fig. 6.
Comparison of RS contents of cooked KDML 105 with four Thai herb juices. Values with different letters are significantly different (p < 0.05).
From Fig. 6, the addition of all four Thai herbs to KDML 105 cooked rice resulted in increased resistant starch content. Among the herbs, Pandan leaf juice exhibited the highest RS content at 3.11 ± 0.17 % in KDML 105 rice, attributed to its high natural dietary fiber content (Zhai et al., 2019). In contrast, cooking KDML 105 rice grown in saline soil without any herb juices yielded only 0.65 % RS content. Furthermore, rice cooked with Pandan leaf extract displayed a decreased glycemic index, suggesting a higher RS content due to the presence of polyphenol compounds in the Pandan leaf juice (Yulianto, 2020). The RS contents of KDML 105 from saline soil cooked with butterfly pea flower juice, Gac fruit juice, and Noni leaf juice were 2.44, 2.19, and 1.39 %, respectively. Chusak et al. (2019) reported that butterfly pea flower extract significantly reduced rapidly digestible starch levels and increased undigested starch levels in cooked rice using an electric rice cooker. Previous studies have indicated that butterfly pea flower extract can influence the presence of resistant starch in cooked rice (Jaiswal, et al., 2020). Resistant starch, which resists digestion in the small intestine and offers various health benefits, can be enhanced by incorporating butterfly pea flower extract into rice. This has potential implications for improving glycemic control, increasing satiety, and supporting weight management. However, further research is required to explore the underlying mechanisms and investigate the specific health benefits associated with the consumption of rice prepared with butterfly pea flower extract.
The combination of pressure cooking, extra virgin coconut oil, citric acid, and local Thai herbs successfully produced RS rice. Tuaño et al. (2021) reported that RS levels of cooked milled rice ranged from 0.15 to 0.99 % (mean = 0.45 %). In our study, all rice samples had higher RS contents than normal milled rice. RS formation indicates a low glycemic index value (Srikaeo and Sangkhiaw, 2014; Kumar et al. 2018).
The findings of this study demonstrated that Sao Hai rice exhibited the highest resistant starch content among the tested rice varieties. Furthermore, the addition of extra virgin coconut oil was found to enhance the RS content more effectively compared to cooking in regular coconut oil. Interestingly, no significant difference in RS content was observed between rice grown in saline soil and rice grown in normal soil conditions. Notably, the utilization of a pressure cooker was identified as a suitable method for increasing the RS content in rice. Additionally, the application of Pandan leaf juice resulted in the highest RS content in rice, suggesting its potential as a valuable ingredient for enhancing RS levels. These findings indicate that various parameters such as rice cookers, oils, and Thai herbs can effectively increase and improve the RS content of rice. The innovative combination of a pressure cooker, extra virgin coconut oil, citric acid, and local Thai herbs successfully produced high RS rice in this study. The outcomes of this research have significant implications for both producers and consumers interested in the development of functional foods with additional health benefits, as well as individuals who prioritize dietary health concerns.
Acknowledgements This research project was financially supported by Mahasarakham University and Thailand Science Research and Innovation (TSRI), Thailand (Grant No. 6506041/2565)
Conflict of interest There are no conflicts of interest to declare.
