Notes
Suppressive effect of doughnuts baked with barley bran obtained by a novel milling method on postprandial blood glucose and insulin levels
2022 Volume 28 Issue 6 Pages 513-519
Details
2022 Volume 28 Issue 6 Pages 513-519
In this study, we developed a functional sweet that has a suppressive effect on plasma glucose elevation while taking the taste characteristics of barley bran obtained through a novel milling method, as well as preference, into account. The experiment was conducted with 10 healthy female subjects using a within-subject, cross-over study design. Baked doughnuts were made using barley bran (test sweet) and wheat flour (control sweet). Compared with the control, once baked doughnuts with barley bran were consumed, there was a reduction in peak plasma glucose elevation and insulin secretion (p < 0.05), as well as a significantly lower area under the curve (p < 0.05). According to sensory evaluation results, no significant difference in subjects' preference for baked doughnuts was observed across all items. The findings imply that highly appealing sweets may aid in the prevention of lifestyle-related diseases such as type 2 diabetes.
Presently, barley bran is discarded as food waste and is frequently used as animal feed. Nevertheless, the recent development of a new method of milling barley results in less astringent and bitter-tasting barley bran. Barley bran is a high-fiber source that is like wheat bran and unpolished rice, contains a lot of polyphenols and β-glucans, a type of soluble dietary fiber. The β-glucans found in barley bran influence gastrointestinal hormones and have been shown to suppress appetite, increase perceived satiety, and decrease perceived hunger (Aoe, 2015). Many studies have also discovered a link between cereal-derived dietary fiber consumption and the onset and mortality of cardiovascular disease (Ciecierska et al., 2019, Threapleton et al., 2013) and diabetes mellitus onset (Ye et al., 2012). Consequently, β-glucans and other dietary fibers have recently attracted the interest of researchers because of their potential role in the prevention of problematic lifestyle-related diseases.
In a previous study (Tanabe et al., 2018), we compared the consumption of shortbread made with barley bran obtained through a novel milling method, which contained the equivalent of 50 g available carbohydrates, and with the consumption of shortbread made with wheat flour. Shortbread containing barley bran resulted in a significant reduction in plasma glucose elevation and insulin secretion at 30 min after ingestion, and as well as a significantly lower area under the curve (AUC). Nonetheless, the color, smell, texture, and taste of barley bran shortbread differed from that of wheat flour shortbread, and the inferior palatability of barley bran was identified as a topic for future research. In addition, the suppressive effects of shortbread containing barley bran on postprandial blood glucose and insulin secretion may be due to inhibition of digestion rather than a delay in digestion. It has already been shown that digestible carbohydrates that inhibit digestion are metabolized into carbon dioxide, hydrogen gas, and short-chain fatty acids by fermentation by intestinal bacteria after reaching the large intestine (Holt et al., 1996, Oku et al., 2009). Our previous study confirmed that sucrose ingested simultaneously with Morus alba leaf extract, which strongly inhibits α-glucosidases, does not elevate postprandial blood glucose and insulin levels in humans and rats, but it does increase the excretion of hydrogen, a specific product of gut microbiota (Nakamura et al., 2009, Oku et al., 2022). Therefore, it is necessary to measure breath hydrogen gas to clarify whether the inhibition of digestion is associated with the inhibitory effect of barley bran-containing sweets on elevated blood glucose levels.
This study aimed to develop functional sweets with an emphasis on palatability for maintaining and improving human health. As in previous studies, this study not only investigated the suppressive effect of barley bran on plasma glucose elevation and insulin secretion, but also the dynamics of breath hydrogen gas excretion. This study also attempted to develop sweets (baked doughnuts) that make the best use of the eating quality characteristics of barley bran obtained through a novel milling method, and to investigate the sensory evaluation of baked doughnuts.
Test materials This study's test material was barley bran obtained through a novel milling method (Ito Seibakujo, Isahaya City, Nagasaki, Japan). This milling method is used to recover barley bran produced in the barley pounding process, in which barley is pounded in stages using multiple pounding machines. The barley bran used in present study was recovered at a yield ratio of between 75% and 85% by weight. This milling method separates and removes the barley hulls, which have a strong astringent and bitter flavor. Table 1 shows the nutritional value of this barley bran.
| (g/100 g) | |
|---|---|
| Water | 9.2 |
| Protein | 13.6 |
| Lipid | 5.7 |
| Ash | 3.5 |
| Available carbohydrate | 51.3 |
| Dietary fiber | 16.7 |
| (β-glucan) | (3.9) |
Sample preparation with barley bran Barley bran was sifted through a 500-µm sieve to remove any lumps or residual hulls before use. To prepare baked doughnuts, eggs and granulated sugar were mixed well, then cow's milk was added and mixed. Baking powder and either soft wheat flour or barley bran were then added and mixed thoroughly. Butter, rapeseed oil and honey were then added, and the mixture was kneaded thoroughly. A thin layer of butter was spread onto doughnut molds (silicon cake molds, 6 per tray, Living & Health K.K., Shiga, Japan), then doughnut dough was poured into the molds and baked in an oven at 180 °C for 15 min. After cooling, the doughnuts were removed from the molds, covered with a syrup prepared with honey, powdered sugar and water, and left in a refrigerator to cool for about 1 h. Doughnuts were then wrapped in wax paper, and stored in a refrigerator. The ingredients and nutritional composition of the baked doughnuts are shown in Table 2. Control samples were made with 100% wheat flour, whereas the test samples were made with barley bran instead of wheat flour. Assuming these sweets would be consumed as a snack between meals, each was designed to contain approximately 200 kcal, or approximately 10% of a typical adult's daily energy intake. Sucrose was also added to ensure that all of the sweets had the same amount of available carbohydrates.
Ethical approval All experiments were compliant with the ethics code of the World Medical Association (Declaration of Helsinki, October 2013). The study protocol involving humans was approved by the Ethical Committee Concerning Research in Humans of Nagoya Women's University (approval no. 29-3). All subjects were asked to provide written informed consent to participate in the study. All experiments were conducted in the laboratory of Public Health Nutrition at Nagoya Women's University.
Subject attributes Ten healthy female university students (mean age: 20.7 ± 0.5 years; height: 157.3 ± 4.6 cm, body weight: 52.2 ± 6.0 kg; body mass index (BMI): 21.1 ± 2.3 kg/m2) with no gastrointestinal diseases, hyperglycemia, or other diseases served as subjects. The subjects were given a thorough explanation of the study's objectives, details of experiments, and test materials, and only subjects who agreed to participate in the study were included.
Experimental protocol and subject management The experimental protocol was based on a within-subject, cross-over design (Tanabe et al., 2018, Tanabe et al., 2020, Nakamura et al., 2020, Tanabe et al., 2021), with subjects consuming control and test foods repeatedly. The subjects consumed test materials for at least 1 week apart from experiments. The subjects were instructed to withdraw promptly from experiments if they became unable to continue participating due to health issues. The subjects were instructed to continue their normal lifestyle habits the day before ingestion of test materials, but to finish their evening meal by 9 p.m. The subjects were only allowed to drink water and tea after 9 p.m. Before ingesting test materials, the subjects' blood pressure and pulse rate were measured in a resting state, and they were asked about their physical condition, with the results recorded on a record sheet. After a health check, blood was drawn while the subject was fasting. Breath hydrogen gas was measured after thoroughly rinsing the mouth, and end-expiratory gas was collected in an expiration sampling bag. The subjects ingested the prepared test sample within 6 min of completing the preceding procedure. The weights of the test samples were approximately 62 g for baked doughnuts. Following the ingestion of each test sample, subjects rinsed their mouth with 100 mL of water to ensure that no residual test material remained in the mouth.
Plasma glucose and insulin concentration measurement The end of each subject's finger was pricked with a Medisafe Finetouch lancing device (Terumo, Tokyo, Japan) dedicated to each subject, and approximately 100 µL of blood was collected in a heparinized hematocrit capillary tube (Fischer Scientific, USA). Blood was collected seven times in total, with the blood sample collected before test sample ingestion serving as the 0 min sample, followed by blood collection at 30, 60, 90, 120, 150, and 180 min after test material ingestion. Following the collection of each blood sample, the hematocrit capillary tube was centrifuged for 5 min at 14 375 ×g and 23 °C in an inverter hematocrit centrifuge (Kubota Corporation, Tokyo), and plasma obtained from centrifugation was tested for plasma glucose and insulin levels. Glucose oxidase was used to measure plasma glucose using the Trinder method (Trinder, 1969). The concentration of insulin in plasma was determined using an enzyme immunoassay method and an ELISA assay kit containing guinea pig antibodies (Morinaga Institute of Biological Science, Inc., Kanagawa, Japan).
End-expiratory gas collection method Subjects were placed at rest in a seated position before collecting end-expiratory gas, and they were restricted from exercise that induces hyperventilation, prohibited from sleep, and prohibited from ingestion of foods other than the designated test foods until the collection of end-expiratory gas was complete. Furthermore, to preclude the normal microbiota of the oral cavity from interfering with the results, the oral cavities were washed with water before the collection of end-expiratory gas. The 0 min sample point was taken from end-expiratory gas collected once before test food ingestion. The end-expiratory gas was then collected at 1 h intervals until 8 h after the test food ingestion, for a total of nine end-expiratory gas collections. For each sampling, 750 mL of end-expiratory gas was collected using a mouthpiece and a sampling bag fitted with a T-piece and non-return valve (Quintron Instruments, USA). The hydrogen and methane concentrations in the end-expiratory gas were determined by gas chromatography (BGA-1000D H2/CH4 Breath Gas Analyzer (Laboratory for Expiration Biochemistry Nourishment Metabolism Co., Ltd., Nara, Japan)). The measurements were carried out in an environment that was kept at a temperature of around 25 °C and a humidity of approximately 70%. For calibration, a standard gas containing 100 ppm hydrogen and 49.7 ppm methane (Taiyo Nippon Sanso Corporation, Tokyo) was used.
Sensory evaluation of baked doughnuts prepared with the barley bran The sensory evaluation was performed by 10 panelists who assessed the baked doughnuts in a randomized order and assigned a numerical value between -4 (extremely unacceptable) to +4 (extremely acceptable) for the following criteria: taste, smell, appearance, texture, mouthfeel, and general impression. Between each sample, drinking water (50 mL) was provided to cleanse the panelists' palates.
Data Analysis and Statistical Analysis The mean and standard deviation for plasma glucose, insulin, and breath hydrogen gas was calculated for each sample at each collection time point after test and control food ingestion. After the normal distribution test, the data were analyzed by repeated measures two-way ANOVA, a paired t-test was used to compare between the test and control food items. Following the normality test, a paired t-test was run on the AUC results for plasma glucose, insulin level, and breath hydrogen gas concentration. The sensory evaluation data were subjected to a Wilcoxon signed-rank test. All statistical analyses were carried out using SPSS ver. 24 (SPSS Inc., Japan) with a < 5% significance probability.
Subject Participation No participants dropped out of the study and experienced side effects. Their health status throughout the entire study period was good.
Plasma glucose elevation and insulin secretion Fig. 1 shows the change in plasma glucose and insulin levels over time following the consumption of baked doughnuts. Plasma glucose peaked 30 min after ingesting the control food (120.9 ± 14.0 mg/dL) and subsequently fell to a fasting level 180 min later (73.3 ± 13.3 mg/dL). Plasma glucose peaked 30 min after ingesting the test food (101.8 ± 9.5 mg/dL) and subsequently fell to reach a fasting level 180 min later (77.8 ± 7.6 mg/dL). Baked doughnuts with barley bran suppressed plasma glucose elevation, with a significant reduction observed 30 min after ingestion compared with the control food at the time of peak plasma glucose elevation (p < 0.05). Plasma glucose levels were lower at 60 and 90 min after ingesting the test food compared to the control food, but there was no statistically significant difference.
The suppressive effect of a baked doughnut containing barley bran obtained through a novel milling method on postprandial plasma glucose and insulin levels
The values are mean ± S.D. (n = 10). *: There were significant differences between test food and control food at each time point with p < 0.05 by paired t-test.
Insulin levels behaved similarly to plasma glucose levels. Insulin peaked 30 min after ingestion of both the control food (2.2 ± 0.5 ng/mL) and the test food (1.4 ± 0.2 ng/mL), and then returned to fasting levels 180 min later (control food: 0.5 ± 0.2 ng/mL, test food: 0.2 ± 0.1 ng/mL). Insulin secretion was suppressed after test food ingestion compared to control food, with significant reductions observed in peak insulin elevation at 30 and 60 min after ingestion (control food: 1.4 ± 0.4 ng/mL, test food: 0.1 ± 0.3 ng/mL) (p < 0.05). No significant difference was subsequently observed, although the insulin concentration after test food ingestion never exceeded the insulin concentration after control food ingestion, implying that replacing wheat flour with barley bran suppressed insulin secretion.
Table 3 shows the AUC for plasma glucose elevation and insulin concentrations for baked doughnuts. When baked doughnuts with barley bran were consumed, there was a reduction in peak plasma glucose elevation and insulin secretion, as well as significantly lower AUC results compared with the ingestion of control food (p < 0.05).
| Plasma glucose (h · mg/dL) | Plasma insulin (h · ng/mL) | Breath hydrogen (h · ppm) | |
|---|---|---|---|
| Wheat flour | 2 469 ± 935 | 118 ± 64 | 432 ± 339 |
| Barley bran | 1 396 ± 921* | 66 ± 25* | 1 334 ± 759* |
The values are mean ± S.D. (n = 10).
Breath hydrogen gas excretion Fig. 2 shows the change in breath hydrogen gas excretion after ingestion of baked doughnuts over time. Breath hydrogen gas excretion after control food ingestion was unchanged from a fasted state up to 3 h after ingestion (4.0 ± 2.4 ppm), before increasing at 4 h after ingestion (8.3 ± 5.7 ppm), and then gradually decreasing after 5 h (6.3 ± 3.8 ppm) and thereafter. Breath hydrogen gas excretion after test food ingestion exhibited similar behavior up to 3 h after ingestion (4.7 ± 2.1 ppm), but continued to increase up to 5 h after ingestion (13.7 ± 8.6 ppm) before gradually decreasing. The amount of hydrogen gas excreted in the breath after ingestion was significantly higher at 7 h and 8 h after ingestion of test food compared with control food (7 h after control food: 5.4 ± 7.4 ppm, test food: 12.8 ± 7.4 ppm, 8 h after control food: 5.3 ± 2.6 ppm, test food: 10.4 ± 6.8 ppm) (p < 0.05).
Change of breath hydrogen gas excretion on a baked doughnut containing barley bran obtained through a novel milling method
The values are mean ± S.D. (n = 10). *: There were significant differences between test food and control food at each time point, with p < 0.05 by paired t-test.
Table 3 shows the AUC for breath hydrogen gas excretion. There was an increase in breath hydrogen gas and a significant increment in AUC after test food ingestion when compared with control food ingestion (p < 0.05).
Sensory evaluation Fig. 3 shows the results of a taste test for baked doughnuts. Even though the cont rol food outperformed the test food in every survey item, none of the differences were statistically significant.
The sensory evaluation of a baked doughnut containing barley bran obtained through a novel milling method using the bipolar rating nine-point method
The values are mean ± S.D. (n = 10).
Barley bran-containing baked doughnuts were found to suppress both plasma glucose elevation and insulin secretion, as well as the associated AUCs. Consumption of baked doughnuts containing barley bran is thus thought to not only to slow, but also to inhibit digestion and absorption. The increase in hydrogen gas in the breath also suggests that food constituents that escaped digestion and absorption are fermented by intestinal bacteria in the large intestine (Oku et al., 2022). In other words, baked doughnuts containing barley bran are thought to resist digestion and absorption in the small intestine, with the undigested matter being transported to the large intestine. Soluble dietary fiber has been shown to resist digestion by digestive enzymes and to reach the large intestine where much of the dietary fiber (particularly soluble dietary fiber) is fermented by intestinal bacteria (Oku and Nakamura, 2014). Hughes et al. (2008) reported that β-glucans, a type of dietary fiber found in barley bran, have a slight prebiotic effect. In an in vitro study using pig feces, Bai et al. (2021) reported that β-glucan was not fermented even after 6 h of reaction. Using the same experimental method as the present study, we previously demonstrated that the maximum level of breath hydrogen gas excretion was about 10 ppm, even after ingestion of 5 g of dietary fiber (Oku and Nakamura 2014). The baked doughnuts used in this study contained 2.9 g of dietary fiber and 0.7 g of β-glucan. Therefore, we consider that the increase in breath hydrogen gas due to the baked doughnut consumption is unlikely to be influenced by fermentation of dietary fiber. It is more reasonable to assume that the increase in breath hydrogen gas observed after ingestion of barley bran-containing baked doughnuts is due to fermentation of undigested material that was prevented from being digested and absorbed in the small intestine. Polyphenols have been reported to inhibit α-glucosidase (Kim et al., 2016). The barley bran used in this study also has about 50% of the polyphenol content of typical barley bran because it is reduced during the pounding process. The polyphenols in barley bran are known to be mainly proanthocyanidins, and one serving of the baked doughnut containing barley bran contained 82.3 mg of polyphenols. A suppressive effect of postprandial blood glucose elevation in healthy subjects by ingestion of proanthocyanidins has been observed at doses of 100 mg or more (Yang and Chan, 2017). Therefore, polyphenols in barley bran may also be involved the suppression of postprandial blood glucose elevation, but the effect may not be pronounced.
β-glucans increase solution viscosity and this viscosity traps undigested material in the gastrointestinal tract. The ability to increase viscosity has a suppressive effect on plasma glucose elevation (Wang and Ellis, 2014). The physical properties of β-glucans, such as solubility and soluble viscosity, are affected by their molecular weight. The molecular weight of β-glucans is influenced by plant species, growth conditions (Andersson and Börjesdotter, 2011; Ajithkumar et al., 2005), and other factors, but it is also reduced by baking (Beer et al., 1997; Kerckhoffs et al., 2003), the Maillard reaction (Wong et al., 2011) with proteins, and reactions with other food constituents (Lazaridou et al., 2003; Tosh et al., 2004). We did not measure the molecular weights of β-glucans present in the barley bran used in this study; thus, further research is required to determine whether molecular weight is an influencing factor, and this should be considered when the number of food ingredients or processing steps is increased.
Future uses of barley bran in sweets will necessitate modifications based on the color and smell of barley bran. When compared with the control foods, the test foods had a darker baked appearance, which survey respondents may have associated with overcooking. Barley bran appears as a light brown flour, but it darkens when water is added and during the baking process, and this color is easily perceived by people as browning caused by baking, but it may also be associated with overcooking. Consequently, baking temperatures and time periods must be adjusted to produce a color that suggests a pleasant taste.
The findings of this study show that when sweets are prepared with an emphasis on palatability, it is possible to achieve a suppressive effect on plasma glucose elevation and insulin secretion with the barley bran obtained by a novel milling method. The ultimate goal of this study is to use waste barley bran to maintain and improve human health. We are interested in understanding the mechanism that prevents plasma glucose elevation and to see how barley bran-containing products can benefit human health through regular consumption.
Acknowledgements The authors thank Ito Seibakujo (Nagasaki, Japan) for providing barley bran obtained through a novel milling method.
Conflict of interest There are no conflicts of interest to declare.
