Mariam Fouda and Alfred Anderson*
Department of Food Science and Nutrition, College of Life Sciences, Kuwait University, Kuwait
Received: 20 September, 2016; Accepted: 26 September, 2016; Published: 28 September, 2016
Alfred Anderson, Department of Food Science and Nutrition, College of Life Sciences, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait, Tel.: +96524633097; Fax: +96522513929; E-mail:
Fouda M, Anderson A (2016) Effect of Substitution of β-Glucans on the Glycemic Response and Thermal Properties of Four Common Starches. Int J Agricultural Sci Food Technology 2(1): 009-015. DOI: 10.17352/2455-815X.000008
© 2016 Fouda M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
β-Glucan; Glycemic Index; Resistant Starch
Diabetes Mellitus (DM) is a major public health concern worldwide which can lead to a series of disabling complications and diseases. β-glucans are non-starch polysaccharides that are being used as food additives for their numerous health benefits including the ability to lower the postprandial glucose response. The aim of this study was to determine the effect of substituting β-glucans on the glycemic response and thermal properties of four commonly consumed starches. Oat β-glucans were added at concentration levels of 0%, 10%, 20%, and 30% (w/w) to each of the four starch types. Composite starches were incubated with α-amylase followed by further incubation with amyloglucosidase. Glucose released was measured using the 3, 5-dinitrosalicylic acid (DNSA) method. Incremental Area under the Curve (iAUC) was used to represent the estimated glycemic response of the β-glucan/starch composites. Thermal analysis of the starch composite starches was conducted using a differential scanning calorimeter. An overall reduction in the amount of glucose released after the addition of β-glucans was observed (p ≤ 0.05). Substituting starches with 20% and 30% β-glucans resulted in a significant reduction in the glucose release rate and thus improved the estimated glucose response of all starches. A marked increase in the enthalpy of gelatinization, ∆H, of all starches was observed. Substitution at 10% β-glucans caused a significant increase in ∆H of blank tapioca starch (p ≤ 0.05). The 20% and 30% β-glucan samples also had significantly higher ∆H than the blank and the 10% β-glucan samples. The data from this study suggest the potential use of β-glucan as a suitable food ingredient in diabetic food products.
Diabetes mellitus (DM) is a major public health concern which has become a primary global healthcare challenge. DM and uncontrolled long-term hyperglycemia are considered key causative factors in the development of irreversible secondary health complications such as diabetic retinopathy, nephropathy, cardiovascular diseases, myocardial infarctions, strokes, and many other chronic diseases that are disabling and often life threatening .
Whereas medicinal therapies are undoubtedly effective, studies have shown that nutrition and lifestyle approaches can be more effective in preventing and controlling the disease [2,3]. The important role of nutrition has been increasingly recognized with medical nutrition therapy (MNT) mainly focusing on normalizing blood glucose levels, blood pressure levels, and lipoprotein profile to improve the glycemic status and reduce associated complications . Besides carbohydrate counting and glycemic load, another very important aspect in managing blood glucose levels is the Glycemic index (GI) of foods. The GI is a ranking system that represents how quickly a carbohydrate is broken down and how fast it raises postprandial blood glucose. Foods with a lower GI are broken down slowly by the body and thus enter the blood stream at a slower rate; this reduces both the glycemic response and the resultant insulin release . On the other hand, foods with a high glycemic index because a rapid increase in blood glucose after a meal, followed by a sudden drop and potential hypoglycemia. This pattern of glycemic response has been reported to have a positive correlation with an increased incidence of type II diabetes .
Low glycemic index foods have been shown to increase the feeling of satiety and can accordingly help in appetite control and weight management . Moreover, since insulin resistance and type II diabetes are commonly associated with excess body weight, the ability to manage weight gain via increased sense of fullness will help in reducing the risk and prevalence of type II diabetes .
β-glucans are non-starch polysaccharides composed of D-glucose units joined together in chains via β-glycosidic bonds . They are naturally found in fungi, yeast, bacteria, and in cereal grains, such as barley and oats, and in fewer amounts in wheat and rye, where they are located in the cell wall of the endosperm [9,10]. Some β-glucans are soluble, such as the ones found in oats and barley, composed of linear (1,3)(1,4) β-glucans, while others are insoluble, such as those found in fungi composed of branched (1,3)(1,6) β-glucans . Similarly, β-glucans vary in other physicochemical characteristics such as viscosity, molecular weight, and fermentation, which determine their potential health benefits in the body [7,10].
Numerous health benefits of β-glucans have been discovered, leading to the increased popularity and consumption of oats and oat products , in addition to their recent use as active food components . Besides having negligible caloric value, beneficial roles of β-glucans include hypoglycemic, antihypercholesterolemic, antihypertensive, immunomodulatory, weight management, and wound healing properties, all of which are either risk factors or consequences of DM .
While several studies have been conducted on the effect of ingesting β-glucans on postprandial biomarkers of glycemic response, little has been reported on in vitro studies on the effect of β-glucan levels on α-amylase activity in relation to glucose release and glycemic index. Although β-glucans have been well-known for their ability to enhance nutritional quality of foods, the majority of research studies focus on the health effect of ingesting food sources of β-glucans, such as oats and barley, while less have been done on the changes that β-glucans impose upon the foods to which they are added.
The aim of this study, therefore, was to evaluate the effect of β-glucan substitution on the glycemic response and changes in thermal properties of four commonly consumed starch sources, namely rice, potato, corn, and tapioca.
Materials and Methods
Potato starch, corn starch, and tapioca starch, and oats β-glucan were obtained from commercial sources. Rice starch was purchased from Sigma-Aldrich (EC 232-679-6, Belgium).
Oat β-glucans were added at four concentration levels of 0%, 10%, 20%, and 30% (w/w) to each of the starch types forming four composites of 50 g individually. The β-glucans concentration levels selected were based on results from preliminary optimization studies. All samples, except for blanks, were prepared in duplicates. Additionally, one β-glucan blank sample (50g β-glucans) was prepared for comparison purposes. β-glucan/starch slurries were prepared by dispersing the specified amounts of soluble β-glucans in distilled water, then adding the starch powders and stirring mildly for 10 minutes at room temperature to avoid lump formation. The latter was then freeze dried to ensure homogeneity of the samples.
In vitro Digestibility of starches
In vitro starch digestion of lyophilized samples was performed based on the procedure of Goni et al. , with slight modifications. Initially, two enzyme solutions for digestion were prepared as follows: (1) α-amylase solution was prepared by dispersing 25 mg α-amylase from Aspergillus oryzae (EC 232-588-1, Sigma-Aldrich) to 10 ml of 0.2 M phosphate buffer (pH 6.9), and (2) amyloglucosidase solution was prepared by suspending 3 mg of amyloglucosidase from Aspergillus niger (70U/mg, EC 232-877-2, Sigma-Aldrich) in 50 ml 0.5 M sodium acetate buffer (pH 4.5). These enzyme solutions were freshly prepared for each digestion analysis.
Samples composing of 250 mg each of the freeze-dried β-glucan/starch composite were incubated with 10 ml of the freshly prepared α-amylase solution (pH 6.9) in a shaking water bath at 37 ºC for 2 hrs. Aliquots (1 ml) were taken at baseline and every 30 minutes for 2 hours. Test tubes were shaken at each interval using a vortex, and aliquots were pipetted into fresh test tubes, and immediately immersed in a boiling water bath for 5 minutes to denature the enzyme. After the 120 minutes incubation period, aliquots were cooled to room temperature and then incubated again with 1 ml amyloglucosidase solution (pH 4.5) in a shaking water bath at 55 ºC for 45 minutes to completely hydrolyze the digested starch. At the end of the incubation period, samples were placed in a boiling water bath for another 5 minutes to denature the amyloglucosidase enzyme. Finally, samples were placed in two ml microfuge tubes and centrifuged at 13000 x g for 1 minute using a microfuge to precipitate the enzymes.
Determination of glucose content
The glucose content in the supernatant was measured colorimetrically using the 3,5-dinitrosalicylic acid (DNSA) method established by Miller . The principle of this method is based on a redox reaction that takes place under alkaline conditions, upon heating, where reducing sugars (such as glucose, fructose, and maltose) reduce 3,5-dinitrosalicylic acid into 3-amino-5-nitrosalicylic acid which has a dark red color and is absorbed at 540 nm. The intensity of the color is directly proportional to the amount of reducing sugars present in an unknown sample . This reaction is reported to be dependent on the type of reducing sugar, since different reducing sugars were reported to result in different color intensities .
For each composite sample, 0.25 ml of sample was pipetted into a test tube containing 4.75 ml distilled water and two ml DNSA reagent. The test tubes were then covered by aluminum foil and incubated in a boiling water bath for five minutes, until color changed to dark red, then immediately immersed in an iced water bath to rapidly cool down the mixture and stop the reaction. Another 13 ml of distilled water was added, diluting the solution to 20 ml. After cooling to room temperature, the absorbance of the supernatant was measured at 540nm using an Agilent UV-Vis-NIR (Cary 5000, USA) Spectrophotometer. A blank solution, containing no sample, was also prepared for the instrument’s calibration purpose. The concentration of reducing sugars was finally determined using a standard curve obtained from a standard glucose solution.
Thermal analysis of the starch composite samples was conducted using a differential scanning calorimeter (NETZSCH DSC 204F1 Phoenix®) calibrated using indium and an empty sealed aluminum crucible as a reference. Duplicates of the β-glucan/starch samples, weighing 5 mg each, were placed in the aluminum crucibles, and sealed before heating in the DSC. Scans were performed from 25 to 300 °C at a controlled constant rate of 10 °C/min. Onset temperature (To), peak temperature (Tp), conclusion temperature (Tc) and enthalpy of gelatinization (ΔH), expressed as J/g dry starch, were automatically generated by the equipment software. The gelatinization temperature range (∆T) was calculated as Tc - To.
IBM SPSS Statistics version 22.0 was used to analyze the data via Repeated Measures Analysis of Variances (ANOVA), in which the effect of different treatments on glycemic response and thermal properties was studied between and within starch types. Tukey’s posthoc test was performed when treatment effects showed significance. An alpha level of ≤ 0.05 was considered statistically significant. Data was expressed as the mean value of duplicate measurements ± standard deviation (SD).
Treatment with different concentrations of β-glucans showed a significant effect on the iAUC after 120 minutes of in-vitro digestion within all starch types (p ≤ 0.05), as shown in Figure 1. The addition of β-glucans resulted in an overall lower glucose release in all of the starch types. Multiple comparison of the effect of substituting different β-glucan concentrations on the iAUC showed that, collectively, starch blanks and samples treated with 10% β-glucans were not significantly different (p ≤ 0.05). Similarly 10%, 20%, and 30% treated samples were not significantly different. However, the effect of treatment was most evident and showed statistical significance between the blank starch samples and the samples substituted with 20% β-glucans (p ≤ 0.05), as well as between the blank and the 30% β-glucan substituted starches. This indicates that substituting starches with 20% and 30% β-glucans resulted in a significant reduction in the glucose release rate and thus improved the estimated glucose response of all starches.