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Article

An Examination of the Possibility of Lowering the Glycemic Index of Oat and Barley Flakes by Minimal Processing

Departments of Applied Nutrition and Food Chemistry, and ^* Food Engineering, Chemical Centre, University of Lund, S-221 00 Lund, Sweden

¹To whom correspondence should be addressed.

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Differences in glycemic responses to various starchy foods are related to differences in the rate of starch digestion and absorption. In this study, the importance of the degree of gelatinization and the product thickness for postprandial glycemic and insulinemic responses to rolled oats and barley were studied in healthy subjects (5 men and 5 women). Thick (1.0 mm) rolled oats were made from raw or preheated (roasted or steamed) kernels. In addition, thin (0.5 mm) rolled oats were made from roasted or roasted and steamed (processed under conditions simulating commercial production) oat kernels. Finally, steamed rolled barley kernels (0.5 or 1.0 mm) were prepared. All thin flakes elicited high glucose and insulin responses [glycemic index (GI), 88–118; insulinemic index (II), 84–102], not significantly different from white wheat bread (P > 0.05). In contrast, all varieties of thick oat flakes gave significantly lower metabolic responses (GI, 70–78; II, 58–77) than the reference bread (P < 0.05). Thick barley flakes, however, gave high glucose and insulin responses (GI, 94; II, 84), probably because the botanical structure underwent more destruction than the corresponding oat flakes. We conclude that minimal processing of oat and barley flakes had a relatively minor effect on GI features compared with the more extensive commercial processing. One exception was thick oat flakes, which in contrast to the corresponding barley flakes, had a low GI.

KEY WORDS: • glycemic index • oats • barley • humans

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
The differing effects of various carbohydrate foods in raising blood glucose and insulin postprandially have long been recognized (Jenkins et al. 1983 , Wolever et al. 1994 ). The selection of foods that induce low glycemic responses after a meal has been shown to improve overall blood glucose control in patients with diabetes mellitus (Brand et al. 1991 , Collier et al. 1988 ) and to reduce total serum cholesterol and triglyceride levels in hyperlipidemic subjects (Jenkins et al. 1985 ) as well as total serum cholesterol in healthy subjects (Jenkins et al. 1987 ). In a recent study (two consecutive 24-d periods) in type 2 diabetic patients, it was reported that a low glycemic index (GI)² diet caused important reductions in plasminogen activator inhibitor, leading to a normalization of the fibrinolytic activity in these patients (Järvi et al. 1999 ). The current international recommendations from FAO/WHO (1997) also suggest increasing the consumption of foods with low glycemic indices.

The postprandial responses to starchy foods may be modified by a variety of factors, including the processing conditions (Björck 1996 ). Thus, processes that gelatinize the starch granules or disrupt the food structure increase the glycemic and insulinemic responses.

A disruption of the structure present in native starch by gelatinization (i.e., swelling of the granules in the presence of heat and water) increases its susceptibility to enzymatic degradation in vitro (Snow and O’Dea, 1981 ) and its availability for digestion and absorption in the small intestine. A more prominent rise in blood glucose and insulin has thus been reported with consumption of cooked as opposed to raw starch. Consequently, glucose and insulin responses in healthy subjects were found to be significantly higher after ingestion of cooked compared with raw starch from wheat (Berthold and Mohamed 1976 ), corn (Collings et al. 1981 ) or potato (Vaaler et al. 1984 ).

For cereals, processing by heat is the most common method for manufacturing consumer products. The starch can thus be expected to be more or less completely gelatinized. An exception is flaking (e.g., steaming and rolling of cereal kernels), which usually results in incomplete gelatinization (Holm et al. 1988b ). However, even at low levels of gelatinization, the rate of amylolysis increases greatly. Moreover, oat flakes, with a comparatively low level of gelatinization [37% measured by differential scanning calorimetry (DSC)], induced a high increment in glucose and insulin after a meal, equal to that after consumption of white bread (Granfeldt et al. 1995 ).

The structure of the food is also a factor in the postprandial responses to starchy foods. Boiled intact cereal grains such as rye, oats, wheat and barley cause low glucose and insulin responses (Granfeldt et al. 1995 , Jenkins et al. 1988). However, when the raw materials were ground into flours before boiling, the postprandial glucose and insulin responses increased significantly compared with boiled intact seeds (Granfeldt et al. 1994 , Liljeberg et al.1992 , O’Dea et al. 1980 , Tovar et al. 1992 ). Even a less extreme disruption of the botanical tissue such as that occurring during rolling of steamed cereal grains is enough to increase blood glucose and insulin responses. We showed previously that rolling of steamed oat grains increased the accessibility of the starch for digestion and absorption compared with boiled intact oat kernels (Granfeldt et al. 1995 ). The glucose and insulin responses increased to values similar to those after consumption of white bread. These high metabolic responses agree with results of other studies with rolled cereals, such as oats (Wolever 1990 ), rye (Hagander et al. 1987 ) and wheat (Fairchild et al, 1996 ).

The nutritional properties of starch in the breakfast meal are of special importance in that the metabolic response after breakfast may influence the glucose and insulin responses also after a subsequent lunch meal. Thus, a slow-release, starchy breakfast meal was followed by a significantly flatter blood glucose response to a standard lunch, compared with a "rapid" breakfast (Jenkins et al. 1982 , Liljeberg et al. 1999 ). Golay et al. (1992) showed that merely switching breakfast from standard cereals to slow-release starch cereals improved blood glucose control all day in diabetic patients. During the low glycemic index breakfast period (raw rolled wheat and white bean flakes), both insulin requirement and daily blood glucose were lower than during the high glycemic index period (corn flakes). Most conventional breakfast cereals such as corn flakes, puffed rice, rice bubbles, shredded wheat, muesli and porridge have high GI, ranging from 80 to 126 (Foster-Powell and Brand Miller 1995 ). To date, few alternative breakfast cereals with low GI exist.

The purpose of this work was to evaluate the nutritional potential of modifying the process conditions used when preparing flaked cereals. Thus, processing conditions were selected to minimize the degree of gelatinization as well as the extent of disruption of the botanical structure. Postprandial glucose and insulin responses were measured in healthy subjects given oat or barley flakes varying in thickness and/or in degree of gelatinization.

	SUBJECTS AND METHODS

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Material

    Oat and barley flakes. Dehulled oats or pearled barley grains were received from, and processed in a small Swedish mill (Vårgårda kvarn, Vårgårda). Before rolling, the grains were treated in one of the following ways: 1) soaking in a small quantity of cold water for 1 h (final content of water in the grains was 13%). 2) Roasting and soaking: the roasting took place in a continuous double-shelled cylindrical drum, heated with steam (0.11 MN/m², 120°C). The processing time was 20–25 min and the final temperature of the grains was 95°C. After roasting the grains were soaked in cold water for 15 h (final content of water in the grains was 17%). 3) Roasting and steaming: the roasting procedure was the same as described above. Thereafter, the grains were steamed (0.11 MN/m², 120°C) for 17–18 min. The final temperature of the grains was 104°C. The steamed grains were still warm when rolled. 4) Steaming, as described above. After the treatment (soaking and/or heat treatment by roasting and/or steaming), the grains were rolled to a product thickness of 0.5 or 1.0 mm, respectively. The processing conditions used are presented in Table 1 .

Methods

    Chemical analysis. Rolled cereals were dried under vacuum (40°C) and milled to a particle size of <0.8 mm (Cyclotec, Tecator, Sweden) before analysis. The rolled cereals were analyzed for available starch (Holm et al. 1986 ); protein content was measured according to the Kjeldahl method, with a conversion factor of 5.83. Fat content was analyzed according to the Schmid-Bondzynski-Ratzlaff method, employing extraction with ether after hydrolyzation in HCl (Croon and Fuchs, 1980 ).

The degree of gelatinization was measured using DSC. Measurements of the degree of gelatinization were performed by comparing the area for the gelatinization peak for the raw sample (degree of gelatinization equal to zero) with that for heat-treated samples (Holm et al. 1988a ). The instrument used was a Perkin-Elmer DSC 2C (Perkin-Elmer, Eden Prairie, MN). Samples were mixed with water to give a dry matter:water ratio of 1:3; 10 mg of this mixture was transferred to preweighed coated aluminum pans, which were sealed and reweighed. The DSC scanning rate was 10°C/min, and the samples were heated in the temperature range 290–400 K with an empty pan as a reference. The dry matter content was determined for each pan after the scan by puncturing the pan and drying it at 105°C for 16 h. The transition enthalpy (H), gelatinization onset temperature (T_o), peak temperature (T_p) and conclusion temperature (T_c) were evaluated as described elsewhere (Eliasson 1986 ). Each value presented is the mean and SD of three measurements.

Evaluation of postprandial blood glucose responses

    Test subjects and methodology. Healthy volunteers (n = 10; 5 men, 5 women) with a mean ± SD age of 38 ± 8 y participated in the study. Their mean body mass indices were normal (21 ± 2 kg/m²). Eight different test meals were given at breakfast after an overnight fast on separate mornings 1 wk apart. The meals were given between 0800 and 0830 h, and were eaten over 15 min. Zero time was set as the time eating began. Finger-prick blood samples were withdrawn using mini-lancets (Clean Chemical Sweden AB, Borlänge Sweden) shortly before and at 30, 45, 70, 95, 120 and 180 min after the test meals. Capillary blood was collected (50 µL) and analyzed for glucose with glucose oxidase/peroxidase reagent. Serum insulin was determined in the blood samples (500 µL) taken shortly before and at 30, 45, 95 and 120 min, employing an enzyme-linked immunoassay kit (Boehringer Mannheim, Germany). The GI (Jenkins et al. 1981 ) was calculated from the 1.5- and 2-h incremental glucose area using white wheat bread as a reference (GI = 100). Glucose values below baseline were considered equivalent to zero. The insulinemic index (II) was calculated in a similar way from the 1.5- and 2-h insulin response curves.

    Test meals. The test meals consisted essentially of either one of the flake products (82.6–94.8 g), or of the reference bread (116.4 g). All meals providing 50 g of starch, 12.0 g of protein and 6.8 g of fat. The meals with barley flakes and white wheat bread were adjusted to contain similar amounts of protein and fat as the oat flakes, with cheese (10.9 and 15.4 g with the barley flakes and the white wheat bread, respectively) and butter (2.9 and 3.2 g with the barley flakes and the white wheat bread, respectively). In addition, 200 mL milk and 150 mL coffee or tea were included in each meal. The energy contents were similar at 1621 kJ.

    Statistical evaluation. Results are expressed as means ± SEM. Significant differences in glucose and insulin responses (P < 0.05) were evaluated by the Wilcoxon test for paired observations using the SPSS/PC + (SPSS, Chicago, IL) program, with each person as his or her own control.

	RESULTS

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Differential scanning calorimetry.

The degree of gelatinization in the heat-treated and rolled grains is shown in Table 2 , together with other information obtained from the DSC thermograms. All flakes were incompletely gelatinized, with a degree of gelatinization ranging from 16 to 27%. The highest values were noted for starch in the products processed under conditions simulating commercial processing. Roasted and steamed rolled oats had a degree of gelatinization of 24%, whereas less heat treatment i.e., roasting or steaming, resulted in a lower degree of gelatinization (16%). The barley flakes processed under conditions simulating commercial processing had a higher degree of gelatinization (27%).

View this table: [in this window] [in a new window]   Table 2. Onset (To) peak (Tp) and conclusion (Tc) temperature, gelatinization enthalpy (H) and degree of gelatinization (%) in the processing of oat and barley flakes1

Both thin oat flakes processed under conditions applied commercially and those mildly heat-treated (roasted) elicited high glucose responses (Fig. 1A and Table 3 ). No significant differences were seen at any time point between these products and the reference bread (0–120 min). All varieties of thick oat flakes, however, elicited lower peak values (30-min values) (Table 4 ) than the bread and the oat flakes processed under conditions simulating commercial processing. At 180 min, all oat products gave higher responses than the white reference bread.

View larger version (44K): [in this window] [in a new window]   Figure 1. Incremental blood glucose (A) and serum insulin (B) concentrations in healthy humans after ingestion of breakfast meals with different oat flakes. Each point is the mean, n = 10. Means with different letters are significantly different (P < 0.05). Error terms and significant differences are given in Table 3 .

The GI was significantly lower for the thick rolled oats compared with bread and the rolled oat product processed under conditions used commercially (Table 4) . The steamed thick and roasted thick oat flakes also gave a lower GI than roasted thin oat flakes. The II were associated with the GI, with the exception of the II of the thick products, which did not deviate from that of the oat flakes processed under conditions simulating commercial production, and in the case of raw thick rolled oats, did not differ from bread. No differences in GI and II were found between the thin rolled oats and the bread.

The mean incremental glucose and insulin concentrations after consumption of both thin and thick barley flakes were high (Fig. 2 and Table 5 ). No significant differences in glucose or insulin responses were seen at any time point between the rolled barley products and the reference bread, with three exceptions, i.e., the glucose response to the thick barley flakes was lower than for bread at 70 and higher at 180 min for both thin and thick flakes. The insulin response to both barley flakes was lower than for bread at 95 min.

View larger version (31K): [in this window] [in a new window]   Figure 2. Incremental blood glucose (A) and serum insulin (B) concentrations in healthy humans after ingestion of breakfast meals with different barley flakes. Each point is the mean, n = 10. Means with different letters are significantly different (P < 0.05). Error terms and significant differences are given in Table 5 .

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

Flakes processed under conditions used commercially.

Usually, the starch in cereal products is completely gelatinized. However, starch in cereal flakes provides an exception. Holm et al. (1988a) showed that the lower and upper limits of processing conditions normally used commercially for rolling of cereals result in a degree of gelatinization ranging from 22 to 65%, also measured by DSC. The cereal kernels are steamed before rolling to make them softer and thus easier to roll. In the case of oats, with a relatively high amount of lipids, the heat treatment is also important for inactivating lipases. The heat treatment of oat grains is therefore more powerful; the grains are steamed twice or roasted and steamed. In this study, rolled oats processed under conditions simulating commercial production and barley were included. Before being rolled to thin (0.5 mm) flakes, the oat grains were steamed and roasted and the barley grains were steamed. These heat treatments resulted in a low degree of gelatinization, i.e., 24 and 27%, respectively, for oat and barley. The postprandial glucose and insulin responses to these thin flakes were high; the GI was 118 and 91, respectively, for oat and barley, and the II between 102 and 87. The few human studies that have been performed previously with rolled cereals confirm high postprandial glucose and insulin responses. Consequently, a GI close to 90 has been reported for commercial rolled oats in different laboratories (Granfeldt et al. 1995 , Wolever 1990 ). Finally, Hagander et al. (1987) found similar metabolic responses to rolled rye and white bread, also indicating rapid features of the starch.

Thin oat flakes (16% gelatinized).

One purpose of this study was to minimize the prior heat treatment of oat grains before rolling to yield flakes with a lower degree of gelatinization. The oat grains were roasted for 20 min, soaked in cold water and finally rolled (0.5 mm). This treatment resulted in a degree of gelatinization of 16% as measured with DSC. In spite of the very low degree of gelatinization from the mild conditions used, the metabolic responses were high, with GI = 97. Although there was a tendency to a lower insulin response (II = 84), the effect was not significant. Thus, the crystallinity of the starch granules seems to be affected sufficiently to render the starch easily available for digestion and absorption in the small intestine, thus resulting in high glucose and insulin responses. This is in accordance with results from a previous study with rats (Holm et al. 1988b ) in which pure wheat starch, also with a low degree of gelatinization (14%, as measured with DSC), raised the glycemic response far above that of raw starch. Thus, we conclude that a low degree of gelatinization is not sufficient to lower glycemic and insulinemic responses to oats.

Thick oat and barley flakes (16 and 27% gelatinized).

In contrast to the high postprandial responses to the thin rolled oats, the thick (1.0 mm) oat flakes, with a similar degree of gelatinization (16%), gave significantly lower responses (GI = 70 and 72; II = 59 and 68). The slower digestion of thick flakes is presumably due to a lowered accessibility to amylase when the outer layer of the endosperm and/or the cell walls are less disrupted. The dependence of the flake thickness on metabolic responses again shows the importance of food structure.

However, although a larger product thickness reduced glucose and insulin responses in the case of oat flakes, no such effect was noted for barley flakes (degree of gelatinization 27%). Thus, the glucose and insulin responses to the thick barley flakes were high (GI = 94; II = 84) and not significantly different from the responses to the corresponding thin barley product (GI = 91; II = 87). Although most of the outer layer of the endosperm and/or the cell walls of the oat flakes with a thickness of 1.0 mm were intact, they probably were destroyed in the corresponding rolled barley (1.0 mm). This result was unexpected and likely due to the different sizes of the grains. Therefore, the length and thickness of some grains were measured. The barley grains were larger than the oat grains (the length was 0.9 and 0.7 cm, respectively for barley and oat and the thickness 0.7 and 0.4 cm, respectively). Consequently, the botanical structure in a 1.0-mm barley flake undergoes greater destruction than a corresponding oat flake.

Thick oat flakes (0% gelatinized).

In our efforts to minimize the extent of heat treatment of the oat grains before rolling, flakes made from soaked oat grains were included. The grains were soaked in cold water before being rolled to thick (1.0 mm) flakes. Although this product displayed lower GI and II (GI = 78; II = 74) than a commercial oat flake of lower product thickness and higher degree of gelatinization, it was not distinguishable from a corresponding partially gelatinized, thick oat flake. It seems that the presence of native granular starch per se does not decrease enzyme availability in oats. However, in contrast, it was shown previously that certain raw starches (corn, wheat, potato) (Bertholdand and Mohamed 1976 , Collings et al. 1981 , Vaaler et al. 1984 ) result in lower postprandial glucose and insulin responses than the corresponding gelatinized starch. Thus, the effect of raw or a low-to-intermediate degree of gelatinization in realistic foods must be studied in more detail.

	FOOTNOTES

 
² Abbreviations used: GI, glycemic index; II, insulinemic index; DSC, differential scanning calorimetry.

Manuscript received September 17, 1999. Initial review completed October 11, 1999. Revision accepted May 5, 2000.

	REFERENCES

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 

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