4.1. Rapidly Available Glucose, Sensory Evaluation, and Physical Properties of Noodles Made from Legumes
The purpose of this study was to provide a functional staple food that is palatable and does not cause a rapid postprandial glucose increase. Four types of noodles were prepared exclusively from legumes. First, an in vitro digestion experiment was performed to predict whether the variety and composition of the legumes used could cause a difference in postprandial blood glucose elevation. At the same time, the prepared noodles were compared with existing commercial gluten-free pasta products and WR.
The results of the in vitro digestion experiment showed no significant difference in RAG level between the legume noodles. RAG is correlated with postprandial blood glucose elevation. RAG is affected by the amount of carbohydrates and the structure of the food. For example, retrograded starch is less susceptible to hydrolysis by amylase, and only a small amount is available as RAG. Similarly, starch granules contained in legumes (mainly resistant starch; RS1 and RS2) are resistant to digestive enzymes [
11,
21,
22]. The RAG values of peas, chickpeas, and lentils as unprocessed cooked beans are reported to be 6, 6, and 8 g/100g as eaten, respectively [
17,
23], which is consistent with the values of YP, YP-U, CP, and LT produced in this study. Furthermore, the carbohydrate content that affects RAG levels was similar among the various samples. This suggests that yellow peas, chickpeas, and lentils can be used to produce foods with similar postprandial blood glucose-elevating properties, regardless of the type of legume, and even if processed into extruded noodles.
To establish legume-based noodles as a staple food, its taste is important. It should be noted that all the noodles produced in this study had better palatability scores than products already on the market. Among the foods, YP, YP-U, and CP had relatively high palatability scores, suggesting that these noodles have the potential to be eaten daily. Furthermore, the overall palatability score of YP was 5.30 ± 0.24; however, it should be noted that five points corresponded to the evaluation of “Neither like nor dislike.” Crucially, if the palatability is too high, it may lead to overeating, while if it is too low, it may be difficult to continue eating. Therefore, we believe a score of this level is preferable for a staple food.
Some subjects commented in the sensory test that YP and YP-U had a unique al dente texture. Therefore, the mechanical properties were examined using a creep meter to quantitatively evaluate the texture. As a result, both breaking stress and breaking strain were found to be significantly larger in YP and YP-U than in other samples. This indicated that chewing noodles made with yellow peas requires more stress and deformation than chewing the other types. On the other hand, in the palatability sensory test, YP and YP-U were not the only foods that scored highly for “hardness” and “taste”, and there was no significant difference between the associated scores for YP, YP-U, and CP. This discrepancy is assumed to be a result of the multiple factors (other than the stress/strain characteristics) that affect palatability assessments.
4.2. Blood Glucose/Insulin Response from Noodles Made from Legumes
Based on these results, YP and YP-U were selected as study foods for measurements of GI and the post-consumption insulin responses in this study. In previous studies, the GI of noodles made with wheat and pea (70% and 30%, respectively) or wheat and lentil (50% each) were reported as 93 and 55, respectively, with a reference food of white bread [
24,
25]. These reports attempted to verify that the GI was reduced by mixing the main ingredient with legume; however, the GI was not significantly lower than the wheat-only control food [
24,
25]. Others have described noodles made exclusively from legumes, but these reports did not define the quality of taste or GI [
26]. In order to increase the knowledge of legume-based products, this study evaluated the sensory, measured GI, and assessed post-consumption insulin response. The noodles made exclusively from yellow pea were shown to be a low GI food, indicating that it could be a staple food option for controlling blood glucose.
Although there was no significant difference in glycemic response between subjects when comparing YP and YP-U, YP was more effective in suppressing the increase in blood glucose. We hypothesized that YP-U would be more effective at suppressing the increase in blood glucose because it contains the seed coat of the legume, and therefore contains more dietary fiber [
27], but no such effect was observed. This may be due to a weakening of the structural strength of the coat when processing into noodles, which made it easier to be digested in the stomach and intestine. The pea seed coat has a cuticle layer with a waxy surface [
28]. It is presumed that the wax reduced the binding strength to the starch and proteins that form the structure of the noodles. If YP-U is easier to be digested, it is assumed that more glucose may be taken into the blood compared to YP. On the other hand, the equivalent RAG levels of YP and YP-U may indicate that the structural strength of the noodles was not relevant to the results, since homogenized samples were used for the in vitro digestion experiment. There was no difference between YP and YP-U in the measurement of mechanical properties, and there was no difference in the scores for the sensory test of texture, so the change in structural strength caused by the seed coat is assumed to be present primarily at a micro level (and undetectable through macro-structural determinations). If strong-bond noodles could be made through future improvements in manufacturing, it could be possible to provide superior foods for controlling blood glucose.
Blood glucose levels are affected by insulin secretion. In this study, blood insulin levels were measured at the same time as blood glucose. Insulin hypersecretion was significantly suppressed by YP compared with WR and YP-U. YP suppressed both blood glucose elevation and insulin hypersecretion, suggesting that it has excellent characteristics for blood glucose control. On the other hand, YP-U resulted in insulin secretion equal to that of WR. A GI study of bread made with chickpea reported that the addition of legumes increased insulin secretion [
29]. Proteins [
30] and the amino acids arginine [
31] and phenylalanine [
32] have been reported as food components, other than blood glucose, that increase insulin secretion. Peas are generally reported to contain a large amount of arginine and phenylalanine [
33]. In this study, the YP-U group contained 2 g more protein than the YP group, which may have affected insulin secretion. YP-U produced the same level of insulin secretion as WR, but the blood glucose elevation was significantly lower than WR. Therefore, although YP-U is not as effective as YP, it is an advantageous food for controlling blood glucose. Further research on foods using pea products for glycemic control is necessary to fully understand their impact and relationship to human health.
In this study, the superiority of noodles made with peas in controlling blood glucose was discussed on the basis of GI, but glycemic load (GL) must also be considered. Many studies have simultaneously examined the relationship between GL and lifestyle-related disease factors in addition to GI [
6]. GI is a “quality” index that indicates how easily sugar is absorbed, whereas GL is calculated by multiplying the amount of carbohydrates actually consumed in one meal by the GI of the food. Therefore, GL is an index that simultaneously represents “quality and quantity” [
34]. In this study, subjects consumed 50 g of carbohydrates to measure GI, but to achieve this level of consumption, an intake of 216 g in wet weight was required for YP and 237 g for YP-U. This amount was approximately 1.5 times greater than the 147 g required for WR. As the difference between the calorific value per weight of YP, YP-U, and WR is small (WR = 624 kJ/100g, YP = 637 kJ/100g, YP-U = 605 kJ/100g), the consumption of these foods may have a more pronounced effect on glycemic control if the intake of YP and YP-U per meal was the same as that for WR.
This study does, however, have a few limitations. First, the sensory tests of noodles made with legumes were performed only by Japanese panelists; therefore, it is undeniable that there is a bias towards Japanese eating habits and preferences. Second, GI and insulin secretion measurements using YP and YP-U were also performed with Japanese patients. Thus, the response to YP and YP-U intake may not account for racial differences in consumers. Future studies on the palatability, the blood glucose levels, and the insulin levels associated with noodles made with yellow peas should be conducted with non-Japanese subjects. Additionally, further research is needed to incorporate these foods into the diet in various parts of the world. Third, we measured GI and insulin secretion in healthy men and women, but we have not been able to verify whether these foods are effective for diabetic or pre-diabetic patients who wish to control blood glucose. Therefore, it is desirable to verify the effect of these foods on glycemic control in non-healthy subjects. Fourth, as the study adopted a single-dose study design, it is not clear how long-term continuous intake would affect glycemic control. Therefore, long-term intervention tests are needed. Finally, in this study, different types of legumes were used to produce noodles, but it may not be possible to optimize the production method. The GI and insulin response may change if the manufacturing conditions change, so further research is needed to evaluate the influence of production method on the development of superior foods.