Gluten free bread from millet extruded flour and their physicochemical properties and antihyperglycemic activity -

Gluten free bread from millet extruded flour and their physicochemical properties and antihyperglycemic activity

Millet (Pennisetum glaucum) is a small gluten-free cereal and an interesting source of dietary fiber, vitamins, minerals, proteins and phenolic compounds superior to the most traditional consumed cereals. Bread is a food product of high acceptance that is traditionally made from wheat, particularly from refined flours. However, it is known to present a high glycemic index and low levels of dietary fiber and naturally contains gluten proteins. In this study developed whole grain bread using blends of raw and extruded flours from millet and to evaluate the physicochemical parameters in comparison to commercial whole bread products with and without gluten. Millet is presented as potential food interesting for the production of bread considered in the diet particularly to those suffering from gluten allergy and diabetes.

Millet bread preparation:

In order to obtain raw whole grain millet flour (RMF) and precooked millet flour (PCMF), whole millet grains were ground in an hammer mill equipped with a sieve opening sieve of 0.8 mm. PCMF was obtained by processing RMF in a twin screw extruder with screws of 25 mm sectional diameter running at 500 rpm, 40:1 length/diameter ratio (L/D), barrel with ten temperature controlled heating zones (25, 40, 60, 80, 110, 120, 120, 120, 140 and 140 ^◦C), fitted with a circular die of 4 holes of 3.8 mm diameter each. The water content of the RMF was adjusted to 15% water content with help of a water feed piston pump. After processing, the extrudates were dried in a fan oven at 45 ^◦C overnight and then shredded in an industrial blender and subsequently packaged in plastic bags and sealed until the later analyzes and elaboration of the products. The bread formulation was developed from previous tests, being the best formulation chosen. Bread making was carried out using 100% RMF, 100% PCMF and 50:50 blend of RMF: PCMF. The other ingredients added, based on 100% flour, were water (35%), dry yeast (5%), salt (2%), fat (8%), whole liquid eggs (35%) and fine granulated white sugar (10%). The bread making process was the direct fermentation of the dough, in which yeast and sugar were dissolved in the liquid phase and added to other ingredients in a planetary mixer equipped with flatter beater until complete homogenization, remaining at rest for 30 min. The dough was placed in rectangular metal trays (15x7x4 cm) previously greased with soybean oil and placed in fermentation chamber at 35 ◦C and 75% humidity for 90 min. Breads were then baked in an electrical oven at 200 ^◦C for 40 min. After cooling, they were stored in polyethylene packages at room temperature until further analysis.

Results and discussion

The bread with the highest specific volume and height and the greatest formation of uniform aovoli was the bread made with 50:50 RMF + PCMF followed by the bread made with 100% RMF and the bread with the smallest specific volume was the made with 100% PCMF. This result is directly related to an analysis of the texture profile, in which the bread made with the 50:50 mixture of flours presents the greatest softness. During the preparation of bread with 100% RMF, an increase in volume could be visually along olfactory characteristics which were similar to commercial wholegrain bread, with visible development of uniform and small cells. The higher amylose content promotes greater gas retention properties and volumes in gluten-free breads prepared with rice flour, thus, it is possible to suggest, that samples prepared with 100% RMF presented high volume due to a higher concentration of amylose in the RMF compared to PCMF. However, despite the high expansion observed in bread dough made with 100% RMF, the presence of fibers may have caused discontinuity of the dough matrix associated with the absence of a network capable of retaining the gas produced from the fermentation. Dough with 100% of precooked flour did not grow during the fermentation and presented spongy texture, probably due to the strong interactions between the hydrogen and water during the preparation and lack network formation capable of retaining the gas released during fermentation. Among the developed formulations (RMF, PCMF and RMF and PCMF (50: 50)) the bread that presented similar characteristics to commercial whole wheat bread was prepared with a mixture of RMF and PCMF (50: 50). As per these similar characteristics the millet bread RMF and PCMF (50: 50) showed high increase volume rate during the fermentation process and maintained these characteristics after cooking.

Breads made with extruded millet flour had better technological properties (volume and height), good texture (low hardness, high resilience) when compared to breads added of raw millet flour, corroborating with results found in this study. Compared to wheat bread, there is a major challenge in the production of gluten-free breads due to deficient gas retention during fermentation produced in the formation of uneven alveoli and breads with low height and volume, which alter the softness of the crumb, where breads with higher specific volumes have greater softness and greater formation of alveoli.

Analysis of the texture profile of bread:

All gluten-free breads had higher hardness value compared to whole-grain wheat bread. PCMF bread had the highest hardness value, probably due to the lower amylose content (in relation to RMF) and due to the interaction between modified starch molecules that formed a rigid network, limiting the mass expansion during the fermentation process. In other words, the thermoplastic extrusion process is an efficient tool that modifies amylose-amylopectin chains forming a structure that could be used in replacement of gluten in gluten-free matrix. A higher content of amylose would lead to greater gas retention properties, thus promoting greater softness of the dough. Bread with 50:50 RMF + PCMF showed hardness values close to commercial whole wheat bread, whereas the RMF showed value close to the gluten-free bread. The elasticity of bread produced from the RMF + PCMF mixture and the bread produced with PCMF were close to commercial gluten-free samples, despite being statistically different. Concerning to resilience, the bread produced with RMF + PCMF mixture (0.269 N) was similar to commercial gluten-free sample (Control 1) (0.267 N), even though showed statistical difference. However, in regard to gumminess and chewiness, RMF + PCMF bread showed values approximately 2-fold higher in comparison to commercial gluten-free bread. This behavior occurs due to the starch conversion and greater fluidity of the sample structures. In the present study, considering same production process, the observed gumminess may have being caused due to the proportion of pre-cooked flour used, which in general presents higher water absorption rate and, as consequence, higher gumminess was observed in 100% PCMF bread.

Conclusion

Pearl millet has the potential to be used in baking with the desired characteristics in terms of gluten mimicking substitute and antioxidant compounds. The use of extrusion technology did not reduce the nutritional content of the breads; it also significantly improved their antioxidant capacity and also contributed to a lower hardness and greater specific volume (RMA + PCMF 50:50). In addition, this addition contributes to obtaining a food with anti-hypoglycemic activity in vitro superior to the hypoglycemic agent used, suggesting that it can be used in diets with carbohydrate restriction. The use of thermoplastic extrusion as a modifying tool applied in whole cereal flour has great potential to replace wheat flour in gluten-free bread formulations.

Reference:

Pessanha, K.L.F., de Menezes, J.P., dos Anjos Silva, A., da Silva Ferreira, M.V., Takeiti, C.Y. and Carvalho, C.W.P., 2021. Impact of whole millet extruded flour on the physicochemical properties and antihyperglycemic activity of gluten free bread. LWT, 147, p.111495.