Effect of Microwave Treatment on Starch Fraction Distribution and In Vitro Digestibility of Red Lentil Starch

Introduction. Starch is widely used in the food industry as a functional ingredient; however, its rapid hydrolysis in the small intestine is accompanied by a sharp release of glucose and hampers the development of low-glycemic-index foods. Accordingly, there is growing interest in resistant starch (RS), which in its physiological action is close to dietary fiber. Physical, chemical, and enzymatic methods are used to modify starch digestibility and increase the proportion of RS; however, the behavior of legume starches, in particular red lentil starch, under microwave treatment, as well as the combined effect of its parameters, remains insufficiently studied.

Purpose. To establish the patterns by which microwave treatment regimes influence the distribution of starch fractions in red lentil starch, in order to substantiate the conditions for the directed formation of resistant starch and its application in functional foods.

Materials and Methods. Starch was isolated from red lentil seeds. Samples with an initial moisture content of 10.0 ± 0.2% were subjected to microwave treatment (2450 MHz) while varying the power (280, 460, and 700 W), duration (60, 90, and 120 s), and moisture content (20, 25, and 30%). After treatment, the samples were slowly cooled and held at 4.0 ± 1.0 °C for 48 h to induce retrogradation and the formation of resistant starch. The contents of the rapidly digestible (RDS), slowly digestible (SDS), and resistant (RS) starch fractions were determined by an in vitro enzymatic method. The data were processed using response surface methodology based on a Box–Behnken design with analysis of variance.

Results. Power and treatment duration, as well as their interaction, significantly affected the distribution of the fractions (for RDS, F = 18.94, p < 0.001; for RS, F = 12.87, p < 0.001). The highest RDS content (56.88%) was achieved at a power of 700 W and a duration of 120 s. The SDS content decreased in all treated samples. The maximum RS content (58.77%) was obtained at a power of 460 W, a duration of 60 s, and a sample moisture content of 20%, which corresponds to an approximately 1.6-fold increase in this fraction relative to the native starch (36.6%) (p < 0.001 for the linear effects of power and duration). The effect of moisture content on RDS was moderate (p ≈ 0.05).

Conclusion. Microwave treatment is an effective method for the directed modification of starch digestibility: by selecting the appropriate regimes, the content of either resistant or rapidly digestible starch can be increased depending on the intended application. The accumulation of resistant starch under moderate regimes indicates amylose retrogradation with the formation of structures resistant to enzymatic hydrolysis. The results are promising for the development of functional foods. At the same time, further research into the structural mechanisms is required, employing modern methods including X-ray diffraction and electron microscopy, together with an assessment of the feasibility of scaling the technology to an industrial level.

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