Investigation of lentil seed behavior under microwave and microwave-infrared ‎thermal ‎treatments and their impact on modifying the physico-chemical and ‎‎functional properties

Abstract

The rise in people's awareness about their dietary patterns on environmental impact has ‎increased their demand to introduce more plant-based food products. Lentil is a pulse crop and ‎affordable plant-based protein source with well-known nutritional, health, and environmental ‎benefits. However, its lengthy cooking time and deficiency in functionality and digestibility ‎limit its consumption and application in other food products. Thermal processing is a ‎recognized method to address these issues and create food ingredients with various ‎functionality attributes. This research is focused on the utilization of microwave and ‎microwave-infrared thermal treatments (cleaner energy) of lentils by providing the information ‎that can be used to develop a suitable design and scale-up consideration for building devices ‎using these methods and also assessing the modification occurred in the flour obtained from the ‎processed grain. Due to the critical role of moisture content on modification purposes, the ‎initial moisture content of seed was tempered (increased) by adding water and preprocessing by ‎steam before the thermal process.‎ The drying kinetics of the tempered grain under different microwave and infrared ‎thermal energy combinations have been investigated by ‎mathematical modeling and comparing ‎the average effective moisture diffusivity. The results revealed that using infrared heating at ‎low power of 0.375 kW combined with the microwave energy at power ‎levels of 0.35 and 0.7 ‎kW significantly increases the average effective moisture diffusivity, leading to a noticeable ‎reduction in drying time. ‎Amongst different thin-layer drying empirical models, the Page ‎model was the most ‎appropriate model to describe the drying behavior of tempered lentils ‎seeds under microwave and microwave-infrared processes. The study also proposed a novel ‎method for modeling variation in dielectric loss factor and effective moisture diffusivity ‎properties of lentils based on their ‎dependent‏ ‏‎(affecting) temperature and moisture content ‎factors. Combining the multi-objective optimization approach ‎and the numerical solution of the ‎governing heat and mass transfer equations is the basis of the method, in which ‎the coefficients ‎of estimated models for loss factor and effective moisture diffusivity were ‎obtained. The ‎developed models demonstrated that while the seed temperature was increasing and moisture ‎content was reducing during the microwave process, ‎the effective moisture diffusivity grew ‎arithmetically‎, and the loss factor generally increased. However, transition points were ‎observed in the trend for the ‎samples tempered up to the 50% moisture content, which were ‎attributed to the starch ‎gelatinization and confirmed how the biochemical reaction would have ‎a noticeable effect ‎on this property, determining the microwave energy absorbance.‎ Starch and protein structure alteration during the modification process of lentil seeds ‎can bring about the desirable changes in the functional and nutritional properties of its flour. ‎The structure alterations have been evaluated in current research by using FT-MIR (Fourier ‎transform mid-infrared spectroscopy) spectroscopy. Starch gelatinization resulted in reducing ‎the degree of order in starch and was mainly affected by the pre-treatment methods. Regarding ‎the protein secondary structure, the unfolding and aggregation of protein molecules were ‎evident because of the denaturation reaction. This phenomenon caused the decline in the β-band ‎and α-helix of thermally treated samples. Also, the rise in β-I (intermolecular β-sheets) and RC ‎‎(random coil) indicated that protein structures transformed from ordered to unordered ‎structures by the modification process. Particle size distribution and SEM (‎scanning electron ‎microscopy) results illustrated that thermal treatment eased the process of breaking down ‎coarse particles during the milling step. On the other hand, the gelatinization of starch and its ‎aggregation to the partially denatured protein increased the portion of middle-size particles in ‎the modified flours. In addition, results showed that WHC (water holding capacity) of modified ‎flours was the only functional property that revealed some improvement by modification ‎process, while no significant variations were observed for OBC (oil binding capacity) and EA ‎‎(emulsifying activity), and PS (protein solubility) and ES (emulsifying stability) were ‎noticeably reduced with respect to raw lentil flour. Modification processes used in the study ‎were also effective for enhancing nutritional properties by increasing in vitro starch and protein ‎digestibility.‎