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.