Effect of ultrasound on protein yield and fate of alkaloids during lupin alkaline extraction process

Abstract

Protein isolates from legumes have attracted scientific attention because they are an efficient option to cover the daily protein requirement. One of the legumes with the highest protein content (20-50%) nowadays is Lupinus spp., with a good amino acid profile and moderate protein digestibility (≈ 70%) but with the presence, in certain species, of quinolizidine alkaloids (QA), toxic to the organism. The most popular method to QA removal is one based in washing with water, but inconvenient because of the high use of water and time. With the aim to propose eventually a new process for lupin detoxification, the objective of this work was the elucidation of the fate of QA during lupin protein extraction process assisted with ultrasound and the evaluation of the nutritional and functional properties of the protein fraction. Proximal characterization, concentration of anti-nutritional compounds, amino acid profile and protein solubility profile of flours from three lupin species were assessed: L. albus, L. angustifolious and L. mutabilis. The result showed a significant difference (p < 0.05) in protein concentration, total alkaloids and particle size between the three species flours. Considering the above, the species L. mutabilis and L. angustifolious were chosen to study the behavior of the protein fraction in terms of functionality (water absorption and nitrogen solubility), composition (protein yield and percentage reduction of alkaloids during the extraction process) and resistance to thermal treatments (displacement of denaturation temperature, change in secondary structure and modification of the electrophoretic profile). The results obtained for L. mutabilis described the ultrasound effect as beneficial for protein yield (14% more than control with 10 min ultrasound) and QA reduction from bagasse (81% less than control with 10 min ultrasound treatment) and protein isolate (50% less than control with 10 min ultrasound treatment). In addition, the change in protein structure and composition modified the functional and thermal properties of the protein, making the isolate a good candidate for food ingredient. In the other hand, L. angustifolious was more resistant to the ultrasound effect with no significant difference between treatments (10 and 15 min) and control but with the lower toxicity (3 ppm of QA in control protein isolate) and better amino acid score (0.85 of 15 min ultrasound treatment). These results will be useful to design processes to assist in the objective for meet protein demand of the population.