Physicochemical and Structural Characterization of Ultrasound-Modified Proteins Recovered from Cassava Wastewater

Agro-industrial residues generate environmental and economic impacts when improperly discarded. Manipueira, a liquid residue from cassava processing (>17 m³/ton of starch), presents high organic load and cyanogenic compounds. Nevertheless, cassava starch has considerable economic relevance in Brazil, with a Gross Production Value of R$ 2.08 billion in 2024 and a projected growth of 20.9% for 2025. In this context, ultrasound emerges as a promising alternative for valorizing this residue by modifying proteins and reducing cyanogenic compounds. Thus, this study aimed to physicochemically characterize proteins recovered from manipueira after ultrasound treatment and evaluate the reduction of cyanogenic compounds. The protein extract was obtained by isoelectric precipitation (pH 3.8), adjusted to neutral pH and freeze-dried. Subsequently, it was resuspended in distilled water (1% w/v) and ultrasound-treated at high, low power and control (700, 300 and 0 W, respectively) for 10 min under controlled temperature. After treatment, physicochemical characterization was performed. Both treatments promoted modifications in protein structure. The samples U300 and U700 showed reductions in enthalpy (−25.42 and −33.65 mJ, respectively) and denaturation temperature (67.38 and 60.87 °C, respectively) compared to the control (−39.69 mJ and 73.44 °C), indicating mild protein denaturation. A decrease in intrinsic fluorescence and an increase in surface hydrophobicity were observed in U300 and U700 (7086.00 and 8058.33 H₀, respectively) compared to the control (3697.45 H₀), indicating partial unfolding and changes in tertiary structure. Regarding secondary structure, treated samples (U300 and U700) showed reduced α-helix content (13.47% and 34.47%) and increased disordered structures (33.53% and 31.27%) compared to the control (80.36% α-helix content and 14.36% from disordered). In addition, a reduction in cyanogenic compounds was observed, from 26.33 μg/g in manipueira to values around 7 μg/g in the protein extracts, meeting the maximum permitted limit of 10 μg/g. These results indicate that ultrasound, even at low power, promotes light protein denaturation, exposing functional groups and suggesting potential improvements in techno-functional properties. Additionally, reducing cyanogenic compounds to safe levels contributes to mitigating environmental impacts and aligns with the United Nations Sustainable Development Goals.