Amino- and Thiol-Functionalized Brewer's Spent Yeast Hydrolysates: Physicochemical and Antioxidant Properties

A significant by-product of beer brewing is brewer's spent yeast (BSY), which is often discarded. However, it has been demonstrated that the feasibility of BSY valorization can be achieved through the extraction and isolation of a range of economically valuable components, including proteins, functional peptides and amino acids, vitamins, bioactive β-glucans, minerals, flavor compounds, and dietary fiber. In the present study, BSY valorization was conducted through hydrolysis with pepsin to obtain HBSY with varying degrees of hydrolysis (DH%). These hydrolysates were then thiolated and aminated using different reagents, including silanization, amidation, thiourea, and glutaraldehyde cross-linking. According to Ellman's reagents, the thiolation (SH; 104.16 mg/100 g for HBSY and 39.37 mg/100 g for BSY) and ninhydrin reagent, amination (NH) contents of BSY (16.66 mg/100 g) and HBSY (41.16 mg/100 g) in silanization were substantially higher than those of other treatments. FTIR depicted an absorption peak around 2048 cm⁻¹ (~45%) and 2347–2438 cm⁻¹ after thiolation, and an augmented N-H bond at 1033 cm⁻¹ and 1625 cm⁻¹ following amination. The amid bands I and II underwent significant changes following thiolation and amidation, which were confirmed by SEM images. CD spectroscopy revealed that the amount of α-helix reduced and converted to random coil structure. A positive correlation (R = 0.99, p < 0.05) was observed with %DH, thiol, and amino surface groups, whereas a negative correlation was observed between surface hydrophobicity (H₀) and thiol (R = −0.94, p < 0.05) and amino groups (R = −0.97, p < 0.05). The thiolated and aminated HBSY significantly enhanced emulsion (78.1% and 91.6%) and foam-forming abilities (83.5% and 98%), respectively, which were more pronounced at basic and acidic pH, respectively. Moreover, the antioxidant capacities (DPPH radical scavenging activity) substantially improved after thiolation (33.6%–79.1%) and amination (21.9%–51.7%) compared to BSY (5.6%–11.7%) and HBSY (12.9%–24.6%). Therefore, the thiol- and amino-functionalization of HBSY allows for the development of novel applications, specifically as a functional food ingredient and as a substrate for fermentation processes.