Soy protein hydrolysate; solubility, thermal stability, bioactivity, and sensory acceptability in a tea beverage.

Abstract

Protein solubility affects the quality of many food products, especially high-protein beverages (>4.2% protein). Beverages formulated with soy protein at >4.2% protein content are currently unavailable in the market due to thermal challenges affecting the protein solubility and also due to flavor challenges. Enzyme hydrolysis of soy protein may lead to enhanced solubility and thermal stability. However, bitter taste caused by hydrolyzing soy protein is a limiting factor to using SPH in food applications. Therefore, controlled and limited hydrolysis of 2-8% is required to minimize the production of bitter peptides. Another advantage of controlled hydrolysis of soy protein is the release of bioactive peptides. Among the many physiological benefits of bioactive peptides, antihypertensive activity has gained much attention. Therefore the objectives of this work were 1) to optimize hydrolysis conditions of soy protein for enhanced solubility, thermal stability and bioactivity, while maintaining low degree of hydrolysis (DH); and 2) to determine the sensory acceptability of lemon flavored iced tea beverages formulated with soy protein hydrolysate at various concentrations. Soy protein isolate (SPI) was hydrolyzed by papain, bromelain, trypsin, and alcalase at various enzyme levels while keeping other hydrolysis conditions constant. Enzymatic activities were optimized based on DH measurements. Degree of hydrolysis (DH) was measured using O-phthaldialdehyde method. Samples were subjected to SDS-PAGE analysis to monitor enzyme selectivity. ACE inhibitory activity was measured using a standardized assay. For determination of solubility and thermal stability, solutions (5% w/v) of hydrolysates and SPI were subjected to heat treatment at 95°C for 60 min followed by centrifugation and determination of protein content in the supernatant using a nitrogen analyzer. Solubility and thermal stability at different protein concentrations (1%, 2.5%, 5%, and 7%) in a black tea beverage were also measured. Alcalase-hydrolyzed SPI was chosen for the sensory study as it had the highest solubility, thermal stability, and antihypertensive activity among the hydrolysates. Nine formulations were prepared, including a control which was a tea with no added protein and the four different protein concentrations containing either SPI or alcalase-hydrolyzed SPI. One-hundred-one subjects rated overall liking, flavor, aroma, appearance, and mouthfeel on an 11-point hedonic scale and rated intensity of bitterness and off flavor on a line scale. Hydrolysates produced using 2.654 GDU of bromelain, 0.012 AU-A of alcalase, 19,680 USP units of papain, and 235,000 U of trypsin had DH of 3.98%, 3.57%, 6.77%, and 2.60%, respectively, thus were selected for further experiments. Use of different enzymes resulted in distinctive differences in hydrolysis patterns. Among the four produced hydrolysates, alcalase-hydrolyzed SPI was highly soluble (75.25%) at relatively high protein concentrations, thermally stable, and possessed the most pronounced antihypertensive activity (IC50=0.263 mg protein/ mL). Lemon flavored iced tea beverages fortified with alcalase-hydrolyzed SPI at various protein concentrations (1 up to 7 % protein) were found to be acceptable. The beverages formulated with the hydrolysate at 1%, 2.5%, and 5% protein content were liked as much as those without added protein. Beverages formulated with up to 5% hydrolysate also had significantly less bitter taste and off flavors than SPI beverages. These findings can be utilized in the development of high protein fortified beverages, which is acceptable with relatively low bitter taste and noted physiological benefits.