Determining the Roel of Astringency Mechanism of Whey Protein at Acidic pHs

Abstract

Whey proteins are used in beverages for a wide range of nutritional benefits. Low pH conditions (< 4.5) enhance clarity and stability; however, they are also associated with increased astringency. The goal of this investigation was to determine if interactions between positively charged whey proteins and negatively charged salivary proteins contribute to the astringency of low pH whey protein beverages. In the first experiment, the effect of protein concentration (0.25-13%) on the astringency and basic tastes of beverages containing -lactoglobulin at pH 3.5 was investigated by a trained sensory panel (Spectrum method). Controls consisted of pH 3.5 phosphate buffers at concentrations equivalent to the amount of acid required to adjust the protein pH to 3.5. Astringency significantly increased with increasing protein concentration (0.25-4% wt/wt). Astringency of controls was not significantly different; indicating that increased astringency was not due to phosphate concentration or phosphate buffering ability but was contributed from increasing protein concentration. At higher concentration (>4% wt/wt), maximum astringency was not significantly different, sourness significantly increased, showing the effect of increased acid concentration. Time intensity studies showed the maximum astringency and duration were different among protein concentrations, while the time to maximum astringency was similar for all protein concentrations. Saliva and β-lactoglobulin interactions, as indicated by turbidity, showed maximum turbidity was between pH 4.0 and 5.5. In the second experiment, astringency of three commercial whey protein isolates (WPI, pI ~ 5.2) and lactoferrin (pI ~ 8.8) at pH 3.5, 4.5, and 7.0 were evaluated At pH 7.0, positively charged lactoferrin was astringent, while all three negatively charged WPIs were not; suggesting that charge interactions play a role in the astringency mechanism. Interactions among whey and saliva proteins were determined. -lactoglobulin or lactoferrin (at pH 2.6, 3.5 and 7.0) and saliva were mixed, centrifuged, and the supernatant and pellet compositions were analyzed by polyacrylamide gel electrophoresis. Results showed the presence of -lactoglobulin or lactoferrin and saliva proteins in the pellet, indicating that saliva decreased the solubility of -lactoglobulin or lactoferrin, presumably through direct interactions. In the third experiment, the effects of increasing sweetness (sucralose) and reducing sugar-base (ribose) modification were investigated. As sucralose concentration was increased from 0.011-0.023% (wt/wt), the astringency was not significantly different at any of the protein concentrations (1, 4, or 14% wt/wt). The addition of ribose to the protein beverages significantly reduced the astringency rating compared to the addition of sucralose to the protein beverages. The addition of ribose was shown to cause a reduction in available amino groups (positive charge). These results support the hypothesis that direct interactions between positively charged whey and negatively charged salivary proteins play a role in the astringency mechanism and modifying the protein may reduce the astringency.