The effect of protein-pectin interactions on the functional properties of soy drinks in coffee applications

Soy beverages often encounter an unappealing issue when introduced in coffee, known as curdling, due to protein aggregation and coagulation in its acidic, high-temperature environment. While stabilizers, such as buffers and hydrocolloids, are commonly used as a coping mechanism, their use is not ideal for organic and clean-label products. Conversely, pectin offers a more suitable alternative, particularly within a soy drink, although its application in this context remains an understudied aspect. This study investigated the impact of pectin addition on curdling mitigation in two commercially available soy drink variants, i.e. drinks with and without added acidity regulator (monopotassium phosphate) and stabilizer (gellan gum). Low methoxyl and high methoxyl pectin, with different degrees of esterification, were added at concentrations of 0.1 %, 0.2 %, 0.4 %, and 0.8 %. The investigation evaluated the curdling susceptibility through visual observations during an acid coffee test (at different pH conditions) and microscopic analysis of the curdled particles in acetate buffer. Additionally, the drinks’ isoelectric point was determined through zeta-potential measurements as a function of pH, while important physicochemical attributes, including viscosity, particle size, and foam characteristics were also examined. The results demonstrated that the addition of 0.8 % of either low or high methoxyl pectin to both soy drink types notably reduced their curdling sensitivity and their particle size at pH 4.0, especially in soy drinks without an acidity regulator. Unexpectedly, the pectin type had a limited impact on the curdling stability but significantly influenced the other examined physicochemical characteristics of the soy drinks.