Chelators are a common ingredient in most laundry detergents. They have a number of different functions such as reducing water hardness, assisting in keeping particulate soil in suspension and the removal of certain stains, thus complementing the action of the anionic surfactants. Another important group of components in a modern liquid detergent is enzymes, mainly proteases and amylases. As the most commonly used enzymes within the detergent industry are dependent on bound calcium ions to maintain conformational stability and function, the presence of both chelators and enzymes in a liquid detergent presents a challenge. The three commonly used Ca2+ chelators: citrate, DTPA (diethylene triamine pentaacetic acid) and HEDP (1-hydroxyethane-1,1-diyl)bis(phosphonic acid), were studied with regard to their impact on protease and amylase stability in buffer and in a model liquid detergent. Enzyme stability was characterized by differential scanning calorimetry (DSC) and activity studies, and correlated to the chelator-Ca2+ interaction properties. The results show that a chelator’s ability to reduce water hardness and its Ca2+ affinity are in reality two separate aspects in the context of their use in liquid detergents. In the presence of DTPA, stoichiometric surplus of free Ca2+ is required to maintain sufficient amylase and protease stability. In the presence of the weaker chelators, HEDP and citrate, the total Ca2+ concentration is more important to protein stability than stoichiometric balancing between chelator and Ca2+. Thus, for these chelators their total concentration only has a minor impact on the Ca2+ concentration required to maintain or improve enzyme storage stability. The results underline the importance of Ca2+ in liquid detergent formulations, and suggest how proper balancing of chelators and Ca2+ can be used to improve overall enzyme stability.
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