Detergent can be classified into a powder detergent and a liquid detergent, based on the form thereof. A liquid detergent is excellent in terms of solubility, in comparison to a powder detergent. Also, the liquid detergent is advantageous in that an undiluted detergent can be directly applied to stains. On the other hand, in the case of such a liquid detergent, since an enzyme such as a protease must be preserved at ordinary temperature in the liquid, it has technical difficulty in terms of stable preservation of the enzyme, differing from a powder detergent.
Moreover, since a liquid detergent contains a surfactant, fatty acid, a solvent, a chelating agent and the like, it imposes extremely strict conditions on the enzyme. For example, it has been conventionally known to add calcium ions as a means for stabilizing the enzyme in a liquid detergent. However, since a chelating agent comprised in such a liquid detergent has a property of taking calcium ions coordinated to the enzyme, it reduces an effect of stabilizing calcium ions and thereby destabilizes the enzyme.
An object of mixing a protease into a detergent is to decompose protein stains on clothes into low molecules, and thus, to promote removal of the stains. As a detergent protease, an alkaline protease with a molecular weight of approximately 43,000, which has a washing performance on composite stains containing lipids as well as proteins, has been developed so far (see Patent Literature 1). Moreover, studies have been conducted to cause alkaline protease to mutate, so as to improve specific activity, or stability, etc., thereby enhancing the washing performance thereof (Patent Literatures 2 and 3).
(Patent Literature 1) WO 99/18218
(Patent Literature 2) JP-A-2010-273673
(Patent Literature 3) JP-B-5202690