A system, computer readable storage medium storing instructions, and computer-implemented method for determining sentiment expressed in documents is disclosed. A document is received from a plurality of documents. A sentence in the document that includes at least one sentiment signature within a predetermined distance of at least one keyword from a list of keywords is identified, wherein the list of keywords is extracted from the plurality of documents and is filtered using a phase transition formula, and wherein the at least one sentiment signature corresponds to an expression of at least one sentiment in the sentence. At least one category corresponding to the at least one keyword of the sentence is determined, wherein the at least one category is included in a list of categories that is generated using the list of keywords. At least one sentiment corresponding to the at least one category is determined based on the at least one sentiment signature.