1. A computer-implemented method for determining sentiment expressed in a document, comprising: receiving a document from a plurality of documents; using at least one processor, identifying 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, wherein the list of keywords is extracted from the plurality of documents and is filtered using a phase transition formula, wherein the phase transition formula is f/N x , where f is a frequency of the keyword in the plurality of documents, N is a number of documents that include the keyword, and x>1, and wherein the at least one sentiment signature corresponds to an expression of at least one sentiment in the sentence; determining at least one category corresponding to the at least one keyword of the sentence, wherein the at least one category is included in a list of categories that is generated using the list of keywords; determining at least one sentiment corresponding to the at least one category based on the at least one sentiment signature.