16. A system to determine sentiment expressed in a document, comprising: at least one processor; memory; and at least one program stored in the memory, the at least one program comprising instructions to: receive a document from a plurality of documents; identify 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; determine 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; and determine at least one sentiment corresponding to the at least one category based on the at least one sentiment signature.
16. A system to determine sentiment expressed in a document, comprising: at least one processor; memory; and at least one program stored in the memory, the at least one program comprising instructions to: receive a document from a plurality of documents; identify 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; determine 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; and determine at least one sentiment corresponding to the at least one category based on the at least one sentiment signature. 17. The system of claim 16 , wherein further comprising instructions to extract the list of keywords, wherein the instructions to extract the list of keywords from the document include instructions to: extract keywords from each document of the plurality of documents; for each keyword, calculate a frequency, f, of the keyword in the plurality of documents and a number of documents, N, that include the keyword; use the phase transition formula to calculate the relevancy of the keyword based on the frequency of the keyword in the plurality of documents and the number of documents that include the keyword; and add the keyword to the list of keywords when the relevancy of the keyword exceeds a predetermined threshold.