Sentence:  For <WSD>protein</WSD> identification , fractions from the column were concentrated by pervaporation against a stream of air at 5 ` C or by negative pressure dialysis in an apparatus which permitted simultaneous concentration of the protein and dialysis against isotonic saline . 
Meaning of the word marked with <WSD> tags:  any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes