Extraction, identification and characterization of the water-insoluble proteins from tobacco biomass

Tobacco leaves are a potential candidate for plant proteins, yielding fourfold more protein per acre than soybeans. However, more than 60% of these proteins are water-insoluble and remain in the residue (referred to as 'tobacco biomass') after aqueous extraction. Efficient extraction of tobacco biomass proteins (TBPs) could boost the development of value-added products from tobaccos.TBPs were resistant to salt extraction at pH 2.0-12.0, but they were readily extracted by organic solvents. A simplified extracting method, including mechanical homogenization, a first extraction with a methanol-water mixture (40/60, v/v) and a second one with water at pH 6.0, recovered at most 68% of the TBPs. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated the presence of both the water-soluble tobacco F(1) protein and the less soluble cell wall proteins in TBPs, while Fourier transform infrared spectra suggested the coexistence of TBPs with polysaccharides (especially pectin). Meanwhile, a higher content of hydrophobic amino acids was found in TBPs compared with water-extractable tobacco proteins. The amino acid score of TBPs was 0.71, with cysteine and methionine being the primary limiting amino acids.Satisfying recovery of TBPs was achieved using a two-step organic solvent extraction. The hydrophobicity and protein-pectin interaction of TBPs explained for this result.