title: The effect of antioxidants on quantitative changes of lysine and methionine in linoleic acid emulsions at different pH conditions.
abstract: BACKGROUND
  Plants are an important source of phenolic compounds. The antioxidant capacities of green tea, thyme and rosemary extracts that contain these compounds have been reported earlier. However, there is a lack of accessible information about their activity against lipid oxidation in emulsions and inhibit the interaction of lipid oxidation products with amino acids. Therefore, the influence of green tea, thyme and rosemary extracts and BHT (butylated hydroxytoluene) on quantitative changes in lysine and methionine in linoleic acid emulsions at a pH of isoelectric point and a pH lower than the isoelectric point of amino acids was investigated.
  
  
  METHODS
  Total phenolic contents in plant extracts were determined spectrophotometrically by using Folin-Ciocalteu's reagent, and individual phenols by using HPLC. The level of oxidation of emulsion was determined using the measurement of peroxides and TBARS (thiobarbituric acid reactive substances). Methionine and lysine in the system were reacted with sodium nitroprusside and trinitrobenzenesulphonic acid respectively, and the absorbance of the complexes was measured.
  
  
  RESULTS
  Extract of green tea had the highest total polyphenol content. The system containing antioxidants and amino acid protected linoleic acid more efficiently than by the addition of antioxidants only. Lysine and methionine losses in samples without the addition of antioxidants were lower in their isoelectric points than below these points. Antioxidants decrease the loss of amino acids. The protective properties of antioxidants towards methionine were higher in a pH of isoelectric point whereas towards lysine in pH below this point.
  
  
  CONCLUSIONS
  Green tea, thyme and rosemary extracts exhibit antioxidant activity in linoleic acid emulsions. Moreover, they can be utilized to inhibit quantitative changes in amino acids in lipid emulsions. However, the antioxidant efficiency of these extracts seems to depend on pH conditions. Further investigations should be carried out to clarify this issue.
authors.affiliation: Department of Food Service and Catering, Poznań University of Life Sciences, Poland.
authors.forename: Marzanna
authors.initials: M
authors.lastname: Hęś
authors.affiliation: Department of Technology and Instrumental Analysis, Poznań University of Economics, Poland.
authors.forename: Anna
authors.initials: A
authors.lastname: Gliszczyńska-Świgło
authors.affiliation: Department of Food Service and Catering, Poznań University of Life Sciences, Poland.
authors.forename: Anna
authors.initials: A
authors.lastname: Gramza-Michałowska
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D002084:Butylated Hydroxytoluene / Q000737:chemistry; D004655:Emulsions; D006863:Hydrogen-Ion Concentration; D019787:Linoleic Acid / Q000032:analysis; D008239:Lysine / Q000032:analysis*; D008715:Methionine / Q000032:analysis*; D010936:Plant Extracts / Q000737:chemistry; D059808:Polyphenols / Q000737:chemistry; D027542:Rosmarinus / Q000737:chemistry; D013662:Tea / Q000737:chemistry; D017392:Thiobarbituric Acid Reactive Substances / Q000032:analysis; D046930:Thymus Plant / Q000737:chemistry
chemical_list: D000975:Antioxidants; D004655:Emulsions; D010936:Plant Extracts; D059808:Polyphenols; D013662:Tea; D017392:Thiobarbituric Acid Reactive Substances; D002084:Butylated Hydroxytoluene; D019787:Linoleic Acid; D008715:Methionine; D008239:Lysine
keywords: environmental pH; green tea; linoleic acid emulsion; lipid oxidation; lysine; methio- nine; rosemary and thyme extracts