Physico-chemical and functional properties of sunflower proteins

Keywords: Sunflower protein, Helianthus<span class=SpellE>annuus</span> ,<span class=SpellE>helianthinin</span>, albumins, solubility, structure,<span class=SpellE>denaturation</span>, pH, temperature, ionic strength,<span class=SpellE>phenolic</span>compounds,<span class=SpellE>chlorogenic</span>acid, foams, emulsions, functionality<o:p></o:p></span>The research described in this thesis deals with the relation between specific sunflower proteins, their structure and their functional properties as a function of extrinsic factors as pH, ionic strength and temperature.</span>S</span><span lang=EN-GB style='font-weight:normal;mso-bidi-font-weight:bold'>unflower protein isolate (SI) devoid of<span class=SpellE>chlorogenic</span>acid (CGA), the main<span class=SpellE>phenolic</span>compound present, was obtained without<span class=SpellE>denaturation</span>of the proteins. Sunflower proteins were found to be composed of two main protein fractions: 2S albumins or sunflower albumins (<span class=SpellE>SFAs</span>) and<span class=SpellE>helianthinin</span>. Subsequently, these protein fractions were<span class=SpellE>biochemically</span>and structurally characterized under conditions relevant to food processing.<o:p></o:p></span>Depending on pH, ionic strength, temperature and protein concentration,<span class=SpellE>helianthinin</span>occurs in the 15-18S (high molecular weight aggregate), 11 S (<span class=SpellE>hexamer</span>), 7S (<span class=SpellE>trimer</span>) or 2-3S (monomer) form. Dissociation into 7S from 11S gradually increased with increasing pH from 5.8 to 9.0. Enhancing the ionic</span><span lang=EN-GB>strength</span><span lang=EN-US style='mso-ansi-language:EN-US;mso-fareast-language:ES'>resulted in stabilization of the 11S form. Heating and lowering the pH resulted in dissociation into the<span class=SpellE>monomeric</span>form of<span class=SpellE>helianthinin</span>. The 11S and 7S form of<span class=SpellE>helianthinin</span>differ in their secondary structure, tertiary structure, and thermal stability. With respect to solubility</span><span lang=EN-GB>as a function of pH</span><span lang=EN-US style='mso-ansi-language:EN-US;mso-fareast-language:ES'>,</span><span class=SpellE><span lang=EN-GB>helianthinin</span></span><span lang=EN-GB>shows a bell shaped curve with a minimum at approximately pH 5.0 at low ionic strength. At high ionic strength,<span class=SpellE>helianthinin</span>is almost insoluble at pH&lt; 5.0.</span><span lang=EN-US style='mso-ansi-language: EN-US;mso-fareast-language:ES'><o:p></o:p></span>The second main sunflower fraction,<span class=SpellE>SFAs</span>, revealed to be very stable against pH changes (pH 3.0 to 9.0) and heat treatment (up to 100</span><span lang=EN-US style='font-family:Symbol;mso-ascii-font-family:"Times New Roman"; mso-hansi-font-family:"Times New Roman";mso-ansi-language:EN-US;mso-fareast-language: ES;mso-char-type:symbol;mso-symbol-font-family:Symbol'><span style='mso-char-type: symbol;mso-symbol-font-family:Symbol'>°</span></span><span lang=EN-US style='mso-ansi-language:EN-US;mso-fareast-language:ES'>C), and their solubility was only marginally affected by pH and ionic strength. The solubility of the SI as a function of pH seems to be dominated by that of<span class=SpellE>helianthinin</span>.<o:p></o:p></span>Foam and emulsion properties of the sunflower isolate as well as those of purified<span class=SpellE>helianthinin</span>,<span class=SpellE>SFAs</span>and combinations thereof were studied at various pH values and ionic strengths, and after heat treatment. Sunflower proteins were shown to form stable emulsions, with the exception of<span class=SpellE>SFAs</span>at alkaline and neutral pH values. Increasing amount of<span class=SpellE>SFAs</span>impaired the emulsifying properties. Regarding foam properties, less foam could be formed from<span class=SpellE>helianthinin</span>than from<span class=SpellE>SFAs</span>, but foam prepared with<span class=SpellE>helianthinin</span>was more stable against<span class=SpellE>Ostwald</span>ripening and drainage than foam prepared with<span class=SpellE>SFAs</span>. Increasing amounts of<span class=SpellE>SFAs</span>had a positive effect on foam volume and a negative one on foam stability and drainage. It was found that treatments that increase conformational flexibility improve the emulsion and foam properties of sunflower proteins