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Peanut protein isolate (PPI) and soy protein isolate (SPI) were obtained by alkaline extraction from their respective defatted flours and phosphorylated with sodium trimetaphosphate (STMP) at three levels (1, 2 and 3 % w/w) in order to find the best percentage in terms of extent of phosphorylation. The use of STMP in peanut protein has been not reported before and a good opportunity of new protein development is foreseen, mainly because this reaction improves some of the most important functional properties when used as food ingredients: solubility, emulsifiability and whippability without detrimental effect in nutritional characteristics. The reactions were executed in an aqueous solution (pH 11.5) at 35°C during 3 h, and the extent of phosphorylation indirectly determined in the supernatant using two methods: titration with EDTA 0.01 M and inductively coupled plasma (ICP). When STMP is used, the reaction yields, by each serine transformed, an equivalent amount of pyrophosphate. The extent of reaction was expressed as pyrophosphate real/ theoretical expressed as percentage and the two methods were compared. ICP was the technique with less variation over the titration counterpart, besides is faster and has not been tested before in vegetable protein phosphorylation. The highest extent of modified serine was reached with 2% of STMP (25.3%). These results were compared with phosphorylation in a soy protein isolate and similar results were obtained opening now the possibility to perform a complete functional evaluation of the obtained products in order to assess the improvement of vegetable protein properties.