Physico-chemical and functional properties of potato proteins

Key words: potato proteins, patatin, protease inhibitors, solubility, structure, pH, temperature, ethanol, ionic strength, phenolic compounds, foams, emulsionsIn potato starch manufacture an aqueous byproduct remains that is called potato fruit juice (PFJ). On a dry matter basis PFJ contains about 20-25 % protein and amino acids, 15 % sugars, 20 % minerals, 14 % organic acids and other components, such as phenolic compounds. Potato protein has a relatively high nutritional quality, comparable to that of whole egg, and it therefore has high potential for utilization in food applications. Protein recovery from industrial PFJ is presently achieved through heat coagulation by steam injection after pH adjustment. This method is very efficient in removing protein from solution. However, it leads to protein precipitates that exhibit a poor solubility, which hampers potential food applications.An economic method to efficiently recover soluble potato protein would considerably increase its possibilities for use in food and add to its commercial value. Therefore, the important question resulting in this study was: can potato proteins be recovered from PFJ in such a way that they retain their functional properties, most importantly their solubility? This recovery method should be applicable at a large scale and result in a high yield. Potato protein recovery was expected to be complicated by the presence of and the interactions with non-protein components in PFJ. The objective in this study was to examine how extrinsic factors like pH, ionic strength and temperature would influence the structure of potato proteins, this in relation to the functionality of the proteins in making and stabilizing foams and emulsions.Three groups of potato proteins can be distinguished in PFJ. Patatin, the major potato tuber protein, comprises 38 % of the protein in PFJ from cultivar Elkana . The protease inhibitors make up about 50 % and other proteins up to 12 % of total protein in PFJ from cultivar Elkana .In Chapter 2 the effects of pH and various additives on the precipitation and (re)solubility at pH 7 of potato proteins from industrial PFJ are studied. Addition of various strong and weak acids caused the same extent of protein precipitation, which comprised at the most 60 % of total protein at pH 3. The use of weak acids, however, resulted in an increase in the resolubility of the precipitates at pH 7, as compared to strong acids. At pH 5 addition of FeCl 3 or ZnCl 2 increased both precipitation and resolubility. The largest increase in precipitation and resolubility was achieved by using organic solvents, resulting in a maximum precipitation (pH 5) of 91 % of total protein and a maximum resolubility of 91 % of precipitated protein. The results described in Chapter 2 lead to the hypothesis that precipitation and resolubilization of potato proteins from PFJ is not so much determined by their isoelectric pH but by their interactions with low molecular weight components.In Chapter 3 it was shown, using DSC and both far-UV and near-UV CD spectroscopy, that potato proteins unfold between 55°C and 75°C. Increasing the ionic strength from 15 to 200 mM generally caused an increase in denaturation temperature. It was concluded that the dimeric protein patatin unfolds either in its monomeric state or that its monomers are loosely associated and unfold independently. Thermal unfolding of the protease inhibitors was correlated with a decrease in protease inhibitor activities and resulted in an ionic strength dependent loss of protein solubility. Potato proteins were best soluble at neutral and strongly acidic pH. At mildly acidic pH the overall potato protein solubility was dependent on ionic strength and the presence of unfolded patatin.In Chapter 4 a protein isolate with a high solubility at neutral pH prepared from industrial PFJ by precipitation at pH 5 in the presence of ethanol is described. The effects of ethanol itself and the effects of its presence during precipitation on the properties of various potato protein fractions were examined. The presence of ethanol significantly reduced the denaturation temperature of potato proteins, indicating that preparation of this potato protein isolate should be done at low temperature to retain a high solubility. In the presence of ethanol the thermal unfolding of the tertiary and the secondary structure of patatin were shown to be almost completely decoupled. Even at 4°C precipitation of potato proteins in the presence of ethanol induced significant conformational changes. These changes did, however, only result in minor changes in the solubility of the potato protein preparations.In Chapter 5 foam forming and stabilizing properties of potato proteins are described; whipping or sparging was used to make foam. The performed whipping tests showed that less foam could be formed from untreated patatin than from the protease inhibitors, but also that patatin foam was much more stable against coalescence, Ostwald ripening and drainage. The foam forming properties of patatin could be strongly improved by partial unfolding of the protein. Whipping tests, at both low (0.5 mg/ml) and high (10 mg/ml) protein concentrations, also indicated that foams made with an ethanol precipitated protein isolate (PPI) were more stable against Ostwald ripening and drainage than those made withβ-casein andβ-lactoglobulin. More generally it was concluded that when proteins are used as a foaming agent, a high concentration is required, because the available protein is inefficiently used. Also, the different methods used to make foam, result in changes in the mutual differences in foaming properties between the various protein preparations and may induce different instabilities to become apparent in foams made at the same conditions.In Chapter 6 emulsions made with various potato protein preparations were characterized with respect to average droplet size, plateau surface excess and the occurrence of droplet aggregation. The average droplet size of the emulsions made with potato proteins appeared to be determined by the lipolytic release of surface active fatty acids and monoglycerides from the tricaprylin oil phase during the emulsification process. It was concluded that only trace amounts of patatin, the lipase activity of which has been strongly underestimated in literature, sufficed to liberate significant amounts of these surfactants. The plateau surface excess of emulsions made with patatin was found to be 2.6 mg/m 2 , while emulsion droplets made with protease inhibitors showed a significantly smaller surface excess. Of the various solvent conditions and treatments applied only heat treatment resulted in a significant increase in surface excess. Droplet aggregation in emulsions made with potato protein preparations other than patatin, could in contrast to at pH 5 and at pH 7 be prevented at pH 3.In Chapter 7 the relations between potato protein structure, solubility and foam and emulsion forming and stabilizing properties are discussed. Also, the different mechanisms by which phenolic compounds may affect protein solubility are discussed in relation to the solubility and resolubility behavior of potato proteins in PFJ and when separated. A summary of the most important differences in the properties of patatin and protease inhibitors is also given.