Abstract :
[en] Peanut proteins have been widely used as an important ingredient in food products, due to its high nutritional value, functional properties, and low cost. Arachin and conarachin, the major component of peanut proteins, which account for about 90% of total seed proteins is responsible for functional properties of peanut proteins. Although it is known that different protein subunit compositions contribute to different protein properties, and that heat treatment is a widespread unit operation in the food industry, there are only very few studies on structure and properties characterization of arachin and conarachin with different subunit compositions during thermal processing. In order to contribute to this knowledge, the aim of this work was to study the subunit composition, thermal properties and interfacial properties of peanut protein fractions and their application in film formation.
Firstly, the effect of heat treatment on secondary structure and interfacial properties of arachin and conarachin were analyzed by infrared spectroscopy, circular dichroism and Langmuir trough technique. For native sample, arachin mainly consist of β-sheet structure, α-helix, random coil while conarachin is predominantly composed by α-helix and β-sheet. With the temperature increasing, arachin shows a decrease of sheet structure while for conarachin, the component of secondary structure was changed irregularly with temperature. The shape of the compression isotherm recorded for arachin is typical of liquid expanded monolayers, while for conarachin, the isotherm exhibits the typical phase transition from liquid expand state, to liquid condensed state, and solid state. Arachin seemed more stable to temperature treatment than conarachin.
Secondly, in order to get more information on arachin, the subunit composition and the thermal stability, steady flow properties, surface hydrophobicity and sulfhydryl content of arachin from six peanut varieties were determined. Two groups of arachin were identified: one containing the 35.5kDa subunit, was more heat-sensitive (Tonset <100℃), had a significant lower initial denaturation temperature, less disulfide bonds and more hydrophobic groups; and the other one without this specific subunit, was less heat-sensitive to temperature, and presented a more compact globular structure. The presence or absence of the 35.5kDa subunit in arachin significantly influenced the thermal and conformation properties of arachin. Furthermore, the 35.5 kDa subunit was sequenced by quadrupole-time of flight and identified as an isoform of Ara h3.
Thirdly, Thermal properties and molecular conformation of two types of arachin was further evaluated. The surface hydrophobicity and sulfhydryl content were determined, the protein interactions were characterized by SDS-PAGE electrophoresis, and the molecule size was measured by SEC-MALLS. Heat treatment at 90 ℃ induced an increase in surface hydrophobicity and sulfhydryl groups due to a partial unfolding of the peanut protein structure and the formation of aggregates linked by disulfide bonds. While heat treatment at 120℃ led to lower surface hydrophobicity due to intramolecular hydrophobic binding of protein lateral chains and burying of the sulfhydryl groups inside the protein. The presence of the 35.5 kDa subunit or not of arachin showed the different molecular structure upon heat treatment, arachin without 35.5 kDa subunit exhibited a more compact structure and a higher ability to suppress the thermal processing than did arachin with 35.5kDa subunit.
Then, a protocol for the 35.5 kDa subunit purification from arachin was developed. Secondary structure and interfacial properties of the arachin subunit upon heat treatment were investigated using circular dichroism and Langmuir trough technique. Heating resulted in a decrease of α-helix content in the subunit with a corresponding increase in random coil. It also led to a decrease in surface pressure of the monolayer film indicating a decrease of the interfacial stability, a similar trend to the one observed for arachin.
Finally, in order to evaluate the film forming property of peanut protein, the structure and mechanical properties of peanut protein isolates film, and the effects of xylose content on protein film were analyzed. According to the above results, heat treatment at 90℃ is beneficial for structure unfolding of protein, and does not affect the interfacial property of peanut fractions. The protein films were successfully developed from peanut protein, but the mechanical properties need to be improved. Compared to pure peanut protein isolate, the incorporation of xylose markedly enhances the mechanical properties and the water resistance of films by increasing the degree of glycosylation and surface hydrophobicity of xylose-modified peanut protein isolate.
Globally, this research contributes to identify peanut varieties with specific functionalities and to provide useful information for the processing of peanut protein products with good thermal stability. It will help to the development and utilization of peanut protein resources.
