Denaturation of Vegetable Proteins

Samples

Commercial seeds from health food store.

Conditions

Measuring Cell: DSC20

Pan: Aluminum standard 40 µl, thermetically sealed 

Reference: Approximately the same amount of water in a sealed pan

Sample preparation: Mill seeds, aqueous extraction at pH 4.5, centrifuge, discard supernatant, re-extract residue at pH 8.5, supernatant evaporated to desired dry substance content under vacuum.

DSC measurement: Heating from 30 °C to 110 °C at 5 K/min

Interpretation

Four different untreated vegetable proteins were investigated. Each curve shows an endothermic peak in the range between 72 °C and 100 °C that is characteristic for protein denaturation. The proteins differ in their thermic stability and enthalpy of denaturation. With comparable sample weights and protein concentrations, the integrals of the peaks are a direct measure of the specific reaction enthalpies. Spelt contains the proteins that are most temperature sensitive and that have the highest enthalpy of denaturation. Whereas the wheat proteins are already extensively denatured at 86 °C, the main fraction of the soybean protein is still native. In this case, denaturation does not set in until 88 °C. The enthalpies of denaturation for grain and oil seed proteins usually lie in the range 3 – 10 J/g protein. The enthalpy changes depend on the phase of the proteins, so that the enthalpy of denaturation of seed proteins in the original condition is less than in the dissolved state in aqueous solutions.

The higher the enthalpy of denaturation of the native protein, the better the measurement of the fractions of denatured protein in processed protein samples by DSC. The free selection of the heating rate with DSC can be of use to help improve the measurement and detection of reactions. The higher the heating rate, the larger the detected signal, i.e. the larger the measured peak. The calculated peak area should of course be the same if the reaction process is the same. However, it should be noted that at high heating rates the reaction is shifted to higher temperatures. Identification of vegetable proteins by DSC is an application that is less likely to be used in practice and would also be complicated since the reaction enthalpy can vary depending on the variety and growth conditions. A native protein must, therefore, always be measured as a reference when estimating the degree of denaturation. The DSC curve is a ‘fingerprint’ of the measured sample. Such fingerprints of protein fractions in foods are characteristic of the condition of the protein.

Evaluation

Conclusions

The measurements discussed show that, in principle, any product which contains protein can be investigated by DSC. This applies all the more to animal proteins (see also egg, blood and muscle protein) owing to the generally higher original protein concentration in animal products (muscle), the greater measurement sensitivity and their occurrence in an aqueous environment.

^{Denaturation of Vegetable Proteins | Thermal Analysis Application No. HB 1002 | Application published in METTLER TOLEDO TA Application Handbook Food}