Gelation of egg white protein (EWP) solutions can be induced as a consequence of freezing and thawing in the presence of a denaturant (urea). This mechanism of cryogelation can be affected by the relative concentrations of protein and urea, as well as freezing conditions including freezing temperature and freezing duration.
The compression peak force (CPF) of the obtained cryogels was measured using large deformation texture analysis to indicate the gel strength of samples. In the range of added urea concentration from 1M to 6.6M the CPF of samples was seen to decrease, whilst increasing EWP concentration from 5% (w/v) to 15% (w/v) resulted in a strengthening of gel structure with increasing CPF. Lower freezing temperatures (e.g. -30oC) caused a decrease in CPF of samples compared to -18oC and samples stored for 68 hours had higher CPF than those stored for 20 hours (at 1M added urea).
Water holding capacity (WHC) of cryogel was determined by measuring the amount of released water from samples gravimetrically. With increasing concentration of urea the WHC of samples was seen to initially decreased, followed by a progressive increase as the concentration of urea was raised. This trend was observed for most protein concentration with the exception of 5% EWP which showed a significant increase at 4M urea, followed by a drop at higher urea concentrations. Increasing EWP concentrations tended to result in higher WHC in general, although samples with 10% and 12% EWP had relatively similar WHC values. WHC of samples frozen at -30oC was higher than those frozen at -18oC at high urea concentrations. However, increasing the frozen storage time did not appear to affect the WHC.
The microstructure of EWP cryogel was observed using scanning electric microscopy (SEM) and transmission electric microscopy (TEM). The SEM data showed a porous structure for all samples. The increase in the concentration of added urea and the decrease in freezing temperature seemed to reduce the porosity and connectivity of the structure especially at low EWP concentrations. There was also some observed difference of the gel wall thickness between some different samples. The TEM data provided a clear distribution of protein and pores within the gel phase.
The effect of urea on the thermal stability of protein molecules was studied using nano differential scanning colorimetry (nano DSC). Results showed that the addition of urea progressively denatured the protein with increasing urea concentration. Proteins appeared to be further denatured as a consequence of the freezing-thawing process.
The effect of urea addition on freezing point depression and ice content was calculated, allowing the protein content in the unfrozen phase to be determined. The relative concentrations of protein, urea, frozen and unfrozen water in the frozen state provided some indications as to how the extent of denaturation coupled with freeze concentration in the unfrozen phase contributed to the cryogels structures being formed. A number of correlations were determined that assisted in developing a mechanistic understanding of the cryogelation effect. Findings demonstrate a potential means of creating food gel structures with novel structural and material characteristics.