| Dehydration is a good process approach for food preservation. However, dried food products may still suffer from deterioration if store in an abused environment such as high humidity (water activity (aw) > 0.6) and/or high temperature (> 45°C). These storage conditions can induce undesirable chemical reactions (disulfide bond interactions, Maillard reaction and/or lipid oxidation), resulting in a significant decrease in food quality.;In this study, the storage stability of a commercial spray-dried egg yolk powder was evaluated. The dried egg yolk powder (DEY) was stored at three temperatures (room temperature, 35°C, and 45°C) and at six aw (0.05, 0.12, 0.37, 0.44, 0.54, 0.66) for at least two months, and several physicochemical changes and extent of protein aggregation were measured.;The overall color change of DEY was that it became slightly darker (decrease of L* value), more red (increase of a* value), and less yellow (decrease of b* value) with increased storage time. The reaction kinetics of the L* value of DEY was also calculated using a first-order hyperbolic model. Its Q10 (rate increase with temperature increase at 10°C) was 2.9, which was more indicative of lipid oxidation, and the Ea (activation energy) was around 83 kJ/mole. The color change was mostly due to the browning pigments that were produced from the Maillard reaction and lipid oxidation. The glucose content went to zero after one-week during storage at 45°C at an aw of 0.66, confirming the occurrence of the Maillard reaction. The peroxide value of DEY storage at 45°C at aw of 0.66 was significantly increased compared to the control (vacuum packaged at -20°C), proving the occurrence of lipid oxidation. In addition, the Maillard reaction products and lipid oxidation products were both detected using the front face fluorescence spectrometer.;After storage at an aw of 0.66 at 45°C for 8 weeks, protein solubility of DEY in TBS-SDS buffer [Tris-buffered saline (TBS: 20 mM Tris and 500 mM sodium chloride, pH 7.5) containing 1% sodium dodecyl sulfate (SDS, g/ml)] decreased to ~ 78% compared with that of the original DEY. Formations of buffer-soluble and --insoluble protein aggregates were discovered using SDS-PAGE. The protein aggregates were mainly formed through unfolded intermediates and unfolded states as well as direct chemical linkages. The proteins in DEY were all denatured after storage at an aw of 0.66 at 45°C for 8 weeks, resulting in numerous unfolded intermediates and states that could interact with each other to form aggregates. The spray drying process during the manufacturing of DEY also caused denaturation of protein, which explained the detection of buffer-insoluble protein aggregates in the original sample. Increases of disulfide bond links and protein-lipid interaction during storage were also found using techniques such as Raman spectrometry, fourier transform infrared spectroscopy, and front-face fluorescence spectrometry, indicating that some of the protein aggregates were induced by chemical reactions.;The high molecular weight protein aggregates (HMWPAs) were further evaluated. Results showed that 32 proteins were involved with formation of buffer-soluble and -insoluble HMWPAs. They were products of natural egg yolk proteins and egg white proteins including serum albumin, vitellogenin, apovitellenin, as well as ovotansferrin, ovalbumin, lysozyme, ovomucoid, and ovastatin. Most of them contain disulfide bonds and some of them contain ligand and fatty acid binding sites, which corresponded with the theory of the direct chemical linkages induced protein aggregates.;Overall, physicochemical changes and protein aggregates were found during the storage of DEY and it is mostly due to three undesirable chemical reactions, i.e., disulfide bond interactions, the Maillard reaction and/or lipid oxidation. Therefore, most effective approaches to reduce and/or inhibit the occurrence of those reactions include adjusting storage temperature and humidity as well as vacuum packaging after drying. |
