Influence and effect mechanism of filler type on the physicochemical properties, microbial numbers, and digestibility of ovalbumin emulsion gels during storage.

This study was designed to investigate the influence and effect mechanism of the filler type on the physicochemical properties, microbial numbers, and digestibility of ovalbumin emulsion gels (OEGs) during storage. Sunflower oil was emulsified with ovalbumin (20 mg mL-1) and Tween 80 (20 mg mL-1) separately to prepare ovalbumin emulsion gels (OEGs) that contained active and inactive fillers, respectively. The formed OEGs were stored at 4 °C for 0, 5, 10, 15, and 20 days. The active filler enhanced the gel hardness, water holding capacity, fat holding capacity, and surface hydrophobicity and decreased the digestibility and free sulfhydryl content during storage compared to control (unfilled) ovalbumin gel, whereas the inactive filler had the opposite effects. Protein aggregation diminished, lipid particle aggregation increased, and the amide A band shifted to a higher wavenumber for all three types of gel during storage, suggesting that the compact network structure of the OEG became rough and disordered with storage. The OEG with the active filler did not inhibit microbial growth, and the OEG with the inactive filler did not significantly promote the development of bacteria. In addition, the active filler delayed the in vitro digestion of the protein in the OEG throughout storage. Emulsion gels containing active filler facilitated the retention of the gel properties during storage, whereas emulsion gels containing inactive filler exacerbated the loss of the gel properties during storage.

Influence and effect mechanism of disulfide bonds linkages between protein-coated lipid droplets and the protein matrix on the physicochemical properties, microstructure, and protein structure of ovalbumin emulsion gels.

In this study, disulfide bonds between the interfacial protein film formed on the lipid particles and the protein in ovalbumin emulsion gels were blocked with 0, 1, 3, 5 and 10 mM of the N-ethylmaleimide (NEM) to explore the influence and effect mechanism of disulfide bonds between the interfacial proteins on the physicochemical properties, microstructure, and protein structure of sunflower oil-ovalbumin emulsion gels. Ovalbumin emulsion gels with NEM-treated ovalbumin emulsion (N-OE) had lower hardness, free sulfhydryl content, water holding capacity (WHC), and surface hydrophobicity, but higher spin-spin relaxation time (T2) than ovalbumin emulsion gels with NEM-treated ovalbumin substrate solution (N-OSS). In addition, N-OE and N-OSS had lower hardness, free sulfhydryl content, WHC and surface hydrophobicity, as well as a more coarse and disordered microstructure than non-NEM treated ovalbumin emulsion gel (control group). The free sulfhydryl content, hardness, WHC, and surface hydrophobicity of the ovalbumin emulsion gels all decreased as the NEM concentration rose (p < 0.05), whereas the amide A band changed to higher wave numbers. These results collectively indicated that the reduction of disulfide between the interfacial layer and the proteins inhibited the hydrophobic effect, the formation of hydrogen bonds, and prevented the formation of larger aggregates. Thus the disulfide bonds between the interfacial proteins contribute to the hardness enhancement and water stabilization of the ovalbumin gel.

Corrigendum: The changes of umami substances and influencing factors in preserved egg yolk: pH, endogenous protease, and proteinaceous substance.

[This corrects the article DOI: 10.3389/fnut.2022.998448.].

The changes of umami substances and influencing factors in preserved egg yolk: pH, endogenous protease, and proteinaceous substance.

The study investigated the changes of nucleotides, succinic acid, and free amino acids amounts in yolk and the causes leading to the changes after pickling to uncover the fundamental umami component of preserved egg yolk. The findings demonstrated that while the contents of 5'-adenosine monophosphate (AMP), 5'-cytidine monophosphate (CMP), 5'-guanosine monophosphate (GMP), 5'-uridine monophosphate (UMP), and succinic acid increased after slightly decreasing aspartic acid (Asp) content in preserved egg yolk increased gradually. The contents of 5'-inosine monophosphate (IMP) and other free amino acids were gradually decreased. Comparing the taste activity value (TAV), it was found that the single umami substance, succinic acid, played a key role in inducing the umami taste. In combination with the Spearman correlation analysis, it was shown that the proteinaceous substance, which is the most significant umami component in preserved egg yolk, tended to condense first and subsequently disintegrate in an alkaline environment. The orthogonal partial least squares analysis (OPLS) found that pH was also affected by the changes in proteinaceous substance. These findings offer suggestions for enhancing the pickling procedure and investigating the optimal pickling period for preserved eggs.

Effects of partial replacement of NaCl by KCl and CaCl2 on physicochemical properties, microstructure, and textural properties of salted eggs.

KCl and CaCl2 were used as partial substitutes for NaCl during pickling salted eggs process in this study. The effects on the physicochemical properties, microstructure, textural properties and sensory quality of the salted eggs were evaluated, while comparing with the 18% NaCl group (Na group). The 3% replacement of NaCl by KCl reduced the Na content (p < 0.05), accelerated the water migration (p < 0.05) in salted eggs and increased the apparent oil yield and oil exudation of salted egg yolk (p < 0.05); but the rheological properties and microstructure of salted egg yolk were minimally affected. The 3% replacement of NaCl by CaCl2 reduced the Na content (p < 0.05), delayed the water migration rate (p < 0.05) in salted eggs and decreased the apparent oil yield and oil exudation of salted egg yolk (p < 0.05). Additionally, the process of egg white thinning and egg yolk hardening were delayed. The results indicate that the partial substitution of NaCl by KCl or CaCl2 during the pickling process of salted eggs could effectively inhibit the infiltration of Na+ , and the presence of KCl could improve the quality of salted eggs. Still, the presence of CaCl2 delay the ripening of salted eggs. PRACTICAL APPLICATION: KCl and CaCl2 as substitutes of sodium salt could play the role of reducing Na content but not affecting saline taste of salted eggs, which is conducive to the development of low-sodium salted eggs.

Isolation and screening of umami peptides from preserved egg yolk by nano-HPLC-MS/MS and molecular docking.

The material basis leading to the rich umami flavor of preserved egg yolk is poorly understood. This study used nano-high-performance liquid chromatography - tandem mass spectrometry (nano-HPLC-MS/MS) to isolate, identify, and screen umami peptides from preserved egg yolk. Five novel umami peptides-AGFMPLP, APYSGY, PPMF, SLSSLMK, and VAMNPVDHPH-were identified. Molecular docking showed that Phe527 on the taste receptor T1R1/T1R3 (T1R1, taste receptor type 1 member 1; T1R3, taste receptor type 1 member 3) was the key interaction site. Hydrogen bonding, electrostatic interactions, and hydrophobic interactions were the main binding forces between T1R1/T1R3 and umami peptides. These results contribute to understanding the umami peptides in preserved egg yolk and the interaction mechanism between umami peptides and umami receptors.

Improvement of gel properties and digestibility of the water-soluble polymer of tea polyphenol-egg white under thermal treatment.

In this study, the improvement of gel properties and digestibility of the water-soluble polymer of tea polyphenol (TP)-egg white protein (TEP) under heat induction (HTEP), was studied. Results indicated that the particle size and turbidity of TEP increased with TP concentration, and the absolute value of ζ-potential decreased. After heat induction, the surface hydrophobicity of HTEP decreased with TP concentration, and the degree of protein aggregation increased. Microstructure and T2 showed that the gel structure became compact and stable, and HTEP had a strong water-binding ability. The ionic and disulfide bonds were the main chemical bonds in HTEP. The hardness and disulfide bond increased, but the digestion of HTEP increased initially and then decreased (caused by the change of gel structure). Infrared spectroscopy indicated the mutual conversion of intermolecular and intramolecular ß-sheets. In short, TP could modify egg white gel through forming stable disulfide bonds and dense gel network structures.

Warming effects on permafrost ecosystem carbon fluxes associated with plant nutrients.

Large uncertainties exist in carbon (C)-climate feedback in permafrost regions, partly due to an insufficient understanding of warming effects on nutrient availabilities and their subsequent impacts on vegetation C sequestration. Although a warming climate may promote a substantial release of soil C to the atmosphere, a warming-induced increase in soil nutrient availability may enhance plant productivity, thus offsetting C loss from microbial respiration. Here, we present evidence that the positive temperature effect on carbon dioxide (CO2 ) fluxes may be weakened by reduced plant nitrogen (N) and phosphorous (P) concentrations in a Tibetan permafrost ecosystem. Although experimental warming initially enhanced ecosystem CO2 uptake, the increased rate disappeared after the period of peak plant growth during the early growing season, even though soil moisture was not a limiting factor in this swamp meadow ecosystem. We observed that warming did not significantly affect soil extractable N or P during the period of peak growth, but decreased both N and P concentrations in the leaves of dominant plant species, likely caused by accelerated plant senescence in the warmed plots. The attenuated warming effect on CO2 assimilation during the late growing season was associated with lowered leaf N and P concentrations. These findings suggest that warming-mediated nutrient changes may not always benefit ecosystem C uptake in permafrost regions, making our ability to predict the C balance in these warming-sensitive ecosystems more challenging than previously thought.

Different soil respiration responses to litter manipulation in three subtropical successional forests.

Aboveground litter inputs have been greatly altered by human disturbances and climate change, which have important effects on soil respiration. However, the knowledge of how soil respiration responds to altered litter inputs is limited in tropical and subtropical forests. We conducted an aboveground litterfall manipulation experiment in three successional forests in the subtropics to examine the soil respiration responses to different litter inputs from January 2010 to July 2012. The soil respiration decreased by 35% in the litter exclusion treatments and increased by 77% in the doubled litter additions across all three forests. The reduction in soil respiration induced by the litter exclusion was greatest in the early successional forest, which may be related to a decrease in the soil moisture and shifts in the microbial community. The increase in soil respiration produced by the doubled litter addition was largest in the mature forest, which was most probably due to its relatively high quantity and quality of litterfall. Our results suggest that the effect of reduced litter inputs on the soil respiration lessened with forest succession but that the doubled litter inputs resulted in a stronger priming effect in the mature forest than in the other two forests.

CO2 enrichment and N addition increase nutrient loss from decomposing leaf litter in subtropical model forest ecosystems.

As atmospheric CO2 concentration increases, many experiments have been carried out to study effects of CO2 enrichment on litter decomposition and nutrient release. However, the result is still uncertain. Meanwhile, the impact of CO2 enrichment on nutrients other than N and P are far less studied. Using open-top chambers, we examined effects of elevated CO2 and N addition on leaf litter decomposition and nutrient release in subtropical model forest ecosystems. We found that both elevated CO2 and N addition increased nutrient (C, N, P, K, Ca, Mg and Zn) loss from the decomposing litter. The N, P, Ca and Zn loss was more than tripled in the chambers exposed to both elevated CO2 and N addition than those in the control chambers after 21 months of treatment. The stimulation of nutrient loss under elevated CO2 was associated with the increased soil moisture, the higher leaf litter quality and the greater soil acidity. Accelerated nutrient release under N addition was related to the higher leaf litter quality, the increased soil microbial biomass and the greater soil acidity. Our results imply that elevated CO2 and N addition will increase nutrient cycling in subtropical China under the future global change.