Effects of deamidation on aggregation and emulsifying properties of barley glutelin

The structure, solubility and emulsifying properties of the deamidated barley glutelin were investigated. The SDS–PAGE, size exclusion chromatography combined with a laser photometer (SEC-LP) and Fourier transform infrared spectroscopy (FTIR) results revealed that glutelin underwent a rapid hydrolysis and unfolding even at the initial stage of deamidation, leading to remarkably improved glutelin solubility at both acidic and neutral pHs. The deamidated glutelin demonstrated a strong tendency to form soluble aggregates of very large molecular weight (10⁶–10⁷ g/mol) probably due to disulphide-crosslinking and hydrophobic interactions. These aggregates played a major role in stabilising the emulsions at a broad range of deamidation levels (2.2–5.6% to 43%). In this way, undesirable property changes which result from excessive deamidation can be avoided, leading to wider applications of deamidated barley glutelin as an emulsifying ingredient in food and non-food applications.     

Effect of Enzymatic Protein Deamidation on Protein Solubility and Flavor Binding Properties of Soymilk

Abstract: The effect of enzymatic deamidation by protein‐glutaminase (PG) on protein solubility and flavor binding potential of soymilk was studied. Treatment of soymilk with PG for 2 h (temperature of 44 °C and enzyme:substrate ratio (E/S) of 40 U/g protein) resulted in high degree of protein deamidation (66.4% DD) and relatively low degree of protein hydrolysis (4.25% DH). Deamidated (DSM) and control soymilks (CSM) did not differ with respect to aroma, but differed in taste characteristics by sensory evaluation. Protein solubility in DSM was enhanced at weakly acidic conditions (pH 5.0), but did not differ from non‐deamidated soymilk at pH values of 3.0 and 7.0. Odor detection thresholds for the flavor compounds vanillin and maltol were approximately 5 and 3 fold lower, respectively, in DSM than in CSM. Dose‐response curves (Fechner's law plots and n exponents from Stevens's power law) further demonstrated that DSM had a lower flavor binding potential than CSM. PG deamidation has the potential to reduce flavor binding problems encountered in high protein‐containing foods and beverages. Practical Application: The findings of this study can help lead to the development of technology to produce protein‐containing foods with improved functional properties, especially protein solubility, and potentially decreased flavor fade problems associated with flavor‐protein interactions, especially with carbonyl containing flavor compounds.     

Modification of rice protein with glutaminase for improved structural and sensory properties

Rice protein is highly nutritive and hypoallergenic but certain physicochemical properties restrict its applications in beverages. This study aimed to tackle this issue by modifying rice endosperm protein with glutaminase via deamidation. Glutaminase treatment facilitated an increase in deamidation degree and mean particle size of rice protein and a continuing decrease in turbidity with time. Favourable changes induced in sensory properties included: improved suspendability (increased by 81.25%), lumpiness (decreased by 84.78%), grittiness (decreased by 72.34%) and overall taste liking (improved by 12.24%). Visual observations of rice protein dispersions and turbidity measurements further confirmed the improved suspendability in water. Fourier transform infrared spectroscopy analysis revealed that the action of glutaminase did not remarkably change the protein structure, but slightly reduced α-helical conformation whilst increased β-sheet and β-turn structures. Glutaminase could be useful for deamidation modification of products with high content of glutamine residues (like rice protein), through inducing the breakdown of some intermolecular and intramolecular bonds including hydrophobic interactions and hydrogen bonding without serious cleavage of the peptide bonds, allowing protein rearrangements to more flexible or extended forms. As a result of protein unfolding with more hydrophobic clusters exposed, certain physicochemical properties of rice protein were improved.     

Effects of interaction between rice glutelin and starch on starch gelatinization in a simulated storage system

Starch and protein are the two major components of rice and interaction between them will occur during storage. However, whether the interaction between starch and protein impacts rice starch gelatinization is still unclear. In this study, the effects of starch-protein interaction on starch gelatinization (japonica rice) were investigated using a glutelin–starch simulation system under high temperature and humidity storage conditions. A glutelin–starch ratio of 1:4 was used in the simulation system and stored in an artificial climate chamber (40 °C, 60% humidity) for 60 d. The interaction had a significant effect on starch gelatinization. Starch granule particles were broken down, and the bonds of starch granules weakened during interaction with glutelin. The secondary structure of glutelin loosened, as the starch disintegrated the α-helix structure. Correlation analyses revealed that peak viscosity (PV), trough viscosity (TV), and final viscosity (FV) had a significant negative correlation with carbonyl content, and disulfide bonds (P < 0.01), as well as a significant positive correlation with the active thiol content (P < 0.01). Moreover, setback (SB) had a significant positive correlation with the α-helix and a negative correlation with the β-turn content (P < 0.05). PV, TV, breakdown (BD), and FV were significantly related to β-sheet in glutelin (P < 0.05). These results suggest that glutelin and starch can interact and affect starch gelatinization.     

Effect of deamidation by protein-glutaminase on physicochemical and functional properties of skim milk

Protein-glutaminase (PG) is an enzyme that catalyzes the deamidation of glutamine residues in proteins, and protein deamidation is a promising method of improving solubility and other functional properties of foods. We investigated the effect of PG on skim milk proteins, finding that their deamidation degree increased dose-dependently. The solubility and relative viscosity of skim milk solution were remarkably improved as the deamidation degree increased, while its turbidity decreased until the milk became translucent; emulsifying capacity also increased, and the mean droplet diameter decreased to <4 μm. Particle size distribution analysis and transmission electron microscopy showed that smaller submicelle particles were produced in highly deamidated skim milk. These observations suggest that PG deamidation induces the dissociation of casein micelle, resulting in an increase of the oil/water surface area. This enzymatic deamidation helped to improve the functionality of skim milk by increasing the electrostatic repulsion of carboxyl groups in caseins.     

脱酰胺对热诱导小麦面筋蛋白构象及凝胶性质的影响

利用蛋白质谷氨酰胺酶(PGase)对小麦面筋蛋白进行处理,研究PGase脱酰胺及热诱导对小麦面筋蛋白构象及其凝胶性质的影响。结果表明,PGase脱酰胺能够在一定程度上抑制小麦面筋蛋白的热诱导聚集行为,提高小麦面筋蛋白热诱导凝胶的强度和保水性。当PGase添加量为3 U/g时,其凝胶强度和保水性均达到最大值,分别为903. 27 N和78. 70%。研究还表明,PGase处理后小麦面筋蛋白与水相互作用增加,从而提高了脱酰胺小麦面筋蛋白热诱导凝胶中非可冻结水含量。扫描电镜结果显示PGase脱酰胺结合热诱导处理的小麦面筋蛋白形成了更加有序的凝胶网络结构。     

Effects of enzymatic modification of wheat protein on the formation of pyrazines and other volatile components in the Maillard reaction

Enzymatically hydrolysed wheat gluten hydrolysate (WGH) was deamidated using glutaminase to produce deamidated wheat gluten hydrolysate (DWGH). Volatile components were analysed in WGH and DWGH thermally reacted with glucose or fructose. In the reaction system containing glucose, 19 pyrazines, 2 furans, and 5 sulphur-containing components were detected in WGH, while 34 pyrazines, 4 furans, and 7 sulphur-containing components were found in DWGH. In the system containing fructose, 24 pyrazines, 3 furans, and 6 sulphur-containing components were identified in the thermal reaction of WGH, whereas 36 pyrazines, 4 furans, and 8 sulphur-containing components were found in DWGH. The volatile components increased in DWGH, both qualitatively and quantitatively, mainly due to free ammonia released by deamidation. More volatiles were also developed in WGH and DWGH with fructose than with glucose. It was found that ammonia released from wheat protein via deamidation participated in the generation of diverse volatile components including pyrazines in the Maillard reaction.     

不同pH条件下米谷蛋白的理化及结构特性研究

为探索溶解大米蛋白的最适pH值,扩大其应用范围,表征了不同pH(pH 3.0,4.0,7.0)条件下大米蛋白中主要成分——米谷蛋白的理化及结构性质。结果表明,与中性条件下相比,酸性条件下米谷蛋白的溶解度和结构性质发生了明显改变,pH 7.0时米谷蛋白分子结合紧密,形成庞大的分子聚集体,溶解度仅(6.24±1.25)%;而在酸性条件下,米谷蛋白逐渐分散,分子间二硫键断裂,呈现分散疏松的小分子体状态,pH 3.0时其溶解度最高,达到(72.47±2.36)%。     

ENZYMATICALLY AND CHEMICALLY MODIFIED ZEIN FOR IMPROVEMENT OF FUNCTIONAL PROPERTIES

Limited specific hydrolysis of zein by trypsin was used to produce 1.42%, 1.70%, and 1.87% hydrolysates (percentage of peptide bonds cleaved), containing three to four polypeptides (by Fast Protein Liquid Chromatography (FPLC)) ranging in size from ～16,000 to 3,500 daltons. The mixtures of polypeptides had substantially increased solubilities at pH below 11, including at the isoelectric point of zein (pH 6–7). In general, the emulsifying activity index was less for the hydrolysates than for native zein; the emulsion stability was higher for the 1.70% hydrolysate than was the emulsion of native zein only at pH above 8. At lower pHs, the stability of the emulsions was similar for all samples. The foamability (% overrun) of the hydrolysates was higher than for native zein; the foam stability was higher for the 1.87% hydrolysate than for the other hydrolysates. Phosphorylation and deamidation of the hydrolysates markedly increased the water solubility in the pH range from 4 to 10. Chemical modification of the native zein improved the solubility at pH 2 to 9 after phosphorylation and pH 6 to 11 after deamidation as compared to the native protein. In general, the emulsifying activity index, as well as its stability, was less for the phosphorylated and deamidated hydrolysates than for native zein but the emulsifying activity index of deamidated native zein was higher at pH 7 to 11. Foaming activity and stability of chemically modified hydrolysates were similar to the 1.87% hydrolysate.     

Functional Properties of Deamidated Oat Protein Isolates

Oat protein isolates were deamidated by mild acid hydrolysis. Amino acid analysis and gel filtraiton chromatography showed no significant cleavage of peptide linkages although the aggregated and oligomeric oat proteins were extensively dissociated. Deamidation led to marked improvement in solubility, emulsifying properties and water and fat binding capacities. The pH for heat-induced gelation was lowered by deamidation, and a firm, elastic gel was produced by mixing egg white with deamidated oat isolates.