Structural changes and dynamic rheological properties of sarcoplasmic proteins subjected to pH-shift method

Contribution of mono- and co-culture of Pseudomonas paralactis, Acinetobacter MN21 and Stenotrophomonas maltophilia to the spoilage of chill-stored lamb

Exploring the contribution of common microorganisms to spoilage is of great significance in inhibiting spoilage in lamb. This work investigated the extent of protein degradation and profile changes of free amino acids (FAAs), free fatty acids (FFAs) and volatile organic compounds (VOCs) in lamb caused by single- and co-culture of the common aerobic spoilage bacteria, P. paralactis, Ac. MN21 and S. maltophilia. Meanwhile, some key VOCs produced by the three bacteria during lamb spoilage were also screened by orthogonal partial least square discriminant analysis and difference value in VOCs content between inoculated groups and sterile group. Lamb inoculated with P. paralactis had the higher total viable counts, pH, total volatile base nitrogen and TCA-soluble peptides than those with the other two bacteria. Some FAAs and FFAs could be uniquely degraded by P. paralactis but not Ac. MN21 and S. maltophilia, such as Arg, Glu, C15:0, C18:0 and C18:1n9t. Co-culture of the three bacteria significantly promoted the overall spoilage, including bacterial growth, proteolysis and lipolysis. Key VOCs produced by P. paralactis were 2, 3-octanedione, those by Ac. MN21 were 1-octanol, octanal, hexanoic acid, 1-pentanol and hexanoic acid methyl ester, and that by S. maltophilia were hexanoic acid. The production of extensive key-VOCs was significantly and negatively correlated with C20:0, C23:0 and C18:ln9t degradation. This study can provide a basis for inhibiting common spoilage bacteria and promoting high-quality processing of fresh lamb.

Evolution of lean meat tenderness stimulated by coordinated variation of water status, protein structure and tissue histology during cooking of braised pork

Tenderness of lean meat in braised pork is of great importance to the consumer palatability and acceptance. The influence of water status, protein structure and histological changes on lean meat tenderness during cooking was investigated. Results indicated that lean meat began to tenderize mainly after 20 min-cooking. In the early period of cooking, the decrease of total sulfhydryl content caused the protein oxidative cross-linking, leading to the gradual unfolding of the protein structure, thus resulting in a decrease of T₂₂ and an increase of centrifugal loss, which decreased the tenderness of lean meat. However, after cooking for 20 min, the β-sheet decreased and random coil increased, thus generating conversion between P₂₁ and P₂₂. The rupture of perimysium structure was observed. Changes in protein structure, water status, and tissue histology could facilitate the initiation and development of lean meat tenderness.

Release mechanism between sarcoplasmic protein-bound and free heterocyclic amines and the effects of dietary additives using an in-vitro digestion model

In this study, the release mechanism between free and sarcoplasmic protein-bound heterocyclic amines (HAs) in an in-vitro digestion model were investigated by comparing changes in the amount of HAs between groups with different enzyme dosages and at different digestion stages. The effects of the addition of onions, peppers, and apples on the release mechanism were also studied. We found that bound HAs cannot be completely converted to free HAs under normal enzyme dosages and that a significant increase in HAs occurred in the intestines. The release rate of bound HAs was 5.99%-43.84%, and the total release rate of HAs was 36.67%. Furthermore, the release rate of β-carbolines was the highest (34.41%-43.84%). The addition of onions, peppers, and apples significantly increased the number of free HAs, with growth rates reaching 56.06%, 43.43%, and 54.44%, respectively. These additives mainly promoted the generation of free harman and norharman.

Synergistic effect of preheating and different power output high-intensity ultrasound on the physicochemical, structural, and gelling properties of myofibrillar protein from chicken wooden breast

The effects of preheating to 50 ℃ and the subsequent application of high-intensity ultrasound (HIU, 20 kHz) at 200, 400, 600, and 800 W on the physicochemical, structural, and gelling properties of wooden breast myofibrillar protein (WBMP) were studied. Results suggested that the WBMP structure expanded to the balanced state at 600 W, and rheological properties exhibit that 600 W HIU (P < 0.05) significantly improved the storage modulus (G') of WBMP. Notably, the WBMP gel (600 W) had the best hardness (65.428 ± 0.33 g), springiness (0.582 ± 0.01), and water-holding capacity (86.11 ± 0.83%). Raman spectra and low-field NMR indicated that 600 W HIU increased the β-fold content (37.94 ± 0.04%) and enlarged the immobilized-water proportion (93.87 ± 0.46%). Scanning electron micrographs confirmed that the gel was uniform and dense at 600 W. Therefore, preheating to 50 ℃ followed by HIU (600 W) helped form a superior WBMP gel.

Protein extraction from catfish byproducts and physicochemical properties of the protein isolates

In order to optimize protein recovery from catfish byproducts by alkaline extraction, the effects of different factors, including particle size, mince-to-water ratio, pH, and extraction time were investigated. It was found that a protein recovery of about 30% could be achieved. Increases in pH (pH 10.5, 11, and 11.5) not only improved protein recovery, but also increased protein denaturation evidenced by decreased solubility, decreased α-helix, increased β-sheet, and increased random coil. The color and texture of gels made from protein isolate were greatly affected by the pH values used for protein extraction. For the gels made from fillet mince, and protein isolates extracted at pH 10.5, 11, and 11.5, the "L" values were 78.96, 60.38, 57.74, and 54.39, the breaking forces were 205, 492, 585, and 458 g, and deformation values were 10.59, 8.07, 6.73, and 5.04 mm, respectively. Electrophoresis revealed protein degradation during alkali-aided extraction with MHC, the most predominant band, showing about 50% decrease in comparison with fillet mince. It also demonstrated that gelation not only caused cross-linking, but also autolysis with 53%, 56%, 59%, and 81% decrease in MHC intensity for fillet mince, protein isolates extracted at pH 10.5, 11, and 11.5, respectively. Fillet mince and protein isolates exhibited different storage modulus patterns during temperature sweep, implying different gelation mechanisms. This study proved the protein extracted from catfish byproducts was potential to be utilized as edible food components especially in gel making. PRACTICAL APPLICATION: Catfish byproducts, which account for 70% of total weight and 50% of total protein of catfish, are normally used as animal feed, fertilizer, or even waste. This study demonstrated the potential of the utilization of catfish wastes to develop edible food components. This could reduce the total processing waste being discarded into the environment and nutrient loss, therefore increasing profitability of catfish industry.

Effects of low voltage electrostatic field on the microstructural damage and protein structural changes in prepared beef steak during the freezing process

This study investigated the effect of low voltage electrostatic field (LVEF) on the microstructure damage and protein structure changes of prepared beef steak during freezing. The scanning electron microscopy results showed that LVEF-assisted freezing (LVEFF) minimized the gaps in the cross section between muscle fibers induced by freezing and thus improved fiber compactness. Furthermore, LVEFF reduced the length of the enlarged sarcomere, repaired the Z-line fractures, and intensified the dismission of the A band in the air-blast freezing (AF) process. The decreased carbonyl content and increased total sulfhydryl content indicated that LVEFF reduced protein oxidation in the freezing process. In addition, the results of Raman spectroscopy and fluorescence spectroscopy revealed that LVEFF minimized the changes in protein secondary and tertiary structures during freezing. In conclusion, utilization of LVEF in the freezing of prepared beef steak could reduce both the microstructure damage and protein structure changes in the freezing process.

Physicochemical characteristics of surimi-like material made from the muscle tissues of freshwater mussels (Sinanodonta woodiana Lea, 1834)

The aim of the study was to determine selected characteristics of surimi-like material (SLM) made from the muscle tissues of Sinanodonta woodiana (Lea, 1834) freshwater mussels. The research material consisted of unwashed mussel muscle homogenate as the control sample—C, mussel muscle tissue twice washed with water (SLM-W) and sample washed with NaCl at a concentration of 0.169 mol/L as well as water (SLM–S). A raw control sample and surimi like-materials were analysed using the SDS-PAGE technique. They were also tested using the DMTA method (dynamic mechanical thermal analysis) while heating the samples up to 80 °C and cooling to 20 °C. The thermal drip and texture of gels after heating (75 °C, 30 min) were also determined. The washing procedure had a significant impact on the protein composition of the SLMs. A significantly (p < 0.05) higher percentage of proteins with molecular weights of 270–273 kDa and 105–110 kDa (corresponding to specific filamin isoforms), as well as 42–43 kDa (corresponding to actin), were found in the SLMs compared to sample C. Ccorrelation analysis confirmed a strong positive relationship between the percentage share of the above proteins and the values of the elasticity modulus (r ≥ 0.84) and firmness (r ≥ 0.88) of SLM gels. The SLM-S sample was characterised by the lowest significant (p < 0.05) thermal drip values. During heating, the rheological traits of all samples changed at two stages: from 20 to 50 °C and over 50 °C. The highest dynamics of variation in the elasticity modulus (G′) value were noticed at temperatures exceeding 50 °C in all samples. The G′ values in the SLM samples were significantly greater than the values in sample C. The analysis revealed a strong correlation (r ≥ 0.81) between the determinants of the texture of the SLM samples and their rheological parameters. Sample SLM-W was the one with the highest firmness and elasticity values. The analysis showed that the textural properties of the SLM samples mainly resulted from the reaction of spatial matrices to mechanical interactions.

Generation of Sarcoplasmic and Myofibrillar Protein-Bound Heterocyclic Amines in Chemical Model Systems under Different Heating Temperatures and Durations

The protein-bound heterocyclic amines (HAs) and their generation pattern are still unclear. Generation of sarcoplasmic (SP)- and myofibrillar protein (MP)-bound HAs under different heating conditions was investigated in chemical model systems using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The results showed that eight types (387.01 ± 37.50 ng/100 mg MP) and six types (452.06 ± 33.32 ng/100 mg SP) of protein-bound HAs were generated at 180 °C/40 min, respectively. MP system can generate either more types or amounts of bound HAs at 90-150 °C. The main categories of protein-bound HAs in two systems were almost the same: β-carbolines, α-carbolines, pyridines, and quinolines, among which nonpolar HAs dominated by β-carbolines (at least 69.2%) are most easily formed. Principal component analysis revealed no significant difference in bound HAs at 90-120 °C but showed a significant increase at 150-180 °C. The higher the temperature, the more significant the differences between samples with different durations at the same temperature.

Effect of Fermentation with Two Molds on Characteristics of Chicken Meat

In the present study, we investigated the characteristics of chicken meat fermented with Penicillium nalgiovense and Penicillium chrysogenum. Hardness and springiness gradually decreased, while gumminess gradually increased during fermentation. Fermentation with P. chrysogenum led to higher hardness and lower gumminess than fermentation with P. nalgiovense. Fermentation with two molds resulted in similar microstructure, such as granule formation and fractured myofibril. The highest percentage of secondary structure was ɑ-helix, and tyrosine residues were buried after fermentation. P. nalgiovense-fermented samples contained more bound water, lower relative content of alkanes, and higher relative content of aldehydes than P. chrysogenum-fermented samples.

Beneficial effects of Jerusalem artichoke powder and olive oil as animal fat replacers and natural healthy compound sources in Harbin dry sausages

This study aimed to improve the nutritional quality of Harbin dry sausages using natural plant-based Jerusalem artichoke powder (JAP) and olive oil as animal fat replacers. Low-fat Harbin dry sausages were manufactured with 2 different formulations containing JAP and olive oil as pork fat replacers. The texture, rheological properties, microstructure, water holding capacity, muscle protein structure, physicochemical indices, microbiological characteristics, and sensory evaluation of the sausages were analyzed. The result showed that Harbin dry sausages with JAP and olive oil were healthier than control sausages based on the lower fat content and improved fatty acid composition. Scanning electron microscopy showed gel network formation in sausages with a high JAP content. Low-field nuclear magnetic resonance illustrated that the water-holding capacity of the modified sausages was improved, suggesting that the replacers enhanced protein gel formation by changes in C-H stretching and bending vibrations, a reduction in α-helixes, and increases in β-sheets and random coils accompanying the exposure of reactive groups and microenvironment of the tertiary structure. Dynamic rheological and texture tests indicated that the replacers improved the elasticity of sausages. The reduction of fat and addition of replacers significantly enhanced lipid oxidative resistance. Overall, JAP and olive oil improved the fatty acid composition, gel structure, lipid oxidative resistance, and sensory quality of the sausages. These results may contribute to the development of healthy meat products to further reduce animal fat.

The influence of endogenous cathepsin in different subcellular fractions on the quality deterioration of Northern pike (Esox lucius) fillets during refrigeration and partial freezing storage

The purpose of this study was to investigate the endogenous cathepsin activity in each subcellular fraction and the effect of this activity on myofibrillar protein and texture during refrigeration and partial freezing storage of northern pike (Esox lucius) fillets. The results showed that fillets stored under the refrigerated condition were more susceptible to oxidation than partial freezing. Endogenous cathepsin activity indicated that partial freezing destroys the integrity of lysosomes more effectively than refrigeration and inhibits the increase in cathepsin B and B + L in lysosomes. The activity of cathepsin B and B + L in lysosomes, mitochondria and myofibrils under the partial freezing conditions was always lower than that under refrigeration. Texture analysis showed that refrigeration had a negative impact on hardness and springiness. In conclusion, the cathepsin activity in each subcellular fraction was effectively inhibited and better textural characteristics were obtained with partial freezing than refrigeration.

Rational design of a multi-valent human papillomavirus vaccine by capsomere-hybrid co-assembly of virus-like particles

The capsid of human papillomavirus (HPV) spontaneously arranges into a T = 7 icosahedral particle with 72 L1 pentameric capsomeres associating via disulfide bonds between Cys175 and Cys428. Here, we design a capsomere-hybrid virus-like particle (chVLP) to accommodate multiple types of L1 pentamers by the reciprocal assembly of single C175A and C428A L1 mutants, either of which alone encumbers L1 pentamer particle self-assembly. We show that co-assembly between any pair of C175A and C428A mutants across at least nine HPV genotypes occurs at a preferred equal molar stoichiometry, irrespective of the type or number of L1 sequences. A nine-valent chVLP vaccine-formed through the structural clustering of HPV epitopes-confers neutralization titers that are comparable with that of Gardasil 9 and elicits minor cross-neutralizing antibodies against some heterologous HPV types. These findings may pave the way for a new vaccine design that targets multiple pathogenic variants or cancer cells bearing diverse neoantigens.

Physicochemical and microstructural attributes of marinated chicken breast influenced by breathing ultrasonic tumbling

Currently, the conventional atmospheric pressure-based and vacuum-based tumbling processes have a limited improvement on the chicken characteristic attributes during the marination process. In view of this, through a breathing (pressure change) tumbling strategy, ultrasonication (40 kHz, 140 W) was applied to improve tenderness, taste, and microstructure of chicken by a redesigned tumbler. The results showed that the tumbling with the breathing action and ultrasonication significantly enhanced the marinating absorptivity, tenderness and taste, and accelerated the degradation of myosin light chain. Free peptides (from 1465.9 ± 34.6 to 4725.7 ± 43.2 μg/mL) and amino acids (from 1.503 ± 0.096 to 2.593 ± 0.109 mg/mL) rose evidently for the control and the breathing tumbling treatment assisted by ultrasound respectively. Raman analysis revealed that strength of disulfide bonds declined from 0.731 ± 0.006 to 0.607 ± 0.011 a.u. and the conversion from α-helix (decreased by 67.23%) into β-fold (increased by 1573%) conformation occurred. Low field NMR analysis indicated that the content of immobilized water increased from 77385 ± 14 to 137011 ± 106 au·ms by integral calculus. Scanning and transmission electron microscopies clearly showed a prospective rupture of myofibers, myofibrils, and lysosomes. Overall, as a potential alternative, the breathing ultrasonic tumbling means improved the marinating efficiency and characteristics of marinated chicken breast.

Substituting sodium by various metal ions affects the cathepsins activity and proteolysis in dry-cured pork butts

Salt contents and compositions are crucial parameters to enzyme activity and might even affect the proteolysis and quality of dry-cured meat. However, the metal ions significantly vary with geographic origin, which would be a determinant in the dry-cured meat manufacture. Therefore, the main salt compositions of KCl, CaCl₂, MgCl₂ and ZnCl₂ were therefore used to partially substitute NaCl while additionally assessing and regulating their function during the dry-cured pork butts processing. Physiochemical properties, cathepsins activity, proteolysis and sensory evaluation were investigated. The results indicated that partial substitution of sodium prevented the dehydration, and accelerated the pH reduction. CaCl₂ and MgCl₂ partial substitution moreover exerted some promoting effect on cathepsin activity whereas ZnCl₂ was a subtle inhibitor. The proteolysis index of CaCl₂ and MgCl₂ substitution were superior to the rest. The metal ions partial substitution reduced saltness, while the presence of KCl and MgCl₂ enhanced bitterness. Further correlation analysis was performed to better understand the interactions between those parameters.

Estimating the structure of sarcoplasmic proteins extracted from pork tenderloin thawed by a high-voltage electrostatic field

The application of high-voltage electrostatic field (HVEF) is a novel method of thawing. To determine if HVEF thawing could lead to sarcoplasmic proteins denaturation, and to provide a theoretical estimation of the structure of the sarcoplasmic proteins, pork tenderloin was thawed by traditional and HVEF methods. The results from protein solubility analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimeter showed that HVEF thawing did not result in more protein denaturation than those thawed under air or running water. From the principal component analysis of FTIR raw spectra (1700-1600 cm^-1, Amide I region), we observed some separations of samples with different thawing treatments. It was found that the proportions of α-helix (1650-1640 cm^-1 spectral bands in the original data) could lead to the differences on the PC2 axis of score plots.

Inhibition of Heat-Induced Flocculation of Myosin-Based Emulsions through Steric Repulsion by Conformational Adaptation-Enhanced Interfacial Protein with an Alkaline pH-Shifting-Driven Method

Protein conformational rearrangement triggered by adsorption to the hydrophobic interface of oil droplets has long been considered as a key factor in emulsification. In this study, an alkaline pH-shifting-driven conformational adaptation enhanced interfacial proteins was used to improve their stability against heat-induced flocculation of myosin emulsions. We used the unfolded myosin at pH 12 to emulsify soy oil and then readjusted the pH of the emulsion to neutral. The corresponding myosin emulsion (0.5% w/v protein, 10% v/v soy oil, and 0.6 M NaCl) almost not flocculated when heated at 75 °C for 30 min. Moreover, after thermal treatment, the particle size of the emulsion was not significantly increased ( P > 0.05) and the emulsion did not exhibit a creaming phenomenon after a week. Based on the circular dichroism and Fourier transform infrared analysis, we speculated the superiority of the emulsion is closely related to the alkaline pH-shifting-driven conformational adaptation enhanced interfacial protein. Additionally, the resulting steric stabilization in overcoming the attractive hydrophobic forces between denatured protein molecules coated droplets might be the main factor for the inhibition of heat-induced flocculation of the emulsion. Our research may have important implications for the formulation of protein-stabilized oil-in-water emulsions.

Nutritional Quality and Physical Characteristics of Soluble Proteins Recovered from Silver Carp

The objective of this study was to evaluate the nutritional quality and physical characteristics of soluble proteins separated from silver carp at 4, 20, and 40 °C. Ground silver carp was diluted, and soluble proteins were separated by centrifugation and dried. The proximate composition (dry wt) of the protein powders averaged 82.42% protein, 3.25% lipid, and 14.50% ash. Average protein recovery yield was 11.78% with the better yields occurring at 20 °C (P < 0.05). Mineral profile revealed greater concentrations of Fe, Mg, P, and Na when compared to the initial homogenate. More saturated and monounsaturated fatty acids were recovered in the 4 °C powder and the least in the 40 °C powder (P < 0.05). Polyunsaturated fatty acids displayed a reverse trend, with the greatest concentration in the 40 °C powder and the least in the 4 °C powder (P < 0.05). The amino acid profile revealed that the protein powder met all FAO/WHO/UNO amino acid requirements for adults. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed high amounts of low and medium molecular weight (MW) proteins (10-15 and 25-50 kDa, respectively). Two-dimensional (2-D) electrophoresis indicated that the low MW proteins possessed a neutral isoelectric point relative to that of the medium MW proteins. The protein powder was significantly less soluble (P < 0.05) than whey protein concentrate 80 at every pH tested (pH 3.0 to 11.0). Similar tendencies were seen when ionic strength was shifted (0.0 to 1.1 I; P < 0.05). Soluble protein powders derived from silver carp are nutrient rich and have physical characteristics resembling whey protein concentrate. Changes in process temperature had limited effects on protein powder composition.

PRACTICAL APPLICATION

Soluble proteins contribute to 20 to 40% of fish protein and are soluble in neutral salt solutions. Much of the sarcoplasmic proteins are lost when they solubilize in processing water and are discarded similarly to how whey protein was once discarded during dairy processing. When government regulations on whey disposal were implemented, the dairy industry responded by repurposing the high-quality protein for human use and it is now a billion dollar industry. The aim of this research project was to verify the composition of an otherwise overlooked protein source.

Evaluation of Acid-treated Fish Sarcoplasmic Proteins on Physicochemical and Rheological Characteristics of Pork Myofibrillar Protein Gel Mediated by Microbial Transglutaminase

Fish sarcoplasmic protein (SP) is currently dumped as waste from surimi industry and its recovery by practical method for being the non-meat ingredient in meat industry would be a strategy to utilize effectively the fish resource. This study was aimed to apply pH treatment for fish SP recovery and evaluated its effect on pork myofibrillar protein (MP) gel. The pH values of fish SP were changed to 3 and 12, and neutralized to pH 7 before lyophilizing the precipitated protein after centrifugation. Acid-treated fish SP (AFSP) showed about 4-fold higher recovery yield than that of alkaline-treated SP and water absorption capacity was also about 1.2-fold greater. Because of the high recovery yield and water absorption capacity, AFSP was selected to incorporate into MP with/without microbial transglutaminase (MTG). The effects of AFSP and MTG on the physicochemical and rheological characteristics of MP and MP gel were evaluated. MTG induced an increase shear stress of the MP mixture and increase the breaking force of MP gels. MP gel lightness was decreased by adding AFSP. MP gel with MTG showed higher cooking loss than that without MTG. A reduction of cooking loss was observed when the AFSP was added along with MTG, where the insoluble particles were found. Therefore, AFSP could be contributed as a water holding agent in meat protein gel.

Changes in physiochemical properties of water-soluble proteins from crucian carp (Carassius auratus) during heat treatment

In order to understand physicochemical properties of water-soluble proteins obtained from crucian carp, turbidity, total sulfhydryl content, hydrophobicity and SDS-PAGE of crucian carp water-soluble proteins during heat treatment were investigated. Turbidity remained unchanged up to 44°C and considerably increased from 46°C to 54°C, one peak of increase rate was found at 50°C; total SH content decreased rapidly when heated from 50°C to 55°C; hydrophobicity increased sharply when heated up to 45°C, indicating the conformation of water-soluble proteins from crucian carp began to unfold and expose the buried nonpolar amino acids at temperatures above 45°C; analysis of SDS-PAGE indicating the formation of disulfide linkage of creatine kinase and glyceraldehy-3-phosphate dehydrogenase when the temperature reached 65°C and 80°C, respectively.

Effects of pH-Shift Processing and Microbial Transglutaminase on the Gel and Emulsion Characteristics of Porcine Myofibrillar System

This study investigated the effects of microbial transglutaminase (MTGase) and pH-shift processing on the functional properties of porcine myofibrillar proteins (MP). The pH-shift processing was carried out by decreasing the pH of MP suspension to 3.0, followed by re-adjustment to pH 6.2. The native (CM) and pH-shifted MP (PM) was reacted with and without MTGase, and the gelling and emulsion characteristics were compared. To compare the pH-shifted MTGase-treated MP (PT), deamidation (DM) was conducted by reacting MTGase with MP at pH 3.0. Rigid thermal gel was produced by MTGase-treated native MP (CT) and PT. PM and DM showed the lowest storage modulus (G') at the end of thermal scanning. Both MTGase and pH-shifting produced harder MP gel, and the highest gel strength was obtained in PT. All treatments yielded lower than CM, and CT showed significantly higher yield than PM and DM treatments. For emulsion characteristics, pH-shifting improved the emulsifying ability of MP-stabilized emulsion, while the treatments had lower emulsion stability. PM-stabilized emulsion exhibited the lowest creaming stability among all treatments. The emulsion stability could be improved by the usage of MTGase. The results indicated that pH-shifting combined with MTGase had a potential application to modify or improve functional properties of MP in manufacturing of meat products.

Investigation of the susceptibility of acid-deamidated wheat gluten to in vitro enzymatic hydrolysis using Raman spectra and free amino acid analysis

BACKGROUND

The number and surface nature of amino acids (AAs) in substrate proteins available to hydrolytic enzymes are critical. Among them, the micro-environmental properties of specific AAs in substrates before hydrolysis would probably dominate the susceptibility of substrates to enzymatic hydrolysis. Fundamental knowledge concerning this regard is lacking. The objective of this work was to investigate the relationship between the exposure level of AAs in acid-deamidated wheat gluten and their susceptibilities to in vitro enzymatic hydrolysis by pancreatin through both high-performance liquid chromatography and Raman spectra. Wheat gluten deamidated with HCl (HDWG), citric acid (CDWG), succinic acid (SDWG) and acetic acid (ADWG) at the same degree of deamidation under the same heat treatment were chosen as the substrates. Substrate characterisations including degree of hydrolysis, surface hydrophobicity and structural characteristics before hydrolysis, together with analysis of free AAs of the corresponding hydrolysates during hydrolysis, were investigated.

RESULTS

Hydrolysates from SDWG had the highest value for the degree of hydrolysis. The susceptibility of CDWG to pancreatin hydrolysis was the lowest, lower than native wheat gluten (CK) after the initial 36 h. Compared with free AAs, the mole increase profiles of CK, Arg production levelled off in HDWG after 12 h whereas it was inhibited in ADWG. For SDWG, Arg release was dramatically inhibited after 12 h and was replaced by Trp. Investigations using Raman spectra of the micro-environment of Cys, Trp, Tyr and His and the mole increase trend of them indicated that the exposure level of these amino acids in substrates was positively related to their susceptibilities to pancreatin hydrolysis especially after 24 h of hydrolysis.

CONCLUSION

Deamidation by four acids has a distinct influence on the structural characteristics of wheat gluten substrates. Although the substrates were selected at the same level of deamidation by the same heat treatment, their resultant conformational differences significantly influenced the exposure level of amino acids for binding to enzymes and the susceptibility of substrates to in vitro enzymatic hydrolysis. Therefore, it had an influence on changing enzyme cutting sites of pancreatin. This information will provide a better understanding of specific behaviour of AAs in wheat gluten during enzymatic hydrolysis from a new perspective.