Light exposure accelerates oxidative protein polymerization in beef stored in high oxygen atmosphere

Studies of the binding mechanism between liquid smoke from tea tree branches and proteins in dry-cured tenderloin using 4D-DIA proteomics, synergistic multispectral analysis, and molecular docking techniques

This research investigates the impact of various concentrations of tea branch liquid smoke (TLS) on the protein structure of dry cured pork tenderloin using multispectral techniques, molecular docking, and 4D-DIA proteomics. The results reveal that TLS enhances the solubility of myofibrillar protein, with varying effects on tryptophan exposure based on the concentration. Notably, at 5 mL/kg, TLS inhibits myofibrillar protein unfolding. Raman spectroscopy demonstrates that higher TLS concentrations mitigate disruptions in hydrogen bonding and hydrophobicity. Guaiacol and furfural in TLS engage in π-stacking interactions with myosin, heightening myosin interaction with its carrier. 4D-DIA proteomics has revealed that TLS can down-regulate the expression of cytoplasmic and mitochondrial proteins, metabolic enzymes, and ligases, playing pivotal roles in metabolism and genetic information processing. These proteins, featuring membrane linkers and phosphatases, potentially impact peptide and amino acid biosynthesis, thereby affecting meat quality modifications.

Light wavelengths that induce oxidation of oxymyoglobin in meat

Light wavelengths that induce meat discoloration and the photoreceptors in the meat were studied. We investigated the effects of the light wavelength on the oxidation rate of myoglobin (Mb) by exposing Mb extracts or model solutions containing Mb to light at specific wavelengths with a bandwidth of 5 nm using a fluorescence spectrophotometer. The wavelengths examined comprised 385, 415, 445, 460, 490, 525, 555, 580, 605, 630,660, and 750 nm. In the Mb extracts, Mb oxidation was induced through exposure to the light at 445 and 580-605 nm; Mb was insensitive to light at 445 nm. Mitochondria, containing cytochrome a and cytochrome a3 with absorption peaks at 448 and 600 nm, and riboflavin with fluorescence at 450 nm were studied as 445 nm receptors. Mitochondria significantly oxidized Mb via cytochrome c oxidation through complex IV activity; however, no 445 nm-specific photo sensitivity effects were observed. In contrast, riboflavin increased the Mb oxidation rate induced via exposure to the light at 450 nm in a concentration-dependent manner (minimum concentration: 38.4 μg L-1). While native mitochondria did not show 445 nm-specific photosensitivity effects on Mb, supernatants of heated mitochondria conferred 445 nm-wavelength sensitivity to Mb. Riboflavin concentration in this supernatant was 182 ± 60 μg L-1. The Mb photosensitivity spectrum with 473 μg L-1 riboflavin had two peaks at 445 nm and 580 nm, which were similar to those of Mb extract. These results suggest that mitochondrial damage affects the meat discoloration through the release of cytochrome c and riboflavin.

Strategies for the incorporation of organosolv lignin in hydroxypropyl methylcellulose-based films: A comparative study

Organosolv lignin extracted from vine pruning residues was added to hydroxypropyl methylcellulose (HPMC)-based films using three strategies: i) lignin incorporated into the film (lignin-based film), ii) lignin nanoparticles (LNPs) incorporated into the film (LNPs-based film), and iii) lignin coated on HPMC films' surface (lignin-coated film). The films obtained were evaluated in terms of morphology, water barrier and mechanical properties, and antioxidant capacity. Results showed that LNPs incorporation did not affect the films´ water vapour permeability (WVP). Nonetheless, the lignin-based and lignin-coated films improved the water barrier properties of HPMC-based films, achieving a 31.5 and 36 % reduction of WVP, respectively. The morphological evaluation, performed by scanning electron microscopy, revealed films' morphology changes with the lignin incorporation, which was more evident in the lignin-based films. Fourier transform infrared spectroscopy (FTIR) showed minor changes in the film's structure using the different lignin incorporation methods. The mechanical properties were improved, including a significant increase in the tensile strength in the lignin-based and lignin-coated films. All films showed high radical scavenging activity (RSA) after 24 h, with a gradual increase in the lignin-coated films over time. The lignin-coated films showed to be the most promising incorporation strategy to improve the HPMC-based film's properties.

Young apple polyphenols confer excellent physical and oxidative stabilities to soy protein emulsions for effective β-carotene encapsulation and delivery

Protein emulsions' poor physical and oxidative stabilities restrict their use in functional foods. Soy protein isolate (SPI) emulsions' physical stability was enhanced by adding young apple polyphenols (YAP) in this study, but decreased when YAP was 0.12%. YAP binding prefolded SPI's structure, which promotes efficient SPI stacking at the interface. YAP also improved SPI emulsions' oxidation resistance in a dose-dependent manner. SPI-YAP interaction promoted more YAP adsorption (>80%) at the interface, which increased emulsions' antioxidant capacities twofold. Furthermore, over 90% of unsaturated fatty acids were preserved, and the oxidation of lipid-SPI-β-carotene appeared to be reduced as YAP increased. In addition, SPI-YAP emulsions were effective in encapsulating and safeguarding β-carotene during emulsion storage and in vitro digestion, leading to a delayed and maximum release of β-carotene. This study improves the understanding of polyphenols inhibition on lipid-protein oxidation through interface strengthening and broadens the potential applications of YAP and SPI in functional foods.

The Oxidative and Color Stability of Beef from Steers Fed Pasture or Concentrate during Retail Display

Beef production in Uruguay is based on pasture (~85%) or concentrate (~15%), resulting in differences in meat quality. The objective of this study was to compare the oxidative stability and color of beef from these two systems during refrigerated retail display. For these purposes, the Semimembranosus muscle was removed from ten Aberdeen Angus steers raised and fed on pasture (130 days prior to slaughter) and from another ten steers fed concentrate (100 days prior to slaughter), sliced. The muscles were placed in a refrigerated showcase for 3, 6, and 9 days. The contents of β-carotene, α-tocopherol, and fatty acids were determined before the meat was placed on display. Lipid and protein oxidation, color, and heme iron content were determined before and during display. The meat from pasture-fed steers had a lower intramuscular fat content (1.78 ± 0.15 vs. 4.52 ± 0.46), lower levels of monounsaturated fatty acids, a lower n-6/n-3 ratio, less lipid and protein oxidation, lower L* and a* values, and higher levels of α-linolenic acid, DHA, total n-3, β-carotene, and α-tocopherol. In conclusion, the meat from pasture-fed steers was more stable during retail display from an oxidative point of view, which may be due to its higher levels of antioxidant compounds such as β-carotene and α-tocopherol and had a healthier fatty acid profile for consumers.

Photo-induced protein modifications: a range of biological consequences and applications

Proteins are the most abundant biomolecules in living organisms and tissues and are also present in many natural and processed foods and beverages, as well as in pharmaceuticals and therapeutics. When exposed to UV-visible light, proteins containing endogenous or exogenous chromophores can undergo direct and indirect photochemical processes, resulting in protein modifications including oxidation of residues, cross-linking, proteolysis, covalent binding to molecules and interfaces, and conformational changes. When these modifications occur in an uncontrolled manner in a physiological context, they can lead to biological dysfunctions that ultimately result in cell death. However, rational design strategies involving light-activated protein modification have proven to be a valuable tool for the modulation of protein function or even for the construction of new biomaterials. This mini-review describes the fundamentals of photochemical processes in proteins and explores some of their emerging biomedical and nanobiotechnological applications, such as photodynamic therapy (PDT), photobonding for wound healing, photobioprinting, photoimmobilization of biosensors and enzymes for sensing, and biocatalysis, among others.

Lipid oxidation in foods and its implications on proteins

Lipids in foods are sensitive to various environmental conditions. Under light or high temperatures, free radicals could be formed due to lipid oxidation, leading to the formation of unstable food system. Proteins are sensitive to free radicals, which could cause protein oxidation and aggregation. Protein aggregation significantly affects protein physicochemical characteristics and biological functions, such as digestibility, foaming characteristics, and bioavailability, further reducing the edible and storage quality of food. This review provided an overview of lipid oxidation in foods; its implications on protein oxidation; and the assessment methods of lipid oxidation, protein oxidation, and protein aggregation. Protein functions before and after aggregation in foods were compared, and a discussion for future research on lipid or protein oxidation in foods was presented.

The influence of piperine on oxidation-induced porcine myofibrillar protein gelation behavior and fluorescent advanced glycation end products formation in model systems

This study investigated the effects of piperine on oxidation-induced porcine myofibrillar protein (MP) gelation behavior and fluorescent advanced glycation end products (fAGEs) formation. Model systems were prepared, lipid oxidation, MP gelling behavior, and fAGEs content were determined daily. The results indicated that lipid oxidation, carbonyl content, S₀, cooking loss, and tryptophan fluorescence intensity of MP significantly decreased, whereas gel strength, WHC, and whiteness markedly increased as the concentration of piperine increased (from 0 to 30 μM/g protein), indicating that piperine could reduce lipid oxidation and oxidative damage to MP. The fluorescence intensity of fAGEs markedly decreased (P < 0.05), from 93.1 ± 4.4 to 61.2 ± 3.0, as the concentration of piperine increased from 0 μM/g protein to 30 μM/g protein after 5 days of incubation. These results in model systems suggest that the presence of piperine has an important role in the inhibition of MP oxidation and fAGEs formation.

Improvement of Color and Oxidative Stabilities in Nellore Bull Dark Meat in High-Oxygen Package by Lactate and Rosemary Oil Extract

This study aimed to improve the color and oxidative stabilities of dark Nellore bull steaks with greater-than-normal ultimate pH (pH_u) by the injection (8% raw wet weight basis) of a solution with L-lactate (2.5%), phosphate (0.3%) and rosemary extract (0.06%), with further packaging in high oxygen atmosphere (HiOx MAP). Longissimus lumborum muscles from pasture-fed Nellore bulls were divided into three pH_u ranges: normal (<5.80), intermediate (5.81-6.19), and high (≥6.2). Muscles were then halved, with sections were randomly assigned to non-enhanced (C, n = 6/pH_u range) or injected (E, n = 6/pH_u range) groups, at 72 h postmortem. Each section was cut into 2 cm-slices, which were HiOx-packed and then stored for 5 days (dark) and displayed for 9 days (fluorescent lighting) at 2 °C. Higher pH_u steaks exhibited greater a*, b*, h*, C* and surface oxymyoglobin and lower surface deoxymyoglobin and oxygen consumption compared to those of normal pH_u between days 0 and 5 (p < 0.05). Over the time, normal-pH_u muscles showed oxidative protection (lower TBARS and greater metmyoglobin reducing ability values, p < 0.05) in enhanced-steaks. Therefore, enhancement and HiOx MAP seem to produce greater-than-normal pH_u Nellore bull steaks with a preferable color and quality, even after display time.

Lipid and Protein Oxidation of Brown Rice and Selenium-Rich Brown Rice during Storage

Selenium-rich rice has become one of the effective ways to increase people's selenium intake. Selenium-containing proteins have higher antioxidant properties, which may lead to selenium-rich brown rice (Se-BR) having better storage stability than ordinary brown rice (BR). By measuring the peroxidation value, fatty acid value, carbonyl value and protein secondary structure, it was found that Se-BR had higher oxidation resistance stability than BR. The biological function of the differential proteins (DEPs) between ordinary brown rice stored for 0 days (BR-0) and 180 days (BR-6) as well as Se-rich brown rice stored for 0 days (Se-0) and 180 days (Se-6) was investigated by using iTRAQ. A total of 237, 235, 113 and 213 DEPs were identified from group A (BR-0/BR-6), group B (Se-0/Se-6), group C (BR-0/Se-0) and group D (BR-6/Se-6), respectively. Kyoto Encyclopedia of Genes and Genomes analysis showed that the DEPs were mainly enriched in glucose metabolism, tricarboxylic acid cycle, fatty acid biosynthesis and degradation, glutathione metabolism, sulfur metabolism, peroxisome and other metabolic pathways. This study provides theoretical support for the study of protein oxidation kinetics and storage quality control of brown rice during storage.

Recent advances on characterization of protein oxidation in aquatic products: A comprehensive review

In addition to microbial spoilage and lipid peroxidation, protein oxidation is increasingly recognized as a major cause for quality deterioration of muscle-based foods. Although protein oxidation in muscle-based foods has attracted tremendous interest in the past decade, specific oxidative pathways and underlying mechanisms of protein oxidation in aquatic products remain largely unexplored. The present review covers the aspects of the origin and site-specific nature of protein oxidation, progress on the characterization of protein oxidation, oxidized proteins in aquatic products, and impact of protein oxidation on protein functionalities. Compared to meat protein oxidation, aquatic proteins demonstrate a less extent of oxidation on aromatic amino acids and are more susceptible to be indirectly oxidized by lipid peroxidation products. Different from traditional measurement of protein carbonyls and thiols, proteomics-based strategy better characterizes the targeted oxidation sites within proteins. The future trends using more robust and accurate targeted proteomics, such as parallel reaction monitoring strategy, to characterize protein oxidation in aquatic products are also given.

Comparison of Effects from Ultrasound Thawing, Vacuum Thawing and Microwave Thawing on the Quality Properties and Oxidation of Porcine Longissimus Lumborum

The effects of vacuum thawing (VT), ultrasound thawing (UT) and microwave thawing (MT) on the quality, protein and lipid oxidation, internal temperature distribution and microstructure of porcine longissimus lumborum were compared. The results showed that a significant decrease (p < 0.05) in quality compared with those of fresh meat (FM) occurred for all of the thawing samples, especially for the MT samples. Changes in quality of the VT and UT samples were less significant than those of the MT samples. The increases in carbonyl content and TBARS value indicated that proteins and lipids in the thawing samples were oxidized. The decreases in uniform degrees of internal temperature distributions of muscles from the thawing samples were analysed by infrared thermography. Scanning electron microscopy images showed that the myofibril arrangements of thawing samples were looser than those of the FM samples with compact and ordered structure, which was proven by the obvious increase in the myofibril gap value of the thawing samples.

Oxidation of Whey Proteins during Thermal Treatment Characterized by a Site-Specific LC-MS/MS-Based Proteomic Approach

Thermal treatment is often employed in food processing to tailor product properties by manipulating the ingredient functionality, but these elevated temperatures may accelerate oxidation and nutrient loss. Here, oxidation of different whey protein systems [α-lactalbumin (α-LA), β-lactoglobulin (β-LG), a mix of α-LA and β-LG (whey model), and a commercial whey protein isolate (WPI)] was investigated during heat treatment at 60-90 °C and a UHT-like treatment by LC-MS-based proteomic analysis. The relative modification levels of each oxidation site were calculated and compared among different heat treatments and sample systems. Oxidation increased significantly in protein systems after heating at ≥90 °C but decreased in systems with higher complexity [pure protein (α-LA > β-LG) > whey model > WPI]. In α-LA, Cys, Met, and Trp residues were found to be most prone to oxidation. In β-LG-containing protein systems, Cys residues were suggested to scavenge most of the reactive oxidants and undergo an oxidation-mediated disulfide rearrangement. The rearranged disulfide bonds contributed to protein aggregation, which was suggested to provide physical protection against oxidation. Overall, limited loss of amino acid residues was detected after acidic hydrolysis followed by UHPLC analysis, which showed only a minor effect of heat treatment on protein oxidation in these protein systems.

Study on the mechanism of mulberry polyphenols inhibiting oxidation of beef myofibrillar protein

In order to explore the mechanism of mulberry polyphenols inhibiting the oxidation of myofibrillar protein (MP), the effect of mulberry polyphenols on the structure and physicochemical properties of MP in the oxidation system was investigated. The results revealed that the content of carbonyl group and sulfhydryl group of MP was notably reduced, while the Zeta potential, storage modulus G' and surface hydrophobicity were improved when the addition of mulberry polyphenol was 0.5%. SDS-PAGE showed an irreducible aggregation of mulberry polyphenols with proteins. Fluorescence spectroscopy and FT-IR analysis manifested that mulberry polyphenols promoted the unfolding of protein structure and the transformation of α-helix to β-turn structure. Scanning electron microscopy (SEM) observed that oxidation with polyphenols facilitated the cross-linking and aggregation of MP more tightly. Nevertheless, excessive addition (≥1.0%) weakened its gel properties. Thus, to maintain the good quality of meat products, both polyphenols addition and oxidation intensity should be controlled simultaneously.

Chemical Stability of Proteins in Foods: Oxidation and the Maillard Reaction

Protein is a major nutrient present in foods along with carbohydrates and lipids. Food proteins undergo a wide range of modifications during food production, processing, and storage. In this review, we discuss two major reactions, oxidation and the Maillard reaction, involved in chemical modifications of food proteins. Protein oxidation in foods is initiated by metal-, enzyme-, or light-induced processes. Food protein oxidation results in the loss of thiol groups and the formation of protein carbonyls and specific oxidation products of cysteine, tyrosine, tryptophan, phenylalanine, and methionine residues, such as disulfides, dityrosine, kynurenine, m-tyrosine, and methionine sulfoxide. The Maillard reaction involves the reaction of nucleophilic amino acid residues with reducing sugars, which yields numerous heterogeneous compounds such as α-dicarbonyls, furans, Strecker aldehydes, advanced glycation end-products, and melanoidins. Both protein oxidation and the Maillard reaction result in the loss of essential amino acids but may positively or negatively impact food structure and flavor. Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Identification and Activity Characterization of γ-Glutamyltransferase from Bovine Milk

It is well known that bovine milk contains γ-glutamyltransferase (GGT) activity. To verify the identity of the GGT and further to characterize the generation of γ-glutamyl peptides, identification of GGT from bovine milk and quantification of kokumi peptides and free amino acids were performed. GGT was purified from skim milk and identified as the bovine protein (G3N2D8), and it reveals that it is composed of two subunits. Sequence alignment with human GGT and molecular mass determination showed that the bovine GGT was glycosylated and contained an N-terminal transmembrane part. Further activity characterization was performed in comparison with GGT from Bacillus amyloliquefaciens in terms of the ability to generate γ-glutamyl peptides from casein hydrolysates. During the transpeptidation reaction catalyzed by both GGT, γ-glutamyl peptides significantly (P < 0.05) increased after γ-glutamylation; addition of glutamine contributed to the generation of γ-glutamyl peptides, suggesting that glutamine could act as a γ-glutamyl donor. This study reveals that the GGT of skim milk membranes is a glycosylated membrane protein that can generate γ-glutamyl peptides.

Effect of Addition of Tryptophan on Aggregation of Apo-α-Lactalbumin Induced by UV-Light

UV-B illumination facilitates aggregation of alpha-lactalbumin (α-LA) by intramolecular disulfide bond cleavage followed by intermolecular thiol-disulfide exchange reactions. However, long term exposure to UV-B illumination may induce undesired oxidative modifications of amino acid residues in the protein. The purpose of this study was to examine the effect of UV-induced aggregation of apo-α-LA (a calcium-depleted form of α-LA) under aerobic and anaerobic conditions and by addition of tryptophan (Trp) as a photosensitizer. The addition of Trp to apo-α-LA illuminated under anaerobic conditions facilitated the highest level of free thiol release and disulfide-mediated aggregation as compared to without addition of Trp under both anaerobic and aerobic conditions. Addition of Trp under aerobic condition resulted in the lowest level of free thiols and disulfide-mediated aggregation and the aerobic conditions caused oxidation of the free Trp with formation of kynurenine and 5-hydroxy-Trp. Minor levels of the Trp oxidation product, 3-hydroxy-kynurenine (2% converted from Trp), was formed in apo-α-LA with added Trp under both aerobic and anaerobic conditions after UV-B treatment.

Tracking structural modifications and oxidative status of myofibrillar proteins from silver carp (Hypophthalmichthys molitrix) fillets treated by different stunning methods and in vitro oxidizing conditions

In order to explain the increased susceptibility of stunning-stressed fillets to oxidative modifications, effect of stunning methods (percussion and gill cut) and in vitro metal-catalyzed oxidation on structural changes and oxidative status of myofibrillar proteins (MPs) from silver carp fillets was examined. In comparison to the percussion group, oxidized MPs (10 mM H₂O₂) from gill cut-stunned fillets exhibited higher extent of structural disintegration as well as elevated carbonyl levels. Using label-free proteomics, isoforms of myosin heavy chain and actin were major proteins underwent oxidative modifications including monooxidation of methionine, dioxidation of aromatic amino acids, adduction of lipid peroxidation products with aliphatic amino acids, and the carbonylation of lysine and arginine into semialdehydes. In addition, amino acids located at the tail portion of myosin were highly accessible to oxidation. Owing to the structural disorganization caused by stunning stress, MPs from gill cut-stunned fillets were more susceptible to oxidation in vitro.

Analysis of Protein Oxidation in Food and Feed Products

Food and feed proteins are subject to oxidation reactions during production, processing, and storage. Several individual oxidized amino acids have been described in model systems and food; however, protein oxidation in food is still mostly assessed by the analysis of protein carbonylation. In the present review, the chemistry of protein oxidation and its implications for protein functionality, food flavor, and nutritional physiology are briefly summarized. Limitations of generic methods targeting redox-relevant functional groups and properties of typical reaction products, such as the determination of protein carbonyls and fluorescence spectroscopy, are presented. Methods for the quantitation of individual oxidation products of susceptible amino acids, such as cysteine, methionine, phenylalanine, tyrosine, and tryptophan, are reported. Special regard is paid to limitations resulting from the required hydrolysis procedures and unintended formation of the analytes during sample pretreatment. If available, results from food analysis obtained by different methods are compared. Suggestions and requirements for future works on protein oxidation in food and nutrition are given.

Production of Taste Enhancers from Protein Hydrolysates of Porcine Hemoglobin and Meat Using Bacillus amyloliquefaciens γ-Glutamyltranspeptidase

To improve the flavor of hydrolysates from porcine hemoglobin and meat, γ-glutamyltranspeptidase (GGT) from Bacillus amyloliquefaciens was added to catalyze the formation of kokumi γ-glutamyl peptides via a γ-glutamyl transfer reaction. Quantitation of free amino acids and γ-glutamyl dipeptides was carried out in combination with sensory analysis. Sensory perception, especially the thick, complex, continuous, and overall kokumi sensation of both hemoglobin and meat hydrolysates, was greatly enhanced by γ-glutamylation. Due to the higher amount of glutamine present in meat hydrolysates, γ-glutamylated hydrolysates from meat contained higher concentrations of γ-glutamyl dipeptides and showed stronger kokumi sensation than the hemoglobin counterpart without the addition of glutamine. For hydrolysates from both raw materials, extra addition of glutamine (10 and 20 mM) was beneficial for obtaining higher concentrations of γ-glutamyl dipeptides but contributed little to the kokumi sensation. This study revealed that the kokumi sensation of protein hydrolysates could be intensified by a γ-glutamyl transfer reaction, and the enhanced kokumi sensation could be related to the generation of γ-glutamyl peptides.

Microporous modified atmosphere packaging to extend shelf life of fresh foods: A review

In recent years, microporous modified atmosphere packaging has been widely concerned because of its adjustable air permeability and low processing cost. With the development and increasing demand of fresh food industry, the limited permeability of film in modified atmosphere packaging can't meet the fresh-keeping requirements of fresh foods, especially vegetables and fruits. Microporous film can flexibly adjust the gas permeability according to the physiological metabolic characteristics of fresh foods, which has gradually become a fresh-keeping technology in the domain of vegetables and fruits. This paper reviewed the research progress of microporous modified atmosphere packaging and its extension on shelf life of fresh foods. The latest applied researches were described in a comprehensive manner, particularly fruits and vegetables. Besides, this article also covered theoretical support and analysis, including the perforation mode, air permeability mechanism and mathematical model of microporous film, the characteristics of fresh foods, pore parameters and traits of film materials. This paper payed attention to the application of environmentally friendly degradable film materials (biological film materials, nano materials) in fruits and vegetables preservation. Research has shown that the degradable material can enlarge the fresh-keeping effect of microporous modified atmosphere packaging, which is worthy of further research and development. Finally, the development trends and directions in the future were discussed.

Combination of light and oxygen accelerates formation of covalent protein-polyphenol bonding during chill storage of meat added 4-methyl catechol

Plant polyphenols applied as natural antioxidant ingredients, are known to bind to cysteine residues on meat proteins. The aim of this study was to examine the effect of light exposure on the formation of cysteine-phenol adduct in meat added 4-methylcatechol (4MC), a model polyphenol, during storage through quantitative LC-MS/MS-based analysis. Cysteine-4-methylcatechol adduct (Cys-4MC) formation in meat added 1500 ppm 4-MC increased significantly (by 50%) when stored under light in oxygen at 4 °C for 7 days as compared to storage in the dark. This was reflected by a significant decrease in thiol concentrations in the same sample. Gel electrophoresis showed loss in myosin heavy chain (MHC), and a resulting increase in cross-linked MHC (CL-MHC) and larger protein polymers in samples added 4MC. Protein blots stained with nitroblue tetrazolium (NBT) showed intensive protein-polyphenol binding in the meat samples added 4MC, but no major differences between storage conditions.

Cleavage of Disulfide Bonds in Cystine by UV-B Illumination Mediated by Tryptophan or Tyrosine as Photosensitizers

Photolytic cleavage of disulfide bonds in proteins by UV light will influence their structure and functionality. The present study aimed to investigate the efficiency of disulfide cleavage by UV-B light in a system without a protein backbone consisting of combinations of cystine (a disulfide) and tryptophan (Trp) or tyrosine (Tyr) under anaerobic and aerobic conditions and to identify oxidation products formed by UV-B light. Cystine was reduced to cysteine (Cys) almost with a 1:1 stoichiometry by photoexcited Trp for anaerobic equimolar aqueous solutions (each 200 μM; pH 7.0), while photoexcited Tyr provided lower concentrations of Cys. The calculation of apparent quantum yields allowed for a comparison between the efficiency of reactions and showed that formation of Cys from disulfide cleavage of cystine was more efficient by photoexcited Trp than by photoexcited Tyr and of cystine alone and that Trp was more sensitive to photodegradation than Tyr and cystine under both aerobic and anaerobic conditions. Increasing the ratio between cystine and Trp to a 1:2 ratio did not increase the efficiency of free thiol formation but caused a more efficient photodegradation of Trp. The free thiol formed from disulfide cleavage of cystine was further oxidized to other unidentified compounds. Trp oxidation products (3-hydroxykynurenine (3-OH-Kyn) and tryptamine) were only identified in minor concentrations following light exposure of cystine and Trp in 1:1 and 1:2 ratios under both aerobic and anaerobic conditions, indicating further photodegradation to unidentified compounds. 3,4-Dihydroxyphenylalanine (DOPA) was formed from the oxidation of Tyr in the illuminated samples of cystine and Tyr in a 1:1 ratio under both aerobic and anaerobic conditions.

Generation of Aggregates of α-Lactalbumin by UV-B Light Exposure

Whey proteins are widely used as ingredients in the form of aggregates to obtain certain functionalities in food applications. The aim of this study was to understand how UV illumination generates aggregates of α-lactalbumin (α-LA) as an alternative to heat treatments traditionally used for industrial production of protein aggregates. Absorption of UV light by α-LA caused cleavage of disulfide bonds and release of thiol groups, which resulted in primarily disulfide-mediated aggregation. This process mediated efficient aggregation with up to 98% monomer conversion into aggregates through formation of intermolecular disulfide bonds, while only minor levels of nonreducible cross-links were observed. SDS-PAGE analysis revealed that illumination led to formation of dimeric, trimeric, and oligomeric forms of α-LA. LC-MS/MS analysis showed that all of the four native disulfide bonds in α-LA were cleaved by UV illumination but to different extents, and the extent of cleavage was found to be higher in the absence of calcium. Seventeen different non-native disulfides were formed after 24 h of UV illumination. Two dityrosine bonds were identified (Tyr103-Tyr103 and Tyr36-Tyr103) alongside ditryptophan (Trp118-Trp118) and tyrosine-tryptophan (Tyr50-Trp60) cross-links. In addition, Trp60, Trp118, Cys73, Cys91, Cys120, Phe80, Met90, His68, and His107 were found to be oxidized up to 12% as compared to a nonilluminated control. Our work illustrates that light exposure can be used for generation of α-LA aggregates, but optimization of the illumination conditions is required to reduce oxidative damage to Trp, Cys, Phe, Met, and His residues.

Quantitation of Protein Cysteine-Phenol Adducts in Minced Beef Containing 4-Methyl Catechol

Thiol groups of cysteine (Cys) residues in proteins react with quinones, oxidation products of polyphenols, to form protein-polyphenol adducts. The aim of the present work was to quantify the amount of adduct formed between Cys residues and 4-methylcatechol (4MC) in minced beef. A Cys-4MC adduct standard was electrochemically synthesized and characterized by liquid chromatography-mass spectrometry (LC-MS) as well as NMR spectroscopy. Cys-4MC adducts were quantified after acidic hydrolysis of myofibrillar protein isolates (MPIs) and LC-MS/MS analysis of meat containing either 500 or 1500 ppm 4MC and stored at 4 °C for 7 days under a nitrogen or oxygen atmosphere. The concentrations of Cys-4MC were found to be 2.2 ± 0.3 nmol/mg MPI and 8.1 ± 0.9 nmol/mg MPI in meat containing 500 and 1500 ppm 4MC, respectively, and stored for 7 days under oxygen. The formation of the Cys-4MC adduct resulted in protein thiol loss, and ca. 62% of the thiol loss was estimated to account for the formation of the Cys-4MC adduct for meat containing 1500 ppm 4MC. Furthermore, protein polymerization increased in samples containing 4MC as evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the polymerization was found to originate from protein-polyphenol interactions as evaluated by a blotting assay with staining by nitroblue tetrazolium.

Plant derived ingredients rich in nitrates or phenolics for protection of pork against protein oxidation

A pork model system containg phenolic extracts (citrus, rosemary, and acerola), traditional Spanish food ingredients (paprika, garlic, and oregano), or natural nitrate sources (beet, lettuce, arugula, spinach, chard, celery, and watercress) were oxidized by an hydrophilic (OX_Hydro, 2,2'-azobis(2-amidinopropane)-dihydrochloride; AAPH) or lipophilic (OX_Lip, 2,2'-azobis(2,4-dimethylvaleronitrile; AMVN) radical initiator. Citrus as well as lettuce and spinach protected almost fully against protein thiol loss and showed efficient radical scavenging activity as determined by ESR spectroscopy in both oxidizing systems. Rosemary was an efficient radical scavenger in both systems, but behaved as a prooxidant on thiols in the OX_Hydro system. Acerola was also found to be prooxidative as determined by increased radical signal intensity especially in the OX_Lip system, assigned to high concentration of ascorbate in the extract. Natural nitrate sources, especially lettuce and spinach, are accordingly potential substitutes for synthetic phenolic antioxidants protecting against protein thiol oxidation and radical formation in pork.