Inactivation of papain by pulsed electric fields in a continuous system

Impact of non-thermal techniques on enzyme modifications for their applications in food

The present review elaborates on the details of the enzyme, its structure, specificity, and the mechanism of action of selected enzymes as well as structural changes and loss or gain of activity after non-thermal treatments for food-based applications. Enzymes are biological catalysts found in various systems such as plants, animals, and microorganisms. Most of the enzymes have their optimum pH, temperature, and substrate or group of substrates. The conformational modification of enzymes either increases or decreases the rate of reaction at different pH, and temperature conditions. Enzymes are modified by different techniques to enhance the activity of enzymes for their commercial applications mainly due to the high cost of enzymes, stability, and difficulties that occur during the use of enzymes in different conditions. On the opposite, enzyme inactivation provides its application to extend the shelf life of fruits and vegetables by denaturation and partial inactivation of enzymes. Hence, the activation and inactivation of enzymes are studied by non-thermal techniques in both the model and the food system. The highly reactive species generated during non-thermal techniques cause chemical and structural modification. The enzyme modifications depend on the type and source of the enzyme, type of technique, and the parameters used.

Effect of electron beam irradiation on the structural characteristics and functional properties of goat's milk casein

The effects of electron beam irradiation (EBI) at different doses (0, 2, 4, 6, 8, and 10 kGy) were investigated on the structural and functional properties of casein, including their interrelationship. A gradual reduction in the α-helix content of the secondary structure (as a stable structure) indicates that casein under EBI treatment mainly undergoes fragmentation and aggregation from a structural perspective. Furthermore, the hydrophobic group and tryptophan in the tertiary structure were exposed, which opened up the internal structure of the protein. In addition, a continuously increasing irradiation dose led to casein aggregation, as confirmed by electron microscopy. The structural changes affected its functional properties, such as solubility, emulsification, foaming, and rheological properties, all of which increased first and subsequently decreased. Finally, at irradiation doses of 4-6 kGy, casein was modified to exhibit optimal functional properties, which enhanced its food processing value and performance.

Impact of Prior Pulsed Electric Field and Chitooligosaccharide Treatment on Trypsin Activity and Quality Changes in Whole and Beheaded Harpiosquillid Mantis Shrimp during Storage in Iced Water

Harpiosquillid mantis shrimp (Harpiosquilla raphidea) (HMS) without and with beheading pretreated with pulsed electric field (PEF) (15 kV/cm, 800 pulses, 5 min) were soaked in chitooligosaccharide (COS) solution at varying concentrations (0, 1 and 2%, w/v) for 20 min and stored for 3 days in iced water. Changes in the trypsin activity, color, texture, protein pattern, TCA soluble peptide content, histological images, protein secondary structure and microbial load were monitored during the storage. The beheaded HMS pretreated with PEF and soaked in 2% COS solution showed the maximum efficacy in inhibiting trypsin activity and proteolysis, thus retaining muscle proteins, especially myosin heavy chain, actin and troponin T as well as shear force up to day 3. Pronounced muscle destruction in the whole HMS was displayed by a decreased mean grey index and fiber gapping. Such changes were lowered by the beheading and PEF/2% COS treatment (2% COS-BH). Nevertheless, no marked change in the secondary structure including α-helix, β-sheets, β-turns and random coil were observed among any of the samples. The microbiological analysis revealed that the total viable count (TVC) was below 6 log CFU/g till day 2 in all samples. Nonetheless, the 2% COS-BH sample had the lowest psychrophilic bacterial count and Enterobacteriaceae count at day 3, compared to the others. Thus, the combination of the prior PEF and 2% COS treatment of beheaded HMS could effectively inhibit proteases, retard the microbial growth and maintain the quality of HMS stored in iced water.

Role of Enzymatic Reactions in Meat Processing and Use of Emerging Technologies for Process Intensification

Meat processing involves different transformations in the animal muscle after slaughtering, which results in changes in tenderness, aroma and colour, determining the quality of the final meat product. Enzymatic glycolysis, proteolysis and lipolysis play a key role in the conversion of muscle into meat. The accurate control of enzymatic reactions in meat muscle is complicated due to the numerous influential factors, as well as its low reaction rate. Moreover, exogenous enzymes are also used in the meat industry to produce restructured products (transglutaminase), to obtain bioactive peptides (peptides with antioxidant, antihypertensive and gastrointestinal activity) and to promote meat tenderization (papain, bromelain, ficin, zingibain, cucumisin and actinidin). Emerging technologies, such as ultrasound (US), pulsed electric fields (PEF), moderate electric fields (MEF), high-pressure processing (HPP) or supercritical CO₂ (SC-CO₂), have been used to intensify enzymatic reactions in different food applications. This review aims to provide an overview of the enzymatic reactions taking place during the processing of meat products, how they could be intensified by using emerging technologies and envisage potential applications.

Impact of Pulsed Electric Fields and pH on Enzyme Inactivation and Bioactivities of Peptic Hydrolysates Produced from Bovine and Porcine Hemoglobin

The production of bioactive peptides from hemoglobin via peptic hydrolysis is a promising alternative to valorizing slaughterhouse blood proteins. Nevertheless, it has some limitations such as low yield, high cost of enzymes, and the use of chemical reagents. The latter is aggravated by the pH increase to inactivate the enzyme, which can affect the bioactivity of the peptides. Thus, this study aimed to evaluate the effect of pulsed electric fields (PEF) on the pepsin inactivation and biological activities (antimicrobial and antioxidant) of hemoglobin hydrolysates. Bovine (Hb-B) and porcine (Hb-P) hemoglobin were hydrolyzed with pepsin for 3 h and treated with PEFs to inactivate the enzyme. The degree of hydrolysis (DH) did not show significant changes after PEF inactivation, whereas peptide population analysis showed some changes in PEF-treated hydrolysates over time, suggesting residual pepsin activity. PEF treatments showed no significant positive or negative impact on antimicrobial and antioxidant activities. Additionally, the impact of pH (3, 7, and 10) on bioactivity was studied. Higher pH fostered stronger anti-yeast activity and DPPH-scavenging capacity, whereas pH 7 fostered antifungal activity. Thus, the use of hemoglobin from the meat industry combined with PEF treatments could fit the circular economy concept since bioactive peptides can be produced more eco-efficiently and recycled to reduce the spoilage of meat products. Nevertheless, further studies on PEF conditions must be carried out to achieve complete inactivation of pepsin and the potential enhancement of peptides’ bioactivity.

Mechanism of trypsin activation by pulsed electric field treatment revealed based on chemical experiments and molecular dynamics simulations

A pulsed electric field (PEF) treatment exhibits different effects on trypsin; however, the mechanism of enzyme activation remains unclear. Herein, chemical experiments combined with molecular dynamics simulations revealed the mechanism of trypsin activation by PEF treatment at the molecular level. The results indicated that compared with the values at 0 kV/cm, the enzyme activity, Vmax, and Kcat at 20 kV/cm increased by 9.30%, 4.74%, and 4.30%, respectively, and Km decreased by 11.14%, indicating an improved interaction between the enzyme and substrate. The simulation results revealed that PEF treatment increased the number of molecular hydrogen bonds and the solvent-accessible surface area, while decreasing the rotation radius and random coil content by 5.00% and 3.37%, respectively. Molecular docking indicated that PEF treatment altered the active center and increased the affinity between the enzyme and substrate. The simulation results were consistent with those of the spectroscopic experiments conducted on trypsin after PEF treatment.

Enhanced synthesis of succinylated whey protein isolate by pulsed electric field pretreatment

The objective of this study was to investigate the feasibility of pulse electric field (PEF) as a pretreatment for whey protein isolate (WPI) before its succinylation. The degree of succinylation (DS) of WPI increased from 88.31% for native WPI to 93.45% for PEF-pretreated WPI (PWPI, initial pH 10.0) for the same succinic anhydride (SA) to WPI ratio (1:1). Fourier transform infrared spectroscopy and scanning electron microscopy analysis proved the successful succinylation of WPI. For PWPIs, the surface hydrophobicity, exposed sulphydryl, and total sulphydryl decreased, which indicates the occurrence of changes in protein structures with more hydrophilic groups and better protein dispersion. Moreover, PEF may expose more amino acid residues binding sites that are present inside the protein, which is more suitable for succinylation. Therefore, the PEF pretreatment of proteins can improve their efficient use that is expected to play a critical role in succinylation industry.

Exploring the Effect of Pulsed Electric Fields on the Technological Properties of Chicken Meat

Pulsed electric field (PEF) is a non-thermal technology which is increasingly drawing the interest of the meat industry. This study aimed at evaluating the effect of PEF on the main technological properties of chicken meat, by investigating the role of the most relevant process parameters such as the number of pulses (150 vs. 300 and 450 vs. 600) and the electric field strength (0.60 vs. 1.20 kV/cm). Results indicated that PEF does not exert any effect on meat pH and just slightly affects lightness and yellowness. Low-intensity PEF treatments improved the water holding capacity of chicken meat by significantly (p < 0.001) reducing drip loss up to 28.5% during 4 days of refrigerated storage, without damaging proteins' integrity and functionality. Moreover, from the analysis of the process parameters, it has been possible to highlight that increasing the number of pulses is more effective in reducing meat drip loss rather than doubling the electric field strengths. From an industrial point of view, the results of this explorative study suggested the potential of PEF to reduce the undesired liquid inside the package, thus improving consumer acceptance.

Understanding the impact of moderate-intensity pulsed electric fields (MIPEF) on structural and functional characteristics of pea, rice and gluten concentrates

Aim

The effect of moderate-intensity pulsed electric fields (MIPEF) was evaluated on vegetable protein concentrates from pea, rice, and gluten.

Methods

Five percent (w/w) suspensions of protein concentrates (pH 5 and 6) were exposed to up to 60,000 MIPEF pulses at 1.65 kV/cm. Both structural modifications (absorbance at 280 nm, free sulfhydryl groups, FT-IR-spectra) and functional properties (solubility, water and oil holding capacity, foamability) were analyzed.

Results

MIPEF was able to modify protein structure by inducing unfolding, intramolecular rearrangement, and formation of aggregates. However, these effects were strongly dependent on protein nature and pH. In the case of rice and pea samples, structural changes were associated with negligible modifications in functional properties. By contrast, noticeable changes in these properties were observed for gluten samples, especially after exposure to 20,000 pulses. In particular, at pH 6, an increase in water and oil holding capacity of gluten was detected, while at pH 5, its solubility almost doubled.

Conclusion

These results suggest the potential of MIPEF to steer structure of proteins and enhance their technological functionality.

Bridging the Knowledge Gap for the Impact of Non-Thermal Processing on Proteins and Amino Acids

Proteins represent one of the major food components that contribute to a wide range of biophysical functions and dictate the nutritional, sensorial, and shelf-life of food products. Different non-thermal processing technologies (e.g., irradiation, ultrasound, cold plasma, pulsed electric field, and high-pressure treatments) can affect the structure of proteins, and thus their solubility as well as their functional properties. The exposure of hydrophobic groups, unfolding followed by aggregation at high non-thermal treatment intensities, and the formation of new bonds have been reported to promote the modification of structural and functional properties of proteins. Several studies reported the reduction of allergenicity of some proteins after the application of non-thermal treatments. The composition and concentration of free amino acids could be changed after non-thermal processing, depending on the processing time and intensity. The present review discusses the effects of different non-thermal treatments on protein properties in detail, and highlights the opportunities and disadvantages of these technologies in relation to protein functionality.

Effect of electron beam irradiation on the structural characteristics and functional properties of rice proteins

A study of the structural and functional changes of rice proteins (RPs) induced by electron beam irradiation (EBI) at 5 kGy, 10 kGy, 20 kGy, and 30 kGy was performed. The microcosmic surface structures of the RPs were changed and fragmented due to irradiation damage occurring on the RP surfaces. The changes in the UV visible spectra, intrinsic fluorescence spectra, surface hydrophobicity and SH and SS group contents indicated that the RPs unfolded after EBI treatment. In addition, the degree of conformational change was increased with increasing EBI treatment doses. FTIR analysis showed that the secondary structure redistributed, showing decreases in α-helices and concomitant increases in β-sheets, β-turns and random coils. The functional properties, emulsifying abilities, water adsorption capacities and oil adsorption capacities of the irradiated RPs improved dose-dependently, with maximums occurring at 30 kGy. The foaming properties were also enhanced by EBI; however, this effect was not dose-dependent. In contrast, all of the samples irradiated by electron beams presented lower emulsion stability than the control (0 kGy). These results provide a theoretical basis for the application of EBI in improving protein properties in the future.

Pulsed Electric Fields-Modified Ferritin Realizes Loading of Rutin by a Moderate pH Transition

Ferritin shares a conserved 24-subunit spherical structure and a unique reversible self-assembly characteristic. In the present work, pulsed electric fields (PEF) technology was used to treat red bean seed ferritin deprived of iron (apoRBF) to fabricate a PEF-modified apoRBF (PEFF). Results indicated that PEF treatment at 20 kV/cm for 7.05 ms retained the spherical structure but decreased the α-helix/β-sheet contents of ferritin. Differential scanning calorimetry (DSC) and UV-vis analyses proved that the thermal stability of the PEFF was decreased. Consequently, PEFF disassembled at pH 3.6 and reassembled when the pH was restored to 7.0, exhibiting a more moderate condition relevant to the traditional approach. Using the pH 3.6/7.0 transition routine, rutin molecules were successfully loaded within PEFF nanoparticle. The rutin-loaded PEFF showed a diameter of 12 nm with an encapsulation ratio of 13.7% (w/w). Moreover, PEFF played a role in protecting the encapsulated rutin molecules upon thermal treatment (20-70 °C). This work will be beneficial for extension of PEF application in protein modification and will improve ferritin functionalization as a carrier for food bioactive molecules by a moderate pH transition method.

Effects of Pulsed Electric Field Processing on Apple and Pear Polyphenoloxidases

The feasibility of inhibiting polyphenoloxidase from apple and pear by pulsed electric field processing was evaluated. These treatments significantly lowered polyphenoloxidase activity of enzyme extracts from apple ( Golden deliciousvar.) and pear ( Blanquillavar.). Exponential decay pulses were generated by a laboratory scale electric pulse generator and applied in bipolar mode. Pulse duration was 0.02 ms and electric field intensities were up to 24.6 kV/cm. The temperature of samples never exceeded 15 ºC during pulsed electric field processing treatments. Polyphenoloxidase activities were reduced up to 3.15% and 38.0% initial value in apple extract at 24.6 kV/cm and pear extract at 22.3 kV/cm both for 6 ms total treatment time, respectively. Apple and pear polyphenoloxidase exposed to pulsed electric field processing diminishes their activities following first order kinetics. Rate constants ranged from 132 to 440 ms 1 for apple polyphenoloxidase, whereas for pear 1 and changed exponentially with the applied electric field intensity. Residual polyphenoloxidase activity was correlated to energy density by an exponential decay model.

Quality-related enzymes in plant-based products: effects of novel food processing technologies part 2: pulsed electric field processing

Pulsed electric field (PEF) processing is an effective technique for the preservation of pumpable food products as it inactivates vegetative microbial cells at ambient to moderate temperature without significantly affecting the nutritional and sensorial quality of the product. However, conflicting views are expressed about the effect of PEF on enzymes. In this review, which is part 2 of a series of reviews dealing with the effectiveness of novel food preservation technologies for controlling enzymes, the scientific literature over the last decade on the effect of PEF on plant enzymes is critically reviewed to shed more light on the issue. The existing evidence indicates that PEF can result in substantial inactivation of most enzymes, although a much more intense process is required compared to microbial inactivation. Depending on the processing condition and the origin of the enzyme, up to 97% inactivation of pectin methylesterase, polyphenol oxidase, and peroxidase as well as no inactivation have been reported following PEF treatment. Both electrochemical effects and Ohmic heating appear to contribute to the observed inactivation, although the relative contribution depends on a number of factors including the origin of the enzyme, the design of the PEF treatment chamber, the processing condition, and the composition of the medium.

Macroporous Biodegradable Cryogels of Synthetic Poly(α-amino acids)

We present an investigation of the preparation of highly porous hydrogels based on biodegradable synthetic poly(α-amino acid) as potential tissue engineering scaffolds. Covalently cross-linked gels with permanent pores were formed under cryogenic conditions by free-radical copolymerization of poly[N(5)-(2-hydroxyethyl)-L-glutamine-stat-N(5)-(2-methacryloyl-oxy-ethyl)-L-glutamine] (PHEG-MA) with 2-hydrohyethyl methacrylate (HEMA) and, optionally, N-propargyl acrylamide (PrAAm) as minor comonomers. The morphology of the cryogels showed interconnected polyhedral or laminar pores. The volume content of communicating water-filled pores was >90%. The storage moduli of the swollen cryogels were in the range of 1-6 kPa, even when the water content was >95%. The enzymatic degradation of a cryogel corresponded to the decrease in its storage modulus during incubation with papain, a model enzyme with specificity analogous to wound-healing enzymes. It was shown that cryogels with incorporated alkyne groups can easily be modified with short synthetic peptides using azide-alkyne cycloaddition "click" chemistry, thus providing porous hydrogel scaffolds with biomimetic features.

Investigation of the protein-protein aggregation of egg white proteins under pulsed electric fields

Egg whites were exposed to pulsed electric fields (PEFs) to investigate the protein-protein aggregation. No insoluble protein aggregate was found when egg whites were exposed to PEFs at 25, 30, and 35 kV/cm for 400 micros. However, atomic force microscopy showed that the sizes of the protein particles increased. Native polyacrylamide gel electrophoresis (PAGE) demonstrated the existence of aggregates under PEFs at 35 kV/cm for 400 micros. Sodium dodecyl sulfate (SDS)-PAGE in the presence and absence of 2-mercaptoethanol further indicated that sulfhydryl-disulfide interchange reactions occurred under PEFs. Differential scanning calorimetry scans showed 400 micros of PEF treatment at 35 kV/cm denatured 16.5% proteins. Insoluble egg white protein aggregates were induced by PEF (35 kV/cm, 800 micros) and heat (60 degrees C, 3.5 min) treatments. Disulfide bonds were the dominant binding forces in the formation of protein aggregates. However, the weakly noncovalent bonds play a much more important role in the protein aggregation forming in heat treatment (60 degrees C, 3.5 min) than that in PEF treatment (35 kV/cm, 800 micros).

Investigation of the mechanisms of pulsed electric fields on inactivation of enzyme: lysozyme

Lysozyme was selected as a model enzyme to investigate the effects of pulsed electric fields (PEF) on its activity and structure. The irreversible inactivation of lysozyme in sodium phosphate buffer (10 mM, pH 6.2) induced by PEF at 35 kV/cm followed a first-order model when the treatment time was longer than 300 micros. Unfolding of lysozyme structure was induced by PEF, accompanied by the cleavage of disulfide bonds and self-association aggregation when the applied PEF dosage was higher than a critical level. The inactivation of lysozyme by PEF was correlated to the loss of alpha-helix in secondary structure. The relative residual activity of PEF-treated lysozyme was in close agreement with the relative molar ellipticity at 208 nm. Both PEF- and heat-induced inactivations of lysozyme were correlated to the alteration of the secondary structure of lysozyme, but the effects of PEF and heat treatment on secondary structure were inconsistent.