Effects of repeated freezing and thawing on myofibrillar protein and quality characteristics of marinated Enshi black pork

Effects of compound emulsifiers on the characteristics and stability of nano-emulsions from pollock bones

To improve the stability of pollock bone broth, compound emulsifiers were employed and evaluated in nano-emulsions from pollock bones (PBNs). The microstructure, creaming index, particle size, zeta potential, and viscosity of PBNs were characterized and the stability of PBNs was investigated. It revealed that the concentration of compound emulsifiers is one of the principal factors for particle size, zeta potential, and viscosity of PBNs, and 0.9% of sodium caseinate and sucrose fatty acid ester (CS-SE) can make the PBN display good stability. Its particle size changed from 81.17 ± 1.33 nm to 19.62 ± 0.21 nm when the temperature ranged from 40 °C to 80 °C, and its creaming index could reach a maximum (90.83%) among all PBNs in 4 months of freeze-thaw assays. PBN stability could be improved by the compound emulsifier (CS-SE), which offers a theoretical basis for the application of pollock bone broth.

Contribution to energy conservation and quality improvement of frozen pork via contact/contactless immersion freezing in NaCl solution

High energy consumption and quality deterioration are major challenges in the meat freezing process. In this study, the energy consumption and qualities of frozen pork were investigated using three freezing methods: nonpackaged pork air freezing (NAF), contactless immersion freezing (PIF) and contact immersion freezing (NIF) with NaCl solution as a refrigerant. The results indicated that NIF could improve the energy conservation and freezing efficiency in >4 freezing treatment-times by increasing the unfrozen water content, decreasing the frozen heat load, shortening the freezing time and reducing evaporation loss. NIF could also increase the a* value of the pork and improve the water-holding capacity by facilitating the conversion of free water to immobilized-water. The two immersion freezing methods could reduce freezing-thawing loss and protein loss by alleviating muscle tissue freezing damage. These results provide a suitable application of immersion freezing with energy conservation, high efficiency and good quality of frozen-pork.

Behavior of Spoilage Bacterial Communities in Different Cuts of Enshi Black Pork under Refrigerated Storage (4 °C)

The Enshi black pig is a Chinese native breed known for its rich nutrition content and high quality, which has notable traction in the consumer market. In this study, the potential impact of the main commercial cuts from Enshi black pork carcasses (ham, loin, and belly) on the bacteria community of spoiled meat under refrigerated storage (4 °C) was assessed by using a high-throughput sequencing method. Moreover, the spoilage potential of isolated strains from spoiled pork was investigated. The results demonstrated significant differences (p < 0.05) in bacterial community diversity and composition among spoiled ham, loin, and belly samples. Linear discriminant analysis effect size (LEfSe) analysis revealed a total of 20 significantly different potential bacterial biomarkers, with the dominant genera of Pseudomonas, Psychrobacter, Shewanella and Carnobacterium. Additionally, C. divergens THT1-5, isolated from spoiled ham samples, displayed cold adaptation and higher spoilage potential in Enshi black pork. These findings are helpful for identifying key factors contributing to spoilage in Enshi black pork and developing strategies to inhibit bacterial growth during preservation.

Oxidized myoglobin: Revealing new perspectives and insights on factors affecting the water retention of myofibrillar proteins

The impact of oxidized myoglobin (Mb) on myofibrillar protein (MP) oxidation and water retention was investigated. Results showed that the oxidation of Mb increased with increasing concentration of oxidized linoleic acid (OLA). In the presence of 100 mmol/L OLA, hemin iron decreased by 62.07 % compared to the control group. Further investigation showed that mild oxidation of Mb (≤10 mmol/L OLA) increased the water retention and the absolute value of the zeta potential of MP, whereas excessive oxidation (>10 mmol/L OLA) decreased these properties. With the increase of Mb oxidation, the carbonyl content in MP increased, and α-helices changed to random helix. And the tertiary structure changed. Pearson correlation analysis suggested that oxidized Mb affected the water retention of MP, which was closely related to hemin iron and non-hemin iron. In conclusion, OLA induced Mb oxidation, further promoted MP oxidation and affected its water retention.

The Protection of Quinoa Protein on the Quality of Pork Patties during Freeze-Thaw Cycles: Physicochemical Properties, Sensory Quality and Protein Oxidative

The present study investigated the impact of quinoa protein (QP) on the physicochemical properties, sensory quality, and oxidative stability of myofibrillar protein (MP) in pork patties during five freeze-thaw (F-T) cycles. It was observed that repeated F-T cycles resulted in a deterioration of pork patty quality; however, the incorporation of QP effectively mitigated these changes. Throughout the F-T cycles, the sensory quality of the QP-treated group consistently surpassed that of the control group. After five F-T cycles, the thiobarbituric acid reactive substance (TBARS) content in the control group was measured at 0.423 mg/kg, whereas it significantly decreased to 0.347 mg/kg in the QP-treated group (p < 0.05). Furthermore, QP inclusion led to a decrease in pH and an increase in water-holding capacity (WHC) within pork patties. Following five F-T cycles, Ca2+-ATPase activity exhibited a significant increase of 11.10% in the QP-treated group compared to controls (p < 0.05). Additionally, supplementation with QP resulted in elevated total sulfhydryl content and reduced carbonyl content, Schiff base content, and dityrosine content within myofibrillar proteins (MPs), indicating its inhibitory effect on MP oxidation. In particular, after five F-T cycles, total sulfhydryl content reached 58.66 nmol/mL for the QP-treated group significantly higher than that observed for controls at 43.65 nmol/mL (p < 0.05). While carbonyl content increased from 2.37 nmol/mL to 4.63 nmol/mL between the first and fifth F-T cycle for controls; it only rose from 2.15 nmol/mL to 3.47 nmol/mL in the QP-treated group. The endogenous fluorescence levels were significantly higher (p < 0.05) in the QP-treated group compared to controls. In conclusion, the addition of QP enhanced the quality of pork patties and effectively inhibited the oxidative denaturation of MP during F-T cycles.

Evaluating the effects of low-frequency alternating magnetic field thawing on oxidation, denaturation, and gelling properties of porcine myofibrillar proteins

The impact of low-frequency alternating magnetic field thawing (LF-MFT) on the physicochemical and gelling properties of porcine myofibrillar proteins (MP) was studied. Results showed that compared to atmosphere thawing (AT), LF-MFT helped in inhibiting the oxidation and denaturation of protein during thawing, thereby maintaining a superior MP gel (P < 0.05). In particular, LF-MFT-4 (LF-MFT at 4 mT) could decrease the oxidation of MP, which might be due to having a higher content of total sulfhydryl and less carbonyl of MP than other thawing treatments. The denaturation of MP was reduced since LF-MFT-4 led to less aggregation and degradation than AT. The gelling properties were also retained, and a compact and homogeneous network structure was formed after LF-MFT-4, resulting in excellent water retention. These findings suggested that LF-MFT-4 improved the gelling properties of MP by inhibiting its oxidation and denaturation, demonstrating a potential application of LF-MFT in meat thawing.

Quinoa protein Pickering emulsion improves the freeze-thaw stability of myofibrillar protein gel: Maintaining protein composition, structure, conformation and digestibility and slowing down protein oxidation

In this work, the effects of quinoa protein Pickering emulsion (QPPE) on protein oxidation, structure and gastrointestinal digestion property of myofibrillar protein gels (MPGs) after freeze-thaw (F-T) cycles are revealed. SDS-PAGE results indicated that 5.0 %-10.0 % QPPE addition slowed down the protein degradation. Meanwhile, 5.0 %-7.5 % QPPE maintained the stability of the protein secondary and tertiary structure of MPGs after F-T cycles. The sulfhydryl group, disulfide bond and dityrosine content increased with QPPE supplementation. The conformations of disulfide bond changed from g-g-t and t-g-t to g-g-g after F-T cycles, and 5.0 %-7.5 % QPPE stabilized the changes of t-g-t conformation. Furthermore, the increase of dityrosine content after F-T cycles was significantly reduced with 7.5 % QPPE addition, indicating its effect to slow down protein oxidation of MPGs. In addition, MPGs with 5.0 % and 7.5 % QPPE showed noticeably higher zeta potential values than other groups, indicating the enhanced electrostatic repulsion and weakened aggregation caused by F-T damage. This work showed that 7.5 % QPPE improved the F-T stability of MPGs and reduced the protein denaturation and oxidation caused by F-T treatments, exerting no side effect on the digestion property of MPGs. QPPE can be used as a green and effective antifreeze in meat industry.

Coexisting with Ice Crystals: Cryogenic Preservation of Muscle Food─Mechanisms, Challenges, and Cutting-Edge Strategies

Cryopreservation, one of the most effective preservation methods, is essential for maintaining the safety and quality of food. However, there is no denying the fact that the quality of muscle food deteriorates as a result of the unavoidable production of ice. Advancements in cryoregulatory materials and techniques have effectively mitigated the adverse impacts of ice, thereby enhancing the standard of freezing preservation. The first part of this overview explains how ice forms, including the theoretical foundations of nucleation, growth, and recrystallization as well as the key influencing factors that affect each process. Subsequently, the impact of ice formation on the eating quality and nutritional value of muscle food is delineated. A systematic explanation of cutting-edge strategies based on nucleation intervention, growth control, and recrystallization inhibition is offered. These methods include antifreeze proteins, ice-nucleating proteins, antifreeze peptides, natural deep eutectic solvents, polysaccharides, amino acids, and their derivatives. Furthermore, advanced physical techniques such as electrostatic fields, magnetic fields, acoustic fields, liquid nitrogen, and supercooling preservation techniques are expounded upon, which effectively hinder the formation of ice crystals during cryopreservation. The paper outlines the difficulties and potential directions in ice inhibition for effective cryopreservation.

Impact of frozen storage on some functional properties and sensory evaluation of goose meat

The objective of this study was to investigate the changes in the functional properties (pH, water holding capacity [WHC], water binding capacity [WBC], cooking losses [CL], defrosting losses [DL]), color parameters (L*, a*, b*, C, h°, ΔE), and sensory evaluation of breast (BM) and leg (LM) muscles from 17-wk-old female White Kołuda geese packaged in a vacuum and stored in frozen conditions at -20°C. During 17 wk, the geese were fed ad libitum on the same complete feed. The samples (18 BM and 18 LM) from the right part of the carcasses were stored for 30, 90, 80, 270, and 365 d. The changes in functional properties were established using a standard method used in the meat industry, according to Wierbicki et al. (1962), Grau and Hamm (1953), and CIE, (1986). Sensory evaluation was established according to defined parameters in PN-ISO 8586-2:2008. The time of frozen storage affected the decrease in WHC and WBC of BM and LM. Moreover, the LM can be characterized by a higher WHC and WBC compared to the values in the BM. It was established that CL and DL, which are the critical quality indicators, negatively increased in BM and LM during frozen storage. Considering the sensory evaluation and L*, a*, b*, C, it was established that changes in BM and LM during frozen storage were unfavorable. The scores given for smell, taste, consistency, and general appearance, as also L*, a*, and b* parameters decreased significantly during frozen storage. In addition, BM received lower scores for general appearance (at 180th and 270th day), and L* (in all frozen storage), than LM. BM and LM characterized the parameter ΔE in the range of 0.44 to 1.45, which allowed us to conclude that slight color differences were visible in these muscles (<2). Based on the study, it can be suggested that the optimal frozen storage time for BM and LM should not be longer than 180 d.

Structural and functional changes on polyhydroxy alcohol-mediated curing pork myofibrillar protein: Experimental and molecular simulation investigations

This study aimed to investigate the structural and functional changes in polyhydroxy alcohol-mediated curing on pork myofibrillar proteins (MP). The results obtained from total sulfhydryl groups, surface hydrophobicity, fluorescence and Raman spectroscopies, and solubility demonstrated that the polyhydroxy alcohols (especially xylitol) significantly modified the MP tertiary structure, making this structure more hydrophobic and tighter. However, no significant differences were detected in the secondary structure. Furthermore, the thermodynamic analysis revealed that polyhydroxy alcohols could develop an amphiphilic interfacial layer on the MP surface, significantly increasing the denaturation temperature and enthalpy of denaturation (P < 0.05). On the other hand, the molecular docking and dynamics simulations showed that polyhydroxy alcohols interact with actin mainly through hydrogen bonds and van der Waals forces. Therefore, this could help reduce the effect of high-content salt ions on MP denaturation and improve the cured meat quality.

Physicochemical and structural changes of myofibrillar proteins in muscle foods during thawing: Occurrence, consequences, evidence, and implications

Myofibrillar protein (MP) endows muscle foods with texture and important functional properties, such as water-holding capacity (WHC) and emulsifying and gel-forming abilities. However, thawing deteriorates the physicochemical and structural properties of MPs, significantly affecting the WHC, texture, flavor, and nutritional value of muscle foods. Thawing-induced physicochemical and structural changes in MPs need further investigation and consideration in the scientific development of muscle foods. In this study, we reviewed the literature for the thawing effects on the physicochemical and structural characters of MPs to identify potential associations between MPs and the quality of muscle-based foods. Physicochemical and structural changes of MPs in muscle foods occur because of physical changes during thawing and microenvironmental changes, including heat transfer and phase transformation, moisture activation and migration, microbial activation, and alterations in pH and ionic strength. These changes are not only essential inducements for changes in spatial conformation, surface hydrophobicity, solubility, Ca2+ -ATPase activity, intermolecular interaction, gel properties, and emulsifying properties of MPs but also factors causing MP oxidation, characterized by thiols, carbonyl compounds, free amino groups, dityrosine content, cross-linking, and MP aggregates. Additionally, the WHC, texture, flavor, and nutritional value of muscle foods are closely related to MPs. This review encourages additional work to explore the potential of tempering techniques, as well as the synergistic effects of traditional and innovative thawing technologies, in reducing the oxidation and denaturation of MPs and maintaining the quality of muscle foods.

Effects of liquid nitrogen freezing at different temperatures on the quality and flavor of Pacific oyster (Crassostrea gigas)

This study aimed to evaluate the effects of different liquid nitrogen freezing (LNF) temperatures (-20, -40, -60, -80, and -100 °C) on the water holding capacity, texture, microstructure, and flavor of Crassostrea gigas (C. gigas). The results showed that -40 °C LNF, -60 °C LNF, and -80 °C LNF improved the water holding capacity of C. gigas (P < 0.05); -60 °C LNF and -80 °C LNF could effectively maintain the hardness of the body trunk and adductor muscles. Compared with -20 °C refrigerator freezing (RF), the LNF group could form smaller ice crystals and thus reduce the damage to the muscle cell structure damage, especially LNF at -80 °C. Gas chromatography-ion mobility spectrometry (GC-IMS) and e-nose results indicated that -80 °C LNF maintained the flavor profile of few aldehydes and alcohols compared to other freezing groups. Therefore, -80 °C LNF effectively improved the quality and maintain the flavor characteristics of frozen C. gigas.

Quality improvement of tilapia fillets by light salting during repeated freezing-thawing: Contribution of structural rearrangement and molecular interactions

The present study evaluated the effects and underlying mechanisms of light salting on quality properties of tilapia fillets during repeated freezing-thawing. Light salting was found to improve water-holding capacity and decelerated texture softening in tilapia fillets during repeated freezing-thawing. Instead of tissue distortion and heterogeneous aggregates in control groups, light salting promoted myofibril disassembly and formation of an ordered protein network with the solubilized myofibrillar proteins. The myofibrils presented an overall amorphous appearance with the loss of M-lines, removing the restraints to myofibril swelling and solubilization from A-binds in salted groups during repeated freezing-thawing. The structural rearrangement caused by light salting facilitated the enlargement of water-holding space, transformation of tissue water, and tissue recoverability, improving water-holding capacity and texture properties of tilapia fillets during freezing-thawing. The finding provided novel insight into the improvement of quality properties of tilapia fillets by light salting when subjected to drastic temperature fluctuations.

Characteristics and potential application of myofibrillar protein from golden threadfin bream (Nemipterus virgatus) complexed with chitosan

The strategies to broaden the applications of proteins involve their modification with polysaccharides. However, the characteristics and application of myofibrillar proteins (MPs) from golden threadfin bream (Nemipterus virgatus) complexed with chitosan (CS) are still unclear. Therefore, the characteristics of MPs complexed with CS and their application were investigated at different MP/CS ratios (100:0-80:20 (w/w)). The turbidity of MP/CS complexes was small at the MP/CS ratio of 95:5 (w/w). Besides, CS addition induced changes in MP structure to make it hydrophilic. Secondary structure analysis showed that α-helix and β-turn interconverted with β-sheet and random coil after the addition of CS. Additionally, the thermal stability of MP/CS mixtures enhanced after the addition of CS and the MP/CS mixtures at the ratio of 95:5 (w/w) had a relatively compact structure. High internal phase emulsions (HIPEs) prepared at the MP/CS ratio of 95:5 (w/w) were relatively stable compared to those at the other ratios. Consequently, MP/CS mixtures at suitable ratios possess the potential ability to prepare HIPEs. These results exhibit that MP/CS mixtures may be applied for constructing food-graded emulsion delivery systems with a high internal phase in the food industry.

Analysis of the water state and microfractal dimension of tilapia fillets during freezing and thawing

To study the effects of freezing and thawing times and freezing temperatures on the water state and microstructure of tilapia fillets, experiments on tilapia fillets were carried out at -4 and -18°C with one to four freezing and thawing cycles (FTCs). Low-field nuclear magnetic resonance (LF-NMR) and nuclear magnetic resonance imaging were used to observe the water state after different treatments, and scanning electron microscopy (SEM) and frozen sections were used to observe the microstructure changes. Fractal dimension (FD) was used to quantitatively characterize the microstructure of the fish tissue, and the correlation between FD and fish fillet quality parameters was studied by principal component analysis (PCA). The findings showed that with the increase of FTCs, the thawing loss increased, and the water holding capacity (WHC) fell. FTCs cause a decrease in immobilized water and an increase in free water in the fillet. This indicates the migration of immobilized water to free water. SEM and frozen slice images showed that the growth of ice crystals led to the destruction of myogenic fibers. A decrease in freezing temperature inhibited ice crystal growth. The FD value dropped in accordance with an increase in FTCs. PCA demonstrated that the WHC, NMR data, and FD value had a strong correlation with the quality changes in the tilapia fillets. Therefore, FD and water state can reflect the quality characteristics of tilapia fillets. PRACTICAL APPLICATION: The water migration in tilapia fillets is detected with LF-NMR, and the microscopic image may be quantified using the FD value. Both approaches can offer fresh perspectives on how to assess the quality of tilapia fillets and reflect changes in their quality.

Effects of Pulsed Pressure Curing on Beef Quality

The study was proposed to investigate the effects of pulsed pressure curing on the beef absorption of the curing solution, cooking loss, moisture content, centrifugal loss, salt content, sensory attributes, texture, microstructures and volatile compounds. Curing methods include the following four treatments: (1) control group 1-static curing (SC); (2) control group 2-vacuum curing (VC); (3) control group 3-pressurized curing (PC); and (4) treatment group-pulsed pressure curing (PPC). The acquired results revealed that pulsed pressure curing significantly boosts the curing efficiency and moisture content, decreases cooking loss in beef, brightens meat color, and enhances texture compared to static curing, vacuum curing, and pressurized curing. Additionally, centrifugal losses were not impaired, and sensory findings revealed that PPC significantly improved the saltiness of beef. TPA results showed that the springiness and cohesiveness of PPC were greatly increased, and hardness and chewiness were significantly reduced. Moreover, PPC significantly reduced the content of 1-octen-3-ol and 1-hexanol. Scanning electron microscopy (SEM) images documented that pulsed pressure curing can effectively increase the tenderness of beef. This study demonstrates that processed meat product efficiency and sensory attributes should be taken into account when selecting a curing technique, and the PPC technique has an advantage in both areas.

The preservable effects of ultrasound-assisted alginate oligosaccharide soaking on cooked crayfish subjected to Freeze-Thaw cycles

To improve the quality of cooked and frozen crayfish after repeated freeze-thaw cycles, the effects of alginate oligosaccharide (1 %, w/v) with ultrasound-assisted (40 W, 3 min) soaking (AUS) on the physicochemical properties were investigated. The AUS samples improved water-holding capacity with 19.47 % higher than the untreated samples. Low-field nuclear magnetic resonance confirmed that mobile water (T₂₂) in the samples after 5 times of freeze-thaw cycles was reduced by 13.02 % and 29.34 % with AUS and without treatment, correspondingly; and with AUS and without treatment, average size of the ice crystals was around 90.26 μm² and 113.73 μm², and average diameter of the ice crystals was 5.83 μm and 8.14 μm, respectively; furthermore, it enhanced the solubility and zeta potential, lowered the surface hydrophobicity, reduced the particle size, and maintained the secondary and tertiary structures of myofibrillar protein (MP) after repeated freeze-thawing. Gel electrophoresis revealed that the AUS treatment mitigated the denaturation of MPs. Scanning electron microscopy revealed that the AUS treatment preserved the structure of the tissue. These findings demonstrated that the AUS treatment could enhance the water retention and physicochemical properties of protein within aquatic meat products during temperature fluctuations..

Effects of tea branch liquid smoke on oxidation and structure of myofibrillar protein derived from pork tenderloin during curing

This study focused on how different concentrations of tea branch liquid smoke (TLS) in the curing solution impacted the physicochemical properties and antioxidant properties of pork tenderloin. Five experimental (1.25 mL/kg, 2.5 mL/kg, 5 mL/kg, 10 mL/kg, 20 mL/kg) and blank groups set up over 4 days, and it was found that the physicochemical indexes, antioxidant capacity, thermal stability and protein network structure of the cured meat using 5 mL/kg of liquid smoke were excellent than the other groups used (P < 0.05). However, concentrations at 20 mL/kg accelerated protein oxidation. Low frequency nuclear magnetic resonance (LFNHR) revealed that TLS also improved the water holding capacity of the cured meat by increasing the percentage of bound water. Additionally, the correlation analysis demonstrated that the inoxidizability of myofibrillar protein was significantly related to cooking loss and water distribution, which were adjusted by changing the usage of liquid smoke.

Impact of frozen storage on fatty acid profile in goose meat

The objective of this study was to investigate the changes of the fatty acid in breast (BM) and leg (LM) muscles from 17-wk-old female White Kołuda geese packaged in a vacuum and stored in freezing conditions at −20°C. During 17 weeks, the geese were fed ad libitum on the same complete feed. The samples (18 LM and 18 BM) from the right part of the carcasses were stored for 30, 90, 80, 270, and 365 d. The changes in the fatty acid profile were established by gas chromatography. In this work, there were also calculated lipid profile indicators such as Σ PUFA n−6/Σ PUFA n−3, Σ UFA/Σ SFA, and Σ PUFA/Σ SFA. Time of frozen storage affected the decrease in Σ SFA, Σ MUFA, and Σ PUFA of BM and LM. The statistical analysis of the obtained data shows that the type of muscle also generally affected the fatty acid profile. The BM are characterized higher proportion of Σ SFA, and the LM are defined as containing more Σ MUFA and Σ PUFA. Extending frozen storage time caused only the deterioration of Σ PUFA n−6/Σ PUFA n−3. The Σ PUFA n-6/Σ PUFA n−3 were the highest in BM and LM on the 365th day of storage. Although the Σ PUFA n−6/Σ PUFA n−3 ratio in muscles stored for 180, 270, and 365 d was higher than the recommended values. The lipid profile indicators (Σ UFA/Σ SFA, and Σ PUFA/Σ SFA) were similar in raw meat and in all frozen storage samples. It means that frozen storage didn't affect this index and the BM and LM have the same quality from the dietary point of view. Leg muscles during frozen storage are characterized by higher Σ UFA/Σ SFA and Σ PUFA/Σ SFA than the breast muscles.

Integrated Lipidomic and Metabolomics Analysis Revealing the Effects of Frozen Storage Duration on Pork Lipids

Frozen storage is an important strategy to maintain meat quality for long-term storage and transportation. Lipid oxidation is one of the predominant causes of the deterioration of meat quality during frozen storage. Untargeted lipidomic and targeted metabolomics were employed to comprehensively evaluate the effect of frozen duration on pork lipid profiles and lipid oxidative products including free fatty acids and fatty aldehydes. A total of 688 lipids, 40 fatty acids and 14 aldehydes were successfully screened in a pork sample. We found that ether-linked glycerophospholipids, the predominant type of lipids, gradually decreased during frozen storage. Of these ether-linked glycerophospholipids, ether-linked phosphatidylethanolamine and phosphatidylcholine containing more than one unsaturated bond were greatly influenced by frozen storage, resulting in an increase in free polyunsaturated fatty acids and fatty aldehydes. Among these lipid oxidative products, decanal, cis-11,14-eicosenoic acid and cis-5,8,11,14,17-dicosapentaenoic acid can be considered as potential indicators to calculate the freezing time of unknown frozen pork samples. Moreover, over the three-month frozen storage, the first month was a rapid oxidation stage while the other two months were a slow oxidation stage.