Boosting VFAs production during the anaerobic digestion of corn stover waste in Northeast China: Freeze-thaw pretreatment and effect of initial pH

doi:10.21203/rs.3.rs-5302487/v1

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Boosting VFAs production during the anaerobic digestion of corn stover waste in Northeast China: Freeze-thaw pretreatment and effect of initial pH

https://doi.org/10.21203/rs.3.rs-5302487/v1

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published 14 Jul, 2025

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The low temperature in Northeastern China is a prospective and exploitable advantage in low energy consumption pretreatment for lignocellulose. The effects of freeze-thaw pretreatment on the microstructure of corn stover hydrolysis characteristics and fermentation acid production were investigated using corn stover as the raw material. The experimental results demonstrated that the reducing sugar release and SCOD values of corn stover following freeze-thaw pretreatment exhibited increases of 15.77–66.96% and 13.89–68.94%, respectively, compared to those of the control group. Freeze-thaw pretreatment effectively alters the microstructure of stover, disrupting the hydrogen bonds between cellulose and hemicellulose in the amorphous zone of treated corn stover and removing lignin. The acid yield of the treated corn stover under optimal pretreatment conditions is enhanced by up to 77.94% compared with the control group. The initial pH pronouncedly influenced the acid yield of anaerobic fermentation of corn stover, with the highest acid yield of 3.78 g/L observed at pH values between 7.5 and 8. This study provides theoretical guidance for the industrial development of low-cost and low-energy consumption pretreatment method in lignocellulose wastes.

corn stover

low-temperature pretreatment

anaerobic fermentation

short-chain fatty acids

The pre-treated solid-liquid mixture of straw is used for anaerobic digestion.
The yield of VFAs after pretreatment is increased by 77.94%.
Freeze-thaw pretreatment destroys straw microstructure, increases the release of SCOD and the reducing sugar.
Freeze-thaw enhanced the acid buffer function of corn straw.

Straw is a kind of cellulosic biomass waste produced during crop production and processing, which has the characteristics of large yield and renewability[1]. In China, the annual production of straw exceeds 1 billion tons. In the northeast region, a significant producer of corn, soybeans, and other grains, the annual production of straw accounts for over 20% of the country's total, the vast majority of which is treated as waste piled up in the field or taken the centralized incineration way to dispose of[2]. Not only does this result in significant resource depletion, but it also leads to severe environmental contamination issues. The abundance of organic matter in crop straw has made it a prominent subject of research and discussion both domestically and internationally about its resource usage. Compared with other resource utilization pathways such as straw return to the field, straw gasification, straw power generation, and straw ethanol production, the anaerobic fermentation of straw has the best "economic-environmental-sustainability" benefits [3]. Volatile fatty acids (VFAs) are critical intermediate products of anaerobic fermentation; VFAs have more economic value and a more comprehensive range of applications than other fermentation products like methane and hydrogen.[4]. In addition to their usage as an extra carbon source in wastewater treatment [5], they also demonstrate significant potential in producing polyhydroxyalkanoate (PHAs) and synthesizing other compounds [6].

Nevertheless, the lignin in straw is firmly attached to cellulose and hemicellulose, creating a physical and chemical barrier at the cellular scale. This barrier makes the straw less prone to microbial degradation and makes it challenging to break down through hydrolysis. As a result, the hydrolysis stage is typically the step that limits the overall rate of anaerobic fermentation of straw[7–9]. Consequently, before anaerobic fermentation, straw is commonly subjected to pretreatment to disintegrate the lignin fiber barrier and enhance the surface area for anaerobic microbes to interact with the organic matter. The total biogas production increased by 33.07% and methane production by 75.57% after pretreatment of corn stover for 15 d with 0.01% of composite fungicide [10]. Compared with conventional technologies such as acid and alkali pretreatment and biological pretreatment, freeze-thaw pretreatment has the advantages of being environmentally friendly, less costly, and without secondary pollution. During the freezing process, water expansion destroys the hydrogen bonds of the straw, which reduces the degree of cellulose polymerization and improves the hydrolysis rate of the straw [11]. Sun et al. discovered that the enzymatic hydrolysis efficiency of cellulose and hemicellulose reached a maximum of 70.66% after being treated at -10°C for 24 hours. This level was 3.13 times greater than the control level [12]. Free-thaw technology is mainly used to promote anaerobic digestion of residual sludge [13, 14], microalgae [15, 16], and food waste[17]. Moreover, most of the researches focus on the use of hydrolysate after freeze-thaw pretreatment of straw in the batch or continuous anaerobic digestion [18–20]. The process of anaerobic f digestion for VFAs production using the solid-liquid mixture of straw with freeze-thaw pretreatment, particularly in the semi-continuous fermentation, is currently a few-explored area of research.

Northeast China has a unique natural cold resource with low winter temperatures, and the average monthly minimum temperature in winter can be below − 20°C. This unique natural cold resource can be combined with the abundant local straw waste resources to achieve "zero energy consumption" freeze-thaw pretreatment of straw waste, which significantly reduces the pre-treatment cost while enhancing the efficiency of straw utilization. In this study, we simulated the outdoor low-temperature environmental conditions in winter in Northeast China and pretreated corn stover by freezing and thawing in the laboratory. The effects of different solid-liquid ratios and different freezing times on the hydrolysis effect of corn stover at -20℃ were investigated. Response surface methodology was used to optimize the pretreatment conditions, and the differences in the response of pretreatment conditions to different indicators were compared and analyzed to assess the performance enhancement of anaerobic fermentation of corn stover after treatment under the optimal pretreatment conditions. As our previous research, the fermentation was significantly affected by the initial pH in batch experiment[21]. Whereas, the effect of initial pH in semi-continuous experiment was not clear. Thus, the fermentation efficiency of corn stover under different initial pH condition was investigated during the semi-continuous experiment.

In this study, the pretreated straw and hydrolysate did not require solid-liquid separation, i.e., they could be directly subjected to semi-continuous fermentation for acid production without significant pH adjustment. This provides theoretical guidance for the industrial development of low-cost, low-energy freeze-thawed cellulose pretreatment technology.

2.1 Materials and inoculum

The corn straw utilized in the experiment was purchased from agricultural land, air-dried, pulverized using a grinder, and filtered through a 20-mesh screen. The resulting material was then gathered in self-sealing bags and stored at room temperature. The inoculated sludge was taken from the anaerobic tank of a sewage treatment plant in Shenyang and stored at 4℃ for use. Table 1 displays the physical and chemical characteristics of the experimental materials.

Table 1

Basic properties of experimental raw materials
Measurement indicators	Total solid(TS)	Volatile solids(VS)	Total chemical oxygen demand(TCOD)	pH
Corn straw	92.70%	78.41%	112%	5.49

2.2 Experimental setup and operation

2.2.1 Pretreatment of corn straw

Weighing 2g of crushed corn straw in a 50 mL centrifuge tube, according to the solid-liquid ratio (w/w) of 1:6, 1:8, 1:10, 1:12, adding 12mL, 16mL, 20mL, and 24mL of distilled water, and then freezing in a refrigerator at -20 ℃ for 6, 12, 24, and 48 hours. Once the corn straw has been frozen, take it out and allow it to thaw at a temperature of 25 ℃ for 12 hours. The unfrozen and thawed samples were utilized as the control (CK). The experiment was replicated three times for each treatment condition.

2.2.2 Optimization of preprocessing conditions

Response surface methodology is a set of mathematical and statistical approaches used to study the correlation between a response and an independent variable. It also helps in creating a mathematical model that represents the functional relationship between the response and the independent variable[22]. The coefficients of the cubic polynomial model were estimated to determine the functional relationship between the response (reducing sugars and soluble chemical oxygen demand (SCOD)) and the factors (A and B).

$$\:{R}_{Response}={\beta\:}_{0}+{\beta\:}_{1}A+{\beta\:}_{2}B+{\beta\:}_{11}{A}^{2}+{\beta\:}_{22}{B}^{2}+{\beta\:}_{12}AB+{\beta\:}_{111}{A}^{3}+\:{\beta\:}_{222}{B}^{3}\:\:\:\:\:\:\left(1\right)$$

A and B represent the pretreatment time and solid-liquid ratio, respectively. 𝛽0, 𝛽1, 𝛽2, 𝛽11, 𝛽22, 𝛽111, and 𝛽222 are the regression intercepts. The pre-processing conditions were optimized by analyzing the experimental results and the quadratic polynomial coefficients.

2.2.3 Semi-continuous anaerobic digestion with corn straw as substrate

The semi-continuous experiments were initiated using a 100mL serum bottle as a reactor with a working volume of 60mL and a stepwise incubation method. Firstly, 30mL (1/2 of the practical volume) of inoculated sludge was added to the anaerobic fermentation reactor. Then corn stover with an organic load of 3.27gVS/(L·d) was added every two days for 1–6 days to initiate the anaerobic fermentation, and the solids content of the substrate (TS) was 16.67 g/L. Following the initiation of the reaction, the feed was conducted at intervals of four days. Following the completion of the reaction startup, feeding and discharging will be conducted at regular intervals of four days. During the feeding process, the maize stover that has undergone treatment will be combined with water and introduced into the system. Simultaneously, an equivalent quantity of solid-liquid mixture will be expelled, and the pH of the system will be modified. The feed parameters were an organic loading rate of 3.27gVS/(L·d), a hydraulic retention time of 12 days, a fermentation temperature of 35 ± 1 ℃, and a rotational speed of 120 rpm. The pH of the fermentation process was systematically modified in 12-day intervals to examine its impact on the generation of acid in anaerobic fermentation.

2.3 Measurement and analytical methods

TS and VS were determined by the weighing method[23]. The samples were centrifuged at 8000 r/min for 5 min, and the supernatant was taken and filtered to determine reducing sugars and SCOD. Reducing sugars were determined using the 3,5-dinitro-salicylic acid method at 540 nm [24], and SCOD was determined using the potassium dichromate method. pH was determined using a pH meter. The gas yield was determined by the drainage method. Volatile fatty acids (VFA) were determined using gas chromatography [25].

2.4 Data analysis

The data recording and statistical analysis were conducted using Excel and Origin2022, while the data optimization for calculation was performed using Design Expert software.

3.1. Effect of freeze-thaw pretreatment on the hydrolysis properties of corn stover

3.1.1 Changes in reducing sugar release from corn stover under different conditions of freeze-thaw pretreatment

Figure 1 illustrates the effect of pretreatment on the release of reducing sugars from corn stover. The pretreatment process destroys the crystal structure of cellulose and releases more free sugars, which leads to an increase in the release of reducing sugars. Therefore, the amount of reduced sugar released is an essential indicator for assessing the effect of pretreatment [26]. The increase in pretreatment time and solid-liquid ratio resulted in an initial rise followed by a subsequent decrease in the release of reducing sugars. The maximum release of reducing sugar was observed at a pretreatment time of 36 hours and a solid-liquid ratio of 1:10. This value was 177.43mg/g, 66.96% higher than the control group. However, as the freezing and thawing time increased to 60 hours, the release of reduced sugar decreased to 151.24mg/g. This may be due to the growth of freeze-thaw time from 18h to 36h; the formation of fine ice crystals from water freezing also increased after 36h of freeze-thaw, fine ice crystals in the internal distribution of the straw, effectively destroying the cellulose structure, and enhance the amount of reducing sugar release[27]. When the duration of freezing and thawing surpassed 36 hours, there was a progressive increase in the size of the tiny ice crystals [28]. Consequently, the ice crystals exhibited a localized physical detrimental impact on the cell wall, decreasing the release of reducing sugars. As the solid-liquid ratio increased from 1:6 to 1:10, there was a gradual increase in the release of reducing sugars. The increase in the solid-liquid ratio can enhance the adsorption of cellulose to the aqueous solution, thereby promoting the lysis of the straw's cell wall and enhancing the hydrolysis of cellulose and hemicellulose [29]. Nevertheless, a notable decrease in the release of reducing sugars was seen when the solid-liquid ratio was elevated to 1:12. The observed decline can be ascribed to the reduction in the number of ice crystals resulting from an excessive presence of water. As a result, there was a drop in the efficiency of cellulose hydrolysis [29].

3.1.2 Changes in SCOD of corn stover before and after freeze-thaw pretreatment

The SCOD value is an important index for evaluating the biodegradation rate during anaerobic fermentation, and it can respond to the degree of hydrolysis of straw in the pretreatment stage [30]. The results depicted in Fig. 2 demonstrate a notable improvement in the release of corn straw SCOD when subjected to freeze-thaw pretreatment compared to the absence of pretreatment. The experimental results indicate that the release of corn straw SCOD exhibited a pattern of initial increase followed by a subsequent decrease in response to variations in freeze-thawing duration and solid-liquid ratio. Specifically, the release of corn straw SCOD reached its peak value of 185.83 mg/g when subjected to a freeze-thawing duration of 36 hours and a solid-liquid ratio of 1:10. This value represents a significant enhancement of 68.94% compared to the control group. Using the SCOD release of corn straw as a measure, it has been observed that a solid-liquid ratio of 1:10 can achieve the most effective pretreatment effect compared to other solid-liquid ratios. Similarly, a pretreatment time of 36 hours has yielded the best pretreatment effect compared to other pretreatment times.

3.1.3 Changes in pH of corn stover before and after freeze-thaw pretreatment

Figure 3 demonstrates that the pH value lowered to varying extents after freeze-thaw pretreatment, with the lowest value being reduced to approximately 5.2 compared to CK. The pH value exhibited a progressive decline as the solid-liquid ratio of the freeze-thaw pretreatment was reduced. As the duration of freezing and thawing was extended from 18 hours to 36 hours, there was a gradual drop in the pH value. However, as the freezing and thawing period was further extended to 60 hours, the pH value experienced a slight recovery, albeit remaining lower than the initial pH value. The cause of this occurrence could be attributed to the pretreatment process, which facilitated the breakdown and dissolution of cellulose and hemicellulose, leading to increased conversion of these substances into volatile fatty acids. The pH of the pretreated maize stover decreased as a consequence [31].

3.2. Optimization analysis of pretreatment conditions

3.2.1 Reducing Sugar Release Response Surface Analysis

The response of reducing sugar release from corn straw under different pretreatment conditions is shown in Fig. 4. The coefficient of determination (R²) for the quadratic term was determined to be 0.9509, suggesting a solid fit between the regression equation and the experimental and projected values of decreasing sugar release. Based on the statistical analysis and ANOVA results (Table 2), the "PROB > F" value was below 0.0500, indicating that the model was statistically significant. The regression equation that was established is as follows:

Table 2

Analysis of variance in response surface method
Source	Sum of squares	df	Mean square	F value	Prob > F
Model	2469.15	9	274.35	12.91	0.0028	Significant
A	733.62	1	733.62	34.52	0.0011
B	607.70	1	607.70	28.60	0.0017
AB	3.545E-004	1	3.545E-004	1.668E-005	0.9969
A²	0.100	1	0.100	4.691E-003	0.9476
B²	122.73	1	122.73	5.78	0.0531
A²B	58.60	1	58.60	2.76	0.1479
AB²	0.73	1	0.73	0.034	0.8589
A³	30.22	1	30.22	1.42	0.2781
B³	435.94	1	435.94	20.51	0.0040
Residual	127.50	6	21.25
Cor total	2596.65	15

$$\:{R}_{TRS}=148.64+27.89\times\:A-39.88\times\:B-0.021\times\:AB-1.11{A}^{2}-7.43\times\:{B}^{2}+6.32\times\:{A}^{2}B-0.65\times\:A{B}^{2}-12.35{\times\:A}^{3}\:+34.97\times\:{B}^{3}\left(2\right)\:$$

The variables A and B represent the duration of freezing and thawing and the ratio of solid to liquid, respectively. The findings from the response surface analysis indicate that the pretreatment condition with the highest theoretical optimality was determined to be 50.97 hours, with a solid-liquid ratio of 1:9.51. The theoretical release of reducing sugar from corn straw was 170.12mg/g. The optimal pretreatment conditions for the experiment were a freeze-thaw time of 36 h and a solid-liquid ratio of 1:10, under which the reducing sugar release was 177.43 mg/g, which was 4.3% higher than that of the theoretically optimal treatment conditions for straw-reducing sugar release.

3.2.2 SCOD response surface analysis

Figure 5 displays the response of corn straw SCOD under various pretreatment settings. The coefficient of determination (R²) for the quadratic term was 0.9711, suggesting a solid fit between the regression equation and the experimental and projected values of reducing sugar release. Based on the statistical analysis and ANOVA results (Table 3), the F value of the model was 22.42, and the "PROB > F" value was below 0.0500, suggesting that the model was significant. The regression equation that has been established is:

Table 3

Analysis of variance in response surface method
Source	Sum of squares	df	Mean square	F value	Prob > F
Model	3749.10	9	416.57	17.89	0.0011	Significant
A	161.20	1	161.20	6.92	0.0390
B	1559.60	1	1559.60	66.99	0.0002
AB	125.35	1	125.35	5.38	0.0594
A²	8.93	1	8.93	0.38	0.5585
B²	426.55	1	426.55	18.32	0.0052
A²B	108.69	1	108.69	4.67	0.0740
AB²	2.39	1	2.39	0.10	0.7596
A³	0.77	1	0.77	0.033	0.8620
B³	1455.13	1	1455.13	62.50	0.0002
Residual	139.69		6	23.28
Cor total	3888.79		15

$$\:{R}_{SCOD}=154.30+13.07\times\:A-63.88\times\:B-12.26\times\:AB-10.47\times\:{A}^{2}-13.85\times\:{B}^{2}+8.60\times\:{A}^{2}B-1.18A{B}^{2}-1.97{A}^{3}+63.89{B}^{3}\:\left(3\right)$$

A and B denote the freezing and thawing time and solid-liquid ratio, respectively. The results of response surface analysis showed that the theoretical optimal pretreatment conditions were a freeze-thaw time of 50.5h, a solid-liquid ratio of 1:9.69, and the theoretical value of corn straw SCOD under this condition was 172.92mg/g.

3.2.3 Reducing Sugar and SCOD Response Ratio Analysis

When comparing reducing sugar to SCOD, the R² value was closer to 1, indicating a stronger relationship. Additionally, the "PROB > F" value was less, making it more appropriate for use as the response factor. SCOD is a response when the theoretical optimal conditions freeze-thaw time is 50.5h, the solid-liquid ratio is 1:9.69, and the theoretical corn straw SCOD under this condition is 172.92mg/g. The SCOD under the optimal pretreatment condition of the test was 185.83 mg/g, which was somewhat higher than the SCOD under the theoretical optimal condition. This could be attributed to the regression equations R² values of 0.9505 and 0.9711 for reducing sugar and SCOD, which were below 1.00. Consequently, the theoretical optimal values for reducing sugar and SCOD were smaller than the experimental ones. The freeze-thaw time of 36 hours and the solid-liquid ratio of 1:10 were selected as the optimal pretreatment conditions.

3.3. Effect of Freeze-Thaw Pretreatment on Microstructural Properties of Corn Stover

3.3.1 Changes in the apparent structure of corn stover

To assess the impact of freeze-thaw pretreatment on the micro-morphological characteristics of maize stover, the surface morphology of corn stover was examined using scanning electron microscopy (SEM) under three different pretreatment circumstances: untreated and three freeze-thaw pretreatment conditions (Fig. 6a-h). The analysis of Figs. 6a and 6b reveals that the stover surface exhibited a smooth texture, structural integrity, and compactness, with no apparent signs of damage in the absence of pretreatment. This is because the lignin in straw is tightly bound to cellulose and hemicellulose, forming a physicochemical barrier outside the structure capable of resisting microbial attack. The surface micro-morphological alterations of corn stover following pretreatment were evident, as depicted in Figs. 5c-h. The pretreated corn stover exhibited twisted and broken external stalks, resulting in noticeable holes, and the destruction of the overall block structure led to numerous irregular fragmented structures. These findings suggest that the freeze-thaw pretreatment process effectively disrupted the dense lignin barrier, increased the specific surface area of the stover, and facilitated complete contact between anaerobic microorganisms and the cellulose hemicellulose. Consequently, the hydrolysis efficiency of corn stover was enhanced. Furthermore, it is noteworthy that the surface morphology of the corn stover exhibited a relatively uniform texture despite the evident disruption of its outer structure caused by the treatment. This phenomenon can be attributed to the freeze-thaw pretreatment's capacity to effectively preserve the cellulose and hemicellulose components within the stover while disrupting the outer lignin structure.

3.4. Effect of freeze-thaw pretreatment on acid production from anaerobic fermentation of corn straw

3.4.1 pH changes during fermentation

The pH within the anaerobic fermentation reaction system can indicate the alterations in acid-base chemical equilibrium resulting from microbial proliferation and metabolic processes. Additionally, it can partially represent the generation of volatile acids within the reaction system [32]. Figure 7 illustrates that both the freeze-thaw treatment group and the control group experienced a significant decrease in pH at the start of fermentation. However, the pH gradually stabilized after 12 days of fermentation. After 36 days, there was a slight increase in pH, which remained stable. This was because, at the early stage of fermentation, the rate of VFA generation in the system was more significant than the rate of consumption, which led to the accumulation of VFA and a decrease in pH. At the end of the fermentation period, the initial pH was increased to 8-8.5 by alkali modulation, which increased pH during the fermentation process. The pH in the anaerobic fermentation system of corn stover after freeze-thaw pretreatment was slightly higher than that in the group, the pH change was more minor, and the chemical environment of the fermentation system was milder, indicating that the freeze-thaw pretreated fermentation played a buffering effect on the acidification process of the system.

3.4.2 Effect of freeze-thaw pretreatment on the concentration of VFAs in the fermentation broth during the fermentation process

The concentration of volatile fatty acids (VFAs) during anaerobic fermentation in the freeze-thaw pretreatment group and the control group under various pH controls is depicted in Fig. 8. The content of volatile fatty acids (VFAs) exhibited a pattern of initial increase followed by subsequent decrease in both groups under varying pH settings. At a pH of 7.5-8, the concentration of VFAs reached its peak at 3.78 g/L. Figure 7 demonstrates that the freeze-thaw pretreatment significantly improved the yield of volatile fatty acids (VFAs), and the maximal acid yield was enhanced by 77.94% compared to the control group.

Following the freeze-thaw pre-treatment, the released reducing sugars and the SCOD values of the corn stover samples were found to be increased by 15.77–66.96% and 13.89–68%, respectively, in comparison to the control group. This process effectively alters the microstructure of the corn stover, breaking the hydrogen bonds between the non-crystalline regions of cellulose and hemicellulose and removing the lignin. The optimal freeze-thaw treatment conditions for corn stover were determined through a response surface analysis, with the optimal conditions being a freeze-thaw time of 36 hours and a solid-liquid ratio of 1:10.The acid production of corn stover following the optimal freeze-thaw treatment was 77% higher than that of the control. When the freeze-thaw pre-treatment was employed, a 94% reduction in the fermentation process's pH was observed. This indicates that the freeze-thaw pre-treatment can act as a buffer, moderating the oxidative process within the system and resulting in a gentler reaction environment. The initial pH of the fermentation medium significantly impacts the acid production of the anaerobic fermentation of corn stover. The highest acid production, at 3.78 g/L, was observed at a pH of 7.5 to 8.

Funding

This research was supported by Doctoral Research Start-up Project of Liaoning Province, China (2023010473-JH3/101) and Youth Research Fund Project of Liaoning University (LDYBJC2301)..

Declaration of generative AI in scientific writing

No generative artificial intelligence (AI) and AI-assisted technologies were used in the writing process.

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published 14 Jul, 2025

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Editorial decision: Reconsider pending major revisions
24 Feb, 2025
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Reviewers invited by journal
27 Oct, 2024
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Boosting VFAs production during the anaerobic digestion of corn stover waste in Northeast China: Freeze-thaw pretreatment and effect of initial pH

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Abstract

Figures

Highlights

1. Introduction

2. Materials and methods

2.1 Materials and inoculum

2.2 Experimental setup and operation

2.2.1 Pretreatment of corn straw

2.2.2 Optimization of preprocessing conditions

2.2.3 Semi-continuous anaerobic digestion with corn straw as substrate

2.3 Measurement and analytical methods

2.4 Data analysis

3. Results and discussions

3.1. Effect of freeze-thaw pretreatment on the hydrolysis properties of corn stover

3.1.1 Changes in reducing sugar release from corn stover under different conditions of freeze-thaw pretreatment

3.1.2 Changes in SCOD of corn stover before and after freeze-thaw pretreatment

3.1.3 Changes in pH of corn stover before and after freeze-thaw pretreatment

3.2. Optimization analysis of pretreatment conditions

3.2.1 Reducing Sugar Release Response Surface Analysis

3.2.2 SCOD response surface analysis

3.2.3 Reducing Sugar and SCOD Response Ratio Analysis

3.3. Effect of Freeze-Thaw Pretreatment on Microstructural Properties of Corn Stover

3.3.1 Changes in the apparent structure of corn stover

3.4. Effect of freeze-thaw pretreatment on acid production from anaerobic fermentation of corn straw

3.4.1 pH changes during fermentation

4. Conclusion

Declarations

References

Status:

Journal Publication

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