Strains producing medium-chain fatty acids and apparatuses and methods for preparing medium-chain fatty acids

Abstract

The present application belongs to the technical field of breeding tailings treatment and resource utilization, and particularly relates to a strain for producing medium-chain fatty acids and a device and method for preparing medium-chain fatty acids. The strain for producing medium-chain fatty acids is Enterobacter YTLJ-N-S18. The Enterobacter YTLJ-N-S18 was preserved in the Guangdong Microbial Culture Collection Center on July 5, 2021, with a preservation number of GDMCC No: 61773, a classification name of Enterobacter sp., and a preservation address of Guangzhou, China. The present application changes the original fermentation process of solid waste such as breeding tailings by using a high-efficiency strain for producing medium-chain fatty acids, and finally realizes a new mode of breeding solid waste disposal.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of breeding tailings treatment and resource utilization, and particularly relates to a strain producing medium-chain fatty acids and a device and method for preparing medium-chain fatty acids. BACKGROUND

[0002] The accumulation of tailings brought by large-scale factory farming and the frequent occurrence of aquatic diseases seriously hinder the development of China's aquaculture industry. At present, there are few technologies for resource utilization of aquaculture tailings, which seriously limits the popularization and application of recirculating aquaculture technology.

[0003] Medium-chain fatty acids (MCFAs) are a kind of fatty acids containing 6-12 carbon atoms. Compared with ordinary long-chain fatty acids, medium-chain fatty acids are rare in nature, but have high nutritional value and special physiological and biochemical functions. Due to the limited natural sources of medium-chain fatty acids, the demand is increasing. Therefore, medium-chain fatty acids are currently obtained by using enzymes or chemical catalysts to esterify between alcohol (glycerol) and fatty acid. However, through comparison and summary, it is found that the above technologies have the disadvantages of high preparation cost and poor economy, which limit the yield of medium-chain fatty acids.

[0004] In recent years, the medium-chain fatty acid production technology has begun to have certain application in solid waste disposal. This technology not only realizes the rapid fermentation of solid waste and plays a role in disposing solid waste, but also serves as a new technical means to obtain high-value products such as medium-chain fatty acids, thereby reducing the cost of solid waste treatment. However, the lack of high-efficiency functional strains also makes this technology face the shortcomings of low yield and unstable system operation. At present, there is little report on the production and preparation of medium-chain fatty acids from aquaculture tailings and the high-value utilization. There is also no report on the production of medium-chain fatty acids from aquaculture tailings and the further fertilization for improving soil and increasing crop yield. SUMMARY

[0005] The purpose of the present application is to overcome the shortcomings of the prior art and provide a strain producing medium-chain fatty acids and a device and method for preparing medium-chain fatty acids.

[0006] To achieve the above purpose, the technical solution adopted by the present application is as follows:

[0007] A strain producing medium-chain fatty acids, the strain producing medium-chain fatty acids is Enterobacter YTLJ-N-S18;

[0008] The Enterobacter YTLJ-N-S18 was preserved in the Guangdong Microbial Culture Collection Center on July 5, 2021, with the preservation number of GDMCC No: 61773, the classification name of Enterobacter sp., and the preservation address of Guangzhou, China.

[0009] The application of a strain that produces medium-chain fatty acids, and the application of said strain in the production of medium-chain fatty acids.

[0010] A microbial agent for producing medium-chain fatty acids, the microbial agent containing the aforementioned Enterobacter YTLJ-N-S18.

[0011] A method for preparing the aforementioned medium-chain fatty acid-producing bacterial agent involves propagating and drying Enterobacter YTLJ-N-S18, then mixing it with an adjuvant at a mass ratio of 1:1-100. The total effective viable count of the bacterial agent is ≥1.0 × 10⁻⁶. 9 per mL.

[0012] The additives are sterilized and dried livestock manure residue, sulfur, iron powder, calcium carbonate powder, La(NO3)3, and Y(NO3)3; wherein the amount of sulfur added is 0.001%-0.01% of the additive mass, the amount of iron powder added is 0.005%-0.05% of the additive mass, the amount of calcium carbonate powder added is 0.5-1.0% of the additive mass, the amount of La(NO3)3 added is 0.001-0.005% of the additive mass, and the amount of Y(NO3)3 added is 0.001-0.005% of the additive mass.

[0013] Among them, the sterilized and dried aquaculture manure residue is fish aquaculture manure residue, and the main indicators are: TSS range 250-280g / L, COD range 2-3g / L, and sCOD range 3000-3500mg / L.

[0014] An apparatus for producing medium-chain fatty acids using the strain or the inoculum, comprising a fermenter, a stirrer, an automatic inoculum dosing system, an automatic ethanol addition system, a pH monitoring and control system, a temperature monitoring and control system, a timer, and an intelligent control platform; wherein, a stirrer is inserted into the fermenter; one side of the automatic inoculum dosing system and the automatic ethanol addition system are respectively connected to the fermenter via wiring, and the other side of the two systems are respectively connected to the intelligent control platform via wiring; the pH monitoring and control system, the temperature monitoring and control system, and the timer are respectively inserted into the fermenter; the automatic inoculum dosing system adds the strain or the inoculum.

[0015] The pH monitoring and control system, temperature monitoring and control system, and timer are connected to the intelligent control platform via lines, and the intelligent control platform is also connected to the fermenter via lines.

[0016] A method for producing medium-chain fatty acids using the aforementioned device: Aquaculture residue is added to the fermenter of the device. The temperature inside the fermenter is controlled at 34-36℃, and the pH is set at 9.5-10. Fermentation is carried out for 5-7 days to produce acid. Then, ethanol is added to the fermenter via an automatic ethanol addition system, and the aforementioned bacterial strain or bacterial agent is added via an automatic bacterial agent addition system. A sulfur-iron-ethanol mixture is then added to the fermenter, and carbon chain extension fermentation is carried out for 2-3 weeks at a pH of 5.0-6.0 and a temperature of 36-38℃. After purification, medium-chain fatty acids are obtained.

[0017] The aquaculture residue is a mixture of feces, uneaten feed, and tailings from the fish farming process. Its main indicators are as follows: TSS range 250-280g / L, COD range 2-3g / L, and sCOD range 3000-3500mg / L.

[0018] During the acid production process, if the pH is not within the optimal range, acid or alkali is added to adjust the pH to 10; if the temperature is not within the optimal range, temperature regulation is initiated to adjust the temperature to 35°C.

[0019] During the carbon chain elongation fermentation process, if the pH is not within the optimal range, acid or alkali is added to adjust the pH to 5.5; if the temperature is not within the optimal range, temperature regulation is initiated to adjust the temperature to 37°C.

[0020] The amount of the bacterial strain or inoculum and ethanol added accounts for 1-5% of the fermentation liquid volume, respectively; the amount of the sulfur-iron-ethanol mixture added accounts for 0.5-5% of the fermentation liquid volume, and the amount of sulfur-modified iron-carbon material added accounts for 10-30% of the mass of the sulfur-iron-ethanol mixture.

[0021] The sulfur-iron-ethanol mixture is prepared by dissolving sulfur-modified iron-carbon material in ethanol. The sulfur-modified iron-carbon material is made by mixing ferrous sulfate, carbon powder, and sodium borohydride, followed by adding sodium sulfide and mixing thoroughly. The mass ratio of ferrous sulfate, carbon powder, and sodium borohydride is 0.5-1:10:1, and the amount of sodium sulfide added is 1-3% of the mass of the carbon powder.

[0022] The purified fermentation residue can be used to prepare fertilizers for soil improvement.

[0023] Advantages of this invention:

[0024] This invention utilizes highly efficient medium-chain fatty acid-producing bacteria to alter the original fermentation process of solid waste such as tailings from aquaculture systems, ultimately achieving a new model for the disposal of aquaculture solid waste. Specifically:

[0025] 1) This invention employs a special novel bacterial strain to enhance the introduction of medium-chain fatty acid-producing bacteria into the fermentation system. This improves the conversion of short-chain fatty acids to medium-chain fatty acids, inhibits methanogenic bacteria from producing methanogens from short-chain fatty acids, and ensures that the organic acids produced are retained in the form of medium-chain fatty acids, avoiding the drawbacks of short-chain fatty acids being easily volatilized and degraded. The medium-chain fatty acids extracted from the fermentation products can be used as fertilizer to improve saline-alkali soils, and the medium-chain fatty acids also act as growth regulators to promote crop growth. This invention provides an eco-friendly solution to the problem of retained aquaculture manure and uneaten feed, reducing environmental pollution.

[0026] 2) This invention is a new low-carbon resource utilization technology for livestock solid waste. This invention changes the traditional anaerobic fermentation process, utilizing the short-chain fatty acids produced during fermentation to further generate medium-chain fatty acids, rather than converting them into methane. This invention adds a sulfur-iron-ethanol mixture and sulfur-modified iron-carbon materials to the medium-chain fatty acid production system to enhance the conductivity of the fermentation system and inhibit methanogenesis, thereby promoting the enrichment of medium-chain fatty acids. The reaction time is rapid, and methane production is avoided. This invention avoids the disadvantages of traditional anaerobic fermentation, such as long fermentation time and the difficulty in centralized treatment and utilization of greenhouse gases such as methane and nitrous oxide, while also overcoming the disadvantages of aerobic fermentation, such as the release of odors and carbon dioxide that pollute the environment. It effectively overcomes many technical difficulties faced by traditional solid waste fermentation treatment technologies.

[0027] 3) This invention utilizes a novel strain to alter the traditional anaerobic fermentation environment during the production of medium-chain fatty acids, thereby reducing nitrogen loss. The novel strain and its fermentation conditions are unfavorable to biological denitrification reactions such as denitrification and anaerobic ammonia oxidation, thus reducing nitrogen loss during fermentation and improving the fertility of the fermentation products. Attached Figure Description

[0028] Figure 1 This is a cluster analysis diagram of Enterobacter YTLJ-N-S18, a medium-chain fatty acid-producing bacterium, with other strains of the same genus, provided in an embodiment of the present invention.

[0029] Figure 2 This is a gas chromatogram of hexanoic acid produced in an embodiment of the present invention, where the horizontal axis represents time (s).

[0030] Figure 3 This is a gas chromatogram of octanoic acid produced in an embodiment of the present invention, where the horizontal axis represents time (s).

[0031] Figure 4 This is a simplified diagram of an apparatus for producing medium-chain fatty acids provided in an embodiment of the present invention, wherein 1 is a fermenter; 2 is a stirrer; 3 is an automatic inoculant dosing system; 4 is an automatic ethanol addition system; 5 is a pH monitoring and control system; 6 is a temperature monitoring and control system; 7 is a timer; and 8 is an intelligent control platform. Detailed Implementation

[0032] The present invention will be further illustrated by the following embodiments; however, the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] Isolation of Enterobacter YTLJ-N-S18:

[0035] The Enterobacter YTLJ-N-S18 was isolated from the coastal environment. The specific isolation process is as follows: 10g of sediment was placed in 50mL of sterile water and kept at 37℃ with constant shaking for 48h. After centrifugation at 8000rpm for 10min, the supernatant was removed. The lower bacterial culture was transferred to a sterile enrichment medium and cultured at 37℃ for 1 week. After dilution culture in LB medium, single colonies were selected and streaked on agar solid medium for isolation to obtain the purified Enterobacter YTLJ-N-S18 strain.

[0036] The enrichment medium used was composed of the following mass ratios: 0.5-1% NH4Cl, 0.1-1% acetic acid, 0.5-5.0% ethanol, 0.05-0.1% FeCl2, 2-5% K2HPO4, 1-3% MgSO4, 1-3% NaCl, 0.01-0.05% FeSO4, 0.01-0.05% MnSO4, 0.001-0.005% La(NO3)3, and 0.001-0.005% Y(NO3)3.

[0037] The Enterobacter YTLJ-N-S18 was deposited at the Guangdong Provincial Center for Microbial Culture Collection on July 5, 2021, with accession number GDMCC No: 61773, classified as Enterobacter sp., and deposited in Guangzhou, China.

[0038] Enterobacter YTLJ-N-S18 was confirmed as a newly discovered species by comparison of its 16S rDNA gene sequence (see [link]). Figure 1 ).

[0039] The 16S rDNA gene sequence of the Enterobacter YTLJ-N-S18 is as follows:

[0040]

[0041] Application of Enterobacter YTLJ-N-S18 in medium-chain fatty acid production systems:

[0042] The isolated and preserved Enterobacter YTLJ-N-S18 pure bacteria were cultured in enrichment medium, and inoculated into simulated fermentation broth at an inoculum size of 10 wt%. The pH of the system was adjusted to 5.5, and fermentation was carried out at 37°C for 2 weeks. This resulted in the enrichment of medium-chain fatty acids, producing hexanoic acid (20 g / L) and octanoic acid (1 g / L) (see [link to relevant documentation]). Figure 2 and Figure 3 ).

[0043] The simulated fermentation broth is prepared by adding acetic acid (20 g / L), propionic acid (0.4 g / L), butyric acid (5 g / L), and valeric acid (0.1 g / L) per liter of water, along with ethanol (100 g / L) and a sulfur-iron-ethanol mixture at a volume ratio of 3% of the fermentation broth.

[0044] The sulfur-iron-ethanol mixture is prepared by dissolving sulfur-modified iron-carbon material in ethanol. The sulfur-modified iron-carbon material is made by mixing ferrous sulfate, carbon powder, and sodium borohydride, followed by adding sodium sulfide and mixing thoroughly. The mass ratio of ferrous sulfate, carbon powder, and sodium borohydride is 0.8:10:1, and the amount of sodium sulfide added is 2% of the mass of the carbon powder.

[0045] Example 2

[0046] For apparatus for producing medium-chain fatty acids, see [link to documentation]. Figure 4 It mainly includes a fermenter 1, a stirrer 2, an automatic microbial agent dosing system 3, an automatic ethanol dosing system 4, a pH monitoring and control system 5, a temperature monitoring and control system 6, a timer 7, and an intelligent control platform 8.

[0047] The device controls the fermentation process in two stages: pre-acidification and carbon chain elongation.

[0048] The device collects fermentation environment data in real time by installing online pH and temperature sensors, and is equipped with an automatic temperature and acid-base control system to intelligently control the process of producing medium-chain fatty acids.

[0049] The device is equipped with a medium-chain fatty acid producing bacterial agent and an automatic ethanol addition system.

[0050] Furthermore, the device includes a fermenter 1, a stirrer 2, an automatic microbial agent dosing system 3, an automatic ethanol addition system 4, a pH monitoring and control system 5, a temperature monitoring and control system 6, a timer 7, and an intelligent control platform 8; wherein, the stirrer 2 is inserted into the fermenter 1; one side of the automatic microbial agent dosing system 3 and the automatic ethanol addition system 4 are respectively connected to the fermenter 1 by wiring, and the other side of the two systems are respectively connected to the intelligent control platform 8 by wiring; the pH monitoring and control system 5, the temperature monitoring and control system 6, and the timer 7 are respectively inserted into the fermenter 1.

[0051] The pH monitoring and control system 5, the temperature monitoring and control system 6, and the timer 7 are connected to the intelligent control platform 8 via lines, and the intelligent control platform 8 is also connected to the fermenter 1 via lines.

[0052] Example 3

[0053] The device system described in the above embodiments is used for the treatment of freshwater fish farming tailings:

[0054] 1) such as Figure 4 System Setup: The aquaculture tailings resource utilization device mainly includes a fermentation tank 1, a stirrer 2, an automatic microbial agent dosing system 3, an automatic ethanol addition system 4, a pH monitoring and control system 5, a temperature monitoring and control system 6, a timer 7, and an intelligent control platform 8.

[0055] The fermentation process of the aquaculture tailings resource utilization device is divided into two stages: pre-acidification and carbon chain elongation.

[0056] The aforementioned aquaculture tailings resource utilization device collects fermentation environment data in real time by installing online pH and temperature sensors, and is also equipped with an automatic temperature and acid-base control system to intelligently control the process of producing medium-chain fatty acids.

[0057] 2) Method for preparing the medium-chain fatty acid producing bacterial agent

[0058] a) Bacterial propagation: Enterobacter YTLJ-N-S18 was inoculated into the propagation medium at a volume of 5 wt%, and then cultured at 37°C for 24 hours. After the culture time was completed, the culture product was collected. The propagation medium consisted of acetic acid (1%), glucose (0.2%), and ethanol (10%) by weight, with the remainder being water.

[0059] b) Preparation of bacterial powder: The Enterobacter YTLJ-N-S18 obtained by propagation was freeze-dried to obtain a single strain of dry powder.

[0060] c) Preparation of additives: The additives are sterilized and dried livestock manure residue, sulfur, iron powder, calcium carbonate powder, La(NO3)3 and Y(NO3)3; wherein, the amount of sulfur added is 0.01% of the mass of the additives, the amount of iron powder added is 0.05% of the mass of the additives, the amount of calcium carbonate powder added is 1.0% of the mass of the additives, the amount of La(NO3)3 added is 0.005% of the mass of the additives, and the amount of Y(NO3)3 added is 0.001% of the mass of the additives.

[0061] Among them, the sterilized and dried aquaculture manure residue is eel aquaculture manure residue, with TSS of 280g / L, COD of 3.0g / L, and sCOD of 3500mg / L.

[0062] d) Preparation of microbial agent: Mix the microbial powder obtained in step 2 with the adjuvant obtained in step 3 in a 1:1 ratio (mass ratio) to prepare a microbial agent for producing medium-chain fatty acids.

[0063] 3) Start-up and operation of the first stage of fermentation (pre-acidification)

[0064] a) Add freshwater fish farming tail residue (a mixture of feces and uneaten feed from eel farming, with TSS of 280g / L, COD of 3.0g / L, and sCOD of 3500mg / L) to the fermentation tank and ferment for 1 week at a certain fermentation time and pH.

[0065] b) Real-time online pH testing: when pH is less than 9.5, alkali addition is initiated to adjust pH to 10; when pH is greater than 10.5, acid addition is initiated to adjust pH to 10.

[0066] c) Online real-time temperature monitoring: when the temperature is below 34°C, heating is initiated and the temperature is adjusted to 35°C; when the temperature is above 36°C, cooling is initiated and the temperature is adjusted to 35°C.

[0067] 4) Start-up and operation of the second stage of fermentation (carbon chain elongation)

[0068] a) After the first stage (pre-acidification stage) lasts for 1 week, the second fermentation stage (carbon chain elongation stage) begins. The system automatically adds the prepared inoculum and ethanol (both added at 5% of the fermentation broth volume), and the reaction time for the second stage is set to 2 weeks. Simultaneously, the fermentation system adds a sulfur-iron-ethanol mixture at 5% of the fermentation broth volume, with the sulfur-modified iron-carbon material accounting for 30% of the mass of the sulfur-iron-ethanol mixture.

[0069] The sulfur-iron-ethanol mixture is prepared by dissolving sulfur-modified iron-carbon material in ethanol. The sulfur-modified iron-carbon material is made by mixing ferrous sulfate, carbon powder, and sodium borohydride for 30 minutes, followed by adding sodium sulfide and stirring for another 10 minutes until homogeneous. The mass ratio of ferrous sulfate, carbon powder, and sodium borohydride is 1:10:1, and the amount of sodium sulfide added is 3% of the mass of the carbon powder.

[0070] b) Online real-time testing and detection of pH and temperature. The optimal pH setting is 5.0-6.0, and the optimal temperature setting is 36-38 degrees Celsius. When the pH is less than 5.0, start adding alkali and adjust the pH to 5.5. When the pH is greater than 6.0, start adding acid and adjust the pH to 5.5.

[0071] c) Monitor the temperature online in real time. When the temperature is below 36℃, start heating and adjust the temperature to 37℃; when the temperature is above 38℃, start cooling and adjust the temperature to 37℃.

[0072] The control group was a conventional anaerobic methanogenic fermentation system (see Bioresource Technology 364 (2022)128083; https: / / doi.org / 10.1016 / j.biortech.2022.128083, which describes anaerobic fermentation. This conventional anaerobic fermentation produces methane after the material is added and the fermentation is sealed). Enterobacter YTLJ-N-S18 was not added and the pH was not adjusted. Other culture conditions were the same as those for medium-chain fatty acid fermentation.

[0073] After the fermentation in the above embodiments of the present invention was completed, compared with the control, the nitrogen loss of the fermentation product was reduced by 30% (the total nitrogen concentration of the medium-chain fatty acid fermentation broth was 3.6 g / kg, and the total nitrogen concentration of the ordinary fermentation broth was 2.5 g / kg). The fermentation product obtained in the embodiments was used to extract medium-chain fatty acids by mineral oil solvent membrane extraction containing 3% tri-n-octylphosphine oxide (TOPO) to produce hexanoic acid (yield 10.9 g / kg) and octanoic acid (yield 0.8 g / kg).

[0074] Subsequently, the fermentation residue (C:N:P mass ratio of 200:5:1) was extracted and used as fertilizer to improve farmland soil. Specifically, the residue was applied at a rate of 40 kg / mu to watermelon or vegetable (rapeseed, tomato, and eggplant) planting areas in southwestern Shandong or Fuqing, Fujian. At the same time, a comparison was made between the same amount of ordinary fertilizer and the above-mentioned fermentation residue. Compared with the control, the yields of watermelon, rapeseed, tomato, and eggplant increased by 30%, 13%, 21%, and 18%, respectively, after using the residue of this invention.

[0075] Example 4

[0076] Treatment of marine fish farming tailings in coastal areas:

[0077] 1) such as Figure 4 System Setup: The aquaculture tailings resource utilization device mainly includes a fermentation tank 1, a stirrer 2, an automatic microbial agent dosing system 3, an automatic ethanol addition system 4, a pH monitoring and control system 5, a temperature monitoring and control system 6, a timer 7, and an intelligent control platform 8.

[0078] The fermentation process of the aquaculture tailings resource utilization device is divided into two stages: pre-acidification and carbon chain elongation.

[0079] The aforementioned aquaculture tailings resource utilization device collects fermentation environment data in real time by installing online pH and temperature sensors, and is also equipped with an automatic temperature and acid-base control system to intelligently control the process of producing medium-chain fatty acids.

[0080] 2) Method for preparing the medium-chain fatty acid producing bacterial agent

[0081] a) Strain propagation: Enterobacter YTLJ-N-S18 was inoculated into the propagation medium at a rate of 1 wt%. After inoculation, the culture was carried out at 37°C for 72 hours. After the culture period, the culture products were collected. The propagation medium consisted of acetic acid (0.1%), glucose (0.02%), and ethanol (1.0%) by weight, with the remainder being water.

[0082] b) Preparation of bacterial powder: The Enterobacter YTLJ-N-S18 obtained by propagation was freeze-dried to obtain a single strain of dry powder.

[0083] c) Preparation of additives: The additives are sterilized and dried livestock manure feed residue, sulfur, iron powder, calcium carbonate powder, La(NO3)3 and Y(NO3)3; wherein, the amount of sulfur added is 0.001% of the mass of the additives, the amount of iron powder added is 0.005% of the mass of the additives, the amount of calcium carbonate powder added is 0.5% of the mass of the additives, the amount of La(NO3)3 added is 0.001% of the mass of the additives, and the amount of Y(NO3)3 added is 0.005% of the mass of the additives.

[0084] Among them, the sterilized and dried aquaculture manure residue was grouper aquaculture manure residue, with TSS of 250g / L, COD of 2.0g / L, and sCOD of 3000mg / L.

[0085] d) Preparation of microbial agents: The microbial powder obtained in step 2 is mixed with the adjuvant obtained in step 3 at a ratio of 1:100 (mass ratio) to prepare a microbial agent for producing medium-chain fatty acids.

[0086] 3) Start-up and operation of the first stage of fermentation (pre-acidification)

[0087] a) Add aquaculture tailings (a mixture of feces and uneaten feed from grouper farming, with 250g / L of organic matter, 2.0g / L of COD, and 3000mg / L of sCOD) to the fermentation tank and ferment for 1 week at a certain fermentation time and pH.

[0088] b) Real-time online pH testing: when pH is less than 9.5, alkali addition is initiated to adjust pH to 10; when pH is greater than 10.5, acid addition is initiated to adjust pH to 10.

[0089] c) Online real-time temperature monitoring: when the temperature is below 34℃, heating is initiated and the temperature is adjusted to 35℃; when the temperature is above 36℃, cooling is initiated and the temperature is adjusted to 35℃.

[0090] 4) Start-up and operation of the second stage of fermentation (carbon chain elongation)

[0091] a) After the first stage (pre-acidification stage) lasts for 1 week, the second fermentation stage (carbon chain elongation stage) begins. The system automatically adds the prepared inoculum and ethanol (both added at a mass ratio of 1%), and the reaction time for the second stage is set to 3 weeks. Simultaneously, 1% sulfur-iron-ethanol mixture is added to the fermentation system. The amount of sulfur-modified iron-carbon material added to the sulfur-iron-ethanol mixture accounts for 10% of the mass of the mixture.

[0092] The sulfur-iron-ethanol mixture is prepared by dissolving sulfur-modified iron-carbon material in ethanol. The sulfur-modified iron-carbon material is made by mixing ferrous sulfate, carbon powder, and sodium borohydride, stirring for 30 minutes, then adding sodium sulfide and stirring for another 10 minutes until well mixed. The mass ratio of ferrous sulfate, carbon powder, and sodium borohydride is 0.5:10:1, and the amount of sodium sulfide added is 1% of the mass of the carbon powder.

[0093] b) Online real-time testing and detection of pH and temperature; when pH is less than 5.0, alkali addition is initiated to adjust pH to 5.5; when pH is greater than 6.0, acid addition is initiated to adjust pH to 5.5.

[0094] c) Monitor the temperature online in real time. When the temperature is below 36℃, start heating and adjust the temperature to 37℃; when the temperature is above 38℃, start cooling and adjust the temperature to 37℃.

[0095] After the fermentation in the above embodiments of the present invention was completed, compared with the control (described in the above embodiments), the nitrogen loss of the fermentation product was reduced by 10%, the total nitrogen concentration of the medium-chain fatty acid fermentation broth reached 2.5 g / kg, and the total nitrogen concentration of the ordinary fermentation broth reached 2.2 g / kg. The fermentation product obtained in the embodiments was used to extract medium-chain fatty acids using a mineral oil solvent membrane extraction method containing 3% tri-n-octylphosphine oxide (TOPO), producing hexanoic acid (7.8 g / kg) and octanoic acid (0.6 g / kg).

[0096] Subsequently, the post-fermentation residue (C:N:P = 100:5:1) was extracted and used as fertilizer to improve saline-alkali soil. Specifically, the residue was applied at a rate of 20 kg / mu to the corn and salt-tolerant vegetable (celery, ice plant, spinach, and zucchini) planting areas in Dongying, Yellow River Delta. At the same time, ordinary fertilizer was used in comparison with the same amount of the above-mentioned fermentation residue. Compared with the control, the yields of corn, celery, ice plant, spinach, and zucchini increased by 10%, 12%, 16%, 20%, and 24% respectively after using the residue.

Claims

1. A strain producing medium-chain fatty acids, characterized by: The strain producing medium-chain fatty acids is Enterobacter (Escherichia) Enterobacter sp. ) YTLJ-N-S18; The Enterobacter YTLJ-N-S18 is preserved in the Guangdong Microbial Culture Collection Center on July 5, 2021, and the preservation number is GDMCC No: 61773, and the preservation address is Guangzhou, China.

2. Use of a strain producing medium-chain fatty acids according to claim 1, characterized in that: The application of the strain in producing medium-chain fatty acids; the medium-chain fatty acids are hexanoic acid or octanoic acid.

3. A microbial agent producing medium-chain fatty acids, characterized by: The bacterial agent contains the Enterobacter YTLJ-N-S18 in claim 1.

4. A method for preparing the bacterial agent producing medium-chain fatty acids according to claim 3, characterized by: The enterobacter YTLJ-N-S18 is expanded and dried, and then mixed with an additive at a mass ratio of 1:1-100 to prepare the bacterial agent, wherein the total effective viable bacterial count of the bacterial agent is ≥1.0×10 9 / mL.

5. The method for preparing the microbial agent for producing medium-chain fatty acids according to claim 4, characterized in that: The additives are breeding fecal residues, sulfur, iron powder, calcium carbonate powder, La(NO3)3 and Y(NO3)3 after sterilization and drying; wherein the addition amount of sulfur is 0.001%-0.01% of the mass of the additives, the addition amount of iron powder is 0.005%-0.05% of the mass of the additives, the addition amount of calcium carbonate powder is 0.5-1.0% of the mass of the additives, the addition amount of La(NO3)3 is 0.001-0.005% of the mass of the additives, and the addition amount of Y(NO3)3 is 0.001-0.005% of the mass of the additives.

6. A method of producing medium-chain fatty acids, characterized by: The residues of aquaculture are added to the fermentation tank of the device, and the temperature in the fermentation tank is controlled at 34-36℃, the pH is set to 9.5-10, and the acid production is treated for 5-7 days, and then ethanol is added to the fermentation tank through an automatic ethanol adding system, the strain in claim 1 or the bacterial agent in claim 3 is added through an automatic bacterial agent adding system, and a sulfur-iron-ethanol mixture is added to the fermentation tank, and the pH is set to 5.0-6.0 and the temperature is set to 36-38℃ for carbon chain extension fermentation for 2-3 weeks, and then medium-chain fatty acids are obtained after purification; The medium-chain fatty acids are hexanoic acid or octanoic acid.

7. The method of producing medium-chain fatty acids according to claim 6, characterized by: The addition amount of the strain in claim 1 or the bacterial agent in claim 3 is 1-5% of the volume of the fermentation liquid; the addition amount of ethanol is 1-5% of the volume of the fermentation liquid; the addition amount of the sulfur-iron-ethanol mixture is 0.5-5% of the volume of the fermentation liquid, and the addition amount of the sulfur-modified iron-carbon material is 10-30% of the mass of the sulfur-iron-ethanol mixture. The sulfur-iron-ethanol mixture is made of ethanol-dissolved sulfur-modified iron-carbon material, wherein the sulfur-modified iron-carbon material is a mixture of ferrous sulfate, carbon powder and sodium borohydride after stirring, and then sodium sulfide is added to the mixture and stirred uniformly for use; wherein the mass ratio of ferrous sulfate, carbon powder and sodium borohydride is 0.5-1:10:1, and the addition amount of sodium sulfide is 1-3% of the mass of the carbon powder.

8. The method of producing medium-chain fatty acids according to claim 6, characterized by: The residues of the fermentation product after purification are used to prepare a fertilizer for improving soil.

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