A method for the continuous synthesis of medium-chain fatty acids

Abstract

The application provides a method for continuous synthesis of medium-chain fatty acids, and belongs to the technical field of biochemistry and resource recycling. The application provides a method for continuous synthesis of medium-chain fatty acids, which comprises the following steps: mixing pretreated vinasse and a polyvinyl alcohol-sodium alginate composite matrix to obtain a fermentation matrix; adding the fermentation matrix to a fermentation system to perform pre-acidification; after the pre-acidification is completed, adjusting the feeding and discharging of the fermentation system every day, adjusting the material residence time to 9-15 days through the discharging amount, controlling the pH of the top region of the fermentation system to be 4.0-5.5 and / or the ORP to be-350--150 mV, and constructing an axial coupling system of the lactic acid production area of the top hydrolysis acidification area and the medium-chain fatty acid synthesis area of the bottom region of the fermentation system. The method does not need to add exogenous inoculum, and can realize the continuous synthesis of medium-chain fatty acids through partition regulation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of biochemistry and resource recycling, and particularly relates to a method for continuous synthesis of medium-chain fatty acids. BACKGROUND

[0002] With the surge in global demand for biomass waste resources, distiller's grains, as the main by-product of the brewing industry, have an annual global production of over 120 million tons. Distiller's grains not only contain rich crude fiber, crude starch, crude protein, organic acids, ethanol, amino acids and polysaccharides, but also are rich in microorganisms such as lactic acid bacteria, acetic acid bacteria and bacillus. The ethanol content ranges from 299.64 to 11667.45 mg / kg·DM (DM is dry matter), the acetic acid content ranges from 1.80 to 7.11 g / kg·DM, and the lactic acid content ranges from 25.41 to 92.90 g / kg·DM.

[0003] Distiller's grains have rich carbon sources and nutrients. The traditional utilization of distiller's grains is mainly as feed or fertilizer, and there is a deficiency in the improvement of functional substances. With the development of science and technology, people begin to explore new ways of using distiller's grains in order to maximize the utilization of resources. Based on carbon chain extension technology, short-chain carbon sources such as ethanol and lactic acid in distiller's grains can be converted into medium-chain fatty acids, which can be used as biofuels, antibacterial agents, and spices. Using distiller's grains as a carbon source for carbon chain extension can not only realize the resource utilization of distiller's grains, but also produce high-value-added products. At present, the carbon chain extension process is dependent on inoculums, and most of them need to add exogenous acetic acid-producing bacteria (such as Clostridium autoethanogenum) and acid-producing bacteria groups, which leads to competition between lactic acid-producing bacteria groups and medium-chain fatty acid-producing bacteria groups, metabolic pathway conflicts, and energy efficiency loss of double bacterial group system. Clostridium kluyveri SUMMARY

[0004] In view of the defects in the prior art, the purpose of the present application is to provide a method for continuous synthesis of medium-chain fatty acids using distiller's grains, which does not need to add exogenous inoculums and can realize continuous synthesis of medium-chain fatty acids through partition control.

[0005] The purpose of the present application is achieved by the following technical solutions:

[0006] The present application provides a method for continuous synthesis of medium-chain fatty acids, comprising the following steps:

[0007] The pretreated distiller's grains are mixed with a polyvinyl alcohol-sodium alginate composite matrix to obtain a fermentation matrix; the solid content of the pretreated distiller's grains is 8% to 10%, the carbon-nitrogen ratio is (25 to 30):1, 90% of the particle diameter is ≤2 mm, and the pH value is 4.4 to 4.6; the mass ratio of the polyvinyl alcohol-sodium alginate composite matrix to the pretreated distiller's grains when mixed is 1:(10 to 20);

[0008] ​adding the fermentation substrate to the fermentation system for pre-acidification; the pre-acidification is performed at a temperature of 37.5-38.5℃; the pre-acidification is performed at a dissolved oxygen of ≤0.5 ppm; the pre-acidification is performed at a pH of 4.0-5.0;

[0009] after the pre-acidification, adjusting the feeding and discharging of the fermentation system on a daily basis, adjusting the residence time of the fermentation system by the amount of discharging to be 9-15 d; controlling the pH of the top region of the fermentation system to be 4.0-5.5 and / or the ORP to be -350--150 mV, to construct an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium-chain fatty acid synthesis zone in the bottom region of the fermentation system;

[0010] the top region of the fermentation system refers to the upper 1 / 3-1 / 4 region of the fermentation system;

[0011] the discharging refers to the discharging from the bottom of the fermentation system; the feeding refers to the feeding to the top of the fermentation system; the feeding refers to adding the fermentation substrate to the fermentation system.

[0012] Preferably, the method for controlling the pH of the top region of the fermentation system to be 4.0-5.5 and / or the ORP to be -350--150 mV comprises: when the pH is <4.0 and the ORP is lower than -350 mV, the feeding and discharging frequency is adjusted to be 48-72 h / time; when the pH is >5.5 and the ORP is higher than -150 mV, the feeding and discharging frequency is adjusted to be 6-12 h / time.

[0013] Preferably, the pre-acidification is performed for 3-7 d.

[0014] Preferably, in the first 7 d of adjusting the feeding and discharging of the fermentation system on a daily basis, 30%-50% of the daily discharging is refluxed to the lower 1 / 3-1 / 4 region of the fermentation system before feeding.

[0015] Preferably, the amount of feeding is less than or equal to the amount of discharging when feeding and discharging on a daily basis.

[0016] Preferably, the polyvinyl alcohol-sodium alginate composite substrate comprises a polyvinyl alcohol-sodium alginate hydrogel.

[0017] Preferably, the material for adjusting the carbon-nitrogen ratio of the pretreated distiller's grains comprises any one or two or more of corn stalks, wheat stalks, urea and pig manure; the distiller's grains comprise any one or two or more of white spirit distiller's grains, beer distiller's grains and aged distiller's grains.

[0018] Preferably, the method for separating the medium-chain fatty acids from the discharging comprises heat treatment.

[0019] Preferably, the heat treatment is performed at a temperature of 80-100℃; the heat treatment is performed for 10 min.

[0020] Preferably, the medium-chain fatty acids comprise any one or two or more of caproic acid, heptanoic acid and octanoic acid.

[0021] Advantages of the present application:

[0022] The application provides a method for continuous synthesis of medium-chain fatty acids, comprising the following steps: mixing pretreated vinasse with a polyvinyl alcohol-sodium alginate composite matrix to obtain a fermentation matrix; the pretreated vinasse has a solid content of 8-10%, a carbon-nitrogen ratio of (25-30):1, 90% of the particle diameter is ≤2 mm, and a pH value of 4.4-4.6; the mass ratio of the polyvinyl alcohol-sodium alginate composite matrix to the pretreated vinasse is 1:(10-20) when they are mixed; adding the fermentation matrix to a fermentation system for pre-acidification; the pre-acidification temperature is 37.5-38.5°C; the pre-acidification dissolved oxygen is ≤0.5 ppm; the pre-acidification pH value is 4.0-5.0; after pre-acidification, the material residence time in the fermentation system is adjusted to 9-15 d by adjusting the feed and discharge amount of the fermentation system every day; the pH value of the top region of the fermentation system is controlled to 4.0-5.5 and / or the ORP is controlled to-350--150 mV, so as to construct an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium-chain fatty acid synthesis zone in the bottom zone of the fermentation system; the top region of the fermentation system is the upper 1 / 3-1 / 4 region of the fermentation system; the discharge is the discharge at the bottom of the fermentation system; the feed is the feed at the top of the fermentation system; and the feed is adding the fermentation matrix to the fermentation system. The method provided by the application establishes a system without external inoculum, and the vinasse itself carries a rich microbial flora, including lactic acid bacteria, acetic acid bacteria and butyric acid bacteria. By regulating the conditions in the fermentation system, the dominant flora in the endogenous microbial community of the vinasse is regulated, and the organic matter in the vinasse is converted into medium-chain fatty acids. The method introduces the polyvinyl alcohol-sodium alginate composite matrix into the extraction system of medium-chain fatty acids, and constructs a "carrier-catalyst" dual-mode synergistic mechanism. The polyvinyl alcohol-sodium alginate composite matrix has the effects of microbial immobilization and selective adsorption-catalysis, and can utilize the polyvinyl alcohol-sodium alginate composite matrix to convert acid-producing functional bacteria (such as Clostridium kluyveri) in the three-dimensional network structure, improve the bacterial density, enhance the stability of the biofilm, thereby promoting the production of medium-chain fatty acids; the polyvinyl alcohol-sodium alginate composite matrix can adsorb the target product in real time in the carbon chain extension zone, and the medium-chain fatty acids can be obtained by removing the polyvinyl alcohol-sodium alginate composite matrix and the fermentation product complex in the discharge of the system, and in the continuous separation and purification process of the medium-chain fatty acids, the polyvinyl alcohol-sodium alginate composite matrix can be fully utilized to reduce the subsequent purification steps and reduce the energy consumption of separation. The method utilizes the partition control release system of the vertical axial partitioned fermentation tank, first, the pH of the anaerobic fermentation of the vinasse is regulated to realize the continuous production of short-chain fatty acids by the hydrolysis and acidification of the vinasse in the top 1 / 4 to 1 / 3 of the fermentation tank; second, the feeding and discharging frequency is regulated to optimize the porosity and material transfer efficiency, so as to ensure that the short-chain fatty acids form a stable mass transfer gradient and penetrate downward, and then the middle 1 / 4 to 1 / 3 of the fermentation tank is used as a transition area; the fermentation system in the bottom 1 / 4 to 1 / 3 of the fermentation tank maintains the pH at 5.5 to 6.0 due to the acetic acid / lactic acid metabolism buffer, which is beneficial to the synthesis of medium-chain fatty acids, forms a carbon chain extension synthesis medium-chain fatty acid area, and produces medium-chain fatty acids more efficiently under the catalysis of the polyvinyl alcohol-sodium alginate composite matrix. In the present application, after the polyvinyl alcohol-sodium alginate composite matrix carrier completes the adsorption of medium-chain fatty acids, it is subjected to heat treatment, so that the physical binding force between the hydrophobic modified layer and the product is weakened, the adsorbed caprylic acid, caproic acid and other target products are released, and the mechanical strength (swelling rate < 10%) and adsorption capacity (after 10 cycles, it still maintains more than 90% of the initial value) of the hydrogel are maintained. Further, the regenerated polyvinyl alcohol-sodium alginate composite matrix is returned to the fermentation tank to realize the in-situ recycling of the carrier material, and the amount of hydrogel added per batch is reduced by 50% (i.e., the regenerated polyvinyl alcohol-sodium alginate hydrogel is returned to the fermentation tank, and after using the recycling method, the amount of fresh polyvinyl alcohol-sodium alginate hydrogel required is 50% of the amount of hydrogel used in the first feeding, and the recycling ratio of polyvinyl alcohol-sodium alginate hydrogel is 50%), which significantly reduces the operating cost. In addition, the method uses the "adsorption-regeneration-readsorption" closed loop design of the hydrogel to minimize material consumption and waste discharge, which conforms to the concept of circular economy and realizes the sustainable use of resources. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, brief introductions will be given to the drawings needed in the embodiments. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0024] Figure 1 Process flow chart of the method for continuous synthesis of medium-chain fatty acids;

[0025] Figure 2 Figure 1 is a schematic diagram of the apparatus for the method of continuous synthesis of medium chain fatty acids. DETAILED DESCRIPTION

[0026] The present application provides a method for continuous synthesis of medium chain fatty acids, comprising the following steps:

[0027] The pretreated vinasse is mixed with a polyvinyl alcohol-sodium alginate composite matrix to obtain a fermentation matrix; the pretreated vinasse has a solid content of 8% to 10%, a carbon-nitrogen ratio of (25-30):1, 90% of the particle diameter ≤2 mm, and a pH value of 4.4 to 4.6; the mass ratio of the polyvinyl alcohol-sodium alginate composite matrix to the pretreated vinasse is 1:(10-20) when mixed;

[0028] The fermentation matrix is added to a fermentation system for pre-acidification; the pre-acidification temperature is 37.5-38.5℃; the pre-acidification dissolved oxygen is ≤0.5 ppm; and the pre-acidification pH value is 4.0-5.0;

[0029] After pre-acidification, the fermentation system is adjusted for daily feeding and discharging, and the material residence time is adjusted to 9-15 d by the amount of discharging; the pH value of the top region of the fermentation system is controlled to be 4.0-5.5 and / or ORP-350--150 mV, so as to construct an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium chain fatty acid synthesis zone in the bottom region of the fermentation system;

[0030] The top region of the fermentation system is the upper 1 / 3-1 / 4 region of the fermentation system;

[0031] The discharging is the discharging at the bottom of the fermentation system; the feeding is the feeding at the top of the fermentation system; and the feeding is the addition of the fermentation matrix to the fermentation system.

[0032] The present application mixes the pretreated vinasse with polyvinyl alcohol-sodium alginate composite matrix to obtain fermentation matrix. In the present application, the pretreated vinasse treatment method comprises: adjusting the carbon-nitrogen ratio and solid content of the vinasse to obtain fermentation raw material; crushing the fermentation raw material and adjusting the pH to 4.4-4.6. As an optional embodiment of the present application, the carbon-nitrogen ratio can be adjusted to (25-30):1, or 25:1, 26:1, 27:1, 28:1, 29:1 or 30:1. In the present application, the raw material for adjusting the carbon-nitrogen ratio of the vinasse can be any one or two or more of corn stalks, wheat stalks, urea and pig manure; the corn stalks can be corn ensiled stalks. The solid content can be adjusted to 8%-10% (w / w), or 8%, 9% or 10% (w / w). After adjusting the solid content, the present application obtains fermentation raw material. After obtaining the fermentation raw material, the present application crushes the fermentation raw material. The crushing method is not particularly limited in the present application, and any conventional crushing method can be used. As an optional embodiment of the present application, the particle size of the crushed material can be ≤2 mm, or 90% of the particles can have a particle size of ≤2 mm, or 1.2 mm. After crushing, the pH of the system is preferably adjusted to 4.4-4.6, or 4.4, 4.5 or 4.6. After adjusting the pH of the system, the present application obtains pretreated vinasse.

[0033] After obtaining the pretreated vinasse, the present application mixes the pretreated vinasse with polyvinyl alcohol-sodium alginate composite matrix to obtain fermentation matrix. As an optional embodiment of the present application, the polyvinyl alcohol-sodium alginate composite matrix comprises polyvinyl alcohol-sodium alginate hydrogel; the mass ratio of the polyvinyl alcohol-sodium alginate composite matrix to the pretreated vinasse can be 1:(10-20), or 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19 or 1:20. The mixing method is not particularly limited in the present application, and any conventional mixing method can be used.

[0034] After obtaining the fermentation matrix, the present application adds the fermentation matrix to the fermentation system for pre-acidification. As an optional embodiment of the present application, the pre-acidification temperature can be 37.5-38.5℃, or 38℃; the dissolved oxygen of the pre-acidification is ≤0.5 ppm; the pH of the pre-acidification is 4.0-5.0, or 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9 or 5.0; the pre-acidification time can be 3-7 days, or 3, 4, 5, 6 or 7 days.

[0035] In the pre-acidification process, acid-producing bacteria such as lactic acid bacteria and acetic acid bacteria in the raw material vinasse mainly undergo hydrolysis and acidification reactions, converting complex organic matter into products mainly including short-chain fatty acids and alcohols such as lactic acid, ethanol, acetic acid, propionic acid, and butyric acid. Low pH inhibits methanogenesis and carbon chain elongation reactions, ensuring the accumulation of acids and alcohols. In the subsequent carbon chain elongation reaction, acetic acid acts as an electron acceptor, and ethanol and lactic acid act as electron donors.

[0036] After the pre-acidification is completed, the present application adjusts the fermentation system to carry out feeding and discharging every day, and adjusts the material residence time in the fermentation system to 9-15 days by adjusting the discharging amount; controls the pH of the top region of the fermentation system to be 4.0-5.5 and / or the ORP to be-350--150mV, to construct an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium-chain fatty acid synthesis zone in the bottom region in the fermentation system; the top region of the fermentation system is the upper 1 / 3-1 / 4 region of the fermentation system. In the present application, the discharging is the bottom discharging of the fermentation system; the feeding is the top feeding of the fermentation system; and the feeding refers to adding the fermentation substrate to the fermentation system.

[0037] As an optional embodiment of the present application, the feeding and discharging are preferably carried out in the order of discharging first and then feeding. As an optional embodiment of the present application, the feeding amount per day is less than or equal to the discharging amount. The present application adjusts the material residence time to 9-15 days by adjusting the discharging amount, which can also be 9, 10, 11, 12, 13, 14, or 15 days. In the present application, the material residence time refers to the residence time of the same batch of material in the fermentation system calculated from the start of feeding and discharging. For example, when the feeding amount is the total volume of the fermentation system, the material residence time in the fermentation tank = the volume of the fermentation tank / the discharging amount per day; or it can be the material residence time in the fermentation tank = the volume of the fermentation system / the discharging amount per day.

[0038] In the present application, in the first 7 days of continuous feeding and discharging, after discharging every day, the discharged material is refluxed to the lower 1 / 3-1 / 4 region of the fermentation system; the refluxed discharging amount can be 30%-50% of the total discharging amount, or 40% can be refluxed; the refluxing is directly refluxing 30%-50% of the discharging amount to the lower 1 / 3-1 / 4 region of the fermentation system through the peristaltic pump from the feeding port.

[0039] The first time the invention discharges, it is the intermediate fermentation liquor that has completed 3-7 days of pre-acidification but has not undergone carbon chain extension. The discharge liquor of the first continuous feeding and discharging mode needs to be refluxed for the first 7 days to make the system reach a stable state. For the first 7 days of the first continuous feeding and discharging mode, 30%-50% of the discharge liquor is refluxed to the medium-chain fatty acid synthesis zone at the bottom of the fermentation system every day, while new fermentation substrate is continuously added to the top of the fermentation system to fill the fermentation device. During the reflux period, the net discharge amount of the system decreases until the 8th day when the reflux stops and the full discharge mode is entered. During the reflux period, the feed still needs to add fermentation substrate to fill the fermentation device according to the established mode, i.e., adding the fermentation substrate obtained by mixing the pretreated vinasse and the polyvinyl alcohol-sodium alginate composite substrate to the top of the fermentation system. This is because the main function of the refluxed discharge liquor is to adjust the ecological environment inside the system, while the addition of fermentation substrate to the feed is to continuously provide raw materials for the fermentation process to maintain the progress of the fermentation reaction. The two complement each other in function and role, and together ensure the stable operation of the fermentation system and the generation of products.

[0040] The present application constructs an axial coupling system of the top hydrolysis acidification lactic acid production zone and the bottom medium-chain fatty acid synthesis zone based on the daily collection amount of the leachate and the porosity of the material. In the present application, the daily collection amount of the leachate (unit: L / (L·d)) refers to the volume of the liquid phase leachate produced and collected in 24 hours per unit volume of the fermentation system; the porosity of the material reflects the proportion of the void volume inside the whole mixed material (including pretreated vinasse, PVA-SA hydrogel and microbial metabolites, etc. solid components) in the total volume of the fermentation system, and is a core parameter reflecting the looseness of the material and the liquid phase flow space in the entire fermentation device. As an optional embodiment of the present application, the daily collection amount of the leachate can be 0.08 L / (L·d); the porosity of the material is the collected material porosity, and the material porosity is 30%-40%. The material porosity of the present application can be obtained by configuring the related parameters of the fermentation system according to the technical solution described above. The material porosity of the fermentation system is stabilized at 30%-40%, which not only ensures the transfer of hydrolysis products from top to bottom to the bottom, but also provides a continuous and stable substrate supply for the carbon chain extension bacteria at the bottom, ultimately realizing the synergistic function of "top hydrolysis acidification lactic acid production and bottom carbon chain extension medium-chain fatty acid production", and completing the construction of the axial coupling system.

[0041] After the feeding and discharging are completed every day, the pH and ORP of the top region of the fermentation system are preferably monitored in real time, so that the pH of the top region of the fermentation system is maintained at 4.0-5.5 and / or the ORP is-150--350 mV, and an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium-chain fatty acid synthesis zone in the bottom region of the fermentation system is constructed. The pH and ORP of the top region of the fermentation tank are monitored in real time every day, preferably starting from changing the feeding mode, that is, the top 1 / 3-1 / 4 region of the fermentation tank. The pH and ORP of the top region of the fermentation system are monitored in real time, and if the pH deviates from 4.0-5.5 and the ORP deviates from-350--150 mV, the feeding and discharging frequency of the fermentation system is adjusted, and the pH and ORP of the top region are adjusted.

[0042] As an optional embodiment of the present application, the method for controlling the pH of the top region of the fermentation system to be 4.0-5.5 and the ORP to be-350--150 mV includes: when the pH is less than 4.0 and the ORP is lower than-350 mV, the feeding and discharging frequency is adjusted to 48-72 h / time, which can be 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71 or 72 h / time; when the pH is greater than 5.5 and the ORP is higher than-150 mV, the feeding and discharging frequency is adjusted to 6-12 h / time, which can be 6, 7, 8, 9, 10, 11 or 12 h / time.

[0043] The present application starts from the daily feeding and discharging, through the regulation of pH and ORP of the top area of the fermentation system, so that the top area of the fermentation system mainly carries out the pre-acidification stage. The main microorganism of the pre-acidification stage is acid-producing bacteria (such as lactic acid bacteria, acetic acid bacteria), and the metabolic products are mainly organic acids such as lactic acid and acetic acid, which leads to the decrease of pH. If the pH of the top area of the fermentation system is less than 4.0 and the ORP is lower than-350 mV, prolonging the residence time of the material can make the methanogens (such as methanobacillus) have more time to convert organic acids (such as acetic acid) into methane (CH4) and carbon dioxide (CO2), which consumes organic acids and thus increases the pH value. In the present application, high pH, i.e. pH>5.5, is caused by excessive consumption of organic acids or accumulation of ammonia nitrogen. If the pH of the top area of the fermentation system is greater than 5.5 and the ORP is higher than-150 mV, shortening the residence time can reduce the degradation of organic acids by methanogens, and at the same time, high-frequency feeding can bring in new fermentable substrates, stimulate the metabolism of acid-producing bacteria (such as butyric acid bacteria and propionic acid bacteria), and re-accumulate organic acids (such as butyric acid and propionic acid), leading to the decrease of pH. Oxidation-reduction potential (ORP) is a key parameter for characterizing the redox state of the system, and the higher the value, the stronger the oxidation of the environment. Methanogens, as strict anaerobic microorganisms, rely on a low ORP environment to maintain their metabolic activity. The ORP abnormal regulation strategy is as follows: when the ORP is higher than-150 mV, it indicates that the anaerobic environment may be damaged, which is usually caused by oxygen input. At this time, the interval between feeding and discharging should be prolonged or the feeding should be stopped, the main purpose of which is to reduce the oxygen that may be introduced in the new material. In the stagnation stage, facultative anaerobic bacteria in the system can quickly consume residual oxygen, and consume electron acceptors (such as O2, NO3 - ), while generating reducing substances (such as H2, reduced ferredoxin), thereby effectively reducing the ORP. When the ORP is lower than-350 mV, it reflects that the system is too strong in reduction, which is often accompanied by excessive proliferation of methanogens. At this time, the interval between feeding and discharging should be shortened, and the over-reduced metabolic products and a large number of methanogens enriched in the system should be discharged by accelerating the rate of material renewal. The input of fresh material can introduce trace amounts of oxidizing components or change the substrate composition, thereby mildly increasing the ORP value within a controllable range.

[0044] After the feeding and discharging are completed, the pH and ORP of the top area of the fermentation tank are adjusted to regulate the metabolism of acid-producing bacteria (such as lactic acid bacteria and acetic acid bacteria) to generate organic acids such as lactic acid and acetic acid, and to inhibit the activity of methanogens and carbon chain elongation bacteria. The original remaining fermentation system is gradually self-maintained at a pH of 5.5-6.0 due to the acetic acid / lactic acid metabolic buffer, and the carbon chain elongation bacteria as the dominant bacterial flora in the fermentation system carry out carbon chain elongation reaction. Under the action of carbon chain elongation microorganisms such as Clostridium kluyveri (C. kluyveri) and the like, carbon chain elongation is carried out to synthesize medium-chain fatty acids such as hexanoic acid and octanoic acid as medium-chain fatty acids with ethanol and lactic acid as electron donors. Clostridium kluyveri

[0045] ​The method utilizes a vertical axial partitioned fermenter partitioned control system, first, by regulating the pH of the upper 1 / 3~1 / 4 of the fermentation system to 4.0~5.5 and the ORP to -350~-150 mV, to realize the continuous production of short-chain fatty acids in the top 1 / 4~1 / 3 of the fermenter, forming a lactic acid-rich zone.

[0046] Secondly, by regulating the feeding and discharging frequency to optimize the porosity and material transfer efficiency, the stable mass transfer gradient of short-chain fatty acids is ensured to penetrate downward, and then the middle 1 / 4~1 / 3 of the fermentation system serves as a phase transition zone.

[0047] The bottom 1 / 4~1 / 3 of the fermentation system maintains a pH of 5.5~6.0 due to the acetic acid and / or lactic acid metabolic buffer system and microbial metabolic feedback mechanism, which is beneficial to the synthesis of medium-chain fatty acids, forming a carbon chain extension medium-chain fatty acid synthesis region, and more efficient production of medium-chain fatty acids under the catalysis of polyvinyl alcohol-sodium alginate composite matrix, forming a carbon chain extension zone.

[0048] In the phase transition zone and the carbon chain extension zone, no real-time monitoring and adjustment of pH and ORP is performed. In the carbon chain extension zone, as the reaction proceeds, metabolic changes occur in organic acids such as acetic acid and lactic acid. On the one hand, acetic acid and lactic acid can participate in subsequent reactions as substrates, for example, in carbon chain extension reactions, they are consumed as electron donors, reducing the concentration of acidic substances in the system. On the other hand, the metabolism of these organic acids by microorganisms can produce some basic byproducts or change the ratio of acidic and basic substances in the system, thereby increasing the pH. The carbon chain extension zone has strong buffering capacity based on the acetic acid and / or lactic acid metabolic buffer system, and through the balance adjustment mechanism of its own weak acid and its conjugate base, under the synergistic action of microbial metabolic feedback, it resists pH changes, thereby maintaining the pH at 5.5~6.0. Under this condition, acid-producing bacteria such as Clostridium kluyveri (Clostridium kluyveri) can be enriched. Clostridium kluyveri Through the reverse beta-oxidation pathway, the C2~C4 precursors are converted into C6~C10 medium-chain fatty acids.

[0049] In the present invention, when short-chain fatty acids penetrate downward from the phase transition zone to the lower region of the fermenter (i.e. the bottom 1 / 4~1 / 3 of the fermenter), hexanoic acid and octanoic acid and other medium-chain fatty acids are synthesized in the lower region, i.e. the carbon chain extension zone.

[0050] In the present invention, the carbon chain extension reaction is mainly carried out by Clostridium (Clostridium) and Megasphaera (Megasphaera). Clostridium Megasphaera ​) drive, through the reverse beta-oxidation pathway to extend short-chain fatty acids to C6~C12, converted into medium-chain fatty acids, such as ethanol, lactic acid and other electron donors, first oxidation to generate acetyl-CoA, and then acetyl-CoA enters the RBO cycle, four steps of condensation, dehydrogenation, dehydration and reduction, each cycle increases 2 carbon atoms, such as two molecules of acetyl-CoA condensation to form acetyl-CoA, and then sequentially to form butyryl-CoA, butyryl-CoA continues to combine with acetyl-CoA to form caproyl-CoA, and finally release caproic acid, octanoic acid and other medium-chain fatty acids. In addition, in the fermentation of distiller's grains, PVA-SA hydrogel adsorbs acid-producing bacteria to form a biofilm structure, accelerates substrate transmission, and provides precursors for carbon chain extension.

[0051] The present application discharges every day, separates the fermentation product in the axial coupling system, and obtains medium-chain fatty acids. As an optional embodiment of the present application, the method for separating medium-chain fatty acids in the discharge includes heat treatment; the temperature of the heat treatment can be 80~100℃, or 80℃, 90℃ or 100℃; and the time of the heat treatment is 10 min. After the medium-chain fatty acids are separated, the polyvinyl alcohol-sodium alginate composite matrix can be recovered. The polyvinyl alcohol-sodium alginate composite matrix can continue to be used as a hydrogel to mix with pretreated distiller's grains for the synthesis of medium-chain fatty acids. In the present application, in order to make the fermentation system more efficient, the recycled polyvinyl alcohol-sodium alginate composite matrix can be used to replace 50% of the polyvinyl alcohol-sodium alginate composite matrix in the fermentation matrix configuration for recycling.

[0052] In the carbon chain extension fermentation process, the separation and purification of the product is a key link. The traditional separation and purification method usually needs to take the fermentation liquid out of the fermentation tank, and through a series of complex separation and purification steps, so as to realize the purpose of separating and purifying the fermentation product, which has the characteristics of complex operation and low separation and purification efficiency. The method of the present application introduces the polyvinyl alcohol-sodium alginate composite matrix, such as polyvinyl alcohol-sodium alginate hydrogel, which is a three-dimensional network structure of polymer material with high specific surface area, adjustable pore size and flexible chemical modification. The polyvinyl alcohol-sodium alginate hydrogel not only can fix functional bacteria groups through physical adsorption or chemical cross-linking to improve the stability of the reactor operation, but also can selectively enrich medium-chain fatty acids through hydrogen bonding, hydrophobic interaction, etc. The application of polyvinyl alcohol-sodium alginate hydrogel in the distiller's grains carbon chain extension system can realize the purpose of in-situ purification, improve the efficiency of the fermentation process and the purity of the product. The present application further returns the regenerated polyvinyl alcohol-sodium alginate composite matrix to the fermentation tank to realize the in-situ recycling of the carrier material, and the fresh polyvinyl alcohol-sodium alginate hydrogel dosage of each batch can be reduced by 50%, which can significantly reduce the operating cost.

[0053] In order to further illustrate the present application, the technical solutions provided by the present application are described in detail below in conjunction with the accompanying drawings and examples, but they should not be understood as limiting the scope of protection of the present application.

[0054] Polyvinyl alcohol-sodium alginate hydrogel was purchased from Delta Biotechnology.

[0055] The material residence time is the time from the start of the pretreated distiller's grains and PVA-SA hydrogel mixed system to the time of entering the fermentation system or the time of leaving the fermentation system, specifically: when calculating the material residence time, the pre-acidification time is not included.

[0056] The process flow diagram of the method for continuous synthesis of medium-chain fatty acids is shown in Figure 1 The device schematic diagram of the method for continuous synthesis of medium-chain fatty acids is shown in Figure 2 The technical solutions of the present application are described in detail below in conjunction with examples.

[0057] Example 1

[0058] A method for continuous synthesis of medium-chain fatty acids, the steps are as follows:

[0059] Step one: raw material pretreatment: take the white spirit distiller's grains, mix the white spirit distiller's grains with crushed corn silage straw, adjust the C / N ratio of the mixed system to 25:1, then adjust the solid content of the mixed system to 10% (w / w), to obtain the fermentation raw material. The fermentation raw material is crushed twice to make the particle size D90≤2 mm, i.e. 90% of the particle diameter ≤2 mm, and the pH is adjusted to 4.5±0.1 to obtain the pretreated distiller's grains.

[0060] After mixing polyvinyl alcohol-sodium alginate hydrogel (PVA-SA hydrogel) and pretreated distiller's grains at a mass ratio of 1:15, add them to the fermentation tank device. Start the fermentation device, and when feeding for the first time, the amount of feed is equal to the effective volume of the fermentation device, i.e. 2L, and pre-acidification is carried out in the fermentation device for 5 days. No feed and discharge during pre-acidification. The fermentation device is maintained at 38℃±0.5℃, the dissolved oxygen is ≤0.5ppm, and the pH value is 4.0. After 5 days of pre-acidification, change the feeding mode to daily feeding and discharging.

[0061] During pre-acidification, acid-producing bacteria such as lactic acid bacteria and acetic acid bacteria in the raw material distiller's grains mainly undergo hydrolysis and acidification reactions, converting complex organic matter into products mainly composed of short-chain fatty acids such as lactic acid, ethanol, acetic acid, propionic acid, butyric acid, and alcohols. Low pH inhibits methanogenesis and carbon chain elongation reactions, ensuring the accumulation of acids and alcohols. In the subsequent carbon chain elongation reaction, acetic acid acts as an electron acceptor, and ethanol and lactic acid act as electron donors, providing raw materials for carbon chain elongation.

[0062] Step two: after pre-acidification is completed, change the feeding mode to daily feeding and discharging.

[0063] The specific method is: the feeding and discharging frequency is 24 h / time, specifically:

[0064] First, discharging is performed, and the material residence time is controlled to be 12 d through the daily discharging amount, and the material residence time of the 2 L mixed system in the fermenter is 12 d by adjusting the daily discharging amount to 167 mL, and the mixed system in the fermenter is not stirred during the fermentation process.

[0065] Based on the collection amount of daily leachate and the porosity of the material, the daily leachate collection amount is controlled to be 0.08 L / (L·d) during the daily feeding and discharging process, and the porosity of the collected material is 30%~40%, so as to construct the axial coupling system of the top hydrolysis acidification lactic acid production zone and the bottom middle-chain fatty acid synthesis zone.

[0066] In the first 7 days of the continuous feeding and discharging mode, the discharging liquid needs to be directly refluxed to the lower 1 / 3 area of the fermentation system through the peristaltic pump every day, and the daily reflux amount of the discharging liquid is 50% of the total discharging amount of the day. After the reflux of the discharging liquid in the first 7 days of the continuous feeding and discharging mode, feeding is performed, and the total volume of the fermentation system is 2 L.

[0067] From the 8th day of the first continuous feeding and discharging mode, there is no need to perform reflux of the discharging liquid, and thereafter, the feeding amount is equal to the discharging amount, and the feeding is stopped.

[0068] From the change of the feeding mode, the pH and ORP of the top area of the fermenter are monitored in real time every day, the top area of the fermenter is the top 1 / 3 area of the fermenter, and the pH and ORP of the top area of the fermenter are monitored to maintain the pH value of the top area of the fermenter in the range of 4.0~5.5 and / or the ORP in the range of -350~-150 mV, and a pre-acidification treatment stage is performed. If the pH deviates from 4.0~5.5 and the ORP exceeds -350~-150 mV, the feeding and discharging frequency is adjusted.

[0069] The control strategy is as follows:

[0070] When the pH is less than 4.0 and the ORP is lower than -350 mV, the feeding and discharging frequency is adjusted to 48~72 h / time.

[0071] When the pH is greater than 5.5 and the ORP is higher than -150 mV: the feeding and discharging frequency is increased to 6~12 h / time.

[0072] After the feeding and discharging are completed, the metabolism of acid-producing bacteria (such as lactic acid bacteria and acetic acid bacteria) is adjusted through the adjustment of the pH and ORP of the top area of the fermenter, to generate lactic acid, acetic acid and other organic acids, while inhibiting the activity of methanogens.

[0073] The original remaining fermentation system maintains the pH in the range of 5.5~6.0 due to the acetic acid / lactic acid metabolic buffer, and performs carbon chain extension reaction. In the Clostridium kluyveri (Clostridium kluyveri ) under the action of carbon chain extension microorganisms, and the medium-chain fatty acids such as hexanoic acid and octanoic acid are synthesized from ethanol and lactic acid as electron donors.

[0074] With the feeding and discharging, the top part (1 / 3 area of the top part of the fermenter) of the fermenter is mainly used for hydrolysis acidification to produce acetic acid, lactic acid and other materials through the regulation of pH and ORP, which is called lactic acid enrichment zone.

[0075] The middle part (1 / 3 area below the top part of the fermenter) of the fermenter is not regulated by pH and ORP, and the porosity and material transfer efficiency are optimized by regulating the feeding and discharging frequency to form a stable mass transfer gradient for short-chain fatty acids to penetrate and diffuse downward. This section is called phase transition zone.

[0076] The lower part (1 / 3 area below the bottom part of the fermenter) of the fermenter is not regulated by pH and ORP, and the fermentation system can maintain the pH at 5.5~6.0 by itself due to the acetic acid / lactic acid metabolic buffer system and microbial metabolic feedback mechanism. The acid-producing bacteria such as Clostridium kluyveri (C. kluyveri) are directionally enriched. The acid-producing bacteria convert C2~C4 precursors into C6~C10 medium-chain fatty acids through reverse beta-oxidation pathway. This section of the fermenter is called carbon chain extension zone. Clostridium kluyveri

[0077] In the actual fermentation process, there is no strict boundary between the lactic acid enrichment zone, the phase transition zone and the carbon chain extension zone.

[0078] Step three: purification of the discharging and continuous recovery of the product. The fermentation broth (discharging) is subjected to 10 minutes of heat treatment operation under temperature control at 80~100℃ to realize controllable release and high-purity recovery of the adsorbed product (medium-chain fatty acid) and simultaneously complete the in-situ regeneration of the hydrogel carrier.

[0079] The treatment method of the discharging is as follows: centrifugation at 4℃ and 8000r / min for 10 minutes, taking the supernatant through a 0.22μm filter membrane to remove microorganisms and suspended impurities, and obtaining the to-be-tested liquid. The to-be-tested liquid is subjected to 10 minutes of heat treatment operation under temperature control at 80~100℃, and after the heat treatment operation, the PVA-SA hydrogel is separated from the aqueous solution, and the acid production in the aqueous solution is detected by GC-MS. The hydrogel obtained by heat treatment can be recycled to replace 50% of the fresh hydrogel.

[0080] The first day of feeding and discharging is counted as the 0th day, and the 1st day is counted after 24 hours of feeding and discharging. The acid production is determined every day after the completion of feeding and discharging, and the concentrations of short-chain fatty acids and medium-chain fatty acids in the discharging are determined to calculate the production of short-chain fatty acids and medium-chain fatty acids every day.

[0081] The acid production rate of each day is calculated from the first day of feeding and discharging, and the results are shown in Table 1. ​

[0082] Table 1 Acid production of Example 1 from 1 to 12 days after continuous feeding and discharging

[0083]

[0084] Note: unit is mg / L.

[0085] From Table 1, the acid production of medium-chain fatty acids (the sum of hexanoic acid, heptanoic acid and octanoic acid) on the second day was 3.93 g / L, the yield of hexanoic acid was 3.93 g / L, the acid production of medium-chain fatty acids on the third day was 7.21 g / L, the yield of hexanoic acid was 5.53 g / L, the acid production of medium-chain fatty acids on the fourth day was 6.64 g / L, the yield of hexanoic acid was 5.19 g / L, the acid production of medium-chain fatty acids on the fifth day was 2.65 g / L, the yield of hexanoic acid was 2.58 g / L, the acid production of medium-chain fatty acids on the sixth day was 2.74 g / L, the yield of hexanoic acid was 2.66 g / L, the acid production of medium-chain fatty acids on the seventh day was 3.15 g / L, the yield of hexanoic acid was 3.06 g / L, the acid production of medium-chain fatty acids on the eighth day was 6.09 g / L, the yield of hexanoic acid was 4.55 g / L, the acid production of medium-chain fatty acids on the ninth day was 6.51 g / L, the acid production of medium-chain fatty acids on the tenth day was 6.2 g / L, the yield of hexanoic acid was 5.74 g / L, the acid production of medium-chain fatty acids on the eleventh day was 7.67 g / L, the yield of hexanoic acid was 7.01 g / L, the acid production of medium-chain fatty acids on the twelfth day was 7.91 g / L, the yield of hexanoic acid was 6.92 g / L. Among them, the acid production of medium-chain fatty acids on the eleventh day was 7.67 g / L, and the yield of hexanoic acid was 7.01 g / L.

[0086] After 72 hours (3 days) of continuous feeding and discharging, the polyvinyl alcohol-sodium alginate hydrogel was regenerated for 5 cycles, and the medium-chain fatty acid adsorption capacity retention rate was 92.4% (initial value 0.42 g / g).

[0087] The initial value was calculated by adsorption experiment: in order to exclude the complex interference of the fermentation process, a simulated liquid volume V (100 mL) with a concentration C0 of the real fermentation liquid was prepared, and a mass m (1 g) of unused PVA-SA hydrogel was weighed and placed in the simulated liquid to stand until adsorption equilibrium (i.e. the hydrogel no longer adsorbed MCFAs). After the reaction was completed, centrifugation, filtration, and taking the supernatant, the fatty acid concentration C1 was detected by GC-MS. The initial adsorption capacity Q0 (g / g) = [(C0- C1) x V] / m, and the calculation result was 0.42 g / g.

[0088] The product and polyvinyl alcohol-sodium alginate hydrogel were released by heat treatment (80-100℃, 10 minutes) of the effluent, and the polyvinyl alcohol-sodium alginate hydrogel was the regenerated hydrogel; the regenerated hydrogel was replaced by 50% of the polyvinyl alcohol-sodium alginate hydrogel, and was re-introduced into the fermentation system to adsorb medium-chain fatty acids, which was the first "adsorption-regeneration" cycle, and the operation was repeated 5 times. The adsorption capacity of the hydrogel after the fifth regeneration was determined according to the above-mentioned initial adsorption capacity determination method (denoted as Q5, unit: g / g). The retention rate = (Q5 / Q1) x 100%, the initial adsorption capacity was 0.42 g / g, Q1 was very small compared with the initial adsorption capacity Q0, the adsorption capacity after 5 times of regeneration was 0.388 g / g, and the retention rate was 92.4%.

[0089] Example 2

[0090] A method for continuous synthesis of medium-chain fatty acids, the steps are as follows:

[0091] Raw material pretreatment: beer lees and aged lees were mixed in a mass ratio of 7:3 to obtain lees, the lees were mixed with urea to adjust the C / N ratio to 30:1, and then the solid content of the mixed system was adjusted to 8% (w / w) to obtain fermentation raw materials. The fermentation raw materials were treated by ball milling to make the particle size D50≤1.2mm, and the pH value was adjusted to 4.5±0.1 to obtain pretreated lees.

[0092] The PVA-SA hydrogel and the pretreated lees were mixed in a mass ratio of 1:10, and then added to the fermentation tank device. Start the fermentation device, the first feeding amount is equal to the effective volume of the fermentation device, that is, 2L, and pre-acidification is carried out in the fermentation device for 3 days. No feeding and discharging during pre-acidification. The fermentation device is maintained at 38℃±0.5℃, the dissolved oxygen is ≤0.5ppm, and the pH value is 4.0. After 3 days of pre-acidification, the feeding mode is changed to feeding and discharging every day.

[0093] Step two: after pre-acidification, change the feeding mode to feeding and discharging every day.

[0094] The specific method is: the feeding and discharging frequency is 24h / time, specifically:

[0095] First, discharge, control the material residence time to be 15d through the daily discharge amount, adjust the daily discharge amount to be 133mL, and then make the material residence time of the 2L mixed system in the fermentation tank to be 15d, and the mixed system in the fermentation tank does not perform stirring during the fermentation process.

[0096] In the first 7 days of the continuous feeding and discharging mode, the effluent needs to be directly refluxed to the lower 1 / 3 area of the fermentation system through the peristaltic pump every day, and the refluxed effluent amount is 50% of the total daily discharge amount. After the effluent is refluxed in the first 7 days of the continuous feeding and discharging mode, feeding is performed. The total volume of the fermentation system is 2L.

[0097] From the 8th day of the first continuous feeding and discharging mode, no backflow of the discharging liquid was needed, and after that, the feeding was stopped when the feeding amount was equal to the discharging amount.

[0098] From the beginning of the change of the feeding mode, the pH and ORP of the top region of the fermenter were monitored in real time every day, in the same manner as in Example 1.

[0099] The middle part of the fermenter (1 / 3 region below the top of the fermenter) and the lower part of the fermenter (1 / 3 region below the bottom of the fermenter) were not subjected to pH and ORP regulation.

[0100] Step three: purification of the discharging liquid and continuous recovery of the product. The fermentation liquid (discharging liquid) was subjected to a 10-minute heat treatment operation under temperature control at 80-100°C, to achieve controllable release and high-purity recovery of the adsorbed product (medium-chain fatty acid), and to simultaneously complete the in-situ regeneration of the hydrogel carrier. The method was the same as in Example 1.

[0101] The acid production rate was calculated every day from the first day of the start of feeding and discharging, and the results are shown below.

[0102] The acid production of medium-chain fatty acid on the 2nd day was 2.23 g / L, and the production of hexanoic acid was 1.77 g / L. The acid production of medium-chain fatty acid on the 3rd day was 3.66 g / L, and the production of hexanoic acid was 2.94 g / L. The acid production of medium-chain fatty acid on the 4th day was 3.99 g / L, and the production of hexanoic acid was 3.30 g / L. The acid production of medium-chain fatty acid on the 5th day was 3.92 g / L, and the production of hexanoic acid was 3.49 g / L. The acid production of medium-chain fatty acid on the 6th day was 4.19 g / L, and the production of hexanoic acid was 3.97 g / L. The acid production of medium-chain fatty acid on the 7th day was 5.10 g / L, and the production of hexanoic acid was 4.95 g / L. The acid production of medium-chain fatty acid on the 8th day was 3.92 g / L, and the production of hexanoic acid was 3.67 g / L. The acid production of medium-chain fatty acid on the 9th day was 3.57 g / L. The acid production of medium-chain fatty acid on the 10th day was 4.85 g / L, and the production of hexanoic acid was 4.35 g / L. The acid production of medium-chain fatty acid on the 11th day was 4.15 g / L, and the production of hexanoic acid was 3.61 g / L. The acid production of medium-chain fatty acid on the 12th day was 3.99 g / L, and the production of hexanoic acid was 3.94 g / L. The average production of hexanoic acid on the 13th, 14th, and 15th days was about 3.48 g / L.

[0103] Comparative Example 1

[0104] A method for continuous synthesis of medium-chain fatty acids, the steps being the same as in Example 1, except that polyvinyl alcohol-sodium alginate hydrogel was not added.

[0105] Specifically: take liquor lees, mix the liquor lees with crushed corn silage straw, adjust the C / N ratio of the mixed system to 25:1, and then adjust the solid content of the mixed system to 10% (w / w) to obtain fermentation raw materials. The fermentation raw materials are crushed twice to make the particle size D90≤2 mm, i.e., 90% of the particle diameter≤2 mm, and the pH is adjusted to 4.5±0.1 to obtain pretreated lees.

[0106] The pretreated lees are directly added to the fermentation tank device. The fermentation device is started, and the first feeding amount is equal to the effective volume of the fermentation device, i.e., 2L, and the pre-acidification is carried out in the fermentation device for 5 days. No feeding and discharging are carried out during the pre-acidification process. The fermentation device is maintained at 38℃±0.5℃, the dissolved oxygen is≤0.5ppm, and the pH value is 4.0. After 5 days of pre-acidification, the feeding mode is changed to feeding and discharging every day.

[0107] After the pre-acidification is completed, the feeding mode is changed to feeding and discharging every day, and 167mL of the discharging amount is discharged every day, and then 50% of the 167mL discharging amount is refluxed to the lower 1 / 3 area of the fermentation system at the top of the fermentation tank by a peristaltic pump. After the reflux is completed, the total volume of the feeding to the fermentation system is 2L. During the first 7 days of the continuous feeding and discharging mode, 50% of the discharging amount needs to be refluxed every day.

[0108] Starting from the 8th day of the first continuous feeding and discharging mode, no reflux of the discharging liquid is needed, and thereafter the feeding amount is equal to the discharging amount, and the feeding is stopped.

[0109] Starting from the day when the feeding mode is changed, the pH and ORP of the top area of the fermentation tank, i.e., the top 1 / 3 area of the fermentation tank, are monitored in real time every day. The pH and ORP of the top area of the fermentation tank are monitored to maintain the pH value of the top area of the fermentation tank in the range of 4.0~5.5 and / or the ORP in the range of -350~-150 mV, and the method is the same as that in Example 1.

[0110] The acid production rate is calculated every day starting from the first day of feeding and discharging, and the results are as follows:

[0111] The acid production rate is calculated every day starting from the first day of feeding and discharging, and the yield of caproic acid is determined from the 1st to the 12th day. The results show that the highest yield of caproic acid is 4.5g / L on the 9th day.

[0112] Comparative Example 2

[0113] A method for continuous synthesis of medium-chain fatty acids, the steps of which are the same as those of Example 1, except that no pre-acidification treatment stage is carried out, specifically:

[0114] Take the distiller's grains, mix the distiller's grains with the crushed corn silage straw, adjust the C / N ratio of the mixed system to 25:1, and then adjust the solid content of the mixed system to 10% (w / w) to obtain fermentation raw materials. The fermentation raw materials are crushed twice to have a particle size D90≤2 mm, i.e., 90% of the particle diameters are ≤2 mm, and the pH is adjusted to 4.5±0.1 to obtain pretreated distiller's grains.

[0115] The polyvinyl alcohol-sodium alginate hydrogel (PVA-SA hydrogel) and the pretreated distiller's grains are mixed at a mass ratio of 1:15, and then added to the fermentation tank device. The fermentation device is started, and the first feeding amount is equal to the effective volume of the fermentation device, i.e., 2 L. On the day of feeding, it is the 0th day, and from the first day, feeding and discharging are performed every day. First, discharging is performed every day, and the discharging amount is 167 mL. Then, 50% of the 167 mL of the discharging liquid is returned to the lower 1 / 3 region of the fermentation system at the top of the fermentation tank by a peristaltic pump. After the return flow is completed, the total volume of the feeding to the fermentation system is 2 L. In the first 7 days of the continuous feeding and discharging mode, 50% of the discharging liquid needs to be returned. Starting from the 8th day of the first continuous feeding and discharging mode, the return flow of the discharging liquid is no longer needed, and after that, the feeding amount is equal to the discharging amount, and the feeding is stopped.

[0116] The pH and ORP of the top region of the fermentation tank, i.e., the top 1 / 3 region of the fermentation tank, are monitored in real time every day, and the pH and ORP of the top region of the fermentation tank are controlled to maintain the pH in the range of 4.0-5.5 and / or the ORP in the range of -350--150 mV, by the method same as in Example 1.

[0117] The acid production rate is calculated from the first day of feeding and discharging, and the caproic acid production is determined from the 1st to the 12th day. The results show that the caproic acid production is the highest on the 3rd day, which is 1.4 g / L.

[0118] Comparative Example 3

[0119] A method for continuous synthesis of medium-chain fatty acids, the steps of which are the same as in Example 1, except that the pH of the pre-acidification process is maintained at 5.5.

[0120] The acid production rate is calculated from the first day of feeding and discharging, and the caproic acid production is determined from the 1st to the 12th day. The results show that the caproic acid production is the highest on the 7th day, which is 3.06 g / L.

[0121] Comparative Example 4

[0122] A method for continuous synthesis of medium-chain fatty acids, the steps of which are the same as in Example 1, except that the feeding and discharging is performed every 2 days.

[0123] The pH and ORP of the top region of the fermenter, i.e. the top 1 / 3 region of the fermenter, are monitored in real time every day, and the pH and ORP of the top region of the fermenter are controlled to maintain the pH of the top region of the fermenter in the range of 4.0-5.5 and / or the ORP of the top region of the fermenter in the range of -350 to -150 mV.

[0124] When the pH is less than 4.0 and the ORP is less than -350 mV, the feeding and discharging frequency is adjusted to 72 h / once.

[0125] When the pH is greater than 5.5 and the ORP is greater than -150 mV, the feeding and discharging frequency is increased to 24 h / once.

[0126] The acid production rate per day is calculated from the first day of feeding and discharging, and the caproic acid production is determined on days 1-12, and the results show that the caproic acid production is the highest on day 7, which is 3.58 g / L.

[0127] Comparative Example 5

[0128] A method for continuous synthesis of medium-chain fatty acids, the steps are the same as in Example 1, except that the mass ratio of polyvinyl alcohol-sodium alginate hydrogel to pretreated vinasse is 1:25 when mixing.

[0129] The acid production rate per day is calculated from the first day of feeding and discharging, and the caproic acid production is determined on days 1-12, and the results show that the caproic acid production is the highest on day 8, which is 4.2 g / L.

[0130] Although the above examples have made a detailed description of the present application, it is only a part of the embodiments of the present application, but not all the embodiments, and people can also obtain other embodiments according to the present embodiments without creativity, and these embodiments all belong to the protection scope of the present application.

Claims

1. A method for the continuous synthesis of medium-chain fatty acids, characterized in that, The method comprises the following steps: The pretreated distiller's grains are mixed with a polyvinyl alcohol-sodium alginate composite matrix to obtain a fermentation matrix; the solid content of the pretreated distiller's grains is 8%-10%, the carbon-nitrogen ratio is (25-30):1, 90% of the particle diameters are ≤2 mm, and the pH value is 4.4-4.6; the mass ratio of the polyvinyl alcohol-sodium alginate composite matrix to the pretreated distiller's grains is 1:(10-20) when they are mixed; The fermentation matrix is added to a fermentation system for pre-acidification; the pre-acidification temperature is 37.5-38.5 ℃; the pre-acidification dissolved oxygen is ≤0.5 ppm; the pre-acidification pH value is 4.0-5.0; and the pre-acidification time is 3-7 days; After the pre-acidification is completed, the material residence time in the fermentation system is adjusted to 9-15 days by adjusting the daily feed and discharge of the fermentation system; the pH value of the top region of the fermentation system is controlled to be 4.0-5.5 and / or the ORP is controlled to be-350--150 mV, so as to construct an axial coupling system of the lactic acid production zone in the top hydrolysis acidification zone and the medium-chain fatty acid synthesis zone in the bottom zone of the fermentation system; In the first 7 days of adjusting the daily feed and discharge of the fermentation system, 30%-50% of the daily discharge is refluxed to the lower 1 / 3-1 / 4 region of the fermentation system, and then the feed is added; The top region of the fermentation system refers to the upper 1 / 3-1 / 4 region of the fermentation system; The discharge refers to the discharge at the bottom of the fermentation system; the feed refers to the feed at the top of the fermentation system; and the feed refers to the addition of the fermentation matrix to the fermentation system.

2. The method of claim 1, wherein, The method for controlling the pH value of the top region of the fermentation system to be 4.0-5.5 and / or the ORP to be-350--150 mV comprises: when the pH value is <4.0 and the ORP is lower than-350 mV, the feed and discharge frequency is adjusted to 48-72 h / time; and when the pH value is >5.5 and the ORP is higher than-150 mV, the feed and discharge frequency is adjusted to 6-12 h / time.

3. The method of claim 1, wherein, When the feed and discharge are performed every day, the feed amount is less than or equal to the discharge amount.

4. The method of claim 1, wherein, The polyvinyl alcohol-sodium alginate composite matrix comprises a polyvinyl alcohol-sodium alginate hydrogel.

5. The method of claim 1, wherein, The material for adjusting the carbon-nitrogen ratio of the pretreated distiller's grains comprises any one or two or more of corn stalks, wheat stalks, urea and pig manure; and the distiller's grains comprise any one or two or more of white spirit distiller's grains, beer distiller's grains and aged distiller's grains.

6. The method of claim 1, wherein, The method for separating the medium-chain fatty acids in the discharge comprises heat treatment.

7. The method of claim 6, wherein, The heat treatment temperature is 80-100 ℃; and the heat treatment time is 10 min.

8. The method of claim 1, wherein, The medium-chain fatty acids comprise any one or two or more of caproic acid, heptanoic acid and octanoic acid.

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