Method for improving lactic acid fermentation saccharic acid conversion rate by using water-soluble citrate as neutralizer
Technical Field
The invention relates to the field of microbial fermentation, in particular to a method for improving the sugar acid conversion rate of lactic acid fermentation by using water-soluble citrate as a neutralizing agent.
Background
Under the global plastic-forbidden background, biodegradable bio-based materials are more and more valued by governments of various countries. The global consumption of polylactic acid (PLA) has increased year by year, and the lactic acid and polylactic acid industry has begun to blow out, and according to prediction, in 2019, the lactic acid production scale in continental china alone has reached 30 million tons/year. The polylactic acid is a polymer obtained by polymerizing lactic acid serving as a main raw material, and a product prepared from the polylactic acid can be biodegraded, and has the characteristics of biocompatibility, high glossiness, high transparency, good hand feeling, good heat resistance and the like, and is convenient to process and very wide in application.
The majority of the industrial production of lactic acid worldwide is currently carried out by microbial fermentation. However, when lactic acid is industrially produced on a large scale by a microbial fermentation method, systematic optimization of middle and downstream of bioengineering, such as process improvement of strain modification, cheap raw material substitution and separation and extraction, is required to achieve the goals of yield increase, purity improvement, cost reduction, benefit improvement, higher tolerance of thalli and the like, and potential is continuously mined from multiple links of a lactic acid production process to improve efficiency. The fermentation process is the key of the lactic acid production process, and the low-cost raw material cost and the high sugar-acid conversion rate are the core of the fermentation process control. At present, the main feasible raw materials comprise corn, cassava, rice and other crude raw materials and corresponding starch raw materials, and the enterprises in the deep processing industry chain of grains have obvious advantages in the aspect of raw material cost. In addition, the homotypic fermentation (generally bacterial fermentation) mode with high saccharic acid conversion rate is also effectively popularized. On the basis of the homotypic fermentation mode, energy consumption control (temperature, stirring and ventilation) and neutralizer screening of the process also become keys with high and low profit points in the industry.
The pH level of lactic acid fermentation is generally controlled to be 5-7, and a neutralizing agent is added in the fermentation process to maintain a stable acid-base environment along with the accumulation of lactic acid. At present, the most neutralizing agent for lactic acid fermentation is calcium carbonate, and the calcium carbonate neutralizing agent can cause the generation of a large amount of calcium sulfate byproducts in the extraction process, so that the research on a proper neutralizing agent has important significance for the subsequent separation cost control.
Disclosure of Invention
The invention aims to solve the problems that a large amount of calcium sulfate byproducts are generated in the extraction process and the sugar-acid conversion rate needs to be improved in the prior art, and provides a method for improving the sugar-acid conversion rate of lactic acid fermentation by using water-soluble citrate as a neutralizing agent. According to the method, a mixed solution of water-soluble citrate and strong base is used as an indirect neutralizer to replace the traditional calcium carbonate neutralizer, and the lactic acid is produced by utilizing facultative anaerobic lactic acid bacteria fermentation, so that C element contained in the water-soluble citrate can participate in metabolic utilization in the metabolic process, the sugar-acid conversion rate in the lactic acid fermentation process is further improved, the solid-waste separation operation is avoided, the separation cost is reduced, and the optical purity of the final lactic acid product is not influenced.
In order to achieve the above object, the present invention provides a method for increasing conversion rate of sugar acid in lactic acid fermentation by using water-soluble citrate as a neutralizing agent, comprising: inoculating the activated lactobacillus into a fermentation culture medium for lactic acid fermentation, controlling the pH value of a fermentation system by using a mixed solution of water-soluble citrate and strong alkali in the fermentation process, and introducing oxygen-containing gas into the fermentation system until the fermentation is finished.
In another aspect, the invention provides the use of a water-soluble citrate solution in lactic acid fermentation.
By the technical scheme, the sugar-acid conversion rate in the lactic acid fermentation process can reach more than 80%, the problem of solid waste in the extraction process is solved, the separation cost is reduced, the optical purity of the lactic acid final product is unchanged, and no obvious residue of citric acid exists after the fermentation is finished.
Detailed Description
The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.
The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.
The inventor of the invention finds that the saccharic acid conversion rate in the lactic acid fermentation process can be improved by using the mixed solution of the water-soluble citrate and the strong base in the lactic acid fermentation process and controlling the air introduction amount within a specific range, and a large amount of byproducts are not generated in the subsequent extraction process, so that the separation cost can be reduced.
The invention provides a method for improving the conversion rate of lactic acid fermentation saccharic acid by taking water-soluble citrate as a neutralizer, which comprises the following steps: inoculating the activated lactobacillus into a fermentation culture medium for lactic acid fermentation, controlling the pH value of a fermentation system by using a mixed solution of water-soluble citrate and strong alkali in the fermentation process, and introducing oxygen-containing gas into the fermentation system until the fermentation is finished.
According to the present invention, the lactic acid bacteria may be facultative anaerobes.
Preferably, the lactic acid bacteria are at least one of lactobacillus plantarum, lactobacillus rhamnosus, pediococcus acidilactici, bacillus coagulans, and lactobacillus thermophilus.
According to a preferred embodiment of the present invention, the method further comprises the step of activating the lactic acid bacteria prior to inoculating the lactic acid bacteria into the fermentation medium. The activation may be any conventional activation method in the art and will not be described herein.
According to a preferred embodiment of the present invention, the activated lactic acid bacteria are inoculated in the form of a suspension of lactic acid bacteria.
According to the invention, the activated lactic acid bacteria suspension OD is expressed in terms of fermentation medium per liter600And the inoculation amount can be 50-100ml for 2-8.
Preferably, the activated lactic acid bacteria suspension OD is one liter of fermentation medium600And (4) =4-5.9, and the inoculation amount is 80-100 ml.
According to the invention, the carbon source in the fermentation medium provides 24-40g/L of carbon element for the medium. Any carbon source that can provide the amount of carbon element can be suitable for use in the methods provided herein.
Preferably, the carbon source in the fermentation medium is glucose. The content of the glucose in the fermentation medium is 60-130 g/L.
According to the invention, the nitrogen source in the fermentation medium provides 2-3.6g/L of nitrogen element for the medium. Any nitrogen source that can provide such amounts of elemental nitrogen is suitable for use in the methods provided herein.
Preferably, the nitrogen source in the fermentation medium is corn steep liquor. The content of the corn slurry in the fermentation medium is 72-130 g/L.
Preferably, the nitrogen source in the fermentation medium is corn steep liquor. The content of the corn steep liquor powder in the fermentation medium is 25-45 g/L.
According to the present invention, the water-soluble citrate solution may be water-soluble citrate commonly used in the art, and particularly, the lactate formed by combining the cation in the citrate with lactate is also water-soluble citrate, such as alkali metal citrate.
Preferably, the water-soluble citrate is sodium citrate and/or potassium citrate.
According to the present invention, there is no particular requirement for the amount of the mixed solution of the water-soluble citrate and the strong base as long as the pH of the fermentation system can be controlled.
Preferably, the strong base is sodium hydroxide and/or potassium hydroxide.
Preferably, the pH of the fermentation system is controlled to be 5.8-6 by using a mixed solution of water-soluble citrate and strong alkali. Wherein, in the mixed solution, the concentration of the water-soluble citrate is 0.5-3mol/L, the concentration of the strong base is 1-2mol/L, and the total adding amount of the water-soluble citrate and the strong base is 1.5-5mol/L, preferably 1.5-3 mol/L.
According to the invention, the mixed solution of the water-soluble citrate and the strong base is added in the following manner: continuously feeding a mixed solution of water-soluble citrate and strong alkali to keep the pH value of the fermentation system stable. Keeping the pH of the fermentation system stable means: the difference between the highest pH and the lowest pH of the fermentation system is not more than 0.2. The flow rate of the mixed solution of the water-soluble citrate and the strong base can be controlled by those skilled in the art, and will not be described in detail herein.
According to a preferred embodiment of the present invention, wherein the oxygen-containing gas may be selected from air for the purpose of convenience of operation and cost reduction.
According to the preferred embodiment of the invention, the aeration mode is that an air flow meter is used for adjusting the aeration time from 6h to 12h after the fermentation is started until the fermentation is finished. During the period, the amount of the oxygen-containing gas introduced is controlled to be 0.002 to 0.2L/(min. L), preferably 0.02 to 0.16L/(min. L), in terms of oxygen. The inventor of the invention finds in research that the aeration mode can further improve the conversion rate of the sugar and acid in the lactic acid fermentation process.
According to the present invention, the lactic acid fermentation conditions can be conventional in the art, and can be adjusted according to the requirements of different lactic acid bacteria on the fermentation conditions.
According to a preferred embodiment of the present invention, wherein the lactic acid fermentation conditions include: the pH value is 5-7, the time is 36-60h, and the temperature is 35-45 ℃.
Preferably, the lactic acid fermentation conditions include: the pH value is 5-7, the time is 36-48h, and the temperature is 35-45 ℃.
More preferably, the lactic acid fermentation conditions include: pH5.8-6, the time is 36-45h, and the temperature is 37-42 ℃.
According to the invention, the pH is detected in real time by using an online pH electrode, and the pH is controlled by feeding a neutralizing agent to stabilize the pH within the pH range in the lactic acid fermentation conditions.
In another aspect, the invention provides the use of a water-soluble citrate salt in lactic acid fermentation. Preferably, the water-soluble citrate is sodium citrate and/or potassium citrate.
According to a preferred embodiment of the present invention, wherein the extraction manner of the fermented lactic acid may be any conventional manner for extracting lactic acid in the art. For example, the fermented lactic acid may be extracted by a conventional acid hydrolysis-crystallization process.
Specifically, the acidolysis-crystallization process can be the acidolysis-crystallization method described in Sun Qiume, QiaoKai, Wang claiming, high-grow, Wang Chonghui, research progress on separation and extraction of lactic acid in fermentation broth [ J ] chemical engineering progress (stage 9): 2656-.
According to a most preferred embodiment of the invention, the method comprises the steps of:
strain: the center for researching the nutrition and health of the Chinese food has the lactobacillus plantarum strain with the preservation number: CGMCC number 16835, a strain disclosed in CN 109504630A.
Culture medium: 125 g/L glucose 115, 95-105 g/L corn slurry, 1.8-2.2g/L sodium acetate, 0.18-0.22g/L magnesium sulfate heptahydrate, 1.8-2.2g/L dipotassium hydrogen phosphate, 0.04-0.06g/L manganese sulfate and 0.8-1.2ml/L Tween 80.
Inoculation amount of lactic acid bacteria: inoculating the seeds into a seed activation culture medium at an inoculation amount of 1.8-2.2ml/L to obtain a seed activation suspension OD600And (5.8-6), and fermenting at the inoculation amount of 88-92ml/L of the seed activation suspension.
And (3) process control: the culture temperature is 38-42 ℃, the initial stirring speed is 145-155 r/min, the mixed solution of 0.8-1.2mol/L sodium citrate, 0.8-1.2mol/L potassium citrate and 0.8-1.2mol/L sodium hydroxide is taken as a neutralizer, the process control pH is 5.9-6.1, the aeration is started for 9.5-10.5h of fermentation, the aeration quantity is adjusted to be 0.78-0.82L/(min. L) by using a manual air flow meter, and the fermentation period is 40-45 h.
The following further explains embodiments of the present invention with reference to specific examples. It should be understood that the examples set forth herein are presented by way of illustration and explanation only and are not intended to limit the present invention.
In the following examples and comparative examples, the mode of measuring the fermentation end point lactic acid was high performance liquid chromatography, and the conditions thereof were as follows:
a chromatographic column: agilent Technologies 1260 Infinity II;
a detector: RID;
separating Column, Aminex HPX-87H Column 300 × 7.8.8 mm;
column temperature: 55 deg.C
Mobile phase: 0.005M sulfuric acid;
flow rate: 0.5 mL/min;
sample introduction amount: 20 mu L of the solution;
the retention time of lactic acid is about 14 min.
The calculation formula of the saccharic acid conversion rate is [ fermentation end lactic acid concentration g/L × (fermentation end volume L + sampling volume L) -fermentation initial lactic acid concentration g/L × fermentation initial volume L ]/fermentation initial glucose weight g × 100%.
In the following examples and comparative examples, the reagents used are all commercially available. Wherein the glucose is AOBOX D (+) -glucose, and the corn starch is AOBOX corn starch (total nitrogen content is 8%).
In the following examples and comparative examples, the fermentation apparatus used was a 1L fermenter of the brand Infors, model MiniBio.
In the following examples and comparative examples, the oxygen-containing gas was introduced as air.
Example 1
Strain: lactobacillus rhamnosus CICC 6141 purchased from CICC China center for Collection of Industrial microorganism strains.
Culture medium: 60g/L glucose, 25 g/L corn steep liquor powder, 2g/L sodium acetate, 0.2g/L magnesium sulfate heptahydrate, 2g/L dipotassium phosphate, 0.05g/L manganese sulfate and 1ml/L Tween-80.
Inoculation amount of lactic acid bacteria: inoculating the seeds into a seed activation culture medium at an inoculation amount of 2ml/L to obtain a seed activation suspension OD600=4, fermentation is carried out with an inoculum size of 80ml/L seed activation suspension.
And (3) process control: the culture temperature is 37 ℃, the initial stirring speed is 150 r/min, the mixed solution of 1mol/L sodium citrate and 2mol/L sodium hydroxide is used as a neutralizer, the process control pH is 5.8, aeration is started when fermentation is carried out for 6 hours, the aeration quantity is adjusted to be 0.5L/(min L) by using a manual air flow meter, and the fermentation period is 36 hours.
As a result: the lactic acid concentration at the fermentation end point is 53g/L, the fermentation liquor has no precipitate, no obvious citric acid residue, uniform fluidity is kept, acidolysis extraction has no solid waste, and the saccharic acid conversion rate is 88.33%.
Example 2
Strain: lactobacillus acidophilus CICC 6096 purchased from CICC China industrial microorganism strain preservation management center.
Culture medium: 100 g/L glucose, 45g/L corn steep liquor powder, 2g/L sodium acetate, 0.2g/L magnesium sulfate heptahydrate, 2g/L dipotassium phosphate, 0.05g/L manganese sulfate and 1ml/L Tween-80.
Inoculation amount of lactic acid bacteria: inoculating the seeds into a seed activation culture medium at an inoculation amount of 2ml/L to obtain a seed activation suspension OD600=4, fermentation is carried out with an inoculum size of 100ml/L seed activation suspension.
And (3) process control: the culture temperature is 42 ℃, the initial stirring speed is 150 r/min, the mixed solution of 1mol/L potassium citrate and 2mol/L potassium hydroxide is used as a neutralizer, the pH value is controlled in the process of 6, the aeration is started for 12h of fermentation, the aeration quantity is adjusted to be 0.1L/(min L) by manually adjusting an air flow meter, and the fermentation period is 48 h.
As a result: the lactic acid concentration at the fermentation end point is 84.5g/L, the fermentation liquor has no precipitate, no obvious citric acid residue, uniform fluidity is kept, the acidolysis extraction has no solid waste, and the saccharic acid conversion rate is 84.5%.
Example 3
Strain: the center for researching the nutrition and health of the Chinese food has the lactobacillus plantarum strain with the preservation number: CGMCC number 16835, a strain disclosed in CN 109504630A.
Culture medium: 120 g/L glucose, 100 g/L corn slurry, 2g/L sodium acetate, 0.2g/L magnesium sulfate heptahydrate, 2g/L dipotassium phosphate, 0.05g/L manganese sulfate and 1ml/L Tween-80.
Inoculation amount of lactic acid bacteria: inoculating the seeds into a seed activation culture medium at an inoculation amount of 2ml/L to obtain a seed activation suspension OD600=5.9, fermentation is performed with an inoculum size of 90ml/L seed activation suspension.
And (3) process control: the culture temperature is 40 ℃, the initial stirring speed is 150 r/min, the mixed solution of 1mol/L sodium citrate, 1mol/L potassium citrate and 1mol/L sodium hydroxide is used as a neutralizer, the pH value is controlled to be 6 in the process, the aeration is started for 10h of fermentation, the aeration quantity is adjusted to be 0.8L/(min L) by using a manual air flow meter, and the fermentation period is 42 h.
As a result: the concentration of the D-lactic acid at the fermentation end point is 115g/L, the fermentation liquor has no precipitate, no obvious citric acid residue, uniform fluidity is kept, the acidolysis extraction has no solid waste, and the saccharic acid conversion rate is 95.8%.
Example 4
Strain: pediococcus acidilactici ATCC 20284 available from Sichuan Ruixiao Biotech Co.
Culture medium: 80 g/L glucose, 130g/L corn slurry, 2g/L sodium acetate, 0.2g/L magnesium sulfate heptahydrate, 2g/L dipotassium phosphate, 0.05g/L manganese sulfate and 1ml/L Tween-80.
Inoculation amount of lactic acid bacteria: to be provided withInoculating the seeds into a seed activation culture medium at an inoculation amount of 2ml/L to obtain a seed activation suspension OD600=3, fermentation is carried out with an inoculum size of 70ml/L seed activation suspension.
And (3) process control: the culture temperature is 45 ℃, the initial stirring speed is 150 r/min, the mixed solution of 0.5mol/L sodium citrate, 0.5mol/L potassium citrate, 1mol/L sodium hydroxide and 1mol/L potassium hydroxide is used as a neutralizer, the process control pH is 5.5, the aeration is started for 12h of fermentation, the aeration quantity is adjusted to be 0.1L/(min. L) by using a manual air flow meter, and the fermentation period is 45 h.
As a result: the lactic acid concentration at the fermentation end point is 65.6g/L, the fermentation liquor has no precipitate, no obvious citric acid residue, uniform fluidity is kept, the acidolysis extraction has no solid waste, and the saccharic acid conversion rate is 82%.
Example 5
The aeration was performed all the way from the start to the end of the fermentation, and the other fermentation methods, the strains used and the fermentation raw materials were exactly the same as those in example 1.
As a result: the lactic acid concentration at the fermentation end point is 49g/L, the fermentation liquid has no precipitate, no obvious citric acid residue, uniform fluidity is kept, and the sugar-acid conversion rate is 81.6%.
Comparative example 1
Initial pH5.8, neutralizing agent using 90g/L calcium carbonate, other fermentation method, using bacterial strain and fermentation raw material and 1 in the embodiment of the same.
As a result: the lactic acid concentration at the fermentation end point is 50g/L, a large amount of calcium lactate is separated out from the fermentation broth, the fermentation broth is integrally agglomerated, a large amount of calcium sulfate is produced in the acidolysis extraction process, and the saccharic acid conversion rate is 83.3%.
Comparative example 2
The whole course of anaerobic fermentation, that is, no air was introduced during the whole course of fermentation, and the other fermentation methods, the strains used and the fermentation raw materials were exactly the same as those in example 1.
As a result: the lactic acid concentration at the end of the fermentation is 47.6g/L, the fermentation liquor has no precipitate and maintains uniform fluidity, the sugar-acid conversion rate is 79.4 percent, and the citric acid residue is 9.4 g/L.
Comparative example 3
The mixed solution of 1mol/L sodium carbonate and 2mol/L sodium hydroxide is used as a neutralizer, the process control pH is 5.8, and other fermentation methods, used strains and fermentation raw materials are completely the same as those in the example 1.
As a result: the lactic acid concentration at the fermentation end point is 49.4g/L, the fermentation liquid has no precipitate and keeps uniform flow, and the sugar-acid conversion rate is 82.3%.
Comparative example 4
The pH of the process was controlled to 5.8 using 2mol/L NaOH solution as a neutralizer, and the other fermentation methods, the strains used and the fermentation raw materials were exactly the same as those in example 1.
As a result: the lactic acid concentration at the end of fermentation is 51.1g/L, the fermentation liquor has no precipitate and keeps uniform flow, and the saccharic acid conversion rate is 85.2%.
The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, numerous simple variants can be made to the technical solution of the invention, comprising the combination of the individual specific technical features in any suitable way. The invention is not described in detail in order to avoid unnecessary repetition. Such simple variations and combinations should be considered as disclosed herein.