Method for producing polyhydroxyalkanoate

Abstract

The problem addressed is to provide a method for producing PHA that can sufficiently remove impurities while reducing the amount of wastewater in a purification process and can realize a practical filtration rate. The problem is solved by a method for producing PHA, the method comprising: a centrifugal separation step in which a PHA aqueous suspension (1) is centrifuged to obtain a PHA aqueous suspension (2) having a predetermined protein content; and a filtration step in which the PHA aqueous suspension (2), which has been adjusted to a pH of from more than 5.5 to 11.0, is subjected to dead end filtration.

Description

Method for producing polyhydroxyalkanoate

[0001] The present invention relates to a method for producing polyhydroxyalkanoates.

[0002] Polyhydroxyalkanoates (hereinafter, sometimes referred to as "PHAs") are known to be biodegradable, and in recent years, their utilization has been promoted from the viewpoint of environmental consideration.

[0003] One of the advantages of PHA is that it can be produced by microorganisms using renewable plant raw materials. To utilize PHA produced by microorganisms, it is first necessary to destroy the cells of the PHA-containing microorganisms or solubilize biological components other than PHA, disperse the PHA in the cells in water to obtain an aqueous suspension of PHA, and then purify the PHA by further removing impurities other than PHA from this aqueous PHA suspension.

[0004] Known techniques for purifying such PHA include a method of centrifuging an aqueous PHA suspension (Patent Documents 1 and 2) and a method of filtering an aqueous PHA suspension (Patent Documents 3 and 4).

[0005] International Publication No. WO2023 / 120193 Japanese Patent Publication No. 2023-86317 Japanese Patent Publication No. 2016-524926 Chinese Patent No. 111500650 Specification

[0006] However, among the conventional PHA purification techniques, the method of centrifuging a PHA aqueous suspension requires multiple centrifugation steps (generally four or more steps) for purification, and the amount of wastewater discharged during the purification process may be excessively large. In other words, there is room for improvement in terms of the amount of wastewater discharged during the purification process. On the other hand, when the number of centrifugation steps is limited in order to reduce the amount of wastewater discharged, it is not possible to sufficiently remove impurities.

[0007] Furthermore, the method of filtering the aqueous PHA suspension has a significantly low filtration rate, making it difficult to use it for industrial purposes (practical use).

[0008] In light of the above-described circumstances, an object of the present invention is to provide a method for producing PHA that can sufficiently remove impurities while reducing the amount of wastewater discharged during the purification process and that can achieve a filtration rate sufficient for practical use.

[0009] As a result of intensive research into solving the above-mentioned problems, the present inventors have discovered a new finding that, in purifying PHA, first an aqueous PHA suspension is centrifuged until a predetermined protein concentration is reached, and then the centrifuged aqueous PHA suspension is subjected to dead-end filtration with the pH adjusted to a predetermined range, thereby making it possible to reduce the amount of wastewater discharged during the purification process, while sufficiently removing impurities and achieving a filtration speed sufficient for practical use, and have completed the present invention.

[0010] That is, one aspect of the present invention is a method for producing a polyhydroxyalkanoate, comprising: a centrifugation step of centrifuging a polyhydroxyalkanoate aqueous suspension (1) to obtain a polyhydroxyalkanoate aqueous suspension (2) having a protein content of 6,000 to 30,000 ppm; and a filtration step of subjecting the obtained polyhydroxyalkanoate aqueous suspension (2) to dead-end filtration, wherein the pH of the polyhydroxyalkanoate aqueous suspension (2) subjected to the filtration step is greater than 5.5 and not greater than 11.0.

[0011] According to one aspect of the present invention, a method for producing PHA can be provided that can reduce the amount of wastewater discharged during the purification process, while sufficiently removing impurities and achieving a filtration rate suitable for practical use.

[0012] An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to the respective configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. All academic literature and patent documents described in this specification are incorporated herein by reference. Furthermore, unless otherwise specified in this specification, the term "A to B" representing a numerical range means "greater than or equal to A (including and greater than A) and less than or equal to B (including and less than B)."

[0013] 1. Method for Producing PHA A method for producing a PHA according to one embodiment of the present invention (hereinafter, the "method for producing a PHA according to one embodiment of the present invention" may be referred to as "the present production method") is a method for producing a PHA, comprising: a centrifugation step of centrifuging an aqueous PHA suspension (1) to obtain an aqueous PHA suspension (2) having a protein content of 6,000 to 30,000 ppm; and a filtration step of subjecting the obtained aqueous PHA suspension (2) to dead-end filtration.

[0014] <Technical Concept of the Present Invention> PHA produced by a microorganism may naturally contain impurities (particularly proteins) derived from the microorganism. The impurities contained in such PHA produced by a microorganism may cause quality degradation, such as contamination with foreign matter or deterioration in color tone, in products made by processing the PHA. For this reason, when using PHA produced by a microorganism in a product application, it is first necessary to remove the impurities contained in the PHA and purify it.

[0015] However, conventional purification of PHA by centrifuging an aqueous PHA suspension requires multiple centrifugation operations to sufficiently remove impurities, resulting in the generation of a large amount of wastewater during the purification process. For example, in the technology of Patent Document 1, PHA is purified by four centrifugation operations (and then dehydrated by filtration), resulting in the generation of a large amount of wastewater.

[0016] The present inventors have focused on a method of purifying PHA by filtration while investigating a method of purifying PHA with a smaller amount of wastewater from the viewpoint of reducing environmental load, etc. However, as described above, it has been difficult to achieve a filtration rate sufficient for practical use in purifying PHA by filtration.

[0017] When the inventors studied the cause of the decrease in filtration rate during the purification of PHA by filtration, they found that one of the causes of the decrease in filtration rate is that the filter material used for filtration is clogged by a large amount of impurities contained in the unpurified PHA aqueous suspension.

[0018] After gaining the above knowledge, the present inventors conducted further research and found that by reducing the amount of impurities in the PHA aqueous suspension to a certain level beforehand and then subjecting it to filtration, clogging of the filter material during the filtration process can be suppressed, thereby enabling the purification of PHA by filtration at a practical filtration rate. Furthermore, they also found that the operation of reducing the amount of impurities in the PHA aqueous suspension to the above level, i.e., to a level that does not cause clogging of the filter material during filtration, can be performed with a relatively small number of centrifugation cycles (i.e., with a small amount of wastewater). In addition, they also found that the impurity removal ability can be further improved by controlling the pH of the PHA aqueous suspension within a predetermined range during filtration.

[0019] Having gained these findings, the present inventors have discovered that by combining a centrifugation step for reducing the amount of impurities in the PHA aqueous suspension to the above-mentioned level with a filtration step for filtering the PHA aqueous suspension with the impurity amount reduced while adjusting the pH to a predetermined range, it is possible to reduce the amount of wastewater compared to filtration by centrifugation alone, achieve a filtration speed sufficient for practical use that is difficult to achieve with filtration by filtration alone, and further, have excellent impurity removal ability, i.e., it is possible to sufficiently remove impurities while reducing the amount of wastewater in the purification process, and achieve a filtration speed sufficient for practical use, thereby completing the present invention.

[0020] This production method can also be said to be a method for purifying PHA (PHA purification method) that can sufficiently remove impurities while reducing the amount of wastewater discharged during the purification process and can be carried out at a filtration rate sufficient for practical use.

[0021] Each step included in this manufacturing method will be described in detail below.

[0022] <Centrifugation Step> The present production method includes a centrifugation step in which the PHA aqueous suspension (1) is centrifuged to obtain a PHA aqueous suspension (2) having a protein content of 6,000 to 30,000 ppm.

[0023] (PHA aqueous suspension (1)) First, the PHA aqueous suspension (1) to be subjected to the centrifugation step will be described. In this specification, the PHA aqueous suspension is intended to mean a solution in which PHA is suspended (dispersed) in water (aqueous medium) and has fluid properties. Furthermore, the PHA aqueous suspension may contain, in addition to water, other solvents (e.g., organic solvents compatible with water), components derived from PHA-producing microorganisms (e.g., cell walls, proteins, etc.), and / or other compounds generated during purification. In other words, the PHA aqueous suspension (1) according to the present production method may contain these components in addition to PHA and water.

[0024] The PHA aqueous suspension (1) in this production method refers to an unpurified PHA aqueous suspension and a substantially unpurified PHA aqueous suspension. In this specification, the term "unpurified PHA aqueous suspension" refers to an aqueous PHA suspension produced by using a culture solution of a PHA-producing microorganism or an aqueous solution containing a PHA-producing microorganism as a raw material, disrupting the PHA-containing microorganism cells in the raw material and / or solubilizing biological components other than PHA, and dispersing the PHA in the cells in water, preferably by disrupting the PHA-containing microorganism cells and solubilizing biological components other than PHA, and which has not undergone any purification treatment, specifically, centrifugation or filtration. The method for destroying the cells of PHA-containing microorganisms and / or solubilizing biological components other than PHA is not particularly limited, and known methods can be used, such as methods using enzymes such as lytic enzymes such as lysozyme and / or proteolytic enzymes such as alcalase.

[0025] In addition, when the purification history of a certain PHA aqueous suspension is unknown, whether the aqueous suspension is unpurified can be determined, for example, based on the amount of impurities, particularly the amount of protein, in the aqueous suspension. Specifically, in this specification, if the protein content of a certain PHA aqueous suspension is 50,000 ppm or more, the aqueous suspension is considered to be an unpurified PHA aqueous suspension. In addition, a PHA aqueous suspension that has undergone some processing but still has a protein content of 50,000 ppm or more is also considered to be a "substantially unpurified PHA aqueous suspension." Needless to say, even if the protein content of a PHA aqueous suspension is less than 50,000 ppm, if the aqueous suspension is unpurified, the aqueous suspension can be used as the PHA aqueous suspension (1) according to the present production method. The protein content of the PHA aqueous suspension is measured by the method described in the Examples.

[0026] The shear viscosity of the PHA aqueous suspension (1) is not particularly limited, but from the viewpoint of improving the filtration rate in the subsequent filtration step, the shear viscosity at 40°C and a shear rate of 10 1 / s is preferably 4 to 15 mPa·s, and more preferably 5 to 10 mPa·s. In this specification, unless otherwise specified, the "shear viscosity of the PHA aqueous suspension" means the shear viscosity at 40°C and a shear rate of 10 1 / s. The shear viscosity of the PHA aqueous suspension is measured by the method described in the Examples.

[0027] The solids concentration of the PHA aqueous suspension (1) (i.e., the concentration of PHA) is not particularly limited, but from the viewpoint of improving the fluidity of the PHA aqueous suspension, it is preferably 5 to 30% by weight, and more preferably 10 to 20% by weight.

[0028] Next, the PHA contained in the PHA aqueous suspension (1) will be described in detail. Note that, since the basic physical properties of the PHA contained in the PHA aqueous suspension (1) do not change in the centrifugation step and the filtration step of this production method, the following description also applies to the specific aspects of the PHA contained in the PHA aqueous suspension (2) made from the PHA aqueous suspension (1) as a raw material.

[0029] PHA "PHA" is a general term for polymers containing hydroxyalkanoate as a monomer unit (monomer repeating unit) and is generally biodegradable. In particular, in this specification, "PHA" refers to a (co)polymer containing hydroxyalkanoate repeating units at 50 mol% or more of the total monomer repeating units (100 mol%), and a resin composed of such a (co)polymer. Specific examples of hydroxyalkanoate repeating units that constitute PHA include 3-hydroxybutanoic acid units, 4-hydroxybutanoic acid units, 3-hydroxypropionic acid units, 3-hydroxypentanoic acid units, 3-hydroxyhexanoic acid units, 3-hydroxyheptanoic acid units, 3-hydroxyoctanoic acid units, and 2-hydroxypropionic acid units. In this specification, the term "(co)polymer" is used to refer to both homopolymers composed of only one type of monomer and copolymers composed of two or more types of monomers.

[0030] Examples of PHAs provided by this production method include poly(3-hydroxyalkanoate) (hereinafter, sometimes referred to as "P3HA"), poly(4-hydroxyalkanoate), etc. Among these, P3HA is preferred because it is suitable for use in molded articles.

[0031] P3HA has the formula: [-CHR-CH ２ -CO-O-] (wherein R is C ｎ H ２ｎ＋１ and n is an integer of 1 or more and 15 or less.) as an essential repeating unit.

[0032] Specific examples of P3HA include poly(3-hydroxybutyrate) (hereinafter, sometimes referred to as "P3HB"), which is a homopolymer of 3HB, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (hereinafter, sometimes referred to as "P3HB3HH"), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (hereinafter, sometimes referred to as "P3HB4HB"), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate ... Examples of suitable polyhydroxybutyrates include poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate), poly(3-hydroxybutyrate-co-3-hydroxydecanoate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-2-hydroxypropionate), and poly(3-hydroxybutyrate-co-3-hydroxypropionate). Among these, P3HB, P3HB3HH, and P3HB4HB are preferred, with P3HB3HH and P3HB4HB being more preferred, in view of ease of industrial production using microorganisms.

[0033] In this specification, "poly(X-co-Y)" refers to a copolymer containing an X repeating unit and a Y repeating unit, and is intended to mean a copolymer obtained by copolymerizing a monomer from which the X repeating unit is derived and a monomer from which the Y repeating unit is derived. As described above, the name of P3HA is determined by the repeating units contained in the P3HA. However, a very small amount (about 1 mol% or less) of a monomer contained in P3HA may not be reflected in the name of the P3HA, provided that it does not significantly affect the physical properties of the P3HA. In other words, P3HA may contain, in addition to the repeating units corresponding to its name, very small amounts of other repeating units.

[0034] When P3HA contains 3HB repeating units, from the viewpoint of the balance between flexibility and strength, the composition ratio of 3HB repeating units to repeating units other than 3HB repeating units (other repeating units) in all monomer repeating units (100 mol%) in the P3HA (3HB repeating units / other repeating units) is preferably 99 / 1 (mol% / mol%) to 60 / 40 (mol% / mol%), more preferably 97 / 3 (mol% / mol%) to 70 / 30 (mol% / mol%), and even more preferably 95 / 5 (mol% / mol%) to 80 / 20 (mol% / mol%). When the composition ratio of 3HB repeating units in P3HA is 60 mol% or more, it has the advantage of being able to provide resin products with superior rigidity. On the other hand, when the composition ratio of 3HB repeating units in P3HA is 99 mol% or less, it has the advantage of being able to provide resin products with superior flexibility. The monomer composition ratio of P3HA can be measured by gas chromatography or the like (see, for example, WO 2014 / 020838).

[0035] P3HA can be produced by microorganisms. Examples of microorganisms capable of producing P3HA include P3HB-producing bacteria, such as Bacillus megaterium, which was first discovered in 1925, as well as other naturally occurring microorganisms such as Cupriavidus necator (formerly classified as Alcaligenes eutrophus and Ralstonia eutropha) and Alcaligenes latus. In these microorganisms, P3HB accumulates intracellularly.

[0036] Furthermore, known bacteria that produce P3HA, a copolymer of 3HB and other hydroxyalkanoic acids, include Aeromonas caviae, which produces P3HB3HH, and Alcaligenes eutrophus, which produces poly(3-hydroxybutyrate-co-4-hydroxybutyrate). In particular, Alcaligenes eutrophus AC32 (FERM BP-6038) (T. Fukui, Y. Doi, J. Bacteriol., 179, pp. 4821-4830 (1997)), into which genes encoding P3HA synthases have been introduced, is preferred for increasing the productivity of P3HB3HH. In addition to the above, genetically modified microorganisms into which various P3HA synthesis-related genes have been introduced can also be used depending on the desired physical properties of P3HA.

[0037] - Method for Producing PHA Aqueous Suspension As the PHA aqueous suspension (1) to be subjected to the centrifugation step, a PHA aqueous suspension derived from a culture broth of a PHA-producing microorganism can be suitably used. Such an aqueous PHA suspension derived from the culture solution of a PHA-producing microorganism can be prepared, for example, by the following methods: (1) culturing a microorganism capable of producing PHA; (2) inactivating the culture solution of the microorganism by heating to obtain an inactivated culture solution; (3) treating the inactivated culture solution with hydrogen peroxide to reduce the viscosity of the culture solution; (4) treating the inactivated culture solution treated with hydrogen peroxide with alkali; (5) adding a lytic enzyme (a cell wall-degrading enzyme) to the alkali-treated inactivated culture solution to lyse the microbial cells and disperse the substances within the cells, including PHA, in the culture solution; (6) adding a protease to the culture solution to decompose substances derived from the microbial cells other than PHA (particularly proteins); (7) further adjusting the pH of the culture solution and adding a surfactant to decompose substances derived from the microbial cells other than PHA (particularly cell membranes).

[0038] The present production method may include, prior to the centrifugation step, a step of culturing a PHA-producing microorganism, which includes one or more of the above-mentioned operations, and preparing a PHA aqueous suspension (1) from the culture solution (PHA aqueous suspension preparation step). Furthermore, when the present production method includes two or more of the above-mentioned operations, the order in which the operations are performed is not limited to the above-mentioned order.

[0039] (Centrifugation Operation) Next, the centrifugation operation performed in the centrifugation step will be described in detail. The centrifugation operation in the centrifugation step is not particularly limited as long as it can remove impurities (particularly proteins) derived from the cells of the PHA-containing microorganism in the PHA aqueous suspension (1) by centrifugation and obtain a PHA aqueous suspension (2) having a protein content of 6,000 to 30,000 ppm, and can be performed by any centrifugation method known in the technical field of the present invention.

[0040] Such known centrifugation methods include, for example, centrifugation using a centrifugal settler or a centrifugal dehydrator.

[0041] Examples of centrifugal settlers that can be used in the centrifugation step include separation plate type (e.g., disk type, self-cleaning type, nozzle type, screw decanter type, skimming type, etc.) and cylindrical type centrifugal settlers. Furthermore, both palindrome type and continuous type centrifugal settlers can be used. Similarly, either palindrome type or continuous type centrifugal dehydrators can be used.

[0042] The number of centrifugation steps in the centrifugation step significantly affects the amount of wastewater generated in this production method. Therefore, from the viewpoint of reducing the amount of wastewater generated by this production method, the number of centrifugation steps in this production method is preferably two or less, and may be one. In this specification, one centrifugation step refers to a series of operations in which a target liquid (e.g., PHA aqueous suspension (1)) is centrifuged under any conditions, followed by removal of a portion of the supernatant (e.g., an amount equivalent to 40% by volume or more of the liquid subjected to centrifugation) or all of the supernatant. When two or more centrifugations are performed, the liquid to be centrifuged may be a concentrated liquid (concentrated PHA aqueous suspension) obtained by removing the supernatant obtained by the above centrifugation operation, or may be a liquid obtained by adding an aqueous solvent to the concentrated liquid and adjusting the concentration to any desired level.

[0043] In conventional PHA purification processes using centrifugation, four or more centrifugation steps are required to sufficiently remove impurities from a PHA aqueous suspension, resulting in the generation of a large amount of wastewater (equivalent to at least four steps). In contrast, the present production method combines a centrifugation step and a filtration step to perform PHA purification, thereby enabling the production of PHA (PHA cake) with sufficiently reduced impurities despite performing only two or fewer centrifugations. Therefore, compared to conventional PHA purification processes using centrifugation, it is possible to reduce the amount of wastewater generated throughout the entire purification process.

[0044] The centrifugation conditions (number of rotations, rotation time) in the centrifugation step are not particularly limited as long as a PHA aqueous suspension (2) having a desired protein content can be obtained by the desired number of centrifugations. For example, the number of rotations per centrifugation may be 3,000 to 6,000 rpm, and the rotation time may be 1 to 30 minutes.

[0045] (PHA aqueous suspension (2)) The PHA aqueous suspension (2) obtained by the centrifugation step is an aqueous PHA suspension derived from the PHA aqueous suspension (1) and has a protein content of 6,000 to 30,000 ppm. As described above, the PHA aqueous suspension (2) is an aqueous PHA suspension derived from the PHA aqueous suspension (1), and therefore the PHA contained in the PHA aqueous suspension (2) has the same composition as that of the PHA aqueous suspension (1).

[0046] The protein content of the PHA aqueous suspension (2) is 6,000 to 30,000 ppm. Having a protein content of 30,000 ppm or less in the PHA aqueous suspension (2) enables a sufficient filtration rate to be achieved in the subsequent filtration step. From the viewpoint of improving the filtration rate in the filtration step, the lower the protein content of the PHA aqueous suspension (2), the more preferable. Specifically, the protein content of the PHA aqueous suspension (2) is preferably 25,000 ppm or less, more preferably 15,000 ppm or less, and even more preferably 10,000 ppm or less. That is, in the centrifugation step, it is preferable to carry out the centrifugation operation so that the protein content of the resulting PHA aqueous suspension (2) falls within the above range.

[0047] The solids concentration of the aqueous PHA suspension (2) (i.e., the concentration of PHA) is not particularly limited, but from the viewpoint of improving the fluidity of the aqueous PHA suspension, it is preferably 5 to 30% by weight, and more preferably 10 to 20% by weight.

[0048] The pH of the PHA aqueous suspension (2) immediately after centrifugation is not particularly limited, but the pH of the PHA aqueous suspension (2) immediately before being subjected to the subsequent filtration step is greater than 5.5 and not greater than 11.0. By adjusting the pH of the PHA aqueous suspension (2) to be subjected to the filtration step to greater than 5.5 and not greater than 11.0, it is possible to improve the efficiency of impurity removal while maintaining a sufficient filtration rate in the filtration step, and as a result, it is possible to efficiently obtain PHA from which impurities have been sufficiently removed.

[0049] The higher the pH of the PHA aqueous suspension (2) subjected to the filtration step, the more the impurity removal efficiency in the filtration step tends to improve. Therefore, from the viewpoint of further improving the impurity removal efficiency, the pH of the PHA aqueous suspension (2) subjected to the filtration step is preferably 6.2 or more, more preferably 6.7 or more, more preferably 7.2 or more, even more preferably 7.7 or more, even more preferably 8.2 or more, even more preferably 8.7 or more, even more preferably 9.0 or more, even more preferably 9.2 or more, even more preferably 9.7 or more, and even more preferably 10.2 or more.

[0050] pH Adjustment Step: In one embodiment of the present production method, the pH of the PHA aqueous suspension (2) immediately after centrifugation may be 5.5 or less, or 11.0 or more. In such cases, it is preferable to adjust the pH of the PHA aqueous suspension (2) to the above-mentioned range, for example, greater than 5.5 and less than 11.0, prior to the filtration step. That is, the present production method preferably includes a pH adjustment step of adjusting the pH of the PHA aqueous suspension (2) to greater than 5.5 and less than 11.0 prior to the filtration step.

[0051] In the pH adjustment step, the method for adjusting the pH of the PHA aqueous suspension (2) to the above range is not particularly limited, but it is preferable to adjust the pH of the PHA aqueous suspension (2) by, for example, adding an acid or alkali.

[0052] The acid used in the pH adjustment step is not particularly limited and may be either an organic acid or an inorganic acid, regardless of whether it is volatile. More specifically, examples of the acid used in the pH adjustment step include sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid.

[0053] The alkali used in the pH adjustment step is also not particularly limited, and examples thereof include alkali metal or alkaline earth metal hydroxides such as sodium oxide and potassium hydroxide; metal carbonates such as sodium carbonate and potassium carbonate; metal phosphates or metal hydrogen phosphates such as sodium phosphate, potassium phosphate, sodium hydrogen phosphate, and potassium hydrogen phosphate; and the like.

[0054] The amount of these acids or alkalis used in the pH adjustment step can be appropriately determined by those skilled in the art according to the pH of the PHA aqueous suspension (2) before pH adjustment and the pH of the PHA aqueous suspension (2) after the desired pH adjustment.

[0055] In another embodiment of the present invention, the pH of the PHA aqueous suspension (1) can be adjusted to more than 5.5 and not more than 11.0 prior to the centrifugation step, thereby directly obtaining an aqueous PHA suspension (2) having a pH of more than 5.5 and not more than 11.0. That is, the timing of the pH adjustment step is not particularly limited as long as it is performed before the filtration step and an aqueous PHA suspension (2) having the desired pH can be provided, and the pH adjustment step may be performed before or after the centrifugation step. However, it is preferable to perform the pH adjustment step after the centrifugation step, as this enables the production of a PHA with a lower impurity content.

[0056] <Filtration Step> The present production method includes a filtration step of subjecting the aqueous polyhydroxyalkanoate suspension (2) obtained in the above-mentioned centrifugation step to dead-end filtration. In this specification, the term "dead-end filtration" means "filtration by the dead-end filtration method."

[0057] The specific mode of the dead-end filtration operation in the filtration step is not particularly limited, and examples thereof include suction filtration, pressure filtration, centrifugal filtration, and gravity filtration.

[0058] The material of the filter medium used in the filtration step is not particularly limited and can be selected from various materials, for example, paper, filter cloth (woven or nonwoven), screen, sintered plate, bisque, polymer membrane, punched metal, wedge wire, etc. From the viewpoints of cost and ease of cleaning, filter cloth is preferably used.

[0059] The air permeability of the filter medium used in the filtration step is not particularly limited, and is, for example, 0.50 cc / cm ２ / sec or less, 0.40cc / cm ２ / sec or less, or 0.30 cc / cm ２ / sec or less, but 0.25 cc / cm ２ / sec or less, and 0.20 cc / cm ２ It is more preferable that the filter medium has an air permeability of 0.25 cc / cm or less. ２ By performing filtration using a filter medium with an air permeability of 0.25 cc / cm or less, leakage of PHA into the filtrate can be suppressed, and the recovery rate (yield) of PHA can be improved. ２ From the above viewpoint, the lower the air permeability of the filter medium used in the filtration step, the more preferable it is. The lower limit is not particularly limited, but it is preferably 0.01 cc / cm ２ / sec or more.

[0060] In this specification, the air permeability of a filter medium is the air permeability of a unit area (cm ) of the target filter medium per second. ２ The air permeability of a filter medium can be measured by the method described in the Examples.

[0061] In this production method, the protein content of the PHA aqueous suspension is reduced to 30,000 ppm or less in the centrifugation step, thereby enabling a practical filtration speed to be achieved in the filtration step. As a result, it is now possible to purify PHA by filtration (remove impurities), which was previously difficult to put into practical use due to the filtration speed.

[0062] In this specification, the filtration rate in the filtration step can be evaluated by the filtrate permeation rate measured under the conditions described in the Examples. The higher the filtrate permeation rate in the filtration step, the faster the filtration rate in the filtration step. More specifically, when the filtrate permeation rate in the filtration step is 100 L / m ２ From the above viewpoint, it can be evaluated that a practical filtration rate has been achieved when the filtrate permeation rate in the filtration step of the present production method is 100 L / m or more. ２ / hr or more, and 150 L / m ２ / hr or more, and more preferably 200 L / m ２ / hr or more, and 250 L / m ２ / hr or more.

[0063] In the filtration step, the PHA aqueous suspension (2) is filtered to obtain a filter cake as a residue. The filter cake undergoes a two-stage purification process consisting of a centrifugation step and a filtration step, thereby becoming a PHA aggregate from which impurities have been sufficiently removed.

[0064] The amount of impurities in the filter cake obtained by this production method can be evaluated based on the protein content of the filter cake. Specifically, when the protein content of the filter cake obtained through the centrifugation step and the filtration step is 5,500 ppm or less, the filter cake can be said to be a filter cake from which impurities have been sufficiently removed. The lower the protein content of the filter cake, the more impurities have been removed from the filter cake. From the above perspective, the protein content of the filter cake obtained by this production method is 5,500 ppm or less, preferably 5,000 ppm or less, and more preferably 4,500 ppm or less.

[0065] Furthermore, in the filtration step, if a filter cake satisfying the above conditions is not obtained by a single filtration operation, the obtained filter cake may be suspended in water or the like and subjected to another filtration operation. That is, the filtration step may include two or more filtration operations. However, since wastewater is generated during the filtration operation, albeit in a smaller amount than in the centrifugation operation, from the viewpoint of reducing the amount of wastewater generated throughout the present production method, the fewer the number of filtration operations performed in the filtration step, the more preferable. For example, it is preferably seven times or less, more preferably six times or less, more preferably five times or less, more preferably four times or less, more preferably three times or less, even more preferably two times or less, and particularly preferably one time. On the other hand, from the viewpoint of removing as many impurities as possible, the more the number of filtration operations is, the more preferable, it is preferably one or more times, more preferably two or more times, and even more preferably three or more times. From the viewpoint of achieving both a reduction in the amount of wastewater and the removal of impurities, the number of filtration operations performed in the filtration step is preferably one to three times.

[0066] By drying the filter cake obtained by the filtration process by a known method, PHA particles or PHA powder with sufficiently reduced impurities can be obtained.Such PHA particles or PHA powder can be used as a molded body by a known molding method, for example, injection molding, extrusion molding, blow molding or compression molding.In addition, by being foamed by a known method and then molded, it can also be used as a foamed molded body.The molded body and foamed molded body made from these PHA particles or PHA powder as raw materials can be used for various purposes such as paper, film, sheet, tube, plate, rod, container (for example, bottle container, etc.), tableware (for example, straw, cutlery, etc.), bag, parts, etc.

[0067] [Others] One aspect of the present invention may include the following configuration.

[0068] [1] A method for producing a polyhydroxyalkanoate, comprising: a centrifugation step of centrifuging a polyhydroxyalkanoate aqueous suspension (1) to obtain a polyhydroxyalkanoate aqueous suspension (2) having a protein content of 6,000 to 30,000 ppm; and a filtration step of subjecting the obtained polyhydroxyalkanoate aqueous suspension (2) to dead-end filtration, wherein the pH of the polyhydroxyalkanoate aqueous suspension (2) subjected to the filtration step is greater than 5.5 and not greater than 11.0.

[0069] [2] The method for producing a polyhydroxyalkanoate according to [1], further comprising a pH adjustment step of adjusting the pH of the aqueous polyhydroxyalkanoate suspension (2) to more than 5.5 and not more than 11.0 before the filtration step.

[0070] [3] The method for producing a polyhydroxyalkanoate according to [2], wherein in the pH adjustment step, the pH is adjusted by adding an acid or alkali to the aqueous polyhydroxyalkanoate suspension (2).

[0071] [4] In the filtration step, the air permeability is 0.50 cc / cm ２ The method for producing polyhydroxyalkanoate according to any one of [1] to [3], wherein dead-end filtration is performed using a filter medium having a flow rate of 100 s or less.

[0072] [5] In the filtration step, the air permeability is 0.25 cc / cm ２ The method for producing polyhydroxyalkanoate according to any one of [1] to [4], wherein dead-end filtration is performed using a filter medium having a flow rate of 100 s / sec or less.

[0073] [6] The method for producing polyhydroxyalkanoate according to any one of [1] to [5], wherein the number of times of centrifugation in the centrifugation step is two or less.

[0074] [7] The method for producing a polyhydroxyalkanoate according to any one of [1] to [6], wherein the protein content of the aqueous polyhydroxyalkanoate suspension (1) is 50,000 ppm or more.

[0075] [8] The method for producing a polyhydroxyalkanoate according to any one of [1] to [7], wherein the shear viscosity of the aqueous polyhydroxyalkanoate suspension (1) at 40°C and 10 1 / s is 4 to 15 mPa·s.

[0076] [9] The method for producing polyhydroxyalkanoate according to any one of [1] to [8], wherein the protein content of the filter cake obtained by the filtration step is 5,500 ppm or less.

[0077]

[10] The method for producing a polyhydroxyalkanoate according to any one of [1] to [9], wherein the pH of the aqueous suspension of polyhydroxyalkanoate (2) subjected to the filtration step is 9.0 to 11.0.

[0078] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0079] [Measurement Method] Measurements in the examples and comparative examples were carried out by the following methods.

[0080] (pH of PHA Aqueous Suspension) The pH of the PHA aqueous suspension (PHA aqueous suspensions (1) and (2)) was measured using a pH meter (9652-10D, manufactured by HORIBA).

[0081] (Shear Viscosity of PHA Aqueous Suspension) The shear viscosity of the PHA aqueous suspension was measured by the following method. Specifically, the shear viscosity was measured in a coaxial double cylinder using an MCR302 manufactured by Anton Paar. The PHA aqueous suspension was poured into a 20 mL cylinder, and the liquid temperature was adjusted to 40°C or 50°C. After reaching the target shear rate (10 1 / s), the viscosity was measured when the change in torque with time became less than 1%.

[0082] (Protein content of PHA aqueous suspension) The protein content of the PHA aqueous suspension was measured using a BCA Protein Assay Kit (manufactured by Thermo Fisher Scientific). Specifically, an amount of PHA aqueous suspension equivalent to 10 mg of PHA in solid content was placed in a 15 mL Falcon tube, 2 mL of the kit's reagent was added, and the mixture was shaken at 60 ° C. for 30 minutes. 30 minutes after the end of shaking, the mixture was cooled to 25 ° C., and the absorbance at a wavelength of 562 nm was measured. The protein content of the PHA aqueous suspension was calculated based on the measured absorbance.

[0083] (Measurement of Air Permeability of Filter Material) The air permeability of the filter material (filter cloth) used in the filtration step was measured using FX3345 Flex Air manufactured by TEXTEST INSTRUMENTS.

[0084] (Filtrate permeation rate in filtration step) Filtration was carried out under the conditions described in the filtration step of each example and comparative example, and the amount of filtrate was measured for 1 minute from the start of the filtration step. The filtrate permeation rate (LMH) was calculated based on the following formula: Filtrate permeation rate (LMH) = Filtrate amount (L) / Filtration area (m ２ ) / filtration time (h).

[0085] (Protein content of filter cake) The protein content of filter cake was measured using BCA Protein Assay Kit (manufactured by Thermo Fisher Scientific). Specifically, filter cake was dried to obtain PHA dry particles, 2 mg of the dry particles was placed in a 15 mL Falcon tube, 2 mL of the reagent of the kit was added, and then the mixture was shaken at 60 ° C for 30 minutes. After 30 minutes from the end of shaking, the mixture was cooled to 25 ° C, and the absorbance at a wavelength of 562 nm was measured. Based on the measured absorbance, the protein content of the filter cake was calculated.

[0086] (PHA Recovery Rate) The PHA recovery rate was measured using the following procedure. The total weight of the filtrate obtained by the filtration process was measured. The filtrate was then thoroughly mixed, and 1 g of the filtrate was dispensed using a dropper. The dispensed filtrate was heated at 105°C using a heat-dry moisture meter ML-50 (manufactured by A&D Co., Ltd.) until the weight change rate fell below 0.05% (W.B.) / min, thereby measuring the solids concentration in the filtrate. The total weight of the filtrate was multiplied by the obtained solids concentration to calculate the amount of solids (g) in the total filtrate. The leakage rate (%) was calculated by dividing the weight of solids (g) in the filtrate by the weight of solids (g) of the PHA aqueous suspension before the filtration process. The leakage rate (%) was then subtracted from 100% to calculate the PHA recovery rate (%).

[0087] Example 1 (Preparation of PHA aqueous suspension (1)) Preparation of bacterial cell culture Ralstonia eutropha described in International Publication No. WO 2019 / 142717 was cultured by the method described in paragraphs

[0041] to

[0048] of the same document to obtain a bacterial cell culture containing PHA-containing bacterial cells. The obtained PHA was a copolymer composed of 3HB repeating units and 3HH repeating units (i.e., poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)), and the composition ratio of the repeating units in the PHA (composition ratio of 3HB units / 3HH units) was 99 / 1 to 92 / 8 (mol / mol).

[0088] The bacterial cell culture solution obtained above was sterilized by heating and stirring at an internal temperature of 60 to 70°C for 7 hours, thereby obtaining an inactivated culture solution. The weight-average molecular weight of PHA in the obtained inactivated culture solution was 1,800,000. The solids concentration of the inactivated culture solution was 30% by weight.

[0089] Hydrogen peroxide treatment: Hydrogen peroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the inactivated culture solution obtained above to a concentration of 0.66%, and the inactivated culture solution was treated with hydrogen peroxide. The shear viscosity of the inactivated culture solution after treatment was 5.01 mPa s at a temperature of 50°C and a shear rate of 10 1 / s.

[0090] Alkali Treatment: A 30% aqueous solution of sodium hydroxide was added to the inactivated culture solution with reduced viscosity obtained above to adjust the pH to 11.0. The solution was maintained at 60°C, and the 30% aqueous solution of sodium hydroxide was added continuously to maintain the pH at 11.0 for 1.5 hours, thereby obtaining an aqueous PHA suspension.

[0091] Neutralization and enzyme treatment: The pH of the resulting aqueous PHA suspension was adjusted to 7.0±0.2 by adding 95% sulfuric acid. The solids concentration of this aqueous PHA suspension was measured and found to be 30% by weight. After adding sulfuric acid, lysozyme (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), an enzyme that decomposes sugar chains (peptidoglycans) in cell walls, was added to a liquid concentration of 10 ppm and maintained at 50 ° C. for 2 hours. Then, Alcalase 2.5L (manufactured by Novozyme), a protease, was added to a liquid concentration of 300 ppm, and then 30% sodium hydroxide was added at 50 ° C., and the mixture was maintained for 2 hours while adjusting the pH to 8.5.

[0092] Surfactant Addition Treatment Sodium dodecyl sulfate (SDS, manufactured by Kao Corporation) was added to the resulting enzyme-treated PHA aqueous suspension to a concentration of 0.6 to 1.0 wt %. The pH was then adjusted to 11.0±0.2 using aqueous sodium hydroxide. After holding the suspension at 40°C for 1 hour, the PHA aqueous suspension was diluted with aqueous sodium hydroxide to obtain PHA aqueous suspension (1) with a solids concentration of 15 wt % and a pH of 11. The protein content of the resulting PHA aqueous suspension (1) is shown in Table 1. The shear viscosity of the resulting PHA aqueous suspension (1) at a liquid temperature of 40°C and a shear rate of 10 1 / s was 8.58 mPa s.

[0093] (Centrifugation step) The obtained PHA aqueous suspension (1) was centrifuged (4500 rpm, 10 minutes), and then an amount of supernatant equivalent to 50% by volume of the aqueous suspension centrifuged was removed to obtain a 2-fold concentrated PHA aqueous suspension. To this PHA aqueous suspension, an amount of sodium hydroxide solution equal to the amount of the removed supernatant was added, and the mixture was centrifuged again (4500 rpm, 10 minutes), and an amount of supernatant equivalent to 50% by volume of the aqueous suspension centrifuged was removed (centrifugation step). A sodium hydroxide solution was added again to the PHA aqueous suspension obtained after these two centrifugation operations, and a PHA aqueous suspension (2) with a pH of 11.0 and a solids concentration of 30 wt% was obtained (pH adjustment step). The protein content of the obtained PHA aqueous suspension (2) is shown in Table 1.

[0094] (Filtration step) For 18.2 g of the obtained pH-adjusted PHA aqueous suspension (2), an air permeability of 0.16 cc / cm ２ Dead-end filtration was performed using a filter cloth of 1 / sec to obtain a filter cake. Specifically, the filter cloth was placed in a filter with an inner diameter of 40 mm, and attached to a suction bottle (2L, manufactured by SHIBATA). Then, while sucking to -90 kPa with a vacuum pump, PHA aqueous suspension (2) was introduced into the filter, and suction filtration was performed. The protein content of the obtained filter cake and the filtration rate in the filtration process (filtration rate 1 minute after the start of filtration) are shown in Table 1.

[0095] Example 2 A PHA purification treatment was carried out in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 7.7 using sulfuric acid. Table 1 shows the viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step.

[0096] Example 3 A PHA purification treatment was carried out in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 7.0 using sulfuric acid. Table 1 shows the viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step.

[0097] Example 4 A PHA purification treatment was carried out in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 6.4 using sulfuric acid. Table 1 shows the viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step.

[0098] Example 5 A PHA purification treatment was carried out in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 9.0 using sulfuric acid. The viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step are shown in Table 1. The PHA recovery rate was measured and found to be 99.1%.

[0099] (Example 6) In the filtration step, the air permeability was 0.30 cc / cm ２ PHA purification treatment was carried out in the same manner as in Example 1, except that a filter cloth with a viscosity of 1000 s / sec was used. The viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step are shown in Table 1. The PHA recovery rate was measured and found to be 91.5%.

[0100] Comparative Example 1 A PHA filter cake was obtained in the same manner as in Example 1, except that the centrifugation step was not performed, i.e., the PHA aqueous suspension (1) was directly subjected to the filtration step. Table 1 shows the viscosity and protein content of the obtained PHA aqueous suspension (1), the air permeability of the filter cloth used, and the filtration rate in the filtration step.

[0101] Comparative Example 2 PHA was purified in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 4.8 using sulfuric acid. Table 1 shows the viscosity of the resulting PHA aqueous suspension (1), the air permeability of the filter cloth used, the protein content of each PHA aqueous suspension and filter cake, and the filtration rate in the filtration step.

[0102] Comparative Example 3 PHA was purified in the same manner as in Example 1, except that the pH of the PHA aqueous suspension (2) to be subjected to the filtration step was adjusted to 3.4 using sulfuric acid. The viscosity of the resulting PHA aqueous suspension (1), the protein content of the filter cake, and the filtration rate in the filtration step are shown in Table 1.

[0103] (Example 7) The filter cake obtained in Example 1 was dispersed in an aqueous sodium hydroxide solution to obtain an aqueous suspension with a solids concentration of 30 wt% and a pH of 11.0. This aqueous suspension was filtered again under the same conditions as in Example 1, thereby obtaining a filter cake that had undergone a total of two filtration operations. This filter cake was further dispersed in an aqueous sodium hydroxide solution to obtain an aqueous suspension with a solids concentration of 30 wt% and a pH of 11.0. This aqueous suspension was filtered again under the same conditions as in Example 1, thereby obtaining a filter cake that had undergone a total of three filtration operations. The protein content of the obtained filter cake and the amount of wastewater generated before obtaining the filter cake (amount per kg of resin (PHA)) are shown in Table 2.

[0104] (Example 8) The filter cake obtained in Example 3 was dispersed in an aqueous sodium hydroxide solution to obtain an aqueous suspension with a solids concentration of 30 wt% and a pH of 7.0. This aqueous suspension was then filtered again under the same conditions as in Example 1, yielding a filter cake that had undergone a total of two filtration operations. This filter cake was then dispersed in an aqueous sodium hydroxide solution to obtain an aqueous suspension with a solids concentration of 30 wt% and a pH of 7.0. This aqueous suspension was then filtered again under the same conditions as in Example 1, yielding a filter cake that had undergone a total of three filtration operations. The protein content of the obtained filter cake and the amount of wastewater (amount per kg of resin (PHA)) generated before obtaining the filter cake were measured. The results are shown in Table 3.

[0105] (Comparative Example 4) The PHA aqueous suspension (1) obtained in Example 1 was centrifuged a total of six times under the same conditions as in Example 1. After each centrifugation, the protein content of the precipitate and the cumulative amount of wastewater (amount per kg of resin (PHA)) were measured. The results are shown in Table 4. The protein content of the precipitate was measured by the method described above in the section (Protein content of filter cake), except that precipitate was used instead of filter cake.

[0106] [Summary] As is clear from Table 1, the protein content of the PHA aqueous suspension (1) obtained by the centrifugation step was 9885 ppm, which indicates that impurities in the PHA aqueous suspension cannot be sufficiently treated by only two centrifugations that take into consideration the reduction of the amount of wastewater. On the other hand, the results of Examples 1 to 6 indicate that by performing a filtration step in addition to two centrifugations, the amount of wastewater can be reduced by minimizing the number of centrifugations, while still sufficiently removing impurities from the PHA aqueous suspension.

[0107] Furthermore, a comparison between Examples 1 to 6 and Comparative Example 1 showed that, while a practically usable filtration rate could not be achieved if a centrifugation step was not performed before the filtration step, by performing a centrifugation step before the filtration step and controlling the protein content in the aqueous suspension, it was possible to purify PHA by filtration at a practically usable filtration rate. Furthermore, a comparison between Examples 1 to 6 and Comparative Examples 2 and 3 showed that, by adjusting the pH of the PHA aqueous suspension (2) to be subjected to the filtration step to more than 5.5 and not more than 11.0, a larger amount of impurities could be removed in the filtration step, and a PHA with a sufficiently reduced amount of impurities could be provided.

[0108] Furthermore, a comparison of Examples 7 and 8 with Comparative Example 4 showed that, while a total of four centrifugation operations are required to provide PHA with a sufficiently reduced amount of impurities using only centrifugation, the present production method, which combines centrifugation and filtration, can provide PHA with a sufficiently reduced amount of impurities with a smaller amount of wastewater. Furthermore, the results of Examples 7 and 8 also showed that by increasing the number of filtrations, it is possible to remove a larger amount of impurities while reducing the amount of wastewater compared to centrifugation alone.

[0109] This production method can reduce the amount of wastewater discharged during the purification process, while sufficiently removing impurities, and can achieve a filtration rate suitable for practical use, making it suitable for use in the production of PHA. PHA produced by this production method can be suitably used, for example, as a molded product in agriculture, fisheries, forestry, horticulture, medicine, sanitary products, clothing, non-clothing, packaging, automobiles, building materials, and other fields.

Claims

1. A method for producing a polyhydroxyalkanoate, comprising: a centrifugation step of centrifuging a polyhydroxyalkanoate aqueous suspension (1) to obtain a polyhydroxyalkanoate aqueous suspension (2) having a protein content of 6,000 to 30,000 ppm; and a filtration step of subjecting the obtained polyhydroxyalkanoate aqueous suspension (2) to dead-end filtration, wherein the pH of the polyhydroxyalkanoate aqueous suspension (2) subjected to the filtration step is greater than 5.5 and not greater than 11.

0.

2. The method for producing polyhydroxyalkanoate according to claim 1, further comprising a pH adjustment step of adjusting the pH of the aqueous polyhydroxyalkanoate suspension (2) to a value greater than 5.5 and equal to or less than 11.0 prior to the filtration step.

3. The method for producing polyhydroxyalkanoate according to claim 2, wherein the pH is adjusted by adding an acid or alkali to the aqueous polyhydroxyalkanoate suspension (2) in the pH adjustment step.

4. In the filtration step, the air permeability is 0.50 cc / cm ２ 2. The method for producing a polyhydroxyalkanoate according to claim 1, wherein dead-end filtration is carried out using a filter medium having a flow rate of 1 / sec or less.

5. In the filtration step, the air permeability is 0.25 cc / cm ２ 2. The method for producing a polyhydroxyalkanoate according to claim 1, wherein dead-end filtration is carried out using a filter medium having a flow rate of 1 / sec or less.

6. The method for producing polyhydroxyalkanoate according to claim 1, wherein the number of centrifugation steps in the centrifugation step is two or less.

7. The method for producing polyhydroxyalkanoate according to claim 1, wherein the protein content of the aqueous polyhydroxyalkanoate suspension (1) is 50,000 ppm or more.

8. The method for producing a polyhydroxyalkanoate according to claim 1, wherein the shear viscosity of the aqueous polyhydroxyalkanoate suspension (1) at 40°C and 10 1 / s is 4 to 15 mPa·s.

9. The method for producing polyhydroxyalkanoate according to claim 1, wherein the protein content of the filter cake obtained by the filtration step is 5,500 ppm or less.

10. The method for producing a polyhydroxyalkanoate according to claim 1, wherein the pH of the aqueous polyhydroxyalkanoate suspension (2) subjected to the filtration step is 9.0 to 11.0.

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