Production process and application of fermented tapioca starch for baking

Abstract

The disclosure discloses a production process and application of fermented tapioca starch for baking, and belongs to the fields of starch deep processing and food processing and production. The disclosure develops a production method of the fermented tapioca starch for baking. The method includes simple steps and greatly shortens a process cycle. By using tapioca starch as a main raw material and adding a specific amount of carbon source and a specific strain, under the action of fermentation and illumination in cooperation, the structure of the starch is improved. By adding the fermented tapioca starch, the effects of increasing the size of gluten-free Mochi bread, increasing pores of the bread and improving the texture and taste of the bread are realized.

Claims

1. A method of preparing fermented tapioca starch for baking, comprising: mixing a carbon source with water to prepare a fermentation medium, wherein the carbon source is molasses, and wherein an initial mass concentration of the carbon source in the fermentation medium 1.0 g/100 g to 2.0 g/100 g; sterilizing the fermentation medium at a temperature of from 115? C. to 121? C. and a pressure of from 0.1 MPa to 0.3 MPa; adding tapioca starch to form tapioca starch milk; and fermenting the tapioca starch milk by adding an inoculum of Lactobacillus plantarum (L. plantarum) CCTCC M2017138 to the tapioca starch milk, wherein the inoculum comprises at least 4.2?10.sup.9 CFU/mL L. plantarum, wherein the tapioca starch milk is first added to the inoculum in an amount of 50% of a mass of the fermentation medium, and then an inoculum size of 10% to 15% by mass of the tapioca starch milk is added to the tapioca starch milk.

2. The method according to claim 1, wherein a mass concentration of the tapioca starch in the tapioca starch milk is 30 g/100 g to 70 g/100 g.

3. The method according to claim 1, wherein fermenting is conducted at a temperature if 30? C. to 40? C., for 12 hours to 120 hours.

4. The method according to claim 2, wherein fermenting is performed at a fermentation temperature of 30? C. to 40? C., for 12 hours to 120 hours.

5. The method according to claim 1, further comprising: illuminating after fermenting.

6. The method according to claim 2, further comprising: illuminating after fermenting.

7. The method according to claim 3, further comprising: illuminating after fermenting.

8. The method according to claim 1, further comprising the following steps performed after fermenting: adjusting pH of the tapioca starch milk to pH 3.5 to 7; adjusting a water content of the tapioca starch milk basis to 30% to 60% by washing the tapioca starch milk; flat spreading the tapioca starch milk on a drying container; and exposing the tapioca starch milk to 100,000 to 300,000 lux illumination for 6 hours to 72 hours.

9. The method of claim 8, wherein the pH is adjusted to 5.0.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 shows gelatinization property curves of fermented tapioca starch obtained from different use amounts of carbon source in Example 2 and raw starch thereof.

(2) FIG. 2 shows gelatinization property curves of fermented tapioca starch obtained from different inoculum sizes of strain in Example 3 and raw starch thereof.

(3) FIG. 3 shows gelatinization property curves of fermented starch from different sources and raw starch.

(4) FIG. 4 shows gelatinization property curves of tapioca starch obtained by acid addition.

(5) FIG. 5 shows gelatinization property curves of fermented tapioca starch obtained by natural fermentation for different times.

DETAILED DESCRIPTION

(6) The implementation scheme of the disclosure is described in detail with reference to Examples as follows. If no specific conditions are indicated in Examples, conventional conditions are adopted. Agents and instruments that are used have no clear indication of manufacturers are all conventional products.

(7) A measurement method for types and contents of organic acids in fermentation liquor comprises the steps: separating the organic acids in the fermentation liquor by a C-18 reverse chromatographic column, and detecting contents of malic acid, lactic acid, formic acid, acetic acid, citric acid, succinic acid, fumaric acid, propionic acid, butyric acid, tartaric acid and ascorbic acid.

(8) A measurement method for a gelatinization property of starch comprises the steps: weighing a certain mass of corn acidified starch sample to be evenly mixed with deionized water in an RVA aluminum specimen box to prepare a suspension with a total mass percentage of 6% (based on a dry basis); performing measurement according to Standard 2 of a stipulated method of American Association of Cereal Chemists (AACC) by steps of heat preserving for 1 min at 50? C., heating to 95? C. at a speed of 6? C./min, heat preserving for 5 min, cooling to 50? C. at a speed of 6? C./min and heat preserving for 2 min; and performing viscosity measurement at a stirring rate of 960 r/min within the first 10 s and then a stirring rate of 160 r/min.

(9) Peak viscosity: rapid viscosity analyzer (RVA), starch mass concentration of 6%.

(10) An expansion property of starch is tested by the following steps: evenly kneading 50 g of starch with 40 mL of boiling water, cutting the dough into pieces with a weight of 10 g, shaping the pieces into a flat and round shape, and baking the pieces with upper fire and lower fire for 20 min at 200? C.; measuring sizes after baking by a millet replacement method in triplicate; and obtaining the expansion property (mL/g) according to sizes after baking/original mass.

(11) The method of the disclosure may be implemented generally as follows.

(12) Fermented tapioca starch for baking and a production method thereof are provided. The production method comprises the steps:

(13) 1, preparing a fermentation medium with a content of 0.1-10 g/100 g from molasses (based on the content of available glucose), and performing high-temperature and high-pressure sterilizing, where the temperature range is controlled to be 115-121? C., the pressure range is controlled to be 0.1-0.3 MPa, and the sterilizing time is controlled to be 20-30 min;
2, obtaining a culture medium of a seed solution as a conventional culture medium that is the most suitable for the strain in this field, which is a Lactic acid bacteria culture medium (MRS broth), where the culture temperature is 30-40? C., the culture time is 10-20 h, and the concentration of Lactic acid bacteria in the culture medium is 1.0?10.sup.9-7.0?10.sup.9 CFU/mL, and after cooling the fermentation medium to 20-50? C., adding tapioca starch under a clean environment at an addition amount of 30-70% of the fermentation medium, stirring to prepare starch milk, and transferring the cultured seed solution into the fermentation medium at an inoculum size of 1-20% of the starch milk (1-30% of the dry basis mass of the starch) at a fermentation temperature of 30-40? C. for a fermentation time of 12-120 h;
3, washing the starch milk to a pH of 3.5-7 after fermentation ends, and adjusting a water content of a fermented tapioca starch basis to 30-60%; and
4, flat spreading the starch milk with an adjusted water content on a drying container at an average illumination intensity of 100,000-300,000 lux for an illumination time of 6-72 h.

Example 1

(14) (1) A fermentation medium with a content of 0.5 g/100 g is prepared from molasses (based on the content of available glucose), and the fermentation medium is sterilized for a sterilizing time of 20 min under conditions of a high temperature (115? C.) and a high pressure (0.2 MPa).

(15) (2) A seed solution with a L. plantarum concentration of 4.2?10.sup.9 CFU/mL is obtained with a L. plantarum culture medium (MRS broth) as a seed culture medium and L. plantarum (CCTCC M2017138) as a strain at a culture temperature of 37? C. for a culture time of 12 h. After the fermentation medium is cooled to 25? C., tapioca starch is added under a clean environment at an addition amount of 50% of a mass of the fermentation medium and stirred to prepare starch milk, and the seed solution is transferred into the fermentation medium at an inoculum size of 10% of a mass of the starch milk at a fermentation temperature of 37? C. for a fermentation time of 96 h.

(16) (3) The starch milk is washed to a pH of 5 of the starch milk after fermentation ends, and the water content of a fermented tapioca starch basis is adjusted to 50%.

(17) (4) The starch milk with adjusted water content is flat spread on a drying container at an average illumination intensity of 150,000 lux for an illumination time of 8 h to obtain fermented tapioca starch, where the production time of the whole process is 5 d.

Example 2

(18) With reference to Example 1, fermented tapioca starch is prepared by changing an addition amount of molasses (based on the content of available glucose) to 0 g/100 g, 1.0 g/100 g, 1.5 g/100 g and 2.0 g/100 g respectively and keeping other conditions unchanged. The production time of each whole process is 5 d. Property results of obtained products are shown in Table 1, Table 2 and Table 3.

(19) Gelatinization viscosity properties of the fermented tapioca starch obtained from different use amounts of carbon source are measured, and results are shown in Table 1. It is known from Table 1 that compared with raw starch, tapioca starch presents obvious drops in peak viscosity, trough viscosity, final viscosity and setback value after fermentation and illumination from 1,651 mPa.Math.s, 796.5 mPa.Math.s, 1,301.0 mPa.Math.s and 504.5 mPa.Math.s to about 1,100 mPa.Math.s, 300 mPa.Math.s, 450 mPa.Math.s and 150 mPa.Math.s respectively. The fermented tapioca starch obtained from different use amounts of carbon source presents small changes in breakdown value, with a drop of about 10-30 mPa.Math.s. It is known from Table 3 by comparison that when the peak viscosity is 1,100-1,200 mPa.Math.s, the expansion property of the starch is on the rise.

(20) TABLE-US-00001 TABLE 1 Measurement of gelatinization viscosity properties of fermented tapioca starch obtained from different use amounts of carbon source Addition Peak Trough Breakdown Final Setback amount of viscosity viscosity value viscosity value molasses (%) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) 0.0 1651.0 ? 7.1 796.5 ? 16.3 854.5 ? 9.2 1301.0 ? 8.5 504.5 ? 24.8 0.5 1120.5 ? 2.1 .sup.284 ? 24.0 836.5 ? 21.9 423.5 ? 31.8 139.5 ? 7.8 1.0 1198.0 ? 5.7 328.5 ? 2.1 869.5 ? 3.5 485.5 ? 2.12 157.0 ? 0.0 1.5 1119.5 ? 34.6 295.5 ? 16.3 824.0 ? 18.4 433.0 ? 24.0 137.5 ? 7.8 2.0 1057.5 ? 12.0 231.0 ? 36.8 826.5 ? 24.7 333.0 ? 58.0 102.0 ? 21.2

(21) Contents of organic acids in fermentation liquor from a fermentation system with different use amounts of carbon source added are measured, and results are shown in Table 2. It is known from Table 2 and Table 3 that when a lactic acid concentration reaches 9,000-10,500 mg/L, the starch has an expansion property close to 6 mg/L and presents an excellent baking property. It is known from Table 3 that when an addition amount of molasses is 1.0 g/100 g, the expansion property is the best, when the addition amount of molasses is less than 1.0 g/100 g, the expansion property is on the rise, and as the addition amount of molasses increases, the expansion property drops instead.

(22) TABLE-US-00002 TABLE 2 Measurement of types and contents of organic acids in fermentation liquor obtained from different use amounts of carbon source Addition amount of Tartaric Oxalic Lactic Citric Succinic Butyric molasses acid acid acid acid acid acid (%) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 0.5 34 ? 8 18 ? 6 3415 ? 148 7 ? 4 203 ? 23 928 ? 46 1.0 59 ? 10 20 ? 4 9097 ? 302 224 ? 13 121 ? 34 2354 ? 125 1.5 54 ? 5 19 ? 3 10359 ? 215 93 ? 3 288 ? 12 1817 ? 96 2.0 57 ? 3 31 ? 12 9083 ? 321 26 ? 16 182 ? 20 860 ? 34

(23) Expansion properties of the fermented tapioca starch obtained from different use amounts of carbon source are measured, and results are shown in Table 3.

(24) TABLE-US-00003 TABLE 3 Measurement of expansion properties of fermented tapioca starch obtained from different use amounts of carbon source Addition amount of molasses (%) 0.0 0.5 1.0 1.5 2.0 Expansion 3.37 ? 0.16 4.17 ? 0.26 6.52 ? 0.35 6.18 ? 0.50 5.85 ? 0.34 property (mL/g)

Example 3

(25) (1) A fermentation medium with a content of 1.0 g/100 mL is prepared from molasses (based on the content of available glucose), and the fermentation medium is sterilized for a sterilizing time of 20 min under conditions of a high temperature (115? C.) and a high pressure (0.2 MPa).

(26) (2) A seed solution with a L. plantarum concentration of 4.2?10.sup.9 CFU/mL is obtained with a lactic acid bacteria culture medium (MRS broth) as a seed solution culture medium and L. plantarum (CCTCC M2017138) as a strain at a culture temperature of 37? C. for a culture time of 12 h. After the fermentation medium is cooled to 25? C., tapioca starch is added under a clean environment at an addition amount of 50% of a mass of the fermentation medium and stirred to prepare starch milk, and the cultured seed solution is transferred into the fermentation medium at an inoculum size of 0.5%, 5%, 15% and 30% of a mass of the starch milk instead at a fermentation temperature of 37? C. for a fermentation time of 96 h.

(27) (3) The starch milk is washed to a pH of 5 after fermentation ends, and the water content of a fermented tapioca starch basis is adjusted to 50%.

(28) (4) The starch milk with adjusted water content is flat spread on a drying container at an average illumination intensity of 150,000 lux for an illumination time of 8 h to obtain fermented tapioca starch.

(29) Gelatinization viscosity properties of the fermented tapioca starch obtained from different inoculum sizes of strain are measured, and results are shown in Table 4. It is known from Table 4 that when the inoculum size is 0.5%, fermentation is quite slow, the gelatinization property of the obtained fermented tapioca starch does not change greatly compared with raw starch, and the expansion property of the starch slightly increases. When the inoculum size is increased to 30%, the viscosity of the obtained starch changes excessively, resulting in the drop of the expansion property of the starch. When the inoculum size is 10-15%, the obtained starch has a high expansion property within a proper viscosity range of 1,100-1,200 mPa.Math.s. To sum up, it is economical and reasonable to make the inoculum size of a fermentation system at 10%.

(30) TABLE-US-00004 TABLE 4 Measurement of gelatinization viscosity properties of fermented tapioca starch obtained from different inoculum sizes of strain Inoculum Peak Trough Breakdown Final Setback size of viscosity viscosity value viscosity value strain (%) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) 0.5 .sup.1364 ? 15.5 653 ? 79.2 711 ? 94.8 1255 ? 182.4 602 ? 103.2 5 1282.5 ? 10.7 584 ? 14.1 698.5 ? 3.5 905.5 ? 20.5.sup. 321.5 ? 6.36.sup. 15 1180.5 ? 26.2 240 ? 26.8 940.5 ? 53.0 358 ? 50.9 118 ? 24.0 30 .sup.963 ? 2.8 222.5 ? 6.4 740.5 ? 3.5 343 ? 2.8 120.5 ? 3.5

(31) Expansion properties of fermented tapioca starch obtained from different inoculum sizes of strain are measured, and results are shown in Table 5.

(32) TABLE-US-00005 TABLE 5 Measurement of expansion properties of fermented tapioca starch obtained from different inoculum sizes of strain Inoculum size of strain (%) 0.5 5 15 30 Expansion 4.24 ? 0.56 5.23 ? 0.87 6.08 ? 0.34 5.34 ? 0.40 property (mL/g)

Example 4

(33) (1) A fermentation medium with a content of 0.5 g/100 mL is prepared from molasses (based on the content of available glucose), and the fermentation medium is sterilized for a sterilizing time of 20 min under conditions of a high temperature (115? C.) and a high pressure (0.2 MPa).

(34) (2) A seed solution with a L. plantarum concentration of 4.2?10.sup.9 CFU/mL is obtained with a lactic acid bacteria culture medium (MRS broth) as a seed solution culture medium and L. plantarum (CCTCC M2017138) as a strain at a culture temperature of 37? C. for a culture time of 12 h. After the fermentation medium is cooled to 25? C., starch such as potato starch, rice starch and corn starch instead is added under a clean environment at an addition amount of 50% of a mass of the fermentation medium and stirred to prepare starch milk, and the cultured seed solution is transferred into the fermentation medium at an inoculum size of 10% of a mass of the starch milk at a fermentation temperature of 37? C. for a fermentation time of 96 h.

(35) (3) The starch milk is washed to a pH of 5 after fermentation ends, and the water content of a fermented tapioca starch basis is adjusted to 50%.

(36) (4) The starch milk with adjusted water content is flat spread on a drying container at an average illumination intensity of 150,000 lux for an illumination time of 8 h to obtain fermented tapioca starch, where a production time of the whole process is 5 d.

(37) Gelatinization viscosity properties and expansion properties of fermented starch obtained from different types of starch are measured, and results are shown in Table 6 and Table 7. It is known from Table 6 and Table 7 that a peak viscosity of the potato starch is too high and reaches 4,890.5 mPa.Math.s, a peak viscosity of the rice starch and a peak viscosity of the corn starch are too low and reach 678.0 mPa.Math.s and 754.5 mPa.Math.s respectively, and expansion properties of raw starch are all quite low. A viscosity of the potato starch obtained after fermentation is further reduced to 2,979.5 mPa.Math.s, the fermented potato starch is likely to be endowed with an expansion property by prolonging a fermentation time, but it consumes time and expansive raw materials, which leads to low production economic benefits.

(38) TABLE-US-00006 TABLE 6 Measurement of gelatinization viscosity properties of starch from different sources and obtained fermented starch Peak Trough Breakdown Final Setback viscosity viscosity value viscosity value Category (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) Potato starch 4890.5 ? 218.5 1881.0 ? 39.6 3009.5 ? 178.9 2300.5 ? 31.8 419.5 ? 7.8 Fermented 2979.5 ? 38.8 1454 ? 29.7 1419.0 ? 72.1 2040.5 ? 34.6 480.0 ? 1.4 potato starch Rice starch 678.0 ? 8.49 380.5 ? 16.3 295.5 ? 7.8 832.5 ? 14.9 452.0 ? 1.4 Fermented rice 611.5 ? 0.7 483.5 ? 3.5 128.0 ? 2.8 643.0 ? 1.4 159.5 ? 5.0 starch Corn starch 754.5 ? 3.5 614.5 ? 10.6 140.0 ? 7.1 802.0 ? 21.2 185.0 ? 31.8 Fermented corn 729.5 ? 74.2 511.5 ? 47.4 218.0 ? 26.9 678.5 ? 55.9 167.0 ? 8.5 starch

(39) TABLE-US-00007 TABLE 7 Measurement of expansion properties of starch from different sources and obtained fermented starch Fermented Fermented Fermented Starch Potato potato Rice rice Corn corn category starch starch starch starch starch starch Expansion 1.62 ? 0.09 1.52 ? 0.09 0.62 ? 0.03 1.31 ? 0.03 0.64 ? 0.02 0.93 ? 0.06 property (mL/g)

Comparative Example 1 Preparation of Fermented Tapioca Starch by Direct Artificial Acid Addition

(40) Production method: The production method comprises the steps: preparing 40% (w/w) tapioca starch and adding lactic acid to the starch for reaction for different times at a reaction temperature of 37? C., washing starch milk to a pH of 5 after reaction ends, adjusting a water content of a fermented tapioca starch basis to 50%, and flat spreading the starch milk with an adjusted water content on a drying container at an average illumination intensity of 150,000 lux for an illumination time of 8 h to obtain the fermented tapioca starch.

(41) Results: The production time of the whole process is 2 d, and the expansion property of the obtained tapioca starch is poor.

(42) Gelatinization viscosity properties of the fermented tapioca starch obtained from acid with different concentrations are measured, and results are shown in Table 8. It is found through experiments that by adding pure lactic acid with a same concentration (1%) to fermentation liquor, the viscosity of the starch may be rapidly reduced within a short time and meanwhile the expansion property of the starch is poor. By adjusting an acid concentration and processing time, the peak viscosity of the starch is within a proper range of 1,100-1,200 mPa.Math.s, and it is found that the expansion property of the starch slightly changes, however, by a small increase amplitude. In conclusion, direct acid addition may shorten a fermentation time, but it fails to lead to an excellent expansion property.

(43) TABLE-US-00008 TABLE 8 Measurement of gelatinization viscosity properties of fermented tapioca starch obtained from acid with different concentrations in Comparative example 1 Peak Trough Breakdown Final Setback viscosity viscosity value viscosity value Category (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) .sup.1%-24 h 417.0 ? 8.5 68 ? 2.8 352.5 ? 10.6 101.0 ? 1.4 34.5 ? 0.7 0.5%-24 h 572.5 ? 12.0 106.0 ? 2.8 466.5 ? 9.2 166.5 ? 3.5 60.5 ? 0.7 1%-0.5 h 655 ? 14.1 106.0 ? 1.4 549.0 ? 12.7 166.0 ? 0.0 60.0 ? 1.4 0.25%-12 h 1143.5 ? 29.0 379.5 ? 23.3 764.0 ? 5.7 580.5 ? 37.5 201.0 ? 14.1 0.25%-24 h 1118.0 ? 19.8 314.5 ? 4.9 803.5 ? 24.7 487.0 ? 8.5 172.5 ? 3.5 0.2%-24 h 1186.5 ? 19.1 356.5 ? 10.6 830.0 ? 8.5 549.0 ? 12.7 192.5 ? 2.1

(44) Expansion properties of the fermented tapioca starch obtained from acid with different concentrations are measured, and results are shown in Table 9.

(45) TABLE-US-00009 TABLE 9 Measurement of expansion properties of fermented tapioca starch obtained in Comparative example 1 Inoculum size of strain (%) 1%-24 h 0.5%-24 h 1%-0.5 h 0.25%-12 h 0.25%-24 h 0.2%-24 h Expansion 3.93 ? 0.20 3.74 ? 0.55 4.11 ? 0.29 3.58 ? 0.27 3.77 ? 0.28 4.57 ? 0.85 property (mL/g)
1%-24 h: 1% of a mass of starch milk participates in lactic acid reaction for 24 h; 0.5%-24 h: 0.5% of the mass of starch milk participates in lactic acid reaction for 24 h; 1%-0.5 h: 1% of the mass of starch milk participates in lactic acid reaction for 0.5 h; 0.25%-12 h: 0.25% of the mass of starch milk participates in lactic acid reaction for 0.5 h; 0.25%-24 h: 0.25% of the mass of starch milk participates in lactic acid reaction for 24 h; and 0.2%-24 h: 0.2% of the mass of starch milk participates in lactic acid reaction for 0.5 h.

Comparative Example 2 Preparation of Fermented Tapioca Starch from Fermentation Liquor Obtained by Natural Fermentation

(46) With reference to Example 1, a seed solution is replaced with the settled and centrifuged fermentation liquor which is prepared from 40% starch milk by natural fermentation for 4 d, and a corresponding culture medium is unprocessed water. Other conditions are not changed, and the fermented tapioca starch is prepared. The production time of the whole process is 10-14 d.

(47) Gelatinization viscosity properties of the tapioca starch fermented by a multi-strain fermentation system are measured, and results are shown in Table 10. It is known from Table 10 that the fermented tapioca starch obtained by fermentation for 4 d has a peak viscosity of 1,160 mPa.Math.s and a proper viscosity range of 1,100-1,200 mPa.Math.s. It is known from Table 11 that the starch has a weak expansion property, which may be caused when amylase and other relevant substances generated by multi-strain fermentation complexly change the starch and thus cause a high final viscosity and increasing setback to the starch. It is known from Table 11 that after fermentation for 4 d, 6 d and 8 d, the expansion property of the starch rises first and then falls, but it is still poor compared with that of single-strain fermentation. After fermentation for 8 d, an obvious starch flocculation phenomenon is observed in an experiment, the peak viscosity falls, the final viscosity obviously rises, and the setback value also rises.

(48) TABLE-US-00010 TABLE 10 Measurement of gelatinization viscosity properties of fermented tapioca starch obtained by multi-strain fermentation system Peak Trough Breakdown Final Setback Fermentation viscosity viscosity value viscosity value time (d) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) (mPa .Math. s) 4 1160.0 ? 113.1 672.5 ? 37.5 487.5 ? 75.7 1061.0 ? 69.3 388.5 ? 31.8 6 1080.5 ? 10.6 556.5 ? 7.8 524.0 ? 18.4 858.5 ? 17.7 302.0 ? 9.9 8 989.0 ? 2.8 738.5 ? 2.1 250.5 ? 4.9 1144.0 ? 7.1 405.5 ? 9.2

(49) Expansion properties of the fermented tapioca starch obtained by a multi-strain fermentation system are measured, and results are shown in Table 11.

(50) TABLE-US-00011 TABLE 11 Measurement of expansion properties of fermented tapioca starch obtained by multi-strain fermentation system in natural fermentation Fermentation time (d) 4 6 8 Expansion 4.05 ? 0.84 5.46 ? 0.32 4.27 ? 0.66 property (mL/g)