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Properties and Composition of Concentrates and Syrup Obtained by Microfiltration of Saccharified Corn Starch Hydrolysate
Journal of Cereal Science 27 (1998) 315–320
Properties and Composition of Concentrates and Syrup Obtained by Microfiltration of Saccharified Corn Starch Hydrolysate N. Singh∗ and M. Cheryan† ∗Department of Agricultural Engineering, University of Illinois, Urbana, IL 61801 and †Department of Food Science and Human Nutrition, University of Illinois, Urbana, IL 61801, U.S.A. Received 3 December 1996
ABSTRACT Flow properties, clarity and composition of concentrates (retentates) and clarified syrup (permeates) obtained by membrane microfiltration of saccharified corn starch hydrolysate were determined. Clarified syrup (permeate) had low color and no suspended solids at all volume concentration ratios (VCRs) up to 100 X. The color and suspended solids in the concentrate increased linearly with increase in volume concentration ratio. Viscosity of the concentrates increased at higher VCR, but the viscosity of permeates remained essentially unchanged. The densities of all retentates and permeates were not significantly different from each other. Dextrose content of retentates and permeates at all VCRs did not change significantly during the microfiltration, averaging 28·90±1·01% w/w. Total solids, nitrogen, fat and ash content of the retentates increased with increase in VCR. In contrast, total solids, nitrogen, fat and ash of the permeates remained the same for all VCRs, averaging 29·44, 0·011, 0·07 and 0·042% respectively. 1998 Academic Press Limited
Keywords: microfiltration, starch hydrolysate, corn syrup.
INTRODUCTION Dextrose syrups are produced by hydrolysis of starch. Starch can be liquified and saccharified using an acid–enzyme or enzyme–enzyme conversion process. The range of products obtained from starch by acid hydrolysis is limited to 30–55 DE (dextrose equivalent)1. Enzyme hydrolysis, on the other hand, can result in dextrose syrups of 95 DE and higher. The liquified starch is saccharified at 60 °C with glucoamylase to about : DE=dextrose equivalents; VCR=volume concentration ratio. Mention of product or trade names does not imply endorsement by University of Illinois. Corresponding author: M. Cheryan, University of Illinois, Agricultural Bioprocess Laboratory, 1302 W Pennsylvania Avenue, Urbana, IL 61801, U.S.A. 0733–5210/98/030315+06 $25.00/0/jc970169
93–96% dextrose (db). This starch hydrolysate is clarified to remove suspended solids and insoluble impurities, which consist of protein and fat (commonly called ‘mud’). Traditionally rotary vacuum filters pre-coated with a filter aid such as diatomaceous earth or carbon are used. The clarified syrup is further refined and processed into various sweeteners. Microfiltration, a membrane separation process, can be used for clarification of hydrolysate more efficiently and economically2–5. The membrane also has a significant impact on the unit operations which follow clarification: ion exchange, decolorization, and enzymatic reactions. Membrane separations can reduce carbon requirements in downstream processing by as much as 60–70% and ionexchange by 20–30%. These reductions, along with the elimination of diatomaceous earth filter aid, result in annual savings of U.S.$2 to $3 million 1998 Academic Press Limited
316
N. Singh and M. Cheryan
Pressure gauge P Valve 2
Permeate
Permeate Membrane module
Membrane module
P
Feed
tank
Valve 1
T
Heat exchanger
Figure 1
Pressure gauge
Thermometer
Centrifugal pump
Schematic of setup used to collect samples of retentates and permeates at various volume concentration ratios.
per year for a 500 gal/min line4. Although it has been claimed that quality of the membraneclarified syrup is superior in terms of turbidity, color and microbiological cleanliness compared to the conventional centrifugation or filtration using filter aid3, there are few data on the composition and properties of membrane-clarified dextrose syrups. This study focused on the degree of clarification, and changes in flow properties and composition of concentrates and syrup obtained by microfiltration of a typical stream of saccharified corn starch hydrolysate.
MATERIALS AND METHODS The saccharified corn starch hydrolysate was obtained from the last stage of saccharification reactor of a mid-west corn processing company. The hydrolysate had a pH of 4·43±0·03 and starch conversion to approximately 95 DE. A laboratoryscale cross-flow microfiltration system was used to clarify the hydrolysate (Fig. 1). The microfiltration system consisted of two ceramic membrane modules, each having a surface area of 0·12 m2 and a nominal pore size of 0·2 lm (Ceramem Corporation, Waltham, MA, U.S.A.). The system
Microfiltered corn starch hydrolysate
HPX-87 column (Bio-Rad, Richmond, VA, U.S.A.) and a refractive index detector. Temperature of the column was 65 °C, and the mobile phase was 0·01 N sulfuric acid with a flow rate of 0·8 mL/min. The elution time for dextrose under these conditions was 6·6 min. Statistical software (SAS Institute, Cary, NC, U.S.A.) was used to analyze the data. One-way analysis of variance (ANOVA) was used to determine the significant differences in the various concentrate and syrup samples. Duncan’s multiple range test was used for multiple comparisons. The probability of a (type I error) was 5% (P<0·05). RESULTS AND DISCUSSION Degree of clarification The feed had a color of 16·63±0·95 whereas the average color of all permeates was 1·01±0·21. The color of the permeates at all volume concentration ratios was not significantly different from each other. The color of the retentates (concentrates) increased linearly with increasing VCR (Fig. 2). A statistical model to correlate color with VCR was determined (R2=0·99): color=15·935 X
(1)
where X is the VCR. The clarified syrup was consistently low in color throughout processing at all VCR values, demonstrating that microfiltration can be effectively used for the clarification of corn starch hydrolysates at high yields.
Color
was operated at a transmembrane pressure of 1·7 bar (25 p.s.i.), a crossflow velocity of 4 m/s, and a temperature of 60 °C. The retentate, containing suspended solids and insoluble impurities, was returned to the feed tank, while permeate (clarified syrup) was continuously removed. Samples of the clarified syrup (permeate) and concentrate (retentate) were taken at volume concentration ratios of 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100. Volume concentration ratio (VCR) is the ratio of the initial feed volume to the volume of retentate (concentrate) remaining at any time during the microfiltration process. VCR of 1 refers to fresh feed. For syrups, the acceptance of the product is contingent on a specified maximum color6. The color of the syrup (permeate) compared to that of concentrate (retentate) is a measure of the degree of clarification. The color of all the permeates and retentates was determined using a spectrophotometer (Spectronic 1001, Bausch and Lomb, Rochester, NY). The color was the difference of absorbances at 450-nm and 600-nm wavelengths per unit cell length, expressed in percentage7. Suspended solids were determined in all samples by centrifugation of 15 mL of sample in graduated glass tubes at 2000 rev/min (640 g) for 60 min and expressed in volume percentage. Since the feed coming out of the saccharification reactor is at a temperature of 60 °C1,8–10, the flow properties (density and viscosity) were determined at 60 °C. Density of all syrups (permeates) and concentrates (retentates) was determined using pre-calibrated pycnometers. Viscosity was measured with a coaxial cylinder sensor system (Rotovisco RV3, Haake, Paramus, NJ, U.S.A.). The rheological measurements were performed up to shear rate of 400/s to check for possible nonNewtonian behavior. Total solids were determined gravimetrically by drying the samples at 60 °C for 24 h, followed by 100 °C for 5 h in a forced-air convection-drying oven11. Nitrogen content was determined using a Kjeldahl nitrogen procedure by block digestion and steam distillation12. Fat content was determined from the samples dried overnight at 100 °C by Soxhlet extraction method. Ash content was determined by charring the samples over a hot plate with the aid of olive oil, and then placing them in muffle furnace at 525 °C for 5 h; the samples were moistened with water after 2 h13. Dextrose content was determined by highperformance liquid chromatography using the
317
1750 1500 1250 1000 750 500 250 0 1.0 0.5 0
Retentate
Permeate 20 40 60 80 Volume concentration ratio (X)
100
Figure 2 Color of concentrates and clarified syrup for all volume concentration ratios (the difference of spectrophotometric absorbances at 450-nm and 600-nm wavelengths per unit cell length, expressed in percentage).
318
N. Singh and M. Cheryan
20
Retentate
Total solids (%)
Suspended solids (%)
25
15 10 5 Permeate
0 0
20 40 60 80 Volume concentration ratio (X)
100
32 31 30 29 28 27 26 25 24 23 22
Retentate Permeate
0
20 40 60 80 Volume concentration ratio (X)
100
Figure 3 Suspended solids of concentrates and clarified syrup for all volume concentration ratios.
Figure 5 Total solids of concentrates and clarified syrup for all volume concentration ratios.
The suspended solids in the retentates increased linearly with increase in VCR (Fig. 3). The concentrate at VCR 100 had suspended solids of 24·56% compared to a feed suspended solids of 0·24%. On the other hand, permeates at all VCR values were free of suspended matter. A statistical model equation was developed to predict suspended solids for all VCRs (R2=0·9999):
determined to predict dynamic viscosity of retentate for all VCRs (R2=0·94).
suspended solids (%)=0·2466 X
(2)
where X is the VCR. Flow properties All retentates and permeates displayed Newtonian flow behavior (i.e. varying shear rates did not change the viscosity resulting in linearity of the flow curve). The dynamic viscosity of the feed was 1·24±0·12 cp and increased with increasing VCR (Fig. 4). A statistical model equation was 2.25 Dynamic viscosity (cp)
2.00 1.50
Permeate
1.00 0.75 0.50 0.25 0
20 40 60 80 Volume concentration ratio (X)
(3)
where X is the VCR. A VCR of 100 implies a yield of clarified syrup of 99%. Since the viscosity of the 100 X retentate was still low (>2 cp), it suggests that VCR of 200 or more with a corresponding yield of 99·5% or higher could be obtained, provided the membrane module and system were properly designed. The dynamic viscosity of permeates at all VCR values averaged 1·35±0·12 cp, and were not significantly different from each other. The viscosity of the permeates was also not significantly different from the viscosity of the feed. The density of retentates (concentrates) and permeates (syrup) at all VCRs were not significantly different from each other, averaging 1·1029 g/cm3. Composition
Retentate
1.75 1.25
dynamic viscosity (cp)=0·0064X+1·3821
100
Figure 4 Dynamic viscosity of concentrates and clarified syrup at 60 °C for all volume concentration ratios.
The corn starch hydrolysate that was obtained from this manufacturer had a total solids of 29·09±0·12% (w/w). The total solids of the permeates at all VCRs were not significantly different. The average total solids for all permeates was 29·44±0·45% (w/w). The total solids of the retentate increased with increasing VCRs (Fig. 5). A statistical model equation to predict total solids from VCRs was determined (R2=0.93). Total solids (w/w)=28·952 X0·0117 where X is the VCR.
(4)
0.07
0.9
0.06
0.8 Rejection coefficient
Nitrogen content (%)
Microfiltered corn starch hydrolysate
0.05 0.04
Retentate
0.03 0.02
Permeate
0.01
319
Nitrogen
0.7 0.6 0.5 Ash
0.4 0.3 0.2 0.1
0
20 40 60 80 Volume concentration ratio (X)
100
0
Figure 6 Nitrogen content of concentrates and clarified syrup for all volume concentration ratios.
The nitrogen content of the feed was 0·0131% and increased with increase in VCR values (Fig. 6). A statistical model equation was determined to predict nitrogen content in retentate for VCRs (R2=0·98). nitrogen content (%)=0·0005 X+0·0134
(5)
where X is the VCR. The nitrogen content for permeates at all VCRs was not significantly different from each other, averaging 0·0114%. The fat content of the feed was 0·08±0·02% (db) whereas the fat content of the retentate at VCR 100 was 0·46±0·07% (db). The fat content of permeates at all concentration ratios was not significantly different from each other, averaging 0·07±0·06% (db). The ash content of the feed was 0·052±0·002% and increased with increase in VCR values (Fig.
20 40 60 80 Volume concentration ratio (X)
Figure 8 Rejection coefficients during microfiltration of corn starch hydrolysate.
7). A statistical model equation to predict ash content in retentate for VCRs was determined (R2=0.98). ash (%)=0·0521e0·0037 X
(6)
where X is the VCR. The ash content of the permeates was difficult to measure causing scattering of the data (Fig. 7). It averaged 0·042% for all VCRs. The average dextrose content of all retentates and permeates was 28·90±1·01% (w/w), and was not significantly different for all VCR values. Rejection characteristics The rejection characteristics of a membrane are described by the rejection coefficients (s) of the various solute species14: r=1–
0.12
100
CP CR
(7)
Ash content (%)
0.10 0.08
Retentate
0.06 0.04
Permeate
0.02 0
20 40 60 80 Volume concentration ratio (X)
100
Figure 7 Ash content of concentrates and clarified syrup for all volume concentration ratios.
where CP is the solute concentration in the permeate and CR is the solute concentration in the retentate or bulk solution. The rejection coefficient of the nitrogen and the ash increases with increase in the VCR during the clarification of hydrolysate using microfiltration membranes (Fig. 8). The statistical model equations to predict rejection coefficients (r) of nitrogen (R2=0·98) and ash (R2=0·99) for all VCRs were determined. rprotein=0·1551n(X)+0·0858
(8)
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N. Singh and M. Cheryan
rash=0·0025 X+0·1935
(9)
where X is the VCR. The rejection coefficient of fat was 0·14 in the fresh feed (VCR of 1) and 0·84 at VCR of 100.
2.
CONCLUSION
3.
Consistent low color of the clarified syrup and the lack of suspended solids in the permeates at all VCRs demonstrate that microfiltration can be effectively used for clarification of corn starch hydrolysates. The flow behavior of all the retentates and permeates was Newtonian. The dynamic viscosity of the retentate increased linearly with increase in VCR, but the viscosity of all the permeates remained constant. Total solids, nitrogen content, fat content, and ash content of the permeates remained essentially unchanged for all VCRs indicating that a high-quality syrup can be obtained by microfiltration of corn starch hydrolysate up to at least a VCR of 100 (99% yield).
10.
Acknowledgements
11.
The authors acknowledge the help provided by Fola Akanbi in the determination of nitrogen and fat content in all samples.
4. 5.
6. 7. 8.
9.
12. 13.
REFERENCES 1. Howling, D. Glucose syrup: production, properties, and applications. In ‘Starch Hydrolysis Products’, (F.W.
14.
Schenck and R.E. Hebeda, eds), VCH Publishers, New York (1992) pp 277–317. Short, J.L. and Skelton, R. Crossflow microfiltration in the food industry. In ‘Effective Industrial Membranes Processes: Benefits and Opportunities’, (M. K. Turner, ed.), Elsevier Applied Science, New York (1991) pp 181– 189. Lancrenon, X., Theoleyre, M. A. and Kientz, G. Mineral membrane filtration for the corn refining industry. International Sugar Journal 96 (1994) 365–367. Graver Separations. Literature No. S-106. Glasgow, DE (1996). Singh, N. and Cheryan, M. Microfiltration for clarification of corn starch hydrolysates. Cereal Foods World 41 (1997) 21–24. Bernhardt, W.O. Color and turbidity in solutions. Food Technology 23 (1969) 30–31. CRA. Standard Analytical Methods. Method E-16. Corn Refiners Association, Washington, DC (1980). Reilly, P. J. Enzyme degradation of starch. In ‘Starch Conversion Technology’, (G.M.A. Van Beynum and J.A. Roels, eds), Marcel Dekker, New York (1985) pp 101–142. Hebeda, R. E. Corn sweeteners. In ‘Corn: Chemistry and Technology’, (S.A. Watson and P.E. Ramstad, eds), American Association of Cereal Chemists, St Paul, MN (1987) pp 501–534. Blanchard, P. H. Technology of Corn Wet Milling and Associated Processes. Elsevier Science Publishers, New York (1992). AOAC. Official Methods of Analysis of the AOAC, 15th edn. Method 925.45. Association of Official Analytical Chemists, Arlington, VA (1990). CRA. Standard Analytical Methods. Method E-52. Corn Refiners Association, Washington, DC (1980). AACC. Approved Methods of the AACC, 8th edn. Method 08–14. American Association Of Cereal Chemists, St Paul, MN (1983). Cheryan, M. Ultrafiltration and Microfiltration Handbook. Technomic Publishing Co., Lancaster, PA (1998).
Properties and Composition of Concentrates and Syrup Obtained by Microfiltration of Saccharified Corn Starch Hydrolysate N. Singh∗ and M. Cheryan† ∗Department of Agricultural Engineering, University of Illinois, Urbana, IL 61801 and †Department of Food Science and Human Nutrition, University of Illinois, Urbana, IL 61801, U.S.A. Received 3 December 1996
ABSTRACT Flow properties, clarity and composition of concentrates (retentates) and clarified syrup (permeates) obtained by membrane microfiltration of saccharified corn starch hydrolysate were determined. Clarified syrup (permeate) had low color and no suspended solids at all volume concentration ratios (VCRs) up to 100 X. The color and suspended solids in the concentrate increased linearly with increase in volume concentration ratio. Viscosity of the concentrates increased at higher VCR, but the viscosity of permeates remained essentially unchanged. The densities of all retentates and permeates were not significantly different from each other. Dextrose content of retentates and permeates at all VCRs did not change significantly during the microfiltration, averaging 28·90±1·01% w/w. Total solids, nitrogen, fat and ash content of the retentates increased with increase in VCR. In contrast, total solids, nitrogen, fat and ash of the permeates remained the same for all VCRs, averaging 29·44, 0·011, 0·07 and 0·042% respectively. 1998 Academic Press Limited
Keywords: microfiltration, starch hydrolysate, corn syrup.
INTRODUCTION Dextrose syrups are produced by hydrolysis of starch. Starch can be liquified and saccharified using an acid–enzyme or enzyme–enzyme conversion process. The range of products obtained from starch by acid hydrolysis is limited to 30–55 DE (dextrose equivalent)1. Enzyme hydrolysis, on the other hand, can result in dextrose syrups of 95 DE and higher. The liquified starch is saccharified at 60 °C with glucoamylase to about : DE=dextrose equivalents; VCR=volume concentration ratio. Mention of product or trade names does not imply endorsement by University of Illinois. Corresponding author: M. Cheryan, University of Illinois, Agricultural Bioprocess Laboratory, 1302 W Pennsylvania Avenue, Urbana, IL 61801, U.S.A. 0733–5210/98/030315+06 $25.00/0/jc970169
93–96% dextrose (db). This starch hydrolysate is clarified to remove suspended solids and insoluble impurities, which consist of protein and fat (commonly called ‘mud’). Traditionally rotary vacuum filters pre-coated with a filter aid such as diatomaceous earth or carbon are used. The clarified syrup is further refined and processed into various sweeteners. Microfiltration, a membrane separation process, can be used for clarification of hydrolysate more efficiently and economically2–5. The membrane also has a significant impact on the unit operations which follow clarification: ion exchange, decolorization, and enzymatic reactions. Membrane separations can reduce carbon requirements in downstream processing by as much as 60–70% and ionexchange by 20–30%. These reductions, along with the elimination of diatomaceous earth filter aid, result in annual savings of U.S.$2 to $3 million 1998 Academic Press Limited
316
N. Singh and M. Cheryan
Pressure gauge P Valve 2
Permeate
Permeate Membrane module
Membrane module
P
Feed
tank
Valve 1
T
Heat exchanger
Figure 1
Pressure gauge
Thermometer
Centrifugal pump
Schematic of setup used to collect samples of retentates and permeates at various volume concentration ratios.
per year for a 500 gal/min line4. Although it has been claimed that quality of the membraneclarified syrup is superior in terms of turbidity, color and microbiological cleanliness compared to the conventional centrifugation or filtration using filter aid3, there are few data on the composition and properties of membrane-clarified dextrose syrups. This study focused on the degree of clarification, and changes in flow properties and composition of concentrates and syrup obtained by microfiltration of a typical stream of saccharified corn starch hydrolysate.
MATERIALS AND METHODS The saccharified corn starch hydrolysate was obtained from the last stage of saccharification reactor of a mid-west corn processing company. The hydrolysate had a pH of 4·43±0·03 and starch conversion to approximately 95 DE. A laboratoryscale cross-flow microfiltration system was used to clarify the hydrolysate (Fig. 1). The microfiltration system consisted of two ceramic membrane modules, each having a surface area of 0·12 m2 and a nominal pore size of 0·2 lm (Ceramem Corporation, Waltham, MA, U.S.A.). The system
Microfiltered corn starch hydrolysate
HPX-87 column (Bio-Rad, Richmond, VA, U.S.A.) and a refractive index detector. Temperature of the column was 65 °C, and the mobile phase was 0·01 N sulfuric acid with a flow rate of 0·8 mL/min. The elution time for dextrose under these conditions was 6·6 min. Statistical software (SAS Institute, Cary, NC, U.S.A.) was used to analyze the data. One-way analysis of variance (ANOVA) was used to determine the significant differences in the various concentrate and syrup samples. Duncan’s multiple range test was used for multiple comparisons. The probability of a (type I error) was 5% (P<0·05). RESULTS AND DISCUSSION Degree of clarification The feed had a color of 16·63±0·95 whereas the average color of all permeates was 1·01±0·21. The color of the permeates at all volume concentration ratios was not significantly different from each other. The color of the retentates (concentrates) increased linearly with increasing VCR (Fig. 2). A statistical model to correlate color with VCR was determined (R2=0·99): color=15·935 X
(1)
where X is the VCR. The clarified syrup was consistently low in color throughout processing at all VCR values, demonstrating that microfiltration can be effectively used for the clarification of corn starch hydrolysates at high yields.
Color
was operated at a transmembrane pressure of 1·7 bar (25 p.s.i.), a crossflow velocity of 4 m/s, and a temperature of 60 °C. The retentate, containing suspended solids and insoluble impurities, was returned to the feed tank, while permeate (clarified syrup) was continuously removed. Samples of the clarified syrup (permeate) and concentrate (retentate) were taken at volume concentration ratios of 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100. Volume concentration ratio (VCR) is the ratio of the initial feed volume to the volume of retentate (concentrate) remaining at any time during the microfiltration process. VCR of 1 refers to fresh feed. For syrups, the acceptance of the product is contingent on a specified maximum color6. The color of the syrup (permeate) compared to that of concentrate (retentate) is a measure of the degree of clarification. The color of all the permeates and retentates was determined using a spectrophotometer (Spectronic 1001, Bausch and Lomb, Rochester, NY). The color was the difference of absorbances at 450-nm and 600-nm wavelengths per unit cell length, expressed in percentage7. Suspended solids were determined in all samples by centrifugation of 15 mL of sample in graduated glass tubes at 2000 rev/min (640 g) for 60 min and expressed in volume percentage. Since the feed coming out of the saccharification reactor is at a temperature of 60 °C1,8–10, the flow properties (density and viscosity) were determined at 60 °C. Density of all syrups (permeates) and concentrates (retentates) was determined using pre-calibrated pycnometers. Viscosity was measured with a coaxial cylinder sensor system (Rotovisco RV3, Haake, Paramus, NJ, U.S.A.). The rheological measurements were performed up to shear rate of 400/s to check for possible nonNewtonian behavior. Total solids were determined gravimetrically by drying the samples at 60 °C for 24 h, followed by 100 °C for 5 h in a forced-air convection-drying oven11. Nitrogen content was determined using a Kjeldahl nitrogen procedure by block digestion and steam distillation12. Fat content was determined from the samples dried overnight at 100 °C by Soxhlet extraction method. Ash content was determined by charring the samples over a hot plate with the aid of olive oil, and then placing them in muffle furnace at 525 °C for 5 h; the samples were moistened with water after 2 h13. Dextrose content was determined by highperformance liquid chromatography using the
317
1750 1500 1250 1000 750 500 250 0 1.0 0.5 0
Retentate
Permeate 20 40 60 80 Volume concentration ratio (X)
100
Figure 2 Color of concentrates and clarified syrup for all volume concentration ratios (the difference of spectrophotometric absorbances at 450-nm and 600-nm wavelengths per unit cell length, expressed in percentage).
318
N. Singh and M. Cheryan
20
Retentate
Total solids (%)
Suspended solids (%)
25
15 10 5 Permeate
0 0
20 40 60 80 Volume concentration ratio (X)
100
32 31 30 29 28 27 26 25 24 23 22
Retentate Permeate
0
20 40 60 80 Volume concentration ratio (X)
100
Figure 3 Suspended solids of concentrates and clarified syrup for all volume concentration ratios.
Figure 5 Total solids of concentrates and clarified syrup for all volume concentration ratios.
The suspended solids in the retentates increased linearly with increase in VCR (Fig. 3). The concentrate at VCR 100 had suspended solids of 24·56% compared to a feed suspended solids of 0·24%. On the other hand, permeates at all VCR values were free of suspended matter. A statistical model equation was developed to predict suspended solids for all VCRs (R2=0·9999):
determined to predict dynamic viscosity of retentate for all VCRs (R2=0·94).
suspended solids (%)=0·2466 X
(2)
where X is the VCR. Flow properties All retentates and permeates displayed Newtonian flow behavior (i.e. varying shear rates did not change the viscosity resulting in linearity of the flow curve). The dynamic viscosity of the feed was 1·24±0·12 cp and increased with increasing VCR (Fig. 4). A statistical model equation was 2.25 Dynamic viscosity (cp)
2.00 1.50
Permeate
1.00 0.75 0.50 0.25 0
20 40 60 80 Volume concentration ratio (X)
(3)
where X is the VCR. A VCR of 100 implies a yield of clarified syrup of 99%. Since the viscosity of the 100 X retentate was still low (>2 cp), it suggests that VCR of 200 or more with a corresponding yield of 99·5% or higher could be obtained, provided the membrane module and system were properly designed. The dynamic viscosity of permeates at all VCR values averaged 1·35±0·12 cp, and were not significantly different from each other. The viscosity of the permeates was also not significantly different from the viscosity of the feed. The density of retentates (concentrates) and permeates (syrup) at all VCRs were not significantly different from each other, averaging 1·1029 g/cm3. Composition
Retentate
1.75 1.25
dynamic viscosity (cp)=0·0064X+1·3821
100
Figure 4 Dynamic viscosity of concentrates and clarified syrup at 60 °C for all volume concentration ratios.
The corn starch hydrolysate that was obtained from this manufacturer had a total solids of 29·09±0·12% (w/w). The total solids of the permeates at all VCRs were not significantly different. The average total solids for all permeates was 29·44±0·45% (w/w). The total solids of the retentate increased with increasing VCRs (Fig. 5). A statistical model equation to predict total solids from VCRs was determined (R2=0.93). Total solids (w/w)=28·952 X0·0117 where X is the VCR.
(4)
0.07
0.9
0.06
0.8 Rejection coefficient
Nitrogen content (%)
Microfiltered corn starch hydrolysate
0.05 0.04
Retentate
0.03 0.02
Permeate
0.01
319
Nitrogen
0.7 0.6 0.5 Ash
0.4 0.3 0.2 0.1
0
20 40 60 80 Volume concentration ratio (X)
100
0
Figure 6 Nitrogen content of concentrates and clarified syrup for all volume concentration ratios.
The nitrogen content of the feed was 0·0131% and increased with increase in VCR values (Fig. 6). A statistical model equation was determined to predict nitrogen content in retentate for VCRs (R2=0·98). nitrogen content (%)=0·0005 X+0·0134
(5)
where X is the VCR. The nitrogen content for permeates at all VCRs was not significantly different from each other, averaging 0·0114%. The fat content of the feed was 0·08±0·02% (db) whereas the fat content of the retentate at VCR 100 was 0·46±0·07% (db). The fat content of permeates at all concentration ratios was not significantly different from each other, averaging 0·07±0·06% (db). The ash content of the feed was 0·052±0·002% and increased with increase in VCR values (Fig.
20 40 60 80 Volume concentration ratio (X)
Figure 8 Rejection coefficients during microfiltration of corn starch hydrolysate.
7). A statistical model equation to predict ash content in retentate for VCRs was determined (R2=0.98). ash (%)=0·0521e0·0037 X
(6)
where X is the VCR. The ash content of the permeates was difficult to measure causing scattering of the data (Fig. 7). It averaged 0·042% for all VCRs. The average dextrose content of all retentates and permeates was 28·90±1·01% (w/w), and was not significantly different for all VCR values. Rejection characteristics The rejection characteristics of a membrane are described by the rejection coefficients (s) of the various solute species14: r=1–
0.12
100
CP CR
(7)
Ash content (%)
0.10 0.08
Retentate
0.06 0.04
Permeate
0.02 0
20 40 60 80 Volume concentration ratio (X)
100
Figure 7 Ash content of concentrates and clarified syrup for all volume concentration ratios.
where CP is the solute concentration in the permeate and CR is the solute concentration in the retentate or bulk solution. The rejection coefficient of the nitrogen and the ash increases with increase in the VCR during the clarification of hydrolysate using microfiltration membranes (Fig. 8). The statistical model equations to predict rejection coefficients (r) of nitrogen (R2=0·98) and ash (R2=0·99) for all VCRs were determined. rprotein=0·1551n(X)+0·0858
(8)
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N. Singh and M. Cheryan
rash=0·0025 X+0·1935
(9)
where X is the VCR. The rejection coefficient of fat was 0·14 in the fresh feed (VCR of 1) and 0·84 at VCR of 100.
2.
CONCLUSION
3.
Consistent low color of the clarified syrup and the lack of suspended solids in the permeates at all VCRs demonstrate that microfiltration can be effectively used for clarification of corn starch hydrolysates. The flow behavior of all the retentates and permeates was Newtonian. The dynamic viscosity of the retentate increased linearly with increase in VCR, but the viscosity of all the permeates remained constant. Total solids, nitrogen content, fat content, and ash content of the permeates remained essentially unchanged for all VCRs indicating that a high-quality syrup can be obtained by microfiltration of corn starch hydrolysate up to at least a VCR of 100 (99% yield).
10.
Acknowledgements
11.
The authors acknowledge the help provided by Fola Akanbi in the determination of nitrogen and fat content in all samples.
4. 5.
6. 7. 8.
9.
12. 13.
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