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A Rheological Description of Amylose–Amylopectin Mixtures
A Rheological Description of Amylose-Amylopectin Mixtures By J.-L. Doublier and G. Llamas LABORATOIRE DE PHYSICOCHIMIE DES MACROMOL~CULES,INRA, BP 527, 44026 NANTES CGDEX 03, FRANCE
1 Introduction Starch is composed 'of two macromolecular components which differ in their chemical structure --mylose (A) is almost totally linear, whereas amylopectin (AP) is highly branched. Amylose accounts for 18-33% of normal starch, depending upon the botanical origin, and it is known to play a major role in starch functionality. It is remarkable, for instance, that cereal starches behave in a totally different way from potato starch despite the amylose content differing only slightly, namely, 27-28% for the former against 22% for the latter. It is also noteworthy that each starch component behaves in a different way in aqueous solution. Amylopectin alone is easily solubilized in neutral aqueous solution, and it yields essentially stable solutions. Amylose, on the other hand, can be solubilized only under drastic conditions, in alkali (1 M) or at an elevated temperature (>130 "C) at neutral pH. Moreover, opaque elastic amylose gels are produced upon neutralization or cooling. Another peculiarity of these two polysaccharides is their thermodynamic incompatibility in aqueous medium; this leads to phase separation, one phase being enriched with amylose while the other contains mostly amylopectin.' The practical consequence of this phenomenon in starch gelation is composite material f ~ r m a t i o n ? ~ Of particular interest is the range of amylose contents lower than 30%. Starting from 0% (amylopectin alone), it is possible to increase gradually the amylose content so that this latter component plays an increasing role until reaching a value where amylose replaces amylopectin in the continuous phase, the latter component then being located in the dispersed phase. This critical point may be defined as a phase inversion point as in emulsions. The position of this point has been estimated to correspond to an amy1ose:amylopectin (A:AP) ratio of the order of 30:70, on the basis of rigidity measurements in compression experiments.* It is thus clear that a low amylose content determines strongly the properties of amylose plus amylopectin mixtures. The object of
J.-L. Doublier and G. Llamas
139
the present work is to describe more accurately the visco-elastic properties of such mixed systems. The overall aim is to understand the physical basis for the properties of starch.
2 Experimental Materials Potato amylose was obtained from Avebe (The Netherlands) and has been characterized in a previous investigation ( [ q ]= 70 mlg-' in KCl (0.33 M); M, = 390000).4 Amylopectin was prepared from a waxy maize starch. Both crude materials were purified by dispersion in 95% dimethylsulfoxide (DMSO), followed by precipitation using pure ethanol. Amylose was then dried under vacuum, while amylopectin was redispersed in boiled water before being freeze-dried.
Preparation of Mixed Systems Amylose was dispersed in hot water (150 "C) as described previ~usly.~ Amylopectin was solubilized in water at 95 "C while stirring. Weighed amounts of amylose and amylopectin solutions were mixed at 50-60 "C in order to yield a final total concentration of 4 wt% and immediately transferred to the measuring system of the rheometer.
Measurements of Turbidity Development of turbidity was monitored by measuring absorbance variations at 25 "C as a function of time at 600nm, using a UV/visible spectrophotometer .
Rheology Small-amplitude oscillatory shear experiments were performed at 25 "C with a Rheometrics Fluids Spectrometer (RFS 11) using the cone-and-plate geometry (diameter, 5 cm; angle, 0.04 radians). For gel-cure experiments, measurements were performed at 1 rads-' for 8 hours, the strain amplitude being fixed at 0.05 or lower. The characteristics of the final system were described by the mechanical spectrum plotted between 0.01 and 100 rad s-l at the same strain amplitude.
3 Results and Discussion Figure 1 illustrates the visco-elastic behaviour of amylopectin and amylose at concentrations of 4 wt YO and 1wt YO, respectively. These concentrations have been chosen since they correspond to the concentrations employed in the present work. The amylose content in the mixture varied between
Rheological Description of Amylose-Amylopectin Mixtures
140
1
lo2 3
10 0 10;1
10
-2
1o - 2
lo-’
a
1 oo (rad s ” )
10’
1 o2
Figure 1 Mechanical spectra of a 4 wt YO amylopectin solution (0,G’;0, G”)and a 1 wt % amylose gel ( 0 ,G‘; A , G”).Moduli are plotted against frequency, u)
0.02 wt YO (A:AP = 5 9 5 ) and 1.08 wt YO (A:AP = 27:73) and hence never exceeded 1.1 wt YO (see below). The total polysaccharide concentration was fixed at 4 wt YO. The mechanical spectrum exhibited by amylopectin is characterized by a strong variation in the storage modulus (G’) and in the loss modulus (G”) as a function of frequency, with G” >> G’ at low frequency, and a trend to a crossover of the curves at high frequency (ca. 100 rads-l). Such behaviour is typical of a macromolecular solution, and it indicates that there are no strong interactions between the macromolecules apart from topological entanglements, as has been reported for other non-gelling polysaccharides. Amylose, on the other hand, exhibited, after ageing for 8 hours, the behaviour typical of a gel, with G’ independent of frequency, and G’ XOG”.’ It is to be emphasized that the final value of G’, at 1 rads-’, is 6.3 Pa suggesting that we are quite close to the lowest concentration (C,) where gelation is possible. This result is consistent with a previous investigation on the same sample, where a Co value of 0.9 wt % was found, although experimental conditions for the preparation of gels were different. Figure 2 shows the evolution of G’ and G” as a function of time for an amylose gel at 1wt %. These rheology traces are plotted together with the turbidity evolution. Details of the rheology of amylose gelation have been previously reported. 498 The present figure shows clearly that gelation takes place after a lag period. The gelation time, -77 min in the present example, is defined as usual by the point G ’ ( w ) and G ” ( w ) crossover. 498
J. -L.
141
v
o
1
2
3
4 5 Time (h)
6
7
8
Figure 2 The storage modulus G' and the loss modulus G" as a function of time for a 1.0 wt % amylose gel. Also shown is the evolution of the turbidity
Beyond this time, turbidity and G' increase simultaneously as a result of the phase separation process.499 Figure 3 shows what is obtained for a mixture with a 22:78 A:AP ratio. The shape of the G' trace is comparable to that for amylose in Figure 1
0
1
2
3
4
5
Time (h)
6
7
8
Figure 3 Plots of G' and G" as a finction of time for a 22.:78 amylose-amylopectin gel (total concentration = 4 wt %). Also shown is the evolution of turbidity
142
Rheological Description of Amylose-Amylopectin Mixtures
despite the actual concentration being lower, i.e. 0.88 wt % as against 1.0 wt YO. The only difference is that gelation took place much earlier, 32 miri as against 77 min. In contrast, the turbidity trace is different. The system became turbid as soon as the mixture was prepared (initial absorbance = 0.15, as against 0.06 for 1wt 740 amylose) and turned progressively opaque. However, the final turbidity of the 8-hour old gel was slightly lower than with amylose alone (absorbance = 0.8 for the mixture, compared to 1.2 for amylose). Figure 4 shows the mechanical spectrum of the 22:78 mixture as compared to that of amylose. It is clear that both systems display the characteristics of a gel. However, although G' is of the same order of magnitude in both cases, the G"(w) variations are completely different. This parameter was found to remain almost constant and very low for amylose alone, whereas a steady increase was seen in the case of the mixture, the slope of the curve being of the order of 0.5. From the comparison of Figures 2, 3, and 4, we infer that phase separation occurs as soon as both macromolecular components are mixed. This is ascribed to the thermodynamic incompatibility of amylose and amylopectin, which leads to the formation of two phases, one enriched with amylose, the other with amylopectin. The fact that the system exhibits unambiguously the visco-elastic behaviour of a gel means that it is amylose that is in the continuous phase. We are thus beyond the phase inversion point. However, the frequency dependence of G" suggests that amylopectin also contributes to the visco-elasticity of the system. Final values of G' are of the same order of magnitude as in 1 w t % amylose gel; this means that amylose has been concentrated beyond Co in the continuous phase.
'02
'
1o-2
10-1
1O0
o (rad s e c - ' )
1o1
1o2
Figure 4 Mechanical spectrum of the 22:78 amylose and amylopectin mixutre ( 0 , G'; D, G") in comparison with that of a 1wt % amylose gel (dashed lines; data from Figure 1 )
143
J.-L. Doublier and G . Llamas
A comparison was made of behaviour at A:AP ratios in the range from 5:95 to 27:73. Two examples of the mechanical spectra are shown in Figure 5. At a low ratio (5:95) (not shown), the G ’ ( w ) and G”(w) traces did not differ from those of amylopectin alone. It is only beyond a 10:90 ratio that the presence of amylose is noticeable. The first effect is a dramatic increase in G’ particularly in the low frequency range. The shape of the G ’ ( w ) curve is modified with a flattening towards the low frequency range. This is classically interpreted as a manifestation of an elastic plateau as a result of network formation. In contrast, the G ” ( w ) curve is modified much more progressively, its shape being similar to that illustrated in Figure 4. Similar trends were exhibited at 15% and 17:83 A:AP ratios. It is also to be mentioned that these systems remained fluid when submitted to shearing, their flow behaviour being typical of suspensions with the appearance of a yield stress. Hence, despite these mixed systems remaining fluid-like, they exhibited the visco-elastic behaviour typical of a gel as the A:AP ratio was increased. Regarding the kinetics of gelation, the same effects as illustrated in Figure 2 were seen. The mixed systems became cloudy more rapidly than with amylose alone at an equivalent concentration, and gelation, as estimated by the G’-G” crossover, took place earlier as the A:AP ratio increased. Table 1 illustrates the dramatic effects of the A:AP ratio on gelation time and G’. The former parameter decreases drastically as the ratio increases from 1535 to 27:83, namely from -180min to 5 min; meanwhile G’ increases from 0.5 to 25 Pa. These dependencies could be expressed as a function of the amylose content using power-law equations with exponents of -3.0 and 6.5 for gelation time and G’, respectively.
lo1 loo n
d
-
Y
10-l
b
b 1 o-2
lo-’
1 oo
0 (rad s - ’ )
10’
Figure 5 Mechanical spectra of two amylose and amylopectin (A:AP = 17233, dashed lines; A:AP = 10:90, 0 , G’; A , G”)
1 o2 mixtures
144
Rheological Description of Amylose-Amylopectin Mixtures
Table 1 Storage modulus G' and gelation time as a function of the amy1ose:amylopectin (A:AP) ratio A:AP 100:o 0: 100 595 10:90 15:85 17:83 20230 22:78 25:75 27~73
Arnylose (wt %)
G'l (Pa)
Gelation time (min)
100 0 0.20 0.40 0.60 0.68 0.80 0.88 1 .oo 1.08
6.4 0.104 0.104 0.196 0.50 1.21 2.46 7.74 14.0 25 .O
77
-
179 116 32 32 17 5
* after 8 h ageing (measured at 1 rads-l) Such dependencies are comparable with data reported for amylose This suggests that the effects are merely to be ascribed to the presence of the amylose component. Moreover, the point of phase inversion can be estimated as being close to the A:AP ratio of 15:85, corresponding to an amylose content of 0.60 wt %. It is thus clear that the presence of amylose induces gelation in the mixture at a content that is much lower than Co of amylose alone. These results confirm that the behaviour of amylose and amylopectin mixtures is governed by a phase separation process. Amylopectin, the major component which accounts for more than 73 wt% of the total polysaccharide content, plays a minor role in the rheology of the system. Its presence, however, effects the G"(o)curve, and it induces cloudiness much earlier due to the onset of phase separation. Amylose, on the other hand, plays a major role as soon as the point of phase inversion is reached (A:AP 15%). It forms a continuous phase which entraps amylopectin molecules, yielding a gel, the properties of which strongly depend upon the A:AP ratio. A weak gel is experienced below A:AP = 22:78, whereas quite strong gels are seen beyond this ratio. These results are to be compared with those obtained with starch, as in Figure 6 for tapioca starch (A:AP- 17:83) and maize starch (A:AP 27:73) gels at a comparable concentration (4 wt %). Both samples were prepared by heating a starch suspension at 95 "C under conditions of slow stirring and immediately transferring the sample to the measuring system of the rheometer. The gel was aged at 25 "C for 8 hours, and then a mechanical spectrum was recorded. It is clear that these starch gels do not compare directly to the mixtures. The spectrum of tapioca starch is characterized by a strong dependence of G' and G" upon frequency and by G' G" over the whole frequency range. This is similar to the data for the amylose and amylopectin mixture at a 10:90 ratio. Such a result suggests that only part of amylose in the starch is effective in forming a gel.
-
-
-
J.-L. Doublier and G . Llamas
145
lo3
n
(0
10'
Figure 6 Mechanical spectra of a 4 wt % maize starch gel (0, G'; A , G") and a 4 wt % tapioca starch gel (-, G'; - - G") 9
Probably, the phase separation is not completed as effectively in the starch as in the mixture owing to an incomplete separation of components during the pasting process. The data obtained with maize starch, on the other hand, are typical of a gel. The behaviour differs, however, from that of the 27:73 mixture in having a higher G' value (of the order of 100Pa against 25 Pa in the mixture) and a low value of G" which did not depend upon frequency as in the mixture. A maize starch gel can therefore be described as a composite, its continuous phase being enriched with amylose.11-13The visco-elastic behaviour of the 27:73 mixed system also reflects the properties of a composite. However, though it appears evident that phase separation does occur in both cases, the redistribution of the components seems to take place in different ways, and to depend upon the preparation procedure.
4 Conclusion We confirm that the properties of starch systems strongly depend upon the amylose:amylopectin ratio. These properties are mainly governed by a phase separation process due to the thermodynamic incompatibility of amylose and amylopectin. On mixing, a phase separation takes place which results in a composite system. The critical range in terms of A:AP ratio is between 15:85 and 22:78, and the point of phase inversion is around 1585. Below 1985, the continuous phase contains predominantly amylopectin playing the major role. Beyond a 22:78 ratio, the amylose is concentrated
146
Rheological Description of Amylose-Amylopectin Mhtures
enough in the continuous phase to form a strong gel. Of particular interest is the intermediate range, where the properties are far removed from those of either amylopectin or amylose. This corresponds, in fact, to the A:AP ratios of tapioca and potato starches. The preaent findings may thus provide a basis for understanding and controlling the rheology of these starches.
References 1. M. T. Kalichevsky and S. G. Ring, Carbohydr. Res., 1987, 162,323. 2. V. M. Leloup, P. Colonna, and A. BulCon, J. Cereal Sci., 1991, 13, 1. 3. M. L. German, A. L. Blumenfeld, Ya. V. Guenin, V. P. Yuryev, and V. B. Tolstoguzov, Carbohydr. Polym., 1992, 18, 27. 4. J.-L. Doublier and L. Choplin, Carbohydr. Res., 1989, 193,215. 5. J.-L. Doublier, I. CotC, G. Llamas, and G. Charlet, Progr. Colloid Polym. Sci., 1992, in press. 6. G. Robinson and S. B. Ross-Murphy, Carbohydr. Res., 1982, 107, 17. 7. A. H. Clark and S. B. Ross-Murphy, Adv. Polym. Sci., 1987, 83,57. 8. J.-L. Doublier, G. Llamas, and L. Choplin, Makromol. Chem. Macromol. Symp., 1990, 39, 171. 9. M. J. Miles, V. J. Morris, and S. G. Ring, Carbohydr. Res., 1985, 135,257. 10. H. S. Ellis and S. G. Ring, Carbohydr. Polym., 1985, 5 , 201. 11. M. J. Miles, V. J. Moms, P. D. Orford, and S. G. Ring, Carbohydr. Res., 1985, 135,271. 12. S . G. Ring, Starch, 1985, 37, 80. 13. J.-L. Doublier, G. Llamas, and G. Le Meur, Carbohydr. Polym., 1987, 7 , 251.
1 Introduction Starch is composed 'of two macromolecular components which differ in their chemical structure --mylose (A) is almost totally linear, whereas amylopectin (AP) is highly branched. Amylose accounts for 18-33% of normal starch, depending upon the botanical origin, and it is known to play a major role in starch functionality. It is remarkable, for instance, that cereal starches behave in a totally different way from potato starch despite the amylose content differing only slightly, namely, 27-28% for the former against 22% for the latter. It is also noteworthy that each starch component behaves in a different way in aqueous solution. Amylopectin alone is easily solubilized in neutral aqueous solution, and it yields essentially stable solutions. Amylose, on the other hand, can be solubilized only under drastic conditions, in alkali (1 M) or at an elevated temperature (>130 "C) at neutral pH. Moreover, opaque elastic amylose gels are produced upon neutralization or cooling. Another peculiarity of these two polysaccharides is their thermodynamic incompatibility in aqueous medium; this leads to phase separation, one phase being enriched with amylose while the other contains mostly amylopectin.' The practical consequence of this phenomenon in starch gelation is composite material f ~ r m a t i o n ? ~ Of particular interest is the range of amylose contents lower than 30%. Starting from 0% (amylopectin alone), it is possible to increase gradually the amylose content so that this latter component plays an increasing role until reaching a value where amylose replaces amylopectin in the continuous phase, the latter component then being located in the dispersed phase. This critical point may be defined as a phase inversion point as in emulsions. The position of this point has been estimated to correspond to an amy1ose:amylopectin (A:AP) ratio of the order of 30:70, on the basis of rigidity measurements in compression experiments.* It is thus clear that a low amylose content determines strongly the properties of amylose plus amylopectin mixtures. The object of
J.-L. Doublier and G. Llamas
139
the present work is to describe more accurately the visco-elastic properties of such mixed systems. The overall aim is to understand the physical basis for the properties of starch.
2 Experimental Materials Potato amylose was obtained from Avebe (The Netherlands) and has been characterized in a previous investigation ( [ q ]= 70 mlg-' in KCl (0.33 M); M, = 390000).4 Amylopectin was prepared from a waxy maize starch. Both crude materials were purified by dispersion in 95% dimethylsulfoxide (DMSO), followed by precipitation using pure ethanol. Amylose was then dried under vacuum, while amylopectin was redispersed in boiled water before being freeze-dried.
Preparation of Mixed Systems Amylose was dispersed in hot water (150 "C) as described previ~usly.~ Amylopectin was solubilized in water at 95 "C while stirring. Weighed amounts of amylose and amylopectin solutions were mixed at 50-60 "C in order to yield a final total concentration of 4 wt% and immediately transferred to the measuring system of the rheometer.
Measurements of Turbidity Development of turbidity was monitored by measuring absorbance variations at 25 "C as a function of time at 600nm, using a UV/visible spectrophotometer .
Rheology Small-amplitude oscillatory shear experiments were performed at 25 "C with a Rheometrics Fluids Spectrometer (RFS 11) using the cone-and-plate geometry (diameter, 5 cm; angle, 0.04 radians). For gel-cure experiments, measurements were performed at 1 rads-' for 8 hours, the strain amplitude being fixed at 0.05 or lower. The characteristics of the final system were described by the mechanical spectrum plotted between 0.01 and 100 rad s-l at the same strain amplitude.
3 Results and Discussion Figure 1 illustrates the visco-elastic behaviour of amylopectin and amylose at concentrations of 4 wt YO and 1wt YO, respectively. These concentrations have been chosen since they correspond to the concentrations employed in the present work. The amylose content in the mixture varied between
Rheological Description of Amylose-Amylopectin Mixtures
140
1
lo2 3
10 0 10;1
10
-2
1o - 2
lo-’
a
1 oo (rad s ” )
10’
1 o2
Figure 1 Mechanical spectra of a 4 wt YO amylopectin solution (0,G’;0, G”)and a 1 wt % amylose gel ( 0 ,G‘; A , G”).Moduli are plotted against frequency, u)
0.02 wt YO (A:AP = 5 9 5 ) and 1.08 wt YO (A:AP = 27:73) and hence never exceeded 1.1 wt YO (see below). The total polysaccharide concentration was fixed at 4 wt YO. The mechanical spectrum exhibited by amylopectin is characterized by a strong variation in the storage modulus (G’) and in the loss modulus (G”) as a function of frequency, with G” >> G’ at low frequency, and a trend to a crossover of the curves at high frequency (ca. 100 rads-l). Such behaviour is typical of a macromolecular solution, and it indicates that there are no strong interactions between the macromolecules apart from topological entanglements, as has been reported for other non-gelling polysaccharides. Amylose, on the other hand, exhibited, after ageing for 8 hours, the behaviour typical of a gel, with G’ independent of frequency, and G’ XOG”.’ It is to be emphasized that the final value of G’, at 1 rads-’, is 6.3 Pa suggesting that we are quite close to the lowest concentration (C,) where gelation is possible. This result is consistent with a previous investigation on the same sample, where a Co value of 0.9 wt % was found, although experimental conditions for the preparation of gels were different. Figure 2 shows the evolution of G’ and G” as a function of time for an amylose gel at 1wt %. These rheology traces are plotted together with the turbidity evolution. Details of the rheology of amylose gelation have been previously reported. 498 The present figure shows clearly that gelation takes place after a lag period. The gelation time, -77 min in the present example, is defined as usual by the point G ’ ( w ) and G ” ( w ) crossover. 498
J. -L.
141
v
o
1
2
3
4 5 Time (h)
6
7
8
Figure 2 The storage modulus G' and the loss modulus G" as a function of time for a 1.0 wt % amylose gel. Also shown is the evolution of the turbidity
Beyond this time, turbidity and G' increase simultaneously as a result of the phase separation process.499 Figure 3 shows what is obtained for a mixture with a 22:78 A:AP ratio. The shape of the G' trace is comparable to that for amylose in Figure 1
0
1
2
3
4
5
Time (h)
6
7
8
Figure 3 Plots of G' and G" as a finction of time for a 22.:78 amylose-amylopectin gel (total concentration = 4 wt %). Also shown is the evolution of turbidity
142
Rheological Description of Amylose-Amylopectin Mixtures
despite the actual concentration being lower, i.e. 0.88 wt % as against 1.0 wt YO. The only difference is that gelation took place much earlier, 32 miri as against 77 min. In contrast, the turbidity trace is different. The system became turbid as soon as the mixture was prepared (initial absorbance = 0.15, as against 0.06 for 1wt 740 amylose) and turned progressively opaque. However, the final turbidity of the 8-hour old gel was slightly lower than with amylose alone (absorbance = 0.8 for the mixture, compared to 1.2 for amylose). Figure 4 shows the mechanical spectrum of the 22:78 mixture as compared to that of amylose. It is clear that both systems display the characteristics of a gel. However, although G' is of the same order of magnitude in both cases, the G"(w) variations are completely different. This parameter was found to remain almost constant and very low for amylose alone, whereas a steady increase was seen in the case of the mixture, the slope of the curve being of the order of 0.5. From the comparison of Figures 2, 3, and 4, we infer that phase separation occurs as soon as both macromolecular components are mixed. This is ascribed to the thermodynamic incompatibility of amylose and amylopectin, which leads to the formation of two phases, one enriched with amylose, the other with amylopectin. The fact that the system exhibits unambiguously the visco-elastic behaviour of a gel means that it is amylose that is in the continuous phase. We are thus beyond the phase inversion point. However, the frequency dependence of G" suggests that amylopectin also contributes to the visco-elasticity of the system. Final values of G' are of the same order of magnitude as in 1 w t % amylose gel; this means that amylose has been concentrated beyond Co in the continuous phase.
'02
'
1o-2
10-1
1O0
o (rad s e c - ' )
1o1
1o2
Figure 4 Mechanical spectrum of the 22:78 amylose and amylopectin mixutre ( 0 , G'; D, G") in comparison with that of a 1wt % amylose gel (dashed lines; data from Figure 1 )
143
J.-L. Doublier and G . Llamas
A comparison was made of behaviour at A:AP ratios in the range from 5:95 to 27:73. Two examples of the mechanical spectra are shown in Figure 5. At a low ratio (5:95) (not shown), the G ’ ( w ) and G”(w) traces did not differ from those of amylopectin alone. It is only beyond a 10:90 ratio that the presence of amylose is noticeable. The first effect is a dramatic increase in G’ particularly in the low frequency range. The shape of the G ’ ( w ) curve is modified with a flattening towards the low frequency range. This is classically interpreted as a manifestation of an elastic plateau as a result of network formation. In contrast, the G ” ( w ) curve is modified much more progressively, its shape being similar to that illustrated in Figure 4. Similar trends were exhibited at 15% and 17:83 A:AP ratios. It is also to be mentioned that these systems remained fluid when submitted to shearing, their flow behaviour being typical of suspensions with the appearance of a yield stress. Hence, despite these mixed systems remaining fluid-like, they exhibited the visco-elastic behaviour typical of a gel as the A:AP ratio was increased. Regarding the kinetics of gelation, the same effects as illustrated in Figure 2 were seen. The mixed systems became cloudy more rapidly than with amylose alone at an equivalent concentration, and gelation, as estimated by the G’-G” crossover, took place earlier as the A:AP ratio increased. Table 1 illustrates the dramatic effects of the A:AP ratio on gelation time and G’. The former parameter decreases drastically as the ratio increases from 1535 to 27:83, namely from -180min to 5 min; meanwhile G’ increases from 0.5 to 25 Pa. These dependencies could be expressed as a function of the amylose content using power-law equations with exponents of -3.0 and 6.5 for gelation time and G’, respectively.
lo1 loo n
d
-
Y
10-l
b
b 1 o-2
lo-’
1 oo
0 (rad s - ’ )
10’
Figure 5 Mechanical spectra of two amylose and amylopectin (A:AP = 17233, dashed lines; A:AP = 10:90, 0 , G’; A , G”)
1 o2 mixtures
144
Rheological Description of Amylose-Amylopectin Mixtures
Table 1 Storage modulus G' and gelation time as a function of the amy1ose:amylopectin (A:AP) ratio A:AP 100:o 0: 100 595 10:90 15:85 17:83 20230 22:78 25:75 27~73
Arnylose (wt %)
G'l (Pa)
Gelation time (min)
100 0 0.20 0.40 0.60 0.68 0.80 0.88 1 .oo 1.08
6.4 0.104 0.104 0.196 0.50 1.21 2.46 7.74 14.0 25 .O
77
-
179 116 32 32 17 5
* after 8 h ageing (measured at 1 rads-l) Such dependencies are comparable with data reported for amylose This suggests that the effects are merely to be ascribed to the presence of the amylose component. Moreover, the point of phase inversion can be estimated as being close to the A:AP ratio of 15:85, corresponding to an amylose content of 0.60 wt %. It is thus clear that the presence of amylose induces gelation in the mixture at a content that is much lower than Co of amylose alone. These results confirm that the behaviour of amylose and amylopectin mixtures is governed by a phase separation process. Amylopectin, the major component which accounts for more than 73 wt% of the total polysaccharide content, plays a minor role in the rheology of the system. Its presence, however, effects the G"(o)curve, and it induces cloudiness much earlier due to the onset of phase separation. Amylose, on the other hand, plays a major role as soon as the point of phase inversion is reached (A:AP 15%). It forms a continuous phase which entraps amylopectin molecules, yielding a gel, the properties of which strongly depend upon the A:AP ratio. A weak gel is experienced below A:AP = 22:78, whereas quite strong gels are seen beyond this ratio. These results are to be compared with those obtained with starch, as in Figure 6 for tapioca starch (A:AP- 17:83) and maize starch (A:AP 27:73) gels at a comparable concentration (4 wt %). Both samples were prepared by heating a starch suspension at 95 "C under conditions of slow stirring and immediately transferring the sample to the measuring system of the rheometer. The gel was aged at 25 "C for 8 hours, and then a mechanical spectrum was recorded. It is clear that these starch gels do not compare directly to the mixtures. The spectrum of tapioca starch is characterized by a strong dependence of G' and G" upon frequency and by G' G" over the whole frequency range. This is similar to the data for the amylose and amylopectin mixture at a 10:90 ratio. Such a result suggests that only part of amylose in the starch is effective in forming a gel.
-
-
-
J.-L. Doublier and G . Llamas
145
lo3
n
(0
10'
Figure 6 Mechanical spectra of a 4 wt % maize starch gel (0, G'; A , G") and a 4 wt % tapioca starch gel (-, G'; - - G") 9
Probably, the phase separation is not completed as effectively in the starch as in the mixture owing to an incomplete separation of components during the pasting process. The data obtained with maize starch, on the other hand, are typical of a gel. The behaviour differs, however, from that of the 27:73 mixture in having a higher G' value (of the order of 100Pa against 25 Pa in the mixture) and a low value of G" which did not depend upon frequency as in the mixture. A maize starch gel can therefore be described as a composite, its continuous phase being enriched with amylose.11-13The visco-elastic behaviour of the 27:73 mixed system also reflects the properties of a composite. However, though it appears evident that phase separation does occur in both cases, the redistribution of the components seems to take place in different ways, and to depend upon the preparation procedure.
4 Conclusion We confirm that the properties of starch systems strongly depend upon the amylose:amylopectin ratio. These properties are mainly governed by a phase separation process due to the thermodynamic incompatibility of amylose and amylopectin. On mixing, a phase separation takes place which results in a composite system. The critical range in terms of A:AP ratio is between 15:85 and 22:78, and the point of phase inversion is around 1585. Below 1985, the continuous phase contains predominantly amylopectin playing the major role. Beyond a 22:78 ratio, the amylose is concentrated
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Rheological Description of Amylose-Amylopectin Mhtures
enough in the continuous phase to form a strong gel. Of particular interest is the intermediate range, where the properties are far removed from those of either amylopectin or amylose. This corresponds, in fact, to the A:AP ratios of tapioca and potato starches. The preaent findings may thus provide a basis for understanding and controlling the rheology of these starches.
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