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The interactions between hydroxypropylcellulose and starch during gelatinization
Food Hydrocolloids Vo],7 no.3 pp.181-193, 1993
The interactions between hydroxypropylcellulose and starch during gelatinization R.E.Cameron, e.M.Sansom and A.M.Donald Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 ORE, UK Abstract. The behaviour of a system of starch, water and hydroxypropylcellulose (HPC) has been studied using optical microscopy, differential scanning calorimetry (DSC) and ultra violet spectrophotometry. Although no discernible difference in the gelatinization behaviour of the starch was found when HPC was added, there was a noticeable change in the precipitation of HPC in the presence of starch granules. The results suggest that, during heating, the HPC molecules preferentially adsorb on the starch granules thereby reducing the effective concentration of the solution. The consequences of this interaction for processing starchy foods with HPC are discussed.
Introduction
Starch provides a high proportion of the world's food energy intake. It is laid down in plants in the form of insoluble granules which have a diameter of -20 urn, These granules are composed of two polymers of glucose units: amylose, which is an essentially linear polymer and amylopectin, which has a highly branched structure. Wheat starch is -30% (w/w) amylose (1). Starch granules are birefringent, showing a characteristic Maltese cross pattern under the polarizing microscope. This is due to a high degree of molecular order within each granule. When starch in excess water is heated to within a certain temperature range, a process known as gelatinization occurs. Water is absorbed into the granules which swell. The structural order is disrupted, the birefringence is lost and amylose leaches from the granules. The exact temperature at which these phenomena occur depends on the heating rate, but for wheat starch gelatinization begins at -51°e. When starch is processed, various components may be added as processing aids. In the case of extruded products, cellulose derivatives have been found to be effective in giving increased output with lower energy costs (2). They have also been shown to alter the breaking stress and water absorption (3). Hydroxypropylcellulose (HPC) is one specific cellulose derivative which is frequently used as a food additive in starch products for both moulding and extrusion. HPC is also added to various manufactured food products to reduce sticking of the product during cooking. HPC is a non-ionic, water soluble cellulose ether, which is physiologically inert, thermoplastic and highly surface active (4). It is produced in several grades of differing molecular weights. HPC is soluble in water below 38°C giving a clear smooth solution but it is insoluble -45°C, precipitating between 40°C and 45°C (5). Its phase diagram in water is published in the literature (6). As indicated above, throughput during extrusion has been shown to increase 181
R.E.Cameron, C.M.Sansom and A.M.Donald
following addition of HPC, and high molecular weight HPC seems to reduce the expansion ratio (7). In contrast, low molecular weight HPC (100 000) has been shown to increase the compressibility and elasticity of the extruding material while increasing the expansion ratio (8). To understand all these effects it is important to understand the interplay between HPC and starch as the starch is gelatinized. Does the HPC affect the extent or mode of gelatinization of the starch? Does the presence of starch granules affect the behaviour of the HPC? How do these effects change the viscosity of the system? The research in this paper explores these questions. Earlier work aimed at investigating HPC-starch interactions focused on changes in viscosity. Figure 1 shows the results of experiments performed with a capillary rheometer at the University of East Anglia and the Institute of Food Research at Norwich (8). Wheat starch samples of 46% (w/w) starch in water were pregelatinized in the barrel for 10 min at 100°C or 15 min at 115°C. After this gelatinization treatment, the piston was set at various speeds, and the viscosity calculated on the basis of the pressure reading after stabilization. Figure 1 shows the viscosities obtained at a shear rate of 4000/s for various levels of HPC addition. Similar trends were seen at the other shear rates. The results suggest that at an HPClstarch ratio somewhere between 0.012 and 0.059 (w/w) there is a minimum in the plot of viscosity against this ratio, The research reported here extends this work, looking at other manifestations of possible starch-HPC interactions, specifically the effect of HPC on starch gelatinization and of the effect of starch on the precipitation of HPC. The effect
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182
Starch-hydroxypropylcellulose interactions
of HPC on starch could have various origins. The HPC might be affecting any of granule swelling , amylose leaching, crystallinity and birefringence loss, thermal properties or granule surface properties-this last was suggested as being the key parameter in reference 1. This paper explores these various alternatives . Materials and methods
Materials
HPC was supplied by Hercules Inc . under the trade name Klucel L and had a quoted molecular weight of 100000. A stock solution of 2.5% (w/w) HPC in distilled water was prepared using the following method . A measured quantity of distilled water was equilibrated at 55°C in a water bath. A known quantity of the HPC powder was added slowly and stirred continuously for -15 min. At this temperature the HPC does not dissolve in water, the purpose of this stage being to separate the particles of HPC without any dissolution taking place. The flask was then removed from the water bath and distilled water at room temperature added, making the solution up to about 2.5%. Wheat starch, a gift from the Institute of Food Research , Norwich, was used throughout. The sample conditions required by the different experimental techniques meant that different ratios of starch:water were necessary for different experiments. However, care was taken to ensure that the samples were in the excess water regime for gelatinization for all experiments. The highest concentration of starch in water used in any of the experiments was 20% (w/w), well within this regime (9) . The phase behaviour of HPC is unchanged by concentration in the HPC/water system at the concentrations of HPC in water used here. Optical microscopy Observation ofgranule swelling. Starch slurries of 0.3 % (w/w) starch in water , or starch in an HPC solution of a known concentration were placed on a dimpled microscope slide and covered with a glass slip. The temperature of the sample was controlled by a Linkam TH600 hot-stage. The sample was observed with a Carl Zeiss Jenapol optical microscope. Series of photographs were taken under the required conditions and printed onto 5" x 7" photographic paper. Measurements could then be taken from the prints using a ruler. In each photograph, 10 granules of representative size and shape were chosen. The longest dimension of each granule was measured. This dimension was compared with that of the same granule before heating and converted into a percentage increase in the longest dimension. The average percentage increase in the size of the longest dimension and the statistical variation in this was calculated . Observation of granule birefringence. Starch slurries in water or HPC solution with 3-6% (w/w) starch were viewed through crossed polars. The temperature of the sample was controlled as above. Constant exposure photographs were
183
R.E.Cameron, C.M.Sansom and A.M.Donald
taken as gelatinization progressed and the birefringence disappeared. The negatives were then image analysed using an Oxford Framestore Applications Ltd image analyser. This instrument divided a chosen section of the photograph into ~40 000 pixels, allocated each a grey level depending on its darkness, and plotted a histogram showing the number of pixels in each grey level. Care was taken to correct for non-linearities in the response of the TV camera on the image analyser. Thus, by studying the integrated area of the spectrum corresponding to the birefringence of the granules (i.e. by choosing only a suitable range of grey levels), the course of the loss of birefringence could be followed.
Observation of HPC precipitation. A known quantity of starch was added to a solution of HPC in water of a particular concentration and a slide prepared. The remaining solution was reweighed and starch added to make a new concentration of starch. Repeating this process yielded a series of samples with constant HPC/water ratios but varying starch concentrations. The microscope was used in transmitted light mode without the use of crossed polars. The samples were heated at 20°C/min to 51°C and held at this temperature for ~6 min. They were then allowed to cool. Constant exposure photographs were taken before treatment, on reaching 51°C, 6 min after reaching 51°C, at -45°C (when any precipitate was disappearing) and after treatment. These photographs were then image analysed, and a histogram of grey levels plotted. The swelling of the granules does not affect the histograms since histograms before and after heating are the same. Precipitation of HPC tended to make the overall image darker on the constant exposure photograph since it blocked out light. The amount of precipate present could therefore be followed using the histograms. At some of the HPC concentrations used, the HPC precipitate observable by eye was too faint to be picked up by the image analysis of the photographs. Observation of amylose leaching by UV spectrophotometry The experimental technique used was based on the 'Blue Value Method' of Radley (10). A stock iodine solution was prepared with 50 mg iodine and 500 mg potassium iodide dissolved in 25 ml water. A chosen weight of HPC solution was added to 20 mg starch in a volumetric flask and the solution made up to 25 ml with distilled water. The flask was then sealed, placed in a water bath at 85°C for 30 min and shaken every few minutes by hand. It was then transferred to a water bath at 25°C for 20 min. Thereafter the solution was poured into a centrifuge tube and spun in a preparative centrifuge for 5 min to separate the starch granules from the leached amylose. The supernatant liquor was carefully removed using a Pasteur pipette. A test solution containing 2 ml supernatant liquor solution, 0.2 ml iodine solution and 20 ml water was prepared and used to fill a 1 em width quartz cuvette. This was placed in a Perkin Elmer Lambda 9 UVNIS/NIR spectrophotometer and run against a reference of water between 300 and 800 nm. 184
Starch-hydroxypropylcellulose interactions
Differential scanning calorimetry
About 20 mg of starch slurry in HPC solution was placed in aluminium sample pans and run in a Perkin Elmer DSC7 at lOoC/min from 32 to 85°C. The enthalpy of the gelatinization transition was calculated using the standard programs on the DSC7 processor and the onset and peak temperatures recorded. 'Several samples were run for the calculation of each experimental point. Results The effect of HPC on the gelatinization of starch Swelling. Figure 2 shows the swelling of the longest dimension of wheat starch at 51 and 53°C. The percentage of starch in water (w/w) is 0.07%. Values of HPC/starch of 0 (i.e. no HPC) and 0.25 are shown. The error bars in the diagram represent the width of the distribution of sizes of the granules rather than an experimental uncertainty. It can be seen that the centre of each distribution and its width is not changed by HPC addition. No effect on the swelling with the addition of HPC is seen. In addition, no effect of HPC addition on swellilng was seen when the granules were held at 55°C or when the granules were heated from 45 to 75°C at 2°C/min.
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185
R.E.Cameron, C.M.Sansom and A.M.Donald
Birefringence loss. These experiments were complicated by the fact that for most samples observed the precipitation of HPC during gelatinization partly or totally obscured the birefringence pattern. It was found from precipitation experiments discussed later that for ratios of HPC/starch <0.017 no precipitate is formed. Figure 3 shows the loss in birefringence of samples of 0.03% starch in water with HPC/starch ratios of 0 (i.e. no HPC) and 0.01 held at SlOe. The experimental uncertainty is rather high but no difference is detectable between them.
Amylose leaching. For all the samples studied by this technique, the percentage of starch in water was approximately the same (0.08%) and well within the range of excess water. Thus, any effects seen should be solely due to different HPC levels. Ratios of HPClstarch ranging from 0.011 to 7.7 were studied. HPC was found to have no effect on the amount of amylose leached from the starch at these additions levels.
Endothermic transition. DSC experiments were performed on samples in which the percentage of starch in water was -20%, within the excess water regime for DSC experiments on gelatinization. No difference was found in the gelatinization enthalpy or range for samples of HPClstarch of 0, 0.014 and 0.02.
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186
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The effect of starch on the precipitation of HPC
Figures 4-6 are constant exposure photographs of a sample of HPC/starch ratio of 0.12 with a percentage of starch in water of 2.3% (this corresponds to a percentage of HPC in water of 0.3%). The sample is shown at room temperature (Figure 4) , upon reaching 51°C (Figure 5) and after 6 min at 51°C (Figure 6). HPC precipitate is seen as an overall darkening of the constant exposure photograph , a uniformity of colour and an obscuring of the granules. A strong precipitate is seen upon reaching 51°C which decreases with time held at that temperature. This qualitative analysis of the photographs is translated into histograms of grey levels in Figure 7. The swelling of the granules does not affect the histograms since histograms before and after heating are the same. Changes in the histogram are therefore solely due to the presence of precipitate. The effects of precipitate are seen as a sharpening of the distribution (indicating greater uniformity) and a shift to higher (and darker) grey levels. Figures 8 and 9 show the distributions for HPC/starch ratio of 0.05 (percentage of starch in water 6%) and 0.013 (percentage of starch in water 19%). At the ratio of .HPC/starch of 0.013 the precipitate is no longer seen. Further experiments suggest that the critical ratio of HPC/starch for samples reaching 51°C lies between 0.014 and 0.020. At HPC/starch composition of 0.020 precipitate is seen upon reaching 51°C, while none is seen for 0.014. The experiment was repeated with HPC solutions of 0.4 , 0.7 and 1% and the critical ratio was found to lie between the same bounds for each. Samples with HPC/starch ratios of 0.14 and 0.20 were examined for 0.4,0.7 and 1% HPC solutions. For each HPC solution at an HPC/starch ratio of 0.020 a precipitate is seen, but none is seen at a ratio of 0.014. Discussion
The results presented here demonstrate that most of the parameters describing starch gelatinization are unchanged by the addition of HPC. However, the alteration in the precipitation behaviour of HPC in the presence of the starch granules, and the occurrence of a critical HPC/starch ratio indicate that there is a specific interplay between the two components. One explanation of these results is that the HPC molecules coat the surface of the granules in preference to forming a precipitate. When the amount of HPC present is less than in a critical ratio of HPC/starch, all the HPC coats the granules and there is therefore no precipitate-the solutions remain clear at temperatures when neat HPC solutions would turn cloudy. At HPC levels greater than this critical ratio there is insufficient granule surface area for all the HPC to coat the granules and free HPC remains in the surrounding solution. This material then forms a precipitate upon heating above 45°C. When held at 51°C the granules swell, creating more granule surface area. Thus, more HPC is able to form a granule coating and the level of precipitate drops. This is obviously a kinetic effect, since it requires the diffusion of HPC to the granule surface, and it takes a finite time for the level of precipitate to drop. The critical ratio will depend on the temperature not only since more granule surface area is created as the granules swell during 187
R.E.Cameron, C.M.Sansom and A.M.Donald
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Fig. 5. The same sample as Figure 4 upon reaching 51°C.
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gelatinization, but also because the solubility limit of the HPC in water may change. The presence of a critical ratio of HPC and starch suggests that there is a limit to the thickness of the coating. One possibility is that a single molecule layer of 190
Starch-hydroxypropylcellulose interactions
HPC is formed at the surface of the starch granule . Simple calculations may be performed to assess the feasibility of this theory. If the granules are assumed to be spherical, the total granule area available is given by total granule area = 47rR 2 n
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where m s is the mass of air dry starch , R; is the radius of an air dry granule and Ps is the density of air dry starch . During gelatinization the number of granules remains constant. The number of molecules of HPC is given by no. HPC molecules
= mHNA
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where mH is the mass of HPC, M H is the molecular weight of HPC and N A is Avogadro 's number. Thus the granule area per molecule of HPC , A , may be written (4) In its crystalline form, HPC is known to exist in the form of a three fold extended helix (11,12). The hydrophobic side groups ensure that the chains prefer to lie side by side. If the coating is visualized as a single layer of hexagonally packed HPC molecules with the axis of the helices perpendicular to the granule then the nearest neighbour distance from helix to helix , I, would be given by
(5) Given that Ps = 1.55 g/crrr' (11); M H = 100000; R; = 11.5 urn (from the measurements from photographs); R/R s = 1.03 on reaching 51°C (Figure 2), and the critical ratio for complete coverage of the granules with HPC on reaching 51°C , mH/ms = 0.017 , the value of I is calculated to be 14k Independent X-ray measurements (11) have suggested an average value of 13A for the typical centre to centre distance between chains. The results of these rough calculations are therefore consistent with the theory that the HPC molecules form a monolayer coating around the granules. Although these experiments were performed on dilute suspensions of starch in water, while the viscosity measurements of Pooley (8) were performed on 191
R.E.Cameron, C.M.Sansom and A.M.Donald
concentrated suspensions, it is to be expected that the interactions between HPC and starch are similar in both regimes. The results in this paper tie in with the earlier observation of a minimum viscosity at a particular HPC/starch ratio, and also with the suggestion made in (1) that the origin of the effect lies in the way the HPC coats the starch granule surface. The difference in the actual value of the critical ratio can be attributed to the different gelatinization treatments applied, since the surface area available for the HPC to adsorb on will depend on the temperature of treatment through the extent of swelling. The role of HPC as a food additive to prevent 'sticking' in starch-containing formulations may thus be attributed at least in part to modification of the starch granule surface. Although this study has not studied how an HPC monolayer affects the surface properties, it is to be expected that changes will occur as the chemistry of the surface is altered. The monolayer coverage would seem to lead to changes in adhesion between starch granules and thus also in elasticity. Conclusions
HPC has little or no effect on the gelatinization of starch at the addition levels studied. This suggests that the effects of the addition of HPC on starch extrusion are not due to differences in the degree of gelatinization of starch. The presence of starch granules, however, inhibits the precipitation of HPC. The findings are consistent with the theory that the HPC coats the surface of the starch granules with a critical ratio of HPC/starch for complete coverage on reaching SloC of 0.017. This critical ratio will presumably depend on temperature since the granule surface area increases as the granules swell during gelatinization. Simple calculations show that these results are consistent with a single layer of HPC molecules coating the starch granules. Such a coating would affect the flow properties of the starch resulting in the modification of the extrusion behaviour. Acknowledgements
The authors are grateful to Andrew Smith of the Institute of Food Research, Norwich and to the Agriculture and Food Research Council for financial support. References 1. Galliard.T. and Bowler,P. (1987) In Galliard,T. (ed.), Starch: Properties and Potential. Wiley, Chichester. 2. Glicksman.M, (1984) In Phillips,G.O., Wedlock,D.J. and Williams,P.A. (eds), Gums and Stabilisers for the Food Industry 2. Pergamon, Oxford, pp. 297-320. 3. Maga,J.A. and Fapojovo,O.O. (1988) Int. J. Food Sci. Tech., 23, 49-56. 4. Chang,S.A. and Gray,D.G. (1978) J. Coli. Interf. Sci., 67, 255. 5. Kluce!: Hydroxypropylcellulose. Chemical and Physical Properties. Hercules Inc, Wilmington, Del., 1981. 6. Werbowyj,R.S. and Gray,D.G. (1980) Macromolecules, 13,69-73. 7. Mitchell,J.R., Berrington,D. and Oliver.J. (1986) In Phillips,G.O., Wedlock,D.J. and Williams,P .A. (eds), Gums and Stabilisers for the Food Industry 3. Pergamon, Oxford, pp. 621627.
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Starch-hydroxypropylcellulose interactions
8. Pooley,L. (1986) The Effect of the Food Additive Hydroxypropylcellulose on Starch Based Materials. Dissertation, University of East Anglia. 9. Blanshard,J.M.V. (1987) In Galliard,T. (ed.), Starch: Properties and Potential. Wiley, Chichester, ch. 2. 10. Radley,J.A. (1976) Examination and Analysis of Starch and Starch Products. Applied Science Publishers Ltd. 11. Atkins,E.D.T., Fulton,W.S. and Miles,M.J. (1980) TAPPI Conference Papers. 5th International Dissolving Pulps, pp. 208-213. 12. Atkins,E. (1986) Int. J. Bioi. Macromol., 8, 323-329.
Received on February 24, 1993; accepted on April 19, 1993
193
The interactions between hydroxypropylcellulose and starch during gelatinization R.E.Cameron, e.M.Sansom and A.M.Donald Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 ORE, UK Abstract. The behaviour of a system of starch, water and hydroxypropylcellulose (HPC) has been studied using optical microscopy, differential scanning calorimetry (DSC) and ultra violet spectrophotometry. Although no discernible difference in the gelatinization behaviour of the starch was found when HPC was added, there was a noticeable change in the precipitation of HPC in the presence of starch granules. The results suggest that, during heating, the HPC molecules preferentially adsorb on the starch granules thereby reducing the effective concentration of the solution. The consequences of this interaction for processing starchy foods with HPC are discussed.
Introduction
Starch provides a high proportion of the world's food energy intake. It is laid down in plants in the form of insoluble granules which have a diameter of -20 urn, These granules are composed of two polymers of glucose units: amylose, which is an essentially linear polymer and amylopectin, which has a highly branched structure. Wheat starch is -30% (w/w) amylose (1). Starch granules are birefringent, showing a characteristic Maltese cross pattern under the polarizing microscope. This is due to a high degree of molecular order within each granule. When starch in excess water is heated to within a certain temperature range, a process known as gelatinization occurs. Water is absorbed into the granules which swell. The structural order is disrupted, the birefringence is lost and amylose leaches from the granules. The exact temperature at which these phenomena occur depends on the heating rate, but for wheat starch gelatinization begins at -51°e. When starch is processed, various components may be added as processing aids. In the case of extruded products, cellulose derivatives have been found to be effective in giving increased output with lower energy costs (2). They have also been shown to alter the breaking stress and water absorption (3). Hydroxypropylcellulose (HPC) is one specific cellulose derivative which is frequently used as a food additive in starch products for both moulding and extrusion. HPC is also added to various manufactured food products to reduce sticking of the product during cooking. HPC is a non-ionic, water soluble cellulose ether, which is physiologically inert, thermoplastic and highly surface active (4). It is produced in several grades of differing molecular weights. HPC is soluble in water below 38°C giving a clear smooth solution but it is insoluble -45°C, precipitating between 40°C and 45°C (5). Its phase diagram in water is published in the literature (6). As indicated above, throughput during extrusion has been shown to increase 181
R.E.Cameron, C.M.Sansom and A.M.Donald
following addition of HPC, and high molecular weight HPC seems to reduce the expansion ratio (7). In contrast, low molecular weight HPC (100 000) has been shown to increase the compressibility and elasticity of the extruding material while increasing the expansion ratio (8). To understand all these effects it is important to understand the interplay between HPC and starch as the starch is gelatinized. Does the HPC affect the extent or mode of gelatinization of the starch? Does the presence of starch granules affect the behaviour of the HPC? How do these effects change the viscosity of the system? The research in this paper explores these questions. Earlier work aimed at investigating HPC-starch interactions focused on changes in viscosity. Figure 1 shows the results of experiments performed with a capillary rheometer at the University of East Anglia and the Institute of Food Research at Norwich (8). Wheat starch samples of 46% (w/w) starch in water were pregelatinized in the barrel for 10 min at 100°C or 15 min at 115°C. After this gelatinization treatment, the piston was set at various speeds, and the viscosity calculated on the basis of the pressure reading after stabilization. Figure 1 shows the viscosities obtained at a shear rate of 4000/s for various levels of HPC addition. Similar trends were seen at the other shear rates. The results suggest that at an HPClstarch ratio somewhere between 0.012 and 0.059 (w/w) there is a minimum in the plot of viscosity against this ratio, The research reported here extends this work, looking at other manifestations of possible starch-HPC interactions, specifically the effect of HPC on starch gelatinization and of the effect of starch on the precipitation of HPC. The effect
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HPC / Starch Fig. 1. Viscosity of pregelatinized wheat starch in a capillary rheometer against the ratio HPC/starch (w/w). The shear rate was 4oo0/s. Closed circles show starch gelatinized and extruded at 100°C, open circles at 115°C. (Data from reference 8.)
182
Starch-hydroxypropylcellulose interactions
of HPC on starch could have various origins. The HPC might be affecting any of granule swelling , amylose leaching, crystallinity and birefringence loss, thermal properties or granule surface properties-this last was suggested as being the key parameter in reference 1. This paper explores these various alternatives . Materials and methods
Materials
HPC was supplied by Hercules Inc . under the trade name Klucel L and had a quoted molecular weight of 100000. A stock solution of 2.5% (w/w) HPC in distilled water was prepared using the following method . A measured quantity of distilled water was equilibrated at 55°C in a water bath. A known quantity of the HPC powder was added slowly and stirred continuously for -15 min. At this temperature the HPC does not dissolve in water, the purpose of this stage being to separate the particles of HPC without any dissolution taking place. The flask was then removed from the water bath and distilled water at room temperature added, making the solution up to about 2.5%. Wheat starch, a gift from the Institute of Food Research , Norwich, was used throughout. The sample conditions required by the different experimental techniques meant that different ratios of starch:water were necessary for different experiments. However, care was taken to ensure that the samples were in the excess water regime for gelatinization for all experiments. The highest concentration of starch in water used in any of the experiments was 20% (w/w), well within this regime (9) . The phase behaviour of HPC is unchanged by concentration in the HPC/water system at the concentrations of HPC in water used here. Optical microscopy Observation ofgranule swelling. Starch slurries of 0.3 % (w/w) starch in water , or starch in an HPC solution of a known concentration were placed on a dimpled microscope slide and covered with a glass slip. The temperature of the sample was controlled by a Linkam TH600 hot-stage. The sample was observed with a Carl Zeiss Jenapol optical microscope. Series of photographs were taken under the required conditions and printed onto 5" x 7" photographic paper. Measurements could then be taken from the prints using a ruler. In each photograph, 10 granules of representative size and shape were chosen. The longest dimension of each granule was measured. This dimension was compared with that of the same granule before heating and converted into a percentage increase in the longest dimension. The average percentage increase in the size of the longest dimension and the statistical variation in this was calculated . Observation of granule birefringence. Starch slurries in water or HPC solution with 3-6% (w/w) starch were viewed through crossed polars. The temperature of the sample was controlled as above. Constant exposure photographs were
183
R.E.Cameron, C.M.Sansom and A.M.Donald
taken as gelatinization progressed and the birefringence disappeared. The negatives were then image analysed using an Oxford Framestore Applications Ltd image analyser. This instrument divided a chosen section of the photograph into ~40 000 pixels, allocated each a grey level depending on its darkness, and plotted a histogram showing the number of pixels in each grey level. Care was taken to correct for non-linearities in the response of the TV camera on the image analyser. Thus, by studying the integrated area of the spectrum corresponding to the birefringence of the granules (i.e. by choosing only a suitable range of grey levels), the course of the loss of birefringence could be followed.
Observation of HPC precipitation. A known quantity of starch was added to a solution of HPC in water of a particular concentration and a slide prepared. The remaining solution was reweighed and starch added to make a new concentration of starch. Repeating this process yielded a series of samples with constant HPC/water ratios but varying starch concentrations. The microscope was used in transmitted light mode without the use of crossed polars. The samples were heated at 20°C/min to 51°C and held at this temperature for ~6 min. They were then allowed to cool. Constant exposure photographs were taken before treatment, on reaching 51°C, 6 min after reaching 51°C, at -45°C (when any precipitate was disappearing) and after treatment. These photographs were then image analysed, and a histogram of grey levels plotted. The swelling of the granules does not affect the histograms since histograms before and after heating are the same. Precipitation of HPC tended to make the overall image darker on the constant exposure photograph since it blocked out light. The amount of precipate present could therefore be followed using the histograms. At some of the HPC concentrations used, the HPC precipitate observable by eye was too faint to be picked up by the image analysis of the photographs. Observation of amylose leaching by UV spectrophotometry The experimental technique used was based on the 'Blue Value Method' of Radley (10). A stock iodine solution was prepared with 50 mg iodine and 500 mg potassium iodide dissolved in 25 ml water. A chosen weight of HPC solution was added to 20 mg starch in a volumetric flask and the solution made up to 25 ml with distilled water. The flask was then sealed, placed in a water bath at 85°C for 30 min and shaken every few minutes by hand. It was then transferred to a water bath at 25°C for 20 min. Thereafter the solution was poured into a centrifuge tube and spun in a preparative centrifuge for 5 min to separate the starch granules from the leached amylose. The supernatant liquor was carefully removed using a Pasteur pipette. A test solution containing 2 ml supernatant liquor solution, 0.2 ml iodine solution and 20 ml water was prepared and used to fill a 1 em width quartz cuvette. This was placed in a Perkin Elmer Lambda 9 UVNIS/NIR spectrophotometer and run against a reference of water between 300 and 800 nm. 184
Starch-hydroxypropylcellulose interactions
Differential scanning calorimetry
About 20 mg of starch slurry in HPC solution was placed in aluminium sample pans and run in a Perkin Elmer DSC7 at lOoC/min from 32 to 85°C. The enthalpy of the gelatinization transition was calculated using the standard programs on the DSC7 processor and the onset and peak temperatures recorded. 'Several samples were run for the calculation of each experimental point. Results The effect of HPC on the gelatinization of starch Swelling. Figure 2 shows the swelling of the longest dimension of wheat starch at 51 and 53°C. The percentage of starch in water (w/w) is 0.07%. Values of HPC/starch of 0 (i.e. no HPC) and 0.25 are shown. The error bars in the diagram represent the width of the distribution of sizes of the granules rather than an experimental uncertainty. It can be seen that the centre of each distribution and its width is not changed by HPC addition. No effect on the swelling with the addition of HPC is seen. In addition, no effect of HPC addition on swellilng was seen when the granules were held at 55°C or when the granules were heated from 45 to 75°C at 2°C/min.
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185
R.E.Cameron, C.M.Sansom and A.M.Donald
Birefringence loss. These experiments were complicated by the fact that for most samples observed the precipitation of HPC during gelatinization partly or totally obscured the birefringence pattern. It was found from precipitation experiments discussed later that for ratios of HPC/starch <0.017 no precipitate is formed. Figure 3 shows the loss in birefringence of samples of 0.03% starch in water with HPC/starch ratios of 0 (i.e. no HPC) and 0.01 held at SlOe. The experimental uncertainty is rather high but no difference is detectable between them.
Amylose leaching. For all the samples studied by this technique, the percentage of starch in water was approximately the same (0.08%) and well within the range of excess water. Thus, any effects seen should be solely due to different HPC levels. Ratios of HPClstarch ranging from 0.011 to 7.7 were studied. HPC was found to have no effect on the amount of amylose leached from the starch at these additions levels.
Endothermic transition. DSC experiments were performed on samples in which the percentage of starch in water was -20%, within the excess water regime for DSC experiments on gelatinization. No difference was found in the gelatinization enthalpy or range for samples of HPClstarch of 0, 0.014 and 0.02.
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186
Starch-hydroxypropylcellulose interactions
The effect of starch on the precipitation of HPC
Figures 4-6 are constant exposure photographs of a sample of HPC/starch ratio of 0.12 with a percentage of starch in water of 2.3% (this corresponds to a percentage of HPC in water of 0.3%). The sample is shown at room temperature (Figure 4) , upon reaching 51°C (Figure 5) and after 6 min at 51°C (Figure 6). HPC precipitate is seen as an overall darkening of the constant exposure photograph , a uniformity of colour and an obscuring of the granules. A strong precipitate is seen upon reaching 51°C which decreases with time held at that temperature. This qualitative analysis of the photographs is translated into histograms of grey levels in Figure 7. The swelling of the granules does not affect the histograms since histograms before and after heating are the same. Changes in the histogram are therefore solely due to the presence of precipitate. The effects of precipitate are seen as a sharpening of the distribution (indicating greater uniformity) and a shift to higher (and darker) grey levels. Figures 8 and 9 show the distributions for HPC/starch ratio of 0.05 (percentage of starch in water 6%) and 0.013 (percentage of starch in water 19%). At the ratio of .HPC/starch of 0.013 the precipitate is no longer seen. Further experiments suggest that the critical ratio of HPC/starch for samples reaching 51°C lies between 0.014 and 0.020. At HPC/starch composition of 0.020 precipitate is seen upon reaching 51°C, while none is seen for 0.014. The experiment was repeated with HPC solutions of 0.4 , 0.7 and 1% and the critical ratio was found to lie between the same bounds for each. Samples with HPC/starch ratios of 0.14 and 0.20 were examined for 0.4,0.7 and 1% HPC solutions. For each HPC solution at an HPC/starch ratio of 0.020 a precipitate is seen, but none is seen at a ratio of 0.014. Discussion
The results presented here demonstrate that most of the parameters describing starch gelatinization are unchanged by the addition of HPC. However, the alteration in the precipitation behaviour of HPC in the presence of the starch granules, and the occurrence of a critical HPC/starch ratio indicate that there is a specific interplay between the two components. One explanation of these results is that the HPC molecules coat the surface of the granules in preference to forming a precipitate. When the amount of HPC present is less than in a critical ratio of HPC/starch, all the HPC coats the granules and there is therefore no precipitate-the solutions remain clear at temperatures when neat HPC solutions would turn cloudy. At HPC levels greater than this critical ratio there is insufficient granule surface area for all the HPC to coat the granules and free HPC remains in the surrounding solution. This material then forms a precipitate upon heating above 45°C. When held at 51°C the granules swell, creating more granule surface area. Thus, more HPC is able to form a granule coating and the level of precipitate drops. This is obviously a kinetic effect, since it requires the diffusion of HPC to the granule surface, and it takes a finite time for the level of precipitate to drop. The critical ratio will depend on the temperature not only since more granule surface area is created as the granules swell during 187
R.E.Cameron, C.M.Sansom and A.M.Donald
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Fig. 5. The same sample as Figure 4 upon reaching 51°C.
188
Starch-hydroxypropylcellulose interactions
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Fig. 6. The same sample as Figures 4 and 5 after 6 min at 51°e.
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189
R.E.Cameron, C.M.Sansom and A.M.Donald
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Fig. 8. Histograms of constant exposure photographs of samples of HPC/starch ratio n.DS (w/w) with a percentage of starch in water of 6%. Open circles show the sample at room temperature; closed circles show the sample on reaching SloC and triangles show the sample after 6 min at Sloe.
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gelatinization, but also because the solubility limit of the HPC in water may change. The presence of a critical ratio of HPC and starch suggests that there is a limit to the thickness of the coating. One possibility is that a single molecule layer of 190
Starch-hydroxypropylcellulose interactions
HPC is formed at the surface of the starch granule . Simple calculations may be performed to assess the feasibility of this theory. If the granules are assumed to be spherical, the total granule area available is given by total granule area = 47rR 2 n
(1)
where R is the radius of the granule and n is the number of granules. The number of granules can be written mass of starch n = --------mass of a single granule
(2)
where m s is the mass of air dry starch , R; is the radius of an air dry granule and Ps is the density of air dry starch . During gelatinization the number of granules remains constant. The number of molecules of HPC is given by no. HPC molecules
= mHNA
MH
(3)
where mH is the mass of HPC, M H is the molecular weight of HPC and N A is Avogadro 's number. Thus the granule area per molecule of HPC , A , may be written (4) In its crystalline form, HPC is known to exist in the form of a three fold extended helix (11,12). The hydrophobic side groups ensure that the chains prefer to lie side by side. If the coating is visualized as a single layer of hexagonally packed HPC molecules with the axis of the helices perpendicular to the granule then the nearest neighbour distance from helix to helix , I, would be given by
(5) Given that Ps = 1.55 g/crrr' (11); M H = 100000; R; = 11.5 urn (from the measurements from photographs); R/R s = 1.03 on reaching 51°C (Figure 2), and the critical ratio for complete coverage of the granules with HPC on reaching 51°C , mH/ms = 0.017 , the value of I is calculated to be 14k Independent X-ray measurements (11) have suggested an average value of 13A for the typical centre to centre distance between chains. The results of these rough calculations are therefore consistent with the theory that the HPC molecules form a monolayer coating around the granules. Although these experiments were performed on dilute suspensions of starch in water, while the viscosity measurements of Pooley (8) were performed on 191
R.E.Cameron, C.M.Sansom and A.M.Donald
concentrated suspensions, it is to be expected that the interactions between HPC and starch are similar in both regimes. The results in this paper tie in with the earlier observation of a minimum viscosity at a particular HPC/starch ratio, and also with the suggestion made in (1) that the origin of the effect lies in the way the HPC coats the starch granule surface. The difference in the actual value of the critical ratio can be attributed to the different gelatinization treatments applied, since the surface area available for the HPC to adsorb on will depend on the temperature of treatment through the extent of swelling. The role of HPC as a food additive to prevent 'sticking' in starch-containing formulations may thus be attributed at least in part to modification of the starch granule surface. Although this study has not studied how an HPC monolayer affects the surface properties, it is to be expected that changes will occur as the chemistry of the surface is altered. The monolayer coverage would seem to lead to changes in adhesion between starch granules and thus also in elasticity. Conclusions
HPC has little or no effect on the gelatinization of starch at the addition levels studied. This suggests that the effects of the addition of HPC on starch extrusion are not due to differences in the degree of gelatinization of starch. The presence of starch granules, however, inhibits the precipitation of HPC. The findings are consistent with the theory that the HPC coats the surface of the starch granules with a critical ratio of HPC/starch for complete coverage on reaching SloC of 0.017. This critical ratio will presumably depend on temperature since the granule surface area increases as the granules swell during gelatinization. Simple calculations show that these results are consistent with a single layer of HPC molecules coating the starch granules. Such a coating would affect the flow properties of the starch resulting in the modification of the extrusion behaviour. Acknowledgements
The authors are grateful to Andrew Smith of the Institute of Food Research, Norwich and to the Agriculture and Food Research Council for financial support. References 1. Galliard.T. and Bowler,P. (1987) In Galliard,T. (ed.), Starch: Properties and Potential. Wiley, Chichester. 2. Glicksman.M, (1984) In Phillips,G.O., Wedlock,D.J. and Williams,P.A. (eds), Gums and Stabilisers for the Food Industry 2. Pergamon, Oxford, pp. 297-320. 3. Maga,J.A. and Fapojovo,O.O. (1988) Int. J. Food Sci. Tech., 23, 49-56. 4. Chang,S.A. and Gray,D.G. (1978) J. Coli. Interf. Sci., 67, 255. 5. Kluce!: Hydroxypropylcellulose. Chemical and Physical Properties. Hercules Inc, Wilmington, Del., 1981. 6. Werbowyj,R.S. and Gray,D.G. (1980) Macromolecules, 13,69-73. 7. Mitchell,J.R., Berrington,D. and Oliver.J. (1986) In Phillips,G.O., Wedlock,D.J. and Williams,P .A. (eds), Gums and Stabilisers for the Food Industry 3. Pergamon, Oxford, pp. 621627.
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Starch-hydroxypropylcellulose interactions
8. Pooley,L. (1986) The Effect of the Food Additive Hydroxypropylcellulose on Starch Based Materials. Dissertation, University of East Anglia. 9. Blanshard,J.M.V. (1987) In Galliard,T. (ed.), Starch: Properties and Potential. Wiley, Chichester, ch. 2. 10. Radley,J.A. (1976) Examination and Analysis of Starch and Starch Products. Applied Science Publishers Ltd. 11. Atkins,E.D.T., Fulton,W.S. and Miles,M.J. (1980) TAPPI Conference Papers. 5th International Dissolving Pulps, pp. 208-213. 12. Atkins,E. (1986) Int. J. Bioi. Macromol., 8, 323-329.
Received on February 24, 1993; accepted on April 19, 1993
193