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soy protein isolate model food systems
G. Charalambous (Ed.), Food Flavors: Generation, Analysis and Process Influence © 1995 Elsevier Science B.V. All rights reserved
1155
Changes in microstructure and thermal properties of thermally processed cornstarch/soy protein isolate model food systems F.A. Nyanzi, J.A. Maga and C. Evans Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, Colorado 80523, U.S.A.
Abstract Fluorescent microscopy and differential scanning calorimetry were used to study the influence of thermal processing on the microstructure and thermal properties of cornstarch/soy protein model food systems. Both analytical methods showed that extrusion thermal processing at 50'C affected the physical characteristics of cornstarch/soy protein samples almost to the same extent as conventional heating at 150**C. For example, the enthalpies of transition were 0.8J/g for samples conventionally processed at IBO'C and 0.6J/g for samples extruded at 50'C. The micrographs also showed that the starch granules were not completely disrupted by conventional thermal processing at 150*C but extrusion thermal processing gelatinized the starch granules at 50'C. Overall, the greatest microstructure modifications of starch or protein were clearly shown to be due to extrusion when compared to conventional thermal processing.
1.
INTRODUCTION
Thermal processing of food systems induces changes in microstructure [1] and the thermal properties [2] of food components. Studies of these changes may contribute to the understanding of how food components can influence the functionality of food systems. Electron microscopy has been used by researchers [1,3-6] to study the microstructure of food systems. Other researchers [7-10] have followed changes in thermal properties of thermally processed samples by the use of differential scanning calorimetry. Fluorescent microscopy has received very limited attention in the study of microstructural changes in processed samples. A study by Goossens et al. [11] showed that fluorescent microscopy is a good method for identifying protein and starch in wheat samples. This method has not yet been shown to be applicable to thermally processed samples. Therefore, the purpose of the present study was to use fluorescent microscopy in conjunction with differential scanning calorimetry to determine the changes in microstructure and thermal properties induced by thermal processing method and temperature.
1156 2.
MATERIALS AND METHODS
2.1. Materials The ingredients consisted of commercial cornstarch (Pure-Dent B700: a gift from Grain Processing Corporation, Muscatine, Iowa) and commercial soy protein isolate (Ardex R: a gift from Archer Daniels Midland Company (ADM), Decatur, Illinois). 2.2
Cornstarch preparation for extrusion A 2kg starch sample was mixed with 188 ml of 0.08N sodium hydroxide solution for 10 minutes using a Hobart mixer. Model D-300 (The Hobart Manufacturing Company, Troy, Ohio, USA) set at the lowest speed. The prepared sample had 20% w/w moisture and pH=7. Samples were sealed in plastic bags and left at room temperature for extrusion processing the following day. 2.3
Protein pellet preparation for extrusion Two ml of 0.3N sodium hydroxide was mixed with 2.5 g of soy protein isolate, and a round pellet was manually made. The prepared pellet had a pH of 7. These pellets were stored overnight at room temperature in sealed plastic containers to prevent moisture loss, for extrusion processing the following day. 2.4
Soy protein isolate and/or cornstarch preparation for conventional processing Samples containing lOg of cornstarch or (75:25) cornstarch:soy protein isolate were mixed manually with a glass rod for 1 minute while adding 2 ml of 1.5N sodium hydroxide. Samples were left in sealed containers overnight at room temperature before an aliquot was packed in stainless steel tubes for processing. Stainless tubes 15cm long with an inside diameter of 0.40cm were used in this method. The samples were packed in the tubes and both ends were tightly sealed with metal screw covers. The tubes, with their respective samples, were temperature- and moisture-equilibrated overnight before thermal processing. 2.5
Extrusion processing Extrusion thermal processing was performed in a single screw Brabender Plasticorder Extruder, Model PL-V500 (C.W. Brabender Instruments, Inc., South Hackensack, New Jersey), equipped with a variable D-C drive unit, a tachometer, and a torque meter. The extruder barrel had a diameter of 19.05mm with a 20:1 length:diameter ratio and eight 0.79 X 3.18mm longitudinal grooves. In addition to a tapered screw with a 3:1 compression ratio, a die with a diameter of 4.8mm was used. The temperature of the extruder barrel was controlled by two electrically heated zones. The first heating zone was in the compression section and the second one in the metering section just next to the die. The dough temperature was maintained by compressed-air-cooled barrel collars. The compressed air was controlled by thermostats found inside the barrel wall. The preconditioned starch was fed manually into the feed zone of the extruder while bringing the screw speed to the processing speed of 120 rpm. After a steady state flow, as shown by a torque variation of +2.5 inch-pound, had been maintained and the processing dough temperature just before the die (either 50'C or 150'C) obtained, an experimental protein pellet was introduced in the feed zone. Immediately after a pellet was introduced, the extrudate strand was cut off and discarded. The subsequent exiting extrudate strand, which contained the protein sample, was
1157 cut off and saved. The presence of protein was determined by observing the difference in color of the exiting strand. 2.6
Conventional thermal processing An oil bath at 160'C was used to process the samples. The tubes to be processed, and another tube containing a thermocouple to monitor the temperature, were placed in metal racks, and immersed in the heated oil-bath. When the thermocouple registered the desired processing temperature (either 50'C or 150'C), the processed tubes were removed and immediately immersed into an icecold water-bath to stop the effects of thermal processing. For each set of tubes to be processed, a temperature-control tube containing the same soy protein and/or cornstarch preparation and a thermocouple was used. 2.7
Fluorescent microscopy Fluorescent microscopy was used to determine the microstructure changes in processed cornstarch/soy protein isolate samples. Samples were prepared by embedding 1 cm sections in paraffin blocks. The embedded samples were sectioned into 5 iim slices with a microtome and mounted on a microscope slide. Acridine orange fluorochrome was used in order to differentiate starch from protein structures as per Goossens et al. [11]. A Carl Zeiss fluorescent microscope equipped with 40X objective lens, 47 background filter, BG12 excitation filter, a 200W mercury arc lamp, and a camera body with automatic exposure control, were used to record microstructure of the samples on Ektachrome 100 ASA film. 2.8
Differential scanning calorimetry The method of Kugimiya and Donovan [12], with minor modifications, was employed to analyze thermal characteristics of experimental samples. A 2 to 3 mg sample of thermally processed cornstarch/soy protein isolate was weighed directly into a tared aluminum hermetic pan. A microsyringe was used to deliver lO^L of distilled water into the pan containing the sample and the pan was sealed by a press. The sealed pans were stored overnight at room temperature before thermal analysis. A DuPont 910 cell base and 9900 computer/thermal analyzer were used. The cell was purged with nitrogen gas at a rate of 40 ml per second. The heating rate was 10"C per minute from 20'C to 120'C. Enthalpies of transition ( A H ) , onset temperature (T^), and peak maximum temperature (Tp) were determined by a DuPont computer program General Analysis Utility Version 2.1.
3. RESULTS AND DISCUSSION 3.1
Microstructure determination by fluorescent microscopy Figure 1 shows the microstructure of the starch-protein sample conventionally processed at 50'C. No observable structural changes were detected with the starch granules or the soy proteins. The starch granules and protein particles appear to be intact.
1158
Figure 1 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by conventional method (oil-bath heating) at 50'C, 20%w/w feed moisture, on the protein and starch microstructure of the sample.
1159
Figure 2 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by extrusion method at 50'C, 20% w/w feed moisture, on the protein and starch microstructure of the sample.
In contrast, a micrograph of an extruded sample at 50'C, shows "melted" starch and a possible aggregation of protein particles (Figure 2). Shear and pressure during extrusion thermal processing, regardless of the temperature levels, appear to drastically alter the microstructure of starch granules.
1160
Figure 3 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by conventional method (oil-bath heating) at 150'C, 20% w/w feed moisture, on the protein and starch microstructure.
Increasing thermal processing temperatures to 150'C, for the conventional method, influenced the microstructure for both starch and protein. Figure 3 shows that most of starch granules have been disrupted and "melted" with a few unaffected granules. Protein particles appear to have been unfolded and spread over the "melted" starch mass.
1161
Figure 4 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by extrusion method at 150'C, 20% w/w feed moisture, on the protein and starch microstructure.
Extrusion thermal processing at 150T causes protein unfolding and realignment as seen in Figure 4. Protein fibers are observed in the micrograph. The micrograph shows that the starch granules were completely "melted" and that some of the starch is intermeshed with the protein strands. Starch appears to be adsorbed to the protein fibers.
1162
m
3 Q. O fi-
IT
ro 3 3: ro o O
TQmpQraturQ (°C) Figure 5 Differential scanning calorimetry thermograms of cornstarch/soy protein isolate (75:25) samples. Thermograms indicated are for samples: Aunprocessed, B and D - conventional method processed at 50'C and 150'C, respectively; C and E extrusion method processed at 50*'C and 150'C, respectively. All samples had 20% w/w feed moisture before processing.
3.2
Differential scanning calorimetry thermoanalysis Thermoanalysis data (Table 1) and DSC thermograms (Figure 5) show thermal properties of cornstarch-soy protein isolate samples thermally processed by both procedures at 50'C and 150'C, along with a non-heat processed control.
A single enthalpy peak occurred for the unprocessed starch/protein blend as shown in thermogram A. The peak had an enthalpy of transition (AH) of 7.4J/g and peak temperature (Tp) of 74.3*'C. Unprocessed starch alone was determined to have an enthalpy of transition of 16.9 J/g which was within the range of 15.5 J/g to 31.8 J/g as determined by Stevens and Elton [13]. The peak appeared in the same temperature range where cornstarch gelatinization enthalpy normally occurs as
1163 reported by Stevens and Elton [13]. Therefore, the peak was attributed to the cornstarch gelatinization endotherm. The thermogram did not show other peaks generally attributed to the enthalpy of protein denaturation [14]. The reason for the absence of the protein peak might have been due to the nature of protein since commercial soy protein isolate had been shown to produce no detectable peaks with DSC thermoanalysis [10]. The thermogram for a starch-protein blend conventionally processed at 50'C (Figure 5, thermogram B) did not produce detectable thermal property changes in the sample components. The samples had virtually the same enthalpy of transition and T as the unprocessed sample. This may have been the result of a very short duration of heating or that heating the sample to 50'C was not enough to cause any thermal changes in the sample. Parsons and Patterson [15] showed that thermal properties of food systems are influenced by both temperature and duration of heat treatment. Therefore, increasing thermal processing duration might have caused some changes in at least the enthalpy peak of transition. The effect of thermal processing on the thermoprofile of a starch-protein blend was demonstrated when the samples were extruded at 50'C (Figure 5). Table 1 showed that the enthalpy of transition was decreased from 7.4 J/g (unprocessed sample) to 0.6 J/g for the extruded sample but only a slight decrease for a sample conventionally processed at 50"C. This showed that extrusion thermal processing affects the thermal properties of a food system differently than conventional thermal processing. The low temperature when coupled with shear and pressure during extrusion caused changes in the thermal properties of a food system. When the starch-protein sample was conventionally heated to 150'C, the enthalpy peak measured by DSC was virtually the same as that of samples extruded at SO^'C (Figure 5 ) . DSC thermograms showed that there was incomplete starch gelatinization even at a processing temperature of 150'C. This observation is in line with the observation that the amount of water [9,16,17] and the presence of protein [18] in a starch sample influence starch gelatinization, thus influencing the size of the enthalpy peak. In contrast, extrusion thermal processing of samples at 150*C caused the elimination of any detectable DSC enthalpy peak. Table 1 Effect of thermal processing method and processing temperature on onset temperature (To), peak maximum temperature (Tp), and transition enthalpy (AH) of All samples were thermally cornstarch/soy protein isolate bend (75:25). processed at 20% w/w feed moisture. Scanning was performed at temperatures form 20''C to 120^C at a heating rate of lO'^C per minute. Processing method Unprocessed Conventional Extrusion Conventional Extrusion
Processing temp. f'C)
50 50 150 150
TJ'C) 69.0 68.5 70.4 70.5
+ + + +
ND
Tp(^C) 0,2 0.5 0.2 0.2
74.3 74.8 75.0 75.1
+ + + +
ND
AH(J/q) 0.1 0.1 0.2 0.1
7.4 + 0.5 6.9 + 0.3 0.62 + 0.05 0.78 + 0.08
ND
1164 ND:
4. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Not detected
REFERENCES L. Jing-ming and Z. Sen-lin, Starch, 42 (1990) 96. C.G. Biladeris, Food Chem. 10 (1983) 239. M.U. Taranto, G.F. Ceqla and K.C. Rhee, J. Food Sci., 43 (1978) 973. M.J. Gomez and J.M. Aguilera, J. Food Sci., 48 (1983) 378. L.C. Lin and T. Ito, J. Food Technol., 21 (1986) 133. M. Bhattacharya and M.A. Hanna, Lebensm. Wiss. Technol., 20 (1987) 195. J.W. Donovan and R.A. Beardslee, J. Biol. Chem., 250 (1975) 1966. A.M. Hermansson, J. Text. Studies, 9 (1978) 33. C.G. Biliaderis, T.J. Maurice and J.R. Vose, J. Food Sci., 45 (1980) 1669. S.D. Arntfield and F.D. Murray, Can. Inst. Food Sci. Technol. J., 14 (1981) 289. J. Goossens, F. Derez and K.H. Bahr, Starch, 40 (1988) 327. M. Kugimiya and J.W. Donovan, J. Food Sci., 46 (1981) 765. D.J. Stevens and G.A.H. Elton, Starch, 23 (1971) 8. F.W. Sosulski, R. Hoover, R.T. Tyler, R.T. Murray and S.D. Arntfield, Starch, 37 (1985) 257. S.E. Parsons and R.L.S. Patterson, J. Food Technol., 21 (1986) 123. J. W. Donovan, Biopolymers, 18 (1979) 263. D.J. Burt and P.L. Russell, Starch, 35 (1983) 354. R.a. Gryzybowski and B.J. Donnelly, J. Food Sci., 42 (1977) 1304.
1155
Changes in microstructure and thermal properties of thermally processed cornstarch/soy protein isolate model food systems F.A. Nyanzi, J.A. Maga and C. Evans Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, Colorado 80523, U.S.A.
Abstract Fluorescent microscopy and differential scanning calorimetry were used to study the influence of thermal processing on the microstructure and thermal properties of cornstarch/soy protein model food systems. Both analytical methods showed that extrusion thermal processing at 50'C affected the physical characteristics of cornstarch/soy protein samples almost to the same extent as conventional heating at 150**C. For example, the enthalpies of transition were 0.8J/g for samples conventionally processed at IBO'C and 0.6J/g for samples extruded at 50'C. The micrographs also showed that the starch granules were not completely disrupted by conventional thermal processing at 150*C but extrusion thermal processing gelatinized the starch granules at 50'C. Overall, the greatest microstructure modifications of starch or protein were clearly shown to be due to extrusion when compared to conventional thermal processing.
1.
INTRODUCTION
Thermal processing of food systems induces changes in microstructure [1] and the thermal properties [2] of food components. Studies of these changes may contribute to the understanding of how food components can influence the functionality of food systems. Electron microscopy has been used by researchers [1,3-6] to study the microstructure of food systems. Other researchers [7-10] have followed changes in thermal properties of thermally processed samples by the use of differential scanning calorimetry. Fluorescent microscopy has received very limited attention in the study of microstructural changes in processed samples. A study by Goossens et al. [11] showed that fluorescent microscopy is a good method for identifying protein and starch in wheat samples. This method has not yet been shown to be applicable to thermally processed samples. Therefore, the purpose of the present study was to use fluorescent microscopy in conjunction with differential scanning calorimetry to determine the changes in microstructure and thermal properties induced by thermal processing method and temperature.
1156 2.
MATERIALS AND METHODS
2.1. Materials The ingredients consisted of commercial cornstarch (Pure-Dent B700: a gift from Grain Processing Corporation, Muscatine, Iowa) and commercial soy protein isolate (Ardex R: a gift from Archer Daniels Midland Company (ADM), Decatur, Illinois). 2.2
Cornstarch preparation for extrusion A 2kg starch sample was mixed with 188 ml of 0.08N sodium hydroxide solution for 10 minutes using a Hobart mixer. Model D-300 (The Hobart Manufacturing Company, Troy, Ohio, USA) set at the lowest speed. The prepared sample had 20% w/w moisture and pH=7. Samples were sealed in plastic bags and left at room temperature for extrusion processing the following day. 2.3
Protein pellet preparation for extrusion Two ml of 0.3N sodium hydroxide was mixed with 2.5 g of soy protein isolate, and a round pellet was manually made. The prepared pellet had a pH of 7. These pellets were stored overnight at room temperature in sealed plastic containers to prevent moisture loss, for extrusion processing the following day. 2.4
Soy protein isolate and/or cornstarch preparation for conventional processing Samples containing lOg of cornstarch or (75:25) cornstarch:soy protein isolate were mixed manually with a glass rod for 1 minute while adding 2 ml of 1.5N sodium hydroxide. Samples were left in sealed containers overnight at room temperature before an aliquot was packed in stainless steel tubes for processing. Stainless tubes 15cm long with an inside diameter of 0.40cm were used in this method. The samples were packed in the tubes and both ends were tightly sealed with metal screw covers. The tubes, with their respective samples, were temperature- and moisture-equilibrated overnight before thermal processing. 2.5
Extrusion processing Extrusion thermal processing was performed in a single screw Brabender Plasticorder Extruder, Model PL-V500 (C.W. Brabender Instruments, Inc., South Hackensack, New Jersey), equipped with a variable D-C drive unit, a tachometer, and a torque meter. The extruder barrel had a diameter of 19.05mm with a 20:1 length:diameter ratio and eight 0.79 X 3.18mm longitudinal grooves. In addition to a tapered screw with a 3:1 compression ratio, a die with a diameter of 4.8mm was used. The temperature of the extruder barrel was controlled by two electrically heated zones. The first heating zone was in the compression section and the second one in the metering section just next to the die. The dough temperature was maintained by compressed-air-cooled barrel collars. The compressed air was controlled by thermostats found inside the barrel wall. The preconditioned starch was fed manually into the feed zone of the extruder while bringing the screw speed to the processing speed of 120 rpm. After a steady state flow, as shown by a torque variation of +2.5 inch-pound, had been maintained and the processing dough temperature just before the die (either 50'C or 150'C) obtained, an experimental protein pellet was introduced in the feed zone. Immediately after a pellet was introduced, the extrudate strand was cut off and discarded. The subsequent exiting extrudate strand, which contained the protein sample, was
1157 cut off and saved. The presence of protein was determined by observing the difference in color of the exiting strand. 2.6
Conventional thermal processing An oil bath at 160'C was used to process the samples. The tubes to be processed, and another tube containing a thermocouple to monitor the temperature, were placed in metal racks, and immersed in the heated oil-bath. When the thermocouple registered the desired processing temperature (either 50'C or 150'C), the processed tubes were removed and immediately immersed into an icecold water-bath to stop the effects of thermal processing. For each set of tubes to be processed, a temperature-control tube containing the same soy protein and/or cornstarch preparation and a thermocouple was used. 2.7
Fluorescent microscopy Fluorescent microscopy was used to determine the microstructure changes in processed cornstarch/soy protein isolate samples. Samples were prepared by embedding 1 cm sections in paraffin blocks. The embedded samples were sectioned into 5 iim slices with a microtome and mounted on a microscope slide. Acridine orange fluorochrome was used in order to differentiate starch from protein structures as per Goossens et al. [11]. A Carl Zeiss fluorescent microscope equipped with 40X objective lens, 47 background filter, BG12 excitation filter, a 200W mercury arc lamp, and a camera body with automatic exposure control, were used to record microstructure of the samples on Ektachrome 100 ASA film. 2.8
Differential scanning calorimetry The method of Kugimiya and Donovan [12], with minor modifications, was employed to analyze thermal characteristics of experimental samples. A 2 to 3 mg sample of thermally processed cornstarch/soy protein isolate was weighed directly into a tared aluminum hermetic pan. A microsyringe was used to deliver lO^L of distilled water into the pan containing the sample and the pan was sealed by a press. The sealed pans were stored overnight at room temperature before thermal analysis. A DuPont 910 cell base and 9900 computer/thermal analyzer were used. The cell was purged with nitrogen gas at a rate of 40 ml per second. The heating rate was 10"C per minute from 20'C to 120'C. Enthalpies of transition ( A H ) , onset temperature (T^), and peak maximum temperature (Tp) were determined by a DuPont computer program General Analysis Utility Version 2.1.
3. RESULTS AND DISCUSSION 3.1
Microstructure determination by fluorescent microscopy Figure 1 shows the microstructure of the starch-protein sample conventionally processed at 50'C. No observable structural changes were detected with the starch granules or the soy proteins. The starch granules and protein particles appear to be intact.
1158
Figure 1 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by conventional method (oil-bath heating) at 50'C, 20%w/w feed moisture, on the protein and starch microstructure of the sample.
1159
Figure 2 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by extrusion method at 50'C, 20% w/w feed moisture, on the protein and starch microstructure of the sample.
In contrast, a micrograph of an extruded sample at 50'C, shows "melted" starch and a possible aggregation of protein particles (Figure 2). Shear and pressure during extrusion thermal processing, regardless of the temperature levels, appear to drastically alter the microstructure of starch granules.
1160
Figure 3 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by conventional method (oil-bath heating) at 150'C, 20% w/w feed moisture, on the protein and starch microstructure.
Increasing thermal processing temperatures to 150'C, for the conventional method, influenced the microstructure for both starch and protein. Figure 3 shows that most of starch granules have been disrupted and "melted" with a few unaffected granules. Protein particles appear to have been unfolded and spread over the "melted" starch mass.
1161
Figure 4 Fluorescent micrograph of cornstarch/soy protein isolate (75:25) sample showing the effect of thermal processing by extrusion method at 150'C, 20% w/w feed moisture, on the protein and starch microstructure.
Extrusion thermal processing at 150T causes protein unfolding and realignment as seen in Figure 4. Protein fibers are observed in the micrograph. The micrograph shows that the starch granules were completely "melted" and that some of the starch is intermeshed with the protein strands. Starch appears to be adsorbed to the protein fibers.
1162
m
3 Q. O fi-
IT
ro 3 3: ro o O
TQmpQraturQ (°C) Figure 5 Differential scanning calorimetry thermograms of cornstarch/soy protein isolate (75:25) samples. Thermograms indicated are for samples: Aunprocessed, B and D - conventional method processed at 50'C and 150'C, respectively; C and E extrusion method processed at 50*'C and 150'C, respectively. All samples had 20% w/w feed moisture before processing.
3.2
Differential scanning calorimetry thermoanalysis Thermoanalysis data (Table 1) and DSC thermograms (Figure 5) show thermal properties of cornstarch-soy protein isolate samples thermally processed by both procedures at 50'C and 150'C, along with a non-heat processed control.
A single enthalpy peak occurred for the unprocessed starch/protein blend as shown in thermogram A. The peak had an enthalpy of transition (AH) of 7.4J/g and peak temperature (Tp) of 74.3*'C. Unprocessed starch alone was determined to have an enthalpy of transition of 16.9 J/g which was within the range of 15.5 J/g to 31.8 J/g as determined by Stevens and Elton [13]. The peak appeared in the same temperature range where cornstarch gelatinization enthalpy normally occurs as
1163 reported by Stevens and Elton [13]. Therefore, the peak was attributed to the cornstarch gelatinization endotherm. The thermogram did not show other peaks generally attributed to the enthalpy of protein denaturation [14]. The reason for the absence of the protein peak might have been due to the nature of protein since commercial soy protein isolate had been shown to produce no detectable peaks with DSC thermoanalysis [10]. The thermogram for a starch-protein blend conventionally processed at 50'C (Figure 5, thermogram B) did not produce detectable thermal property changes in the sample components. The samples had virtually the same enthalpy of transition and T as the unprocessed sample. This may have been the result of a very short duration of heating or that heating the sample to 50'C was not enough to cause any thermal changes in the sample. Parsons and Patterson [15] showed that thermal properties of food systems are influenced by both temperature and duration of heat treatment. Therefore, increasing thermal processing duration might have caused some changes in at least the enthalpy peak of transition. The effect of thermal processing on the thermoprofile of a starch-protein blend was demonstrated when the samples were extruded at 50'C (Figure 5). Table 1 showed that the enthalpy of transition was decreased from 7.4 J/g (unprocessed sample) to 0.6 J/g for the extruded sample but only a slight decrease for a sample conventionally processed at 50"C. This showed that extrusion thermal processing affects the thermal properties of a food system differently than conventional thermal processing. The low temperature when coupled with shear and pressure during extrusion caused changes in the thermal properties of a food system. When the starch-protein sample was conventionally heated to 150'C, the enthalpy peak measured by DSC was virtually the same as that of samples extruded at SO^'C (Figure 5 ) . DSC thermograms showed that there was incomplete starch gelatinization even at a processing temperature of 150'C. This observation is in line with the observation that the amount of water [9,16,17] and the presence of protein [18] in a starch sample influence starch gelatinization, thus influencing the size of the enthalpy peak. In contrast, extrusion thermal processing of samples at 150*C caused the elimination of any detectable DSC enthalpy peak. Table 1 Effect of thermal processing method and processing temperature on onset temperature (To), peak maximum temperature (Tp), and transition enthalpy (AH) of All samples were thermally cornstarch/soy protein isolate bend (75:25). processed at 20% w/w feed moisture. Scanning was performed at temperatures form 20''C to 120^C at a heating rate of lO'^C per minute. Processing method Unprocessed Conventional Extrusion Conventional Extrusion
Processing temp. f'C)
50 50 150 150
TJ'C) 69.0 68.5 70.4 70.5
+ + + +
ND
Tp(^C) 0,2 0.5 0.2 0.2
74.3 74.8 75.0 75.1
+ + + +
ND
AH(J/q) 0.1 0.1 0.2 0.1
7.4 + 0.5 6.9 + 0.3 0.62 + 0.05 0.78 + 0.08
ND
1164 ND:
4. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Not detected
REFERENCES L. Jing-ming and Z. Sen-lin, Starch, 42 (1990) 96. C.G. Biladeris, Food Chem. 10 (1983) 239. M.U. Taranto, G.F. Ceqla and K.C. Rhee, J. Food Sci., 43 (1978) 973. M.J. Gomez and J.M. Aguilera, J. Food Sci., 48 (1983) 378. L.C. Lin and T. Ito, J. Food Technol., 21 (1986) 133. M. Bhattacharya and M.A. Hanna, Lebensm. Wiss. Technol., 20 (1987) 195. J.W. Donovan and R.A. Beardslee, J. Biol. Chem., 250 (1975) 1966. A.M. Hermansson, J. Text. Studies, 9 (1978) 33. C.G. Biliaderis, T.J. Maurice and J.R. Vose, J. Food Sci., 45 (1980) 1669. S.D. Arntfield and F.D. Murray, Can. Inst. Food Sci. Technol. J., 14 (1981) 289. J. Goossens, F. Derez and K.H. Bahr, Starch, 40 (1988) 327. M. Kugimiya and J.W. Donovan, J. Food Sci., 46 (1981) 765. D.J. Stevens and G.A.H. Elton, Starch, 23 (1971) 8. F.W. Sosulski, R. Hoover, R.T. Tyler, R.T. Murray and S.D. Arntfield, Starch, 37 (1985) 257. S.E. Parsons and R.L.S. Patterson, J. Food Technol., 21 (1986) 123. J. W. Donovan, Biopolymers, 18 (1979) 263. D.J. Burt and P.L. Russell, Starch, 35 (1983) 354. R.a. Gryzybowski and B.J. Donnelly, J. Food Sci., 42 (1977) 1304.