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State behavior and crystal growth kinetics of sucrose and corn syrup mixtures
Journal of Food Engineering 161 (2015) 1–7
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State behavior and crystal growth kinetics of sucrose and corn syrup mixtures Jiahui Chen a,b, Christine Nowakowski a,b, Dan Green c, Richard W. Hartel b,⇑ a
General Mills, Inc., 330 University Ave. SE, Minneapolis, MN 55414, USA Department of Food Science, University of Wisconsin, 1605 Linden Drive, Madison, WI 53706, USA c General Mills, Inc., 9000 Plymouth Ave N, Golden Valley, MN 55427, USA b
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Article history: Received 27 December 2014 Received in revised form 24 March 2015 Accepted 28 March 2015 Available online 3 April 2015 Keywords: Sucrose Corn syrup Crystal growth State diagram Growth rate dispersion
a b s t r a c t Effects of 63DE (dextrose equivalent) commercial corn syrup on the state behavior and crystal growth rate of sucrose and corn syrup mixtures in thin films (1.08 mm) were investigated using DSC and polarized light microscopy. Glass transition temperature (Tg) and solubility temperature increased significantly as moisture content decreased from 16.75% to 3.75%. Additionally, higher levels of corn syrup depressed solubility temperature but at the levels studied, showed no apparent effect on Tg. Addition of corn syrup significantly decreased sucrose crystal growth rate. For systems containing 14% and 30% corn syrup, growth rates measured between 40 and 120 °C ranged from 5.6 to 3400 lm2/min and 1.8 to 470 lm2/min, respectively. Growth rate dispersion (GRD) was observed for all conditions and the extent of the GRD increased with increasing growth rate. By overlaying crystallization rate zones on the state diagram, the competing effects of supersaturation driving force and molecular mobility inhibition on sucrose crystal growth rates were clearly observed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The physical characteristics of sucrose systems are dependent on the formulation and processing conditions under which they were prepared. State behavior such as crystallization and glass transition are important to the quality of many food products. The glass transition temperature (Tg) plays a critical role in many food product’s quality and storage stability. The understanding of glass transitions of food systems has allowed food material characterization and prediction of their behavior at high solids contents and in the frozen state at varying temperatures and water contents (Roos, 2010). State diagrams, or supplemented phase diagrams (Slade and Levine, 1991), provide useful maps for the observation of changes in glass transition as a function of water content or varying levels of freeze-concentration (Roos and Karel, 1991b). The state diagram is a map of the different states of a food as a function of water or solids content and temperature (Roos, 1995; Rahman, 2006). As the temperature of a glass increases above the glass transition temperature (Tg), the system becomes unstable and collapses into a liquid-like, rubbery state. Here the system is supersaturated and in a metastable state bounded by the solubility curve and the Tg curve. As the temperature and concentration of ⇑ Corresponding author. Tel.: +1 (608) 263 1965; fax: +1 608 262 6872. E-mail address: [email protected] (R.W. Hartel). http://dx.doi.org/10.1016/j.jfoodeng.2015.03.032 0260-8774/Ó 2015 Elsevier Ltd. All rights reserved.
such a system approach the solubility curve, the rate of crystallization increases initially and decreases after reaching a maximum due to competition between molecular mobility and supersaturation (Hartel, 2001). Sucrose crystallization is encountered in many food and pharmaceutical applications. Food products where sucrose crystallization is important include refined sugar, confections, ready to eat cereals, and some snack foods. During processing, nuclei are either formed in situ or added as seeds. Once formed or added, crystal grow at a rate dependent on conditions in their surrounding environment in a series of steps (Mullin, 2001). In sucrose crystallization, diffusion of the sucrose from the bulk solution to crystal surface and integration of the sucrose molecule into the lattice structure are the typical rate-limiting steps (Van Hook, 1981; Hartel, 2013). Many factors can affect the growth rate, including temperature, supersaturation, agitation, and impurities (Hartel and Shastry, 1991). Molecular mobility also influences growth rate. For instance, in a hard candy matrix, crystals imbedded within the metastable glass matrix do not grow, despite the highly supersaturated condition (Hartel et al., 2008). Growth rate dispersion (GRD) describes the phenomenon whereby individual crystals grow at different rates under identical conditions of supersaturation, temperature and hydrodynamics. It was first seen by White and Wright (1971) in sucrose batch crystallization. Liang et al. (1987a,b) also confirmed that GRD occurs
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in sucrose-water system. Speculations on GRD have centered on two primary mechanisms; differences in the density of dislocation steps on the surface of each crystal, and variations of the crystal perfection (internal lattice strain) of each crystal (Berglund and Murphy, 1986; Hartel, 2001; Pantaraks, 2004). The primary goal of this work was to study the effects of processing conditions on Tg, solubility temperature and sucrose crystal growth kinetics for sucrose-corn syrup systems. Parameters studied include corn syrup solids level, cooking temperature (moisture or initial concentration inversely), and isothermal crystallization temperature. 2. Materials and methods Sugar solutions were produced with extra fine granulated sugar (Royal Ingredients, Alkmaar, Netherlands), 63DE (dextrose equivalent) corn syrup (Cargill, Minneapolis, MN), and deionized water. Three formulations were studied, with ratios of 86/14, 78/22, and 70/30 sucrose/corn syrup on wt/wt dry basis. Batch sizes were 25 grams each. All formulations were prepared using the same cooking method. The ingredients were mixed in a 30 ml beaker, heated to completely dissolve all sugar crystals, and then boiled to different temperatures on a PC-420 Stirrer Hot-Plate (Corning Incorporated, Corning, NY). Cooking temperatures of 113 °C, 118 °C, 124 °C, 132 °C, 143 °C, and 154 °C were used to generate different water contents with each level of corn syrup solids. 2.1. Moisture content A Karl Fischer (KF) Aquametry instrument (795 KFT Titrino, Metrohm Ltd., Herisau, Switzerland) with an automatic pump for the Karl Fischer reagent (Hydranal Composite 5, Riedel deHaën, Sigma–aldrich, Co., St Louis, MO) was used to measure the moisture content of sugar samples. This technique involved titration of the Karl Fischer reagent into a 150 mL titration vessel containing the sample pre-dissolved in a solvent. The solvent, a 4:3 ratio of formamide (Fisher Scientific, Fair Lawn, NJ) and Karl Fischer grade methanol (low water content 0.006%) solution, was used to completely dissolve the sugar glass and release the incorporated water. Three replicates were done for each sample. 2.2. Glass transition and solubility temperature Measurement of glass transition temperature (Tg) and solubility temperature using differential scanning calorimeter (DSC) is based on the dynamic relationship between enthalpy and temperature (Mohan et al., 2002). In this study, a DSC 8500 (Perkin-Elmer, Inc., Shelton, CT) was used to determine the Tg and solubility temperature of the sugar/corn syrup mixtures cooked to different temperatures. The DSC was connected to a refrigeration system and used the Pyris software program (Version 11, Perkin–Elmer, Inc., Shelton, CT) for analysis. Approximately 20–30 mg of each sugar/ syrup blend sample was added to each of three tared, O-ring sealed, large volume stainless steel DSC pans (Perkin–Elmer, Inc., Shelton, CT) and then sealed with a Universal Crimper Press (Perkin-Elmer, Inc., Shelton, CT). The heating and cooling rates were 5 °C/min and 40 °C/min, respectively. The thermal profiles chosen for all conditions were based on their physical properties, which depended on cooking temperatures. The first scan started from a temperature about 25 °C lower than the expected Tg and the highest temperature was 200 °C for all conditions. To eliminate the effects of sample thermal history, each sample was heated through a first heating cycle, cooled quickly back to the start temperature, with Tg and solubility temperature obtained from the second heating scan. The scan temperature range varied depending
on sample characteristics, which depended on cooking temperatures. For instance, the thermal profile used for sample cooked to 154 °C was: hold for 1 min at 10 °C, heat from 10 °C to 80 °C, cool from 80 °C to 10 °C and then heat from 10 °C to 200 °C. 2.3. Sucrose crystal growth rates Amorphous sugar samples were prepared and, while still hot and fluid, were placed in the center of metal washers on preheated microscope slides (Fisher Scientific, Hanover Park, IL) and enclosed with a cover glass (Levenson and Hartel, 2005). The microscope slide coupled with a washer and cover glass were heated in a hot stage (Analysa Peltier-LTS120, Linkam Scientific Instruments, Guildford, UK) prior to the start of each experiment. The washers were used to maintain a uniform volume and height of sugar sample on each slide and had a height of 1.08 mm and an inner diameter of 8.96 mm. A syringe (BD Brand, Franklin Lakes, NJ) was used to draw approximately 1 ml of the hot sugar syrup, which was injected in the center of a washer. The washer was rapidly enclosed with a cover glass and pressed to provide an even sample surface with no exposure to ambient air and a constant volume and height. The slide was then quickly set in the hot stage and put under the microscope for further image analysis. This process generally took no more than one minute. Based on observations, the sugar samples had already nucleated during the process, so that once injected on the slide, crystal growth was dominant and no additional nuclei were formed. A Nikon polarized light microscope (Labphot-2, Tokyo, Japan) coupled with a hot stage and a polarizer was used at 410 magnification. A digital camera (QICAM Fast1394, QImaging, Surrey, BC, Canada) and QCapture software (QCapture Pro 5.1, QImaging, Surrey, BC, Canada) were used to record crystal growth. The prepared microscope slides were placed in the hot stage set at either 40, 60, 80, 100, or 120 °C, the maximum temperature allowed with this system. A minimum of 3 slides of each formulation and concentration were studied under each temperature. The slides were observed under the microscope and images taken at different time intervals (determined by preliminary experiments) chosen to capture the initial growth rate. For each condition, at least 30 individual crystals were tracked. In order to calculate their surface area by Image Pro Plus software (Version 7.0, Media Cybernetics, Inc., Bethesda, MD), these crystals were manually digitized by outlining them in Paint (Version 6.1, Microsoft Corporation, Redmond, WA). A macro, ‘‘Area’’, written in Image Pro Plus macro language streamlined the process and automatically calculated the surface area. The surface area vs. time was found to be linear and the slope of each line was the growth rate (lm2/min) for that crystal. 2.4. Data analysis To determine significant differences in the data, an analysis of variance was conducted with ANOVA using a 95% confidence interval by SAS software (Enterprise Guide V 5.1, SAS Institute Inc., Cary, NC). This analysis was performed for moisture content, Tg, solubility, and growth rate using a comparison of cooking temperature, observation temperature and formulation used. Software R (version 2.15.2, The R foundation for Statistical Computing) was used to generate linear regression models for moisture content, Tg and solubility temperature as a function of cooking temperature and formulation. Linear regression models for growth rates as a function of crystallization temperature, cook temperature and formulation were also performed by R. Growth rates contour plots were generated by JMP software with Contour Plot command in the Graph menu (Pro 10.0.0, SAS Institute Inc., Cary, NC).
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3. Results and discussion State behavior such as crystallization and glass transition are important to the quality of many food products. The physical characteristics of sucrose systems are dependent on the formulation (e.g., adding corn syrup) and processing conditions under which they were prepared. In this study, glass transition (Tg) and solubility temperatures of the sugar mixtures cooked with three different sucrose/corn syrup ratios, 86/14, 78/22 and 70/30 (sucrose weight/corn syrup weight ratio on a dry solids basis), were determined to understand their state behavior. In addition, crystal growth rates were also measured to quantify the crystal growth kinetics under different composition-temperature conditions. 3.1. State diagram There was an inverse relationship between cooking temperature and moisture. Increasing the cooking temperature caused a decrease in the moisture content due to the boiling point elevation as water was removed. Boiling point elevation is inversely related to the molecular weight of a solute (Pancoast and Junk, 1980). Based on the colligative effect, molecules with lower molecular weight result in higher boiling points. Monosaccharides such as glucose and fructose will lead to a greater increase of boiling point of water than disaccharides, such as sucrose and maltose. Since the average molecular weight of 63DE corn syrup (Mw = 295) is lower than sucrose, when cooked to the same temperature, samples containing higher levels of this corn syrup (lower average molecular weight) should have had higher moisture content. However, there was no significant difference (p > 0.05) between samples with different sucrose to corn syrup ratio. Apparently, the differences in composition were small enough to have no significant effect on boiling point elevation and moisture content. Glass transition temperature, Tg, decreased as water content increased due to plasticization, in agreement with previous work (Roos and Karel, 1990; Nowakowski and Hartel, 2002). Fig. 1 also indicates that there was no significant compositional effect on Tg (p > 0.05). Tg decreased as cooking temperature decreased (increased water content), but there was no apparent trend between corn syrup level and Tg in these samples. In general, the addition of 63 DE corn syrup solids might have been expected to cause a slight decrease in Tg, with higher level having a greater effect due to the decrease in average molecular weight of the sugar mixture by adding corn syrup. However, no significant effects were found at the corn syrup solid levels used here. The relationship between moisture content and solubility temperature for the three formulation systems is also shown in Fig. 1. The solubility temperature decreased significantly as water
Fig. 1. The relationship between moisture content, Tg (open symbols) and solubility temperature (filled symbols) under different sucrose/corn syrup solids ratios 86/14 (square), 78/22 (triangle) and 70/30 (circle). Some error bars are within the size of the symbols.
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content increased (p < 0.05). This result agreed well with literature values (Pancoast and Junk, 1980; Bubnik et al., 1996). In addition, increasing the corn syrup level of the mixture resulted in a slight, but significant (p < 0.05), decrease in solubility due to the competition between corn syrup saccharides and sucrose for hydrogen bonding sites with water molecules (Hartel, 2001). These results agreed with the general trends in the literature (Tjuradi and Hartel, 1995; Jonathan, 2004; Bonifacio et al., 2008). 3.2. Crystal growth rate Typical plots of crystal surface area vs. time are shown in Fig. 2 (8 crystals out of 30 crystals were shown as an example). The change in area of each growing crystal was approximately linear so the slope (change in surface area per minute) was taken as the growth rate of each individual crystal. Each individual crystal had a different growth rate (different slope), a phenomenon called growth rate dispersion (GRD), which is a well-known characteristic of sucrose crystals (Larson et al., 1985; Liang et al., 1987a; Jones et al., 2000; Iswanto et al., 2006). For each condition, at least 30 crystals were tracked, with mean growth rate calculated for each condition. In the current study, the standard deviation of the distribution of growth rates for one condition was taken as the extent of growth rate dispersion for that condition. Fig. 3 illustrates the trends between the growth rate of sucrose crystals and concentration, as governed by the temperature to which the sugar syrup was cooked. For most storage temperatures, increasing concentration resulted in an initial increase in growth rate due to the increasing supersaturation. At higher concentrations, however, the limited molecular mobility caused a decrease in growth rate despite the even higher supersaturation. The peak in growth rate moved to higher concentration at higher crystallization temperature. At the lowest temperature, the growth rate decreased with increasing concentration, probably because the concentration range in this study was not large enough to see the whole trend. In many foods, cooling affects crystallization. For example, fondant is first cooked to about 118 °C to reach a moisture content of
Fig. 2. (A) Change in surface area of sucrose crystals in sugar mixture cooked at 118 °C and observed under 60 °C with sucrose/corn syrup solids ratio 78/22. Each line captures the size progression of an individual crystal. (B) Pictures were chosen at the beginning (0 min), middle (2.25 min) and end of observation (3.75 min). 3 out of 8 crystals were shown as an example. Crystal #1, 2, 3 are ordered from left to right (the leftmost one was not taken into account due to irregular shape).
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Fig. 3. Growth rates (GR) as a function of concentration at different temperatures(40–120 °C) under different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30. Error bars represent the standard deviation of the growth rate distribution.
Fig. 4. Growth rates (GR) as a function of temperature at different concentration (83.3–96.3%) under different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/ 30. Error bars represent the standard deviation of the growth rate distribution.
about 10%. It is then cooled statically to a temperature between 40 and 50 °C, a range often considered to be optimal for promoting crystallization. At this temperature, the sugar mass is agitated intensely to promote crystallization with the specific aim of creating many small crystals for a smooth texture (Hartel, 2001; Hartel et al., 2011). Fig. 4 highlights the effects of crystallization temperature on growth rate in this study. As the sugar syrups were cooled from cook temperature (which governed the total solids content) to different hold temperatures, growth rate initially increased as crystallization temperature decreased. However, after reaching a peak, the growth rate began to decrease again as temperature was lowered further. The initial decrease in temperature below solubility led to an increase in supersaturation, resulting in this increase in the growth rate. A further decrease in temperature resulted in reduced molecular mobility as the system approached Tg, and thus decreased the growth rate. At the highest concentrations, no decreases in growth rate were observed, most likely due to the limited temperature range evaluated (higher temperature could not be reached due to the limit of the hot stage). From Fig. 4, temperatures of peak growth rate fell between 80 °C to 100 °C, significantly higher than the optimal temperature, noted earlier, of 40–50 °C for fondant production (Hartel, 2001).
This was undoubtedly due to the difference in crystallization conditions, primarily in the agitation rate. In this study, crystals grew in a quiescent environment, completely limited by diffusion processes, whereas in fondant, intense agitation is utilized to promote crystallization. Previous work also demonstrated temperature was a very influential factor on crystal growth in thin films. Howell and Hartel (2001) proved that growth rates increased by a factor of 4.5 as temperature increased from 40 to 70 °C. Mazzobre et al. (2003) indicated that there was a clear trend of increasing crystal growth rate when increasing heating temperature. 3.3. Overlay of state diagram and growth kinetics To put the previous results into context, the growth rate data were combined on the state diagram in the form of contour plots, as shown in Fig. 5. The overall growth rate was expected to be highest somewhere in the crystallization zone between the Tg and solubility lines. As these contour plots document, the growth rates descended progressively from a peak zone of fastest growth in all directions of changing temperatures and concentrations. This way of looking at crystallization kinetics relative to the state diagram provides a map that may be used as a predictive tool in
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with previous work (Gabarra and Hartel, 1998; Laos et al., 2007). Note that there were slight differences between solubility temperatures among the three systems. However, the effect of composition on growth rate far outweighed the influence of the difference in solubility temperatures. The inhibition of growth rates of sucrose crystals by adding corn syrup has been attributed to the combination of oligosaccharides (Dorow, 1993). Interference of sucrose crystal growth by adsorption of corn syrup saccharides (monosaccharides, glucose, fructose, and glucose polymers) onto the crystal lattice structure has been proposed for crystallization of sucrose in solution (Bamberger et al., 1980; Tjuradi and Hartel, 1995; Hartel, 2001).
3.4. Growth rate dispersion (GRD)
Fig. 5. Growth rate contour plots for different sucrose/corn syrup solids ratios 86/ 14, 78/22 and 70/30, solubility and glass transition temperature curves are represented by triangle and circle symbols, respectively. Note that maximum growth rate zones per each graphic are not equal.
industrial applications to understand the effects of process conditions on crystallization. An interesting point seen in Fig. 5 is that the peak of crystallization does not fall at the midway point, in either temperature or solids concentration, between solubility and glass transition. In fact, the peak growth rate occurs at a significantly higher temperature than the midpoint between Ts and Tg. Again, this is most likely due to the diffusion-controlled growth studied in these experiments. Specifically, Tg of this sugar mixture is an average of the higher-molecular weight saccharides and the lower molecular weight water. Mobility of the sucrose component of this glass is limited at much higher temperatures than the average, resulting in the peak growth rate being well above the mid-point. Also seen in Fig. 5 is the effect of corn syrup on the positioning of the peak growth zone. The addition of corn syrup dramatically reduced the magnitude of growth rates and diminished the regions of high growth rates (dark zones), with higher addition level having greater effects. In general, the addition of corn syrup significantly decreased the growth rate of sucrose crystals (p < 0.001), with higher level having a greater effect. This result agreed well
GRD was first seen by White and Wright (1971) in batch sucrose crystallization. Typically, changing the solution supersaturation or crystallization temperature will lead to the change of GRD. In this study, GRD was observed under all conditions with each formulation, even though they were grown under the same external conditions. Fig. 6 illustrates the effect of composition on GRD. In general, a lower degree of GRD was observed with an increase in the level of corn syrup. Moreover, all the individual crystals had different growth rates and showed no trend with respect to initial size of the respective crystals (data not shown). The contour plots in Fig. 7 show the combined impact of temperature and concentration on sucrose crystal GRD. The GRD descended progressively from the dark zone to the light zone in all directions of changing temperatures and concentrations. These contour plots were generally in agreement with the growth rate contour plots, which indicate that the GRD increased proportionally with increased growth rates. Howell and Hartel (2001) found a similar trend, where the variability of growth rate increased with increasing temperature, which was attributed to the extent of increase in the GRD with higher supersaturation. This finding also agrees with Liang et al. (1987a), who showed that the GRD of sucrose crystals in stirred and stagnant situations increased with the square of the average growth rate. This relationship can also be seen in Fig. 8 for sucrose/corn syrup solids ratio 78/22; systems 86/14 and 70/30 followed the same trend. Furthermore, based on informal macroscopic observation of the slides, the differences in GRD had no apparent influence on the appearance of thin films between different levels of corn syrup. By overlaying GRD zones on the state diagram, the competing effects of supersaturation driving force and molecular mobility inhibition on sucrose crystal GRD were clearly observed. This graph can be used to predict optimal coating process conditions based on different state behaviors to deliver desired sensory attributes and processing behavior in food systems.
Fig. 6. Growth rate dispersion for sugar mixtures cooked to 124 °C and observed under 120 °C for different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30.
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4. Conclusions In this study, moisture content decreased as cooking temperatures increased. In addition, there were no significant differences in moisture content between the three formulation systems, indicating the difference in sucrose to corn syrup ratio had no significant effect on boiling point elevation. As expected, Tg and solubility temperature decreased significantly as moisture content increased, which agrees with previous research. The impact of sucrose to corn syrup ratio was more evident on solubility temperature than Tg. No significant differences in Tg were detected among the three systems, whereas the solubility temperatures were notably depressed as the corn syrup level increased. Sucrose crystal growth rates were highly related to temperature, concentration, and sucrose to corn syrup ratio. Concentration and temperature significantly influenced sucrose crystal growth rates, with increases in concentration and temperature initially giving higher growth rates; however, after reaching a maximum point, growth rates started to decrease as either supersaturation or molecular mobility decreased. The addition of corn syrup reduced the magnitude of growth rates and diminished the regions of high growth rates (maximum zones per each graphic are not equal in contour plots), with higher addition level having greater effects. Evidence of GRD was found in all experiments for systems of sucrose, corn syrup and water, showing that individual crystals grown under identical conditions do not have the same growth rates. The growth rate distributions of all crystals in this study were approximately normally distributed. The extent of the GRD increased with increasing growth rate. Similar trends were seen from the contour plots of GRD as concentration and temperature changed. In general, a lower degree of GRD was observed with an increase in the level of corn syrup, related to the reduction in average growth rate. Conflict of interest The authors declare no conflict of interest. Fig. 7. Growth rate dispersion (GRD: standard deviation of growth rate distribution) contour plot for different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30, solubility and glass transition temperature curves are represented by triangle and circle symbols, respectively. Note that maximum growth rate zones per each graphic are not equal.
Acknowledgement We acknowledge the financial support for this work from General Mills, Inc and figure revision done by Jamie Gutkowski. References
Fig. 8. Growth rate dispersion (GRD: standard deviation of growth rate distribution) as a function of growth rate (GR) for sucrose/corn syrup solids ratio 78/ 22.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
State behavior and crystal growth kinetics of sucrose and corn syrup mixtures Jiahui Chen a,b, Christine Nowakowski a,b, Dan Green c, Richard W. Hartel b,⇑ a
General Mills, Inc., 330 University Ave. SE, Minneapolis, MN 55414, USA Department of Food Science, University of Wisconsin, 1605 Linden Drive, Madison, WI 53706, USA c General Mills, Inc., 9000 Plymouth Ave N, Golden Valley, MN 55427, USA b
a r t i c l e
i n f o
Article history: Received 27 December 2014 Received in revised form 24 March 2015 Accepted 28 March 2015 Available online 3 April 2015 Keywords: Sucrose Corn syrup Crystal growth State diagram Growth rate dispersion
a b s t r a c t Effects of 63DE (dextrose equivalent) commercial corn syrup on the state behavior and crystal growth rate of sucrose and corn syrup mixtures in thin films (1.08 mm) were investigated using DSC and polarized light microscopy. Glass transition temperature (Tg) and solubility temperature increased significantly as moisture content decreased from 16.75% to 3.75%. Additionally, higher levels of corn syrup depressed solubility temperature but at the levels studied, showed no apparent effect on Tg. Addition of corn syrup significantly decreased sucrose crystal growth rate. For systems containing 14% and 30% corn syrup, growth rates measured between 40 and 120 °C ranged from 5.6 to 3400 lm2/min and 1.8 to 470 lm2/min, respectively. Growth rate dispersion (GRD) was observed for all conditions and the extent of the GRD increased with increasing growth rate. By overlaying crystallization rate zones on the state diagram, the competing effects of supersaturation driving force and molecular mobility inhibition on sucrose crystal growth rates were clearly observed. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The physical characteristics of sucrose systems are dependent on the formulation and processing conditions under which they were prepared. State behavior such as crystallization and glass transition are important to the quality of many food products. The glass transition temperature (Tg) plays a critical role in many food product’s quality and storage stability. The understanding of glass transitions of food systems has allowed food material characterization and prediction of their behavior at high solids contents and in the frozen state at varying temperatures and water contents (Roos, 2010). State diagrams, or supplemented phase diagrams (Slade and Levine, 1991), provide useful maps for the observation of changes in glass transition as a function of water content or varying levels of freeze-concentration (Roos and Karel, 1991b). The state diagram is a map of the different states of a food as a function of water or solids content and temperature (Roos, 1995; Rahman, 2006). As the temperature of a glass increases above the glass transition temperature (Tg), the system becomes unstable and collapses into a liquid-like, rubbery state. Here the system is supersaturated and in a metastable state bounded by the solubility curve and the Tg curve. As the temperature and concentration of ⇑ Corresponding author. Tel.: +1 (608) 263 1965; fax: +1 608 262 6872. E-mail address: [email protected] (R.W. Hartel). http://dx.doi.org/10.1016/j.jfoodeng.2015.03.032 0260-8774/Ó 2015 Elsevier Ltd. All rights reserved.
such a system approach the solubility curve, the rate of crystallization increases initially and decreases after reaching a maximum due to competition between molecular mobility and supersaturation (Hartel, 2001). Sucrose crystallization is encountered in many food and pharmaceutical applications. Food products where sucrose crystallization is important include refined sugar, confections, ready to eat cereals, and some snack foods. During processing, nuclei are either formed in situ or added as seeds. Once formed or added, crystal grow at a rate dependent on conditions in their surrounding environment in a series of steps (Mullin, 2001). In sucrose crystallization, diffusion of the sucrose from the bulk solution to crystal surface and integration of the sucrose molecule into the lattice structure are the typical rate-limiting steps (Van Hook, 1981; Hartel, 2013). Many factors can affect the growth rate, including temperature, supersaturation, agitation, and impurities (Hartel and Shastry, 1991). Molecular mobility also influences growth rate. For instance, in a hard candy matrix, crystals imbedded within the metastable glass matrix do not grow, despite the highly supersaturated condition (Hartel et al., 2008). Growth rate dispersion (GRD) describes the phenomenon whereby individual crystals grow at different rates under identical conditions of supersaturation, temperature and hydrodynamics. It was first seen by White and Wright (1971) in sucrose batch crystallization. Liang et al. (1987a,b) also confirmed that GRD occurs
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in sucrose-water system. Speculations on GRD have centered on two primary mechanisms; differences in the density of dislocation steps on the surface of each crystal, and variations of the crystal perfection (internal lattice strain) of each crystal (Berglund and Murphy, 1986; Hartel, 2001; Pantaraks, 2004). The primary goal of this work was to study the effects of processing conditions on Tg, solubility temperature and sucrose crystal growth kinetics for sucrose-corn syrup systems. Parameters studied include corn syrup solids level, cooking temperature (moisture or initial concentration inversely), and isothermal crystallization temperature. 2. Materials and methods Sugar solutions were produced with extra fine granulated sugar (Royal Ingredients, Alkmaar, Netherlands), 63DE (dextrose equivalent) corn syrup (Cargill, Minneapolis, MN), and deionized water. Three formulations were studied, with ratios of 86/14, 78/22, and 70/30 sucrose/corn syrup on wt/wt dry basis. Batch sizes were 25 grams each. All formulations were prepared using the same cooking method. The ingredients were mixed in a 30 ml beaker, heated to completely dissolve all sugar crystals, and then boiled to different temperatures on a PC-420 Stirrer Hot-Plate (Corning Incorporated, Corning, NY). Cooking temperatures of 113 °C, 118 °C, 124 °C, 132 °C, 143 °C, and 154 °C were used to generate different water contents with each level of corn syrup solids. 2.1. Moisture content A Karl Fischer (KF) Aquametry instrument (795 KFT Titrino, Metrohm Ltd., Herisau, Switzerland) with an automatic pump for the Karl Fischer reagent (Hydranal Composite 5, Riedel deHaën, Sigma–aldrich, Co., St Louis, MO) was used to measure the moisture content of sugar samples. This technique involved titration of the Karl Fischer reagent into a 150 mL titration vessel containing the sample pre-dissolved in a solvent. The solvent, a 4:3 ratio of formamide (Fisher Scientific, Fair Lawn, NJ) and Karl Fischer grade methanol (low water content 0.006%) solution, was used to completely dissolve the sugar glass and release the incorporated water. Three replicates were done for each sample. 2.2. Glass transition and solubility temperature Measurement of glass transition temperature (Tg) and solubility temperature using differential scanning calorimeter (DSC) is based on the dynamic relationship between enthalpy and temperature (Mohan et al., 2002). In this study, a DSC 8500 (Perkin-Elmer, Inc., Shelton, CT) was used to determine the Tg and solubility temperature of the sugar/corn syrup mixtures cooked to different temperatures. The DSC was connected to a refrigeration system and used the Pyris software program (Version 11, Perkin–Elmer, Inc., Shelton, CT) for analysis. Approximately 20–30 mg of each sugar/ syrup blend sample was added to each of three tared, O-ring sealed, large volume stainless steel DSC pans (Perkin–Elmer, Inc., Shelton, CT) and then sealed with a Universal Crimper Press (Perkin-Elmer, Inc., Shelton, CT). The heating and cooling rates were 5 °C/min and 40 °C/min, respectively. The thermal profiles chosen for all conditions were based on their physical properties, which depended on cooking temperatures. The first scan started from a temperature about 25 °C lower than the expected Tg and the highest temperature was 200 °C for all conditions. To eliminate the effects of sample thermal history, each sample was heated through a first heating cycle, cooled quickly back to the start temperature, with Tg and solubility temperature obtained from the second heating scan. The scan temperature range varied depending
on sample characteristics, which depended on cooking temperatures. For instance, the thermal profile used for sample cooked to 154 °C was: hold for 1 min at 10 °C, heat from 10 °C to 80 °C, cool from 80 °C to 10 °C and then heat from 10 °C to 200 °C. 2.3. Sucrose crystal growth rates Amorphous sugar samples were prepared and, while still hot and fluid, were placed in the center of metal washers on preheated microscope slides (Fisher Scientific, Hanover Park, IL) and enclosed with a cover glass (Levenson and Hartel, 2005). The microscope slide coupled with a washer and cover glass were heated in a hot stage (Analysa Peltier-LTS120, Linkam Scientific Instruments, Guildford, UK) prior to the start of each experiment. The washers were used to maintain a uniform volume and height of sugar sample on each slide and had a height of 1.08 mm and an inner diameter of 8.96 mm. A syringe (BD Brand, Franklin Lakes, NJ) was used to draw approximately 1 ml of the hot sugar syrup, which was injected in the center of a washer. The washer was rapidly enclosed with a cover glass and pressed to provide an even sample surface with no exposure to ambient air and a constant volume and height. The slide was then quickly set in the hot stage and put under the microscope for further image analysis. This process generally took no more than one minute. Based on observations, the sugar samples had already nucleated during the process, so that once injected on the slide, crystal growth was dominant and no additional nuclei were formed. A Nikon polarized light microscope (Labphot-2, Tokyo, Japan) coupled with a hot stage and a polarizer was used at 410 magnification. A digital camera (QICAM Fast1394, QImaging, Surrey, BC, Canada) and QCapture software (QCapture Pro 5.1, QImaging, Surrey, BC, Canada) were used to record crystal growth. The prepared microscope slides were placed in the hot stage set at either 40, 60, 80, 100, or 120 °C, the maximum temperature allowed with this system. A minimum of 3 slides of each formulation and concentration were studied under each temperature. The slides were observed under the microscope and images taken at different time intervals (determined by preliminary experiments) chosen to capture the initial growth rate. For each condition, at least 30 individual crystals were tracked. In order to calculate their surface area by Image Pro Plus software (Version 7.0, Media Cybernetics, Inc., Bethesda, MD), these crystals were manually digitized by outlining them in Paint (Version 6.1, Microsoft Corporation, Redmond, WA). A macro, ‘‘Area’’, written in Image Pro Plus macro language streamlined the process and automatically calculated the surface area. The surface area vs. time was found to be linear and the slope of each line was the growth rate (lm2/min) for that crystal. 2.4. Data analysis To determine significant differences in the data, an analysis of variance was conducted with ANOVA using a 95% confidence interval by SAS software (Enterprise Guide V 5.1, SAS Institute Inc., Cary, NC). This analysis was performed for moisture content, Tg, solubility, and growth rate using a comparison of cooking temperature, observation temperature and formulation used. Software R (version 2.15.2, The R foundation for Statistical Computing) was used to generate linear regression models for moisture content, Tg and solubility temperature as a function of cooking temperature and formulation. Linear regression models for growth rates as a function of crystallization temperature, cook temperature and formulation were also performed by R. Growth rates contour plots were generated by JMP software with Contour Plot command in the Graph menu (Pro 10.0.0, SAS Institute Inc., Cary, NC).
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3. Results and discussion State behavior such as crystallization and glass transition are important to the quality of many food products. The physical characteristics of sucrose systems are dependent on the formulation (e.g., adding corn syrup) and processing conditions under which they were prepared. In this study, glass transition (Tg) and solubility temperatures of the sugar mixtures cooked with three different sucrose/corn syrup ratios, 86/14, 78/22 and 70/30 (sucrose weight/corn syrup weight ratio on a dry solids basis), were determined to understand their state behavior. In addition, crystal growth rates were also measured to quantify the crystal growth kinetics under different composition-temperature conditions. 3.1. State diagram There was an inverse relationship between cooking temperature and moisture. Increasing the cooking temperature caused a decrease in the moisture content due to the boiling point elevation as water was removed. Boiling point elevation is inversely related to the molecular weight of a solute (Pancoast and Junk, 1980). Based on the colligative effect, molecules with lower molecular weight result in higher boiling points. Monosaccharides such as glucose and fructose will lead to a greater increase of boiling point of water than disaccharides, such as sucrose and maltose. Since the average molecular weight of 63DE corn syrup (Mw = 295) is lower than sucrose, when cooked to the same temperature, samples containing higher levels of this corn syrup (lower average molecular weight) should have had higher moisture content. However, there was no significant difference (p > 0.05) between samples with different sucrose to corn syrup ratio. Apparently, the differences in composition were small enough to have no significant effect on boiling point elevation and moisture content. Glass transition temperature, Tg, decreased as water content increased due to plasticization, in agreement with previous work (Roos and Karel, 1990; Nowakowski and Hartel, 2002). Fig. 1 also indicates that there was no significant compositional effect on Tg (p > 0.05). Tg decreased as cooking temperature decreased (increased water content), but there was no apparent trend between corn syrup level and Tg in these samples. In general, the addition of 63 DE corn syrup solids might have been expected to cause a slight decrease in Tg, with higher level having a greater effect due to the decrease in average molecular weight of the sugar mixture by adding corn syrup. However, no significant effects were found at the corn syrup solid levels used here. The relationship between moisture content and solubility temperature for the three formulation systems is also shown in Fig. 1. The solubility temperature decreased significantly as water
Fig. 1. The relationship between moisture content, Tg (open symbols) and solubility temperature (filled symbols) under different sucrose/corn syrup solids ratios 86/14 (square), 78/22 (triangle) and 70/30 (circle). Some error bars are within the size of the symbols.
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content increased (p < 0.05). This result agreed well with literature values (Pancoast and Junk, 1980; Bubnik et al., 1996). In addition, increasing the corn syrup level of the mixture resulted in a slight, but significant (p < 0.05), decrease in solubility due to the competition between corn syrup saccharides and sucrose for hydrogen bonding sites with water molecules (Hartel, 2001). These results agreed with the general trends in the literature (Tjuradi and Hartel, 1995; Jonathan, 2004; Bonifacio et al., 2008). 3.2. Crystal growth rate Typical plots of crystal surface area vs. time are shown in Fig. 2 (8 crystals out of 30 crystals were shown as an example). The change in area of each growing crystal was approximately linear so the slope (change in surface area per minute) was taken as the growth rate of each individual crystal. Each individual crystal had a different growth rate (different slope), a phenomenon called growth rate dispersion (GRD), which is a well-known characteristic of sucrose crystals (Larson et al., 1985; Liang et al., 1987a; Jones et al., 2000; Iswanto et al., 2006). For each condition, at least 30 crystals were tracked, with mean growth rate calculated for each condition. In the current study, the standard deviation of the distribution of growth rates for one condition was taken as the extent of growth rate dispersion for that condition. Fig. 3 illustrates the trends between the growth rate of sucrose crystals and concentration, as governed by the temperature to which the sugar syrup was cooked. For most storage temperatures, increasing concentration resulted in an initial increase in growth rate due to the increasing supersaturation. At higher concentrations, however, the limited molecular mobility caused a decrease in growth rate despite the even higher supersaturation. The peak in growth rate moved to higher concentration at higher crystallization temperature. At the lowest temperature, the growth rate decreased with increasing concentration, probably because the concentration range in this study was not large enough to see the whole trend. In many foods, cooling affects crystallization. For example, fondant is first cooked to about 118 °C to reach a moisture content of
Fig. 2. (A) Change in surface area of sucrose crystals in sugar mixture cooked at 118 °C and observed under 60 °C with sucrose/corn syrup solids ratio 78/22. Each line captures the size progression of an individual crystal. (B) Pictures were chosen at the beginning (0 min), middle (2.25 min) and end of observation (3.75 min). 3 out of 8 crystals were shown as an example. Crystal #1, 2, 3 are ordered from left to right (the leftmost one was not taken into account due to irregular shape).
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Fig. 3. Growth rates (GR) as a function of concentration at different temperatures(40–120 °C) under different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30. Error bars represent the standard deviation of the growth rate distribution.
Fig. 4. Growth rates (GR) as a function of temperature at different concentration (83.3–96.3%) under different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/ 30. Error bars represent the standard deviation of the growth rate distribution.
about 10%. It is then cooled statically to a temperature between 40 and 50 °C, a range often considered to be optimal for promoting crystallization. At this temperature, the sugar mass is agitated intensely to promote crystallization with the specific aim of creating many small crystals for a smooth texture (Hartel, 2001; Hartel et al., 2011). Fig. 4 highlights the effects of crystallization temperature on growth rate in this study. As the sugar syrups were cooled from cook temperature (which governed the total solids content) to different hold temperatures, growth rate initially increased as crystallization temperature decreased. However, after reaching a peak, the growth rate began to decrease again as temperature was lowered further. The initial decrease in temperature below solubility led to an increase in supersaturation, resulting in this increase in the growth rate. A further decrease in temperature resulted in reduced molecular mobility as the system approached Tg, and thus decreased the growth rate. At the highest concentrations, no decreases in growth rate were observed, most likely due to the limited temperature range evaluated (higher temperature could not be reached due to the limit of the hot stage). From Fig. 4, temperatures of peak growth rate fell between 80 °C to 100 °C, significantly higher than the optimal temperature, noted earlier, of 40–50 °C for fondant production (Hartel, 2001).
This was undoubtedly due to the difference in crystallization conditions, primarily in the agitation rate. In this study, crystals grew in a quiescent environment, completely limited by diffusion processes, whereas in fondant, intense agitation is utilized to promote crystallization. Previous work also demonstrated temperature was a very influential factor on crystal growth in thin films. Howell and Hartel (2001) proved that growth rates increased by a factor of 4.5 as temperature increased from 40 to 70 °C. Mazzobre et al. (2003) indicated that there was a clear trend of increasing crystal growth rate when increasing heating temperature. 3.3. Overlay of state diagram and growth kinetics To put the previous results into context, the growth rate data were combined on the state diagram in the form of contour plots, as shown in Fig. 5. The overall growth rate was expected to be highest somewhere in the crystallization zone between the Tg and solubility lines. As these contour plots document, the growth rates descended progressively from a peak zone of fastest growth in all directions of changing temperatures and concentrations. This way of looking at crystallization kinetics relative to the state diagram provides a map that may be used as a predictive tool in
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with previous work (Gabarra and Hartel, 1998; Laos et al., 2007). Note that there were slight differences between solubility temperatures among the three systems. However, the effect of composition on growth rate far outweighed the influence of the difference in solubility temperatures. The inhibition of growth rates of sucrose crystals by adding corn syrup has been attributed to the combination of oligosaccharides (Dorow, 1993). Interference of sucrose crystal growth by adsorption of corn syrup saccharides (monosaccharides, glucose, fructose, and glucose polymers) onto the crystal lattice structure has been proposed for crystallization of sucrose in solution (Bamberger et al., 1980; Tjuradi and Hartel, 1995; Hartel, 2001).
3.4. Growth rate dispersion (GRD)
Fig. 5. Growth rate contour plots for different sucrose/corn syrup solids ratios 86/ 14, 78/22 and 70/30, solubility and glass transition temperature curves are represented by triangle and circle symbols, respectively. Note that maximum growth rate zones per each graphic are not equal.
industrial applications to understand the effects of process conditions on crystallization. An interesting point seen in Fig. 5 is that the peak of crystallization does not fall at the midway point, in either temperature or solids concentration, between solubility and glass transition. In fact, the peak growth rate occurs at a significantly higher temperature than the midpoint between Ts and Tg. Again, this is most likely due to the diffusion-controlled growth studied in these experiments. Specifically, Tg of this sugar mixture is an average of the higher-molecular weight saccharides and the lower molecular weight water. Mobility of the sucrose component of this glass is limited at much higher temperatures than the average, resulting in the peak growth rate being well above the mid-point. Also seen in Fig. 5 is the effect of corn syrup on the positioning of the peak growth zone. The addition of corn syrup dramatically reduced the magnitude of growth rates and diminished the regions of high growth rates (dark zones), with higher addition level having greater effects. In general, the addition of corn syrup significantly decreased the growth rate of sucrose crystals (p < 0.001), with higher level having a greater effect. This result agreed well
GRD was first seen by White and Wright (1971) in batch sucrose crystallization. Typically, changing the solution supersaturation or crystallization temperature will lead to the change of GRD. In this study, GRD was observed under all conditions with each formulation, even though they were grown under the same external conditions. Fig. 6 illustrates the effect of composition on GRD. In general, a lower degree of GRD was observed with an increase in the level of corn syrup. Moreover, all the individual crystals had different growth rates and showed no trend with respect to initial size of the respective crystals (data not shown). The contour plots in Fig. 7 show the combined impact of temperature and concentration on sucrose crystal GRD. The GRD descended progressively from the dark zone to the light zone in all directions of changing temperatures and concentrations. These contour plots were generally in agreement with the growth rate contour plots, which indicate that the GRD increased proportionally with increased growth rates. Howell and Hartel (2001) found a similar trend, where the variability of growth rate increased with increasing temperature, which was attributed to the extent of increase in the GRD with higher supersaturation. This finding also agrees with Liang et al. (1987a), who showed that the GRD of sucrose crystals in stirred and stagnant situations increased with the square of the average growth rate. This relationship can also be seen in Fig. 8 for sucrose/corn syrup solids ratio 78/22; systems 86/14 and 70/30 followed the same trend. Furthermore, based on informal macroscopic observation of the slides, the differences in GRD had no apparent influence on the appearance of thin films between different levels of corn syrup. By overlaying GRD zones on the state diagram, the competing effects of supersaturation driving force and molecular mobility inhibition on sucrose crystal GRD were clearly observed. This graph can be used to predict optimal coating process conditions based on different state behaviors to deliver desired sensory attributes and processing behavior in food systems.
Fig. 6. Growth rate dispersion for sugar mixtures cooked to 124 °C and observed under 120 °C for different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30.
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4. Conclusions In this study, moisture content decreased as cooking temperatures increased. In addition, there were no significant differences in moisture content between the three formulation systems, indicating the difference in sucrose to corn syrup ratio had no significant effect on boiling point elevation. As expected, Tg and solubility temperature decreased significantly as moisture content increased, which agrees with previous research. The impact of sucrose to corn syrup ratio was more evident on solubility temperature than Tg. No significant differences in Tg were detected among the three systems, whereas the solubility temperatures were notably depressed as the corn syrup level increased. Sucrose crystal growth rates were highly related to temperature, concentration, and sucrose to corn syrup ratio. Concentration and temperature significantly influenced sucrose crystal growth rates, with increases in concentration and temperature initially giving higher growth rates; however, after reaching a maximum point, growth rates started to decrease as either supersaturation or molecular mobility decreased. The addition of corn syrup reduced the magnitude of growth rates and diminished the regions of high growth rates (maximum zones per each graphic are not equal in contour plots), with higher addition level having greater effects. Evidence of GRD was found in all experiments for systems of sucrose, corn syrup and water, showing that individual crystals grown under identical conditions do not have the same growth rates. The growth rate distributions of all crystals in this study were approximately normally distributed. The extent of the GRD increased with increasing growth rate. Similar trends were seen from the contour plots of GRD as concentration and temperature changed. In general, a lower degree of GRD was observed with an increase in the level of corn syrup, related to the reduction in average growth rate. Conflict of interest The authors declare no conflict of interest. Fig. 7. Growth rate dispersion (GRD: standard deviation of growth rate distribution) contour plot for different sucrose/corn syrup solids ratios 86/14, 78/22 and 70/30, solubility and glass transition temperature curves are represented by triangle and circle symbols, respectively. Note that maximum growth rate zones per each graphic are not equal.
Acknowledgement We acknowledge the financial support for this work from General Mills, Inc and figure revision done by Jamie Gutkowski. References
Fig. 8. Growth rate dispersion (GRD: standard deviation of growth rate distribution) as a function of growth rate (GR) for sucrose/corn syrup solids ratio 78/ 22.
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