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Interactions and Contributions of Stabilizers and Emulsifiers to Development of Structure in Ice-cream
Interactions and Contributions of Stabilizers and Emulsifiers to Development of Structure in Ice-cream By H. Douglas Goff DEPARTMENT OF FOOD SCIENCE, UNIVERSITY OF GUELPH, GUELPH, ONTARIO N1G 2W1, CANADA
1 Introduction Ice-cream is a complex food colloid that consists of air bubbles, fat globules, ice crystals, and an unfrozen serum phase. The air bubbles are usually lined with fat globules' and the fat globules are coated with a protein-emulsifier layer2. The serum phase consists of the sugars and high-molecular-weight polysaccharides in a freeze-concentrated solution. Various steps in the manufacturing process, including pasteurization, homogenization, ageing, freezing, and hardening, contribute to the development of this structure. Two categories of additives are usually incorporated into ice-cream mix formulations, the emulsifiers and the stabilizers. Each of these contributes to the final structure and texture of the ice-cream, but their mode of action is very different. Emulsifiers (mainly small-molecule surfactants) are used to promote dryness during extrusion and a smooth texture with slow meltdown. Stabilizers (mainly polysaccharides) are used in frozen foods such as ice-cream to protect the product from the development of a coarse texture during temperature fluctuations which may occur during storage and distribution. The objectives of this research were to examine the mode of action of these two additives.
2 Materials and Methods The action of the emulsifiers was explored using various combinations of proteins and surfactants in both mixes and model systems by measuring the fat-serum interfacial tension with a du Nuoy ring surface tensiometer; measuring the surface excess on the emulsion droplets by centrifugal removal of the fat globules and determining adsorbed protein by difference (Kjeldahl) ; examining interfacial layers by transmission electron microscopy (TEM) and image analysis; and measuring emulsion destabilization by
72
Development of Structure in Ice-cream
spectrometry during freezing of ice-cream m i x e ~ . ~The - ~ action of the stabilizers was studied using various concentrations and types of polysaccharides, in both mixes (11% fat, 11% milk solids, non-fat, 12% sucrose, 4% 42DE corn syrup solids) and model systems,.by measuring the glass transition temperature T , , the amount of frozen water, and the onset of melting by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA); measuring the viscosity at subzero temperatures by TMA parallel-plate rheometry ; and examining the ice-crystal size distributions as a function of storage times and temperatures by low-temperature (cryo-) scanning electron microscopy (LT-SEM) and image analysis. 596
3 Results and Discussion The Behaviour of the Surfactants The adsorption of milk proteins to fat globules lowered the interfacial tension of the fat-serum interface from 8.26 mN m-l to 5.5 mN m-l. However, the addition of a surfactant (polysorbate 80) lowered the interfacial tension further than was accomplished by the proteins alone, to 2.24 mNm-' in the presence of the milk protein, thus favouring its preferential adsorption to the €at globule surface. This led to a reduction in the amount of protein which was adsorbed, from 15.9 wt YO of the total protein in the ice-cream mix to 7.8 wt YO in the presence of the polysorbate 80. TEM techniques demonstrated that there were significantly more casein micelles adsorbed to the fat globules in the absence of the surfactant (3.82 f 0.25 per fat globule section) than in the presence (2.20 +. 0.18 per fat globule section). The emulsifier had no effect on the size distribution of the globules in the mix. The lowering of the surface excess causes the ice-cream-mix emulsion to be less stable to the shear forces encountered in the barrel freezer which result from the formation of ice crystals and the mechanical action of the knives and dashers. This results in a controlled amount of fat globule aggregation (emulsion destabilization) around the aif bubbles. Partial coalescence imparts desirable structure and texture to the ice-cream.
The Behaviour of the Polysaccharides The controlling effect of the stabilizers on ice crystal growth was demonstrated by LT-SEM. Ice-cream mixes without stabilizer and with 0.15 wt% locust bean gum and 0.02 wt YO carrageenan added, which were continuously frozen and blast hardened (-25 "C), were compared after preparation and after 24 weeks of storage at temperatures fluctuating daily between -25 "C and -10 "C. Image analysis of the micrographs from this study illustrated the growth in size of the ice crystals that occurred as a
H . D . Goff
73
function of time during the temperature-abusive storage, and also demonstrated the protective effect that the stabilizers provided. Both the initial size of the crystals and the rate of growth of the crystals were reduced in the presence of the stabilizers. The effect of polysaccharides on the phase behaviour of sucrose solutions was examined to determine whether differences existed in low temperature thermal events occurring in DSC or TMA scans, i.e. the glass transition temperature T, of the maximally freeze-concentrated solution (a glass being defined as an amorphous metastable solid with a high viscosity of Pas), the onset temperature of the melting endotherm, the peak maximum temperature, or the enthalpy of the melting endotherm (hence the amount of frozen water). The polysaccharide was found to have no effect on the thermal behaviour of the sucrose solution. The viscosity of the ice-cream mixes with and without added stabilizer at sub-ambient temperatures was calculated from sample deformation measurements in the parallel plate rheometer attachment of the TMA. A large divergence in the two samples was seen at temperatures below -14 "C. At a temperature of -18 "C, the viscosity of the stabilized sample was 2.7 X lo6 Pas as compared with 1 x lo6 Pas in the unstabilized sample at the same temperature. Although the polysaccharides have been shown to provide beneficial sensory attributes in frozen systems, it has been difficult to demonstrate their mode of action.' Recent research on the subject of cryostabilization8y9 suggests that kinetic mobility is the overriding mechanism in frozen food stability. The results presented here show that polysaccharides increase viscosity at temperatures above T , thus reducing molecular mobility in the freeze-concentrated visco-elastic serum phase and providing resistance to recrystallization and structural collapse.
Acknowledgements The author wishes to acknowledge the contributions of M. Liboff (Cornell University), J. Kinsella (University of California- Davis), K. Caldwell (Ault Foods), and M. Sahagian (University of Guelph) for their contributions to this research, and to thank the Wisconsin Milk Marketing Board, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Ministry of Agriculture and Food for support of this project.
References 1. K. B. Caldwell, H. D. Goff, and D. W. Stanley. Food Struct., 1992, 11, 1. 2. H. D. Goff, M. Liboff, W. K. Jordan, and J. E. Kinsella. Food Microstruct., 1987, 6, 193. 3. H. D. Goff and W. K. Jordan. J. Dairy Sci., 1989, 72, 18. 4. H. D. Goff, J. E. Kinsella, and W. K. Jordan. J. Dairy Sci., 1989, 72, 385. 5. K. B. Caldwell, H. D. Goff, and D. W. Stanley. Food Struct., 1992, 11, 11.
74
Development of Structure in Ice-cream
6. H. D. Goff, K. B. Caldwell, D. W. Stanley, and T. J. Maurice. J . Dairy Sci., 1992, submitted. 7. A . H. Muhr and J. M. V. Blanshard. J . Food Technol., 1986, 21, 683. 8. H. Levine and L. Slade, in ‘Thermal Analysis of Foods’, ed. V. R. Harwalker and C. Y. Ma, Elsevier Applied Science, New York, 1990, p. 221. 9. H. D. Goff.Food Res. Int., 1992, accepted.
1 Introduction Ice-cream is a complex food colloid that consists of air bubbles, fat globules, ice crystals, and an unfrozen serum phase. The air bubbles are usually lined with fat globules' and the fat globules are coated with a protein-emulsifier layer2. The serum phase consists of the sugars and high-molecular-weight polysaccharides in a freeze-concentrated solution. Various steps in the manufacturing process, including pasteurization, homogenization, ageing, freezing, and hardening, contribute to the development of this structure. Two categories of additives are usually incorporated into ice-cream mix formulations, the emulsifiers and the stabilizers. Each of these contributes to the final structure and texture of the ice-cream, but their mode of action is very different. Emulsifiers (mainly small-molecule surfactants) are used to promote dryness during extrusion and a smooth texture with slow meltdown. Stabilizers (mainly polysaccharides) are used in frozen foods such as ice-cream to protect the product from the development of a coarse texture during temperature fluctuations which may occur during storage and distribution. The objectives of this research were to examine the mode of action of these two additives.
2 Materials and Methods The action of the emulsifiers was explored using various combinations of proteins and surfactants in both mixes and model systems by measuring the fat-serum interfacial tension with a du Nuoy ring surface tensiometer; measuring the surface excess on the emulsion droplets by centrifugal removal of the fat globules and determining adsorbed protein by difference (Kjeldahl) ; examining interfacial layers by transmission electron microscopy (TEM) and image analysis; and measuring emulsion destabilization by
72
Development of Structure in Ice-cream
spectrometry during freezing of ice-cream m i x e ~ . ~The - ~ action of the stabilizers was studied using various concentrations and types of polysaccharides, in both mixes (11% fat, 11% milk solids, non-fat, 12% sucrose, 4% 42DE corn syrup solids) and model systems,.by measuring the glass transition temperature T , , the amount of frozen water, and the onset of melting by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA); measuring the viscosity at subzero temperatures by TMA parallel-plate rheometry ; and examining the ice-crystal size distributions as a function of storage times and temperatures by low-temperature (cryo-) scanning electron microscopy (LT-SEM) and image analysis. 596
3 Results and Discussion The Behaviour of the Surfactants The adsorption of milk proteins to fat globules lowered the interfacial tension of the fat-serum interface from 8.26 mN m-l to 5.5 mN m-l. However, the addition of a surfactant (polysorbate 80) lowered the interfacial tension further than was accomplished by the proteins alone, to 2.24 mNm-' in the presence of the milk protein, thus favouring its preferential adsorption to the €at globule surface. This led to a reduction in the amount of protein which was adsorbed, from 15.9 wt YO of the total protein in the ice-cream mix to 7.8 wt YO in the presence of the polysorbate 80. TEM techniques demonstrated that there were significantly more casein micelles adsorbed to the fat globules in the absence of the surfactant (3.82 f 0.25 per fat globule section) than in the presence (2.20 +. 0.18 per fat globule section). The emulsifier had no effect on the size distribution of the globules in the mix. The lowering of the surface excess causes the ice-cream-mix emulsion to be less stable to the shear forces encountered in the barrel freezer which result from the formation of ice crystals and the mechanical action of the knives and dashers. This results in a controlled amount of fat globule aggregation (emulsion destabilization) around the aif bubbles. Partial coalescence imparts desirable structure and texture to the ice-cream.
The Behaviour of the Polysaccharides The controlling effect of the stabilizers on ice crystal growth was demonstrated by LT-SEM. Ice-cream mixes without stabilizer and with 0.15 wt% locust bean gum and 0.02 wt YO carrageenan added, which were continuously frozen and blast hardened (-25 "C), were compared after preparation and after 24 weeks of storage at temperatures fluctuating daily between -25 "C and -10 "C. Image analysis of the micrographs from this study illustrated the growth in size of the ice crystals that occurred as a
H . D . Goff
73
function of time during the temperature-abusive storage, and also demonstrated the protective effect that the stabilizers provided. Both the initial size of the crystals and the rate of growth of the crystals were reduced in the presence of the stabilizers. The effect of polysaccharides on the phase behaviour of sucrose solutions was examined to determine whether differences existed in low temperature thermal events occurring in DSC or TMA scans, i.e. the glass transition temperature T, of the maximally freeze-concentrated solution (a glass being defined as an amorphous metastable solid with a high viscosity of Pas), the onset temperature of the melting endotherm, the peak maximum temperature, or the enthalpy of the melting endotherm (hence the amount of frozen water). The polysaccharide was found to have no effect on the thermal behaviour of the sucrose solution. The viscosity of the ice-cream mixes with and without added stabilizer at sub-ambient temperatures was calculated from sample deformation measurements in the parallel plate rheometer attachment of the TMA. A large divergence in the two samples was seen at temperatures below -14 "C. At a temperature of -18 "C, the viscosity of the stabilized sample was 2.7 X lo6 Pas as compared with 1 x lo6 Pas in the unstabilized sample at the same temperature. Although the polysaccharides have been shown to provide beneficial sensory attributes in frozen systems, it has been difficult to demonstrate their mode of action.' Recent research on the subject of cryostabilization8y9 suggests that kinetic mobility is the overriding mechanism in frozen food stability. The results presented here show that polysaccharides increase viscosity at temperatures above T , thus reducing molecular mobility in the freeze-concentrated visco-elastic serum phase and providing resistance to recrystallization and structural collapse.
Acknowledgements The author wishes to acknowledge the contributions of M. Liboff (Cornell University), J. Kinsella (University of California- Davis), K. Caldwell (Ault Foods), and M. Sahagian (University of Guelph) for their contributions to this research, and to thank the Wisconsin Milk Marketing Board, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Ministry of Agriculture and Food for support of this project.
References 1. K. B. Caldwell, H. D. Goff, and D. W. Stanley. Food Struct., 1992, 11, 1. 2. H. D. Goff, M. Liboff, W. K. Jordan, and J. E. Kinsella. Food Microstruct., 1987, 6, 193. 3. H. D. Goff and W. K. Jordan. J. Dairy Sci., 1989, 72, 18. 4. H. D. Goff, J. E. Kinsella, and W. K. Jordan. J. Dairy Sci., 1989, 72, 385. 5. K. B. Caldwell, H. D. Goff, and D. W. Stanley. Food Struct., 1992, 11, 11.
74
Development of Structure in Ice-cream
6. H. D. Goff, K. B. Caldwell, D. W. Stanley, and T. J. Maurice. J . Dairy Sci., 1992, submitted. 7. A . H. Muhr and J. M. V. Blanshard. J . Food Technol., 1986, 21, 683. 8. H. Levine and L. Slade, in ‘Thermal Analysis of Foods’, ed. V. R. Harwalker and C. Y. Ma, Elsevier Applied Science, New York, 1990, p. 221. 9. H. D. Goff.Food Res. Int., 1992, accepted.