- Email: [email protected]
Formation and stabilisation of structure in ice-cream and related products
Current Opinion in Colloid and Interface Science 7 (2002) 432–437
Formation and stabilisation of structure in ice-cream and related products H. Douglas Goff* Department of Food Science, University of Guelph, Guelph, ON,Canada N1G 2W1
Abstract Partial coalescence of the fat emulsion and its control by manipulation of the oilywater interface continues to be an active area of ice cream research and understanding at both the basic and applied levels have greatly improved. Interactions between all the discrete phases are increasingly being studied in more complex systems, leading to an appreciation of such things as the effect of air bubble size distribution on ice recrystallization. The importance of protein polysaccharide phase separation in the freezeconcentrated unfrozen phase and its effect on ice recrystallization has also been recognised. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Emulsion; Foam; Partial coalescence; Aeration; Fat; Protein; Polysaccharides; Phase separation
1. Introduction Ice cream and related aerated frozen desserts are complex-colloidal systems comprised of, in their frozen state: ice crystals; air bubbles; partially-coalesced fat globules and aggregates; all in discrete phases surrounded by an unfrozen continuous matrix of sugars, proteins, salts, polysaccharides and water (Fig. 1). Their manufacture usually begins by formulating, pasteurising, homogenising and cooling an emulsion premix, followed by aerating and freezing this premix under high shear conditions in a scraped surface freezer. The aeration and freezing process involves numerous physical changes including: the action of proteins and surfactants in forming and stabilising the foam phase; partial coalescence of the fat emulsion causing both absorption of fat at the air interface and formation of fat globule clusters that stabilise the lamellae between air bubbles; and freeze concentration of the premix by the removal of water from solution in the form of ice. Colloidal aspects of ice cream were reviewed in 1997 w1x and numerous colloid related papers can be found in the proceedings of an international ice cream conference published in 1998 w2x. This paper will discuss recent advances in colloidal aspects of ice cream published since these reviews, with a particular emphasis on the period since 2000. It will categorise specific developments along the lines of the structural elements, viz. fat, air, ice and the *Tel.: q1-519-824-4120 x3878; fax: q1-519-824-6631. E-mail address: [email protected] (H.D. Goff).
serum phase, although it should be recognised that many aspects are inter-related and thus categorisation becomes somewhat arbitrary. 2. Fat globules The optimal formation of fat structure in ice cream is responsible for many desirable properties including dryness and shape retention after scraped surface freezing and slowness of meltdown and smooth eating textural properties after hardening w1,3x. Several recent papers have discussed fat sources w4x, non-fat sources to provide fat-like properties, the so-called ‘fat replacers’ w5,6●,7x and the effect of fat on sensory properties w8–10x and flavour perception w11x. Research work related to colloidal fat structure has focused on: homogenisation of the pre-mix w12–14●●,60x; protein surfactant interactions at the fat interface w15,16●,17●●,18,19●,20●,21–24x; partial coalescence w25x and fat structure formation during freezing w26–28●●x; fat partial coalescence measurement in ice cream w17●●,29x; the effect of fat structure on ice cream meltdown w14●●x; and the effect of low temperature extrusion of ice cream on fat partial coalescence and structure formation w30,31x. From the work of Koxholt et al. w14●●x, Barfod w20●x and Goff et al. w28●●x, our understanding of partiallycoalesced fat structures has been advanced. Analogies to fat partial coalescence and fat structure formation in whipped cream, although in some respects similar, have
1359-0294/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 Ž 0 2 . 0 0 0 7 6 - 6
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
433
Fig. 1. The structure of ice cream mix, ice cream and melted ice cream. (a) Ice cream mix as viewed by thin section transmission electron microscopy. f, Fat globule; c, casein micelle; arrow, crystalline fat within the globule; bar wshown in (d)xs0.5 mm. (b) Close-up of an air bubble in ice cream as viewed by low temperature scanning electron microscopy. a, Air bubble; f, fat globule adsorbed to the bubble surface; bar wshown in (d)xs10 mm. (c) Ice cream as viewed by thin section transmission electron microscopy with freeze substitution and low temperature embedding. a, Air bubble; f, fat globule; fc, fat cluster; bar wshown in (d)xs1 mm. (d) Melted ice cream as viewed by thin section transmission electron microscopy. c, Casein micelle; fn, fat network; bars5 mm. For methodology, see Goff et al. w28●●x.
been shown to be incomplete. While whipped cream is typically stabilised at maximum firmness by almost complete coverage of the air interface with agglomerated fat, such is not the case with ice cream. Rather, fat agglomerates have been shown to provide structure to the lamellae between air bubbles offering resistance to collapse during meltdown and, in conjugation also with the development of thin lamellae, to ice recrystallization. Fat at the air interface tends to be more in the form of discrete droplets. Improved colloidal understanding of meltdown phenomena w14●●,17●●x has led to a proposed simple method for fat structure formation analysis, based on mass and fat drip loss during melting w17●●,29x. The composition of the fat interface and its role in partial coalescence also continues to receive considerable research attention. Bolliger et al. w17●●x showed a
direct relationship between protein content (mg my2), resulting from displacement by emulsifiers, and partial coalescence. Davies et al. w18,19●x suggested that the effect of saturated vs. unsaturated emulsifiers on partial coalescence might be more than one of protein displacement but might also effect fat crystal habit within the globules. Innocente et al. w21x suggested that the proteose-peptone fraction of whey proteins might serve to replace conventional emulsifiers in ice cream applications. The work of Segall and Goff w16●,22,23x has focused on a minimal coverage of whey protein at the fat interface, as created by selective homogenisation, as a means of achieving optimal partial coalescence in formulations with no added emulsifiers. Koxholt et al. w14●●x also showed that selective homogenisation can produce acceptable fat globule size distributions.
434
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
Ice cream is conventionally removed from a scraped surface freezer at about y5 8C. However, further freezing under low shear conditions in a twin-screw extruder makes extrusion at temperatures as low as y12 to y15 8C possible w30x. The effect of this low temperature extrusion process on the formation of structural elements suggests that formulation changes are required to optimise fat partial coalescence w31x. 3. Air bubbles Ice cream and related products are generally aerated and characterised as frozen foams. The gas phase volume varies greatly from a high of greater than 50% to a low of 10–15%. Air is distributed in the form of numerous small air bubbles of size range 20–50 mm. Whilst a great deal is known about protein surfactant interactions at the fat interface, as discussed previously, it seems that far less work has been done on the air interface. More work has been done on non-frozen systems, as reviewed in 1999 w32x. This review focused on stabilisation of air bubbles by structure formation through partial coalescence and also looked at protein surfactant films and displacement studies at both the oil water and air water interface in aerated emulsions. Recent colloidal research on foaming aspects of ice cream and related frozen products has focused on: development of the air phase w13,33x; morphology and channelling of the air bubbles w34x; interactions of fat and air in stabilising the air phase w20●,28●●,35x; and measurement of air bubble structures w34,36●,61x. The scraped surface freezer is responsible for ice crystal formation, but also for air comminution into a fine distribution of bubbles, which are thought to be correlated with enhanced sensory perception of smoothness. Koxholt et al. w13x addressed the question of optimal residence time in the scraped surface freezer by suggesting that opposing phenomena were involved: smallest ice crystal size distributions suggest minimal residence times but smallest air bubble size distributions resulted from longer residence times. They advocated pre-aeration of the mix prior to freezing to optimise both independently. Minimisation of both size distributions has also been claimed to be a benefit of low temperature extrusion w30x. The effect of small air bubble size distributions, as enhanced by surfactants, on minimising ice recrystallization has also recently been shown by Barfod w20●x. Our understanding of the stability of the air phase has been enhanced by the work of Turan et al. w34,35x. They have shown that loss of the discrete nature of the gas bubbles and channelling, leading to a continuous network of coalesced bubbles, is related to volume collapse (shrinkage) and can be measured by examining the response of frozen ice cream to fluctuating pressures. Discrete and independent air
bubbles correlate to expectations based on the gas laws while channelled air networks do not. 4. Ice crystals Ice crystals form another discrete phase in ice cream and it is well recognised that formulations and manufacturing and handling procedures that lead to numerous small, discrete ice crystals also lead to enhanced smoothness in texture. Ice crystal formation in ice cream is generally by secondary nucleation in the scraped surface freezer w37x. However, small ice crystals rapidly undergo recrystallization phenomena, especially in the presence of temperature fluctuations (reviewed in w38,39x). This can be controlled by the maintenance of low constant temperature and by the presence in the formulation of stabilising agents such as polysaccharide gums w40,41x. Since recrystallization leads to rapid loss of quality, much research emphasis has been devoted to its control, particularly polysaccharide functionality w40–45x. However, a thorough review of ice recrystallization is outside the scope of this paper. Rather, discussion here will focus on the relationship between fat and air structures and ice recrystallization w13,20●,30x and on the effect of freeze concentration on polysaccharide interactions, as discussed in the next section. Somewhat unique in the colloidal context is the work by Barfod w20●x showing the interrelationships between fat structure formation, air phase volume, air bubble size distribution and ice recrystallization. Optimisation of fat structure and minimisation of air bubble size gives rise to enhanced protection against ice recrystallization. This might also be accomplished through low temperature extrusion w30x. 5. Serum phase As the premix becomes freeze-concentrated, components that are either dissolved or dispersed in water are increasingly brought together as temperature decreases and water is removed from solution and this also can cause numerous physical and chemical changes to serum-phase components. Two of the important hydrocolloid components in the serum include the polysaccharides and the casein micelles, caseins and whey proteins from the milk sources. Protein functionality in ice cream has recently been reviewed w3,46x. In both the pre-mix and in the unfrozen solution phase of the frozen product, proteins and polysaccharides phase separate due to solution incompatibility. This phenomenon has also been reviewed recently w47●,48● x. After extensive freeze concentration, due to both low temperature and high viscosity, components of the unfrozen phase can enter into the glassy state w49x. In keeping with the colloid focus of this review, recent research to be noted includes: that related to source of polysaccharide stabi-
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
lisers for ice cream use w50,51x; polysaccharide solution properties in situations relevant to ice cream manufacture w42–45,52–55x; properties of proteins in the freezeconcentrated serum phase in ice cream w56x; and protein polysaccharide phase separations under conditions relevant to ice cream manufacture w57–59x. In the highly concentrated state, it has been shown by Goff and co-workers that some polysaccharides used as ice cream stabilisers can form a weak gel that is formed and strengthened by temperature fluctuations causing partial or complete freeze–thaw cycles w42,44,45,55x. Despite the structural role that such a cryo-gel in the unfrozen phase might contribute, it has not, however, been shown to correlate by itself with ice recrystallization inhibition. Rather, it appears that protein polysaccharide phase separation in the unfrozen phase can offer enhanced ice recrystallization inhibition compared to independent polysaccharide action w40– 42,44,45x. Casein micelles, which are not affected structurally by freeze concentration w56x, separate readily from polysaccharides w57–59x. Regand and Goff w44x showed by microscopic techniques that proteins, not polysaccharides, resided in closest proximity to ice crystals after scraped surface freezing of model ice cream solutions. It thus appears that ice recrystallization inhibition results from a highly concentrated, viscous and heterogeneous layer that surrounds ice crystals. Such a layer would affect mobility of water and sugars (which must be in equilibrium at the surface of the ice crystal to invoke freezing and thawing along the equilibrium freezing curve with temperature fluctuation w44,45x), if not molecular diffusion w52x, thus promoting thawing and refreezing on existing ice crystals. 6. Conclusions The complexity and practicality of colloidal structure in ice cream and related products continues to be a source of research interest, as evidenced by the fact that nearly half of the references cited in this paper are from the year 2000 and later and all have been published since 1997. Although the importance of fat partial coalescence in ice cream has long been recognised w1x, our understanding of the morphology and functionality of fat agglomerates has improved in the last few years, and we now have a more complete picture of the interaction of the various structural elements. Manipulating the fat interface by surfactants, proteins and fractions thereof will give us broader means of structure formation. We have also come to realise the importance of protein polysaccharide incompatibility and phase separation, especially in the freeze-concentrated matrix, in the last few years and this will undoubtedly continue to be an area of fruitful research. The phase that lags behind in terms of improvements in fundamental understanding is the air phase. While some progress has been
435
made in understanding the wider implications of the dispersed nature of the gas phase, we know far less about the composition and competition amongst constituents of the air interface than we do about the fat interface. References ● of special interest ●● of outstanding interest w1x Goff HD. Colloidal aspects of ice cream—a review. Int Dairy J 1997;7:363 –73. w2x Buchheim W. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. w3x Walstra P, Jonkman M. The role of milkfat and protein in ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 17 – 24. w4x Abd El-Rahman AM, Madkor SA, Ibrahim FS, Kilara A. Physical characteristics of frozen desserts made with cream, anhydrous milk fat, or milk fat fractions. J Dairy Sci 1997;80:1926 –35. w5x Ohmes RL, Marshall RT, Heymann H. Sensory and physical properties of ice creams containing milk fat or fat replacers. J Dairy Sci 1998;81:1222 –8. w6x Adapa S, Dingeldein H, Schmidt KA, Herald TJ. Rheological ● properties of ice cream mixes and frozen ice creams containing fat and fat replacers. J Dairy Sci 2000;83:2224 –9. This paper measured viscoelastic properties of ice cream mix and ice cream as a function of fat content and the presence of fat replacers. It was shown that the amount of fat in ice cream and the degree of fat destabilization affected elasticity, but that the addition of protein-based and carbohydrate-based fat replacers did not enhance the elastic properties of the ice creams, but rather increased the viscous properties. One unique aspect of this work was the method used to measure viscoelastic properties in the frozen ice creams. Product from the scraped surface freezer was spread on to petri dishes and then hardened. A hollow cylinder was then used to cut disks of frozen product that were then transferred on to the precooled rheometer plate. w7x Aime DB, Arntfield SD, Malcolmson LJ, Ryland D. Textural analysis of fat reduced vanilla ice cream products. Food Res Int 2001;34:237 –46. w8x Guinard J-X, Zoumas-Morse C, Mori L, Uatoni B, Panyam D, Kilara A. Sugar and fat effects on sensory properties of ice cream. J Food Sci 1997;62:1087 –94. w9x Roland AM, Phillips LG, Boor KJ. Effects of fat content on the sensory properties, melting, color and hardness of ice cream. J Dairy Sci 1999;82:32 –8. w10x Prindiville EA, Marshall RT, Heymann H. Effect of milk fat on the sensory properties of chocolate ice cream. J Dairy Sci 1999;82:1425 –32. w11x Li Z, Marshall RT, Heymann H, Fernando L. Effect of milk fat content on flavor perception of vanilla ice cream. J Dairy Sci 1997;80:3133 –41. w12x Thomsen M, Holtsborg J. The effect of homogenisation pressure and emulsifier type on ice cream mix and finished ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 105 – 11. w13x Koxholt M, Eisenmann B, Hinrichs J. Effect of process parameters on the structure of ice-cream. Eur Dairy Mag 2000;12(1):27 –30.
436
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
w14x Koxholt MMR, Eisenmann B, Hinrichs J. Effect of the fat ●● globule sizes on the meltdown of ice cream. J Dairy Sci 2001;84:31 –7. These authors systematically studied the effect of homogenisation pressure and resultant fat globule size distributions on fat partial coalescence and ice cream melt down properties. They suggested that homogenisation pressures of at least 10 MPa were sufficient to produce fat agglomeration and appropriate melt down properties and that two-stage homogenisation produced no further improvements. These pressure recommendations are considerably lower than those conventionally used in ice cream manufacturing, and could lead to substantial energy savings. They also suggested that selective homogenisation of fat with only a portion of the serum was also effective at producing optimal size fat globules and aggregates in the mix and ice cream, respectively, and that such a process could also lead to energy saving or to the ability to add shear-sensitive ingredients to the non-homogenised portion (see also Segall and Goff w16●x). They proposed a model to explain the effects of fat aggregates on shape retention and meltdown. They suggested that once fat aggregates reach the size of the lamellae between air bubbles, collapse of the foam during meltdown is prevented and structure becomes stabilised, but coverage of the air interface by fat is not required. This model is supported by the microstructural evidence presented in Goff et al. w28●●x. w15x Pelan BMC, Watts KM, Campbell IJ, Lips A. The stability of aerated milk protein emulsions in the presence of small molecule surfactants. J Dairy Sci 1997;80:2631 –87. w16x Segall KI, Goff HD. Influence of adsorbed milk protein type ● and surface concentration on the quiescent and shear stability of butteroil emulsions. Internat Dairy J 1999;9:683 –91. In this paper and the two related ones w22,23x, the concept of creating fat emulsion droplet interfaces with optimal protein loads for perikinetic stability but orthokinetic instability is explored. This was accomplished by selectively homogenising the fat in the presence of only a small portion of whey protein, followed by addition of micellar casein and additional whey protein posthomogenisation. Such a process could lead to optimal partial coalescence in the absence of emulsifier addition. w17x Bolliger S, Goff HD, Tharp BW. Correlation between col●● loidal properties of ice cream mix and ice cream. Internat Dairy J 2000;10:303 –9. In this paper, good correlation was shown between the quantity of protein absorbed at the fat globule interface (mg my2), which was varied by varying emulsification levels in the mix, and several measures of fat partial coalescence including turbidity, solvent extractable fat and integrated laser light scattering methods. Correlation was also shown between fat content in the dripped portion of a meltdown test and fat partial coalescence determined by the above methods. This gives rise to a new, simple method for measurement of fat structure formation and it has also been shown to be independent of the viscous effects of added stabiliser w29x. w18x Davies E, Dickinson E, Bee RD. Shear stability of sodium caseinate emulsions containing monoglyceride and triglyceride crystals. Food Hydrocolloids 2000;14:145 –53. w19x Davies E, Dickinson E, Bee RD. Orthokinetic destabilization ● of emulsions by saturated and unsaturated monoglycerides. Internat Dairy J 2001;11:827 –36. In this paper and a related one w18x, three emulsifier systems: glycerol monooleate; glycerol monopalmitate; and glycerol monostearate, were studied in protein stabilised emulsions relevant to ice cream. Fat partial coalescence after shear decreased in the order of GMO)GMP)GMS, due to two factors, competitive displacement of proteins from the fat interface and fat crystal morphology. The best combination for quiescent stability and shear instability was a 1ow level of protein displacement and creation of spiky spherulites made up of crystals that can penetrate the fat droplet surface. A combination of GMOqGMS or GMOqGMP was
found to be superior in this regard to GMO alone. A cautionary note was also given, though. This study used a combination of peanut oil and tristearin crystals as the discrete phase in a model emulsion but its applicability to more complex food systems with more conventional fat sources was not studied. w20x Barfod N. The emulsifier effect. Dairy Industries Internat ● 2001;66:32 –3. In addition to the conventional functionality of emulsifiers in fat destabilization, these authors showed that emulsifiers increased air bubble stability and resulted in a finer distribution of air bubbles. These factors, especially when associated with a higher total air phase volume, protected the ice cream from excessive ice crystal growth during heat shock. They attributed the reduction in ice recrystallization to enhanced formation of a partially coalesced fat network that retarded movement of structural elements. w21x Innocente N, Comparin D, Corrandini C. Proteose-peptone whey fraction as emulsifier in ice-cream preparation. Internat Dairy J 2002;12:69 –74. w22x Segall KI, Goff HD. A modified processing routine for ice cream that promotes fat destabilization in the absence of added emulsifier. Internat Dairy J 2002, in press. w23x Segall KI, Goff HD. Secondary adsorption of milk protein from the continuous phase to the oil–water interface in dairy emulsions. Internat Dairy J 2002, in press. w24x Sourdet S, Relkin P, Fosseux P-Y, Aubry V. Composition of fat protein layer in complex food emulsions at various weight ratios of casein-to-whey proteins. Le Lait 2002, in press. w25x Vanapalli SA, Coupland JN. Emulsions under shear—the formulation and properties of partially coalesced lipid structures. Food Hydrocolloids 2001;15:507 –12. w26x Campbell IJ, Pelan BMC. The influence of emulsion stability on the properties of ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 25 –36. w27x Kokubo S, Sakurai K, Iwaki S, Tomita M, Yoshida S. Agglomeration of fat globules during the freezing process of ice cream manufacturing. Milchwissenschaft 1998;53:206 –9. w28x Goff HD, Verespej E, Smith AK. A study of fat and air ●● structures in ice cream. Internat Dairy J 1999;9:817 –29. Cryo-scanning and freeze substitution transmission electron microscopy techniques were used to examine the fine structure of partially coalesced fat networks in ice cream. Varying degrees of fat partial coalescence were induced by varying emulsifier type, concentration and shear levels. Increasing fat destabilization levels were seen as increasing partially coalesced fat agglomerates extending from the air interface into the serum phase and in the serum phase itself and enhanced adsorption of discrete fat droplets at the air interface. Even at the highest levels of fat destabilization the air interface was not completely covered by fat (see also w14●●x). w29x Goff HD, Spagnuolo P. Effect of stabilisers on fat destabilization measurements in ice cream. Milchwissenschaft 2001;56:450 –3. w30x Wildmoser H, Windhab EJ. Impact of flow geometry and processing parameters in ultra low temperature ice-cream extrusion (ULTICE) on ice-cream microstructure. Eur Dairy Mag 2001;13(10):26 –32. w31x Bolliger S, Kornbrust B, Goff HD, Tharp BW, Windhab EJ. Influence of emulsifiers on ice cream produced by conventional freezing and low temperature extrusion processing. Int Dairy J 2000;10:497 –504. w32x Leser ME, Michel M. Aerated milk protein emulsions—new microstructural aspects. Curr Opin Colloid Interface Sci 1999;4:239 –44. w33x Chang YH, Hartel RW. Development of air cells in a batch ice cream freezer. J Food Eng 2002;55:77 –8.
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437 w34x Turan S, Bee RD. Measurement of gas phase morphology in ice cream. In: Campbell GM, Webb C, Pandiella SS, Niranjan K, editors. Bubbles in Food. St. Paul, MN: Eagen Press, 1999. p. 183 –9. w35x Turan S, Kirkland M, Trusty PA, Campbell I. Interaction of fat and air in ice cream. Dairy Industries Int 1999;64:27 – 31. w36x Chang Y, Hartel RW. Measurement of air cell distributions ● in dairy foams. Internat Dairy J 2002;12:463 –72. These authors quantitatively measured air bubble size distributions by both low temperature light microscopy and cryo-scanning electron microscopy, with good correlations. Their development of a method for the measurement of air cells by light microscopy is novel and provides a rapid and relatively easy technique for the quantification of an important structural element in ice cream. w37x Russell AB, Cheney PE, Wantling SD. Influence of freezing conditions on ice crystallization in ice cream. J Food Eng 1999;39:179 –91. w38x Hartel RW. Mechanisms and kinetics of recrystallization in ice cream. In: Reid DS, editor. The Properties of Water in Foods ISOPOW 6. New York: Blackie Academic and Professional, 1998. p. 287 –328. w39x Adapa S, Schmidt KA, Jeon IJ, Herald TJ, Flores RA. Mechanisms of ice crystallization and recrystallization in ice cream: a review. Food Reviews Internat 2000;16:259 –71. w40x Flores AA, Goff HD. Ice crystal size distributions in dynamically frozen model solutions and ice cream as affected by stabilisers. J Dairy Sci 1999;82:1399 –407. w41x Flores AA, Goff HD. Recrystallization in ice cream after constant and cycling temperature storage conditions as affected by stabilisers. J Dairy Sci 1999;82:1408 –15. w42x Goff HD, Ferdinando D, Schorsch C. Fluorescence microscopy to study galactomannan structure in frozen sucrose and milk protein solutions. Food Hydrocolloids 1999;13:353 –64. w43x Bolliger S, Wildmoser H, Goff HD, Tharp BW. Relationships between ice cream mix visco-elasticity and ice crystal growth in ice cream. Int Dairy J 2000;10:791 –7. w44x Regand A, Goff HD. Effect of biopolymers on structure and ice recrystallization in dynamically-frozen ice cream model systems. J Dairy Sci 2002, in press. w45x Regand A, Goff HD. Structure and ice recrystallization in frozen stabilised ice cream model systems. Food Hydrocolloids 2002, in press. w46x Goff HD. Ice cream. In: Fox PF, McSweeney PLH, editors. Advanced Dairy Chemistry—1. Proteins, 3rd ed. New York: Kluwer Academic 2002, in press. w47x Syrbe A, Bauer WJ, Klostermeyer H. Polymer science ● concepts in dairy systems—an overview of milk protein and food hydrocolloid interaction. Int Dairy J 1998;8:179 –93. Although ‘wheying off’ has been recognised as a defect in ice cream mix and ice cream for many years, since the addition of polysaccharide stabilisers became common, and it has long been recognised that the addition of k-carrageenan can control this problem, it has only been in recent years that the underlying
437
mechanisms of protein polysaccharide phase separation due to thermodynamic incompatibility have been recognised. This is an excellent review, along with Doublier et al. w48●x, of the related phenomena. w48x Doublier J-L, Garnier C, Renard D, Sanchez C. Protein— ● polysaccharide interactions. Curr Opin Colloid Interface Sci 2000;5:202 –14. This is another excellent review, along with Syrbe et al. w47●x, of the various interactions to be found in polysaccharide protein mixed systems. It focuses on underlying mechanisms and also microscopic, rheological and light scattering techniques to study phase separations. w49x Goff HD. Measurement and interpretation of the glass transition in frozen foods. In: Erickson MC, Hung YC, editors. Quality in Frozen Foods. New York: Chapman and Hall, Inc, 1997. p. 29 –50. w50x Turquois T, Gloria H. Determination of the absolute molecular weight averages and molecular weight distributions of alginates used as ice cream stabilisers by using multiangle laser light scattering measurements. J Agric Food Chem 2000;48:5455 –8. w51x Balyan DK, Tyagi SM, Singh D, Tanwar VK. Effect of extraction parameters on the properties of Fenugreek mucilage and its use in ice cream as a stabilizer. J Food Sci Technol India 2002;38:171 –4. w52x Martin DR, Ablett S, Darke A, Sutton RL, Sahagian ME. An NMR investigation into the effects of locust bean gum on the diffusion properties of aqueous sugar solutions. J Food Sci 1999;64:46 –9. w53x Dunstan DE, Chen Y, Liao M-L, Salvatore R, Boger DV, Prica M. Structure and rheology of k-carrageenanylocust bean gum gels. Food Hydrocolloids 2001;15:475 –84. w54x Ikeda S, Nishinari K. ‘Weak gel’-type rheological properties of aqueous dispersions of nonaggregated k-carrageenan helices. J Agric Food Chem 2001;49:4436 –41. w55x Patmore JV, Goff HD, Fernandes S. Cryo-gelation of galactomannans in ice cream model systems. Food Hydrocolloids 2002, in press. w56x Jonkman MJ, Walstra P, van Boekel MAJS, Cebula DJ. Behaviour of casein micelles at conditions comparable to those in ice cream. Int Dairy J 1999;9:201 –5. w57x Bourriot S, Garnier C, Doublier J-L. Phase separation, rheology and microstructure of micellar casein–guar gum mixtures. Food Hydrocoll 1999;13:43 –9. w58x Schorsch C, Clark AH, Jones M, Norton IT. Behavior of milk proteinypolysaccharide systems in high sucrose. Colloids Surfaces B 1999;12:317 –29. w59x Schorsch C, Jones M, Norton IT. Thermodynamic incompatibility and microstructure of milk proteinylocust bean gumy sucrose systems. Food Hydrocoll 1999;13:89 –99. w60x Ruger PR, Baer RJ, Kasperson KM. Effect of double homogenization and whey protein concentrate on the texture of ice cream. J Dairy Sci 2002;85:1684 –92. w61x Chang Y, Hartel RW. Stability of air cells in ice cream during hardening and storage. J Food Eng 2002;55:59 –70.
Formation and stabilisation of structure in ice-cream and related products H. Douglas Goff* Department of Food Science, University of Guelph, Guelph, ON,Canada N1G 2W1
Abstract Partial coalescence of the fat emulsion and its control by manipulation of the oilywater interface continues to be an active area of ice cream research and understanding at both the basic and applied levels have greatly improved. Interactions between all the discrete phases are increasingly being studied in more complex systems, leading to an appreciation of such things as the effect of air bubble size distribution on ice recrystallization. The importance of protein polysaccharide phase separation in the freezeconcentrated unfrozen phase and its effect on ice recrystallization has also been recognised. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Emulsion; Foam; Partial coalescence; Aeration; Fat; Protein; Polysaccharides; Phase separation
1. Introduction Ice cream and related aerated frozen desserts are complex-colloidal systems comprised of, in their frozen state: ice crystals; air bubbles; partially-coalesced fat globules and aggregates; all in discrete phases surrounded by an unfrozen continuous matrix of sugars, proteins, salts, polysaccharides and water (Fig. 1). Their manufacture usually begins by formulating, pasteurising, homogenising and cooling an emulsion premix, followed by aerating and freezing this premix under high shear conditions in a scraped surface freezer. The aeration and freezing process involves numerous physical changes including: the action of proteins and surfactants in forming and stabilising the foam phase; partial coalescence of the fat emulsion causing both absorption of fat at the air interface and formation of fat globule clusters that stabilise the lamellae between air bubbles; and freeze concentration of the premix by the removal of water from solution in the form of ice. Colloidal aspects of ice cream were reviewed in 1997 w1x and numerous colloid related papers can be found in the proceedings of an international ice cream conference published in 1998 w2x. This paper will discuss recent advances in colloidal aspects of ice cream published since these reviews, with a particular emphasis on the period since 2000. It will categorise specific developments along the lines of the structural elements, viz. fat, air, ice and the *Tel.: q1-519-824-4120 x3878; fax: q1-519-824-6631. E-mail address: [email protected] (H.D. Goff).
serum phase, although it should be recognised that many aspects are inter-related and thus categorisation becomes somewhat arbitrary. 2. Fat globules The optimal formation of fat structure in ice cream is responsible for many desirable properties including dryness and shape retention after scraped surface freezing and slowness of meltdown and smooth eating textural properties after hardening w1,3x. Several recent papers have discussed fat sources w4x, non-fat sources to provide fat-like properties, the so-called ‘fat replacers’ w5,6●,7x and the effect of fat on sensory properties w8–10x and flavour perception w11x. Research work related to colloidal fat structure has focused on: homogenisation of the pre-mix w12–14●●,60x; protein surfactant interactions at the fat interface w15,16●,17●●,18,19●,20●,21–24x; partial coalescence w25x and fat structure formation during freezing w26–28●●x; fat partial coalescence measurement in ice cream w17●●,29x; the effect of fat structure on ice cream meltdown w14●●x; and the effect of low temperature extrusion of ice cream on fat partial coalescence and structure formation w30,31x. From the work of Koxholt et al. w14●●x, Barfod w20●x and Goff et al. w28●●x, our understanding of partiallycoalesced fat structures has been advanced. Analogies to fat partial coalescence and fat structure formation in whipped cream, although in some respects similar, have
1359-0294/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 Ž 0 2 . 0 0 0 7 6 - 6
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
433
Fig. 1. The structure of ice cream mix, ice cream and melted ice cream. (a) Ice cream mix as viewed by thin section transmission electron microscopy. f, Fat globule; c, casein micelle; arrow, crystalline fat within the globule; bar wshown in (d)xs0.5 mm. (b) Close-up of an air bubble in ice cream as viewed by low temperature scanning electron microscopy. a, Air bubble; f, fat globule adsorbed to the bubble surface; bar wshown in (d)xs10 mm. (c) Ice cream as viewed by thin section transmission electron microscopy with freeze substitution and low temperature embedding. a, Air bubble; f, fat globule; fc, fat cluster; bar wshown in (d)xs1 mm. (d) Melted ice cream as viewed by thin section transmission electron microscopy. c, Casein micelle; fn, fat network; bars5 mm. For methodology, see Goff et al. w28●●x.
been shown to be incomplete. While whipped cream is typically stabilised at maximum firmness by almost complete coverage of the air interface with agglomerated fat, such is not the case with ice cream. Rather, fat agglomerates have been shown to provide structure to the lamellae between air bubbles offering resistance to collapse during meltdown and, in conjugation also with the development of thin lamellae, to ice recrystallization. Fat at the air interface tends to be more in the form of discrete droplets. Improved colloidal understanding of meltdown phenomena w14●●,17●●x has led to a proposed simple method for fat structure formation analysis, based on mass and fat drip loss during melting w17●●,29x. The composition of the fat interface and its role in partial coalescence also continues to receive considerable research attention. Bolliger et al. w17●●x showed a
direct relationship between protein content (mg my2), resulting from displacement by emulsifiers, and partial coalescence. Davies et al. w18,19●x suggested that the effect of saturated vs. unsaturated emulsifiers on partial coalescence might be more than one of protein displacement but might also effect fat crystal habit within the globules. Innocente et al. w21x suggested that the proteose-peptone fraction of whey proteins might serve to replace conventional emulsifiers in ice cream applications. The work of Segall and Goff w16●,22,23x has focused on a minimal coverage of whey protein at the fat interface, as created by selective homogenisation, as a means of achieving optimal partial coalescence in formulations with no added emulsifiers. Koxholt et al. w14●●x also showed that selective homogenisation can produce acceptable fat globule size distributions.
434
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
Ice cream is conventionally removed from a scraped surface freezer at about y5 8C. However, further freezing under low shear conditions in a twin-screw extruder makes extrusion at temperatures as low as y12 to y15 8C possible w30x. The effect of this low temperature extrusion process on the formation of structural elements suggests that formulation changes are required to optimise fat partial coalescence w31x. 3. Air bubbles Ice cream and related products are generally aerated and characterised as frozen foams. The gas phase volume varies greatly from a high of greater than 50% to a low of 10–15%. Air is distributed in the form of numerous small air bubbles of size range 20–50 mm. Whilst a great deal is known about protein surfactant interactions at the fat interface, as discussed previously, it seems that far less work has been done on the air interface. More work has been done on non-frozen systems, as reviewed in 1999 w32x. This review focused on stabilisation of air bubbles by structure formation through partial coalescence and also looked at protein surfactant films and displacement studies at both the oil water and air water interface in aerated emulsions. Recent colloidal research on foaming aspects of ice cream and related frozen products has focused on: development of the air phase w13,33x; morphology and channelling of the air bubbles w34x; interactions of fat and air in stabilising the air phase w20●,28●●,35x; and measurement of air bubble structures w34,36●,61x. The scraped surface freezer is responsible for ice crystal formation, but also for air comminution into a fine distribution of bubbles, which are thought to be correlated with enhanced sensory perception of smoothness. Koxholt et al. w13x addressed the question of optimal residence time in the scraped surface freezer by suggesting that opposing phenomena were involved: smallest ice crystal size distributions suggest minimal residence times but smallest air bubble size distributions resulted from longer residence times. They advocated pre-aeration of the mix prior to freezing to optimise both independently. Minimisation of both size distributions has also been claimed to be a benefit of low temperature extrusion w30x. The effect of small air bubble size distributions, as enhanced by surfactants, on minimising ice recrystallization has also recently been shown by Barfod w20●x. Our understanding of the stability of the air phase has been enhanced by the work of Turan et al. w34,35x. They have shown that loss of the discrete nature of the gas bubbles and channelling, leading to a continuous network of coalesced bubbles, is related to volume collapse (shrinkage) and can be measured by examining the response of frozen ice cream to fluctuating pressures. Discrete and independent air
bubbles correlate to expectations based on the gas laws while channelled air networks do not. 4. Ice crystals Ice crystals form another discrete phase in ice cream and it is well recognised that formulations and manufacturing and handling procedures that lead to numerous small, discrete ice crystals also lead to enhanced smoothness in texture. Ice crystal formation in ice cream is generally by secondary nucleation in the scraped surface freezer w37x. However, small ice crystals rapidly undergo recrystallization phenomena, especially in the presence of temperature fluctuations (reviewed in w38,39x). This can be controlled by the maintenance of low constant temperature and by the presence in the formulation of stabilising agents such as polysaccharide gums w40,41x. Since recrystallization leads to rapid loss of quality, much research emphasis has been devoted to its control, particularly polysaccharide functionality w40–45x. However, a thorough review of ice recrystallization is outside the scope of this paper. Rather, discussion here will focus on the relationship between fat and air structures and ice recrystallization w13,20●,30x and on the effect of freeze concentration on polysaccharide interactions, as discussed in the next section. Somewhat unique in the colloidal context is the work by Barfod w20●x showing the interrelationships between fat structure formation, air phase volume, air bubble size distribution and ice recrystallization. Optimisation of fat structure and minimisation of air bubble size gives rise to enhanced protection against ice recrystallization. This might also be accomplished through low temperature extrusion w30x. 5. Serum phase As the premix becomes freeze-concentrated, components that are either dissolved or dispersed in water are increasingly brought together as temperature decreases and water is removed from solution and this also can cause numerous physical and chemical changes to serum-phase components. Two of the important hydrocolloid components in the serum include the polysaccharides and the casein micelles, caseins and whey proteins from the milk sources. Protein functionality in ice cream has recently been reviewed w3,46x. In both the pre-mix and in the unfrozen solution phase of the frozen product, proteins and polysaccharides phase separate due to solution incompatibility. This phenomenon has also been reviewed recently w47●,48● x. After extensive freeze concentration, due to both low temperature and high viscosity, components of the unfrozen phase can enter into the glassy state w49x. In keeping with the colloid focus of this review, recent research to be noted includes: that related to source of polysaccharide stabi-
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
lisers for ice cream use w50,51x; polysaccharide solution properties in situations relevant to ice cream manufacture w42–45,52–55x; properties of proteins in the freezeconcentrated serum phase in ice cream w56x; and protein polysaccharide phase separations under conditions relevant to ice cream manufacture w57–59x. In the highly concentrated state, it has been shown by Goff and co-workers that some polysaccharides used as ice cream stabilisers can form a weak gel that is formed and strengthened by temperature fluctuations causing partial or complete freeze–thaw cycles w42,44,45,55x. Despite the structural role that such a cryo-gel in the unfrozen phase might contribute, it has not, however, been shown to correlate by itself with ice recrystallization inhibition. Rather, it appears that protein polysaccharide phase separation in the unfrozen phase can offer enhanced ice recrystallization inhibition compared to independent polysaccharide action w40– 42,44,45x. Casein micelles, which are not affected structurally by freeze concentration w56x, separate readily from polysaccharides w57–59x. Regand and Goff w44x showed by microscopic techniques that proteins, not polysaccharides, resided in closest proximity to ice crystals after scraped surface freezing of model ice cream solutions. It thus appears that ice recrystallization inhibition results from a highly concentrated, viscous and heterogeneous layer that surrounds ice crystals. Such a layer would affect mobility of water and sugars (which must be in equilibrium at the surface of the ice crystal to invoke freezing and thawing along the equilibrium freezing curve with temperature fluctuation w44,45x), if not molecular diffusion w52x, thus promoting thawing and refreezing on existing ice crystals. 6. Conclusions The complexity and practicality of colloidal structure in ice cream and related products continues to be a source of research interest, as evidenced by the fact that nearly half of the references cited in this paper are from the year 2000 and later and all have been published since 1997. Although the importance of fat partial coalescence in ice cream has long been recognised w1x, our understanding of the morphology and functionality of fat agglomerates has improved in the last few years, and we now have a more complete picture of the interaction of the various structural elements. Manipulating the fat interface by surfactants, proteins and fractions thereof will give us broader means of structure formation. We have also come to realise the importance of protein polysaccharide incompatibility and phase separation, especially in the freeze-concentrated matrix, in the last few years and this will undoubtedly continue to be an area of fruitful research. The phase that lags behind in terms of improvements in fundamental understanding is the air phase. While some progress has been
435
made in understanding the wider implications of the dispersed nature of the gas phase, we know far less about the composition and competition amongst constituents of the air interface than we do about the fat interface. References ● of special interest ●● of outstanding interest w1x Goff HD. Colloidal aspects of ice cream—a review. Int Dairy J 1997;7:363 –73. w2x Buchheim W. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. w3x Walstra P, Jonkman M. The role of milkfat and protein in ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 17 – 24. w4x Abd El-Rahman AM, Madkor SA, Ibrahim FS, Kilara A. Physical characteristics of frozen desserts made with cream, anhydrous milk fat, or milk fat fractions. J Dairy Sci 1997;80:1926 –35. w5x Ohmes RL, Marshall RT, Heymann H. Sensory and physical properties of ice creams containing milk fat or fat replacers. J Dairy Sci 1998;81:1222 –8. w6x Adapa S, Dingeldein H, Schmidt KA, Herald TJ. Rheological ● properties of ice cream mixes and frozen ice creams containing fat and fat replacers. J Dairy Sci 2000;83:2224 –9. This paper measured viscoelastic properties of ice cream mix and ice cream as a function of fat content and the presence of fat replacers. It was shown that the amount of fat in ice cream and the degree of fat destabilization affected elasticity, but that the addition of protein-based and carbohydrate-based fat replacers did not enhance the elastic properties of the ice creams, but rather increased the viscous properties. One unique aspect of this work was the method used to measure viscoelastic properties in the frozen ice creams. Product from the scraped surface freezer was spread on to petri dishes and then hardened. A hollow cylinder was then used to cut disks of frozen product that were then transferred on to the precooled rheometer plate. w7x Aime DB, Arntfield SD, Malcolmson LJ, Ryland D. Textural analysis of fat reduced vanilla ice cream products. Food Res Int 2001;34:237 –46. w8x Guinard J-X, Zoumas-Morse C, Mori L, Uatoni B, Panyam D, Kilara A. Sugar and fat effects on sensory properties of ice cream. J Food Sci 1997;62:1087 –94. w9x Roland AM, Phillips LG, Boor KJ. Effects of fat content on the sensory properties, melting, color and hardness of ice cream. J Dairy Sci 1999;82:32 –8. w10x Prindiville EA, Marshall RT, Heymann H. Effect of milk fat on the sensory properties of chocolate ice cream. J Dairy Sci 1999;82:1425 –32. w11x Li Z, Marshall RT, Heymann H, Fernando L. Effect of milk fat content on flavor perception of vanilla ice cream. J Dairy Sci 1997;80:3133 –41. w12x Thomsen M, Holtsborg J. The effect of homogenisation pressure and emulsifier type on ice cream mix and finished ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 105 – 11. w13x Koxholt M, Eisenmann B, Hinrichs J. Effect of process parameters on the structure of ice-cream. Eur Dairy Mag 2000;12(1):27 –30.
436
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437
w14x Koxholt MMR, Eisenmann B, Hinrichs J. Effect of the fat ●● globule sizes on the meltdown of ice cream. J Dairy Sci 2001;84:31 –7. These authors systematically studied the effect of homogenisation pressure and resultant fat globule size distributions on fat partial coalescence and ice cream melt down properties. They suggested that homogenisation pressures of at least 10 MPa were sufficient to produce fat agglomeration and appropriate melt down properties and that two-stage homogenisation produced no further improvements. These pressure recommendations are considerably lower than those conventionally used in ice cream manufacturing, and could lead to substantial energy savings. They also suggested that selective homogenisation of fat with only a portion of the serum was also effective at producing optimal size fat globules and aggregates in the mix and ice cream, respectively, and that such a process could also lead to energy saving or to the ability to add shear-sensitive ingredients to the non-homogenised portion (see also Segall and Goff w16●x). They proposed a model to explain the effects of fat aggregates on shape retention and meltdown. They suggested that once fat aggregates reach the size of the lamellae between air bubbles, collapse of the foam during meltdown is prevented and structure becomes stabilised, but coverage of the air interface by fat is not required. This model is supported by the microstructural evidence presented in Goff et al. w28●●x. w15x Pelan BMC, Watts KM, Campbell IJ, Lips A. The stability of aerated milk protein emulsions in the presence of small molecule surfactants. J Dairy Sci 1997;80:2631 –87. w16x Segall KI, Goff HD. Influence of adsorbed milk protein type ● and surface concentration on the quiescent and shear stability of butteroil emulsions. Internat Dairy J 1999;9:683 –91. In this paper and the two related ones w22,23x, the concept of creating fat emulsion droplet interfaces with optimal protein loads for perikinetic stability but orthokinetic instability is explored. This was accomplished by selectively homogenising the fat in the presence of only a small portion of whey protein, followed by addition of micellar casein and additional whey protein posthomogenisation. Such a process could lead to optimal partial coalescence in the absence of emulsifier addition. w17x Bolliger S, Goff HD, Tharp BW. Correlation between col●● loidal properties of ice cream mix and ice cream. Internat Dairy J 2000;10:303 –9. In this paper, good correlation was shown between the quantity of protein absorbed at the fat globule interface (mg my2), which was varied by varying emulsification levels in the mix, and several measures of fat partial coalescence including turbidity, solvent extractable fat and integrated laser light scattering methods. Correlation was also shown between fat content in the dripped portion of a meltdown test and fat partial coalescence determined by the above methods. This gives rise to a new, simple method for measurement of fat structure formation and it has also been shown to be independent of the viscous effects of added stabiliser w29x. w18x Davies E, Dickinson E, Bee RD. Shear stability of sodium caseinate emulsions containing monoglyceride and triglyceride crystals. Food Hydrocolloids 2000;14:145 –53. w19x Davies E, Dickinson E, Bee RD. Orthokinetic destabilization ● of emulsions by saturated and unsaturated monoglycerides. Internat Dairy J 2001;11:827 –36. In this paper and a related one w18x, three emulsifier systems: glycerol monooleate; glycerol monopalmitate; and glycerol monostearate, were studied in protein stabilised emulsions relevant to ice cream. Fat partial coalescence after shear decreased in the order of GMO)GMP)GMS, due to two factors, competitive displacement of proteins from the fat interface and fat crystal morphology. The best combination for quiescent stability and shear instability was a 1ow level of protein displacement and creation of spiky spherulites made up of crystals that can penetrate the fat droplet surface. A combination of GMOqGMS or GMOqGMP was
found to be superior in this regard to GMO alone. A cautionary note was also given, though. This study used a combination of peanut oil and tristearin crystals as the discrete phase in a model emulsion but its applicability to more complex food systems with more conventional fat sources was not studied. w20x Barfod N. The emulsifier effect. Dairy Industries Internat ● 2001;66:32 –3. In addition to the conventional functionality of emulsifiers in fat destabilization, these authors showed that emulsifiers increased air bubble stability and resulted in a finer distribution of air bubbles. These factors, especially when associated with a higher total air phase volume, protected the ice cream from excessive ice crystal growth during heat shock. They attributed the reduction in ice recrystallization to enhanced formation of a partially coalesced fat network that retarded movement of structural elements. w21x Innocente N, Comparin D, Corrandini C. Proteose-peptone whey fraction as emulsifier in ice-cream preparation. Internat Dairy J 2002;12:69 –74. w22x Segall KI, Goff HD. A modified processing routine for ice cream that promotes fat destabilization in the absence of added emulsifier. Internat Dairy J 2002, in press. w23x Segall KI, Goff HD. Secondary adsorption of milk protein from the continuous phase to the oil–water interface in dairy emulsions. Internat Dairy J 2002, in press. w24x Sourdet S, Relkin P, Fosseux P-Y, Aubry V. Composition of fat protein layer in complex food emulsions at various weight ratios of casein-to-whey proteins. Le Lait 2002, in press. w25x Vanapalli SA, Coupland JN. Emulsions under shear—the formulation and properties of partially coalesced lipid structures. Food Hydrocolloids 2001;15:507 –12. w26x Campbell IJ, Pelan BMC. The influence of emulsion stability on the properties of ice cream. In: Buchheim W, editor. Ice Cream. Brussels: Internat Dairy Federation Special Issue 9803, 1998. p. 25 –36. w27x Kokubo S, Sakurai K, Iwaki S, Tomita M, Yoshida S. Agglomeration of fat globules during the freezing process of ice cream manufacturing. Milchwissenschaft 1998;53:206 –9. w28x Goff HD, Verespej E, Smith AK. A study of fat and air ●● structures in ice cream. Internat Dairy J 1999;9:817 –29. Cryo-scanning and freeze substitution transmission electron microscopy techniques were used to examine the fine structure of partially coalesced fat networks in ice cream. Varying degrees of fat partial coalescence were induced by varying emulsifier type, concentration and shear levels. Increasing fat destabilization levels were seen as increasing partially coalesced fat agglomerates extending from the air interface into the serum phase and in the serum phase itself and enhanced adsorption of discrete fat droplets at the air interface. Even at the highest levels of fat destabilization the air interface was not completely covered by fat (see also w14●●x). w29x Goff HD, Spagnuolo P. Effect of stabilisers on fat destabilization measurements in ice cream. Milchwissenschaft 2001;56:450 –3. w30x Wildmoser H, Windhab EJ. Impact of flow geometry and processing parameters in ultra low temperature ice-cream extrusion (ULTICE) on ice-cream microstructure. Eur Dairy Mag 2001;13(10):26 –32. w31x Bolliger S, Kornbrust B, Goff HD, Tharp BW, Windhab EJ. Influence of emulsifiers on ice cream produced by conventional freezing and low temperature extrusion processing. Int Dairy J 2000;10:497 –504. w32x Leser ME, Michel M. Aerated milk protein emulsions—new microstructural aspects. Curr Opin Colloid Interface Sci 1999;4:239 –44. w33x Chang YH, Hartel RW. Development of air cells in a batch ice cream freezer. J Food Eng 2002;55:77 –8.
H.D. Goff / Current Opinion in Colloid and Interface Science 7 (2002) 432–437 w34x Turan S, Bee RD. Measurement of gas phase morphology in ice cream. In: Campbell GM, Webb C, Pandiella SS, Niranjan K, editors. Bubbles in Food. St. Paul, MN: Eagen Press, 1999. p. 183 –9. w35x Turan S, Kirkland M, Trusty PA, Campbell I. Interaction of fat and air in ice cream. Dairy Industries Int 1999;64:27 – 31. w36x Chang Y, Hartel RW. Measurement of air cell distributions ● in dairy foams. Internat Dairy J 2002;12:463 –72. These authors quantitatively measured air bubble size distributions by both low temperature light microscopy and cryo-scanning electron microscopy, with good correlations. Their development of a method for the measurement of air cells by light microscopy is novel and provides a rapid and relatively easy technique for the quantification of an important structural element in ice cream. w37x Russell AB, Cheney PE, Wantling SD. Influence of freezing conditions on ice crystallization in ice cream. J Food Eng 1999;39:179 –91. w38x Hartel RW. Mechanisms and kinetics of recrystallization in ice cream. In: Reid DS, editor. The Properties of Water in Foods ISOPOW 6. New York: Blackie Academic and Professional, 1998. p. 287 –328. w39x Adapa S, Schmidt KA, Jeon IJ, Herald TJ, Flores RA. Mechanisms of ice crystallization and recrystallization in ice cream: a review. Food Reviews Internat 2000;16:259 –71. w40x Flores AA, Goff HD. Ice crystal size distributions in dynamically frozen model solutions and ice cream as affected by stabilisers. J Dairy Sci 1999;82:1399 –407. w41x Flores AA, Goff HD. Recrystallization in ice cream after constant and cycling temperature storage conditions as affected by stabilisers. J Dairy Sci 1999;82:1408 –15. w42x Goff HD, Ferdinando D, Schorsch C. Fluorescence microscopy to study galactomannan structure in frozen sucrose and milk protein solutions. Food Hydrocolloids 1999;13:353 –64. w43x Bolliger S, Wildmoser H, Goff HD, Tharp BW. Relationships between ice cream mix visco-elasticity and ice crystal growth in ice cream. Int Dairy J 2000;10:791 –7. w44x Regand A, Goff HD. Effect of biopolymers on structure and ice recrystallization in dynamically-frozen ice cream model systems. J Dairy Sci 2002, in press. w45x Regand A, Goff HD. Structure and ice recrystallization in frozen stabilised ice cream model systems. Food Hydrocolloids 2002, in press. w46x Goff HD. Ice cream. In: Fox PF, McSweeney PLH, editors. Advanced Dairy Chemistry—1. Proteins, 3rd ed. New York: Kluwer Academic 2002, in press. w47x Syrbe A, Bauer WJ, Klostermeyer H. Polymer science ● concepts in dairy systems—an overview of milk protein and food hydrocolloid interaction. Int Dairy J 1998;8:179 –93. Although ‘wheying off’ has been recognised as a defect in ice cream mix and ice cream for many years, since the addition of polysaccharide stabilisers became common, and it has long been recognised that the addition of k-carrageenan can control this problem, it has only been in recent years that the underlying
437
mechanisms of protein polysaccharide phase separation due to thermodynamic incompatibility have been recognised. This is an excellent review, along with Doublier et al. w48●x, of the related phenomena. w48x Doublier J-L, Garnier C, Renard D, Sanchez C. Protein— ● polysaccharide interactions. Curr Opin Colloid Interface Sci 2000;5:202 –14. This is another excellent review, along with Syrbe et al. w47●x, of the various interactions to be found in polysaccharide protein mixed systems. It focuses on underlying mechanisms and also microscopic, rheological and light scattering techniques to study phase separations. w49x Goff HD. Measurement and interpretation of the glass transition in frozen foods. In: Erickson MC, Hung YC, editors. Quality in Frozen Foods. New York: Chapman and Hall, Inc, 1997. p. 29 –50. w50x Turquois T, Gloria H. Determination of the absolute molecular weight averages and molecular weight distributions of alginates used as ice cream stabilisers by using multiangle laser light scattering measurements. J Agric Food Chem 2000;48:5455 –8. w51x Balyan DK, Tyagi SM, Singh D, Tanwar VK. Effect of extraction parameters on the properties of Fenugreek mucilage and its use in ice cream as a stabilizer. J Food Sci Technol India 2002;38:171 –4. w52x Martin DR, Ablett S, Darke A, Sutton RL, Sahagian ME. An NMR investigation into the effects of locust bean gum on the diffusion properties of aqueous sugar solutions. J Food Sci 1999;64:46 –9. w53x Dunstan DE, Chen Y, Liao M-L, Salvatore R, Boger DV, Prica M. Structure and rheology of k-carrageenanylocust bean gum gels. Food Hydrocolloids 2001;15:475 –84. w54x Ikeda S, Nishinari K. ‘Weak gel’-type rheological properties of aqueous dispersions of nonaggregated k-carrageenan helices. J Agric Food Chem 2001;49:4436 –41. w55x Patmore JV, Goff HD, Fernandes S. Cryo-gelation of galactomannans in ice cream model systems. Food Hydrocolloids 2002, in press. w56x Jonkman MJ, Walstra P, van Boekel MAJS, Cebula DJ. Behaviour of casein micelles at conditions comparable to those in ice cream. Int Dairy J 1999;9:201 –5. w57x Bourriot S, Garnier C, Doublier J-L. Phase separation, rheology and microstructure of micellar casein–guar gum mixtures. Food Hydrocoll 1999;13:43 –9. w58x Schorsch C, Clark AH, Jones M, Norton IT. Behavior of milk proteinypolysaccharide systems in high sucrose. Colloids Surfaces B 1999;12:317 –29. w59x Schorsch C, Jones M, Norton IT. Thermodynamic incompatibility and microstructure of milk proteinylocust bean gumy sucrose systems. Food Hydrocoll 1999;13:89 –99. w60x Ruger PR, Baer RJ, Kasperson KM. Effect of double homogenization and whey protein concentrate on the texture of ice cream. J Dairy Sci 2002;85:1684 –92. w61x Chang Y, Hartel RW. Stability of air cells in ice cream during hardening and storage. J Food Eng 2002;55:59 –70.