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Enhancement of fat colloidal interactions for the preparation of ice cream high in unsaturated fat
International Dairy Journal 21 (2011) 540e547
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Enhancement of fat colloidal interactions for the preparation of ice cream high in unsaturated fat Carlos Méndez-Velasco, H. Douglas Goff* Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2010 Received in revised form 7 March 2011 Accepted 10 March 2011
Ice cream high in unsaturated fat lacks the quality of that with higher saturated fat. However, the quality may be improved if the fat is structured differently in the product. Emulsions containing saturated fat or unsaturated oil were combined in the preparation of ice cream; solid fat droplets contributed to the structure-forming properties and stability while protein-stabilised liquid droplets acted as inert fillers, producing so called two-stream ice creams. The latter were compared with ice creams containing droplets in which the two fat sources were homogenised together. No difference was evident at first in particle size or melt stability. Increasing the emulsifier content on the solid droplets led to the highest destabilisation levels and overall higher G0 of the fat/air phase in the ice cream. The increase in particle size as a function of emulsifier concentration correlated with higher G0 . Overrun decreased consistently with greater destabilisation in the ice cream. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In the last decade, increasing trends towards the consumption of more healthy foods have forced processors of high fat products to shift their formulations to higher proportions of unsaturated or “healthier” fats. Ice cream is a good example, where the replacement of saturated fat with healthier fractions may appeal to the consumer. However, saturated fat, also here referred to as solid fat due to low temperatures of ice cream processing, plays a key role in creating the structure responsible for the smooth and creamy texture of ice cream (Crilly, Russell, Cox, & Cebula, 2008; Goff, 1997). Unsaturated or liquid fats are unable to create a similar structure, and result in a product with less body, which is also perceived as less creamy (Barfod, 2001; Crilly et al., 2008; Goh, Ye, & Dale, 2006). The widely different functional properties of crystalline and liquid fat are the key feature determining ice cream quality. Partially crystalline emulsion droplets aggregate readily during ice cream processing to form increasingly large fat networks that extend throughout the aerated emulsion. This process, known as partial coalescence, builds up the microstructure and enhances the viscosity (Goff, 2001), which in turn increases the perceived creaminess of oil-in-water emulsions (Akhtar, Murray, & Dickinson, 2006). Consequently, the development of the desired mouthfeel in
* Corresponding author. Tel.: þ1 519 824 4120x53878; fax: þ1 519 824 6631. E-mail address: [email protected] (H.D. Goff). 0958-6946/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2011.03.008
ice cream is largely dependent on the ability of the fat droplets to interact together to partially coalesce. In addition, adsorption of partially coalesced fat at the airewater interface provides ice cream with air cell stability and thus additional melt resistance (Dalgleish, 2006; Eisner, Wildmoser, & Windhab, 2005; Zhang & Goff, 2005). The interconnected droplets also form bridges among the air cells and thereby contribute to the formation of a mechanically stable mousse-like foam (Koxholt, Eisenmann, & Hinrichst, 2001). In the absence of solid fat, the droplets lack the necessary strength to retain their identity during aggregation (Thivilliers, Laurichesse, Saadaoui, Leal-Calderon, & Schmitt, 2008). Instead, they promptly fuse together during collisions (coalescence), becoming larger in size and causing a reduction in the overall number of droplets (McClements, 2005, chap. 7). Coalesced fat lacks the ability to efficiently stabilise the air surface, resulting in air cells prone to fusion (Muse & Hartel, 2004). This will lead to rapid loss of air and inevitably faster melt rate. Hence the fat coalescence mechanism is said to act as a “structure breaker” during the manufacture of ice cream. Hence, the replacement of solid fat with “healthier” or liquid fractions is a major challenge in the production of high quality ice cream because the texture, probably the most sought after quality in this product, is highly compromised. This study aimed to produce acceptable ice cream with lower amounts of solid fat in higher unsaturated fat formulations. However, simply replacing the saturated fat with unsaturated oil has been shown to result in detrimental quality (Goh et al., 2006), suggesting that changes in the solid/liquid fat ratio of droplets should be developed differently.
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The current approach seeks to arrange fat droplets in a way that facilitates the preparation of ice cream high in unsaturated fat yet with the mouthfeel and stability found in ice cream high in saturated fat. To do this, low volumes of solid fat droplets and higher quantities of liquid droplets must coexist together, each with different functional properties. Maintaining the saturated fat in separate droplets could enhance structure formation around the air cells during freezing/ whipping. Adsorption of emulsifier on the surface of this group of droplets will make them shear-sensitive due to displacement of bulky protein from their interface (Eisner, Jeelani, Bernhard, & Windhab, 2007; Goff & Jordan, 1989). To prevent coalescence of the unsaturated fat, homogenisation of these droplets with milk proteins in the absence of emulsifier will provide sufficient steric/ electrostatic stability to avoid their close contact. Previous work has shown that preparation of ice cream in the absence of emulsifier produces droplets that remain as separate entities throughout the freezing/whipping process (Barfod, 2001; Granger, Langendorff, Renouf, Barey, & Cansell, 2004). The liquid droplets would hence remain dispersed in the ice cream as inert fillers. The production of high quality ice cream rich in unsaturated fat would therefore require: (a) aggregation of the solid droplets for optimum structuring during processing and consequently the formation of a final product with melt resistance; and (b) keeping the unsaturated fat from damaging the microstructure. To do this, two ice cream mixes were prepared separately: one containing the shear-sensitive solid fat and the other with protein-stabilised liquid fat. Combination of the different mixes into one mix prior to freezing, here referred to as a two-stream process, was used to deliver the two different types of fat into the ice cream. However, questions arise as to the effect that surface-active material in one stream could have on the other during processing, e.g., displacement of interfacial protein from the liquid droplets by emulsifier on the solid fat. This would be a major concern given that liquid droplets devoid of the stability conferred by the milk proteins would readily coalesce and hinder the formation of appropriate structure. As a precautionary step, several surface protein concentration analyses on the individual and combined emulsions were carried out at different emulsifier concentrations. The aim of this was to choose appropriate monoglyceride concentrations high enough to displace protein from the solid droplets, however, with care to prevent alteration of the liquid droplets. Ice creams were characterised with static light scattering, rheology and the measurement of overrun and melt stability. 2. Materials and methods 2.1. Ice cream preparation 2.1.1. Mix formulation A variety of mixes were prepared and analysed for surface protein concentration to select the appropriate emulsifier levels in the solid fat stream of the ice creams. The base formulation for all mixes consisted of 10% skim milk powder (Parmalat, London, ON, Canada), 12% sucrose (Lantic Sugar Limited, Montreal, QC, Canada), 4% crystalline corn syrup solids (42DE, Casco Inc., Etobicoke, ON, Canada), 0.15% guar gum and 0.02% carrageenan (Danisco Inc., Scarborough, ON, Canada). The mixes differed in the levels of glycerol monooleate (GMO) (Danisco) as well as the composition and concentration of fat. Fractionated palm kernel oil (PKO) (ACH Food Companies Inc., Cordova, TN, USA) was used as source of saturated fat while high oleic sunflower oil (HOSO) (Trisun 80, Nealanders International Inc., Mississauga, ON, Canada) was the source of unsaturated fat. Based on the assumption that 0.15% emulsifier is suitable for the destabilisation of 10% fat ice cream (Sung & Goff, 2010), GMO
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concentrations were calculated to maintain this ratio for the mixes containing different PKO levels. The monoglyceride content was then raised gradually for each mix up to a point where the surface load remained unchanged and the lowest GMO concentration from this plateau was chosen so as to prevent the use of emulsifier in excess amounts. Based on the data provided by the surface protein load analyses (results not shown), the following mixes containing PKO or HOSO were prepared and combined at 1:1 ratio to produce 10% fat formulations:
4% PKO mix ð0:105% GMOÞ þ 16% HOSO mix ð0% GMOÞ ¼ 2% PKO ice cream ð0:052% GMOÞ 6% PKO mix ð0:135% GMOÞ þ 14% HOSO mix ð0% GMOÞ ¼ 3% PKO ice cream ð0:067% GMOÞ 8% PKO mix ð0:165% GMOÞ þ 12% HOSO mix ð0% GMOÞ ¼ 4% PKO ice cream ð0:082% GMOÞ The above ice creams are hereafter referred to as two-stream ice creams. Control ice creams consisted of the same formulations, though they were prepared differently (see Section 2.1.2.). Regular ice creams, prepared in the same way as control ice creams, all contained 0.15% GMO. 2.1.2. Mix preparation All ingredients, except for the monoglyceride, were blended in water at room temperature (w23 C) and continuously stirred while heating gradually. The monoglyceride was added to the premix at 45 C to avoid sticking to the container surfaces at lower temperatures. Pasteurisation was carried out at 75 C for 15 min. Thereafter, the mixes were pre-homogenised with a high-speed mixer (Silverson L4RT, Silverson Machines, Inc., UK) at 7500 rpm for 5 min to ensure complete melting and dispersion of the emulsifier and the fat. Afterwards, the coarse emulsions were passed twice through a two-stage homogeniser (31MR Laboratory Homogenizer, APV Gaulin Inc., Everett, MA, USA) with the first stage operating at 20 MPa and second stage at 7 MPa. Following homogenisation, the mixes were cooled to 25 C and aged at 4 C overnight. After ageing, the PKO and HOSO mixes were combined accordingly (1:1) to produce two-stream mix and the latter was stored for a further 30 min at 4 C before freezing/whipping. Control mixes were prepared by dispersing both the solid and liquid fats together in the mix and emulsifying all the fat with GMO. Thus, two-stream and control mixes consisted of identical compositions but differed in the way the droplets were arranged in the mix. Regular and control mixes were prepared identically and frozen/whipped after ageing. 2.1.3. Freezing/whipping Ice cream mix (1.5 L) was frozen in a batch freezer (Model 104, Taylor Company, Rockton, IL, USA) for 6 min (draw temperature w5 C) and whipped for 4.5 min for a total 10.5 min freezing/ whipping. Extruded ice creams were collected into 250 mL containers and hardened at 30 C in a blast freezer (Series 6899780, Foster Refrigeration, Drummondville, QC, Canada) for at least 24 h before further testing was performed. 2.2. Determination of particle size Approximately 10 g ice cream samples were melted for 2 h at 4 C before analysis. The weighted average particle diameter (d4,3) was measured with a Malvern Mastersizer 2000 (Malvern Instruments, Malvern, Worcestershire, UK). Dilution of the samples during
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measurement was approximately 1:1000 with Milli-Q water in the Mastersizer chamber. The refractive index for the fat and the dispersing medium was 1.46 and 1.33, respectively, with absorbance of 0.001 and obscuration value in the range of 12e18%. 2.3. Overrun The weight of ice cream mix was calculated from a cup with a fixed volume of 227 mL assuming a density for the mix of 1.1 kg L1. Ice creams were weighed into the same cup and overrun percentage was determined by
% overrun ¼ ½ðg ice cream mix g ice creamÞ=g ice cream 100 ð1Þ 2.4. Melt stability
Fig. 1. Accumulated % mass loss of control (open), two-stream (patterned) and regular (filled) ice creams with increasing solid fat following 90 min meltdown. Control and two-stream ice creams were prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively. Regular ice creams consisted of 0.15% GMO at all fat ratios. Error bars define the standard error of duplicates.
During ice cream extrusion, samples were collected into 250 g containers for the purpose of meltdown testing. After hardening, ice creams were tempered at 15 C for 24 h prior to testing the melt stability at room temperature (w23 C). Ice creams were placed on rectangular stainless steel wire mesh screens (15 11.5 cm, hole size of 2.5 2.5 mm) supported on tripods. The melted serum dripping through the mesh was collected and weighed every 10 min for a period of 90 min. Plots were constructed of % mass loss versus time and the accumulated % mass loss after completion of the test was determined.
analyses were performed with GraphPad Prism 5 (GraphPad Software Inc., San Diego, CA, USA). Analysis of variance was obtained with One-way analysis of variance (ANOVA) routines and multiple comparisons of means were conducted using t tests. Statistical significance is given by P values, with differences at the 95% confidence interval (P < 0.05) considered statistically significant.
2.5. Rheology
3.1. Characterisation of ice cream structure by melt stability and particle size
Small-deformation oscillatory tests were performed using a controlled stress rheometer (AR 2000, TA instruments, New Castle, DE, USA) fitted with 20 mm parallel-plate geometry (2000 mm gap), following a method similar to that of Granger et al. (2004) and Wildmoser, Scheiwiller, and Windhab (2004). The storage modulus G0 , which reflects the solid-like properties of the material, was recorded with increasing temperature from 10 C to 30 C with heating rate of 0.5 C min1 at fixed shear stress and frequency of 1 Pa and 1 Hz, respectively. The applied stress was selected based on the linear viscoelastic range (LVR) so the temperature ramp applied to 2% PKO two-stream ice cream required lower shear stress of 0.5 Pa. Ice creams were cut into samples with dimensions of 20 mm diameter and 3e5 mm thickness at 20 C with a cylindrical cutting tool. The samples were rapidly transferred in a steel container to the rheometer and placed on the peltier plate set at 10 C. The gap was adjusted and ice creams were simultaneously covered with a styrofoam movable hood, to prevent temperature exchange with the environment, and left to equilibrate on the plate for 2 min at 10 C before the experiment. The gap adjusted to the frozen ice cream was slightly smaller than its thickness to account for volume loss during ice cream meltdown on the plate and thus maintain contact between geometry and sample at all times. The rheology of the fat/air phase was measured following the complete melting of ice. The G0 values corresponding to the fat/air structures were calculated from the average of measurements taken at 3, 6, 9, 12, 15, 18 and 21 C for each ice cream. This temperature range was chosen because PKO would mainly exist in the solid state thereby avoiding collapse of the fat/air structures. Rheological measurements are reported as the average of four replicates from ice cream prepared twice. 2.6. Statistics Results are reported as the mean from ice creams prepared in duplicate with error bars representing the standard error. Statistical
3. Results and discussion
Regular ice creams exhibited significantly lower % mass loss than two-stream or control ice creams at any given fat ratio (P < 0.05) with no significant difference between two-stream and control ice creams for a given fat ratio or with increasing solid fat (Fig. 1). The effect of the latter on the meltdown behaviour of regular ice creams is discussed in further detail below. Ice creams were analysed for particle size, revealing a correlation with melt stability (Fig. 2). Regular ice creams, with substantially lower melt rate, exhibited larger particles than two-stream or control ice creams (P < 0.05) while the latter two, with highly similar particle size, melted equally fast. Just as the melt stability remained the same for two-stream and control ice creams with increasing PKO levels, so did the particle size. Destabilisation of regular ice creams with increasing PKO is discussed below. The large difference in particle size between control and regular ice creams points to the greater destabilisation reached simply by
Fig. 2. Weighted mean particle diameter (d4,3) of control (open), two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Control and twostream ice creams were prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively. Regular ice creams contained 0.15% GMO at all fat ratios. Error bars define the standard error of duplicates.
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increasing the GMO levels. Unsaturated monoglycerides have been shown to promote optimum aggregation among droplets. Some workers attributed this to greater protein displacement at increasing GMO concentrations (Davies, Dickinson, & Bee, 2000, 2001) while others put more emphasis on the interactions between unsaturated monoglycerides and hard vegetable fats for the formation of highly interacting droplets (Barfod, Schrader, & Buchheim, 2000; Granger, Leger, Barey, Langendorff, & Cansell, 2005a; Granger, Schöppe, Leger, Barey, & Cansell, 2005b). Adsorption of partially coalesced aggregates at the airewater interface imparts high air stability because localised fat clumps in the lamellae between air cells block water drainage during meltdown and so retard bubble coalescence (Chang & Hartel, 2002; Goff, 2001; Koxholt et al., 2001; Sofjan & Hartel, 2004). Larger aggregates slow down the melt rate more efficiently because of their greater ability to retain water and air cells together (Muse & Hartel, 2004; Pelan, Watts, Campbell, & Lips, 1997). The much faster meltdown in control and two-stream ice creams is attributed to poor structuring around the air cells as evidenced by their smaller particle size when compared with regular ice creams. Although the former two had in common the lack of structure, the colloidal interactions may have varied in these ice creams because they were structured very differently. In control ice creams, the GMO concentrations were probably insufficient to render the droplets shear-sensitive. Conversely, two-stream ice creams with localised GMO on the solid droplets may have favoured stronger interactions. Yet, with PKO accounting for less than half of the total fat volume it is possible that the droplets may have not been present in sufficient amounts to aggregate into fat networks reinforcing the ice cream matrix. Having observed no differences between preparation methods, the GMO concentration was further raised on the PKO droplets of two-stream ice creams. This time, the approach was to generate sufficient interactions between the unsaturated monoglyceride and the fat to enhance aggregation. Interactions between GMO and hard vegetable fats at the oilewater interface have been shown to create highly deformed globules resembling the structures of crystal platelets (Barfod, 2001; Barfod et al., 2000). These fat particles are prone to setting larger fat networks than the more globular fat in addition to providing extremely high air cell stability (Barfod, 2001; Persson, 2009). To this end, mix containing 8% PKO and 0.3% GMO in addition to all other ingredients was prepared, showing a monomodal droplet size distribution following homogenisation (results not shown). Interestingly, only ageing was required to create substantial changes in the droplet arrangements of the mix (Fig. 3). Even before freezing/whipping, the monodispersed emulsion turned into what seemed like a partially coalesced mix. Dilution with sodium dodecyl sulphate (SDS) or ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA) was performed to verify the nature of the colloidal interactions. Both SDS and EDTA affect the droplet size distribution differently based on whether fat particles connect through protein or fat. If the flocs are protein-bound, the almost complete disruption of micellar casein into caseinates with EDTA (Horne, 2009) at 1.7 wt% (Goh et al., 2006) inhibits micellar-induced bridging flocculation, leading to re-dispersion of the agglomerates. When the droplets are held together by protruding fat crystals, adsorption of SDS (2 wt%) alters the wettability of the latter, causing expulsion of the crystals into the aqueous phase and, with no crystals to maintain the droplets closely associated, inevitable re-dispersion (Boode & Walstra, 1993). The ability of SDS to also disrupt casein micelles required the use of EDTA alone to separate the fat crystaland protein-binding effects. More detailed experiments on the effect of SDS and EDTA on a variety of fat aggregates will soon be published.
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Fig. 3. Droplet size distribution of aged 8% PKO (0.3% GMO) mix diluted with water (C), 2% SDS (:) or 1.7% buffered EDTA (B).
Disrupting the casein micelles with EDTA did not reverse the droplet size distribution. On the other hand, the aggregate-disrupting effect of SDS led to the appearance of smaller particles, supporting the concept that its ability to break up micellar casein was not related to its particle size-reduction power but is indicative of the presence of partial coalescence. These findings suggest that enhanced GMOePKO interactions triggered the formation of fat networks in the mix even before shearing. Given its high tendency to destabilisation, this mix (8% PKO, 0.3% GMO) was diluted with mix containing HOSO droplets to produce two-stream ice creams. The GMO concentration came to 0.15% or less as a result of dilution to reach 2, 3 or 4% PKO formulations. The GMO levels varied across the 2, 3 and 4% PKO mixes, yet the surface properties and functionality of the PKO droplets remained the same across the formulations because the solid droplets all originated from the same mix. After freezing/whipping, two-stream ice cream was compared with the previously reported regular ice cream for particle size and melt stability. Two-stream ice creams with 2 or 3% PKO exhibited lower melt resistance than regular ice creams, with the difference significant for 2% PKO (P < 0.05) (Fig. 4). At 4% PKO, the stability of two-stream ice cream improved greatly, melting at the same rate as regular ice cream. With increasing solid fat, % mass loss was significantly reduced at 4% PKO for both regular and two-stream ice creams (P < 0.05). Overall, the light scattering studies parallelled melting behaviour (Fig. 5). At 2 and 3% PKO, regular ice creams showed larger particles than two-stream ice creams (P < 0.05). The opposite effect was observed at 4% PKO, with the two-stream ice cream destabilising more (P < 0.05). The shift towards larger particles with solid
Fig. 4. Accumulated % mass loss of two-stream (patterned) and regular (filled) ice creams with increasing solid fat following 90 min meltdown. Regular ice creams consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
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Fig. 5. Weighted mean particle diameter (d4,3) of two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Regular ice creams consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
fat content was significant at 3 and 4% PKO for regular ice creams and at 4% PKO for two-stream ice creams (P < 0.05). The two-stream process promoted greater destabilisation at 4% PKO. However, the performance in lower PKO volumes (2 and 3%) was lacking, suggesting a critical concentration above which the aggregating droplets exist in sufficient quantities to develop structure. The low performance in two-stream ice creams with 2 or 3% PKO is not attributed to coalescence of the unsaturated fat, which could have occurred upon substantial displacement of interfacial protein from the liquid droplets by excess emulsifier on the solid fat. The appearance of peaks in the droplet size distributions of two-stream ice creams in the range of 0.3e1 mm confirmed the stability of the unsaturated droplets against aggregation (results not shown). Despite greater destabilisation in the 4% PKO two-stream ice cream its melt stability did not surpass that of the corresponding regular ice cream. It is worth remarking that the melted serum in both these ice creams was clear in appearance, indicating the absence of air cells and fat and that either level of fat destabilisation was optimum to retain the structural fat/air networks together in a stable foam. It is noteworthy that static light scattering theory assumes the dispersed fat particles are spherical and non-interacting. Hence, the particle size data reported here can only be used as an index of the relative size of aggregates rather than their actual size.
3.2. Overrun Overrun decreased significantly at 3 and 4% PKO in two-stream ice creams and at 3% PKO in regular ice creams (P < 0.05). Differences between two-stream and regular ice creams were not significant (Fig. 6). Greater aggregation increasingly hindered the incorporation of air cells, revealing a relationship between particle size and foamability. Previous work has shown enhanced foamability in milkfat ice cream when the saturated fat is partially replaced with olein-rich fractions (Bazmi & Relkin, 2009). However, the effect observed in this study could be more related to the particle size than the actual fat compositions. This may be concluded from the differences, though not significant, observed between two-stream and regular ice creams for equal fat ratios. Milk proteins are known for their excellent foaming properties (Zhang & Goff, 2004). Fat particles, on the other hand, hinder foaming (Dickinson, 1992, chap. 5), yet they confer excellent air cell stabilising properties when held together in the form of aggregates (Dalgleish, 2006). It is possible that extensive partial coalescence in the ice creams with higher destabilisation may have displaced protein from the airewater interface
Fig. 6. Development of overrun in two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Regular ice creams with increasing solid fat consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
during freezing/whipping, thus reducing the foamability while also rendering the air cells more stable to coalescence. Although air cell size was not studied here, it has been reported that larger aggregates promote the formation of smaller air cells and narrower air cell size distributions, less prone to fusion (Eisner et al., 2005; Relkin, Sourdet, Smith, Goff, & Cuvelier, 2006). On the contrary, the caseins and whey proteins despite being better foaming agents cannot stabilise air cells to the same extent as the fat clusters. Milk protein-stabilised bubbles favour air cell growth as they coalesce during freezing/whipping. This explains the faster meltdown of ice cream with higher overrun and thus less destabilisation. 3.3. Rheology of the fat/air phase According to previous research, G0 values above the ice melting point of ice cream measure the response of the fat/air structures and relate to creaminess in that higher G0 contributes to greater perception of this attribute (Wildmoser et al., 2004). For their rheological characterisation, ice creams were divided into two groups: those with low or high GMO concentrations. Control and two-stream ice creams prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively, are hereafter referred to as ice creams with “low” GMO. Two-stream ice creams derived from diluting 8% PKO mix containing 0.3% GMO, and regular ice creams are hereafter referred to as ice creams with “high” GMO. 3.3.1. Low glycerol monooleate concentrations The average G0 corresponding to the fat/air phase of two-stream and control ice creams is shown in Fig. 7. At 2% PKO, control ice cream exhibited significantly larger G0 than two-stream ice cream while the opposite effect occurred at 3% PKO (P < 0.05). At 4% PKO, two-stream ice cream also revealed higher G0 , although the difference was not significant. Solid fat content gave significantly greater G0 at 4% PKO among control ice creams, and at 3 and 4% PKO among two-stream ice creams (P < 0.05). It has been consistently reported that greater destabilisation enhances G0 in the temperature region where the fat/air structures are measurable (Barfod et al., 2000; Eisner et al., 2005; Granger et al., 2004, 2005a; Wildmoser et al., 2004). The current experiments, however, do not fully agree. While particle size remained the same across all samples with low GMO, G0 varied considerably under the same conditions (see Figs. 2 and 7), increasing with higher PKO and thus suggesting a greater effect of solid fat than particle size on G0 . 3.3.2. High GMO concentrations The rheological characterisation of high GMO ice creams also pointed to greater G0 in two-stream ice creams (Fig. 8), yet this
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Fig. 7. Average storage modulus of the fat/air structures in two-stream (patterned) and control (filled) ice creams with low GMO. Average G0 was obtained in the temperature range of 3e21 C. Error bars define the standard error of four replicates from ice cream prepared in duplicate.
trend was significant only at 3% PKO (P < 0.05). The G0 of regular ice cream dropped from 2 to 3% PKO (P < 0.05) followed by a large increase at 4% PKO (P < 0.05). The same effect, although not significant, was found for two-stream ice cream from 2 to 3% PKO, with G0 rising substantially again at 4% PKO (P < 0.05). The 2 and 3% PKO two-stream ice creams showed larger G0 than the more destabilised 2 and 3% PKO regular ice creams (see Figs. 5 and 8). Moreover, increasing the PKO from 2 to 3% in regular ice cream resulted in larger particles but also lower G0 . These results are not in agreement with previous literature showing a correlation of destabilisation with G0 . Unlike ice creams with low GMO, increasing the solid fat in high GMO ice creams lacked a constant increase in G0 . The decrease in G0 from 2 to 3% PKO could be partly explained by differences in the shape retention properties of regular ice cream during meltdown. While 2% PKO regular ice cream left a collapsed mass of foam on the mesh after melting, 3% PKO regular ice cream did not do this. Instead, serum flowed steadily out of the ice cream leaving very little standing on the mesh. A large mass of solid-like foam was once again visible after meltdown of 4% PKO regular ice cream. This indicates that the fat/ air microstructure at 3% PKO was of a different nature to that created at 2 and 4% PKO. The settling of ice cream into a collapsed foam after meltdown is characteristic of optimum droplet interactions and high air cell stability (Eisner et al., 2005; Koxholt et al., 2001). The absence or alterations of these properties at 3% PKO may be associated with lower G0 . Interestingly, changes in particle size as influenced by GMO concentration were found to alter the storage moduli. This is supported by comparing equally-processed ice creams for a given fat ratio as a function of low versus high GMO (see Figs. 7 and 8 for low versus high GMO two-stream ice cream or control versus regular ice cream). Overall, higher % GMO promoted larger G0 , especially at
Fig. 8. Average storage modulus of the fat/air structures in two-stream (patterned) and regular (filled) ice creams with high GMO. Average G0 was obtained in the temperature range of 3e21 C. Error bars define the standard error of four replicates from ice cream prepared in duplicate.
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2% PKO in both types of ice cream process and at 3% PKO in twostream ice creams (P < 0.05). This is attributed to an increase in the size of the fat aggregates and suggests that large aggregates per se may not necessarily behave in the same way. This is proposed based on the effect of particle size on G0 ; the formation of larger particles with increasing solid fat or a change in processing did not increase G0 . Only larger particles created by higher levels of emulsifier led to higher G0 . The different effects caused by aggregates of similar (large) size may be attributed to differences in their shape/ conformations. This property seems to affect the way droplets arrange to produce texture, and vary according to the condition responsible for inducing the aggregation process, e.g., processing technique (two-stream versus regular), solid fat content or emulsifier concentration. Thus, the condition favouring fat destabilisation may be equally important as the size of aggregates in relation to the structure and so the development of texture. To better understand the elastic nature of the fat/air phases, changes in the loss modulus (G00 ), which reflects the liquid-like properties of the material, were evaluated in relation to G0 . The ratio of G00 /G0 is defined by the tangent of the phase angle d (tan d). This parameter indicates whether the viscous or elastic property dominates in the material as stress is applied. tan d Values close to or above 1 indicate the predominance of the viscous properties versus the elastic. As tan d approaches 0 the material is said to behave more like a solid. The storage modulus of each ice cream was plotted with the corresponding tan d (Fig. 9). Two distinct rheological behaviours are depicted in this plot; the first one illustrates a growing influence of G0 over G00 as G0 increases up to about 150 and tan d decreases constantly. Up to this point the ice creams become increasingly elastic. After G0 of 150, tan d remains constant with increasing G0 . The greater stiffness in the ice creams is in this case attributed to a constant rise in both the elastic and viscous properties. That is, G00 increased simultaneously with G0 . Unfortunately, similar treatment of the data for G0 and tan d has not been previously reported. Full tan d patterns in relation to G0 during temperature ramps have been reported (Granger et al., 2004, 2005a). Plotting tan d, as such, gives good correlation with the developing G0 but it is not until the responses of different ice creams are plotted together with their corresponding tan d that a better relation of G0 to G00 is observed. Ice creams with 4% PKO, regardless of the preparation method or GMO concentration, mark the appearance of steady tan d with increasing G0 . However, it is not clear why an increase in % solid fat leads to the formation of stronger structures with enhanced viscous character. 3.3.3. Fat/air interactions model It has been suggested that deposition of fat aggregates in the lamellae between air cells is the mechanism by which air cells
Fig. 9. Development of tan d with the corresponding G0 of ice creams. Average G0 was obtained in the temperature range of 3e21 C.
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Fig. 10. From left to right: schematic representation of a) liquid (open spheres) and solid (filled spheres) droplets adsorbed at the airewater interface forming “micro-bridges” in the lamellae between air cells, b) blockage of the lamellae by partially crystalline aggregates and c) fat platelet aggregates (filled structures) in the presence of liquid droplets (open spheres).
become more stable to rupture (Koxholt et al., 2001). According to these workers, the fat, if present in sufficiently large clusters, blocks the lamellae and prevents drainage, reducing the chances of air cell coalescence. The predominance of higher G0 in two-stream ice creams when compared with those prepared conventionally may depart from this hypothesis. If the fat aggregates in the lamellae are to act as “micro-bridges” among air cells, then it is likely that the properties of the aggregates would influence the shear-induced response of the ice cream. These properties, other than particle size, include composition as well as conformation/shape. In the case of two-stream ice creams, adsorption of fully solid fat clusters at the air cell surface may yield greater resistance to shear (Fig. 10a), even when clusters are present in lower amounts and sometimes smaller size. In contrast, the oil-containing aggregates, as in the case of conventional ice creams, may have facilitated further compression of the fat/air phase due to the “softer” character of the droplets, regardless of their larger amount and sometimes larger size (Fig. 10b). The greater G0 associated with high GMO ice creams may be the result of alterations in the shape/conformation of the aggregates due to enhanced GMOePKO interactions. As previously described, the interactions between these two ingredients drastically affects the spherical shape of droplets, turning them into interacting platelets capable of stabilising the airewater interface. It is therefore possible that platelet aggregates of this kind in the lamellae formed blockages of greater strength more resistant to stress (Fig. 10c). It should be noted that the diagrams represented here attempt to depict the rheological behaviour as influenced by adsorbed fat only. The surface of air bubbles is also composed of other surfaceactive materials such as protein and emulsifiers.
4. Conclusions Particle size and melt stability correlated well, with larger particles increasing the melt resistance. At low GMO concentrations, both these properties remained the same despite processing method or solid fat content. The G0 in ice cream with limited structure seems mainly determined by % solid fat. Increasing GMO caused drastic changes in the stability and structure formation of ice cream from either process. Restricting the emulsifier to droplets intended for aggregation, via the twostream approach, caused greater destabilisation than regular processing at 4% PKO. Most probably, at lower PKO the amount of
interacting droplets for structure build-up was limited. The equally low melt rate of regular and two-stream ice creams at 4% PKO indicates that no more serum is retained among air cells above optimum aggregation levels. Particle size correlated inversely with overrun as increasingly larger particles most probably reduced the availability of proteins at the airewater interface. Higher GMO favours larger droplets with a G0 -enhancing effect. Other variables also capable of raising particle size showed no such rheological behaviour. This fact points to the effect of formulation/processing on the formation of aggregates with different functionality, which seems associated with their composition/conformations other than just size. The shearinduced response of ice cream may originate from the structure of the air cell lamellae, which in turn is defined by the arrangement of fat. The decrease in tan d followed by a plateau with increasing G0 indicates a shift in the rheological properties of ice cream from elastic to increasingly viscoelastic. The higher G0 resulting from two-stream processing may alter sensory attributes, favouring the formation of desirable ice cream, especially when considering the allegedly established link between G0 and creaminess. Acknowledgements The authors would like to thank Nestec, Inc. for financial support, especially Drs. Virginie de Boishebert and Max Puaud, Nestle Research Centre, Beauvais, France, for their helpful discussions on the completion of this work. References Akhtar, M., Murray, B. S., & Dickinson, E. (2006). Perception of creaminess of model oil-in-water dairy emulsions: influence of the shear-thinning nature of a viscosity-controlling hydrocolloid. Food Hydrocolloids, 20, 839e847. Barfod, N. M. (2001). The emulsifier effect. Dairy Industries International, 66, 32e34. Barfod, N. M., Schrader, K., & Buchheim, W. (2000). Water continuous fat crystal networks in ice cream mix with unsaturated monoglycerides. In Second international symposium on food rheology and structure, March 12e16, 2000, Zurich, Switzerland. Bazmi, A., & Relkin, P. (2009). Effects of processing conditions on structural and functional parameters of whipped dairy emulsions containing various fatty acid compositions. Journal of Dairy Science, 92, 3566e3574. Boode, K., & Walstra, P. (1993). Partial coalescence in oil-in-water emulsions 1. Nature of the aggregation. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 81, 121e137. Chang, Y., & Hartel, R. W. (2002). Development of air cells in a batch ice cream freezer. Journal of Food Engineering, 55, 71e78. Crilly, J. F., Russell, A. B., Cox, A. R., & Cebula, D. J. (2008). Designing multiscale structures for desired properties of ice cream. Industrial and Engineering Chemistry Research, 47, 6362e6367.
C. Méndez-Velasco, H.D. Goff / International Dairy Journal 21 (2011) 540e547 Dalgleish, D. G. (2006). Food emulsions e their structures and structure-forming properties. Food Hydrocolloids, 20, 415e422. Davies, E., Dickinson, E., & Bee, R. (2000). Shear stability of sodium caseinate emulsions containing monoglyceride and triglyceride crystals. Food Hydrocolloids, 14, 145e153. Davies, E., Dickinson, E., & Bee, R. D. (2001). Orthokinetic destabilization of emulsions by saturated and unsaturated monoglycerides. International Dairy Journal, 11, 827e836. Dickinson, E. (1992). An introduction to food colloids. Oxford, UK: Oxford University Press. Eisner, M. D., Jeelani, S. A. K., Bernhard, L., & Windhab, E. J. (2007). Stability of foams containing proteins, fat particles and nonionic surfactants. Chemical Engineering Science, 62, 1974e1987. Eisner, M. D., Wildmoser, H., & Windhab, E. J. (2005). Air cell microstructuring in a high viscous ice cream matrix. Colloids and Surfaces A: Physicochemical Engineering Aspects, 263, 390e399. Goff, H. D. (1997). Colloidal aspects of ice cream e a review. International Dairy Journal, 7, 363e373. Goff, H. D. (2001). Emulsion partial coalescence and structure formation in dairy systems. In N. Widlak, R. Hartel, & S. Narine (Eds.), Crystallization and solidification properties of lipids (pp. 200e212). Champaign, IL, USA: AOCS Press. Goff, H. D., & Jordan, W. K. (1989). Action of emulsifiers in promoting fat destabilization during the manufacture of ice cream. Journal of Dairy Science, 72, 18e29. Goh, K. K. T., Ye, A., & Dale, N. (2006). Characterisation of ice cream containing flaxseed oil. International Journal of Food Science and Technology, 41, 946e953. Granger, C., Langendorff, V., Renouf, N., Barey, P., & Cansell, M. (2004). Short communication: impact of formulation on ice cream microstructures: an oscillation thermo-rheometry study. Journal of Dairy Science, 87, 810e812. Granger, C., Leger, A., Barey, P., Langendorff, V., & Cansell, M. (2005a). Influence of formulation on the structural networks in ice cream. International Dairy Journal, 15, 255e262. Granger, C., Schöppe, A., Leger, A., Barey, P., & Cansell, M. (2005b). Influence of formulation on the thermal behavior of ice cream mix and ice cream. Journal of the American Oil Chemists’ Society, 82, 427e431.
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Horne, D. S. (2009). Casein micelle structure and stability. In A. Thompson, M. Boland, & H. Singh (Eds.), Milk proteins: From expression to food (pp. 133e162). New York, NY, USA: Academic Press. Koxholt, M. M. R., Eisenmann, B., & Hinrichst, J. (2001). Effect of the fat globule sizes on the meltdown of ice cream. Journal of Dairy Science, 84, 31e37. McClements, D. J. (2005). Food emulsions: Principles, practices and techniques (2nd ed.). Boca Raton, FL, USA: CRC Press. Muse, M. R., & Hartel, R. W. (2004). Ice cream structural elements that affect melting rate and hardness. Journal of Dairy Science, 87, 1e10. Pelan, B. M. C., Watts, K. M., Campbell, I. J., & Lips, A. (1997). The stability of aerated milk protein emulsions in the presence of small molecule surfactants. Journal of Dairy Science, 80, 2631e2638. Persson, M. (2009). Nutritionally optimized ice cream fats. Lipid Technology, 21, 62e64. Relkin, P., Sourdet, S., Smith, A. K., Goff, H. D., & Cuvelier, G. (2006). Effects of whey protein aggregation on fat globule microstructure in whipped-frozen emulsions. Food Hydrocolloids, 20, 1050e1056. Sofjan, R. P., & Hartel, R. W. (2004). Effects of overrun on structural and physical characteristics of ice cream. International Dairy Journal, 14, 255e262. Sung, K. K., & Goff, H. D. (2010). Effect of solid fat content on structure in ice creams containing palm kernel oil and high-oleic sunflower oil. Journal of Food Science, 75, 274e279. Thivilliers, F., Laurichesse, E., Saadaoui, H., Leal-Calderon, F., & Schmitt, V. (2008). Thermally induced gelling of oil-in-water emulsions comprising partially crystallized droplets: the impact of interfacial crystals. Langmuir, 24, 13364e13375. Wildmoser, H., Scheiwiller, J., & Windhab, E. J. (2004). Impact of disperse microstructure on rheology and quality aspects of ice cream. Lebensmittel Wissenschaft und Technologie, 37, 881e891. Zhang, Z., & Goff, H. D. (2004). Protein distribution at air interfaces in dairy foams and ice cream as affected by casein dissociation and emulsifiers. International Dairy Journal, 14, 647e657. Zhang, Z., & Goff, H. D. (2005). On fat destabilization and composition of the air interface in ice cream containing saturated and unsaturated monoglyceride. International Dairy Journal, 15, 495e500.
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International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj
Enhancement of fat colloidal interactions for the preparation of ice cream high in unsaturated fat Carlos Méndez-Velasco, H. Douglas Goff* Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2010 Received in revised form 7 March 2011 Accepted 10 March 2011
Ice cream high in unsaturated fat lacks the quality of that with higher saturated fat. However, the quality may be improved if the fat is structured differently in the product. Emulsions containing saturated fat or unsaturated oil were combined in the preparation of ice cream; solid fat droplets contributed to the structure-forming properties and stability while protein-stabilised liquid droplets acted as inert fillers, producing so called two-stream ice creams. The latter were compared with ice creams containing droplets in which the two fat sources were homogenised together. No difference was evident at first in particle size or melt stability. Increasing the emulsifier content on the solid droplets led to the highest destabilisation levels and overall higher G0 of the fat/air phase in the ice cream. The increase in particle size as a function of emulsifier concentration correlated with higher G0 . Overrun decreased consistently with greater destabilisation in the ice cream. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In the last decade, increasing trends towards the consumption of more healthy foods have forced processors of high fat products to shift their formulations to higher proportions of unsaturated or “healthier” fats. Ice cream is a good example, where the replacement of saturated fat with healthier fractions may appeal to the consumer. However, saturated fat, also here referred to as solid fat due to low temperatures of ice cream processing, plays a key role in creating the structure responsible for the smooth and creamy texture of ice cream (Crilly, Russell, Cox, & Cebula, 2008; Goff, 1997). Unsaturated or liquid fats are unable to create a similar structure, and result in a product with less body, which is also perceived as less creamy (Barfod, 2001; Crilly et al., 2008; Goh, Ye, & Dale, 2006). The widely different functional properties of crystalline and liquid fat are the key feature determining ice cream quality. Partially crystalline emulsion droplets aggregate readily during ice cream processing to form increasingly large fat networks that extend throughout the aerated emulsion. This process, known as partial coalescence, builds up the microstructure and enhances the viscosity (Goff, 2001), which in turn increases the perceived creaminess of oil-in-water emulsions (Akhtar, Murray, & Dickinson, 2006). Consequently, the development of the desired mouthfeel in
* Corresponding author. Tel.: þ1 519 824 4120x53878; fax: þ1 519 824 6631. E-mail address: [email protected] (H.D. Goff). 0958-6946/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2011.03.008
ice cream is largely dependent on the ability of the fat droplets to interact together to partially coalesce. In addition, adsorption of partially coalesced fat at the airewater interface provides ice cream with air cell stability and thus additional melt resistance (Dalgleish, 2006; Eisner, Wildmoser, & Windhab, 2005; Zhang & Goff, 2005). The interconnected droplets also form bridges among the air cells and thereby contribute to the formation of a mechanically stable mousse-like foam (Koxholt, Eisenmann, & Hinrichst, 2001). In the absence of solid fat, the droplets lack the necessary strength to retain their identity during aggregation (Thivilliers, Laurichesse, Saadaoui, Leal-Calderon, & Schmitt, 2008). Instead, they promptly fuse together during collisions (coalescence), becoming larger in size and causing a reduction in the overall number of droplets (McClements, 2005, chap. 7). Coalesced fat lacks the ability to efficiently stabilise the air surface, resulting in air cells prone to fusion (Muse & Hartel, 2004). This will lead to rapid loss of air and inevitably faster melt rate. Hence the fat coalescence mechanism is said to act as a “structure breaker” during the manufacture of ice cream. Hence, the replacement of solid fat with “healthier” or liquid fractions is a major challenge in the production of high quality ice cream because the texture, probably the most sought after quality in this product, is highly compromised. This study aimed to produce acceptable ice cream with lower amounts of solid fat in higher unsaturated fat formulations. However, simply replacing the saturated fat with unsaturated oil has been shown to result in detrimental quality (Goh et al., 2006), suggesting that changes in the solid/liquid fat ratio of droplets should be developed differently.
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The current approach seeks to arrange fat droplets in a way that facilitates the preparation of ice cream high in unsaturated fat yet with the mouthfeel and stability found in ice cream high in saturated fat. To do this, low volumes of solid fat droplets and higher quantities of liquid droplets must coexist together, each with different functional properties. Maintaining the saturated fat in separate droplets could enhance structure formation around the air cells during freezing/ whipping. Adsorption of emulsifier on the surface of this group of droplets will make them shear-sensitive due to displacement of bulky protein from their interface (Eisner, Jeelani, Bernhard, & Windhab, 2007; Goff & Jordan, 1989). To prevent coalescence of the unsaturated fat, homogenisation of these droplets with milk proteins in the absence of emulsifier will provide sufficient steric/ electrostatic stability to avoid their close contact. Previous work has shown that preparation of ice cream in the absence of emulsifier produces droplets that remain as separate entities throughout the freezing/whipping process (Barfod, 2001; Granger, Langendorff, Renouf, Barey, & Cansell, 2004). The liquid droplets would hence remain dispersed in the ice cream as inert fillers. The production of high quality ice cream rich in unsaturated fat would therefore require: (a) aggregation of the solid droplets for optimum structuring during processing and consequently the formation of a final product with melt resistance; and (b) keeping the unsaturated fat from damaging the microstructure. To do this, two ice cream mixes were prepared separately: one containing the shear-sensitive solid fat and the other with protein-stabilised liquid fat. Combination of the different mixes into one mix prior to freezing, here referred to as a two-stream process, was used to deliver the two different types of fat into the ice cream. However, questions arise as to the effect that surface-active material in one stream could have on the other during processing, e.g., displacement of interfacial protein from the liquid droplets by emulsifier on the solid fat. This would be a major concern given that liquid droplets devoid of the stability conferred by the milk proteins would readily coalesce and hinder the formation of appropriate structure. As a precautionary step, several surface protein concentration analyses on the individual and combined emulsions were carried out at different emulsifier concentrations. The aim of this was to choose appropriate monoglyceride concentrations high enough to displace protein from the solid droplets, however, with care to prevent alteration of the liquid droplets. Ice creams were characterised with static light scattering, rheology and the measurement of overrun and melt stability. 2. Materials and methods 2.1. Ice cream preparation 2.1.1. Mix formulation A variety of mixes were prepared and analysed for surface protein concentration to select the appropriate emulsifier levels in the solid fat stream of the ice creams. The base formulation for all mixes consisted of 10% skim milk powder (Parmalat, London, ON, Canada), 12% sucrose (Lantic Sugar Limited, Montreal, QC, Canada), 4% crystalline corn syrup solids (42DE, Casco Inc., Etobicoke, ON, Canada), 0.15% guar gum and 0.02% carrageenan (Danisco Inc., Scarborough, ON, Canada). The mixes differed in the levels of glycerol monooleate (GMO) (Danisco) as well as the composition and concentration of fat. Fractionated palm kernel oil (PKO) (ACH Food Companies Inc., Cordova, TN, USA) was used as source of saturated fat while high oleic sunflower oil (HOSO) (Trisun 80, Nealanders International Inc., Mississauga, ON, Canada) was the source of unsaturated fat. Based on the assumption that 0.15% emulsifier is suitable for the destabilisation of 10% fat ice cream (Sung & Goff, 2010), GMO
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concentrations were calculated to maintain this ratio for the mixes containing different PKO levels. The monoglyceride content was then raised gradually for each mix up to a point where the surface load remained unchanged and the lowest GMO concentration from this plateau was chosen so as to prevent the use of emulsifier in excess amounts. Based on the data provided by the surface protein load analyses (results not shown), the following mixes containing PKO or HOSO were prepared and combined at 1:1 ratio to produce 10% fat formulations:
4% PKO mix ð0:105% GMOÞ þ 16% HOSO mix ð0% GMOÞ ¼ 2% PKO ice cream ð0:052% GMOÞ 6% PKO mix ð0:135% GMOÞ þ 14% HOSO mix ð0% GMOÞ ¼ 3% PKO ice cream ð0:067% GMOÞ 8% PKO mix ð0:165% GMOÞ þ 12% HOSO mix ð0% GMOÞ ¼ 4% PKO ice cream ð0:082% GMOÞ The above ice creams are hereafter referred to as two-stream ice creams. Control ice creams consisted of the same formulations, though they were prepared differently (see Section 2.1.2.). Regular ice creams, prepared in the same way as control ice creams, all contained 0.15% GMO. 2.1.2. Mix preparation All ingredients, except for the monoglyceride, were blended in water at room temperature (w23 C) and continuously stirred while heating gradually. The monoglyceride was added to the premix at 45 C to avoid sticking to the container surfaces at lower temperatures. Pasteurisation was carried out at 75 C for 15 min. Thereafter, the mixes were pre-homogenised with a high-speed mixer (Silverson L4RT, Silverson Machines, Inc., UK) at 7500 rpm for 5 min to ensure complete melting and dispersion of the emulsifier and the fat. Afterwards, the coarse emulsions were passed twice through a two-stage homogeniser (31MR Laboratory Homogenizer, APV Gaulin Inc., Everett, MA, USA) with the first stage operating at 20 MPa and second stage at 7 MPa. Following homogenisation, the mixes were cooled to 25 C and aged at 4 C overnight. After ageing, the PKO and HOSO mixes were combined accordingly (1:1) to produce two-stream mix and the latter was stored for a further 30 min at 4 C before freezing/whipping. Control mixes were prepared by dispersing both the solid and liquid fats together in the mix and emulsifying all the fat with GMO. Thus, two-stream and control mixes consisted of identical compositions but differed in the way the droplets were arranged in the mix. Regular and control mixes were prepared identically and frozen/whipped after ageing. 2.1.3. Freezing/whipping Ice cream mix (1.5 L) was frozen in a batch freezer (Model 104, Taylor Company, Rockton, IL, USA) for 6 min (draw temperature w5 C) and whipped for 4.5 min for a total 10.5 min freezing/ whipping. Extruded ice creams were collected into 250 mL containers and hardened at 30 C in a blast freezer (Series 6899780, Foster Refrigeration, Drummondville, QC, Canada) for at least 24 h before further testing was performed. 2.2. Determination of particle size Approximately 10 g ice cream samples were melted for 2 h at 4 C before analysis. The weighted average particle diameter (d4,3) was measured with a Malvern Mastersizer 2000 (Malvern Instruments, Malvern, Worcestershire, UK). Dilution of the samples during
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measurement was approximately 1:1000 with Milli-Q water in the Mastersizer chamber. The refractive index for the fat and the dispersing medium was 1.46 and 1.33, respectively, with absorbance of 0.001 and obscuration value in the range of 12e18%. 2.3. Overrun The weight of ice cream mix was calculated from a cup with a fixed volume of 227 mL assuming a density for the mix of 1.1 kg L1. Ice creams were weighed into the same cup and overrun percentage was determined by
% overrun ¼ ½ðg ice cream mix g ice creamÞ=g ice cream 100 ð1Þ 2.4. Melt stability
Fig. 1. Accumulated % mass loss of control (open), two-stream (patterned) and regular (filled) ice creams with increasing solid fat following 90 min meltdown. Control and two-stream ice creams were prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively. Regular ice creams consisted of 0.15% GMO at all fat ratios. Error bars define the standard error of duplicates.
During ice cream extrusion, samples were collected into 250 g containers for the purpose of meltdown testing. After hardening, ice creams were tempered at 15 C for 24 h prior to testing the melt stability at room temperature (w23 C). Ice creams were placed on rectangular stainless steel wire mesh screens (15 11.5 cm, hole size of 2.5 2.5 mm) supported on tripods. The melted serum dripping through the mesh was collected and weighed every 10 min for a period of 90 min. Plots were constructed of % mass loss versus time and the accumulated % mass loss after completion of the test was determined.
analyses were performed with GraphPad Prism 5 (GraphPad Software Inc., San Diego, CA, USA). Analysis of variance was obtained with One-way analysis of variance (ANOVA) routines and multiple comparisons of means were conducted using t tests. Statistical significance is given by P values, with differences at the 95% confidence interval (P < 0.05) considered statistically significant.
2.5. Rheology
3.1. Characterisation of ice cream structure by melt stability and particle size
Small-deformation oscillatory tests were performed using a controlled stress rheometer (AR 2000, TA instruments, New Castle, DE, USA) fitted with 20 mm parallel-plate geometry (2000 mm gap), following a method similar to that of Granger et al. (2004) and Wildmoser, Scheiwiller, and Windhab (2004). The storage modulus G0 , which reflects the solid-like properties of the material, was recorded with increasing temperature from 10 C to 30 C with heating rate of 0.5 C min1 at fixed shear stress and frequency of 1 Pa and 1 Hz, respectively. The applied stress was selected based on the linear viscoelastic range (LVR) so the temperature ramp applied to 2% PKO two-stream ice cream required lower shear stress of 0.5 Pa. Ice creams were cut into samples with dimensions of 20 mm diameter and 3e5 mm thickness at 20 C with a cylindrical cutting tool. The samples were rapidly transferred in a steel container to the rheometer and placed on the peltier plate set at 10 C. The gap was adjusted and ice creams were simultaneously covered with a styrofoam movable hood, to prevent temperature exchange with the environment, and left to equilibrate on the plate for 2 min at 10 C before the experiment. The gap adjusted to the frozen ice cream was slightly smaller than its thickness to account for volume loss during ice cream meltdown on the plate and thus maintain contact between geometry and sample at all times. The rheology of the fat/air phase was measured following the complete melting of ice. The G0 values corresponding to the fat/air structures were calculated from the average of measurements taken at 3, 6, 9, 12, 15, 18 and 21 C for each ice cream. This temperature range was chosen because PKO would mainly exist in the solid state thereby avoiding collapse of the fat/air structures. Rheological measurements are reported as the average of four replicates from ice cream prepared twice. 2.6. Statistics Results are reported as the mean from ice creams prepared in duplicate with error bars representing the standard error. Statistical
3. Results and discussion
Regular ice creams exhibited significantly lower % mass loss than two-stream or control ice creams at any given fat ratio (P < 0.05) with no significant difference between two-stream and control ice creams for a given fat ratio or with increasing solid fat (Fig. 1). The effect of the latter on the meltdown behaviour of regular ice creams is discussed in further detail below. Ice creams were analysed for particle size, revealing a correlation with melt stability (Fig. 2). Regular ice creams, with substantially lower melt rate, exhibited larger particles than two-stream or control ice creams (P < 0.05) while the latter two, with highly similar particle size, melted equally fast. Just as the melt stability remained the same for two-stream and control ice creams with increasing PKO levels, so did the particle size. Destabilisation of regular ice creams with increasing PKO is discussed below. The large difference in particle size between control and regular ice creams points to the greater destabilisation reached simply by
Fig. 2. Weighted mean particle diameter (d4,3) of control (open), two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Control and twostream ice creams were prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively. Regular ice creams contained 0.15% GMO at all fat ratios. Error bars define the standard error of duplicates.
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increasing the GMO levels. Unsaturated monoglycerides have been shown to promote optimum aggregation among droplets. Some workers attributed this to greater protein displacement at increasing GMO concentrations (Davies, Dickinson, & Bee, 2000, 2001) while others put more emphasis on the interactions between unsaturated monoglycerides and hard vegetable fats for the formation of highly interacting droplets (Barfod, Schrader, & Buchheim, 2000; Granger, Leger, Barey, Langendorff, & Cansell, 2005a; Granger, Schöppe, Leger, Barey, & Cansell, 2005b). Adsorption of partially coalesced aggregates at the airewater interface imparts high air stability because localised fat clumps in the lamellae between air cells block water drainage during meltdown and so retard bubble coalescence (Chang & Hartel, 2002; Goff, 2001; Koxholt et al., 2001; Sofjan & Hartel, 2004). Larger aggregates slow down the melt rate more efficiently because of their greater ability to retain water and air cells together (Muse & Hartel, 2004; Pelan, Watts, Campbell, & Lips, 1997). The much faster meltdown in control and two-stream ice creams is attributed to poor structuring around the air cells as evidenced by their smaller particle size when compared with regular ice creams. Although the former two had in common the lack of structure, the colloidal interactions may have varied in these ice creams because they were structured very differently. In control ice creams, the GMO concentrations were probably insufficient to render the droplets shear-sensitive. Conversely, two-stream ice creams with localised GMO on the solid droplets may have favoured stronger interactions. Yet, with PKO accounting for less than half of the total fat volume it is possible that the droplets may have not been present in sufficient amounts to aggregate into fat networks reinforcing the ice cream matrix. Having observed no differences between preparation methods, the GMO concentration was further raised on the PKO droplets of two-stream ice creams. This time, the approach was to generate sufficient interactions between the unsaturated monoglyceride and the fat to enhance aggregation. Interactions between GMO and hard vegetable fats at the oilewater interface have been shown to create highly deformed globules resembling the structures of crystal platelets (Barfod, 2001; Barfod et al., 2000). These fat particles are prone to setting larger fat networks than the more globular fat in addition to providing extremely high air cell stability (Barfod, 2001; Persson, 2009). To this end, mix containing 8% PKO and 0.3% GMO in addition to all other ingredients was prepared, showing a monomodal droplet size distribution following homogenisation (results not shown). Interestingly, only ageing was required to create substantial changes in the droplet arrangements of the mix (Fig. 3). Even before freezing/whipping, the monodispersed emulsion turned into what seemed like a partially coalesced mix. Dilution with sodium dodecyl sulphate (SDS) or ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA) was performed to verify the nature of the colloidal interactions. Both SDS and EDTA affect the droplet size distribution differently based on whether fat particles connect through protein or fat. If the flocs are protein-bound, the almost complete disruption of micellar casein into caseinates with EDTA (Horne, 2009) at 1.7 wt% (Goh et al., 2006) inhibits micellar-induced bridging flocculation, leading to re-dispersion of the agglomerates. When the droplets are held together by protruding fat crystals, adsorption of SDS (2 wt%) alters the wettability of the latter, causing expulsion of the crystals into the aqueous phase and, with no crystals to maintain the droplets closely associated, inevitable re-dispersion (Boode & Walstra, 1993). The ability of SDS to also disrupt casein micelles required the use of EDTA alone to separate the fat crystaland protein-binding effects. More detailed experiments on the effect of SDS and EDTA on a variety of fat aggregates will soon be published.
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Fig. 3. Droplet size distribution of aged 8% PKO (0.3% GMO) mix diluted with water (C), 2% SDS (:) or 1.7% buffered EDTA (B).
Disrupting the casein micelles with EDTA did not reverse the droplet size distribution. On the other hand, the aggregate-disrupting effect of SDS led to the appearance of smaller particles, supporting the concept that its ability to break up micellar casein was not related to its particle size-reduction power but is indicative of the presence of partial coalescence. These findings suggest that enhanced GMOePKO interactions triggered the formation of fat networks in the mix even before shearing. Given its high tendency to destabilisation, this mix (8% PKO, 0.3% GMO) was diluted with mix containing HOSO droplets to produce two-stream ice creams. The GMO concentration came to 0.15% or less as a result of dilution to reach 2, 3 or 4% PKO formulations. The GMO levels varied across the 2, 3 and 4% PKO mixes, yet the surface properties and functionality of the PKO droplets remained the same across the formulations because the solid droplets all originated from the same mix. After freezing/whipping, two-stream ice cream was compared with the previously reported regular ice cream for particle size and melt stability. Two-stream ice creams with 2 or 3% PKO exhibited lower melt resistance than regular ice creams, with the difference significant for 2% PKO (P < 0.05) (Fig. 4). At 4% PKO, the stability of two-stream ice cream improved greatly, melting at the same rate as regular ice cream. With increasing solid fat, % mass loss was significantly reduced at 4% PKO for both regular and two-stream ice creams (P < 0.05). Overall, the light scattering studies parallelled melting behaviour (Fig. 5). At 2 and 3% PKO, regular ice creams showed larger particles than two-stream ice creams (P < 0.05). The opposite effect was observed at 4% PKO, with the two-stream ice cream destabilising more (P < 0.05). The shift towards larger particles with solid
Fig. 4. Accumulated % mass loss of two-stream (patterned) and regular (filled) ice creams with increasing solid fat following 90 min meltdown. Regular ice creams consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
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Fig. 5. Weighted mean particle diameter (d4,3) of two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Regular ice creams consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
fat content was significant at 3 and 4% PKO for regular ice creams and at 4% PKO for two-stream ice creams (P < 0.05). The two-stream process promoted greater destabilisation at 4% PKO. However, the performance in lower PKO volumes (2 and 3%) was lacking, suggesting a critical concentration above which the aggregating droplets exist in sufficient quantities to develop structure. The low performance in two-stream ice creams with 2 or 3% PKO is not attributed to coalescence of the unsaturated fat, which could have occurred upon substantial displacement of interfacial protein from the liquid droplets by excess emulsifier on the solid fat. The appearance of peaks in the droplet size distributions of two-stream ice creams in the range of 0.3e1 mm confirmed the stability of the unsaturated droplets against aggregation (results not shown). Despite greater destabilisation in the 4% PKO two-stream ice cream its melt stability did not surpass that of the corresponding regular ice cream. It is worth remarking that the melted serum in both these ice creams was clear in appearance, indicating the absence of air cells and fat and that either level of fat destabilisation was optimum to retain the structural fat/air networks together in a stable foam. It is noteworthy that static light scattering theory assumes the dispersed fat particles are spherical and non-interacting. Hence, the particle size data reported here can only be used as an index of the relative size of aggregates rather than their actual size.
3.2. Overrun Overrun decreased significantly at 3 and 4% PKO in two-stream ice creams and at 3% PKO in regular ice creams (P < 0.05). Differences between two-stream and regular ice creams were not significant (Fig. 6). Greater aggregation increasingly hindered the incorporation of air cells, revealing a relationship between particle size and foamability. Previous work has shown enhanced foamability in milkfat ice cream when the saturated fat is partially replaced with olein-rich fractions (Bazmi & Relkin, 2009). However, the effect observed in this study could be more related to the particle size than the actual fat compositions. This may be concluded from the differences, though not significant, observed between two-stream and regular ice creams for equal fat ratios. Milk proteins are known for their excellent foaming properties (Zhang & Goff, 2004). Fat particles, on the other hand, hinder foaming (Dickinson, 1992, chap. 5), yet they confer excellent air cell stabilising properties when held together in the form of aggregates (Dalgleish, 2006). It is possible that extensive partial coalescence in the ice creams with higher destabilisation may have displaced protein from the airewater interface
Fig. 6. Development of overrun in two-stream (patterned) and regular (filled) ice creams with increasing solid fat. Regular ice creams with increasing solid fat consisted of 0.15% GMO at all fat ratios. Two-stream ice creams were obtained from diluting 8% PKO mix (0.3% GMO) with the corresponding HOSO mixes to generate 2, 3 and 4% PKO ice creams. Error bars define the standard error of duplicates.
during freezing/whipping, thus reducing the foamability while also rendering the air cells more stable to coalescence. Although air cell size was not studied here, it has been reported that larger aggregates promote the formation of smaller air cells and narrower air cell size distributions, less prone to fusion (Eisner et al., 2005; Relkin, Sourdet, Smith, Goff, & Cuvelier, 2006). On the contrary, the caseins and whey proteins despite being better foaming agents cannot stabilise air cells to the same extent as the fat clusters. Milk protein-stabilised bubbles favour air cell growth as they coalesce during freezing/whipping. This explains the faster meltdown of ice cream with higher overrun and thus less destabilisation. 3.3. Rheology of the fat/air phase According to previous research, G0 values above the ice melting point of ice cream measure the response of the fat/air structures and relate to creaminess in that higher G0 contributes to greater perception of this attribute (Wildmoser et al., 2004). For their rheological characterisation, ice creams were divided into two groups: those with low or high GMO concentrations. Control and two-stream ice creams prepared with 0.052, 0.067 and 0.082% GMO at 2, 3 and 4% PKO, respectively, are hereafter referred to as ice creams with “low” GMO. Two-stream ice creams derived from diluting 8% PKO mix containing 0.3% GMO, and regular ice creams are hereafter referred to as ice creams with “high” GMO. 3.3.1. Low glycerol monooleate concentrations The average G0 corresponding to the fat/air phase of two-stream and control ice creams is shown in Fig. 7. At 2% PKO, control ice cream exhibited significantly larger G0 than two-stream ice cream while the opposite effect occurred at 3% PKO (P < 0.05). At 4% PKO, two-stream ice cream also revealed higher G0 , although the difference was not significant. Solid fat content gave significantly greater G0 at 4% PKO among control ice creams, and at 3 and 4% PKO among two-stream ice creams (P < 0.05). It has been consistently reported that greater destabilisation enhances G0 in the temperature region where the fat/air structures are measurable (Barfod et al., 2000; Eisner et al., 2005; Granger et al., 2004, 2005a; Wildmoser et al., 2004). The current experiments, however, do not fully agree. While particle size remained the same across all samples with low GMO, G0 varied considerably under the same conditions (see Figs. 2 and 7), increasing with higher PKO and thus suggesting a greater effect of solid fat than particle size on G0 . 3.3.2. High GMO concentrations The rheological characterisation of high GMO ice creams also pointed to greater G0 in two-stream ice creams (Fig. 8), yet this
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Fig. 7. Average storage modulus of the fat/air structures in two-stream (patterned) and control (filled) ice creams with low GMO. Average G0 was obtained in the temperature range of 3e21 C. Error bars define the standard error of four replicates from ice cream prepared in duplicate.
trend was significant only at 3% PKO (P < 0.05). The G0 of regular ice cream dropped from 2 to 3% PKO (P < 0.05) followed by a large increase at 4% PKO (P < 0.05). The same effect, although not significant, was found for two-stream ice cream from 2 to 3% PKO, with G0 rising substantially again at 4% PKO (P < 0.05). The 2 and 3% PKO two-stream ice creams showed larger G0 than the more destabilised 2 and 3% PKO regular ice creams (see Figs. 5 and 8). Moreover, increasing the PKO from 2 to 3% in regular ice cream resulted in larger particles but also lower G0 . These results are not in agreement with previous literature showing a correlation of destabilisation with G0 . Unlike ice creams with low GMO, increasing the solid fat in high GMO ice creams lacked a constant increase in G0 . The decrease in G0 from 2 to 3% PKO could be partly explained by differences in the shape retention properties of regular ice cream during meltdown. While 2% PKO regular ice cream left a collapsed mass of foam on the mesh after melting, 3% PKO regular ice cream did not do this. Instead, serum flowed steadily out of the ice cream leaving very little standing on the mesh. A large mass of solid-like foam was once again visible after meltdown of 4% PKO regular ice cream. This indicates that the fat/ air microstructure at 3% PKO was of a different nature to that created at 2 and 4% PKO. The settling of ice cream into a collapsed foam after meltdown is characteristic of optimum droplet interactions and high air cell stability (Eisner et al., 2005; Koxholt et al., 2001). The absence or alterations of these properties at 3% PKO may be associated with lower G0 . Interestingly, changes in particle size as influenced by GMO concentration were found to alter the storage moduli. This is supported by comparing equally-processed ice creams for a given fat ratio as a function of low versus high GMO (see Figs. 7 and 8 for low versus high GMO two-stream ice cream or control versus regular ice cream). Overall, higher % GMO promoted larger G0 , especially at
Fig. 8. Average storage modulus of the fat/air structures in two-stream (patterned) and regular (filled) ice creams with high GMO. Average G0 was obtained in the temperature range of 3e21 C. Error bars define the standard error of four replicates from ice cream prepared in duplicate.
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2% PKO in both types of ice cream process and at 3% PKO in twostream ice creams (P < 0.05). This is attributed to an increase in the size of the fat aggregates and suggests that large aggregates per se may not necessarily behave in the same way. This is proposed based on the effect of particle size on G0 ; the formation of larger particles with increasing solid fat or a change in processing did not increase G0 . Only larger particles created by higher levels of emulsifier led to higher G0 . The different effects caused by aggregates of similar (large) size may be attributed to differences in their shape/ conformations. This property seems to affect the way droplets arrange to produce texture, and vary according to the condition responsible for inducing the aggregation process, e.g., processing technique (two-stream versus regular), solid fat content or emulsifier concentration. Thus, the condition favouring fat destabilisation may be equally important as the size of aggregates in relation to the structure and so the development of texture. To better understand the elastic nature of the fat/air phases, changes in the loss modulus (G00 ), which reflects the liquid-like properties of the material, were evaluated in relation to G0 . The ratio of G00 /G0 is defined by the tangent of the phase angle d (tan d). This parameter indicates whether the viscous or elastic property dominates in the material as stress is applied. tan d Values close to or above 1 indicate the predominance of the viscous properties versus the elastic. As tan d approaches 0 the material is said to behave more like a solid. The storage modulus of each ice cream was plotted with the corresponding tan d (Fig. 9). Two distinct rheological behaviours are depicted in this plot; the first one illustrates a growing influence of G0 over G00 as G0 increases up to about 150 and tan d decreases constantly. Up to this point the ice creams become increasingly elastic. After G0 of 150, tan d remains constant with increasing G0 . The greater stiffness in the ice creams is in this case attributed to a constant rise in both the elastic and viscous properties. That is, G00 increased simultaneously with G0 . Unfortunately, similar treatment of the data for G0 and tan d has not been previously reported. Full tan d patterns in relation to G0 during temperature ramps have been reported (Granger et al., 2004, 2005a). Plotting tan d, as such, gives good correlation with the developing G0 but it is not until the responses of different ice creams are plotted together with their corresponding tan d that a better relation of G0 to G00 is observed. Ice creams with 4% PKO, regardless of the preparation method or GMO concentration, mark the appearance of steady tan d with increasing G0 . However, it is not clear why an increase in % solid fat leads to the formation of stronger structures with enhanced viscous character. 3.3.3. Fat/air interactions model It has been suggested that deposition of fat aggregates in the lamellae between air cells is the mechanism by which air cells
Fig. 9. Development of tan d with the corresponding G0 of ice creams. Average G0 was obtained in the temperature range of 3e21 C.
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Fig. 10. From left to right: schematic representation of a) liquid (open spheres) and solid (filled spheres) droplets adsorbed at the airewater interface forming “micro-bridges” in the lamellae between air cells, b) blockage of the lamellae by partially crystalline aggregates and c) fat platelet aggregates (filled structures) in the presence of liquid droplets (open spheres).
become more stable to rupture (Koxholt et al., 2001). According to these workers, the fat, if present in sufficiently large clusters, blocks the lamellae and prevents drainage, reducing the chances of air cell coalescence. The predominance of higher G0 in two-stream ice creams when compared with those prepared conventionally may depart from this hypothesis. If the fat aggregates in the lamellae are to act as “micro-bridges” among air cells, then it is likely that the properties of the aggregates would influence the shear-induced response of the ice cream. These properties, other than particle size, include composition as well as conformation/shape. In the case of two-stream ice creams, adsorption of fully solid fat clusters at the air cell surface may yield greater resistance to shear (Fig. 10a), even when clusters are present in lower amounts and sometimes smaller size. In contrast, the oil-containing aggregates, as in the case of conventional ice creams, may have facilitated further compression of the fat/air phase due to the “softer” character of the droplets, regardless of their larger amount and sometimes larger size (Fig. 10b). The greater G0 associated with high GMO ice creams may be the result of alterations in the shape/conformation of the aggregates due to enhanced GMOePKO interactions. As previously described, the interactions between these two ingredients drastically affects the spherical shape of droplets, turning them into interacting platelets capable of stabilising the airewater interface. It is therefore possible that platelet aggregates of this kind in the lamellae formed blockages of greater strength more resistant to stress (Fig. 10c). It should be noted that the diagrams represented here attempt to depict the rheological behaviour as influenced by adsorbed fat only. The surface of air bubbles is also composed of other surfaceactive materials such as protein and emulsifiers.
4. Conclusions Particle size and melt stability correlated well, with larger particles increasing the melt resistance. At low GMO concentrations, both these properties remained the same despite processing method or solid fat content. The G0 in ice cream with limited structure seems mainly determined by % solid fat. Increasing GMO caused drastic changes in the stability and structure formation of ice cream from either process. Restricting the emulsifier to droplets intended for aggregation, via the twostream approach, caused greater destabilisation than regular processing at 4% PKO. Most probably, at lower PKO the amount of
interacting droplets for structure build-up was limited. The equally low melt rate of regular and two-stream ice creams at 4% PKO indicates that no more serum is retained among air cells above optimum aggregation levels. Particle size correlated inversely with overrun as increasingly larger particles most probably reduced the availability of proteins at the airewater interface. Higher GMO favours larger droplets with a G0 -enhancing effect. Other variables also capable of raising particle size showed no such rheological behaviour. This fact points to the effect of formulation/processing on the formation of aggregates with different functionality, which seems associated with their composition/conformations other than just size. The shearinduced response of ice cream may originate from the structure of the air cell lamellae, which in turn is defined by the arrangement of fat. The decrease in tan d followed by a plateau with increasing G0 indicates a shift in the rheological properties of ice cream from elastic to increasingly viscoelastic. The higher G0 resulting from two-stream processing may alter sensory attributes, favouring the formation of desirable ice cream, especially when considering the allegedly established link between G0 and creaminess. Acknowledgements The authors would like to thank Nestec, Inc. for financial support, especially Drs. Virginie de Boishebert and Max Puaud, Nestle Research Centre, Beauvais, France, for their helpful discussions on the completion of this work. References Akhtar, M., Murray, B. S., & Dickinson, E. (2006). Perception of creaminess of model oil-in-water dairy emulsions: influence of the shear-thinning nature of a viscosity-controlling hydrocolloid. Food Hydrocolloids, 20, 839e847. Barfod, N. M. (2001). The emulsifier effect. Dairy Industries International, 66, 32e34. Barfod, N. M., Schrader, K., & Buchheim, W. (2000). Water continuous fat crystal networks in ice cream mix with unsaturated monoglycerides. In Second international symposium on food rheology and structure, March 12e16, 2000, Zurich, Switzerland. Bazmi, A., & Relkin, P. (2009). Effects of processing conditions on structural and functional parameters of whipped dairy emulsions containing various fatty acid compositions. Journal of Dairy Science, 92, 3566e3574. Boode, K., & Walstra, P. (1993). Partial coalescence in oil-in-water emulsions 1. Nature of the aggregation. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 81, 121e137. Chang, Y., & Hartel, R. W. (2002). Development of air cells in a batch ice cream freezer. Journal of Food Engineering, 55, 71e78. Crilly, J. F., Russell, A. B., Cox, A. R., & Cebula, D. J. (2008). Designing multiscale structures for desired properties of ice cream. Industrial and Engineering Chemistry Research, 47, 6362e6367.
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