Effects of emulsifiers on vegetable-fat based aerated emulsions with interfacial rheological contributions

Food Research International 53 (2013) 342–351

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Effects of emulsifiers on vegetable-fat based aerated emulsions with interfacial rheological contributions Hyun-Jung Kim ⁎, Arjen Bot, Isabel C.M. de Vries, Matt Golding 1, Eddie G. Pelan Unilever R&D Vlaardingen, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands

a r t i c l e

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Article history: Received 11 February 2013 Accepted 28 April 2013 Keywords: Whipping emulsion properties Emulsifier Oil and water interfacial properties Partial coalescence

a b s t r a c t A relationship between oil/water interfacial properties and the whipping performance of vegetable-fat based o/w emulsions was investigated for two emulsifier mixtures: lactic acid esters of mono and diglycerides (LACTEM), and partially unsaturated/saturated monoglyceride mixture (PUS/SMG). The whipping emulsions were evaluated on storage viscosity, overrun, and firmness. The model interfacial layer between water and oil with LACTEM gave higher shear modulus and the emulsion showed low bulk viscosity, high overrun and high firmness after whipping. The interfacial modulus with PUS/SMG was lower and the emulsion provided higher bulk viscosity, lower overrun and lower firmness. Microscopy and particle size distribution in sheared unwhipped emulsions showed individual fat globules without aggregation in the case with LACTEM, while globules with PUS/SMG became highly aggregated. These suggest that the weaker interfacial layer with PUS/SMG can be more susceptible to the fat crystal mediated partial coalescence of fat globules: Partial coalescence can happen already during quiescent storage. The subsequent whipping led to further aggregation then a strong fat network can help to stabilize air bubbles. The more stable interfacial layer in LACTEM-based emulsions leads to a lower tendency to coalesce during quiescent conditions, and only to aggregation and coalescence during the whipping stage. Fat aggregation was confirmed to be essential in achieving a good aerated structure but there were differences depending on the surface layer properties of fat globules induced by emulsifiers. This caused the rheological differences observed during storage. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction With increased needs to deliver consumer health benefits, dealing with obesity, health, nutrition and weight management is seen as an important driver of the food industry for long. Practical approaches taken by food manufacturers include reducing the necessity of “negatively” perceived ingredients, such as amount of fat, salt and sugar, in combination with fortification of food with functional and nutritional actives, such as minerals, vitamins, and antioxidants (Golding & Pelan, 2008). Whipping creams are aerated emulsions with overruns typically ranging from 100% to ~ 300%, which contain more than 30% of milk fat. The whipped structure should have good foam stability and firmness. For consumers' benefits through lower level of fats, whipping emulsions with lower amount of fat from vegetable sources have been developed for the market as an alternative to traditional whipping creams of 30–36% dairy fat. These are often referred to as non-dairy creams, indicating that the milk fat has been replaced by vegetable fat. These are developed to be more effective in ⁎ Corresponding author. Tel.: +31 10 460 6779; fax: +31 10 460 6747. E-mail address: [email protected] (H.-J. Kim). 1 Current address: Institute of Food, Nutrition and Human Health, Massey University. Palmerston North 4442, New Zealand. 0963-9969/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodres.2013.04.027

structuring at lower fat concentrations than dairy whipping creams (Berger, 1988; Nesaretnam, Robertson, Basiron, & Machphie, 1993; Shamsi, Man, Yusoff, & Jinap, 2002). Hence, the mechanism, by which a stable foam structure can be generated during whipping for both dairy and non-dairy emulsions, has been of considerable academic and commercial interest for more than couple of decades. Various aspects have received attention, such as the functionality of emulsifiers, stabilizers, fats and the crystals, proteins and process conditions (Allen, Dickinson, & Murray, 2006; Allen, Dickinson, & Murray, 2008; Allen, Murray, & Dickinson, 2008; Bazmi, Duquenoy, & Relkin, 2007; Hotrum, Cohen Stuart, van Vliet, Avino, & van Aken, 2005; Leser & Michel, 1999; Relkin & Sourdet, 2005; van Aken, 2001; Vanapalli & Coupland, 2001). It is generally accepted that a good whipped emulsion structure requires both fat globule adsorption to the air–water interface in the foam and the creation of aggregated fat globule network in the continuous phase through various structuring mechanisms. Examples could be in the case of homogenized dairy whipping emulsion through mainly casein–calcium bridge network (Besner & Kessler, 1998) or for non-dairy/dairy whipping emulsion, mainly by fat crystal induced partial coalescence and agglomeration (Boode & Walstra, 1993a, 1993b; Brooker, 1993; Buchheim, Barfod, & Krog, 1985). For aerated emulsions, it is necessary to add small molecular emulsifiers, e.g., monoglycerides, to the formulation to ensure appropriate

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structuring takes place (Boode & Walstra, 1993a, 1993b; Bruhn & Bruhn, 1987; Flack, 1985; Goff, 2002; Pelan, Watts, Campbell, & Lips, 1997; Walstra, 1993). Proteins are essential to produce a stable emulsion during preparation of the emulsion (Dickinson 1999; McClements, 2005; Walstra, 1993). However, if the adsorbed protein layer is too stable, it prevents adsorption of globules to the air–water interface by providing over-stability to fat globules against partial coalescence during the whipping process (Pelan et al., 1997; Zhang & Goff, 2005). Together with the fact that most food emulsifiers have the ability to displace protein from the interface of emulsion droplets (Ananthapadmanabhan, 1993; Bos & van Vliet, 2001; Kiokias & Bot, 2006; Mackie & Wilde, 2005; Patino et al., 2001), it should be noted that the composition and nature of the interface, and the consequent properties of whipping emulsion during storage and after whipping can vary significantly depending on the specific emulsifier(s) and concentration used (Allen, Dickinson, & Murray, 2008; Allen, Murray, & Dickinson, 2008; Bazmi et al., 2007; Davies, Dickinson, & Bee, 2000, 2001; Krog, 1997; Pelan et al., 1997). More and more, studies on the impact of interfacial composition and rheological properties have also been reported (Davies et al., 2000, 2001; Thivilliers-Arvis, Laurichesse, Schmitt, & Leal-Calderon, 2010). With respect to the work with emulsifiers, various results have been reported mainly with focus on the efficiencies in displacing milk protein at the interface and its contribution to stabilizing bubbles in aerated structure (Barfod, Krog, Larsen, & Buchheim, 1991; Bazmi et al., 2007; Davies et al., 2000, 2001; Krog, 1997; Pelan et al., 1997; Zhang & Goff, 2005). Among the small molecular emulsifiers, the role of commonly used monoglycerides depends on their saturation level or concentration. (Barfod et al., 1991; Davies et al., 2001; Krog, 1997; Pelan et al., 1997). With the displacement of milk protein by emulsifiers, the interfacial composition and behavior will control fat crystal network formation and/or adsorption of fat globules at a/w interface (Murray, 2002). There have been many studies addressing the mechanism and contributions of ingredients. It is sometimes with inconsistency and debates (e.g., impact of emulsifiers by its concentrations, Pelan et al., 1997; Miura, Yamamoto, & Konishi, 2002; Miura, Yamamoto, & Sato, 2002), but also general trends have been abstracted (e.g., the encounter frequency and the capture efficiency of partial coalescence, Fredrick, Walstra, & Dewettinck, 2010). However, the contribution of fat globule interface to the whipping emulsion properties and performance has attracted less attention (Pawar, Caggioni, Hartel, & Spicer, 2012). In this study, we aim to get an understanding on the effect of emulsifiers on o/w interfacial properties and on the final product properties with low fat (18%) non-dairy whipping emulsions. Preliminary work was done with a large number of whipping emulsions of various emulsifier mixtures and emulsifier concentrations. Emulsions of acceptable whipping cream properties (pourability of emulsion, foaming properties, and structural firmness) which are similar to those of dairy whipping cream with 36% fat in the market were selected. Among those, two emulsions with different interfacial properties were selected for the present study: emulsion with lactic acid esters of mono and diglycerides (LACTEM) and emulsion with partially unsaturated/saturated monoglyceride mixture (PUS/SMG). These emulsions have exactly the same base ingredients and processconditions, with differences only in emulsifier type and its concentration. Thus, the interfacial properties of the fat globules and potential differences in fat crystals induced by the emulsifiers as crystal habit modifier should be the main contributor to the differences in aeration and the structuring properties of the emulsions. In this study, we aim to relate the effect of the interfacial properties induced by emulsifiers to the properties of whipping emulsion by monitoring 1. the interfacial properties by the emulsifiers at a model liquid oil–water interface where no fat crystals are formed, 2. emulsion properties under shear without aeration to know the impact

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of fat globule aggregation by shear, and 3. whipping properties and performance. 2. Materials and methods 2.1. Materials The products have 18% w/w fat and ~ 82% w/w water phase, including all the water soluble components. Water soluble components were milk powder (5.25% w/w in product as a mixture of skim milk powder and butter milk powder from Nordmilch (Oldenburg, Germany)) and biopolymeric stabilizers (0.16% w/w of product as a mixture of locust bean gum, guar gum and kappa carageenan from Danisco, Copenhagen, Denmark). The fat was composed with interesterified vegetable fat from palm oil (Unilever in house production). The fat consists of a hard fat mixed with liquid oil, resulting in a composition containing 47% saturated fatty acids (mainly C16:0), 39% C18:1 and 14% polyunsaturated fats. X-ray diffraction and differential scattering calorimetry have been applied to verify that the emulsifiers under investigation did not affect fat crystallization properties. Polymorphism, crystallization speed and crystal size were comparable to each other irrespective of emulsifier mixed. Still, to test without potential misinterpretation from the impact of fat crystal induced by different types of emulsifiers (Berger, 1990), the same composition of whipping emulsions but with liquid oil (sunflower oil from local market), were also prepared by the same process. Oil soluble emulsifiers were from supplier (Danisco, Copenhagen, Denmark). Two different types of emulsifiers were studied: One is a commercial mixture of lactic acid esters of mono and diglycerides (LACTEM). This emulsifier, which has melting point of ~ 45 °C and is fully saturated with iodine value (IV) of ~ 2, was used as 1% w/w in total. The other emulsifier mixture used in 0.45% w/w in total was a commercially blended unsaturated monoglyceride and saturated monoglyceride (PUS/SMG) from Danisco. This mixture has a melting point of about 60 °C and IV ~ 60. To monitor interfacial properties of model system, mineral oil (cat # 16,140-3) was purchased from Sigma-Aldrich. This was to remove potential interference from surface active impurities in commercial food grade liquid oils. 2.2. Methods 2.2.1. Analysis of interfacial properties of model interface 2.2.1.1. Interfacial properties. Interfacial shear rheology was measured with an Anton Paar Physica Rheometer (MCR 300) using a bi-conical bob. The outer radius of the bi-cone is 34.14 mm, the inner radius of the cylindrical sample vessel is 40 mm. The distance between these two, 5.85 mm, is the width of the ring-shaped protein and emulsifier adsorption layer which is subjected to the shear applied. To see the interfacial properties solely by emulsifiers at the interface without artifacts from e.g., crystals from fat or surface active impurities, high purity of lab grade liquid oil was chosen. Each emulsifier mixture of test was dissolved in the oil at 80 °C for complete melting of emulsifiers. LACTEM was at the concentration of 1% w/w in oil and PUS/SMG was 0.45% w/w in oil which were to match the ratio used in the final whipping emulsions. The water phase was prepared with milk powder (5.25% w/w) after heating the solution at 80 °C for 30 min. This was to mimic the condition which surface active components in water would experience during the production process of whipping emulsion. After cooling down the water phase to 20 °C, the protein solution was poured into the vessel. The bob was positioned at the exact water surface with software monitoring. The oil phase of 80 °C, containing the dissolved emulsifier, was carefully poured on top of the water phase and the bob positioned. Sufficient time was taken to allow the emulsifiers to adsorb at the interface, by cooling down the oil phase to 20 °C. Then, the changes

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in interfacial viscoelasticity with temperature cycling at 20, 10 and 20 °C were recorded to see the sensitivity of the sample to temperature cycling around the temperature of the whipping process. For this, linear viscoelastic regime was determined by a strain sweep with a constant frequency of 1 Hz or 0.5 Hz. The range of the sweep was from 0.001% to 1% strain with each interface. The sweeps were done at 10 °C and 20 °C respectively to ensure that the condition of oscillation should be within the linear region. Finally, 0.01% strain at 0.5 Hz was chosen for all the measurements. After whole measurements, the resulting oscillatory properties were calculated into interfacial term using the software provided by the supplier, with density, viscosity and volume parameters of each solution (Erni, Fisher, & Windhab, 2003; Erni, Fisher, Windhab, Kusnezof, et al., 2003; Kragel, Derkatch, & Miller, 2008) together with the consideration of the vessel geometry. Because the viscosity of oil phase which contains emulsifier varies with temperature, the viscosity of the oil with emulsifiers was measured respectively at each temperature and used for the calculations. 2.2.1.2. Microscopic observation. Both water and oil phases around the interface from interfacial shear measurements was carefully collected and slightly agitated to make coarse emulsion droplets at 20 °C. A drop of this emulsion was placed for a microscope and the microstructure of the coarse emulsion was observed using conventional optical microscopy (Zeiss, Germany). 2.2.2. Whipping emulsion production The fully melted fat phase was prepared by mixing the vegetable fat at 65 °C with LACTEM or PUS/SMG. This fat phase was added to the water phase which contains all the water soluble ingredients at 85 °C and mixed with the Ultra-turrax (IKA, USA). The mixture was pasteurized at 87 °C for 12 s and continuously processed with high pressure homogenizer at 55 °C (100 bar, GEA Niro Soavi, US), cooled immediately to 5 °C. This was put in a plastic bottle and stored at 5 °C. To see the impact of interfacial properties without fat crystal on the fat globule aggregation induced by different emulsifiers, emulsions with sunflower oil were produced by the same process. 2.2.3. Whipping emulsion properties before whipping All analyses were performed after 1 week of storage at 5 °C after the whipping emulsions production. The criteria for “acceptability” with whipping emulsion performance were defined through a consumer preference study by the combination of the physical properties; (1) for the pourability, apparent viscosity before whipping should be in the range of 400 mPa s to 600 mPa s by the method below, (2) overrun by whipping should be between 120% to 150% based on the overrun definition below and (3) firmness of the whipped cream should be between ~196 mN to 392 mN by the same method below. Based on these, the levels of emulsifiers in the whipping emulsions were chosen in this study. The solid fat content (SFC) was analyzed by NMR (Bruker Minispec MQ20). The apparent viscosity of whipping emulsion was measured by Brookfield viscometer with a spindle RV2 at 60 rpm (Brookfield, Ochten, NL). Dynamic oscillatory measurements were performed to monitor the rheological properties of unwhipped emulsion. A Bohlin rheometer (CVO) was used for this with cone and plate geometry, in which the cone was 40 mm in diameter with 2° angle, and the gap between the plate and cone was set at 70 μm. The plate was temperature controlled by Peltier to keep the temperature at 5 °C during the measurements. A spoonful of unwhipped emulsion after 1 week-storage at 5 °C was put on the plate. Linear regime was determined by a strain sweep from 0.001 to 1 at a constant frequency of 1 Hz. Then with the chosen shear strain of 0.01, frequency swept (starting with 10 Hz to 0.01 Hz and back to 10 Hz). The storage modulus (G′) and the loss modulus (G″) were obtained with this condition and the values at initial 10 Hz were presented as a representative strength in Table 1.

2.2.3.1. Properties of whipping emulsions for structural understanding before and after shear without air. Immediately after the oscillatory measurement above, a shear sweep from 10 s −1 to 1000 s −1 was applied to the sample at 5 °C in 4 min of duration. This was to mimic the whipping process without air incorporation in order to see the impact of shear on the destabilization of fat globules. 2.2.3.2. Particle size distribution. Each whipping emulsion of original and after shearing was collected to measure particle-size distributions using a Malvern Mastersizer (MS 2000 laser light-scattering analyzer). To avoid both multiple scattering effects and any fat or emulsifiers melting during measurements, the emulsions were diluted to a particle concentration of ~0.005wt.% using 5 °C double distilled water and stirred continuously throughout the measurements to ensure that the samples were homogenous. The mean particle size is reported as the volume-weighted mean diameter, D4,3(=Σni di4/Σni di3) or surface-weight mean diameter, D3,2(=Σni di3/Σni di2), where ni is the number of particles with diameter di (Alderliesten, 1991). D4,3 is more sensitive to the larger volume particles, therefore provided more information about particle aggregation than D3,2. (Hunter, 1986). To a highly aggregated emulsion after shear, mild sonication (25% magnitude for 10 s) was applied in the measurement cell before measurement. This was to break up weak aggregates. 2.2.3.3. Microscopic observation. Whipping emulsions of original and the collected after shear on rheometer were diluted with 5 °C ice water to avoid undesirable melting of any fat crystal or emulsifiers. The appearance of the emulsion was observed using conventional optical microscopy (Zeiss, Germany). 2.2.4. Whipping performance and structural properties After one-week aging at 5 °C, emulsions were whipped for 3.5 min by a Kenwood mixer (KM 220) at its maximum speed 5. The overrun of the whipped emulsion was calculated by measuring the weight of it which was put in a volume-standardized cup, before and after whipping, with the Eq. (1). After whipping, the whipped emulsion temperature was about 15 °C by natural increase.  % Overrun ¼ ðweight of unwhipped emulsion–weight of whipped emulsionÞ =weight of whipped emulsiong  100:

ð1Þ The firmness (mN) of the whipped emulsion was measured by a Stevens Texture analyzer model LFR (Stable Micro Systems), with a cylinder type probe of 25.4 mm diameter. The penetration depth was 4 mm and penetration speed was 1 mm/s. All whipping emulsion production and experiments were done at least three times in separate tests and averaged. 3. Results and discussion 3.1. Interfacial shear rheology at model oil and water interface and its microscopic observation In the presence of emulsifiers, both interfaces showed higher elasticity contribution (storage modulus) than viscosity contribution (loss modulus) (both Gi′LACTEM > Gi″LACTEM and Gi′PUS/SMG > Gi″PUS/SMG), and were considered as both are solid-like. Fig. 1 compares the interfacial storage modulus by LACTEM and by PUS/SMG at the oil– water interface. Comparing the two emulsifiers, the interface with PUS/SMG clearly showed much lower strength (Gi′ at 13–18 mN/m, at different temperatures) than those of LACTEM interface, e.g., Gi′ at 110–150 mN/m at different temperatures (Gi′PUS/SMG b b Gi′LACTEM). The changes induced by temperature cycling were also significantly lower for PUS/SMG than for LACTEM. Results indicated that the LACTEM

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emulsifier was adsorbed at the interface and produced a highly solid-like and structured interface especially at 10 °C (Gi′ at ~150 mN/m). This interfacial layer was sensitive to temperature variation, decreasing the modulus to ~110 mN/m at 20 °C but with good recovery upon cooling to 10 °C (Fig. 1A). The high values for Gi′ were thought to be due to crystallization of the emulsifier at the oil and water interfacial layer (Krog & Larsson, 1992). LACTEM is with saturated monoglycerides so it provides solid-like property to the interface at the experimental temperature. PUS/SMG contained more unsaturated monoglycerides. Together with the fact that lower concentration of PUS/SMG was used than that of LACTEM in this experiment, the higher level of unsaturation in the molecules of PUS/SMG were concluded to have contributed to the lower layer strength. Following the interfacial rheological measurements, a part from the interfacial layer of oil phase and water phase was carefully collected and a coarse emulsion was prepared by manual agitation. LACTEM produced small and separated oil droplets (Fig. 2A1). Fig. 2A2 is the same sample, pictured with polarized light to visualize crystals. Crystals, if any, will strongly reflect light when polarized light pass through its lattice. In this sample, the only source of crystal should be emulsifiers. Therefore one could confirm the presence of crystals in a polarized image, provided the amount exceeds the detection limit. The fact that the polarized image of the coarse emulsion with LACTEM showed strong polarized light implied that the emulsifier induced plenty of random crystals. On the other hand, the coarse emulsion with PUS/SMG showed intermingledlooking droplets in Fig. 2B1. Observations in literature are that unsaturated monoglycerides do not actually crystallize at the interface, per se: rather the emulsifier forms a soft condensed layer (Golding & Sein, 2004). The soft condensed layer was understood to occur because the emulsifier is capable of forming mesophases in an aqueous phase (Stauffer, 1999). These are to be considered “sticky” in nature and interfacial layers will readily fuse together when they come into contact. The PUS/SMG stabilized emulsion showed little signs of emulsifier crystallization associated with the droplet interfaces. There were big crystal-like structures observed in the aqueous phase surrounding the droplets in the polarized picture in Fig. 2B2, possibly as a result of the rearrangement of PUS/SMG crystals during mixing process (Boode & Walstra, 1993a, 1993b; van Boekel, 1980). This contrasts with the observations for LACTEM in Fig. 2A2, which does confirm high amount of crystallization at the interface. These rheological properties and microscopic pictures suggested relevance of the interfacial moduli of the surface layer of emulsion droplets to their aggregation behavior. The contribution of the emulsifiers to the surface layer of emulsion droplets cannot be isolated from that of the protein, including displacement phenomena at the interface (Allen, Murray, & Dickinson, 2008; Dickinson & Tanai, 1992a, 1992b). Nevertheless, the interfacial attributes studied will still be valid for the behaviors of the individual fat globules and their stability in the whipping emulsion. In our preliminary study, SMP in water and pure oil interface showed higher loss modulus than storage modulus, meaning more liquid-like interface. This might be because the layer is formed by the mixture of surface active molecules in SMP including milk phospholipids and caseins of flexible structure, without tangible intermolecular interactions at the layer. The elasticity contribution became bigger with the adsorption of crystalline emulsifiers. With the measurement we applied, we cannot know how much SMP was displaced by LACTEM or by PUS/SMG. However, the interfaces were thought to be still occupied by both SMP and emulsifiers, as a complete displacement of proteins does not likely happen even when emulsifiers are in the same water phase (Mackie & Wilde, 2005). Finally, it will be good to note that the interfacial rheological measurements with small molecular emulsifiers should be done very carefully as the layer is sensitive to various factors, such as the presence and the type of proteins,

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their interactions, adsorption behavior, temperature and mechanical history. 3.2. The properties of whipping emulsion and whipping performance 3.2.1. Properties related to aeration performance In Table 1, the solid fat contents by NMR were similar in both emulsions. The two vegetable-fat emulsions had the same solid fat content (~ 12%) after 1 week of 5 °C storage, meaning that the added emulsifiers in this study had little effect on solid fat content. The solid fat content is among the important factors for whipping emulsion properties (Riaublanc et al., 2005; Walstra, 1987) but the impact of solid fat content on whipping emulsion performance is not taken into further consideration in the remainder of this study. The apparent viscosities of unwhipped emulsions showed clear difference by the emulsifiers used (Table 1). The viscosity of the emulsion with saturated monoglyceride based LACTEM was 142 mPa s and more pourable, whereas the emulsion with partially unsaturated monoglyceride mixtures PUS/SMG had a viscosity of 852 mPa s. The rheological measurements on emulsions indicated that both showed more solid-like (G′) than liquid-like (G″) properties, as can be seen in Table 1. All unwhipped emulsions were shown having more solid-like structure at 5 °C, e.g. G'>G", 5.5 Pa > 4.1 Pa in emulsion with LACTEM, and 33.8 Pa > 13.2 Pa in emulsion with PUS/SMG. The higher values of G′ with the emulsion of PUS/SMG suggested that it formed much stronger network than the one with LACTEM (G′PUS/SMG 33.8 Pa > G′LACTEM 5.5 Pa) during quiescent and chilled storage. After aeration, the overrun (301% vs 230%) and the firmness (196 mN vs 127 mN) of whipped emulsion were significantly higher in the emulsion with LACTEM than in the emulsion with PUS/SMG. This can be interpreted that LACTEM is more effective in generating and holding the aerated structure than emulsion with PUS/SMG, albeit the emulsion was less structured before whipping. It suggested also that whipped emulsion properties cannot be directly inferred from the properties of the unwhipped emulsion, because other factors like the efficiency of holding bubbles may affect the ultimate structure. It is also worth noting that, while both of the emulsions showed higher overrun compared to the general “acceptability” criteria (see Materials and methods section), the emulsion with LACTEM has much lower viscosity during storage than the criterion. The emulsion with PUS/SMG was higher in the storage viscosity, but showed lower firmness than each criterion after whipping. 3.2.2. Aggregation behavior of emulsion droplets sheared without air incorporation It is well accepted that aerated structure is achieved by partial coalescence of fat globules which are adsorbed to the bubble surfaces (Fredrick et al., 2010; Leal-Calderon, Thivilliers, & Schmitt, 2007; Mendez-Velasco & Goff, 2012). We monitored the changes in fat globules and liquid oil droplets after shear without air incorporation.

Table 1 Whipping performances and properties of whipping emulsions with each emulsifier.a

LACTEMb emulsion PUS/SMGc emulsion a b c d e f g

Viscosity (mPa s)d

Oscillationd G′ and G″(Pa)f

Overrung (%)

Firmnessg (mN)

Solid fat contente (%)

142 ± 25

G′: 5.5 ± 1.4 G″: 4.1 ± 0.8 G′: 33.8 ± 24 G″: 13.2 ± 13

301 ± 9

196 ± 44

12 ± 0.7

230 ± 4

127 ± 20

12 ± 0.5

852 ± 458

Each value represents the mean ± standard deviation of more than duplicate analyses. Lactic acid ester of mono and diglyceride. Partially unsaturated and saturated monoglyceride. Measured at 5 °C. Measured at 20 °C, immediately after taking sample stored at 5 oC. At 10 Hz. Measured at 15 °C.

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B 200

25 20 15 10 5

Gi' Temp

0

0

100

200

300

25

180 160 140 120 100 80 60 40 20 0

20 15 10 Gi' Temp

5

Temperature (oC)

200 180 160 140 120 100 80 60 40 20 0

Temperature (oC)

Gi' (mN/m)

A

Gi' (mN/m)

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0 0

Time (min)

100

200

Time (min)

Fig. 1. Differences in interfacial shear rheology depending on the interfacial components. A: Heat treated milk powder (5.25 w/w %) in water–LACTEM (1 w/w %) in mineral oil interface. B: Heat treated milk powder (5.25 w/w %) in water–PUS/SMG (0.45 w/w %) in mineral oil interface, both with temperature cycling between 10 °C and 20 °C.

The rationale for this test is that: Aggregation of fat globules could be caused by the role played by fat crystals during shear. The liquid oil droplets will be free from the impact of the presence of crystals and any changes will be due to the emulsifiers located at the oil droplets surface. The particle size distribution and mean particle diameter of each emulsion with solid fat before and after shear on the rheometer showed clear differences as shown in Fig. 3. The fat globules in whipping emulsion containing LACTEM were not much aggregated, showing both mean diameters of ~ 1.1–1.2 μm in D3,2 before and after shear (1. 7 μm and 4.9 μm respectively in D4, 3, Fig. 3A). In contrast, the

distribution and mean particle size of the whipping emulsion with PUS/SMG apparently increased after shear, showing D3,2 from 3.5 μm to 13 μm, (D4,3 from 11 μm to 48 μm in Fig. 3B). The sheared sample with PUS/SMG was ultrasonically treated in order to break up any weakly bound aggregates. A slight decrease in mean droplet size (13 μm to 8.1 μm in D3,2) was observed after the sonication but the size distribution was almost unchanged as shown in Fig. 3B. The irreversibility of aggregation by the sonication of the emulsion with PUS/SMG suggested that this aggregation was due to the surface layer and fat crystals' mediating interaction among fat globules.

A1

A2

B1

B2

Fig. 2. Microscopic observation of coarse oil droplets with each emulsifier. A. LACTEM (1 w/w %) in mineral oil–water droplets at room temperature under microscope (×400): A1, normal light, A2: the picture of A1 with polarized light to visualize crystals. B: PUS/SMG (0.45 w/w %) in mineral oil-water droplets (0.01% ß-lactoglobulin) at room temperature under microscope (×400): B1, normal light, B2, the picture of B1 with polarized light to visualize crystals.

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A 12

B 12 Before shear After shear

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8

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6 4

8 6 4 2

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0.1

1

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Fig. 3. Susceptibility of whipping emulsion droplets to shear depending on the use of emulsifiers shown by light scattering. A. Emulsion with LACTEM. B. Emulsion with PUS/SMG.

Microscopic observation at 5 °C confirmed the morphology of the fat globules and the aggregates in the emulsions: although both emulsions had the same content of solid fat (12%) and similar crystal morphology to each other, the fat globules with PUS/SMG showed aggregation after shear, while those with LACTEM kept individual

A1

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globules without aggregation (Fig. 4). These results by shear suggested that the surface layer differences induced by emulsifiers would affect the behavior of fat globule aggregation. The same set of experiments with liquid oil emulsions was done in order to see how the surface layer properties of emulsion droplets

A2

B2

Fig. 4. Susceptibility of whipping emulsion droplets to shear depending on the use of emulsifiers: microscopic observation. A. Emulsion with LACTEM (×400): A1 before shear, A2 after shear. B. Emulsion with PUS/SMG (×1000): B1 before shear, B2 after shear.

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respond to shear when there is no involvement of fat crystals (Figs. 5 & 6). In both liquid oil emulsions with LACTEM or PUS/SMG, there were no aggregates formed after shear (Fig. 5A2 and B2). The mean droplet sizes (D3,2) of LACTEM emulsion and PUS/SMG emulsion before shear were 1.0 μm and 0.8 μm, respectively (Fig. 6). After shear, both of them decreased to 0.6 μm. This indicated that in both cases, neither aggregates nor destabilization of emulsion happened upon shearing with the absence of fat crystals, although there should be clear differences in the surface layer properties of emulsion droplets indicated by interfacial rheology in Fig. 1. The smaller mean droplet sizes after shear and the similar size distribution profile to that before shear, only with differences in volume intensity (Fig. 6), suggested that shear introduced breaking up of flocculated droplets or weakly bound droplets, e.g., by proteins attached to the oil droplets. It confirmed that droplet aggregation did not happen. In the case of liquid oil emulsion with PUS/SMG, some of the bigger oil droplets (Fig. 5B2) were observed after shear, implying that this emulsion was more sensitive to coalescence by droplet collision than the liquid oil emulsion with LACTEM emulsifier. These studies with solid fat and liquid oil showed that the fat crystals in whipping emulsion were essential contributors to fat aggregation by shear. When other factors such as process, ingredients, solid fat contents or fat crystal morphology were kept similar, the emulsifiers and the resulting interfacial properties of fat globules determined the solid fat aggregation.

3.3. A relationship between the interfacial properties and the whipping emulsion properties Overall comparison on the results is summarized in Table 2.

3.3.1. Emulsion with LACTEM The properties of whipping emulsion with LACTEM showed high overrun and firmness (Table 1), low viscosity during storage (Table 1) and high shear stability when no air was introduced (Figs. 3A and 4A). LACTEM formed strong solid-like interfacial layer between model liquid oil and water phases (Fig. 1A). The surface layer of fat globules with LACTEM was thought to protect the integrity of individual fat globules and to prevent partial coalescence from taking place during lateral shearing (Figs. 3 and 4). It has been proposed that at sufficiently high concentrations of emulsifier, LACTEM would build multilayers (Westerbeek, 1989). This hydrated multilayer structure is considered to possess short range hydration forces which counterbalance van der Waals interactions between droplets. Consequently, the stability of the emulsion under quiescent conditions is greater than that of PUS/SMG emulsion, and the LACTEM-stabilized emulsions were less prone to thickening. The layered crystalline interface of high modulus was thought to work collaboratively with fat crystals and air during whipping process, providing high overrun and firmness. LACTEM has been reported to be α-tending without further polymorphic transitions of fat crystals, having a positive effect on whipping properties by stabilizing the alpha phase of triglycerides (Whitehurt, 2004). However, this whipping emulsion showed mostly β-crystals in our study, so the aerated structure was not thought specifically by α-crystals. The natural temperature increase during whipping to ~15 °C was thought to influence the melting of LACTEM so as to play a favorable role for whipped structure, as it showed high sensitivity on temperature in interfacial shear rheology. Allen, Murray and Dickinson (2008) demonstrated that LACTEM has a high aerating capacity as well as providing firm foam structures with emulsions of entirely solid fat droplets. The solid droplets became aggregated after aeration, whereas, liquid oil droplets stabilized by LACTEM did not show whipping properties. In our study, liquid

A1

A2

B1

B2

Fig. 5. Susceptibility of liquid oil droplets on shear depending on the use of emulsifiers: microscopic observation. A. Emulsion with LACTEM: A1 before shear, A2 after shear. B. Emulsion with PUS/SMG: B1 before shear, B2 after shear.

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Fig. 6. Susceptibility of liquid oil droplets to shear depending on the use of emulsifiers shown by light scattering. A. Emulsion with LACTEM. B. Emulsion with PUS/SMG.

oil droplets with LACTEM were intact by shear. All confirmed in a consistent way that the interaction of the interfacial layer and fat crystals was a governing factor to the development of the firm whipped structure. Related to this highly solid-like interfacial properties given by the emulsifiers, one might be curious whether the completely saturated monoglyceride at the same concentration as LACTEM would give similar whipping emulsion properties by its crystallization behavior at the oil and water interface. Saturated monoglyceride, in general has a melting point ~55–60 °C with similar Iodine Value with LACTEM shows strongly solid-like interface in our interfacial rheology measurements (data not shown). A collective answer from our work and the studies published about whipping emulsion mechanisms is “no”. From interfacial layer point, if it is rigid and stable, regardless of the emulsifier source e.g., saturated monoglyceride (Pelan et al., 1997) or rich with whey protein which builds highly structured solid-like interface (Zhang & Goff, 2005), whipping performance would not be as good as with less structured layers, while these emulsions will be mostly stable during storage. In the case with LACTEM, it is due to the specific multilayer building properties of this material at the oil and water interface that there is more interaction between the air and fat crystals. Overall, for a good quiescent status stability together with good whipping performance, the surface layer of the fat globules should be inert to the stress raised during storage, but should allow the interaction with air and fat crystals.

3.3.2. Emulsion with PUS/SMG The emulsion with PUS/SMG was high in viscosity (Table 1) under quiescent state at 5 °C storage. This higher emulsion viscosity appeared to have originated from a relatively weak and soft gel-like interface (Figs. 1B and 2B). The interfacial layer by these emulsifiers during storage was observed sufficient to cause fat globule interaction with fat crystals at/through the surface layer and to form network with close proximity. It is not surprising then that the fat globules were more prone to aggregation by shear (Figs. 3B and 4B droplet aggregation after shear). The structures of whipping emulsion and whipped emulsion with this emulsifier mixture were formed through the ease of partial coalescence and the resulting rearrangement of crystals among droplets and at the bubble surfaces. These also suggested that whipping emulsion with PUS/SMG will be more influenced by fat type, fat crystallization, its orientation or growth than emulsion with LACTEM. The properties of PUS/SMG emulsion will be also more affected by environmental stresses than those of LACTEM emulsion. Our results on rheological measurements of the emulsion with PUS/SMG in Table 1 and in the globule sizes in Fig. 3 showed large standard deviations. In the tables, the values were the averages after characterizing each emulsion several times in separate productions. After each production, emulsions could experience slightly different stresses during transportation and/or by temperature fluctuation. By the nature of the interfacial layer, this is thought to induce

Table 2 Comparisonal overview on the characteristics of emulsifiers at o/w interface and the behaviors of each emulsion. Liquid oil and water interface

Emulsion stabilized by milk powders and emulsifiers in the left hand column Crystallizing triglyceride fat

Liquid sunflower oil

LACTEM (IV ~ 2)

• High interfacial shear modulus • Crystalline • Multilayereda

Upon shearing • No droplet aggregation

• Emulsifier's own properties and the ability of interacting with air and solid fats during aeration

PUS/SMG (IV ~ 60)

• Low interfacial shear modulus • Non-crystalline • Soft-solid condensed layerb

• Low bulk viscosity Upon shearing • No droplet aggregation Upon aeration • High overrun • High firmness of whipped emulsion • High bulk viscosity Upon shearing • Aggregation/partial coalescence of droplets (hardly reversible) Upon aeration • Low overrun • Low firmness of whipped emulsion

Upon shearing • No droplet aggregation

• Interactions of fat crystals with any in close proximity through the surface layer of fat globules

a b

From Westerbeek, 1989. From Golding & Sein, 2004.

Dominant factor on structuring

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noticeable variations in product quality, forming different degrees of network or fat aggregation. Some additional experiments confirmed the hypothesis with PUS/SMG type emulsifier mixture. When more unsaturated monoglyceride was used, it showed more liquid-like interfacial properties in the interfacial rheology in our preliminary studies. The emulsion under cold storage became even more viscous to show the acceptable firmness in whipped emulsion. And it showed larger variations in thickness batch by batch. When the emulsifier was changed to a fully liquid system which displayed no mesophase behaviors, such as polyglycerol polyricinoleate, the interface was completely liquid-like but intermingled appearance under microscope was not observed. With this emulsifier, the whipping emulsion during storage showed lower viscosity than those with mesophase forming emulsifiers. The overrun was acceptable but the firmness was too low to be a product in the market. 4. Conclusion This study led us to deepen the knowledge on the whipping cream structure by stressing the contribution of the surface layer properties of fat globules introduced by emulsifiers. The interfacial shear rheology characterizes the model oil and water interface with PUS/SMG as being much weaker in elastic modulus than that with LACTEM. This supports that PUS/SMG-based emulsion follows the classic model of whipping emulsion structuring: partial coalescence by crystalline fat and its interaction through weak surface layer of fat globules, happened already during quiescent storage. The emulsion with LACTEM shows that the firm aerated structure is formed during the whipping process. The data obtained in this study are valuable for understanding the influence of emulsifier, its contribution to surface layer properties of fat globules and whipped structure relationship. This will help in formulating better whipping emulsions with lower amount of saturated fat but with a higher quality. References Alderliesten, M. (1991). Mean particle diameters. Part II: Standardisation of nomenclature. Particle and Particle Systems Characterization, 8, 237–241. Allen, K. E., Dickinson, E., & Murray, B. (2006). Acidified sodium caseinate emulsion foams containing liquid fat: A comparison with whipped cream. LWT- Food Science and Technology, 39, 225–234. Allen, K. E., Dickinson, E., & Murray, B. S. (2008a). Development of a model whipped cream: Effects of emulsion droplet liquid/solid character and added hydrocolloid. Food Hydrocolloids, 22, 690–699. Allen, K. E., Murray, B. S., & Dickinson, E. (2008b). Whipped cream-like textured systems based on acidified caseinate-stabilized oil-in-water emulsions. International Dairy Journal, 18, 1011–1021. Ananthapadmanabhan, K. P. (1993). Protein–surfactant interactions. In E. D. Goddard, & K. P. Ananthapadmanabhan (Eds.), Interactions of surfactants with polymers and proteins. Boca Raton: CRC Press. Barfod, N. M., Krog, N., Larsen, G., & Buchheim, W. (1991). Effects of emulsifiers on protein–fat interaction in ice cream mix during ageing. I. Quantitative analysis. Fat Science Technology, 93, 24–29. Bazmi, A., Duquenoy, A., & Relkin, P. (2007). Aeration of low fat dairy emulsions: Effects of saturated-unsaturated triglycerides. International Dairy Journal, 17, 1021–1027. Berger, K. G. (1988). The use of palm and palm kernel oil in ice cream and whipped cream products. Palm oil development conference, Kuala Lumpur, Malaysia, October, 11–15. Berger, K. G. (1990). Ice cream. In K. Larsson, & S. Friberg (Eds.), Food emulsions (pp. 367–435). New York: Marcel Dekker. Besner, H., & Kessler, H. G. (1998). Model for foam stabilization of homogenized cream by comparative examinations with non-homogenized cream. Milchwissenschaft-Milk Science International, 53, 609–612. Boode, K., & Walstra, P. (1993a). Kinetics of partial coalescence in oil in-water emulsions. In E. Dickinson, & P. Walstra (Eds.), Food colloids and polymers: Stability and mechanical properties (pp. 23–30). Cambridge, UK: Royal Society of Chemistry. Boode, K., & Walstra, P. (1993b). Partial coalescence in oil-in-water emulsions. 1. Nature of the aggregation. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 81, 121–137. Bos, M. A., & van Vliet, T. (2001). Interfacial rheological properties of adsorbed protein layers and surfactants: A review. Advances in Colloid and Interface Science, 91, 437–471. Brooker, B. E. (1993). The stabilization of air in foods containing air—A review. Food Structure, 12, 115–122.

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