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Orthokinetic destabilization of emulsions by saturated and unsaturated monoglycerides
International Dairy Journal 11 (2001) 827–836
Orthokinetic destabilization of emulsions by saturated and unsaturated monoglycerides Emma Daviesa, Eric Dickinsona,*, Rodney D. Beeb a
Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK b Unilever Research, Colworth House, Sharnbrook, Bedford MK44 1LQ, UK Received 14 March 2001; accepted 30 May 2001
Abstract The influence of pure and mixed monoglycerides on the orthokinetic stability of fine sodium caseinate-stabilized oil-in-water emulsions (1 g 100 g–1 protein, 40 mL 100 mL–1 groundnut oil, average droplet size o0.5 mm), with tristearin (TS) crystals (12.5 g 100 g–1) dispersed in the oil phase, has been investigated under strictly controlled shearing conditions. The presence of pure glycerol monostearate (GMS) was found to produce a highly viscous flocculated state which was resistant to shear-induced partial coalescence. Emulsions containing pure glycerol monopalmitate (GMP) were more shear-sensitive, although the destabilization was not so extensive as to generate the large well-defined viscosity increases found previously for emulsions containing pure glycerol monooleate (GMO). Different levels of (in)stability for the three pure monoglycerides can be attributed to differences in the competitive protein displacement behaviour and in the fat crystal type. A GMO-rich commercial emulsifier was found to produce emulsions having both quiescent stability and shear sensitivity, but the emulsifier concentration range giving this behaviour was extremely narrow. Certain binary mixtures of GMO+GMS or GMO+GMP could also generate emulsions that were orthokinetically unstable but perikinetically stable. Overall, these results indicate that a small change in the amount of added saturated monoglyceride can be used to provide sensitive control over the orthokinetic stability of milk protein-based emulsions. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Monoglycerides; Fat crystallization; Protein displacement; Shear sensitivity; Sodium caseinate; Orthokinetic stability
1. Introduction A complex food colloid such as a whipped topping or an ice-cream contains a mixture of two kinds of surfaceactive agentsFmilk proteins and small-molecule surfactants (emulsifiers). A key step during manufacture is the destabilization of the milk protein-stabilized oil-in-water emulsion during the whipping of the topping or the freezing/aeration of the ice-cream mix. This orthokinetic destabilization produces a network of partially coalesced droplets which contributes to the stabilization of the air cells during the aeration process and also in the final product (Barfod, Krog, Larsen, & Buchheim, 1991; Gelin, Poyen, Courthaudon, Le Meste, & Lorient, 1994; Goff, 1997; Pelan, Watts, Campbell, & Lips, 1997). The rate of clumping of fat globules during shearing is *Corresponding author. Tel.: +44-1132-332956; fax: +44-1132332982. E-mail address: [email protected] (E. Dickinson).
dependent inter alia on the solid fat content of the oil phase, the properties of the fat crystal network within the partially crystallized oil phase, and the type and amount of emulsifier(s) present in the formulation (Walstra, 1987; Boode & Walstra, 1993; Goff & Jordan, 1989). Typically, the active surfactant species is a commercial monoglyceride emulsifier consisting of a mixture of saturated and unsaturated monoglycerides. The presence of monoglycerides in dairy-type emulsions can affect both the solid fat content of the droplets and the amount of protein adsorbed at the oil–water interface. Although the emulsification of triglyceride oils has an inhibiting effect upon fat crystallization, the addition of emulsifiers can enhance crystallization in the droplets, resulting in higher effective solid fat content. Monoglyceride crystallization can also lead to proteins being displaced from the droplet surface (Krog, 1997). Competitive adsorption of milk proteins by emulsifiers reduces the adsorbed layer surface coverage and the protein’s effectiveness as a steric stabilizer, and this
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makes the protective stabilizing layer more susceptible to disruption during shear processing (Dickinson, 1997, 1999; Euston, 1997). The protein surface coverage in an ice-cream mix can actually be used as a predictive measure of fat destabilization during freezing (Bolliger, Goff, & Tharp, 2000). We described recently (Davies, Dickinson, & Bee, 2000) the development of a model emulsion system with sodium caseinate as the protein stabilizing agent and groundnut oil containing tristearin crystals as the dispersed phase. In the presence of the appropriate amount of the added emulsifier, pure glycerol monooleate (GMO), this type of emulsion system can be formulated so as to possess quiescent stability over long periods of time, whilst being susceptible to orthokinetic destabilization when the shear stress exceeds a certain value. The critical shear stress for emulsion destabilization, as measured in a controlled stress rheometer, was found to be sensitive to both the triglyceride crystal content and the added GMO concentration. For a concentrated emulsion made with 1 g 100 g–1 protein and 40 mL 100 mL–1 triglyceride oil containing 5–15% tristearin crystals, it was found that small changes in added GMO concentration (in the range 2–3.5 g 100 g–1) produced large changes in the rate of shear-induced destabilization. On the basis of our previous results using GMO as emulsifier (Davies et al., 2000), we suggested that this same rheological approach might provide a useful systematic procedure for comparing different emulsifiers with respect to their ability to control the stability of emulsions towards partial coalescence in shear flow. In order to understand fully the different degrees of effectiveness of emulsifiers in controlling emulsion clumping in real products, we shall need first to compare the destabilizing abilities of different pure monoglycerides and their mixtures under well-defined conditions. Saturated monoglycerides have a greater ability than partially unsaturated ones to act as initiators of fat crystallization (Berger, 1990), and thus to increase the proportion of solid fat in partially crystalline emulsions. Barfod et al. (1991) showed that, whilst the presence of either the saturated glycerol monostearate (GMS) or the unsaturated GMO initiated more crystallization than in control emulsions without added emulsifier, the solid fat content of the GMS-containing emulsions was always greater than that of the equivalent emulsions with GMO. The molecular structure of the emulsifier also influences its competitive adsorption behaviour with milk proteins. For instance, monoglycerides of shortchain fatty acids are powerful destabilizers, whereas those with longer-chain fatty acids are less effective (Kloser & Keeney, 1959); and partially unsaturated monoglycerides are considered better than saturated monoglycerides at displacing protein from the interface (Barfod et al., 1991; Goff & Jordan, 1989).
This paper compares the orthokinetic destabilization of model emulsion systems containing pure GMS (C18 : 0) or pure glycerol monopalmitate (GMP; C16 : 0) with that of the equivalent pure GMO formulations already reported. We also consider the effectiveness of binary monoglyceride mixtures of known composition, GMS+GMO and GMP+GMO, as well as that of a multicomponent commercial monoglyceride emulsifier. As previously, the emulsions were prepared at a relatively low constant protein : oil ratio, giving sufficient protein surface coverage to stabilize the systems quiescently, but allowing good sensitivity to partial coalescence under unfavourable orthokinetic conditions.
2. Materials and methods The pure monoglycerides GMO, GMS and GMP (p3% other monoglycerides, 3–5% non-monoglycerides) were obtained from Danisco (Brabrand, Denmark), as was the commercial emulsifier Dimodan (72% monoolein, 11% monopalmitin, 11% monostearin, o1% C17 or shorter, o1% C20 or higher). Spraydried sodium caseinate (5.2 g 100 g–1 moisture, 0.05 g 100 g–1 calcium) was from DeMelkindustrie (Vegel, Netherlands). The groundnut oil (Safeways supermarket) was passed through a Fluorisil column to remove surface-active impurities. The tristearin (TS) was from Sigma Chemicals (St. Louis, USA). The 20 mm imidazole buffer was adjusted to pH 7.0 with 0.1 m HCl or NaOH. Fine oil-in-water emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil containing 12.5 g 100 g–1 tristearin) were prepared at 601C using a Shields S-500 high-pressure laboratory homogenizer operating in continuous mode at 40 MPa for 15 min (corresponding on average to each volume element passing six times through the homogenizer valve). The homogenizer was pre-heated by circulating water at 901C. Prior to mixing, the oil phase was heated to 801C and the aqueous phase to 601C. Different types and amounts of monoglycerides were added to the oil phase prior to emulsification. Droplet-size distributions were measured using a Malvern Mastersizer MS20 (presentation code 0505). For freshly made (stable) emulsions containing pure GMS or pure GMP, the average droplet diameters were d32 ¼ 0:3570:01 mm and d43 ¼ 0:4570:01 mm. For emulsions containing mixed GMO+GMS or GMO+ GMP, the average diameters were d32 ¼ 0:3570:01 mm and d43 ¼ 0:4870:01 mm. Using the measured specific surface area Asp (m2 per cm3 of oil), the surface coverage of protein at the oil droplet surface was determined following centrifugation (2.1 104 g, 2 h, 21C) by the depletion method as described previously (Davies et al.,
E. Davies et al. / International Dairy Journal 11 (2001) 827–836
(2000). Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer 7 series thermal analysis system with an air-filled cell as reference. The sample was cooled from 1001C to 01C at 101C min–1. Emulsions were subjected to controlled shear stability testing in the double-gap geometry cell (DG 40/50) of a Bohlin CVO Rheometer. After transferring to the preheated rheometer cell (851C), a freshly made emulsion sample was cooled to 01C at 11C min–1 using the rheometer water bath and temperature program. The emulsion was then left undisturbed for 1 h at 51C. Increasing stresses of duration 10 min were then applied to the sample over the range from 0.073 to 15 Pa (30 values on a logarithmic scale). The apparent viscosity was recorded following each stress application.
3. Results 3.1. Crystallization in systems containing pure monoglycerides The bulk groundnut oil used as the main component of the emulsion dispersed phase remains liquid over the experimental range of this study. Any crystallization occurring in the oil phase is due to the tristearin (TS), whose apparent crystallization temperature in the groundnut oil differs from its bulk crystallization temperature (471C) according to its concentration and the concentration and type of the added emulsifier. To assess the temperature to which samples needed to be cooled in the rheometer in order to ensure oil crystallization, the DSC peak temperatures were determined under conditions of strictly controlled cooling rate. The oil crystallization temperatures determined from DSC peak temperatures are recorded in Table 1 for the addition of 1.25 g 100 g–1 pure monoglyceride to groundnut oil systems containing 12.5 g 100 g–1 TS. We can see that, for the bulk oil phase, the type of added monoglyceride (GMO, GMS or GMP) has little effect on the crystallization temperature (B301C). Upon emulsification, however, there is a substantial difference in crystallization temperature for the systems containing
Table 1 Normalized crystallization (nucleation) temperatures of bulk and emulsified oil systems containing 12.5 g 100 g–1 TS and 1.25 g 100 g–1 monoglyceride in the groundnut oil as indicated by the DSC peak temperature (cooling rate 10 K min–1) Monoglyceride
GMO GMS GMP
Crystallization temperature (1C) Bulk
Emulsion
29.6 30.6 30.0
5.5 21.5 22.0
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saturated and unsaturated monoglycerides. That is, the degree of dispersed phase supercooling is considerably less for the saturated monoglycerides (B91C for GMS, and B81C for GMP) than for the partly unsaturated GMO (B251C). Saturated monoglycerides are known to be good initiators of fat crystallization (Barfod et al., 1991). Since GMP and GMS crystallize themselves at higher temperatures than GMO, they can be considered more effective fat crystal nucleators for TS crystal growth. Previously, it was suggested (Davies et al., 2000) that the liquid GMO may act as a template for TS crystallization, encouraging crystals to grow towards and through droplet surfaces. Such a template effect should be greatly enhanced by the crystallization of the monoglyceride before that of the triglyceride. Due to the match in saturation and C number (i.e., number of carbon atoms in the chain), it is assumed that GMS is especially readily incorporated into growing TS crystals. The slight mismatch in C number for GMP is rather less favourable, leading to the possibility of crystal defects (Smith, Cebula, & Povey, 1994). In the case of GMO, the presence of the kink in the C chain due to the double bond means that it fits less well into growing TS crystals than does GMS.
3.2. Shear sensitivity of emulsions containing pure GMS Emulsion systems (40 mL 100 mL–1 oil, 1 g 100 g–1 sodium caseinate) with 12.5 g 100 g–1 TS and varying GMS concentrations in the oil phase were subjected to steadily increasing shear forces in the rheometer. In the absence of GMS, the emulsions were of low viscosity (ca. 30 mPa s), they were insensitive to shearing, and they were quiescently stable for at least four weeks, with no discernible change in droplet-size distribution. Addition of GMS was found to lead to an increase in the low-stress apparent viscosity. For systems containing from 2 to 5 g 100 g–1 GMS in the oil phase, Fig. 1 shows the influence of applied shear stress on the apparent viscosity. The application of shear stresses up to 15 Pa to emulsions containing 2–3.5 g 100 g–1 GMS in the oil phase was found to lead to a reduction in apparent viscosity by some five orders of magnitude. This shear-thinning behaviour was found to be reversible: on leaving a sample undisturbed for 1 h after application of the original 15 Pa shear stress, the same steady reduction in apparent viscosity with the applied stress was reproducibly observed (within up to, say, one order of magnitude) (Fig. 2). Even when the TS content of the oil phase was increased to 25 g 100 g–1, there was found to be no shear-induced destabilization of the emulsions at the higher stresses as had been observed previously (Davies et al., 2000) with the equivalent GMO-containing emulsions.
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3.3. Shear sensitivity of emulsions containing pure GMP
Fig. 1. Effect of GMS on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various GMS concentrations (g 100 g–1) in the oil phase: &, 2; E, 2.5; n, 2.75; K, 3; J, 3.5; , 5.
Fig. 2. Reversibility of shear-thinning of GMS-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS+2.75 g 100 g–1 GMS in oil phase). Apparent viscosity is plotted against shear stress: &, no pre-shear; E, pre-shear+1 h quiescent storage.
The thinning of the emulsions under shear, and the subsequent quiescent rethickening, implies that the original high viscosity of GMS-containing emulsions was not due to irreversible partial coalescence, but rather to some kind of flocculation. As there was no change in the visible appearance or rheology of the emulsions upon extended storage, it appears that the high viscosity of the flocculated network structure prevents destabilization of the emulsions by gravitational separation or droplet coalescence. Droplet-size distributions of emulsion systems containing o5 g 100 g–1 GMSFas measured before, during, or after shearing, and following quiescent storage for four weeksFwere both narrow and monomodal. This means that the flocculation was unchanged in strength with time and weak enough to be completely disrupted by dilution and stirring in the Mastersizer water-bath.
Emulsions containing GMP were found to become quiescently unstable at far lower monoglyceride concentrations than those with added GMO or GMS. The presence of 0.75 g 100 g–1 GMP in the oil phase of the standard emulsion formulation (40 mL 100 mL–1 oil, 1 g 100 g–1 sodium caseinate, 12.5 g 100 g–1 TS) gave rise to systems that were both perikinetically and orthokinetically unstable. Increasing the concentration of sodium caseinate did not make the emulsions significantly more stable. Above the crystallization temperature, emulsions prepared with added GMP had a narrow droplet-size distribution and low viscosity. Once the temperature dropped below 221C, however, the viscosity of emulsions containing >0.75 g 100 g–1 GMP in the oil phase increased greatly, and droplet-size distributions became distinctly bimodal, reflecting the quiescent instability of the emulsions. Fig. 3 shows the bimodal distribution PðdÞ of an emulsion sample with 1.25 g 100 g–1 GMP in the oil phase. Increasing the GMP content further led to distributions with greater proportions of large particles. Fig. 3 shows that the main feature of PðdÞ for an emulsion with 2 g 100 g–1 GMP is a peak centred at B5 mm. When sheared in the rheometer, emulsion samples with >0.75 g 100 g–1 GMP in the oil phase led to an apparent shear-thinning as indicated in Fig. 4. Close inspection of the samples revealed, however, that some partial coalescence had occurred, with clumps pushed to the outside of the cell and therefore not contributing to the measured viscosity. Hence, the system was presumably exhibiting ‘slip flow’ on the inner cylinder. This shear-thinning behaviour was in sharp contrast to the dramatic increases in viscosity that were induced in GMO-containing emulsions when applied shear forces exceeded critical values (Davies et al., 2000). The shear-induced clumping of emulsions containing GMP meant that they could not be diluted to form
Fig. 3. Particle-size distribution PðdÞ of GMP-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase): F, 1.25 g 100 g–1 GMP in oil phase; F F F, 2 g 100 g–1 GMP in oil phase.
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Table 2 Apparent protein surface coverage (G) of sodium caseinate emulsions with and without added monoglycerides (2 g 100 g–1) in the oil phase of the emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 oil, 12.5 g 100 g–1 TS in oil phase) and mass fraction of protein adsorbed (Fads ). The specific surface area (Asp ) is based on the d32 value determined following the cooling of the emulsions. Each listed value is the average of duplicate sets of measurements.
Fig. 4. Effect of GMP on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various GMP concentrations (g 100 g–1) in the oil phase: &, 0.75; n, 1.25; E, 1.75; J, 3.
Fig. 5. Low-stress apparent viscosity (0.07 Pa) of GMP-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase) as a function of the GMP content in the oil phase.
stable uniform emulsions of low viscosity as could the equivalent emulsions with added GMS. The clumping also disturbed the homogeneity of the undiluted emulsions, hence preventing the viscosity returning to its original high value following shear-induced thinning. Increasing the GMP content led to an increase in the apparent viscosity of the freshly made emulsion as shown in Fig. 5. 3.4. Protein displacement from emulsion droplet surface Table 2 shows how 2 g 100 g–1 monoglyceride added to the groundnut oil phase substantially reduces the apparent surface coverage of sodium caseinate at the emulsion droplet surface. Even without added monoglyceride, the protein surface coverage in these emulsions is rather low (G ¼ 1:1 mg m–2) due to the deliberately low overall protein : oil ratio used here. The presence of the monoglyceride leads to a very substantial reduction in the fraction of protein adsorbed (Fads ) inferred from the measured protein content in the emulsion serum phase after centrifugation. However,
Monoglyceride
Asp (m2 cm–3)
G (mg m–2)
Fads (%)
F GMO GMS GMP
14.3 15.6 14.0 11.4
1.08 0.45 0.17 0.05
71 32 11 3
the very small values of apparent protein surface coverage for GMP and GMS appear somewhat dubious, since they imply that the protein makes almost a negligible contribution to adsorbed layer composition. The actual values of G and Fads quoted in Table 2 for the monoglyceride-containing systems may, in reality, be underestimated due to the effect of partial coalescence during centrifugation, which may liberate previously unadsorbed protein into the bulk aqueous phase, as pointed out by Segall and Goff (1999). On the other hand, if significant partial coalescence were to have occurred during centrifugation, the high local interdroplet forces generated might also have been expected to destabilize the semi-crystalline emulsions with no added monoglycerideFwhereas the experimentally determined value of Fads (>70%) for the monoglyceridefree reference emulsion is, in fact, sensibly high. Based on the data in Table 2, and placing the issue of partial coalescence during centrifugation aside, we infer that the saturated monoglycerides are better at displacing sodium caseinate from the droplet surface than is the unsaturated GMO, and that GMP is even more effective than GMS. The situation here in these model emulsions seems to be more clear-cut than in experiments reported for real ice-cream emulsions: Pelan et al. (1997) could not find any clear difference in displacing ability for saturated and unsaturated monoglycerides, and Krog (1997) states that it is difficult to conclude whether there are significant differences between saturated and unsaturated monoglycerides in relation to protein desorption during ageing. Conversely, Barfod et al. (1991) reported a higher level of protein desorption in ice-cream emulsions with added GMO than in those containing GMS. Anyway, differences amongst the action of various monoglycerides often appear to depend markedly on temperatureFthis is presumably because small changes in temperature affect the sensitive balance of competing protein–lipid and lipid–lipid interactions, with consequences for the adsorbed layer properties as well as for the chain crystallization behaviour.
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3.5. Behaviour of emulsions containing commercial monoglyceride emulsifier The differences described above in the perikinetic and orthokinetic stability of model emulsions containing the pure GMO, GMP and GMS raises interesting questions regarding the behaviour of commercial emulsifiers composed of mixtures of these three monoglycerides. Against this background, we carried out experiments with one particular commercial emulsifier, Dimodan, containing 72% GMO, 11% GMS and 11% GMP. Emulsions containing Dimodan (1.63–1.85 g 100 g–1 in oil phase) were found to crystallize at B191C, corresponding to supercooling at B31C. The GMP and GMS are assumed to crystallize before the GMO or TS. We infer that the presence of crystalline monoglycerides at the droplet surface encourages nucleation and crystallization of the other components, thereby greatly reducing the high levels of supercooling found with the pure monoglycerides. The range of Dimodan concentrations able to produce quiescently stable emulsions that were unstable under shear was much narrower than previously found with pure GMO (Davies et al., 2000). Systems with p1.63 g 100 g–1 Dimodan in the oil phase were found to be both perikinetically and orthokinetically stable, with narrow monomodal droplet-size distributions maintained during prolonged storage. Systems with between 1.75 and 1.88 g 100 g–1 were quiescently stable and shear-sensitive. Fig. 6 shows that, over this narrow emulsifier concentration range, there is a systematic change in the critical destabilizing stress. Fig. 7 shows that the apparent droplet-size distributions of these emulsions were observed to broaden and shift to larger sizes as the oil phases crystallized on cooling, but then to remain unchanged on quiescent storage. Above 1.88 g 100 g–1 emulsifier, the emulsions were destabilized so
Fig. 6. Effect of Dimodan emulsifier on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various emulsifier concentrations (g 100 g–1) in the oil phase: &, 1.63; E, 1.75; n, 1.83; K, 1.88.
Fig. 7. Particle-size distributions PðdÞ of Dimodan-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS+1.83 g 100 g–1 Dimodan in oil phase): K, pre-cool; J, post-cool+1 h storage; ’, post-cool+7 days storage.
much following cooling that transfer to the rheometer cell was impossible. This observation, that relatively small amounts of saturated monoglycerides in a GMO-rich commercial emulsifier can substantially affect emulsion shear sensitivity, has led us to explore further the properties of well-defined model systems containing the binary monoglyceride mixtures GMO+GMS and GMO+ GMP. 3.6. Behaviour of emulsions containing GMO+GMS or GMP Oil crystallization temperatures determined from DSC peak temperatures are recorded in Table 3 for emulsions containing binary mixtures of monoglycerides. The bulk oil phases composed of 12.5 g 100 g–1 TS, 1.75 g 100 g–1 GMO and 0.5 g 100 g–1 GMS or 0.5 g 100 g–1 GMP in groundnut oil were found to crystallize at 29.370.11C and 29.570.11C, respectively. As indicated in Table 3, the emulsification of these oil mixtures leads to substantial supercooling: B121C for emulsions with added GMP, and B161C for emulsions with added GMS. By way of comparison, we note from Table 1 that the degree of supercooling for the equivalent emulsion containing 1.25 g 100 g–1 pure GMO in groundnut oil is B251C. Hence, as with the Dimodan emulsions, the presence of crystallizing saturated monoglyceride at the droplet surface, as well as GMO in the oil phase, enables triglyceride nucleation and crystallization within emulsion droplets to occur at a temperature higher than in droplets containing GMO alone. The stability characteristics of emulsions containing various concentrations of GMO (1.0–2.0 g 100 g–1) and GMP or GMS (0.25–1.0 g 100 g–1) are summarized in Tables 4 and 5, respectively. The ‘ideal’ emulsion for our purposes is one that is quiescently stable and shearsensitive (U+QS). However, most of the shear-sensitive emulsions containing the mixed monoglycerides are also
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E. Davies et al. / International Dairy Journal 11 (2001) 827–836 Table 3 Crystallization temperatures of emulsions (1 g 100 g–1 sodium caseinate, 40 vol% groundnut oil, 12.5 g 100 g–1 TS) containing GMO (1.25–1.75 g 100 g–1)+GMS or GMP (0.5 g 100 g–1) in the oil phase GMO content (g 100 g–1)
1.25 1.50 1.75 a
Crystallization temperature (1C)a +GMS
+GMP
18.0 18.0 17.0
13.3 13.8 13.5
Experimental error 70.11C.
quiescently unstable (U+QU). Whereas shear-sensitive emulsions with GMO present as the sole emulsifier are very stable towards extended storage, the addition of a small quantity of GMP or GMS would seem to remove this desirable characteristic. However, this negative effect is less pronounced for systems with higher GMO concentrations. Fig. 8 shows the level of GMS or GMP required to create shear-sensitive emulsions as a function of the GMO content. The combination of GMO+GMS is more effective than GMO+GMP in producing shearsensitivity over a wider range of GMO concentrations. With increasing GMO content, the amount of extra monoglyceride required to induce shear-sensitivity decreases. Addition of critical amounts of GMP or GMS to emulsions containing GMO has a dramatic influence on the orthokinetic emulsion stability (Fig. 9).
Fig. 8. Influence of GMO concentration in the oil phase on the concentration of GMS or GMP required to produce shear-sensitive emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase): ’, GMS; &, GMP.
We recall that a critical content of 2.13 g 100 g–1 GMO was required to create shear-sensitive emulsions with pure GMO as emulsifier (Davies et al. (2000)). With emulsifier mixtures containing GMO+saturated monoglyceride, however, a lower total monoglyceride concentration is effective. So, for example, a system containing 1 g 100 g–1 GMO was found to become shearsensitive on addition of either 0.75 g 100 g–1 GMS or 1.0 g 100 g–1 GMP. The results in Fig. 9 show that the
Table 4 Stability of emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS) containing different concentrations of both GMO and GMP in the oil phase. Key: QS=quiescently stable; QU=quiescently unstable; S=shear stable; U=shear unstable; X=too unstable for transfer to rheometer cell GMP content (g 100 g–1)
0.25 0.5 0.75 1.0
GMO content (g 100 g–1) 1.0
1.25
1.5
1.75
2.0
S+QS S+QS S+QS U+QU
S+QS S+QS U+QU X
S+QS S+QS U+QS X
S+QS S+QS U+QU X
U+QS U+QS U+QS X
Table 5 Stability of emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS) containing different concentrations of both GMO and GMS in the oil phase. Key: QS=quiescently stable; QU=quiescently unstable; S=shear stable; U=shear unstable; X=too unstable for transfer to rheometer cell GMS content (g 100 g–1)
0.25 0.5 0.75 1.0
GMO content (g 100 g–1) 1.0
1.25
1.5
1.75
2.0
S+QS S+QS U+QS X
S+QS S+QS U+QU X
S+QS U+QS U+QU X
S+QS U+QS U+QU X
U+QS U+QS X X
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Fig. 9. Effect of individual monoglyceride concentrations on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase) containing GMO+GMP or GMO+GMS. Apparent viscosity is plotted against shear stress for various GMO concentrations (1, 1.25, 1.5, 1.75 and 2 g 100 g–1) in the oil phase: K, 0.25 g 100 g–1 GMS; ’, 0.5 g 100 g–1 GMS; m, 0.75 g 100 g–1 GMS; J, 0.25 g 100 g–1 GMP; &, 0.5 g 100 g–1 GMP; n, 0.75 g 100 g–1 GMP; B, 1 g 100 g–1 GMP.
critical destabilizing stress is a well-defined quantity which is reproducibly and consistently dependent on the contents of both the GMO and the GMP (or GMS).
4. Discussion Sodium caseinate-stabilized oil-in-water emulsions containing tristearin crystals and pure GMS or pure GMP in the groundnut oil phase have been found to be quiescently unstable with respect to flocculation, with a large viscosity increase occurring as the fat phase of the emulsion droplets crystallizes. The GMS-containing emulsions were orthokinetically stable, whereas emulsions with added GMP were more shear-sensitive, with clumps appearing as partial coalescence took place. The level of shear-induced destabilization in these emulsions was not sufficiently extensive, however, to produce the same sort of sharp shear-induced viscosity increase, as
was found with emulsions containing pure GMO. This behaviour is broadly consistent with the generally held view that unsaturated monoglycerides are better than saturated monoglycerides in inducing shear-sensitivity in emulsions (Goff & Jordan, 1989). Possibly the higher viscosity of emulsions containing GMS or GMP protects them from shear-induced destabilization by reducing the number of droplet collisions during shearing and/or the magnitude of local critical stresses forcing the droplets together. Alternatively, the explanation may be associated with the differences in properties of adsorbed layers (Lucassen-Reynders, 1993), with the more close-packed layers of the saturated monoglycerides at the oil–water interface leading to more highly viscoelastic adsorbed layers which may provide greater protection against shear destabilization. Differences in quiescent stability of the emulsions containing the different pure monoglycerides can be explained, partly at least, in terms of the protein displacement behaviour. Addition of GMP gave rise to the highest level of protein displacement and also the most quiescently unstable emulsions. The GMS-containing systems showed better storage stability, with no irreversible clumping taking place. Addition of GMO led to the lowest level of protein displacement and to quiescently stable emulsions. Using confocal microscopy, Heertje (1993) has reported that GMS is more effective than GMO at displacing protein from an oil–water interface at monoglyceride concentrations above 0.25 g 100 g–1, and this was attributed to the closer molecular packing of the saturated chains at the interface. On the other hand, at low monoglyceride concentrations (o0.25 g 100 g–1), the GMO was found to be a more effective displacer (Heertje, 1993), and this was ascribed to the unsaturated GMO having a higher free energy of adsorption than GMS. The double bond in GMO means that fewer molecular configurations are possible than in GMS, and so it is argued that the reduction in entropy on adsorption is not as great for the unsaturated monoglyceride as for the saturated one. However, as the monoglyceride concentration is increased, leading to higher levels of adsorption, the greater lowering of the surface free energy due to the close packing of saturated fatty acid chains becomes a more important factor than the configurational entropic considerations. In the study of Barfod et al. (1991), the monoglyceride concentrations were as low as 0.2 g 100 g–1, i.e. below the threshold described by Heertje (1993), so helping to explain why, in their experiments, the unsaturated GMO was a more effective displacer than GMS. Emulsion stability is also affected by the type of fat crystals formed within droplets (Walstra, 1987). When there is a high defect density on the crystal surface, more
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crystals may grow from these defect points, leading to spherulites (Smith, 1995). Under the normal light microscope, we have observed that mixtures of GMO+TS give rise to a low density of spiky spherulites that may contribute significantly to partial coalescence. The low density of spherulites could reflect a retardation of nucleation caused by the presence of the unsaturated GMO which is unable to fit perfectly into the growing saturated TS crystals. This explanation is consistent with the observed reduction in crystallization temperature with increasing GMO concentration. Mixtures of GMP+TS have been observed to produce fairly a large and rounded spherulites, with fewer crystals appearing to have the ability to rupture droplet surfaces. Mixtures of GMS+TS have been observed to crystallize into rings of small crystals; the low level of crystallization and small crystal size helps to explain the lack of shearsensitivity in the corresponding emulsions. We may speculate that the low number density and the spikiness of crystals formed in mixtures of GMO+TS allow the corresponding emulsion systems to be both quiescently stable and shear-sensitive. While the edges of spherulites are not forced close against the protein-coated droplet surface simply by Brownian collisions, the application of shear forces does allow the crystals to pierce the oil–water interface, causing partial coalescence (Boode & Walstra, 1993). The presence of the larger, more rounded spherulites found with GMS+TS or GMP+TS could mean that some crystals are close to the droplet surface, leading to perikinetic protein-mediated aggregation, but that they are not sufficiently needle-like to pierce the droplet surface on shearing and allow partial coalescence to take place. Results for these model systems containing mixtures of saturated and unsaturated monoglycerides can therefore help to explain why the emulsions made with the GMO-rich commercial emulsifier Dimodan have substantially different stability properties from those made with pure GMO. Orthokinetic stability is reduced by the presence of a small amount of GMS or GMP in addition to GMO. Whilst few binary monoglyceride-containing emulsions studied could be characterized as being both shear-sensitive and quiescently stable (as was ideally sought), there is apparently better quiescent stability with GMO+GMS than with GMO+GMP. At each chosen GMO content, increasing the amount of added GMP or GMS above a certain critical level leads to an increase in shear-sensitivity.
with GMO. The presence of pure GMS leads to extensive flocculation which subsequently seems to ‘protect’ the emulsion systems against the destabilizing effects of shear forces. Emulsions containing pure GMP are more shear-sensitive, although the destabilization is not extensive enough to give the substantial viscosity increases that we previously observed with emulsions containing pure GMO. The different levels of (in)stability for the three pure monoglycerides can be attributed to a combination of two factors: the extent of competitive protein displacement and the fat crystal type. It would appear that the best combination for quiescent stability and shear sensitivity is a relatively low level of protein displacement and the creation of spiky spherulites made up of crystals that can penetrate the droplet surface. Use of the commercial emulsifier Dimodan can generate emulsions having both quiescent stability and shear sensitivity, but the emulsifier concentration range giving these favourable properties is extremely narrow, even though the emulsifier contains >70% GMO. The use of a simple binary mixture of monoglycerides, GMO+GMS or GMO+GMP, has been shown to provide better stability control. The combination of GMO+GMS gives a range of emulsions that are both quiescently stable and shear-sensitive, particularly at the higher GMO concentrations. Shear sensitivity can be induced at a lower total monoglyceride concentration than when GMO is present alone. The critical destabilizing stress can be manipulated by making very small adjustments in the concentration of the saturated monoglyceride, giving a powerful degree of control over the orthokinetic stability of the emulsions. Finally, a cautionary remark seems appropriate. The trends described here relate to stability behaviour of carefully chosen model emulsions that are considerably simpler in microstructure and composition than those typically encountered in food products such as ice-cream and whipped toppings. While we have shown that, by changing the concentrations and combinations of monoglycerides in our model systems, we can sensitively control emulsion shear stability, it is possible that with a different set of variables (droplet size, type of fat phase, etc.) there could be substantially different results. Further experiments will therefore be required to establish whether our recipe for making emulsions that are both shear-sensitive and quiescently stable has broad applicability in the formulation of commercial dairybased whipping emulsions.
5. Conclusions
Acknowledgements
Comparisons have been made between stability properties of semi-crystalline model emulsion systems with added GMS, GMP, and binary mixtures of these
We acknowledge the award of a BBSRC CASE Studentship in collaboration with Unilever, Colworth House.
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References Barfod, N. M., Krog, N., Larsen, G., & Buchheim, W. (1991). Effects of emulsifiers on protein/fat interaction in ice-cream mix during ageing. 1. Quantitative analyses. Fat Science Technology, 93, 24–29. Berger, K. G. (1990). Ice cream. In K. Larsson, & S. Friberg (Eds.), Food emulsions (pp. 367–435). New York: Marcel Dekker. Bolliger, S., Goff, H. D., & Tharp, B. W. (2000). Correlation between colloidal properties of ice cream mix and ice cream. International Dairy Journal, 10, 303–309. Boode, K., & Walstra, P. (1993). Kinetics of partial coalescence in oilin-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. Davies, E., Dickinson, E., & Bee, R. (2000). Shear stability of sodium caseinate emulsions containing monoglyceride and triglyceride crystals. Food Hydrocolloids, 14, 145–153. Dickinson, E. (1997). Properties of emulsions stabilized with milk proteins. Journal of Dairy Science, 80, 2607–2619. Dickinson, E. (1999). Adsorbed protein layers at fluid interfaces: interactions, structure and surface rheology. Colloids and Surfaces B, 15, 161–176. Euston, S. R. (1997). Emulsifiers in dairy products and dairy substitutes. In L. Hasenhuettl, & R. W. Hartel (Eds.), Food emulsifiers and their applications (pp. 173–210). New York: Chapman & Hall. Gelin, J.-L., Poyen, L., Courthaudon, J.-L., Le Meste, M., & Lorient, D. (1994). Structural changes in oil-in-water emulsions during the manufacture of ice-cream. Food Hydrocolloids, 8, 299–308.
Goff, H. D. (1997). Instability and partial coalescence in whippable dairy emulsions. Journal of Dairy Science, 80, 2620–2630. 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, 18–29. Heertje, I. (1993). Microstructural studies in fat research. Food Structure, 12, 77–94. Kloser, J. J., & Keeney, P. G. (1959). A study of some of the variables that affect fat stability and dryness in ice cream. Ice Cream Reviews, 42, 36–38. Krog, N. (1997). Use of emulsifiers in ice-cream. In W. Buchheim (Ed.), Ice cream, Proceedings of the International Symposium, Athens, Greece, 18–19 September (pp. 37–44). Northfield, IL: International Dairy Federation. Lucassen-Reynders, E. H. (1993). Interfacial viscoelasticity in emulsions and foams. Food Structure, 12, 1–12. 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, 2631–2638. Segall, K. I., & Goff, H. D. (1999). Determination of protein surface concentration for emulsions containing a partially crystalline dispersed phase. Food Hydrocolloids, 13, 291–297. Smith, P. (1995). The molecular basis for crystal habit modification in triglycerides. Ph.D. thesis, University of Leeds. Smith, P. R., Cebula, D. J., & Povey, M. J. W. (1994). The effect of lauric-based molecules on trilaurin crystallization. Journal of American Oil Chemists Society, 71, 1367–1372. Walstra, P. (1987). Physical principles of emulsion science. In J. M. V. Blanshard, & P. Lillford (Eds.), Food structure and behaviour (pp. 87–106). New York: Academic Press.
Orthokinetic destabilization of emulsions by saturated and unsaturated monoglycerides Emma Daviesa, Eric Dickinsona,*, Rodney D. Beeb a
Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, UK b Unilever Research, Colworth House, Sharnbrook, Bedford MK44 1LQ, UK Received 14 March 2001; accepted 30 May 2001
Abstract The influence of pure and mixed monoglycerides on the orthokinetic stability of fine sodium caseinate-stabilized oil-in-water emulsions (1 g 100 g–1 protein, 40 mL 100 mL–1 groundnut oil, average droplet size o0.5 mm), with tristearin (TS) crystals (12.5 g 100 g–1) dispersed in the oil phase, has been investigated under strictly controlled shearing conditions. The presence of pure glycerol monostearate (GMS) was found to produce a highly viscous flocculated state which was resistant to shear-induced partial coalescence. Emulsions containing pure glycerol monopalmitate (GMP) were more shear-sensitive, although the destabilization was not so extensive as to generate the large well-defined viscosity increases found previously for emulsions containing pure glycerol monooleate (GMO). Different levels of (in)stability for the three pure monoglycerides can be attributed to differences in the competitive protein displacement behaviour and in the fat crystal type. A GMO-rich commercial emulsifier was found to produce emulsions having both quiescent stability and shear sensitivity, but the emulsifier concentration range giving this behaviour was extremely narrow. Certain binary mixtures of GMO+GMS or GMO+GMP could also generate emulsions that were orthokinetically unstable but perikinetically stable. Overall, these results indicate that a small change in the amount of added saturated monoglyceride can be used to provide sensitive control over the orthokinetic stability of milk protein-based emulsions. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Monoglycerides; Fat crystallization; Protein displacement; Shear sensitivity; Sodium caseinate; Orthokinetic stability
1. Introduction A complex food colloid such as a whipped topping or an ice-cream contains a mixture of two kinds of surfaceactive agentsFmilk proteins and small-molecule surfactants (emulsifiers). A key step during manufacture is the destabilization of the milk protein-stabilized oil-in-water emulsion during the whipping of the topping or the freezing/aeration of the ice-cream mix. This orthokinetic destabilization produces a network of partially coalesced droplets which contributes to the stabilization of the air cells during the aeration process and also in the final product (Barfod, Krog, Larsen, & Buchheim, 1991; Gelin, Poyen, Courthaudon, Le Meste, & Lorient, 1994; Goff, 1997; Pelan, Watts, Campbell, & Lips, 1997). The rate of clumping of fat globules during shearing is *Corresponding author. Tel.: +44-1132-332956; fax: +44-1132332982. E-mail address: [email protected] (E. Dickinson).
dependent inter alia on the solid fat content of the oil phase, the properties of the fat crystal network within the partially crystallized oil phase, and the type and amount of emulsifier(s) present in the formulation (Walstra, 1987; Boode & Walstra, 1993; Goff & Jordan, 1989). Typically, the active surfactant species is a commercial monoglyceride emulsifier consisting of a mixture of saturated and unsaturated monoglycerides. The presence of monoglycerides in dairy-type emulsions can affect both the solid fat content of the droplets and the amount of protein adsorbed at the oil–water interface. Although the emulsification of triglyceride oils has an inhibiting effect upon fat crystallization, the addition of emulsifiers can enhance crystallization in the droplets, resulting in higher effective solid fat content. Monoglyceride crystallization can also lead to proteins being displaced from the droplet surface (Krog, 1997). Competitive adsorption of milk proteins by emulsifiers reduces the adsorbed layer surface coverage and the protein’s effectiveness as a steric stabilizer, and this
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makes the protective stabilizing layer more susceptible to disruption during shear processing (Dickinson, 1997, 1999; Euston, 1997). The protein surface coverage in an ice-cream mix can actually be used as a predictive measure of fat destabilization during freezing (Bolliger, Goff, & Tharp, 2000). We described recently (Davies, Dickinson, & Bee, 2000) the development of a model emulsion system with sodium caseinate as the protein stabilizing agent and groundnut oil containing tristearin crystals as the dispersed phase. In the presence of the appropriate amount of the added emulsifier, pure glycerol monooleate (GMO), this type of emulsion system can be formulated so as to possess quiescent stability over long periods of time, whilst being susceptible to orthokinetic destabilization when the shear stress exceeds a certain value. The critical shear stress for emulsion destabilization, as measured in a controlled stress rheometer, was found to be sensitive to both the triglyceride crystal content and the added GMO concentration. For a concentrated emulsion made with 1 g 100 g–1 protein and 40 mL 100 mL–1 triglyceride oil containing 5–15% tristearin crystals, it was found that small changes in added GMO concentration (in the range 2–3.5 g 100 g–1) produced large changes in the rate of shear-induced destabilization. On the basis of our previous results using GMO as emulsifier (Davies et al., 2000), we suggested that this same rheological approach might provide a useful systematic procedure for comparing different emulsifiers with respect to their ability to control the stability of emulsions towards partial coalescence in shear flow. In order to understand fully the different degrees of effectiveness of emulsifiers in controlling emulsion clumping in real products, we shall need first to compare the destabilizing abilities of different pure monoglycerides and their mixtures under well-defined conditions. Saturated monoglycerides have a greater ability than partially unsaturated ones to act as initiators of fat crystallization (Berger, 1990), and thus to increase the proportion of solid fat in partially crystalline emulsions. Barfod et al. (1991) showed that, whilst the presence of either the saturated glycerol monostearate (GMS) or the unsaturated GMO initiated more crystallization than in control emulsions without added emulsifier, the solid fat content of the GMS-containing emulsions was always greater than that of the equivalent emulsions with GMO. The molecular structure of the emulsifier also influences its competitive adsorption behaviour with milk proteins. For instance, monoglycerides of shortchain fatty acids are powerful destabilizers, whereas those with longer-chain fatty acids are less effective (Kloser & Keeney, 1959); and partially unsaturated monoglycerides are considered better than saturated monoglycerides at displacing protein from the interface (Barfod et al., 1991; Goff & Jordan, 1989).
This paper compares the orthokinetic destabilization of model emulsion systems containing pure GMS (C18 : 0) or pure glycerol monopalmitate (GMP; C16 : 0) with that of the equivalent pure GMO formulations already reported. We also consider the effectiveness of binary monoglyceride mixtures of known composition, GMS+GMO and GMP+GMO, as well as that of a multicomponent commercial monoglyceride emulsifier. As previously, the emulsions were prepared at a relatively low constant protein : oil ratio, giving sufficient protein surface coverage to stabilize the systems quiescently, but allowing good sensitivity to partial coalescence under unfavourable orthokinetic conditions.
2. Materials and methods The pure monoglycerides GMO, GMS and GMP (p3% other monoglycerides, 3–5% non-monoglycerides) were obtained from Danisco (Brabrand, Denmark), as was the commercial emulsifier Dimodan (72% monoolein, 11% monopalmitin, 11% monostearin, o1% C17 or shorter, o1% C20 or higher). Spraydried sodium caseinate (5.2 g 100 g–1 moisture, 0.05 g 100 g–1 calcium) was from DeMelkindustrie (Vegel, Netherlands). The groundnut oil (Safeways supermarket) was passed through a Fluorisil column to remove surface-active impurities. The tristearin (TS) was from Sigma Chemicals (St. Louis, USA). The 20 mm imidazole buffer was adjusted to pH 7.0 with 0.1 m HCl or NaOH. Fine oil-in-water emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil containing 12.5 g 100 g–1 tristearin) were prepared at 601C using a Shields S-500 high-pressure laboratory homogenizer operating in continuous mode at 40 MPa for 15 min (corresponding on average to each volume element passing six times through the homogenizer valve). The homogenizer was pre-heated by circulating water at 901C. Prior to mixing, the oil phase was heated to 801C and the aqueous phase to 601C. Different types and amounts of monoglycerides were added to the oil phase prior to emulsification. Droplet-size distributions were measured using a Malvern Mastersizer MS20 (presentation code 0505). For freshly made (stable) emulsions containing pure GMS or pure GMP, the average droplet diameters were d32 ¼ 0:3570:01 mm and d43 ¼ 0:4570:01 mm. For emulsions containing mixed GMO+GMS or GMO+ GMP, the average diameters were d32 ¼ 0:3570:01 mm and d43 ¼ 0:4870:01 mm. Using the measured specific surface area Asp (m2 per cm3 of oil), the surface coverage of protein at the oil droplet surface was determined following centrifugation (2.1 104 g, 2 h, 21C) by the depletion method as described previously (Davies et al.,
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(2000). Differential scanning calorimetry (DSC) was carried out on a Perkin-Elmer 7 series thermal analysis system with an air-filled cell as reference. The sample was cooled from 1001C to 01C at 101C min–1. Emulsions were subjected to controlled shear stability testing in the double-gap geometry cell (DG 40/50) of a Bohlin CVO Rheometer. After transferring to the preheated rheometer cell (851C), a freshly made emulsion sample was cooled to 01C at 11C min–1 using the rheometer water bath and temperature program. The emulsion was then left undisturbed for 1 h at 51C. Increasing stresses of duration 10 min were then applied to the sample over the range from 0.073 to 15 Pa (30 values on a logarithmic scale). The apparent viscosity was recorded following each stress application.
3. Results 3.1. Crystallization in systems containing pure monoglycerides The bulk groundnut oil used as the main component of the emulsion dispersed phase remains liquid over the experimental range of this study. Any crystallization occurring in the oil phase is due to the tristearin (TS), whose apparent crystallization temperature in the groundnut oil differs from its bulk crystallization temperature (471C) according to its concentration and the concentration and type of the added emulsifier. To assess the temperature to which samples needed to be cooled in the rheometer in order to ensure oil crystallization, the DSC peak temperatures were determined under conditions of strictly controlled cooling rate. The oil crystallization temperatures determined from DSC peak temperatures are recorded in Table 1 for the addition of 1.25 g 100 g–1 pure monoglyceride to groundnut oil systems containing 12.5 g 100 g–1 TS. We can see that, for the bulk oil phase, the type of added monoglyceride (GMO, GMS or GMP) has little effect on the crystallization temperature (B301C). Upon emulsification, however, there is a substantial difference in crystallization temperature for the systems containing
Table 1 Normalized crystallization (nucleation) temperatures of bulk and emulsified oil systems containing 12.5 g 100 g–1 TS and 1.25 g 100 g–1 monoglyceride in the groundnut oil as indicated by the DSC peak temperature (cooling rate 10 K min–1) Monoglyceride
GMO GMS GMP
Crystallization temperature (1C) Bulk
Emulsion
29.6 30.6 30.0
5.5 21.5 22.0
829
saturated and unsaturated monoglycerides. That is, the degree of dispersed phase supercooling is considerably less for the saturated monoglycerides (B91C for GMS, and B81C for GMP) than for the partly unsaturated GMO (B251C). Saturated monoglycerides are known to be good initiators of fat crystallization (Barfod et al., 1991). Since GMP and GMS crystallize themselves at higher temperatures than GMO, they can be considered more effective fat crystal nucleators for TS crystal growth. Previously, it was suggested (Davies et al., 2000) that the liquid GMO may act as a template for TS crystallization, encouraging crystals to grow towards and through droplet surfaces. Such a template effect should be greatly enhanced by the crystallization of the monoglyceride before that of the triglyceride. Due to the match in saturation and C number (i.e., number of carbon atoms in the chain), it is assumed that GMS is especially readily incorporated into growing TS crystals. The slight mismatch in C number for GMP is rather less favourable, leading to the possibility of crystal defects (Smith, Cebula, & Povey, 1994). In the case of GMO, the presence of the kink in the C chain due to the double bond means that it fits less well into growing TS crystals than does GMS.
3.2. Shear sensitivity of emulsions containing pure GMS Emulsion systems (40 mL 100 mL–1 oil, 1 g 100 g–1 sodium caseinate) with 12.5 g 100 g–1 TS and varying GMS concentrations in the oil phase were subjected to steadily increasing shear forces in the rheometer. In the absence of GMS, the emulsions were of low viscosity (ca. 30 mPa s), they were insensitive to shearing, and they were quiescently stable for at least four weeks, with no discernible change in droplet-size distribution. Addition of GMS was found to lead to an increase in the low-stress apparent viscosity. For systems containing from 2 to 5 g 100 g–1 GMS in the oil phase, Fig. 1 shows the influence of applied shear stress on the apparent viscosity. The application of shear stresses up to 15 Pa to emulsions containing 2–3.5 g 100 g–1 GMS in the oil phase was found to lead to a reduction in apparent viscosity by some five orders of magnitude. This shear-thinning behaviour was found to be reversible: on leaving a sample undisturbed for 1 h after application of the original 15 Pa shear stress, the same steady reduction in apparent viscosity with the applied stress was reproducibly observed (within up to, say, one order of magnitude) (Fig. 2). Even when the TS content of the oil phase was increased to 25 g 100 g–1, there was found to be no shear-induced destabilization of the emulsions at the higher stresses as had been observed previously (Davies et al., 2000) with the equivalent GMO-containing emulsions.
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3.3. Shear sensitivity of emulsions containing pure GMP
Fig. 1. Effect of GMS on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various GMS concentrations (g 100 g–1) in the oil phase: &, 2; E, 2.5; n, 2.75; K, 3; J, 3.5; , 5.
Fig. 2. Reversibility of shear-thinning of GMS-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS+2.75 g 100 g–1 GMS in oil phase). Apparent viscosity is plotted against shear stress: &, no pre-shear; E, pre-shear+1 h quiescent storage.
The thinning of the emulsions under shear, and the subsequent quiescent rethickening, implies that the original high viscosity of GMS-containing emulsions was not due to irreversible partial coalescence, but rather to some kind of flocculation. As there was no change in the visible appearance or rheology of the emulsions upon extended storage, it appears that the high viscosity of the flocculated network structure prevents destabilization of the emulsions by gravitational separation or droplet coalescence. Droplet-size distributions of emulsion systems containing o5 g 100 g–1 GMSFas measured before, during, or after shearing, and following quiescent storage for four weeksFwere both narrow and monomodal. This means that the flocculation was unchanged in strength with time and weak enough to be completely disrupted by dilution and stirring in the Mastersizer water-bath.
Emulsions containing GMP were found to become quiescently unstable at far lower monoglyceride concentrations than those with added GMO or GMS. The presence of 0.75 g 100 g–1 GMP in the oil phase of the standard emulsion formulation (40 mL 100 mL–1 oil, 1 g 100 g–1 sodium caseinate, 12.5 g 100 g–1 TS) gave rise to systems that were both perikinetically and orthokinetically unstable. Increasing the concentration of sodium caseinate did not make the emulsions significantly more stable. Above the crystallization temperature, emulsions prepared with added GMP had a narrow droplet-size distribution and low viscosity. Once the temperature dropped below 221C, however, the viscosity of emulsions containing >0.75 g 100 g–1 GMP in the oil phase increased greatly, and droplet-size distributions became distinctly bimodal, reflecting the quiescent instability of the emulsions. Fig. 3 shows the bimodal distribution PðdÞ of an emulsion sample with 1.25 g 100 g–1 GMP in the oil phase. Increasing the GMP content further led to distributions with greater proportions of large particles. Fig. 3 shows that the main feature of PðdÞ for an emulsion with 2 g 100 g–1 GMP is a peak centred at B5 mm. When sheared in the rheometer, emulsion samples with >0.75 g 100 g–1 GMP in the oil phase led to an apparent shear-thinning as indicated in Fig. 4. Close inspection of the samples revealed, however, that some partial coalescence had occurred, with clumps pushed to the outside of the cell and therefore not contributing to the measured viscosity. Hence, the system was presumably exhibiting ‘slip flow’ on the inner cylinder. This shear-thinning behaviour was in sharp contrast to the dramatic increases in viscosity that were induced in GMO-containing emulsions when applied shear forces exceeded critical values (Davies et al., 2000). The shear-induced clumping of emulsions containing GMP meant that they could not be diluted to form
Fig. 3. Particle-size distribution PðdÞ of GMP-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase): F, 1.25 g 100 g–1 GMP in oil phase; F F F, 2 g 100 g–1 GMP in oil phase.
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Table 2 Apparent protein surface coverage (G) of sodium caseinate emulsions with and without added monoglycerides (2 g 100 g–1) in the oil phase of the emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 oil, 12.5 g 100 g–1 TS in oil phase) and mass fraction of protein adsorbed (Fads ). The specific surface area (Asp ) is based on the d32 value determined following the cooling of the emulsions. Each listed value is the average of duplicate sets of measurements.
Fig. 4. Effect of GMP on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various GMP concentrations (g 100 g–1) in the oil phase: &, 0.75; n, 1.25; E, 1.75; J, 3.
Fig. 5. Low-stress apparent viscosity (0.07 Pa) of GMP-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase) as a function of the GMP content in the oil phase.
stable uniform emulsions of low viscosity as could the equivalent emulsions with added GMS. The clumping also disturbed the homogeneity of the undiluted emulsions, hence preventing the viscosity returning to its original high value following shear-induced thinning. Increasing the GMP content led to an increase in the apparent viscosity of the freshly made emulsion as shown in Fig. 5. 3.4. Protein displacement from emulsion droplet surface Table 2 shows how 2 g 100 g–1 monoglyceride added to the groundnut oil phase substantially reduces the apparent surface coverage of sodium caseinate at the emulsion droplet surface. Even without added monoglyceride, the protein surface coverage in these emulsions is rather low (G ¼ 1:1 mg m–2) due to the deliberately low overall protein : oil ratio used here. The presence of the monoglyceride leads to a very substantial reduction in the fraction of protein adsorbed (Fads ) inferred from the measured protein content in the emulsion serum phase after centrifugation. However,
Monoglyceride
Asp (m2 cm–3)
G (mg m–2)
Fads (%)
F GMO GMS GMP
14.3 15.6 14.0 11.4
1.08 0.45 0.17 0.05
71 32 11 3
the very small values of apparent protein surface coverage for GMP and GMS appear somewhat dubious, since they imply that the protein makes almost a negligible contribution to adsorbed layer composition. The actual values of G and Fads quoted in Table 2 for the monoglyceride-containing systems may, in reality, be underestimated due to the effect of partial coalescence during centrifugation, which may liberate previously unadsorbed protein into the bulk aqueous phase, as pointed out by Segall and Goff (1999). On the other hand, if significant partial coalescence were to have occurred during centrifugation, the high local interdroplet forces generated might also have been expected to destabilize the semi-crystalline emulsions with no added monoglycerideFwhereas the experimentally determined value of Fads (>70%) for the monoglyceridefree reference emulsion is, in fact, sensibly high. Based on the data in Table 2, and placing the issue of partial coalescence during centrifugation aside, we infer that the saturated monoglycerides are better at displacing sodium caseinate from the droplet surface than is the unsaturated GMO, and that GMP is even more effective than GMS. The situation here in these model emulsions seems to be more clear-cut than in experiments reported for real ice-cream emulsions: Pelan et al. (1997) could not find any clear difference in displacing ability for saturated and unsaturated monoglycerides, and Krog (1997) states that it is difficult to conclude whether there are significant differences between saturated and unsaturated monoglycerides in relation to protein desorption during ageing. Conversely, Barfod et al. (1991) reported a higher level of protein desorption in ice-cream emulsions with added GMO than in those containing GMS. Anyway, differences amongst the action of various monoglycerides often appear to depend markedly on temperatureFthis is presumably because small changes in temperature affect the sensitive balance of competing protein–lipid and lipid–lipid interactions, with consequences for the adsorbed layer properties as well as for the chain crystallization behaviour.
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3.5. Behaviour of emulsions containing commercial monoglyceride emulsifier The differences described above in the perikinetic and orthokinetic stability of model emulsions containing the pure GMO, GMP and GMS raises interesting questions regarding the behaviour of commercial emulsifiers composed of mixtures of these three monoglycerides. Against this background, we carried out experiments with one particular commercial emulsifier, Dimodan, containing 72% GMO, 11% GMS and 11% GMP. Emulsions containing Dimodan (1.63–1.85 g 100 g–1 in oil phase) were found to crystallize at B191C, corresponding to supercooling at B31C. The GMP and GMS are assumed to crystallize before the GMO or TS. We infer that the presence of crystalline monoglycerides at the droplet surface encourages nucleation and crystallization of the other components, thereby greatly reducing the high levels of supercooling found with the pure monoglycerides. The range of Dimodan concentrations able to produce quiescently stable emulsions that were unstable under shear was much narrower than previously found with pure GMO (Davies et al., 2000). Systems with p1.63 g 100 g–1 Dimodan in the oil phase were found to be both perikinetically and orthokinetically stable, with narrow monomodal droplet-size distributions maintained during prolonged storage. Systems with between 1.75 and 1.88 g 100 g–1 were quiescently stable and shear-sensitive. Fig. 6 shows that, over this narrow emulsifier concentration range, there is a systematic change in the critical destabilizing stress. Fig. 7 shows that the apparent droplet-size distributions of these emulsions were observed to broaden and shift to larger sizes as the oil phases crystallized on cooling, but then to remain unchanged on quiescent storage. Above 1.88 g 100 g–1 emulsifier, the emulsions were destabilized so
Fig. 6. Effect of Dimodan emulsifier on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase). Apparent viscosity is plotted against shear stress for various emulsifier concentrations (g 100 g–1) in the oil phase: &, 1.63; E, 1.75; n, 1.83; K, 1.88.
Fig. 7. Particle-size distributions PðdÞ of Dimodan-containing emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS+1.83 g 100 g–1 Dimodan in oil phase): K, pre-cool; J, post-cool+1 h storage; ’, post-cool+7 days storage.
much following cooling that transfer to the rheometer cell was impossible. This observation, that relatively small amounts of saturated monoglycerides in a GMO-rich commercial emulsifier can substantially affect emulsion shear sensitivity, has led us to explore further the properties of well-defined model systems containing the binary monoglyceride mixtures GMO+GMS and GMO+ GMP. 3.6. Behaviour of emulsions containing GMO+GMS or GMP Oil crystallization temperatures determined from DSC peak temperatures are recorded in Table 3 for emulsions containing binary mixtures of monoglycerides. The bulk oil phases composed of 12.5 g 100 g–1 TS, 1.75 g 100 g–1 GMO and 0.5 g 100 g–1 GMS or 0.5 g 100 g–1 GMP in groundnut oil were found to crystallize at 29.370.11C and 29.570.11C, respectively. As indicated in Table 3, the emulsification of these oil mixtures leads to substantial supercooling: B121C for emulsions with added GMP, and B161C for emulsions with added GMS. By way of comparison, we note from Table 1 that the degree of supercooling for the equivalent emulsion containing 1.25 g 100 g–1 pure GMO in groundnut oil is B251C. Hence, as with the Dimodan emulsions, the presence of crystallizing saturated monoglyceride at the droplet surface, as well as GMO in the oil phase, enables triglyceride nucleation and crystallization within emulsion droplets to occur at a temperature higher than in droplets containing GMO alone. The stability characteristics of emulsions containing various concentrations of GMO (1.0–2.0 g 100 g–1) and GMP or GMS (0.25–1.0 g 100 g–1) are summarized in Tables 4 and 5, respectively. The ‘ideal’ emulsion for our purposes is one that is quiescently stable and shearsensitive (U+QS). However, most of the shear-sensitive emulsions containing the mixed monoglycerides are also
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E. Davies et al. / International Dairy Journal 11 (2001) 827–836 Table 3 Crystallization temperatures of emulsions (1 g 100 g–1 sodium caseinate, 40 vol% groundnut oil, 12.5 g 100 g–1 TS) containing GMO (1.25–1.75 g 100 g–1)+GMS or GMP (0.5 g 100 g–1) in the oil phase GMO content (g 100 g–1)
1.25 1.50 1.75 a
Crystallization temperature (1C)a +GMS
+GMP
18.0 18.0 17.0
13.3 13.8 13.5
Experimental error 70.11C.
quiescently unstable (U+QU). Whereas shear-sensitive emulsions with GMO present as the sole emulsifier are very stable towards extended storage, the addition of a small quantity of GMP or GMS would seem to remove this desirable characteristic. However, this negative effect is less pronounced for systems with higher GMO concentrations. Fig. 8 shows the level of GMS or GMP required to create shear-sensitive emulsions as a function of the GMO content. The combination of GMO+GMS is more effective than GMO+GMP in producing shearsensitivity over a wider range of GMO concentrations. With increasing GMO content, the amount of extra monoglyceride required to induce shear-sensitivity decreases. Addition of critical amounts of GMP or GMS to emulsions containing GMO has a dramatic influence on the orthokinetic emulsion stability (Fig. 9).
Fig. 8. Influence of GMO concentration in the oil phase on the concentration of GMS or GMP required to produce shear-sensitive emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase): ’, GMS; &, GMP.
We recall that a critical content of 2.13 g 100 g–1 GMO was required to create shear-sensitive emulsions with pure GMO as emulsifier (Davies et al. (2000)). With emulsifier mixtures containing GMO+saturated monoglyceride, however, a lower total monoglyceride concentration is effective. So, for example, a system containing 1 g 100 g–1 GMO was found to become shearsensitive on addition of either 0.75 g 100 g–1 GMS or 1.0 g 100 g–1 GMP. The results in Fig. 9 show that the
Table 4 Stability of emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS) containing different concentrations of both GMO and GMP in the oil phase. Key: QS=quiescently stable; QU=quiescently unstable; S=shear stable; U=shear unstable; X=too unstable for transfer to rheometer cell GMP content (g 100 g–1)
0.25 0.5 0.75 1.0
GMO content (g 100 g–1) 1.0
1.25
1.5
1.75
2.0
S+QS S+QS S+QS U+QU
S+QS S+QS U+QU X
S+QS S+QS U+QS X
S+QS S+QS U+QU X
U+QS U+QS U+QS X
Table 5 Stability of emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS) containing different concentrations of both GMO and GMS in the oil phase. Key: QS=quiescently stable; QU=quiescently unstable; S=shear stable; U=shear unstable; X=too unstable for transfer to rheometer cell GMS content (g 100 g–1)
0.25 0.5 0.75 1.0
GMO content (g 100 g–1) 1.0
1.25
1.5
1.75
2.0
S+QS S+QS U+QS X
S+QS S+QS U+QU X
S+QS U+QS U+QU X
S+QS U+QS U+QU X
U+QS U+QS X X
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Fig. 9. Effect of individual monoglyceride concentrations on the shear stability of concentrated emulsions (1 g 100 g–1 sodium caseinate, 40 mL 100 mL–1 groundnut oil, 12.5 g 100 g–1 TS in oil phase) containing GMO+GMP or GMO+GMS. Apparent viscosity is plotted against shear stress for various GMO concentrations (1, 1.25, 1.5, 1.75 and 2 g 100 g–1) in the oil phase: K, 0.25 g 100 g–1 GMS; ’, 0.5 g 100 g–1 GMS; m, 0.75 g 100 g–1 GMS; J, 0.25 g 100 g–1 GMP; &, 0.5 g 100 g–1 GMP; n, 0.75 g 100 g–1 GMP; B, 1 g 100 g–1 GMP.
critical destabilizing stress is a well-defined quantity which is reproducibly and consistently dependent on the contents of both the GMO and the GMP (or GMS).
4. Discussion Sodium caseinate-stabilized oil-in-water emulsions containing tristearin crystals and pure GMS or pure GMP in the groundnut oil phase have been found to be quiescently unstable with respect to flocculation, with a large viscosity increase occurring as the fat phase of the emulsion droplets crystallizes. The GMS-containing emulsions were orthokinetically stable, whereas emulsions with added GMP were more shear-sensitive, with clumps appearing as partial coalescence took place. The level of shear-induced destabilization in these emulsions was not sufficiently extensive, however, to produce the same sort of sharp shear-induced viscosity increase, as
was found with emulsions containing pure GMO. This behaviour is broadly consistent with the generally held view that unsaturated monoglycerides are better than saturated monoglycerides in inducing shear-sensitivity in emulsions (Goff & Jordan, 1989). Possibly the higher viscosity of emulsions containing GMS or GMP protects them from shear-induced destabilization by reducing the number of droplet collisions during shearing and/or the magnitude of local critical stresses forcing the droplets together. Alternatively, the explanation may be associated with the differences in properties of adsorbed layers (Lucassen-Reynders, 1993), with the more close-packed layers of the saturated monoglycerides at the oil–water interface leading to more highly viscoelastic adsorbed layers which may provide greater protection against shear destabilization. Differences in quiescent stability of the emulsions containing the different pure monoglycerides can be explained, partly at least, in terms of the protein displacement behaviour. Addition of GMP gave rise to the highest level of protein displacement and also the most quiescently unstable emulsions. The GMS-containing systems showed better storage stability, with no irreversible clumping taking place. Addition of GMO led to the lowest level of protein displacement and to quiescently stable emulsions. Using confocal microscopy, Heertje (1993) has reported that GMS is more effective than GMO at displacing protein from an oil–water interface at monoglyceride concentrations above 0.25 g 100 g–1, and this was attributed to the closer molecular packing of the saturated chains at the interface. On the other hand, at low monoglyceride concentrations (o0.25 g 100 g–1), the GMO was found to be a more effective displacer (Heertje, 1993), and this was ascribed to the unsaturated GMO having a higher free energy of adsorption than GMS. The double bond in GMO means that fewer molecular configurations are possible than in GMS, and so it is argued that the reduction in entropy on adsorption is not as great for the unsaturated monoglyceride as for the saturated one. However, as the monoglyceride concentration is increased, leading to higher levels of adsorption, the greater lowering of the surface free energy due to the close packing of saturated fatty acid chains becomes a more important factor than the configurational entropic considerations. In the study of Barfod et al. (1991), the monoglyceride concentrations were as low as 0.2 g 100 g–1, i.e. below the threshold described by Heertje (1993), so helping to explain why, in their experiments, the unsaturated GMO was a more effective displacer than GMS. Emulsion stability is also affected by the type of fat crystals formed within droplets (Walstra, 1987). When there is a high defect density on the crystal surface, more
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crystals may grow from these defect points, leading to spherulites (Smith, 1995). Under the normal light microscope, we have observed that mixtures of GMO+TS give rise to a low density of spiky spherulites that may contribute significantly to partial coalescence. The low density of spherulites could reflect a retardation of nucleation caused by the presence of the unsaturated GMO which is unable to fit perfectly into the growing saturated TS crystals. This explanation is consistent with the observed reduction in crystallization temperature with increasing GMO concentration. Mixtures of GMP+TS have been observed to produce fairly a large and rounded spherulites, with fewer crystals appearing to have the ability to rupture droplet surfaces. Mixtures of GMS+TS have been observed to crystallize into rings of small crystals; the low level of crystallization and small crystal size helps to explain the lack of shearsensitivity in the corresponding emulsions. We may speculate that the low number density and the spikiness of crystals formed in mixtures of GMO+TS allow the corresponding emulsion systems to be both quiescently stable and shear-sensitive. While the edges of spherulites are not forced close against the protein-coated droplet surface simply by Brownian collisions, the application of shear forces does allow the crystals to pierce the oil–water interface, causing partial coalescence (Boode & Walstra, 1993). The presence of the larger, more rounded spherulites found with GMS+TS or GMP+TS could mean that some crystals are close to the droplet surface, leading to perikinetic protein-mediated aggregation, but that they are not sufficiently needle-like to pierce the droplet surface on shearing and allow partial coalescence to take place. Results for these model systems containing mixtures of saturated and unsaturated monoglycerides can therefore help to explain why the emulsions made with the GMO-rich commercial emulsifier Dimodan have substantially different stability properties from those made with pure GMO. Orthokinetic stability is reduced by the presence of a small amount of GMS or GMP in addition to GMO. Whilst few binary monoglyceride-containing emulsions studied could be characterized as being both shear-sensitive and quiescently stable (as was ideally sought), there is apparently better quiescent stability with GMO+GMS than with GMO+GMP. At each chosen GMO content, increasing the amount of added GMP or GMS above a certain critical level leads to an increase in shear-sensitivity.
with GMO. The presence of pure GMS leads to extensive flocculation which subsequently seems to ‘protect’ the emulsion systems against the destabilizing effects of shear forces. Emulsions containing pure GMP are more shear-sensitive, although the destabilization is not extensive enough to give the substantial viscosity increases that we previously observed with emulsions containing pure GMO. The different levels of (in)stability for the three pure monoglycerides can be attributed to a combination of two factors: the extent of competitive protein displacement and the fat crystal type. It would appear that the best combination for quiescent stability and shear sensitivity is a relatively low level of protein displacement and the creation of spiky spherulites made up of crystals that can penetrate the droplet surface. Use of the commercial emulsifier Dimodan can generate emulsions having both quiescent stability and shear sensitivity, but the emulsifier concentration range giving these favourable properties is extremely narrow, even though the emulsifier contains >70% GMO. The use of a simple binary mixture of monoglycerides, GMO+GMS or GMO+GMP, has been shown to provide better stability control. The combination of GMO+GMS gives a range of emulsions that are both quiescently stable and shear-sensitive, particularly at the higher GMO concentrations. Shear sensitivity can be induced at a lower total monoglyceride concentration than when GMO is present alone. The critical destabilizing stress can be manipulated by making very small adjustments in the concentration of the saturated monoglyceride, giving a powerful degree of control over the orthokinetic stability of the emulsions. Finally, a cautionary remark seems appropriate. The trends described here relate to stability behaviour of carefully chosen model emulsions that are considerably simpler in microstructure and composition than those typically encountered in food products such as ice-cream and whipped toppings. While we have shown that, by changing the concentrations and combinations of monoglycerides in our model systems, we can sensitively control emulsion shear stability, it is possible that with a different set of variables (droplet size, type of fat phase, etc.) there could be substantially different results. Further experiments will therefore be required to establish whether our recipe for making emulsions that are both shear-sensitive and quiescently stable has broad applicability in the formulation of commercial dairybased whipping emulsions.
5. Conclusions
Acknowledgements
Comparisons have been made between stability properties of semi-crystalline model emulsion systems with added GMS, GMP, and binary mixtures of these
We acknowledge the award of a BBSRC CASE Studentship in collaboration with Unilever, Colworth House.
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