A rheological analysis of structured water-in-olive oil emulsions

Journal of Food Engineering 107 (2011) 296–303

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A rheological analysis of structured water-in-olive oil emulsions Francesca R. Lupi a, Domenico Gabriele a,⇑, Bruno de Cindio a, Maria C. Sánchez b, Crispulo Gallegos b a b

University of Calabria, Department of Engineering Modelling, Via P. Bucci, Cubo 39C, I-87036 Rende (CS), Italy University of Huelva, Department of Chemical Engineering, Campus de ‘‘El Carmen’’, E-21071 Huelva, Spain

a r t i c l e

i n f o

Article history: Received 18 February 2011 Received in revised form 6 June 2011 Accepted 9 July 2011 Available online 23 July 2011 Keywords: W/O structured emulsions Mono- and di-glycerides of fatty acids Cocoa butter Olive oil Organogel Crystallisation

a b s t r a c t Structured emulsions are widely used in the food industry. In the case of water-in-oil emulsions, an oil phase structuration is achieved by the creation of a saturated fat crystalline network inside which water droplets are entrapped. Traditional technology based on the hydrogenation of vegetable oils, leads to the formation of saturated trans-fatty acids, considered unhealthy owing to their potential contribution to cardio-vascular diseases. As a consequence, nowadays the use of hydrogenated fatty acids has been reduced and the consumption of healthy oils has increased. However oils need to be properly structured to be used as solid fat replacers. The present work deals with the rheological study of W/O emulsions, structured through the oil phase crystallisation by organogelator agents (mono- and di-glycerides of fatty acids). The oil phase was prepared by blending a high-oleic-acid-containing oil (olive oil) with a natural saturated fatty acids source (cocoa butter). A highly structured network is obtained by rapidly cooling the molten oil phase at low shear rates. The emulsions prepared were compared with commercial margarines and they showed rheological properties suitable to a potential application as ‘‘solid fats’’. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Structured multiphase systems are adopted in different food applications such as dairy emulsion, mousse, margarine, spreadable fats and so on. Determined by their nature and application, the desired texture is usually achieved by structuring the water phase with hydrophilic gelling agents and/or hydrocolloids (Gabriele et al., 2009; Rodríguez-Abreu and Lazzari, 2008), structuring the oil phase by fat crystallisation (Coupland, 2002; Pernetti et al., 2007), or by concentrating the dispersed phase up to volume fractions much higher than the limit value /⁄, corresponding to the maximum packing fraction of an equivalent suspension of hard spheres (Leal-Calderon et al., 2007; Pal, 1998). Regarding water-in-oil emulsions, oil phase crystallisation is the most common technique used to increase their hardness in order to produce ‘‘solid fats’’, such as shortenings, margarine and other biphasic systems for the food industry. Margarine is one of the best known fats for leavening and baking products. Its production process passes through different steps aimed at obtaining W/O emulsions with a dispersed phase fraction usually lower than 20% (w/w) (Vaisey-Genser, 2003). The specific rheological characteristics are revealed thanks to the high melting point of saturated fats, in the oil phase, whose organisation in crystals aggregates form an

⇑ Corresponding author. Tel.: +39 0984 496687; fax: +39 0984 494009. E-mail address: [email protected] (D. Gabriele). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.07.013

interacting ordered structure surrounding and stabilising the water droplets, to confer a hard gel-like character to these foods. The typical method, currently employed in food processing industries in order to structure the oil phase, is the partial hydrogenation of vegetable oils (Ghotra et al., 2002). A catalytic three phase (gas–solid–liquid) process is carried out in a batch autoclave at 110–190 °C (Singh et al., 2009). The process may imply the movement of double bonds in their positions on the fatty acid carbon chain, producing positional and geometrical isomers, trans-fatty acids (TFAs), which have numerous negative health effects (Blanco Muñoz, 2004; Marangoni, 2009). These include increased incidence of heart disease or high cholesterol levels. As a consequence, in order to reduce the presence of hydrogenated fatty acids in the diet, many governments are introducing laws strictly limiting the amount of TFAs in foods. First in 2003, Denmark established a limit of 2% (w/w) of TFAs in fats and oils destined for human consumption; many other countries, in recent years, have been changing their legislation to limit or ban TFAs (Marangoni, 2009). Moreover, recent studies have confirmed that a greater consumption of polyunsaturated fatty acids, in place of TFAs, would significantly reduce rates of coronary heart disease (Mozaffarian et al., 2010). These new legislative requirements and scientific results are forcing the food industry to find alternative ways to produce structured oil phases. These novel solutions are mainly based either on the gelation of edible oils (rich in unsaturated components) by adding suitable organogelator systems, such as triacylglycerols (TAGs), diacylglycerols (DAGs), monoacylglycerols (MAGs), fatty

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acids, fatty alcohols, waxes, etc. (Marangoni, 2009; Pernetti et al., 2007), or on mixtures of high- and low-melting fats (Higaki et al., 2003; Marangoni, 2009). Among potential organogelator systems acylglycerols seem to be particularly interesting, because they are already used as common emulsifiers in the food industry. Mono- and di-glyceride of fatty acids are lipophilic emulsifiers (HLB value of about 3.7) allowing the formation of W/O emulsions (Constantinides and Yiv, 1994; Friberg, 1997). They are produced by the reaction of glycerol with vegetable oils and fats, whose composition is strictly determined by the characteristics of the native fruit from which the oil is obtained (Clogston et al., 2000). Ojijo et al. (2004) achieved rheological properties close to conventional fat spreads by cooling a mixture of monoglycerides (MAGs) and olive oil. A three-dimensional gel network was obtained when the MAGs volume fraction was higher than a critical low limit, and its properties studied as a function of cooling and shear rate. On the other hand, Skogerson et al. (2007) proposed the emulsification of W/O emulsions with a glyceride emulsifier containing a large di-glyceride fraction, especially useful in preparing puff pastry products. The interactions between emulsifiers and fat crystals are not well clarified yet. Rousseau and Hodge (2005), by using atomic force microscopy and X-ray diffraction, found that the presence of the emulsifier (mono-olein) affected wax crystals morphology in water-in-oil emulsions, by varying the network structure and, as a consequence, their physical properties. A probable stabilisation mechanism of fat particles linked to the emulsifier molecules was explained by Garti et al. (1998). Thus, emulsifiers are the molecular bridges that allow the fat crystals to be linked to the dispersed phase droplets where they play an important role in the Pickering stabilisation mechanism. The use of mixtures of saturated (or hydrogenated) fats rich in stearic acid and high-oleic acid oils seems particularly interesting, owing to the neutral effects of stearic acid on cholesterol levels and the beneficial effects of oleic acid on human health (Marangoni, 2009). Even though gel-like fat mixtures were obtained by using only high- and low-melting fats (Higaki et al., 2003), it seems that more interesting properties can be obtained either by fat interesterification or by the addition of suitable emulsifiers and stabilising agents. Marangoni (2009) showed interesting results for interesterified mixtures of hydrogenated canola oil in high in oleic acid sunflower oil, whereas, in his patent, Jahaniaval (2005) suggested a procedure for the preparation of healthy margarine and butter substitutes, based on liquid oils (at room temperature), i.e. olive oil, and phospholipids as stabilisers. The final consistency of the resulting margarine was increased by mixing, at high temperature, oil and cocoa butter (a pale yellow pure edible vegetable fat, extracted from the cocoa bean, containing a total amount of butter of 50–60% w/w and with a high content of stearic acid) (Salas et al., 2011). The complexity and flexibility of the triglyceride molecules allow different crystalline packing of the same ensemble of molecules, leading to the existence of crystalline arrangements, or polymorphs, which exhibit significantly different melting temperatures (Himawan et al., 2006; McClements, 1999; Narine and Marangoni, 1999). The polymorphism of most fats is based around three main forms: a, b0 , and b, even though, depending on the fat composition, other metastable polymorphs were observed (Narine and Marangoni, 1999). The thermodynamic stability of the three forms decreases in the order b > b0 > a. Nevertheless, triacylglycerols often crystallise in one of the metastable states because they have lower activation energy of nuclei formation; however, they progressively evolve to the most stable form with ageing (McClements, 1999). For foodstuffs such as margarine, the oil phase should crystallise in the b0 form (Norton et al., 2009). This form possesses smooth mouthfeel, gives hardness to the final margarine and, also, traps a large amount of liquid oil because of its spherulitic nature

(Borwankar et al., 1992; Schenk and Peschar, 2004; Wiederman, 1978). Nevertheless, Garti et al. (1998) affirm that, in the presence of food emulsifiers, W/O emulsions (like margarines) can be stabilised by a-form (mixed with b0 -form) submicron crystals. Hindle et al. (2000) studied emulsified cocoa butter crystallised in the a-form, while Coupland (2002) studied palm oil and lard emulsions with b-crystals. It is worth pointing out that a slow cooling rate can lead, in a quiescent state, directly to the polymorph b0 , but slow cooling rates and low annealing temperatures allow a more disordered network to be produced, yielding a final product with a small elastic modulus (Fessas et al., 2005). The aim of the present work was the characterisation of W/O emulsions, with a structured continuous oil phase, having rheological properties similar to commercial solid fats. In order to produce a structured fat that could be considered ‘‘healthy’’, the main ingredients of these innovative emulsions are olive oil, a typical element in the Mediterranean diet, cocoa butter, with its high stearic acid content, and mono- and di-glycerides of fatty acids, a common food emulsifier. The investigation on emulsion properties was preceded by a preparatory rheological and microstructural characterisation of the oil phase; the effects of a constant amount of solid fat and emulsifier on crystallisation and structuration phenomena in olive oil/cocoa butter systems were evaluated as a function of cooling rate and shear. Afterwards, different emulsions were prepared, followed by an investigation of the effects of both the emulsion preparation method and the oil phase composition on their mechanical properties. Finally, the rheological behaviour of selected emulsions was compared to that of commercial solid fats, in order to know their potential application as ‘‘healthy’’ shortenings replacers. 2. Materials and methods 2.1. Samples ingredients and manufacture The raw materials used for samples preparation were: distilled water and a virgin olive oil (Carbonell, Spain), as the main constituents of the two phases; cocoa butter (average composition in Table 1) (Icam S.P.A., Italy); mono- and di-glycerides of fatty acids, Myverol 18–04 K (mainly composed of monoglycerides, kindly supplied by Kerry Group, Ireland) as emulsifiers (referred to as Myverol throughout the paper); and NaCl (Panreac, Spain), at 0.1 M aqueous concentration, in order to identify the type of emulsion (W/O or O/W) through electric conductivity measurements. All ingredients were used without further purification with the aim of investigating the behaviour of commercial products potentially suitable for industrial applications. The preparatory investigation of Myverol effects on oil phase crystallisation phenomena, was carried out on sample SO (see Table 2) prepared by adding simultaneously (Garti et al., 1998) to the oil (94.3% w/w), at 70 °C, cocoa butter (2.3% w/w) and Myverol (3.4% w/w); the system was continuously stirred with a magnetic agitator (Agimatic E, Selecta, Spain). Once the cocoa butter and

Table 1 Average composition of the cocoa butter used (ICAM, 2008). Fatty acid

Composition (% w/w)

Stearic acid Oleic acid Palmitic acid Linoleic acid Arachidic acid Palmitoleic acid

32–37 30–37 23–30 2–4 <1 <1

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Table 2 Compositions and manufacturing conditions for the different investigated samples (concentrations based on total mass formulation). The last column refers to the homogenisation steps conditions. The reported temperature is that of the sample. SCR is ‘‘slow cooling rate’’ of the oil phase, while FCR is ‘‘fast cooling rate’’. Sample

SO E1 E2 E3 E4 E5 E6

Oil phase composition Olive oil (% w/w)

Cocoa butter (% w/w)

Myverol (% w/w)

94.3 94.3 94.3 94.3 94.3 85.2 73.9

2.3 2.3 2.3 2.3 2.3 11.4 22.7

3.4 3.4 3.4 3.4 3.4 3.4 3.4

Oily phase/aqueous phase ratio ()

Cocoa butter in emulsion (% w/w)

Homogenisation procedure

– 7.3 7.3 7.3 7.3 7.3 7.3

– 2 2 2 2 10 20

– 1st 70 °C + 2nd 25 °C 25 °C, SCR 20 °C, FCR, gently mixed 20 °C, FCR, static 20 °C, FCR, gently mixed 20 °C, FCR, gently mixed

Myverol were completely melted (under visual inspection), mixing was prolonged for a further 5 min. Different emulsions (batches of 200 g) were prepared by varying sample manufacture protocol or the oil/cocoa butter ratio (see Table 2 for detailed information) without changing either the oil phase/aqueous phase ratio (approximately 7.3:1) or the total Myverol concentration (3/100 g emulsion or 3.4/100 g oil phase) kept constant at the same value adopted for sample SO. The aqueous phase was obtained by dissolving the corresponding amount of NaCl in water, at room temperature, under agitation (5 min). Subsequently, the aqueous phase was added to the oil phase (prepared according to the previously described procedure and having the same composition as the SO sample); the emulsion was then manufactured using a rotor–stator turbine (Ultra-Turrax T 50, IKA, Germany) equipped with a G 45 F dispersing element. Emulsion E1 (all the emulsified samples are indentified by the capital letter ‘E’) was prepared by adding the aqueous to the oil phase to be homogenised at 7600 rpm and 70 °C, for 5 min. The emulsions were highly unstable at these emulsifying conditions. Therefore, a second homogenisation step was necessary (at 25 °C, for 5 min) to produce a stable emulsion. Sample E2 was prepared by mixing the two phases at 25 °C. Thus, the oil phase was slowly cooled from 70 to 25 °C (the hot oil phase was stored, at room temperature, for the time necessary to reach thermal equilibrium). Sample E3 was produced after applying a fast cooling process to the gently stirred oil phase, quenching it in a thermostatic cold bath (at 0 °C) up to the final temperature of 20 °C. In this case, the oil phase was poured into a round aluminium flask (/ = 15 cm), where the final thickness of the cold hard fatty phase reached a maximum value of about 1 cm, in order to obtain a fast and, approximately, uniform cooling. Afterwards the water phase was added to the cold oil phase and homogenised according to the previously described procedure. Sample E4 was prepared in the same way, but the oil phase was cooled in static conditions before water addition. Samples E5 and E6 were prepared by using the same procedure and step sequence adopted for E3, however, the cocoa butter fraction, in the oil phase, was increased up to 11.4% (w/w) (corresponding to a total amount of 10/100 g of emulsion) in E5 and 22.7% (w/ w) (corresponding to a total amount of 20/100 g of emulsion) in E6 (see Table 2). As benchmarks for expected rheological properties, two commercial emulsions, ‘‘Flora’’ (Unilever, Spain) and ‘‘Vallè’’ (18% w/ w butter, France), were also studied. 2.2. Optical microscopy In order to study the effect of shear rate on the final internal microstructure of the samples, an optical microscopy analysis was also performed. Even though crystals size distribution could not be analysed, a qualitative inspection of images allowed a reasonable interpretation of the rheological data obtained. Micrographs were taken on samples recovered directly from the rheometer plate after

rheological test completion at 20 °C, by using optical microscopy (MX5300H, MEIJI, Japan, magnification 20). 2.3. Rheological characterisation The rheological characterisation of sample SO was performed with a controlled-strain rheometer ARES-RFS (TA Instruments, USA), using a plate–plate geometry (/ = 50 mm, gap 0.9 ± 0.1 mm). Temperature was controlled with a Peltier system (±0.1 °C). In order to observe the evolution of the linear viscoelastic properties with temperature, a temperature sweep test, in oscillatory shear and inside the linear viscoelastic domain, was carried out at 1 Hz frequency. With this aim, temperature was decreased from 70 to 20 °C, using both a fast (5 °C/min) and a slow (1 °C/min) ramp rate. The applied strain was modified during the test according to the temperature change, in order to guarantee the linear viscoelastic domain (preliminarily investigated by strain sweep tests at 1 Hz and different temperatures). The flow behaviour of the SO sample was studied by carrying out temperature sweep tests at different constant shear rates (1, 5, 10, 100 s1), from 70 to 20 °C, at 5 °C/min. However, a slower cooling rate (1 °C/min) was also selected for the test performed at 10 s1. Potential slippage issues were investigated by carrying out preliminary flow tests (data not shown) by using different gaps; according to the literature (see for example Ma and Barbosa-Canovas, 1995) flow curves obtained in parallel plate geometry are independent of gaps selected if there is no slippage. No difference in measured viscosity was observed, confirming the absence of slipping phenomena. The rheological characterisation of emulsions and commercial margarines was carried out with a HAAKE MARS controlled-stress rheometer (Thermo Scientific, Germany), using a serrated plate– plate geometry (/ = 35 mm, gap 2 ± 0.1 mm). Temperature was controlled by means of a thermostatic bath. Stress sweep tests, at 1 Hz, in order to determine the linear viscoelastic range, and frequency sweep tests, within the linear viscoelastic region, in the frequency range 0.1–10 Hz, were carried out at 20 °C. On the other hand, the viscous flow behaviour of some emulsions (E1E4), in the shear rate range 0.01–1000 s1, was characterised at 20 °C. The remaining emulsions (E5 and E6) and the commercial margarines showed wall-slip and some instability phenomena in a wide range of shear rates (results not shown). Each sample was prepared independently and the results presented are the average values of the rheological properties measured for each one; differences of measurement are shown by standard deviations. 2.4. Characterisation of emulsion type The type of emulsion (O/W or W/O) was characterised by performing electrical conductivity analysis, in order to guarantee that water-in-oil emulsions had been obtained.

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In this sense, all samples were prepared with a 0.1 M NaCl solution of known conductivity value (approximately 10.7 mS/cm at 25 °C) (Eicke et al., 1989; Ozawa et al., 1997). The electrical conductivity was measured by using a conductimeter inoLab pH/Cond Level 1 (WTW, Germany).

3. Results and discussion 3.1. Oil phase characterisation The rheological characteristics of W/O emulsions, with a structured oil phase, are strictly related to the formation of fat crystals within the oil phase (Coupland, 2002; Macierzanka et al., 2009; McClements, 1999). Thus, the rheological characterisation of sample SO, with the same oil phase composition used to manufacture samples E1–E4, was accomplished as a preliminary investigation step to determine the potential operating conditions (in terms of cooling rate and mixing speed) more proper for emulsion preparation. The influence of fat crystallisation on the viscoelastic characteristics of SO is reported in Fig. 1, where downward temperature sweep tests in oscillatory shear are presented for two different cooling rates (1 and 5 °C). A liquid-like behaviour, confirmed by loss tangent values greater than one, is noticed at temperatures above 42 °C (critical temperature for fat crystallisation onset) and, afterwards, a solid-like behaviour, evidenced by a sharp decrease in the loss tangent values, is observed, which also corresponds to the temperature at which a relevant increase in the G⁄ modulus was encountered (Ojijo et al., 2004). On the other hand, the experimental data obtained demonstrate that the complex modulus, below the threshold temperature for crystallisation, increases with the cooling rate, whereas the loss tangent decreases, evidencing that a faster ramp yields a more solid-like material. These results are in agreement with previously reported data, and can be attributed to the fact that slower cooling rates yield more disordered and less dense crystal networks (Wiking et al., 2009), and, consequently, a final product with smaller elastic modulus values (Fessas et al., 2005; Pérez-Martínez et al., 2007). As revealed by previous works on fats crystallisation (Campos et al., 2002; Wiking et al., 2009), the microstructure of rapidly crystallised fats shows a granular morphology composed of a large number of small crystals with a larger surface area. This favours the development of a more rigid network yielding a larger number of interactions and, consequently, much higher oil phase consistency. This is related to the fact that, when slow cooling rates are used, nucleation occurs at higher temperature and with slower rates than those observed with faster thermal ramps (McClements, 1999; Wiking et al., 2009). As a consequence, few nuclei, which can grow in larger crystals, are formed in these conditions. The viscous flow behaviour, below the critical temperature for fat crystallisation, is also affected by cooling rate. As can be observed in Fig. 2, where downward temperature sweep tests, at 10 s1, are shown for the two different cooling rates previously mentioned, viscosity values are larger for the sample cooled at 5 °C/min. In the high temperature region (from 70 to 43 °C), sample SO is in the molten state and the viscosity increases due only to kinetic effects. This behaviour may be described by an activated mechanism derived from the Arrhenius law: Ea

gðT; 10 s1 Þ ¼ gr ekT

ð1Þ

where g is the viscosity of the oil at a given temperature T, Ea the activation energy, k the Boltzmann constant, and gr a constant (Tarabukina et al., 2009). The activation energy computed for the present system (Ea = 4.22  1020 ± 1  1021 J/mol) is in good agreement

Fig. 1. Temperature sweep in oscillatory shear (at 1 Hz) of sample SO (cooling rate 1 °C/min (full symbols) and 5 °C/min (open symbols); complex modulus (circles) and loss tangent (diamonds)).

Fig. 2. Evolution of sample SO viscosity (at 10 s1) with temperature (cooling rate 1 °C/min (full symbols) and 5 °C/min (open symbols)).

with the literature values for a different vegetable oil (5.1  1020 J/mol for palm oil) (Tarabukina et al., 2009). Below this ‘‘molten state’’ region, a huge increase in viscosity is encountered for both cooling rates selected and, at low temperature conditions (T < 30 °C), a crossover between both curves is noticed, followed by a larger increase in viscosity for the fastest cooling rate curve as temperature decreases further. It is worth remarking that the crystallisation temperature obtained is the same for both types (viscous and oscillatory shear) of tests (approximately 43 °C), suggesting that the onset of crystallisation is not affected by the kinematic conditions adopted. In order to verify that crystallisation phenomena are strongly related to the Myverol presence, a sample prepared without the emulsifier was analysed and no crystallisation was observed in the considered temperature range (see Fig. 3); the sample exhibited only an almost linear increase in viscosity, between 70 and 20 °C, owing to kinetic effects. These data confirm that oil crystallisation is caused only by Myverol, which promotes the formation of a fat crystal network at relatively high temperature, as compared with pure fat crystallisation temperature. These experimental results are in good agreement with data reported in the literature. Thus, Pérez-Martínez et al. (2007) found that, for different cocoa butter–vegetable oil blends, crystallisation temperatures were a function of oil nature and being always lower than 20 °C (onset temperature for pure cocoa butter). On the other hand, Ojijo et al. (2004), investigating the rheological properties of olive oil– monoglyceride systems, reported that relevant crystallisation phenomena, yielding a sharp increase in G0 during cooling, were found only when monoglyceride (MAG) volume fraction was higher than

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Fig. 3. Evolution of sample SO viscosity (at 10 s1), with (open symbols) and without (full symbols) Myverol with temperature (cooling rate 5 °C/min).

Fig. 4. Evolution of sample SO viscosity with temperature (cooling rate 5 °C/min), as a function of shear rate.

0.013 and the onset temperature ranged between 35 and 50 °C, depending on MAG concentration. With the aim of achieving an understanding of the effects of shear during the crystallisation process, temperature sweep tests (at a cooling rate of 5 °C/min) were carried out at different constant shear rates. As can be observed in Fig. 4, different regions (delimited by broken lines) are noticed, whose limits are determined by temperature and shear rate, denoting relevant microstructural modifications, more evident at high shear rates. At high temperature (above 43 °C), low viscosity values, independent of the applied shear rate, are observed. When temperature is decreased down to, approximately, 43 °C, the onset of the first crystallisation stage (To, independent of shear rate) occurs, as shown by a sharp change in viscosity (region ‘‘A’’). Further reductions in temperature evidence two additional changes in the slope of viscosity vs. temperature (delimiting the ‘‘B’’ and ‘‘C’’ regions). Finally, a plateau region ‘‘D’’ is noticed. The viscosity values obtained in these regions are always shear-rate-dependent.

A similar behaviour was described by Tarabukina et al. (2009) for palm oil, cooled down to 10 °C under constant shear rate and hold at isothermal conditions for a further period. The results obtained were attributed to sample microstructural changes due to different crystallisation phenomena. Thus, at the onset of primary crystallisation (To), crystals (a form) and spherulites are formed, whose size and number increase progressively during cooling (region ‘‘A’’). This occurs even though high shear rates seem to hinder crystal aggregation, yielding a reduction in the slope of viscosity vs. temperature (region ‘‘B’’). During cooling, crystallisation proceeds through a nucleation and successive a ? b0 polymorphic transformations. Shear has no effect on the onset of a crystallisation (1st crystallisation), but affects the polymorphic transition (2nd crystallisation) because at low shear rates, aggregates of a nuclei have time to be formed, delaying a ? b0 transformation (Mazzanti et al., 2005; Tarabukina et al., 2009). On the contrary, at larger shear rates, a nuclei remain as individual crystals, which can more easily be transformed to b0 crystals. This can explain why region ‘‘C’’, which corresponds to the second crystallisation, appears sooner when shear rate is increased (Mazzanti et al., 2005; Tarabukina et al., 2009) and suggest that the amount of b0 crystals is larger in final samples cooled under high shear rate. Finally, crystal aggregates have a slower movement due to collaborative motion, and, after their growth is accomplished, all the aggregates can connect together and form a network, having different structures depending on the nature of the present crystal form (either a or b0 ) and on the shear rate that can induce destructuration phenomena. According to Himawan et al. (2006), different polymorphs exhibit various morphologies: the a-form produces a mass of very tiny crystals while the b0 -form is generally a bulky shape or spherulitic. Microphotographs taken at 1 and 100 s1 evidence, at low shear rates (Fig. 5A), the formation of many small crystals and aggregates, confirming the presence of a larger fraction of a crystals, which are the interconnecting unities for a dense crystalline network, having, as a consequence, a great final viscosity (Servais et al., 2002). The sample at high shear rate (Fig. 5B) is characterised by the presence of a small number of bigger and less polydisperse aggregates, based on spherulitic b0 crystals, poorly interacting with each other. As a consequence a lower viscosity is observed in these conditions.

3.2. Emulsion dynamic data analysis The structured oil phase can be considered as a three-dimensional network formed by the assembly of organogelator molecules (Myverol in present work) and fat crystals. During emulsification, an ‘‘emulsion gel’’ is obtained, where the structured oil phase entraps the water drops, yielding a stable water-in-oil emulsion similar to other organogel products reported in the literature (Marangoni, 2009).

Fig. 5. Optical micrographs for SO, taken after submitting the sample to steady-state shear, at 1 s1 (A) and 100 s1 (B). Reference bars correspond to 50 lm.

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Table 3 Weak-gel model parameters for the commercial and model emulsions studied. Sample

A (Pa s1/z)

z ()

Flora Vallè E1 E2 E3 E4 E5 E6

52,200 ± 600 37,500 ± 400 1170 ± 20 2480 ± 40 3830 ± 50 3330 ± 30 20,830 ± 60 960,000 ± 10,000

12 ± 1 14 ± 2 6.0 ± 0.3 6.8 ± 0.5 7.8 ± 0.5 8.5 ± 0.5 11.2 ± 0.3 15 ± 2

These emulsions can be described as weakly structured systems, the consequence of the development of a three-dimensional network with rheological ‘‘units’’ connected by weak bonds. Many foods are characterised by a similar structure and, from a macroscopic point of view, they behave as solid under small deformations while they flow under large stresses owing to the weak bonds breakage (weak gel behaviour). From a rheological point of view, they exhibit a power law relaxation mechanism (Gabriele et al., 2001; Ng and McKinley, 2008) similar to that of the so called ‘‘critical gel’’, i.e. a material at the sol–gel transition (Winter and Chambon, 1986). Their rheological behaviour, in small amplitude oscillations, can be described, in a limited frequency range, by a power law relationship between the dynamic complex modulus, G⁄, and the oscillation frequency, x:

G ðT; xÞ ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ðG0 Þ2 þ ðG00 Þ2 ¼ AðTÞ  xzðTÞ

Fig. 6. Frequency sweep in oscillatory shear, at 20 °C, for sample E1 (storage modulus (d), loss modulus (s) and loss tangent (D)).

ð2Þ

where T is the temperature, z the network extension, related to the number of interacting rheological units within the 3-D network, and A is the strength of the interactions. When A increases, the interaction forces within the network increase, whereas a high z value indicates a large number of interacting units cooperating and increasing the network connectivity. It is worth recalling that the rheological behaviour of foods, as weakly structured materials, is described by this power law model in a limited frequency window, usually ranging between 0.1 and 100 Hz, whereas for lower frequencies different relaxation mechanisms should be considered (Gabriele et al., 2001). 3.3. Emulsion characterisation In order to define a suitable range of rheological functions values for model emulsions, a rheological characterisation of commercial margarines was performed. Both commercial samples show an almost linear trend, in a log–log plot, of the dynamic moduli vs. frequency (data not shown) in the frequency range tested, showing a predominant elastic behaviour (the elastic modulus being always greater than the loss modulus by almost an order of magnitude). Experimental data were analysed by fitting the complex modulus with the ‘‘weak gel’’ model (see Fig. 9) by using a commercial software (Table Curve 4, Jandel Scientific, USA), and obtained parameters are reported in Table 3. As can be observed, z values are quite similar for both samples, whereas, a slightly higher value of A was obtained for sample ‘‘Flora’’. The electrical conductivity analysis, carried out on samples E1–E6, did not show any significant conductivity value confirming that oil was the dispersing phase. The frequency dependence of the dynamic moduli for sample E1 is shown in Fig. 6, evidencing a predominant elastic behaviour. A similar trend was found for all the emulsions tested. Accordingly, the experimental data were always analysed by using the ‘‘weak gel’’ model (Table 3); data reported in Figs. 7 and 9a, in terms of complex modulus, for samples E1–E4 and samples E5–E6, respectively, show that the model adequately describes the experimental

Fig. 7. Frequency dependence of the complex modulus at 20 °C, for sample E1 (d), E2 (s), E3 (D), E4 (). Symbols are experimental data, lines represent the weak gel model fitting.

data, even though at the lowest frequencies a slight deviation from the power law can be noticed for samples E1 and E2. Both ‘‘weak gel’’ model parameters, for E1 and E2, are lower than the reference values obtained for commercial margarines. However, it seems that homogenisation at low temperature is more suitable than the high temperature procedure to yield more consistent emulsions. Probably the fat crystal network, formed during oil phase cooling, is able to entrap and stabilise the water drops, whereas mixing at high temperature and consequent cooling could hinder network formation, yielding a non-stable emulsion and requiring a further processing step. According to the preliminary oil phase characterisation, the crystal network formation is promoted by high cooling rates and low shear rates. Therefore, E3 and E4 were prepared by quenching the oil phase and by using, respectively, static conditions or a gently mixing. If the ‘‘weak gel’’ parameters for these samples are compared with those for E2, a relevant increase in network strength is observed (Table 3), a consequence of the presence of a more structured fat crystal network interacting with water droplets. Moreover, it is known that smaller crystals (formed during fast cooling) are likely to provide better coverage on the droplets surface than larger crystals, giving more stable Pickering emulsions (Rousseau, 2000). Mild mixing conditions (E3) improve emulsion consistency in comparison with static conditions, probably because they yield a more uniform temperature profile (and therefore crystallisation phenomena) inside the oil phase during cooling. Moreover, mixing

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Fig. 8. Evolution of viscosity with shear rate, at 20 °C, for sample E1 (d), E2 (s), E3 (D), E4 (). Symbols are experimental data, lines represent the Sisko model fitting.

Table 4 Sisko model parameters for some of the model emulsions studied (E1–E4). Sample

g1 (Pa s)

k (Pa sn)

n ()

E1 E2 E3 E4

0.26 ± 0.01 0.26 ± 0.01 0.28 ± 0.01 0.27 ± 0.01

8.0 ± 0.5 16 ± 2 36 ± 2 31.4 ± 0.8

0.15 ± 0.02 0.08 ± 0.02 0.040 ± 0.007 0.045 ± 0.009

allows a better dispersion of the produced nuclei, favouring the nucleation step and enhancing a ? b0 transformation that results in a more consistent system (De Graef et al., 2009). The flow properties of the samples E1E4 were also tested and a linear trend, in a log–log plot, was obtained, up to high shear rate values, where a Newtonian plateau was reached (Fig. 8). Thus, the Sisko equation, used in other works to model the flow behaviour of emulsions (Barnes et al., 1989), was considered a suitable model for data-fitting:

gðc_ Þ ¼ g1 þ kc_ n1

Fig. 9. Frequency dependence of the complex modulus (A) and loss tangent (B), at 20 °C, for different model (E5 (s) and E6 (d)) and commercial emulsions (Flora (D) and Vallè (})). Symbols are experimental data, solid lines represent the weak gel model fitting for complex modulus (A), dotted lines represent only a guide for eyes for loss tangent (B).

ð3Þ

Parameters k and n are consistency and flow indices, respectively, whereas g1 is the asymptotic viscosity value at high shear rates. The parameter k can be considered as an index of the material ‘‘consistency’’, whereas n, always positive and lower than one, can be considered a measure of the structural breakdown rate. From a physical point of view, the lower the n value the higher is the curve slope, meaning a sharper decrease in viscosity when increasing shear rate (greater tendency to in-flow destructuration). Sisko’s model parameters (Table 4) were computed by fitting experimental data (Table Curve 4, Jandel Scientific, USA), and a good agreement was observed (see Fig. 8). As it can be seen, the highshear-rate-limiting viscosity, which is related to the complete breakdown of the network (McClements, 1999), is quite similar for all the samples studied. On the other hand, an increase in the consistency index, k, was observed for samples E1, E2 and E3, whereas sample E4 showed a lower k value if compared with E3. Therefore, the flow behaviour confirms the results obtained in oscillatory shear. The flow index values, n, slightly decrease, evidencing a different dependence of the emulsion network on shear flow conditions. The homogenisation procedure used for E3 was selected to manufacture emulsions having a higher cocoa butter concentration, aiming at increasing the solid fat content and, therefore, emulsion consistency. It can be seen that the parameter related to the strength of the network, A, increases several orders of magnitude by increasing cocoa butter concentration in emulsion from 2.3% (E4) up to 10% (E5) or 20% (E6), whilst the change in z is less relevant (approximately from 8 to 15). This behaviour can be attributed to the higher saturated fat content, which should yield a denser and more structured 3D network entrapping the water drops.

Finally, Fig. 9A and B show the frequency dependence of the complex modulus and loss tangent, respectively, for E5 and E6 as well as for the commercial margarines studied. It is worth noting that the commercial products exhibit viscoelastic properties, at 20 °C, inside the range defined by the two last model emulsions studied in this work (E5 and E6). As a consequence, it seems that similar rheological properties to those shown by commercial shortenings can be obtained by properly adjusting cocoa butter concentration in the oil phase, depending on the required product. Even though obtained rheological properties seem suitable for commercial applications, it is worth recalling that customer acceptability of new food products is mainly related to the ‘‘texture’’, usually defined as a complex combination of sensorial properties perceived during food consumption (Bourne, 1982). Texture derives from the structure of the food and, therefore, it falls, partly, within the field of rheology and partly outside this field (Bourne, 1982). As a consequence, issues such as the taste or the appearance should be considered, before commercial production, to assess properly the global potential customer satisfaction. Moreover chemical stability issues, related to potential chemical reactions involving unsaturated fatty acids should be analysed, whereas physical stability (i.e. resistance to oil–water separation) should be properly guaranteed by the dispersing phase high viscosity (Marangoni, 2009).

4. Conclusions In this work, the manufacture of structured W/O emulsions based on high-oleic acid oil (olive oil), suitable for solid fat replacement, is investigated by performing different rheological tests.

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The oil phase was structured by using a commercial emulsifier (Myverol) to promote oil gelation, and by adding cocoa butter (rich in stearic acid) to increase the saturated fat content. The characterisation of the oil phase evidenced the relevant role played by Myverol to obtain a stable network structure and, also, it was useful to identify the optimal operating conditions (i.e. cooling and shear rate) necessary to enhance oil structuration. It was found that high cooling rates and low shear conditions, which favour the production of a large amount of small fat crystals, yield a strong and extended crystalline network able to entrap and stabilise the water droplets during the emulsification. Emulsions based on a structured oil phase, containing between 10% and 20% (w/w) cocoa butter, were compared to some commercial products. Model emulsions showed rheological properties suitable for their potential use as ‘‘solid fats’’ even though other textural issues (such as taste and appearance) should be investigated before the commercial applications. Finally, the dramatic influence of cocoa butter concentration on emulsion rheological properties and, consequently, on the desired final texture was proved. References Barnes, H.A., Hutton, J.F., Walters, K., 1989. An Introduction to Rheology, first ed. Elsevier Science Publishers B.V., Amsterdam. Blanco Muñoz, M.A., 2004. Olive oil in food spreads. Grasas y Aceites 55, 92–94. Borwankar, R.P., Frye, L.A., Blaurock, A.E., Sasevich, F.J., 1992. Rheological characterization of melting of margarines and tablespreads. Journal of Food Engineering 16, 55–74. Bourne, M.C., 1982. Food Texture and Viscosity, first ed. Academic Press, New York (Chapter 1). Campos, R., Narine, S.S., Marangoni, A.G., 2002. Effect of cooling rate on the structure and mechanical properties of milk fat and lard. Food Research International 35, 971–981. Clogston, J., Rathman, J., Tomasko, D., Walker, H., Caffrey, M., 2000. Phase behaviour of a monoacylglycerol (Myverol 18–99 K)/water system. Chemistry and Physics of Lipids 107, 191–220. Constantinides, P.P., Yiv, S.H., 1994. Particle size determination of phase-inverted water-in-oil microemulsions under different dilution and storage conditions. International Journal of Pharmaceutics 115, 225–234. Coupland, J.N., 2002. Crystallization in emulsions. Current Opinion in Colloid and Interface Science 7, 445–450. De Graef, V., Van Puyvelde, P., Goderis, B., Dewettinck, K., 2009. Influence of shear flow on polymorphic behavior and microstructural development during palm oil crystallization. European Journal of Lipid Science and Technology 111, 290– 302. Eicke, H.-F., Borkovec, M., Das-Gupta, B., 1989. Conductivity of water-in-oil microemulsions: a quantitative charge fluctuation model. Journal of Physical Chemistry A 93, 314–317. Fessas, D., Signorelli, M., Schiraldi, A., 2005. Polymorphous transitions in cocoa butter a quantitative DSC study. Journal of Thermal Analysis and Calorimetry 82, 691–702. Friberg, S.E., 1997. Emulsion stability. In: Friberg, S.E., Larsson, K. (Eds.), Food Emulsions, third ed. Marcel Dekker Inc., New York. Gabriele, D., de Cindio, B., D’Antona, P., 2001. A weak gel model for foods. Rheologica Acta 40, 120–127. Gabriele, D., Migliori, M., Di Sanzo, R., Oliviero Rossi, C., Ruffolo, S.A., de Cindio, B., 2009. Characterisation of dairy emulsions by NMR and rheological techniques. Food Hydrocolloids 23, 619–628. Garti, N., Binyamin, H., Aserin, A., 1998. Stabilization of water-in-oil emulsions by submicrocrystalline a-form fat particles. Journal of American Oil Chemists’ Society 75, 1825–1831. Ghotra, B.S., Dyal, S.D., Narine, S.S., 2002. Lipid shortenings: a review. Food Research International 35, 1015–1048. Higaki, K., Sasakura, Y., Koyano, T., Hachiya, I., Sato, K., 2003. Physical analyses of gel-like behavior of binary mixtures of high- and low-melting fats. Journal of the American Oil Chemists’ Society 80, 263–270. Himawan, C., Starov, V.M., Stapley, A.G.F., 2006. Thermodynamic and kinetic aspects of fat crystallization. Advances in Colloid and Interface Science 122, 3–33. Hindle, S., Povey, M.J.W., Smith, K., 2000. Kinetics of crystallization in n-hexadecane and cocoa butter oil-in-water emulsions accounting for droplet collisionmediated nucleation. Journal of Colloid and Interface Science 232, 370–380. ICAM, 2008. Technical Bulletin, 42/001A. Lecco, Italy.

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