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Effect of κ-carrageenan on rheological properties, microstructure, texture and oxidative stability of water-in-oil spreads
LWT - Food Science and Technology 43 (2010) 843–848
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Effect of k-carrageenan on rheological properties, microstructure, texture and oxidative stability of water-in-oil spreads Raluca I. Alexa a, b, John S. Mounsey a, *, Brendan T. O’Kennedy a, Jean C. Jacquier b a b
Teagasc, Moorepark Food Research Centre, Fermoy, Co. Cork, Ireland School of Agriculture, Food Science & Veterinary Medicine, Agriculture & Food Science Centre, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2009 Received in revised form 24 September 2009 Accepted 8 October 2009
The effect of k-carrageenan concentration (0-7.5 g kg1) on the rheology, microstructure, texture and oxidative stability of water-in-oil (W/O) spreads (600 g fat kg1 emulsion) was examined over 60 days storage time. Results showed that increasing the k-carrageenan concentration to 7.5 g kg1 significantly increased the viscosity of the aqueous phase (to 42.7 mPa s at 60 C) resulting in gelation of the aqueous phase on cooling. The microstructure of the spreads was disrupted by higher levels of k-carrageenan, resulting in a less homogeneous distribution of the aqueous phase. Melt temperature (where tan d > 1) decreased significantly from 62 to 56.2 C with increasing k-carrageenan concentration from 0 to 7.5 g kg1. The firmness and the G0 at 6 C for all samples were significantly increased after 60 days storage with only small effects due to k-carrageenan levels. Oxidation of the fat phase was evident by the significant increases in peroxide values of all spreads on storage, with k-carrageenan exhibiting no antioxidant behaviour. While increased k-carrageenan levels modified the microstructure of W/O spreads in terms of the droplet size of the aqueous phase and its distribution few changes were evident in the continuous fat phase. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Water-in-oil spreads k-Carrageenan Rheology Microstructure Oxidation
1. Introduction Spreadable fats are emulsions of the W/O type and were introduced as an economical, functional and low calorie alternative of butter (Caponio & Gomes, 2004; Laia, Ghazalia, Cho & Chong, 2000). Replacement of the fat with water alters the rheological properties and structural characteristics of spreads, which are mainly given by the shape and the size of the fat crystals (Kasapis, 2000). A number of biopolymers are used in low-fat formulations as fat mimics and as stabilisers of the aqueous phase (Chronakis & Kasapis, 1995a; Chronakis, 1997) through network stabilisation as well as stabilisation via interfacial action (Benichou, Aserin & Garti, 2002; Dickinson, 2003). Polysaccharides such as k-carrageenan have been shown to enhance the sensorial properties of reduced-fat spreads (Clegg, Moore & Jones, 1996). k-Carrageenan has the capacity to gel on cooling through a disordered-ordered transition forming intermolecular double helices and subsequent aggregation and gelation under specific conditions (Oakenfull & Scott, 1990; Heyraud, Rinaudo, & Rochas, 1990). In a previous study by Mounsey, Stathopoulos, Chockchaisawasdee, O’Kennedy, Gee and Doyle (2008)
on the fortification of W/O spreads containing k-carrageenan, authors found that the increase in the gel strength of the aqueous phase upon addition of transition metals altered the microstructure of the W/O spreads. The purpose of the present study was to assess the effect of k-carrageenan on the rheology of the aqueous phase and on the microstructure, texture, and rheology of the experimental W/O spreads. Studies with reference to the antioxidant activities of sulphated polysaccharides extracted from brown and red seaweeds have been published in the literature (Sirendi, Gohtani, & Yamano, 1998; de Souza, Marques, Dore, da Silva, Rocha, & Leite, 2007; Wang, Liu, Zhang, Zhang, Qi, & Li, 2009). In-vitro studies showed that sulphated polysaccharides presented activity in inhibiting free radicals. The degree of sulphation was found to be directly related to the radical scavenging activity, with k-carrageenan exhibiting mild antioxidant properties (de Souza et al., 2007). The effect of k-carrageenan on the oxidative degradation of the W/O spreads was also investigated in the present work. 2. Materials and methods 2.1. Materials
* Corresponding author. Tel.: þ353 2542443. E-mail address: [email protected] (J.S. Mounsey). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.10.003
A commercial source of k-carrageenan (Grindsted Carrageenan CL 107, Danisco, Denmark) was used without further purification.
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The vegetable oil blend was obtained from DairyGold (Mitchelstown, Ireland). The fatty acid composition of the oil blend was 480 g kg1 monounsaturated, 320 g kg1 saturated, 190 g kg1 polyunsaturated and 10 g kg1 trans fatty acids. The monoglyceride emulsifier (Paalsgaard 0291) was supplied by Grinstead A/S, Braband, Denmark. All other solvents and chemicals used were of analytical grade and purchased from Sigma–Aldrich, St. Louis, MO 63103, USA. Deionised water was used for the aqueous phase. 2.2. Manufacture of W/O spreads Water-in-oil spreads (batches of 10 kg) were prepared in duplicate following margarine production technology using a Perfector scraped surface heat exchanger (Gerstenberg and Agger, Copenhagen, Denmark) as described previously (Mounsey et al., 2008). A control spread was prepared by mixing an aqueous phase (water; 400 g kg1 of the final product) and the fat (or oil) phase contained the oil blend and the emulsifier (600 g kg1 of the final product) at approximately 55 C. The preliminary mixing of the oil and aqueous phase was performed for 1 min at 1500 rpm using a Silverson mixer (model AX3, Silverson Machines Ltd., Waterside, Chesham, Bucks, UK). The formed W/O emulsion was transferred to the jacketed tank connected to the Perfector and pasteurized at 75 C for 15–20 s and immediately cooled down to 65 C. The emulsion (w55 C) was pumped through two scraped-surface coolers (at 432 rpm) bringing the temperature to 12 C before the spread was filled into 454 g plastic tubs and stored at 4.5 C prior to testing. The spreads containing k-carrageenan (1.56, 3.12, 6.25, 12.5 or 18.8 g L1 in the aqueous phase) were produced in order to give a final k-carrageenan concentration of 0.625, 1.25, 2.5, 5, and 7.5 g kg1 reported to the total quantity. 2.3. Rheology of the aqueous phase Small-scale deformation measurements were carried out on the aqueous phase of the spreads using cup and bob systems in an AR 2000 Rheometer (TA Instruments, UK). All measurements were made at a frequency of 1 Hz and a maximum strain of 0.500%. Parameters such as the storage modulus (G0 ), loss modulus (G00 ) and tan d (G0 /G0 ) were monitored during cooling from 60 C to 6 C at a rate of 1 C min1 using a Peltier heating element, followed by reheating to 60 C at the same ramp rate. The samples (15 g) were loaded at 60 C and covered with n-Tetradecane (Sigma Chemica, Co., St. Louis, MO, USA) to minimise evaporation. Viscosity of the aqueous phase of the spreads was measured using the same geometry of the AR 2000 Rheometer as above. A shear rate sweep from 0.1 to 500 s1 was applied for 5 min. The apparent viscosity (mPa s) was taken at 60 C and a shear rate of 100 s1. 2.4. Rheology of W/O spreads A controlled strain AR-2000 rheometer (TA Instruments, New Castle, Delaware) was used in the dynamic mode for small-scale deformation measurements. Disc-shaped samples of spread (25 mm diameter, 2.5 mm in thickness) were prepared using a 25 mm diameter cork borer. A 25 mm diameter serrated parallel plate geometry was used with a serrated lower plate. Samples were placed on the lower plate and compressed with a normal force of 0.5 N to prevent slippage, with 3 min for temperature equilibration and stress relaxation prior to testing. Measurements were taken at a frequency of 1 Hz and a strain of 0.2 %. Samples were loaded at 6 C before n-Tetradecane (Sigma Chemical Co., St. Louis, MO, USA) was added to the side in order to avoid evaporation. The change in G0 , G00 and tan d were measured during
heating from 6 to 60 C at a rate of 1 C min1. Tests were carried out in triplicate. 2.5. Texture Profile Analysis (TPA) Compression tests were performed using a TA-XT2 Texture Analyser from Stable Microsystems (UK). Cylinders of spreads of 25 25 mm were cut and allowed to equilibrate at 4.5 C for 4 h. Samples were compressed to 50% of their initial height (12.5 mm) at a crosshead speed of 1 mm s1 using a 5 kg load cell and a 75 mm diameter plate. Six samples were analysed from each spread tub. The hardness values, expressed in Newtons (N) were measured at the point of fracture. 2.6. Scanning Electron Microscopy (SEM) Samples were prepared for cryo-SEM by mounting them into copper rivets and plunged into nitrogen slush (207 C). Samples were then transferred under vacuum into the preparation chamber, freeze fractured with a cold blade, etched at 88 C for 5 min and then sputtered coated with gold (10 mA for 60 s). Samples were then transferred under vacuum onto the cold stage which was maintained at 125 C and imaged using FE-SEM (Zeiss Supra Gemini, Darmstadt, Germany). Images were acquired at 2.00 kV with 2000–20,000 magnification. 2.7. Determination of the peroxide values (PV) The PV test of melted fat (mEq O2.kg1) was derived from the International Dairy Federation (IDF) Standard 74:1974. The W/O spread sample was melted at 62 C and 0.1 mL of melted fat was dissolved into 10 mL of chloroform/methanol (70:30) mixture, followed by addition of ammonium thiocyanate and ferrous chloride, respectively. Using glass stoppers, the tubes were inverted and placed in dark cupboard for 10 min. Simultaneously, a blank test with only reagents and no sample was carried out. The absorbance of the samples was read at 505 nm on a Varian Cary Scan 1 instrument. After calibration, the blank value was subtracted from the sample values (1) and the PVs were calculated. OD [ Abssample L Absstandard
(1)
where OD is the optical density. Samples were analysed after 1, 5, 7, 15, and 31 days of storage. 2.8. Statistical analysis The rheology, texture and oxidative stability data of W/O spreads containing k-carrageenan were statistically tested by analysis of variance (ANOVA) using SigmaStat (version 3.0; Jandel Scientific, Corte Madera, CA, USA). Differences among treatments were determined by Student–Newman–Keuls pairwise-comparison test. Treatment means were considered significantly different at *P < 0.05. 3. Results and discussion 3.1. Rheology of the aqueous phase of W/O spreads 3.1.1. Viscosity at 60 C The effect of k-carrageenan concentration on the apparent viscosity of the aqueous phase of W/O spreads is presented in Fig. 1. The sample containing 1.56 g kg1 k-carrageenan (in the aqueous phase) had a low viscosity (4.31 mPa s) whereas the apparent
R.I. Alexa et al. / LWT - Food Science and Technology 43 (2010) 843–848 10000
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G' at 6°C (Pa)
Apparent viscosity at 60°C (mPa.s)
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viscosity increased significantly (***P < 0.001) to 42.7 mPa s for sample containing 18.8 g kg1 k-carrageenan. At low concentrations the coils of k-carrageenan are free to move in the solution while the apparent viscosity proportionally increases with the concentration (Morris, 1984; Mounsey et al., 2008). Nonetheless, kcarrageenan remained on a non-gelled state at 60 C, even at concentrations as high as 18.8 g kg1.
3.1.2. Gelling properties at 6 C The effect of increasing k-carrageenan concentration on the gelation properties of the aqueous phase of the W/O spreads was monitored on cooling from 60 to 6 C. The G0 values of samples at 6 C are shown in Fig. 1. The aqueous phases containing 3.12 and 6.25 g kg1 k-carrageenan showed no gelation on cooling. By increasing the concentration of k-carrageenan to 12.5 g kg1 the aqueous phase had a gelling temperature (Tg) of 23 C and a relatively weak gel was obtained at 6 C (53 Pa). Further increasing k-carrageenan concentration of the aqueous phase to 18.8 g kg1, Tg increased to 28.3 C and a strong gel of 1.49 kPa was obtained at 6 C. 3.2. Small scale deformation of W/O spreads 3.2.1. Rheology at 6 C Small-scale rheological assessment was performed in order to investigate the structure of the experimental W/O spreads under non-destructive conditions. The effect of k-carrageenan concentration as well as the length of storage on the elastic modulus (G0 ) of the W/O spread is presented in Fig. 2. Statistical interpretation showed that the G0 values did not change significantly (*P > 0.05) with increased levels of k-carrageenan. On the other hand, the G0 of the control at 6 C had substantially greater values (*P < 0.05) after 60 days of storage, as well as the G0 of the k-carrageenan containing samples. The increase in the G0 over time may be due to the increase in the amount of crystallinity in the oil phase, thus increasing the solidness of the spreads at 6 C. Borwankar, Frye, Blaurock & Sasevich (1992) found that the rheology of W/O spreads is strongly associated with the degree of fat crystallisation. An additional rheological parameter used to describe the viscoelastic profile of a material is the tangent of the phase angle (tan d (¼G00 /G0 )). The effect of k-carrageenan on tan d (measured as a function of storage is presented in Fig. 3). The tan d values of the W/O spreads showed little change with the addition of k-carrageenan (*P > 0.05), but increased sharply with the storage. The tan d values of the control significantly increased (*P < 0.05) during
0.62
1.25 2.5 κ-carrageenan (g.kg-1)
5
7.5
Fig. 2. Effect of k-carrageenan concentration on the G0 (at 6 C) of W/O spreads as a function of storage time after 2 ( ), 15 ( ), and 60 ( ) days. Letters a, b, c represent significant differences within treatment means between 2, 15 and 60 days. Letters x, y, z represent significant differences between treatment means after 2 days. Means with the same letter do not differ significantly at *P < 0.05.
storage time, indicating a more plastic behaviour of the spreads given by a large viscous constituent in the system (Chronakis & Kasapis, 1995b). 3.3. Rheology on heating The melt temperature – where tan d > 1 or the temperature at which the total solid fat content is zero (Himawan, Starov & Stapley, 2006) of all samples increased over time (Table 1), possibly due to modification in the structure of fat crystals (a, b0 , and b polymorph types) found in the network. It was shown by Ojijo et al. (2004) using polarized light microscopy that the fat crystal network of an olive oil/monoglyceride mixture suffered a progressive growth in crystal clusters during storage, due to a more compact geometry of hydrocarbon chains. The melting temperature of the spreads significantly decreased (*P < 0.05) with k-carrageenan concentration from 62.4 C (control) to 55.9 C (sample containing 7.5 g kg1 k-carrageenan). This may be probably due to the variation in the emulsion properties, with k-carrageenan having a destabilisation effect. After 60 days of storage the spreads did not melt in the range of temperature examined. The melting profile of the experimental W/O spreads was comparable to that observed by Borwankar et al. (1992) in a study on melting characterisation of margarine and table spreads containing gelatine, which gelled in the aqueous phase at low
c
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Fig. 1. The effect of k-carrageenan on the apparent viscosity of the aqueous phase of W/O spreads at 60 C and 100 s1 shear rate ( ) and the storage modulus (G0 ) at 6 C ( ). Note: 18.8 g kg1 in the aqueous phase corresponds to 7.5 g kg1 in the final spread. Each point on the curve represents the mean of triplicate trials. Vertical bars show standard deviation of the means.
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Fig. 3. Effect of k-carrageenan on tan d (at 6 C) of W/O spreads as a function of storage time after 2 ( ), 15 ( ), and 60 ( ) days. Letters a, b, c represent significant differences within treatment means between 2, 15 and 60 days. Letters x, y, z represent significant differences between treatment means after 2 days. Means with the same letter do not differ significantly at *P < 0.05.
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Table 1 The effect of k-carrageenan on melt temperature (temperature where tan d > 1) of W/O spreads as a function of storage time.
k-Carrageenan [g kg1] 0 0.625 1.25 2.5 5 7.5
Melt t 2 Days
15 Days
60 Days
56a 54.5ab 53.7ab 53.8ab 54.5ab 52.8bc
62.4a 62.3ab 58.8bc 55.9cd 56.3ce 55.9cf
– – – – – –
For each column, means with the same letter do not differ significantly at *P < 0.05.
temperatures and promoted meltability by destabilising the formed emulsion. 3.4. Texture analysis of W/O spreads The texture of plastic materials such as fat spreads is mainly given by the amount of solid fat (Deman & Beers, 1987; Bot & Vervoort, 2006). Thickening the aqueous phase using hydrocolloids may improve the spreadability and reduce the loss of moisture normally caused by the large amounts of water contained by these products (Bot & Vervoort, 2006). The effect of k-carrageenan on the textural stability of W/O spreads was investigated through large deformation techniques (compression) and presented in Fig. 5 The hardness at fracture (N) of the W/O spreads was measured during 60 days of storage and gave an indication of the sample resistance to deformation. Texture evaluation showed that the hardness of the W/O spreads increased over time. The hardness of the control sample (no k-carrageenan) had considerably increased (*P < 0.05) from 11.13 N to 13.20 N during storage. There was also observed an increase of hardness in time for samples containing k-carrageenan.
This could be possibly due to the slow post-crystallisation processes and development of bonds within the fat crystal network that took place during storage, which resulted in a more solid structure and therefore, more force was required to fracture. Both, force at fracture and G0 of the W/O spreads increased in time. A relationship between the storage modulus of a fat system and its hardness was indicated by Narine and Marangoni (1999) who suggested that the G0 varies with the solid fat content of the network. Generally, the hardness of spreads was inconsistent with increasing k-carrageenan levels. High k-carrageenan concentrations (5-7.5 g kg1) had a slight weakening effect, probably due to the increase in the aqueous droplet sizes (as shown by SEM), making the spreads more compressible than the samples containing smaller aqueous droplets (control). Although addition of k-carrageenan enhanced the gel strength of the aqueous phase, the hardness of spreads was mainly given by the fat crystallisation. de Bruijne and Bot (1999) suggested that the properties of W/O spreads with reduced level of fat (600 g kg1) are not considerably affected by the aqueous phase. Keogh (2006) and Mounsey et al. (2008) also found that the texture of W/O spreads was less affected by the viscosity of the aqueous phase. Furthermore, while the W/O spreads became rancid with storage, the change in the chemical composition of the fatty acids did not seem to have a clear effect on the texture of spreads. 3.5. Microstructure of W/O spreads In our study, SEM was employed to visualise the microstructure of W/O spreads in terms of water droplet size distribution. The control sample (Fig. 4a, b.) presented a well emulsified and homogeneous structure, with many globular water droplets evenly distributed within the continuous fat structure. Samples containing k-carrageenan presented comparable microstructure, although
Fig. 4. Scanning electron images of control W/O spread at (a) low magnification, (b) high magnification and sample containing 7.5 g kg1 k-carrageenan at (c) low magnification and (d) high magnification.
R.I. Alexa et al. / LWT - Food Science and Technology 43 (2010) 843–848
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Fig. 5. Effect of k-carrageenan on the hardness of W/O spreads at time 6 ( ), 15 ( ), 30 ( ), and 60 ( ) days of storage at 4.5 C. Each bar represents the mean of duplicate trials. Vertical bars show standard deviation of the means. Letters a, b, c represent significant differences within treatment means between 6, 15, 30, and 60 days. Letters v, w, x, y represent significant differences between treatment means at day 6. Means with the same letter do not differ significantly at *P < 0.05.
addition of 7.5 g kg1 k-carrageenan in the overall product resulted in a less homogeneous aqueous phase distribution, with slightly larger and clustered aqueous phase droplets (Fig. 4d). k-Carrageenan increased the viscosity of the aqueous phase and most likely induced gelation of the droplets during spread manufacture, as previously shown by Mounsey et al. (2008). Similar results were obtained by Clegg et al. (1996) in spreads containing 40 g kg1 gelatine. 3.6. Oxidative stability of W/O spreads Lipid oxidation significantly affects the quality characteristics of spreadable fats due to formation of rancid flavours and secondary oxidised compounds which are detrimental for the health (Frankel, 1998). Chaiyasit, Elias, McClements & Decker (2007) showed that the concentration of peroxides is directly associated with the oxidative stability of fats. The effect of k-carrageenan on the oxidative stability of W/O spreads, as measured by the PV test, was monitored during 31 days and presented in Fig. 6. The PV is a measure of the hydroperoxide concentration in the early stages of oxidation (Kochhar, 2003). In all tests, PV increased in time. The PV of the control increased noticeably (*P < 0.05) from 0.72 to 2.33 mEq O2 kg1 during storage time, although the spreads were resistant against oxidation for 5 days. Samples containing levels of
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up to 2.5 g kg1 k-carrageenan were not significantly (*P > 0.05) more oxidised after 7 days of storage compared to the control. These results indicate that all samples became rancid during storage and the oxidation proceeded more rapidly in samples containing high levels of k-carrageenan. It was anticipated that k-carrageenan would have potential lipid antioxidation effect, such as other polysaccharides (Matsumura et al., 2003; de Souza et al., 2007). As k-carrageenan increased the viscosity of the aqueous phase, forming gel droplets upon cooling (Mounsey et al., 2008), it was hoped that the oxidation would be delayed by decelerating the activity of reactants. Basaran, Coupland and McClements (1999) showed that the viscosity of the aqueous phase had no effect in delaying the motion of small molecules through the polysaccharide gel network. Polysaccharides may have a possible antioxidant effect through a metal ion chelation mechanism or hydrogen donation (McClements & Decker, 2000). In our study, k-carrageenan did not retard the oxidation of W/O spreads. It is possible that the oxidation was promoted by the trace amounts of metallic cations contained by k-carrageenan source and implicit, higher levels present in samples prepared with increased concentration of k-carrageenan in the aqueous phase; or due to incorporation of air and commencement of oxidation of the oil-blend during emulsification process. 4. Conclusions This study showed that increased levels of k-carrageenan resulted in increased viscosity at 60 C as well as gelation on cooling of the aqueous phase of model W/O spreads. The textural hardness and dynamic rheological parameters (G0 and tan d at 6 C) were not significantly modified by increased k-carrageenan addition. A less homogeneous structure was associated to an improved meltability of the spreads containing increased levels of k-carrageenan. Nonetheless, k-carrageenan did not inhibit oxidation of the W/O spreads. k-Carrageenan has application in controlling the rheology of the aqueous phase of W/O spreads during spread formation, storage and subsequent re-heating. Acknowledgements This research has been funded by Department of Agriculture and Food under the Food Institutional Research Measure (National Development Plan). The authors thank J. Roche for his help in the preparation of W/O spreads. Vivian Gee of the National Food Imaging Centre is gratefully acknowledged for help with collection of the SEM images presented.
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Fig. 6. Effect of k-carrageenan on the oxidative stability of W/O spreads after 1 ( ) 5 ( ), 7 ( ), 15 ( ), and 31 ( ) days storage at 4.5 C. Each bar represents the mean of duplicate trials. Vertical bars show standard deviation of the means. Letters a, b, c, d, e represent significant differences within treatment means between 1, 5, 7, 15 and 31 days. Letters v, w, x, y represent significant differences between treatment means at day 1. Means with the same letter do not differ significantly at *P < 0.05.
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Matsumura, Y., Egami, M., Satake, C., Maeda, Y., Takahashi, T., Nakamura, A., et al. (2003). Inhibitory effects of peptide-bound polysaccharides on lipid oxidation in emulsions. Food Chemistry, 83, 107–119. McClements, D. J., & Decker, E. A. (2000). Lipid oxidation in oil-in-water emulsions: impact of molecular environment on chemical reactions in heterogeneous food systems. Journal of Food Science, 65(8), 1270–1281. Morris, E. R. (1984). Rheology of hydrocolloids. In G. O. Phillips, D. J. Wedlock, & P. A. Williams (Eds.), Gums and stabilisers for the food industry 2. Oxford: Pergamon Press (pp. 57–78). Mounsey, J. S., Stathopoulos, C. E., Chockchaisawasdee, S., O’Kennedy, B. T., Gee, V., & Doyle, J. (2008). Effect of zinc fortification on rheological properties and microstructure of water-in-oil spreads containing k-carrageenan. European Food Research and Technology, 227, 675–681. Narine, S. S., & Marangoni, A. G. (1999). Relating structure of fat crystal networks to mechanical properties: a review. Food Research International, 32, 227–248. Oakenfull, D., & Scott, A. (1990). The role of the cation in the gelation of kappacarrageenan. In G. O. Phillips, D. J. Wedlock, & P. A. Williams (Eds.), Gums and stabilisers for the food industry 5. Oxford: Oxford University Press (pp. 507–510). Ojijo, N. K. O., Kesselman, E., Shuster, V., Eichler, S., Eger, S., Neeman, I., et al. (2004). Changes in microstructural, thermal, and rheological properties of olive/monoglycerides networks during storage. Food Research International, 37, 385–393. Sirendi, M., Gohtani, S., & Yamano, Y. (1998). Effect of some polysaccharides on oxidative stability of methyl linoleate in emulsions. Journal of Dispersion Science and Technology, 19, 679–694. de Souza, M. C. R., Marques, C. T., Dore, C. M., da Silva, F. R. F., Rocha, H. A. O., & Leite, E. L. (2007). Antioxidant activities of sulfated polysaccharides from brown and red seaweeds. Journal of Applied Phycology, 19, 153–160. Wang, J., Liu, L., Zhang, Q., Zhang, Z., Qi, H., & Li, P. (2009). Synthesized oversulphated, acetylated and benzoylated derivatives of fucoidan extracted from Laminaria japonica and their potential antioxidant activity in vitro. Food Chemistry, 114, 1285–1290.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Effect of k-carrageenan on rheological properties, microstructure, texture and oxidative stability of water-in-oil spreads Raluca I. Alexa a, b, John S. Mounsey a, *, Brendan T. O’Kennedy a, Jean C. Jacquier b a b
Teagasc, Moorepark Food Research Centre, Fermoy, Co. Cork, Ireland School of Agriculture, Food Science & Veterinary Medicine, Agriculture & Food Science Centre, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2009 Received in revised form 24 September 2009 Accepted 8 October 2009
The effect of k-carrageenan concentration (0-7.5 g kg1) on the rheology, microstructure, texture and oxidative stability of water-in-oil (W/O) spreads (600 g fat kg1 emulsion) was examined over 60 days storage time. Results showed that increasing the k-carrageenan concentration to 7.5 g kg1 significantly increased the viscosity of the aqueous phase (to 42.7 mPa s at 60 C) resulting in gelation of the aqueous phase on cooling. The microstructure of the spreads was disrupted by higher levels of k-carrageenan, resulting in a less homogeneous distribution of the aqueous phase. Melt temperature (where tan d > 1) decreased significantly from 62 to 56.2 C with increasing k-carrageenan concentration from 0 to 7.5 g kg1. The firmness and the G0 at 6 C for all samples were significantly increased after 60 days storage with only small effects due to k-carrageenan levels. Oxidation of the fat phase was evident by the significant increases in peroxide values of all spreads on storage, with k-carrageenan exhibiting no antioxidant behaviour. While increased k-carrageenan levels modified the microstructure of W/O spreads in terms of the droplet size of the aqueous phase and its distribution few changes were evident in the continuous fat phase. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Water-in-oil spreads k-Carrageenan Rheology Microstructure Oxidation
1. Introduction Spreadable fats are emulsions of the W/O type and were introduced as an economical, functional and low calorie alternative of butter (Caponio & Gomes, 2004; Laia, Ghazalia, Cho & Chong, 2000). Replacement of the fat with water alters the rheological properties and structural characteristics of spreads, which are mainly given by the shape and the size of the fat crystals (Kasapis, 2000). A number of biopolymers are used in low-fat formulations as fat mimics and as stabilisers of the aqueous phase (Chronakis & Kasapis, 1995a; Chronakis, 1997) through network stabilisation as well as stabilisation via interfacial action (Benichou, Aserin & Garti, 2002; Dickinson, 2003). Polysaccharides such as k-carrageenan have been shown to enhance the sensorial properties of reduced-fat spreads (Clegg, Moore & Jones, 1996). k-Carrageenan has the capacity to gel on cooling through a disordered-ordered transition forming intermolecular double helices and subsequent aggregation and gelation under specific conditions (Oakenfull & Scott, 1990; Heyraud, Rinaudo, & Rochas, 1990). In a previous study by Mounsey, Stathopoulos, Chockchaisawasdee, O’Kennedy, Gee and Doyle (2008)
on the fortification of W/O spreads containing k-carrageenan, authors found that the increase in the gel strength of the aqueous phase upon addition of transition metals altered the microstructure of the W/O spreads. The purpose of the present study was to assess the effect of k-carrageenan on the rheology of the aqueous phase and on the microstructure, texture, and rheology of the experimental W/O spreads. Studies with reference to the antioxidant activities of sulphated polysaccharides extracted from brown and red seaweeds have been published in the literature (Sirendi, Gohtani, & Yamano, 1998; de Souza, Marques, Dore, da Silva, Rocha, & Leite, 2007; Wang, Liu, Zhang, Zhang, Qi, & Li, 2009). In-vitro studies showed that sulphated polysaccharides presented activity in inhibiting free radicals. The degree of sulphation was found to be directly related to the radical scavenging activity, with k-carrageenan exhibiting mild antioxidant properties (de Souza et al., 2007). The effect of k-carrageenan on the oxidative degradation of the W/O spreads was also investigated in the present work. 2. Materials and methods 2.1. Materials
* Corresponding author. Tel.: þ353 2542443. E-mail address: [email protected] (J.S. Mounsey). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.10.003
A commercial source of k-carrageenan (Grindsted Carrageenan CL 107, Danisco, Denmark) was used without further purification.
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The vegetable oil blend was obtained from DairyGold (Mitchelstown, Ireland). The fatty acid composition of the oil blend was 480 g kg1 monounsaturated, 320 g kg1 saturated, 190 g kg1 polyunsaturated and 10 g kg1 trans fatty acids. The monoglyceride emulsifier (Paalsgaard 0291) was supplied by Grinstead A/S, Braband, Denmark. All other solvents and chemicals used were of analytical grade and purchased from Sigma–Aldrich, St. Louis, MO 63103, USA. Deionised water was used for the aqueous phase. 2.2. Manufacture of W/O spreads Water-in-oil spreads (batches of 10 kg) were prepared in duplicate following margarine production technology using a Perfector scraped surface heat exchanger (Gerstenberg and Agger, Copenhagen, Denmark) as described previously (Mounsey et al., 2008). A control spread was prepared by mixing an aqueous phase (water; 400 g kg1 of the final product) and the fat (or oil) phase contained the oil blend and the emulsifier (600 g kg1 of the final product) at approximately 55 C. The preliminary mixing of the oil and aqueous phase was performed for 1 min at 1500 rpm using a Silverson mixer (model AX3, Silverson Machines Ltd., Waterside, Chesham, Bucks, UK). The formed W/O emulsion was transferred to the jacketed tank connected to the Perfector and pasteurized at 75 C for 15–20 s and immediately cooled down to 65 C. The emulsion (w55 C) was pumped through two scraped-surface coolers (at 432 rpm) bringing the temperature to 12 C before the spread was filled into 454 g plastic tubs and stored at 4.5 C prior to testing. The spreads containing k-carrageenan (1.56, 3.12, 6.25, 12.5 or 18.8 g L1 in the aqueous phase) were produced in order to give a final k-carrageenan concentration of 0.625, 1.25, 2.5, 5, and 7.5 g kg1 reported to the total quantity. 2.3. Rheology of the aqueous phase Small-scale deformation measurements were carried out on the aqueous phase of the spreads using cup and bob systems in an AR 2000 Rheometer (TA Instruments, UK). All measurements were made at a frequency of 1 Hz and a maximum strain of 0.500%. Parameters such as the storage modulus (G0 ), loss modulus (G00 ) and tan d (G0 /G0 ) were monitored during cooling from 60 C to 6 C at a rate of 1 C min1 using a Peltier heating element, followed by reheating to 60 C at the same ramp rate. The samples (15 g) were loaded at 60 C and covered with n-Tetradecane (Sigma Chemica, Co., St. Louis, MO, USA) to minimise evaporation. Viscosity of the aqueous phase of the spreads was measured using the same geometry of the AR 2000 Rheometer as above. A shear rate sweep from 0.1 to 500 s1 was applied for 5 min. The apparent viscosity (mPa s) was taken at 60 C and a shear rate of 100 s1. 2.4. Rheology of W/O spreads A controlled strain AR-2000 rheometer (TA Instruments, New Castle, Delaware) was used in the dynamic mode for small-scale deformation measurements. Disc-shaped samples of spread (25 mm diameter, 2.5 mm in thickness) were prepared using a 25 mm diameter cork borer. A 25 mm diameter serrated parallel plate geometry was used with a serrated lower plate. Samples were placed on the lower plate and compressed with a normal force of 0.5 N to prevent slippage, with 3 min for temperature equilibration and stress relaxation prior to testing. Measurements were taken at a frequency of 1 Hz and a strain of 0.2 %. Samples were loaded at 6 C before n-Tetradecane (Sigma Chemical Co., St. Louis, MO, USA) was added to the side in order to avoid evaporation. The change in G0 , G00 and tan d were measured during
heating from 6 to 60 C at a rate of 1 C min1. Tests were carried out in triplicate. 2.5. Texture Profile Analysis (TPA) Compression tests were performed using a TA-XT2 Texture Analyser from Stable Microsystems (UK). Cylinders of spreads of 25 25 mm were cut and allowed to equilibrate at 4.5 C for 4 h. Samples were compressed to 50% of their initial height (12.5 mm) at a crosshead speed of 1 mm s1 using a 5 kg load cell and a 75 mm diameter plate. Six samples were analysed from each spread tub. The hardness values, expressed in Newtons (N) were measured at the point of fracture. 2.6. Scanning Electron Microscopy (SEM) Samples were prepared for cryo-SEM by mounting them into copper rivets and plunged into nitrogen slush (207 C). Samples were then transferred under vacuum into the preparation chamber, freeze fractured with a cold blade, etched at 88 C for 5 min and then sputtered coated with gold (10 mA for 60 s). Samples were then transferred under vacuum onto the cold stage which was maintained at 125 C and imaged using FE-SEM (Zeiss Supra Gemini, Darmstadt, Germany). Images were acquired at 2.00 kV with 2000–20,000 magnification. 2.7. Determination of the peroxide values (PV) The PV test of melted fat (mEq O2.kg1) was derived from the International Dairy Federation (IDF) Standard 74:1974. The W/O spread sample was melted at 62 C and 0.1 mL of melted fat was dissolved into 10 mL of chloroform/methanol (70:30) mixture, followed by addition of ammonium thiocyanate and ferrous chloride, respectively. Using glass stoppers, the tubes were inverted and placed in dark cupboard for 10 min. Simultaneously, a blank test with only reagents and no sample was carried out. The absorbance of the samples was read at 505 nm on a Varian Cary Scan 1 instrument. After calibration, the blank value was subtracted from the sample values (1) and the PVs were calculated. OD [ Abssample L Absstandard
(1)
where OD is the optical density. Samples were analysed after 1, 5, 7, 15, and 31 days of storage. 2.8. Statistical analysis The rheology, texture and oxidative stability data of W/O spreads containing k-carrageenan were statistically tested by analysis of variance (ANOVA) using SigmaStat (version 3.0; Jandel Scientific, Corte Madera, CA, USA). Differences among treatments were determined by Student–Newman–Keuls pairwise-comparison test. Treatment means were considered significantly different at *P < 0.05. 3. Results and discussion 3.1. Rheology of the aqueous phase of W/O spreads 3.1.1. Viscosity at 60 C The effect of k-carrageenan concentration on the apparent viscosity of the aqueous phase of W/O spreads is presented in Fig. 1. The sample containing 1.56 g kg1 k-carrageenan (in the aqueous phase) had a low viscosity (4.31 mPa s) whereas the apparent
R.I. Alexa et al. / LWT - Food Science and Technology 43 (2010) 843–848 10000
300
1000
250
b
10 10
b b
b
a
b
b
b
a
200 G′(kPa)
100
G' at 6°C (Pa)
Apparent viscosity at 60°C (mPa.s)
100
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b
x a
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ab
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x a
x a
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1
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-1
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0
κ-carrageenan (g.kg )
viscosity increased significantly (***P < 0.001) to 42.7 mPa s for sample containing 18.8 g kg1 k-carrageenan. At low concentrations the coils of k-carrageenan are free to move in the solution while the apparent viscosity proportionally increases with the concentration (Morris, 1984; Mounsey et al., 2008). Nonetheless, kcarrageenan remained on a non-gelled state at 60 C, even at concentrations as high as 18.8 g kg1.
3.1.2. Gelling properties at 6 C The effect of increasing k-carrageenan concentration on the gelation properties of the aqueous phase of the W/O spreads was monitored on cooling from 60 to 6 C. The G0 values of samples at 6 C are shown in Fig. 1. The aqueous phases containing 3.12 and 6.25 g kg1 k-carrageenan showed no gelation on cooling. By increasing the concentration of k-carrageenan to 12.5 g kg1 the aqueous phase had a gelling temperature (Tg) of 23 C and a relatively weak gel was obtained at 6 C (53 Pa). Further increasing k-carrageenan concentration of the aqueous phase to 18.8 g kg1, Tg increased to 28.3 C and a strong gel of 1.49 kPa was obtained at 6 C. 3.2. Small scale deformation of W/O spreads 3.2.1. Rheology at 6 C Small-scale rheological assessment was performed in order to investigate the structure of the experimental W/O spreads under non-destructive conditions. The effect of k-carrageenan concentration as well as the length of storage on the elastic modulus (G0 ) of the W/O spread is presented in Fig. 2. Statistical interpretation showed that the G0 values did not change significantly (*P > 0.05) with increased levels of k-carrageenan. On the other hand, the G0 of the control at 6 C had substantially greater values (*P < 0.05) after 60 days of storage, as well as the G0 of the k-carrageenan containing samples. The increase in the G0 over time may be due to the increase in the amount of crystallinity in the oil phase, thus increasing the solidness of the spreads at 6 C. Borwankar, Frye, Blaurock & Sasevich (1992) found that the rheology of W/O spreads is strongly associated with the degree of fat crystallisation. An additional rheological parameter used to describe the viscoelastic profile of a material is the tangent of the phase angle (tan d (¼G00 /G0 )). The effect of k-carrageenan on tan d (measured as a function of storage is presented in Fig. 3). The tan d values of the W/O spreads showed little change with the addition of k-carrageenan (*P > 0.05), but increased sharply with the storage. The tan d values of the control significantly increased (*P < 0.05) during
0.62
1.25 2.5 κ-carrageenan (g.kg-1)
5
7.5
Fig. 2. Effect of k-carrageenan concentration on the G0 (at 6 C) of W/O spreads as a function of storage time after 2 ( ), 15 ( ), and 60 ( ) days. Letters a, b, c represent significant differences within treatment means between 2, 15 and 60 days. Letters x, y, z represent significant differences between treatment means after 2 days. Means with the same letter do not differ significantly at *P < 0.05.
storage time, indicating a more plastic behaviour of the spreads given by a large viscous constituent in the system (Chronakis & Kasapis, 1995b). 3.3. Rheology on heating The melt temperature – where tan d > 1 or the temperature at which the total solid fat content is zero (Himawan, Starov & Stapley, 2006) of all samples increased over time (Table 1), possibly due to modification in the structure of fat crystals (a, b0 , and b polymorph types) found in the network. It was shown by Ojijo et al. (2004) using polarized light microscopy that the fat crystal network of an olive oil/monoglyceride mixture suffered a progressive growth in crystal clusters during storage, due to a more compact geometry of hydrocarbon chains. The melting temperature of the spreads significantly decreased (*P < 0.05) with k-carrageenan concentration from 62.4 C (control) to 55.9 C (sample containing 7.5 g kg1 k-carrageenan). This may be probably due to the variation in the emulsion properties, with k-carrageenan having a destabilisation effect. After 60 days of storage the spreads did not melt in the range of temperature examined. The melting profile of the experimental W/O spreads was comparable to that observed by Borwankar et al. (1992) in a study on melting characterisation of margarine and table spreads containing gelatine, which gelled in the aqueous phase at low
c
0.3
c c
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c
b
c
b
b
b
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Tan δ
Fig. 1. The effect of k-carrageenan on the apparent viscosity of the aqueous phase of W/O spreads at 60 C and 100 s1 shear rate ( ) and the storage modulus (G0 ) at 6 C ( ). Note: 18.8 g kg1 in the aqueous phase corresponds to 7.5 g kg1 in the final spread. Each point on the curve represents the mean of triplicate trials. Vertical bars show standard deviation of the means.
0
b x a
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x a
x a
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1.25 2.5 κ-carrageenan (g.kg-1)
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7.5
Fig. 3. Effect of k-carrageenan on tan d (at 6 C) of W/O spreads as a function of storage time after 2 ( ), 15 ( ), and 60 ( ) days. Letters a, b, c represent significant differences within treatment means between 2, 15 and 60 days. Letters x, y, z represent significant differences between treatment means after 2 days. Means with the same letter do not differ significantly at *P < 0.05.
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Table 1 The effect of k-carrageenan on melt temperature (temperature where tan d > 1) of W/O spreads as a function of storage time.
k-Carrageenan [g kg1] 0 0.625 1.25 2.5 5 7.5
Melt t 2 Days
15 Days
60 Days
56a 54.5ab 53.7ab 53.8ab 54.5ab 52.8bc
62.4a 62.3ab 58.8bc 55.9cd 56.3ce 55.9cf
– – – – – –
For each column, means with the same letter do not differ significantly at *P < 0.05.
temperatures and promoted meltability by destabilising the formed emulsion. 3.4. Texture analysis of W/O spreads The texture of plastic materials such as fat spreads is mainly given by the amount of solid fat (Deman & Beers, 1987; Bot & Vervoort, 2006). Thickening the aqueous phase using hydrocolloids may improve the spreadability and reduce the loss of moisture normally caused by the large amounts of water contained by these products (Bot & Vervoort, 2006). The effect of k-carrageenan on the textural stability of W/O spreads was investigated through large deformation techniques (compression) and presented in Fig. 5 The hardness at fracture (N) of the W/O spreads was measured during 60 days of storage and gave an indication of the sample resistance to deformation. Texture evaluation showed that the hardness of the W/O spreads increased over time. The hardness of the control sample (no k-carrageenan) had considerably increased (*P < 0.05) from 11.13 N to 13.20 N during storage. There was also observed an increase of hardness in time for samples containing k-carrageenan.
This could be possibly due to the slow post-crystallisation processes and development of bonds within the fat crystal network that took place during storage, which resulted in a more solid structure and therefore, more force was required to fracture. Both, force at fracture and G0 of the W/O spreads increased in time. A relationship between the storage modulus of a fat system and its hardness was indicated by Narine and Marangoni (1999) who suggested that the G0 varies with the solid fat content of the network. Generally, the hardness of spreads was inconsistent with increasing k-carrageenan levels. High k-carrageenan concentrations (5-7.5 g kg1) had a slight weakening effect, probably due to the increase in the aqueous droplet sizes (as shown by SEM), making the spreads more compressible than the samples containing smaller aqueous droplets (control). Although addition of k-carrageenan enhanced the gel strength of the aqueous phase, the hardness of spreads was mainly given by the fat crystallisation. de Bruijne and Bot (1999) suggested that the properties of W/O spreads with reduced level of fat (600 g kg1) are not considerably affected by the aqueous phase. Keogh (2006) and Mounsey et al. (2008) also found that the texture of W/O spreads was less affected by the viscosity of the aqueous phase. Furthermore, while the W/O spreads became rancid with storage, the change in the chemical composition of the fatty acids did not seem to have a clear effect on the texture of spreads. 3.5. Microstructure of W/O spreads In our study, SEM was employed to visualise the microstructure of W/O spreads in terms of water droplet size distribution. The control sample (Fig. 4a, b.) presented a well emulsified and homogeneous structure, with many globular water droplets evenly distributed within the continuous fat structure. Samples containing k-carrageenan presented comparable microstructure, although
Fig. 4. Scanning electron images of control W/O spread at (a) low magnification, (b) high magnification and sample containing 7.5 g kg1 k-carrageenan at (c) low magnification and (d) high magnification.
R.I. Alexa et al. / LWT - Food Science and Technology 43 (2010) 843–848
20
b c abc abc
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Fig. 5. Effect of k-carrageenan on the hardness of W/O spreads at time 6 ( ), 15 ( ), 30 ( ), and 60 ( ) days of storage at 4.5 C. Each bar represents the mean of duplicate trials. Vertical bars show standard deviation of the means. Letters a, b, c represent significant differences within treatment means between 6, 15, 30, and 60 days. Letters v, w, x, y represent significant differences between treatment means at day 6. Means with the same letter do not differ significantly at *P < 0.05.
addition of 7.5 g kg1 k-carrageenan in the overall product resulted in a less homogeneous aqueous phase distribution, with slightly larger and clustered aqueous phase droplets (Fig. 4d). k-Carrageenan increased the viscosity of the aqueous phase and most likely induced gelation of the droplets during spread manufacture, as previously shown by Mounsey et al. (2008). Similar results were obtained by Clegg et al. (1996) in spreads containing 40 g kg1 gelatine. 3.6. Oxidative stability of W/O spreads Lipid oxidation significantly affects the quality characteristics of spreadable fats due to formation of rancid flavours and secondary oxidised compounds which are detrimental for the health (Frankel, 1998). Chaiyasit, Elias, McClements & Decker (2007) showed that the concentration of peroxides is directly associated with the oxidative stability of fats. The effect of k-carrageenan on the oxidative stability of W/O spreads, as measured by the PV test, was monitored during 31 days and presented in Fig. 6. The PV is a measure of the hydroperoxide concentration in the early stages of oxidation (Kochhar, 2003). In all tests, PV increased in time. The PV of the control increased noticeably (*P < 0.05) from 0.72 to 2.33 mEq O2 kg1 during storage time, although the spreads were resistant against oxidation for 5 days. Samples containing levels of
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up to 2.5 g kg1 k-carrageenan were not significantly (*P > 0.05) more oxidised after 7 days of storage compared to the control. These results indicate that all samples became rancid during storage and the oxidation proceeded more rapidly in samples containing high levels of k-carrageenan. It was anticipated that k-carrageenan would have potential lipid antioxidation effect, such as other polysaccharides (Matsumura et al., 2003; de Souza et al., 2007). As k-carrageenan increased the viscosity of the aqueous phase, forming gel droplets upon cooling (Mounsey et al., 2008), it was hoped that the oxidation would be delayed by decelerating the activity of reactants. Basaran, Coupland and McClements (1999) showed that the viscosity of the aqueous phase had no effect in delaying the motion of small molecules through the polysaccharide gel network. Polysaccharides may have a possible antioxidant effect through a metal ion chelation mechanism or hydrogen donation (McClements & Decker, 2000). In our study, k-carrageenan did not retard the oxidation of W/O spreads. It is possible that the oxidation was promoted by the trace amounts of metallic cations contained by k-carrageenan source and implicit, higher levels present in samples prepared with increased concentration of k-carrageenan in the aqueous phase; or due to incorporation of air and commencement of oxidation of the oil-blend during emulsification process. 4. Conclusions This study showed that increased levels of k-carrageenan resulted in increased viscosity at 60 C as well as gelation on cooling of the aqueous phase of model W/O spreads. The textural hardness and dynamic rheological parameters (G0 and tan d at 6 C) were not significantly modified by increased k-carrageenan addition. A less homogeneous structure was associated to an improved meltability of the spreads containing increased levels of k-carrageenan. Nonetheless, k-carrageenan did not inhibit oxidation of the W/O spreads. k-Carrageenan has application in controlling the rheology of the aqueous phase of W/O spreads during spread formation, storage and subsequent re-heating. Acknowledgements This research has been funded by Department of Agriculture and Food under the Food Institutional Research Measure (National Development Plan). The authors thank J. Roche for his help in the preparation of W/O spreads. Vivian Gee of the National Food Imaging Centre is gratefully acknowledged for help with collection of the SEM images presented.
e
Peroxide value (mEq O2.kg-1)
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c wx b
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7.5
Fig. 6. Effect of k-carrageenan on the oxidative stability of W/O spreads after 1 ( ) 5 ( ), 7 ( ), 15 ( ), and 31 ( ) days storage at 4.5 C. Each bar represents the mean of duplicate trials. Vertical bars show standard deviation of the means. Letters a, b, c, d, e represent significant differences within treatment means between 1, 5, 7, 15 and 31 days. Letters v, w, x, y represent significant differences between treatment means at day 1. Means with the same letter do not differ significantly at *P < 0.05.
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