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Effect of some emulsifiers on the structure of extrudates with high content of fat
Journal of Food Engineering 79 (2007) 1351–1358 www.elsevier.com/locate/jfoodeng
Effect of some emulsifiers on the structure of extrudates with high content of fat Teresa De Pilli *, Barbara F. Carbone, Anna G. Fiore, Carla Severini Department of Food Science – University of Foggia, Via Napoli 25, 71100 Foggia, Italy Received 25 November 2005; accepted 12 April 2006 Available online 19 May 2006
Abstract A study on extruded foods with wheat flour, almond flour, water and four kinds of emulsifiers (soy lecithin (SL), sucrose esters (SE), mono-glycerides (MG) and mono- and di-glycerides (MDG) of fatty acids) was carried out. In particular, the effects of these additives on the oil loss, which occurs during extrusion processing, and the structural characteristics of extrudates were evaluated. Results showed that the sucrose esters were the most suitable emulsifier to reduce oil loss and to give the best structure of the extrudates (highest percentage porosity and smallest breaking strength). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Emulsifier; Extrusion; Fatty flour; Almond; Sucrose esters
1. Introduction It is already known that extrusion-cooking is used to produce expanded snacks and shaped foods. This cooking processing carried out using high temperatures and short time of treatment, gives finished product with high quality (high digestibility and nutritional value) and reduces degradation reactions that occur during thermal processing (for example, loss of nutrients). In the extruders, the components are mixed, sheared and subjected to elevated temperatures and pressures. So that the dough shows a plastic consistency that favours the expansion of product at the exit of the die. In fact, the sudden drop in pressure at the exit of extruder causes the release of gases entrapped in the product, determining the swelling of extrudates (Best, Fayard, Holz, & Vanacker, 1999). Many factors affect to expansion the extrudates: type of plant, screw configuration, moisture content of the dough, residence time, operating pressure, temperature
*
Corresponding author. Tel./fax: +39 881 589245. E-mail address: [email protected] (T. De Pilli).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.054
profile of the barrel, screw speed and feed capacity (Meuser & Van Lengerich, 1992). Also the type of ingredients used to produce extrudates could have a considerable effect on expansion degree. The presence of lipids for example leads to a compact structure as biscuits because of the reduction of the starch dispersion with consequent decrease of product expansion (Guy, 1994). Furthermore, a content of lipids greater than 16% in the flour involves also technological drawbacks: migration of fatty fraction outside of dough (due to operating temperature and pressure) and, subsequently, percolation oil at the die (De Pilli, Severini, Guidolin, & Massini, 2000). Oil loss is not advisable to preserve both the extrudates (quickly oxidation of oil on the extrudate surface) and the hygienic condition of the plant. The oil loss that occurs during extrusion processing of flours with high content of lipids, can be reduced by using an emulsifier (De Pilli et al., 2001). However, the addition of this ingredient could influence the structure of the extrudates (Ryu, Neumann, & Walker, 1994). Emulsifiers are usually added to cereal flours in order to produce specific desired characteristics such as soft structure. In fact, during extrusion, emulsifiers form complexes
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with amylose that affect texture, cell distribution, and density of the extruded products (Galloway, Biliaderis, & Staneley, 1989; Harper, 1981). Furthermore, emulsifiers act as lubricants for the melted dough (Ryu & Walker, 1994) reducing the specific mechanical energy (Carrera, 1978). Emulsifiers show both hydrophilic and hydrophobic properties on the same molecule. Stutz, Del Vecchio, and Tenney (1973) discussed the role of emulsifiers in foods as substances that modify the surface behaviour of liquids in which they are dispersed at low concentration. The lipophilic portions of emulsifiers are believed to form a complex with the amylose fraction of starch during cooking, retarding starch gelatinization and decreasing swelling. The aim of this research was to investigate the effect of four kinds of emulsifiers (soy lecithin (SL), sucrose esters (SE), mono-glycerides (MG) and mono- and di-glycerides (MDG) of fatty acids) on oil loss in extrusion processing and on structural characteristics of extrudates obtained through extrusion of mixtures containing almond and wheat flours in a co-rotating twin-screw extruder. 2. Materials and methods 2.1. Flours Wheat flour was supplied by Cereal Destrine (Cadelbosco Sopra, Reggio Emilia, Italy) and almond flour by Alimentaristica Pugliese (Cerignola, Foggia, Italy). The chemical composition of both flours is shown in Table 1. 2.2. Chemical analysis of flours and extrudates The content of moisture, ash, protein and fat were determined according to the American Association of Cereal Chemists (2003). 2.3. Snack formulas The doughs were prepared according to the formulas reported in Table 2. The percentages of ingredients were chosen according to a previous work (De Pilli, Derossi, Giuliani, & Severini, 2004). The emulsifiers supplied by Danisco Cultor (Grindsted, Denmark) are soy lecithin (SL), sucrose esters (SE), monoglycerides (MG) and mono- and di-glycerides (MDG) of fatty acids, were chosen because commonly used in confectionary.
Table 1 Chemical composition of flours used for the extrusion experiments Raw material
% Moisture
% Ash
% Protein
% Fat
Wheat flour Almond flour
15.00 ± 0.04 3.89 ± 0.12
1.51 ± 0.01 2.90 ± 0.05
9.30 ± 0.01 21.66 ± 0.04
0.90 ± 1.05 62.53 ± 1.14
Table 2 Formulas used in extrusion experiments Formulas
Wheat Almond Saccharose Sodium Emulsifiers flour (%) flour (%) (%) chloride (%) (%)
Control A B C D E F G H
66.0 65.8 65.3 65.8 65.3 65.8 65.3 65.8 65.3
25 25 25 25 25 25 25 25 25
8 8 8 8 8 8 8 8 8
1 1 1 1 1 1 1 1 1
0 0.2 0.7 0.2 0.7 0.2 0.7 0.2 0.7
SL SL MDG MDG SE SE MG MG
2.4. Extruder A BC-21 CLEXTRAL (Firminy, France) co-rotating twin-screw extruder was used. The screw geometrical features were the following: diameter 25 mm and length 900 mm, distance between shafts 21 mm (L/D = 36:1). The used screw configuration (from inlet to the die) was reported in Table 3. During the extrusion experiments, the flour feed rate was maintained constant at 10 kg/hr using a volumetric gravity feeder; while the moisture content of the dough was adjusted using a water pump. Water was pumped to the first zone of the extruder. The extruder was divided into nine zones, which can be independently controlled and adjusted to desired temperature. The first five zones were kept at room temperature whereas the last four zones were adjusted at the temperatures of 100 °C (95 °C temperature of product at the die). The pressure at the die during extrusion was measured with a pressure trasducer (Dynisco PT462). The processing variables used during the extrusion experiments were: 25% dough moisture (dry basis), 250 rpm screw speed and 5 bar pressure at the die. Moreover, according to equation reported by Bhattacharya and Choudhury (1994), the specific mechanic energy (SME) estimated was 175 kJ/kg. Values of processing variables were selected according to preliminary experiments. The used die had a spherical shape (diameter of 60 mm) and was provided of two spherical holes with a diameter of 5 mm. All tests were performed in triplicate (27 extrusion experiments).
Table 3 Screw profile used for extrusion experiments Type of screw element
Screw element details (pitch/length)
Total length (mm)
Forward pitch
50/50 50/33
150 100
Kneading block
50/25 50/33
200 100
Decreased pitch
50/25 50/16
100 250
T. De Pilli et al. / Journal of Food Engineering 79 (2007) 1351–1358
Extrudates were cut, at the exit of the die, in sticks (20 mm in length and 5 mm in diameter) with a cutter (210 rpm). Extrudates were put on aluminium trays and dried in an oven (DeLonghi Combi & Functions Convection, Milano, Italy) at 80° for 10 min. The dried extrudates with a moisture content of about 8% were stored in polyethylene bags and analysed. 2.5. Analyses All analyses were replicated at least in triplicate. 2.6. Oil loss % The percentage of oil loss was calculated as a difference between oil content in raw mixtures and oil content in extrudates and expressed as g of oil loss/100 g of product. Before extraction, extrudates were finely grinded (particles <300 lm) by mill BUHLER ML 1204 (Germany).The extraction time (hours) was chosen after preliminary tests to obtain maximum yield in oil. Oil was extracted with n-hexane (Carlo Erba Reagenti, Milano, Italy) by Soxhlet model B-811 (Switzerland) apparatus. 2.7. Breaking strength and deformability A dynamometer stable Micro System TA-HDi Texture Analyser (ENCO s.r.l., Venezia, Italy) with a small wedge (0.5 mm/s speed) was used for the rheological tests. Results were expressed as breaking strength (N/mm2), i.e., the strength applied to breaking extrudate, and deformability (mm), i.e., the measure of extrudate deformability before rupture. 2.8. Volume of extrudates Sample volume was measured using the method described by Barber, Ortola, Barber, and Fernandez (1992). This analysis was carried out trough a measuring jug with a volumetric scale (pycnometer). The pycnometer was filled with flax seeds up to maximum level and then weighed. After, the container was emptied, filled with 10 g of sample and flax seeds up to maximum level and then weighed. Volume of the extrudates was expressed in millilitres and calculated according to the following formula: Volume ðmlÞ ¼
½wc þ wf ðwcfs ws Þ df
where wc is weight of container; wf, is weight of flax seeds; ws, is weight of sample; wcfs, is weight of container, flax seeds and sample; df, is density of flax seeds. For all tests were weighted 10 g of extrudates.
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2.9. Image analysis Air cell structures of extrudates were measured using the method described by Barrett and Ross (1990). Fifteen extrudates were selected from all samples. Each extrudate was cross-sectioned out. The section was pressed against a desk pad to blacken cell structures. The sections of extrudates were photographed using a dv-camera (CANON MV6i MC, Japan) with a 275 mm lens (0.8 Megapixel, MICRO NICON). Images were analysed using the OPTIMAS ver. 6.1 software (Bioscan, USA). This software allowed to evaluate the surface porosity by applying a grey level value (i.e., position between black and white) to each pixel in the image. Threshold levels were selected to construct a binary (black and white) image in which pixels having grey levels between the thresholds were black, and pixels having grey levels below these values were white. The area of the black object within the binary image was measured. Results were expressed as percentage ratio between surface of pores and surface of all extrudate section (porosity %). 2.10. Statistical analysis The variance analysis (ANOVA) was carried out on results obtained from the physical analysis of extruded products. ANOVA was carried out using software StatSoft ver. 6.0 (Statsoft, Tulsa, USA). The means of these results were compared by the Fisher’s test. 3. Results and discussion Fig. 1 shows the percentages of oil loss obtained during extrusion processing of samples containing 0%, 0.2% and 0.7% of four studied emulsifiers. According to ANOVA, the type and concentrations of emulsifiers had a significant effect on oil loss (Table 4). In particular, doughs with added emulsifiers showed a smaller oil loss than the control (Fig. 1). Moreover, oil loss from doughs containing 0.7% of MDG and MG was greater than formulas with 0.2% of emulsifiers. Probably, the high temperature of extrusion processing and the increase of emulsifier concentration favoured the transition from o/w to w/o emulsions (Gasˇic´, Jovanovic´, & Jovanovic´, 2002; Kruglyakov & Nushtayeva, 2004; Ruckenstein, 1999; Sajjadi, Jahanzad, & Yianneskies, 2004). The best results were obtained for doughs with added SE that did not show oil loss. This phenomenon can be explained by considering the HLB index (Hydrophile/Lipophyle Balance) of the emulsifiers. The HLB system is based on the concept that some molecules have hydrophilic groups, other molecules have lipophilic groups, and some have both. Weight percentage of each type of group, in a molecule or in a mixture, can predict what behaviour the molecular structure will exhibit. The scale as originally proposed by Griffin (1949) ranged
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Fig. 1. Oil loss % (mean and standard deviation) occurring during extrusion process of extrudates added with 0%, 0.2% and 0.7% of different emulsifiers.
Table 4 Variance analysis related to oil loss % data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Degree Mean of freedom square
F
Intercept Type of emulsifiers Concentration of emulsifiers Error
901.2004 49.4057
1 3
901.2004 345.5404 <0.0001 16.4686 6.3144 0.0019
353.6216
2
176.8108
78.2427 30
2.6081
67.7932
p
0.0001
from 0–20, with the low end signifying an emulsifier that is much more soluble in oil than in water, and the high end signifying the reverse. As a general rule, w/o emulsions are stabilized by HLB values in the 3–6 range, o/w emul-
sions are stabilized by HLB values in the 11–15 range, and intermediate HLB values give good wetting properties but not good emulsion stability (Vecchio, 2002). In a practical system that includes other ingredients such as sugar, salt, protein and other food components, the optimal HLB might be somewhat different. In our case, SE, that have a HLB number between 8 and 15, despite the presence of other ingredients and the high processing temperature, were able to retain the fatty fraction with no oil loss. Instead, as expected, MDG, SL and MG having an HBL index between 2.8 and 4.3 were less effective in oil loss reduction because they favoured the formation of water in oil emulsions. Fig. 2 shows the breaking strength of extrudates with different type and percentage of emulsifiers. Also in this
Fig. 2. Breaking strength (N/mm2) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
T. De Pilli et al. / Journal of Food Engineering 79 (2007) 1351–1358
case the type and concentrations of emulsifiers showed a significant effect on break strength (Table 5). It can be observed that the samples with emulsifiers had a breaking strength values smaller than control (Fig. 2). In particular, extruded products containing sucrose esters (SE) showed the smallest breaking strength values and then resulted more fragile than samples added with other emulsifiers. However, the values of extrudate breaking strength increased with increasing of emulsifier percentages at 0.7%. The samples with emulsifiers showed lower values of breaking strength than control probably because of capacity of these additives to incorporate and stabilize air bubbles in dough (Richardson, Langton, Faldt, & Hermanson, 2002) that favours the formation of homogeneous porosity structure of the extrudates. This capacity is more effective with sucrose esters (Ebeler & Walker, 1984). The sucrose esters interacting with the flour proteins by means of hydrophilic and/or hydrophobic bindings, favour the development of a more flexible gluten network (Wijnans, Baal, & Vianen, 1993). The sudden drop of pressure, which is subjected flexible network at the exit of the die, causes an instantaneous evaporation of water that gives a more
Table 5 Variance analysis related to break strength (N/mm2) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Intercept Type of emulsifiers Concentration of emulsifiers Error
28.43237 1.26983
Degree of freedom
Mean square
F
p
1 3
28.43237 0.42328
722.0910 10.7498
<0.0001 0.0001
1.05567
2
0.52783
13.4053
0.0001
1.18125
30
0.03938
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Table 6 Variance analysis related to porosity % data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Intercept Type of emulsifier Concentration of emulsifier Error
3396.74 1273.66
Degree of freedom
Mean square
F
p
1 3
3396.74 424.55
94.7884 11.8475
<0.0001 <0.0001
308.06
2
154.03
4.2983
0.030
645.03
18
35.83
expanded and porous structure to the extrudate. So results obtained from image analysis showed that samples added with emulsifiers have had a porosity value compared with samples without emulsifier (Fig. 3). Moreover, ANOVA results showed that type and concentration of emulsifier had a significant effect on porosity extrudate (Table 6). The samples that showed the highest values of porosity percentages were those added with sucrose esters in both percentages (0.2% and 0.7%). This behaviour could be explained considering that almond oil coming outside of almond flour, during extrusion processing, had a negative impact on air incorporation. The addition to dough of an oil soluble emulsifier as sucrose esters, could favour the formation of a solid film at the oil/water interface, which encapsulates the oil and prevents collapse of the foam (Richardson, Bergenstahl, Langton, Stading, & Hermansson, 2004). Deformability of the extrudates is an index of the energy opposed to the product at the strength applied. The deformability of extrudates added with 0.2% and 0.7% of different emulsifiers is reported in Fig. 4. Results of ANOVA showed that concentration of emulsifier did not have a significant effect on extrudate deformability; instead type of
Fig. 3. Porosity % of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
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Fig. 4. Deformability (mm) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
emulsifier had a significant effect on this parameter (Table 7). In particular, the mono-glycerides added with 0.2% showed the smallest value of deformability. It is known that
deformability is correlated to degree of gelatinization (Case, Hamann, & Schwartz, 1992) and then small values of this index could be due to the retard of starch granule swelling
Table 7 Variance analysis related to deformability (mm) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%)
Table 8 Variance analysis related to volume (ml) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%)
Effect
Sum square
Intercept Type of emulsifiers Concentration of emulsifiers Error
0.539981 0.002411
Degree of freedom
Mean square
F
p
Effect
Sum square
1 3
0.539981 0.000804
2550.816 3.797
<0.0001 0.02
4313.030 55.460
0.000043
2
0.000022
0.102
0.006351
30
0.000212
Intercept Type of emulsifiers Concentration of emulsifiers Error
0.90
Degree of freedom
Mean square
F
p
1 3
4313.030 18.487
1375.956 5.898
<0.0001 0.003
83.433
2
41.716
13.308
0.0001
94.037
30
3.135
Fig. 5. Volume (ml) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
T. De Pilli et al. / Journal of Food Engineering 79 (2007) 1351–1358
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Table 9 Regression results obtained from correlation of extrudate properties (oil loss, break strength, deformability, volume and porosity) Properties of extrudates
Regression equation 2
Oil loss %: break strength (N/mm ) Oil loss %: deformability (mm) Oil loss %: volume (ml) Oil loss %: porosity % Break strength (N/mm2): deformability (mm) Break strength (N/mm2): volume (ml) Break strength (N/mm2): porosity % Deformability (mm): volume (ml) Deformability (mm): porosity % Volume (ml): porosity %
*
y = 0.63712 + 0.06535 x y = 0.11645 + 0.000691*x y = 10.853 0.0346*x y = 17.358 2.136*x y = 0.10513 + 0.01549*x y = 7.9255 + 3.2616*x y = 38.757 33.35*x y = 0.29944 + 88.081*x y = 5.709 + 132.89*x y = 1.757 + 1.0984*x
r
p-level
0.82 0.14 0.04 0.58 0.28 0.28 0.72 0.41 0.154 0.27
0.007* 0.725 0.924 0.104 0.453 0.465 0.027* 0.275 0.691 0.473
r: Correlation coefficient. * Significant at a p < 0.05 level.
and, thus, starch gelatinization. The reduction of starch gelatinization could be, probably, related to the formation of lipid starch complexes that are derived from the lipophilic portions of emulsifiers with the amylose fraction of starch during cooking. Starch granule swelling and solubility decline with an increase in the complex formation (Eliasson, 1985; Ghiasi, Varriano-Marston, & Hoseney, 1982). Fig. 5 shows the extrudate volume of different types and percentages of emulsifiers. Both variables had a significant effect on extrudate volume (Table 8). The most of samples added with 0.2% of emulsifiers had volume values similar to control sample, with exception of extruded products with soy lecithin that showed a volume value lower than control sample. It can suppose that the presence of lecithin, probably, reduced the water absorption of dough that caused a decrease of the starch gelatinization degree (Knightly, 1989). With increasing of emulsifier percentages, there are significant differences between the volume values of different samples. In Table 9 regression results obtained from correlation of extrudate properties (oil loss, break strength, deformability, volume and porosity) are shown. Break strength is positively correlated with oil loss and negatively with porosity. The positive correlation between oil loss and break strength could be due, as hypothesized by Guy (1994), to the presence of high free fat fraction that reduced starch dispersion with consequent decrease of product expansion that imparts a compact structure, as biscuits. Instead, the negative correlation between break strength and porosity is obviously because of the porous structure makes extrudate more fragile. 4. Conclusion Results showed that sucrose esters are the most suitable emulsifiers to reduce oil loss during extrusion processing. In fact, samples added with sucrose ester did not show oil loss during extrusion processing already at the smallest percentage of this emulsifier. Moreover, the addition of emulsifiers significantly affected the rheological characteristics of extruded prod-
ucts. In particular, the samples added with sucrose esters showed the best structure (highest porosity % and smallest breaking strength). The increase of emulsifier percentages always determined a decrease in expansion degree of extrudates. References American Association of Cereal Chemists. (2003). Approved methods of the AACC (10th ed.) Methods 44-15A, 08-01,46-10, 30–25. St. Paul, MN: The Assn. Barber, B., Ortola, C., Barber, S., & Fernandez, F. (1992). Storage of packaged white bread. III Effects of sourdough and addition of acids on bread characteristics. Zeitschrift fu¨r Lebensmittel-Untersuchung und -Forschung, 194, 442–449. Barrett, A., & Ross, E. W. (1990). Correlation of extrudate infusibility with bulk properties using image analysis. Journal of Food Science, 55(5), 1378–1382. Best, E., Fayard, G., Holz, K. & Vanacker, P. (1999). Processing for obtaining extruded food products having high die shape conformity and reduced adhesion. US Patent 5,976,596,2. Bhattacharya, S., & Choudhury, G. S. (1994). Twin-screw extrusion of rice flour: effect of extruder length-to-diameter ratio and barrel temperature on extrusion parameters and product characteristics. Journal of Food Process and Preservation, 18, 389–406. Carrera, J. (1978). Extrusion cooking of wheat starch: Effects of pH and emulsifiers. M.Sc. Thesis. Kansas State University: Manhattan, USA. Case, S. E., Hamann, D. D., & Schwartz, S. J. (1992). Effect of starch gelatinization on physical properties of extruded wheat- and cornbased products. Cereal Chemistry, 69, 401–404. De Pilli, T., Severini, C., Guidolin, E., Massini, R. (2000). Studio sul processo di estrusione applicato a farina di semi oleosi – In: Ricerche ed Innovazione nell’Industria Alimentare (Vol. IV, pp. 722–730) Chiriotti Editori, Pinerolo – Italia. De Pilli, T., Derossi, A., Giuliani, R., Severini, C. (2004). Using of different emulsifiers for fatty flours extrusion. Book of abstract ICEF9, International conference engineering and food, (p. 206) Montpellier (France). De Pilli, T., Severini, C., Baiano, A., Guidolin, E., Legrand, J., & Massini, R. (2001). Application du proce´de´ d’extrusion aux farines grasses: cas de la farine d’amande. Science des Aliments, 21, 519–536. Ebeler, S. E., & Walker, C. E. (1984). Effects of various sucrose fatty acid ester emulsifiers on high–ratio white layer cakes. Journal of Food Science, 49, 380–387. Eliasson, A. C. (1985). Starch gelatinization in the presence of emulsifier. A morphological study of wheat starch. Starch, 37, 411–415. Galloway, G. I., Biliaderis, C. G., & Staneley, D. W. (1989). Properties and structure of amylose-glyceryl monostereate complexes formed in
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Effect of some emulsifiers on the structure of extrudates with high content of fat Teresa De Pilli *, Barbara F. Carbone, Anna G. Fiore, Carla Severini Department of Food Science – University of Foggia, Via Napoli 25, 71100 Foggia, Italy Received 25 November 2005; accepted 12 April 2006 Available online 19 May 2006
Abstract A study on extruded foods with wheat flour, almond flour, water and four kinds of emulsifiers (soy lecithin (SL), sucrose esters (SE), mono-glycerides (MG) and mono- and di-glycerides (MDG) of fatty acids) was carried out. In particular, the effects of these additives on the oil loss, which occurs during extrusion processing, and the structural characteristics of extrudates were evaluated. Results showed that the sucrose esters were the most suitable emulsifier to reduce oil loss and to give the best structure of the extrudates (highest percentage porosity and smallest breaking strength). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Emulsifier; Extrusion; Fatty flour; Almond; Sucrose esters
1. Introduction It is already known that extrusion-cooking is used to produce expanded snacks and shaped foods. This cooking processing carried out using high temperatures and short time of treatment, gives finished product with high quality (high digestibility and nutritional value) and reduces degradation reactions that occur during thermal processing (for example, loss of nutrients). In the extruders, the components are mixed, sheared and subjected to elevated temperatures and pressures. So that the dough shows a plastic consistency that favours the expansion of product at the exit of the die. In fact, the sudden drop in pressure at the exit of extruder causes the release of gases entrapped in the product, determining the swelling of extrudates (Best, Fayard, Holz, & Vanacker, 1999). Many factors affect to expansion the extrudates: type of plant, screw configuration, moisture content of the dough, residence time, operating pressure, temperature
*
Corresponding author. Tel./fax: +39 881 589245. E-mail address: [email protected] (T. De Pilli).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.04.054
profile of the barrel, screw speed and feed capacity (Meuser & Van Lengerich, 1992). Also the type of ingredients used to produce extrudates could have a considerable effect on expansion degree. The presence of lipids for example leads to a compact structure as biscuits because of the reduction of the starch dispersion with consequent decrease of product expansion (Guy, 1994). Furthermore, a content of lipids greater than 16% in the flour involves also technological drawbacks: migration of fatty fraction outside of dough (due to operating temperature and pressure) and, subsequently, percolation oil at the die (De Pilli, Severini, Guidolin, & Massini, 2000). Oil loss is not advisable to preserve both the extrudates (quickly oxidation of oil on the extrudate surface) and the hygienic condition of the plant. The oil loss that occurs during extrusion processing of flours with high content of lipids, can be reduced by using an emulsifier (De Pilli et al., 2001). However, the addition of this ingredient could influence the structure of the extrudates (Ryu, Neumann, & Walker, 1994). Emulsifiers are usually added to cereal flours in order to produce specific desired characteristics such as soft structure. In fact, during extrusion, emulsifiers form complexes
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with amylose that affect texture, cell distribution, and density of the extruded products (Galloway, Biliaderis, & Staneley, 1989; Harper, 1981). Furthermore, emulsifiers act as lubricants for the melted dough (Ryu & Walker, 1994) reducing the specific mechanical energy (Carrera, 1978). Emulsifiers show both hydrophilic and hydrophobic properties on the same molecule. Stutz, Del Vecchio, and Tenney (1973) discussed the role of emulsifiers in foods as substances that modify the surface behaviour of liquids in which they are dispersed at low concentration. The lipophilic portions of emulsifiers are believed to form a complex with the amylose fraction of starch during cooking, retarding starch gelatinization and decreasing swelling. The aim of this research was to investigate the effect of four kinds of emulsifiers (soy lecithin (SL), sucrose esters (SE), mono-glycerides (MG) and mono- and di-glycerides (MDG) of fatty acids) on oil loss in extrusion processing and on structural characteristics of extrudates obtained through extrusion of mixtures containing almond and wheat flours in a co-rotating twin-screw extruder. 2. Materials and methods 2.1. Flours Wheat flour was supplied by Cereal Destrine (Cadelbosco Sopra, Reggio Emilia, Italy) and almond flour by Alimentaristica Pugliese (Cerignola, Foggia, Italy). The chemical composition of both flours is shown in Table 1. 2.2. Chemical analysis of flours and extrudates The content of moisture, ash, protein and fat were determined according to the American Association of Cereal Chemists (2003). 2.3. Snack formulas The doughs were prepared according to the formulas reported in Table 2. The percentages of ingredients were chosen according to a previous work (De Pilli, Derossi, Giuliani, & Severini, 2004). The emulsifiers supplied by Danisco Cultor (Grindsted, Denmark) are soy lecithin (SL), sucrose esters (SE), monoglycerides (MG) and mono- and di-glycerides (MDG) of fatty acids, were chosen because commonly used in confectionary.
Table 1 Chemical composition of flours used for the extrusion experiments Raw material
% Moisture
% Ash
% Protein
% Fat
Wheat flour Almond flour
15.00 ± 0.04 3.89 ± 0.12
1.51 ± 0.01 2.90 ± 0.05
9.30 ± 0.01 21.66 ± 0.04
0.90 ± 1.05 62.53 ± 1.14
Table 2 Formulas used in extrusion experiments Formulas
Wheat Almond Saccharose Sodium Emulsifiers flour (%) flour (%) (%) chloride (%) (%)
Control A B C D E F G H
66.0 65.8 65.3 65.8 65.3 65.8 65.3 65.8 65.3
25 25 25 25 25 25 25 25 25
8 8 8 8 8 8 8 8 8
1 1 1 1 1 1 1 1 1
0 0.2 0.7 0.2 0.7 0.2 0.7 0.2 0.7
SL SL MDG MDG SE SE MG MG
2.4. Extruder A BC-21 CLEXTRAL (Firminy, France) co-rotating twin-screw extruder was used. The screw geometrical features were the following: diameter 25 mm and length 900 mm, distance between shafts 21 mm (L/D = 36:1). The used screw configuration (from inlet to the die) was reported in Table 3. During the extrusion experiments, the flour feed rate was maintained constant at 10 kg/hr using a volumetric gravity feeder; while the moisture content of the dough was adjusted using a water pump. Water was pumped to the first zone of the extruder. The extruder was divided into nine zones, which can be independently controlled and adjusted to desired temperature. The first five zones were kept at room temperature whereas the last four zones were adjusted at the temperatures of 100 °C (95 °C temperature of product at the die). The pressure at the die during extrusion was measured with a pressure trasducer (Dynisco PT462). The processing variables used during the extrusion experiments were: 25% dough moisture (dry basis), 250 rpm screw speed and 5 bar pressure at the die. Moreover, according to equation reported by Bhattacharya and Choudhury (1994), the specific mechanic energy (SME) estimated was 175 kJ/kg. Values of processing variables were selected according to preliminary experiments. The used die had a spherical shape (diameter of 60 mm) and was provided of two spherical holes with a diameter of 5 mm. All tests were performed in triplicate (27 extrusion experiments).
Table 3 Screw profile used for extrusion experiments Type of screw element
Screw element details (pitch/length)
Total length (mm)
Forward pitch
50/50 50/33
150 100
Kneading block
50/25 50/33
200 100
Decreased pitch
50/25 50/16
100 250
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Extrudates were cut, at the exit of the die, in sticks (20 mm in length and 5 mm in diameter) with a cutter (210 rpm). Extrudates were put on aluminium trays and dried in an oven (DeLonghi Combi & Functions Convection, Milano, Italy) at 80° for 10 min. The dried extrudates with a moisture content of about 8% were stored in polyethylene bags and analysed. 2.5. Analyses All analyses were replicated at least in triplicate. 2.6. Oil loss % The percentage of oil loss was calculated as a difference between oil content in raw mixtures and oil content in extrudates and expressed as g of oil loss/100 g of product. Before extraction, extrudates were finely grinded (particles <300 lm) by mill BUHLER ML 1204 (Germany).The extraction time (hours) was chosen after preliminary tests to obtain maximum yield in oil. Oil was extracted with n-hexane (Carlo Erba Reagenti, Milano, Italy) by Soxhlet model B-811 (Switzerland) apparatus. 2.7. Breaking strength and deformability A dynamometer stable Micro System TA-HDi Texture Analyser (ENCO s.r.l., Venezia, Italy) with a small wedge (0.5 mm/s speed) was used for the rheological tests. Results were expressed as breaking strength (N/mm2), i.e., the strength applied to breaking extrudate, and deformability (mm), i.e., the measure of extrudate deformability before rupture. 2.8. Volume of extrudates Sample volume was measured using the method described by Barber, Ortola, Barber, and Fernandez (1992). This analysis was carried out trough a measuring jug with a volumetric scale (pycnometer). The pycnometer was filled with flax seeds up to maximum level and then weighed. After, the container was emptied, filled with 10 g of sample and flax seeds up to maximum level and then weighed. Volume of the extrudates was expressed in millilitres and calculated according to the following formula: Volume ðmlÞ ¼
½wc þ wf ðwcfs ws Þ df
where wc is weight of container; wf, is weight of flax seeds; ws, is weight of sample; wcfs, is weight of container, flax seeds and sample; df, is density of flax seeds. For all tests were weighted 10 g of extrudates.
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2.9. Image analysis Air cell structures of extrudates were measured using the method described by Barrett and Ross (1990). Fifteen extrudates were selected from all samples. Each extrudate was cross-sectioned out. The section was pressed against a desk pad to blacken cell structures. The sections of extrudates were photographed using a dv-camera (CANON MV6i MC, Japan) with a 275 mm lens (0.8 Megapixel, MICRO NICON). Images were analysed using the OPTIMAS ver. 6.1 software (Bioscan, USA). This software allowed to evaluate the surface porosity by applying a grey level value (i.e., position between black and white) to each pixel in the image. Threshold levels were selected to construct a binary (black and white) image in which pixels having grey levels between the thresholds were black, and pixels having grey levels below these values were white. The area of the black object within the binary image was measured. Results were expressed as percentage ratio between surface of pores and surface of all extrudate section (porosity %). 2.10. Statistical analysis The variance analysis (ANOVA) was carried out on results obtained from the physical analysis of extruded products. ANOVA was carried out using software StatSoft ver. 6.0 (Statsoft, Tulsa, USA). The means of these results were compared by the Fisher’s test. 3. Results and discussion Fig. 1 shows the percentages of oil loss obtained during extrusion processing of samples containing 0%, 0.2% and 0.7% of four studied emulsifiers. According to ANOVA, the type and concentrations of emulsifiers had a significant effect on oil loss (Table 4). In particular, doughs with added emulsifiers showed a smaller oil loss than the control (Fig. 1). Moreover, oil loss from doughs containing 0.7% of MDG and MG was greater than formulas with 0.2% of emulsifiers. Probably, the high temperature of extrusion processing and the increase of emulsifier concentration favoured the transition from o/w to w/o emulsions (Gasˇic´, Jovanovic´, & Jovanovic´, 2002; Kruglyakov & Nushtayeva, 2004; Ruckenstein, 1999; Sajjadi, Jahanzad, & Yianneskies, 2004). The best results were obtained for doughs with added SE that did not show oil loss. This phenomenon can be explained by considering the HLB index (Hydrophile/Lipophyle Balance) of the emulsifiers. The HLB system is based on the concept that some molecules have hydrophilic groups, other molecules have lipophilic groups, and some have both. Weight percentage of each type of group, in a molecule or in a mixture, can predict what behaviour the molecular structure will exhibit. The scale as originally proposed by Griffin (1949) ranged
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Fig. 1. Oil loss % (mean and standard deviation) occurring during extrusion process of extrudates added with 0%, 0.2% and 0.7% of different emulsifiers.
Table 4 Variance analysis related to oil loss % data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Degree Mean of freedom square
F
Intercept Type of emulsifiers Concentration of emulsifiers Error
901.2004 49.4057
1 3
901.2004 345.5404 <0.0001 16.4686 6.3144 0.0019
353.6216
2
176.8108
78.2427 30
2.6081
67.7932
p
0.0001
from 0–20, with the low end signifying an emulsifier that is much more soluble in oil than in water, and the high end signifying the reverse. As a general rule, w/o emulsions are stabilized by HLB values in the 3–6 range, o/w emul-
sions are stabilized by HLB values in the 11–15 range, and intermediate HLB values give good wetting properties but not good emulsion stability (Vecchio, 2002). In a practical system that includes other ingredients such as sugar, salt, protein and other food components, the optimal HLB might be somewhat different. In our case, SE, that have a HLB number between 8 and 15, despite the presence of other ingredients and the high processing temperature, were able to retain the fatty fraction with no oil loss. Instead, as expected, MDG, SL and MG having an HBL index between 2.8 and 4.3 were less effective in oil loss reduction because they favoured the formation of water in oil emulsions. Fig. 2 shows the breaking strength of extrudates with different type and percentage of emulsifiers. Also in this
Fig. 2. Breaking strength (N/mm2) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
T. De Pilli et al. / Journal of Food Engineering 79 (2007) 1351–1358
case the type and concentrations of emulsifiers showed a significant effect on break strength (Table 5). It can be observed that the samples with emulsifiers had a breaking strength values smaller than control (Fig. 2). In particular, extruded products containing sucrose esters (SE) showed the smallest breaking strength values and then resulted more fragile than samples added with other emulsifiers. However, the values of extrudate breaking strength increased with increasing of emulsifier percentages at 0.7%. The samples with emulsifiers showed lower values of breaking strength than control probably because of capacity of these additives to incorporate and stabilize air bubbles in dough (Richardson, Langton, Faldt, & Hermanson, 2002) that favours the formation of homogeneous porosity structure of the extrudates. This capacity is more effective with sucrose esters (Ebeler & Walker, 1984). The sucrose esters interacting with the flour proteins by means of hydrophilic and/or hydrophobic bindings, favour the development of a more flexible gluten network (Wijnans, Baal, & Vianen, 1993). The sudden drop of pressure, which is subjected flexible network at the exit of the die, causes an instantaneous evaporation of water that gives a more
Table 5 Variance analysis related to break strength (N/mm2) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Intercept Type of emulsifiers Concentration of emulsifiers Error
28.43237 1.26983
Degree of freedom
Mean square
F
p
1 3
28.43237 0.42328
722.0910 10.7498
<0.0001 0.0001
1.05567
2
0.52783
13.4053
0.0001
1.18125
30
0.03938
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Table 6 Variance analysis related to porosity % data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%) Effect
Sum square
Intercept Type of emulsifier Concentration of emulsifier Error
3396.74 1273.66
Degree of freedom
Mean square
F
p
1 3
3396.74 424.55
94.7884 11.8475
<0.0001 <0.0001
308.06
2
154.03
4.2983
0.030
645.03
18
35.83
expanded and porous structure to the extrudate. So results obtained from image analysis showed that samples added with emulsifiers have had a porosity value compared with samples without emulsifier (Fig. 3). Moreover, ANOVA results showed that type and concentration of emulsifier had a significant effect on porosity extrudate (Table 6). The samples that showed the highest values of porosity percentages were those added with sucrose esters in both percentages (0.2% and 0.7%). This behaviour could be explained considering that almond oil coming outside of almond flour, during extrusion processing, had a negative impact on air incorporation. The addition to dough of an oil soluble emulsifier as sucrose esters, could favour the formation of a solid film at the oil/water interface, which encapsulates the oil and prevents collapse of the foam (Richardson, Bergenstahl, Langton, Stading, & Hermansson, 2004). Deformability of the extrudates is an index of the energy opposed to the product at the strength applied. The deformability of extrudates added with 0.2% and 0.7% of different emulsifiers is reported in Fig. 4. Results of ANOVA showed that concentration of emulsifier did not have a significant effect on extrudate deformability; instead type of
Fig. 3. Porosity % of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
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Fig. 4. Deformability (mm) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
emulsifier had a significant effect on this parameter (Table 7). In particular, the mono-glycerides added with 0.2% showed the smallest value of deformability. It is known that
deformability is correlated to degree of gelatinization (Case, Hamann, & Schwartz, 1992) and then small values of this index could be due to the retard of starch granule swelling
Table 7 Variance analysis related to deformability (mm) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%)
Table 8 Variance analysis related to volume (ml) data of extrudates added with different type and percentages of emulsifiers (0%; 0.2%; 0.7%)
Effect
Sum square
Intercept Type of emulsifiers Concentration of emulsifiers Error
0.539981 0.002411
Degree of freedom
Mean square
F
p
Effect
Sum square
1 3
0.539981 0.000804
2550.816 3.797
<0.0001 0.02
4313.030 55.460
0.000043
2
0.000022
0.102
0.006351
30
0.000212
Intercept Type of emulsifiers Concentration of emulsifiers Error
0.90
Degree of freedom
Mean square
F
p
1 3
4313.030 18.487
1375.956 5.898
<0.0001 0.003
83.433
2
41.716
13.308
0.0001
94.037
30
3.135
Fig. 5. Volume (ml) of extrudates (mean and standard deviation) added with 0%, 0.2% and 0.7% of different emulsifiers.
T. De Pilli et al. / Journal of Food Engineering 79 (2007) 1351–1358
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Table 9 Regression results obtained from correlation of extrudate properties (oil loss, break strength, deformability, volume and porosity) Properties of extrudates
Regression equation 2
Oil loss %: break strength (N/mm ) Oil loss %: deformability (mm) Oil loss %: volume (ml) Oil loss %: porosity % Break strength (N/mm2): deformability (mm) Break strength (N/mm2): volume (ml) Break strength (N/mm2): porosity % Deformability (mm): volume (ml) Deformability (mm): porosity % Volume (ml): porosity %
*
y = 0.63712 + 0.06535 x y = 0.11645 + 0.000691*x y = 10.853 0.0346*x y = 17.358 2.136*x y = 0.10513 + 0.01549*x y = 7.9255 + 3.2616*x y = 38.757 33.35*x y = 0.29944 + 88.081*x y = 5.709 + 132.89*x y = 1.757 + 1.0984*x
r
p-level
0.82 0.14 0.04 0.58 0.28 0.28 0.72 0.41 0.154 0.27
0.007* 0.725 0.924 0.104 0.453 0.465 0.027* 0.275 0.691 0.473
r: Correlation coefficient. * Significant at a p < 0.05 level.
and, thus, starch gelatinization. The reduction of starch gelatinization could be, probably, related to the formation of lipid starch complexes that are derived from the lipophilic portions of emulsifiers with the amylose fraction of starch during cooking. Starch granule swelling and solubility decline with an increase in the complex formation (Eliasson, 1985; Ghiasi, Varriano-Marston, & Hoseney, 1982). Fig. 5 shows the extrudate volume of different types and percentages of emulsifiers. Both variables had a significant effect on extrudate volume (Table 8). The most of samples added with 0.2% of emulsifiers had volume values similar to control sample, with exception of extruded products with soy lecithin that showed a volume value lower than control sample. It can suppose that the presence of lecithin, probably, reduced the water absorption of dough that caused a decrease of the starch gelatinization degree (Knightly, 1989). With increasing of emulsifier percentages, there are significant differences between the volume values of different samples. In Table 9 regression results obtained from correlation of extrudate properties (oil loss, break strength, deformability, volume and porosity) are shown. Break strength is positively correlated with oil loss and negatively with porosity. The positive correlation between oil loss and break strength could be due, as hypothesized by Guy (1994), to the presence of high free fat fraction that reduced starch dispersion with consequent decrease of product expansion that imparts a compact structure, as biscuits. Instead, the negative correlation between break strength and porosity is obviously because of the porous structure makes extrudate more fragile. 4. Conclusion Results showed that sucrose esters are the most suitable emulsifiers to reduce oil loss during extrusion processing. In fact, samples added with sucrose ester did not show oil loss during extrusion processing already at the smallest percentage of this emulsifier. Moreover, the addition of emulsifiers significantly affected the rheological characteristics of extruded prod-
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