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Effect of sugar, citric acid and egg white type on the microstructural and mechanical properties of meringues
Journal of Food Engineering 108 (2012) 453–462
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of sugar, citric acid and egg white type on the microstructural and mechanical properties of meringues Fabio Licciardello b, Pierangelo Frisullo a, Janine Laverse a, Giuseppe Muratore b, Matteo Alessandro Del Nobile a,c,⇑ a
Department of Food Science, University of Foggia, Via Napoli, 25, 71100 Foggia, Italy Department of Agricultural and Food Productions (DISPA), Section of Food Technology and Microbiology, University of Catania, Via Santa Sofia, 98, 95123 Catania, Italy Istituto per la Ricerca e le Applicazioni Biotecnologiche per la Sicurezza e la Valorizzazione dei Prodotti Tipici e di Qualità, Università degli Studi di Foggia, BIOAGROMED, Via Napoli, 52, 71100 Foggia, Italy
b c
a r t i c l e
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Article history: Received 7 March 2011 Received in revised form 2 August 2011 Accepted 19 August 2011 Available online 6 September 2011 Keywords: Food microstructure Egg white Mechanical properties Meringues X-ray microtomography
a b s t r a c t Meringues are characterized by a predominant air phase and their overall quality is intimately related with microstructure. The formation of meringues microstructure relies on the capacity of egg white (EW) proteins to form voluminous and stable foams and it is ultimately related with the chemical properties of proteins and with the addition of ingredients such as sugars, salts, acids and surfactants. The study aimed at assessing the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties of meringues. Meringues prepared with different sugar/EW, citric acid level and different EW type were subjected to microstructural analysis by X-ray microtomography and to mechanical assessment by compression tests. Results demonstrate the ability of X-ray microtomography to reconstruct the 3D microstructure of meringues allowing the measurement of porosity, size, shape and distribution of pores. Citric acid, sugar concentration and EW type play a fundamental role on meringues microstructural parameters and mechanical properties. Low sugar/EW ratios as well as increasing citric acid levels increase the air phase and result in a softer texture of meringues. Moreover, low sugar/EW ratios and increasing citric acid in the meringue result in a reduction of pore size and also influence the shape of pores. Meringues microstructural and mechanical properties are affected by the EW quality: fresh and pasteurized EWs and EWs stored at refrigerated temperatures scored the highest structural and mechanical performances, while powdered and frozen EWs and albumens from old eggs showed the worst results. Not only the balance among ingredients but also the choice of raw materials can strongly affect the final quality of meringues. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Meringues are confectionery products simply made of EW sugar and small amounts of salts and acids added as foam stabilizers, characterized by a predominant (>80%) air phase. Such characteristic relies on the capacity of EW proteins to form voluminous and stable foams. Meringues, due to their simple composition, represent a model system to study the behavior of EW froths, which are the basis for other, more complex, bakery products. The characteristics of meringues are related to their microstructure that, in turn, depends on the composition and on the balance among ingredients. The formation of food microstructure is a complex phenomenon which, in the specific case of EW froths, relies on a structure-building phase, i.e. whipping, and on the opposite concurring stage of structure disruption, i.e. foam drainage (Kato et al., ⇑ Corresponding author at: Department of Food Science, University of Foggia, Via Napoli, 25, 71100 Foggia, Italy. Tel./fax: +39 881 589 242. E-mail addresses: [email protected], [email protected] (M.A. Del Nobile). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.08.021
1983; Campbell and Mougeot, 1999; Foegeding et al., 2006). Such phases are regulated by foamability and foam stability, respectively, two properties related with the chemical properties of proteins and with the addition of other ingredients such as sugars, salts, acids and surfactants (Li-Chan and Nakai, 1989; Raikos et al., 2007). Meringues are subject to a heat treatment aimed at ‘‘freezing’’ the formed structure by converting the foam from a liquid to a solid. The 3D microstructural information can provide a better understanding of the physical structure of the product and from an engineering perspective; knowledge about the microstructure of foods can be used to identify the important processing parameters that affect the quality of a product. Structure–property relationships can strongly affect the physiochemical, sensorial, technological and even nutritional properties of foods. Processes are no longer designed from a macroscopic level; knowing the properties of foods on the micro scale determines the process specification. Given the enormous success of X-ray microtomography (lCT) in medical applications, material science, chemical engineering,
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Table 1 Scheme of the experimental plan for meringues preparation.
A1 A2 A3 A4 = C1 A5 B1 B2 B3 = C7 C1 = A4 C2 C3 C4 C5 C6 C7 = B3 a
Sugar/ EW
Citric acid EW type (% on EW)
0.5:1 1:1 1.5:1 2:1 2.5:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1
1 1 1 1 1 0 0.5 1 1 1 1 1 1 1 1
Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 2 Pasteurized 2 Pasteurized 2 Pasteurized 1 From fresh eggs Not from fresh eggs (>28 days) Not from fresh eggs (>28 days), stored at 4 °C Powdereda Frozen Pasteurized 2
Reconstituted with water at 1:7 (w:w) ratio.
geology and biology, it is not surprising that in recent years much attention has been focused on extending this imaging technique to food science, as a useful technique to aid in the study of food microstructure. X-ray microtomography has been successfully used to observe the stability of gas bubbles in dough during the bread making process (Whitworth and Alava, 1999), the microstructure of foams (Lim and Barigou, 2004), three-dimensional quantitative analysis of breadcrumb (Falcone et al., 2005), the study of bread porous structure (Falcone et al., 2004) and ice crystals within frozen foods (Mousavi et al., 2005). Recently this technique has also been used to study the bubble size distribution in
Fig. 2. Three-dimensional reconstructions of meringue microstructure: (a) type A, (b) type B, (c) type C.
wheat flour dough (Bellido et al., 2006), the effect of far-infrared radiation assisted drying on microstructure of banana slices (Léonard et al., 2007), three-dimensional pore space quantification of apple tissue (Mendoza et al., 2007), the role of sugar and fat in sugar-snap cookies (Pareyt et al., 2009), the study of processed meat microstructure and the assessment of intramuscular fat level and
Fig. 1. Gray level reconstructed cross section images of meringues acquired by lCT: (a) type A, (b) type B, (c) type C.
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(a) 100
(b)
POV A a b
90
OSVR A
0.05 0.045
b
0.04 80
c
70
0.035
c
0.03
OSVR
% POV
60 50 40
c
A4
A5
0.025 0.02 0.015
30
c
a
a,b
A1
A2
b
0.01
20
0.005
10
0
0 A1
A2
A3
A4
A5
variety
variety
Test Duncan;variabile Var2 (Spreadsheet10) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,00000, gl = 10,000
Test Duncan;variabile Var2 (Spreadsheet5) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= 23,470, gl = 10,000
Var1 Cella N. 4 5 3 2 1
POV A4 POV A5 POV A3 POV A2 POV A1
Var1
Var2 1 2 3 Media 52,74765 **** 53,34641 **** 72,65410 **** 73,41983 **** 82,59266 ****
Cella N. 1 2 3 5 4
(c) 9
A3
OSVR A1 OSVR A2 OSVR A3 OSVR A5 OSVR A4
Var2 1 2 3 Media 0,007202 **** 0,009692 **** **** 0,011222 **** 0,020786 **** 0,022144 ****
SMI A
a
8 7
SMI
6 5
b,c
b,c
b,c
b
4 3 2 1 0 A1
A2
A3
A4
A5
variety Test Duncan;variabile Var2 (Spreadsheet17) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= 2,1101, gl = 10,000
Var1 Cella N. 3 2 5 4 1
SMI A3 SMI A2 SMI A5 SMI A4 SMI A1
Var2 Media 2,157400 2,714216 3,977610 4,018926 5,493312
1
2
**** **** **** **** **** **** **** ****
Fig. 3. (a–c) Microstructural parameters for type A meringue samples: POV (a), OSVR (b), and SMI (c).
distribution in beef muscles (Frisullo et al., 2009) and the sugar crystals distribution and size and cocoa matter content in cold-processed chocolate (Frisullo et al., 2010). Previous works on EW
foaming properties used conventional image analysis techniques (Stevenson et al., 2007) or confocal laser scanning microscopy (Pernell et al., 2002; Raikos et al., 2007) to study the microstructure
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parameters in EW foams while, as far as we are concerned, tomographic techniques have never been used to study the microstructure of meringues. In this paper, the pore microstructural and mechanical properties of different types of meringues were studied. Accordingly, this work aimed at demonstrating the effectiveness of X-ray microtomography as a useful tool to study the microstructure of meringues, and at assessing the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties. 2. Materials and methods 2.1. Meringues preparation Meringues were prepared by a professional confectioner using different sugar/EW ratio, citric acid content and EW type, as indicated in Table 1. Fresh eggs were bought from a local retailer and were used within 7 days from laying. The other eggs were bought 35 days in advance to meringues preparation, one batch (not from fresh eggs) was stored at room temperature while another batch (not from fresh eggs stored at 4 °C) was stored at refrigerated conditions. Both batches were bought from the same retailer and stored for 35 days. pH of fresh EW was 7.40 and that of not fresh EW was 9.18 and 8.75 for those stored at room temperature and at 4 °C, respectively. Pasteurized EWs, packed in 1-l multilayer briks, distributed by Eurovo S.r.l. (Mordano, BO, Italy) were bought from a local retailer. Samples named ‘‘pasteurized 1’’ and ‘‘pasteurized 2’’ differed in the shelf life of the product, 35 days for the former and 6 months for the latter, as a result of different pasteurization process applied. In particular, ‘‘pasteurized 2’’ egg whites were obtained by a modern pasteurization process called ‘‘liottization’’, protected by patent. pH of pasteurized 1 and 2 egg whites was 9.0 and 9.1, respectively. Standard-grade, powdered EWs were supplied by Eurovo S.r.l. (Mordano, BO, Italy), and were used after reconstitution with water at 1:7 (w:w) ratio, following instructions of the manufacturer. The dry matter of reconstituted EWs was 11.5%, as calculated from the labeled value of the powder (92%), and was coincident with the dry matter values of other EW types. EWs were whipped into a steel bowl at 660 rpm at room temperature using a professional planetary mixer (mod. Plutone 7, Sirman Spa, Pieve di Curtarolo, PD, Italy). Refrigerated and frozen EWs were brought to room temperature before use. Citric acid was added to the EWs before beating. Sugar, added with a small amount of water (1% of the EW volume), was melt and brought to the ‘‘soft ball’’ stage (115 °C) before mixing with the whipped
EWs. Meringues were shaped in balls of about 3 cm diameter using a manual pastry bag provided with a head having diameter of 1.5 cm and they were cooked in oven (G-Therm 075, Galli, Milan, Italy) at 80 °C for 2 h. 2.2. Tomographic analysis The samples were imaged under the same conditions, using the Skyscan 1172 high-resolution desktop X-ray microtomography system (Skyscan, Belgium). A sample was placed on a round metal plate; the source and the detector were fixed, while the sample was rotated during measurement. Power settings of 29 kVp and 175 lA were used. A CCD camera with 4000 2096 pixels was used to record the transmission of the conical X-ray beam through all samples. The distance source–object-camera was adjusted to produce images with a pixel size of 2.8 lm. Four-frame averaging, a rotation step of 0.60° and an exposure time of 589 ms were chosen to minimize the noise, covering a view of 180°. Smoothing and beam-hardening correction steps were applied to suppress noise and beam hardening artifacts, respectively. Beam hardening correction was only moderately applied, set to 25% within NRecon and a fast ring artifacts reduction (set to 25 within NRecon) was also applied. Once initial parameters were set, the acquisition step was completely automated and did not require operator assistance. Scan time, on average, required 70 min. A set of flat cross section images, was obtained for each sample after tomographical reconstruction by the reconstruction software NRecon (Skyscan). Three-dimensional reconstructions of samples were created by effectively stacking all 2D tomographs, a total of 420 slice images with a slice spacing of 0.069 mm. For image processing and analysis, the Skyscan software, CTAnalyser (CTAn) was used. Image segmentation was firstly carried out on the smoothed 8-bit gray-scale images obtained from the reconstruction step, using CTAn (Skyscan) software. Segmentation is the process of converting the gray-scale images into black and white images by assigning the value 1 to all pixels whose intensity was below a given gray tone value and 0 to all the others. For this, an automatic threshold based on the entropy of the histogram (Sahoo et al., 1988) was calculated for each image. The lower gray threshold (0) and upper gray threshold (60) values were identified; each sample was processed under the same conditions. For data analysis, prior to 3D reconstruction, a component-labeling algorithm, available within CTAn, was used to isolate the largest 3D connected structures. All reconstructions where created in CTAn (Skyscan) using an adaptive rendering (locality 10 and tolerance 0.25) algorithm A2
A5
50
A4 A3
Force in N
40
30
20
10
0
0
2
4
6
8
10
Path in mm Fig. 4. Typical compression curves for type A meringue samples.
12
A1
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POV B
100
0.05
90
0.045
80
0.04
70
0.035
60
OSVR B
(b)
a
a
a
50 40
a
a a
0.03
OSVR
% POV
(a)
0.025 0.02
30
0.015
20
0.01 10
0.005 0 B1
B2
0
B3
B1
variety Test Duncan;variabile Var2 (Spreadsheet4) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= 59,438, gl = 6,0000
Var1 Var2 Media Cella N. 1 POV B1 37,55408 POV B2 40,06950 2 POV B3 44,58951 3
B3
Test Duncan;variabile Var2 (Spreadsheet12) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,00002, gl = 6,0000
1
Var1 Var2 Media Cella N. 3 OSVR B3 0,024012 OSVR B2 0,026868 2 1 OSVR B1 0,031030
**** **** ****
9
B2
variety
1 **** **** ****
SMI B
(c)
8 7
SMI
6 5 4 3
a
a
a
2 1 0 B1
B2
B3
variety Test Duncan;variabile Var2 (Spreadsheet19) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,25601, gl = 6,0000
Var1 Var2 Media Cella N. 1 SMI B1 1,250178 SMI B3 1,387836 3 SMI B2 1,640356 2
1 **** **** ****
Fig. 5. (a–c) Microstructural parameters for type B samples: POV (a), OSVR (b), and SMI (c).
and saved as P3G surface model (SkyScan model format). P3G models were then imported into CT vol software (Skyscan) for visualization. The following microstructural parameters were used for this study: (1) the percentage object (pore) volume, i.e. the geometric parameter POV obtained by lCT analysis, was calculated for each sample as a representation of the percentage total pore content within the sample. (2) The surface/volume ratio (OSVR) of the
pores present in the samples; a low OSVR value indicates high porosity and the formation of larger pore sizes at the cost of smaller ones. (3) The fragmentation index (FI) parameter is the index of connectivity and is a measure of the connectivity of the total solid surface (Lim and Barigou, 2004). A negative FI signifies better-connected solid lattices while a positive FI indicates a more disconnected solid structure. (4) The degree of anisotropy (DA) is a measure of the 3D structural symmetry, i.e. in this case it indicates
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50
B1
B2
B3
Force in N
40
30
20
10
0 0
2
4
6
8
10
12
Path in mm Fig. 6. Typical compression curves for type B meringue samples.
the presence or absence of preferential alignment of the pores present along a particular direction (Lim and Barigou, 2004). A value of 0 would correspond to total isotropy, whereas a value of 1 would indicate total anisotropy. (5) The SMI parameter, that is a topological index, gives an estimate of the characteristic shape in terms of plates and cylinders composing the 3D structure (Hildebrand and Ruegsegger, 1997) and is calculated using a differential analysis of triangulated surface of the structure under examination. The SMI assumes integer values of 0, 3 and 4 for ideal plates, cylinder and spheres, respectively. 2.3. Determination of mechanical properties Mechanical tests were carried out using a Zwick Roell BT1FR0.5TN.D14 texture analyzer. Compression curves were obtained using a load cell of 50 N and setting test rate at 10 mm/min. Slopes and shapes of compression curves were used as indicators of the overall mechanical properties of meringues: slopes represent the hardness of samples, while shape (i.e. curve from smooth to rough) gives an indication of the micro and macro-cracks suffered by the samples during compression. 2.4. Statistical analysis All parameters obtained were submitted to one-way analysis of variance (ANOVA) and Duncan’s test (p < 0.05) through the statistical package Statistica for Windows (Statsoft, Tulsa, USA).
in which the brighter regions correspond to the higher level of attenuation, i.e. higher density region. It can be assumed from these figures that the darkest areas represent the holes (air) as it has a lower absorption coefficient with respect to structure. Fig. 2 shows as examples the three-dimensional reconstructions of the samples from which the geometrical parameters were calculated using the CTAn software (Skyscan). From these images the different types of microstructure and the number of holes in the samples can be noted. Results (data not shown) highlight negative FI values for all the three groups of meringues investigated in this work, hence it can be stated that in general the pores present in meringues have connected solid lattices. Moreover, basing on DA results (data not shown), pores present in all the groups of meringue samples are isotropic in nature. Results on POV, OSVR and SMI will be analyzed with respect to each variable considered. With regards to mechanical tests, the slope and shape of compression curves were analyzed to acquire information on the overall mechanical properties of meringues: steeper slopes correspond to hard and stiff samples, while the shape, i.e. presence of peaks in the curves are related to micro and macro-cracks during compression, hence to the micro and macro-porosity of meringues. Meringue topography, with special regards for sample contact area at first contact, affects mechanical measurement, being that the first fracture point in the compression curves varies with the sample contact area at first contact; on the other hand, the slope and the number of fracture points are more reliable indexes of hardness.
3. Results and discussion
3.1. Effect of sugar/EW ratio
Since the aim of the work is to assess the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties of meringues, results will be divided in three paragraphs, each one analyzing the effects of one variable on the meringue’s microstructural and mechanical parameters. Firstly, some general comments on the microstructural and mechanical parameters are reported. With regards to lCT images, Fig. 1 shows as examples the gray level reconstructed cross section images of the samples acquired by lCT, the contrasts in these images are based on the differences in absorption of X-rays by the constituents of the sample (e.g. structure and air). This contrast is produced by a variation of density and a change in composition of the sample and is based exclusively on the detection of an amplitude variation of X-rays transmitted through the sample itself. The obtained image is a map of the spatial distribution of the X-rays
Fig. 3a shows the percentage POV values for all type A meringue samples. It can be noted from this figure that an increase in the sugar/EW ratio within this group (from A1 to A5) results in a decrease in POV and as sample A1 has the lowest sugar content it therefore has the highest percentage volume of pores present. On the basis of confectioner’s indications, we considered as standard meringues the ones produced using a sugar/EW ratio of 2:1 (samples A4): this ratio gave samples with the lowest porosity (i.e. lower POV), while a reduction of such ratio is effective in obtaining products with higher porosity. As it can be inferred from Fig. 3b, there is an increment of OSVR with the increase of the sugar/EW ratio, for example, samples A4 and A5 that have the highest sugar/EW ratio have the highest and statistically equal OSVR values hence have a higher volume of smaller pores compared to the other three samples within the group; on the other hand, sample A1 and A2 have the lowest and statistically equal OSVR values. Results therefore
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(a)
POV C
0.05
100
a
0.04 0.035
a b,a
b,c
b,a
c,b
b,a
OSVR
a
70
% POV
a
0.045
90 80
OSVR C
(b)
60
c
50
0.03
b,c
b,c
b,c
0.025
b 0.02
40
0.015
30 20
0.01
10
0.005 0
0
C1
C2
C3
C4
C5
C6
C7
C1
C2
C3
variety
Var1 POV C3 POV C5 POV C7 POV C6 POV C2 POV C1 POV C4
Var2 Media 32,69755 38,10280 44,58951 44,65867 46,58883 52,74765 54,00388
C5
C6
C7
variety
Test Duncan;variabile Var2 (Spreadsheet8) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= 32,558, gl = 14,000
Cella N. 3 5 7 6 2 1 4
C4
1
2
Test Duncan;variabile Var2 (Spreadsheet15) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,00002, gl = 14,000
3
Var1 Cella N. 4 2 1 7 6 3 5
**** **** **** **** **** **** **** **** **** **** ****
9
(c)
OSVR C4 OSVR C2 OSVR C1 OSVR C7 OSVR C6 OSVR C3 OSVR C5
Var2 Media 0,015775 0,018386 0,022144 0,024012 0,024931 0,034192 0,034630
1
2
3
**** **** **** **** **** **** **** **** **** ****
SMI C
8 7
SMI
6 5
a
4
b b,c
3
d
c,d
b,c,d
C5
C6
C7
2 1 0 C1
C2
C3
C4
variety Test Duncan;variabile Var2 (Spreadsheet21) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,13625, gl = 14,000
Var1 Cella N. 5 6 7 4 3 2 1
SMI C5 SMI C6 SMI C7 SMI C4 SMI C3 SMI C2 SMI C1
Var2 Media 0,785192 1,047562 1,387836 1,394600 1,499624 1,785068 4,018926
1
2
3
4
**** **** **** **** **** **** **** **** **** **** **** **** ****
Fig. 7. (a–c) Microstructural parameters for type C meringue samples: POV (a), OSVR (b), and SMI (c).
suggest that an increase in the sugar/EW ratio determines higher volumes of very small pores that are evenly disperse. The results for the SMI parameter, Fig. 3c, suggest that on average the pores present in this group are cylindrical to spherical in nature. Although there is not a clear trend for the SMI parameter in this group, it can be noted that the sample with the lowest sugar/EW ratio (A1) and those with the highest sugar/EW ratio (A4 and A5)
have SMI values (above 3) suggesting the presence of truly spherical pores. As it can be inferred from Fig. 4, an increase of the sugar/EW ratio (from A1 to A5) determines a change in the mechanical behavior of meringues. The compression curve relative to A1 highlights small peaks which indicate numerous internal micro-cracks during compression; moreover, the curve has a slight slope, indicating
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C6
C3
C5
C2
C4
C1
40
Force in N
C7
30
20
10
0
0
2
4
6
8
10
12
14
Path in mm Fig. 8. Typical compression curves for type C meringue samples.
that a minimum strength is required to compress the probe and that the meringue structure does not oppose resistance to compression, that is to say that meringue is rather soft. The curve of sample A3 appears more discontinuous, being characterized by some sharp drops that can be related to internal structural collapses due to compression. The curve shape together with its slope, slightly higher for A4 samples, indicates a higher stiffness of structure and a more crunchy texture. Finally, the curve relative to A5 indicates higher hardness, as it is possible to infer from the very high slope of the compression curve. For all samples the mechanical test was considered concluded as soon as the compression force reached the cell load threshold value (i.e. 50 N). The results confirm that the sugar content is strongly related to the microstructure and hence the textural properties of meringues. Indeed, as can be inferred from Figs. 3a, b and 4, an increase in sugar/ EW ratio results in the decrease of the POV parameter, in the increase of OSVR and therefore an increase in hardness. These results confirm what has been reported by McGee (1984) and Labensky and Hause (1999) who state that the amount of sugar depends on the final application of the meringue and the lightness required, being that soft meringues have sugar at less or equal the amount of EW and that hard ones have twice the amount of sugar than EW. It is known that sugar acts as stabilizing for EW foam (Campbell and Mougeot, 1999), therefore the lower POV values observed for meringues with higher sugar/EW ratio should be a consequence of a reduced foamability compared with those obtained with low sugar/ EW ratios. Maache-Rezzoug et al. (1998) highlighted the effect of sugar on bakery products, reporting that sucrose acts as a hardening agent in biscuits by crystallizing after cooling, making products crispy and fragile. To sum up, mechanical properties depend on microstructure; meringues that have low POV values and high OSVR values are stiffer due to the fact that they have a low air phase and smaller pores while meringues with a high POV and low OSVR are softer as they have a higher air phase and bigger pores. 3.2. Effect of citric acid addition As observed from Fig. 5a for meringues group B, there is a slight increase in the POV values (although not statistically significant) as a result of an increase of citric acid level, therefore the citric acid does have a slight effect on the percentage pore volume of the meringue. Hammershoj et al. (1999) and Li-Chan and Nakai (1989) demonstrated that foaming properties of egg white proteins are optimal near their isoelectric points, and that the addition of acid to egg white improves foaming properties by decreasing pH, such that it is closer to the isoelectric point. Similar results have been
obtained with whey proteins (Davis et al., 2004; Phillips et al., 1990). On the other hand, it can be noted from Fig. 5b that there is also a slight decrease in OSVR (again not statistically significant) with the increase of citric acid: as it is evident from a comparison among the samples within this group (B1, B2 and B3), B1 has the highest OSVR whilst sample B3 has the lowest OSVR value. Therefore an increase of citric acid slightly increases the pore size and hence the percentage porosity. With regards to the SMI parameter, Fig. 5c, it can be noted that the general shape of the pores is elongated to cylindrical in nature and there does not seem to be any trend with the increase of citric acid. The addition of citric acid (increasing from B1 to B3) determines a progressive distension of the compression curves (Fig. 6), which for samples B1 stops at 4.5 mm path. Moreover, the height and number of sharp peaks, which represent macro-cracks of the meringues during compression test, decrease, as to testify a less rigid structure characterized by high porosity. The test highlights that the addition of citric acid affects the microstructure and the textural properties of meringues. Indeed, as can be inferred from Figs. 5a and b and 6, an increase in the citric acid level results in the slight increase of the POV parameter, slight decrease of OSVR and hardness. As stated in the previous paragraph, the mechanical properties depend on the microstructure, being that meringues, which are less porous, i.e. high volume of smaller pores, are stiffer than meringues with higher air phase and bigger pores, i.e. higher POV. 3.3. Effect of EW type Fig. 7a shows the POV values for the meringue samples of group C; as it can be noted from this figure, sample C4 and C1 have the highest POV value and sample C3 the lowest POV value. Whereas with regards to Fig. 7b relative to the OSVR parameter, samples C3 and C5 have the highest statistically equal OSVR values and sample C4 the lowest. Therefore it can be stated that the EW type does have an effect on the POV and OSVR parameters. Fig. 7c shows that only meringues prepared with pasteurized EW 1 (sample C1) score SMI values corresponding to ideal spheres, while the other samples obtain SMI values ranging between 1 and 2, while only sample C5 (meringues prepared with powdered EW) scored values <1. Different EW types can dramatically influence the mechanical properties of meringues (Figs. 8 and 9). This is especially true for C5 and C6 samples, made from powdered and frozen EWs, respectively, compared with C1, C2 and C7. The former samples are characterized by compression curves with steep slopes, reaching the maximum threshold for the test (50 N) after 2–4 mm path, and by sharp peaks, indicating macro-cracks during compression and
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a stiff structure. On the other hand, the latter meringues show comparable curves, all of them reaching at least 9 mm of path and characterized by a variable number of sharp drops. Among the latter samples, C7 shows the smoothest curve, while C3 the roughest. The results show that EW quality clearly affects the microstructure and the textural properties of meringues. Indeed, as it can be inferred from Figs. 7a, b and 8, the use of better quality EWs (i.e. fresh or pasteurized) results in higher POV values, lower OSVR and hardness, while low quality EWs (i.e. powdered, frozen or coming from old eggs) make stiff meringues, with lower air phases. Meringues produced with not fresh eggs stored at 4 °C (C4) score values similar to those obtained with good quality EWs, this is probably due to the fact that storage at low temperature preserves their technological properties despite the long storage time. With regards to powdered EWs, this is in agreement with previous studies that state the effect of the drying process of EWs on their technological properties (Lechevalier et al., 2007). Once more, it was confirmed that mechanical properties depend on microstructure, being that meringues that are less porous (low POV) and characterized by smaller pore sizes are stiffer than meringues with higher air phase and bigger pores. Results suggest that a strong negative correlation ( 0.91, p 6 0.05) exists between the microstructural parameters POV and
OSVR; the former being the percentage measure of the overall air content and the latter being the pores surface/volume ratio therefore an indication of the average pore size: porosity is therefore inversely correlated with pore size, that is to say that meringues with higher void volumes have larger pores. Results of the present study have therefore allowed estimation of the effect of sugar concentration, citric acid and EW quality on meringues microstructure and mechanical properties, but further work is needed to tell whether such ingredients and their relative concentration affect the final microstructural and mechanical properties of meringues by influencing the foam power, the foam stability or both of such properties.
4. Conclusions X-ray microtomography was shown to be effective in acquiring full 3D images of meringue samples with a very high resolution and image analysis of the full 3D microstructure allowed the measuring of size, shape, distribution and porosity. This technique was able to quantify the structural alteration of the microstructure of meringues produced by different types and quantities of ingredients.
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Results give information about the effect of sucrose, citric acid and EW quality on the microstructure of meringues and how these changes in microstructure determine changes also in their mechanical properties. Citric acid and sugar concentration play a fundamental role in meringues structure formation, being that low sugar/EW ratios and high citric acid levels determine an increase in the average pore size hence a higher porosity. Samely, it was demonstrated by mechanical tests that sucrose addition contributes to hardness and brittleness of meringues and that citric acid addition contributes to make softer meringues. EW quality plays a determinant role in meringues microstructure and mechanical properties: better quality EWs (i.e. fresh or pasteurized) result in higher POV values, lower OSVR and hardness, while low quality EWs (i.e. powdered, frozen or coming from old eggs) make stiff meringues, with lower air phases. Findings of this research should encourage the use of fresh raw materials compared to dried or frozen albumen, which are frequently used in patisserie due to higher ease of use, and to suitably choose the right proportions for obtaining meringues with desired characteristics. References Bellido, G.G., Scanlon, M.G., Page, J.H., Hallgrimsson, B., 2006. The bubble size distribution in wheat flour dough. Food Research International 39, 1058–1066. Campbell, G.M., Mougeot, E., 1999. Creation and characterization of aerated food products. Trends in Food Science and Technology 10, 283–296. Davis, J.P., Foegeding, E.A., Hansen, F.K., 2004. Electrostatic effects on the yield stress of whey protein isolate foams. Colloids and Surfaces B: Biointerfaces 34, 13–23. Falcone, P.M., Baiano, A., Zanini, F., Mancini, L., Tromba, G., Dreossi, D., Del Nobile, M.A., 2005. Three-dimensional quantitative analysis of bread crumb by X-ray microtomohraphy. Journal of Food Science 70 (3), 265–272. Falcone, P.M., Baiano, A., Zanini, F., Mancini, L., Tromba, G., Montanari, F., Del Nobile, M.A., 2004. A novel approach to the study of bread porous structure: phasecontrast X-ray micro-tomography. Journal of Food Science 69 (1), 38–43. Foegeding, E.A., Luck, P.J., Davis, J.P., 2006. Factors determining the physical properties of protein foams. Food Hydrocolloids 20, 284–292. Hammershoj, M., Prins, A., Qvist, K.B., 1999. Influence of pH on surface properties of aqueous egg albumen solutions in relation to foaming behaviour. Journal of the Science of Food and Agriculture 79, 859–868. Frisullo, P., Laverse, J., Marino, R., Del Nobile, M.A., 2009. X-ray computer tomography to study processed meat micro-structure. Journal of Food Engineering 94, 283–289. Frisullo, P., Licciardello, F., Muratore, G., Del Nobile, M.A., 2010. Microstructural characterization of multi-phase chocolate using X-ray microtomography. Journal of Food Science 75 (7), 469–476.
Hildebrand, T., Ruegsegger, P., 1997. A new method for the model independent assessment of thickness in three dimensional images. Journal of Microscopy 185, 67–75. Kato, A., Takahashi, A., Matsudomi, N., Kobayashi, K., 1983. Determination of foaming properties of proteins by conductivity measurements. Journal of Food Science 48, 62–65. Labensky, S.R., Hause, A.M., 1999. On Cooking: A Textbook of Culinary Fundamentals. Prentice Hall, Upper Saddle River, NJ. Lechevalier, V., Jeantet, R., Arhaliass, A., Legrand, J., Nau, F., 2007. Egg white drying: influence of industrial processing steps on protein structure and functionalities. Journal of Food Engineering 83, 404–413. Léonard, A., Blacher, S., Nimmol, C., Devahastin, S., 2007. Effect of far-infrared radiation assisted drying on microstructure of banana slices: an illustrative use of X-ray microtomography in microstructural evaluation of a food product. Journal of Food Engineering 85, 154–162. Li-Chan, E., Nakai, S., 1989. Biochemical basis for the properties of egg white. CRC Critical Reviews in Poultry Biology 2, 21–58. Lim, K.S., Barigou, M., 2004. X-ray micro-tomography of cellular food products. Food Research International 37, 1001–1012. Maache-Rezzoug, Z., Bouvier, J.-M., Allaf, K., Patras, C., 1998. Effect of principal ingredients on rheological behaviour of biscuit dough and on quality of biscuits. Journal of Food Engineering 35 (1), 23–42. McGee, H., 1984. On Food and Cooking: The Science and Lore of the Kitchen. MacMillian, New York. Mendoza, F., Verboven, P., Mebatsion, H.M., Kerckhofs, G., 2007. Three-dimensional pore space quantification of apple tissue using X-ray computed microtomography. Planta 226, 559–570. Mousavi, R., Miri, T., Cox, P.W., Fryer, P.J., 2005. A novel technique for ice crystal visualization in frozen solids using X-ray micro-computed tomography. Journal of Food Science 70 (7), 437–442. Pareyt, B., Talhaoui, F., Kerckhofs, G., Brijs, K., Goesaert, H., Wevers, M., Delcour, J.A., 2009. The role of sugar and fat in sugar-snap cookies: structural and textural properties. Journal of Food Engineering 90, 400–408. Phillips, L.G., Schulman, W., Kinsella, J.E., 1990. PH and heat treatment effects on foaming of whey-protein isolate. Journal of Food Science 55, 1116–1119. Pernell, C.W., Foegeding, E.A., Luck, P.J., Davis, J.P., 2002. Properties of whey and egg white protein foams. Colloids and Surfaces A: Physicochemical and Engineering Aspects 204, 9–21. Raikos, V., Campbell, L., Euston, S.R., 2007. Effects of sucrose and sodium chloride on foaming properties of egg white proteins. Food Research International 40, 347– 355. Sahoo, P.K., Soltani, S., Wong, A.K.C., Chen, Y.C., 1988. A survey of thresholding techniques. Computer Vision, Graphics and Image Processing 41 (2), 233–260. Stevenson, P., Mantle, M.D., Hicks, J.M., 2007. NMRI studies of the free drainage of egg white and meringue mixture froths. Food Hydrocolloids 21, 221–229. Whitworth, M.B., Alava, J.M., 1999. The imaging and measurement of bubble in bread doughs. In: Campbell, G.M., Webb, C., Pandiella, S.S., Niranjan, K. (Eds.), Bubbles in Foods. St. Eagan Press, Paul, MN, pp. 221–231.
Contents lists available at SciVerse ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of sugar, citric acid and egg white type on the microstructural and mechanical properties of meringues Fabio Licciardello b, Pierangelo Frisullo a, Janine Laverse a, Giuseppe Muratore b, Matteo Alessandro Del Nobile a,c,⇑ a
Department of Food Science, University of Foggia, Via Napoli, 25, 71100 Foggia, Italy Department of Agricultural and Food Productions (DISPA), Section of Food Technology and Microbiology, University of Catania, Via Santa Sofia, 98, 95123 Catania, Italy Istituto per la Ricerca e le Applicazioni Biotecnologiche per la Sicurezza e la Valorizzazione dei Prodotti Tipici e di Qualità, Università degli Studi di Foggia, BIOAGROMED, Via Napoli, 52, 71100 Foggia, Italy
b c
a r t i c l e
i n f o
Article history: Received 7 March 2011 Received in revised form 2 August 2011 Accepted 19 August 2011 Available online 6 September 2011 Keywords: Food microstructure Egg white Mechanical properties Meringues X-ray microtomography
a b s t r a c t Meringues are characterized by a predominant air phase and their overall quality is intimately related with microstructure. The formation of meringues microstructure relies on the capacity of egg white (EW) proteins to form voluminous and stable foams and it is ultimately related with the chemical properties of proteins and with the addition of ingredients such as sugars, salts, acids and surfactants. The study aimed at assessing the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties of meringues. Meringues prepared with different sugar/EW, citric acid level and different EW type were subjected to microstructural analysis by X-ray microtomography and to mechanical assessment by compression tests. Results demonstrate the ability of X-ray microtomography to reconstruct the 3D microstructure of meringues allowing the measurement of porosity, size, shape and distribution of pores. Citric acid, sugar concentration and EW type play a fundamental role on meringues microstructural parameters and mechanical properties. Low sugar/EW ratios as well as increasing citric acid levels increase the air phase and result in a softer texture of meringues. Moreover, low sugar/EW ratios and increasing citric acid in the meringue result in a reduction of pore size and also influence the shape of pores. Meringues microstructural and mechanical properties are affected by the EW quality: fresh and pasteurized EWs and EWs stored at refrigerated temperatures scored the highest structural and mechanical performances, while powdered and frozen EWs and albumens from old eggs showed the worst results. Not only the balance among ingredients but also the choice of raw materials can strongly affect the final quality of meringues. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Meringues are confectionery products simply made of EW sugar and small amounts of salts and acids added as foam stabilizers, characterized by a predominant (>80%) air phase. Such characteristic relies on the capacity of EW proteins to form voluminous and stable foams. Meringues, due to their simple composition, represent a model system to study the behavior of EW froths, which are the basis for other, more complex, bakery products. The characteristics of meringues are related to their microstructure that, in turn, depends on the composition and on the balance among ingredients. The formation of food microstructure is a complex phenomenon which, in the specific case of EW froths, relies on a structure-building phase, i.e. whipping, and on the opposite concurring stage of structure disruption, i.e. foam drainage (Kato et al., ⇑ Corresponding author at: Department of Food Science, University of Foggia, Via Napoli, 25, 71100 Foggia, Italy. Tel./fax: +39 881 589 242. E-mail addresses: [email protected], [email protected] (M.A. Del Nobile). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.08.021
1983; Campbell and Mougeot, 1999; Foegeding et al., 2006). Such phases are regulated by foamability and foam stability, respectively, two properties related with the chemical properties of proteins and with the addition of other ingredients such as sugars, salts, acids and surfactants (Li-Chan and Nakai, 1989; Raikos et al., 2007). Meringues are subject to a heat treatment aimed at ‘‘freezing’’ the formed structure by converting the foam from a liquid to a solid. The 3D microstructural information can provide a better understanding of the physical structure of the product and from an engineering perspective; knowledge about the microstructure of foods can be used to identify the important processing parameters that affect the quality of a product. Structure–property relationships can strongly affect the physiochemical, sensorial, technological and even nutritional properties of foods. Processes are no longer designed from a macroscopic level; knowing the properties of foods on the micro scale determines the process specification. Given the enormous success of X-ray microtomography (lCT) in medical applications, material science, chemical engineering,
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Table 1 Scheme of the experimental plan for meringues preparation.
A1 A2 A3 A4 = C1 A5 B1 B2 B3 = C7 C1 = A4 C2 C3 C4 C5 C6 C7 = B3 a
Sugar/ EW
Citric acid EW type (% on EW)
0.5:1 1:1 1.5:1 2:1 2.5:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1
1 1 1 1 1 0 0.5 1 1 1 1 1 1 1 1
Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 1 Pasteurized 2 Pasteurized 2 Pasteurized 2 Pasteurized 1 From fresh eggs Not from fresh eggs (>28 days) Not from fresh eggs (>28 days), stored at 4 °C Powdereda Frozen Pasteurized 2
Reconstituted with water at 1:7 (w:w) ratio.
geology and biology, it is not surprising that in recent years much attention has been focused on extending this imaging technique to food science, as a useful technique to aid in the study of food microstructure. X-ray microtomography has been successfully used to observe the stability of gas bubbles in dough during the bread making process (Whitworth and Alava, 1999), the microstructure of foams (Lim and Barigou, 2004), three-dimensional quantitative analysis of breadcrumb (Falcone et al., 2005), the study of bread porous structure (Falcone et al., 2004) and ice crystals within frozen foods (Mousavi et al., 2005). Recently this technique has also been used to study the bubble size distribution in
Fig. 2. Three-dimensional reconstructions of meringue microstructure: (a) type A, (b) type B, (c) type C.
wheat flour dough (Bellido et al., 2006), the effect of far-infrared radiation assisted drying on microstructure of banana slices (Léonard et al., 2007), three-dimensional pore space quantification of apple tissue (Mendoza et al., 2007), the role of sugar and fat in sugar-snap cookies (Pareyt et al., 2009), the study of processed meat microstructure and the assessment of intramuscular fat level and
Fig. 1. Gray level reconstructed cross section images of meringues acquired by lCT: (a) type A, (b) type B, (c) type C.
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Test Duncan;variabile Var2 (Spreadsheet5) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= 23,470, gl = 10,000
Var1 Cella N. 4 5 3 2 1
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Fig. 3. (a–c) Microstructural parameters for type A meringue samples: POV (a), OSVR (b), and SMI (c).
distribution in beef muscles (Frisullo et al., 2009) and the sugar crystals distribution and size and cocoa matter content in cold-processed chocolate (Frisullo et al., 2010). Previous works on EW
foaming properties used conventional image analysis techniques (Stevenson et al., 2007) or confocal laser scanning microscopy (Pernell et al., 2002; Raikos et al., 2007) to study the microstructure
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parameters in EW foams while, as far as we are concerned, tomographic techniques have never been used to study the microstructure of meringues. In this paper, the pore microstructural and mechanical properties of different types of meringues were studied. Accordingly, this work aimed at demonstrating the effectiveness of X-ray microtomography as a useful tool to study the microstructure of meringues, and at assessing the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties. 2. Materials and methods 2.1. Meringues preparation Meringues were prepared by a professional confectioner using different sugar/EW ratio, citric acid content and EW type, as indicated in Table 1. Fresh eggs were bought from a local retailer and were used within 7 days from laying. The other eggs were bought 35 days in advance to meringues preparation, one batch (not from fresh eggs) was stored at room temperature while another batch (not from fresh eggs stored at 4 °C) was stored at refrigerated conditions. Both batches were bought from the same retailer and stored for 35 days. pH of fresh EW was 7.40 and that of not fresh EW was 9.18 and 8.75 for those stored at room temperature and at 4 °C, respectively. Pasteurized EWs, packed in 1-l multilayer briks, distributed by Eurovo S.r.l. (Mordano, BO, Italy) were bought from a local retailer. Samples named ‘‘pasteurized 1’’ and ‘‘pasteurized 2’’ differed in the shelf life of the product, 35 days for the former and 6 months for the latter, as a result of different pasteurization process applied. In particular, ‘‘pasteurized 2’’ egg whites were obtained by a modern pasteurization process called ‘‘liottization’’, protected by patent. pH of pasteurized 1 and 2 egg whites was 9.0 and 9.1, respectively. Standard-grade, powdered EWs were supplied by Eurovo S.r.l. (Mordano, BO, Italy), and were used after reconstitution with water at 1:7 (w:w) ratio, following instructions of the manufacturer. The dry matter of reconstituted EWs was 11.5%, as calculated from the labeled value of the powder (92%), and was coincident with the dry matter values of other EW types. EWs were whipped into a steel bowl at 660 rpm at room temperature using a professional planetary mixer (mod. Plutone 7, Sirman Spa, Pieve di Curtarolo, PD, Italy). Refrigerated and frozen EWs were brought to room temperature before use. Citric acid was added to the EWs before beating. Sugar, added with a small amount of water (1% of the EW volume), was melt and brought to the ‘‘soft ball’’ stage (115 °C) before mixing with the whipped
EWs. Meringues were shaped in balls of about 3 cm diameter using a manual pastry bag provided with a head having diameter of 1.5 cm and they were cooked in oven (G-Therm 075, Galli, Milan, Italy) at 80 °C for 2 h. 2.2. Tomographic analysis The samples were imaged under the same conditions, using the Skyscan 1172 high-resolution desktop X-ray microtomography system (Skyscan, Belgium). A sample was placed on a round metal plate; the source and the detector were fixed, while the sample was rotated during measurement. Power settings of 29 kVp and 175 lA were used. A CCD camera with 4000 2096 pixels was used to record the transmission of the conical X-ray beam through all samples. The distance source–object-camera was adjusted to produce images with a pixel size of 2.8 lm. Four-frame averaging, a rotation step of 0.60° and an exposure time of 589 ms were chosen to minimize the noise, covering a view of 180°. Smoothing and beam-hardening correction steps were applied to suppress noise and beam hardening artifacts, respectively. Beam hardening correction was only moderately applied, set to 25% within NRecon and a fast ring artifacts reduction (set to 25 within NRecon) was also applied. Once initial parameters were set, the acquisition step was completely automated and did not require operator assistance. Scan time, on average, required 70 min. A set of flat cross section images, was obtained for each sample after tomographical reconstruction by the reconstruction software NRecon (Skyscan). Three-dimensional reconstructions of samples were created by effectively stacking all 2D tomographs, a total of 420 slice images with a slice spacing of 0.069 mm. For image processing and analysis, the Skyscan software, CTAnalyser (CTAn) was used. Image segmentation was firstly carried out on the smoothed 8-bit gray-scale images obtained from the reconstruction step, using CTAn (Skyscan) software. Segmentation is the process of converting the gray-scale images into black and white images by assigning the value 1 to all pixels whose intensity was below a given gray tone value and 0 to all the others. For this, an automatic threshold based on the entropy of the histogram (Sahoo et al., 1988) was calculated for each image. The lower gray threshold (0) and upper gray threshold (60) values were identified; each sample was processed under the same conditions. For data analysis, prior to 3D reconstruction, a component-labeling algorithm, available within CTAn, was used to isolate the largest 3D connected structures. All reconstructions where created in CTAn (Skyscan) using an adaptive rendering (locality 10 and tolerance 0.25) algorithm A2
A5
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Var1 Var2 Media Cella N. 1 POV B1 37,55408 POV B2 40,06950 2 POV B3 44,58951 3
B3
Test Duncan;variabile Var2 (Spreadsheet12) Gruppi Omogenei, alfa = ,05000 Errore: MS Tra grp= ,00002, gl = 6,0000
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Var1 Var2 Media Cella N. 1 SMI B1 1,250178 SMI B3 1,387836 3 SMI B2 1,640356 2
1 **** **** ****
Fig. 5. (a–c) Microstructural parameters for type B samples: POV (a), OSVR (b), and SMI (c).
and saved as P3G surface model (SkyScan model format). P3G models were then imported into CT vol software (Skyscan) for visualization. The following microstructural parameters were used for this study: (1) the percentage object (pore) volume, i.e. the geometric parameter POV obtained by lCT analysis, was calculated for each sample as a representation of the percentage total pore content within the sample. (2) The surface/volume ratio (OSVR) of the
pores present in the samples; a low OSVR value indicates high porosity and the formation of larger pore sizes at the cost of smaller ones. (3) The fragmentation index (FI) parameter is the index of connectivity and is a measure of the connectivity of the total solid surface (Lim and Barigou, 2004). A negative FI signifies better-connected solid lattices while a positive FI indicates a more disconnected solid structure. (4) The degree of anisotropy (DA) is a measure of the 3D structural symmetry, i.e. in this case it indicates
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the presence or absence of preferential alignment of the pores present along a particular direction (Lim and Barigou, 2004). A value of 0 would correspond to total isotropy, whereas a value of 1 would indicate total anisotropy. (5) The SMI parameter, that is a topological index, gives an estimate of the characteristic shape in terms of plates and cylinders composing the 3D structure (Hildebrand and Ruegsegger, 1997) and is calculated using a differential analysis of triangulated surface of the structure under examination. The SMI assumes integer values of 0, 3 and 4 for ideal plates, cylinder and spheres, respectively. 2.3. Determination of mechanical properties Mechanical tests were carried out using a Zwick Roell BT1FR0.5TN.D14 texture analyzer. Compression curves were obtained using a load cell of 50 N and setting test rate at 10 mm/min. Slopes and shapes of compression curves were used as indicators of the overall mechanical properties of meringues: slopes represent the hardness of samples, while shape (i.e. curve from smooth to rough) gives an indication of the micro and macro-cracks suffered by the samples during compression. 2.4. Statistical analysis All parameters obtained were submitted to one-way analysis of variance (ANOVA) and Duncan’s test (p < 0.05) through the statistical package Statistica for Windows (Statsoft, Tulsa, USA).
in which the brighter regions correspond to the higher level of attenuation, i.e. higher density region. It can be assumed from these figures that the darkest areas represent the holes (air) as it has a lower absorption coefficient with respect to structure. Fig. 2 shows as examples the three-dimensional reconstructions of the samples from which the geometrical parameters were calculated using the CTAn software (Skyscan). From these images the different types of microstructure and the number of holes in the samples can be noted. Results (data not shown) highlight negative FI values for all the three groups of meringues investigated in this work, hence it can be stated that in general the pores present in meringues have connected solid lattices. Moreover, basing on DA results (data not shown), pores present in all the groups of meringue samples are isotropic in nature. Results on POV, OSVR and SMI will be analyzed with respect to each variable considered. With regards to mechanical tests, the slope and shape of compression curves were analyzed to acquire information on the overall mechanical properties of meringues: steeper slopes correspond to hard and stiff samples, while the shape, i.e. presence of peaks in the curves are related to micro and macro-cracks during compression, hence to the micro and macro-porosity of meringues. Meringue topography, with special regards for sample contact area at first contact, affects mechanical measurement, being that the first fracture point in the compression curves varies with the sample contact area at first contact; on the other hand, the slope and the number of fracture points are more reliable indexes of hardness.
3. Results and discussion
3.1. Effect of sugar/EW ratio
Since the aim of the work is to assess the influence of sugar/EW ratio, citric acid and EW type on the microstructural and mechanical properties of meringues, results will be divided in three paragraphs, each one analyzing the effects of one variable on the meringue’s microstructural and mechanical parameters. Firstly, some general comments on the microstructural and mechanical parameters are reported. With regards to lCT images, Fig. 1 shows as examples the gray level reconstructed cross section images of the samples acquired by lCT, the contrasts in these images are based on the differences in absorption of X-rays by the constituents of the sample (e.g. structure and air). This contrast is produced by a variation of density and a change in composition of the sample and is based exclusively on the detection of an amplitude variation of X-rays transmitted through the sample itself. The obtained image is a map of the spatial distribution of the X-rays
Fig. 3a shows the percentage POV values for all type A meringue samples. It can be noted from this figure that an increase in the sugar/EW ratio within this group (from A1 to A5) results in a decrease in POV and as sample A1 has the lowest sugar content it therefore has the highest percentage volume of pores present. On the basis of confectioner’s indications, we considered as standard meringues the ones produced using a sugar/EW ratio of 2:1 (samples A4): this ratio gave samples with the lowest porosity (i.e. lower POV), while a reduction of such ratio is effective in obtaining products with higher porosity. As it can be inferred from Fig. 3b, there is an increment of OSVR with the increase of the sugar/EW ratio, for example, samples A4 and A5 that have the highest sugar/EW ratio have the highest and statistically equal OSVR values hence have a higher volume of smaller pores compared to the other three samples within the group; on the other hand, sample A1 and A2 have the lowest and statistically equal OSVR values. Results therefore
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Fig. 7. (a–c) Microstructural parameters for type C meringue samples: POV (a), OSVR (b), and SMI (c).
suggest that an increase in the sugar/EW ratio determines higher volumes of very small pores that are evenly disperse. The results for the SMI parameter, Fig. 3c, suggest that on average the pores present in this group are cylindrical to spherical in nature. Although there is not a clear trend for the SMI parameter in this group, it can be noted that the sample with the lowest sugar/EW ratio (A1) and those with the highest sugar/EW ratio (A4 and A5)
have SMI values (above 3) suggesting the presence of truly spherical pores. As it can be inferred from Fig. 4, an increase of the sugar/EW ratio (from A1 to A5) determines a change in the mechanical behavior of meringues. The compression curve relative to A1 highlights small peaks which indicate numerous internal micro-cracks during compression; moreover, the curve has a slight slope, indicating
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that a minimum strength is required to compress the probe and that the meringue structure does not oppose resistance to compression, that is to say that meringue is rather soft. The curve of sample A3 appears more discontinuous, being characterized by some sharp drops that can be related to internal structural collapses due to compression. The curve shape together with its slope, slightly higher for A4 samples, indicates a higher stiffness of structure and a more crunchy texture. Finally, the curve relative to A5 indicates higher hardness, as it is possible to infer from the very high slope of the compression curve. For all samples the mechanical test was considered concluded as soon as the compression force reached the cell load threshold value (i.e. 50 N). The results confirm that the sugar content is strongly related to the microstructure and hence the textural properties of meringues. Indeed, as can be inferred from Figs. 3a, b and 4, an increase in sugar/ EW ratio results in the decrease of the POV parameter, in the increase of OSVR and therefore an increase in hardness. These results confirm what has been reported by McGee (1984) and Labensky and Hause (1999) who state that the amount of sugar depends on the final application of the meringue and the lightness required, being that soft meringues have sugar at less or equal the amount of EW and that hard ones have twice the amount of sugar than EW. It is known that sugar acts as stabilizing for EW foam (Campbell and Mougeot, 1999), therefore the lower POV values observed for meringues with higher sugar/EW ratio should be a consequence of a reduced foamability compared with those obtained with low sugar/ EW ratios. Maache-Rezzoug et al. (1998) highlighted the effect of sugar on bakery products, reporting that sucrose acts as a hardening agent in biscuits by crystallizing after cooling, making products crispy and fragile. To sum up, mechanical properties depend on microstructure; meringues that have low POV values and high OSVR values are stiffer due to the fact that they have a low air phase and smaller pores while meringues with a high POV and low OSVR are softer as they have a higher air phase and bigger pores. 3.2. Effect of citric acid addition As observed from Fig. 5a for meringues group B, there is a slight increase in the POV values (although not statistically significant) as a result of an increase of citric acid level, therefore the citric acid does have a slight effect on the percentage pore volume of the meringue. Hammershoj et al. (1999) and Li-Chan and Nakai (1989) demonstrated that foaming properties of egg white proteins are optimal near their isoelectric points, and that the addition of acid to egg white improves foaming properties by decreasing pH, such that it is closer to the isoelectric point. Similar results have been
obtained with whey proteins (Davis et al., 2004; Phillips et al., 1990). On the other hand, it can be noted from Fig. 5b that there is also a slight decrease in OSVR (again not statistically significant) with the increase of citric acid: as it is evident from a comparison among the samples within this group (B1, B2 and B3), B1 has the highest OSVR whilst sample B3 has the lowest OSVR value. Therefore an increase of citric acid slightly increases the pore size and hence the percentage porosity. With regards to the SMI parameter, Fig. 5c, it can be noted that the general shape of the pores is elongated to cylindrical in nature and there does not seem to be any trend with the increase of citric acid. The addition of citric acid (increasing from B1 to B3) determines a progressive distension of the compression curves (Fig. 6), which for samples B1 stops at 4.5 mm path. Moreover, the height and number of sharp peaks, which represent macro-cracks of the meringues during compression test, decrease, as to testify a less rigid structure characterized by high porosity. The test highlights that the addition of citric acid affects the microstructure and the textural properties of meringues. Indeed, as can be inferred from Figs. 5a and b and 6, an increase in the citric acid level results in the slight increase of the POV parameter, slight decrease of OSVR and hardness. As stated in the previous paragraph, the mechanical properties depend on the microstructure, being that meringues, which are less porous, i.e. high volume of smaller pores, are stiffer than meringues with higher air phase and bigger pores, i.e. higher POV. 3.3. Effect of EW type Fig. 7a shows the POV values for the meringue samples of group C; as it can be noted from this figure, sample C4 and C1 have the highest POV value and sample C3 the lowest POV value. Whereas with regards to Fig. 7b relative to the OSVR parameter, samples C3 and C5 have the highest statistically equal OSVR values and sample C4 the lowest. Therefore it can be stated that the EW type does have an effect on the POV and OSVR parameters. Fig. 7c shows that only meringues prepared with pasteurized EW 1 (sample C1) score SMI values corresponding to ideal spheres, while the other samples obtain SMI values ranging between 1 and 2, while only sample C5 (meringues prepared with powdered EW) scored values <1. Different EW types can dramatically influence the mechanical properties of meringues (Figs. 8 and 9). This is especially true for C5 and C6 samples, made from powdered and frozen EWs, respectively, compared with C1, C2 and C7. The former samples are characterized by compression curves with steep slopes, reaching the maximum threshold for the test (50 N) after 2–4 mm path, and by sharp peaks, indicating macro-cracks during compression and
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a stiff structure. On the other hand, the latter meringues show comparable curves, all of them reaching at least 9 mm of path and characterized by a variable number of sharp drops. Among the latter samples, C7 shows the smoothest curve, while C3 the roughest. The results show that EW quality clearly affects the microstructure and the textural properties of meringues. Indeed, as it can be inferred from Figs. 7a, b and 8, the use of better quality EWs (i.e. fresh or pasteurized) results in higher POV values, lower OSVR and hardness, while low quality EWs (i.e. powdered, frozen or coming from old eggs) make stiff meringues, with lower air phases. Meringues produced with not fresh eggs stored at 4 °C (C4) score values similar to those obtained with good quality EWs, this is probably due to the fact that storage at low temperature preserves their technological properties despite the long storage time. With regards to powdered EWs, this is in agreement with previous studies that state the effect of the drying process of EWs on their technological properties (Lechevalier et al., 2007). Once more, it was confirmed that mechanical properties depend on microstructure, being that meringues that are less porous (low POV) and characterized by smaller pore sizes are stiffer than meringues with higher air phase and bigger pores. Results suggest that a strong negative correlation ( 0.91, p 6 0.05) exists between the microstructural parameters POV and
OSVR; the former being the percentage measure of the overall air content and the latter being the pores surface/volume ratio therefore an indication of the average pore size: porosity is therefore inversely correlated with pore size, that is to say that meringues with higher void volumes have larger pores. Results of the present study have therefore allowed estimation of the effect of sugar concentration, citric acid and EW quality on meringues microstructure and mechanical properties, but further work is needed to tell whether such ingredients and their relative concentration affect the final microstructural and mechanical properties of meringues by influencing the foam power, the foam stability or both of such properties.
4. Conclusions X-ray microtomography was shown to be effective in acquiring full 3D images of meringue samples with a very high resolution and image analysis of the full 3D microstructure allowed the measuring of size, shape, distribution and porosity. This technique was able to quantify the structural alteration of the microstructure of meringues produced by different types and quantities of ingredients.
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Results give information about the effect of sucrose, citric acid and EW quality on the microstructure of meringues and how these changes in microstructure determine changes also in their mechanical properties. Citric acid and sugar concentration play a fundamental role in meringues structure formation, being that low sugar/EW ratios and high citric acid levels determine an increase in the average pore size hence a higher porosity. Samely, it was demonstrated by mechanical tests that sucrose addition contributes to hardness and brittleness of meringues and that citric acid addition contributes to make softer meringues. EW quality plays a determinant role in meringues microstructure and mechanical properties: better quality EWs (i.e. fresh or pasteurized) result in higher POV values, lower OSVR and hardness, while low quality EWs (i.e. powdered, frozen or coming from old eggs) make stiff meringues, with lower air phases. Findings of this research should encourage the use of fresh raw materials compared to dried or frozen albumen, which are frequently used in patisserie due to higher ease of use, and to suitably choose the right proportions for obtaining meringues with desired characteristics. References Bellido, G.G., Scanlon, M.G., Page, J.H., Hallgrimsson, B., 2006. The bubble size distribution in wheat flour dough. Food Research International 39, 1058–1066. Campbell, G.M., Mougeot, E., 1999. Creation and characterization of aerated food products. Trends in Food Science and Technology 10, 283–296. Davis, J.P., Foegeding, E.A., Hansen, F.K., 2004. Electrostatic effects on the yield stress of whey protein isolate foams. Colloids and Surfaces B: Biointerfaces 34, 13–23. Falcone, P.M., Baiano, A., Zanini, F., Mancini, L., Tromba, G., Dreossi, D., Del Nobile, M.A., 2005. Three-dimensional quantitative analysis of bread crumb by X-ray microtomohraphy. Journal of Food Science 70 (3), 265–272. Falcone, P.M., Baiano, A., Zanini, F., Mancini, L., Tromba, G., Montanari, F., Del Nobile, M.A., 2004. A novel approach to the study of bread porous structure: phasecontrast X-ray micro-tomography. Journal of Food Science 69 (1), 38–43. Foegeding, E.A., Luck, P.J., Davis, J.P., 2006. Factors determining the physical properties of protein foams. Food Hydrocolloids 20, 284–292. Hammershoj, M., Prins, A., Qvist, K.B., 1999. Influence of pH on surface properties of aqueous egg albumen solutions in relation to foaming behaviour. Journal of the Science of Food and Agriculture 79, 859–868. Frisullo, P., Laverse, J., Marino, R., Del Nobile, M.A., 2009. X-ray computer tomography to study processed meat micro-structure. Journal of Food Engineering 94, 283–289. Frisullo, P., Licciardello, F., Muratore, G., Del Nobile, M.A., 2010. Microstructural characterization of multi-phase chocolate using X-ray microtomography. Journal of Food Science 75 (7), 469–476.
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