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Methodological developments in crispness assessment: Effects of cooking method on the crispness of crusted foods
LWT 41 (2008) 1252–1259 www.elsevier.com/locate/lwt
Methodological developments in crispness assessment: Effects of cooking method on the crispness of crusted foods Paula Varela, Ana Salvador, Susana M. Fiszman Instituto de Agroquı´mica y Tecnologı´a de Alimentos (CSIC), Apartado de Correos 73, 46100 Burjassot, Valencia, Spain Received 5 March 2007; received in revised form 1 June 2007; accepted 23 August 2007
Abstract A new method was developed to assess the texture of crispy-crusted foods with a soft, high-moisture core. This method, applied to commercial pre-cooked chicken nuggets, combines characteristics derived from the force/displacement curves of the whole sample with characteristics of the simultaneously emitted sound. The use of a not-sharp blade probe to perform the test proved to be an effective technique for characterizing the texture of chicken nuggets after different final cooking processes. The force curves of the samples differed with the cooking process. Deep-fried samples and those cooked in a conventional oven presented jagged force curves and acoustic signals with many peaks, both characteristics of crispy products. The curve profiles of microwaved samples were drastically different and typical of tough, gummy products. The use of a susceptor package in microwave heating improved the crispness of the samples. The number of sound peaks was the acoustic parameter that best discriminated between the samples. It was found that although the moisture and fat contents of the core and crust are closely related to the texture characteristics, samples with similar water contents can have very dissimilar crispness characteristics. The fat content of the core did not change significantly with the final cooking process in any of the samples. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Nuggets; Crispness; Frying; Oven; Microwave; Susceptor
1. Introduction Crispness is a highly valued textural characteristic; in particular, breaded and battered foods like fish, seafood, poultry, cheese or vegetables are favoured by consumers and have become very popular over the last two decades (Antonova, Mallikarjunan, & Duncan, 2003, 2004; Dogan, Sahin, & Sumnu, 2005). These products are very common both in high-convenience consumer societies and in developing countries. Using batter and breading on chicken is a trend that has increased since the 1980s and constitutes a large segment of the processed poultry market (Mukprasit, Herald, Boyle, & Rausch, 2000). Appearance, colour, texture, adhesion and flavour are important factors in consumer perceptions of coated foods and crispness is the most critical property that determines consumer acceptance, as the crisp outer layer contrasts with the soft Corresponding author. Tel.: +34 963 90 0022.
E-mail address: sfi[email protected] (S.M. Fiszman).
interior (Loewe, 1993; Maskat & Kerr, 2002). Crispness is a typical textural property of coated fried products which depends on the ingredients, formulation and process. The coating reduces dehydration, aids browning and improves the texture by providing crispness (Altunakar, Sahin, & Sumnu, 2004). One problem associated with the consumption of battered and breaded fried foods is the high amount of oil absorbed during the pre-frying and frying processes; recently there has been a trend to reduce the fat content in these foods by changing the formulations or developing new processing methods to avoid one of the frying steps (Altunakar et al., 2004; Mellema, 2003; Salvador, Sanz, & Fiszman, 2005). Apart from the nutritional aspect, the growing tendency to spend less time on food preparation has lead to a great demand for time-saving ‘‘ready-to-heat’’ frozen food products for microwave-heating. Unfortunately, considerable differences appear when frozen prefried foods which have been fried, baked or microwaved as their final cooking method are compared, particularly in
0023-6438/$34.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.08.008
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
microwaved ones, which tend to be undesirably soggy (Lenchin & Bell, 1987). Antonova et al. (2004) found significant differences in the sensory crispness of chicken nuggets: fried samples were significantly crisper than chicken nuggets cooked in either a convection oven or a microwave oven. One of the more recent developments that can be applied to processing pre-fried products is the use of susceptor packaging materials to maintain the crispy texture of the crust in microwave heating; susceptors change the product heating pattern to counteract problems associated with cooking in microwave ovens: uneven cooking, lack of crispness and lack of browning, reaching temperatures of about 200 1C (Anon, 1996; Pszczola, 2005). The importance of crispness in the acceptability of crusted foods has created the need to define and measure this perception. However, studies related to crispness in products with a high moisture core have been limited and inconclusive (Antonova et al., 2004). Some works have measured the crust separated from the core after frying, whether by puncturing it (Baixauli, Sanz, Salvador, & Fiszman, 2003; Maskat & Kerr, 2002; Salvador et al., 2005) or with a Kramer shear cell (Ling, Gennadios, Hanna, & Cuppet, 1998). Other studies have measured batter, fried alone or on a metal substrate, by puncturing it with a cylinder or a conical probe (Fan, Singh, & Pinthus, 1997; Matsunaga, Kawasaki, & Takeda, 2003; Mohamed, Hamid, & Hamid, 1998). The limitation of these methods is that they only measure the outer layer of the product. In crusted products, the deformation of the outer layer while eating is certainly a result of the mechanical behaviour of the crust, but it also depends on the mechanics of the noncrispy core, where shear and compression forces are mixed. The mechanical, morphological and compositional differences between the layers of this sandwich-like structure make it difficult to determine the contribution of each part (Luyten, Plijter, & Van Vliet, 2004), but the mechanics of both layers stacked together are essential for the understanding of the food piece texture as a whole. Some other studies have analyzed the texture of chicken nuggets in the entire crusted food piece. Antonova et al. (2003) used a Kramer shear-compression cell, measuring the maximum force peak and the total energy. Altunakar et al. (2004) and Dogan et al. (2005) also measured the maximum force peak by penetration with a conical probe. The limitation of these methods is that the maximum force peak and the work of compression are related to hardness and toughness, which in turn are related to crispness in some aspects but are not direct measurements of the crispness of the crusted product. Drake (1963), and Vickers and Bourne (1976) related the perception of crispness to auditory sensations. Since then, two principal approaches have been taken in the study of crispy/crunchy textures using acoustic techniques: one measures the perception of air-conducted sounds to establish their contribution to the sensation of crispness, the other records the sounds while performing a mechan-
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ical test. The latter in particular, the methods in which simultaneous texture and acoustic measurements are performed, have proved very useful (Chen, Karlsson, & Povey, 2005; Varela, Chen, Fiszman, & Povey, 2006). However, these techniques have been used particularly for solid, brittle foods. Maskat and Kerr (2002) tried to characterize fried chicken breasts with different breading particle sizes by recording the acoustic data during a compression test and determining the jaggedness of the amplitude versus time curves; they were not able to discriminate between samples using that technique. The aims of this work were to (a) develop a new method to assess the crispness of crusted products with a high moisture core (chicken nuggets) by means of an integrated approach, measuring the force/displacement curves during cutting and the sound emitted simultaneously; (b) study the relation between the crispness of nuggets after final cooking by four different processes and their moisture and fat contents. 2. Materials and methods 2.1. Testing materials Three different brands of commercial precooked frozen chicken nuggets, purchased in a local supermarket, were used in this study. Two of them were breaded products (samples 1 and 2). Sample 1 is sold as special for microwave heating; the manufacturer recommends cooking two units of the product in a microwave oven at 800 W for 1.15 min; the dimensions of sample 1 were 65 37 19 mm3. The manufacturer of sample 2 recommends deep-frying (DF) at 180 1C for 3 min; the dimensions of sample 2 were 55 41 18 mm3. Sample 3 was a precooked battered chicken nugget which the manufacturer recommended DF at 180 1C for 3 min, or cooking in a conventional oven (CO) at 225 1C for 10–12 min; the dimensions of sample 3 were 53 36 13 mm3. These dimensions are the mean values of digital calliper measurements of 10 nuggets from each sample, which presented an average standard deviation of 2 mm. 2.2. Cooking procedures Deep-frying (DF): Two nuggets at a time, 180 1C, 3 min, in sunflower oil in a 3 L domestic fryer (Fritaurus Professional 4, Barcelona, Spain). Conventional oven (CO): Two nuggets at a time, 225 1C, with convection, 11 min in a conventional domestic convection oven (Fagor 2H 114, Mondrago´n, Guipuzcoa, Spain). Microwave oven (MW): Two nuggets at a time, maximum power 800 W, measured power output 691 W (International Standard IEC 60705, IEC Publication 705, 1988), 1.15 min, in a domestic MW oven (Samsung M1727, Hospital de Llobregat, Barcelona, Spain). After cooking
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the sample was left to rest for 1 min before performing the measurements. Microwave oven+susceptor (MW+S): One nugget at a time wrapped in Neocrisps laminate susceptor (Mondi Packaging, Nyborg, Denmark), in the same MW oven, maximum power, 1.15 min. The wrapping material was a safety flexible laminated susceptor (PET+aluminium+ paper). After cooking the package was removed and the sample was left to rest for 1 min before performing the measurements. 2.3. Compositional analyses: moisture and total fat contents After cooking, crust and core were separated to analyze the moisture and fat contents of each part. Four determinations were performed for each analysis. Moisture was determined by vacuum drying at 95 1C to constant weight, AOAC method 950.46 (Association of Official Analytical Chemists (AOAC), 2000). Total fat content was determined by direct extraction with ethyl ether for 12 h in a Soxhlet extractor (AACC, 1967). 2.4. Instrumental analyses: texture and sound emission A TA-XT plus Texture Analyser (Stable Micro Systems, Godalming, UK) was used for force/displacement measurement with a 25 kg load cell and a Perspex blade (60 mm wide, 401 angle and 3 mm thickness, tip polished to blunt the edge). The samples were placed on the HDP/90 Heavy Duty Platform with a slotted blade insert, with the major axis of the nugget placed perpendicular to the blade. The test settings were: test speed 1 mm/s, trigger force 5 g, distance 20 mm (in order to cut completely through the sample). An Acoustic Envelope Detector (AED) described in detail elsewhere (Chen et al., 2005; Varela et al., 2006) was used for sound recording, with the corresponding software (Texture Exponent 32). The gain of the AED was set at one. A Bruel and Kjaer free-field microphone (8-mm diameter), calibrated using an Acoustic Calibrator Type 4231 (94 and 114 dB SPL-1000 Hz) was positioned at a 4 cm distance and a 451 angle to the sample. A built-in low pass (anti-aliasing) filter set the upper calibrated and measured frequency at 16 kHz. Ambient acoustic and mechanical noise was filtered by the use of a 1 kHz high pass filter. The AED operates by integrating all the frequencies within the band pass range generating a voltage proportional to the sound pressure level (SPL). The data acquisition rate was 500 points per second for both force and acoustic signals. All tests were performed in a laboratory with no special soundproofing facilities, a relative humidity of around 2571% and a temperature of 2272 1C. Six replications were performed on nuggets from each sample and cooking method. Force vs. displacement and SPL vs. displacement were plotted, the parameters extracted from the curves, with their units of measurement, are displayed in Table 1.
Table 1 Definition of the instrumental parameters extracted from the force/ displacement and sound pressure level (SPL) curves Parameter
Definition
Units
Area
Area of the force curve (related to the work of compression) Number of peaks of force, using a threshold value of 0.0981 N Number of peaks of the sound plot, using a threshold value of 2.5 at 55–57.5 dB Maximum peak intensity of the sound pressure level (SPL)
Ns
Number of force peaks Number of sound peaks Max SPL
– – dB
2.5. Statistical analyses One-way analysis of variance (ANOVA) was performed on the instrumental parameters for each sample and on the compositional parameters for each sample and zone (core and crust) using the SPSS 12 package (SPSS inc., Chicago). The first and second derivatives of the force curves (using 11-point Savitzky–Golay smoothing) were used to study the correlation with the sound plots (Chen et al., 2005; Varela et al., 2006). The derivative analysis was performed with OriginPro 7.5 (OriginLab Corporation, Northampton). 3. Results and discussion 3.1. Compositional analyses: moisture and total fat contents The effect of water or lipids as plasticizers and their influence on crispness as a result of plasticization has been discussed in some works. The effect of these components could be misunderstood if they are considered as total contents in the whole product without taking into account their distribution in the product (Luyten, Plijter, & Van Vliet, 2003; Luyten et al., 2004); in crusted foods it could be especially important to consider the moisture and lipid contents of each of the different layers. In the present study, in none of the samples did the fat content of the core change significantly with the different cooking procedures. Table 2 shows the fat content (dry basis) of the crust. Microwaved samples and those microwaved with susceptors had much smaller fat contents: evident oil residue was found on the heating plate or on the susceptor wrapping. The deep-fried samples presented the highest amount of fat, due to the additional oil uptake during frying. Antonova et al. (2003) obtained similar results in nuggets that were deep-fried, microwaved, cooked in an oven and then placed under a heat lamp to maintain crispness; the fat content of the core did not change with the cooking procedure, the microwaved samples had the lowest fat content in the crust and the fried ones the highest. With regard to the moisture content, some studies have measured the moisture content separately in crust and core
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259 Table 2 Mean values of the fat content of the crust (% dry basis) and the moisture content (%) of the core and crust of samples 1, 2 and 3 subjected to the four cooking procedures Cooking procedure
Sample 1
Sample 2
Sample 3
Fat (% dry basis) Crust DF CO MW MW+S
31.7c 26.7a 29.7b,c 29a,b
27.2c 21.4b 20.9b 18.9a
36.7c 33.5a,b 33.7b 31.2a
Moisture (%) Core DF CO MW MW+S
67.7a 67.8a 59.4b 47.9c
69.0a 66.8b 55.8c 53.1d
63.6a 62.1b 56.0c 43.0d
Crust DF CO MW MW+S
27.1a 23.1b 27.5a 20.6c
31.6a 28.2b 28.8b 22.2c
28.4b 22.1d 34.3a 26.4c
Identical letters in the same part of each sample indicate that there is no significant difference at p40.05 (Tukey’s test).
after cooking and related the two measurements to crispness loss with aging, as moisture migrates from the core to the crust (Antonova et al., 2003; Norme´n, Rovedo, & Singh., 1998). Other works have directly related the moisture content of the crust to the perceived crispness, hypothesizing that a crisper batter or breading contains less moisture; this proved to be true in the case of comparisons between different coating formulations subjected to the same frying conditions (Maskat & Kerr, 2002) or the same product fried for different times or at different temperatures (Ling et al., 1998; Matsunaga et al., 2003). Sanz, Primo-Martı´ n, and van Vliet (2007) used of a near-infrared ray reflectance moisture meter to measure the moisture of the outer layer of French fries, with or without pre-frying, fried for different times, finding a correlation with the instrumental crispness measured by texture and acoustic means. An increase in the content of a plasticizer agent (water or other) causes an increase in the mobility of the macromolecules, lowering the glass transition temperature and the stiffness of the product, increasing the energy dissipation not related to fracturing and so decreasing fracture speed and crispy/crunchy perception (Luyten et al., 2004). However, the relation between the crispness and total moisture content of the crust does not appear so clear when different cooking methods are compared. Nuggets subjected to the different cooking procedures presented significant differences in the moisture content of the core and crust (Table 2). With regard to the crust, deepfried and microwaved chicken nuggets had the highest moisture contents. Nuggets heated in a microwave oven have a varying amount of internal water evaporation due to the increased thermal energy; this moisture reaches the
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surface, which is surrounded with cold air, causing a humidification of the crust. When a susceptor is used, it generates heat outside the nugget and counteracts this effect by allowing the moisture to evaporate and also by drying the surface. Though related, the total moisture content of the crust is not an index of crispness, as with similar moisture contents deep-fried nuggets were very crisp and microwaved nuggets were not at all crisp. The states in which water is present, the distribution within the crust and what fraction of the water affects the energy dissipation during the deformation and fracturing process need to be differentiated (Luyten et al., 2003). The crust of baked and fried products is a result of the changes during the cooking process, but the real crust region is difficult to define as the moisture loss is more extreme in the outer layers, the internal regions of the batter or breading being moist and tough rather than crisp (Ling et al., 1998; Luyten et al., 2004). In the samples analyzed, the crust was quite easy to remove completely from the nugget core and moisture determinations were then performed on the whole crust; however, it was possible to observe that the water distribution in the crust was not homogeneous, as the internal part of the crust was moister than the outer layers in some of the samples. However, it is virtually impossible to delimit these zones and cut them apart in order to perform a moisture determination in the different layers of the crust with standard procedures (oven drying). Then, even measuring separate moisture contents in the coating would not suffice to characterize the crust and its relation to crispness. Crust formation also depends on other physical and chemical phenomena apart from water migration during cooking. Physical processes such as oil migration from the frying oil to the product (in the case of DF) or fat from the product to the outside also influence plasticization and are interrelated with moisture loss. Chemical changes include denaturing of proteins (already taking place at temperatures lower than 100 1C), nonenzymatic browning (Maillard) and caramelization of sugars, pyrolisis and oxidation of different compounds and the most important process in crust formation, the gelatinization of starch (Pokorny, 1999). Moisture and crispness are not always related; Primo-Martı´ n et al. (2006) also reached this conclusion in the case of a different crusted product, bread: at high relative humidity crispness is directly related to moisture uptake but at certain lower relative humidities, water is lost instead of gained and crust hardening on staling plays an important role in the loss of crispness. Regarding the moisture content of the nugget core, significant differences with cooking procedure were found in all the samples (Table 2). Microwave heating dried the core considerably, particularly when the susceptor was used. Because of their composition, susceptor packaging materials convert microwave energy into heat (Anon, 1996). The very thin layer of conductive material (aluminium) has significant electrical resistance and generates localized resistance heating where they are exposed to
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
a rapidly oscillating electric field. Thin metal susceptors interact primarily with the electrical component of the energy field and provide sensible heat to the food (Bohrer & Brown, 2001). This causes the temperature to rise beyond the temperatures produced by boiling water into steam (100 1C in normal microwave heating), so the system reaches temperatures comparable to a CO (about 200 1C); this effect, combined with the microwave action, caused extensive water evaporation and a product that was drier in both core and crust was obtained. Samples that were deepfried or cooked in a CO maintained a higher nugget core moisture. 3.2. Instrumental analyses: texture and acoustics 3.2.1. Selection of the method The intention was to test the samples with an imitative test, as though they were being cut with a knife, bitten or chewed. Based on preliminary tests, the Perspex blade probe and the test parameters used were selected to achieve a compromise: reproducible and discriminative force curves where the fracture events and the sound peaks could be obtained and distinguished. Preliminary tests included:
Penetration with a Delrin cylinder 0.5 in diameter: the results did not discriminate between samples. Penetration with the Volodkevich tooth: the texture plots were discriminative but the number of sound peaks was low even for the crisper samples. Compression of half a nugget with a 35-mm aluminium cylinder: although discriminative, it was not very reproducible, probably because of the structural damage caused when cutting the samples. Compression of the whole nugget with a 75-mm aluminium compression plate: this was a good method, being discriminative and reproducible, but was not very representative of a real-life procedure. Cutting with the Perspex blade. Good discrimination and reproducibility. Sound peaks could be distinguished well.
3.2.2. Profiles of the texture curves Fig. 1 exemplifies the texture profiles (force vs. displacement of the probe plots) of sample 1 subjected to the four different cooking procedures; the curve profiles obtained for samples 2 and 3 had similar shapes when subjected to the same cooking procedures (data not shown). The force curves of the deep-fried sample (DF) and of the one cooked in a CO were very similar. The curves presented two regions. In the first part, which is a function of the stiffness of the crust, the sample underwent deformation but the blade did not start to cut, so the force values increased with the displacement but no major force drop occurred. The second part of the curve began at the point where the blade started to cut the sample, when the force values showed a sudden decrease, depending on the fracturability of the
30 25 20 Force (N)
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15 10 5 0 0
5
10 Distance (mm)
15
20
Fig. 1. Force (N) versus distance (mm) curves, sample 1 subjected to the four different cooking procedures: DF (thin grey line), CO (thin black line), MW (thick grey line), and MW+S (thick black line).
material. The fracture propagated until inhibited by the presence of a crack stopper (heterogeneity), then started again as the nugget was further deformed. A highly jagged curve profile with many peaks due to numerous fracture events is often produced by products that are perceived as crispy or crunchy (Varela, Aguilera, & Fiszman, 2007; Varela et al., 2006; Varela, Salvador, & Fiszman, 2007; Vincent, 1998). The force plot of the microwaved sample (MW) was dramatically different from the others: it did not present drops in force, no fracture event happened during the test, the sample was not crisp but gummy and tough and until almost the end of the test the sample was deformed under compression without being cut. The use of the susceptor in microwave heating (sample MW+S) imparted crispness to the crust and the plot presented ups and downs in force, although the curve was not as jagged as for the DF and CO samples. 3.2.3. Texture and acoustics Fig. 2 shows the SPL and force plot versus distance for sample 1 subjected to DF. The curves are displayed together as they appeared in the computer screen when performing the test; the signals were synchronized, allowing real-time comparison of the force and sound data. Both the force and acoustic signals were very jagged. Sound peaks were already present in the plot at the beginning of the test, when the crust began to deform, and lasted in quantity and loudness until the end of the test. All the major structural breakdowns in force were accompanied by peaks of high SPL, showing that the DF samples were extremely crisp. In general, when a crisp material is broken, it is hypothesized that each fracture event corresponds to an acoustic event (Chen et al., 2005; Varela et al., 2006; Vincent, 1998). This would happen in solid crisp food items but in the case of this sandwich-like structure (crisp on the
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
6
65
4 60
2 0
55 0
5
10 Distance (mm)
15
20
Fig. 2. Sound pressure level (SPL; grey line) and force (black line) versus distance. Sample 1, DF.
outside, soft on the inside) it should be remembered that not all the drops in force will correspond to fractures in the crust, as many may be related to other heterogeneities such as air pockets in the interface between crust and core or bubbles in the core mass (in general composed of an extruded or thermoformed preparation). It can be observed that in certain parts of the plot there were some minor breakdowns without associated sound peaks; these probably correspond to the deformation of the soft core and, by not producing sound, do not contribute to the perception of crispness. Previous works have demonstrated that the peaks do not have to be correlated one-to-one, as the sound emission is the result of a sudden release of energy while the force curve is a reflection of the energy applied to the sample. To correlate them, it would be interesting to define how much of the energy applied to the material is released as a sound pulse. The second derivative of the force is a measure of the rate of change of elastic energy in the sample as it is stressed. The release of energy is reflected in the negative values of the second derivative of the force curve and it is clear from the mechanisms involved in the creation of crispy sensations that the crack events that generate these perceptions will be correlated in time with energy release (Chen et al., 2005; Varela et al., 2006). In the present work, the second derivative of the force curves was calculated but the correlations did not improve. The energy supplied during the deformation of a solid material can be stored elastically, dissipated as fracture energy or dissipated in other forms. The amount of each component in energy release depends on the material; energy dissipative processes other than fracturing affect the energy available for fracturing and for sound emission (Luyten et al., 2003). Foods composed of layers with different textural characteristics have a particular behaviour, as the deformation of the outer crispy layer, as well as being a result of the mechanical behaviour of the crust, also depends on the mechanics of the ‘‘bed’’ on which it lies. Much of the energy supplied in the test is probably stored elastically in the soft core.
3.2.4. Analysis of the mechanical and acoustic parameters As can be appreciated in Fig. 6a, the area of the force curves did not give a good indication of the crispness of the samples cooked by the different methods. For all three samples, DF, CO and MW procedures did not present significant differences in area despite having very different crispness characteristics. Both compressive (uniaxial and biaxial) and shear forces are involved in this test. The area 85
12
80
10 8
75
6
70
4
Force (N)
8
70
65 2 60 0 0
5
10 Distance (mm)
15
20
Fig. 3. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, CO.
20
85 80
15 75 70
10
65
Force (N)
SPL (dB)
10
Force (N)
12 75
SPL (dB)
14
80
Fig. 3 displays the force and SPL plots for sample 1 cooked in a CO. It can be appreciated that although the force curve was very similar to sample 1 DF the sound signal obtained was not so jagged; the CO nugget would be likely to be perceived as less crisp. Fig. 4 shows the force and SPL plots for sample 1 cooked in microwave oven. The acoustic signal was very quiet: almost no sound peaks were detected. The enhanced crispness of the crust with the use of the susceptor packaging can be supported by Fig. 5; the SPL plot showed various fracture events accompanied by sound emission.
SPL (dB)
16
85
1257
5
60 55
0 0
5
10 Distance (mm)
15
20
Fig. 4. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, MW.
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
1258
30
85
140 25
15 70
Force (N)
SPL (dB)
20 75
10
65 0
5
10 Distance (mm)
15
d
c
40
0
0
b
b, c b
b
a
a
Sample 1
Sample 2
a
Sample 3
86 81 Max SPL (dB)
b
d
60
20
b b
80
5
Fig. 5. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, MW+S.
300
100
20
350
c
c
120 n° Sound Peaks
80
b
b b
b
b
b
b
b
76 b
71 66
Area (N.s)
250 200
61
a a
150
a
a a
100
56
a
a
Sample 1
a
Sample 2
Sample 3
a
Fig. 7. Mean values of the instrumental parameters extracted from the SPL/distance plots: (a) number of sound peaks and (b) maximum sound pressure level (dB). Identical letters within a sample indicate that there is no significant difference at p40.05 (Tukey’s test). Deep frying, conventional oven, microwave+susceptor, ’ microwave.
50 0 Sample 1
a
a a
Sample 2
Sample 3
140 c
n° Force Peaks
120 100
c
80 60
c c
40 b
b
b
20 Sample 1
a
a
a
0
b
c
Sample 2
Sample 3
Fig. 6. Mean values of the instrumental parameters extracted from the force/distance plots: (a) area (N s) and (b) number of force peaks. Identical letters within a sample indicate that there is no significant difference at p40.05 (Tukey’s test). Deep frying, conventional oven, microwave+susceptor, ’ microwave.
behind the curve can be associated with the total work and thus can be related to the toughness of the whole nugget. Sample MW+S had a much higher area value, indicating its increased toughness, which can be explained because of the massive water evaporation from both core and crust observed in this sample.
Fig. 6b displays the mean values of the number of force peaks for the three samples subjected to the different cooking procedures, which proved a good parameter for characterizing crispness, in agreement with previous works that had already used it to characterize other crispy/ crunchy food items (Chen et al., 2005; Varela et al., 2006; Vincent, 1998). Those previous works also extracted two parameters from the SPL versus distance plots, as in Fig. 7. The number of sound peaks (Fig. 7a) is the better discriminator between the samples and is directly related to crispness, as it measures only the mechanical events that produces sound. The maximum SPL is a measure of the sound level or the loudness of the sound events; it can be appreciated that this parameter did not discriminate between the three crispy samples (Fig. 7b) but did characterize the sound emitted, which in none of the cases rose above 81 dB. 4. Conclusions A new methodology was developed to assess the texture of crisp-crusted foods with a soft, high-moisture core. The use of a Perspex blade probe to perform a test on the whole
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
sample (crust and core) while simultaneously measuring the sound pressure level of the emitted sound proved to be an effective method for characterizing chicken nuggets cooked by different procedures. Although the moisture and fat contents of the core and crust are related to the texture characteristics of the crusted samples, generalizations cannot be made as samples with similar moisture contents in the crust can have very dissimilar crispness characteristics and, at all events, crispness would depend on the water distribution within the crust, a more difficult parameter to measure in this kind of product. Acknowledgements The authors are indebted to the Comisio´n Interministerial de Ciencia y Tecnologı´ a (Spain) for financial support (Project AGL-2006-11653-C02-01) and to the Ministerio de Educacio´n y Ciencia (Spain) for the grant awarded to the author Paula Varela. References AACC. American Association of Cereal Chemists. (1967). Cereal laboratory methods. Method 30-20. Anon. (1996). Packaging plays an active role. Food Review, 23(8), 16–25. Association of Official Analytical Chemists. (2000). Official methods of analysis (17th ed.). Gaithersburg, MD: AOAC. Antonova, I., Mallikarjunan, P., & Duncan, S. E. (2003). Correlating objective measurements of crispness in breaded fried chicken nuggets with sensory crispness. Journal of Food Science, 68(4), 1308–1315. Antonova, I., Mallikarjunan, P., & Duncan, S. E. (2004). Sensory assessment of crispness in a breaded fried food held under a heat lamp. Foodservice Research International, 14, 189–200. Altunakar, B., Sahin, S., & Sumnu, G. (2004). Functionality of batters containing different starch types for deep fat frying of chicken nuggets. European Food Research and Technology, 218, 318–322. Baixauli, R., Sanz, T., Salvador, A., & Fiszman, S. (2003). Effect of the addition of dextrin or dried egg on the rheological and textural properties of batters for fried foods. Food Hydrocolloids, 17, 305–310. Bohrer, T., & Brown, R. (2001). Packaging techniques for microwaveable foods. In A. Datta, & R. Anantheswaran (Eds.), Handbook of microwave technology for food applications (pp. 397–467). New York: Marcel Dekker, Inc. Chen, J., Karlsson, C., & Povey, M. (2005). Acoustic envelope detector for crispness assessment of biscuits. Journal of Texture Studies, 36, 139–156. Dogan, S. F., Sahin, S., & Sumnu, G. (2005). Effects of soy and rice flour addition on batter rheology and quality of deep-fat fried chicken nuggets. Journal of Food Engineering, 71, 127–132. Drake, B. K. (1963). Food crushing sounds. An introductory study. Journal of Food Science, 28, 233–241. Fan, J., Singh, P., & Pinthus, E. (1997). Physicochemical changes in starch during deep-fat frying of a moulded corn starch patty. Journal of Food Processing and Preservation, 21, 443–460. IEC Publication 705. (1988). Methods of measuring the performance of microwave ovens for household and similar purposes (2nd ed.). Geneva: International Electrotechnical Commission.
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Lenchin, J. M., & Bell, H. (1987). Batters containing high amylose flour for microwaveable pre-fried foodstuffs A23L1/00; A23L1/315; A23L1/00; A23L1/315; (IPC1-7): A21D10/00. Assignee: National Starch & Chemical Corp. (US). Ling, D., Gennadios, A., Hanna, M. A., & Cuppet, S. (1998). Quality evaluation of deep-fat fried onion rings. Journal of Food Quality, 21, 95–105. Loewe, R. (1993). Role of ingredients in batter systems. Cereal Foods World, 38(9), 673–677. Luyten, H., Plijter, J., & Van Vliet, T. (2003). Understanding the sensory attributes crispy and crunchy: An integrated approach. In: P. Fischer, I. Marti, E. J. Windhab (Eds.), Proceedings of the third international symposium on food rheology and structure, Zurich (pp. 379–384). ISBN 3-905609-19-3. Luyten, A., Plijter, J. J., & Van Vliet, T. (2004). Crispy/crunchy crusts of cellular solid foods: A literature review with discussion. Journal of Texture Studies, 35, 445–492. Maskat, M. Y., & Kerr, W. L. (2002). Coating characteristics of fried chicken breasts prepared with different particle size breading. Journal of Food Processing Preservation, 26, 27–38. Matsunaga, K., Kawasaki, S., & Takeda, Y. (2003). Influence of physicochemical properties of starch on crispness of tempura fried batter. Cereal Chemistry, 80(3), 339–345. Mellema, M. (2003). Mechanism and reduction of fat uptake in deep-fat fried foods. Trends in Food Science and Technology, 14, 364–373. Mohamed, S., Hamid, N. A., & Hamid, M. A. (1998). Food components affecting the oil absorption and crispness of fried batter. Journal of the Science of Food and Agriculture, 78, 39–45. Mukprasit, A., Herald, T. J., Boyle, D. L., & Rausch, K. D. (2000). Poultry Science, 79, 1356–1363. Norme´n, E., Rovedo, C. O., & Singh., R. P. (1998). Mechanical properties of an immersion fried potato starch–gluten gel during postfrying period. Journal of Texture Studies, 29, 681–697. Pokorny, J. (1999). Changes of nutrients at frying temperatures. In D. Boskou, & I. Elmadfa (Eds.), Frying of food (pp. 69–103). Pennsylvania: Technomic Publishing Company, Inc. Primo-Martı´ n, C., van de Pijpekamp, A., van Vliet, T., de Jongh, H. H. J., Plijter, J. J., & Hamer, R. J. (2006). The role of the gluten network in the crispness of bread crust. Journal of Cereal Science, 43(3), 342–352. Pszczola, D. (2005). New batters and breadings. Go beyond just crumbs. Food Technology, 50(10), 49–60. Salvador, A., Sanz, T., & Fiszman, S. (2005). Effect of the addition of different ingredients on the characteristics of a batter coating for fried seafood prepared without a pre-frying step. Food Hydrocolloids, 19, 703–708. Sanz, T., Primo-Martı´ n, C., & van Vliet, T. (2007). Characterization of crispness of French fries by fracture and acoustic measurements, effect of pre-frying and final frying times. Food Research International, 40, 63–70. Varela, P., Aguilera, J. M., & Fiszman, S. M. (2007). Quantification of fracture properties and microstructural features of roasted Marcona almonds by image analysis. LWT, 20, 311–320. Varela, P., Chen, J., Fiszman, S., & Povey, M. (2006). Crispness assessment of roasted almonds by an integrated approach to texture description: Texture, acoustics, sensory and structure. Journal of Chemometrics, 20, 311–320. Varela, P., Salvador, A., & Fiszman, S. (2007). Changes in apple tissue with storage time. Rheological, textural and microstructural analyses. Journal of Food Engineering, 78, 622–629. Vickers, Z., & Bourne, M. C. (1976). A psycho-acoustic theory of crispness. Journal of Food Science, 41, 1158–1164. Vincent, J. F. V. (1998). The quantification of crispness. Journal of the Science of Food and Agriculture, 78, 162–168.
Methodological developments in crispness assessment: Effects of cooking method on the crispness of crusted foods Paula Varela, Ana Salvador, Susana M. Fiszman Instituto de Agroquı´mica y Tecnologı´a de Alimentos (CSIC), Apartado de Correos 73, 46100 Burjassot, Valencia, Spain Received 5 March 2007; received in revised form 1 June 2007; accepted 23 August 2007
Abstract A new method was developed to assess the texture of crispy-crusted foods with a soft, high-moisture core. This method, applied to commercial pre-cooked chicken nuggets, combines characteristics derived from the force/displacement curves of the whole sample with characteristics of the simultaneously emitted sound. The use of a not-sharp blade probe to perform the test proved to be an effective technique for characterizing the texture of chicken nuggets after different final cooking processes. The force curves of the samples differed with the cooking process. Deep-fried samples and those cooked in a conventional oven presented jagged force curves and acoustic signals with many peaks, both characteristics of crispy products. The curve profiles of microwaved samples were drastically different and typical of tough, gummy products. The use of a susceptor package in microwave heating improved the crispness of the samples. The number of sound peaks was the acoustic parameter that best discriminated between the samples. It was found that although the moisture and fat contents of the core and crust are closely related to the texture characteristics, samples with similar water contents can have very dissimilar crispness characteristics. The fat content of the core did not change significantly with the final cooking process in any of the samples. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Nuggets; Crispness; Frying; Oven; Microwave; Susceptor
1. Introduction Crispness is a highly valued textural characteristic; in particular, breaded and battered foods like fish, seafood, poultry, cheese or vegetables are favoured by consumers and have become very popular over the last two decades (Antonova, Mallikarjunan, & Duncan, 2003, 2004; Dogan, Sahin, & Sumnu, 2005). These products are very common both in high-convenience consumer societies and in developing countries. Using batter and breading on chicken is a trend that has increased since the 1980s and constitutes a large segment of the processed poultry market (Mukprasit, Herald, Boyle, & Rausch, 2000). Appearance, colour, texture, adhesion and flavour are important factors in consumer perceptions of coated foods and crispness is the most critical property that determines consumer acceptance, as the crisp outer layer contrasts with the soft Corresponding author. Tel.: +34 963 90 0022.
E-mail address: sfi[email protected] (S.M. Fiszman).
interior (Loewe, 1993; Maskat & Kerr, 2002). Crispness is a typical textural property of coated fried products which depends on the ingredients, formulation and process. The coating reduces dehydration, aids browning and improves the texture by providing crispness (Altunakar, Sahin, & Sumnu, 2004). One problem associated with the consumption of battered and breaded fried foods is the high amount of oil absorbed during the pre-frying and frying processes; recently there has been a trend to reduce the fat content in these foods by changing the formulations or developing new processing methods to avoid one of the frying steps (Altunakar et al., 2004; Mellema, 2003; Salvador, Sanz, & Fiszman, 2005). Apart from the nutritional aspect, the growing tendency to spend less time on food preparation has lead to a great demand for time-saving ‘‘ready-to-heat’’ frozen food products for microwave-heating. Unfortunately, considerable differences appear when frozen prefried foods which have been fried, baked or microwaved as their final cooking method are compared, particularly in
0023-6438/$34.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.08.008
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microwaved ones, which tend to be undesirably soggy (Lenchin & Bell, 1987). Antonova et al. (2004) found significant differences in the sensory crispness of chicken nuggets: fried samples were significantly crisper than chicken nuggets cooked in either a convection oven or a microwave oven. One of the more recent developments that can be applied to processing pre-fried products is the use of susceptor packaging materials to maintain the crispy texture of the crust in microwave heating; susceptors change the product heating pattern to counteract problems associated with cooking in microwave ovens: uneven cooking, lack of crispness and lack of browning, reaching temperatures of about 200 1C (Anon, 1996; Pszczola, 2005). The importance of crispness in the acceptability of crusted foods has created the need to define and measure this perception. However, studies related to crispness in products with a high moisture core have been limited and inconclusive (Antonova et al., 2004). Some works have measured the crust separated from the core after frying, whether by puncturing it (Baixauli, Sanz, Salvador, & Fiszman, 2003; Maskat & Kerr, 2002; Salvador et al., 2005) or with a Kramer shear cell (Ling, Gennadios, Hanna, & Cuppet, 1998). Other studies have measured batter, fried alone or on a metal substrate, by puncturing it with a cylinder or a conical probe (Fan, Singh, & Pinthus, 1997; Matsunaga, Kawasaki, & Takeda, 2003; Mohamed, Hamid, & Hamid, 1998). The limitation of these methods is that they only measure the outer layer of the product. In crusted products, the deformation of the outer layer while eating is certainly a result of the mechanical behaviour of the crust, but it also depends on the mechanics of the noncrispy core, where shear and compression forces are mixed. The mechanical, morphological and compositional differences between the layers of this sandwich-like structure make it difficult to determine the contribution of each part (Luyten, Plijter, & Van Vliet, 2004), but the mechanics of both layers stacked together are essential for the understanding of the food piece texture as a whole. Some other studies have analyzed the texture of chicken nuggets in the entire crusted food piece. Antonova et al. (2003) used a Kramer shear-compression cell, measuring the maximum force peak and the total energy. Altunakar et al. (2004) and Dogan et al. (2005) also measured the maximum force peak by penetration with a conical probe. The limitation of these methods is that the maximum force peak and the work of compression are related to hardness and toughness, which in turn are related to crispness in some aspects but are not direct measurements of the crispness of the crusted product. Drake (1963), and Vickers and Bourne (1976) related the perception of crispness to auditory sensations. Since then, two principal approaches have been taken in the study of crispy/crunchy textures using acoustic techniques: one measures the perception of air-conducted sounds to establish their contribution to the sensation of crispness, the other records the sounds while performing a mechan-
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ical test. The latter in particular, the methods in which simultaneous texture and acoustic measurements are performed, have proved very useful (Chen, Karlsson, & Povey, 2005; Varela, Chen, Fiszman, & Povey, 2006). However, these techniques have been used particularly for solid, brittle foods. Maskat and Kerr (2002) tried to characterize fried chicken breasts with different breading particle sizes by recording the acoustic data during a compression test and determining the jaggedness of the amplitude versus time curves; they were not able to discriminate between samples using that technique. The aims of this work were to (a) develop a new method to assess the crispness of crusted products with a high moisture core (chicken nuggets) by means of an integrated approach, measuring the force/displacement curves during cutting and the sound emitted simultaneously; (b) study the relation between the crispness of nuggets after final cooking by four different processes and their moisture and fat contents. 2. Materials and methods 2.1. Testing materials Three different brands of commercial precooked frozen chicken nuggets, purchased in a local supermarket, were used in this study. Two of them were breaded products (samples 1 and 2). Sample 1 is sold as special for microwave heating; the manufacturer recommends cooking two units of the product in a microwave oven at 800 W for 1.15 min; the dimensions of sample 1 were 65 37 19 mm3. The manufacturer of sample 2 recommends deep-frying (DF) at 180 1C for 3 min; the dimensions of sample 2 were 55 41 18 mm3. Sample 3 was a precooked battered chicken nugget which the manufacturer recommended DF at 180 1C for 3 min, or cooking in a conventional oven (CO) at 225 1C for 10–12 min; the dimensions of sample 3 were 53 36 13 mm3. These dimensions are the mean values of digital calliper measurements of 10 nuggets from each sample, which presented an average standard deviation of 2 mm. 2.2. Cooking procedures Deep-frying (DF): Two nuggets at a time, 180 1C, 3 min, in sunflower oil in a 3 L domestic fryer (Fritaurus Professional 4, Barcelona, Spain). Conventional oven (CO): Two nuggets at a time, 225 1C, with convection, 11 min in a conventional domestic convection oven (Fagor 2H 114, Mondrago´n, Guipuzcoa, Spain). Microwave oven (MW): Two nuggets at a time, maximum power 800 W, measured power output 691 W (International Standard IEC 60705, IEC Publication 705, 1988), 1.15 min, in a domestic MW oven (Samsung M1727, Hospital de Llobregat, Barcelona, Spain). After cooking
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the sample was left to rest for 1 min before performing the measurements. Microwave oven+susceptor (MW+S): One nugget at a time wrapped in Neocrisps laminate susceptor (Mondi Packaging, Nyborg, Denmark), in the same MW oven, maximum power, 1.15 min. The wrapping material was a safety flexible laminated susceptor (PET+aluminium+ paper). After cooking the package was removed and the sample was left to rest for 1 min before performing the measurements. 2.3. Compositional analyses: moisture and total fat contents After cooking, crust and core were separated to analyze the moisture and fat contents of each part. Four determinations were performed for each analysis. Moisture was determined by vacuum drying at 95 1C to constant weight, AOAC method 950.46 (Association of Official Analytical Chemists (AOAC), 2000). Total fat content was determined by direct extraction with ethyl ether for 12 h in a Soxhlet extractor (AACC, 1967). 2.4. Instrumental analyses: texture and sound emission A TA-XT plus Texture Analyser (Stable Micro Systems, Godalming, UK) was used for force/displacement measurement with a 25 kg load cell and a Perspex blade (60 mm wide, 401 angle and 3 mm thickness, tip polished to blunt the edge). The samples were placed on the HDP/90 Heavy Duty Platform with a slotted blade insert, with the major axis of the nugget placed perpendicular to the blade. The test settings were: test speed 1 mm/s, trigger force 5 g, distance 20 mm (in order to cut completely through the sample). An Acoustic Envelope Detector (AED) described in detail elsewhere (Chen et al., 2005; Varela et al., 2006) was used for sound recording, with the corresponding software (Texture Exponent 32). The gain of the AED was set at one. A Bruel and Kjaer free-field microphone (8-mm diameter), calibrated using an Acoustic Calibrator Type 4231 (94 and 114 dB SPL-1000 Hz) was positioned at a 4 cm distance and a 451 angle to the sample. A built-in low pass (anti-aliasing) filter set the upper calibrated and measured frequency at 16 kHz. Ambient acoustic and mechanical noise was filtered by the use of a 1 kHz high pass filter. The AED operates by integrating all the frequencies within the band pass range generating a voltage proportional to the sound pressure level (SPL). The data acquisition rate was 500 points per second for both force and acoustic signals. All tests were performed in a laboratory with no special soundproofing facilities, a relative humidity of around 2571% and a temperature of 2272 1C. Six replications were performed on nuggets from each sample and cooking method. Force vs. displacement and SPL vs. displacement were plotted, the parameters extracted from the curves, with their units of measurement, are displayed in Table 1.
Table 1 Definition of the instrumental parameters extracted from the force/ displacement and sound pressure level (SPL) curves Parameter
Definition
Units
Area
Area of the force curve (related to the work of compression) Number of peaks of force, using a threshold value of 0.0981 N Number of peaks of the sound plot, using a threshold value of 2.5 at 55–57.5 dB Maximum peak intensity of the sound pressure level (SPL)
Ns
Number of force peaks Number of sound peaks Max SPL
– – dB
2.5. Statistical analyses One-way analysis of variance (ANOVA) was performed on the instrumental parameters for each sample and on the compositional parameters for each sample and zone (core and crust) using the SPSS 12 package (SPSS inc., Chicago). The first and second derivatives of the force curves (using 11-point Savitzky–Golay smoothing) were used to study the correlation with the sound plots (Chen et al., 2005; Varela et al., 2006). The derivative analysis was performed with OriginPro 7.5 (OriginLab Corporation, Northampton). 3. Results and discussion 3.1. Compositional analyses: moisture and total fat contents The effect of water or lipids as plasticizers and their influence on crispness as a result of plasticization has been discussed in some works. The effect of these components could be misunderstood if they are considered as total contents in the whole product without taking into account their distribution in the product (Luyten, Plijter, & Van Vliet, 2003; Luyten et al., 2004); in crusted foods it could be especially important to consider the moisture and lipid contents of each of the different layers. In the present study, in none of the samples did the fat content of the core change significantly with the different cooking procedures. Table 2 shows the fat content (dry basis) of the crust. Microwaved samples and those microwaved with susceptors had much smaller fat contents: evident oil residue was found on the heating plate or on the susceptor wrapping. The deep-fried samples presented the highest amount of fat, due to the additional oil uptake during frying. Antonova et al. (2003) obtained similar results in nuggets that were deep-fried, microwaved, cooked in an oven and then placed under a heat lamp to maintain crispness; the fat content of the core did not change with the cooking procedure, the microwaved samples had the lowest fat content in the crust and the fried ones the highest. With regard to the moisture content, some studies have measured the moisture content separately in crust and core
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259 Table 2 Mean values of the fat content of the crust (% dry basis) and the moisture content (%) of the core and crust of samples 1, 2 and 3 subjected to the four cooking procedures Cooking procedure
Sample 1
Sample 2
Sample 3
Fat (% dry basis) Crust DF CO MW MW+S
31.7c 26.7a 29.7b,c 29a,b
27.2c 21.4b 20.9b 18.9a
36.7c 33.5a,b 33.7b 31.2a
Moisture (%) Core DF CO MW MW+S
67.7a 67.8a 59.4b 47.9c
69.0a 66.8b 55.8c 53.1d
63.6a 62.1b 56.0c 43.0d
Crust DF CO MW MW+S
27.1a 23.1b 27.5a 20.6c
31.6a 28.2b 28.8b 22.2c
28.4b 22.1d 34.3a 26.4c
Identical letters in the same part of each sample indicate that there is no significant difference at p40.05 (Tukey’s test).
after cooking and related the two measurements to crispness loss with aging, as moisture migrates from the core to the crust (Antonova et al., 2003; Norme´n, Rovedo, & Singh., 1998). Other works have directly related the moisture content of the crust to the perceived crispness, hypothesizing that a crisper batter or breading contains less moisture; this proved to be true in the case of comparisons between different coating formulations subjected to the same frying conditions (Maskat & Kerr, 2002) or the same product fried for different times or at different temperatures (Ling et al., 1998; Matsunaga et al., 2003). Sanz, Primo-Martı´ n, and van Vliet (2007) used of a near-infrared ray reflectance moisture meter to measure the moisture of the outer layer of French fries, with or without pre-frying, fried for different times, finding a correlation with the instrumental crispness measured by texture and acoustic means. An increase in the content of a plasticizer agent (water or other) causes an increase in the mobility of the macromolecules, lowering the glass transition temperature and the stiffness of the product, increasing the energy dissipation not related to fracturing and so decreasing fracture speed and crispy/crunchy perception (Luyten et al., 2004). However, the relation between the crispness and total moisture content of the crust does not appear so clear when different cooking methods are compared. Nuggets subjected to the different cooking procedures presented significant differences in the moisture content of the core and crust (Table 2). With regard to the crust, deepfried and microwaved chicken nuggets had the highest moisture contents. Nuggets heated in a microwave oven have a varying amount of internal water evaporation due to the increased thermal energy; this moisture reaches the
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surface, which is surrounded with cold air, causing a humidification of the crust. When a susceptor is used, it generates heat outside the nugget and counteracts this effect by allowing the moisture to evaporate and also by drying the surface. Though related, the total moisture content of the crust is not an index of crispness, as with similar moisture contents deep-fried nuggets were very crisp and microwaved nuggets were not at all crisp. The states in which water is present, the distribution within the crust and what fraction of the water affects the energy dissipation during the deformation and fracturing process need to be differentiated (Luyten et al., 2003). The crust of baked and fried products is a result of the changes during the cooking process, but the real crust region is difficult to define as the moisture loss is more extreme in the outer layers, the internal regions of the batter or breading being moist and tough rather than crisp (Ling et al., 1998; Luyten et al., 2004). In the samples analyzed, the crust was quite easy to remove completely from the nugget core and moisture determinations were then performed on the whole crust; however, it was possible to observe that the water distribution in the crust was not homogeneous, as the internal part of the crust was moister than the outer layers in some of the samples. However, it is virtually impossible to delimit these zones and cut them apart in order to perform a moisture determination in the different layers of the crust with standard procedures (oven drying). Then, even measuring separate moisture contents in the coating would not suffice to characterize the crust and its relation to crispness. Crust formation also depends on other physical and chemical phenomena apart from water migration during cooking. Physical processes such as oil migration from the frying oil to the product (in the case of DF) or fat from the product to the outside also influence plasticization and are interrelated with moisture loss. Chemical changes include denaturing of proteins (already taking place at temperatures lower than 100 1C), nonenzymatic browning (Maillard) and caramelization of sugars, pyrolisis and oxidation of different compounds and the most important process in crust formation, the gelatinization of starch (Pokorny, 1999). Moisture and crispness are not always related; Primo-Martı´ n et al. (2006) also reached this conclusion in the case of a different crusted product, bread: at high relative humidity crispness is directly related to moisture uptake but at certain lower relative humidities, water is lost instead of gained and crust hardening on staling plays an important role in the loss of crispness. Regarding the moisture content of the nugget core, significant differences with cooking procedure were found in all the samples (Table 2). Microwave heating dried the core considerably, particularly when the susceptor was used. Because of their composition, susceptor packaging materials convert microwave energy into heat (Anon, 1996). The very thin layer of conductive material (aluminium) has significant electrical resistance and generates localized resistance heating where they are exposed to
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a rapidly oscillating electric field. Thin metal susceptors interact primarily with the electrical component of the energy field and provide sensible heat to the food (Bohrer & Brown, 2001). This causes the temperature to rise beyond the temperatures produced by boiling water into steam (100 1C in normal microwave heating), so the system reaches temperatures comparable to a CO (about 200 1C); this effect, combined with the microwave action, caused extensive water evaporation and a product that was drier in both core and crust was obtained. Samples that were deepfried or cooked in a CO maintained a higher nugget core moisture. 3.2. Instrumental analyses: texture and acoustics 3.2.1. Selection of the method The intention was to test the samples with an imitative test, as though they were being cut with a knife, bitten or chewed. Based on preliminary tests, the Perspex blade probe and the test parameters used were selected to achieve a compromise: reproducible and discriminative force curves where the fracture events and the sound peaks could be obtained and distinguished. Preliminary tests included:
Penetration with a Delrin cylinder 0.5 in diameter: the results did not discriminate between samples. Penetration with the Volodkevich tooth: the texture plots were discriminative but the number of sound peaks was low even for the crisper samples. Compression of half a nugget with a 35-mm aluminium cylinder: although discriminative, it was not very reproducible, probably because of the structural damage caused when cutting the samples. Compression of the whole nugget with a 75-mm aluminium compression plate: this was a good method, being discriminative and reproducible, but was not very representative of a real-life procedure. Cutting with the Perspex blade. Good discrimination and reproducibility. Sound peaks could be distinguished well.
3.2.2. Profiles of the texture curves Fig. 1 exemplifies the texture profiles (force vs. displacement of the probe plots) of sample 1 subjected to the four different cooking procedures; the curve profiles obtained for samples 2 and 3 had similar shapes when subjected to the same cooking procedures (data not shown). The force curves of the deep-fried sample (DF) and of the one cooked in a CO were very similar. The curves presented two regions. In the first part, which is a function of the stiffness of the crust, the sample underwent deformation but the blade did not start to cut, so the force values increased with the displacement but no major force drop occurred. The second part of the curve began at the point where the blade started to cut the sample, when the force values showed a sudden decrease, depending on the fracturability of the
30 25 20 Force (N)
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15 10 5 0 0
5
10 Distance (mm)
15
20
Fig. 1. Force (N) versus distance (mm) curves, sample 1 subjected to the four different cooking procedures: DF (thin grey line), CO (thin black line), MW (thick grey line), and MW+S (thick black line).
material. The fracture propagated until inhibited by the presence of a crack stopper (heterogeneity), then started again as the nugget was further deformed. A highly jagged curve profile with many peaks due to numerous fracture events is often produced by products that are perceived as crispy or crunchy (Varela, Aguilera, & Fiszman, 2007; Varela et al., 2006; Varela, Salvador, & Fiszman, 2007; Vincent, 1998). The force plot of the microwaved sample (MW) was dramatically different from the others: it did not present drops in force, no fracture event happened during the test, the sample was not crisp but gummy and tough and until almost the end of the test the sample was deformed under compression without being cut. The use of the susceptor in microwave heating (sample MW+S) imparted crispness to the crust and the plot presented ups and downs in force, although the curve was not as jagged as for the DF and CO samples. 3.2.3. Texture and acoustics Fig. 2 shows the SPL and force plot versus distance for sample 1 subjected to DF. The curves are displayed together as they appeared in the computer screen when performing the test; the signals were synchronized, allowing real-time comparison of the force and sound data. Both the force and acoustic signals were very jagged. Sound peaks were already present in the plot at the beginning of the test, when the crust began to deform, and lasted in quantity and loudness until the end of the test. All the major structural breakdowns in force were accompanied by peaks of high SPL, showing that the DF samples were extremely crisp. In general, when a crisp material is broken, it is hypothesized that each fracture event corresponds to an acoustic event (Chen et al., 2005; Varela et al., 2006; Vincent, 1998). This would happen in solid crisp food items but in the case of this sandwich-like structure (crisp on the
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
6
65
4 60
2 0
55 0
5
10 Distance (mm)
15
20
Fig. 2. Sound pressure level (SPL; grey line) and force (black line) versus distance. Sample 1, DF.
outside, soft on the inside) it should be remembered that not all the drops in force will correspond to fractures in the crust, as many may be related to other heterogeneities such as air pockets in the interface between crust and core or bubbles in the core mass (in general composed of an extruded or thermoformed preparation). It can be observed that in certain parts of the plot there were some minor breakdowns without associated sound peaks; these probably correspond to the deformation of the soft core and, by not producing sound, do not contribute to the perception of crispness. Previous works have demonstrated that the peaks do not have to be correlated one-to-one, as the sound emission is the result of a sudden release of energy while the force curve is a reflection of the energy applied to the sample. To correlate them, it would be interesting to define how much of the energy applied to the material is released as a sound pulse. The second derivative of the force is a measure of the rate of change of elastic energy in the sample as it is stressed. The release of energy is reflected in the negative values of the second derivative of the force curve and it is clear from the mechanisms involved in the creation of crispy sensations that the crack events that generate these perceptions will be correlated in time with energy release (Chen et al., 2005; Varela et al., 2006). In the present work, the second derivative of the force curves was calculated but the correlations did not improve. The energy supplied during the deformation of a solid material can be stored elastically, dissipated as fracture energy or dissipated in other forms. The amount of each component in energy release depends on the material; energy dissipative processes other than fracturing affect the energy available for fracturing and for sound emission (Luyten et al., 2003). Foods composed of layers with different textural characteristics have a particular behaviour, as the deformation of the outer crispy layer, as well as being a result of the mechanical behaviour of the crust, also depends on the mechanics of the ‘‘bed’’ on which it lies. Much of the energy supplied in the test is probably stored elastically in the soft core.
3.2.4. Analysis of the mechanical and acoustic parameters As can be appreciated in Fig. 6a, the area of the force curves did not give a good indication of the crispness of the samples cooked by the different methods. For all three samples, DF, CO and MW procedures did not present significant differences in area despite having very different crispness characteristics. Both compressive (uniaxial and biaxial) and shear forces are involved in this test. The area 85
12
80
10 8
75
6
70
4
Force (N)
8
70
65 2 60 0 0
5
10 Distance (mm)
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20
Fig. 3. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, CO.
20
85 80
15 75 70
10
65
Force (N)
SPL (dB)
10
Force (N)
12 75
SPL (dB)
14
80
Fig. 3 displays the force and SPL plots for sample 1 cooked in a CO. It can be appreciated that although the force curve was very similar to sample 1 DF the sound signal obtained was not so jagged; the CO nugget would be likely to be perceived as less crisp. Fig. 4 shows the force and SPL plots for sample 1 cooked in microwave oven. The acoustic signal was very quiet: almost no sound peaks were detected. The enhanced crispness of the crust with the use of the susceptor packaging can be supported by Fig. 5; the SPL plot showed various fracture events accompanied by sound emission.
SPL (dB)
16
85
1257
5
60 55
0 0
5
10 Distance (mm)
15
20
Fig. 4. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, MW.
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
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30
85
140 25
15 70
Force (N)
SPL (dB)
20 75
10
65 0
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10 Distance (mm)
15
d
c
40
0
0
b
b, c b
b
a
a
Sample 1
Sample 2
a
Sample 3
86 81 Max SPL (dB)
b
d
60
20
b b
80
5
Fig. 5. Force (black line) and sound pressure level (SPL; grey line) versus test distance. Sample 1, MW+S.
300
100
20
350
c
c
120 n° Sound Peaks
80
b
b b
b
b
b
b
b
76 b
71 66
Area (N.s)
250 200
61
a a
150
a
a a
100
56
a
a
Sample 1
a
Sample 2
Sample 3
a
Fig. 7. Mean values of the instrumental parameters extracted from the SPL/distance plots: (a) number of sound peaks and (b) maximum sound pressure level (dB). Identical letters within a sample indicate that there is no significant difference at p40.05 (Tukey’s test). Deep frying, conventional oven, microwave+susceptor, ’ microwave.
50 0 Sample 1
a
a a
Sample 2
Sample 3
140 c
n° Force Peaks
120 100
c
80 60
c c
40 b
b
b
20 Sample 1
a
a
a
0
b
c
Sample 2
Sample 3
Fig. 6. Mean values of the instrumental parameters extracted from the force/distance plots: (a) area (N s) and (b) number of force peaks. Identical letters within a sample indicate that there is no significant difference at p40.05 (Tukey’s test). Deep frying, conventional oven, microwave+susceptor, ’ microwave.
behind the curve can be associated with the total work and thus can be related to the toughness of the whole nugget. Sample MW+S had a much higher area value, indicating its increased toughness, which can be explained because of the massive water evaporation from both core and crust observed in this sample.
Fig. 6b displays the mean values of the number of force peaks for the three samples subjected to the different cooking procedures, which proved a good parameter for characterizing crispness, in agreement with previous works that had already used it to characterize other crispy/ crunchy food items (Chen et al., 2005; Varela et al., 2006; Vincent, 1998). Those previous works also extracted two parameters from the SPL versus distance plots, as in Fig. 7. The number of sound peaks (Fig. 7a) is the better discriminator between the samples and is directly related to crispness, as it measures only the mechanical events that produces sound. The maximum SPL is a measure of the sound level or the loudness of the sound events; it can be appreciated that this parameter did not discriminate between the three crispy samples (Fig. 7b) but did characterize the sound emitted, which in none of the cases rose above 81 dB. 4. Conclusions A new methodology was developed to assess the texture of crisp-crusted foods with a soft, high-moisture core. The use of a Perspex blade probe to perform a test on the whole
P. Varela et al. / LWT - Food Science and Technology 41 (2008) 1252–1259
sample (crust and core) while simultaneously measuring the sound pressure level of the emitted sound proved to be an effective method for characterizing chicken nuggets cooked by different procedures. Although the moisture and fat contents of the core and crust are related to the texture characteristics of the crusted samples, generalizations cannot be made as samples with similar moisture contents in the crust can have very dissimilar crispness characteristics and, at all events, crispness would depend on the water distribution within the crust, a more difficult parameter to measure in this kind of product. Acknowledgements The authors are indebted to the Comisio´n Interministerial de Ciencia y Tecnologı´ a (Spain) for financial support (Project AGL-2006-11653-C02-01) and to the Ministerio de Educacio´n y Ciencia (Spain) for the grant awarded to the author Paula Varela. References AACC. American Association of Cereal Chemists. (1967). Cereal laboratory methods. Method 30-20. Anon. (1996). Packaging plays an active role. Food Review, 23(8), 16–25. Association of Official Analytical Chemists. (2000). Official methods of analysis (17th ed.). Gaithersburg, MD: AOAC. Antonova, I., Mallikarjunan, P., & Duncan, S. E. (2003). Correlating objective measurements of crispness in breaded fried chicken nuggets with sensory crispness. Journal of Food Science, 68(4), 1308–1315. Antonova, I., Mallikarjunan, P., & Duncan, S. E. (2004). Sensory assessment of crispness in a breaded fried food held under a heat lamp. Foodservice Research International, 14, 189–200. Altunakar, B., Sahin, S., & Sumnu, G. (2004). Functionality of batters containing different starch types for deep fat frying of chicken nuggets. European Food Research and Technology, 218, 318–322. Baixauli, R., Sanz, T., Salvador, A., & Fiszman, S. (2003). Effect of the addition of dextrin or dried egg on the rheological and textural properties of batters for fried foods. Food Hydrocolloids, 17, 305–310. Bohrer, T., & Brown, R. (2001). Packaging techniques for microwaveable foods. In A. Datta, & R. Anantheswaran (Eds.), Handbook of microwave technology for food applications (pp. 397–467). New York: Marcel Dekker, Inc. Chen, J., Karlsson, C., & Povey, M. (2005). Acoustic envelope detector for crispness assessment of biscuits. Journal of Texture Studies, 36, 139–156. Dogan, S. F., Sahin, S., & Sumnu, G. (2005). Effects of soy and rice flour addition on batter rheology and quality of deep-fat fried chicken nuggets. Journal of Food Engineering, 71, 127–132. Drake, B. K. (1963). Food crushing sounds. An introductory study. Journal of Food Science, 28, 233–241. Fan, J., Singh, P., & Pinthus, E. (1997). Physicochemical changes in starch during deep-fat frying of a moulded corn starch patty. Journal of Food Processing and Preservation, 21, 443–460. IEC Publication 705. (1988). Methods of measuring the performance of microwave ovens for household and similar purposes (2nd ed.). Geneva: International Electrotechnical Commission.
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