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ARTICLE ORIGINAL ORIGINAL PAPER
Mechanical and sensory characterisation of dried bread-crumbs: application of fractal concept
J. Scher*1, B. Berton, J. Hardy
RÉSUMÉ
Caractérisation mécanique et sensorielle de chapelures : application du concept de géométrie fractale
La formulation de nouveaux produits alimentaires utilise de plus en plus des matières premières reconstituées (viandes, poissons, etc.). Afin d’augmenter la qualité sensorielle des produits tout en masquant les défauts d’aspect liés au mode de reconstitution, ces aliments sont très souvent enrobés par de la chapelure. Ainsi, depuis quelques années les quantités de chapelure fabri- quées sont en constante augmentation. Peu de travaux scientifiques ont été publiés sur la caractérisation de la texture de ces produits d’enrobage, du fait, d’une part, de leur faible valeur ajoutée mais également de leur carac- tère très hétérogène.
La chapelure peut être caractérisée en termes mécaniques par sa fragilité, sa friabilité ou sa résistance à la compression et en termes sensoriels par des descripteurs croustillant ou dur.
L’objectif du présent travail est ainsi d’étudier les propriétés mécaniques et de structure de la chapelure et de les corréler au caractère « croustillant » qui correspond à la première sensation en bouche ressentie par le consom- mateur lors de la consommation de produits pannés ou enrobés. Pour les essais d’indentation, la géométrie fractale est utilisée pour quantifier l’irrégu- larité des enregistrements. La dimension fractale apparente calculée sur les différents produits utilisés, est ensuite comparée aux descripteurs texturaux, quantifiés par un jury d’analyse sensorielle.
Ainsi, des corrélations significatives ont été mises en évidence entre la dureté sensorielle et la résistance à la compression et entre le caractère croustillant et la dimension fractale apparente.
1. Laboratoire de physico-chimie et génie alimentaires – École nationale supérieure d’agronomie et des industries alimentaires (ENSAIA) – Institut national polytechnique de Lorraine (INPL), 2, avenue de la Forêt-de-Haye, 54505 Vandœuvre-lès-Nancy, France.
* Author to whom correspondence should be addressed/
Correspondance : Joel.Scher@ensaia.inpl-nancy.fr
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Globalement cette étude psycho-rhéologique montre que les techniques mathématiques de modélisation des systèmes complexes peuvent apporter une contribution intéressante à la science des matériaux alimentaires pulvé- rulents.
Mots clés
chapelure, dureté, croustillant, évaluation sensorielle, dimension fractale apparente.
SUMMARY
In the food industry dried bread-crumbs is an ingredient more and more used in the current food formulations. These products can be considered very complex materials. An analytical method based on a fractal geometry concept was propose for quantification of the “crispness” descriptor of dried bread-crumbs. An universal testing machine was used to determine com- pression and penetration forces. The graphs were irregularly shaped so that usual interpretation was made not possible. Nevertheless, the irregular shape or “roughness” displays auto-similarity properties, which can be inter- preted as apparent fractal dimension of the texture. Sensorial evaluation of products was described by a panel trained to quantify the “hardness” and the “cripness” descriptors. The apparent fractal dimension, calculated from the dilatation function obtained by image analysis, permit to erase the irreg- ular of the shapes. High correlations were obtained between sensorial “hard- ness” versus resistance to compression, and between “crispness” versus apparent fractal dimension.
Key-words
dried bread-crumbs, hardness, crispness, sensorial evaluation, apparent fractal dimension.
1 – INTRODUCTION
To day, the formulations of new food products often use more and more reconstituted raw materials mainly from by-products of meats or fishes. These products are often coated with dried bread-crumbs to improve the sensory quality and to mask the lack of food structure and product texture.
Thus, for some years the made quantities of dried bread-crumbs are in strong increase.
Manufacturing processes linked to the composition of the product provide a wide range of very different products. This diversity renders the quantitative determination of the textural quality of products, in terms of “hardness” and
“crispness” descriptors for example, extremely difficult. In addition, during processing, it is extremely important, though very difficult, to preserve product quality because food powders are biological substances and as such, they are
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susceptible to various changes (moisture uptake, softening, changes in granu- larity…)
Few studies have been published about the texture characterisation of dried bread-crumbs (BOTTIN and VAN HECKE, 1999). Generally, food powders are essentially studied in terms of flow properties (AGUILERA and DEL VALLE, 1994;
DE JONG et al.,1999).
For these materials, the exploitation of the mechanical tests (penetration for example) is difficult. The graphs (Force versus deformation) were irregularly shaped so that usual interpretation was made not possible. Nevertheless, the irregular shape, or “roughness” displays auto-similarity properties, which can be interpreted in terms of apparent fractal dimension. It seems to be interesting to tackle the problem by non-classical methods and consider the food materials responses from a mathematical and physical point of view.
In the present work, the quantification of irregularities was then made by an image analysis system allowing determination of apparent fractal dimension.
The products were evaluated by a trained sensory panel for their characteris- tics. A correlations was attempted between sensory properties and apparent fractal dimensions
2 – MATERIALS AND METHODS
2.1 Materials
Two commercially available dried bread-crumbs (A and B) were purchased in a local supermarket. The composition of dried bread-crumbs is presented in table 1.
Table 1
Composition of dried bread-crumbs A and B.
Dried bread-crumb “A” Dried bread-crumb “B”
Wheaten flour Wheaten flour
Moisture: 5.50 % Water activity: 0.35
Moisture:7.00 % Water activity: 0.38
Yeast Yeast
Animal and vegetable fat Animal and vegetable fat
Sugar Sugar
Salt Salt
Malt Antioxidant E 300
Antioxidant E 300 Emulsifiers E 322 et 478
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2.2 Conditioning
One kilogram of bread-crumb was put in the middle of two boxes until water equilibrium was reached (aw = 0.11, 0.33 and 0.43) (GREENSPAN, 1977). The method used was based on equilibration with saturated salt solutions (LAN- DROCK and PROCTOR, 1951) and adapted by Hardy and Steinberg (HARDY and STEINBERG, 1984).
The temperature chamber was of 20˚C and the duration to obtain the equi- librium was of 15 days.
2.3 Particle size distribution
The size distribution of each type of dried bread-crumbs is studied by LASER diffraction with a granulometer MASTERSIZER S., (Malvern Instruments, UK). Results obtained are diameters of equivalent spheres expressed in volume.
The value d10, d50 and d90 are notified, those values mean that 10%, 50% and 90% of particles have a diameter lower than this value. Size of the distribution is also calculated: span = (d90 - d10) / d50 (MELCION, 2000; ORTEGA-RIVAS, 2001).
All results were the mean of triplicate measurements.
2.4 Mechanical measurements
The mechanical measurements of the texture were realised by penetration and by compression on a volume of 80 ml of dried bread-crumbs confined in a container (diameter: 5 cm).
The Instron Universal machine, model 1122 (Instron Corp., Canton, MA) equipped with a 1000N (± 0.005%) load cell was used.
The samples of dried bread-crumbs were compressed by a 1 cm diameter stainless steel rod for penetration tests and 4 cm diameter stainless steel rod for compression tests. The rod was uniaxially driven into the samples at a crosshead speed of 5 mm.min-1 (± 0.005%).
All the tests were performed in triplicate
2.5 Determination of the apparent fractal dimensions
The apparent fractal dimensions (DT) were determined as follows (SCHER and HARDY, 2002; BARRETT et al., 1992; NORMAND and PELEG, 1988). The signal (recordings penetration) was normalised by a four-polynome degree to remove the transition phase and magnify the roughness. The normalised curve was dilated 50 times by the principle of Minkowski (COSTER and CHERMANT, 1989) using an image analysis software (Visilog 3.6 by Noesis). The algorithm of Minkowski was applied to a curve and led to the « saucisse » of Minkowski increasing at each iteration and developed by many authors working with fractal dimensions (GOUYET, 1992; TRICOT et al., 1988). For each dilatation n, we calcu- lated the length L of the curve (figure 1).
L = Area/2n
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The slope of the linear region of the Richardson plot (log L versus log n) gave the apparent fractal dimension of the curve (PELEG and NORMAND, 1993).
This fractal dimension was considered as an apparent fractal dimension because it was limited in a range of length scale and only described macro- scopic details.
The validity of the results of fractal dimension determination was tested with the Weierstrass function, generated by a computer, whose fractal dimension was known. It was demonstrated that as long as the resolution along the two axes was the same, the calculated fractal dimension provided an absolute, rather than a relative, measure of the relationships jaggedness (PELEG and NOR- MAND, 1993; MAURER and HARDY, 1996).
The Weierstrass function was defined as:
where λ (λ > 1) and ε (0 < ε < 1) are constant.
2.6 Sensory analysis
2.6.1 Panel and sensory methodology
A panel of 19 members was selected, based on their ability to describe texture an classify products. Criteria for selection were as described by Civille and Szczesniak
n = 1 n = 2 n = 4 n = 6
n = 8
n = 10
Time(s)
Normalised force (N)
Figure 1
Example of normalised and dilated curves by the principle of Minkowski.
P
DT = 1+ P
fw( )x [sin(λix) λ⁄ εi]
i=1
∑
∞=
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(CIVILLE and SZCZESNIAK, 1973). The panel was composed of 14 women and 4 men ranging in age from 20 to 30 years. It consisted of students, food scientists and tech- nologists. The judges were trained to evaluate texture during ten hourly sessions. The training consisted of discussions and demonstrations of basic techniques for textural evaluation. The sessions of sensory analysis were realised from 12h to 13h in a nor- malised room (AFNOR, 1985). All the sensory parameters were evaluated descrip- tively and transformed into a 1 - 5 numerical scale for data analysis. Each value corresponded to the mean of sensory intensities given by the 19 members. Six sam- ples were evaluated at each session. Three identical sessions were held to test the reproducibility of panel results. Standard deviation was below 10% for each member.
2.6.2 Vocabulary development
A sensory vocabulary for description of bead-crumbs was developed (SZCZES- NIAK, 1963; AFNOR, 1988). Two final terms were selected for this study: “hard- ness” (sensory perception tactile) and “crispness” (sensory perception in mouth).
3 – RESULTS AND DISCUSSION
The intensities of the sensory attributes, the size distribution and the rheo- logical characterisation of dried bread-crumbs are summarised in Table 2.
Table 2
Evolution of the sensory attributes, the size distribution and the rheological characterisation versus water activity (dried bread-crumbs A and B).
Dried bread - crumbs « A » Dried bread - crumbs « B » aw
0.11
aw 0.33
aw 0.43
aw 0.11
aw 0.33
aw 0.43 d50 (µm)
mean size
±753 15
±762 15
±780 15
935±15
±950 15
±970 15
d10 (µm) 630
± 15
±647 15
±670 15
±856 15
±878 15
±906 15
d90 (µm) 856
± 15
±878 15
±902 15
±980 15
1023± 15
1025± 15 span
(d90-d10 )/d50
0.30 0.30 0.30 0.13 0.15 0.12
Apparent fractal dimension (DT)
1.173
± 0.005
1.122
± 0.005
1.091
± 0.005
1.227
± 0.005
1.153
± 0.005
1.101
± 0.005 Slope of
compression resistance
±1.69 0.05
±0.56 0.03
±0.41 0.04
±1.81 0.05
±0.69 0.04
±0.55 0.03
Tactile hardness 4.00
± 0.08
±1.30 0.05
±1.10 0.05
±4.30 0.08
±1.70 0.05
±1.50 0.05 Crispness
(in mouth)
±4.50 0.09
±3.50 0.05
±2.50 0.05
±4.80 0.08
±3.70 0.05
±2.75 0.05
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Some examples of the compression and penetration curves obtained are shown in figure 2.
Water activities were chosen to keep the hardness and crispness attributes in products. The values are also close to the original value.
The standard deviation of sensory attributes, size distribution and rheologi- cal characterisation are weak and allow for the repeatability of the methods.
The statistical analysis of the results for all the techniques used exhibited signif- icant differences between both dried bread-crumbs on one hand, and accord- ing to the water activity for every dried bread-crumbs on the other hand.
Thus, it was shown that dried bread-crumb B was made of bigger particles (25% bigger) and presented a particle size distribution narrower than dried bread-crumbs A (span = 0.30 for A and 0.15 for B).
A small increase was also observed (6 %) on the average size of particles versus water activity. Conversely, the size of the distribution (span) remained identical. An attempt was made about a slight swelling of all particles with the increase of water activity.
The particles of dried bread-crumbs B were characterised by the sensory panel as the hardest and the most crispy at same water activity.
The correlations between sensory analysis and instrumental measurements are displayed in figures 3 and 4.
Sensorial hardness and slope of compression resistance curve were highly correlated. Crispness character and apparent fractal dimension were also shown, (r2 = 0.93).
Nevertheless, figure 3 shows 2 populations of samples: the first one for a water activity of 0.11 (high tactile hardness) and the other one for other water activities (weak tactile hardness). Therefore, the interpretation of the correla- tions between slope of compression resistance curve and tactile hardness remained still difficult.
The correlations between hardness and apparent fractal dimension (r2 = 0.80) on one hand and crispness character and slope of compression resistance curve (r2 = 0.82) on the other hand were less satisfactory.
Deformation (mm) Deformation (mm)
Force (N) Force (N)
aw 0.11 aw 0.43
Figure 2
Examples of the penetration and slope of compression resistance curves of dried bread-crumbs. (aw of 0.11 and 0.43).
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The use a compression test to quantify the tactile hardness of dried bread- crumbs was found valuable as well as the use of the fractal geometry to quan- tify the crispness character of such products.
This psycho-rheological study should be now completed by taking into account the thermal treatment of dried bread-crumbs as well as their stuffing
0.0
1 2 3 4 5
0.5 1.0 1.5 2.0
0.0
1.0 1.2
Hardness
Hardness
Slope of compression
Slope of compression
1.4 1.6 1.8
0.2 0.4 0.6 0.8
r2 = 0.90
aw 0.11
aw 0.33 and 0.43
Figure 3
Slope of compression resistance as a function of the hardness (sensory perception tactile).
1.3
1.2
1.1
1.0
2 3 4 5
Crispness DT
r2 = 0.93
Figure 4
Apparent fractal dimension as a function of crispness (sensory perception in mouth).
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(meat, fish,…). The study of the particle’s shape would be also interesting (BOT- TIN and VAN HECKE, 1999).
The apparent fractal dimension of the normalised curves was a modelisation of the irregular mechanical signatures of the dried bread-crumbs products and the relationship between sensory analysis and apparent fractal dimension was an interesting tool for the quantitative prediction of crispness aspect.
Modelling mathematics methods for complex systems allow useful contribu- tion to Food Science.
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