A review: Crispness in dry foods and quality measurements based on acoustic–mechanical destructive techniques

Journal of Food Engineering 105 (2011) 387–399

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Review

A review: Crispness in dry foods and quality measurements based on acoustic–mechanical destructive techniques Mayyawadee Saeleaw a,b, Gerhard Schleining a,⇑ a b

Department of Food Sciences and Technology, Food Physics Lab, BOKU-University of Natural Resources and Life Sciences, Muthgasse 18, A-1190 Vienna, Austria Faculty of Home Economics Technology, Rajamangala University of Technology Krungthep, Bangkok 10120, Thailand

a r t i c l e

i n f o

Article history: Received 4 January 2011 Received in revised form 10 March 2011 Accepted 12 March 2011 Available online 17 March 2011 Keywords: Acoustic Sound emission Crispness Crunchiness

a b s t r a c t Research during the past years has focused on crispness because of the great interest of consumers towards crispy foods. Currently crispness is measured with sensory, mechanical and morphological variables. Recently, the analysis of acoustic recordings with mechanical testing results appears to be an interesting technique of crisp foods. The first part of this article reviews briefly the definition and the determination of crispness by different test methods. The second part summarizes the results of studies conducted worldwide on the crispness which use destructive acoustic–mechanical methods. Ó 2011 Elsevier Ltd. All rights reserved.

Contents 1.

2.

3.

Crispness and crunchiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Determination of crispness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1. Sensory methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2. Instrumental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3. Morphological methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acoustic–mechanical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Determination of sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Acoustic–mechanical methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Set-up of acoustic–mechanical monitoring systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Interpretation of acoustic–mechanical data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Force signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Sound emission signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Influence factors on sound in dry crisp products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Processing conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Ingredients and hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. Others finished product properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Influence factors on sound in wet crisp products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +43 1 47654 6294; fax: +43 1 47654 6289. E-mail address: [email protected] (G. Schleining). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.03.012

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Table 1 Definition of crispness and crunchiness. Attribute

Definition

References

Crispness

The degree to which the rupture is heard during the first bite Firm and brittle, snaps easily, emitting a typical sound upon deformation The perceived force with which the product separates into two or more distinct pieces during a single bite with the incisors. An abrupt and complete failure of the product is required (sensory description) The characteristic exhibited by a firm tissue with a linear elastic fracture behavior (fruit and vegetables) A combination of noise produced and the breakdown of the product as it is bitten entirely through with the back of molar A light and thin texture producing a sharp clean break with a high-pitch sound when force is applied, mainly during the first bite with front teeth A combination of the type of sound. i.e. short snapping and longer cracking sounds and the force to bite and chew as perceived on the first bite One sound event perceived as a sharp, clean, fast, high pitched sound

Brennan (1988) Szczesniak (1988) Barrett et al. (1994)

Firm and brittle, snaps easily with a typical sound. Sound has a lower pitch, is less loud and longer lasting than for crisp Complex failure mechanism that involves repetitive deformation and fracturing of a cell structure. Necessary are structural subunits, especially cells, with brittle cell walls. Continuous fracture during chewing Temporal aspects of the sensory feedback during mastication are important for the crunchy sensation. It is independent from hardness A hard and dense texture that fracture without prior deformation

VIickers (1984) Barrett et al. (1994)

Crunchiness

Fractures after applying a higher force on the product than for crispness on the first chew with molars High pitched sound, light sound, longer sounding Multiple lower pitched sounds perceived as a series of small events

1. Crispness and crunchiness

Alvarez et al. (2000) Duizer (2001) Fillion and Kilcast (2002) Duizer and Winger, (2006) Chauvin et al. (2008)

Brown et al. (1998) Fillion and Kilcast (2002) Vincent et al. (2002) Dijksterhuis et al. (2007) Chauvin et al. (2008)

In this review various aspects that are relevant to crispy/crunchy behavior of food products are discussed.

1.1. Definitions Crispy and crunchy textures are a desirable quality and contribute to our enjoyment of foods (Vickers, 1983). Crispness is synonymous of freshness. For crispy products, the crispness loss due to the adsorption of ambient moisture or due to the water mass transfer from neighboring components is a major cause of rejection by the consumers (Piazza et al., 2007). Many texture studies have been published on crispness because of the great interest of consumers towards crispy foods. The sensations ‘‘crisp’’ and ‘‘crunch’’ are difficult to describe especially when different food products are involved. For both wet and dry crisp products the sensations crispness and crunchiness are found to be related to the same type of product properties, i.e. hard, brittle and producing a typical sound at fracturing (Luyten et al., 2004). Panelists as well as consumers found it very difficult to define crispy and crunchy terms and needed constant probing. However, all insisted that they could perceive a difference between the two, but then struggled to describe this difference, indicating that it was only slight. Common aspects included a certain resistance to the teeth when biting or chewing, the emission of a sound and brittleness. In brief, crispy and crunchy are words that are used to describe products that break rather than deform, and the way in which they fracture under the application of a force (Fillion and Kilcast, 2002). The definition of crispness is not yet completely understood with only the established generalized concept. In literature we can see lots of definitions about both of them. The examination of these definitions (Table 1, including two earlier studies) shows a large diversity of meaning. 1.2. Determination of crispness Although many types of experiments have been done to determine crispness, the best measurements are still open to doubt. However, the properties related to crispness were able to disclose the complexity of crispness and its association with other similar sensory attributes, e.g. brittleness, hardness, crackliness or crunchiness.

1.2.1. Sensory methods Sensory analysis is widely used to evaluate the perceived characteristics of dry cellular food products like extrudates, chips, or puff cereal products and results often compared to physical measurement (Chaunier et al., 2005). Consumers are able to describe the differences between crisp and crunch by judging the sound: a crisp sound is short, a crunchy sound more long lasting (Fillion and Kilcast, 2002). Sensory descriptive test are among the most sophisticated tools used by sensory scientist. Descriptive analysis training for crispness and crackliness evacuation focuses on parameters such as structure of the intact food, sound emitted at fracture, the force needed to crush the food, the collapse of the food at fracture and the appearance of sample observed and perceived following fracture (Roudaut et al., 2002). From the sensory results, Primo-Martín et al. (2010b) found that panellists perceive a higher sound intensity from crispier foods . 1.2.2. Instrumental methods Although sensory analysis gives a more complete description of the texture, there has been a great interest in developing instrumental techniques to assess crispness. Instrumental techniques present some advantages, especially in industrial environments where quick and easy-to-use methods are in great demand and economically more profitable, Crispness being described as a concept with kinesthesic and auditory components, it is not surprising that the instrumental methods developed to evaluate it, have focused on the measurements of these properties singularly or in combination (Roudaut et al., 2002). 1.2.2.1. Mechanical measurements. Due to fact that mastication is a highly destructive process, mechanical tests are the most popular to simulate biting. Mechanical properties are thought to tell about structural properties of materials by means of resistance to compression by a probe or to a tensile fixture that pulls the structure of food material apart by using a universal testing machine or a texture analyzer. Several types of probes or attachments can be installed with those machines according to the sample type and the

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Fig. 1. An imaginary force–deformation curve of a crispy/crunchy material, showing the different properties (Luyten et al., 2004).

purpose of test. Results of such a tests like texture profile analysis, compression, tension, twist, three-point bending show correlation with properties of a material thus can used for its crispness or other texture properties evaluation. The mechanical properties measurement is easier to conduct than that of structural properties, determined with the Scanning Electron Microscopy (SEM). Szczesniak (1963) explained that the mechanical variables are extracted from a force–deformation or stress–strain curve and shown in the chart recorder; force is used to determine brittleness and hardness. Seymour and Hamann (1988) investigated that the area under the peak force is used to determine the energy required to bite or chew the products or toughness. Some of the researchers found good correlation between sensory crispness and the ratio of work to fracture to total work. Mohamed et al. (1982) studied the crispness of the biscuits and get these positive correlations too. But (Seymour, 1985) used a Kramer shear cell in an Instron to crush samples of several dry crisp foods altered in crispness by humidification. Results from this experiment had negative correlations between crispness and mechanical parameters such as maximum force at failure and work done to failure. Mechanical tests are relatively quicker and easier methods but they have not produced enough correlation with sensory crispness. Also there is another disadvantage when mechanical tests are used; many crisp foods cannot be tested because of their irregular sizes, shapes or part of a food that also consist of noncrisp parts. The deformation and fracture behavior of crispy/crunchy food products are often studied as a function of time at lower rates of deformation in order to be able to study the different fracture events separately (Peleg, 1994; Vincent, 1998). A highly schematically example can be seen in Fig. 1. In general, each fracture event goes along with a drop in force and an acoustic event. The force at this maximum depends on the material composition and the structure, including the size of the inherent defects where fracture starts. After this maximum in the curve the fracture propagates at a certain (high) speed until it stops. This can be due to the presence of crack stoppers (e.g. holes in the structure) or fragmentation of the piece of material studied or eaten. For foods that are not crispy or crunchy or have these properties to a lesser extent, energy dissipation may be important in this context (Van Vliet and Luyten, 1995).

1.2.2.2. Acoustic measurements. Mohamed et al. (1982) investigated the correlation of instrumental and sensory properties of fried foods; they stated that the sounds produced while eating are important for both evaluation and enjoyment of crisp foods. Closely related to the fracture properties of a food are the sounds made while it is bitten and chewed, which add to the eating experience. Drake (1963) observed differences between chewing sounds produced when biting different foodstuffs. A generic aspect of many food products with crispy/crunchy nature is that they are cellular, e.g. they contain gas cells or are built of plan cells. Dry cellular products contain air filled cavities surrounded by brittle walls. When a continuous force or deformation is applied to these walls, they bend and then break. The remaining cell walls and any fragments produced during breakage, snap back to their original shape. This movement sets off vibrations, which generate a sound pressure wave (Duizer, 2001). In cellular crisp products, the sound produced may be due to the rupture of one cell or many cells. In non-cellular dry crisp products, such as potato chips, it is the repeated fracture of the thin chips which contributes to the sound (Duizer, 2001). While mechanical tests are a wellexperienced approach in food science to analyze the structure of foods, the relationship with the consequent acoustic emission, in particular for crisp products, have still to be fully developed (Piazza et al., 2007). 1.2.3. Morphological methods Tesch et al. (1996) stated that, in order to fully understand the relationship between acoustics and instrumental measures of products with an acoustic component, there must be an understanding of how mechanical failure occurs as well as of the resulting sound waves that occur with this failure. The structural properties of the products contribute to this failure and also have an impact on the texture of the product (Bouvier et al., 1997). (Primo-Martin et al., 2008a) showed the influence of morphology on sensory crispness. Cellular foods that contain only air within their cells, such as biscuits or chips, are designated dry crisp foods, while foods that contain fluids within their cells, such as fruits and vegetables, are called wet crisp foods (Chauvin et al., 2008). Some dry fried products contain both air and liquid oil; examples are crisps and fried battered snacks. For the type of crispy/crunchy products, different morphological structures that are of importance for food product behavior and the sensory perception

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Table 2 The effect of the structure on the mechanical and fracture behavior depends on the structural level considered. Going downwards the characteristic length of structural level diminished from mm to nm. Product structure also affects the recorded sound (Luyten et al., 2004). Structural level

Link to mechanical and fracture properties

Link to acoustic emission and sound produced

Crust–core

Local deformation versus overall deformation, sandwich structure End of fracture event End of fracture event Fracture start at voids Fracture propagation, fracture path

Damping

Layers in crust Larger pores Beams/columns and voids therein Distribution polymers determines the Plasticizer distribution Chemical aspects

Mechanical properties and fracture propagation Mechanical properties

(Luyten et al., 2004). The morphology of a cellular material can be characterized by (Luyten et al., 2004):  the porosity or the relative density of the material, which is related to the volume fraction of air in the material;  the size and shape of the cells, and the distribution in these properties;  the local absence of cell walls because of gas cell coalescence, causing the cellular structure to be open. Structure has a large impact on the sounds produced when biting into products with crisp, crunchy and crackly textures (Duizer, 2001). Primo-Martín et al. (2010b) stated that the cellular morphology of dough and bread is an important quality characteristic that affects final bread quality like loaf volume and shape, the texture of the crumb and the mechanical properties of the crust. The latter affects crispness. Thus the acoustic emission during the deformation (or eating) of food products with a crispy/crunchy crust depends on the structure of the food, as is also illustrated in Table 2. Future work in this area should combine acoustics and microstructure (Tunick, 2010). Various microscopic techniques are available for characterizing surfaces and internal structures. Scanning Electron Microscopy (SEM) is often used to study the morphology of the cell walls more closely, for instance, the bubble connectivity in foamed powders. There are many advantages of using of using SEM for this purpose (Marzec et al., 2007). Recently, X-ray microtomography (lCT) has been used to characterize cellular morphologies (Babin et al., 2006; Cheng et al., 2007; Lim and Barigou, 2004; Primo-Martín et al. 2010b). It proved to be a very useful technique in view of its non-invasive character (Primo-Martín et al. 2010b). Fig. 2 is an example picture from used X-ray microtomography. The technique enjoys a number of advantages over other methods such as light or electron microscopy, including: (i) the ability to investigate samples in their natural state at atmospheric pressure and temperature, free from artefacts that plague electron microscope sample preparation, for example; (ii) the much greater spatial resolution of X-ray microscopy over other types of microscopy; (iii) the ability of X-rays to penetrate through any material and, in most cases, capture three-dimensional (3D) details of the inner microstructure (Lim and Barigou, 2004). Moreover, samples may be viewed in their natural state without prior preparation in ESEM (Environmental Scanning Electron Microscopy), making it applicable to almond crispness studied (Varela et al., 2006). 2. Acoustic–mechanical measurements 2.1. Introduction The mechanics and sound it is likely that other stimuli may be important for the sensation of ‘‘crisp’’ and ‘‘crunch’’ (Luyten et al.,

Damping Vibration and size/shape of broken pieces Vibration and size/shape of broken pieces Type of material that really fractures frequency of the acoustic emission Energy dissipation affects stress waves

2004). An important reason for it is that there is no single receptor for texture, but that always several senses are involved (Szczesniak, 2002). The different supposed interrelations are shown schematically in Fig. 3. As described in Fig. 1, crispness has an auditory component. Recently, combining the analysis of acoustic recordings with mechanical testing results appears to be an interesting technique to characterize crispness of snack food. It is therefore not surprising that a rising technique in material analysis is the use of acoustic emission (AE) to detect permanent microstructural changes in the material (Chen et al., 2005; Piazza et al., 2007; Salvador et al., 2009) during instrumental crushing or during mastication. Fortunately, there is an increasing number of acoustic–mechanical studies but they are still not enough. Besides mechanical properties, the degree to which the rupture is heard during the first bite and further mastication affects the sensation of crispness and crunchiness (Luyten et al., 2004). Chakra et al. (1996) suggested that the association between mechanical fracture and sound emission is the first and second law of thermodynamic principles. They believe that the mechanical approach of the phenomenon can ascertain the link existing between the induced acoustical emission and the elastic behavior of the product. In fact, the brittle sample under deformation reaches just before rupture a non-equilibrium stage from which it liberates instantaneously upon rupture; inter-atomic bonds in this case tend to behave as little springs causing the atoms to interact in vibration with the surrounding air. The moment of rupture reflects the moment of destruction of the inter-atomic bonds at a given point (e.g. crack or defect) and the instantaneous liberation of other atoms who will want to regain their initial stage due to and via the elasticity of the body. So it is this perturbation in the structure that produces the sound wave. The existence of a relationship between crispness and characteristics of crushing sounds has been hypothesized by several researchers. Method for quality estimation of crispy products such as chips or crispbread offer an intriguiging problem (Winquist et al., 1999). Many authors have presented methods to analyze the fracture and acoustic behavior. In early investigation, Drake (1963) was one of the first researcher who studied sounds produced by crispy foods and found that the breakdown of the structure of foodstuff during normal chewing was decline in the average amplitude of successive burst of mastication sound. The loudness of crushing sounds differed between crispy and less-crispy products. Later in the 1970s and 1980s, acoustic measurement in food research gained more attention in the characterization of soundrelated texture attributes, e.g. crispness, crunchiness. For example, Vickers and Bourne (1976) studied tape-recorded sounds from biting wet and dry crispy foods and found that the crisper samples showed higher sound amplitude and a greater number of sounds. Mohamed et al. (1982) concluded that combination of sound and mechanical properties predicted sensory crispness better than

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Fig. 2. An example of X-ray microtomography images of (a) extrusion products (Cheng et al., 2007) and (b) bread (Primo-Martín et al., 2010b).

Fig. 3. Schematic overview of the different stimuli affecting the sensations: Crispness and crunchiness (Luyten et al., 2004).

mechanical properties alone. Hi et al. (1988) found that sounds from fresh samples were louder and had a greater amount of high frequency components than those from stale potato and tortilla chips. The development of acoustic measurement occurred in the late 1990s. Until now numerous of research has been done on the combination of force/displacement measurement and acoustic

detection of food materials (Arimi et al., 2010; Castro-Prada et al., 2007; Chakra et al., 1996; Chen et al., 2005; Cheng et al., 2007; Dacremont, 1995; Marzec et al., 2007; Piazza et al., 2007, 2008; Primo-Martín et al., 2009, 2010a,b; Roudaut et al., 1998; Saeleaw and Schleining, 2010; Salvador et al., 2009; Sanz et al., 2007; Taniwaki et al., 2010; Tesch et al., 1996; Varela et al., 2006, 2008; Zdunek et al., 2010a,b). This combination possesses the advantage techniques and should be able to reveal much more information about the crispness of food than either technique alone. Dacremont (1995) divided dry foods into crunchy, crispy and crackly according to the spectral characteristics of sounds emitted during biting. Crispy foods generate high pitched sounds with frequencies higher than 5 kHz, crunchy foods yield low pitched sounds with a characteristics peak on frequency range of 1.25–2 kHz, and crackly foods emit low pitched sounds with a high level of bone conduction. Numerous authors like, Tesch et al. (1996) and latter on Roudaut et al. (1998) investigated the effect of water content on acoustic properties of products and show that the intensity of emitted sound was strong related to the moister of the materials. Chaunier et al. (2005), Chen et al. (2005) and Varela et al. (2006) found that crispness is correlated to mechanical-sound properties measured during compression of cornflakes, biscuits and roasted almond, respectively. Marzec et al. (2007) observed changes in an acoustic emission signal of extruded flat bread with changes of moisture using a texturometer equipped with a contact microphone. In the most recent years, Salvador et al.

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(2009) indicated that sensory crispness of potato chip is positively related to the number of fracture and sound events, pressure level maximum. Primo-Martín et al. (2009) stated that crispness of the crust model was characterized by a low work of fracture and high number of sound and force event. Recently the sound emitted during fracture of the crust of the crispy food was studied as an indicator of instrumental crispness (Arimi et al., 2010; Primo-Martín et al., 2010a,b; Saeleaw and Schleining, 2010; Taniwaki et al., 2010). As a summary, studies have shown that auditory sensations and its measurements are an important for evaluating crispness. If they can be studied together with mechanical measurements, results were more close to reality. In addition, researches showed that using both acoustic and mechanical variables gave good correlations with sensory crispness. These parameters have allowed us to classify crispness and crunchiness from each others. 2.2. Determination of sound Sensory evaluation provides a direct measure of food crispness but it is not a convenient routine test method. An alternative for food crispness evaluation is to analyze the sounds produced during mechanical load with an instrument. At present instrumental acoustic methods are becoming more and more popular for the investigation of food product properties. Few researches have combined mechanical-sound measurements with sensory evaluation. Sensory perception of crispness, pitch and crumbiness of cornbased extrudates, significantly correlated to the fractal analysis dimensions of amplitude-time data was discussed by Duizer et al. (1998). Chen et al. (2005) reported that the instrumental acoustic assessments based on the total acoustic events, on the acoustic events per unit area or on the acoustic events per unit time all showed very good correlation with the sensory results, although the frequency of acoustic events seemed to be more effective in distinguishing the biscuits. According to this research, they also showed that the sound produced by the panelists was not as loud as that produced from the instrumental tests. Varela et al. (2006) reported that sensory crispness of roasted almonds is highly correlated with the rate of emission and size of acoustic peaks emitted by cracks. The use of instrumental analysis is more convenient than that of sensory evaluation. The determination of texture by instrumental tests is easy to perform, simple to reproduce and less time consuming. However, studying sound-mechanical instrumental data corelated with sensory score may help to fully understand crispness perception by individuals.

Measuring the sounds produced by manipulation of a food product can be done by destructive and non-destructive testing. Acoustic methods are divided into methods measuring sound emission, absorption and methods determining phase oscillation, as all parameters change caused by the medium through which the wave is passing (Duizer, 2004). Ultrasound technology provides one of non-destructive quality measurement in various products such as porous food products and fruits and vegetable. Ultrasonic energy will propagate through a material until the sound wave encounters an impedance change, which means that there are some changes in the material density or/and the velocity of the sound wave (Mizrach, 2008). Juodeikiene and Basinskiene (2004) successfully used a non-destructive acoustic technique in wafer sheets, crisp bread, crackers and ring-shaped rolls which were estimated according to the magnitude of the amplitude of a penetrated acoustic signal. They found good correlations between penetrated acoustic signals and the structural and mechanical properties (density, surface porosity, mechanical strength). This review will focus on the destructive method. For destructive testing, recording chewing sounds has been one of the widely used methods for evaluating food textures, and it has been used for analyzing the texture of crispy food as shown in Fig. 4a (Duizer, 2004). It is advantageous in providing an evaluation that is more objective than traditional sensory evaluations. De Belie et al. (2003) recorded the chewing sounds by placing a microphone over the ear canal at the side where the snacks would be chewed. Different sound bursts can be discerned in the bite, which is typical for a dry crisp product. However, many factors maybe responsible for the large variation in the crisp, crunchy or crackly sound. For example, the chewing sounds depend on the person who is chewing a food sample, particularly the sizes of his/her head cavity and mandible (Taniwaki et al., 2010), the amount of contact surface between the teeth and the food (Chauvin et al., 2008) and the pitch of the sound emitted when biting into a product hence depends on the mouth’s configuration, and whether the mouth is closed or open (Chauvin et al., 2008; Fillion and Kilcast, 2002). A solution to this problem is the instrumental measurement of food texture. Recording the sound emission produced via instrumental means an experimental set-up as shown in Fig 4b. The difference between them is the way of crushing the food product. First one is crushing by eating by a person and second one is crushing by instrumental load (e.g. compression). The mechanical test is performed with a texture analyzer and acoustic emission is

Fig. 4. Sound input techniques for measuring texture (a) bitting with teeth and (b) testing by instrument compression (Duizer, 2004).

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measured simultaneously with an acoustic envelope detector (AED) (Piazza et al., 2007). Human beings mainly use three organs to evaluate food texture: the teeth, the nerves and the brain. We masticate food using teeth, detect the texture using nerves and process the signals and perceive the texture of food using the brain. Therefore, a device for measuring food texture should be equipped with these three parts (Taniwaki et al., 2006). In Fig 4b the probe represents the teeth, the sensor the nerves, the microphone the ear and the computer the brain. 2.2.1. Acoustic–mechanical methods Piazza et al. (2007) reported that mechanical measurements of crispness are performed on instruments originally developed for material science, providing physical parameters with fundamental significance in terms of rheological properties, i.e. the materials’ response to the applied force. The texture analyzers typically have a crosshead containing a load cell, which is driven vertically at a range of constant speed. Probes can be attached to the crosshead for penetrating, shearing or crushing food. The reaction is recorded and usually displayed as force–deformation plot. Compression is the most commonly employed mode because of its similarities with the mastication process. When a force is applied to a crisp item, its structure is stressed until a critical point is reached: the action of external force causes the rupture of the brittle walls of the cellular structure which start to vibrate. The vibration is transmitted through the air as acoustic waves, which generates the sound (Piazza et al., 2007). The mechanical and sound emission measuring system consist of an electrically carrier with a force transducer and a microphone. The measurement of sound essentially involves three basic components: the generated sound wave, the detection equipment used to capture sound signals and the processing and interpretation of the collected data. Sound waves are produced by sudden movement in stressed materials. The process of generation and detection is illustrated in Fig. 5. The measurements were controlled by two systems. The first one served to direct the texture analyzer and record the mechanical data. The second system served to control the acoustic measurement to measure the energy detected as sound across a known frequency range and to record how this energy level varies over time. The acoustic emission monitoring system consists of an electroacoustic transducer, a preamplifier, a signal conditioning system and a data acquisition system. Fig. 5 shows an example for mechanical-sound instrument setting. A microphone is used to convert the acoustic energy into electrical voltage during compression of food sample Fig. 5a. Chaunier et al. (2005) record the sound during shear and compression with a Kramer cell as shown in Fig. 5b. 2.2.2. Set-up of acoustic–mechanical monitoring systems There are many factors that have to be optimised to get the best set-up for each product. Varela et al. (2006) stated that the sound emission in compression depends on the mechanics of the fracture, which in turn depends on the parameters of the compression. The strength of the sound depends on the strength of the vibration of the original source, the travel distance and available sound paths (Castro-Prada et al., 2007). One should be especially aware of artefacts in the acoustic measurements. The microphone should be directed at the right angle to the test piece, taking into account the angle over which the microphone is sensitive, while the distance should be standardized. Chen et al. (2005) tested three different angles (0°, 45° and 90°) and three distances for each angle (1, 5 and 10 cm) and they found that the angle of microphone had small effect to the acoustic signal, but the signal intensity showed a significance decrease with the increase distance between the microphone and breaking point. Piazza et al. (2007) placed a microphone as close as possible to the sample in order to further improve the acquisition of the acoustic signal. High sampling rates

Fig. 5. Scheme of a data acquisition system (a) Setting up example with compression method (Varela et al., 2006) and (b) with Kramer shear/compression cell (Chaunier et al., 2005).

are needed to be able to analyze the high frequency part of the emitted sound spectra, much higher than the 500 points/s for a commercial available instrument like the TA.XT.plus Texture Analyzer (Stable Micro Systems [SMS], Godalming, UK) (Chen et al., 2005; Piazza et al., 2008). Moreover, Castro-Prada et al. (2007) and Chen et al. (2005) stated that the acoustic measurement, not only the sampling rate and the quality of the A/D conversion is essential, but also the optimal capturing of sound by the microphone. Chen et al. (2005) pointed out that this could be because of the low resolution of the counting system (500 pps) and also Norton et al. (1998) suggested that a resolution above 200 pps is needed for crispness determination. There are several studies describing the importance of high frequencies for crispness perception (Vickers, 1985). According to Kapur (1971) the resonance frequency of the jaw is about 160 Hz. The bone conducted sound travelling through teeth and jaws to the ear are therefore amplified at this frequency. Also, Drake (1963) had located the peak frequency of the mouthopening sounds at around 160 Hz. The soft tissues in the mouth tend to absorb or damp, especially the higher frequencies of the sound (Vickers, 1991). Dacremont et al. (1991) studied the contribution of air and bone conduction to the transmission of chewing noises to the inner ear. Each panelist had to reconstitute sounds he truly heard during eating, by mixing the air and bone conduction records together. Both bone and air conduction records had to be attenuated at around 160 Hz and air conduction records had to be amplified at around 3.5 kHz. Others indicated the importance of low frequencies in the sound spectrum measures during biting, e.g. from 300 Hz up to 4 kHz for rice cracker and apple crispness (De Belie et al., 2002).

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For the perception of crispiness and crunchiness sound transmitted both through the air and through the teeth has a significant role. Acoustic transducers such as microphones only allow the measurement of the sound that is transmitted though the air. The very common envelope detector from Stable Micro Systems e.g. captures sound across a frequency range of 3–20 kHz (Stable Micro Systems, 2008), which is different from the frequency range mentioned above. Also the kind of sound creation (by compression, penetration, cutting, bending, etc.) must be considered for the set-up of the test. It should be similar to the kind of how people create the sound during consumption. In addition, the effect of contact area between probe and test piece on observed fracture and sound behavior was studied. Castro-Prada et al. (2007) showed that the positioning of biscuit also affected both the mechanical values (average value of maximum force, number of force events and fracture duration) and acoustic values (sound pressure, number of sound events and sound duration). Therefore the selection of apparatus to measure the sound properties of crispy food plays an important role. 2.3. Interpretation of acoustic–mechanical data 2.3.1. Force signal In order to understand the implications of the interpretation in a combined analysis of fracture behavior and acoustic emission, it is necessary to understand some of the basics of fracture mechanics. (Hecke et al., 1998) stated that dry crispy products, the force– time curve would show as many peaks as the number of cell wall ruptures, without any force accumulation but for wet non-crispy extrudate, there would be no cell wall rupture because of the pliability of cell walls; the force time curve would then present a continuous force increase without any peak. Among the several approaches for the objective quantification of texture during compression, the slope of the line joining the origin and the peak force or maximum force has been termed crispness for roasted almond (Varela et al., 2006). The same has been termed stiffness by Vincent (1998). The area under the graph bears some relationship (depending on the work of fracture of the material) to the total work done; the size of the force drop is also related to the area of the fracture surface generated by the release of strain energy associated with the reduction in force (Vincent, 1998). 2.3.2. Sound emission signal When force or deformation is applied to a crisp item, its structure is stressed until a critical point is reached: the action of external load causes the rupture of the brittle walls of the cellular structure which start to vibrate. The vibration is transmitted through the air as acoustic wave, which generates the sound (Piazza et al., 2007). The loudness and pitch of the audible sound and the high frequency non-audible sound emitted during fracturing depends among others on the local material that fractures (Luyten et al., 2004). Therefore an analysis of the sound emitted can give information on the fracture process, like.  The start of the damaging process, which is often far before a maximum stress is reached and falling apart of the food can be seen.  The amount of damage, which is related to the amount of sound energy and the loudness.  The pitch of the emitted sound (frequency), which is related to the type of fracturing process and the behavior of the material involved.  The sizes of broken pieces, which probably affect both the amount of sound and the pitch.

Various properties of sound waves can be measured for crispness and crunchiness evaluation of foods, the most common being the amplitude and frequency. The amplitude is a measure of loudness; the frequency is the number of cycles per second and is perceived as the pitch of the sound. Sensory research has strongly supported the idea that the loudness and/or the total amount of sound is very important for crispness (Drake, 1965), which seems to be similar to a mean height of peaks (Edmister and Vickers, 1985; Vickers, 1987); number of sound bursts (Vickers and Bourne, 1976); and sound duration (Edmister and Vickers, 1985; Vickers, 1987). Also, a combination of these descriptors has been used, including the product of mean height of peaks (amplitude) and number of peaks (Edmister and Vickers, 1985); the product of the number of acoustic bursts (events) and mean amplitude (Vickers, 1983); the product of the number of acoustic bursts (events) and mean amplitude per sound burst duration (Vickers, 1983); and the logarithm of the product of the number of acoustic bursts (events) and mean amplitude (Edmister and Vickers, 1985). Alchakra et al. (1996) and Mohamed et al. (1982) used acoustic energy to characterize sounds emitted by food. Seymour and Hamann (1988) used mean sound pressure or acoustic intensity, while Dacremont (1995) and Lee et al. (1988) used Fast Fourier Analysis to determine frequencies most evident during biting and chewing. Fractal analysis has been used by Barrett et al. (1992), Tesch et al. (1996) and Duizer et al. (1998). Other descriptors that are used for the evaluation of crispness and crunchiness include: sound energy (area) (Harker et al., 2002) or the power within characteristic frequency bands (De Belie et al., 2002). Fig. 6 shows the typical example of acoustic emission and force data versus time of a crusted food (Varela et al., 2006). We can see visual difference apparent between the two curves. Both the force and acoustic signals were jagged. The crisp product has more peaks as well as having peaks of higher amplitude than the less crisp product. 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. During a fracture the crack speed accelerates from zero to a certain value, depending on the deformation or stress applied. 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. The extracted parameters from the curves are summarized in Table 3. 2.4. Influence factors on sound in dry crisp products The sound emitted by crisp food is closely related to their textural attributes. The change of process technology or a change in the recipe has a direct influence on the rheological characteristics of products and on the quality of the end product and its texture. Parameters controlling sound of crisp products can be subdivided in three groups: processing conditions, ingredients (especially hydration) and structure. 2.4.1. Processing conditions The relations between process and sound have been studied considering different processing conditions. But most of the studies are based on extrusion cooking works, investigating the role of water content, screw speed, torque, pressure and temperature. These parameters interact to a large extent and therefore, depending on the combination chosen, their action on texture may be variable. The molecular structure of components may change because of heating (baking or deep-frying). This treatment affects the basic material properties of the crispy/crunchy food crusts, e.g. by starch gelatinization, protein denaturation, Maillard reactions, etc.

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Fig. 6. Force (black line) and sound pressure level (SPL; grey line) versus distance in compression: (a) raw almond and (b) 6-min roasted (Varela et al., 2006).

Table 3 Instrument parameters extracted from acoustic–mechanical curves. Typology

Name

References

Mechanical

Area Number of peaks Average stress, mean force Maximum force

Piazza et al. (2007), Primo-Martín et al. (2008b) and Salvador et al. (2009) Piazza et al. (2007), Primo-Martín et al. (2008b, 2009), Saeleaw and Schleining (2010) and Salvador et al. 2009 Piazza et al.(2007) and Saeleaw and Schleining (2010) Primo-Martín et al. (2008b) and Primo-Martín et al. (2009)

Acoustic

Number of sound peaks, number of sound event Max. sound peak Mean sound peaks, average sound pressure level Fractal dimension

Piazza et al. (2007), Piazza et al. (2008), Primo-Martín et al. (2009), Saeleaw and Schleining (2010), Salvador et al. (2009), Van Loon (2005) and Varela et al. (2008) Primo-Martín et al. (2008b), Saeleaw and Schleining (2010), Salvador et al. (2009), Varela et al. (2008) Piazza et al. (2007, 2008, Primo-Martín et al. (2009), Saeleaw and Schleining (2010) Barrett et al. (1992), Tesch et al. (1996) and Duizer et al. (1998)

The exact degree of starch gelatinization will depend on the frying or baking time and the amount of water locally available. The crispness development during the process has been investigated for frying, baking and toasting. Upon frying, crispness of chips increases as porosity increases and moisture decreases (Kawas and Moreira, 2001). For deep-fried food systems the presence and distribution of oil may be of importance for the behavior of the food product (Luyten et al., 2004). Sanz et al. (2007) recorded the force and sound during the fracture process for the different pre-frying times and a final frying time. They

stated that an increase in pre-frying time produced an increase in the jaggedness of the force curves, together with an increase in the number of higher amplitude peaks. For the sound signal a significant increase in the number of sound events was found with increasing pre-frying time. The non-pre-fried potatoes did not emit sound during fracturing; only the background noise produced by the texture analyzer can be observed (Fig. 7). Both the increase in the number of high amplitude force events and in the number and magnitude of sound events reflect an increase in crispness.

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Fig. 7. Force (left side) and sound pressure (right side) as a function time during the first 125 ms of deformation for various pre-frying times (indicated). Final frying time was 5 min (Sanz et al., 2007).

Varela et al. (2006) investigated the effect of roasting time on the acoustic signal; they found that the number of peaks of sound plot and the number of peaks of the second derivative plot had similar trends, increasing with roasting. Furthermore, the

maximum of the sound pressure level (SPL) is significantly higher for roasted almonds than raw almonds (Fig. 6). Cheng et al. (2007) found that adding WPI (whey protein isolate) at different levels had significant effect on mechanical–acoustic properties.

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2.4.2. Ingredients and hydration 2.4.2.1. Ingredients. The quality of expanded snacks depends on many factors, such as the quality of raw materials, the formulation and the process condition (Saeleaw and Schleining, 2010). Starch plays a very important role in controlling the texture and appearance of final products (Wang, 1997). Our previous studies (Saeleaw and Schleining, 2010) found that the kind of flour mixtures of cracker have a significant effect on texture and sound properties. Primo-Martín et al. (2008b) found that addition of lipase, amylase, glucose oxidase and hydroxypropyl methylcellulose (HPMC) increased the number of force and sound events, indicative of higher crispness in bread crust. The number of fracture and sound events correlated negatively with the water content and positively with the porosity of the crust. Both properties are affected by the use of enzymes/additives. Piazza et al. (2007) stated that when the sugar content increased, toasted sliced bread modify their crispness character by two expected mechanisms. First, the ability of sucrose crystals to generate zones of concentrated tension that is not balanced by the matrix around them. Second, the ability of sucrose crystals to disperse the starch granules and the proteins and therefore make the matrix more incoherent. 2.4.2.2. Hydration. Most low moisture baked or extruded products such as breakfast cereals, wafers, biscuits and snacks have a crispy texture. Most products follow a sigmoidal curve for loss of crispness as a function of water activity or water content (Roudaut et al., 1998). If the moisture content of these products increases, due to water sorption from the atmosphere or by mass transport from neighboring components or phases, a loss of crispness is observed (Nicholls et al., 1995). A great number of studies has been published on this topic with a view to characterizing and predicting the effects of water on crispness or to suggest the physical basis for such effects (Arimi et al., 2010; Dacremont, 1995; Duizer et al., 1998; Gondek et al., 2006; Marzec et al., 2007; Tesch et al., 1996). Gondek et al. (2006) found that the number of acoustic events was strongly affected by the water activity (aw). At low aw, compression of corn flakes was accompanied by generation of about 1300 acoustic events during 10 s. Increasing aw caused a linear decrease in the number of acoustic events, and at aw = 0.9, the compressed product did not show any acoustic activity. Moreover, Arimi et al. (2010) found that both the number of force peaks and sound peaks recorded during the puncture test decreased with increasing aw with the latter following a sigmoid curve (Fig. 8). This shows crispiness decreased with increasing water activity due to plasticisation of the Crackerbread by water. Similar losses in jaggedness of force–displacement and sound amplitude–time curves with increasing moisture content or aw of crispy products has been reported (Harris and Peleg, 1996; Heidenreich et al., 2004; Marzec et al., 2007). The decrease in maximum sound pressure at aw > 0.635 is in agreement with published data that showed crisp products such as cheese balls and croutons became silent when fractured after equilibrating to aw > 0.65 (Tesch et al., 1996). In addition, the mechanical resistance of starch-based crisp products may be related to their glass transition temperature (Tg) of starch (Peleg, 1994). Piazza et al. (2007) investigated the effect of water as a plasticizer on the mechanical and acoustic behavior of toasted sliced bread and found that the products became practically silent to the acoustic measure over 10% of water content, due to the transition from a glass and rigid structure to a rubber one. 2.4.3. Others finished product properties We have stated that the sound emission also depend on the size, shape, density, porosity, structure and microstructure of the sample. Thus, study of food physical properties in relation of fracture mechanic and sound emission are of great interest. In our previous study (Saeleaw and Schleining, 2010) we found a positive

397

Fig. 8. The effect of aw on the number of sound peaks (s) and force peaks (d) generated during puncturing of Crackerbread pre-equilibrated to different aw (Arimi et al., 2010).

correlation between linear expansion and the numbers of sound peaks in cassava crackers. Luyten et al. (2004) reported that the number of acoustic events is a function of the cell number density. There are only a few studies directly related to the sound emission and structure of crispy products. 2.5. Influence factors on sound in wet crisp products Finally we will mention few studies about the influence factors on sound in wet crisp products. The plant cell wall is a key determinant of texture in fruit and vegetables. In the case of plant tissues, mechanical properties will depend on contributions from the different levels of structure, and how they interact with one another. These levels are summarized in Fig. 9. (Waldron et al., 1997). Many researchers agree that crispness should result from structural properties of food (Barrett et al., 1994; Barrett and Peleg, 1992; Bouvier et al., 1997; Gao and Tan, 1996; Mohamed et al., 1982; Stanley and Tung, 1976; Vickers and Bourne, 1976). Wet cellular products, such as apples and lettuce, are composed of turgid cells with elastic cell walls (Duizer, 2001). This turgidity is due to the liquid within the cell pressing outward on the cell wall, while the wall opposes this force with strength and elasticity. When the cells are broken, the contents expand rapidly when released, and a sound pressure wave is produced. This resulting sound is responsible for the perception of crispness (Duizer, 2001). An increase in the turgidity of the cells is associated with an increase in crispness. A preliminary study of Harker et al. (1997) suggested that chewing sounds could also differentiate between different textures in apples. Fruit and vegetables produce a sound when cell walls burst (De Belie et al., 2002). The crispness of wet-crisp foods is also affected by the degree of pectin solubilization and enzymatic pectin degradation, which both cause a decrease of cell–cell adhesion (Zdunek et al., 2010a,b). Unfortunately there are not many acoustic–mechanical studies on wet-crisp products. 3. Conclusions and recommendations In conclusion, when a crisp food is broken or crushed characteristic sounds are produced due to the brittle fracture of the cell walls. Many types of experiments have been done to determine crispness, the best measurements are still open to doubt. Most fundamental studies published on the crispy behavior of food are limited to an aspect of the whole complex behavior and application for the establishment of relationships among measurement parameters and crispness is still limited. These sounds (acoustic emission) have been used to try to quantify sensory crispness. This information

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Fig. 9. Schematic representation of the level of structure that contribute to the mechanical properties of plant tissue (Waldron et al., 1997).

combined with crispness and measurement can be helpful to develop ideas for research of dry foods. Until now, most of the literature has focused on dry foods such as crisps and biscuits. There is, therefore, a need to focus research on the understanding and the measurement of crispness and associated textures in the context. Acknowledgements The authors would like to express their thanks to Royal Thai Government Scholarship, Rajamangala University of Technology Krungtep Thailand and Institute of Food Sciences, Universität für Bodenkultur Wien for funding support. We gratefully acknowledge the valuable work of all researchers in the references. Reference Alchakra, W., Allaf, K., Jemai, A.B., 1996. Characterization of brittle food products: application of the acoustical emission method. Journal of Texture Studies 27, 327–348. Alvarez, M.D., Saunders, D.E.J., Vincent, J.F.V., Jeronimids, G., 2000. An engineering method to evaluate the crisp texture of fruit and vegetables. Journal of Texture Studies 31 (4), 457–473. Arimi, J.M., Duggan, E., O’Sullivan, M., Lyng, J.G., O’Riordan, E.D., 2010. Effect of water activity on the crispiness of a biscuit (Crackerbread): mechanical and acoustic evaluation. Food Research International 43 (6), 1650–1655. Babin, P., Della Valle, G., Chiron, H., Cloetens, P., Hoszowska, J., Pernot, P., Réguerre, A.L., Salvo, L., Dendievel, R., 2006. Fast X-ray tomography analysis of bubble growth and foam setting during breadmaking. Journal of Cereal Science 43 (3), 393–397. Barrett, A.H., Cardrello, A.V., Lesher, L.L., Taub, I.A., 1994. Cellularity, mechanical failure, and textural perception of corn meal exudates. Journal of Texture Studies 25 (1), 77–95. Barrett, A.H., Peleg, M., 1992. Extrudate cell structure–texture relationships. Journal of Food Science 57 (5), 1253–1257. Barrett, A.M., Normand, M.D., Peleg, M., 1992. Characterization of the jagged stress– strain relationships of puffed extrudates using the fast Fourier transform and fractal analysis. Journal of Food Science 57 (1), 227–232. 235. Bouvier, J.M., Bonneville, R., Goullieux, A., 1997. Instrumental methods for the measurement of extrudate crispness. Agro-Food Industry Hi-Technology January/February 16, 19. Brennan, J.G., 1988. Texture perception and measurement. In: Piggott, J.R. (Ed.), Sensory Analysis of Foods, second ed. Elsevier, pp. 69–102. Brown, W.E., Langley, K.R., Braxton, D., 1998. Insight into consumers’ assessments of biscuit texture based on mastication analysis: hardness versus crunchiness. Journal of Texture Studies 29 (5), 481–497. Castro-Prada, E.M., Luyten, H., Lichtendonk, W., Hamer, R.J., Vliet, T., 2007. An improved instrumental characterization of mechanical and acoustic properties of crispy cellular solid food. Journal of Texture Studies 38 (6), 698–724. Chakra, W., Allaf, K., Jemai, A.B., 1996. Characterization of brittle food products: application of the acoustic emission method. Journal of Texture Studies 27 (3), 327–348.

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