ACOUSTIC EMISSION IN PUNCTURE TEST OF APPLES DURING SHELF-LIFE

Artur Zdunek¹, Zbigniew Ranachowski^2
1 Institute of Agrophysics, Polish Academy of Sciences, Lublin, Poland
² Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland

ABSTRACT

Sound is important property used for consumer for food quality evaluation. Most of the sound created during eating is bone-conducted. In this research a contact acoustic emission together with puncture test are used for observation of changes of apples quality during shelf-life.

The new device for acoustic emission measurements during puncturing of apple tissue allows obtaining acoustic parameters simultaneously with firmness and toughness. The acoustic events (number of bursts) and acoustic energy correlates with the mechanical parameters, however they are more sensitive for shelf-life storage than the mechanical one. It was confirmed that fresher and stored in better conditions apples emit louder (higher AE energy) and more dense (higher number of events) sound. The sound created during puncturing has wide range of frequencies. In general, the relationships obtained are similar using audio or ultra sound band. However, using both of them gives advantage of controlling unexpected background noise affecting measurements.

Key words: Acoustic emission, crispness, texture, apple.

INTRODUCTION

Food crushing sound is one of the main factors used for food quality evaluation. Crispness and crunchiness are attributes of high quality product and are usually pointed on the top of a list of consumer preferences [5]. However, the meaning of crispness and crunchiness is imprecise [5, 17]. Its perception varies from country to country and from individual to individual [5, 9]. Luyten et al. [17] stated that for crispness at least four and for crunchiness at least five different definitions in literature can be found [14]. However there is a general consensus that crispy and crunchy sensation is related to fracture properties. Crispy product is mechanically brittle, firm and acoustically noisy as a result of large number of small fractures [6, 23, 25]. Crunchiness is probably related to events (fractures) occurring on subsequent layers in a cell structure what gives the sense of extension of sound duration in time.

In spite of sensory and subjective nature of food quality evaluation by human senses, a big effort is put for objective sound properties analysis during biting and chewing and for developing instrumental methods for human independent food evaluation. The first instrumental analysis of sound was done by Drake [11] who found that crisper products emit louder sound and average amplitude of successive bursts during mastication decreases. The number of sound burst in a bite n can be used for judging a chewing sound, similar as the mean amplitude of the burst A or the products of these values nA or nA/sound duration [6, 11, 24]. The first hypothesis was that the sense of crispness is an auditory phenomena, i.e. the sound is air-conducted. However, work done by Christiansen and Vickers [7] showed that crispness may be a vibratory phenomena, i.e. the sound is bone-conducted.

Most of the work on crispness and crunchiness concern dry food products. However, this problem has been noticed also for fruits and vegetables called as wet food products. Fillion and Kilcast [12] stated that crispness and crunchiness are very complex concept containing sound, fracture characteristic, density and geometry of the product. They found that crispy wet food would refer to a light and thin texture producing a sharp clean break with a high-pitch sound mainly during the first bite with the front teeth. Crunchy wet food would be hard and has a dense texture producing loud, low-pitch sound that occurs over successive chews. De Belie et. al [8, 9] have measured chewing sound of apples and have applied FFT to detect differences in crispness. It has been stated that Fourier-transformed chewing sound may be used for crispness evaluation and mealy and crispy apples could be distinguished using principal component analysis [8]. The best correlation with sensory crispness of apples was found by combining the information from different frequency bands [9].

Many instrumental methods have been applied for crispness and crunchiness evaluation of fruits and vegetables. Due to fact that mastication is a highly destructive process, a mechanical tests are the most popular to simulate the biting. Results of such tests like texture profile analysis [2, 29], compression [1, 28], tension [10, 14], twist [13], three-point bending [3, 26, 27] show correlation with properties of a material thus can be used for its crispness or other textural properties evaluation. One of the simplest is a puncture test where probe is pushed into tissue till 8 mm and a maximum force is used as a firmness value. The maximum force usually correlates well (positively) with crispness [13]. However, no relation between fruit firmness and consumer preferences are reported also [4]. On the other hand, Konopacka and Plocharski [16] have found that for apples there is an optimal firmness value for consumers that changes with varieties and storage conditions. This would relate to fact that consumers usually do not like both too soft and too hard food.

As it was obtained by Christiansen and Vickers [7] crispness may by the bone-conducted phenomena. Therefore, acoustic emission method where a sensor is in a contact with material investigated is adapted to food properties evaluation [19]. In general, the acoustic emission monitors deformation of a material and if a cracking and possible further friction of material pieces occur, it would generate elastic waves that can be recorded by a sensor on the sample surface. This phenomenon is called acoustic emission and is widely used for monitoring different materials and constructions [18]. The information carried in acoustic waves like its amplitude, frequency, energy, etc. describes a source of signal, thereby the properties of the material in the source. In the wet food like tissue of fruits and vegetables deformation causes cell-cell debonding, stretching cell walls and eventual cracking because of intracellular pressure [20, 21]. These two processes can be the sources of acoustic emission. Zdunek and Konstankiewicz [28] showed that in potato tissue the source of ultrasound during deformation is cell wall breaking and defined new mechanical parameters: the critical stress and the critical stress when the tissue starts to crack. The critical stress, the critical strain and a total counts (number of bursts) was analysed for potato and apple tissue in texture profile analysis [29]. It has been shown that apple and potato tissue become more brittle and more acoustically noisy if it has higher turgor.

In this paper, a new method with using acoustic emission in audible and ultrasound frequency range will be applied together with puncture test to apples during shelf-life. The goal of the research is checking if the acoustic parameters are sensitive for quality decline during apples shelf-life.

MATERIALS AND METHODS

The apple cultivars Champion, Idared and Jonagold harvested at different dates were used. The experiment was performed on March 2006. The apples prior to testing were stored in different condition. Champion and Idared were stored in a cold storage (6-8°C is commonly used in small farms in Poland [10]) without atmosphere controlling. These apples were sorted by hand and packed separately for selling as a first quality. They had a uniform size around of 6-8 cm in diameter and a weight 120-150 g. Jonagold variety was stored in control atmosphere, i.e. 2°C, 2% O₂ and 2% CO₂. The Jonagold apples were larger than two other cultivars and they had around 9-12 cm in diameter and a mean weight 220 g. In a laboratory apples were stored for 1 week in black plastic bags in a refrigerator at temperature 2°C.

25 apples of each cultivar were picked up and they were weighted. The batch of 25 apples was divided equally into 5 series. Next, the apples were stored without any cover in a room temperature (20-22°C and humidity of 30-60%) for 10 days. Measurements were performed on each series containing 5 apples in 1, 3, 6, 8 and 10 days after picking up from refrigerator. Before mechanical testing, each apple was weighted again in order to obtain a weigh loss during the shelf-life. Additionally, after a mechanical test an osmolality of apple juice was measured by Osmomat 030-D (Gonotec, Germany).

Puncture test. Apples were cut in half for testing. The skin of thickness of around 2mm was peeled from the place of contact with a probe (fig. 1). Puncture test was performed using probe of 7.94 mm in diameter with spherical ending of 5.16 mm of radius of curvature. The speed of puncturing was 20 mm/min and the probe was pushed into apple flash up to 8 mm. The probe was attached to Lloyd LRX (Lloyd Instruments Ltd, Hampshire, UK) using a 500N load cell and the program Nexygen (Lloyd Instruments Ltd, Hampshire, UK) provided with the apparatus. Two parameters from force-displacement (FD) curve were calculated by the Nexygen software: maximum force F_max called firmness and work to limit called toughness. The work to limit is an area under the FD curve.

The device for AE measuring. Acoustic emission (AE) during the puncture test was recorded using a head with two sensors. The scheme of the head is shown in figure 1. The head consists two parts. Top part is made of ertacetal and the bottom part is made of duraluminium. They are screwed to each other. To the top surface of the metal part two acoustic emission sensors are glued. 4381V (Bruel&Kjear, Narum, Denmark) sensor works in audible range 1-16 kHz. S9223 (Physical Acoustic Limited, Cambridge, UK) sensor has maximum sensitivity in ultrasound range 25-100 kHz. The sensors are connected by 2 m cable long to two AE Signal Amplifiers (EA System S.C., Warsaw, Poland) with adjustable amplifying. The amplifier for audible channel has high-pass filter at 1 kHz and low-pass filter at 20 kHz. The amplifier for ultrasound channel has high-pass filter at 10 kHz and low-pass filter at 900 kHz. Next, the analogue signals are converted into digital one by two A/D boards: Adlink PCI 9112 and Adlink PCI 9118HG (Adlink Technology Inc., Taipei, Taiwan) for audible and ultrasound signal, respectively. Two channels of each A/D boards are used what gives following sampling rates per channel: 44 000 S/s and 150 000 S/s (S/s means samples per second) at a resolution of 16 bit per ±1.25 V and ±5 V for audible and ultrasound signals, respectively. The real gains of the signal were checked using 1 mV signal with frequency 2 kHz or 20 kHz for audio and ultrasound channel, respectively. The real gains are H_f2kHz = 320 for audio channel and H_f20kHz = 1240 for ultrasound channel. The second channel of each card is used for recording an analogue signal of force delivered from Lloyd LRX machine. Data from each channel is recorded and converted to separate files (acoustic signals in “WAV” format and force in “TXT” format).

Calculating of AE parameters. A typical record of a time dependence of acoustic emission signal is presented in figure 2. A section of the signal where measurable oscillations are detected is called AE event. Within a time period of an event the consecutive AE signal amplitudes exceed the preset threshold called AE discrimination level. A basic assumption taken after practical observations is approximation of the registered AE signal produced by the impulse AE source to a shape of damped sinusoid. It is possible to preset a certain signal threshold and register (count, in other words) every moment when current amplitude of AE signal would cross that threshold. That strategy of tracing the activity of AE source is called AE count processing. Time period when the consecutive AE signal amplitudes belonging to the single AE impulse exceed the preset threshold is called AE event (fig. 2) [22].

Registered AE signal, presented in figure 2 in amplitude – time coordinates can be characterised by a descriptor E, AE energy. Assuming that a signal event of duration t₁ and of peak amplitude V was recorded during the investigation then AE event energy can be calculated in units [J] as:

E = 0.5 V² t₁                     (1)

The AE events and AE energy is calculated separately in audible and ultrasound frequency range.

RESULTS AND DISCUSSION

Weight loss and osmolality. Apples stored at a room temperature lost their weight linearly (fig. 3). The fastest weight loss was for Champion cultivar, while the slowest for Idared. The experiment was performed in March, thus 2 months after optimum storage period for Champion and still in time of optimal storage period for Idared. For Jonagold the optimum storage period ended in the month of performing the experiment. This would be the reason of differences in the weight loss rate between these cultivars. Measurements of osmolality which is a concentration of osmoticaly active substances showed no changes in this parameter during the shelf-life (P > 0.05, tables 1 and 3). However for Idared, slight but significant increase of the osmolality was observed (P < 0.05, tab. 2). This suggests that during the shelf-life there is weight loss as a result of water evaporation, although simultaneously with this, due to metabolism, a concentration of intracellular juices has remained the same. Only for Idared, a balance between water evaporation and metabolism is disturbed.

Firmness and Toughness. In tables 1-3 matrixes of correlation coefficients of linear regression are presented for three cultivars, respectively. For all cultivars firmness correlates very well with toughness. Thus, in figure 4 the apple firmness is only shown during 10 days shelf-life; a relationship for toughness looks similar. Both firmness and toughness have decreased significantly (P < 0.01, tab. 1) only for Champion cultivar. For Jonagold there is a slight decrease, however it is not significant (P > 0.05, tab. 3). For Idared both parameters did not change during shelf-life at all (tab. 2). It confirms good storage characteristic for Idared even during 10 days shelf-life. Fast decrease in firmness and toughness of Champion cultivar would be explained as a result of too long storage.

Acoustic parameters. In figure 5 typical acoustic events recorded during puncturing of apple flesh are shown. The AE signal starts just from the moments of touching puncture probe to tissue. The number of events increases progressively up to a moment when FD curve yields. At this moment the whole curved part of the probe is in a contact with the tissue. Thus it can be said that an acoustic activity increases together with increase of the contact area between probe and material investigated. When the probe goes deeper into apple tissue the acoustic activity decreases. This could be result of damping of acoustic waves by surrounding tissue and already damaged tissue layers under the probe. Taking into account possible tissue damage modes, i.e. breaking of cell walls and cell-cell debonding they can be both sources of acoustic emission [20, 28]. Also, a friction between probe-tissue and tissue-tissue can be important source of acoustic signal.

Correlation coefficients in tables 1-3 show high positive correlation among acoustic parameters within given frequency range and also between these ranges. Therefore, any of AE parameter can be used for apples characterisation during shelf-life, in general. It suggests that the same bursts occurring during puncturing create acoustic signal in a wide range of frequencies from 1 kHz up to 75 kHz. In figure 6, spectrum of the signal is presented in both channels in a form of acustograms. Colours in the acustograms represent a power of the signal at given time and at given frequency. A few dominant frequencies can be noticed: 5.5 kHz, 7.5 kHz, 9.5 kHz,15 kHz, 28 kHz, 35 kHz, 44 kHz and 52 kHz. They are characteristic for the system used because no changes in the dominant frequencies during shelf life were observed. The only one change during storage was overall decrease in energy of the signal that occurred uniformly in all dominant frequencies.

In figure 6, is visible that after amplification the energy in ultra channel is stronger than in audio one. However, considering real amplification H_f and calculating after these two AE descriptors, i.e. events and energy in both channel it is visible in figure 7 than much lower values of AE parameters for ultrasound channel were observed than for audio channel. The difference in the AE energy is about 100 times and in the number of AE events is about 4 times between audible and ultrasound channels. We calibrated system without sensors due to have real amplification H_f. However, properties of AE signal depend also on sensors sensitivity that are different, discrimination level, and on properties of AE sources itself. The discrimination levels were also different for two channels because we wanted to record all AE events with amplitude just above noise level which was different due to different noise level in the channels. Therefore, the first two factors make impossible to relate EA parameters obtained in two bands, i.e. it is not possible to state about real acoustic energy in AE source.

In figure 7 AE events and AE energy obtained in puncture test are shown during apples shelf-life. The parameters in audible channel are noted as AUDIO and in ultrasound channel as ULTRA. The figure 7 and analysis done in tables 1-3 show different shelf-life acoustic properties for cultivars investigated.

The most significant (P < 0.01) decrease in AE parameters was observed for Champion. For this cultivar the correlation coefficients are around -0.8 and they are very close to that obtained for firmness and toughness (tab. 1). Jonagold also emits less and less AE energy during shelf-life and less number of AE events is noticeable. However, the decrease is stopped after 6 days of storage. In result, the correlation coefficient is lower than for Champion, but still significant at P < 0.01 (tab. 3). It should be pointed out that firmness and toughness do not change during shelf-life for Jonagold (P > 0.05, tab. 3). For Idared cultivar AE parameters in audio channel increase significantly (positive correlation coefficient, P < 0.01 in tab. 2) which is opposite to other cultivars. It seems that in the 8^th day the measurements in audio channel were surprisingly too high. This was not noticeable for ultrasound channel where results during shelf-life do not change significantly (P > 0.01, tab. 2). It can be concluded from this that different relationships for Idared can be caused by influence of background noise in audio range in the 8^th day. In summary, we suppose that for Idared acoustic properties do not change during shelf-life similar to its firmness and toughness.

Among cultivars investigated only Jonagold was stored in controlled atmosphere. This way of storage is used for keeping high quality of apples. Usually, KA storage causes slower decrease in firmness and slower decrease in crispness of apples [8, 15]. The results in figure 4 show that firmness is similar for apples from cold storage (Champion and Idared) and from controlled atmosphere (Jonagold). It would be property of given cultivar therefore it is difficult to confirm in this experiment the positive influence of KA on firmness. On the other hand figure 7 shows significant difference in acoustic parameters between Jonagold and two other cultivars just after removing material from refrigerator. During the first 3 days of storage Jonagold emits louder sound that is presented as higher AE energy and higher number of AE events. This suggests that crispness of Jonagold would be also higher as it is expected as the result of KA storage. After 6 days of shelf-life Jonagold equals to Idared but is still higher than for Champion.

CONCLUSIONS

The new device for acoustic emission measurements during puncturing of apple tissue allows completing properties of material investigated. It is possible to obtain acoustic parameters simultaneously with firmness and toughness. The acoustic events and acoustic energy correlates with the mechanical parameters, however as it was shown for Jonagold cultivar they are more sensitive for shelf-life storage than the mechanical one. The experiment has confirmed that fresher and stored in better conditions apples emit louder (higher AE energy) and more dense (higher number of AE events) sound.

The sound created during puncturing has wide range of frequencies. In general, the relationships obtained are similar using audio or ultra sound band. However, using both of them gives advantage of controlling unexpected background noise affecting measurements.

The proposed method can be used for crispness evaluation. The puncturing together with the contact acoustic emission is closer to real way of food evaluation by consumer because its measure both force required for breaking a product and a vibratory sound. The present work shows that implementation of the acoustic emission to fruits and vegetables should be continued. In a future work results obtained by acoustic emission will be compared with sensory evaluation. It will allow correlate crispness and crunchiness with acoustic parameters as it is usually done for firmness parameter obtained in puncture test.

ACKNOWLEDGEMENTS

This scientific work was financed from national budget for science in years 2005-2008 as the research project No 2 P06T 089 28.

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Accepted for print: 24.11.2006

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