JFS:
Food Engineering and Physical Properties
Quality Control of Cheese Maturation
and Defects Using Ultrasonics
J. BENEDITO, J. CARCEL, M. GISBERT, AND A. MULET
ABSTRACT: Ultrasonic velocity in Mahon cheese increased during maturation ranging from 1624 to 1737 m/s. A
quadratic equation was used to describe this increase. This equation could be used to assess the maturation time
under specific maturing conditions in order to classify the pieces in different categories. Through-transmission and
pulse-echo techniques were used to detect cracks within the pieces formed by abnormal fermentations. In pulseecho mode it was possible to detect all the cracked pieces and also to assess the distance to the surface. These
ultrasonic measurements are nondestructive, allowing control over all the pieces of a batch.
Key words: quality control, maturity, Mahon cheese, nondestructive testing, ultrasonics
Introduction
F
OOD CHARACTERIZATION IS FREQUENTLY CARRIED OUT IN
Food Engineering and Physical Properties
order to classify products according to different qualities. The techniques used for that purpose often involve the
destruction of the samples. At present, many techniques are
emerging to assess food quality nondestructively (Berg and
others 1997; Defernez and Wilson 1997), involving physicochemical properties or the detection of anomalies within the
product such as voids, metals or hollow hearts. Techniques
reported for that purpose are, among others: Vibration rheometers, visible/near infrared spectroscopy, small displacement probes and electronic noses.
Ultrasound has been used to assess the quality of many
foods nondestructively (McClements 1997; Mulet and others
1999a). Ultrasonics were applied to determine rheological
and textural properties of cheese (Lee and others 1992;
Benedito and others 2000) and to assess the curd firmness to
determine the optimum cut time for cheese making (Gunasekaran and Ay 1996).
Characterization of products that undergo a ripening or
maturation process is particularly difficult due to the physicochemical and microbiological phenomena that take place
within the food. Examples of these products are fruits,
cheese and cured meat and fish. Due to their changes in density, rheology, physical state and microstructure, ultrasound
can be used for quality assessment of these products. Ripeness characteristics have been addressed in avocados, mangos and melons (Mizrach and others 1994; Mizrach and others 1996, 1997).
Nondestructive techniques have also been used to detect
anomalies (foreign bodies, voids, hollow hearts, and so forth)
within food products. Taubert and Stuempel (1997) conducted studies to detect absences and foreign bodies in pork and
pig carcasses using ultrasonic imaging techniques. Hollow
hearts in potatoes were detected using ultrasounds on the
basis of ultrasonic power transmitted through the potato
(Cheng and Haugh 1994).
Mahon cheese (Minorca, Spain) matures in temperature
and humidity controlled chambers and during the maturation process there is a water loss as well as a change in the
textural properties (Benedito and others 1998b; Frau and
others 1999). Very often it is difficult for producers to know
the actual maturity state of a batch due to uncontrolled vari-
100 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 1, 2001
ables such as seasonal variation of milk or cow feeding. Furthermore, retailers are also interested in controlling the
quality (state of maturation) of the purchased product. The
Regulatory Council of Mahon Cheese classifies the state of
maturation of the cheese depending on the time it has been
stored in the chambers. Three different categories are established; fresh cheese must be stored from 21 to 60 d; half ripened from 61 to 150 and ripened more than 150 d. Moreover,
during the maturation process abnormal fermentations can
take place, ending up in a swelling of the pieces. If the cheese
stays in the chamber, the swelling decreases and turns into a
crack that is impossible to detect either by pressing or visually. These cracks are not allowed by the Regulatory Council
and the batch where the pieces were found must be sorted
out. The aim of this study is to relate ultrasonic velocity in
dry-cured cheese to the storage time under specific maturation conditions. The feasibility of detecting cracks within
cheese pieces using ultrasonic techniques will also be addressed.
Materials and Methods
Raw material
Pieces of cheese were parallelepiped-shaped, with dimensions of approximately 20 × 20 × 8 cm. They were matured in
the chamber of 2 different companies, where environmental
conditions of relative humidity and temperature were maintained at 83% and 12 C, respectively. 145 pieces were used for
the ultrasonic measurements, twenty of which were opened
to visually check out the existence of cracks.
Ultrasonic measurements
Figure 1 shows a scheme of the equipment that was used
for the ultrasonic measurements. It consisted of a couple of
narrow-band ultrasonic transducers (1MHz, 0.75'’ dia crystal, Model A314S-SU, Panametrics, Waltham, Mass., U.S.A.), a
pulser-receiver (Toneburst Computer Controlled, Model
PR5000-HP, Matec Instruments, Northborough, Mass.,
U.S.A.), and a digital storage oscilloscope (Tektronix TM TDS
420, Tektronix, Inc. Wilsonville, Oreg., U.S.A.) linked to a personal computer using a GPIB interface. The distance was
measured with a digital height gage (Electronic Height Gage,
Model 752A, Athol, Mass., U.S.A.). In order to calculate the
© 2001 Institute of Food Technologists
Ultrasonic Cheese Quality Control . . .
system delay, pulse transit times measurements were taken
over a set of calibration cylinders of different thicknesses.
The delay time was obtained from the plot time against
thickness. This delay was introduced in the velocity computation. To measure velocity both transducers shown in Figure
1 were used in the through transmission mode. The ultrasonic wave sent from the emitter traveled across the sample
width arriving at the receiver, the time of flight was computed by averaging 5 signal acquisitions. The gain (logarithmic
increase of the signal amplitude) of the receiver was set to 40
(a)
dB in order to obtain a clear and repeatable signal for all the
measured cheeses, a higher gain would provide an increase
of noise with no signal resolution improvement.
The ultrasonic velocity measurements were carried out by
placing the ultrasonic device in the maturing chambers
where a mean temperature of 12 ºC was maintained. Velocity
was measured on 3 points located in the central part of the
cheese (previously marked with a stencil). Five measurements were performed on each point and then averaged. To
improve the matching between the sample-transducer interface, olive oil was used as a couplant.
Attenuation measurements were not carried out due to
the presence of small holes within the cheese. These holes
made that the number of holes of a particular cheese,
masked the possible changes of attenuation during maturation.
Two techniques were used to detect cheese cracks;
through transmission (TT) and pulse-echo (PE). The ultrasonic signal taken for the velocity computation in TT mode
was digitized to quantify the received energy, the area under
the curve of the rectified voltage (modulus) versus time was
computed (Integral). For the PE technique, either a pair of
transducers placed on the same face of the cheese or a single
transducer acting as an emitter and a receiver were used.
For the latter experiments, the gain was set to 20dB in order
to reduce the noise from the excitement of the emitter. Eight
thousand points were digitized for each signal at a sampling
rate of 100 Msamples/s.
Results and Discussion
Maturity assessment
(b)
Figure 1—Ultrasonic set-up used for ultrasonic measurements (a). Piece of cheese (b)
In Figure 2, the results for 138 pieces from a total of 145
analyzed are shown. It was not possible to determine velocity for 7 of the pieces due to some anomaly within the cheese
that will be discussed later. As observed, the ultrasonic velocity increased with the curing time. The ultrasonic velocity
was related to the square root of the elastic modulus (Povey
and McClements 1988). During cheese maturation the loss of
water and the change in the microstructure affect the cheese
Figure 2—Relationship between the ultrasonic velocity and
the maturation time
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101
Food Engineering and Physical Properties
Ultrasonic crack detection
Ultrasonic Cheese Quality Control . . .
texture increasing the elastic modulus and consequently the
ultrasonic velocity (Benedito and others 1998b). Figure 2
shows the 3 time intervals established by the Regulatory
Council for the classification of Mahon cheese. It can be observed that the ultrasonic velocity in fresh cheese is always
lower than 1670 m/s and higher than 1680 m/s in ripened
pieces. Experimental data were fitted to a quadratic equation
shown in Figure 2. Using this quadratic equation 85% of the
analyzed pieces were correctly classified within the 3 considered intervals. As observed, as long as the storage time increased, the dispersion did too. This is probably due to the
differences in the maturation conditions, which differ not
only from one producer to another but also within the company. These differences became more noticeable with longer
storage times. For specific environmental conditions, where
the raw materials as well as the handling procedures would
be more uniform, a more precise relationship could be obtained. This could be used by the Regulatory Council, producers and retailers to classify the inspected cheeses nondestructively according to the established intervals. Similar relationships could be developed for other types of dry cured
cheeses, ultrasonic measurements being a very helpful tool
for nondestructive classification. Temperature greatly affected velocity measurements (Benedito and others 1998a; Mulet
and others 1999b), therefore for temperatures others than 12
C, new relationships should be developed.
Crack detection
Figure 3a shows the ultrasonic signal for through transmission corresponding to a piece without cracks. Figure 3b
shows the signal obtained for the sample shown in Figure 3c.
As observed, the cracks reflected the ultrasonic wave and no
noticeable signal was received from the transducer. Pieces
without cracks had integral values always higher than 5 V s.
In these experiments, it was found that 3 of the opened
crack-free pieces had similar signals to Figure 3b (Integral <
Food Engineering and Physical Properties
(c)
Figure 3—Through transmission measurements. (a) Ultrasonic signal for a cheese without cracks. (b) Ultrasonic
signal for a cracked cheese shown in (c)
102 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 1, 2001
Figure 4—Pulse-echo measurements with a pair of transducers. (a) Ultrasonic signal for a cheese without cracks.
(b) Ultrasonic signal for a cracked cheese shown in Figure
3 (c)
5 V s). This was due to the presence of many (more than
usual) small holes that scattered the ultrasonic wave, producing a similar energy absorption to that of cheese with
Figure 5—Integral of the ultrasonic signal for the pulseecho technique using two transducers. Cracked and non
cracked cheeses
cracks. Therefore, using TT mode it was not possible to distinguish cheeses with small holes, that are allowed by the
Council, from those with cracks.
Pulse-echo techniques were used to sort out cracked
pieces. Figure 4a shows a signal obtained using 2 transducers
placed on the same face of the cheese and Figure 4b the signal for a cheese with a crack (Figure 3c). For this type of
cheese the received ultrasonic signal was much higher. Figure 5 shows the energy content of the ultrasonic signal for
the examined pieces. As observed, all the cheeses with cracks
had an integral higher than 3V s. Cheeses with many small
holes had ultrasonic signals similar to the one shown in Figure 4a (PE mode). Therefore, the pieces that had signals similar to Figure 3b (no energy content) in TT mode and similar
to Figure 4a (Integral < 3V s) in PE mode, were identified as
pieces with many small holes. This technique was shown to
be useful in detecting these cheeses with cracks but to carry
out the experiments, the ultrasonic setup should include 3
transducers, increasing the complexity and cost of the ultrasonic equipment.
To solve the problem, pulse-echo experiments with a single transducer were also carried out. Figure 6a shows the ultrasonic signal for a cheese without cracks, the dotted line
was obtained increasing the maximum signal values for all
the cheeses without cracks analyzed in the study by 50%. As
observed, the ultrasonic signal for a cheese with cracks (Figure 6b) crossed the dotted line as a consequence of the energy that is reflected on the crack surface and returns to the
transducer. Takai and others (1994) used the pulse-echo
technique to evaluate the size and number of voids in kamaboko by counting the number of ultrasonic echo pulses on
the oscillograms. Using this technique, all the cracked cheeses were identified. Furthermore, it was also possible to determine the distance of the crack from the surface supposing a
range of velocities that included the maximum (1740 m/s)
and minimum (1620 m/s) values found in this study. The calculated range for the cheese shown in Figure 3c was 1.841.98 cm that agreed with the distance measured with a digital
gage (1.9 cm).
Conclusions
T
HE ULTRASONIC VELOCITY CAN BE USED TO DETERMINE THE
degree of cheese maturity. Velocity increases for Mahon
cheese from 1620 to 1740 m/s with the increase in the maturation time and is mainly due to the water loss. Using pulseecho techniques, it was also possible to detect cracks within
the cheese. The magnitude and position of cracks could also
be assessed.
These measurements allow control over all the pieces of 1
batch, enabling to sort out those that do not match specifications. Presently, the automation of these techniques is allowing to carry out quick, reliable and on-line quality control
measurements.
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Figure 6—Pulse-echo measurements with a single transducers. (a) Ultrasonic signal for a cheese without cracks.
(b) Ultrasonic signal for a cracked cheese shown in Figure
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MS 19991219
This research was funded by contract ALI96-1180 from the Comisión Interministerial de
Ciencia y Tecnología, Ministerio de Educación y Ciencia, Spain.
Authors Benedito, Carcel, and Mulet are with the Depto. de Tecnología de
Alimentos, Univ. Politécnica de Valencia, Camino de Vera s/n, 46071, Spain.
Author Gisbert is with AINIA. Instituto Tecnológico Agroalimentario, Valencia.
Spain. Direct inquiries to author Mulet (E-mail: [email protected])