WO2008090242A1

WO2008090242A1 - Non-invasive device for determining the temperature and ice content of frozen foods

Info

Publication number: WO2008090242A1
Application number: PCT/ES2007/070208
Authority: WO
Inventors: Pedro Dimas SANZ MARTÍNEZ; Cristina APARICIO PEÑA; Antonio MOLINA GARCÍA; Laura María OTERO GARCÍA; Bérengère GUIGNON
Original assignee: Consejo Superior De Investigaciones Científicas
Priority date: 2007-01-25
Filing date: 2007-12-10
Publication date: 2008-07-31

Abstract

The invention relates to a non-invasive device for determining the temperature and ice content of frozen foods using ultrasound, comprising a 1-10 MHz frequency receiver/reflector and transmitter. The method uses a program to perform calculations as a function of the thickness of the sample and the flight time of the wave which is determined using conventional methods, with the type of material (fruit - vegetable or meat - fish) and the initial freezing point thereof being known. The more specific the aforementioned parameters, the more precise the final measurement.

Description

TITLE

NON-INVASIVE DEVICE FOR DETERMINING TEMPERATURE AND

ICE CONTENT OF A FROZEN FOOD.

SECTOR OF THE TECHNIQUE

Human food, freezing industry, frozen storage.

STATE OF THE TECHNIQUE Freezing and subsequent maintenance in the frozen state constitute one of the main resources currently used for food preservation. The volume of business involved in this activity, including the related industries of the production of equipment for freezing, maintenance and control, is considerable.

Freezing consists in the reduction of the temperature of a system with a variable aqueous content, so that part of the water changes state, solidifying. The main effect that favors conservation is the effective reduction of the amount of free water when part is transformed into ice. Free water is necessary for microbial growth and is involved in chemical reactions between the different components of the system. Various micro-organisms cannot develop below certain water activity values. On the other hand, chemical reactions and a series of physical processes that also require mobility depend on the amount of free water and its speed decreases with this amount. Additionally, the reduction of the temperature also acts against the microbial activity as well as slows the chemical reactions.

The fraction of water that has been converted to ice can be determined by various analytical procedures (for example, DR Heldman, 1973, "Predicting the Relationship between Unfrozen Water Fraction and Temperature during Food Freezing Using Freezing Point Depression", ASAE Paper No. 72-890, and Chen, CS., 1985, "Thermodynamic Analysis of the Freezing and Thawing of Foods: Ice Contení and Mollier Diagram ", Journal of Food Science, 50, 1163-1166) or experimental, being the most accurate measurement by differential scanning calorimetry. This method is highly invasive, since it requires that a small fraction of the product is taken and packaged to be introduced into the instrument, except in those cases where the freezing conditions can be reproduced correctly in the calorimeter, the result may be different from reality, since it is difficult to fragment and package the sample without altering its ice content In any case, the process is slow, invasive and not very adaptable to situations of industrial interest.

In all aqueous systems with components in solution (which includes all foods) the ice content varies depending on the temperature. The concentration of solutes, according to a widely studied dependence (as, for example, shown in the two previous quotes) determines the temperature at which the freezing begins. However, the formed ice expels from its bosom the solute molecules that would distort the crystalline network. Thus, as the ice formation proceeds, a cryoconcentration process takes place, increasing the amount of solutes contained in the fraction of water still in a liquid state. This causes a decrease in the temperature of the solid-liquid equlibrio. Thus, different systems have a different ice content depending on the temperature at which they are maintained. AC. Miles, 1974, in his work "The Ice Contained of Frozen Meat and its Measurement Using Ultrasonic Waves", in "Meat Freezing: Why and How", edited by CL. Cutting, pgs. 15.1-15.7, Langford, Bristol, Great Britain suggests that the amount of ice could be estimated using ultrasound in a meat sample. The present invention applies to any food that has an initial freezing temperature greater than -2.4 ⁰ C (which are practically all of interest). In addition he needs to introduce; the value of the speed of sound at ⁰ ° C in the sample and the fraction of water of the sample, while the one that is now proposed, these initial values are not necessary. The authors H. Sigfusson, GR Ziegler and JN Coupland, 2004, in the publication "Ultrasonic monitoring of food freezing" in Journal of Food Engineering 62, pages 263-269) they measure the flight time of the ultrasounds to know when the food is completely frozen. They detect the ice sheet that has formed. However, in the present invention it is possible to know the percentage of ice and the temperature of the food at any time. On the other hand, not all the water contained in a system is frozen. A part may be linked in various ways to components such as proteins, carbohydrates, etc. This fraction of non-freezing water is not always known or estimated beforehand, and it can be considerable, so that the calculations made theoretically between the temperature of the system and its ice content can be wrong. For example, in the two aforementioned bibliographic citations, that fraction is not taken into account. The procedure described below allows the determination of the ice content without having prior knowledge of these water fractions, and in certain cases, also contributing to its calculation.

Freezing is carried out in most cases by inducing a heat flow to the outside of the system, through the wall or walls that separate it from the cooling medium. In products of complex geometry, in motion, or of a certain thickness, the temperature during freezing is not really constant, since the limited thermal conductivity of many systems means that the temperature in the center or the furthest point of the cold wall is at occasions very different from that of this one. An adequate monitoring of the progress of the amount of ice formed cannot be obtained with precision only from the heat exchange temperature, but a complete map of the temperature distribution in the product is required. However, the procedure described below allows the continuous monitoring of the ice content and the temperature during freezing / defrosting irrespective of the temperature at which they are actually performed.

In food, the final temperature of the freezing process and that at which frozen products are stored is usually usually set between -18 and -2O ⁰ C. During said storage, the temperature is not always constant, due to fluctuations in the freezing equipment (compressor cycles, defrosts) and incidents such as opening doors, changing containers, entry and exit of product batches, etc. In these variable conditions, knowledge of the ice content and its temperature may be of interest to control the final quality and to predict the time that a given product can continue to be stored in that state.

Recrystallization is the growth of larger ice crystals at the expense of smaller ones and occurs in all frozen systems that retain a fraction of water in a liquid state, as this mediates in the process. Recrystallization is the main limiting process of the shelf life of frozen systems (as can be seen in MN Martino and NE Zaritzky, 1989, "Ice Recrystallization in a Model System and in Frozen Muscle Tissue", Cryobiology, 26, 138-148 ). This speed, as indicated, is a function of the amount of free water, and therefore of the ice, present in the system. Therefore, a monitoring of this amount by the procedure described below may estimate the speed at which the recrystallization process will take place and, therefore, help the prediction of the shelf life of frozen products.

The fluctuations in the temperature of a container of frozen products are reflected in their ice content. These fluctuations accelerate the recrystallization process and the water output of the product, so they are undesirable. But the variation of the outside temperature has a variable reflection in the temperature of various regions of the product. Especially in those packaged in portions of large mass and small surface area, the reflection of these fluctuations in the central temperature of the portion will depend largely on the thermal conductivity of the product, which in turn depends on the ice content thereof. Therefore, the direct measurement of the ice content and its temperature, as allowed by the procedure described below, may be more reliable and convenient than mere external temperature fluctuation to determine the degree of alteration that the product is actually suffering.

Storage in the frozen state implies a subsequent defrosting of the product. Depending on the type and uses of the same, the processes must be conducted in various speed conditions, to maintain the properties and quality of the product. A suitable way to monitor this process is through the use of this patent, which allows to obtain a reliable average of the temperature in the same food.

Among the non-classical processes of freezing (and defrosting) are those carried out at high pressure (PD Sanz, 2005, "Freezing and Thawing under Pressure", in "Novel Food Processing Technologies", edited by GV Barbosa-Cánovas, MS Tapia, and MP Cano, pgs. 233-260, Marcel Dekker CRC Press). The freezing temperature of the water decreases considerably with increasing pressure, so that this variation can be used to carry out a series of innovative processes with energy advantages and maintenance of the quality of the products. The information that can be reached on a system that is being subjected to a freezing or thawing process in a vessel with thick steel walls is limited. The procedure described below allows, however, in an easy and non-invasive manner, with minimal installation that does not imply great technical difficulties, knowledge of the amount of ice formed in a system and its temperature. Of special interest is the case of pressure freezing (PD Sanz and L. Otero, 2005, "High Pressure Freezing", in "Emerging Technologies for Food Processing", Food Science & Technology International Series, pgs. 627-652 , Elsevier Ltd), in which the temperature of a system is reduced under pressure to the lowest point at which the freezing does not take place. Then the pressure is abruptly released and that results in the freezing of a considerable fraction of the free water, instantaneously. Both the absence of ice before the expansion and the amount of ice formed after it and its temporal evolution are of special interest in the study and control of these processes, as well as the evolution of the average temperature of the product and its freezing point.

The characteristics of this patent for the determination of the temperature and the ice content of a food, allow its operation in non-equilibrium conditions, regardless of the existence of all kinds of flows, changes and inhomogeneities in the system. For example, during freezing or thawing, where the percentage of ice changes over time and where the distribution of temperatures between surface and center of the product can be very irregular. Thus, this patent allows the monitoring of these processes.

Finally, we can mention situations where freezing is an unwanted event. This is the case of the preservation of food in a refrigerated state, in which it is tried to avoid the formation of ice that can alter the properties of certain products.

BRIEF DESCRIPTION OF THE INVENTION

This invention is characterized by allowing to determine the temperature and the ice content of a frozen food whose nature is known and its initial freezing temperature.

The determination is made by measuring the speed of ultrasound propagation through a portion of the food of known thickness, which may or may not be packaged, or inside containers of various types, among other possibilities. The measurement of the speed of ultrasound propagation is carried out by means of the emission of a signal of characteristics between those considered generally as low power ultrasound, and its subsequent reception, the speed being calculated as the ratio of the distance traveled by the ultrasound ( the thickness of the sample) divided by the time elapsed between transmission and reception of the signal (flight time). Both the emitter and the receiver are formed by transducers or piezoelectric. Alternatively, instead of the second element, an ultrasonic reflection system is located. The emission is carried out by means of a pulse generating equipment and the reception by means of an oscilloscope or equivalent device. The equipment that generates and displays the received signal is equipment commonly available in the market.

The measurements must be carried out in such a way that the emission signals do not overlap or that the signal is not interfered with and the reception of the signals, in the case of the use of reflectors The emission frequency and therefore the nature of the piezoelectric or Transducers must be included among those corresponding to what is generally considered low power ultrasound. In this patent, the frequency range used is 1 to 10 MHz, with the best results being frequencies between 1 and 3 MHz.

DESCRIPTION OF THE FIGURE

Figure 1, Diagram of the device (1) food, (2) Thermostatic bath, (3) Oscilloscope, (4) PC, (5) Pulse generator (6) Magnetic stirrer)

Figure 2, Experimental determination of the freezing point of the hake where T represents the temperature, t, the time, N the freezing point and P the freezing plate.

Figure 3, Relationship between the speed of ultrasound propagation (v _s (m / s)), the ice content (X ₁ (%)), expressed as a molar fraction: number of moles of ice divided by the number of moles of total water) and the temperature (T ( ⁰ C)) determined by the calculation program for a hake sample.

DESCRIPTION OF THE INVENTION This invention consists of a device created from an experimental development (Figure 1) and a computer program by means of which the temperature and the ice content of a food can be determined from the measurement of the ultrasonic propagation speed within it. The requirements to determine the ice content of a food are: that two piezoelectric or transducers are arranged (or in an alternative experimental design, a piezoelectric or transducer and an ultrasonic reflector) with their faces parallel and facing both ends of a fraction shows. The food must have a minimum thickness of 1 mm and a maximum of 500 mm, preferably between 5 mm and 200 mm, losing precision in the measure if these limits are exceeded. The thickness should be constant or of known variation. This variation may be absorbed by the calibration process if the conditions under which it is performed are physically equivalent to those of measurement. The piezoelectric, transducers or reflectors should be in intimate contact with the food a or separated by materials that little attenuate the intensity of the ultrasounds, such as quartz, glass or metals, being soft materials such as rubber or rubber being inadvisable because they hinder the diffusion of the waves as well as materials with discontinuities.

Foods whose temperature and ice content can be determined in this way include fruit, vegetables, fish or meat that have a freezing temperature greater than -2.4 ⁰ C. They can be packaged or not, by systems that do not contribute significantly to Ia ultrasound attenuation. Thus, thick wraps of elastic plastic materials, such as rubber or rubber, will be inadvisable. It is also not convenient for the system to contain air or bubbles. They may be contained between rigid walls of metals, glass or glass. It may be at rest or in motion, this movement being in solidarity with the measurement assembly (as it could happen in a transported system), flowing in a pipe or otherwise, between walls or without walls that separate it from the piezoelectric, transducers or reflectors, or being agitated by mechanical systems, or of another type or by mere thermal agitation or convection.

The process can be carried out in a variety of installations, the non-exhaustive list being: experimental and industrial freezing facilities, inside chambers, trucks, warehouses and other enclosures for frozen storage of food; in defrosting facilities, etc.

The complete procedure would be as follows:

1 ^o ) The freezing point of the food is determined by thermal measurements.

2 ^o ) The measuring device determines the flight time by means of the pulse transmission technique, placing the parallel emitters-receivers facing both sides of the product section considered. The distance between both elements is between 1 mm and 500 mm (optimally 5 mm and 200 mm) and measured accurately. This section of the product may or may not be contained in a container or a rigid container, or it may be generated when the emitter-receivers are introduced into holes such that the desired portion is included between them. These emitters-receivers are two piezoelectric or commercially available transducers, suitable for emitting ultrasound frequencies between those generally considered low power ultrasound. Alternatively, it is possible to work with the pulse-echo technique, where one of the transducers or piezoelectric can be replaced by an ultrasound reflector, a rigid surface, flat and parallel to the other transducer or piezoelectric.

3 ^o ) Ultrasonic pulses are emitted, of adequate duration and intensity for their correct reception, by means of the pulse generator of the device, connected to one of the transducers or piezoelectric.

This device determines the interval of time elapsed between emission and reception (flight time), the distance between them and the speed of ultrasound propagation.

The device is complemented by a computer program that obtains the temperature and the percentage of ice existing in the food from the speed of sound. As inputs, it only requires parameterizing the type of food (fruit-vegetable or meat-fish). However, if its composition is known, that is, its free water content and its bound water content, the accuracy of the program is accentuated. If the data on its bound water content is not available, it can be substituted, introducing its protein and carbohydrate content.

This program runs directly from any PC that has the Matlab ^® 7 programming language installed

(http://www.mathworks.es/company/aboutus/), or higher. In order for it to run on other PCs, it is necessary to install a series of libraries and instructions that are attached with the program.

The advantages of the method for using the device of this invention over existing techniques are:

That the procedure is non-invasive, and can be adapted to all or most of the imaginable situations.

That the procedure is independent, under the conditions described, of the knowledge of the temperature of the product. That the procedure is independent of the fact that the distribution of the product temperature is known.

That the procedure is independent of both internal and external movements suffered by the product. For example, the product is defrosting or is in a production or storage chain.

That the procedure is universal and comparable in many cases and applications.

That the procedure is simple, fast and can be performed continuously. That the procedure can be carried out with minimal intrusion at the measurement sites.

That the procedure can be adapted to a wide variety of environments. That the data of temperature or ice content, thus obtained, can be used to obtain a better indication of the state of quality or deterioration of the frozen food, which would be achieved from the mere measurement of the external temperature of the product .

EXAMPLES OF EMBODIMENT OF THE INVENTION EXAMPLE 1

A first test is carried out to obtain the relationship between the speed of ultrasound propagation through a slice of frozen hake at different temperatures and its ice content under the same conditions.

1 ^o ) Two opposite and parallel piezoelectric devices were used, connected one to a pulse generator and the other to an oscilloscope, acting as a receiver.

2 ^o ) The thickness of the sample, contained in a cell, was 12.4 mm.

3 ^o ) Pulses of frequency 1 MHz and maximum amplitude 12 volts were emitted. The periodicity of the emitted pulse was 200 Hz and its duration was 1 μs.

4 ^o ) The flight time was determined and the propagation speed of the ultrasound was obtained, by dividing the distance of the transmitter-receiver separation between this time.

5 ^o ) The freezing point of the hake was determined experimentally, which turned out to be -0.9 ⁰ C. The procedure is detailed in Figure 2,

6 ^o ) By placing the complete measurement system (shows in its container more emitter and receiver) inside a cryostatic bath of finely regulated temperature, determinations of the speed of ultrasonic propagation were made, as described, at various temperatures , all of them below the freezing point of the product. 7 ^o ) By means of the calculation program explained above, the ice content and the temperature of the system were obtained, by measuring the speed of ultrasound propagation (Figure-3)

The quality of the results can be seen from the relationship between the ice content and the temperature obtained experimentally for another muscular system, of the same water activity, using another (invasive) technique Woolrich, W.R. Secondary Refrigerations. In "Handbook of Refrigeration Engineering" VoI.1, pp 358-361. AVI Publ., Westport, CT, 1965) v

EXAMPLE 2

The test carried out to obtain the relationship between the speed of ultrasound propagation through a slice of frozen hake at different temperatures and its ice content under the same conditions is cited.

2 ^o ) The thickness of the sample, contained in a cell, was 25 mm.

3 ^o ) Pulses of frequency 3MHz and maximum amplitude 12 volts were emitted. The periodicity of the emitted pulse was 200 Hz and its duration was 1 μs.

5 ^o ) The one indicated in Pham, 1987, "Calculation of bound water in frozen food" vol. 52, p. 210., being 0.888 °

6 °) Placing the complete measurement system (shows in its container more emitter and receiver) inside a cryostatic temperature bath finely regulated, ultrasonic propagation speed determinations were made, as described, at various temperatures, all of them below the freezing point of the product.

7 ^o ) By means of the calculation program explained above, the ice content and the temperature of the system were obtained, by measuring the speed of ultrasound propagation.

Claims

CLAIMS1 Device for the determination of the temperature and ice content of frozen foods characterized by comprising: • an ultrasound transmitter-receiver in a frequency range between 1 MHz and 10 MHz. • Pulse generator of the device, connected to one of the transducers or piezoelectric • A receptacle for samples with a thickness between 1 mm and δOO mm • A temperature control system such as a cryostatic bath, a peltier system, a thermostatic chamber • Not to be invasive • Optionally it can have a magnetic stirrer2 Device for determining the temperature and ice content of frozen foods according to claim 1 characterized in that the Transmitter-Receiver are transducers or piezoelectric, parallel and placed at the ends of the sample, using the transmission technique3 Device for Ia determination of temperature and ice content of frozen foods according to claims 1 and 2 characterized in that the Emitter-Receiver are transducers or piezoelectric, parallel and placed at the ends of the sample, using the transmission technique4 Device for determining the temperature and ice content of frozen foods according to claims 1 and 2 characterized in that the Emitter-Receiver is an ultrasound emitter, either a transducer or piezoelectric, and an ultrasound reflector, rigid, flat surface parallel to the other transducer or piezoelectric and placed at the ends of the sample , using the eco pulse technique. Procedure for determining the temperature and ice content of frozen foods characterized by using the device of claims 1 to 4 and by • Determining the freezing point of the food either by thermal measurements or by estimation in tables • Carry out measurements on foods that have a freezing temperature greater than -2.40C (fruits or vegetables, fish or meat) with a thickness of the sample between 1 mm and 500 mm • Issue ultrasonic pulses, of adequate duration and intensity for proper reception, by means of the generator Pulses of the device, connected to one of the transducers or piezoelectric • Determine the time interval between emission and reception (flight time), the distance between them, calculating and the speed of ultrasound propagation Procedure for determining the temperature and ice content of frozen foods according to the claims

1.2.3 and 5 characterized in that the propagation speed of the ultrasound is calculated as the quotient of the separation distance of two ultrasonic emitters-receivers, divided by the time interval elapsed between the emission and the reception of the signal.

Procedure for determining the temperature and ice content of frozen foods according to the claims

1.3.4 and 5 characterized in that the propagation speed of the ultrasound is calculated as the ratio of twice the separation distance of an ultrasound emitter, and a reflector, divided by the time interval elapsed between the emission and the reception of Ia reception of the signal