Electromagnetic Characterization of Beers: Methodology, Results, Limitations, and Applications

ELECTROMAGNETIC CHARACTERIZATION OF BEERS: METHODOLOGY, RESULTS, LIMITATIONS, AND APPLICATIONS

8

Tom De Paepe⁎,†, Isabel Expósito⁎, Alejandro Cuevas⁎, Iñigo Cuiñas⁎, Jo Verhaevert† ⁎

Department Teoría do Sinal e Comunicacións, Universidade de Vigo, Vigo, Spain Department of Information Technology, Ghent Universit—IMEC, Ghent, Belgium

†

8.1 Application The beer market has grown significantly during the last years, presenting a different (and sometimes an opposite) evolution from the wine market. Although middle-aged and old people still drink wine in all situations, younger people seem to associate wine with more sophisticated events. In contrast, drinking beer results to be a softer alcoholic alternative (i.e., with a lower alcohol percentage content) to any social occasion, especially in a period of more severe alcohol control by policemen in order to reduce the number of drunk driving accidents. Parallel to the increase of beer consumption, the quality and the variety of it has also risen. More and more varieties are now available at all supermarkets anywhere. The challenge of maintaining high-quality standards and compete at the same time in a globalized market, moves the breweries and bottleries to implement different kinds of control mechanisms. However, these companies commonly base themselves on different tastings performed by experts on randomly selected bottles and cans. When one of the tasting experts detects a problem, the company managers remove the entire batch from the delivery store and finally destroy the product. This procedure is not only very expensive, nor guarantees that a less high-quality product reaches the final consumer: as the selection is rather random and initiated by human experts, there is a realistic probability of failure. Alcoholic Beverages. https://doi.org/10.1016/B978-0-12-815269-0.00008-8 © 2019 Elsevier Inc. All rights reserved.

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This fact opens an important thesis: the beer industry needs an automatic real-time method to continuously control the quality of the production. The proposal described in this chapter is the use of a radio link crossed by a pipeline carrying the beer to be brewed and/or bottled. It is based on the free-space transmission technique, which is an extended method to determine the shielding behavior and propagation characteristics of electromagnetic waves passing through a material (Morari and Bălan, 2015; Dvurechenskaya et al., 2010; Ferreira et al., 2017). Of course, the radio link must be tuned to one or more dedicated frequencies at which the electromagnetic behavior of the beer under test changes when the product is not in the desired conditions. One of the most fundamental parameters to measure the change of electromagnetic behavior seems to be the dielectric constant. Depending on the dielectric constant of the liquid, the radio wave will suffer more or less attenuation when transmitting across the pipeline. Measuring the received signal power in one of the ends of the radio link would be enough to detect changes in the dielectric constant, thus to act as fast as possible to isolate the liquid with minor quality and to solve the production problem. Based on this principle, the proposed quality tracking system is schematized depicted in Fig. 8.1. But prior to this proposed system design, the first step for that development would be a precise electromagnetic characterization of the beer itself. Due to the large amount of possible factors involved in the electromagnetic performance of a material (the physical components, the temperature, the presence of the bubbles or solid particles, the foam production, etc.), the characterization has to be done for each beer variety and even for each brand in a separate way. It is worth indicating that electromagnetic characterization of materials is a well-known and extended discipline within the radio propagation and channel modeling research community. Earlier published works mainly focus on the characterization of solid materials, including entities that define the propagation environment. Their characterization provides scientific support to channel simulation proposals (Alejos et  al., 2008; Cuiñas and Sánchez, 2000; Cuiñas et  al., 2001; Cuiñas and Sánchez, 2002; Cuiñas et al., 2007; Feitor et al., 2011; Sagnard and

Transmitter

Beer pipeline

Fig. 8.1  Scheme for a quality tracking system.

Receiver

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Zein, 2005; Pinhasi et  al., 2008; Tesserault et  al., 2007; Lahteenmaki and Karttaavi, 1996; Landron et al., 1993; Langen et al., 1994; Protiva et al., 2011). However, studies on characterization of liquids have had less diffusion, probably due to its reduced interest among the radio frequency engineering community in terms of radio planning, but also due to the complexity of gathering confident and useful results. Separating the liquid behavior from the effect of its vessel, air inside the recipient, and others results to be an additional issue in such measurement tasks. The possibility of applying the electromagnetic knowledge on liquid behavior for quality control in real-time and on-site situations and by means of nondestructive technologies, definitely opens a new window of opportunity for such liquid-centered experiments. Breweries and bottleries can use this knowledge to improve and speed up the detection of faults during the different steps of the production process, as changes in the electromagnetic behavior are typically related to variations in the liquid composition, the quality, and/or any other production parameter. Thus, the knowledge of the electromagnetic behavior of beverages will be a key factor for innovation in real-time quality control in every step of the production process of breweries and/or bottleries. There are different methods for the accurate electromagnetic characterization of liquids, from performing a direct analysis of extracted samples to combining different parameters from data indirectly gathered in the flow of the liquid. Among the various procedures designed for measuring electromagnetic characteristics of liquids, the probe reflection method stands out as one of the most promising techniques in terms of accuracy in high-loss materials, but also in manageability of equipment (like the sample preparation, the measurement bandwidth …). This method, based on a vector network analysis, provides easiness in measuring the S11 parameter, which compares the amount of energy impinging on a body (in this case, a volume of beer) with the energy reflected by this body. This scattering parameter indicates how many electromagnetic energy scatters or rebounds on the liquid under test and relates to the dielectric constant of that material or liquid. This dielectric constant, commonly identified as ε, is an intrinsic parameter of a certain material or liquid. This parameter can unfortunately not be measured directly, but the measurement of the S11 however allows us indirectly to determine that dielectric constant. The previously proposed procedure to measure and to check the quality of beers during production stage is based on the possibility of tracking this dielectric constant (which is in fact a more easily detectable related parameter in comparison to the received power) and on relating in real-time possible variations to production problems or mismatches.

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Once measured, the complex valued dielectric constant of the liquid under test can be calculated and derived from the measured S11 parameter. Taking into account the broadband character, the requirement of only small sample sizes, the easy sample preparation and temperature monitoring and the nondestructive nature of the procedure (Venkatesh and Raghavan, 2005; Bois et al., 1999; Moussa et al., 2003), this method has been the most successful among other alternatives. After accurate calibration of the measurement equipment, the complete measurement procedure is as simple as immersing a waveguide probe within the liquid under test, then use this “under-liquid” probe to gather the waves needed to obtain the S11 parameter, which is followed by calculating the dielectric constant. This direct measurement procedure opens a new world in the analysis and test of liquids during their production and storage stages: the electromagnetic characterization may be a method to detect irregularities during such processes. As quality and safety of food products (and beverages are included) concern both producers and authorities, a noninvasive and almost instantaneous method to continuously detect damages or problems can become a huge success. Although the well-known probe reflection method fits both important requirements (noninvasive and fast), it shows some limitations when dealing with beer characterization, related to its intrinsic natural foam and bubble generation. This method is analyzed in this chapter, together with its advantages and drawbacks and is completely validated by extensive measurement campaigns. The rest of the chapter is organized as follows. Section 8.2 describes the different electromagnetic characterization techniques valid for analyzing the behavior of liquids. Section 8.3 is devoted to experimental work: setup, procedure, and the description of the different beers involved in the measurement campaign, whereas Section 8.4 shows the measurement results, including those obtained just after opening the cans. Also the effect of the time going by and the effect of the temperature are handled. Section 8.5 provides an analysis of those results, as well as the detected limitations of the described experimental method. Finally, Section 8.6 outlines the conclusions and possible applicability of the research done.

8.2  Electromagnetic Characterization Techniques There already exist many ways to measure the dielectric properties of liquids (Venkatesh and Raghavan, 2005). However, an ideal method able to measure the electromagnetic behavior of all liquids on all frequencies with a predefined desirable confidence does not exist and

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so, depending on the required information a different method should be selected. Next paragraphs contain brief descriptions of each of the most used procedures. The slotted line reflection system is a broadband method that needs moderate (medium) size samples and provides low accuracy for high-loss materials (and very low accuracy for low-loss materials). The measured parameter is permittivity, which can be a useful parameter, but its focus is pointed to solid or semisolid materials. This slotted line reflection system presents several problems, as it is a destructive test and it badly controls the temperature during the process. Nowadays, no commercial vendor provides a system following this technique, which is obviously a limitation. The guided-wave transmission system is valid for measuring moderate-sized samples, which are destructed, at certain bands and it provides both permittivity and permeability. The difficulty in monitoring the temperature and also in preparing the samples limits its practical application. The accuracy is moderately independent of the type of material under test in terms of losses. In favor, there are some vendors providing resources for such method. As it works only with solid samples, the method is hence not valid for our purposes. The free-space transmission system owns the advantage of an easy control of the environmental conditions and the simplicity in the preparation of the different samples, in this case large flat sheets of material. Besides, it also conserves the integrity of the samples during the entire process. However, its accuracy is moderate for any kind of material. There are also commercial applications of such techniques, but unfortunately they do not manage conveniently liquid samples. The filled cavity resonance system is a single-frequency technique that does not work with high-loss materials, but it provides very high accuracy when measuring low-loss samples. It measures the permittivity and permeability of different kind of materials: solids, semisolids, and liquids, but unfortunately the samples are very difficult to prepare. Although it allows an easy temperature monitoring, its destructive character and the fact that no commercial vendor exists limits its application in the here proposed case. The partially filled cavity resonance system has some characteristics similar to the previous one: single frequency, very easy temperature monitoring, complex sample preparation, measuring permittivity and/or permeability, and sample destructiveness. As differences however, the sample size is very small and it allows measuring high-loss materials, although with low accuracy. However, as it only manages solid samples and there is no commercial equipment using this method, it is also not valid for the above-described measurement campaign.

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Finally, the probe reflection system is a broadband system valid for measuring the permittivity of small samples of solid, semisolid, or liquid materials in a nondestructive way. The system presents easy temperature monitoring and sample preparation and provides high accuracy when measuring high-loss materials (and low accuracy for low losses). Besides that, there are different commercial vendors offering equipment to develop such techniques in a professional way. The probe reflection system uses a coaxial cable and basically is a modification of the well-known transmission line method. The coaxial line has a tip that senses the signal reflected from the material. The tip is brought into contact with the substance by touching a flat surface of a solid material with the probe or by immersing it in a liquid. Taking into account the different above-mentioned options, this system appears as the most adequate for the proposed research and is also commercially available. These are the reasons that support the selection of the method used in the experiments described along next sections.

8.3  Experimental Work As the proposal to set a quality control system during the beer production stage is based on the electromagnetic behavior of the product under study, the first and main task to be implemented is the characterization of different beers in terms of their electromagnetic performance. Therefore, this section describes the extensive measurement campaigns developed to obtain as many real-world data as possible, including a description of the measurement setup, the procedure followed, and the considered beer varieties and brands.

8.3.1  Measurement Setup and Procedure Fig. 8.2 depicts the setup used in our measurements. It is based on a vector network analyzer (VNA) Agilent N5222A, which is remotely controlled by a laptop running DAK (Dielectric Assessment Kit) software (DAK, 2015). This system follows the probe reflection method as it was concluded in Section 8.2 to be the most appropriated to perform the intended measurements. As measurement probe, a DAK-3.5 is used. It is an open-ended coaxial probe with the capability to measure liquids within a frequency range from 200 MHz to 20 GHz. Its operation temperature varies between 0°C and 60°C. The probe is made of stainless steel and it is hence resistant to all kind of corrosive materials. This device is capable of characterizing electromagnetically different materials, both liquids (Baer et al., 2015; Tokaya et al., 2017), moist solids (Said and Hamdy, 2015; Hamdy and Said, 2016; O’Reilly et al., 2016) or even solids (Naghar et al., 2014; Wang and Jia, 2014; Ahmed et al.,

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(A)

(B) Fig. 8.2  Measurement setup: (A) scheme; (B) actual configuration.

2017), and other special materials (Jithin et al., 2016; Nadir et al., 2016; Nikolayev et al., 2016; Odit et al., 2016; Seo and Wang, 2017; Steward et  al., 2017). The wide usage in such different experimental works within various areas of knowledge reinforces the decision of applying this technique along this work. For our measurements, we have connected the probe via a high-quality low-loss cable to one of the ports of the VNA and then it was immersed in each of the different liquids (i.e., beer) at a depth of 3 cm from the top of a normal plastic cup and at least 1 cm within the liquid under test. We have configured the VNA to sweep a broad frequency band from 200 MHz to 20 GHz, with a frequency step of 100 MHz. The measurement probe transmitted the waves generated by the VNA and

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r­ eceived the reflection at the beer interface, gathering the S11 parameter at the port of the analyzer. Once gathered the S11 as a function of the frequency, the DAK software calculated the frequency-dependent dielectric constants. The measurement campaign allows us to obtain the dielectric constant as a function of frequency for all the considered beers in various conditions, as well as to identify the limitations of the probe reflection method with the material under study. The measurement configuration follows several successive steps, which are listed in the following scheme: (1) Calibrate the system with tap water as reference material and at a temperature of 20°C. The DAK software gives several choices for the material to be used in the calibration. We chose tap water as it was the easier to obtain and is a liquid like the other samples in the study. (2) Open the can of beer, cool in the fridge for having a temperature near 0°C, and serve the beer into a clean cup. (3) Measure the exact temperature of the beer and wait until the temperature of the liquid has increased 0.5°C and is stabilized, by using a contact thermometer in this task. (4) Gather the S11 parameter of the beer and compute the related dielectric constant. The time required by the equipment for this task depends on the parameters configured in the analyzer. It is not only dependent on the number of the frequencies under study, but also on the averaging of the measurements and the chosen filter in the VNA. These two parameters should be carefully chosen to achieve reliable results, but without being more time consuming than needed. (5) Repeat steps (3) and (4) at different temperatures between 0°C and 30°C, with steps of 5°C approximately. (6) Put the cup of beer into the fridge, wait for 6 h and then repeat steps (3), (4), and (5). (7) Put the cup of beer into the fridge, wait for 24 h after opening the can of beer and then repeat steps (3), (4), and (5). (8) Put the cup of beer into the fridge, wait for 48 h after opening the can of beer and then repeat steps (3), (4), and (5). The measurement procedure followed the same scheme as previously applied in (Gaspard et  al., 2017), where it has been demonstrated its high performance in measuring complex beverages.

8.3.2  Measured Liquid Samples To have a representative number of samples we repeated this measurement procedure using 11 different beer types from 7 different breweries or brands, all over the world. Table 8.1 summarizes the considered beers, organized according their alcohol contents (from stronger to lighter).

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Table 8.1  List of the Analyzed Beers and Their Main Characteristics Brand

Country

Alcohol Content (%)

Type

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

Spain Spain Denmark The Netherlands Germany Czech Rep. Argentina Germany Czech Rep. England The Netherlands

6.5 5.5 5.0 5.0 5.0 5.0 4.9 4.8 4.4 3.6 0.0

Amber lager (Vienna) Pale lager Pale lager Premium Am. Lager Hefeweizen Bohemian pilsner Pale lager German pilsner Bohemian pilsner English brown ale Alcohol-free

We bought all beers on the same day in a regular supermarket and they have similar dates of preferred consumption. Although there were a large selection of bottles of different volumes and cans, the decision was to buy only canned beers. This recipient was chosen to minimize the differences in the electromagnetic properties that could come due to the effect the container can cause during the time of storage (e.g., the size, the shape, the color, and the dimensions of the used glass bottles) and then to assure that all beers were kept in similar conditions and in the same volume. We also tried to consider a wide variety of beers, looking for common trends instead of specific brand or type impacts. The brand selection was limited by the availability in the local shop, but despite this fact we also arranged to have well-known traditional beers and some local, but very popular samples. There are three pale lager samples, one amber lager (Vienna style), three pilsner (German and Bohemian styles), one premium amber lager, one English brown ale, and one white wheat beer. With this selection of beers, we consider that most of the market is covered, and that the set takes into account people likes. The selection of an alcohol-free beer allows us to identify possible effects due to the presence of alcohol, by comparing results with those provided by regular beers. As stated above tap water is used as reference material for the calibration of the system. It is easy to obtain and is a very suitable

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r­ eference because beers are composed mostly of water. Normally, the tap water reference had an average temperature of 20°C, but its behavior as function of the temperature was also studied. To check whether the performed calibration is correct or something has changed with time, a measurement of the dielectric constant of a water sample is performed and the obtained values are compared with the ones stored in the reference tables from the software in order to assure that there are no errors in this step. This is important as calibration is very sensitive to any aberration in the setup: the analyses should have reached its stable working temperature; any accidental movement of the cable connecting the probe and the analyzed beer requires a new calibration procedure …

8.3.3  Measurement Uncertainty Any measurement has associated a certain uncertainty coming from the instrumentation used in the measurement, the method, the technician doing the procedure, and even from the device/material under test (GUM, 2008). In the case considered in this chapter, a characterization of the uncertainty of the measured magnitude is required to assure that the differences among the analyzed beers are due to the specific characteristics of each one and not the result of uncertainties associated to the process. In many cases, calibrated instruments provide their own error figures, and from these records the uncertainty data are derived. DAK system gives the dielectric constant as the result of the measurement process, after computing it from S11 gathered data. However, as the exact mathematical procedure the system applies to calculate the dielectric constant is not precisely stated, the possibility of obtaining an uncertainty figure from S11 accuracy is out of scope, and so the uncertainty is achieved by statistical methods. In order to get knowledge about the uncertainties of the measurement system and of the used calibration process, 40 measurements of the same sample of regular tap water at room temperature were performed, to conduct a statistical analysis of the uncertainty. In order to have a complete characterization, the whole process was repeated, that means not only the measurement of the dielectric parameters but also the calibration. The number of observations (repetitions) should be large enough to ensure the reliability of the estimation. In our case, with 40 repetitions, the experimental standard deviation of the mean, this is, the “uncertainty of the uncertainty,” is about 11%. Here we should remark that all measurements were performed by the same operator with the same instrument, and all the temperatures of the item under test are specified, otherwise additional factors could be contributing to the uncertainty.

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A zoom of 40 measurement results for tap water is shown in Fig. 8.3, meanwhile indicating in dashed lines the uncertainty intervals. Uncertainty was analyzed in the whole frequency range but only a small region is presented to improve clarity, as water presents a high variation of the dielectric constant in the range under study. We have first calculated the average of the measurements performed (striped line in Fig.  8.3). That is done in order to estimate the positive root square of the variance and to compute how much an individual measurement differs from this mean value, this is the uncertainty of an individual result. The results indicate a slight variation between samples being all of them in the region defined by their mean ± an average deviation of 0.1493 (uncertainty with a coverage factor of 3 or an interval having a level of confidence of approximately 99%). This means that the deviation of the measurements with respect to the mean is less than 5%. On Fig.  8.3 it can be seen that most of the samples are located around their average indicating the stability of this measurement.

8.4  Measurement Results The result of a wave impinging a particular liquid is a division of energy: part of the incident energy is rejected as a reflection, part of it is absorbed in the liquid by warming itself and a third part of the energy is transmitted through the liquid under test. The permittivity drives these three different mechanisms and it is expressed as a complex value in F/m. As permittivity in all media (solid, liquid, and gaseous) is also larger than those related to the vacuum, it is usually expressed

Fig. 8.3  Zoom on the 40 measurements performed to compute the uncertainty (dashed lines) at tap water data.

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in relative terms, compared to vacuum permittivity ε0 (Engelder and Buffler, 1991) and it is called relative permittivity or dielectric constant. This relative permittivity results hence in a dimensionless value and the according complex value is defined as ε* = ε′ + jε″. In this equation, the real part ε′ of the dielectric constant is related to the energy storage in that medium and the imaginary part ε″ of the dielectric constant to the losses of that medium. For the rest of this chapter, only relative permittivity (or dielectric constant) is the parameter under analysis. After the measurement campaign, the obtained amount of data is massive: real and imaginary values of dielectric constants of 11 beers and tap water (the reference), at 198 different frequency spots, at seven temperatures (0–30°C with a 5°C step and two additional measurements at 40°C and 60°C) and taken at four moments in time (just at opening the cans and 6, 24, and 48 h after opening). Figs. 8.4–8.7 represent examples of such data, containing the real and imaginary parts of the dielectric constants at 20°C, measured 24 and 48 h after opening the cans and for the 11 beer varieties together with water. Observing Fig.  8.4, we can recognize some common trends that always appear in the electromagnetic behavior of the different beers as function of the frequency. The real part of the dielectric constant at lower frequencies (around 1 GHz) is always between 80 and 85

Fig. 8.4  Real part of dielectric constant of different beers at 20°C, 24 h after opening.

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Fig. 8.5  Imaginary part of dielectric constant of different beers at 20°C, 24 h after opening.

Fig. 8.6  Real part of dielectric constant of different beers at 20°C, 48 h after opening.

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Fig. 8.7  Imaginary part of dielectric constant of different beers at 20°C, 48 h after opening.

and then this parameter decreases monotonously as the frequency increases toward values between 45 and 50 at 15 GHz. Then, most of the beers show different behaviors in something like resonance oscillations between 15 and 20 GHz, the maximum measured frequency. These oscillations lead to variations of more than 10 units in the real part of the dielectric constant. The tap water measurement, as a reference, follows the same trends. The imaginary part of the dielectric constant, as can be observed in Fig. 8.5, shows a behavior opposite to that of real part in all cases: the traces are monotonically increasing as the frequency increases, faster at lower frequencies (beginning near 0 at 200 MHz) and asymptotically toward a value between 30 and 35 at frequencies over 13 GHz. Finally, between 15 and 20 GHz, some ripple is observed and values from different beers move away from each other. The imaginary part of the dielectric constant of water is completely different in comparison to the above-described beer samples. The according values not only indicate a ripple, but results also in slightly higher values. Twenty-four hours after the previous case, when hence 48 h passed from the moment of opening the cans, the general trends seem to be similar to the ones observed in Figs. 8.4 and 8.5, but there are some

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effects mainly detectable in samples with less alcoholic content. Based on the previous figures, we have already observed the trend of the electromagnetic behavior of beers seeming to be almost similar to that shown by tap water (which is the calibration reference): the real part of the dielectric constant has similar values at the beginning and it is decreasing with frequency, whereas the imaginary part of the dielectric constant follows an increasing trend as a function of the frequency, resulting in more ripples and in slightly different results for the highest frequencies. In Fig. 8.6 (and thus after 48 h opening), all the different beers show similar trends, except the one corresponding to the alcohol-­ free beer, which shows higher real parts of the dielectric constant values at lower frequencies. We can assume that its electromagnetic behavior is dissimilar compared to other regular beers due to the absence of alcohol content, which probably involves a different brewing and/or bottling compared to the rest of the samples. Besides this regular and similar monotonously decrement as a function of the frequency of the real part of dielectric constant, as ­occurred in measurements taken 24 h before, most beers present some differences among them at frequencies higher than 15 GHz. At that frequency band (from 15 to 20 GHz), we can observe larger dissimilarities (up to 10 units) among the various types of beers. Besides, Pilsner and/or other beers with lower alcoholic content percentage show a significant augmentation around 12–13 GHz. Although the observation does not provide a clear dependence neither on alcoholic contents nor on the type of beer, these variations moved us to put the focus on this higher part of the electromagnetic spectrum when analyzing the dielectric constants of different beer specialties as a way to detect problems in the quality of the product. Anyway, the frequency band below 15 GHz seems to provide less insight in the specific characteristics of each product and so it could be superfluous when the objective is a deep analysis of a specific kind of beer. This consideration supports that we focus during the rest of the analysis on frequencies above 15 GHz, which represents an interesting first step for future research in this topic. Nevertheless, before this analysis, observing what happens just after opening the container will provide insight in the limitations of such a characterization. The imaginary part of the dielectric constant, as can be observed in Fig. 8.7, follows a similar trend to the measurement performed 24 h before: its behavior is opposite to the real part as traces monotonically increase as frequency increases, asymptotically toward a value between 30 and 35 at frequencies larger than 13 GHz. As a difference to the 24 h values of Fig. 8.5 and probably related to oxidation processes due to the longer time in contact with atmospheric air, the ripple between 15 and 20 GHz appears to be stronger for Pilsner and/or other beers with lower alcoholic content percentage.

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Tap water results also deserve a comment. We included the water sample as a reference in Figs. 8.4–8.7. Results on tap water are in good agreement to those published in Ogunlade et al. (2006) in a measured frequency band from 8 to 12 GHz, which improves the confidence on the measurement system used here. After this initial observation, we could indicate that over general standard behaviors, we have observed different specific trends at certain frequencies that would help to detect variations in brands and types of beers and also to check evolutions in their electromagnetic characteristics. So, next subsections are devoted to observing more in deep possible trends and effects. The first subsection focuses on the effects just after opening the cans. The second subsection describes the variations induced by time after opening the cans and the third subsection comments on changes as temperature varies.

8.4.1  Results Just After Opening the Containers Data gathered just after opening the cans are especially significant for this work; although the frequency behavior results to be similar to that observed at different times after opening, the just-after-opening gathered values are completely different and sometimes out of ­physical explanation. Fig.  8.8 for instance plots the real part of the ­dielectric

Fig. 8.8  Real part of dielectric constant, John Smith’s beer, just after opening the can.

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constant of the beer John Smith’s at different temperatures, but all immediately after opening. We can observe large variations in the dielectric constant as a function of the frequency for almost all traces. We also remark that these variations are less deep when temperature increases (as the time from opening the can, when it is around 0°C, also increases). In fact, only the last measured trace, with the beer at a temperature of 60°C, is comparable to the results previously shown in Figs. 8.3 and 8.5. This occurred only some minutes after the opening of the can, which is the time needed to perform the whole set of measurements: we removed the beer from the refrigerator, opened it and measured at 0°C, heated the beer to a temperature of 5°C, and then measured again and so on until the complete range of temperatures was evaluated. Other brands also present unexpected variations when measuring the electromagnetic behavior just after opening the cans. Fig. 8.9 shows plots with the real part of the dielectric constant for the Estrella Galicia beer, whereas Fig. 8.10 does the same for the Heineken beer and Fig. 8.11 for the Pilsner Urquell beer. In the previous figures, we can observe peak values that reveal instability in the liquid under study, as can be noticed for example in Fig. 8.11 for a measurement temperature around 15°C.

Fig. 8.9  Real part of dielectric constant, Estrella Galicia beer, just after opening the can.

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Fig. 8.10  Real part of dielectric constant, Heineken beer, just after opening the can.

Fig. 8.11  Real part of dielectric constant, Pilsner Urquell beer, just after opening the can.

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Another issue to consider is that there is an influence of temperature at first sight, but taking an in-depth look there is no pattern in the behavior (just after opening, in Section 8.4.3 the influence of temperature is analyzed in a more stabilized situation). What occurs in this specific situation that could explain such variations? When serving a glass of beer, just after opening the can or the bottle, it is well-known that a foam layer develops on top of the liquid. Located in the foam are particles of wort protein, yeast, and hop residue, making the consistency and shape characteristic of the foam differ with each type or variety of beer and even with each brand. This is something well-known by all beer lovers. In addition, when immersing the probe under the liquid surface, a set of bubbles moves toward the probe walls, covering its entire surface and hence influencing the obtained measurements. This is what we called the “probe reaction” and a descriptive photograph is shown in Fig. 8.12. Fig. 8.12 presents the measurement probe immersed in beer and completely covered by bubbles. Depending on the location and the size of the bubbles, the effect on the measurements can be dramatic. In the limit, if one or more bubbles occupy the complete aperture of

Fig. 8.12  Photograph of bubbles around the probe, what is called the “Probe reaction.”

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the probe, the gas of the bubbles instead of the liquid is measured. And of course this presence of bubbles leads to mistakes in the dielectric constant estimation. Let us try to detect how such strange dielectric constants are computed. Due to the initial evolution of beer within a cup, creating foam near the surface and bubbles around the measurement probe, the S11 parameter gathered at those initial steps shows trends inconsistent to the general behavior of the beer. Bubbles move inside the liquids and some of them stitch up to the surface of the probe. These effects modify the contact environment of the probe. Due to the quite fast movement of the bubbles, these variations result in a fast-changing environment, even during the frequency scan performed for one measurement. Thus, these effects strongly influence the measurement results immediately after opening, explaining the clearly different behavior of these early measurements compared to those after some time of inactivity. Fig. 8.13 shows the evolution of the S11 parameter in a Smith chart for the beer Estrella Galicia, at a measurement temperature of 20°C. Five different traces are visualized in that figure, corresponding to five different ways of using the measurement probe. The “Foam” trace isolates the foam effect as much as possible, with the probe uniquely immersed in the foam layer: this counter-clockwise trend is probably the strangest one. “Foam and probe” combines the presence of foam and bubbles on the probe surface, showing a trace almost completely outside the Smith chart, whereas “Probe” corresponds to a measurement performed below the foam layer, but with the probe almost completely covered by bubbles. Only after a few time, when the probe reaction (the bubbles on the probe surface) has mitigated, the foam has disappeared and the measured liquid is stable, the trace of S11 seems to be representative and physically consistent (it is labeled as “Beer” in Fig.  8.13). Finally, the trace “Water” depicts the measurements performed as a reference, using tap water. Obviously, water does not generate foam or bubbles and its behavior follows the general trend already described earlier. We can observe a normal clockwise trace as the frequency increases, except for the “Foam” reaction. This clockwise trace seems to be the general behavior of liquids, as observed at presented charts and all other gathered data. Additional measured data also show no variations in the trace sense because of temperature increment. The irregular trends observed (data out of Smith chart and/or opposite rotating sense) indicate that the probe reflection method presents a lack of precision when foam and bubbling effects appear in the liquids. Those irregular S11 parameter values lead to out of physical sense computed dielectric constants, as this calculation is no more than mathematical operations made on the measured S11 parameter. The nonsense dielectric constants invalidate the use of such a measuring method for this specific detected case. This fact suggests that it is very important

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   263

Fig. 8.13  Smith chart of evolution of S11 parameters for Estrella Galicia beer, at 20°C, just after opening the can.

to wait a certain time for the liquid to stabilize before measuring. This period will of course depend on the beer itself: its bubble content and the width of foam layer it generates and seem to depend on the variety and the brand of beer. Another strategy can be degasifying the liquids, instead of waiting for the natural stabilization.

8.4.2  Evolution of Dielectric Constant With Time Now, we have detected the limitation of the measurement procedure just after opening the cans and identified the frequency band in which larger differences among beer varieties and environmental

264  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

conditions seem to appear. Therefore, we can analyze the possibility of using the electromagnetic characterization to identify differences in beer evolution and we can decide the most suitable frequency to implement such a quality control system. Figs. 8.14–8.17 show the real part of the dielectric constant obtained at 20°C, at four different times from opening the cans and with four different beers, selected as samples of the whole process: John Smith’s, Pilsner Urquell, Heineken, and Estrella Galicia. We considered a shorter band, from 15 to 20 GHz, being more representative as was concluded from the general results at the beginning of this section. Once the cans were opened, an oxidation process is assumed to have started in the beer, as the liquid comes in contact to atmospheric air. As beer is composed by a complex mixture of elements, chemical changes will occur in beer when time goes on (Guido, 2016). The evolution and the speed of such process probably drive the rhythm at which the beer quality decreases. Thus, we look for data at different times after opening the cans, considering that an oxidized beer would have the most similar outcome to a beer with minor quality that we could generate in an electromagnetic laboratory. With this information, we could analyze the effect of beer degradation on the dielectric constant.

Fig. 8.14  Real part of dielectric constant, John Smith’s beer, at 20°C, different times after opening.

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   265

Fig. 8.15  Real part of dielectric constant, Pilsner Urquell beer, at 20°C, different times after opening.

Fig. 8.16  Real part of dielectric constant, Heineken beer, at 20°C, different times after opening.

266  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

Fig. 8.17  Real part of dielectric constant, Estrella Galicia beer, at 20°C, different times after opening.

In Fig. 8.14, we can see that the peaks appearing in the measurement after opening the can are probably coming from the movement of the bubbles and/or their accumulation in the probe. This effect does not exist in the measurement performed 6 hours and more after opening. We can also notice an increment in the value of the real part of the dielectric constant, when the beer cans are opened longer. However, it seems that a certain process is still happening in the beer as this dielectric constant decreases again as time goes by. After 24 h, we can observe some stabilization as the values agree well with the measurement taken after 48 h. The process that drives the variations in the real part of the dielectric constant values seems to stop or, at least, to slow down. Again, as already explained in the previous situation and in Fig.  8.14, we can observe in Fig.  8.15 that, after 6 h, the value of the real part of the dielectric constant grows in the complete frequency range and after 24 h decreases again. This time, the agreement in the 24and 48 h measurements does not happen. We can conclude that the above-described stabilization process may not have finished in this situation. Heineken beer seems to be a less complex variety. Observing Fig. 8.16, the stabilization process seems to be finished after 6 h, as the other two measurements performed later agree well with it and the

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   267

obtained values after 6 h are not higher than the ones just after opening. The shapes in the graph could show that there is also an important fast effect of the bubbles at opening that ends quite quickly when moving to a further stabilized stage. The case observed in Fig. 8.17, for Estrella Galicia, is very similar to the one presented in Fig. 8.14: increased value of the dielectric constant after 6 h and stabilization and agreement after 24 and 48 h. The main difference with the previous described case is that here the value after 6 h is not so linear, so maybe a particular, stronger chemical process is taking place in this situation. In all cases, the real part of the dielectric constant after opening and even 6 h after opening seems to be different from that after 24 or 48 h. In fact, in most of the presented cases (except Pilsner Urquell, see Fig. 8.15), there are really small differences between the results after 24 and 48 h, which could indicate that the degradation process influences the dielectric constant until a certain level, and depending on the beer itself, it needs more or less time to reach this level. Another interesting observation is that differences in the real part of the dielectric constant are clearly observable at some frequencies, but other frequencies do not provide clear information. In order to summarize the effect on the rest of the brands, Tables 8.2–8.4 show the values at 20°C, different times after opening and at 15, 17.5, and 20 GHz, respectively. Data for Franziskaner at opening

Table 8.2  Computed Dielectric Constant Values at 20°C, Different Times After Opening and at 15 GHz Time After Opening Brand

Opening

6 h

24 h

48 h

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

41.32 40.8 48.62 52.43

44.13 53.71 48.07 48.96 48.62 52.28 52.05 49.23 54.55 53.68 42.65

50.27 49.86 46.95 48.23 48.53 49.06 50.37 48.43 49.86 49.24 51.42

49.21 48.26 48.72 48.72 49.04 49.51 49.69 49.43 49.46 48.82 48.09

51.12 43.69 43.23 49.86 40.99 38.13

268  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

Table 8.3  Computed Dielectric Constant Values at 20°C, Different Times After Opening and at 17.5 GHz Time After Opening Brand

Opening

6 h

24 h

48 hours

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

36.25 36.25 40.68 57.95

33.36 64.96 39.75 44.72 45.02 50.80 50.26 47.36 53.30 52.03 36.25

47.12 45.65 41.18 43.42 43.40 47.63 50.33 43.96 47.84 46.64 48.60

46.37 44.22 44.21 44.40 45.29 40.54 43.18 46.23 36.87 45.15 44.57

47.82 35.93 34.98 43.93 32.81 28.27

Table 8.4  Computed Dielectric Constant Values at 20°C, Different Times After Opening and at 20 GHz Time After Opening Brand

Opening

6 h

24 h

48 h

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

25.44 24.27 40.45 43.23

28.62 44.22 37.93 38.62 38.43 46.52 45.88 39.31 49.05 47.98 27.64

39.43 39.21 35.26 37.70 37.72 38.85 40.73 37.21 39.76 38.85 40.22

39.50 38.21 38.35 38.30 29.20 39.89 40.42 39.17 39.55 38.94 36.92

38.60 28.47 29.93 36.06 26.18 22.74

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   269

time are not available as no reliable results were obtained due to the strong presence of foam and bubbles, despite having repeated the measurement several times. Eight of the 11 beer types studied show an increase in the value of the real part of the dielectric constant 6 h after opening the can. In this group, with the exception of the nonalcoholic beer, the dielectric constant decreases again after 24 h and the values agree well with the ones measured at 48 h. That could reveal that there is some effect during this first hours that affect the real part of the dielectric constant probably related to some chemical reaction that does not exist (or evolves at lower rates) after 24 h. In addition to the oxidation process, we should also take into account that the probe reaction and its influence decreases after some time, depending on the variety of the beers considered (in some cases it does not disappear completely). This is because the air/gas bubbles are not added to the beer, but are created in the fermentation part of the production process. In the Heineken variety, this reaction seems to finish before the 6 h and thus, the measurement at this time and the one at 24 and 48 h are quite similar. Carlsberg has a totally different tendency: values of the real part of the dielectric constant decrease during the first hours and then increase again. These variations are not high in value, but we cannot assure it has reached stability after 48 h. Maybe this variety requires a deeper study to detect a tendency. These results lead us to a proposal for a specific study on each variety and brand before using extracted data for quality applications in that beer. Anyway, variations of the real part of the dielectric constant as observed (10 units or even more) would lead to important attenuations in the radio waves crossing the beer pipeline in a factory, as already was proposed in Fig. 8.1. This attenuation will reflect in a reduction of the received power in the receiver end of the system, which would allow to report a problem and to immediately detect production failures. The conclusion is that most of the beer varieties and brands follow a common trend as a function of the time after opening the cans, with specific electromagnetic behaviors in terms of degradation in atmospheric contact and, probably, in terms of evolution against production faults. However, the differences in dielectric constant values (and, in some cases also in the trend) make it impossible to express a general rule, which can be valid for all beers. Consequently, the design of the complete quality control system must be of course tailor-made, as well as the determination of threshold values for received power considered within the standard values of a valid product.

270  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

8.4.3  Evolution of Dielectric Constant With Temperature In order to observe the effect of the temperature in the measured real parts of the dielectric constants of beers, Figs. 8.18–8.21 show such data at 10°C and 20°C, 24 h after opening the cans, for the same varieties considered in the previous subsections. There is no big difference in the real part of the dielectric constant with the temperature after 24 h, which is the time when most of the beers seem to be in a stable state. This differs from the situation after opening presented in the previous sections. The temperature, in this case a difference of 10°C, does not produce in the showed traces clear changes in the values and/or the trend as a function of the frequency. The following tables, from Table  8.5 to 8.7, summarize the temperature effects on the real part of the dielectric constant gathered 24 hours after opening, as a basis for analyzing that influence at 15, 17.5, and 20 GHz, respectively. Except for the Carlsberg variety, we can observe that at 15 GHz all measured real parts of the dielectric constant suffered an increase in value as temperature increases. This is also valid for all brands at 17.5 GHz. However, at 20 GHz it depends on the brand of measured beer. Anyway, the differences in terms of the real part of the dielectric constant with frequency are not noticeable once 24 h have passed after opening the beer cans.

Fig. 8.18  Real part of dielectric constant, Estrella Galicia beer, at two temperatures, measured 24 h after opening.

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   271

Fig. 8.19  Real part of dielectric constant, John Smith’s beer, at two temperatures, measured 24 h after opening.

Fig. 8.20  Real part of dielectric constant, Carlsberg beer, at two temperatures, measured 24 h after opening.

272  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

Fig. 8.21  Real part of dielectric constant, Heineken beer, at two temperatures, measured 24 h after opening.

Table 8.5  Gathered Dielectric Constant Values, Measured 24 h After Opening, at 15 GHz Temperature (°C) Brand

10

20

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

48.95 49.61 47.40 46.92 48.15 48.00 49.30 47.72 49.05 48.25 48.71

50.27 49.86 46.95 48.23 48.53 49.06 50.37 48.43 49.84 49.24 51.42

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   273

Table 8.6  Gathered Dielectric Constant Values, Measured 24 h After Opening, at 17.5 GHz Temperature (°C) Brand

10

20

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

45.05 44.89 41.87 41.53 43.19 46.23 48.68 43.67 46.73 45.15 44.33

47.12 45.65 41.18 43.42 43.40 47.63 50.33 43.96 47.84 46.64 48.60

Table 8.7  Gathered Dielectric Constant Values, Measured 24 h After Opening, at 20 GHz Temperature (°C) Brand

10

20

1906 Estrella Galicia Carlsberg Heineken Franziskaner Staropramen Quilmes Krombacher Pilsner Urquell John Smith’s Buckler

38.10 38.64 36.84 35.97 37.96 37.70 39.87 37.68 39.16 38.30 36.24

39.43 39.21 35.26 37.70 37.72 38.85 40.73 37.31 39.76 38.85 40.22

274  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

8.5 Analysis Results of the measurement campaign provided in Section  8.4, give us a number of trends that would help to learn in depth about the electromagnetic behavior of the different considered beers, which is analyzed in this section. The behavior with frequency could be summarized, in all cases, as monotonically increasing as frequency grows in terms of the real part of the dielectric constant; and monotonically decreasing in terms of the imaginary part of it. To observe differences and tendencies in the values of the dielectric constant for the different beers we have to choose specific frequencies of interest. Most of the traces are almost similar at frequencies below approximately 15 GHz, which means that the electromagnetic behavior of beers is more or less the same in that range. Differences appear at frequencies between 15 and 20 GHz and so that this would be the frequency band of interest for conceiving, designing, and implementing future quality control system. As it is not possible to select a unique frequency for detecting the evolution with temperature, or with time after opening the can, valid for all considered beers, the installation of the proposed system would need a previous ad-hoc measurement of dielectric constant for the specific beer under production. The measurements would help in selecting the most suitable frequency for detecting changes in the electromagnetic behavior of the particular beer. Then, transmitter and receiver would be tuned to that exact frequency, and placed one in front of the other with a plastic pipe transporting the liquid just in the middle. This tailor-made quality control system will detect changes in the dielectric constant as they would be reflected in changes in the received power or the measured electric field strength. During the analysis of the results we have also found that for one variety was difficult to observe a clear trend with the parameters considered in this study, so maybe some specific cases would require a deeper effort. We should also remark the importance of calibrating the system with well-known reference samples and adapted to each variety. In the same line of error avoidance, there is also the need of a complete characterization of the potential sources of error, this means, an evaluation of the uncertainty of the measuring system and method. On the other side, the analysis of the probe effect, related to the foam and the bubbles created by the beer once served in a glass, imposes another limitation in the proposal: in order to have accurate and repeatable measurements and to assure a good performance, they must be made without any bubble attached to the probe. Related to this proposal, this only means that the tuning process of the system has to be made with data gathered in stable conditions and not just after opening the can. This last reflection is reinforced by the data

Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS   275

obtained at different times after opening the can: in general, 24 h after opening the cans, the beer presents a perfect and confident condition. And depending on the brand, it can sometimes be earlier. Besides that, temperature seems to introduce some differences in the gathered data, but it does not force changes in the general tendencies.

8.6 Conclusion This chapter extensively outlines the feasibility of a proposal for using the electromagnetic behavior of beers during their production to detect possible failures in real time and therefore to provide a quality control system which tracks the complete production and not only a number of random samples as now are done. We have presented the results of a large measurement campaign with the aim of electromagnetically characterizing different beers. The measurements were performed by using the probe reflection method and we have observed that measurements below 15 GHz provide small differences between different varieties and brands. At frequencies higher than 15 GHz, a differential analysis is possible. All measured data series followed a monotonically decreasing trend, lowering its real part of the permittivity as the frequency increases. However, in some cases peaks were detected over such general trends, with different and sometimes specific motivations. Another consideration to be made is related to a detected limitation in the performance of such a method in specific situations. Immediately after serving a glass of beer, a layer of foam develops on top and bubbles rise from bottom to top, looking for the walls and other objects within the liquid (as the measurement probe itself ) to stitch themselves. The measurement data gathered in the presence of foam and bubbles lack reliability, as it could not have been accurately physically explained. After some time, which depends on the characteristics of each specific beer itself, foam and bubble movements tend to disappear (or at least to slow down drastically) and the reliable electromagnetic behavior of the liquid can be measured. In these conditions, the trend observed for the real part of the dielectric constant is monotonically decreasing as the frequency increases, with some irregularities depending on the variety or type of beer. So that a calm and reposed liquid is needed (waiting a time after opening the can or bottle and pouring it into the glass) to obtain confident measured data. Taking into account such a limitation, we observed the effect of oxidation (maintaining a can open for some time, in normal contact to the atmospheric air) on both the real and imaginary part of the dielectric constant, as a simulation of other actual production problems that

276  Chapter 8  ELECTROMAGNETIC CHARACTERIZATION OF BEERS

could arise in breweries and/or bottleries. This effect is noticeable and moves to the variations in the values of electromagnetic parameters, which reinforces our proposal. We observed that each beer variety and brand presents its own specific electromagnetic behavior in terms of degradation in atmospheric contact and, probably, in terms of evolution against production faults. All the variations observed could lead to changes in the radio waves propagation with the transmission/reception setup proposed at the beginning of this chapter, giving the key to report a problem in the production of beer. The conclusion is that the implementation of a quality control system in breweries and/or bottleries based on electromagnetic waves seems to be possible. However, the design of the system (concerning the oscillator frequency, the transmitted power, and the antennas with their radiation patterns, and so on) must be tailor-made, adapted to the specific characteristics of the beer under production. Moreover, the same is valid for the definition of power thresholds in which the received power could be considered as related to a valid product.

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