Dielectric Spectroscopy for the Non-Destructive Characterization of Biomaterials: Fundamentals, Techniques, and Experimentations

Abstract

This chapter provides an overview of research on the dielectric properties of fresh food and their applications in assessing food quality and freshness. Non-destructive methods, including dielectric techniques such as dielectric spectroscopy (DS) and bioelectrical impedance spectroscopy (BIS), have gained importance in assessing food quality without damaging the products. The importance of external appearances, such as color, size, brightness, and hardness, in determining the freshness of fruits and vegetables is emphasized. Several dielectric techniques, such as impedance, capacitance, and electrical conductivity measurements, are studied to assess quality at distinct stages of the supply chain. These techniques can detect defects, diseases, and mechanical damage and facilitate storage quality control and processing quality evaluation. Accurate measurements and instrumentation advancements are crucial for effectively implementing these techniques. The study of dielectric properties offers promising prospects for evaluating food quality and ensuring freshness. Further research and technological advances in this field can enhance the monitoring and maintaining optimal conditions for fresh produce throughout the food supply chain, reducing food waste and improving consumer satisfaction.

1. Introduction

The rapid advancement of technology has broadened its field of application, improving the daily lives of users and their food. Currently, research is being done on systems that will enhance food production to meet the high demand, in addition to studying its quality and improvements in transport conditions [1]. The current trend is to achieve a healthy diet, to which seasonal fruit and vegetable products significantly contribute [2]. Fresh foods are essential in the human diet due to their high nutrients, vitamins, minerals, and fiber content. The quality of these foods can be evaluated in several ways, considering their external appearances, such as color, size, brightness, smell, and hardness, or internal such as taste, texture, water content, and nutritional content. The external appearance is considered an essential criterion to consider some fruit as “fresh” and implies creating expectations about its internal quality. Manufacturers seek a balance between the appearance and maturity of the fruit in the highest qualities, such as color and hardness for consumption, in addition to achieving maximum economic performance in its marketing, including freshness and durability. In the commercialization phase, durability is essential for its processing and commercialization until it arrives at homes in optimal conditions of internal and external quality.

Before marketing, the transfer of fruits and vegetables must be controlled from the orchard to the distribution warehouses and the shops for sale. Depending on the product and the distance, the product must be moved in refrigerated chambers, where temperature and humidity affect fruits and vegetables differently. If it is not moved at the proper temperature, it causes cold damage, not arriving in the right conditions optimal for shops. Therefore, cold damage is investigated, looking for combinations of products with a wide temperature range to be transported with others of a more demanding range but of more excellent economic value [1].

Horticultural engineering has evolved a lot, and higher quality products are increasingly sought, but at the same time more resistant to transport, production, and storage conditions. More research is being conducted on post-harvest treatment, where innovative technologies make it possible to extend the useful life of the products so that they arrive in the best conditions at the consumers’ tables. Constant product quality monitoring is essential at every stage of the supply chain, and it is crucial to utilize portable tools in industrial and field environments [3].

The tendency to consume high-quality fresh products has motivated the producers and marketers of these products to encourage the study of quality detection methods, optical, acoustic, and mechanical systems have been developed to assess quality based on sensory perceptions such as sight, smell, and touch [4]. Quality parameters such as nutritional value, health, and safety are less tangible for consumers because they require measurements. The results of these measurements are shown to consumers on their labels and are a claim for the most demanding consumers or who seek high-quality products in a healthy diet.

In the last decade, non-destructive methods are advancing evaluating food quality since they can be assessed without destroying the examined product. Within non-destructive methods, the main application is those based on images [5], thermal imaging [6], and spectroscopy [7]. However, many systems are based on the simulation of smell, such as the electronic nose or taste, the electronic tongue [8], and sound as acoustic systems [9].

Among other methods of assessing quality, one can consider the measurement of dielectric properties, which refer to the ability of materials to store and transmit electrical energy in an electrical field [10]. The technique known as dielectric spectroscopy (DS) refers to the interaction between a material and an externally applied electric field. In the case of fruits and vegetables, non-destructive methods such as bioelectrical impedance spectroscopy (BIS) are utilized for quality control purposes [11]. Measurement of dielectric properties offers helpful insights into the quality and freshness of food at different stages of the food chain, as well as the presence of defects or diseases [12], storage quality control, and processing quality evaluation [13]. The measurement of these properties can be done using different dielectric techniques, such as DS (which measures the dielectric response of materials at varying frequencies), electrical impedance (a method used to measure the electrical resistance of materials), capacitance (a technique that measures the electrical energy storage capacity of materials) and electrical conductivity (a technique that assesses the ability of materials to conduct electrical current).

DS determines the dielectric properties utilizing a multifrequency impedance analyzer, which observes the electrical response when current passes through the tissue for certain measurement conditions. The approach entails the assessment of the dielectric constant and dielectric loss, which indicate the material’s ability to store and discharge electrical energy. Electric permittivity encompasses the interaction between electromagnetic waves and substances, as well as the determination of charge density when subjected to an external electric field [14]. The first documentation of dielectric properties for grains was reported by Nelson in 1965. Permittivity depends on the dielectric constant and a loss factor [15], explained in the next section for varied materials ranging from solids to liquids and gases.

At present, the dielectric characteristics of many foods are known, being one of the factors that affect the measure of moisture content. Also, certain mineral substances and organic acids have the potential to undergo dissociation, producing elevated electrolytic conductivity in food.

This review presents an overview of the current research on fresh food’s dielectric properties. It delves into the various dielectric techniques employed to measure these properties and explores their applications in evaluating food quality and freshness.

2. Techniques

2.1 Theory

For decades, many industrial applications have succeeded in taking advantage of the potential discovery of radio frequency to establish empirical correlations between the physical properties in materials and the dielectric properties and develop techniques for the rapid and non-destructive study of physical properties. Measurement, modeling, and applications of the dielectric properties is a unique scientific journey in dielectrics that reveals as many challenges as rewards [16].

By dielectric properties of materials are designated the electric characteristics determining the interaction of materials with electric fields. In this concern, a particular approach defines the interaction of the electric field component of the electromagnetic waves when it comes to food materials, agricultural products, and other dielectric materials.

Being dependent on the composition of material, frequency, and temperature, dielectric properties have been opted as bases for developing sensors. These rapid and nondestructive methods can be valid for assessing the physical properties of materials [17].

In discussing the applications of dielectric properties, some simplified definitions are helpful. Primarily, the propagation of electromagnetic energy in free space at the velocity of light (c) is the fundamental characteristic of all forms of electromagnetic energy. The electromagnetic parts of a material have a markable influence on the speed of propagation (V) of electromagnetic energy in that material, and this velocity (V) is given by Eq. (1), such as the magnetic permeability is given by $\mathrm{\mu }$ and the electric permittivity by $\mathrm{\epsilon }$.

$$\mathrm{V}=\frac{1}{\sqrt{\mathrm{\mu \epsilon }}}$$E1

The same equation defined in free space becomes as follows in Eq. (2), where μ₀ is the vacuum permeability y of free space, and ε₀ is vacuum permittivity.

$$\mathrm{V}=\frac{1}{\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}}$$E2

To interpret the molecular mechanism, the interaction of electromagnetic waves with any material is studied by Maxwell’s equation [18]. The relative complex permittivity (ε*) is represented in Eq. (3), where ε′ and ε″ are commonly called the dielectric constant and loss factor, respectively, and j = √ − 1.

$${\mathrm{\varepsilon }}^{\ast }={\mathrm{\varepsilon }}^{´}-{\mathrm{j\varepsilon }}^{´´}$$E3

Relatively to free space, the relative complex permittivity $\left({\mathrm{\epsilon }}_{\mathrm{r}}^{\ast }\right)$ will be presented as follows in Eq. (4).

$${\mathrm{\epsilon }}_{\mathrm{r}}^{\ast }=\frac{{\mathrm{\epsilon }}_{\mathrm{a}}}{{\mathrm{\epsilon }}_0}=\mathrm{\epsilon }{´}_{\mathrm{r}}-\mathrm{j\epsilon }´{´}_{\mathrm{r}}$$E4

Where ε₀ is 8854 × 10⁻¹² F/m, and $\mathrm{\epsilon }{´}_{\mathrm{r}},$ and $\mathrm{\epsilon }´{´}_{\mathrm{r}}$ are the relative dielectric constant and loss factor respectively; hence, this latter the two quantities of interest that are the real part of the complex permittivity ε referred to as the dielectric constant and the complex part given by (ε referred to as loss factor).

As a material classified as “dielectric”, this reflects its ability to store energy under the application of an external electric field. When subjected to an electric field, the real part, ε′, describes the ability of a material to store energy and influences the electric field distribution and the phase of waves traveling through the material; its value is related to chemical structure and intermolecular interaction [19].

The amount of energy lost or dissipated by the material under an external electric field or in any polarization mechanism generating heat is measured by the loss factor, which is the imaginary part of the permittivity [20]. Nevertheless, this loss factor is always proportional to the amount of conversion of thermal energy in food [21].

The loss factor is always positive and has smaller values than the dielectric constants. The values of dielectric constants are usually higher than the loss factor that, is commonly limited and positive and is returned to the energy dissipation such that a material is lossless when this loss factor is null [22].

Relaxation time (τ) measures the mobility of the molecules (dipoles) in a material. It is the time required for a displaced system aligned in an electric field to return to 1/e of its random equilibrium value. It is also called the time needed for dipoles to become oriented in an electric field.

The alternating electric field is slow enough for the dipoles to keep pace with the field variations at frequencies below the relaxation frequency. Since the polarization can fully develop, the loss factor, εr″, is directly proportional to the frequency. As the frequency increases, εr″ continues to increase, but εr′ decreases due to the phase lag between the dipole alignment and the electric field. Above the relaxation frequency, both εr″ and εr´ decrease as the electric field is too fast to affect the dipole rotation, causing the orientation polarization to disappear.

Figure 1 shows the dielectric properties of distilled water at 25°C [23]. Many liquids and solids have been experimented with through the various models of Debye. Thermal agitation is the most significant in liquids where thermal agitation is maximal, thus producing more considerable losses at higher frequencies [24].

Figure 1.

The frequency-dependent dielectric characteristics of water.

It is crucial to obtain the optimum frequency range at which the material under test will have appropriate dielectric properties to develop a proper heating process based on electromagnetic energy [25]. The interaction between the material and high-frequency electromagnetic energy is crucial for defining the desired properties. Consequently, it is essential to carefully select food and meals to facilitate MW propagation [26, 27, 28].

2.2 Measurement systems

Building measurement systems rely on providing equipment for the abovementioned properties. Through the past years, worldwide experiments have witnessed a variety of technologies implemented for that purpose. The built systems differ according to the material under test and the properties such as resonant frequency and quality factor from another perspective. Knowing the physical dimensions of the material and measuring the transmission and/or reflection can characterize the permittivity and permeability. At low frequencies, impedance analyzers and LCR were considerably used by simulating the material with an AC source and monitoring the capacitance and dissipation factors [27]. The impedance data analysis starts with two complex yet critical steps. The first stage involves investigating the physical circuit and estimating the necessary equations. The mathematical methods are evaluated at a second stage, allowing the model parameters to be extracted [29].

Depending on the material’s nature, the fixture’s choice was made so that the sample holder would fit the solid, liquid, powder, or gaseous nature of the tested material, along with appropriate software modeling the interaction of the fixture with the said material. The impedance measurement method is commonly applied by applying a current across electrodes and observing the voltage. Another widespread practice for many applications is the self-balancing bridges developed by Agilent. Such bridges under a higher voltage can give a better signal-to-noise ratio higher than 10–300 mV. Such applications are robust, simple, and more practical than the old manual bridges. With advances in experiments comes the urge for more complex systems to give accurate results and cover more material. This led to a fast improvement in techniques. Moreover, high microwave frequencies applied during experiments were to produce radiation loss, especially considering the expensive design of a microwave network analyzer. Throughout the past decades, a variety of equipment has been accessible in the market for impedance measurements. However, network analyzers emerged as the most widely employed option due to their capacity to handle large electrode systems and cover a wide frequency range. Notably, vector network analyzers like PNS, PNA-L, ENA, and ENA-L are capable of sweeping high-frequency stimulus responses, ranging from 300 kHz to 110 GHz or even 324 GHz. Impedance and scalar network analyzers [30] are known for their cost-effectiveness compared to Vector Network Analyzers (VNAs). However, their usage is restricted to the frequency range associated with the α and β dispersions.

The experimental setup involves utilizing the open-ended probe technique, which includes a coaxial probe and dedicated software, to conduct a range of experiments using the network analyzer. Keysight Technologies came out with spectrum and network analyzers with high measurement integrity.

The S11 reflection parameter determines the surface of contact between the probe and the tested material using the VNA that is meant to be a probe with specialized software. The phase shift corresponding to the change in the magnitude ratio is called S11. The illustrations of reflection and transmission probes are highlighted in Figure 2. Some exemplary models are incorporated into the supplied software to collect the real and imaginary parts of the dielectric permittivity.

Figure 2.

Reflection and transmission types of probes [31].

For less expensive solutions, the 85070E probe is replaced with another subminiature version (SMA) coaxial connector that can perform correctly at some good frequency ranges [32].

Conversely, coaxial transmission lines or waveguides are employed to evaluate the S21 parameter, which represents the ratio between the transmitted signal through the material under test and the incident signal. The magnetic permeability and dielectric permittivity analysis determine the S21 parameter [33].

In the reflective operating mode, the VNA produces a sinusoidal signal spanning a wide frequency range from 20 kHz to 8 GHz. The incident and reflected signal separation happen at this point [34]. From the reflected signal of the probe, dielectric properties obtained are chargeable to derive the phase and the amplitude. Theoretical models are also used to derive the frequency spectra for permittivity factors ε′(f) and ε″(f). Moreover, some elements should be counted as a distribution parameter system for modeling purposes, especially since the real part of ε*(f) requires applying high frequencies [35].

2.3 Measuring techniques

The test fixture was selected depending on the nature of the material to be experimented with. Some significant features behind the considered elected option are the frequency range, the required accuracy level, the measurement volume, and the experiments’ budget [31]. Consequent to the dielectric material under test, the design of the sample holder, the measurement system, and the technique was designated.

Figure 3 illustrates the main methodologies employed in the agricultural and food industry in relation to frequency.

Figure 3.

Methods for characterizing dielectric properties in the agri-food sector across different frequency ranges [31].

The coaxial probe method is a simple and convenient method for liquids and semi-powders where the material is measured by emerging the probe into the liquid and such a system consists of the probe, the network analyzer, the software that is included in the 85070E probe kit. No external computer is needed for the PNA generations. At the same time, it is not the case for other analyzers that rely on the 82357A USB (universal serial bus) to GPIB (General Purpose Interface Bus) interface. A common technique is the Agilent 85070E open coaxial probe, which has been used immensely in biological materials, liquids, and other mixtures, where the permittivity can be derived at a frequency ranging from 0.2 to 50 GHz [31]. The vector network analyzer accompanying such experiments is the popular Agilent 87070 [31]. This system is premeditated to be highly priced but was extensively used in the industry, being one of the most commercial measurement systems adapted for solids and liquids.

Most of the food industry experiments in the last decade were executed using the 85070E Dielectric Probe Kit, including probe and software. The complete system used in process analytic technologies is based on a network analyzer measuring the dielectric parameters of the material (dielectric constant, loss factor, loss tangent, and Cole-Cole). It is connected to the Vector network analyzer Agilent N9912A.

Currently, a commercially available THz-TDS system (TeraView TPS 4000) is employed, which incorporates a pair of GaAs photoconductive antennas (emitter and receiver). This system utilizes a femtosecond laser module to generate laser pulses that are divided into two separate beams (pump and probe) using a beam splitter [30].

The transmission line technique is primarily employed for machinable solids that can be inserted into the transmission line. This broadband method is limited in frequency coverage only by the size of the sample holder.

A technique that requires no contact is the free space approach. This method relies on antennas to focus microwave energy through a slab of the material under elevated temperature, mainly used at millimeter wave frequency.

Within a specific frequency range, the cavity perturbation technique has been widely employed for liquid samples at both low and high temperatures. The resonant cavities at a specified frequency are influenced by the material sample, allowing for the calculation of permittivity. Notable options include Keysight’s 85072A 10 GHz split-cylinder resonator and split-post dielectric resonators [27].

The fundamental inexpensive technique widely applied is the parallel plate capacitor method, where the material is placed between the electrodes forming a capacitor. An impedance analyzer or even an LCR meter can be adopted for a typical system. Another method relies on inductance measurement to derive the permeability where the material under test is wrapped with a wire, and inductance is evaluated with respect to the ends of the wire. For this technique, the Keysight 16454A magnetic material test fixture is available. When a toroidal core is put in, this probe does not flux and thus is considered ideal for single-turn inductors. Reflection-based measurements offer a versatile approach to investigate solids and liquids over a broad frequency spectrum. Alternatively, by employing the Fourier transform of the sensor’s reflectogram resulting from the applied forcing pulse, the frequency spectrum of the complex dielectric permittivity can be obtained. To construct a dielectric permittivity sensor, two or three stainless steel rods can be strategically positioned within the material under examination, forming a segment of a parallel waveguide.

Graphical representation depicted in Figure 4 illustrates the system’s measurement suite, classified according to the material type being tested and the frequency range employed for measurements.

Figure 4.

Measurement apparatus used for the measurement of materials [12].

Some genuine dielectric properties experiments were recently witnessed at the Institute of Agrophysics in Lublin. A web server was used to store data concerning the soil moisture, temperature, and salinity of the soil. Such an implementation has some advantages in lowering power consumption in the long term [36].

While extensive scientific research on dielectric properties explores the testing of various objects, achieving accurate characterization across a wide frequency range through dielectric spectroscopy remains challenging. Therefore, improving dielectric measurements and developing advanced sensors can expand the field’s investigation scope. Additionally, designing equipment for radiofrequency and MW dielectric heating applications holds significant importance [37].

3. Applications

Although it has extended to medicine and material engineering, the main field of research for dielectric characterization has been the horticultural field. The number of applications of these techniques is extensive, and they are usually classified according to the technique used, appearing in two large divisions, dielectric characterization studies and BIS measurement studies [38].

The quality factors of fruits and vegetables that are notably worth highlighting are moisture content, maturity defects, and imperfections. All these factors help harvest, classify, and package the products, improving their uniformity and quality [39]. Conducting this type of measurement using simple and innocuous techniques has been a subject of constant development in recent years, with dielectric characterization being a handy tool in food engineering and technology. Initially, the quality studies contemplated the properties of fruit and vegetables [40]. Subsequently, they have spread to other tissues of animal origin, such as meat [41] or fish [42].

The dielectric properties correlate with the quality parameters of the objects, as well as with their physical characteristics and chemical properties [12]. Quality parameters encompass firmness, pH, soluble solids, moisture content, and electrical conductivity. It is essential to ascertain the ideal frequency range that yields the desired dielectric properties, the depth of penetration reached in a particular food, and its variability with frequency or temperature to characterize varied materials.

Table 1 summarizes the main advances established in studies and applications of dielectric characterization in recent years.

Table 1.

Dielectric characterization applications and studies in recent years (1949–2023).

Electrical impedance spectroscopy is a method employed to evaluate the electrical characteristics of substances through the application of alternating electrical signals at various frequencies, followed by the measurement of the resulting response signals. This technique is safe, non-invasive, fast, portable, low cost, and easy to use, thereby holding significant potential for monitoring quality processes within the food industry [107].

The structure and composition of the materials determine the dielectric properties, while the frequency and temperature are related to the maturity of the agricultural product. Furthermore, ionic components exert notable repercussions on dielectric properties. Density is one more crucial element affecting the interaction between the electromagnetic field and the mass involved. The storage time of agricultural products also plays a vital role in their dielectric properties, as maturation processes occurring during storage can affect them.

The focal point of this investigation involves examining the interrelationships between the dielectric properties and additional chemical and physical attributes of the analyzed substance. The utilization of rapid and non-destructive quality assessment techniques for agricultural products is of utmost importance when evaluating dielectric spectroscopy methodologies in the field of agrophysics [9]. These techniques offer a clean and non-destructive approach that is imperative for effective management systems. They can provide a means to simultaneously examine changes in fruit tissue during ripening and intracellular and extracellular behaviors. Impedance spectroscopy has succeeded in a very high-frequency range, reaching up to 6.25 MHz.

Among the main BIS applications, it is worth highlighting the determination of freshness in fruits of different kinds, such as apples [108] and bananas [109]. As well as the measure of aging [110] or the ripening of apples [111], mangos [112], citrus [113] and pears [114], and other fruits. This technique can also build an immunosensor that looks for virus selection in products [115] and discriminates between healthy and infected samples [116]. Determining acidity in citrus using dielectric impedance has been particularly interesting in recent years as it is a non-destructive method [117]. The effects of temperature on kakis [118], nectarines [119], and their content of soluble solids [120] have been established.

BIS can monitor the concentration of elements such as nitrogen in plants [121]. Therefore, it is considered a non-destructive technique for estimating the impacts of other ions on plant tissue, achieving higher crop yields. Additionally, BIS enables monitoring of plant conditions, including early developmental stages, and facilitates quality control of the final products.

Impedance measurement has recently been employed to investigate the isolated effect of viruses [122]. The findings reveal that the resistance of the leaves of the infected plant is lower than that of the leaves of the healthy plants. Therefore, the impedance measurement is a fast and intelligent method to diagnose the existence of diseases in plants [123], the relaxation processes [10], or the diffusion and interaction of biomolecules with water [124].

BIS is an intriguing method to assess the condition or composition of human organs and different types of biological tissues in vivo [125]. Given the significance of food disinfection for public health and safety, numerous related applications exist. An example of this is the development of an innovative methodology for the real-time detection of the amount of disinfectant in a sample by determining the characteristic frequencies and the dielectric permittivity spectra within a specific frequency range. This method establishes a correlation between the characteristic frequencies and disinfectant concentration with acceptable precision [126].

Among postharvest applications, radiofrequency (RF) and microwave (MW) heating affects the dielectric properties of materials. Permittivity and moisture content are closely correlated in food with high water content. It is essential to find the dielectric properties of partially frozen materials to assess the heating rates and uniformity during microwave (MW) thawing [51]. Water in the microwaved material can increase heating efficiency, as water is a strong microwave absorber that converts microwave energy into heat. However, when the moisture content is too high, there can be a negative effect on heating efficiency, as the water can absorb so much energy that thermal overload occurs in the material, leading to degradation or even carbonization. Therefore, the relationship between moisture content and microwave heating efficiency is intricate and influenced by many factors, such as the composition and structure of the material, the frequency and intensity of microwaves, and the duration of heating. These properties are essential to detect processing conditions or food quality [127].

Dielectric heating has shown its applicability for fruit drying, often achieving quickly and providing higher quality dried products with low energy consumption [128]. Through RF and dielectric heating applied in the drying of agricultural products, it improves thermal efficiency, the quality of the final product and reduces drying time compared to conventional drying. Using RF and dielectric heating, food can be protected from insects inside the nuts [129]. The dielectric heating methodology can be used to reduce the necessary times of heat treatment that entail the control of pests present in food after harvest [130].

The pasteurization and sterilization of fruit juice concentrate are processes where the proper choice of frequency is essential to obtain a uniform pasteurization [131].

In this context, considering the significance of the characterization process of fruits and vegetables from harvest to cold storage, along with the accelerated technological advancements and the availability of mathematical methods, as well as the numerous research studies conducted worldwide, there is a significant demand to review the existing electrical characterization methods in the horticultural field.

4. Conclusions

The robust association between the dielectric response and the chemical and physical composition of agro-physical materials enables the identification of specific properties and quality indicators related to them. One of the primary applications of DS and BIS is the assessment of fruit ripeness and quality. As fresh produce ripens, its dielectric properties change, utilized as indicators of ripeness and quality. The detection of dielectric properties of fresh fruit can also be used to detect disease and mechanical damage. These techniques can also be used to determine the quality of the products during storage, and therefore their control prevents loss of quality and freshness. In addition, they can be used to evaluate product quality during processing to meet quality and freshness standards since techniques such as pasteurization or freezing affect their dielectric properties.

The development of particular and advanced techniques and instrumentation improves the capabilities of these non-destructive characterization techniques, as they are valuable for environmental and economic scientific evaluation within the food industry. Accuracy is indispensable in the measurement of dielectric and bioelectrical impedance spectroscopy since some essential characteristics must be maintained during the execution of the measurement. Factors such as the probe’s altitude, the material’s manufacture, and the quality and size of the beaker used must be considered in the experimentation and measurement. In addition, the amount of liquid under the test in which the probe is fused, the agreement with the amount of water under which the calibration has been performed, and an essential parameter such as the constant depth of the measurement must be considered. Advancements in understanding the dielectric characteristics of materials have led to further progress in the sensing field, enabling the monitoring of fresh produce’s properties. The objective is to maintain the optimal conditions of fresh produce from production and harvesting to consumption.

Despite the antiquity of applying these non-destructive techniques, they are still experimental techniques under development, with a solid technological impact and providing numerous novelties. The traditional field of research has been the fruit and vegetable sector, although nowadays, the study has been extended to different areas, thus establishing new challenges and future perspectives.