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Review Article | Open Access
Claudia Hernández-Aguilar, Arturo Domínguez-Pacheco, Alfredo Cruz-Orea, Rumen Ivanov, "Photoacoustic Spectroscopy in the Optical Characterization of Foodstuff: A Review", Journal of Spectroscopy, vol. 2019, Article ID 5920948, 34 pages, 2019. https://doi.org/10.1155/2019/5920948
Photoacoustic Spectroscopy in the Optical Characterization of Foodstuff: A Review
In this review, the application of the photoacustic spectroscopy (PAS) is presented as an option to evaluate the quality of food. This technique is a type of spectroscopy based on photothermal phenomena, which allow spectroscopic studies. According to the literature review, it was found that its application is increasing in several countries. Spectroscopic studies carried out by employing PAS in the food industry include, among others, fruit, vegetables, condiments, grains, legumes, flours, “tortillas,” milk, water, eggs, etc. Additionally, this technique has been used to evaluate adulterated, irradiated, and contaminated food and so on. The literature review has shown the applicability of PAS to one of the problems of the real world, i.e., food quality assessment. Therefore, PAS can contribute in the future with a wide potential for new applications in the food agroindustry.
One of the problems worldwide is the quality and quantity of food. In developing countries, this is even more evident, causing several chronic diseases such as cancer and malnutrition, according to Hernández et al. . Therefore, the development of technologies to improve global food production is necessary since one of the main challenges of our time is to feed a growing worldwide population [2–6]. In this order of ideas, it is also mandatory to develop technologies that evaluate food quality, before the direct impact that they have on population’s health and consequently on their quality of life. Several authors and public health professionals indicate a relationship between dietary behaviour and the food quality associated with the risk of some cancers and other existing chronic diseases [7, 8]. In this sense, the quality of the food is very important; in the case of cancer diseases, a close relationship between the diet and the different types of cancers has been reported [9–11]. Cancer, among other factors, could be due to the intake of compounds in food that initiate or promote it. The food and its substances which are consumed provide the nutritional support for an organism and help for the disease prevention. However, sometimes they increase chemical substances, and instead of helping, they damage the organism . This highlights the need for a greater level of food control: equal in quantity and quality (i.e., sometimes foodstuff contains both substances that harm and benefit human health. That is to say, to evaluate food in order to avoid consuming contaminated or degraded food (chemically or biologically) and to promote the consumption of food rich in phytonutrients (increasing the proposals for food integrated with superfood and/or fibre), etc. This, among other aspects, is relevant to the prevention of diseases . In food production chain, it is known that contamination and/or degradation can occur at any stage due to contaminants: environmental, agricultural, or incorporated during some agroindustrial process or storage . On the one hand, it is essential to be aware of not consuming food that exceeds allowed limits of mycotoxins, nitrates, and nitrites, harmful fats, addition of preservatives, dyes, or sugar solutions. On the other hand, it is essential to consume food that provides health benefits, such as super-food, cholesterol free, rich source of proteins, minerals, iron, etc. Food rich in phytochemical may play an important role in the reduction of mortality. For the aforementioned facts, the need of a rapid and reliable quantification of compounds in food that contains disease-preventing constituents or food constituents that cause them is urgent, as it is recognized by the food processing industry .
Then, the development of technologies that support the evaluation of food quality every day becomes more relevant, due to the increase of diseases. Among the technologies for food analysis and determination of compounds, the photothermal techniques stand out. In particular, photoacoustic spectroscopy (PAS) is considered by some authors as a “green” technology for the food analysis , i.e., a method with less chemical waste and a minimal sample amount and a nondestructive technique . PAS has some additional characteristics such as the fact that it does not require extraction or sample preparation and it does not use solvents, among others . The reduction or elimination of the use of solvents is very important. For example, in the process of manufacturing, the use and disposal of chemical products and many toxic materials that are dangerous to humans and to the environment are frequent. In fact, these techniques are promising because they can be carried out by nondestructive analysis and without the use of solvents .
Currently, PAS technique, thanks to the technological advances, could be a convenient option to be incorporated in the agrofood industry, for example, in food quality assessment systems, with diverse specific applications (according to production systems and specific food of each case). PAS technique allows to obtain optical qualities of food, depending on its colour, which is the most useful parameter in the agrofood industry since quality and food flavours are closely associated with its colour . Therefore, our objective is to perform a literature review of PAS applications in the food characterization from its origin to recent advances. In this way, it will be possible to know the current state, what has been done and what still needs to be done to reach the application of it in the real world. The PAS experimental setup continues to be optimized and focused on the specific problems of the real world where it could serve as a supportive technique, in order to improve and have attainable techniques for the evaluation of food quality and their respective control in the production process. It is one of the great worries of humanity, both to increase the food production and to take care of its quality, being this a key element in the development and life quality of societies.
1.1. Era before Photoacoustic Spectroscopy in Agriculture and Food
Isaac Newton in 1666, using a prism, observed and recorded the dispersion of white (visible) light into its constituent colours  to describe the colours of the rainbow. He used the word “spectrum” for the first time in history. More than 100 years later, in 1802, Hyde Wollaston expanded Newton’s earlier observation by showing that sunlight possesses discrete bands of light, rather than a continuous spectrum. Wollaston became one of the most famous scientists for his observations of dark lines in the solar spectrum, which eventually led to the discovery of the Sun elements. In 1814, Fraunhofer discovered over 500 bands of sunlight, afterward called “Fraunhofer lines.” In 1859, Kirchoff and Bunsen invented the spectroscope , and they were the ones who developed the chemical analysis by using spectral lines [21, 22].
1.2. Photoacoustic Spectroscopy History
The photoacoustic (PA) effect was discovered, according to Rosencwaig [23, 24], by Tyndall, Röntgen, and Alexander Graham Bell, in 1881. Bell was working together with Charles Summer Tainter in the photophone. Bell discovered that selenium (and other solid materials) emits a sound when illuminated by a modulated light, which was achieved by passing it through a rotating disk with holes. Bell, using the spectrophotometer, discovered that the emitted sound intensity depends on the wavelength or colour of the incident light and that therefore the effect should be attributed to an optical absorption process .
Fifty years after its discovery, the PA effect was used in gas studies. It has ever since become a well-established technique for gas analysis and was well understood  with some applications also in environmental and food areas. However, the PA’s effect in solids was apparently ignored for 90 years until 1973, when Rosencwaig began his study of the PA effect in solids. Probably, this delay was due to the lack of sensitive sound detectors and high-power light sources .
The first photoacoustic spectra obtained by Rosencwaig were specifically of carbon-black, powder of Cr2O3 (normalized), a Cr2O3 crystal, rhodamine-B in a glycerol solution, and rhodamine-B powder . Photoacoustic spectroscopy as a new tool for solid research was presented by Rosencwaig . Since that time, he pointed out the main advantages of photoacoustic spectroscopy, paraphrasing him: “The principal advantage of photoacoustic spectroscopy is that it enables to obtain similar spectra on any type of solid or semisolid material, whether it be crystalline, powder, amorphous, smear, gel, etc. Furthermore, since only the absorbed light is converted to sound, light scattering (a very serious problem when dealing with many solid materials by conventional spectroscopic techniques) presents no difficulties in PAS.” In this sense, the PAS applications were divided under three main headings: bulk, surface, and de-excitation studies.
Also, pioneer applications of PAS in biology were made by Rosencwaig . He obtained the photoacoustic spectra of smears of whole blood, of red blood cells freed from plasma, and of haemoglobin extracted from red blood cells, using the spectral region from 200 to 800 nm. Also, PAS spectra of guinea pig epidermis (250–650 nm) in different conditions were obtained. He also reported a block diagram of single-beam photoacoustic spectrometer with digital data acquisition, integrated by: Xe Lamp, monochromator, chopper, photoacoustic cell, lock-in amplifier, voltage frequency converter, and multichannel analyzer. The first commercial spectrometer (Model 6001) was manufactured in 1980 by Princeton Applied Research Corporation [26, 28].
Other results were obtained with dried solids containing several other hemoproteins, including both soluble ones, such as cytochrome c and insoluble or membrane-bound ones such as cytochrome P-450. Further experiments showed that it is possible to identify absorbing substances (including some drugs) in dried urine samples (e.g., drops of urine over the filter paper).
Regarding the area of the agrofood industry, the first photoacoustic spectra in plants were obtained in flowers by Harshbarger and Robin  among others.
1.3. Applications of PAS
Spectroscopy is the study of the interaction of electromagnetic radiation with atoms and molecules to provide qualitative and quantitative chemical and physical (structural) information, that is contained within the wavelength or frequency spectrum of energy that is either absorbed or emitted . According to Sunandana ; Photoacoustic spectroscopy (PAS), the oldest form of photothermal techniques, is a type of spectroscopy and its name “photoacoustic” (PA) generally implies a particular technique or mechanism of detecting and measuring the optical absorption of opaque and diffuse materials, among others. The basic principle of photothermal spectroscopy is the detection of heat produced in a sample due to nonradiative de-excitation processes resulting from the absorption of intensity-modulated light (wave of pulsed light) by the sample. Thus, according to its basic principle, the PAS has been applied in Biology, Biophysics, Physics, Medicine, and in the Agrofood areas , rescuing an old technology for today’s needs.
Bicanic  mentioned that PAS is a sort of spectroscopy, nondestructive based on photothermal phenomena, which allows spectroscopic studies. The basic configuration uses Xe lamps, mainly in the UV-VIS range. This conventional configuration has been applied to the foodstuff analysis (obtaining PA spectra, as a function of wavelength) including plants, seeds, etc. Among the foodstuff that have been investigated by using PAS are grains and legumes (Zea mays L., Triticum, Hordeum vulgare, Phaseolus vulgaris L., and coffee), vegetables (spinach, lettuce, Raphanus sativus L., Solanum lycopersicum L., and Capsicum annuum), marine vegetables (algae and phytoplankton), fruit (açai, cupuaçu, Brazil nut, persimmon, mango, and strawberries), other liquids or semiliquid food (e.g., milk, water, juice, mustard, and ketchup), flours (maize, wheat, soybeans, peas, white bread flour, and rye), “tortillas” (maize (white and blue), wheat flour (integral or not integral), maize and “nopal,” linseed and “nopal,” etc.), condiments (turmeric and “chile pasilla”), powder (gelatins, curry, and cacao), food with coloring additives, etc. Furthermore, adulterated food and fortified food, among others, have been analysed by using PAS technique.
The first PA spectra (in plants or food) were obtained in black-eyed susan petals, red rose petals, green leaf, and chloroplast of lettuce, marine algae, and spinach [27, 28, 29, 33, 34], among others. Harshbarger and Robin  reported photoacoustic spectra (PA or optoacoustic) of flower petals. With regard to susan blackeyed petals, the authors obtained an optical absorbance spectral band corresponding to carotenoids and another band in the ultraviolet region, related to the content of flavonol glucosides. The photoacoustic spectrum of a rose petal had two maximums, at 530 and 340 nm; the first maxima is due to cyanine absorbance in the flower, and the second one must be due to some other ultraviolet-absorbing compound in the petal.
Meanwhile, Rosencwaig showed photoacoustic spectrum of an intact green leaf with all the optical characteristics of leaf chloroplasts, including Soret’s peak (420 nm), carotenoids (450–550 nm), and chlorophylls (600–700 nm) bands. He points out that PAS can be used to observe secondary metabolites. Species of air-dried marine algae were also evaluated by Rosencwaig and Hall . The authors showed that PAS can be used to estimate the amount of certain metabolites, and they also suggested that PAS could reduce the amount of material required for the screening of such substances (since extraction procedures generally require more material) and that it can greatly reduce the time required for the identification of plant components. Adams et al.  studied spinach leaf, where he demonstrated that the major absorbing components in the spinach were the chlorophylls. The chlorophylls are similar to the hemoproteins; they contain a porphyrin ring, this being chelated to magnesium at the ring centre. Then, the technique allowed it to be useful to determine quickly and easily the spinach components, directly and only using a small piece of spinach (10 mm), in the spectral region from 250 to 700 nm, finding spectral peaks at 450 and 650 nm. Other photoacoustic spectra were also obtained, in the initial era of photoacoustic applications for this purpose, in cotyledons, Raphanus pigments, Tradescantia leaves, etc. [35–37].
Since the initial PAS applications in agriculture and food until now, different spectral regions have been used, from ultraviolet to far infrared, including UV (200–400 nm), visible light (400–700 nm), and near infrared radiation (750–1100 nm). Also, it is important to take into account the lamp power, and there are several studies that indicate that the optimal Xe lamp power ranges from 300 to 1600 W.
According to the present review on PAS applications in food and plants, from the PA spectrum obtained by PAS, it is possible to determine concentrations or presence of compounds: rutin, red beet (in case of adulterated food), flavonoids and flavonols, carotenoids (lycopene, capsanthin, capsorubin, carotene, zeaxanthin, cryptoxanthin, lutein, etc.), basic amino acids (tryptophan, lysine, leucine, phenylalanine, etc.), anthocyanins, peroxide, and lead tetraoxide, among others. Also, by using PAS, it is possible to detect changes in seeds due to induced radiation effects, use of dyes, differences in sanitary qualities, adulterated food, etc. In this sense, for some researchers, PAS is considered as an analytical method.
1.4. PAS Applications in Food and Agrofood Industry
One of the industries which could be benefited by the use of PAS technology would be the milk industry. Martel et al.  carried out measurements by PAS of milk products. They analysed whole milk, 3.25% fat, skim milk, part skim, milk 2% fat, mild cheddar cheese, aged cheddar cheese, plain yogurt, and strawberry-flavoured yogurt drink. Their obtained spectra were in the ultraviolet region. They found a strong absorption peak at 280 nm for all products. For cheese samples, they observed in the spectra a tail, corresponding to fat presence, from 250 to 260 nm. Photoacoustic signal increases when protein concentration increases; the authors relate the UV absorbance band with aromatic amino acids (tryptophan, tyrosine, and phenylalanine), as a measure of protein content. They demonstrated the applicability of PAS to study different milk products, highlighting their utility for the milk industry.
PA spectra of tablets, made out of lyophilized raw milk, showed an absorption peak at 280 nm, corresponding to the absorption of proteins and a smaller absorbance band in the visible (400–500 nm) that might be assigned to milk carotenoids. When the tablets were heated, they gradually turn brown, which contributed to the changes in the PA spectra, appearing to a new band around 335 nm as a consequence of the Maillard reactions. The spectra became broader, to the red side of the spectrum. This could be the sign of many other reactions occurring in the sample according to Nsoukpog-Kossi et al. , demonstrating another possible utility of the photoacoustic technique.
Another use of PAS in the milk industry has been the possibility to measure different powdered milk protein concentrates, enriched with Fe in the form of ferrogluconate at different concentrations. Dóka et al.  obtained PA spectra, in these powdered samples, as a function of ferrogluconate concentration, obtaining an increase in the photoacoustic signal in the UV spectral region. The peaks, at 348, 380, and 552 nm, varied depending on the Fe concentration, resulting in a nonlinear relationship between the ferrogluconate content and the PA signal. In this way, the authors demonstrated that PAS measurements (in the UV-visible range) on milk protein concentrates are capable of determining the Fe content in ferrogluconate form. This demonstrates another possible application of PAS. As with the other applications to detect adulterated milk, it has been proven useful, for example, to detect skimmed milk adulterated with whey powder, when analyzing PA spectra at 370 nm wavelength .
PAS application in milk analysis was also reported in other studies, for example, milk (fresh and oxidized) was evaluated by using PAS. In these investigations, Dóka et al.  used fresh whole milk exposed to UV-C radiation and heat. The PA spectra, obtained by PAS, encompassed the spectral region from 200 to 500 nm. It was reported, absorption peaks at 290 nm (for all evaluated cases), which is associated with the presence of aromatic amino acids in the milk powders. Spectral changes, induced by the accelerated oxidative treatment, were detectable in the 320–360 nm absorbance band (absorbance changes in this range are due to the reaction of aldehydes with a variety of amino compounds). The oxidation of whole milk powder and browning processes were mutually interrelated (i.e., if the oxidation took place, then the color of the powder would turn brown). The authors recommended PAS as a method for routine and rapid assessment of peroxide values in oxidized whole milk powder.
Another PAS application, useful in foodstuff area, is in the assessing of induced radiation effects. For example, irradiated egg powders were evaluated by PAS, finding two peaks, corresponding to the absorbance maxima, in the optical spectrum. One centered at 275 nm, which is related with the aromatic amino acids content in the sample. While the other peak, centered at 480 nm, is related to the presence of carotenoids. It is interesting that PA signal at 480 nm suggests a carotenoid decomposition due to the irradiation . In summary, the foodstuff irradiation processes is another potential area for PAS applications.
On the other hand, the usefulness of PAS has been demonstrated to identify adulterated samples with lead tetraoxide (also called minium or red lead). Dóka et al.  obtained the PA spectra of pure paprika, red lead, and all adulterated samples, in the wavelength range from 320 to 700 nm. The normalised PA signal in a wavelength range from 600 to 700 nm was generally lower; a weak signal was observed at 670 nm. The PA signal from pure red lead was substantially larger than those obtained from adulterated samples. The PA spectra of the four adulterated samples show a peak at 545 nm. In this case, the potential of PAS, as a candidate method for rapid detection of gross amounts of red lead (Pb304) adulterant, in a ground sweet red paprika, was demonstrated. Although the authors recognize that the performance of this method was undoubtedly inferior to that of advanced methods, the PAS method is very practical and rapid in routine situations.
Other food sample types studied by PAS have been reported by Bicanic et al. , who mentioned that PAS technique could be used to detect red beet, added as a colorant to tomato ketchup. The associated changes of colour, resulting in changes of optical absorbance, were monitored in the 500 nm region, corresponding to the absorbance maxima of lycopene. Also, Bicanic  indicates that the argon laser line at 514 nm has been used for lycopene measurements because there is a high absorbance of lycopene and low interference of betacarotene. It is noteworthy that Bicanic (1943–2018) made a notable contribution to photoacoustic and photothermal science with numerous applications in agriculture, environmental science, and food quality, among other issues .
1.5. Grains and Legumes
PA spectra, as a function of wavelength, allow to obtain information about the sample. Also, it is possible to characterize samples regarding its atomic or molecular composition according to De Oliveira et al. . In the case of corn grains, Dominguez et al.  obtained the PA spectrum of maize, in the 300–800 nm wavelength range. They found absorbance bands associated with different natural pigments. This group used white, yellow, and blue maize; in the case of white maize seed, a broad absorbance band was observed in the UV region, from 300 to 400 nm, with a signal peak around 360 nm. While for the yellow and blue maize seeds, the band of PA signal decays around 435 nm. This band could be due to the presence of flavonoids and flavonols. In the case of yellow and blue maize seeds, they have an absorbance band ranging from 470 to 540 nm, being this band associated with the presence of carotenoids. Specifically, for blue maize seed, an absorbance spectrum ranging from 500–690 nm was observed, which is due to the presence of anthocyanins.
Another characteristic of corn seed is its structure type, crystalline or floury. From the photoacoustic signal, Hernández-Aguilar et al.  found the optical absorption coefficient (β) and optical penetration length (lβ), as a function of wavelength. The floury seed variety had a higher β value at 650 nm. In this sense, the authors showed that by means of the optical absorption coefficient, differences between maize varieties of different structures are observed. The PA signal amplitude is higher for floury seeds. Similarly, significant statistical differences were found in the optical absorption coefficient spectra of white maize seeds (of different white), with an absorbance band ranging from 325 to 425 nm wavelength. Also, other authors found differences between the spectra of the first derivative obtained from the β values . Other researches, such as De Oliveira et al. , have indicated that the PA signal amplitude is directly proportional to the concentration of absorbing analytes, where analyte is a component (element, compound, or ion) of analytical interest on a sample. According to Dóka et al. , PAS could be an analytic technique and also a fast and relatively cheap technique.
Other authors have used mathematical analysis on the PA signals, such as the first and second derivatives or mobile standard deviation. This has allowed to distinguish better the maximum peaks of maize grains with different pigmentations, identifying differences of the corn seeds . The use of derivatives in spectra enhances the identification of differences among spectra, resolves overlapping bands, and especially improves the detectability of weaker spectral shoulders. In this sense, PAS could be used in quantitative analyses of compounds . Also, photoacoustic spectroscopy is useful to study dyed samples, not only with natural pigments.
Other studies have pointed out the role of PAS: by using different light modulation frequencies, it is possible to explore different seed depths, e.g., Hernández-Aguilar et al.  obtained the PA spectra of maize seeds (Zea mays L.) at different frequencies (17, 30, and 50 Hz). They compared these spectra with the ones obtained from the phase-resolved method, used to separate the spectra of the seed pericarp and endosperm. Also, photoacoustic spectra, of separate structural components of the seed, were obtained (pericarp, aleuronal layer, and endosperm) and compared with those obtained by the phase-resolved method. The authors indicated that the absorbance band from 550 to 750 nm is due to the anthocyanins in the aleurone layer. So, the PAS technique has a potential for depth profile analysis on complex specimens with different structural components and also, through the absorbance bands, to determine the associated components.
Moreover, PAS has been applied to study wheat, barley, and beans among other grains and legumes, where from PA spectra, it is possible to analyse the differences of the characteristic spectra obtained among the evaluated materials. For example, Dóka et al.  by using PAS in buckwheat found PA spectra, as a function of wavelength and observed two absorbance peaks, at 275 and 378 nm, related to the protein content and rutin, respectively. PA signal appears to be proportional to the rutin content of the samples across the entire wavelength range. Thus, the authors reported that UV-PAS could be an analytical tool for rapid and simple quantification of rutin in buckwheat, and they found a decrease of the time required for the analysis of buckwheat samples when a calibrated curve, of known rutin content, is used.
Photoacoustic spectrum decreases as a function of the frequency, and differences are obtained in the spectra of the deteriorated and nondeteriorated grains. The authors reported lower PA signal in the young seeds when compared with the older ones, due to deterioration in the older seeds because of the presence of fungi or bacteria during storage. This fact produces dark regions and, as a consequence, a higher signal, pointing out another possibility of PAS use, to evaluate sanitary quality of grains .
1.6. Flours and “Tortilla”
Other potential applications that some authors have proposed for PAS are for quality control in the food processing industry. For example, Favier et al.  determined the PA spectra (350–700 nm) of white bread flours, dried pea flour, rye flour, and bread flour. PAS technique appears to be capable of producing reproducible spectra of powdered food samples. The PA spectra of white bread flours have absorbance bands around 370, 385, and 410 nm. For wavelengths above 410 nm, the PA signal decreases rapidly and drops to a nearly zero amplitude at 700 nm. Unlike this, the dried pea flour is the only sort that has a maximum signal at 410 nm. Soya flour exhibits a broader spectrum, whereas rye flour resembles that of the bread flour and also produced the highest signal of all the samples. On this basis, the researchers propose PAS as a viable method for the determination of basic amino acids present in biological samples.
Dóka et al.  obtained PA spectra, in the range from 250 to 550 nm, of sorghum (Sorghum bicolor L.) grain flour. They related the PA spectrum with the presence of aromatic amino acids, flavonoids, and phenolic compounds due to the absorbance peaks located at 285 and 335 nm; they also found that the PA signal decreases when the wavelength is increased. The authors indicated that the main advantage of PAS technique, with respect to a conventional analysis method, is that it is possible to study directly powdered samples, i.e., as they are, without sample preparation. This fact greatly reduces the time needed for its analysis. On the other hand, determination of water contents in the wheat flour (soft and hard), corn starch, and potato starch by PAS also have been evaluated .
Different types of “tortillas” elaborately corn (white and blue) and wheat flour (integral and not integral)—manually processed or not, fortified, and/or supplemented with “nopal,” linseed, “epazote,” and spinach—among others, were analysed by PAS. From the photoacoustic signal, it was possible to obtain the optical absorption coefficient, which was decreasing with the increase in wavelength . In general, photoacoustic spectroscopy is a sensitive technique to characterize inhomogeneous materials.
1.7. Fruit, Vegetables, and Condiments
In Brazilian tropical fruit and vegetables, carotenoids and flavonoids were identified by PAS. Biomolecules of β-carotenes and flavonoids were identified in acerola, pumpkin, broccoli, cabbage, cauliflower, spinach, purple-cabbage, orange tangerine, mango, rucula, and cuité. In addition to the biomolecules of beta-carotene and flavonoids, chlorophyll was also found in watercress and lettuce. Regarding β-carotene, lycopene, lutein, lutein 5, and 6 epoxide were identified in carrots. β-carotene and lycopene were determined at tomato; β-carotene, chlorophyll, and zeaxanthin were found in maize leaves; and β-carotene, lycopene, and possible capsanthin were found in red pepper . Finally, PAS technique can contribute to select and classify fruit, leaves, and other vegetables according to their phytotherapeutic and nutritive properties. Lima and Filho  reported that PAS is a rapid, direct, and efficient analytical method in biomaterials, particularly in the promising field of photochemistry and photobiology.
Other authors have shown the potential of photoacoustic spectroscopy in the assessment of stages of maturity of strawberries using the spectral ratio of anthocyanin and protein bands. Characteristic bands were found: a major one at 278 nm, related to proteins, and a second band around 510 nm attributed to anthocyanin pigments. The authors highlight that PAS is a nondestructive technique that might be extended to other horticultural crops .
In this way, PAS is a type of absorption spectroscopy, which allows to obtain optical absorbance spectra, as a function of wavelength, which provides information about the optical absorbance processes that occur in the sample. It is also possible to characterize samples regarding its atomic or molecular composition according to De Oliveira et al. . Over the years, different methods have been used for the analysis of signals by PAS (Table 1): methods of subtraction, statistical analysis, correlation, variance analysis, derivatives (1 and 2), Gaussian deconvolutions, regression model, multivariable analysis, etc. Using these methods, the extraction of information of the PA signal has been improved. Some researchers have validated this by the use of other conventional techniques such as the UV-Vis spectrophotometer with an integrating sphere.
λ: wavelength; MF: modulation frequency, CA: centers absorbing, P: power, Xe: xenon, β: optical absorption coefficient, nm: nanometers, PA: photoacoustic; PAS: photoacoustic spectroscopy.
In general, according to Dóka et al. , PAS offers several advantages over other analytical techniques: it is nondestructive, requires no pre-preparation of the sample, and is applicable to specimens such as powders as well as optically opaque and gelatinous samples.
Table 1 summarizes the reached progress regarding the applications of PAS, according to the literature review carried out, from its origin to the last years, as a result of several scientific activities around the world in this area. It is possible to observe different food types and agricultural material, which have been evaluated by PAS, using conventional instrumentation, to obtain its optical spectra. The meaning of the different columns is as follows: (0) type of sample, (1) some characteristics of the experimental condition and/or sample preparation, (2) the spectral region used for the sample investigation, (3) the applied lamp power and/or light modulation frequency, (4) wavelengths of the absorbance peaks or spectral region, (5) applied mathematical methods, and finally (6) significant results reported in the literature.
Photoacoustic spectroscopy can be said to have been applied successfully in foodstuff analysis. Figure 1 shows the regions or absorbance peaks related to compounds (e.g., Figure 1) by photoacoustic spectroscopy, which have served to relate the absorbance spectra with these compounds. Even for some compounds, through calibration curves and mathematical analysis, the concentration of the compound has been obtained.
PAS using the conventional configuration, xenon lamp, lock-in, photoacoustic cell, chopper, etc. It has been used, since its first applications and up to date, among other purposes, to obtain photoacoustic spectra of plants and then of foodstuff. There is evidence of a potential application in the future, since its use has increased as can be seen in Figure 2. It is possible to observe that, at the beginning of the application of PAS in foodstuff and plants, there were fewer scientific reports than those that exist now. According to the present literature review, it was found that, in the decade of the 70's, there were only four articles (in this area and in order to obtain only absorption spectra, the motive of the present review), in comparison of the recent included period (2010–2018), where there were 24 articles (considering only those analyzed in the summary of applications of photoacoustic spectroscopy in foodstuff and plants in this review).
It is possible to observe a positive tendency of PAS applications in the foodstuff area, in this particular case, to characterize foodstuff through optical absorbance spectra, making calibrations and mathematical analysis of data. It is known that different photoacoustic configurations have diverse applications in several areas of the knowledge and with the possibility of being used for the obtaining of spectra not only as a function of wavelength, but as well as a function of light modulation frequency. It is worth mentioning that not only amplitude spectra but also phase ones and signals depending on the frequency can be obtained, which would lead to the application of other methods and mathematical analysis to obtain nonradiative relaxation times and sample depth analysis [95–100], among other optical and thermal parameters.
It has been proven that PAS is a kind of “green technology,” in the sense that it is possible to use it minimizing the use of solvents, as well as without the need for sample preparation and using only a small amount thereof. In relation to the state of the sample, it is possible to evaluate it in solid, liquid, or powder forms. The current trend is to continue exploring different applications, defining the concentration of foodstuff components, differentiating them and evaluating the quality of them in relation to the added chemicals (harmful) or phytochemicals (favourable to human health) identified at certain wavelengths, depending on the absorbance centres of the substances contained therein. Application PAS portable systems in the sanitary quality (fungi, mycotoxins, etc.) and safety of foodstuff will be relevant in the coming years.
On the other hand, thanks to technological advances, it is possible to replace xenon lamps with white light LEDs, RGB, or arrangements of switched LEDs or only LEDs at specific wavelengths. Also, the use of laser diodes allows an improved function in different PAS applications.
Regarding photoacoustic cells, on the other hand, one would expect and is already venturing into the creation of photoacoustic cells with methodologies for rapid prototyping as 3D printing. This will reduce the time of construction and particularized designs for different specific applications. An important trend, in which some research groups are already working, is in the replacement of the lock-in through controller cards and a laptop for the acquisition of data.
According to the aforementioned, this would lead to the portability of photoacoustic spectroscopy systems and to the cost reduction, making the technique available to interested people, who could have a support system in the evaluation of foodstuff quality, resulting in a better decision for the consumption of food and impacting people’s quality life, without forgetting the possible incursion of PAS, in the internet of things, with the advancement in technology. Therefore, PAS can be technologically updated, and in this way it can be applied to specific needs and continue its use, rescuing an old proposal to the new necessities of our time in the real world. In this sense, it is necessary to be aware of the need to generate knowledge in this area in a transdisciplinary perspective, among institutions, researchers, and society, to produce results in improving the quality of people’s life.
According to the present review of the scientific literature, it is possible to glimpse the technology of photoacoustic spectroscopy, an old technology with ample potential for new applications in food agroindustry. PAS is an alternative technology to face the problem of evaluating food to consume better quality of foodstuff. Among the main features of the photoacoustic spectroscopy are the size of sample required is very small, due to the small volume of the cell; no special sample preparation is required; it reduces the number of analysis steps; it is a green method with less use of chemical substances, and it is nondestructive. Over the time, it has been observed that the applications of photoacoustic spectroscopy are increasing in the food area.
The authors alone are responsible for the content and writing of the paper.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors are grateful for the support of the National Polytechnic Institute through the projects SIP, EDI, and COFFA. Claudia Hernández-Aguilar thanks the collaboration of educational institutions and research centers that have been allowed to collaborate in research with her for more than ten years: Colegio de Postgraduados-Montecillo, Texcoco, and Cinvestav, through the Department of Physics in particular to Esther Ayala, for his assistance and support during the learning and use of photoacoustic spectroscopy in the laboratories of Cinvestav, Mexico. Thanks are due to all for sharing the research path towards the transdisciplinarity. The present research and publication was funded by means of the projects (SIP 20181534 and SIP 20181645) supported by the Instituto Politécnico Nacional.
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