Comparison of Mango ( Mangifera indica ) Dehydration Technologies: A Systematic Review

: The convective hot-air drying technology can cause physicochemical, nutritional, and organoleptic losses in the mango ( Mangifera indica ). The present Systematic Review was carried out with the objective of comparing mango dehydration technologies to identify the effects on the physicochemical, nutritional, and organoleptic properties of the fruit. Through a review of published scientific and conference papers in the Scopus database, adjusted to the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) methodology, a total of 134 documents dated between 2000 and December 6 of 2022 were obtained; 76 of these documents were finally included in the bibliographic and theoretical analysis. Selection parameters emphasizing the relationship between the articles and the research topic, evidenced by including at least one of three dehydration technologies and the fruit of interest with an experimental or theoretical approach to the dehydration subject; review articles and surveys were excluded. Correlation graphs of bibliographic variables were made using the data mining software VantagePoint (version 15.1), which was graphically restructured in Microsoft Excel with the support of statistical analysis. Of the resulting articles, it was found that the countries with authors who participated most in scientific production like India, Brazil, Colombia, the United States, and Thailand, were those related to mango production or importation. Furthermore, the freeze-drying technology allows operating at lower temperatures than convective hot-air drying, contributing to the preservation of ascorbic acid, among other compounds. The refractance window has the shortest operation time to obtain moisture values between 10 and 20%. The dehydrated samples using the refractance window are smooth, homogeneous, non-porous, and comparable to the color obtained with freeze-drying, which is acceptable for industrial applications.


Introduction
Fruits have economic importance in the market; according to the Food and Agriculture Organization of the United Nations (FAO) [1], "globally, pineapple, avocado, and mango continued to be the three most significantly traded tropical fruits in terms of their export quantities in 2021", of which mango represents 29% with 2.3 million tons exported (between mango, mangosteen, and guava) in the same year, and an average growth rate of 1.9% compared to the immediately preceding year [1].
Due to the perishable nature of fruits, microbiological spoilage, transportation problems, marketing, and, in general, the absence of optimal handling conditions, there are postharvest losses with economic damage for producers, distributors, and, naturally, the agricultural sector [2][3][4][5].According to FAO estimates, globally [6], "13.8 percent of food produced in 2016 was lost from the farm up to, but excluding, the retail stage", of which nearly 21% corresponds to fruits and vegetables.
It is due to the importance of the fruit in the market and its natural tendency to decompose that dehydration as a conservation method [7] plays a relevant role since it allows the production of fruit that has a longer shelf life than fresh fruit [8], which can reduce product and economic losses.
The problem with fruit preservation is that traditional dehydration processes, such as convective hot-air drying, can impact the physicochemical, nutritional (nutritional value), and organoleptic (color, texture, flavor, smell) properties of the fruit [2] due to the loss of volatile and heat-sensible substances because of the exposure of fruit to high temperatures [9]; and the transfer of soluble compounds from the solid to the medium through airflow drag [10].
Therefore, differences between products dehydrated by convective hot-air drying and by alternatives such as freeze-drying and the refractance window should be identified.
The freeze-drying technology works under ultra-vacuum using a previously frozen material [11], from which water is removed by sublimation of ice to steam without passing through the liquid state [12].
Refractance window technology consists of using heat transfer by radiation from hot water through a transparent film in contact with the food to be dehydrated at atmospheric pressure [8,13]; therefore, "it is based on the use of water as the main means for power transfer" [14], a translated quote from its original in Spanish], the mentioned film corresponds to a Mylar ® film.
In this sense, the purpose of the study is to compare mango (Mangifera indica) dehydration technologies through a Systematic Literature Review (SLR) in order to identify the effects on physicochemical, nutritional, and organoleptic properties of the fruit, which are representative of its quality [15]; and additionally, to correlate bibliographic information regarding keywords, authors, year of publication of articles, variety of mango used, products, and applications of interest.
This article intends to be useful for industrialists and business owners in the sense that they have a basic guide to identify the most appropriate technology based on the desired characteristics of the product to be produced and the operating conditions of the process.

Materials and Methods
The research was conducted through a rigorous process, development identification, and screening phases based on the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) methodology [16]; PRISMA methodology is a tool that improves transparency in systematic reviews that consist of 27 items that guide the work structure for reduce risk bias and facilitate repeatability [16] (see Figure 1).

Planning the Review
Following the methodology, the following keywords were chosen to guide a representative search in the Scopus database according to the objective of the research: drying, Mangifera indica, mango, refractance window, freeze-drying, and hot air.Published scien-

Planning the Review
Following the methodology, the following keywords were chosen to guide a representative search in the Scopus database according to the objective of the research: drying, Mangifera indica, mango, refractance window, freeze-drying, and hot air.Published scientific and conference papers (of indexed journals), dated between 2000 and 6 December 2022, were obtained using the search equation: (TITLE-ABS-KEY (drying) AND TITLE-ABS-KEY ("Mangifera indica" OR mango) AND TITLE-ABS-KEY ("refractance window" OR "freeze drying" OR "hot air")) A sequential methodology with concrete inclusion and exclusion criteria was implemented to minimize the risk of bias for each study.When there were doubts about classification or categories, the co-author (G.H.L.) reviewed the inclusion and exclusion criteria.A priori selection criteria emphasis to evaluate each paper was placed on the relationship between the article and the research topic, evidenced by including at least one of the three dehydration technologies and the fruit of interest with an experimental or theoretical approach to the dehydration subject.

Inclusion and Exclusion Criteria for Studies
A total of 134 records were obtained.In the first stage, reviews and surveys were excluded.In the screening and eligibility stage, the titles and abstracts of 127 papers were read; nine of these were excluded because the work team did not have access to the complete documents.The remaining 118 full-text articles were read.A posteriori, the main exclusion criteria for papers were as follows:

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Does not contain information on fruit dehydration technologies  Focus on leaves/tree, not fruit • After the screening process, 76 papers were selected for inclusion in the bibliographic and theoretical analyses.

Documentation and Analysis of Review
Data extraction, like the screening process, was performed by one investigator (L.C.L.); bibliographic information was registered on a log search in a Microsoft Excel ® (version 2407) spreadsheet and Mendeley Cite Software (version 2.84.0) to support reference files.The identified categories of each article were title, year, authors, journal name, Scimago Quartiles, and journal country.In addition, we collected data about specific parts of the fruit investigated (pulp, peel, seed), products of interest made with the fruit, participant dehydration technologies, and variety (cv.) of mangoes in studies; no additional restrictions were required for the purpose of the research.
Correlation graphs of bibliographic variables were made using the data mining software VantagePoint, which were graphically restructured in Microsoft Excel ® with the support of statistical analysis.The studies were grouped for the syntheses by type of dehydration technology: hot-air drying, freeze-drying, and refractance window drying.
For the theoretical extraction of information, three spreadsheets were made, one by each technology.Main categories were defined around specific input and output operation conditions (operating temperature and pressure, final moisture of sample, drying air velocity, sample thickness, relative humidity of drying air, dehydration time, degrees Brix), physicochemical properties (microstructure/porosity, water activity, rehydration capacity), nutritional properties (ascorbic acid, phenol content in sample), and organoleptic properties of fruit (flavor, aftertaste, chewiness, sample color, texture, smell); relevant parameters (compared with formal defined categories [15]) were chosen since the reading of the papers' abstracts was completed within the full-text reading (based on disponibility of tabulated values from papers).To report the principal values or ranges of main parameters by dehydration technology, a synthesis table was made; some intervals indicated are regarding the form mean ± standard deviation of reported data samples; the last, calculated by category with statistical functions of Microsoft Excel ® , to guide the comparison of technologies.

Systematic Literature Review (SLR)
Of the 134 articles retrieved using the search equation, 76 were selected and analyzed as a result of the screening based on the established selection and exclusion parameters.
It was identified that 46 articles included the convective hot-air drying topic in their research, 40 freeze-drying, and nine included the refractance window in an inclusive way since some of the papers compared the technologies, as shown in Figure 2.There were 29 articles exclusively on convective hot-air drying (38.2%), 22 on freeze-drying (28.9%), and six on refractance window (7.9%).Additionally, 16 papers included the comparison between convective hot-air drying and freeze-drying (21.1%), two compared freeze-drying and refractance window (2.6%), and one compared convective hot-air drying and refractance window (1.3%).However, none of these studies compared these three technologies.It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.

Keywords
Figure 3 presents a word cloud of the main keywords of the selected pap of each word is directly proportional to its frequency in the SLR; that is, the word is repeated, the bigger it will appear in the figure.

Keywords
Figure 3 presents a word cloud of the main keywords of the selected papers.The size of each word is directly proportional to its frequency in the SLR; that is, the more a keyword is repeated, the bigger it will appear in the figure .Some of the words with the largest size are the terms "hot air drying", "freeze drying", "mango", and "refractance window", which shows coherence between the search equation and the screening carried out of the articles.

Keywords
Figure 3 presents a word cloud of the main keywords of the selected papers.The size of each word is directly proportional to its frequency in the SLR; that is, the more a keyword is repeated, the bigger it will appear in the figure.Some of the words with the largest size are the terms "hot air drying", "freeze drying", "mango", and "refractance window", which shows coherence between the search equation and the screening carried out of the articles.

Authors
There were 323 authors registered in the SLR, all of whom contributed at least one other author of the same or different nationality.
It was identified that India, Brazil, the United States, Colombia, and Thailand were the five countries with authors who participated most in the development of scientific articles under the SLR parameters (See Figure 4).This is consistent with the importance of the fruit of interest, mango, as a tropical fruit in producing countries such as India, Brazil, and Thailand, in importing countries such as the United States [1], and in exporting countries such as Colombia [17].

Authors
There were 323 authors registered in the SLR, all of whom contributed at least one other author of the same or different nationality.
It was identified that India, Brazil, the United States, Colombia, and Thailand were the five countries with authors who participated most in the development of scientific articles under the SLR parameters (See Figure 4).This is consistent with the importance of the fruit of interest, mango, as a tropical fruit in producing countries such as India, Brazil, and Thailand, in importing countries such as the United States [1], and in exporting countries such as Colombia [17].In support of the above, Figure 5 shows a graph of the annual production of mangoes, mangosteens, and guavas between 2000 and 2021 for the five mentioned countries, based on values from the FAOSTAT online statistical database [18].In support of the above, Figure 5 shows a graph of the annual production of mangoes, mangosteens, and guavas between 2000 and 2021 for the five mentioned countries, based on values from the FAOSTAT online statistical database [18].In support of the above, Figure 5 shows a graph of the annual production of mangoes, mangosteens, and guavas between 2000 and 2021 for the five mentioned countries, based on values from the FAOSTAT online statistical database [18].India has exceeded the production of all the countries listed in that category, followed by Thailand (between 2000 and 2017), Brazil, Colombia (with an average production close to 296,000 tons per year [18]), and, lastly, the United States as the main importing country, with the lowest reported maximum production of 3157 tons in 2001 [18].
According to the market analysis for the year 2021, "India is by far the largest mango producer in the world" ( [1], translated quote from its original in Spanish).Regarding Colombia, as a country that produces for the domestic market and the agroindustry, according to the Ministry of Agriculture and Rural Development [17]: "the result of processed India has exceeded the production of all the countries listed in that category, followed by Thailand (between 2000 and 2017), Brazil, Colombia (with an average production close to 296,000 tons per year [18]), and, lastly, the United States as the main importing country, with the lowest reported maximum production of 3157 tons in 2001 [18].
According to the market analysis for the year 2021, "India is by far the largest mango producer in the world" ( [1], translated quote from its original in Spanish).Regarding Colombia, as a country that produces for the domestic market and the agroindustry, according to the Ministry of Agriculture and Rural Development [17]: "the result of processed mango exports is constantly rising, ranking fifth internationally"; Colombia "exports to more than ten countries, including Canada, France, the Netherlands and Belgium (. ..), exporting to the world [in 2020] near US$2 million, mainly to Aruba, Curaçao, and Panama" ( [19], translated quote from its original in Spanish).
According to the Ministry of Commerce, Industry, and Tourism [19], in 2020, the United States imported US$493 million of mango (549,000 tons), which is related to the demand for that product in the country.Brazil provides 10% of this demand as exports, and as of November 2021, Colombia as well, under the requirements of the U.S. health authority.

Year of Publication of Articles
Figure 6 shows that since 2011, the scientific production of mango dehydration technologies has been a growing trend.Between 2000 and 2005, only one article (published in 2000) that studied freeze-drying technology was identified.This range is not included in the figure for visualization purposes.
It was identified that papers on refractance window technology began to be published in 2012, in a lower volume than the other two technologies, indicating the novelty of this technology.Although it was patented in 1986 by MCD Technologies Inc. in Washington [13,14], its application in mango dehydration research remains under development.It was identified that papers on refractance window technology began to be published in 2012, in a lower volume than the other two technologies, indicating the novelty of this technology.Although it was patented in 1986 by MCD Technologies Inc. in Washington [13,14], its application in mango dehydration research remains under development.

Mango Varieties
Figure 7 shows the seven varieties of mango (Mangifera indica) most used in the research reported in the literature.It is highlighted that all the mango varieties registered in the figure were used for studies with convective hot-air drying technology.For the refractance window, the Carabao, Langra, and Tommy Atkins varieties were used, which was 78 and 82% less than in convective hot-air drying and freeze-drying, respectively.The Tommy Atkins mango is the most used mango variety in SLR research.

Mango Varieties
Figure 7 shows the seven varieties of mango (Mangifera indica) most used in the research reported in the literature.It is highlighted that all the mango varieties registered in the figure were used for studies with convective hot-air drying technology.For the refractance window, the Carabao, Langra, and Tommy Atkins varieties were used, which was 78 and 82% less than in convective hot-air drying and freeze-drying, respectively.The Tommy Atkins mango is the most used mango variety in SLR research.

Products and Applications
Processed mangoes have the potential to be used in the industry for applications in the food, cosmetics, and nutrition sectors, among others [20], due to the diversity of presentations that dehydration allows in a material supported by other operations such as spraying and sieving.
An association between mango products and their cultural importance in different countries was identified.In the SLR, mango leather is regarded by researchers from India, Malaysia, and the Philippines, where it is traditionally consumed principally in the form of mango leather, which is a roll of pure mango that was dehydrated [21,22].
The Trade and Development Report 2018, under the United Nations Conference on Trade and Development (UNCTAD), suggests that under the infrastructure and skilled labor parameters, a relationship between exports of countries and the technological intensity of products is possible, in which investment in technological improvements for manufactured products could reflect improvements in exports, based on analyses of emerging

Products and Applications
Processed mangoes have the potential to be used in the industry for applications in the food, cosmetics, and nutrition sectors, among others [20], due to the diversity of presentations that dehydration allows in a material supported by other operations such as spraying and sieving.
An association between mango products and their cultural importance in different countries was identified.In the SLR, mango leather is regarded by researchers from India, Malaysia, and the Philippines, where it is traditionally consumed principally in the form of mango leather, which is a roll of pure mango that was dehydrated [21,22].
The Trade and Development Report 2018, under the United Nations Conference on Trade and Development (UNCTAD), suggests that under the infrastructure and skilled labor parameters, a relationship between exports of countries and the technological intensity of products is possible, in which investment in technological improvements for manufactured products could reflect improvements in exports, based on analyses of emerging economies in Asia and the BRICS (Brazil, Russia, India, China, and South Africa) [23].
Implementing technology in the agro-industrial production system represents a strategy for strengthening the economy of producing countries [24], which can support the reduction of postharvest losses and optimize the production of high-value-added food products.
Figure 8 shows a graph relating the main parts of the fruit (pulp, peel, and seeds) to the individual dehydration technologies used in their processing.The pulp is the part of the mango that has been most used to experiment with dehydration processes, followed by the peel and, in small quantities, by the seeds.According to the records, the seeds were processed through freeze-drying and convective hot-air drying only.
Dehydrated mango pulp can be manufactured like a healthy snack or like powder ingredients for the industrial sector; therefore, the fruit can be processed in a continuous or batch system, depending on the volume of production.
Since 2012, research has focused on dehydrating mango peels, which generally represent a residue from pulp processing, to extract their aromatic compounds and enable applications in cosmetic and food industries [25], to value carotenoids as phytochemical compounds, and to benefit from the dietary fiber, protein, and phenolic compounds they contain [26].Hence, in the literature, the individual freeze-drying technology is mainly used for processing peels, followed by convective hot-air drying and, in a very low proportion, refractance window, as shown in Figure 8.

Comparison of Dehydration Technologies
Table 1 summarizes the parameters of operating conditions and physicochemical, nutritional, and organoleptic properties of dehydration technologies.
For some mango properties and operating conditions of the dehydration process, data are presented in the (x ± SD) form, where x is the mean, and SD is the standard deviation calculated based on the available SLR values.The pulp is the part of the mango that has been most used to experiment with dehydration processes, followed by the peel and, in small quantities, by the seeds.According to the records, the seeds were processed through freeze-drying and convective hot-air drying only.
Dehydrated mango pulp can be manufactured like a healthy snack or like powder ingredients for the industrial sector; therefore, the fruit can be processed in a continuous or batch system, depending on the volume of production.
Since 2012, research has focused on dehydrating mango peels, which generally represent a residue from pulp processing, to extract their aromatic compounds and enable applications in cosmetic and food industries [25], to value carotenoids as phytochemical compounds, and to benefit from the dietary fiber, protein, and phenolic compounds they contain [26].Hence, in the literature, the individual freeze-drying technology is mainly used for processing peels, followed by convective hot-air drying and, in a very low proportion, refractance window, as shown in Figure 8.

Comparison of Dehydration Technologies
Table 1 summarizes the parameters of operating conditions and physicochemical, nutritional, and organoleptic properties of dehydration technologies.
For some mango properties and operating conditions of the dehydration process, data are presented in the (x ± SD) form, where x is the mean, and SD is the standard deviation calculated based on the available SLR values.Yellow.Highlighted as good and excellent by panelists [62,68] Texture Rough; irregular surface [22,75] Regular surface [35] Smooth [60] Smell --Weighted as "medium" by panelists [68] (-): Does not record.N/A: Not applicable.Some intervals indicated in the table and in previous analyses are regarding the form (Mean ± standard deviation of reported data samples).

Convective Hot-Air Drying
It is important to note that the articles found respond to the following distinctions: those that are essentially experimental, those that focus on determining the drying kinetics, and those that are oriented toward operating conditions.
Convective drying technology traditionally employs hot air to exercise heat and mass (water) transfer in the system: heat transfer from the environment to the solid surface, from the outside to the inside of the solid, and mass transfer from the inside to the solid to the surface by diffusion, and from the surface to the environment (by evaporation of water) [9,10].

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Operating Conditions The drying air temperature data were registered for 29 articles (which report several experiments), indicating that their mode is 60 • C, with an approximate value of 65 ± 15 • C for the average operating conditions used in this technology.It was identified that experiments had been carried out with temperatures from 40 to 100 • C, with 60 to 80 • C being the most common range, finding a non-linear inverse relation between the drying air temperature and sample processing time [46].
Regarding the drying air velocity, an average value was obtained from the 14 articles from which this data was retrieved: approximately 1.70 ± 0.87 m/s, with a mode of 1 m/s.The extreme values of this drying air velocity were also identified: between 0.6 and 4 m/s, with relative humidity of air between 15 and 70%, predominating values close to 24% [35,36].
The experimental samples, mainly mango pulp, were dehydrated to a final value between 3.8 and 20% on a wet basis.
The Sindri, Carabao, Samar Bahisht Chaunsa, and Tommy Atkins varieties were dehydrated to values close to the 10-15% range of moisture.For the Langra variety, average values of around 20% of final moisture on a wet basis were reported.
The above conditions are consistent with the common operation of dehydration of mango and other tropical fruits at drying air temperatures between 40 and 80 • C, with an air velocity between 0.2 and 2 m/s, to reduce the moisture content of the solid up to 15% on a wet basis [46].This is relevant to the theoretical references because, according to Kaur et al. [69], "for the prolonged shelf life of intermediate moisture foods, the moisture content needs to be in the range of 15% to 25% (wet basis) or 17.65% to 33.33% (dry basis)".
Regarding the thickness of the samples to be dried, Mugodo and Workneh [50] identified that with mango pulp films of 3 mm thickness at a drying temperature of 70 • C, acceptable results are achieved that preserve the basic quality of the product and are better than those obtained with 6 mm and 9 mm thicknesses.This can be explained by the fact that "size and shape of sample is said to affect the mass transfer kinetics because of variation in specific surface area to thickness ratio" [76].Experimentally, according to the literature, mango samples with thicknesses between 1 and 9 mm have been used [32,50,76]; the 5 mm thick films (cut parallel to the fibers), pulp [35,36,42,75], peel, and seed have been the most used [30].Mango puree films have similar thickness ranges between 2 and 8 mm [21,47].
The drying time for peel and seeds was reported as 4 and 8 h, respectively [30], and for the pulp, between 5 and 24 h, identifying an approximate average of 11.5 ± 6.1 h, 12 h being the most registered value [35,36,38,39].
As the drying process progresses, the moisture content of samples decreases until reaching the desired final moisture or an operational limit of the product [76].The literature identifies that increasing the drying temperature increases the moisture removal rate from samples and, therefore, reduces the drying time [49].
The above behavior is consistent with experiments reported in the papers, but depending on the mango variety used, it may have quantitative variations, although not in mode.For example, according to Mukhtar et al. [46], mangoes of the Sindri, Tommy Atkins, and Samar Bahisht Chaunsa varieties, when dehydrated at an air velocity of 1 m/s at 60 • C, differ in drying times between 11.8 and 24.1%, which can be attributed to differences in the initial moisture content and • Brix of each mango variety, without identifying a linear correspondence between them.

• Physicochemical Properties
An important aspect of determining the quality of dehydrated products is the rehydration capacity, defined as "the ratio of the drained weight of the rehydrated sample to the weight of the dry sample used for rehydration" [83].Mugodo and Workneh [50] found in their research that the rehydration capacity was reduced for thicker mango samples (6 and 9 mm) at the same temperature, which suggests damage to the cell wall, reducing the rehydrated mass of the dry product compared to the original mass [84]; thus, the relation between the two variables is inverse.Additionally, the rehydration capacity decreases as the drying air temperature increases [41].Marques et al. [66] state that the mango pulp drying through convective hot-air drying causes a "loss of rehydration potential" of the samples.The calculated average value of the rehydration capacity for samples dehydrated at 70 • C using this technology is 3.11 ± 0.72.
The key indicators of browning (i.e., the phenomenon through which fruits darken into a brown color [21]) found in the literature are variations in the CIE-L*a*b* color space parameters measured in research.The color parameters correspond mainly to "L* indicates the lightness, a* indicates chromaticity on a green (−) to red (+) axis, and b* chromaticity on a blue (−) to yellow (+) axis" [21], and ∆E is the total color difference [43].Thus, browning was characterized by an increase in the red color of the a* parameter, a decrease in the yellow color of the b* parameter, a reduction in the L* parameter, and an increase in the color difference ∆E [43].
According to Guarte et al. [21] and Khuwijitjaru et al. [85], the main causes of mango mesocarp browning are enzymatic and non-enzymatic reactions that cause carotenoid degradation during prolonged dehydration.
Enzymatic or oxidative browning is a process in which phenols, in the presence of oxygen and enzymes (polyphenol oxidases) generate quinones that produce molecules with brown pigments through polymerization [86].This last process occurs after the destruction of the mesocarp cellular tissue, such as slicing, maceration, and crushing of the fruit [21,86], and between 30 and 40 • C, which is the optimal range for enzyme activity [87]; therefore, at ambient temperature, this phenomenon can occur.
In contrast, non-enzymatic browning "is a purely chemical darkening phenomenon characterized by the presence of brown polymers called melanoidins" [88].The nonenzymatic reactions occurring in the system correspond to the phenomena of mango sugar caramelization [31,47,51,62], Maillard reactions or condensation of melanoidin through sample heating, and acid ascorbic degradation [88].
Studies have identified that non-enzymatic browning kinetic constants of Arrhenius equation-type models increase with treatment temperature and are influenced by • Brix content [88]; thus, at high temperatures, in drying processes, the occurrence of this phenomenon in fruit samples could be favored.
The temperature and drying time have a direct impact on the color of dried samples and, therefore, on their quality because when extending the drying time at high temperatures (>80 • C), the resulting products darken, which is unwanted since "the appearance and color of the dried product are the important quality factor for the consumer acceptance" [45].
The sample mean and standard deviation of initial moisture and • Brix of fresh mango were 82.58 ± 4.38% moisture on a wet basis and 16.74 ± 5.06 • Brix, respectively.
The water activity (a w ), which is a measure of the availability of water in the product for internal reactions [3], for fresh mango without dehydration, was reported to be higher than 0.85, reaching values up to 0.99 [47].After the drying process, the average value of the water activity (0.468 ± 0.155) was calculated, identifying that the median of the a w data was 0.479, a value close to the mean.The mode of these data is 0.6, a theoretical value of stability at ambient temperature, which is recommended to avoid dried fruit product deterioration during storage [21,28,38].
The glass transition temperature (T g ) is a parameter of interest related to the porosity of the structure of the samples after the dehydration process.It is defined as "the temperature at which an amorphous system changes from the glassy to the rubbery state.This property has a strong effect on the stability of food since water below this temperature is immobilized, avoiding the occurrence of degradation reactions" [48], thus making the dehydrated food product more stable.It should be added that the higher the water content in the sample, the lower the glass transition temperature [74].
Dehydration through convective hot-air drying can produce samples with little porosity and dense structures, with the presence of collapse caused by long drying times [45].

• Nutritional Properties
According to Guarte et al. [21], "in the temperature range of 50 to 60 • C and at 40 • C, only 55 and 42% of the β-carotene content could be retained, respectively", indicating that at lower temperatures, β-carotene degradation by the enzymatic way can occur, and at high temperatures, as the enzymes in the biological material are deactivated, β-carotenes can be degraded by thermal action [21].
Ascorbic acid is a compound used as a nutritional quality index of fruit because it is unstable (in the presence of temperature) compared to other nutrients [38].Jödicke et al. [43] identified that the highest retention of that compound could be achieved at specific drying conditions (40 • C, 32% relative humidity, and 0.9 m/s air velocity), which were different from the highest point of polyphenols retention (94 • C, 3% moisture and 0.9 m/s air velocity).
According to Mishra et al. [49], the ascorbic acid content present in mango samples (Dasheri variety), dehydrated through convective hot-air drying, decreased as the drying air temperature increased, showing the capacity of ascorbic acid to be reduced between 5.4 and 7.7% by increasing the temperature by 10 • C. Mishra et al. [49] obtained approximate values for the ascorbic acid content in the dehydrated fruit between 29.81 to 34.17 mg/100 g of sample on a dry basis (at 50 and 70 • C, respectively).Sarkar et al. [81] report values for phenol and carotenoid contents of 705 mg GAE/100 g and 8.21 mg/100 g, respectively, both expressed based on a dry basis for the Himsagar mango variety.Thus, the operating conditions of dehydration can modify the properties of the resulting products.Furthermore, obtaining high-quality products with high added value is desirable for the industry and requires technical selection according to each company's specific requirements.

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Organoleptic Properties In the SLR, only one article was found that presented sample evaluation by panelists for this technology.Sarkar et al. [22] state that products dehydrated through convective hot-air drying were those that stood out the most in terms of sweetness (obtaining the highest score in this attribute) compared to other technologies, such as freeze-drying.

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Operating Conditions Mawilai et al. [20] identify three main stages during the freeze-drying process: the initial stage is freezing, where the temperature and pressure of the samples are lowered (subzero temperatures and vacuum pressure); in the second stage, a progressive heating ramp is initiated, still at vacuum pressure (at constant value); in the third stage, the temperature is increased up to a final point.
In the SLR, the freezing stage was carried out at temperatures between −10 and −80 • C for 16 to 24 h [37,39,[55][56][57]60,61]. In some studies, liquid nitrogen was used to freeze the samples by immersion [63,65,66].On average, the temperature of this stage was −39.74 ± 18.08 • C, with a mode and median of −40 • C, coinciding with the mean of the data.
The subsequent stage, when the sublimation starts (primary drying), was conducted under vacuum conditions at pressures lower than 0.1 mbar, with values as low as 0.0007 mbar [55] in order to operate in a ramp at low temperatures (sub-zero).Lastly, when the process ends, the temperature is gradually increased to the ambient temperature.To preserve the color, phenolic compounds, and antioxidant capacity of heat-sensitive samples such as mango, it is suggested not to exceed ambient temperature in the last stage of freeze-drying [34].In the literature, it was usually worked up to values of 10, 20, and 25 • C [29,58,62] with a maximum thermal increase of 40 • C [20].
The specific temperature configurations reported in the literature for the freezing process-primary drying-secondary drying are (−20 For this configuration, the benefit of freeze-drying was identified, which refers to the conservation (lower loss of compounds after dehydration [57]) of antioxidant compounds (total phenolic content), compared to convective hot-air drying technology [89].
Freeze-drying has a technical limitation: the vacuum operation, which involves considerations of continuously requiring the vacuum pump since the process duration is high, approximately 35.7 ± 29.8 h, with extreme ranges of 11 and 120 h.The final moisture of the samples was identified as 4.4 ± 1.1% on a wet basis, on average.
The thickness of the sample to be dehydrated is important for obtaining homogeneous results in the freezing process and full sublimation [90].An average thickness of 7.83 ± 4.61 mm was identified, with 5 mm as the mode and median.

•
Physicochemical Properties Given the nature of sublimation, pores are formed in the samples dehydrated with this technology, which previously contained frozen water removed from the solid under the operating conditions of the process [60].The experimental products have greater smoothness and uniformity, which are important elements in the stability of the dried sample [60].Moreover, it is desirable to obtain smaller pores with this technology since shrinkage and collapse of the material are avoided.This can be achieved with a high freezing ratio (ratio of variation of the system temperature from 0 • C to the freezing temperature with respect to the time interval in which this variation is reached) [67].
Based on the information recorded from six articles, the average and standard deviation were calculated for the water activity of mango samples dehydrated using this technology; 0.278 ± 0.082 values were obtained with a median of 0.293.The mean a w is 53.6% lower than that recommended (0.6) in the literature.
Zotarelli et al. [53] found that with this technology, a rehydrated product is obtained with the same rheological behavior as the original pulp without dehydration, with a rehydration capacity between 3.62 [72] and 3.79 [65], which is beneficial for potential applications in the food sector; for example, powder for preparing beverages, desserts, among others, considering that mango powder has hygroscopic nature [56].It is, therefore, essential to monitor its stability over time when developing a product constituting a mixture of ingredients.
The average and sample standard deviation of the initial moisture and • Brix of fresh mango were calculated, obtaining values of 80.62 ± 5.88% moisture on a wet basis and 18.66 ± 3.79 • Brix, respectively.

• Nutritional Properties
From the literature, the total phenolic content of dehydrated mangoes was identified as 848 mg GAE/100 g of sample on a dry basis [81] and 987 mg GAE/100 g of sample on a dry basis [54].The pulp, peel, and seeds contain phenols in their nutritional content [30], and the whole fruit is useful for various applications.Sarkar et al. [81] report carotenoid content for dehydrated mango (Himsagar variety) 9.63 mg/100 g of sample on a dry basis.
Likewise, ascorbic acid values were identified in dehydrated mango at 959 mg/100 g of sample on a dry basis for the Ataulfo mango variety [54] and 333 to 513 mg/100 g of mango on a dry basis for the Tommy Atkins, Keitt, and Sugar varieties [56].For the peel and seeds, values of 75.48 and 65.73 mg of ascorbic acid/100 g of sample on a dry basis were reported for the Tommy Atkins variety [30].Marques et al. [66] report data for dehydrated mango through this technology that allows identifying that the dehydration process can reduce the content of this vitamin by 37.97% compared to fresh fruit.

• Organoleptic Properties
According to Sogi et al. [30], the color obtained from peel and seed samples for application in food products (as a flavor enhancer and supplier of fiber) is acceptable.In the panelists' evaluation, reported and statistically analyzed by Sarkar et al. [22], it was found that the mango leather samples collected through freeze-drying obtained the highest score regarding rigidity and aftertaste properties, compared to those collected with other technologies, such as convective hot-air drying.

•
Operating Conditions During the dehydration process with this technology, the temperature of the water bath must be below the boiling point of water at the system conditions because if it boils, "bubbles and turbulence that interfere with heat transfer through the film" ( [91], translated quote from its original in Spanish) are generated.From the records obtained for this technology, it was identified that an average heating water temperature of 87.7 ± 12.5 • C was used, with a value of 95 • C for mode and median.
There is a distinction between the temperature of the heating water mentioned above and the temperature reached by the sample (puree layer) during the dehydration process since, according to Shende and Datta [47], "product temperature was maintained lower than 75 • C during drying experiment almost all the time"; this is to prevent deterioration due to thermal effects [51].The temperature range of the samples on the clear Mylar ® film was reported to be 71 to 75 • C [60,69].The refractance window system is open to the atmosphere (it can have a fume cupboard on top of the equipment).Literature reports air at ambient temperature (24-25 • C [70,71]) with an approximate speed of 0.7 m/s, "with a relative humidity ranging from 50-52% was applied on the surface of the puree to facilitate moisture removal" [60].However, there are also experiments in ambient air conditions with a relative humidity of 73% [71] and 58-68% [70].
The thickness of the samples is inversely related to the drying speed because the lower the amount of matter, the faster the moisture transfer from the samples to the medium will occur.
In the literature, the thickness of the fruit layer was between 0.5 and 5 mm, with an average value of approximately 2.3 ± 1.4 mm, with 2 mm as the mode and median.
It is highlighted that according to Zoratelli et al. [70], the refractance window technology "is a very efficient drying process, even if the relative importance of radiation heat transfer is negligible.In fact, this study clearly established that radiative heat transfer contributes to less than 5% of the total thermal energy delivery to food".The importance of this technology lies, among other elements, in the capacity to evaporate water from a thin layer of material that represents a large effective drying surface area [60].For example, with a water bath at 95 • C, researchers dehydrated 2 mm pulp, presenting an evaporation capacity of up to 10 kg of water m −2 h −1 [70].
A common dimension among the refractance window equipment used in the experiment of two studies of the literature was, precisely, the surface area of transfer, with a value of 1.10 m 2 .In contrast, the length of the contact surface area can have several values, from 10 cm to 1.83 m [62,69], with or without conveyor belt movement (batch operation [47]), considering that work with laboratory-scale equipment was identified.
From the above, it is important to clarify that heat transfer through radiation is significant when the thickness of the fruit puree layer is small (1 mm); when its thickness is high, heat transfer occurs with a more significant contribution by conduction due to the additional thickness of solid [78].Thus, the transfer of thermal energy from hot water to the fruit layer can occur predominantly through radiation and conduction mechanisms [62].Hence, to favor the dominance of energy transfer by radiation, it is preferable to use a maximum product thickness of 1 mm for dehydration and films of appropriate materials (factory-made, e.g., Mylar ® ).

• Physicochemical Properties
According to Kaur et al. [69], "the change in color increases with drying time up to 20 min of drying", this is based on the use of dimensionless parameters of CIE-L*a*b* as indicators of "visible darkening of the sample within a short period of five minutes (from 20 to 25 min of drying)" [69].
It was identified that the mango samples used in the literature for this technology had an initial moisture content of 81 to 86% on a wet basis [47,68,71].The average time of the dehydration process was 27.5 ± 18.9 min, and the shortest processing times were reported for lower puree layer thickness (close to 2 mm) [47].
The value of the glass transition temperature [74] depends on the initial moisture content of the samples and the temperature they reach during the drying process.Caprino et al. [74] studied mango samples above and below the T g , finding that when the temperature of the sample is higher than the glass transition temperature, the microstructure of dried fruit has a rubbery, "liquid-ish" viscoelastic appearance, i.e., it presents thermal plasticization.However, when the temperature of the material is lower than T g , the microstructure of the sample has a more solid, rigid, and fragile behavior.Based on the characteristics desired of the final product (more rigid or chewy finished food), it is convenient to adapt the temperature of the water bath so that the samples are at the target point with respect to the T g .This behavior is attributable to polymers, and in the case of mangoes, to carbohydrates, which are natural polymers formed by a high content of sugars, such as sucrose, glucose, and fructose [52,63,76,92].Shende and Datta [47] report that the firmness of mango leather obtained with the refractance window is in the range that generates chewable samples under optimum dehydration values of 95 • C for water bath temperature and 2.49 mm as the thickness of the puree layer.
The average and standard deviation of the initial moisture and • Brix were calculated for fresh fruit in the reported studies of this technology, obtaining values of 82.98 ± 2.16% moisture on a wet basis and 15.82 ± 2.77 • Brix, respectively.
The water activity of the dehydrated samples, for 50% of the records of this technology, presented an average value of 0.412 ± 0.172.For this system, the final moisture values were identified as between 1.7 and 20.5% on a dry basis [60,62,68,69,74] and between 5 and 20% on a wet basis [47,71], with 15 and 20% moisture (on a wet basis) as a commonly used range.

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Nutritional Properties Regarding the nutritional properties of dehydrated mango through this technology, it was found that the samples have, according to Shende and Datta's [47] experiment, 62.66 mg of ascorbic acid/100 g of sample in dry weight for the Langra mango variety.
A low volume of records of quantitative content for nutritional compounds in SLR was identified, which represents a research opportunity in this area.This is because, in general, the numerical value of these compounds varies with the temperature at which samples are processed, their thickness, and the mango variety used [47,56].

•
Organoleptic Properties Due to the mango puree layout in the refractance window equipment (high surface area in contact with the clear film and, therefore, higher heat and mass transfer in the system), the dehydrated products have a visible smooth finish that, at the microstructural level translates into smoother, non-porous, and highly uniform surfaces, thus increasing storage time in good conditions, by limiting the spaces where oxidation can occur [60].
Mango leather was analyzed by twenty panelists in the Shende et al. [68] study to judge color, aroma, flavor, and mouthfeel under five sensory factors on a five-point scale (unsatisfactory, fair, average, good, excellent).In all categories, most of the panel judges rated the samples dehydrated through the refractance window as "good" and "excellent", except for aroma (rated as "average" by 45% of the panelists).This allows inferring a general approval of the product evaluated in the categories analyzed.

Parallel between Technologies
The following is a comparison of convective hot-air drying, freeze-drying, and refractance window dehydration technologies based on the information identified in the SLR.

• Differences
The literature reports that in mango peel samples processed with freeze-drying technology, a greater number of carotenoids, about 47% more, was obtained compared to convective hot-air drying [26].
The refractance window technology takes less time to reach sample moisture contents of 10 to 20% on a wet basis than convective hot-air drying.The refractance window has an average processing time of 0.46 ± 0.31 h, while the convective hot-air drying has 11.5 ± 6.1 h for temperatures between 40 and 80 • C. The freeze-drying technology takes the longest time, around 35.7 ± 29.8 h, to dehydrate mango (being able to reach low moisture contents of samples, between 5 and 10% on a wet basis).
When dehydrating samples through freeze-drying, porous products are obtained; with convective hot-air drying, fewer porous products are obtained [79]; and with refractance window technology, more uniform, smoother, and non-porous products are obtained.Through convective hot-air drying, darker products can be produced [82] with less yellow pigmentation (and more orange and brown colors), contrary to freezedrying and the refractance window.Additionally, it is noted that products dried by convective hot-air drying have a rough texture with an irregular surface [22,75], contrary to freeze-drying samples, which have a more regular surface finish without deformations or breaks [22,72].
It is identified that the refractance window technology operates at sample surface temperatures below 75 • C [60,69] and water bath conditions of approximately 95 • C, usually at atmospheric pressure [69].Freeze-drying operates at low pressure (vacuum), starting with a freezing stage between −10 and −80 • C, followed by a primary drying stage, which is operated at low temperatures, and lastly, with ramps of gradual temperature increase up to values close to ambient temperature, without reaching it (10-25 • C), with long dehydration times (between 12 h to 5 days).In turn, convective hot-air drying uses drying air temperatures between 40 and 80 • C at atmospheric pressure [93].However, Freeze-drying can operate at lower temperatures than convective hot-air drying, which favors fruit care as a sensible material against thermal effects [9,10].Moreover, refractance window samples are prevented from further darkening and loss of compounds due to thermal effects [47,51], with the advantage of having the shortest dehydration time required.
Regarding the rehydration capacity of the samples, it was identified that processing them through freeze-drying favors the rehydration process of products, especially powders, thereby exposing the rheological properties of the fresh fruit [53].On the contrary, samples dehydrated by convective hot-air drying tend to lose their rehydration potential [66].Higher values of rehydration capacity were identified for freeze-dried samples than for samples dried by convective hot-air drying.
The refractance window and freeze-drying showed higher favorability in the nonoccurrence of deterioration due to caramelization compared to convective hot-air drying [31,47,51,62].
The evaluation panel judges the organoleptic properties of mango leather samples and prefers the product dehydrated through the refractance window rather than through convective hot-air drying, considering properties such as color, aroma, taste, and mouthfeel [68].

• Similarities
The experiments in the literature were carried using laboratory equipment, scalable to the industrial level [20], to which temperature control systems can be included to maintain stable operating conditions.
The water activity is, in all cases, below 0.6, which is desirable since "products with a water activity below 0.5 are stable at ambient temperature provided that they are bottled in high barrier materials" ( [38], translated quote from its original in Spanish).Moreover, a value of 0.6 for this parameter represents a standard of stability and hygienic safety for the moisture content [21,28].
The previous concept obeys the principle of "exposing microorganisms to a hostile environment (. ..) to prevent or delay their growth" ( [2], translated quote from its original in Spanish) since the unitary operation of dehydration aims precisely to decrease water activity [3].
It is noted in the literature that drying air temperatures between 60 and 80 • C for the drying times of convective hot-air drying and temperatures below 75 • C on the sample surface for the refractance window technology are recommended to protect the samples from further darkening and loss of compounds due to thermal effects [47,51].This effect tends to benefit the refractance window due to shorter processing times.
In general, for the technologies of interest, the topic of the samples' sensory attributes was not reported in a high volume in the SLR.Therefore, a research opportunity was identified regarding the evaluation of dehydrated mango samples by panelists to identify organoleptic properties such as flavor, color, aftertaste, smell, and texture of the samples, as well as to determine the variables influencing consumer favorability of a dehydrated product.

Conclusions
The seventy-six articles resulting from the SLR were subjected to a basic analysis of their bibliographic information, showing that 38.2% of the articles include only the convective hot-air drying technology, 28.9% freeze-drying, and 7.9% refractance window technology.Of the total articles, 21.1% compared convective hot-air drying and freezedrying technologies, 2.6% compared freeze-drying and the refractance window, and the remaining 1.3% compared drying technologies through convective hot-air drying and dehydration through the refractance window.These articles had keywords representative of the research carried out with the proposed search equation.
It was found that the countries with authors who participated the most in the scientific production of mango were those related to the production or importation of the fruit and that, globally, applications related to local products with added value or inputs for cosmetic and food industries were projected.
Since 2011, the publication of articles on mango dehydration technologies with the search equation has had a growing trend, with a low volume of records retrieved on the refractance window, showing a late implementation in the scientific literature with respect to its patent.
The Tommy Atkins mango variety was the most used in the research; furthermore, it was identified that mango pulp is used more than peel or seeds.
Based on the SLR, the operating conditions of the technologies and their effects on the physicochemical, nutritional, and organoleptic properties of the fruit of interest were identified and compared.It was found that contrary to convective hot-air drying, the freezedrying technology allows operating at lower temperatures of the sample.The processing times of each technology were different, with the refractance window being the fastest technology (0.46 ± 0.31 h), followed by convective hot-air drying (11.5 ± 6.1 h) and, lastly, freeze-drying (35.7 ± 29.8 h) to obtain final moisture values of mango samples between 10 and 20%.This contributes to the reduction of loss of compounds of important nutritional value, such as carotenoids, polyphenols, and ascorbic acid, in freeze-drying and refractance window technologies compared to convective hot-air drying.
Dehydration of mango samples with thicknesses between 2 and 8 mm was identified, with the use of 5 mm thickness for freeze-drying, between 3 and 5 mm for convective hot-air drying, and below 1 mm for the refractance window being more common.With these technologies, products that meet the stability standard value at ambient temperature to preserve food during storage are obtained.
Regarding the structure of the dehydrated samples, the refractance window is the one with the non-porous, smoothest, and most homogeneous microstructure.Freezedrying is comparable to the previous method in terms of color, which is acceptable for applications in the food industry.With freeze-drying, porose samples can be produced without collapse under controlled operating conditions.The products dehydrated through convective hot-air drying are the darkest and most irregular, with deformations and slightly porous microstructures.Furthermore, it was identified that the specific number of compounds with nutritional value present in dehydrated mango products depends on the variety of fruits used and the operating conditions.
When the nutritional value of aliments represents a relevant selection reason for consumers, it is important to increase research interest in non-conventional and alternative postharvest processing like dehydration for refractance window, how it was also identified in the study.The last talk on the direction of the tendencies for the future focuses on this theme of investigation.
Thus, refractance window and freeze-drying technologies are projected as alternatives to conventional drying technology through convective hot-air drying for producing dehydrated mango products with important physicochemical, nutritional, and organoleptic attributes.By implementing them in industrial systems worldwide, operating and quality benefits can be obtained.

iEngineering 2024, 6 Figure 2 .
Figure 2. Venn diagram of the number of papers using dehydration technologies.

Figure 2 .
Figure 2. Venn diagram of the number of papers using dehydration technologies.

Figure 2 .
Figure 2. Venn diagram of the number of papers using dehydration technologies.

Figure 3 .
Figure 3. Word cloud of paper keywords.

Figure 3 .
Figure 3. Word cloud of paper keywords.

Figure 4 .
Figure 4. Bubble chart of dehydration technologies per country of paper authors with more scientific production.The bubble sizes are directly proportional to the number of articles that combine the respective categories.

Figure 4 .
Figure 4. Bubble chart of dehydration technologies per country of paper authors with more scientific production.The bubble sizes are directly proportional to the number of articles that combine the respective categories.

Figure 4 .
Figure 4. Bubble chart of dehydration technologies per country of paper authors with more scientific production.The bubble sizes are directly proportional to the number of articles that combine the respective categories.

Figure 6
Figure 6 shows that since 2011, the scientific production of mango dehydration technologies has been a growing trend.Between 2000 and 2005, only one article (published in 2000) that studied freeze-drying technology was identified.This range is not included in the figure for visualization purposes.

Figure 6 .
Figure 6.Bubble chart of dehydration technologies per year of publication of articles.

Figure 6 .
Figure 6.Bubble chart of dehydration technologies per year of publication of articles.

Figure 7 .
Figure 7. Bubble chart of dehydration technologies according to the most reported mango variety in papers.S.B. Chaunsa: Samar Bahisht Chaunsa.

AgriEngineering 2024, 6 2702 Figure 8 .
Figure 8. Bubble chart of dehydration technologies according to the parts of the fruit investigated.

Figure 8 .
Figure 8. Bubble chart of dehydration technologies according to the parts of the fruit investigated.

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Technology different from those investigated •

Table 1 .
Summary of the main parameters of comparison between dehydration technologies.