Phenolic Compounds in Mesoamerican Fruits—Characterization, Health Potential and Processing with Innovative Technologies

Diets rich in phenolic compounds have been associated to reducing the risk of metabolic syndrome and its derived disorders. Fruits are healthy components of the human diet because of their vitamin, mineral, fiber and phenolic profile. However, they have a short shelf-life which is limited by microbiological growth and enzymatic activity. Innovative preservation methods such as high hydrostatic pressure, pulsed electric fields, ultrasound, microwave, cold plasma and ultraviolet light have become popular for the processing of fruits because they can preserve nutritional quality. In this review, the phenolic profile and health potential of 38 Mesoamerican fruits were assessed. Phenolic compounds were classified based on their contribution to the diet as flavonoids, phenolic acids, tannin, lignins and stilbenoids. Due to this composition, fruits showed a wide range of bioactivities which included anti-inflammatory, anti-diabetic, anti-hypertensive and anti-obesity activities, among others. Phenolic content in fruits submitted to innovative food processing technologies depended on parameters such as enzymatic activity, antioxidant capacity, microstructure integrity and cell viability. Innovative technologies could increase phenolic content while assuring microbiological safety by (i) promoting the release of bound phenolic compounds during processing and (ii) inducing the synthesis of phenolic compounds by activation of phenylpropanoid pathway during storage.


Introduction
A significant event in human history was the transformation from a hunting-gathering economy to an agricultural economy. This transformation occurred independently in at least six regions of the world, mainly in tropical and subtropical areas with high biological and cultural diversity [1]. Mesoamerica is considered one of the world's primary centers of domestication where species such as maize (Zea mays L.), beans (Phaseolus spp.) and squash (Cucurbita spp.) were domesticated and integrated into a multi-crop system. Mesoamerica is an historical region in North America which extends from the center of Mexico, through Belize, Guatemala, El Salvador, Honduras, Nicaragua and to the north of Costa Rica. From 2500 BC to AD 1521 Mesoamerica was a cultural area defined by its indigenous cultures such as the Aztecs, Mayas, Olmecs, among many others. Mesoamerica is an important center of genetic diversity and is recognized because of its role in plant domestication.
Fruits have become a fundamental part of the human diet and are of great relevance from a nutritional and economic perspective and they are rich sources of vitamins, minerals and dietary fiber. From a nutritional point of view, interest in fruits has increased in the last years because of their association with the prevention of chronic diseases attributed to their antioxidants such as phenolic compounds. Epidemiological studies have shown that daily intake of plant-derived foods plays a role in reducing the risk of some types of cancer, cardiovascular diseases and diabetes [2]. Fruits are important sources of phenolic compounds such as phenolic acids, flavonoids, anthocyanins, tannins, lignins and stilbenoids that have shown a diversity of health benefits related to metabolic syndrome [3]. Much work has been carried out internationally regarding the characterization of specific families of phenolic compounds in the 38 fruits of Mesoamerican origin included in this review. By integrating the data reported in 105 studies, we provide an integral view of the complete phenolic profile that has been studied until now, as well as the bioactivity of mentioned fruits which is related to reducing the risk of obesity and metabolic syndrome-derived disorders.
Despite their healthy attributes, fruits products are perishable systems with a limited shelf-life due to the microbiological, biochemical and enzymatic reactions taking place during storage. Fruit products such as fresh-cut fruit, juices, beverages, nectar, puree and jams are widely consumed in today's market where consumers are demanding healthy processed foods. Food preservation technologies are becoming more sophisticated in response to the growing demand for food quality, extended shelf-life and high-quality products with nutritional and functional characteristics [4]. The requirements of the preservation of foods have gradually changed throughout the years from seeking innocuous products with a long shelf life to include a high content of nutrients and antioxidants [5].
Thermal treatments such as pasteurization sterilization and concentration, are traditionally used to extend the shelf life of fruits and fruit-based-products. Nevertheless, thermal processing technologies may reduce the overall quality of the product including nutritional and sensory changes.
Due to the high consumer demand for minimally processed products with high nutritional and sensory quality, alternative preservation methods have gained relevance [4]. These include high hydrostatic pressure (HHP), pulsed electric fields (PEF), ultrasound (US), microwave (MW), cold plasma (CP) and ultraviolet light (UV).
Numerous international studies have reported the nutritional, phenolic composition and health benefits of some fruits of Mesoamerican origin. However, the extended and detailed phenolic composition in these fruits is yet to be compiled and analyzed. In addition, the association of phenolic compounds in these fruits with health benefits related to lowering the risk of metabolic syndrome requires further attention. Furthermore, the effects of innovative technologies on phenolic compounds in Mesoamerican fruits has been only reported for selected cultivars and the mechanisms for the observed changes in phenolic content often remains unexplained. This paper aims to provide a detailed review of the micronutrient composition, phenolic profile and health benefits of Mesoamerican fruits, as well as a critical overview of the effects of innovative food processing technologies on phenolic content after treatment and during storage as well as the mechanisms behind each technology. By recommending technologies or treatment intensities that could assure microbiological safety in fruit products while preserving or increasing phenolic content, we expect to contribute to future production of health-promoting fruit products.

Description and Geographical Region
Mesoamerica is a cultural region that influenced Mexican cultures in the pre-Columbian era and is considered an important center of genetic diversity. The scientific name, description and geographical origin of the 38 Mesoamerican fruits included in this review is shown in Table 1. These include 11 tropical fruits, 5 fruits from cactus, 8 members of the Capsicum annuum L. specie (peppers), 3 that are usually classified as culinary vegetables (tomato, squash and zucchini), among others. The domestication of the zucchini in Mexico is considered the first domestication of plants in America (10,000) years ago [18]. Other fruits that are native to the region of Mexico include chagalapoli, peppers of the Capsicum annum L. species, Mexican hawthorn, dragon fruit, sapodilla, cactus berry, prickly pear, canistel, capulin and avocado. Tomatoes, despite being originated in ancestral Peru-Ecuador, were also domesticated in the region of Mexico from where their cultivated forms were later disseminated [30]. Avocados are wild progenitors of eastern and central highlands of Mexico through Guatemala to the Pacific coast of Central America and were domesticated in pre-Hispanic Mexico. In other terms, cactus fruits originated from central and southern Mexico where they were domesticated and include prickly pears, cactus berries, dragon fruit, pitaya and sour prickly pears (xoconostle).
Tropical fruits from the Annonaceae family such as cherimoya, annona, soursop, custard apple and sugar apple are native to Central America and northern South America [7]. Similarly, cashew apple, nance, cainito and mamoncillo are also native to southern Mesoamerica and northern South America (Table 1).

Macronutrient Composition
The macronutrient composition of Mesoamerica fruits is shown in Table 2. Fruits such as peppers, zucchini, tomatillo, squash and tomato possess a high water content (90-95%). Meanwhile, the chagalapoli has a considerably higher protein content (8.6 g protein/100 g fresh fruit) the other fruits which range from 2.7 to 0.2 g protein/100 g fresh fruit. Regarding the fat content of fruits, avocado has a 15% fat content which consists of monounsaturated fatty acids (71%), polyunsaturated fatty acids (13%) and saturated fatty acids (16%), which have been associated with healthy blood lipid profiles and with better bioavailability of fat soluble vitamins and carotenoids [32].
In other terms, fruits such as mamey, custard apple, sugar apple, canistel and Mexican hawthorn possess a higher carbohydrate content (24-36%). Dietary fiber content is an important fraction of total carbohydrates because it is associated with a reduced risk of diabetes, heart disease and some types of cancer [33][34][35]. The American Dietetic Association recommends 14 g of dietary fiber per 1000 kcal or 25 and 38 g for adult women and adult men, respectively [33]. Of the Mesoamerican fruits described in Table 2, nance, avocado, guava, mamey, black sapote and sapodilla all contain high dietary fiber content were a consumption of 100 g could account for 21-31% and 14-20% of the total dietary intake for adult woman and men, respectively.

Micronutrient Composition
The mineral and vitamin content of Mesoamerican fruits is shown in Table 3. Potassium was the most abundant mineral in Mesoamerican fruits and ranged from 125 to 660 mg/100 g contributing from 3.5 to 19% of the daily recommended intake for adults [44]. Fruits that are rich in calcium include nance, Mexican hawthorn and prickly pears (45-56 mg/100 g).
Regarding vitamins, fruits such as nance, bell pepper, jalapeño pepper, poblano pepper, habanero pepper, chilaca pepper, Mexican hawthorn, papaya and guava are rich sources of vitamin C showing from 1.4 to 5 times higher content than orange juice. Foods such as avocado and squash contain about 42% and 48%, respectively, of the total folate content of spinach which is a product known for its high content of this vitamin. In other terms, fruits such as nance, bell pepper, jalapeño pepper, serrano pepper, papaya and tomato contained from 6% to 9% the retinol activity equivalents of carrots [37].

Phenolic Compounds
Phenolic compounds are secondary metabolites that are widely distributed in nature and influence the taste, flavor and appearance of vegetable foods. They consist of an aromatic ring with one or more hydroxyl groups and their structures vary from a simple molecule to a high molecular mass polymer. Phenolic compounds can be classified based on their contribution to the human diet [3]. Mentioned classification consists of phenolic acids (hydroxycinnamic and hydroxybenzoic acids) which represent 1/3 of the daily intake, flavonoids (anthocyanins, flavonols, flavanols, flavones, flavanones, isoflavones and proanthocyanidins) which contribute to 2/3 of the daily intake and others (tannins, lignans and stilbenes) which contribute in minimal amounts to the regular intake of phenolic compounds.
The total phenolic content of fruits described in Table 4 were mainly quantified colorimetrically by the Folin-Ciocalteu method. Although advanced chromatography techniques can also provide total phenolic content, these are determined by the sum of the identified and quantified compounds which are sometimes limited to a particular family of phenolic compounds. In Table 4, total phenolic contents ranged from 11 to 1056 mg/100 g fresh weight. The fruits with the highest phenolic content were namely, cactus berry, chagalapoli, chilaca pepper and zucchini. The high variability in phenolic content reported by various authors could be due to different extraction and analysis methods. In addition, different maturity stages and differences among the cultivars could also play a role in mentioned variability.

Phenolic Acids
Phenolic acids are made up of two carbon frameworks. The two families of phenolic acids, hydroxycinnamic and hydroxybenzoic acids, have a different position hydroxyl groups on the aromatic ring. Hydroxybenzoic acids are less common than hydroxycinnamic acids but can form tannins (gallotannins and ellagitannins) and act as intermediates in lignin biosynthesis [152].
According to this revision, the profile in phenolic acids of all fruits (except annona and yellow mombin) has been reported ( Table 4). Most fruits showed the presence of phenolic acids. p-coumaric, caffeic, ferulic and sinapic acid (hydroxycinnamic acids) and p-hydroxybenzoic, vanillic, syringic and protocatechuic acids (hydroxybenzoic acids). Caffeic acid is usually one of the most abundant phenolic acids in fruits representing up to 75% to 100% of the total hydroxycinnamic acid content [153,154]. Of the 38 fruits included in this review, 63%, 38%, 35% and 31% of them contained caffeic, p-coumaric, ferulic and sinapic acid, respectively. Meanwhile, p-hydroxybenzoic, vanillic, syringic and protocatechuic acids were present in 40%, 19%, 6% and 38% of the fruits of Table 4, respectively. The presence of phenolic acids in fruits is related to increases in bile secretion, reduction of blood cholesterol, lipid levels, as well as antimicrobial activity as major health benefits.

Flavonoids
The major flavonoids for all fruits included in this review were reported (except mamoncillo). Flavonoids are the most widely distributed phenolic compounds in foods as they make up 2/3 of the dietary intake. They are made up by phenylbenzopyran that includes a C 15 (C 6 -C 3 C 6 ) skeleton joined to a chroman ring. Flavonoids can be further subcategorized according to their structure as flavonols, flavanones, flavones, anthocyanins, flavonols and isoflavones [3].
Flavonols are the most abundant flavonoids in foods found as glycosylated compounds associated to glucose or rhamnose. Kaempferol and quercetin are common types of flavonols. In Mesoamerican fruits, kaempferol is present in soursop, chagalapoli, nance, Capsicum annuum L., zucchini, dragon fruit (pitahaya), pitaya, picky pear, avocado, capulin, guava, tomato and mombin; while quercetin is present in 67% of the Mesoamerican fruits of Table 4.
Flavones are typically less abundant in foods, where luteolin and apigenin are examples of flavones that can be mainly found as glycoside forms. In Table 4, luteolin was present in bell pepper, Capsicum annuum L., zucchini and squash, while apigenin was reported in Capsicum annuum L., papaya, black zapote, squash and tomato (Table 4).
Anthocyanins are colored compounds that refer to the glycoside or acyl-glycoside of anthocyanidins. They are found in high quantities in berries (black raspberries, elderberries, chokeberry and blackberries) and are responsible for their characteristic color. Anthocyanins reported in Table 4 were found in colored fruits such as cashew apple, chagalapoli, capulin, nance and in purple verities of tomatillo (Table 4). Their main biological activities include anti-inflammatory, antioxidant and chemoprotective activity. Cyanidin is the most common anthocyanin, followed by delphinidin, malvidin and peonidin [155].

Tannins
Tannins are groups of phenylpropanoid compounds that are condensed to polymers of different lengths which can be classified as proanthocyanidins, hydrolysable tannins, phlorotannins and complex tannins based on their chemical structures and constitutive monomers. Proanthocyanidins can further be subclassified as condensed tannins and are the polymerized product of flavan-3-ols (catechins) and flavan-3,4-diols or both [156,157]. Typically, fruits such as berries are the major sources of proanthocyanidins in the human diet. In Table 4, cactus berry, avocado, mamey, capulin and guava had proanthocyanidins content. In other terms hydrolysable tannins refer to gallotannins and ellagitannins and upon hydrolysis, yield gallic acid or ellagic acid, respectively. In Table 4, mentioned hydrolysable tannins were reported in fruits such as red and yellow cashew apple, cherimoya, green and purple sugar apple, chagalapoli, guava, green and purple caimito, Mexican hawthorn, zucchini, black zapote, dragon fruit (pitahaya), cactus berry and canistel.

Lignans
Lignans were reported in green, red and yellow bell pepper and Capsicum annuum L. (Table 4). Mentioned compounds are made up of two phenylpropane units and are one of the major sources of phytoestrogens in plants. They have significantly lower contribution to the human diet compared to flavonoids and phenolic acids [157]. They can be transformed by the intestinal microbiota to enterolignans which contribute to reducing the risk of certain cancers and cardiovascular diseases [3].

Stilbenes
Like lignans, stilbenes also contribute little to the dietary intake of phenolic compounds. Resveratrol is the most widely studied stilbene which has shown antioxidant, anti-inflammatory, estrogenic, cardioprotective, anti-tumor and anti-viral activities [158]. Resveratrol, besides being found in grapes and grape derived products, can also be found in the fruits such as mamoncillo (Table 4).

Health Benefits of Mesoamerican Fruits
The study of bioactive compounds such as phenolics in fruits has become of great interest from biological, medical, and nutritional points of view because they contribute to the reduction of risk factors of diseases related to metabolic syndrome. The health potential of fruits is mostly studied by assessing the bioactivity of a certain extract that contains health-promoting constituents such as phenolic compounds. In this review, 42 articles were identified because they focus on the health potential of phenolic extracts of the edible fraction of fruits of Mesoamerican origin (Table 4). Extensive evidence has demonstrated that dietary phenolic compounds can act as antioxidant and anti-inflammatory agents by increasing thermogenesis and energy expenditure and reducing oxidative stress [159]. Most of the studies included in this review are related to reducing the risk of obesity and metabolic syndrome-related disorders. Currently, the health potential of subproducts of the fruit processing industry or non-edible fractions has gained great interest but is outside the scope of the present review.
Regarding the in vitro studies, the most reported bioactivities were anticancer (10 fruits), anti-inflammatory (7 fruits) and antidiabetic (5 fruits). Mentioned in vitro bioactivity studies of phenolic-rich extracts showed anticancer activity for cherimoya, guava, tomato and black sapote; antidiabetic activity for annona; antibacterial and anti-inflammatory activities for peppers; antidiabetic, anti-inflammatory and anticancer activities for peppers; antiproliferative and anti-inflammatory activities for papaya; antihypertensive and anticancer activities for green and purple cainito; relaxant activity for Mexican hawthorn; anticancer and anti-inflammatory activities for pitahaya; anti-inflammatory, antidiabetic and anticancer activities for cactus berry; anticancer, anti-inflammatory and antidiabetic activity for green, purple, red and yellow prickly pear; anticancer, anti-inflammatory, antidiabetic activities for avocado; and anti-inflammatory activity for canistel (Table 4).
In other terms, the most reported in vivo studies included antidiabetic (10 fruits), antioxidant (6 fruits) and anti-obesity (4 fruits) activity. In vivo bioactivity studies of phenolic compounds in red and yellow cashew apples showed anti-diabetic, antioxidant, anti-obesity, anti-inflammatory and wound-healing activity. In green sugar apple, the presence of dietary phenolic compounds contributed to its antidiabetic and antioxidant activity. Meanwhile spicy peppers showed antidiabetic, hypocholesterolemic, cardioprotective and antiobesity activity. Phenolic extracts from green caimito showed antihypertensive and gastroprotective activity; antidiabetic, wound healing and antihypertensive activity for pitahaya; antitumor, anti-obesity and antidiabetic activity for sapodilla; antidiabetic and renal protective activity for cactus berry; and antidiabetic, antioxidant and kidney protective activity for prickly pears. Sour prickly pears (xoconostle) extracts have been studied regarding their antidiabetic and antioxidant activity. Furthermore, avocado extracts have shown anti-obesity and antidiabetic activity. Other fruits showed hepatoprotective and antioxidant activity for canistel; antihypertensive and vasorelaxant activity for capulin; antihypertensive, cardioprotective, antidiabetic, antiulcer and hepatic injury protective activity for squash; gastroprotective and ulcer healing activity for yellow mombin; and antioxidant activity for red mombin (Table 4).

Effects of Innovative Technologies on Phenolic Compounds in Fruits
Microbiological growth and enzymatic activity are the most important limiting factors in the shelf life of fruit-derived products. The main concerns in the fruit processing industry are related to contamination of yeasts (i.e., lactic and acetic acid bacteria), molds (i.e., Byssochlamys, Talaromyces and Neosartorya) and pathogenic microorganisms (i.e., Escherichia coli O157:H7, Cryptosporidium parvum and Salmonella spp. [160]. Furthermore, enzymes such as polyphenoloxidase (PPO), peroxidase (POD), pectin methylesterase (PME), lipoxygenase and catalase are the main enzymes responsible for fruit product quality changes (color, texture and flavor) during storage [161].
Innovative food processing technologies are currently being studied for (i) assuring food safety and stability, (ii) as pre-treatments in the manufacturing of food products to reduce energy consumption (i.e., prior to drying, freezing, extraction, distillation, etc.) and (iii) for the obtaining of extracts and nutraceutical development. The present review solely focuses on the use of innovative food processing technologies for the purpose of (i) assuring food safety and stability of fruits and fruit products. Furthermore, the study of these technologies should also include the effects on phenolic compounds and/or on parameters that affect the stability of phenolic compounds in foods (i.e., enzymatic activity, antioxidant capacity, microstructure integrity, color and cell viability).
In this review, 41 published articles with mentioned characteristics were identified. These included only 14 of the 38 fruits of Mesoamerican origin characterized previously. Mentioned fruits included custard apple, avocado, bell pepper, jalapeño pepper, cashew apple, guava, papaya, pitaya, prickly pear, sapodilla, soursop, mamey, dragon fruit and tomato. The fruit products studied in mentioned articles were juices, beverages, sliced fruits, pulps, purees, and jams.

High Hydrostic Pressure (HHP)
High Hydrostic Pressure (HHP) is a nonthermal preservation technology that can extend the shelf life of foods with little or no alterations to its sensory and nutritional characteristics. It is the most commercialized nonthermal technology with sales increasing annually by $10 billion USD and the number of high-pressure units growing exponentially at an annual rate [162]. HHP has shown a wide range of applications in the processing of fruits because these foods are highly susceptible to browning and color change by thermal treatments.
The processing of foods by HHP consists of introducing hermetically sealed products in a thermally insulated airtight vessel and subjecting them to high pressure (100-600 MPa). The pressure is transmitted inside the vessel instantaneously and uniformly by a liquid medium such as water. This uniform pressure (isostatic principle) causes microbiological death and enzymatic inactivation.
The effects of HHP on phenolic compounds and parameters related to their stability in fruits are shown in Table 5. The Mesoamerican fruits that have been treated with HHP include avocado, bell pepper, cashew apple, guaya puree, papaya, dragon fruit, prickly pears, sapodilla and tomato. The main mechanism which led to a higher phenolic content after processing with HHP was an increase in extractability of bound phenolic compounds.   Enhanced extractability (processing). [174,175] 400-600 -Phenolic content ↑ 20%. Enhanced extractability (processing) [176] Sapodilla jam 400 -Phenolic content ↑ 27%. Enhanced extractability (processing). [177] Tomato juice 250 -Antioxidant capacity was retained post treatment and during storage (30 days at 25 • C).
HHP is the most effective technology to stabilize and extend the shelf-life of avocado pulp [164,166,179,180]. Until the last few decades, avocado derived products could not be successfully commercialized due to browning caused by the oxidative effect of PPO. Today, HHP-treated avocado paste has become one of the most successful products treated with this technology and has played a fundamental role in the boosting of commercial HHP units.
However, the degradation of phenolic compounds may also occur as shown for bell pepper treated at 100 MPa and accompanied by a lower antioxidant capacity and significant microstructural damage to cells [167]. Papaya and pitaya beverages processed at 400-600 MPa also showed lower phenolic content post treatment [170,171].

Pulsed Electric Fields (PEF)
Pulsed Electric Fields (PEF) is based on the application of external electric fields (1-50 kV/cm) for a short time (microseconds to milliseconds) to biological material and is based on the principle of electroporation.
Tomatoes were the only fruits of Mesoamerican origin that have been studied using PEF technology and report its effect on phenolic compounds (Table 6). PEF treatments in tomato fruits have a beneficial effect on phenolic content, particularly during storage, due to the activation of the phenylpropanoid pathway which leads to the synthesis of phenolic compounds as a mean of abiotic stress. In tomatoes, increases in phenolic compounds (19-57%) following PEF treatments have been observed after 24 h at 4 • C [181,182]. Mentioned studies showed that the increased number of pulses at 1.2 KV/cm promoted the synthesis of polyphenols in tomatoes as a stress response. This response was induced by the recognition of a stimulus at the cellular level (changes in electrical potential differences of the membranes) which influenced the voltage-gated ion channels and increased membrane permeability for Ca 2+ at the cellular level. This was followed by a quick influx of Ca 2+ through cation channels. Afterwards, Ca 2+ -dependent protein kinase (CDPK) phosphorylates PAL which regulates the phenylpropanoid metabolism that leads to the synthesis of new phenolic compounds [183]. CDPK can also increase the reactive oxygen species (ROS) which are endogenous signal components required for the synthesis of secondary metabolites [184].
In addition, it has been shown that (similar to HHP) PEF can promote the release of intracellularly bound phenolic compounds and contribute to a higher phenolic content as a secondary effect of electroporation [5]. Contrarily, high intensity PEF treatments for extended times tend to induce the degradation of phenolic compounds in fruits.

Ultrasound (US)
Ultrasound (US) for food processing uses inaudible sound waves at a frequency superior to 20 kHz. Ultrasound produces the cavitation of dissolved gas inside the liquid which causes the generation and evolution of microbubbles in a liquid medium. Once these microbubbles reach a critical size, they implode violently and return to their original size which causes the sudden release of all of the accumulated energy while instantly producing increases in local temperature (these are dissipated without substantially raising the temperature of the liquid) [185].
Ultrasound is used because of its effectiveness against undesired microorganisms found in liquid foods and can reach a 5-log reduction of some pathogens such as E. coli in fruit juices [186]. The energy that is released and the mechanical shock affects the microstructure of the cells or cell fragments in a liquid medium (i.e., puree, juice). Large cells are usually more sensitive to ultrasound than smaller ones and gram-negative bacteria are more susceptible to inactivation than Gram-positive bacteria.
Ultrasound treatments enhanced the extractability of phenolic compounds in Mesoamerican fruits such as cashew apple puree, custard apple juice, guava juice and prickly pear juice (Table 6). This effect was mainly attributed to the further rupture and modification of cell fragments in the processed fruits due to cavitation. Cavitation was also responsible for the increased water diffusivity in the samples which contributed to the extraction of phenolic compounds. In other terms, treating tomatoes at 45 kHz increased phenolic content 40% after 15 days of storage at 10 • C [187], which could be due to synthesis of these metabolites as a response to abiotic stress.
In the studies reporting the use of ultrasound in fruits of Mesoamerican origin, there were no reports on the degradation of phenolic compounds. However, a disadvantage of ultrasound treatments is that compared to HHP or PEF, a lower inactivation of microorganisms can be achieved. Furthermore, ultrasound treatments are most effective in juices and puree fruit products and less effective on solid foods.

Microwave (MW)
The use of microwaves as a food processing technique for microbial inactivation consists of two mechanisms: ionic interaction and dipolar rotation [188]. During microwave treatments, ionic polarization is induced by an electrical field. This electrical field causes ions in the food (mainly water) to move at an accelerated pace because of their inherent charge and collide with other ions. These molecular collisions convert kinetic energy into thermal energy.
For fruits, the use of microwaves has become important worldwide in the market of dehydrated products because of its ability to lower processing time. Similar to HHP, PEF and US, microwave treatments are also able to release bound phenolic compounds in fruits and hence increase antioxidant activity [189,190].
The fruits of Mesoamerican origin that have been processed with microwaves and report phenolic content or related parameters include avocado, guava, jalapeño pepper and mamey (Table 6). On one hand, avocado puree treated at 11 W/g resulted in a higher phenolic content (29%) post treatment and remained stable for 4 weeks stored at 4 • C [191] (Table 6). On the other hand, treating jalapeño peppers with ultrasound resulted in a degradation of phenolic compounds (21%) [192]. When compared to nonthermal technologies such as HHP and US, thermal treatments such as microwaves can have a more negative effect on phenolic content even with short processing times. Enhanced extractability (processing). [197]  -Increase in viscosity due to release of soluble pectin.

Cold Plasma (CP)
Cold plasma is one of the most recent nonthermal technologies for the preservation of foods, particularly as a sterilization treatment. Plasma is produced by applying electromagnetic fields to gas (usually O 2 or N 2 ) by generating a mixture of electrons, ions, atomic species, UV photons and charged particles that react with the food substrate. This can target microorganisms by releasing the stored energy [161]. The main parameters for processing with cold plasma are the gas feed, electric field, surrounding media and exposure time [210]. The cellular damages and surface modifications caused by plasma support its potential for increasing the extractability of hydrophilic compounds by decreasing the resistance to diffusion of internal molecules [211].
Cold plasma treatment resulted in a higher phenolic content in cashew apple juice, dragon fruit (pitahaya) and tomato beverage ( Table 6). Treating cashew apple juice at 80 kHz resulted in a higher flavonoid (120%) and polyphenol content (128%) [205]. In dragon fruit treated at 60 kV, a higher total phenolic content of 28% was observed, accompanied by increases in gallic acid, protocatechuic acid and p-coumaric acid after 36 h storage at 15 • C [206]. Similarly, treating pitaya fruit with cold plasma at 60 kV, induced a similar response during storage as observed for PEF where there was an amplified signal role of ROS and the phenylpropanoid metabolism was activated leading to the synthesis of phenolic compounds. The use of cold plasma for the preservation of fruit products offers several advantages over other technologies such as it requires little energy and short treatment times, reactive gas species revert back to original gas within minutes to hours after treatment and it is a dry process that can be adaptable to a food manufacturing environment.

Ultraviolet Light (UV)
Ultraviolet light radiation (UV-C) consists of the applying nonionizing light (200-280 nm) to decontaminate the surface of fruits. The principle of UV-C decontamination is related to the damage of DNA. This kind of processing technology is easy to use, requires inexpensive equipment, does not leave residues and is lethal to most microorganisms (bacteria, viruses, protozoa, yeast, molds and algae [212,213]. In other terms, ultraviolet light radiation (UV-B) (280-315 nm) is also used in fruits and vegetables as a mean of postharvest abiotic stress for the accumulation of health-promoting compounds such as phenolic compounds [214] (Figure 1). In Mesoamerican fruits such as dragon fruit (pitaya) juice (pitaya) and tomato beverages treated with UV-C light, no differences in phenolic content were found after treatment (Table 6). However, in prickly pear fruits treated with UV-B light at 6.4 W/m 2 phenolic content increased 100% in whole pulp and 25% in wounded pulp after 24 h stored at 16 • C [209]. One of the disadvantages of this technology is that it may cause significant changes in the textural characteristics of the stressed tissue that may not be ideal for certain fruit-based products. However, the stressed tissue can be used as raw material to produce functional foods or for the further extraction and purification of compounds with applications in the pharmaceutical and dietary supplement industry.

Mechanisms of Innovative Technologies on Phenolic Compounds
As mentioned previously, innovative food processing technologies can affect phenolic compounds in fruit products. Excessive food processing intensities and time, as well as the partial inactivation of enzymes and food spoilage can lead to the degradation of most antioxidants. However, is applied properly, innovative food preservation technologies can simultaneously (i) assure microbiological safety while (ii) increasing/preserving phenolic content in foods. The main ways that phenolic compounds can increase during food processing or in storage conditions is by their (i) enhanced release (during processing) and (ii) synthesis (during storage).
Phenolic compounds can be found in soluble and insoluble-bound forms. Soluble phenolic compounds are localized in the vacuoles of plant cells where they are contained. Meanwhile, insoluble-bound phenolic compounds are attached to the cell wall matrix to macromolecules such as structural proteins, cellulose and pectin [215]. Phenolic content in foods can increase during food processing because of the release of insoluble-bound phenolic compounds from the cell walls (enhanced release/extractability). Innovative food processing technologies such as HHP, PEF, US, CP and MW can cause microstructural changes in vegetable cells that can promote the release of insoluble phenolic compounds from cell walls and macromolecules ( Figure 1A).

Mechanisms of Innovative Technologies on Phenolic Compounds
As mentioned previously, innovative food processing technologies can affect phenolic compounds in fruit products. Excessive food processing intensities and time, as well as the partial inactivation of enzymes and food spoilage can lead to the degradation of most antioxidants. However, is applied properly, innovative food preservation technologies can simultaneously (i) assure microbiological safety while (ii) increasing/preserving phenolic content in foods. The main ways that phenolic compounds can increase during food processing or in storage conditions is by their (i) enhanced release (during processing) and (ii) synthesis (during storage).
Phenolic compounds can be found in soluble and insoluble-bound forms. Soluble phenolic compounds are localized in the vacuoles of plant cells where they are contained. Meanwhile, insoluble-bound phenolic compounds are attached to the cell wall matrix to macromolecules such as structural proteins, cellulose and pectin [215]. Phenolic content in foods can increase during food processing because of the release of insoluble-bound phenolic compounds from the cell walls (enhanced release/extractability). Innovative food processing technologies such as HHP, PEF, US, CP and MW can cause microstructural changes in vegetable cells that can promote the release of insoluble phenolic compounds from cell walls and macromolecules ( Figure 1A).
During pressurization by HHP, vegetable tissues suffer structural modifications which favor a compact form. This causes changes in the fruit tissue on a cellular level such as changes in cell morphology, cell wall thickness and the rearrangement of cells [216] that promote the release of organelle-bound phenolic compounds. Meanwhile in PEF treatments, electroporation decreases the resistance to diffusion of phenolic compounds and promotes their extractability [178] as a mass transfer process. Ultrasound treatments at high intensities can disrupt cells, inhibit enzymes and enhance the yield of extraction of phenolic compounds by means of cavitation. The intensity required for cavitation to occur depends on the physical and chemical characteristics of the liquid media (vapor pressure, tensile strength, solid concentration and dissolved gas) [160]. In cold plasma, charged molecule interactions play a fundamental role in enhancing the extractability of phenolic compounds by increasing the diffusivity of the solvent. The cellular damages and surface modifications caused by plasma support its potential for increasing the extractability of hydrophilic compounds by decreasing the resistance to diffusion of internal molecules [211]. In microwave treatments, dipolar rotation can increase water diffusivity and the concentration of solids, hence, contributing to the release of phenolic compounds during processing. However, extended processing times can lead the degradation of these antioxidants.
In other terms, recent studies have shown that innovative food processing technologies could act as stress factors that may lead to a burst of reactive oxygen species (ROS) either by signaling (UV light and PEF) or by causing damage to vegetable cells (HHP, US, CP) ( Figure 1B). These endogenous signal components are required for synthesis of secondary metabolites (i.e., phenolic compounds) as a defense response of plants to stress. This application of nonthermal processing technologies as abiotic stress elicitors to induce the accumulation of nutraceuticals in horticultural crops has been proposed as an innovative tool to obtain healthier fruits and vegetables [217].

Conclusions
This review provided a detailed compilation of the nutrient composition, phenolic profile and health benefits of 38 Mesoamerican fruits, as well a critical overview of the effects of innovative technologies on phenolic content.
We provided a complete overview on the phenolic composition in mentioned fruits by analyzing and selecting a total of 63 scientific articles. Phenolic compounds were classified from a nutritional point of view as phenolic acids (contribute to 1/3 of the diet), flavonoids (contribute to 2/3 of the diet) and tannins, lignins and stilbenoids (contribute in minor amounts to the diet). Cactus berry, chagalapoli, chilaca pepper and zucchini had the highest phenolic content.
In addition, the available information on the health potential of these fruits was compiled from 42 scientific articles that studied their phenolic-rich edible fractions. Most of the reported bioactivities in fruits were related to reducing the risk of disorders related to obesity and metabolic syndrome such as anti-inflammatory, anti-diabetic, anti-hypertensive, and anti-obesity activities.
Of the 38 fruits included in this review, the effects of innovative technologies on phenolic compounds and/or related parameters has only been studied in 14 fruits. A total of 41 studies were selected for comparing the different effects of these processing techniques on different fruit products such as juices, beverages, sliced fruits, pulps, purees, and jams. Phenolic content after food processing and during storage depended on parameters such as enzymatic activity, antioxidant capacity, microstructure integrity, color, and cell viability. Increases in phenolic content could be observed due to two main mechanisms (i) release during processing and (ii) synthesis during storage. HHP, PEF, US, CP, and MW could affect phenolic compounds release during processing by different mechanisms. UV and PEF could induce the synthesis of phenolic compounds by signaling. Similarly, HHP, US and CP could induce the synthesis of phenolic compounds by cell injury.
Fruits of Mesoamerican origin contain an abundant variety of phenolic compounds which contribute to their health potential. The adequate processing of fruits with innovative technologies is capable of simultaneously achieving food safety as well as preserving these antioxidant compounds. There is still a need for further research regarding the effects of innovative technologies on phenolic compounds in Mesoamerican fruits.
Funding: Authors Escobedo-Avellaneda and Welti-Chanes acknowledge the support from Tecnológico de Monterrey.

Conflicts of Interest:
The authors declare no conflict of interest.