Maple Syrup: Chemical Analysis and Nutritional Profile, Health Impacts, Safety and Quality Control, and Food Industry Applications

Maple syrup is a delicacy prepared by boiling the sap taken from numerous Acer species, primarily sugar maple trees. Compared to other natural sweeteners, maple syrup is believed to be preferable to refined sugar for its high concentration of phenolic compounds and mineral content. The presence of organic acids (malic acid), amino acids and relevant amounts of minerals, such as potassium, calcium, zinc and manganese, make maple syrup unique. Given the growing demand for naturally derived sweeteners over the past decade, this review paper deals with and discusses in detail the most important aspects of chemical maple syrup analyses, with a particular emphasis on the advantages and disadvantages of the different analytical approaches. A successful utilization on the application of maple syrup in the food industry, will rely on a better understanding of its safety, quality control, nutritional profile, and health impacts, including its sustainability issues.


Introduction
Maple syrup is a delicacy prepared by boiling the sap taken from different Acer species, mainly sugar maple (Acer saccharum Marsh.) trees [1]. Agriculture and Agri-Food Canada [2] reports Canada as the world's largest producer of maple products and it is responsible for nearly 71% of the maple syrup production in the world. In 2017, Quebec produced approximately 92% of all the maple syrup in Canada and is home to more than 13,300 maple syrup producers [3].
Of the many natural sweeteners, maple syrup is recognized as a much superior alternative to refined sugar for not only its mineral content, but also for its high concentration of phenolic compounds with bioactivity properties, i.e., anti-mutagenic, anti-radical, antioxidant, and anti-cancer [4][5][6]. Compared to dextrose, corn syrup and brown rice syrup, maple syrup brings about lower glucose and insulin responses, which make it a healthier substitute for refined sugars in our diet [4,7].
Maple tapping often begins late in winter or early in spring. It only lasts a few weeks because of the weather. To make maple syrup, sweet watery xylem sap is collected and concentrated. As a result of the pressure build-up caused by the freeze-thaw cycle, this sap pours out of maple tree trunks. To make one liter of maple syrup, around 40 liters of sap (containing 2-3% sugar) are required (66% sugar). Other than sucrose, which is Processing Grade Maple syrup: the maple syrup called processing grade is also obtained from maple sap concentration, but does not respect at least one of the quality parameters defined for Grade A or more.
To ensure its stability, maple sugar must not contain more than 10% moisture. Grade A maple syrup can differ in the four color classes, defined by either a transmittance value or the ratio between the intensity of the light passing through samples and that of the light emerging from them [23]. The higher the transmittance value, the clearer and more transparent maple syrup is. The lower the transmittance value, the darker and denser maple syrup is.
It is nature itself that characterizes maple syrup nuances. When harvest begins, syrup tends to be clear, and its sweetness is slight. As the season progresses, syrup becomes darker in color and displays distinct aromatic connotations. Indeed, this natural sweetener presents a range of differing aromatic components, including flavors such as vanilla, hazelnut, floral, coffee and spicy aromas.
All the color classes are characterized by a denomination and accompanied by a note about taste [23]: • Gold (delicate taste); • Amber (rich taste); • Dark (strong taste); • Very dark (robust taste).
All in all, maple syrup quality is driven mainly by its physico-chemical and microbial features. Thus, in order to verify that maple syrup has the appropriate characteristics, namely in terms of color, density and flavor, simple physico-chemical tests are routinely performed. For instance, maple syrup color has been set by measuring the percentage of light transmittance at 560 nm, while sucrose content is determined using a refractometer [24]. These methods are convenient because they provide immediate results [24]. Yet analyses involving more complex techniques that lead to more data, and greater sensitivity, accuracy, and precision in the results are essential to develop processes that allow maple syrup's functional profile to improve and adulterations and contaminants to be detected. Several studies were conducted to deal with the analysis of the physico-chemical and microbiological parameters of maple syrup. Table 1 presents an overview of the followed analytical techniques and the obtained results.
Maple syrup xylem sap contains naturally occurring molecules and process-derived compounds that are generated during sap evaporation [20]. This means that it contains more than 250 compounds other than sucrose, which is the major maple syrup component [13]. Its minor components include minerals and trace elements, amino acids, other carbohydrates, organic acids, phenolic compounds, sulfur compounds, and pyrazines. Table 2 summarizes the research works carried out as part of a study of maple syrup's inorganic and organic constituents. Regarding its mineral profile, maple syrup contains considerable amounts of Ca, K and Mg, along with other minerals and trace elements like Zn, P, Mn, Na and Fe [10,[25][26][27][28]. Several techniques have been used to determine minerals in maple syrup, including flame and furnace atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Regarding amino acids, maple syrup is remarkably rich and particular emphasis is placed on D-alanine and other D-amino acids, which have been shown to be generated during the Maillard reaction [29]. By means of a sap samples analysis by metabolomics, the noteworthy work by Garcia et al. [30] reports that amino acid composition varies with season, which is the case of glutamic acid and histidine, whose content is liable to lower as the season progresses, while that of methionine and asparagine tends to grow. The last two have been reported as precursors of the compounds responsible for off-flavors developing in syrup [30]. High-performance liquid chromatography (HPLC) is the gold standard for determining many types of compounds present in maple syrup, namely non-volatile ones, because HPLC allows swift, sensitive, specific, and accurate measurements to be taken, but other equally sound techniques can be used. For example, Pätzold and Brückner [29] employed gas chromatography (GC) coupled with mass detection (MS) to study the amino acids profile of maple syrup. In this case, the polar nature of amino acids required a derivatization step prior to the analysis to make them more volatile and to improve their chromatographic performance. Applying MS is advantageous for its sensitivity and ability to provide structural data [31].
Concerning carbohydrates, in addition to sucrose, monosaccharides fructose, and glucose, different oligosaccharides and polysaccharides are found. For instance, Sato et al. [32] developed a method based on hydrophilic interaction chromatography coupled with charged aerosol detection (HILIC-CAD). It allows analyses of up to hepta-saccharides in only 30 min. It enabled the separation and quantification of fructosyl oligosaccharides in maple syrup for the first time. Refractive index (RI) and pulsed amperometric detectors (PAD) are widely used in sugar analyses, and although the RI type is often utilized to analyze known substances, it does not exhibit high sensitivity [32]. PAD yields a highresolution analysis of multiple sugars [32], but this entails employing an anion exchange column and sodium hydroxide solution as the mobile phase (e.g., [10,33]). Desalting is necessary, which makes identifying new compounds more difficult. So CAD has become increasingly popular [32]. An alternative to chromatographic methods to separate sugars is capillary electrophoresis (CE), which requires a derivatization step to make them electrically charged. This was shown by Taga and Kodama [34]. CE is an appealing option as it incurs moderate operating costs compared to HPLC, employs less solvent and is easily automatable. However, its robustness still raises doubts [35]. To determine organic acids, phenolics and vitamins in maple syrup, HPLC coupled with UV-Vis or diode-array detection (DAD) are some analytical approaches of choice. Compared to UV-Vis detectors, the speed, sensitivity and resolution of DAD are superior despite it being more susceptible to noise interferences [31]. Phenolics are one of the maple syrup constituents to which the most attention has been paid for their numerous health benefits [36,37]. To date, more than 100 phenolics have been identified [13,20,38,39] thanks to the application of techniques such as nuclear magnetic resonance (NMR), which enables the swift analysis of complex mixtures without having to perform separation and/or purification steps, which makes it ideal to analyze maple syrup (e.g., [20]).
One aspect that deserves our attention is that, as the frequency of fraud resulting from admixing inexpensive sugars in maple syrup increases, the development of detection methods is more pressing [40]. Table 3 summarizes the studies performed in this field. Some tools, such as infrared (IR) spectroscopy, are noteworthy. It requires minimal sample pretreatment, is non-destructive [40] and provides reliable results, as shown in the work by Paradkar et al. [41] which successfully reported the addition of beet and cane sugars to maple syrup. Isotope ratio mass spectrometry (IRMS) is another technique with a huge potential for detecting the same type of adulteration, as proved by Tremblay and Paquin [16]. This technique exhibits high precision and the required sample is smaller than that in NMR [40].
As a final remark, along with developing analytical tools, the possibility of employing elemental maple syrup content as a strong marker for fingerprinting maple syrup against other syrups must be actively investigated. A literature analysis backs the possibility of identifying percentages by allowing the detection of adulterations to maple syrup with inexpensive syrups employing contents of element. Nevertheless, wide fluctuations in metal contents are reported, which hinders making consistent comparisons, while the possible release of metals from instruments can interfere with acquiring accurate data. These techniques has been used in other foods, such as honey and coffee [42,43] Jointly, the progress made with new analytical techniques can help with the detection of elemental maple syrup content as a solid marker for fingerprinting this appreciated product, unlike other syrups with lower quality compositions, which should be more exhaustively studied. The ATP bioluminescence measurement of sap allowed a good maple syrup color assessment. In general, lighter syrups were produced from the saps with a low level of microbial contamination, while those with darker colors came from the saps with a high contamination level.

Non-volatiles
Amino acids 2 Canada (state not specified) Gas chromatography-mass spectrometry (GC-MS) Sample preparation: 1 g of sample was diluted with water (5 mL). pH was adjusted to 2.3. Ion exchange solid-phase extraction was applied, followed by drying, redissolving and redrying. Pentafluoropropionic acid anhydride was used as the derivatizing agent.  Across all the grade syrup samples, no significant differences were observed in glucose (0.670-0.810 g/L), fructose (0.088-0.255 g/L) or in the total reducing sugars (0.870-0.878 g/L).
[26] The following phenolics were detected and identified from a medium-grade maple syrup:  The following phenolics were detected and identified from a very dark-grade maple syrup.
(1) Lignans: (a) lyoniresinol a ; (b) secoisolariciresinol a ; (c) dehydroconiferyl alcohol a ;  The total pyrazine content in different maple syrup classes differed markedly, with "medium" grade maple syrups exhibiting the highest contents (68 ng/g), whilst "amber" grade syrups showed the lowest levels (48.89 ng/g).   The mean δ 13 C of the maple syrup sample sugars was −24.07‰, while that of malic acid was −26.71‰, which agree with the stable carbon isotopic ratios characteristic of C 3 plants. The correlation between sugars and malic acid was good, i.e., r = 0.34. This proves that malic acid is an appropriate internal standard. A new calculation method was developed and applied to improve the decision limit of maple syrup adulteration according to the correlation between the δ 13 C malic acid and the δ 13 C sugars -δ 13 C malic acid (r = 0.704). The theoretical LoD markedly lowered when this technique was applied compared to the usual two standard deviation (SD) method, particularly for the beet sugar-adulterated maple syrup (24 ± 12% vs. 48 ± 20%).
[16] The dynamic rheological method applied detected, with adequate sensitivity, changes in viscosity caused by the addition of polymers such as cellulose gum, wherefore this technique can be successfully employed to detect this type of adulteration. [44] A-Authentic; NA-Non-authentic (fraudulent or adulterated); LoD-Limit of detection.

Nutritional Profile and Health Impacts
Boiling maple sap concentrates carbohydrates and other elements, which is how maple syrup is made. The timing of collection affects the amounts of micronutrients, macronutrients and phenolics in maple sap, which result in variations in maple syrup [56]. Maple syrup is high in phytochemicals, macronutrients (sucrose) and micronutrients [7,11,57], and sucrose is its main component (96%), with a small amount of hexoses. Much lower concentrations of minerals, trace elements, organic acids, phytochemicals (lignan, stilbene, coumarin, phenolic compounds) and vitamins are found than in sugars [8,11,18,20,38,39,51,58]. St Pierre et al. [7] compared chemical maple syrup components to those of other natural sweeteners, including honey, molasses, blue agave syrup, brown rice syrup and golden corn syrup (abscisic acid [ABA], carbohydrates and phenolics). This research concluded that when compared to brown rice syrup, corn syrup, and pure dextrose, maple syrup significantly reduced the peak and total responses of glucose, insulin, amylin, and gastric inhibitory polypeptide (GIP). Molasses and agave syrup both had similar metabolic effects to maple syrup, however, honey increased the peak responses of insulin, amylin, and GIP. The elemental composition of maple syrup and the metabolic reactions to it in rats suggest that it is a healthy natural substitute for refined sugar.
Thériault et al. [21], Legault et al. [67], Kamei et al. [68], Maisuria et al. [69] and Liu et al. [70] have found that maple sap and the phenolic-rich extracts of maple syrup perform antiproliferative, antiradical, antimicrobial, antimutagenic and antioxidant activities. From maple sap and syrup, Thériault et al. [21] identified aglycone phenolic and glycosylated molecules. Glycosylated sap/syrup components have stronger antioxidant and antiradical properties than aglycones. SOS induction suppression in Salmonella typhimurium TA1535/pSK1002 that contained fusion gene umuC-lacZ was used to investigate each chemical's antimutagenic activity. Glycosylated phenolic compounds' antimutagenic properties are optimal for 25% of the season for syrup and for 75% of the season for sap at different times of the year. Aglycones in sap present the greatest antimutagenic feature for 75% of the season, whereas aglycones in syrup do so for 25-100% of the season.
Li and Seeram [20] applied the DPPH (2,2-diphenyl-1-picrylhydrazyl) experiment to separate phenolics from MS-BuOH and to compare their antioxidant activities to a positive antioxidant control (butylated hydroxytoluene). Coumarins have stronger antioxidant capacity than stilbenes and lignans among isolated phenolics. Zhang et al. [25] investigated the biological activity and safety characteristics of maple syrup extract. In vitro, the extract demonstrated anti-inflammatory and antioxidant (DPPH test) properties, and inhibited glucose intake (by HepG2 cells). The study by Liu et al. [70] found that phenolicsenriched maple syrup extract (61.7 g/mL) scavenged ∼50% DPPH and decreased free radical production by 20% throughout the glycation process.
The biological effects of an organic phenolics-rich, sugar-reduced maple syrup extract employed as a new dietary component high in phenolics were studied by Nahar et al. [58]. With a lipopolysaccharide-stimulated RAW 264.7 murine macrophage cell model, anti-inflammatory MS-EtOAc activity and its purified isolates were investigated. MS-EtOAc reduced nitric oxide (NO) and prostaglandin E2 (PGE2) generation by lowering NO synthase (iNOS) levels, while upregulating the protein expression of cyclooxygenase 2 (COX-2) mRNA. The most effective inhibitor of PGE2 and NO was (E)-3,3 -Dimethoxy-4,4 -dihydroxystilbene. In a mouse model of Alzheimer's disease, a phenolics-enriched maple syrup extract presented anti-neuroinflammatory actions [71]. The expression of several inflammatory proteins, including Alzheimer's disease risk-associated proteins, was reduced by maple syrup extract. The impact that maple syrup extract had on hepatic gene expression in mice on a high-fat diet has been reported by Kamei et al. [68] and Kamei et al. [72]. According to changes in the expression of the genes associated with lipid metabolism and immune response, maple syrup extract can help to attenuate hepatic inflammation in mice.
The antiproliferative effects of botj MS-EtOAc (Grades C and D) extracts and purified phenolics against non-tumorigenic (CCD-18Co) and human tumorigenic (HCT-116, HT-29, CaCo-2) colon cells have been investigated by González-Sarrías et al. [5]. Extracts MS-EtOAc, MS-BuOH and MS-MeOH proved more effective against tumorigenic colon cells than non-tumorigenic colon cells. The most active compounds were gallic acid, syringaldehyde, catechaldehyde and catechol, whose presence in Grade D MS-BuOH extract could explain its anticancer properties. Cancer apoptosis was not caused by extracts, but they did cause cell cycle arrest. The synergistic action of various phenolics might explain the high activity of MS-BuOH extract. González-Sarrías et al. [73] studied the anti-proliferative effects of ginnalins A-C on tumorigenic and non-tumorigenic colon (HCT-116) and breast (MCF-7) cells. Ginnalins A-C were twice as active against tumorigenic vs. non tumorigenic cells. This finding indicates that their selectivity for cancer cells is stronger. Ginnalin A was more active than ginnalins B and C. Maple phenolics may have a cancer chemopreventive effect via cell cycle arrest, as well as their direct cytotoxic effect. Yamamoto et al. [74] investigated the effects of dark-colored maple as a drug for gastrointestinal cancer therapy. It suppressed protein kinase B phosphorylation and further decreased cell proliferation by limiting protein kinase B activation. According to Yamamoto et al. [75], MS-EtOAc reduced cell proliferation, migration, and invasion in pancreatic cancer cells.
St-Pierre et al. [7] found several maple syrup components with health-promoting effects on glucose homeostasis. α-glucosidase inhibitory activity has been found in maple syrup phenolics [7,20,76]. ABA is a phytohormone in maple with promising anti-diabetic properties [7,[77][78][79][80]. Furthermore, ABA has been shown to protect against Type-2 diabetes [78,80,81]. The impact of MS-EtOAc and MS-BuOH extracts on inhibiting carbohydrates by hydrolyzing enzymes (i.e., α-glucosidase) has been studied by Apostolidis et al. [82], where MS-BuOH exhibited more marked inhibitory action than EtOAc and was posed as a potential adjuvant with antihyperglycemics for Type-2 diabetes management. How phenolics-rich maple syrup extract affects Type-2 diabetes model mice has been evaluated by Toyoda et al. [81]. In Type-2 diabetic mice livers, treating rats with maple syrup extract inhibited fat accumulation by down and upregulating lipolysis hepatic enzymes and lipogenesis. The same research group [83] revealed how maple syrup extract can help with some dyslipidemia symptoms by another experiment.
In healthy rats, St-Pierre et al. [7] examined maple syrup metabolic reactions against other sweeteners. Dextrose corn syrup and brown rice syrup resulted in lower peak responses insulin, glucose, amylin, and gastric inhibitory polypeptide than maple syrup. Maple syrup's unique properties, plus metabolic reactions to its consumption in animals, indicate that it might be a healthy alternative to other sugars. Dupuy and Tremblay [84] examined the effects that maple-sweetened beverages (sap or syrup) have on cognitive flexibility while practicing high-intensity exercise using a commercial sports drink, water, and glucose. The glycemic index of maple products was lower than the glycemic index of glucose alone.
Antimicrobial potential and a significant synergistic effect with various antibiotics were found in a phenolics-rich maple syrup extract against Gram-negative microorganisms (Escherichia coli, Proteus mirabilis and Pseudomonas aeruginosa). Catechol was effectively combined with antibiotics and other phenolics in a phenolics-rich maple syrup extract to inhibit microbial growth [70].
In summary, we would like to remark that, in addition to the main constituent sucrose, maple products also include phenolics, pyrazines, vitamins, minerals, organic acids, and phytohormones. These bioactive substances have the potential to be valuable due to their positive impacts on health, such as their antioxidant, antiproliferative, and antimutagenic properties. It is proposed that quebecol, lariciresinol, and secoisolariciresinol serve as distinctive markers for maple products and are uncommon in syrups made from other plants [56].

Food Safety
Food safety is a key determinant in the quality of maple syrup, wherefore aspects related to it should be seriously considered. In particular, the risk of contamination with metals and toxic microorganisms, namely fungi [49,85] given the potential for occurrence of mycotoxins even at low water activity levels (aw) [86], which still needs to be investigated. Table 4 summarizes the main findings in terms of the occurrence of contaminants in maple syrup. Eurotium herbariorum was the most prevalent fungus found in the maple syrup samples. It was followed by Penicillium chrysogenum, three Aspergillus species (A. penicillioides, A. restrictus, A. versicolor) and two Wallemia species (W. muriae and W. sebi). Cladosporium cladosporioides was also isolated.

Quality Control
As in other natural sweeteners obtained from vegetables, maple syrup is collected from maple sap trees in some regions of eastern Canada and northwestern USA (Figure 1). It being a seasonal product with given harvest dates means that it can be collected in about 35 days. Its characteristics depend on the production region where it is harvested, and it is affected by both the weather and its extraction process (boiling), performed to obtain the final syrup. For these reasons, Canadian and US maple syrup production can fluctuate yearly due to changes in the weather. The principal Canadian maple syrup production concentrates in the eastern province of Quebec. Therefore, Quebec is the leading maple syrup producer in Canada, and has by far the most maple farms, taps and, as a result, the most maple syrup [87].
Quebec is the province with the highest maple syrup production levels in Canada, which leaves the second highest producing province, New Brunswick, far behind (Table 5). New Brunswick's production levels are the same as that of Vermont in the USA, which is by far the nation's biggest maple syrup producer, followed by New York (Table 5).
Although there are only four species of maple trees used to collect maple sap and to obtain maple syrup, there are actually over 150 maple tree species worldwide [88]. The more important producing species are Acer saccharum (70%) and Acer rubrum (29%). Nevertheless, other silver species, such as silver maple Acer saccharinum and black maple Acer nigrum (1%), can be considered as maple sap-producing species.
We briefly summarize the maple sap-producing description. Maple sap is normally collected in February-March when weather conditions make it easy to collect sap. This is done by boring holes in maple tree trunks. The encrusted tap permits sap to flow by pipelines to buckets. Sap is then concentrated by boiling it down into maple syrup [89]. For Canada, maple sup products (sugar and maple butter, maple syrup and taffy) are very interesting in economic and cultural terms because maple product exports have constantly increased and exported maple products now amount to more than 385 million Canadian dollars.

The Maple Syrup Quality Standard
Maple trees accumulate starch as they grow, which is converted into sugar during spring thaws and mixes with water absorbed by tree roots to create maple sap, which generally flows between February and April every year [90]. Producers employ tubing systems, RO, and high-performance evaporators to collect sap before boiling it down to obtain maple syrup. Canadian maple syrup products range from traditional maple syrup to maple butter, maple candy and maple sugar, plus a wide range of maple syrup-containing products.
Canadian maple syrup takes two grade names: "Canada Grade A" (further graded into four color classes-"Golden, Delicate Taste", "Amber, Rich Taste", "Dark, Robust Taste" and "Very Dark, Strong Taste"-that typically reach consumers and commercial markets); and "Canada Processing Grade", which has no color classes and is frequently applied to large-scale commercial applications [22]. In 2020, they harvested 13.2 million gallons. Thanks to favorable spring weather and more taps, higher yields account for more production. Prices in other maple-producing provinces are set by producers. As a result, they can substantially vary. Prices in Quebec are controlled by the Régie des Marchés Agricoles et Agroalimentaires du Québec. This organization helps to stabilize prices from year to year. In 2020, the price in Quebec remained at $38.55/gallon, and the total maple products value was $509.2 million [90]. Maintaining final product quality is most important. Thus, for economic performance to improve, the increase in maple farmers and maple taps denotes the profitability of such activities [68]. There are several quality guidelines for the production and commercialization of maple syrup from Canada and the US [91,92], but they are all generic guidelines for food safety without a focus on specific hazards.

Factors That Can Influence Final Maple Syrup Composition
Unlike other sugar sources, maple syrup has a unique characteristic flavor that depends on its composition, e.g., organic compounds (sugars, alcohols, ketones, aldehydes), micronutrients and phytochemicals, of which more than 200 compounds appear in maple syrup that are either natural or are transformed during processing [13]. Hence this unique composition can be used for either detecting possible fraud and adulteration with other syrups of lesser quality, such as cane sugar, beet, and corn [13], or for confirming those classified according to strict Canadian and US regulations.
Thus, many essential and non-essential metals are present in maple sap, and their numbers may rise during maple sap concentration done by boiling [13]. Several studies have shown the utility of determining some essential elements, such as salt content, to be used in maple syrup characterization and for differentiating it from other syrups depending on the relation of these compounds.
Several authors, such as Lagacé et al. [49], have studied maple sap during different harvest periods to show variations in its organic composition: sugar (sucrose, fructose, and glucose), organics acid, and phenolic compounds. This means having to change some intrinsic factors, such as maple tree sap flowing in trunks. Moreover, other changes could modify its composition given the microbial maple sap population (fungi and total aerobics), which progressively increase during the season [9,93]. After maple sap is collected, it is contaminated by microorganisms, which are responsible for sucrose hydrolysis and the final presence of fructose and glucose in maple syrup [50].

Applications in the Food Industry and Sustainability Issues
Maple syrup comprises sugar, trace amounts of organic acids, free amino acids, protein, minerals, and phenolic compounds [11]. These trace components allow maple syrup's taste profile to be distinguished from that of sucrose. They are what contributes to its potential health benefits when compared to sucrose [96]. Maple syrup can be manufactured from a combination of corn syrup, maple coloring and flavoring. However, maple syrup in its natural state contains minerals like calcium and potassium, which may

Applications in the Food Industry and Sustainability Issues
Maple syrup comprises sugar, trace amounts of organic acids, free amino acids, protein, minerals, and phenolic compounds [11]. These trace components allow maple syrup's taste profile to be distinguished from that of sucrose. They are what contributes to its potential health benefits when compared to sucrose [96]. Maple syrup can be manufactured from a combination of corn syrup, maple coloring and flavoring. However, maple syrup in its natural state contains minerals like calcium and potassium, which may not be at the same levels when it is manufactured [97]. Pure maple syrup possesses specific standards for clarity, density, flavor, and color properties, along with descriptors that typically include woody, vanilla, caramel, floral, fruit and herbaceous [11,19].
When considering maple syrup applications as an ingredient in the food industry, the chemical analysis and nutritional profile of maple syrup are essential, as discussed in the previous Sections 2 and 3. A better understanding of the physico-chemical and microbiological analyses, organic and inorganic constituents, adulterants, and contaminants of maple syrup will help to ensure its uniformity and quality standards in the food industry.
According to the International Maple Syrup Institute, pure maple syrup has a small market share in the USA, Canada and elsewhere overseas. For example, in the USA, maple syrup, along with honey, represents 1% of all the sweeteners delivered for food and beverage uses [98]. Maple syrup is not only employed as a pancake topping, but its unique characteristics are mediated by bioactive compounds, which makes its suitable for several culinary and industrial applications.
Several other maple syrup applications are found in the food and beverages industry and have been paid some attention as part of the culinary education guide compiled by Kimball [99]. They include the following: -Maple butter: It is thick, but spreadable, and is also called maple cream or spread. It is a whipped version of pure maple syrup; -Clear maple: It begins as maple syrup. It is then altered by adding a processing aid, which is removed later to create higher invert sugar content. The resulting product is a product with a honey-like consistency made from pure maple syrup; -Pure maple syrup concentrate: It is produced after removing almost 50% of the sucrose content in pure maple syrup; -(Medium or coarse) maple flakes: They are made with pure maple syrup that has been dehydrated by means of a unique exclusive process; -Maple jelly: It is made from pure maple syrup with a jelly-like structure and can be used for culinary purposes. -Maple sugar: It is made from pure maple syrup by dehydration into granulated sugar crystals. It can be replaced at 1:1 with regular granulated sugar in the majority of formulae and recipes; -Maple vinegar: It is produced from pure maple syrup by alcoholic fermentation and acetic fermentation processes. Adding maple vinegar creates a signature salad dressing.
Maple syrup can be employed in diverse menu items to sweeten tea, lemonade, regular coffee, and café lattes. Acorn squash or sweet potatoes can be glazed with maple syrup. Maple can be used to create a sweet and savory barbecue sauce, and drizzled on pears, walnuts, and gorgonzola pizzas. Baked maple-kissed goods, ice-cream and desserts can be prepared to gain a better taste. For a wider application, there is a need to utilize maple syrup on an industrial scale that will ensure that more consumers gain from its naturalness and health benefits.
Maple syrup processing mainly involves water removal to increase its viscosity. The main industrial processing techniques are based on conventional evaporation and reverse osmosis (RO), as described by Ramadan et al. [56].
The two processing methods shown in Figure 2 will have impacts on the quality characteristics of the obtained maple syrup. When comparing the two processes, evaporation can result in varying sensory attributes, such as color and flavor, while RO performed at room temperature does not change maple syrup's chemical properties [56]. The heating and evaporation steps of maple sap are critical maple syrup processing stages. Flavor and color are essential factors that affect the maple syrup grade, which range from very dark-colored strong-flavored syrup to very light-colored delicate-flavored syrup [11]. Maple syrup flavor is also influenced by the regions where sugar maple trees grow. The amount of nitrogen compounds, phenolic compounds, flavonoids, and organic acids in maple sap may vary throughout the maple syrup season, and also from one season to the next, according to the region, and even from one maple tree to the next [99]. When comparing RO to conventional evaporation, which requires high energy use, RO can be used to concentrate maple sap to cut energy costs [100]. In industry, most RO techniques are extensive processes involving many steps [101]. To obtain 1 kg of maple syrup (68% sugar), 34 kg of maple sap (2% sugar) are required, plus conventional evaporation. Conversely, when RO concentrates maple saps up to 20%, only 3.5 kg of sap are necessary to obtain 1 kg of maple syrup [102].
It is crucially important to note that quality standards of the obtained maple syrup products are affected by the processing parameters; the phytochemical profile of these products will also influence flavor and color when they are considered for applications in the food industry. Recently, it was suggested that quebecol, lariciresinol, and secoisolariciresinol are distinct markers for maple products since they are not common in other plant-derived syrups [56]. Another significant limitation in the quest for further industrial application of maple syrup is microbial contamination of the maple sap which will affect maple product quality. The application of a continuous heat treatment on buddy syrups for 2 h at 104.5 °C was able to remove the buddy off-flavor by reducing the volatile dimethyl disulfide content in maple syrup which is responsible for this off flavor [52]. Henceforth, it is important to conduct further research on how processing techniques and environmental conditions affect the phytochemicals profile and biological effects of industrially produced maple food products.
With increasing climate change awareness, there is contention about evidence for a climate optimum for syrup production based on a standardized protocol for collecting sap from individual trees under natural conditions. There are indications that there will be shorter sap flow with warming winter temperatures if traditional tapping schedules are maintained [103]. Modeling the relations among climate, sap flow and sugar concentration is necessary to gain an understanding of the basic eco-physiological responses underlying climate effects on syrup production [104]. As Duchesne and Houle [105], Collins et al. [106], and Bal et al. [107] misrepresented this, further research is necessary [104]. In Canada, producers must adhere to the strict standards and guidelines set by Canadian Law and the Federation throughout the maple syrup production process. It is important that the maple syrup value chain is sustainable. The role of maple syrup as When comparing RO to conventional evaporation, which requires high energy use, RO can be used to concentrate maple sap to cut energy costs [100]. In industry, most RO techniques are extensive processes involving many steps [101]. To obtain 1 kg of maple syrup (68% sugar), 34 kg of maple sap (2% sugar) are required, plus conventional evaporation. Conversely, when RO concentrates maple saps up to 20%, only 3.5 kg of sap are necessary to obtain 1 kg of maple syrup [102].
It is crucially important to note that quality standards of the obtained maple syrup products are affected by the processing parameters; the phytochemical profile of these products will also influence flavor and color when they are considered for applications in the food industry. Recently, it was suggested that quebecol, lariciresinol, and secoisolariciresinol are distinct markers for maple products since they are not common in other plant-derived syrups [56]. Another significant limitation in the quest for further industrial application of maple syrup is microbial contamination of the maple sap which will affect maple product quality. The application of a continuous heat treatment on buddy syrups for 2 h at 104.5 • C was able to remove the buddy off-flavor by reducing the volatile dimethyl disulfide content in maple syrup which is responsible for this off flavor [52]. Henceforth, it is important to conduct further research on how processing techniques and environmental conditions affect the phytochemicals profile and biological effects of industrially produced maple food products.
With increasing climate change awareness, there is contention about evidence for a climate optimum for syrup production based on a standardized protocol for collecting sap from individual trees under natural conditions. There are indications that there will be shorter sap flow with warming winter temperatures if traditional tapping schedules are maintained [103]. Modeling the relations among climate, sap flow and sugar concentration is necessary to gain an understanding of the basic eco-physiological responses underlying climate effects on syrup production [104]. As Duchesne and Houle [105], Collins et al. [106], and Bal et al. [107] misrepresented this, further research is necessary [104]. In Canada, producers must adhere to the strict standards and guidelines set by Canadian Law and the Federation throughout the maple syrup production process. It is important that the maple syrup value chain is sustainable. The role of maple syrup as a non-timber forest product, an alternative to extractive forest timber activities within the community will contribute to subsistence needs and help diversify and supplement rural incomes [108]. Generally, harvesting is carried out in such a way that sugar maple trees are tapped in a different area from that of the year before to preserve tree health. For instance, the Canadian "Preservation of Agricultural Land and Agriculture Activities Act" forbids felling a whole maple tree in an agricultural area [57]. While maple syrup demand as a natural and healthy sweetener alternative in the food industry increases, the entire value chain's sustainability is an important criterion from production to consumption.

Conclusions
This review explores the potential of maple syrup as a natural sweetener to be used in human diet. As consumers are showing considerable interest in demanding more natural ingredients in their food items, it critically examines maple syrup, along with its quality characteristics, and nutritional and health impacts. In fact, current scientific evidence indicates that phenolic compounds play a key role in the body's defense, protecting it from damage caused by reactive oxygen species known to be involved in the genesis of various pathologies, cardiovascular, oncological, autoimmune, degenerative, etc. That said, the potential of maple syrup, derived from Acer saccharum Marsh., as a source of nutrients and bioactive compounds is immense and deserves to be highlighted.
Therefore, the objective of this paper was to perform a global review on maple syrup as an interesting sweetener with application in the food sector. Furthermore, this review also aims to contribute to the improvement of food availability in a sustainable way and to provide also economic welfare. Finally, this sweetener can offer an important contribution for the development of new food products in the future and can contribute to decisive improvements in public health.  Acknowledgments: The authors are very grateful to their families and friends for all the support they provided.

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