Chemical Composition and Biological Effects of Maple Syrup - ACS

Chapter 18

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Chemical Composition and Biological Effects of Maple Syrup Liya Li and Navindra P. Seeram* Bioactive Botanical Research Laboratory, Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, University of Rhode Island, Rhode Island, 02881, U.S.A. *E-mail: [email protected]

Pure maple syrup is unique in that it is the largest commercially produced and consumed plant natural product that is obtained entirely from the sap of trees. It is a natural sweetener produced by concentrating the colorless watery sap collected from certain maple (genus, Acer) species. The natural maple tree sap contains minerals, oligosaccharides, peptides, amino and organic acids, phytohormones, and phenolics, apart from sucrose which is its predominant sugar. During the intensive heating process required to transform sap to syrup, a complex cocktail of both native (originally present in the xylem sap), and derived (formed through chemical reactions during processing) phenolic compounds ultimately ends up in maple syrup. From a human health perspective, this is interesting considering that phenolics have attracted significant research attention for their potential role in the prevention and treatment of several chronic human diseases. Here we review current scientific knowledge of the chemical constituents, in particular phenolics, present in maple syrup. The biological activities of these compounds, in relation to the health benefits that may result from maple syrup consumption, are also briefly discussed.

© 2012 American Chemical Society In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Introduction Plant foods and their derived products have attracted tremendous scientific attention for their biological effects and possible human health benefits. While plants contain several essential macro- and micronutrients, such as protein, carbohydrates, fat, fiber, minerals and vitamins, research has shown that their secondary metabolites i.e. phytochemicals, may impart health benefits beyond basic nutrition. Moreover, it is now well accepted that consumption of a phytochemical-rich diet contributes towards reducing the risk of several oxidative-stress and inflammatory mediated diseases such as certain cancers, heart, and other chronic human illnesses. Phytochemical-rich foods include fruits, vegetables, whole grains, spices, beverages, and their derived products such as juices, tea, coffee and wine. Interestingly, among plant foods, maple syrup, which is the subject of this review, stands out in that it is the largest commercially available and consumed natural product which is obtained entirely from the sap of deciduous trees. Maple syrup is a natural sweetner produced by concentrating the sap collected from certain maple species (genus, Acer) (1, 2). The main maple species used for this purpose include the sugar maple (A. saccharum), red maple (A. rubrum), and black maple (A. nigrum) trees which are all native to North America (1, 2). These maple species are best utilized for maple syrup production because their sap contains a higher sugar content compared to that of other maples (1). Maple sap is collected in the late winter to spring months when freeze/thaw cycles causes the sweet sap to rise and flow from taps made in the tree trunk (1, 2). Maple syrup is primarily produced for commercial purposes in the northeastern regions of Canada and the United States. Remarkably, Canada (85%; mainly the province of Quebec) leads the world production of maple syrup. Thus, apart from its economic importance, maple syrup production is of great cultural significance to this region of the world. Maple syrup is obtained by the thermal evaporation of the colorless watery sap and about 40 L of sap is required to produce 1 L of MS (1). During the concentration process of transforming sap to syrup, the characteristic flavor, color, and odor of MS develops. Typically, the color of the syrup becomes darker as the season progresses, and based on Canadian standards, MS is graded as extra light (grade AA), light (grade A), medium/amber (grade B), and dark (grade C) (2). Given the worldwide popularity and consumption of maple syrup, knowledge of its chemical constitutents is of great scientific interest. This is especially relevant from a human health perspective given the aforementioned attention that phytochemicals have attracted. Recent research published by our laboratory (3, 8), and others (4–7), have revealed a wide diversity of phenolic compounds present in pure maple syrup. Here we review the chemical compounds, particularly phenolics, identified in maple syrup, to date, and discuss the biological effects and potential health benefits that may result from maple syrup consumption. It should be noted that certain constituents of maple syrup, specifically minerals, monosaccharides and oligosaccharides, organic and amino acids, peptides, and proteins are well established (1, 2). Thus, this review primarily focuses on the 324 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

phytochemical constituents present in maple syrup, specifically those which are phenolic in nature.


Chemical Constituents of Maple Syrup The chemistry of maple syrup is complicated due to the presence of naturally occurring compounds, originally present in the xylem sap, as well as process-derived compounds which are formed during the intensive heating process required to transform sap into syrup. While increasing the complexity of the final product, these in situ reactions also give maple syrup its characteristic color, odor, and flavor (1). Ultimately, due to a combination of these factors, a complex cocktail of compounds ends up in maple syrup. It should be noted that the chemistry of maple syrup may be further complicated depending on the geographical location and the particular maple species (or combination thereof) used for sap collection, etc. To date, the chemical constituents reported in maple syrup are shown in Table 1 and the corresponding structures of these compounds are shown in Figure 1. The majority of the compounds reported in maple syrup are phenolic in nature and include lignans (1-7), coumarins (8-9), stilbene (10), benzoic acid (11-19) and benzaldehyde (20-22) derivatives, phenylpropanoids (23-32), and flavonoids (3339). Apart from these phenolic compounds, several nitrogen containing molecules, namely pyrazines (47-55), and an assortment of other compounds (56-69), have also been reported in maple syrup. Maple syrup is produced under intensive heating conditions required to transform sap to syrup. Thus, it is not surprising that maple syrup contains naturally occurring phenolics (present in the xylem sap) as well as non-natural, artefacts or process-derived compounds (formed by chemical reactions during its production). In addition, Maillard reactions occur between amino acids and reducing sugars, and polycarbonyl compounds are formed (1). Apart from the natural plant phenolics reported in maple syrup, we have recently reported the isolation of a non-natural, process-derived phenolic compound in maple syrup from Canada (8). The compound, 2,3,3-tri-(3-methoxy-4-hydroxyphenyl)-1-propanol (70) was assigned the common name of quebecol. Examination of the maple sap using LC-MS analyses failed to identify quebecol therein, confirming that it is formed during the syrup and/or extract preparation (8). Phenolics are the predominant phytochemical constituents found in maple syrup. They are also among the most ubiquitous and abundant phytochemicals present in plant foods and thus, in human diet. These compounds have attracted significant research attention for their biological activities, in particular their role as antioxidants and activities beyond antioxidation (reviewed in (9, 10)). The role of these compounds in imparting potential biological effects and health benefits to maple syrup is further discussed below. Naturally occurring plant phenolics exhibit considerable structural diversity including varying types and levels of oxidation as well as varying substitution patterns of hydroxylation and glycosylation. The main classes of monomeric 325 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


natural phenolics that are commonly found in plant foods include flavonoids, lignans, stilbenes, coumarins, and phenolic acids (9). However, it should be noted that process-derived compounds can also be formed from reactions occurring among these compounds (8). Interestingly, maple syrup contains a cocktail of all of the aforementioned sub-classes of naturally occurring phenolics several of which are thought to be formed as degradation products of lignin components in sap (1). Thus, further research to comprehensively isolate and elucidate the structures of all of the chemical constituents present in maple syrup is warranted.

326 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. Structures of chemical constituents identified in maple syrup. Compounds 1- 46, and 70 are phenolics and compounds 47-69 are non-phenolic constituents.


Table 1. Chemical constituents identified in maple syrup Name

Molecular Formula

Molecular weight

Reference


Lignans 1

Lyoniresinol

C22H28O8

420

(3)

2

Secoisolariciresinol

C20H26O6

362

(3)

3

Dihydrodehydrodiconiferyl alcohol

C20H24O6

360

(1)

4

5-methoxy-trans-dihydrodehydrodiconiferyl alcohol

C21H26O7

390

(3)

5

Guaiacylglycerol β-coniferyl ether

C20H24O6

360

(3)

6

Guaiacylglycerol-β-O-4′dihydroconiferyl alcohol

C20H26O7

378

(3)

7

3-[(4-[(6-deoxy-α-L-mannopyranosyl)oxy]-3-methoxyphenyl)-5(3,4-dimethoxyphenyl)dihydro-3-hydroxy-4-(hydroxymethyl)-2(3H)-furanone

C27H34O12

550

(3)

Coumarins 8

Fraxetin

C10H8O5

208

(3)

9

Scopoletin

C10H8O4

192

(3)

C16H16O4

272

(3)

Stilbene 10

(E)-3,3′-dimethoxy-4,4′-dihydroxy stilbene

Benzoic acids 11

Gallic acid

C7H6O5

170

(7)

12

Methyl gallate trimethyl ether

C11H14O5

226

(3)

13

Syringic acid

C9H10O5

198

(7)

14

Protocatechuic acid

C7H6O4

154

(7)

15

Vanillic acid

C8H8O4

168

(7)

16

Benzoic acid

C7H6O2

122

(7)

17

1-O-galloyl-β-D-glucose

C13H16O10

332

(7)

Continued on next page.


Table 1. (Continued). Chemical constituents identified in maple syrup Benzoic acids 18

Resorcylic acid

C7H6O4

154

(7)

19

Gentisic acid

C7H6O4

154

(7)


Benzaldehydes 20

Catechaldehyde

C7H6O3

138

(3)

21

Vanillin

C8H8O3

152

(6)

22

Syringaldehyde

C9H10O4

182

(6)

Phenylpropanoids 23

p-coumaric acid

C9H8O3

164

(7)

24

Ferulic acid

C10H10O4

194

(7)

25

4-methoxycinnamic acid

C10H10O3

178

(7)

26

Sinapic acid

C11H12O5

224

(7)

27

Chlorogenic acid

C16H18O9

354

(7)

28

Syringenin

C11H14O4

210

(3)

29

Coniferyl alcohol

C10H12O3

180

(6)

30

C-veratroylglycol

C10H12O5

212

(3)

31

Dihydroconiferyl alcohol

C10H14O3

182

(5)

32

Syringoyl methyl ketone

C11H12O5

224

(1)

Flavonoids 33

Catechin

C16H16O5

288

(7)

34

Epicatechin

C16H16O5

288

(7)

35

Astragalin

C21H20O11

448

(7)

36

Kaempferol 3-O-galacotoside

C21H20O11

448

(7)

37

Isoquercetrin

C21H20O12

464

(7)

38

Quercitrin

C21H20O11

448

(7)

39

Rutin

C27H30O16

610

(7)

Other phenolic compounds 40

2-Hydroxy-3′,4′-dihydroxyacetophenone

C8H8O4

168

(3)

41

1-(2,3,4-trihydroxy-5-methylphenyl)ethanone

C9H10O4

182

(3)

42

2,4,5-Trihydroxyacetophenone

C8H8O4

168

(3)

43

3′,4′,5′-Trihydroxyacetophenone

C8H8O4

168

(3)

Continued on next page.


Table 1. (Continued). Chemical constituents identified in maple syrup Other phenolic compounds 44

Catechol

C6H6O2

110

(3)

45

Syringol

C8H10O3

154

(1)

46

Homovanillic acid

C9H10O4

182

(6)


Pyrazines 47

2,6-Dimethylpyrazine

C6H8N2

108

(1)

48

Methylpyrazine

C5H6N2

94

(1)

49


C6H8N2

108

(1)

50


C6H8N2

108

(1)

51

2,5-Dimethyl-3,6-diisobutylpyrazine

C14H24N2

220

(1)

52

Ethylpyrazine

C6H8N2

108

(1)

53

2-Ethyl-6-methylpyrazine

C7H10N2

122

(1)

54

2-Butylpyrazine

C8H12N2

136

(1)

55

2,3-Dimethyl-5-isopropylpyrazine

C9H14N2

150

(1)

Other compounds 56

3-Hydroxy-2-pyrone

C5H4O3

112

(1)

57

2,3-Dihydro-3,5-dihydroxy-6-methyl-4pyranone

C6H8O4

144

(1)

58

2-Methylbenzoquinone

C7H6O2

122

(1)

59

3-Methylmaleic anhydride

C5H4O3

112

(1)

60

Homosotolone

C5H4O3

142

(1)

61

Furaneol

C6H8O3

128

(1)

62

2-Methyl-2-cyclopentenone

C6H8O

96

(1)

63

2-Hydroxymethyl-2-cyclopentenone

C6H8O2

112

(1)

64

Cyclotene

C6H8O2

112

(1)

65

2-Hydroxymethylcyclopent-2-en-1-ol

C6H10O2

114

(1)

66

Hexanoic acid

C6H12O2

116

(1)

67

2-Ethylhexanoic acid

C8H16O2

144

(1)

68

Acetoin

C4H8O2

88

(1)

69

Isopropenyl methyl ketone

C5H8O

84

(1)

426

(8)

Process-derived compounds 70

C24H26O7

Quebecol



Biological Effects of Maple Syrup Unfortunately, there is a scarity of reports on the biological evaluation of maple sap and syrup extracts. A study showed that phenolic-enriched extracts of maple sap and syrup, collected at different periods in the season, had antioxidant and antimutagenic activities (11). The later collection periods of sap results in the darker grades of maple syrup which have correspondingly higher levels of phenolics. These authors found that the total polyphenol concentration (in gallic acid equivalents, GAEs) increased from 16.51-8.51 g GAE/100g to 24.6 g GAE/ 100g for samples collected later in the season. Further, in that study the authors concluded that the phenolic constituents which are present in maple syrup in their glycosylated forms were more active than their corresponding aglycones. In a separate study, the antioxidant activity, inhibition of nitric oxide (NO) overproduction in RAW264.7 cells (to measure anti-inflammatory activity), and human cancer cell antiproliferative effects of maple sap and syrup extracts were evaluated (12). For that study, maple sap and syrup were collected from 30 producers at different periods of harvest from three different regions of Quebec, Canada. These authors reported that later collections of sap which had more phenolics also exhibited better activity. Overall, the ethyl acetate extracts of these different samples of maple sap and syrup were found to significantly inhibit the lipopolysaccharide-induced NO overproduction in the RAW264.7 murine macrophages. The authors found that the maple syrup extracts were significantly more active than maple sap extracts and concluded that the transformation of maple sap into syrup increases NO inhibition activity. Notably, the highest NO inhibition induced by the maple syrup extracts was observed at the end of the season. In addition, the darker grade of maple syrup was found to be more active than clear maple syrup. Thus, the authors concluded that ‘colored oxidized’ compounds could be responsible for the observed biological activity. Also, the maple syrup extracts showed selective in vitro antiproliferative activity against a panel of lung, colon, breast, prostate and brain human cancer cells. Finally, recent research in our laboratory showed that a maple syrup butanol extract, and several of its individually purified phenolic constituents, have potent antioxidant activities (3). Using the diphenylpicrylhydrazyl (DPPH) free radical scavenging assay, the antioxidant activities of the maple syrup compounds 1-9, 11, 13, 20, 22, 29, 30, 33, 34, 40, 41, 44 were comparable to vitamin C and the commercial synthetic antioxidant, butylated hydroxytoluene (BHT). The IC50 of the antioxidant activities of the different compounds ranged from 20-1400 µM. Compounds 8, 11, 20, 40, 41 showed superior antioxidant activities when compared to vitamin C (IC50 = 58 µM). Among the diverse phenolic subclasses of compounds identified in the maple syrup butanol extract, the general trend in antioxidant activity was flavonols > benzoic acids, benzaldehyde > coumarins > other simple phenolics > phenylpropanoids > stilbene, lignans. Thus all of the above studies (3, 11, 12), suggest that the phenolic constituents may be responsible in large part for the observed biological effects of maple syrup. It should be noted that phenolics have been implicated in the prevention of several chronic human diseases mediated by oxidative stress and inflammation (reviewed in (9, 10)). Oxidative stress can cause damage to biomolecules 331 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


including lipids, proteins, and DNA, resulting in an increased risk for several chronic human diseases. These include inflammatory and cardiovascular diseases, certain cancers, diabetes, Alzheimer’s disease, and age-related functional decline. It is possible the wide range of phenolic antioxidants found in maple syrup may help to protect cellular systems from oxidative damage thereby also lowering the risk of certain chronic human diseases. However, while the wide array of chemical constituents, both natural and process-derived, found in maple syrup may impart biological effects and potential health benefits to this natural sweetener, in vivo studies to evaluate these properties are needed to confirm this.

Conclusions Maple syrup is the largest commercially produced and consumed natural product which is obtained entirely from the sap of deciduous trees. During the process of tapping of maple trees, natural phenolic compounds present in the tree sap ultimately ends up (and are concentrated 40X) in maple syrup. In addition, because of the intensive heating conditions required to transform sap to syrup, process-derived compounds are also formed and are present in maple syrup. Thus, maple syrup contains a wide diversity of bioactive chemical constituents in combination with minerals, amino acids, organic acids, phytohormones, peptides and natural sugars. The complex cocktail of maple syrup constituents may impart potential health benefits to maple syrup, as suggested by in vitro studies, but more research, particularly in vivo studies, would be needed to confirm this. In conclusion, given the wide diversity of bioactive compounds present in maple syrup, evaluation of the health benefits of this natural sweetener holds great promise.

Acknowledgments This work was supported by the Conseil pour le développement de l’agriculture du Québec (CDAQ), with funding provided by Agriculture and Agri-Food Canada’s Advancing Canadian Agriculture and Agri-Food (ACAAF) program.

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Potter, T. L.; Fagerson, I. S. Phenolic compounds in maple sap. In Phenolic compounds and their effects on health; Ho, C. T., Lee, C. Y., Huang, M. T., Eds.; ACS Symposium Series 506; American Chemical Society: Washington, DC, 1992; Part I. 6. Kermasha, S.; Goetghebeur, M.; Dumont, J. Determination of phenolic compound profiles in maple products by high performance liquid chromatography. J. Agric. Food Chem. 1995, 43, 708–716. 7. Abou-Zaid, M. M.; Nozzolillo, C.; Tonon, A.; Coppens, M.; Lombardo, A. D. A. High performance liquid chromatography characterization and identification of antioxidant polyphenols in maple syrup. Pharm. Biol. 2008, 46, 117–125. 8. Li, L.; Seeram, N. P. Quebecol, a novel phenolic compound isolated from Canadian maple syrup. J. Functional Foods 2011, in press, doi:10.1016/ j.jff.2011.02.004. 9. Seeram, N. P. Bioactive Polyphenols from Foods and Dietary Supplements: Challenges and Opportunities. In Herbs: Challenges in Chemistry and Biology; ACS Symposium Series No. 925; Ho, C. T, Wang, M., Sang, S., Eds; American Chemical Society: Washington, DC, 2006; Chapter 3, p 25−38. 10. Seeram, N. P.; Heber, D. Impact of berry phytochemicals on human health: Effects beyond antioxidation. In Antioxidant Measurement and Applications; Ho, C. T, Shahidi, F. S., Eds.; ACS Symposium Series No. 956; American Chemical Society: Washington, DC, 2007; Chapter 21. 11. Theirault, M.; Caillet, S.; Kermasha, S.; Lacroix, M. Antioxidant, antiradical and antimutagenic activities of phenolic compounds present in maple products. Food Chem. 2006, 98, 490–501. 12. Legault, J.; Girard-Lalancette, K.; Grenon, C.; Dussault, C.; Pichette, A. Antioxidant activity, inhibition of nitric oxide overproduction, and in vitro antiproliferative effect of maple sap and syrup from Acer saccharum. J. Med. Food. 2010, 13, 460–468.


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Chemical Composition and Biological Effects of Maple Syrup - ACS

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