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Dr. Winfried Behr

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Dietetic and Pharmaceutical Raw Materials

MAPLE SYRUP

Maple syrup is manufactured by the evaporative concentration of the sap of certain American maple species (mainly Acer saccharum) (MORSELLI, 1988). Sucrose is stored in the root tissue of maples in the fall in solid form. In spring when the temperature rises above freezing point during the day while there is still frost in the night, water is absorbed by the roots forming a more or less concentrated aqueous solution which, driven by osmotic pressure, under further absorption of water (dilution) expands into the tree. Rising of the sap occurs before any signs of vegetation are visible in a still snow covered lanscape. Formerly the sap flowing from taps was collected in zinc galvanized buckets which were fixed to the individual trees. Nowadays the sap is harvested through a plastic pipe system connected to the individual taps and pumped to the sugar cabin. It has a sucrose concentration ranging from 2% (in younger trees) to 6% (in older ones). The conversion into syrup is achieved by evaporation in wood fired oblong pans with corrugated bottoms. A solid content in the surup of 66% to 67% is established as below 63% fermentation and above 70% crystallization may occur. The solid content of maple syrup consists in principle of sucrose with a minor and varying percentage of glucose and fructose (that is of hydrolysed sucrose). A haziness in untreated maple syrup consists of calcium malate; it is normally removed by a filtration step. Maple syrup has a light to dark brown colour. The colour develops from the colourless (and tasteless) maple sap during evaporation. It is partly the result of caramelization and partly caused by a Maillard reaction between amino acids and (the small percentage of) monosaccharides. The Maillard reaction can only take place if at least some of the non-reducing sucrose is converted to the reducing monosaccharides glucose and fructose. The colour intensity depends therefore both of the processing conditions and the characteristics of the maple sap. Higher ambient temperature (toward the end of the tapping season), the use of lower concentrated sap of younger trees, any delay before evaporation, and geographical influences may cause stronger hydrolysis of the sucrose and/or the presence of higher concentrations of amino acids, and thus a deepenimg of the colour. Colour is used as a basis for the grading of maple syrup; it is measured in a photometer. The attenuation of a light beam at 560 nm wave length is determined after it has passed a maple syrup filled cuvette of a width of 1 cm. The percentage of light transmitted specifies the various gradations:
 
Canadian gradation Light transmission of a 1 cm layer at 560 nm European gradation
No. 1, extra light 75,0% AA
No. 1, light 60,5%-75,0% A
No. 1, medium 44,0%-60,6% B
No. 2, amber 26,0%-43,0% C
No. 3, dark max 25,0% D
End consumers prefer lighter qualities and therefore only grade AA to C (in particular B and C) are sold in bottles and small containers. The taste intensity, however, increases with the colour of the maple syrup (especially a caramel flavour) wherefore grade D is a useful raw material for industrial applications (in flavour blends for icecream, for the aromatization of tobacco, and for meat curing).

The flavour of maple syrup develops like the colour also during evaporation. In gas chromatograms of dichloromethane extracts more than 133 substances have been observed, 48 peaks were identified. 41 components were phenol derivatives which represented the bulk (about 70%) of the extract. In principle two types of flavour bearing constituents occur:

(1) Thermal sugar degradation products as 3-methyl-2-hydroxy-cyclopenten-2-on (cyclotene), 2,5-dimethyl-4-hydroxy-2(2H)-furanon (furaneol) and related compounds. There is little variation in the concentration of these compounds in syrup samples of different provenances and qualities.

(2) Derivatives of the lignin precursors coniferyl, dihydroconiferyl, and dihydrosinapyl alcohol, in particular the derivatives vanillin and syringaldhyde. It is remarkable that also in maple syrup (as of course in maple sap) these precursors are extant in higher concentrations than the actual flavour bearing constituents. (Maple syrup does contain thus a flavour reserve which theoretically could be activated by further air oxidation.) The concentration of the lignin derivatives does - in contrast to the sugar degradation compounds - vary considerably depending on provenance and processing history (POTTER, 1991; DUMONT, 1995; BELFORD, 1992). The component syringaldehyde appears to be of particular importance in the meat fermentation by maple syrup (DELAQUIS, 1993).

The high sugar concentration of maple syrup renders it stable against microbiological degradation. Certain osmophilic moulds may, however, attack maple syrup. Therefore maple syrup is heated to 80°C before being filled into containers which are then sealed while the syrup is still hot. This pasteurization as well as the part vacuum which develops after cooling have the consequence that maple syrup processed in this way has a practically unlimited storage life as long as the container remains sealed. After opening of the containers the product should be used up in a short period or it should be stored at appr. 5°C to avoid deterioration.

Up to the 19th century maple syrup remained the most important sweetening agent in North America. When cane sugar became more economical maple syrup lost its importance but retained the reputation of a traditional sweetener in traditional dishes. Today, maple syrup surrogates consisting in principle of flavoured cane sugar are used predominantly in North America. Such products are perfectly legal as long as the composition is properly labelled. Efficient methods to determine potential adulteration have been developed*). They are based on the fact that the ratio of the carbon isotope ratio 13C to 12C in the sugars depends on the metabolic pathway of the carbon dioxide assimilation. It differs between the monocotyledonous sugar cane and the dicotyledonous maple. In addition there exist variations of the hydrogen and oxygen isotope distribution which can be used for a tracing of adulterations. (HILLAIRE-MARCEL, 1977, 1986)

References:

BELFORD, A.L. et al., 1992: Bound vanillin in maple sap. Flavour and Fragrance Journal, 1992, 7: 1, 9-13

DELAQUIS, P.J., et al., 1993: Maple syrup as carbohydrate source in dry sausage fermentation. Journal of Food Science, 1993, 58: 5, 981-982, 990

DUMONT, J., 1995: Variations of the Quality and Flavour of Maple Syrup over the Tapping Season, in: Proceedings of the 1st International Symposium on Sap Utilization (ISSU), Bifuka, Hokkaido, Japan, April 10-12, 1995, ed. M. Terazawa, Y. Tamai, C.A. Macleod, p. 87 HILLAIRE-MARCEL, C. et al.: 1977: Composition Isotopique 601360C/601260C du Saccharose et du Glucose de Diverses Origines et Contrôle de L'Authenticité des Sirops et Sucres D'Érable. J.Inst. Can. Sci. Technol. Aliment. 10(4),1977, 333-335

HILLAIRE-MARCEL, G.: 1986: Isotopes and Food, in Handbook of Environmental Isotope Geochemistry. Ed. P. Fritz and J. CH. Fontes. MORSELLI F.M. et al., 1988: Ahornsirup - eine Übersicht. Z. Lebensmittel. Unters. Forsch. (1988) 186:6-10

POTTER, Th. L., 1991: Phenolic Compounds in Maple Syrup. ACS Symposium series 506, Phenolic compounds in Food, p 192-199

*)Laboratories which carry out isotope analyses are:

(1) COASTAL SCIENCE LABORATORIES, 6000 Mountain Shadows Drive, Austin, TX 78735,Tel. 001-512-288-5533, Fax 001-512-288-5533

(2) EUROFINS, Site de la Géraudière, CP 4001, F-44073 Nantes Cedex 03, France, Tel. +33-251832100, Fax 0033-251832111
 
 

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