The sap produced by maple trees which is used for the production of maple syrup is a sterile liquid that provides the maple tree with water and nutrients prior to buds opening and leaf appearance. When collected in late winter, sap is a clear and colorless liquid with a faint sweet taste. Differences in the biochemical composition of sap determines the grade of syrup produced each day. While each syrup is unique, the quality of all syrup grades must meet state and provincial inspection standards. Many factors affect changes in sap biochemistry, both in the sugarbush and in the storage tank before evaporation. These partly determine the shades of amber coloration within and between syrup color grades, as well as the flavor.

Chemical Composition of Maple Sap

Not all the biochemical components of sugar maple sap have been identified. However, it is important to identify and understand the roles and interactions of sap precursors of maple syrup colors and flavors. This can help minimize syrup off-flavors and control syrup grade production.

Table A-2.1 gives the range of values of the organic compounds in maple sap. Sap in individual trees ranges from 1% to 10% of the total solids content (predominately sugars), with an average content of 2-2.5% in sap collected from a sugarbush. Conventionally, sap sweetness or sugar concentration is measured in degree Brix(° Brix) which is the food industry accepted measurement for sweetness of fruit juices and other syrups. Sucrose is the most prevalent sugar, comprising 98-99.9% of the dry matter of sap. This leads to a misunderstanding by the casual observer that sap is just sugar water. If that were true, sap would not sustain the life of the tree. It is that small percentage (2.0% or less) of amino acids, organic acids, phenolic compounds, hormones, minerals and salts, and other components that allows sap to be the physiological liquid, with the right pH and buffering capacity, responsible for helping initiate growth within the tree.

Table A-2.1. Organic Composition of Sugar Maple Sap1. Total Solids 1.00 - 5.40% Sucrose 98.00 - 99.99% Invert sugar (glucose) 0.00 - 0.17% Phenolic compounds 0.00 - 4.55 ppm Primary amines 0.53 - 36.10 ppm Peptides 0.40 - 18.57 ppm Amino acids (free) 0.00 - 11.30 ppm Proteins 0.00 - 50.90 ppm Organic acids 2.03 - 45.00 ppm pH 3.9 - 7.9 1Morselli, 1980 (unpublished)

Table A-2.2. Sugars in Maple Sap and Syrup1. Sugarsa Sap Percent Sap (dry weight) Percent Syrup (dry weight) Percent Hexoses 0.0 0.0 0-12 Sucrose 1.44 96.00 88-99 Raffinose and a glycosyl .00021 .014 — Oligosaccharidesb I .00018 .013 — II .00020 .014 — III .00042 .028 — aTypical values, not averages. bThe oligosaccharides have been isolated by chromatography but have not been identified. 1Willits and Hills, 1976, p.66.

It is the interaction of these compounds during the boiling process that accounts for the flavor associated with pure maple syrup. The precursors of syrup colors are mainly sugars (Table A-2.2). Amino acids (Table A-2.3) and some organic acids (Tables A-2.4, A-2.5) are responsible for the characteristic "maple" flavor as well as for some of the off-flavors, such as "buddy" (Willits and Hills, 1976; Morselli and Whalen 1986b). The role of phenolic compounds on syrup grade differences is still unknown.

Table A-2.3. Free Amino Acids in Sterile Maple Sap^a.

	1977			1979
	3/9	3/15	3/27	3/27	4/11	4/13	4/20
Taurine	tr
Urea	*	*	*	*	*	*	*
Aspartic Acid	*	*	tr	*	*	*	*
Theonine						*	*
Serine	*					*	*
Asparagine/Glutamine	*	*	*	*	*	*	*
Glutamic Acid	tr		tr		*	*	*
Citrulline						*
Glycine	*	tr	tr		tr
Alanine			tr				*
Valine						*	*
Methionine						*	*
Isoleucine					*	*	*
Leucine						*	*
Tyrosine							*
Phenylalanine							*
B-Aminoisobutyric Acid						?	?
-Aminobutyric Acid	?	?	?	*	?
Ornithine		tr
Ammonia	*	*	*	*	*	*	*
Hisidine							*
a Free amino acids in sterile (0-1 CFU/m1) sap of two representative maple trees during the 1977 and 1979 season (Morselli and Whalen, unpublished). KEY: * = quantifiable amount; tr = non quantifiable amount; ? = uncertain, but most likely non quantifiable amount.

Table A-2.4. Organic Acids in Sugar Maple Sap (per 100 mL) Throughout a Maple Season¹.

Flow datea 1980	Sample size (mL)	Oxalic	Succinic	Fumaric	1-Malic	Tartaric	cis/Aconitic	Citric/Aconitic	Total
Parts per Billion (ppb)
3/20	156	760	2300	200	4300	95	100	140	7895
3/23	151	250	600	55	1400	46	___b	46	2407
3/27	101	130	740	1300	12000	110	160	150	14590
3/31	111	__b	340	30	45000	36	__b	__b	45406
4/07	148	__b	570	6800	12000	6	320	100	19867
Individual Acids as % of Total Acids
3/20		9.6	29.0	2.8	54.0	1.2	1.3	1.3
3/23		11.0	25.0	2.3	58.0	1.9	___	___
3/27		0.9	5.2	8.8	82.0	0.8	1.1	1.1
3/31		___	0.7	<0.1	99.0	<0.1	___	___
4/07		___	2.9	35.0	59.0	0.3	1.6	1.6
aFive sap collecting dates, representing early-to-late sap flow season, from one sugar maple, one termination. bNot detected using the above given sample size (quantitation limit = 15 ppb). 1Mollica and Morselli, 1984, p.1127.

Table A-2.5. Organic Acids in Sugar Maple Sap (per 100 mL) in Three Individual Sugar Maples¹.

Treea	Sample size (mL)	Oxalic	Succinic	Fumaric	1-Malic	Tartaric	cis/Arconitic	Citric/Shikimic	Total
Parts per Billion (ppb)
1	151	260	600	55	1400	46	__b	46	2407
2	150	350	240	300	880	79	__b	190	2039
3	154	__b	710	1000	10000	45	120	91	12466
Individual acids as % of Total Acids
1		11.0	25.0	2.3	58.0	1.9	___	1.9
2		17.2	11.8	14.7	43.2	3.9	___	9.3
2		___	5.7	11.3	80.9	-.4	1.0	0.7
aOne sap collection date (3/23/90), representing early sap flow, one determination per tree. bNot detected using the above given sample size (quantitation limit = 15 ppb). 1Mollica and Morselli, 1984, p. 1128.

The dissolved inorganic contents of sap are also important. As the sugar in sap is concentrated by heat some of the inorganic compounds will precipitate out according to the natural laws of solubility. Some elements will be bonded to other elements forming salts, while others will be bound to sugars, phenolic compounds and other sap constituents. It is this precipitate from sap processing to syrup that is called "sugar sand or niter/nitre" (Table A-2.6). Sugar sand varies in color and texture. It can be puffy, white to brown or black sandy-looking, or it can be viscous and sticky. Sugar sand is considered a nuisance by the syrup producer because it may burn onto the evaporator pan, clog filters, etc. Yet, in the future, research may find a use for it.

Table A-2.6. Composition of Sugar Sand1.
Sugar sand (in run)	percent	0.05-1.42
pH		6.30-7.20
Ca	percent	0.61-10.91
K	do	0.146-0.380
Mg	do	0.011-0.190
Mn	do	0.06-0.29
P	do	0.03-1.18
Fe	p.p.m.	38-1,250
Cu	p.p.m.	7-143
B	p.p.m.	3.4-23
Mo	p.p.m.	0.17-2.46
Free Acid	percent	0.07-0.37
Total malic acid	do	0.76-38.87
Acids other than malic	do	0.08-2.62
Undetermined material	do	6.94-34.16
Calcium malate	do	1.30-49.41
Sugars in dried samples	do	33.90-85.74
Sugar sand in dried samples	do	14.26-66.09
1Willits and Hills, 1976, p. 66.