Mineral content of tree sap from the subcarpathian region [Zawartość składników mineralnych w sokach drzewnych z terenu Podkarpacia]

Abstract

Tree sap has been used for centuries not only as a source of nutrients available in early spring but also as medicinal substance in folk medicine. Traditionally, it was used to treat various conditions, mostly anaemia and chronic fatigue. This study has been designed to establish the content of metallic elements (sodium, potassium, calcium, magnesium, zinc and copper) in sap collected from eight different species (silver birch, downy birch, hornbeam, Norway maple, boxelder maple, black walnut, black alder and white willow) and to identify which sap has mineral content which would be most beneficial for human health. We measured concentrations of calcium, magnesium, sodium, potassium and zinc on an atomic absorption spectrometer equipped with single-element, hollow cathode lamps and an air/acetylene burner. The content of copper was determined using an atomic absorption spectrometer with Zeeman correction equipped with an electrothermal atomizer and argon as inert gas. White willow sap was determined to have the highest concentrations of magnesium, zinc, calcium and potassium. Moreover, this sap contained the lowest concentration of sodium among all the tree species. The sap with the lowest detected concentration of the metallic elements originated from black alder and boxelder. In conclusion, tree sap can be a valuable source of metallic elements, namely copper, zinc and magnesium, in human diet. Tree sap tapped from white willow is the most valuable in terms of its mineral content. Moreover, the most popular sap of sliver birch has proven to be a rich source of magnesium and zinc.

Full text

Bilek M., Stawarczyk K., Gostkowski M., Olszewski M., Kędziora K.M., Cieślik E.
2016. Mineral content of tree sap from the Subcarpathian region. J. Elem.,
21(3): 669-679. DOI: 10.5601/jelem.2015.20.4.932
Journal of Elementology ISSN 1644-2296
MINERAL CONTENT OF TREE SAP
FROM THE SUBCARPATHIAN REGION
Maciej Bilek1, Kinga Stawarczyk2, Michał Gostkowski3,
Marcin Olszewski4, Katarzyna M. Kędziora5, Ewa Cieślik6
1Department of Food and Agriculture Production Engineering
2Department of Botany, Institute of Applied Biotechnology and Basic Science
University of Rzeszów
3Department of Econometrics and Statistics
Warsaw University of Life Sciences
4Department of Molecular Biotechnology and Microbiology
Gdańsk University of Technology
5Division of Cell Biology
The Netherlands Cancer Institute
6Department of Nutrition Technology and Consumption
Malopolska Centre of Food Monitoring
University of Agriculture in Kraków
AbstrAct
Tree sap has been used for centuries not only as a source of nutrients available in early spring
but also as medicinal substance in folk medicine. Traditionally, it was used to treat various
conditions, mostly anaemia and chronic fatigue. This study has been designed to establish the
content of metallic elements (sodium, potassium, calcium, magnesium, zinc and copper) in sap
collected from eight different species (silver birch, downy birch, hornbeam, Norway maple, boxel-
der maple, black walnut, black alder and white willow) and to identify which sap has mineral
content which would be most benecial for human health. We measured concentrations of cal-
cium, magnesium, sodium, potassium and zinc on an atomic absorption spectrometer equipped
with single-element, hollow cathode lamps and an air/acetylene burner. The content of copper
was determined using an atomic absorption spectrometer with Zeeman correction equipped with
an electrothermal atomizer and argon as inert gas. White willow sap was determined to have
the highest concentrations of magnesium, zinc, calcium and potassium. Moreover, this sap con-
tained the lowest concentration of sodium among all the tree species. The sap with the lowest
detected concentration of the metallic elements originated from black alder and boxelder. In
conclusion, tree sap can be a valuable source of metallic elements, namely copper, zinc and ma-
gnesium, in human diet. Tree sap tapped from white willow is the most valuable in terms of its
mineral content. Moreover, the most popular sap of sliver birch has proven to be a rich source
of magnesium and zinc.
Keywords: tree sap, minerals, intraspecies variability, interspecies variability, nutrition standards.
Maciej Bilek, PhD, Department of Food and Agriculture Production Engineering, University of
Rzeszów, Zelwerowicza 4 street, 35-601 Rzeszów, Poland, e-mail: mbilek@univ.rzeszow.pl

670
INTRODUCTION
Tree sap has been used for centuries in traditional medicine to treat di-
verse conditions such as anaemia, general fatigue, especially in early spring,
or skin lesions (Zyryanova et al. 2010, Svanberg et al. 2012, PaPP et al. 2014).
The fact that several minerals, important for human health, such as magne-
sium, zinc and copper are found in tree sap justies its medicinal use. Inte-
restingly, the presence of bioavailable minerals in birch sap was appreciated
as early as in the 1930s. Until now, the mineral content of sap has been de-
scribed for several species of the genera Betula and Acer, namely silver birch
– Betula pendula (Viškelis, Rubinskienė 2011, kūka et al. 2013), Japanese
white birch – Betula platyphylla (Jeong et al. 2012), painted maple – Acer
mono (Chung et al. 1995, Moon et al. 2004, Jeong et al. 2011), Norway maple
– Acera platanoides (kūka et al. 2013) and Korean maple – Acer pseudosie-
boldianum (Chung et al. 1995). Moreover, it has been shown that tracing
changes in the content of tree saps may be of benet to general environmen-
tal monitoring (haJru, huldén 1990, greenough et al. 2010). Furthermore, it
has been observed that the mineral content of sap determines the quality of
syrups produced from it (MorSelli et al. 1985, MorSelli, Whalen 1987).
Out intention has been to describe the mineral content of sap from eight
different tree species. Six of the above species have never been evaluated in
this regard before. Another aim of this study has been to identify a species
provides sap that is most benecial for human health. For this purpose, sap
samples were collected from trees growing close to one another, which meant
to eliminate the growth conditions as a source of variability.
MATERIAL AND METHODS
Samples of sap were collected from ve individual trees belonging to
each of the species: silver birch (Betula pendula Roth.), downy birch (Betula
pubescens Ehrh.), hornbeam (Carpinus betulus L.), Norway maple (Acer pla-
tanoides L.), boxelder (Acer negundo L.), black walnut (Juglans nigra L.),
black alder (Alnus glutinosa Gaertn.) and white willow (Salix alba L.). All
sampled trees grew on the ground owned by the Institute of Applied Biotech-
nology and Basic Sciences of the University of Rzeszow (Werynia, Podkar-
packie Province, 50.2500° N, 21.8333° E) – Figure 1.
Samples were collected between 26 of February and 16 of March 2014.
They were obtained in an area that is not submitted to any agricultural ac-
tivity, is far from any residential estates, main roads or railways. Trees of
the above species selected for tapping grew in close proximity from one an-
other, which minimized environmental variability. All the sampled trees had
a diameter larger than 25 cm, which is consistent with published recommen-

671
dations (laroChelle et al. 1998). The age of the trees was estimated with
a noninvasive method, based on their diameter measured at the height of
1.3 m (Połucha, Dzioba 2013) – Table 1.
We established a protocol for sample collection according to some previ-
ously published methods (yoon et al. 1992). We focused on minimizing the
risk of microbial contamination, detrimental for samples. Before sap collec-
tion, a fragment of bark was removed from a tree with a sterile chisel, addi-
tionally disinfected with Octenisept (Schülke&Mayr, Norderstedt, Germany).
Spots of tapping were selected on the south side of a trunk, 50 cm above the
ground. Collecting spots were sterilized with 70% ethanol (POCH, Gliwice,
Poland) for 30 s. Holes (10 mm diameter, 4-6 cm deep, 30° angle) were bored
with a sterile, autoclaved drill and cleaned of wooden shavings with a sterile
scalpel (Schreiber, Fridingen, Germany). Collection of sap started 5 min after
the ow began, once the area surrounding a given hole has been sterilized
with 96% ethanol (POCH, Gliwice, Poland) and amed. Saps were gathered
Fig. 1. Map showing the localisation of the tapped trees

672
into 0.015 dm-3 sterile and amed test-tubes, which were again amed before
closing. Thed holes in the tree trunks were sealed with Koro-Derma pruning
ointment (Bros, Poznań, Poland).
Sap samples were frozen (-20ºC) immediately after collection. A whole
batch of frozen samples was transferred to the Malopolska Centre of Food
Monitoring, where the samples were thawed and analyzed with atomic ab-
sorption spectroscopy.
Standard solutions were prepared from 1000 mg dm-3 stock solution
(CertiPUR, Merck KGaA, Germany) of sodium, potassium, calcium, magne-
sium, zinc and copper by dilution with double deionised water (Milli-Q Milli-
pore 18.2 MΩ cm-1 resistivity). Nitric acid (65%) Suprapur reagent was pur-
chased from Merck KGaA (Germany). All the plastic and glassware were
cleaned by soaking in dilute HNO3 (1:9) and were rinsed with distilled water
prior to use.
A Varian AA240FS (Varian, Mulgrave, Victoria, Australia) atomic ab-
sorption spectrometer equipped with single-element, hollow cathode lamps
and an air/acetylene burner was used for determinations of the content of
calcium, magnesium, sodium, potassium and zinc. The instrumental param-
eters were set according to the manufacturer’s recommendations (Table 2).
The determination of copper was performed using an atomic absorption
spectrometer Varian AA240Z (Varian, Mulgrave, Victoria, Australia) with
Zeeman correction equipped with a Varian GTA-120 electrothermal atomizer
coupled with an automatic sample dispenser. Argon was used as inert gas.
The determination of copper was done under the conditions specied in
Table 2. A MARS Xpress microwave closed system (CEM, Matthews, North
Carolina, USA) was used. About 2 g of each sample was digested in
0.010 dm-3 of nitric acid (65%, Suprapur, Merck KGaA, Germany) in a micro-
wave digestion system and diluted to 0.025 dm-3 with double deionised water.
Blank digest was obtained in the same way. Instrumental parameters and
settings were: 5 min for 600 W at 100°C, 5 min for 1200 W at 180°C, 15 min
for 1200 W at 200°C and 5 min vent.
Determinations of the content of investigated components were carried
out in duplicate, and average results were calculated as mg of element per
kg of fresh matter.
Table 1
Approximate age of trees, the source of studied tree sap
Age of
the tree
(years)
No. White
willow
Black
alder Hornbeam Black
walnut
Downy
birch
Silver
birch Boxelder Norway
maple
1 80 60 20 70 20 20 60 50
2 50 40 30 60 30 20 50 30
3 80 50 20 90 20 20 50 20
4 120 70 30 90 30 30 40 20
5 100 60 30 70 30 30 40 20

673
Statistical analysis was performed as a one-way analysis of variance
(one-way ANOVA), with a tree species being a differentiating factor. Statis-
tica v. 10.0 sofwater was used. In order to nd signicantly different means,
a post-hoc analysis with the Tukey’s honest signicance test was used. The
differences were deamed statistically signicant at the cutoff level  = 0.05.
RESULTS AND DISCUSSION
We detected signicant variability in concentrations of the elements be-
tween individual trees of all the analyzed species (Figure 2). However, no
signicant differences were conrmed in the concentrations of copper (p = 0.23)
or calcium (p = 0.23) between the the tree species.
The distribution of particular elements in the eight tree species was found
to be as follows. The highest average concentration of copper was detected
in the sap from hornbeam (1.26 mg dm-3) and white willow (1.02 mg dm-3).
The highest average concentration of calcium was found in the sap of white
willow (75 mg dm-3) and black walnut (65.24 mg dm-3). Furthermore, the hi-
ghest average concentration of zinc was detected in white willow sap (3.04 mg
Table 2
Experimental parameters used in the atomic absorption spectrometry measurements
Conditions for FAAS
Element acetylene
(L min-1)
air
(L min-1)
wavelength
(nm) slit width (nm)
Ca 2.0 13.5 427.7 0.5
Mg 2.0 13.5 285.2 0.5
Na 2.0 13.5 589.0 0.5
K 2.0 13.5 766.5 0.5
Zn 2.0 13.5 213.9 1.0
Conditions for GFAAS
Instrumental conditions Cu
Argon ow (mL min-1) 300
Sample volume (μL) 20
Modier (μL) 5
Wavelength (nm) 327.4
Heating program:
Drying 1 95
Drying 2 120
Ashing 900
Atomization 2300
Cleaning 2300

674
dm-3) and silver birch sap (2.62 mg dm-3). The highest average concentration of
potassium was detected in the sap of white willow (154.53 mg dm-3) and
Norway maple (128 mg dm-3), while the highest average concentration of
sodium was found in the samples taken from silver birch (4.9 mg dm-3) and
black alder (1.71 mg dm-3). The highest average concentration of magnesium
was measured in the sap of white willow (19.98 mg dm-3) and black walnut
(16.43 mg dm-3).
In brief, the sap of white willow was characterized by the highest avera-
ge concentration of magnesium, zinc, calcium and potassium. Noteworthy,
Fig. 2. Graphs show the mean content of metallic elements in sap collected
from different tree species: a, b, c – the same letters indicate a lack of statistically signicant
differences between the studied species (p-value < 0.05)

675
the same sap was the one that contained the least sodium. On the other
hand, the sap with the lowest average concentrations of the metallic ele-
ments (magnesium, zinc, potassium, copper and calcium) was tapped from
black alder and boxelder (Figure 3).
Fig. 3. Graphs show the mean content of different metallic elements in sap
of all studied tree species

676
When all the types of tree sap (40 samples) were pooled together, we did
not detect any correlation between the age of trees and the content of metal-
lic elements in their sap (Figure 4). However, the samples obtained from a
single species underwent a similar analysis, a positive correlation between
the tree age and the content of selected elements appeared. The concentra-
tion of copper in sap correlates with the age of white birch (R2 = 0.63) and
black walnut (R2 = 0.6). The zinc concentration strongly correlates with the
age of black walnut trees (R2 = 0.9). Moreover, analogous positive correla-
tions were found for calcium in boxelder (R2 = 0.86) and Norway maple
(R2 = 0.6), and magnesium in boxelder trees (R2 = 0.73). We did not nd any
correlation between the amount of sodium or potassium in sap and the age
of any of the trees.
Our results are in agreement with measurements of other authors who
used the same analytical method. For example, there is a visible resemblan-
ce in the composition of the silver birch sap between our samples and the
ones collected in Lithuania (Viškelis, Rubinskienė 2011). Moreover, while
comparing our observations with the published data, we noticed a similarity
in the composition sap from trees belonging to the same genus. Namely, the
downy birch sap we analyzed resembles the sap collected from Japanese whi-
te birch in South Korea (Jeong at al. 2012). Furthermore, the mineral con-
tent of the boxelder sap is similar to Korean and Manchurian maples (Chung
et al. 1995).
Tree sap as part of a diet can be a valuable source of nutrients and mi-
nerals, especially in early spring season. In this study, we measured the
concentrations of six different elements in sap. Four of them are classied as
mineral elements and two as electrolytes, according to Polish nutrition stan-
dards (JaroSZ 2012). We can conclude that all the studied types of tree sap
are a valuable source of copper. Namely, consumption of one liter of sap from
black walnut, white willow and hornbeam covers accordingly 100%, 113%
and 140% of the copper recommended dietary allowance. On the other hand,
the sap of downy birch, silver birch and white willow proved to be a rich so-
urce of zinc. Consumption of one liter of the above sap covers respecti-
vely 25%, 33% and 38% of the daily recommended intake of this element.
However, the tree sap we studied cannot be treated as a primary source
of magnesium. The highest concentration of this element was found in sap of
white willow, black walnut and silver birch. However, one liter of this sap
covers between 3.7% to 6.3% of the daily recommended magnesium intake.
Fig. 4. Coefcients of determination (R2) showing the lack of correlation between the age
of the trees and the concentration of different elements for all species pooled together

677
Moreover, sap of the studied tree species cannot be considered a signicant
source of sodium, potassium or calcium (Figure 5).
The question which tree species produce sap with thecomposition which
is most benecial for human health has been addressed in literature. Kūka
and colleagues compared the content of calcium and potassium in sap of
silver birch and Norway maple and concluded that as far as mineral content
is concerned, the later one is a better choice (kūka et al. 2013). Our study
shows that among the investigated species, the sap of white willow contains
the highest concentrations of mineral elements, namely magnesium, zinc,
calcium and potassium (Figures 3 and 5). Interestingly, white willow is rare-
ly used as a source of sap in Central and Eastern Europe. Ethnobotanical
reports mention its use in the region of Siberia (Svanberg et al. 2012).
We observed high intraspecies variability in the composition of collected
sap. However, the samples analyzed in this study were collected from indivi-
* RDA – Recommended Dietary Allowances, ** AI – Adequate Intake
Fig. 5. The graphs show a comparison between the mineral content of one liter of sap (mean
value calculated for ve individual trees within any given species) as a percentage of the rec-
ommended dietary allowances or adequate intake for an adult person

678
dual trees growing in close proximity to one another. Therefore, environmen-
tal factors (especially different types of soil or climate) can be excluded as a
source of the observed variability. Differences in the composition of sap de-
pending on the conditions in which trees were growing have been reported in
literature. For example, Hajru and Hulden studied the amount of metallic
elements in sap of silver birches growing at increasing distances from sulde
deposits. They observed that the concentration of these elements decreased
as the distance from deposits increased (haJru, huldén 1990). Moreover,
Morselli detected signicant differences in the concentration of sodium in
sap collected from maple trees growing in the middle of a forest and at its
edge, in close proximity to roads deiced with sodium chloride (MorSelli, Wha-
len 1987). Also, there are studies focusing on plant physiology which conrm
the dependence between the composition of soil and the concentration of
inorganic elements in sap of different tree species (SChneider et al. 1996,
White, broadley 2001, tagliavini, Millard 2005). However, our study shows
signicant differences in the composition of sap collected from individual
trees of the same species, which were growing in the same conditions. This
emphasizes the role of inter-individual variability in the constitution of tree
sap, which has to be taken into consideration in studies addressing the qu-
ality of tree sap or their use in environmental monitoring.
CONCLUSIONS
1. There is signicant variability in the content of metallic elements both
between and within the studied tree species.
2. The examined tree sap may be considered to be a valuable dietary
supplement, as it is particularly rich in copper, zinc and magnesium.
3. The highest content of minerals (magnesium, zinc, calcium and potas-
sium) was detected in the sap of white willow.
4. The sap from silver birch trees, most popular in Poland and also com-
mercially available as readymade soft drink, can be considered to be a signi-
cant source of magnesium and zinc.
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