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2014, vol. 72, 103–111
http://dx.doi.org/10.12657/denbio.072.009
Łukasz Łuczaj, Artur Adamczak, Magdalena Duda
Tannin content in acorns (Quercus spp.) from
Poland
Received: 19 November 2013; Accepted: 20 May 2014
Abstract: Oak acorns used to be an important human food, up until recent times. The major factor inhib-
iting their use in modern nutrition is their high tannin content. Polish oak trees were screened in order to
nd out whether there are any major geographical or interspecic differences in tannin and total phenolic
contents in acorns, which might help us to establish further directions in the search for low-tannin individ-
uals. We studied the level of phenolic compounds using the standard Polish and European Pharmacopoeia
method, with Folin-Ciocalteu reagent and hide powder. Altogether 49 acorn samples of Quercus robur, 13 of
Q. petraea, 1 of Q. pubescens (all native) and 12 of Q. rubra (introduced) were collected in different regions
of Poland. The amount of investigated phenolics in Q. robur and Q. petraea acorns was similar, and tannins
constituted the main component. The Q. pubescens sample was distinguished by the lowest tannin and total
phenolic content and a relatively high amount of non-tannin phenolics. Q. rubra had a slightly lower tannin
content than Q. robur and Q. petraea acorns, but the level of other phenolics was much higher. The results
for Q. robur suggest geographical variability of phenolic content as well as a relationship between the phy-
tochemical and biometric parameters of oak acorns.
Additional key words: polyphenols, oak acorns, phytochemistry, food security
Addresses: Ł. Łuczaj, University of Rzeszów, Institute of Applied Biotechnology and Basic Sciences,
Department of Botany, Werynia 502, 36-100 Kolbuszowa, Poland, e-mail: lukasz.luczaj@interia.pl
A. Adamczak, Institute of Natural Fibres and Medicinal Plants, Department of Botany, Breeding and
Agricultural Technology, Kolejowa 2, 62-064 Plewiska near Poznań, Poland
M. Duda, University of Rzeszów, Institute of Applied Biotechnology and Basic Sciences, Department of
Botany, Werynia 502, 36-100 Kolbuszowa, Poland
Introduction
Oaks (Quercus spp.) are one of the key species in
temperate ecosystems in the Northern hemisphere.
They not only harbor a large fauna of invertebrates,
which feed on their leaves and acorns (Southwood
1961; Kennedy and Southwood 1984), but they have
constituted one of the most sought after sources of
calories for vertebrates (e.g. boar, rodents, jays etc.)
and for humans in foraging economies (Maurizio
1927; Bainbridge 1986; Johns and Duquette 1991;
Mason 1995, 2000; Massei et al. 1996; Moerman
1998; Mason and Nesbitt 2009; Turner et al. 2011).
Oak acorns were also used as emergency food in
war times in societies, which had previously aban-
doned their use. Oak kernels (Quercus semen), espe-
cially roasted ones (Quercus semen tostum), were also
applied in traditional medicine throughout Europe
104 Łukasz Łuczaj, Artur Adamczak, Magdalena Duda
(e.g. Paluch 1984; Rakić et al. 2007). Their past im-
portance in human and animal nutrition, as well as
in phytotherapy, has caused some interest in their
chemical composition (Ofcarcik and Burns 1971;
Cantos et al. 2003; Rakić et al. 2004, 2005, 2006,
2007; Kobs 2008).
The limiting factor in the use of acorns in human
and animal nutrition is their high tannin content
(Smallwood and Peters 1986; Rakić et al. 2007; Pour
et al. 2010). Plant raw materials with high levels of
tannins are widely used in phytotherapy, and they
show antibacterial, astringent, antihemorrhagic and
antidiarrhoeal properties. Internally, they are used to
treat catarrh in the intestines and stomach (e.g. oak
bark), as well as diarrhea (Kozłowski et al. 2009), but
in large quantities, tannins are toxic to humans and
disturb the ingestion of nutrients from the digestive
tract (Chung et al. 1998). At high concentrations,
tannins can also damage the gut epithelium and liv-
er and kidney tissues (Singleton and Kratzer 1973;
Chung-MacCoubrey et al. 1997). Finally, tannins
at high levels can incur metabolic costs because of
increased detoxication requirements (Chung-Mac-
Coubrey et al. 1997). High levels of tannins also give
food a bitter taste, making it unattractive for con-
sumption. Some indigenous methods of removing
tannins from acorns, or at least decreasing tannin
content, have been described (Moerman 1998). These
are soaking in running water, leaching, and cooking
with clay as an adsorbent. In Poland and other Eastern
European countries, acorns were commonly used to
make a coffee substitute until the mid-20th century,
and this tradition is reviving now (e.g. Łuczaj 2008;
Dénes et al. 2012; Kalle and Sõukand 2012). They
were also commonly used as an admixture to our
to make bread, but only within the context of food
scarcity (Maurizio 1927; Łuczaj and Szymański 2007;
Łuczaj 2008). Within Europe, acorns had much more
importance in the Mediterranean than in northern
Europe. In some parts of the Mediterranean, such as
Spain and Sardinia, the acorns of Q. ilex L. are still
eaten even nowadays (Johns and Duquette 1991;
Tardío et al. 2006; Pignone and Laghetti 2010), and
the use of roasted Q. virgiliana (Ten.) Ten. acorns was
also recorded in southern Italy (Pieroni et al. 2005),
whereas in northern Europe, where Q. robur is the
commonest species, they have been forgotten even
as famine food (Łuczaj 2011; Dénes et al. 2012; Kalle
and Sõukand 2012; Łuczaj et al. 2012). It is not un-
likely that their larger importance in the south stems
from the fact that Q. ilex, Q. virgiliana and Q. pubes-
cens, the commonest oaks in the Mediterranean, are
perceived as sweeter and less bitter than Q. robur, the
dominant species in northern Europe. Additionally,
Mazuelos Vela et al. (1967) found a very low tannin
content in Q. ilex acorns (0.4–0.5%). The acorns of
Quercus have also been used as food in Asia (Pember-
ton and Lee 1996; Hu 2005; Kang et al. 2012, 2013).
As oaks are often components of climax vegetation,
growing varieties of native oaks for food could enable
food production in semi-natural woodlands. Howev-
er, oaks are slow-growing plants, thus it is difcult to
produce new cultivars. Intraspecic variability in the
content of secondary plant metabolites is widespread
in nature (e.g. Sidjimova et al. 2011; Adamczak et al.
2012a; Mirgos et al. 2012; O’Reilly-Wapstra 2013),
and perceived by animals as they make their feed-
ing choices (Smallwood and Peters 1986; Kimball
et al. 2012). Phytochemical differentiation, even in
closely related species, is also very widespread (e.g.
Nowak 2005; Adamczak 2012b; Çaliskan et al. 2012;
Edwards et al. 2012). There is quite a lot of data
on tannin content in oak acorns in North America
(Trimble 1896; Ofcarcik and Burns 1971; Fleck and
Layne 1990; see also Mason 1992 for an overview
of some of these papers), although very few such
studies were performed in Europe (Rakić et al. 2004,
2005, 2006, 2007), and the tannin content of many
common species (e.g. Q. petraea) has never been
studied before. No screening of oak populations has
been performed on a larger scale to study the natural
variation in their levels of tannins. All previous stud-
ies of oak acorns were limited to samples from one or
few localities. Our aim was to ll this gap and study
the diversity of tannin and total phenolic content in
acorns originating from Poland.
Only two native species of oaks and one exotic
taxon are common in Poland. The English oak (Q. ro-
bur L.) can be found in all parts of Poland, except for
higher elevations, and it is very common in a variety
of habitats, from dry to moist. It is both an important
forestry tree and a component of parks and gardens.
The sessile oak (Q. petraea (Matt.) Liebl., syn. Q. ses-
silis Ehrh.) occurs mainly in the lowlands, except for
some parts of NE Poland. It is not as common as the
English oak, and only occasionally becomes sub-dom-
inant in forests. This species is mainly restricted to
drier habitats, where it usually grows together with
Scots pine (Pinus sylvestris) or English oak. The third
taxon of oak, the pubescent oak (Q. pubescens Willd.),
can be found only in one small location in NW Po-
land, in the Odra valley (Chybicki et al. 2012). It is
a sub-Mediterranean species widely distributed in
southern and south-central Europe, mainly in calcar-
eous soils. All three native oaks belong to the Quercus
subgenus (=Lepidobalanus Endl. pro parte) and Quer-
cus section (Boratyński et al. 2006). After the English
oak, the second most common oak species in Poland
is the introduced red oak (Q. rubra L.), extensively
planted in forests and parks and often naturalized.
As oaks of the subgenus of Erythrobalanus (Spach)
Oerst. (e.g. Q. rubra) are regarded as more bitter than
other oaks (e.g. Ofcarcik and Burns 1971), our hy-
pothesis was that the introduced Q. rubra would have
Tannin content in acorns (Quercus spp.) from Poland 105
the most bitter acorns. In North America the mean
content of tannins found in the Erythrobalanus oaks
(“red oaks”) was 9.8% (ranging from 2.9 to 20.3%),
and in Lepidobalanus oaks (“white oaks”) it was 7.0%
(ranging from 3.3 to 10.9%) (Mason 1992). Howev-
er we also assumed that some differences would be
found between Q. robur and Q. petraea, or between
geographical races/forms of Q. robur.
Material and Methods
Plant material
Oak acorns were collected, in October and Novem-
ber 2011 and 2012, from under individual trees. For
each sample 20–30 acorns were usually taken, air-
dried at room temperature (23–25°C), then shelled
out and powdered. For phytochemical analysis, oak
kernels – seeds (Quercus semen) were used. A variety
of measurements were also made (weight before and
after air-drying, and after shelling, as well as length
and width) for acorns collected in 2012. Altogether,
49 acorn samples of Quercus robur, 13 of Q. petraea,
1 of Q. pubescens and 12 of Q. rubra were collected
in various parts of Poland (Fig. 1, Appx 1). As most
of our Q. robur samples came either from western
or south-eastern Poland, we compared these two
groups of localities in some of our analyses (Fig. 1).
The single sample of Q. pubescens (which is extremely
rare in Poland) was collected from the Adam Mick-
iewicz University Botanical Garden in Poznań, from
a tree originating from the only native population of
this species in Poland (from the Bielinek reserve).
Phytochemical analysis
Phytochemical analysis was conducted according
to the Polish and European Pharmacopoeia (2006,
2008, 2010). Binding by raw hide was used as a way
of determining tannin content. Use of collagen-rich
raw hide powder is an easy way to estimate the lev-
el of tannins, due to their protein-binding properties
(Seigler et al. 1986). The amounts of total polyphe-
nols (phenolics) and polyphenols unadsorbed on
hide powder (non-tannin phenolics) were deter-
mined spectrophotometrically with Folin-Ciocalteu
reagent, for 1.0 g of air-dried, powdered oak seeds
after 30 min of hot water extraction in a water bath.
The absorbance was measured at λ=760 nm. The
tannin content (expressed as pyrogallol equivalent)
was calculated as the difference between total phe-
nolics and non-tannin phenolics (Rakić et al. 2006;
Adamczak et al. 2012a). The obtained results were
calculated for dry matter (DM). The moisture content
[%] in the raw material was measured after drying it
at 105°C to constant mass. Folin-Ciocalteu reagent
and sodium carbonate decahydrate were purchased
from POCh. Pyrogallol (1,2,3-trihydroxybenzene)
and hide powder were obtained from Sigma-Aldrich.
All analytical procedures were performed with
protection from light. Acorn samples were extracted
with 150 ml of water for 30 min in a boiling water
bath, then the ask with extract was cooled and made
up with water to 250 ml. After the sedimentation of
plant material, the water extract was ltered, and the
rst 50 ml of ltered liquid was rejected. Total poly-
phenols were determined from 5 ml of ltrate diluted
with water to 25 ml. To 2 ml of this solution 1 ml of
Folin-Ciocalteu reagent was added, then 10 ml of wa-
ter, and the ask was lled up to 25 ml with sodium
carbonate solution (290 g/l). The absorbance was
measured after 30 min of incubation in darkness, us-
ing water for compensation. The content of non-tan-
nin phenolics was determined from 10 ml of ltrate
with addition of 0.1 g hide powder, after shaking (60
min) and ltering of the solution. From this solution,
5 ml was made up with water to 25 ml and the fur-
ther procedure was repeated as above. The reference
standard solution was prepared from 50 mg pyrogal-
lol in water diluted up to 100 ml. Afterwards, 5 ml of
this solution was diluted with water to 100 ml, and 2
ml was used for further analysis with Folin-Ciocalteu
reagent, as described above. The tannin content (%)
was calculated with the following formula:
where: X – tannin content; A1 – absorbance of total
phenolics; A2 – absorbance of phenolics unadsorbed
on hide powder; A3 – absorbance of reference solu-
tion of pyrogallol; m1 – mass of investigated sample
[g]; m2 – mass of pyrogallol [g].
Statistical analysis
The Shapiro-Wilk test was applied to check the
normality of variable distribution, and F-test and
Levene’s test were used to analyse the homogeneity
of variances. For the skewed distribution of variables,
square root, logarithmic and inverse proportion
transformations of data were performed. In the anal-
ysis of intra- and interspecic differences, Student,
Mann-Whitney and Kruskal-Wallis tests were used.
Pearson’s correlation and Spearman’s rank correla-
tion were applied to evaluate the relationships be-
tween variables. An open access statistical program
PAST (Hammer et al. 2001; PAST 2012) and Statisti-
ca 7.1 software (Statistica 2005) were used for anal-
yses.
X = 62.5×(A1−A2)×m2/A3×m1
106 Łukasz Łuczaj, Artur Adamczak, Magdalena Duda
Fig. 1. Distribution of acorn samples
1 – Police near Szczecin (Q. petraea), 2 – Strzekęcino near Koszalin (Q. robur), 3 – Tylkowo near Olsztyn (Q. robur), 4 – Skórka near Piła
(Q. robur), 5 – Śmiardowo Złotowskie near Złotów (Q. robur), 6 – Dorotowo near Złotów (Q. robur), 7 – Klempicz near Czarnków (Q.
robur), 8 – Gościejewo near Rogoźno (Q. robur), 9 – Wągrowiec (Q. robur), 10 – Roszkowo near Wągrowiec (Q. robur), 11–12 – Międzychód
(Q. robur), 13 – Obrzycko near Szamotuły (Q. petraea), 14–15 – Obrzycko near Szamotuły (Q. robur), 16 – Jaryszewo near Szamotuły (Q.
petraea), 17 – Biedrusko near Poznań (Q. robur), 18–20 – Biedrusko near Poznań (Q. petraea), 21 – Zielonka near Poznań (Q. robur), 22 –
Adam Mickiewicz University Botanical Garden in Poznań (Q. pubescens), (22) – the Bielinek reserve (the original locality of Q. pubescens),
23–24 – Adam Mickiewicz University Botanical Garden in Poznań (Q. rubra), 25 – Dendrological Garden of University of Life Sciences
in Poznań (Q. rubra), 26 – Dendrological Garden of University of Life Sciences in Poznań (Q. petraea), 27–28 – Poznań Stare Miasto (Q.
rubra), 29–30 – Poznań Nowe Miasto (Q. robur), 31 – Poznań Nowe Miasto (Q. rubra), 32–33 – Koninko near Poznań (Q. robur), 34–35
– Daszewice near Poznań (Q. petraea), 36 – Daszewice near Poznań (Q. robur), 37–38 – Rogalin near Mosina (Q. robur), 39 – Dusina
near Gostyń (Q. robur), 40 – Bojanowo near Leszno (Q. robur), 41 – Zielona Góra (Q. robur), 42 – Modlęcin near Świebodzice (Q. robur),
43–44 – Bąków near Kluczbork (Q. robur), 45 – Bąków near Kluczbork (Q. rubra), 46 – Warszawa Rembertów (Q. robur), 47 – Pilawa near
Warszawa (Q. robur), 48 – Lublin Śródmieście (Q. robur), 49 – Lublin Konstantynów (Q. robur), 50 – Lublin Konstantynów (Q. rubra),
51 – Porąbka near Bielsko-Biała (Q. rubra), 52 – Kęty near Bielsko-Biała (Q. petraea), 53–54 – Kraków Bronowice (Q. robur), 55 – Kraków
Czyżyny (Q. robur), 56 – Kraków Grzegórzki (Q. robur), 57 – Kraków Stare Miasto (Q. robur), 58 – Raciborowice near Kraków (Q. rubra),
59 – Suchy Grunt near Mielec (Q. robur), 60 – Mielec (Q. robur), 61–62 – Nowa Dęba (Q. petraea), 63 – Werynia near Kolbuszowa (Q.
robur), 64 – Leszcze near Kolbuszowa (Q. petraea), 65 – Rzeszów (Q. rubra), 66 – Gnojnica near Ropczyce (Q. robur), 67–68 – Rzepnik
near Krosno (Q. robur), 69 – Ustrobna near Krosno (Q. robur), 70 – Krosno (Q. robur), 71 – Krosno (Q. rubra), 72 – Haczów near Krosno
(Q. robur), 73 – Chorkówka near Krosno (Q. robur), 74 – Wróblik Królewski near Krosno (Q. robur), 75 – Mymoń near Krosno (Q. robur).
Geographical coordinates of acorn samples – Appx 1.
Tannin content in acorns (Quercus spp.) from Poland 107
Results
Our research showed that oak acorns lose about
30% of their weight after air-drying, then acorn shells
contain an average of 6.9% water, and acorn kernels
(seeds) – 6.4%. Acorns of Q. robur (English oak) and
Q. petraea (sessile oak) were characterized by a high
variability of size and weight, but they did not differ
from each other. Acorns of Q. rubra (red oak) were
signicantly different from the oak species native to
Poland, and they showed a low variation in size and
weight. It is interesting that the percentage share of
seeds at the air-dry weight of Q. rubra acorns was
signicantly lower than in the case of Q. robur and
Q. petraea acorns. Although Q. rubra acorns were on
average heavier than those of the native oaks, their
mean seed mass was slightly lower than that of the
native species (Table 1). The investigated samples of
oak acorns indicated a wide range of phytochemical
variability. Q. pubescens acorns were distinguished by
the lowest content of total phenolics (2.61% DM)
and tannins (0.97%), and by a relatively high amount
of non-tannin phenolics (1.64%). The level of total
phenolics and tannins in the case of other species did
not differ signicantly. Tannins, especially for Q. ro-
bur and Q. petraea acorns, constituted the main com-
ponent of polyphenols. Attention should be paid to
the relatively low tannin content of Q. rubra acorns in
relation to the level of non-tannin phenolics (Table
2). The total amounts of phenolics and tannins in
Q. robur acorns collected in 2012 were signicantly
higher in SE Poland compared to W Poland. The geo-
graphical origin of samples did not affect the content
of non-tannins or the ratio of tannins to non-tannins.
Acorns from western Poland were characterized by a
slightly larger average size and weight, but these dif-
ferences were not statistically signicant (Table 3).
For Q. robur we found negative correlations be-
tween the size and weight of acorns and the lev-
el of the investigated groups of active compounds,
but these correlations were not strong. The highest
correlation was detected between the total level of
phenolics and the air-dry weight of acorn seeds. Tan-
nin content was most strongly associated with seed
weight. In the case of the level of non-tannins, the
highest correlation was found with acorn width. On
the other hand, the ratio of tannins to non-tannins
was not connected with the size and weight of Q.
robur acorns. Similarly, the shape index of acorns
(length/width) was not correlated with the amount
of chemical compounds (Table 4).
Discussion
Our biometric studies of acorns indicate a high
variability of Q. robur and Q. petraea, and lower dif-
ferentiation of Q. rubra (Table 1). According to the
literature, both native species are characterized by
a large morphological variation. The length of Q. ro-
bur acorns ranges from 1.5 to 4.0 cm, width: 1.0–2.0
cm and weight: 2.00–6.90 g. In the case of Q. petraea
acorns it is: 1.6–3.5 cm, 0.8–1.6 cm and 1.54–7.58
g, respectively (Boratyńska et al. 2006). The smaller
variability of Q. rubra from Poland may be due to the
fact that it is an alien species (the so-called “founder
Table 1. Size and weight variability of oak acorns collected in 2012 (mean as well as minimum and maximum given in
brackets)
Variables\Species Q. robur (n=36) Q. petraea (n=11) Q. rubra (n=9)
acorn length [cm]*** 2.71 (2.26–3.66) 2.63 (1.96–3.06) 2.14 (1.98–2.28)
acorn width [cm]*** 1.48 (1.16–1.73) 1.56 (1.32–1.85) 1.87 (1.75–2.02)
length/width of acorns*** 1.83 (1.48–2.23) 1.69 (1.48–1.81) 1.14 (1.02–1.21)
air-dry weight of acorns [g]* 3.15 (1.16–5.32) 3.12 (1.36–4.39) 3.86 (3.09–4.57)
air-dry weight of seeds [g]N.S. 2.63 (0.85–4.55) 2.52 (1.05–3.33) 2.39 (1.98–2.82)
seed share in the acorn weight [%]*** 83 (73–87) 81 (75–85) 62 (55–68)
The length and width of acorns were determined on the basis of 20–30 measurements for each sample, while the weight of acorns and
seeds – on the basis of one measurement for the whole sample. Kruskal-Wallis test for interspecic differentiation of the size and weight
of acorns: N.S. – not signicant (p>0.05); * – p<0.05; *** – p<0.001; n=56.
Table 2. Mean content of total phenolics, tannins and non-tannin phenolics in the seeds of oak acorns collected in the
years 2011–2012 (the minimum and maximum given in brackets)
Variables\Species Q. robur (n=49) Q. petraea (n=13) Q. rubra (n=12) Q. pubescens (n=1)
total phenolics [% DM]N.S. 4.33 (3.05–6.58) 4.29 (3.36–6.66) 4.58 (3.48–5.80) 2.61
tannins [% DM]N.S. 3.48 (2.43–5.17) 3.39 (2.57–4.75) 2.97 (2.10–4.26) 0.97
non-tannins [% DM]*** 0.86 (0.57–1.89) 0.90 (0.58–2.06) 1.60 (1.29–2.00) 1.64
tannins/non-tannins*** 4.25 (1.84–6.26) 4.39 (1.25–5.98) 1.90 (1.19–2.77) 0.59
Total phenolics, tannins and non-tannins – expressed as pyrogallol equivalent; tannins/non-tannins – the ratio of tannins to non-tannins;
DM – dry matter of raw material. Kruskal-Wallis test for interspecic differentiation of the content of phenolic compounds (without Q.
pubescens): N.S. – not signicant (p>0.05); *** – p<0.001; n=74.
108 Łukasz Łuczaj, Artur Adamczak, Magdalena Duda
effect”, or “bottle-neck effect”). The high similarity
of acorn size and weight, as well as phenolic content,
in Q. robur and Q. petraea (Table 1–2) conrms that
both species belong to one section and subsection
of Quercus and are closely related (Boratyński et al.
2006). As expected, the acorns of Q. pubescens (Gallif-
erae subsection), were low in tannin content, however
not as low as Q. ilex (Mazuelos Vela et al. 1967). On
the other hand it was surprising that Q. rubra acorns
were not characterized by a higher level of tannins
than Q. robur and Q. petraea (Table 2). The oaks of
the Erythrobalanus subgenus, to which the red oak (Q.
rubra) belongs, are generally regarded as more bitter
and tannin-rich than other oaks (Ofcarcik and Burns
1971; Weckerly et al. 1989; Mason 1992).
The obtained results of phytochemical investi-
gations are difcult to compare with the literature
data, due to different methods of drying, extraction
and analysis of acorn samples. Tannins are a very ver-
satile group of usually polymeric compounds, and
different analytical methods may exclude or include
certain subgroups of this category of compounds. We
used the hide powder method, as it is widely used in
many countries (Seigler 1986; Rakić et al. 2006; Kobs
2008), although detailed procedures varied between
the studies.
According to Rakić and coauthors (2005, 2006,
2007), the tannin content in Q. robur acorns varies,
depending on the method of drying and extraction
of samples, from 7.76 to 20.4%. It is interesting that
the ratio of tannins to non-tannins was similar to
our results (Table 2) and ranged from 2.78 to 10.73
(Rakić et al. 2006, 2007). On the other hand, Small-
wood and Peters (1986) indicate that the tannin level
in the white oak group Lepidobalanus is much lower
and ranges from 0.5 to 2.5%. Pour et al. (2010) also
show a low level of tannins in oak kernels (1.76%).
Older American studies of acorn composition usually
report larger tannin contents, which may be the re-
sult of different procedures used in analyses.
Comparative analysis of Q. robur samples collected
in one year from western and south-eastern Poland
showed signicant differences in the total content of
phenolics and tannins between these regions (Table
3). This may indicate the geographical variability of
the level of investigated chemical compounds in oak
acorns. It was interesting that these samples were
different, although only slightly, in terms of size and
weight, too (Table 3). Therefore, the relations be-
tween phytochemical and biometric parameters of Q.
robur acorns were studied (Table 4). The results obta-
ined suggest that the amount of polyphenols is con-
nected with the size and weight of acorns. This may
be a clue towards the gathering of lower tannin fru-
its. However, it is important to nd out whether this
phenomenon is caused by the fact that oaks growing
in better habitat (climatic) conditions produce larger
and lower-tannin acorns or whether genetic factors
are responsible for this relationship.
Further research on the tannin content of south
European oak acorns should be carried out in order
to nd out whether the larger popularity of acorns
as food in southern Europe rather than in the north
stems from differences in tannin and polyphenol con-
tents. This hypothesis is supported by the low tannin
content of Q. pubescens acorns, originating from the
only native Polish population of this species (Table
2), and in the case of some samples of Q. ilex (Mazu-
elos Vela et al. 1967 cited by Mason 1992). We pro-
Table 3. Differentiation of the polyphenol content and the biometric parameters of Quercus robur acorns collected in 2012
from sides in two regions of Poland (mean ± SD)
Variables Western Poland South-Eastern Poland p-value
total phenolics [% DM]a3.89 ± 0.45 4.35 ± 0.45 0.0127
tannins [% DM]a3.15 ± 0.39 3.56 ± 0.45 0.0146
non-tannins [% DM]a0.74 ± 0.12 0.78 ± 0.12 0.3677
tannins/non-tanninsb4.32 ± 0.67 4.67 ± 0.93 0.3627
acorn length [cm]a2.75 ± 0.27 2.67 ± 0.26 0.4360
acorn width [cm]a1.52 ± 0.12 1.47 ± 0.11 0.3021
length/width of acornsa1.81 ± 0.13 1.82 ± 0.20 0.8823
air-dry weight of acorns [g]a3.28 ± 0.73 3.15 ± 0.56 0.5901
air-dry weight of seeds [g]a2.76 ± 0.61 2.62 ± 0.52 0.5126
seed share in the acorn weight [%]b84 ± 1.8 83 ± 2.4 0.2737
Total phenolics, tannins and non-tannins – expressed as pyrogallol equivalent; DM – dry matter of raw material; SD – standard deviation.
Statistical tests: a – Student’s test; b – Mann-Whitney test; n=28.
Table 4. Correlation between the polyphenol content and
the biometric parameters of Quercus robur acorns
Variables total
phenolics tannins non-
tannins
acorn lengtha–0.37* –0.33*
acorn widtha–0.44** –0.38* –0.43**
air-dry weight of acornsa–0.45** –0.42* –0.38*
air-dry weight of seedsa–0.48** –0.44** –0.39*
seed share in the acorn weightb–0.39*
a – Pearson’s correlation; b – Spearman’s rank correlation; p-value:
*** – p<0.001, ** – p<0.01, * – p<0.05, n=36.
Tannin content in acorns (Quercus spp.) from Poland 109
pose further research into the chemistry and nutri-
tional value of acorns, as oaks have been key species
during food crises in temperate and Mediterranean
climates and have huge potential as food plants, both
as emergency food and an alternative major source of
starch. This is proven by the fact that the Dary Na-
tury company (from Koryciny, NE Poland) sells large
amounts of tannin-deprived Q. robur our in health
food shops in Poland. A tannin removal technique
(the rm’s secret) was developed by Mr Mirosław
Angielczyk, the owner of the company (http://www.
ziolowyzakatek.sklep.pl/maka.z.zoledzi.eko.1kg.
dary.natury.html). As oaks harbor a rich insect ora,
and acorns are very susceptible to insect and patho-
gen invasions, another branch of oak research should
be the study of tannin-removal techniques. Efcient
tannin removal from acorns may be a more economi-
cally viable option than growing low-tannin cultivars.
Acknowledgments
The authors would like to thank Prof. Waldemar
Buchwald for sharing his laboratory and to Hanna
Zalińska (Institute of Natural Fibres and Medicinal
Plants in Poznań) – for her help with phytochemical
analysis. We thank Prof. Justyna Wiland-Szymańs-
ka (Adam Mickiewicz University Botanical Garden
in Poznań) and Dr. Tomasz Maliński (Dendrological
Garden of University of Live Sciences in Poznań) for
access to the garden to sample collecting. We would
also like to thank the following persons for their help
in collecting some of the acorn samples: Ms. Małgor-
zata Adamczak (Adam Mickiewicz University), Dr.
Piotr Androsiuk (University of Warmia and Mazury
in Olsztyn), Ms. Agnieszka Gryszczyńska (Institute
of Natural Fibres and Medicinal Plants in Poznań),
Dr. Alicja Kolasińska (Adam Mickiewicz University
Botanical Garden in Poznań), Mr. Maciej Łochyński
(Biotope, Poznań), Mr. Romuald Mordalski (Institute
of Natural Fibres and Medicinal Plants in Poznań),
Mr. Marek Podsiedlik (University of Live Sciences in
Poznań), Mr. Przemysław Susek (Regional Inspector-
ate for Environmental Protection in Zielona Góra),
Dr. Joanna Trubicka (Children’s Memorial Health
Institute in Warszawa) and Ms. Marika Węgrzynek
(University of Rzeszów).
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