China et al. Journal of Leather Science and Engineering
https://doi.org/10.1186/s42825-021-00055-2
(2021) 3:13
RESEARCH
Journal of Leather Science
and Engineering
Open Access
Tanning capacity of Tessmannia burttii
extracts: the potential eco-friendly tanning
agents for the leather industry
Cecilia R. China1* , Stephen S. Nyandoro2, Joan J. E. Munissi2, Mihayo M. Maguta3, Michael Meyer4 and
Michaela Schroepfer4
Abstract
In the present study, the tannins from stem and root barks of Tessmannia burttii Harms (Caesalpiniaceae), a plant
species abundantly growing in Tanzania and other parts of Africa, were investigated for their suitability in hides
tanning. Tannin powder was extracted at selected temperatures (30, 50 and 80 °C) and the influence of each
temperature on the crosslinking capacity was evaluated. The interaction mechanism between hide powder collagen
and the tannins was studied by Differential Scanning Calorimetry (DSC), trinitrobenzensulfonic (TNBS) acid assay and
amino acid hydrolysis methods. Extraction temperatures showed low influence on crosslinking capacity of the
tannins. However, extract obtained at 50 °C exhibited best performance in terms of gap size between Tonset and
Tpeak. The stem bark extract yield was higher than that from the root bark, but both were within the recommended
ranges. The tannin content (61%) of T. burttii stem bark extract was above recommended value (10%), whereas its
total phenolic content and total flavonoic content were found to be above that of commercial Acacia mearnsii
tannin. The study of cross-linking parameters as a function of pH showed cross-linking to occur via a covalent
mechanism at the basic amino groups. However, the bonds were not resistant to acid hydrolysis. The observed
interaction mechanism indicated that tannins from stem and root barks of T. burttii belong to the condensed
tannin, similar to A. mearnsii (black wattle), a commercial tannin source that was used in this study as a reference.
Findings from this study depict that T. burttii extracts are auspicious eco-friendly alternative source of vegetable
tannins to overcome the use of chromium salts in the leather industry.
Keywords: Tessmannia burttii, Caesalpiniaceae, Leather industry, Vegetable tanning, Tanning capacity
1 Introduction
Currently, chromium (Cr) tanning technology is taking a
lead in the leather industry, contributing to 90% of the
leather tanned worldwide [1–3]. However, this technology poses waste disposal challenges to the leather industry associated with public health and environmental
concerns. The situation calls for the growing demand to
embark on sustainable and green manufacturing processes [4]. As such vegetable tannins have sparked the
* Correspondence: rolencec@gmail.com
1
Division of Textile and Leather Technologies, Tanzania Industrial Research
and Development Organization, P. O. Box 2355, Dar es Salaam, Tanzania
Full list of author information is available at the end of the article
search for more sources that guarantee unlimited supply
for sustainability of the leather industry globally. Use of
vegetable tannins as tanning agents is deemed to be economical, environmentally friendly and sustainable because they are cheap, non-toxic and renewable [5, 6],
hence considered as an alternative to chromium tanning.
Vegetable tanning takes place either through hydrogen
or covalent bonding between the tannins and the functional groups of the skin collagen, which in turn stabilizes the hide and convert it into leather [6]. It is well
known that the reactivity of functional groups in collagen towards tannins is highly dependent on the pH of
the reaction medium. Additionally, it is acknowledged
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China et al. Journal of Leather Science and Engineering
Page 2 of 9
(2021) 3:13
that cations in the solution preferentially react with
carboxyl groups in collagen fibers while anions selectively react with amino groups [7]. Since vegetable tannins are anions in nature [8], they react preferably with
amino groups, either protonated or unprotonated rather
than carboxyl groups of the collagen [9]. Although several studies have reported the tanning capacity and
interaction mechanism of the extract from different
plant species [9–13], they represent infinitesimal fraction
of available potential of the vegetable tannin sources.
Therefore, there is huge reservoir of unexploited plants
from different parts of the world with significant amount
of tannins that need to be studied for their chemistry as
well as their potential industrial applications including in
the leather industry.
Tanzania is endowed with plethora of plants with their
applications largely being limited to traditional medicine
and other non-industrial economic utilizations [14],
while their use in leather processing is seldomly known.
Currently, only wattle tree is considered as a sole source
of tannins in the country and its demand in the world
market is high, thus depriving low income countrymen
with effective access to the tree [15]. Exploring new
plant materials for vegetable tanning is essential for the
sustainability of the leather industry of Tanzania and beyond. In this regard, a plant species Tessmannia burttii
Harms (Caesalpiniaceae) was included in our on-going
investigations of vegetable tannins from the locally available plants.
T. burttii is among the dominating plant species found
in the coastal forests of Tanzania [16]. The species is
also distributed in other African countries such as
Zambia, Zimbabwe and Democratic Republic of Congo
[17]. Inspired by the presence of catechinoids among
other constituents from the stem and root barks of the
T. burttii (unpublished work) suggesting a high number
of polyphenols (tannins), the extracts therefrom were
screened for their potential tanning capacity for stabilizing collagen of the hide in leather processing. The
screening in this study focused on studying the influence
of extraction temperature on crosslinking capability of
the tannins from T. burttii root and stem bark extracts.
The tannin-collagen interaction mechanisms of the extracts were also explored to further establish the tannin
type of the same extracts.
2 Materials and methods
2.1 Materials
2.1.1 Plant materials and hide powder
Barks from both roots and stems of T. burttii used for
extraction of tannins were collected on 23rd November,
2014 from Zeraninge Forest Reserve (Altitude 300 m;
GPS 37 M 0454993 UTM 9321617), Bagamoyo District
in Pwani Region, Tanzania. The plant was identified by
Frank M. Mbago in the field then authenticated at the
Herbarium, Botany Department of the University of Dar
es Salaam where the voucher specimen is deposited and
given the reference number FMM 3686. Acacia mearnsii
stem bark extract used as reference in this study are as
those reported in our previous work [15]. Hide powder
was kindly bestowed by FILK - Forschungsinstitut für
Leder und Kunststoffbahnen (Research Institute of Leather and Plastic Sheeting), Freiberg, Germany.
2.1.2 Chemicals and reagents
All used chemicals and reagents which included 2,4,6trinitrobenzenesulphonic acid, sodium carbonate, hydrochloric acid, citric acid, disodium hydrogen phosphate,
Mcllvain buffer, hydrochloric acid AR (38%), monosodium phosphate, disodium phosphate, sodium hydrogen
carbonate, lithiumcitrate buffer, ninhydrin, FolinCiocalteu’s phenol reagent, gallic acid, aluminium chloride, catechin and sodium nitrite were of analytical grade
purchased from Sigma Aldrich, Germany.
2.2 Methods
All analyses reported in this study were conducted in
the analytical chemistry laboratory at the FILK-Research
Institute of Leather and Plastic Sheeting, Freiberg,
Germany.
2.2.1 Preparation of root and stem barks
The roots and stems were debarked then chopped into
small pieces. The bark pieces were dried under the shade
at Chemistry Department, University of Dar es Salaam
for 2 weeks. The dried barks were pulverized to a particle size of equal or less than 1 mm.
2.2.2 Extraction of tannins
Pulverized stem and root barks of T. burttii were subjected to extraction. Maceration extraction method [18]
was employed with minor modifications. Approximately
20 g of the root and stem bark samples were separately
sopped in 200 mL of distilled water in a glass beaker
shielded with aluminium foil to preclude water evaporation. The mixture was placed on a water bath ready for
the extraction exercise. Extraction was done at three distinct temperatures (30, 50 and 80 °C), the procedure
which was adopted from our previous work [15]. The
sample mixture was continuously stirred using an overhead stirrer which was connected to the beaker’s opening through a hole created on the aluminium foil used
to cover the sample. Extraction process was performed
for 4 h and then the filtrates were collected. The bark
residue were again subjected to second round of extraction for 4 h. Therefore, 8 h were spent to leach tannins
from one batch of the barks. Time for extraction for
each step was chosen to be 4 h to allow a longer contact
China et al. Journal of Leather Science and Engineering
time between the milled barks and water. Extracts were
lyophilized and resultant powder (tannin) was used for
studying the influence of extraction temperature and pH
on crosslinking capacity, and interaction mechanism
with hide collagen. Preparation of A. mearnsii extract
and resulting powder used as reference in this study was
achieved as recently reported [15]. The extracted tannins
powder was analyzed for extract yield, tannin content
(TC), total phenolic content (TPC) and total flavonoid
content (TFC).
Extract yield was determined using the following
equation;
%Extract yield ¼
Page 3 of 9
(2021) 3:13
Extract obtained ðg Þ
100
Amount of moisture free barks used ðg Þ
2.2.3 Determination of tannin content
A standard procedure as described by Atkin and
Thompson [19] was used to determine tannin content.
In this method, unfiltered tannin solution was detanned
by lightly chromed Freiberg Hide Powder batch number
‘VK 383’. In order to prevent the hide powder from passing through the capillary, a little dry cotton-wool was
placed at the upper part of the bell. The neck was fixed
with rubber stopper carrying capillary glass tube bent
twice at right angles. Afterwards, 7 g of hide powder was
filled in the bell and pressed outward onto the bell’s wall
to prevent channels that may allow tannin solution to
pass through undetanned solution. The prepared filter
bell was placed in 200 mL beaker and the latter was
filled with tannin solution and placed in the water bath
maintained at 18 °C. When the tannin solution was ready
absorbed by hide powder up to the neck, moderate suction was applied to the capillary limb until liquid flows
out gradually at the rate of 8–10 drops per minute. It
was observed that, the detanned solution didn’t produce
turbidity with gelatin-salt reagent. The first 30 mL of the
detanned solution was discarded and 50 mL of the next
60 mL was evaporated and dried to constant weight in
order to establish the non-tannin content. The tannin
content of the resultant extract was determined using
the following formula:
Tannin content ¼ Soluble substances − non tannins
2.2.4 Determination of Total phenolic content
To determine the Total Phenolic Content (TPC), a previously established method was used [20]. To 0.2 mL of
extract solution, 5 mL of 10% Folin-Ciocalteu’s phenol
reagent was added and thoroughly mixed followed by
addition of 4 mL of 7.5% sodium carbonate after 6 min.
The mixture was diluted to 25 mL with deionized water
and incubated for 90 min. UV-VIS spectrophotometer
(Evolution 201) was used to record absorbance at
nm. The calibration curve was obtained using gallic
where the used concentrations were 50, 75, 100,
200 mg mL− 1. TPC was presented as mg gallic
equivalent per g dry weight of the barks and then
sented as percentage based on bark dry weight.
760
acid
125,
acid
pre-
2.2.5 Determination of Total flavonoid content
The total flavonoid content in the resultant extract was
attained using previously instituted procedure [21]. To
0.5 mL of extract solution 4 mL of deionized water was
added followed by 0.3 mL of 5% NaNO2 solution. After
5 min, 0.3 mL of 10% AlCl3 solution was added. The solution was allowed to stand for 6 min, then 2 mL of 1 M
NaOH solution was added. Lastly, the volume was made
to 10 mL using deionized water and left to stand for 15
min followed by absorbance measurement at 510 nm
using UV-VIS-spectrophotometer (Evolution 201). The
calibration curve was obtained using 20, 40, 60, 80, 100
mg mL− 1 concentrations of catechin. The TFC was
expressed as mg catechin equivalent per g dry weight of
the barks and then presented as percentage based on
bark dry weight.
2.2.6 Preparation of hide powder crosslinked with extract
leached at different temperatures
Hide powder was treated with the tannin powder
prepared from the studied barks by employing an
already established procedure [9]. In brief, approximately 1 g of hide powder was sodden in 20 mL of
0.4 M Mcllvail buffer at pH 5 for 1 h followed by
addition of 5% w/v of the tannin powder extracted
at three distinct temperatures. The solution was then
shaken in the mechanical shaker at 35 °C for 5 h
followed by filtration using a vacuum pump. The filtrates were then discarded and treated hide powder
was washed three times with excess water, soaked in
phosphate buffer at pH 7 and was reserved for further analysis.
2.2.7 Preparation of hide powder crosslinked at different pH
The tannin powder prepared from studied barks was
used to crosslink hide powder as per protocol described
by Schropfer and Meyer [9]. About 1 g of hide powder
was soaked in 20 mL 0.4 M Mcllvail buffer at varying pH
(2.5–9.0) for 1 h. About 5% w/v of the tannin powder extracted at 50 °C was added, and the solution was shaken
in the mechanical shaker at 35 °C for 5 h. The solution
was then filtered using a vacuum pump. Filtrates were
discarded and treated hide powder was washed three
times with excess water, soaked in phosphate buffer at
pH 7 and was reserved for further analysis.
China et al. Journal of Leather Science and Engineering
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(2021) 3:13
2.2.8 Differential scanning calorimetry (DSC) analysis of
crosslinked hide powder
The denaturation temperature of the crosslinked hide
powder was measured using DSC 1 device (Mettler-Toledo) to determine the crosslinking capacity of the tannins against pH of the tanning solution. About 6 mg
(calculated on dry weight) of wet cross-linked hide powder was placed in an aluminium pan and then hermetically closed. Temperature scans were run from 10 to
125 °C at a rate of 5 °C min− 1. Finally, the onset
temperature (Tonset) and peak temperature (Tpeak) were
calculated based on the endotherms.
2.2.9 TNBS assay
2,4,6-Trinitrobenzenesulfonic (TNBS) acid assay was
carried out using the experimental protocol developed
by Schroepfer and Meyer [9] to determine the amount
of basic amino groups crosslinked with the tannins from
investigated plant extracts. Approximately 5 mg of the
cross-linked hide powder (pH 7) was incubated in 200
mL of 0.5 M sodium hydrogen carbonate buffer at pH 8
and then 200 mL of 0.5% TNBS acid was added into the
mixture. The samples were incubated at 60 °C for 4 h to
provide a room for TNBS acid to bind on the free primary amines (deprotonated) in the hide powder collagen. Afterwards, the mixture was hydrolyzed with 6 N
HCl at 80 °C for 1.5 h, then diluted with 1 mL of distilled
water and centrifuged at 14000 rpm. Then photometric
measurements of supernatants absorption was performed at 400 nm using UV-VIS-spectrophotometer
(Evolution 201). Quantification of crosslinked amino
groups was performed by calibration with an alanine
standard. The number of free primary amines of crosslinked and non-cross-linked hide powder samples were
used as the basis to calculate the number of primary
amines bound by the tannins using the equation below:
%of crosslinked amino groups
100 − free amines ðμmol per g dry sampleÞ
¼
100
mean value free amines ðμmol per g dry hide powderÞ
2.2.10 Amino acid hydrolysis
The perdormance of amino acid analysis aimed to identify the resistance of the basic amino groups crosslinked
by tannins from the plant extracts under investigation to
acid hydrolysis. Crosslinked hide powder samples were
hydrolysed with 6 N HCl at 110 °C for 20 h. Afterward,
hydrolized samples were dried and dissolved in lithiumcitrate buffer. An amino acid analyser (Biochrom 30+)
was employed to determine the amino acid composition
in the cross-linked and non-cross-linked samples using
pre-column derivatisation with ninhydrin according to
the standard protocols [22]. The amount of primary
amines formed an acid hydrolysis stable bond was calculated from the area under the lysine, hydroxylysine or arginine peaks and standardized to the area under the
peaks from alanine and valine (these were not involved
in cross-linking). The resulting percent of crosslinked
amino groups were related to that calculated from noncross-linked sample.
2.2.11 Quality control and assurance
The quality and accuracy of the analytical results were
monitored by analysing each sample in duplicate as well
as analysing reference materials. Double deionized water
was used for solution preparation, extraction of tannins
and washing glassware to ensure that the solutions and
tannins are free from contaminants. In order to monitor
contamination, the reagent blank was analysed parallel
with all samples analysed in this study. Barks were subjected to two extraction steps to ensure extraction efficiency. After extraction, extract solution was stored at
4 °C in the refrigerator to prevent bio-degradation of
tannins that would lead to the loss of tanning ability.
The dried extract powder was stored in an opaque container to prevent photo-oxidation of the tannins.
3 Results and discussion
3.1 Effect of extraction temperature on crosslinking
capacity
Temperature applied during extraction of tannins from
plant materials is an important parameter to consider as
it determines the availability of phenolics and other reactive species [23]. The effect of temperature on crosslinking capacity was determined by crosslinking hide
powder with the tannin powder extracted at different
temperatures (30 °C, 50 °C and 80 °C) and the results are
presented in Fig. 1a. The influence of the extraction
temperature on crosslinking capacity was observed to be
insignificant for both root and stem bark within the
studied temperature range. The observations were similar to those of commercial tanning source (A. mearnsii
bark extract) used in this study as a reference plant extract (Fig. 1b). Moreover, the results are in accordance
with those we recently reported for other plant extracts
as potential sources of tannining agents [15]. However,
in terms of Tpeak and Tonset intervals, it is worth noting
that the extract obtained at 50 °C is better than the
others because it leads to a crosslinked hide powder with
the lowest interval between the two temperature points,
suggesting high crosslinking capacity (Table 1). The
value is very close to that of hide powder treated with A.
mearnsii bark extract leached at 50 °C (Table 1). It is
well known that the interval between Tonset and Tpeak is
an indicative of the extent of crosslinking process homogeneity, whereby the lower the interval the higher the
homogenous the crosslinking process. This also gives a
China et al. Journal of Leather Science and Engineering
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Fig. 1 Denaturation Temperature Tonset of hide powder cross-linked with extract of Tessmannia burttii (a) and Acacia mearnsii (b) at pH 5 with
dependence to temperature of extraction
clue on the crosslinking capacity of the tanning materials
[24, 25]. It is therefore expected that the extract recovered from samples under investigation at 50 °C is likely
to perfom better in tanning than others, thereby bringing about desired tanning capacity.
Previous studies reported the loss of tannins, especially
hydrolysable ones, when temperature of extraction is
above 65 °C since these tannins tend to decompose at elevated temperatures [6, 26]. It has also been reported
that extraction at elevated temperatures can lead to coextraction of carbohydrates including gums and pectins
in undesirable quantities [26]. Thus, tannins from the
root and stem barks of T. burttii obtained at 50 °C or
lower could be considered in preparation of such vegetable tannins be it hydrolysable or condensed.
It is also important to note that, like other tannin
sources, extraction of tannins from T. burttii root and
stem barks is possible using water as a sole solvent,
which proved to be effective in tanning capacity as discussed in the subsequent section. Use of water for tannin production is cost effective and environmentally safe
for sustainable manufacturing of leather [6, 27]. The
only limitation would be the quantity of water consumed
during extraction, whereby large quantity is required to
achieve the intended goal. Despite this limitation, water
is still a solvent of choice for tannins preparation at
industrial scale [27, 28]. Furthermore, extraction with
water leads to slightly acidic extract [27] principally
matching with pH for the maximum crosslinking ability
as described in the next section of this article. Hence no
acidification is required prior to tanning.
3.2 The extract yield, tannin content, total phenolic and
flavonoid contents of the Tessmannia burttii extracts
Table 2 presents the extract yield, tannin content, total
phenolic content, total flavonoid content and non-solubles
present in the T. burttii extracts leached at 50 °C. The extracts yield of both stem and root barks were found to be
below that of A. mearnsii. However, they are within the
ranges of other recommended tannin source materials [15,
29]. The tannin content of the stem bark of the studied
plant is lower than that of A. mearnsii, but higher than recommended value, which is 10% [30]. On the other hand,
the total phenolic and flavonoid contents is higher than that
of A. mearnsii. Tannins contain phenolic and flavonoid
compounds that carry reactive groups to bind collagen fibers during tanning [31]. Presence of sufficient quantity of
these ingredient is the key to the successful tanning process
[15]. Having phenolic and flavonoid content in a quantity
higher in T. burtii stem bark extract than commercial
source of tannin, A. mearnsii, is an evident that T. burttii
extracts can be a suitable source for tannins production.
Table 1 The interval between Tonset and Tpeak of hide powder cross-linked with extract of Tessmannia burttii and Acacia mearnsii at
pH 5 in dependence of Temperature of extraction
Extraction
temperature
(°C)
Denaturation temperature (°C)
T. burttii stem bark
T onset
Tpeak
T. burttii root bark
Interval (Tpeak - Tonset) Tonset
Tpeak
A. mearnsii stem bark
Interval (Tpeak - Tonset) Interval (Tpeak - Tonset)
30
81.16 ± 0.57 85.64 ± 0.05 4.48
80.93 ± 0.09 85.42 ± 0.20 4.49
3.44
50
81.62 ± 0.41 85.36 ± 0.14 3.75
81.47 ± 0.22 85.22 ± 0.22 3.75
3.71
80
81.34 ± 0.07 85.36 ± 0.40 4.02
81.18 ± 0.00 85.34 ± 0.01 4.16
3.83
China et al. Journal of Leather Science and Engineering
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Table 2 Tannin content, total phenolic content and total flavonoid content of the T. burttii extracts extracted at 50 °C
Plant
Extract yield (%)
Tannin content (%)
Total phenolic content (%)
Total flavonoid content (%)
Tesmmannia burttiia
39.4 ± 1.0 (stem bark)
32.4 ± 1.2 (root bark)
61.2 ± 0.2
58.5 ± 1.0
32.8 ± 0.0
Acacia mearnsii stem bark
56.02 ± 0.3
84.3
32.8 ± 0.5
22.5 ± 0.1
a
Tessmannia burtii root bark extract was not analysed for tannin content, total phenolic and flavonoid contents due to limited amount of resources
3.3 The interaction mechanism between vegetable
tannins from Tessmannia burttii and collagen of the hide
Interactions between tannins and protein have been
studied for more than 50 years because of their unique
characteristics and potential use in various industries
[32]. Recently, the study on the interaction of tannins
with collagen of the skin has also gained attention due
to the role that vegetable tannins play in attaining green
production and eco-friendly tanning goals. Similary, the
The present work studied the interaction mechanism between collagen of the hide powder and the tannins from
the root and stem bark of T. burttii by employing three
different methods. The purpose was to understand the
underlying chemistry and confirming the tannins type.
To achieve this purpose, hide powder were treated with
extracts under the same conditions. The treated hide
powder was analyzed for denaturation temperature at
different pH using DSC technique, amount of crosslinked amines at basic pH before acid hydrolysis and
after acid hydrolysis using TNBS acid assay and amino
acid analysis, respectively.
3.3.1 DSC analysis
The results for DSC analysis are presented in Fig. 2a and
b, where denaturation temperature denotes the extent of
interaction/crosslinking activity of tannins on the hide
powder collagen. It was noted that the crosslinking activity was sharply increasing from lower pH towards
high pH up to 4, above which the denaturation
temperature remained constant (Fig. 2a). Similar trend
was observed for A. mearnsii (Fig. 2b). This indicates
that binding preferentially occurs at unprotonated amino
groups via nucleophilic addition reaction [33–35], which
gives a first hint, that T.burtii contains mainly condensed tanning agents. The DSC measurements were
performed in a buffer at pH 7. All purely electrostatic interactions with protonated amino groups are preferably
eliminated at this pH value, which would be the case
with hydrolysable tanning agents. This and the increasing cross-linking activity with increasing pH value suggest the presence of mainly condensed tanning agents.
With hydrolysable tannins preferentially hydrogen
bonding occurs when phenolic hydroxyl interacts with
protonated amino group or peptide oxygen of collagen
[9, 26]. Since at acid pH less than 4 the interaction observed was low, it might be inferred that tannins in the
investigated samples contain fewer hydroxy groups to
form hydrogen bond with collagen. It has been reported
that high crosslinking activity observed at neutral basic
pH is mainly due to covalent bonding between unprotonated amino groups in the collagen and either quinone
or aldehyde groups in the tannin molecule [9, 36, 37].
Thus, TNBS acid assay was performed in this work to
indicate possible presence of the covalent bonding due
to the interaction between tannins and unprotonated
amino groups.
Fig. 2 Denaturation Temperature Tonset of hide powder cross-linked with extract of Tessmannia burttii (a) and Acacia mearnsii (b) extracted at
50 °C with pH dependence
China et al. Journal of Leather Science and Engineering
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Fig. 3 Amount of bound amino-groups of hide powder cross-linked with the extracts of Tessmannia burttii (a) and Acacia mearnsii (b) extracted
at 50 °C with pH dependence before acid hydrolysis
3.3.2 TNBS acid assay
3.3.3 Amino acid hydrolysis
Since DSC results revealed probable formation of covalent bonds that occurred at pH above 4, it was essential to characterize the amount of amino groups
that participated in the crosslinking activity. The
crosslinked hide powder were derivertised with
TNBS acid and their percentage crosslinked is presented in Fig. 3a and b. Tannins from stem bark revealed high amount of crosslinked amino groups
even for the samples treated at acidic pH. Root bark
tannins showed significant amount of crosslinked
amino groups, but were slightly low at acidic pH
(Fig. 3a). A. mearnsii extracts which belong to condensed tannins exhibited similar trend (Fig. 3b). Previous investigations have also acknowledged the
presence of interactions between tannins and amino
groups of protein preferentially at neutral to alkaline
pH [9, 15, 32].
According to Zhang et al., the postulated chemical
pathway for covalent bond formation between unprotonated amino groups and tannins involves initial oxidization of phenolic structures to form quinone
intermediates under alkaline conditions that can readily
react with nucleophiles from reactive amino acid groups
in protein chain [38]. On the other hand, glutaldelyde
and secoiridoids have also been reported to depict a
similar trend except that the reactive group in these two
compounds is aldehyde and not quinones as it is the
case for condensed tannins [9, 36, 37]. To differentiate
between the two possible crosslinking mechanisms, one
must undertake amino acid hydrolysis on the crosslinked
collagen material based on the fact that the bonds
formed due to quinone are not stable on acid hydrolysis
as opposed to those due to aldehyde [9, 36, 37].
To implicitly determine whether the formation of covalent linkages between tannins from studied plant extracts powder and hide powder collagen was due to
either quinone or aldehyde groups, amino acid hydrolysis method was employed. As for A. mearnsii treated
hide powder (Fig. 4b), high percent of crosslinked amino
groups observed in TNBS acid assay dropped to less
than 2% for both root and stem bark extracts treated
hide powder (Fig. 4a). These few crosslinks observed
might be due to electrostatic interactions. As it was pinpointed earlier, the interaction of this nature is characteristic of condensed tannins of A. mearnsii. In our
previous work we had similar observation for the extracts from three plants namely Acacia xanthophloea,
Euclea divinorum and E. racemosa [15]. Therefore, it
can be concluded that T. burttii contain condensed tannins, being in agreement with already identified catechinoids from the same plant stem and root barks
(unpublished work). This is good for production of durable and thermally stable leather as previuosly confirmed
that covalent bonds between phenolic compounds and
proteins are more rigid and thermally stable than other
interactions [38].
4 Conclusions
The present work reports the investigation of the
crosslinking capacity of the extracts (tannins) from
stem and root barks of T. burttii as a function of extraction temperature and pH. The work was further
extended to study the corresponding interaction
mechanism between the extracts with collagen of the
skin in comparison to A. mearnsii extracts. The
China et al. Journal of Leather Science and Engineering
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Fig. 4 Amount of bound amino-groups of hide powder cross-linked with the extracts of Tessmannia burttii (a) and Acacia mearnsii (b) extracted
at 50 °C with pH dependence after acid hydrolysis
extraction temperature exhibited little effect on crosslinking capacity, but extraction at 50 °C yielded the
best results in relation to Tonset to Tpeak interval. The
extract yield is within the recommended range while
the tannin content is above the recommended values.
T. burtii stem bark extract total phenolic and flavonoid contents are above that of commercial source, A.
mearnsii. Minimum pH for the maximum crosslinking
capacity was established to be 4. The study has established that the interaction between the studied tannins with collagen of the skin could preferentially
occur via covalent bonding with basic amino groups
and the bonds formed are not stable upon acid treatment. Based on the nature of tannin-collagen interactions, the observations that were comparable to those
of A. mearnsii extract used as a reference, it was
ascertained that tannins from both stem and root
barks of T. burttii belong to condensed type. Therefore, T. Burttii extracts have the same tanning potential as A. mearnsii. Considering the environmental
aspects, this work has uncovered a promising alternative eco-friendly source of vegetable tannins deployable in sustainable leather manufacturing. Further
investigatory parameters on their cross-linking capacity such as structural characterization, and participation of the carboxyl and guanidino groups in the
reaction are highly recommended.
Acknowledgements
Authors wish to thank Michael Kramer and Heidrun Berthold for technical
assistance during laboratory work. Mr. Frank M. Mbago, a senior taxonomist
and curator at the Herbarium of the Department of Botany, University of Dar
es Salaam is highly appreciated for locating and identifying the plant species
reported in this investigation.
Authors’ contributions
SSN, MS and CRC conceptualized the research. MS designed experiments to
collect data. CRC and MMM drafted the manuscript and edited all
corrections and comments from co-authors. The rest of the authors
participated fully in reviewing and editing the manuscript. The author(s) read
and approved the final manuscript.
Funding
J.J.E.M. is grateful to the International Foundation for Sciences (IFS) [grant
numbers Grant No. J/5528-1] for financial support involved in the plant
collection.
Availability of data and materials
The datasets used and/or analysed during the current study are available
from the corresponding author on reasonable request.
Declaration
Competing interests
The authors declare that they have no competing interests.
Author details
1
Division of Textile and Leather Technologies, Tanzania Industrial Research
and Development Organization, P. O. Box 2355, Dar es Salaam, Tanzania.
2
Chemistry Department, College of Natural and Applied Sciences, University
of Dar es Salam, P.O. Box 35061, Dar es Salaam, Tanzania. 3Department of
Environmental Planning, Institute of Rural Development Planning (IRDP), P.O.
Box 138, Dodoma, Tanzania. 4FILK - Research Institute of Leather and Plastic
Sheeting, Meißner Ring 1-5, 09599 Freiberg, Germany.
Received: 28 July 2020 Accepted: 15 March 2021
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