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J Biomed Biotechnol. 2004 December 1; 2004(5): 326–331. doi: 10.1155/S1110724304403052. | PMCID: PMC1082888 |
Quantification and Purification of Mulberry Anthocyanins With
Macroporous Resins Xueming Liu,* Gengsheng Xiao, Weidong Chen, Yujuan Xu, and Jijun Wu The Sericultural Research Institute, Guangdong Academy of Agricultural Sciences, 133 Yihenglu, Dongguanzhuang, Tianhe District, Guangzhou 510610, China Received March 3, 2004; Revised June 24, 2004; Accepted July 18, 2004. Total anthocyanins in different cultivars of
mulberry were measured and a process for the industrial
preparation of mulberry anthocyanins as a natural food colorant
was studied. In 31 cultivars of mulberry, the total anthocyanins,
calculated as cyanidin 3-glucoside, ranged from 147.68 to
2725.46  mg/L juice. Extracting and purifying with
macroporous resins was found to be an efficient potential method
for the industrial production of mulberry anthocyanins as a food
colorant. Of six resins tested, X-5 demonstrated the best
adsorbent capability for mulberry anthocyanins (91 mg/mL
resin). The adsorption capacity of resins increased with the
surface area and the pore radius. Residual mulberry fruit juice
after extraction of pigment retained most of its nutrients,
except for anthocyanins, and may provide a substrate for further processing. Edible pigments are important food additives which can increase
the acceptability of a food product. The safety of synthetic
pigments has been questioned, leading to a reduction in the
number of permitted colorants [ 1]. Due to this limitation
and to the worldwide tendency towards the consumption of natural
products, the interest in natural colorants has increased
significantly [ 2]. Anthocyanins provide attractive colors
such as orange, red, and blue. They are water-soluble, which
facilitates their incorporation into aqueous food systems. These
qualities make anthocyanins attractive natural food colorants.
Besides their color attributes, anthocyanins have been reported
to be beneficial to health as potent antioxidants and to improve
visual acuity [ 3]. They have also been observed to possess
antineoplastic, radiation-protective, vasotonic, vasoprotective,
anti-inflammtory, chemo- and hepato-protective activities
[ 3]. The literature provides an abundance of data on most of
the plant species considered as potential sources of anthocyanin
food colors. Bridle and Timberlake
[ 4] suggested several sources including some of
the principal commercially available anthocyanin colorants, that
is, grape ( Vitis sp.), elderberries ( Sambucus
nigra), red cabbage ( Brassica oleracea), roselle
( Hibiscus sabdariffa), and other sources including blood
orange ( Citrus sinensis), black chokeberry
( Aronia melanocarpa), and sweet potato ( Ipomoea
batatas). In many countries, especially in China, mulberry ( Morus
spp., Moraceae) is used for its foliage to feed the silkworm
( Bombyx mori L.). As with many other forage crops,
mulberry breeding has focused on enhancing foliage production
through heterosis breeding [ 5]. Mulberry fruit is rich in
anthocyanins and can be considered as a potential source for
production of a natural red food colorant. The major anthocyanins
identified in the fruit extract are cyanidin 3-glucoside and
cyanidin 3-rutinoside [ 6]. Due to its small size, relatively
low output, and short storage life, this fruit received relatively
little attention. However, application of new technologies in
breeding and processing may offer a commercially profitable
production of anthocyanins from mulberry fruit. Adsorbent resins are durable nonpolar or slightly hydrophilic
polymers having high adsorption capacity with possible recovery
of the adsorbed molecules, relative low cost, and easy
regeneration. They are currently used for adsorption of
flavonoids and other components extracted from many plants.
Nonpolar styrene-divinylbenzene (SDVB) resins have been used to
recover hesperidin from citrus peel or the wastewater flowing
from centrifuges of essential oil separation (yellow water)
[ 7, 8]
and anthocyanins from pulp wash of pigmented oranges
[ 9]. They have also been used to remove naringin and limonin
from citrus juices and to recover cold pressed grapefruit oil from
wastewater [ 10]. Resins are effective adsorbent material for
anthocyanins from different sources and have been widely used in
research and in the production of anthocyanins
[ 4, 9,
11, 12,
13]. The objective of this study was to evaluate the relative
anthocyanin contents in different cultivars of mulberry, and to
investigate a potential industrial process for the extraction of
mulberry anthocyanins for use as a natural food colorant.
Six resins, having different chemical and physical properties,
were investigated for adsorption of mulberry anthocyanins. Plant material and sample treatment Samples for the evaluation of anthocyanin content in various cultivars Mulberry fruits were obtained from South China Mulberry Resource
Garden in Dafeng Agricultural Experimental Base of the Guangdong
Academy of Agricultural Sciences (Guangdong, China). For each
cultivar, 250–300 g fully mature mulberry fruits were
harvested and stored at 4 °C until further treatment.
All treatments were performed on the day of harvesting. After
weighing, mulberry fruits were mixed in an electric blender,
filtered with a nylon filter cloth, and centrifuged at 5000g
for 15 minutes. The supernatant was collected and assayed for
anthocyanin content, soluble solid substances, and total acid content. Samples for evaluation of adsorbent capacities of resins Mulberry fruits for juice production and anthocyanin purification
were obtained from two mulberry bases: Doumen Base and Huadou
Base, situated in Doumen District, Zhuhai and Huadou District,
Guangzhou, China, respectively. Fruits were collected at the
fully mature stage. In order to maintain freshness, mulberry
juice factories were constructed in the vicinity of the mulberry
bases. Mulberry fruits were rinsed with 0.9% saline water and
then by tap water, before being squeezed. The resulting juice was
centrifuged, pasteurized, and stored in sterilized bags at
4°C (for immediate further processing) or at
−18°C (for later processing). Resins: their pretreatment and activation The resins tested were D3520, D4020, X-5, NKA-9, D101A, and AB-8
(Chemical Industrial Company affiliated to Nankai University,
Tianjin, China). All resins were cross-linked polystyrene
copolymers. Their physical and chemical properties are shown in
Table 1. | Table 1Physical and chemical properties of resins. |
The resins were pretreated and activated according to the
manufacturer's recommendation. Firstly, they were rinsed with
distilled water and filtered with nylon filter cloth to retain
those with a particle diameter larger than 0.3 mm. They were
then soaked overnight in 2 bed volumes (BV) of 95% ethanol.
After soaking, the resins were introduced into a glass column and
rinsed with a further 2 BV of 95% ethanol. Subsequently they
were rinsed with 2 BV of distilled water to dispel the ethanol, 1
BV of 4% (w/v) sodium hydroxide, 2 BV of distilled water, 1
BV 4% (v/v) hydrochloric acid, and finally by distilled water
until the pH of the eluent became neutral. Purification of mulberry anthocyanins Raw mulberry juice was centrifuged at 5000 g for 15 minutes
to produce a bright (nonturbid) supernatant. Different volumes of
the supernatant were passed through resin columns depending on
the adsorbent capability of each resin. Anthocyanins and other
phenolics were adsorbed onto the column; sugar, acids, and other
water-soluble compounds were eluted with more than 2 BV of
distilled water until the wash water was clear. The adsorbed
material was then eluted with acidified ethanol
(0.5% (v/v) of hydrochloric acid) until there was no color
in the eluent. The eluent was concentrated on a rotary evaporator
under reduced pressure at 60 °C and the resulting
concentrate was lyophilized to form a pigment powder. In order to select the best resin for capturing mulberry
anthocyanins, the anthocyanins in the eluates were tested
spectrophotometrically as a function of time. The resins were
considered to be saturated when, during column loading, the
anthocyanin contents of the juice and the eluent were equal. To determine the optimum ethanol concentration for elution of the
adsorbed anthocyanins, 5 g aliquots of resin saturated with
anthocyanins were added to a range of 250 mL flasks with
different concentrations of acidified ethanol, and incubated on a
shaker for 1 hour at 25 °C, 125 rpm. The
contents of anthocyanins in these eluates were measured spectrophotometrically. Total anthocyanin content The total anthocyanin content was determined using the pH
differential method [ 14]. An F755B UV spectrophotometer
(Shanghai, China) and 1 cm path length disposable cuvettes
were used for spectral measurements at 420, 538, and 700 nm.
Pigment content was calculated as cyanidin 3-glucoside (cyd
3-glc), using an extinction coefficient of 29 600 L cm −1
mg −1 and molecular weight of 448.8. Determination of pH, titratable acidity, and soluble solid contents The pH measurements were made using a digital pH meter (pB-20,
Sartorius, Germany) calibrated with pH 4 and 7 buffers.
Titratable acidity was measured by the titrimetric method. Titratable acidity of mulberry was
expressed as % citric acid. Soluble solid contents were expressed as refractive
index and were measured with a 2 WAJ Abbe refractometer (Shanghai, China). HPLC analysis of mulberry anthocyanins The HPLC analysis of anthocyanins was performed
as described by Konczak-Islam et al [ 15].
The pigment solutions were filtered through a
0.45 μm syringe-driven filter unit (Millipore
Corporation Bedford, Mass). The HPLC system
consisted of two LC-10AD pumps, SPD-M10A diode array detector, CTO-10AS column
oven, DGV-12A degasser, SIL-10AD autoinjector, and SCL-10A system
controller (Shimadzu, Japan) equipped with Luna (3 μm
C18(2), 4.6 mm × 100 mm, Phenomenex,
Calif) column at 35 °C. The following
solvents in water with a flow rate of 1 mL/min were used: A
(1.5% phosphoric acid) and B (1.5% phosphoric acid, 20% acetic
acid, 25% acetonitrile). The elution profile was a linear
gradient elution with 25%–85% solvent B in solvent A for
100 minutes. The chromatograms were monitored at 530 nm
and recorded, and the relative concentrations of individual
pigments were calculated from the peak areas. Total anthocyanin content in different cultivars of mulberry Thirty-one cultivars of mulberry were analyzed with respect to
their fruit weight, soluble solid substances ( °Brix),
total acids, and total anthocyanins. These characteristics varied
between cultivars despite all samples having been collected at a
mature stage and from the same place ( Table 2). The
total anthocyanin content of the evaluated cultivars varied
between 147.68 (cultivar Yuiyou-26) and 2725.46 mg/L
juice (cultivar 7403). The anthocyanin content was also found to
depend on climate and production area. It was observed that
anthocyanin content of the same mulberry cultivar was much higher
on a sunny day than on a rainy day (data not shown). As was
observed with respect to the anthocyanin content, other mulberry
fruit parameters such as average fruit weight, soluble solid
substances, and total acids varied across the different cultivars studied. | Table 2Average fruit weight, content of soluble solid substances, total acids, and total anthocyanins in fruit of various mulberry cultivars. |
Adsorbent capability of different resins to mulberry pigments The single strength juice was used as the starting material for
the production of purified mulberry pigment. The columns were
loaded with juices to obtain a complete saturation, rinsed with
deionized water, and subsequently eluted with acidified ethanol
solution. The adsorption capacities of the resins were evaluated
based on the spectrophotometrical measurement of anthocyanins
levels in the eluates (). The 6 resins,
ranked according to their adsorption capacities (highest to
lowest), were as follows: X-5>AB-8>D4020>D101A>D3520>NKA-9. The ranking appears
to reflect the different physicochemical properties of these resins. Among the nonpolar resins tested, the mulberry anthocyanin
adsorbing capabilities primarily depended on the pore radius and,
to a lesser extent, on the surface area. Resin X-5, with the
largest pore radius (290~300 Å) and surface area
(500~600 m 2/g), had the best adsorption capability.
Its total adsorption capacity (91 mg/mL of resin) was more
than double that of any other resin. The total
adsorption capacity of the X-5 resin evaluated under the
conditions of this experiment was more than 100 times that
reported by Di Mauro et al [ 9].
Similarly the nonpolar resin D3520, with the smallest pore radius
(85~90 Å), showed the second lowest adsorption
capacity and it is suggested that too small pore size did not
allow penetration of anthocyanins into the reticule of resin
[ 9]. NKA-9, the only polar resin tested and with the
smallest surface area (250~290 m 2/g), had the lowest
mulberry anthocyanin adsorption capacity. These findings are consistent with the results reported by
Di Mauro et al [ 9] although there
are some differences in the exact ranges of pore
radius and surface area studied. Eluent efficacy of different concentrations of acidified ethanol In devising a potential industrial production process for
mulberry anthocyanins as an edible colorant, it is necessary to
take into consideration not only the resin, but also the effluent
solvent and its concentration. As shown in ,
the acidified ethanol could effectively elute anthocyanins from
the resin at concentration higher than 30% (v/v). The
solutions with low concentrations of ethanol have a higher
boiling point and are therefore more difficult to concentrate
than those with high concentrations. In order to design an
accelerated anthocyanin recovery process, a higher concentration
of ethanol is recommended. Elution of mulberry pigment with acidified ethanol Acidified 80% (v/v) ethanol was found to be able to elute
most of the anthocyanins adsorbed by the resin. Using this eluent, the
overall recovery rate was higher than 99%. Of a total of
600 mL eluent, the first 150 mL contained only 1.5% of
the total anthocyanins (due to washing water still present in
resins); the middle 250 mL contained 96%, while the last
200 mL contained 2.5% (). Although high
concentrations of ethanol can effectively elute the pigment
adsorbed by the resins, the product cannot be completely
dissolved in the water, suggesting a presence of some impurities.
In order to obtain a water-soluble pigment, different concentrations of acidified
ethanol (10~80%) (v/v) were evaluated and the resulting
eluent was concentrated, dried, and solubility in water was
tested. The results showed that pigment eluted with less than
30% (v/v) acidified ethanol was completely water-soluble;
however, it accounted for about 80% of the total weight of pigments. Comparison of mulberry pigment before and after purification with resin Crude mulberry juice is rich in anthocyanins and, after
concentration, can be directly used as a natural pigment.
However, the concentration of anthocyanins in such juice is low,
with a color value usually less than 4.0. That is due to the
presence of nonpigment components such as mono-, di-, and
polysaccharides, minerals, proteins, or organic acids in large
proportions. Instead of the concentrated juice, pigment powder
can be produced by spray drying, but the resulting product is
very hygroscopic, becomes sticky and hard to dissolve in water.
These products have a relatively short shelf life (data not presented). Contrary to the crude mulberry juice, the eluent of pigment
purified with resins can be easily concentrated to obtain a high
color value, usually more than 100, and lyophilize easily.
During the purification with resins, most of the impurities are
removed. Removal of the impurities can also decrease the
enzymatic and the nonenzymatic reaction causing the browning of
pigment and thus increase the stability of potential food colorant. When tested by HPLC under the same conditions (520 nm), the
HPLC profiles of the purified and crude pigment extracts were
identical, consisting of four peaks (). This
suggests that pigment purification using the resins did not
change the composition of the anthocyanin mixture. Comparison of raw mulberry juice before and after extracting mulberry pigment with resin Besides the anthocyanin pigments the crude mulberry juice contains
also other components. If in an industrial setting the crude
juice was used only for the production of pigment, environmental
waste would be generated. We have evaluated the total sugar,
total acids, total anthocyanins, some vitamins, and the pH of the
mulberry juice before and after purification with X-5 resin under
unsaturated adsorption conditions ( Table 3). | Table 3Components in raw mulberry juice before and after purification with X-5 resin. |
The results indicated that almost all anthocyanins were adsorbed
by the resins, while total sugar, total acids, and vitamins
(vitamin B1) remained in the raw juice under unsaturated
adsorption. Therefore, it can be considered after removal of the
anthocyanins the residual juice to be fermented in order to
produce products such as juice, wine, and sauce, enhancing the
overall value of the mulberry fruit. Our results indicate that the utilization of mulberry
anthocyanins as a natural food colorant is possible and it may
enhance the overall profitability of mulberry plant from being
only the source of foliage for silkworm to a promising pigment
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