Minerals in buckwheat flours subjected to enzymatic digestion

Abstract

The nutritional characteristic of seven essential minerals , i.e., zinc, copper, manganese , calcium, magnesium, potas­ sium and phosphorus, in buckwheat flour was studied for seven different cultivars. The composition ofthe seven min­ erals in buckwheat flour was analyzed, and the release of the minerals in a soluble form after in vitro enzymatic diges­ tion of the buckwheat flours was analyzed. There was a variation in the mineral composition among the buckwheat flours examined. The present study has shown the great part of the zinc, copper and potassium is released in a soluble form from the buckwheat flour by enzymatic digestion. Nutritional implications for the present findings are discussed.

Full text

Fagopyrum 18:
45-48
(2001)
Minerals in buckwheat flours subjected to enzymatic digestion
Sayoko IKEDA*I, Kazue TOMURAI, Yoshiko YAMASHITA] and Ivan KREFT2
1Faculty
of
Nutrition, Kobe Gakuin University, Nishi-ku, Kobe
65/
-2180, Japan
2Biotechnical Faculty, University 0/Ljubljana, 1001 Ljubljana, Slovenia
Received June 11, 2001; accepted in revised form July 25, 200 I.
Key
words:
buckwheat flour, calcium, copper, essential mineral, magnesium, manganese, phosphorus, potassium, zinc
ABSTRACT
The nutritional characteristic of seven essential minerals, i.e., zinc, copper,manganese, calcium, magnesium, potas-
sium and phosphorus, in buckwheat flour was studied for seven different cultivars. The composition
of
the seven min-
erals in buckwheat flour was analyzed, and the release of the minerals in a soluble form after in vitro enzymatic diges-
tion
of
the buckwheat flours was analyzed. There was a variation in the mineral composition among the buckwheat
flours examined. The present study has shown the great part of the zinc, copper and potassium is released in a soluble
form from the buckwheat flour by enzymatic digestion. Nutritional implications for the present findings are discussed.
INTRODUCTION
Increasing attention to buckwheat (Fagopyrum escu-
lentum Moench) as a functional food (Bonafaccia and
Kreft, 1998; Mazza, 1998) has been currently paid.
Buckwheat flour contains various kinds of essential
nutrients including protein, starch and essential minerals.
Thus buckwheat is widely utilized as an important source
of
these essential nutrients (Ikeda and Ikeda, 1999).We
have conducted the study on the nutritional function of
essential minerals in buckwheat and its products (Ikeda
and Yamashita, 1994; Ikeda, 1996; Ikeda et aI., 2001).
Nutritional characteristics of essential minerals in buck-
wheat, however, have not still been fully clarified.
Humans need many kinds
of
essential minerals
throughout life. Current evidence has shown that sixteen
or more kinds
of
minerals are essential in human nutri-
tion (Suzuki and Wada, 1994). Each essential mineral
has many diverse physiological functions. Recommend-
ed dietary allowances for essential minerals have been
established in many countries in recent years. Recently,
the recommended dietary reference intakes for essential
minerals for Japanese have been established (MHWJ,
1999). The evaluation
of
diets for the adequacy
of
minerals requires knowledge of both the amount and the
bioavailability for intestinal absorption of the minerals.
Thus clarifying both the amounts
of
minerals in foods
and their bioavailability is a subject of great interest.
Although minerals widely distributed in foods, variation
in the bioavailability
of
minerals was found among
foods (Yasumoto, 1994). The bioavailability
of
food
minerals is considered to be affected by various factors
such as proteins, phytic acid, other minerals, and dietary
fiber (Yasumoto, 1994). These findings suggest that the
*Corresponding author
45
chemical form in which dietary minerals are presented
to the intestinal absorptive cells may have a profound
influence on the bioavailability of minerals.
The present study has been undertaken to analyze the
composition
of
seven essential minerals, i.e., zinc, cop-
per, manganese, calcium, magnesium, potassium and
phosphorus, in the flours of various buckwheat varieties,
and to clarify the release of the seven minerals as solu-
ble forms after the in vitro enzymatic digestion
of
the
buckwheat flours.
MATERIALS AND METHODS
Materials
Seven different cultivars
of
common buckwheat
(Fagopyrum esculentum Moench) were selected for this
investigation: three diploid varieties from Japan, i.e.,
'Kitawase-soba' cultivated in Hokkaido-prefecture (prf.),
'Hashikami-wase' cultivated in Aomori-prf. and 'Hitachi-
akisoba' cultivated in Ibaragi-prf.; two tetraploid varie-
ties from Japan, i.e., 'Miyazaki-ootsubu' cultivated in
Miyazaki-prf. and 'Shinshu-oosoba' cultivated in Naga-
no-prf.; and two varieties from Slovenia, i.e., diploid
'Siva' and tetraploid 'Petra' cultivated in Slovenia. Ker-
nels
of
these buckwheat varieties were subjected for
analysis. Before analysis, the buckwheat kernels were
milled to fine flours in our laboratory in order to mini-
mize contamination of hull refuse into resultant flour by
the following procedure: the kernels were firstly milled
to coarse flours with a mortar; the resultant coarse flours
were sieved by four successive steps with four different
sieves with different sieve size
of
opening for preventing
from contamination of hull refuse; the coarse flours ob-
tained were milled an electric-driven mill machine

46 Ikeda et al.
(MK-52M, National Co., Ltd, Japan), then followed by
sieving with a 381 urnsieve.
Determination of essential minerals, protein and
moisture
Seven essential minerals, i.e., zinc, copper, manganese,
calcium, magnesium, potassium and phosphorus were
assayed in this study. These essential minerals, except for
phosphorus, of buckwheat flour samples were deter-
mined with a Hitachi Z-5300 polarized Zeeman atomic
absorption spectrophotometer (Hitachi Ltd., Hitachi-naka,
Japan). Phosphorus was assayed by the colorimetric
method
of
Fiske and Subbarow (1925). Prior to deter-
mining the total contents
of
these essential minerals, the
buckwheat flour samples were wet-ashed with sulfuric
acid and 30% hydrogen peroxide. The protein in the solid
samples was analyzed by the micro-Kjeldahl method
(AOAC, 1984) (N x 6.25). Protein concentration in solu-
tion samples was assayed by the colorimetric method
of
Bradford (1976) with Coomassie brilliant blue G 250.
Moisture content was determined by the drying method
for 3 hr at 135°C (RCSTAJ, 2000).
In vitro proteolytic digestion
Raw buckwheat flour samples were subjected to in
vitro enzymatic digestion with pepsin plus pancreatin
according to the method described previously (Ikeda,
1984; Ikeda, 1990; Ikeda and Murakami, 1995). In brief,
pepsin digestion was performed in 0.06N hydrochloric
acid for 3 hr at 37°C with an enzyme-to-protein weight
ratio
of
I:I00. Immediately after peptic digestion, an
appropriate volume
of
2M Tris-HCI buffer (pH 8.0)
was added to the digestion mixtures to adjust to pH 8.0.
Toluene was added to the buffer to prevent growth
of
microorganisms to a final concentration of 0.0013%.
Pancreatin solution with deoxycholate was then added to
their digestion mixtures at an enzyme-to-protein weight
ratio
of
I:20, and subsequently incubated for an addi-
tional 20 hr at
37°e
(pH 8.0). The final concentration
of
Tris-HCI buffer in the digestion mixtures was 0.2M.
Deoxycholate was added to the digestion medium to a
final concentration of 0.1% to accelerate the digestion
of
fatty components. Immediately after digestion, the sus-
pensions were placed in an ice-cold vessel to diminish
enzymatic action and then clarified by centrifugation at
10,000 rpm for 20 min. The supernatants obtained were
subjected for the analysis
of
minerals.
Statistical analysis
Statistical analysis was performed using a personal
computer with the programs Excel (Microsoft Co., USA)
and Excel Statistics (Esumi Co. Ltd., Tokyo, Japan).
RESULTS AND DISCUSSION
Composition of essential minerals in buckwheat flours
and nutritional contribution of buckwheat flours for
their essential minerals
Table I shows the composition
of
seven essential
minerals, i.e., zinc, copper, manganese, calcium, magne-
sium, potassium and phosphorus, and total protein con-
tent in the flours of seven buckwheat varieties including
diploid and tetraploid buckwheat. There were variations
in the contents
of
some minerals among the varieties
examined. The zinc content per 100 g dry matter (DM)
of buckwheat flours ranged from 2.54 to 3.38 mg with
an average of 2.89 mg; copper content, from 0.49 to
0.91 mg with an average of 0.61 mg; manganese, from
1.16 to 2.48 mg with an average
of
1.84mg; calcium,
from 15.1 to 72.7 mg with an average
of29
.7 mg; magne-
sium, from 254 to 302 mg with an average of 274 mg;
potassium, from 453 to 530 mg with an average of
Table 1. Composition
of
seven essential minerals and total protein in the Hours
of
varous buckwheat varieties"
Buckwheat Zn Cu Mn Ca Mg KP Protein
samples mg/IOO g OM g/IOOg OM
Kitawase-soba 2.72 ± 0.06 0.54 ± 0.02 2.43 ± 0.05 15.1 ± 0.3
254±
I 453 ± 6
444±
57.68 ± 0.27
Hashikami-wase 2.76 ± 0.01 0.59 ± 0.01 1.52 ± 0.01 25.8 ± 0.8 280 ± I 511 ± 3 500 ± 12 8.32 ± 0.47
Hitachi-akisoba 2.54 ± 0.02 0.52 ± 0.02 1.85 ± 0.02 20.0 ± 0.5 257 ± 3 471 ± 3 489 ± 7 9.11 ± 0.31
Miyazaki-ootsubu 2.88 ± 0.07 0.49 ± 0.01 1.44 ± 0.03 20.7 ± 0.6
302±
7 490
±4
38 1 ± 7 6.18 ± 0.22
Shinshu-oosoba 3.38 ± 0.06 0.91 ± 0.02 1.97 ± 0.05 25.8 ± 0.7 271
±4
515 ± 4 554 ± 5 11.40 ± 0.30
Siva 2.8 1 ± 0 .03 0.67 ± 0.01
1.16±0.01
72.7 ± 1.7 255 ± 8 530 ± 7 437 ± 7 9.28 ± 0.33
Petra 3.16 ± 0.05 0.58 ± 0.01 2.48 ± 0.04 27.6
±0
.7 296 ± 5 514
±4
537 ± 9 7.71 ± 0.32
Means [n =7] 2.89 ± 0.29 0.61 ± 0.14 1.84 ± 0.50 29.7 ± 19.5 274
±20
498 ± 28 477 ± 61 8.53 ± 1.64
CY2) [%] 10.0 23.0 27.2 65.7 7.3 5.6 12.8 19.2
I) Values are means ± S.D . [n =4] on a dry weight basis.
2) CY, coefficient
of
variation .

Minerals in buckwheat 47
498 mg; phosphorus, from 381 to 554 mg with an aver-
age
of
477 mg.
The present study has shown that buckwheat flour
contains various kinds
of
essential minerals (Table I).
In this connection, recommended dietary allowances for
essential minerals have recently been established in
many countries.
MHWJ
(1999) established the recom-
mended dietary reference intakes for essential minerals
(RDA) for Japanese.Based on both the analytical data
of
Table I and the RDA (MHWJ, 1999), we evaluated
nutritional contribution
of
buckwheat flour for the seven
minerals: our evaluation suggested that 100 g
of
buck-
wheat flour can provide about 21 to 28% for zinc, about
30 to 39% for copper, about 40 to 53% for manganese,
about 4% for calcium, about 75 to 100% for magnesium,
about 22% for potassium and about 59% for phosphorus
of
the RDA . Thus buckwheat flour can provide as poten-
tial sources
of
zinc, copper, manganese, magnesium,
potassium and phosphorus, but except for calcium, for
theRDA.
Arelatively higher coefficient
of
variation was found
for the contents
of
calcium, copper and manganese
among the seven buckwheat varieties examined (Table 1).
On the other hand, a relatively lower coefficient
of
vari-
ation was found for the contents
of
potassium, magne-
sium and zinc among the seven buckwheat varieties ex-
amined (Table I). These findings suggest that there
may
be a higher variation for calcium, copper and magne-
sium contents among buckwheat varieties, whereas
there may be a lower variation for potassium, magne-
sium and zinc contents among buckwheat varieties.
Essentia
l
minerals
released
after
in
vitro
enzymatic
digestion
of
buckwheat
flours
Table 2 shows the proportions to their original total
contents
of
the amounts
of
each essential mineral, i.e.,
zinc, copper, manganese, calcium, magnesium, potassium
and phosphorus, released as soluble forms after the
pepsin plus pancreatin digestion
of
the flours
of
various
buckwheat varieties. A large variation in the proportions
of
the minerals released on the enzymatic digestion was
found among seven minerals (Table 2): a higher propor-
tion
of
the released amount after the enzymatic digestion
was found for zinc, copper and potassium, whereas a
lower proportion
of
the released amount after the enzy-
matic digestion was found for calcium and magnesium.
In general, information on both the amount
of
food
minerals and their bioavai lab ility for intestinal tract is
important for evaluation
of
exact mineral adequacy
of
diets. Thus clarifying the bioavailability
of
food miner-
als is a research subject
of
much current interest.
It
appears that the bioavailability
of
minerals in foods for
Table 2. Proportion to their original total contents
of
the amounts
of
seven essentialminerals released as soluble forms
after the pepsin plus pancreatin digestion
of
the flours
of
various buckwheat varieties"
Zn Cu Mn Ca Mg K P
Buckwheat samples
/-lg/IOO
g food mg/IOO g food
(%) (%)
Kitawase-soba 1704 ± 19 332 ± 10 265 ± 5 1.11 ± 0.05 67.4 ± 1.6
267±4
117 ± 1
(72.1) (70.0) ( 12.5) (8.4) (30.5) (67.8) (30.4)
Hashikami-wase 1762 ± 28 311 ± 8 149
±2
1.38 ± 0.03 107.7 ± 1.6 291
±2
113 ± 1
(73 .1) (60.2) (11.3) (6.1) (44.1) (65.3) (25 .9)
Hitachi-akisoba 1539 ± 24 315 ± 9 201
±2
1.73 ± 0.06 68 .0 ± 1.4 270 ± 5 115
±2
(70.1) (69 .8) (12.6) (10.0) (30 .6) (66.3) (27 .3)
Miyazaki-ootsubu 1701
±46
287 ± 8
174±2
1.78 ± 0.04 75.3 ± 2.3 289 ± 2 125
±2
(67.8) (67.7) (13.9) (9.9) (28.6) (67 .7) (37.7)
Shinshu-oosoba 18
16±
14 564 ± 3 173
±4
1.75 ± 0.03 75.3 ± 1.3 270 ± 1 134 ± 2
(61.4) (71.0) (10.0) (7.8) (31.8) (60.1) (27 .8)
Siva 1744 ± 7 406 ± 11 123
±2
4.80 ± 0.14 61.5 ± 1.3 301 ± 3 113 ± 2
(70.6) (69.2) (12.1) (7.5) (27.5) (64.6) (29.4)
Petra 1999 ± 8 326 ± 3 203 ± 2 1.80 ± 0.06 70.6 ± 1.5 307
±4
132±2
(71.5) (63.3) (9.3) (7.4) (26.9) (67.5 ) (27.9)
Means [n =7] 1752 ± 139 363 ± 96 184 ± 45 2.05 ± 1.24 75.1 ± 15.2 285 ± 16 121
±9
(69.5 ± 4.0) (67 .3 ± 4.0) (11.7 ± 1.6) (8
.2±
1.4) (31.4 ± 5.9) (65.6 ± 2.7) (29.5 ± 3.9)
CV2)[%] 7.9 26.4 24.5 60.5 20.2 5.6 7.4
(5.8) (5.9) ( 13.7) (17 .1) (18.8) (4.1) (13.2)
1) Values are
means
±S.D. on a wet weight basis [n =4]. Values in parenthesis indicate
means
of
proportion (%)
of
each
minera
lreleased on the digestion
of
buckwheat flour to each total mineralcontents.
2) CV, coefficient
of
variation.

48 Ikeda et al.
intestinal absorption may be closely associated with
their solubilization in the intestinal tract. In this connec-
tion, we have reported enzymatic-digestion method
evaluating the availability
of
minerals with some foods,
and characteristics
of
their minerals (Ikeda, 1984; Ikeda,
1990; Ikeda et aI., 1990; Ikeda and Murakami, 1995).
The present study on buckwheat flour (Table 2) has
shown that the enzymatic digestion enables the large
proportions of three minerals, i.e., zinc, copper and
potassium, to be released as soluble forms, but smaller
proportions of the two minerals, i.e., calcium and man-
ganese.
In conclusion, the present findings (Table I) have
shown that buckwheat flour contains high levels
of
zinc,
copper, manganese, magnesium, potassium and phos-
phorus, but except for calcium. Our evaluation has sug-
gested that buckwheat flour can be important sources of
the above six minerals, providing about 20 to 100% of
the RDA for the above six minerals, but except for
calcium. On the other hand, the present findings (Table 2)
have shown that the enzymatic digestion enables the
large proportions of three minerals, i.e., zinc, copper and
potassium, to be released as soluble forms, but smaller
proportions
of
the two minerals, i.e., calcium and man-
ganese. These findings (Table 2) have suggested a possi-
bility that the three minerals, i.e., zinc, copper and potas-
sium, in buckwheat Hour may be available for intestinal
absorption. Therefore, the present study (Tables I and 2)
has suggested that buckwheat flour contains a high level
of
zinc, copper and potassium, perhaps with high avail-
ability. However, the detailed mechanism involved re-
mains largely uncertain. In addition, the present study
has been performed with raw buckwheat Hours. Effect
of cooking on the release of mineral after the enzymatic
digestion
of
buckwheat Hour will be an interesting sub-
ject in future. Research is currently in progress in our
laboratory to characterize minerals in buckwheat and its
products in view of their bioavailability.
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