and percent elongation at breakage were mea- sured using at least five specimens for each com-
position or treatment tested. The reported results are averages of the experimental values. Due to
the limitations pointed by ASTM D-1708 we did not intend to measure the modulus. Tensile tests
were performed after temperature and relative humidity of stabilisation of specimens.
It was necessary to measure the moisture con- tent of samples before mechanical testing below
1 because of the R.H. conditioned was different from the standard condition of 50.
2
.
6
. Statistic results The results were analysed by means of the
Student’s t-test.
3. Results
In each case the starch employed was obtained from the same vegetal species and purified. The
characterisation of the chemical structure showed that the white bean starch has a ratio of amylose
to amylopectin structure of 7030, while for the black bean the ratio is 955. The degree of
branching points in the black bean starch was found to be 2.9. It was obtained from the rela-
tionship between the l max and the percentage of
a 1,6- linked glucose. The 2.9 value founded in the
black bean can be more accurately interpreted by comparison with the value of the degree of
branching points of rabbit liver glycogen 9.4, phytoglycogen 7.1, amylopectin 5.7 and amylose
of sweet corn maize 1.4 Tolmasky and Krisman, 1987. This relations suggest the presence of an
amylose with a relatively high degree of branching points.
The absorption spectrum of the predominant glucopolysaccharide presents l max = 580 nm in
the presence of CaCl
2
– I
2
– KI reagent in both cultivars. The preparation of blends for molding
from defatted soy whole flour black and white bean whole flour was performed under the same
conditions and proportions used for the protein preparation.
When ultimate tensile strength of the plastic materials from whole flours of black and white
beans are compared with those from the protein soluble fractions, it is found that the tensile
strength MPa is significantly higher for protein materials BP, P B B 0.001 and WP, P B 0.001
with respect to the whole flour materials BWF and WWF. This effect was common to both
varieties of beans Fig. 1A.
When elongation at breakage is analysed, a notable difference between both varieties of beans
can be observed. While the protein plastic com- pound of black bean BP reaches a high elonga-
tion respect of BWF, P B B 0.001, the white plastic bean variety shows better elongation in
their whole flour compound WWF, P B B 0.001 than the protein plastic compound WP Fig.
1B.
Moreover, the elongation in the BP com- pound is seven times higher than WP compound,
0.01 B P B 0.001. The water absorption of these
Table 1 Water absorption of materials
a
Blend Blend ba
Blendi s
s s
0.70 Black bean
17 0.91
31 1.00
27 BWF
BP 22
0.45 22
1.50 21
0.40 White bean
WWF 29
2.00 30
3.00 20
0.06 0.69
18 1.20
21 0.70
WP 21
a
Black bean whole flour BWF; black bean protein preparation BP; White bean whole flour WWF; White bean protein preparation WP; Blend: protein preparation or defatted whole flour; 11.7 starch; 21.7 glycerol; 28 water; Blend ba: blend
with 3 boric acid; Blendi: blend gamma irradiated to 50 kGy.
Fig. 1. Mechanical properties of plastic materials of soluble protein preparation of black bean BP, soluble protein preparation of white bean WP, black bean whole flour BWF and white bean whole flour WWF. A: tensile strength expressed in MPa. B:
elongation . Blend: isolated or defatted whole flour + 11.7 starch, + 21.7 glycerol.
Fig. 2. Effect of boric acid. Mechanical properties of soluble protein preparation of black bean BP, soluble protein preparation of white bean WP, black bean whole flour BWF and white bean whole flour WWF. Tensile strength expressed in MPa.:
Elongation . Blend. isolate or defatted whole flour + 11.7 starch + 21.70 glycerol + 3 boric acid.
same materials can be seen in Table 1. Whole flour materials BWF, 27 s = 1.0; WWF, 29
s = 2.0 absorb 20 9 5 more water than protein materials BP, 22 s = 0.45; WP, 21 s =
0.69; in all cases water absorption was below 30.
3
.
1
. Boric acid treatment The effect on mechanical properties, tensile
strength and elongation by the addition of boric acid in a concentration of 3 to the blends con-
taining 11.7 starch and 21.7 glycerol are shown in Fig. 2.
The result obtained show that neither the ten- sile strength nor the elongation favourably
influenced in plastic compounds based on black bean. There is also no positive influence in water
absorption of these materials, as can be seen in Table 1.
When the results of mechanical properties of white bean plastics materials are analysed, a dif-
ferent behaviour can be appreciated. The tensile strength only increases in the whole flour material
from WWF, 2.5 – 4.5 MPa P B B B 0.001. The elongation resulted improved in the WWF
from 1.8 to 5.6 P B B 0.001; in WP from 2.5 to 3.5 P B B B 0.001, as can be seen by comparing
Fig. 1B and Fig. 2. There was no positive influ- ence in water absorption.
3
.
2
. The gamma radiation The effect of gamma radiation on bean plastics
can be seen in Fig. 3. By comparing with Fig. 1A, B tensile strength shows an increase in the case of
black bean whole flours compound BWF and white bean whole flours compound WWF, 3 – 6
MPa, 0.01 B P B 0.001 and 2.6 – 9.3 MPa, P B B
0.001, respectively.
Regarding percentage
of elongation,
only WWF was improved from 2.5 to 10.5, P B B
0.001. The water absorption was reduced in both
whole flour plastic compounds BWF, 17, and WWF, 20.
4. Discussion
