garis L., from which there is evidence about their prehistoric existence in South America. Their
greatest production is in the North West of Argentina.
The black bean used is a cultivar noticeable for its high resistance to viral diseases of warm areas
and it is eventually of high yield Vizgarra, 1995. We have utilised the white bean cultivar be-
cause it is commercially important and has also been submitted to phyto-improvement by agro-
nomic experiments. The seeds, which are typical of Argentina, were
defatted; their polysaccharides, proteins and fiber isolated, purified and quantified.
Starch is a mixture of two polysaccharides: amylose in which predominates linear a-1,4 glu-
can, and amylopectin highly branched a-1,6 glu- can with branching points occurring through
a -1,6 linkages.
The two components: amylose and amylopectin are present in varied ratios, depending on the
vegetal source Salmoral et al., 1993. In our case, the starch was used in addition to glycerol as
plasticizers and was selected in order not to re- duce the biodegradability of biopolymers.
Boric acid is reported to be able to react with glycerol and starch forming a network among
starch chains and glycerol molecules. It has been suggested the existence of crosslinks among
starch, glycerol and boric acid. Crosslinks be- tween OH- of the components caused by boric
acid would enlarge the molecules and so prevent the segments for easy moving Yu et al., 1998.
In the present work, the addition of boric acid to the blend in a dose non toxic was studied. We
examined the effect of gamma radiation on blends of proteins and defatted whole flour with respect
to the mechanical properties and the water ab- sorption of the molded plastics.
Gamma radiation has been reported to be use- ful in the crosslinking of proteins Ressouany et
al., 1998, which would mean an increase in the tensile strength of the product.
On the other hand, ionizing radiation acts on polysaccharides by breaking C – C bonds and
forming acid and reductive groups Pruzinek and Hola, 1987
The objective was to compare two different varieties of crop products of legumes of Phaseolus
6 ulgaris L.
2. Materials and methods
2
.
1
. Materials
2
.
1
.
1
. Culti6ation Phaseolus 6ulgaris L., black bean Tuc 500 and
white bean Alule 91 were obtained from the Ex- perimental Station of the National Institute of
Agricultural Technology INTA, from Tucuma´n and Salta, Argentina.
2
.
1
.
2
. Soluble protein preparation Black bean soluble protein BP and white bean
soluble protein WP patent pending were pre- pared in the laboratory using pulverized seeds;
ground in a mill and defatted by the Soxhlet method. After treatment with 0.001M NaHSO
3
and the homogenization at pH not higher than 6. The material was filtered to separate the cellulose
material. The supernatant was decanted and the rest of the suspension was concentrated by evapo-
ration and finally lyophillized. It was kept at 4°C, and the protein content determined 50.
2
.
1
.
3
. Whole flours Black bean whole flour BWF and white bean
whole flour WWF were prepared in the labora- tory by crushing the seeds, defatting the powder
by the Soxhlet method and pulverizing the mate- rial with help of a mill until it could be sieved
through a 230 ASTM mesh. It was kept at 4°C and the protein content determined BWF; 32,
WWF; 30.
2
.
1
.
4
. Glucopolysaccharides In the process of isolation the pulverized seeds
were soaked in 0.005 M NaHSO
3
, homogenized and squeezed in cheesecloth thus obtaining a su-
pernatant that was decanted. The precipitated was submitted to several washes with: NaCl in
aqueous solution, NaCl in aqueous solution toluene 140:1, ethanolwater 1:3, 2:3 and
3:1 successively. The starch obtained was finally
washed, dried and defatted by the Soxhlet method.
The purification process: the a1,4 – a1,6 gluco- polysaccharides were purified in Biogel P6 100 –
200 mesh
before analysing
its structural
characteristics the wavelength of maximum ab- sorption and the polysaccharide spectra were
analysed in
a UV-VIS
Beckman spectro-
photometer. The structural analysis of the a1,4-a1,6 linked
glucopolysaccharides, was described in a previous paper Tolmasky and Krisman, 1987; Salmoral et
al., 1993. The proteins were quantified by Kjeldhal
method and in the washing solutions by the Lowry method Lowry et al., 1951.
2
.
2
. Preparation and plasticization of mixtures
2
.
2
.
1
. Blend The starch 11.7 of total weight gelatinized
with water and the glycerol 21.7 of total weight were incorporated to either the protein
soluble fraction or the whole flour material. The water content of these blends was 28. All pro-
portions are ww in total batch basis. The blends were kept at 4°C for 24 h in closed containers for
stabilisation
previously to
the compression
process.
2
.
2
.
2
. Treatment with boric acid The blend was prepared with the addition of
boric acid 3 ww in total batch basis.
2
.
2
.
3
. Specimen preparation Specimens were prepared from bean products
soluble protein preparation and whole flour preparation following the same technique em-
ployed for soybean products described elsewhere Salmoral et al., 1998.
The carefully homogenised blends were left to stand to reach a temperature of 23°C in closed
containers and were immediately submitted to compression-moulding under the following condi-
tions: compression load, 20 MPa; molding tem- perature, 120 – 130°C and molding time 7 min.
Plates 0.7 mm thick were obtained. Specimens for microtensile tests according to ASTM D-1708
ASTM 1984 were cut out of the plates and conditioned at 23°C, 70 relative humidity for 48
h. The borders and edges were polished as well as
the surfaces with abrasive sandpaper. This kind of specimens was chosen in order to be able to
prepare large numbers of specimens with rela- tively small amounts of material, for each kind of
composition tested. The mechanical properties were measured in this work only for comparative
purposes among the different materials employed, not intending to define basic properties of the
materials.
2
.
3
. Irradiation Irradiation of some of the blends was carried
out with gamma beams from a Co-60 source, at a dose rate of 9.5 kGyh to a total dose of 50 kGy.
The samples were packed in polyethylene flasks in the presence of air. The temperature in the irradi-
ation chamber never surpassed 30°C. The dosime- try was performed by means of dichromate
dosimeters for high doses. The homogeneity of dose within the samples was of 30.
Irradiation was always performed after 24 h of blend preparation. The time from irradiation to
compression moulding was also of about 24 h. In all cases irradiated and non-irradiated samples
were processed and tested in the same batch.
2
.
4
. Water absorption Water absorption was measured according to
ASTM D 570 95 ASTM, 1995. Samples 8.0 mm × 15 mm and 1.5mm thick were conditioned
in an oven for 24 h at 50 9 3°C, then cooled in desiccator and weighed to a nearest 0.001 g. The
samples were immersed in distilled water for 2 h at 23°C and immediately weighed to determine
the absorbed water; process utilised for materials having a relatively high rate of absorption.
2
.
5
. Mechanical testing Tensile tests were performed by means of an
Instron Model 1122 testing system. Crosshead speed was 1 mmmin. Tensile strength at breakage
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
