Abstract: Starches from three varieties of rice of high amylose (indica), intermediate amylose (japonica) and almost negligible amylose (japonica waxy) were isolated. They were acetylated by using four times their weight of acetic anhydride (11% sodium hydroxide as 50% aqueous solution as catalyst) for 15, 30 and 60 min.The acetyl content varied from 30 to 41% and degree of substitution (DS) from 1.63 to 2.55. Under similar conditions of acetylation, waxy rice starch showed highest acetyl content and highest DS. Dimethyl formamide was the solvent used for dispersing these modified starches. Films of indica modified starch of 30% acetyl content and 1.63 DS showed highest strength compared to other varieties of modified starch. Glass transition temperature (Tg) was highest in waxy rice starch (~ 232°C) followed by japonica non-waxy and indica rice starch. After acetylation, the Tg values increased in waxy modified ones, remained almost same in indica modified ones, decreased to some extent in japonica non-waxy modified ones compared to their respective native starches. Difference in specific heat capacity between starting and ending of glass transition point of indica and its modified ones remained almost same, increased in japonica and its derivatized ones, decreased in waxy modified one compared to their respective native starches. Among the native starches, melting point was highest in waxy starch. Modification increased this property in all the starches under all periods of acetylation. Increment was highest in japonica waxy acetylated ones. Degradation or decomposition of these starches occurred at 3-10°C higher than their respective melting points. Heat evolution was very high in japonica native (~ 203 mJ mg-1) and heat absorption was highest in indica 15 min modified one (~ 265 mJ mg-1). Difficulties encountered while determining the dynamic viscosity using the equipment of the synthetic polymer have been explained.
INTRODUCTION
Generally all over the globe research is in progress with regard to preparation of biodegradable plastics by making use of various kinds of starches, popularly corn starch, followed by high amylose corn starch and to some extent different types of starches like tuber (potato), root (cassava/tapioca) depending on the availability of the commodities in the respective places. Lawton (1996) prepared films by cooking native starches and other ingredients in a jet cooker, poured them on a heated glass plate and exposed them to different relative humidities and he concluded that high amylose corn starch is the best. Generally amylopectin and whole starches are acetylated to form triester of degree of substitution (DS) 3 and was found to form brittle film (Jarowenko, 1986). Mullen and Pacsu (1942) prepared different esters from root, tuber and cereal starches. They studied even rice starch. But the details of these have not been touched by them. Triester of corn starches were prepared by Wolf et al. (1951). They used various fatty acids and informed that acetic anhydride was the best one for acetylation of starches. They prepared acetates using pyridine as a catalyst which presently is not economical. Mark and Mehltretter (1970) prepared acetylated high amylose corn starch by acetic anhydride and pyridine method. Mark and Mehltretter (1972) had prepared the acetylated starches by making use of alkali as a catalyst, but duration of the experiment was about 4 h to get higher degree substituted starches. Shogren (1996) prepared high amylose corn starch acetate of higher DS (around 2.5) by using alkali as a catalyst. Singh (1996) isolated starches from various cereals, pulses and acid modified them and studied their properties at granular and molecular level. Singh and Ali (2000) have also studied the starch types and their degradation upon acid modification. Singh et al. (2000) have studied the thermal and physico-chemical properties of rice grain, flour and starch. Singh and Ali (2006) have also studied the comparative changes between acid and enzyme modified starches and reported that enzyme modification is always fast. Singh et al. (2006) have also studied the crystalline changes of native and different acid modified starches from various food grains. As corresponding studies on various kinds of rice starches for the preparation of high degree substituted acetylated starches are not available, this experiment was planned to isolate starches from different varieties of rice, acetylate them to higher degree using an economically feasible catalyst and study their properties for the preparation of biodegradable plastics/films.
MATERIALS AND METHODS
Materials
Commercial Thailand rice (indica) was procured from market. Brown rices
of Japonica viz., Nipponbare and Japonica waxy viz., Himenomochi were procured
from the near by Agricultural Research Station which was stated to be stored
around 5 to 6°C, harvested in 1997. All chemicals were of analytical grade
and for all the studies deionised water was used, unless otherwise stated.
Methods
Brown rice was polished to about 10% degree of polish or 90% milling yield
using a Yamamoto rice polisher viz., Rice Pal 31, by keeping a flow rate of
4 and resistance at 3. Thailand rice was slightly polished, as it was available
as polished rice. Starch was isolated from these rice varieties by following
the procedure of Singh and Ali (1987). The wet residue prepared was dried at
40°C for about 18-24 h with stirring the mass at regular intervals in order
to dry uniformly. It was ground using a Retsh centrifugal ultra mill using 1
mm screen. Protein content of these starches were estimated by micro Kjeldhal
method, which varied from 0.2-0.9% (w.b). Careful steps were taken to isolate
starch from japonica rice. Amylose equivalent or amylose content in these starches
were estimated as per the procedure of ISO 6647 (1987).
Acetylation
To start with 50 g (w.b) starch of around 11% moisture content was taken.
Depending on the moisture content, its weight was converted to dry basis. Acetic
anhydride, four times of this weight, was taken. Around 178 g acetic anhydride
(~165 mL) was taken in a round bottom flask. The contents were stirred with
a bead and starch was added slowly. This round bottom flask was placed in a
heating mantle (100 volts, 400 watts, maximum temperature 400°C). It was
fixed with a 3 necked stopper, for fixing the mechanical stirrer, reflux condenser
and a thermometer with the latter two, on both the sides of mechanical stirrer.
Sodium hydroxide (11%, w/w starch) catalyst was added as 50% aqueous solution,
drop by drop with constant stirring. Heater was put on, the regulator of the
mantle was adjusted between 2.5 and 3.0. When the temperature reached 122 to
123°C, the time of reaction being considered from the point when refluxing
began. The reaction was carried out for different timings 15, 30 and 60 min.
All care were taken for proper combination of the reactants. Carefully the product
was transferred to a mixer containing ice cold water. It was ground for a minute
as suggested by Mark and Mehltretter (1972). The starch acetate was precipitated
as a coarse wet mass. It was filtered in a Buchner funnel. The residue was re-suspended
in water, pH adjusted to neutral, washed several times with water and dried
in an oven at 40°C for about 10 h after breaking the lumps. Dried lumps
were powdered in a Retsch ultra centrifugal mill using a 1 mm screen.
Assay of Acetyl Content
Acetyl contents were determined as per the procedure of Mullen and Pacsu,
(1942). Acetyl content and degree of substitution (DS) were estimated as per
the procedure of Wurzburg (1964):
Solubility
The acetates so prepared were tested for their solubility in the following
solvents. Ethyl acetate, dimethyl sulphoxide, dimethyl formamide, 1-propanol,
2-propanol, isobutanol, acetone, chloroform, ethanol, acetonitrile, benzene,
ethyl ether, toluene and acetic acid. A pinch of sample was taken and with above
solvents, where about 5 to 10 mL of the solvent was used and solubility was
confirmed.
Film Preparation
A small concentration of modified starch (5-6%) (Whistler and Hilbert, 1944)
was dispersed in the selected solvent for 45 to 60 min by using a magnetic bead
and stirred on a magnetic stirrer. This dispersed material was kept in a water
bath at 76°C and heated for 15 min. Initially, after heating for 7 min,
required amount of plasticizer, about 20%, (here glycerol) was added and heating
was continued for another 8 min. The dispersion was stirred with a glass rod
at regular intervals. After this the sample was removed outside and kept aside
for 2 to 3 min, to avoid air bubbles. Later it was poured on a glass plate of
sizes 6x6 or 3x3 inches and spread with a glass rod and placed in a good ventilated
oven at 50°C for 2 to 2.5 h. Next it was removed from the oven and kept
at room temperature for 1 to 4 h. The edges of the film was wetted with few
drops of water and removed carefully with the help of a sharp edged knife and
kept in a desiccator containing saturated solution of calcium nitrate (Ca(NO3)2
4H2O) which gives an RH of 51% as suggested by Lawton (1996).
Strength of Film
The films so prepared after storing in desiccator at 51% relative humidity
for about 10 days were removed and cut into standard rectangular sizes of 20x40
mm. Minimum three pieces and maximum 5 pieces were prepared. Each film was held
in the holder or adopter of Tensipresser Model TTP-50 BX II from Taketomo Electric
Company, Japan. Strength of film was measured. Various parameters like Young's
modulus, maximum point etc were noted down. This testing was carried out at
around 23-25°C temperature, for each test around 5 min was required, (starting
from fixing the film, till the end of testing).The clamp distance was around
18 inches. The thickness of the film varied from 0.05 to 0.07 mm.
Flow Tester
To study the dynamic properties of these native and modified starches, a
device namely Shimadzu Flow Tester CFT-500, a machine of capillary extrusion
type was used. About 1.5 g of native starch (or about 1 g of modified starch)
was weighed and poured into a hollow cylinder. Slowly the sample was compressed
with a plunger by hand and later in a compressor using the fly wheel supplied
by the manufacturer (to get a compact sample). This device was removed and kept
on a hollow cylinder and again the plunger was compressed by the fly wheel at
a stretch, by which the compacted sample fell down. This compact sample was
used for the experiment. The instrument was programmed with minimum and maximum
position of the plunger, temperature was programmed which was equal to the melting
point of material which was determined from differential scanning calorimeter.
Required pressure was added as load and the value was programmed. Preheat time
of 50 sec was also programmed. In the Flow meter, a hollow cylinder, above which
a die of 0.5 (length)x1.00 mm (diameter) was placed. This whole system was fixed
in the heating compartment aperture of Flow tester. The die aperture was closed
at the bottom with the screw supplied by the manufacturers.
When the required temperature was reached in the device, the sample was inserted at the top aperture, immediately another plunger was slowly fixed and compressed from the top with the load. Now the instrument was activated by pressing the start button. As soon the preheating time was reached in the heating compartment, the plug was released. The melted sample extruded through the die of cross section as mentioned above, the equipment measured the samples dynamic viscosity between the two setting positions. Immediately position was noted on the display arrangement of the instrument and other dynamic properties like shearing force in dynes cm-2, shearing velocity in per second and flowing quantity in mL/sec were noted down.
Differential Scanning Calorimetry (DSC)
Thermal analysis was performed with a DSC 220C instrument from Seiko Instruments
Incorporation (Japan). Silver crucible, P/N 560-004 AG15 capsule was used for
the experiment. Initial probing studies gave an hint that the melting point
of high DS starches were higher than 200°C. Hence temperature range was
fixed from 100°C onwards. Also to reach 100°C from room temperature,
the system took about 60 to 90 sec. About 5 mg of sample was used in each experiment.
The sample was transferred into the silver cup (it was placed only at the center,
without spreading on the sides of the crucible). It was covered with the silver
cover and sealed very carefully with the help of a sealer supplied by the manufacturers.
Now the crucible with sample was weighed. Difference between the initial empty
cup weight and this gave the weight of the sample. Another empty cup was used
as reference with sealing as before and heating was carried out from 100 to
400°C at the rate of 10°C per min. Initiation of glass transition, glass
transition (Tg), end point of glass transition, melting point and
dextrinization or degradation or decompostion point were noted down. For all
calculations, the Tg was calculated in the peak just before the melting
peak as suggested by Maurice and co-workers (1985). For the purpose of comparison
the same procedure was followed in all the calculations.
RESULTS AND DISCUSSION
Degree of Substitution
Table 1 shows the amylose equivalent of native starches,
the acetyl percentage and degree of substitution of three varieties of rice
starches at three different periods of acetylation. Some interesting features
of the data were, acetylation was car ried out under identical conditions for
different periods, the values indicated that the Thailand starch i.e., indica
had shown lowest acetyl content and lowest degree of substitution under all
periods of modification and highest exhibited by waxy rice starch and in between
was by japonica starch and its modified ones.
| Table 1: | Amylose equivalent (db) of native starches, percent acetyl contents and degree of substitution with different periods of acetylation |
| Table 2: | Solubility of starch acetates |
In other words, higher the acetyl content, higher will be the degree of substitution. The amylose equivalent content in indica starch was around 35%, japonica rice starch was around 23% and in japonica waxy starch was negligible (less than l%). Now in the present work it was observed that higher the amylose equivalent content lower was the acetyl content and lower degree of substitution. Under the conditions employed even at 15 min period acetylation, the acetyl content was 30% in indica starch whereas ~ 39% in waxy starch, indicating the differences in the granular, molecular make up of each type of rice starch granule (in each variety of rice starches studied, the molecule of amylose and amylopectin, or the extent of amorphous and crystalline content will vary in each granule of the starches studied). Even at the end of 30 min acetylation the acetyl content and DS remained almost the same in the waxy rice starch, but its increase was not significant even at the end of 60 min acetylation, where as in indica there was a gradual increase from 15 to 60 min and the increase was to a lesser extent in japonica starch. So it can be seen from this data, that with low amylose equivalent content, increase in acetyl content and DS was not effective after 15 min of acetylation.
Solubility
Table 2 shows the solubility pattern of starch acetates
in general. Solvents in which the starch acetate was completely soluble were
considered here for discussions, the behaviour of other solvents are clearly
seen from the data in Table 2. It is observed that some solvents
were suitable, but handling them was a problem. (Dimethyl sulphoxide is generally
used to disperse native starches with little proportion of water 5 to 15% (French,
1984), but the same solvent dissolves the starch acetates without any addition
of water, but handling this solvent is not easy, as the preparation of film
using this solvent needs very high temperature. While dispersing the material
and drying the films, longer duration is required. It is also a carcinogenic
solvent hence it was not considered to be a suitable. Chloroform was a good
solvent, but because of its anaesthetic effect, this was also not considered.
Benzene being inflammable and dangerous to handle, this too was not considered.
Finally, dimethyl formamide (DMF) was found to be a suitable solvent as it was
easy to disperse, temperature of heating in water bath, spreading on the glass
plate and drying in the ventilated oven were within limits, hence this solvent
was selected for further studies on film preparation.
Differential Scanning Analysis
Differential scanning calorimetry thermograms of these native rice starch
and different period modified starches have been shown in Fig.
1. Table 3 shows some of the parameters measured by the
DSC instrument. Initially, each rice starch and their respective modified ones
will be analysed, afterwards a comparison among these will be made.
In indica starch, difference between starting point of glass transition and
ending point of glass transition (for convenience this will be abbreviated as
A) in native starch and 15 min acetylated starch were almost same i.e., ~ 12°C.
Degradation or deformation or breakdown of native starch occurred just 1.6°C
higher than the melting point, but in 15 min acetylated starch it was 7.4°C
higher than its melting point. In 30 and 60 min acetylated ones, though the
thermogram appeared with several peaks (clearly seen by expanding the x-axis
scale), only the first peak for Tg calculations and second peak for
melting point determination were taken into consideration. The A value was 7.5°C
for 30 min and around 9°C for 60 min acetylated ones, indicating the transition
completed in a gap of p ~ 8 to 9°C in 30 and 60 min modified ones.
| Table 3: | Some parameters noted down from the differential scanning thermograms of various native and modified starches |
| Note: Mel pt.-melting point, Deg.pt.: Degradation/decompostion/dextrinisation point. T1: Starting point of glass transition, Tg :Glass transition point, T2: Endpoint of glass transition point. T2-T1 :Difference between T2 and T1, named in the text as A | |
| Table 3a: | Enthalpy or heat content changes in native and various period modified starches of various rices |
| Fig. 1: | Differential scanning calorimetry thermograms of the high degree substituted acetylated rice starches, their periods of acetylation are shown on each Figure. a) Indica starch and its modified ones, b) Japonica starch and its modified ones, c) Japonica waxy starch and its modified ones |
The degradation occurred about 5°C higher than the melting point in 30 and 60 min modified ones. Glass transition point appeared to be almost same in native and modified ones with an exception of 15 min modified one where it came down by around 5.5°C. Difference in heat capacity at Tg appeared to be almost same starting from native to all the modified starches, being slightly low in 15 and in 60 min modified one and slightly greater in 30 min modified ones. Melting point increased from native to 60 min modified ones. Much increase was not there between native and 15 min one, but there was an increase in melting point in others i.e., 14 to 15°C, indicating with higher content of acetyl group from 15 to 60 min, the hardness in the derivatives have increased and hence melting point and degradation point have increased in indica starch which had highest amylose equivalent content among the three varieties of starches used for the experiment.
The behaviour was quite different in japonica (non-waxy) compared to that of indica. In native starch the A value was 9.2°C, after modification this value increased in all periods of acetylation, highest being in 30 min acetylated one, indicating a gap of ~ 6.4°C between native and this. Glass transition was highest in native, after modification this value decreased in all periods of acetylation indicating acetyl group makes the molecule of amylose and amylopectin bulky. [These bulky groups were of flexible nature, hence the Tg value decreased (Armeniades and Baer, 1977)]. Melting point increased with all periods of modification, the increase was around 10°C in 15 min modified ones but ~ 32 to 35°C in 30 and 60 min modified ones. These results suggested that esterification made the japonica starch molecules harder and harder, hence a higher amount of thermal energy was required to melt the modified ones. Degradation started within 3 to 10°C higher than the corresponding melting point in native as well as acetylated ones. This degradation can be considered as a decomposition of starch acetate molecules just after melting but at almost same temperature as in native and different period acetylated ones. In 15 min modified one it occurred around 10°C higher than the melting point, but in 30 and 60 min about 3 to 5°C higher than the melting point. Even degradation point increased from native to different period modified ones by 17 to 36°C as in the case of melting point. There was increase in heat capacity (CP) after modification compared to native, which means the heat holding capacity or enthalpy of japonica acetylated ones were higher than the native, the order of increasing heat capacity follows 30, 60 and 15 min, indicating the significance of 15 min modified one. Even lower acetyl content in 15 min modified one (~ 35%, Table 1) has shown a higher Cp value whereas higher acetyl content shown in 30 and 60 min acetylated ones (~ 38% and 40%, Table 1) have exhibited lower Cp values. These values are quite interesting and need in depth studies in future.
Waxy starch, almost completely a branched molecule in its starch granule had thermograms which were different in nature. The value of A was highest in native and it decreased continuously with different period of acetylation except the degree of decrease is reverse in 30 and 60 min. Among Tg values, there is continuous increase from native to different periods in acetylated ones. Much difference was not observed between native and 15 min modified one. But the value rose tremendously in higher periods of acetylation, indicating the special nature of acetylated amylopectin molecule in the waxy starch. Also according to Armeniades and Baer (1977) these acetyl groups which replaces -OH groups in several chains of amylopectin molecules of starch will be bulky or cross links occurs between the different chains of amylopectin molecule (as this starch contains less than 1-2 % linear portion), which decreases the mobility of the amylopectin acetate molecule and hence increases the Tg. Specific heat capacity decreased with modification continuously and was least in 60 min modified one, indicating japonica waxy starch after acetylation loses its heat holding capacity. In a branched molecule like japonica waxy starch acetate, why, how and where this phenomenon occurs is to be thought over with further research. The melting points increased continuously with modification, as in the case of japonica rice starch. Waxy starch which has a huge number of branches in its molecules (amylopectin), the acetyl content even at 15 min modification, was very high, about 39%, indicating amylopectin in particular undergoes acetylation to a greater extent compared to other varieties of starches studied. The degradation or decomposition of modified starches occurred just 5°C above the melting point of each waxy acetylated starch.
Comparison Among the Three Native and Respective Modified Starches
The Tg was highest in japonica waxy and lowest in indica and
in between for japonica non-waxy. The A value was highest in indica and almost
same in other two natives, after acetylation the A values decreased gradually
in indica, drastically in japonica waxy but increased to various extents in
japonica non-waxy. The melting points of all the native starches remained almost
same with an exception in waxy japonica where about 4°C higher value was
noticed. After modification melting points increase was noticed in all three
varieties but the extent of increase was high in waxy japonica. Highest specific
heat capacity was in indica followed by waxy and least in japonica. Acetylation
imparted least changes in indica, drastic reduction in japonica waxy and tremendous
increase in japonica non-waxy. Glass transition temperature remained almost
same in indica and its modified ones, but in japonica the values decreased after
modification indicating acetyl groups were flexible in nature, but in waxy acetylated
starches there was increase in Tg specially in higher period of modification,
reasons are as informed before. The melting point difference between native
and 60 min modified ones was highest in waxy japonica (42.6°C) and lowest
in indica (~ 15°C) and in between occupied by japonica (34.5°C). But
in waxy modified ones specially in the 60 min one there was a jump about 43°C
compared to its native (Further detail research in these aspects are required).
Thus we can conclude each type of starch behaves differently after acetylation. It appears amylose equivalent content in each, plays a special role. Glass transition temperature appears to increase to a greater extent in waxy rice starch after acetylation but the same appears to decrease in non-waxy rice starches. Thus special behaviour of japonica waxy is noticed in all the properties.
Heat Contents and Their Changes after Modification
Table 3a shows the enthalpy changes in the DSC melting
as well as endothermic peaks. The top peaks of Fig. 1 were
of exothermic and bottom peaks were of endothermic in nature.
In indica native 59 mJ mg-1 was the heat evolved during melting. After 15 min acetylation heat lost by the system was more, up to an extent of 50 mJ mg-1 extra compared to the native (-110.4-(-59)). But in 30 and 60 min, the values of ΔH increased and heat holding capacity had increased compared to native. In other words there was an absorption of heat during melting i.e., 17.6-(-59) = +32 mJ mg-1. Under endothermic peaks, in 15 min modified one, good amount of heat absorption was seen but after 30 and 60 min acetylation the absorption capacity decreased indicating the special role of substituted acetyl groups in these modified starches. Thus 15 min acetylated indica starch show special behaviour with respect to exothermic and endothermic compared to 30 and 60 min acetylated ones.
In japonica native, heat evolved was ~203 mJ mg-1, which was highest among all the native starches implying the fact that the heat content in native japonica was highest. After modification the evolution of heat decreased gradually from 15 to 60 min indicating the heat holding capacity had increased after acetylation i.e., -185.5-(-202.8) = 17.3 implying that holding of heat in 15 min modified one was high compared to its native. Similarly this heat capacity was high with 30 min as well as 60 min aceylated ones. Quite interesting observation was that endothermic changes were reducing very gradually from native to the 60 min modified one, whole absorption of heat was least in 60 min acetylated one. This indicates the special nature or property of acetyl groups in the starch acetate molecule of japonica starch.
In Japonica waxy native heat evolved was very low, after modification the values increased, showing the special behaviour of acetylated amylopectin molecule of starch. In 15 min acetylated one, highest evolution of heat was observed but after 30 and 60 min the evolution decreased. Heat absorption capacity was least in 60 min modified one and also absorption decreased gradually except in japonica waxy 30 min modified one.
Comparison
Among the three native starches, highest evolution of heat was seen in japonica
starch followed by indica native and least by japonica waxy, which implies that
the japonica varieties behaved differently compared to indica starch. After
acetylation the evolution of heat came down continuously in indica over a period
from 15 to 60 min. In japonica the same phenomenon occurred but the extent of
decrease was less compared to indica modified ones; but in the modified japonica
waxy, there was more evolution of heat compared to its native. In endothermic
peaks, enormous changes were seen only in 15 min indica acetylated one, but
less changes in 30 and 60 min acetylated ones. Japonica and Japonica waxy and
their modified ones behaved almost in similar way. Endothermic changes are also
predominant in japonica native followed by indica and japonica waxy. Indica
acetylated for 15 min, showed highest change followed by japonica and japonica
waxy. Abnormality was seen in other two periods of acetylation.
To conclude, exothermic changes are predominant in japonica (non-waxy) native, 15 min acetylated ones brought out major changes in all the starches and abnormality was seen in higher periods of acetylated ones only in the case of japonica non-waxy and waxy ones. Under endothermic peaks, gradation was observed in japonica waxy and non-waxy varieties but abnormality was seen in indica acetylated ones.
Film Properties
Figure 2 clearly shows the films prepared from these modified
starches. Among the films prepared following modified ones could give good films:
Indica starch of 15, 30 and 60 min acetylated. In Japonica starch only 15 and 30 min modified ones. Waxy acetylated could form film but, after drying, it could not be peeled off from the plate or cracks use to appear while drying. (It is well known from the previous researchers that amylopectin starch is not suitable for preparation of films).
Strength of Film
All the films were tested in the Tensipresser tensile strength unit. Table
4 shows some of the properties measured in these films. Indica modified
for 15, 30 and 60 min as well as japonica modified for 15 and 30 min showed
Young's modulus. Among the films tested high value for Youngs modulus is shown
by indica 15 min modified one followed by 60 min and least by indica 30 min.
Among japonica modified ones, the one with 15 min could register higher Youngs
modulus and lesser by japonica 30 min, thus indicating lower period of acetylated
starch has higher Youngs modulus compared to higher period of acetylation. Braking
force again followed same pattern, requiring highest force in the case indica
15 min and least in japonica 30 min modified one. Maximum strain of 1.5x10-3
was seen in indica 15 min modified one whereas in japonica 15 and 30 min modified
one the same phenomena occurred at 0.25x10-3 and at 0.05x10-3,
respectively implying that the weaker films are formed by japonica modified
ones. Work involved i.e., the area under the curve from starting point to the
maximum point followed same pattern as in Youngs modulus and braking force.
Highest work was involved in indica 15 min modified one and least in japonica
30 min. Presence of higher quantity of linear component (amylose equivalent)
plays a key role for the strength of film. Among the studied samples, indica
starch with ~ 35% amylose equivalent content, with 1.63 DS is very good, compared
to higher degree substituted starches. Hence we can conclude that DS of 2 and
above is not good to obtain film of proper strength.
| Fig. 2: | Films prepared from high degree substituted rice starches. a) Indica rice starch 15, 30 and 60 min, respectively acetylated, b) Japonica rice starch 15 and 30 min acetylated |
| Table 4: | Some properties from the Tensipresser with respect to the strength of the films |
Indica l5, 30 and 60 are indica rice starch modified for these periods in min, Jap.15 and 30 are japonica non-waxy rice starch modified for these periods. The unit of Young's modulus is in N m-2; Maximum point is in dynes and work is in Joules To conclude, among the films studied for their properties, it has been noted that either in indica or japonica, 15 min acetylated one appears to be stronger compared to higher period modified ones.
Flow Meter or Flow Tester
Initially to get a proper film, proportion of material as well as quantity
of material required is standardized while manufacturing films. After knowing
these parameters, before making the sheet, another important property namely
dynamic flow is also required to be studied. This could be studied by making
use of a Flow tester or a flow meter.
From Table 5 it is clear, that for every type of starch,
value of maximum and minimum position of the plunger plays a key role. As seen
the value for indica starch was different compared to japonica waxy rice starch.
| Table 5: | Conditions applied on the instrument and response given by the Flow meter/Flow tester |
| Note: Min. posn.-Minimum position of the plunger, Max.posn.-Maximum position of the plunger, Preht.: Preheat time in seconds, Viscosity of flow: viscosity of the material, in other words dynamic flow of the material which flows through the die, Sh. stress: shearing stress of the material in dynes/cm2, Shearing velocity: per second, Flowing quantity: when the sample melts, quantity of flow in mL sec-1 | |
The temperature fixed or programmed was always 5°C higher than the melting point of the material which was known from the DSC instrument. The quantity of flow was 0.42 mL sec-1 in the case of indica starch but it was different in the case of waxy starch. Two values of waxy starch have been shown, where we observe changes in viscosity, shearing velocity and flowing quantity though minimum and maximum positions fixed for waxy starch was same. Other details, standardization of experiments are to be carried out in future, in order to understand the dynamic flow behaviour of various starches in native form. There were difficulties in measuring dynamic viscosity of modified ones, which needs further studies and standardization of techniques in this instrument.
CONCLUSIONS
Starches were isolated from three types of rices and they were acetylated for different periods. Native and modified starches found to be easily soluble in dimethyl formamide than other solvents and hence films were prepared from this combination. Under the conditions employed waxy japonica had highest acetyl content as well as highest DS. Japonica waxy and its modified ones found to be toughest by the Differential Scanning Calorimetry studies. Enthalpy changes found to be significant in lower periods of acetylation than in higher periods of acetylation. Film forming ability and strength of the film appeared to be good in lower periods of acetylated indica than in other japonica and its modified starches.
ACKNOWLEDGMENTS
Vasudeva Singh is grateful to the United Nations University, Japan, for the award of the UNU-KIRIN Fellowship (1998-99) and T. Yoshino, at National Food Research Institute, for helping in DSC measurements.