Different studies have suggested that around
one-third of the food produced for human consumption is lost globally[1].
Food waste sources include household, commercial, industrial, and agricultural
residues[2],
while the compositional matrix of food wastes varies broadly based on source
and type[3].
Even though numerous nations have established rules for minimizing food waste,
they have not been able to implement them due to numerous technical obstacles.
For instance, European Union guidelines for handling food waste specify that it
should be delivered preferentially to animals, but this practice was made
illegal due to fears of disease transmission to animals[4].
Therefore, it is essential to utilize the food waste by transforming it into
various products with added value, such as fertilizers, bio-gas, and pesticides[5].
Now, food waste can be converted into
bioplastics as an alternative to commodity plastics, which are an environmental
burden due to their lengthy degradation time. Due to their durability, low
cost, and adaptability, commodity plastics are widely used in our daily lives[6].
However, the manufacture of current commodity plastics utilizes non-renewable
petrochemical compounds. In contrast, biodegradable polymers may be produced
from various renewable sources[7–9],
and they can be decomposed by heat, chemicals, microorganisms, and enzymes;
this is why bioplastics have attracted much interest in recent years[7].
In addition, the polymer degradation process also alters the physicochemical
features such as tensile strength, water absorption capacity, shape, molecular
weight, and color.
Similarly, several different bioplastics may
be made from agricultural waste, such as cellulose and protein[9],
as well as starch[10]
by extracting them from the aforementioned agricultural waste. Because of its
intrinsic biodegradability, overwhelming availability[11],
and annual renewability[12],
starch-based bioplastics have been widely used in food packaging[13],
agriculture[14],
pharmaceutical sectors, etc., as a replacement for existing plastics. Many
renewable resources, such as cassava, jute[15],
corn[16], potato peels[17],
pineapple peels[18],
mango seed[19],
litchi seeds[20],
banana peels[21] etc., can be used to extract this chemical
compound. Potato peel is one of the essential bio-waste materials for
bioplastic manufacturing as around 70-140 thousand tons of potato peels are
produced each year throughout the world[22].
Bioplastics can be synthesized from potato peels because of their high
concentration of starch-containing polymer chains such as amylose and
amylopectin[23].
Although starch is not a thermoplastic material, the starch granules melt and
flow at moderate temperatures (90-180°C) to produce thermoplastic starch[24].
Because starch-based bioplastics prepared without plasticizers are rigid and
brittle, a water-soluble plasticizer is added to the mix when the bioplastics
are being made.
Another advantage of using plasticizers in
polymer manufacturing is that they are a valuable class of low molecular
weight, comparatively non-volatile organic compounds[9].
Using plasticizers is projected to reduce modulus, tensile strength, hardness
and density, and the glass transition temperature while simultaneously
enhancing flexibility, elongations at the break, and toughness[25].
Research on a variety of plasticizers, such as polyvinyl alcohol (PVA),
glycerol, ethylene glycol(EG), propylene glycol(PG), Tri-ethylene glycol(TEG),
and diethylene glycol(DEG), as well as other biodegradable polymers, has been
conducted using biodegradable polymer. Thermoplastic starch (TPS) degradation
was shown to be reduced by the addition of glycerol, a biodegradable polymer[25,26].
As part of this work, we investigated the effect of glycerol concentration on
the physicochemical properties of starch-based bioplastics by varying the
concentration of glycerol plasticizers.
2. Materials and Methods
2.1 Materials
Solanum tuberosum (potato) was purchased from the local market
of Kathmandu in September 2020 and was cultivated in Tokha, Kathmandu. Glycerol
(99%), sodium metabisulphite (Na2S2O5), HCl
(36%), and NaOH (97%) were manufactured by Thermo Fisher Scientific India Pvt.
Ltd., India, supplied from Science House, Tripureshwor, Kathmandu and used
without further purification.
2.2 Method
i) Extraction of
Starch from Potato Peels
The
potato peels were washed with distilled water, sliced into small pieces, soaked
for 30 minutes in a beaker containing 1% sodium metabisulphite solution, and
then ground into a paste using a blender with minor adjustment[27].
The ground paste was filtered after being rinsed with distilled water. The
filtrate was collected in a beaker and left for one hour to allow sedimentation
and decantation of brownish-white particles with a muddy appearance. The
remaining bottom particles were washed four times with distilled water[28].
The obtained starch powder was dried in an oven at 50 °C for two hours.
ii) Preparation of
starch-based bioplastics
2.5
grams of extracted starch was dissolved in 25 milliliters of deionized water
with continuous stirring. After adding 3 mL of 0.5 N HCl and 2 mL of glycerol
as a plasticizer, 0.5 N NaOH was added to neutralize the acidic solution. The
neutralized solution was heated for 15 minutes before being placed onto Petri
dishes. The prepared sample was heated at 80 °C for five hours before being
air-dried for three days. The same procedure was followed to prepare
starch-based bioplastics by altering the glycerol concentration[20,24].
2.3 Characterization
Technique
The
Fourier Transform Infrared spectroscopy (FTIR) analysis was carried out using
the IR Prestige-21 model, SHIMADZU, Japan. The FTIR spectrum of the sample was
obtained at the wavenumber in the range of 4000-400 cm-1.
The
bioplastics were cut in size approximately 30×10 mm and dried in an oven at
about 50 °C. Then the dried bioplastics were weighed before being kept in water
at room temperature. The weight of the bioplastic was recorded every two hours
at intervals in day time while deep in water to till 50 hours. The amount of
water absorption was calculated according to the equation[29,30].
............... (i)
where,
Ww = wet weight (after absorption), Wd = dry weight
(before absorption)
The
soil burial test experiment was performed in the laboratory by taking the moist
soil in a 250 mL beaker. The bioplastic was cut in 30×30 mm size and weighted,
and then the bioplastic was buried in the soil at about 4 cm in depth. The
degradation of bioplastic was inspected at every five days intervals for 64
days and was calculated by using equation[30,31].
……………..(ii)
Where
Wd= dry weight of film after being washed with distilled water
Wi= initial dry weight
of specimen.
2.6 Acid-Base
Resistance Test
All
prepared bioplastics were cut into 30×10 mm dimensions and the weighted sample
was placed in the beaker containing 20 mL of 0.5 N HCl. The weight of each
bioplastic was noted every three-hour interval till 42 hours in the daytime.
The base resistance test was carried out in a similar procedure using a 0.5 N
NaOH solution. The acid-base resistance property of each sample was measured by
using the equation[32]:
........................(iii)
...............................(iv)
Where, w1=
initial weight of sample film, w2= weight of sample after absorption
3. Result and Discussion
The
functional groups present in commercial starch and extracted starch from potato
peels were analyzed using FTIR spectroscopy to demonstrate a substantial
similarities between them. The FTIR spectra of commercial starch (CS) and
starch isolated from potato peels (PS) are shown in Figure 1. Figure 1
demonstrates that the spectra of PS and CS were nearly identical, with strong
peaks detected at approximately 3278 cm-1, 2931 cm-1,
1643 cm-1, 1348 cm-1, 1157 cm-1, 1080 cm-1,
978 cm-1, and 856 cm-1. The peak at 3278 cm-1
is related to the OH stretching vibration, whereas the peak at 2931 cm-1
corresponds to the C-H stretching vibration. Similarly, the peak at 1643 cm-1
was attributed to C=O bonding associated with OH, indicative of bond water
molecules within starch and the peak at 1348 cm-1 represented C-H
symmetric bonding.
Figure 1: FTIR spectra of extracted potato starch (PS)
and commercial starch (CS)
The
peak at 1157 cm-1 was assigned to C-O-C asymmetric stretching, which
indicated the presence of α-1, 6-linkage in amylopectin. The peak at 1080 cm-1
corresponded to the stretching vibration of C-O, and the peak at 978 cm-1
indicated the C-O-C linkage of α-1, 4-glycosidic linkages in amylose. A similar
result was reported in the characterization of irradiated starch[33].
In addition, the spectra at 856 cm-1 are the result of axial or
equatorial C-H bonds to the ring in pyranose sugar. Thus, the FTIR spectra of
commercial starch (CS) and extracted starch (PS) from potato peels confired the
presence of amylopectin, amylose and further pyranose sugar.
The FTIR spectra of the synthesized
bioplastics with different concentrations of plasticizers are shown in Figure
2.
Figure 2: FTIR spectra of
starch-based bioplastic using
various concentrations of glycerol
Table
1 displays the FTIR spectra of starch-based bioplastics with varying
concentrations of glycerol, where the peaks at 3278 cm-1
corresponding to the stretching vibration band of the free O-H group, and 2924
cm-1 represents the stretching of the C-H bond which indicating the
interaction between glycerol and starch. Similarly, the 1643 cm-1
and 995 cm-1 FTIR spectra were caused by C=O stretching and C-O
stretching, respectively. The parallel readings were consistent with the
research conducted on corn starch[34–38].
The band between 704 and 1014 cm-1 reflected the stretching of the
C-O band, while the peak at 3270 cm-1 represented the stretching of
the -OH groups. Table 1 summarizes the principal absorption peaks obtained from
FTIR spectra for starch-based bioplastics with varying glycerol contents.
In addition, similar peak intensities were discussed in
the literature[28,29]
for the characterization of starch-based bioplastic using glycerol as a
plasticizer. Thus the FTIR spectra of each bioplastics confirmed the formation
of good linkage between the glycerol and starch.
Table1: Functional group concerning the wavenumber of
starch-based bioplastics at different concentrations of glycerol.
|
Functional Group
|
Wavenumber (cm-1)
|
|
|
2.4%
|
24.3%
|
36.5%
|
48.6%
|
97.2%
|
|
O-H
|
3278
|
3278
|
3255
|
3278
|
3278
|
|
C-H
|
2924
|
2924
|
2924
|
2924
|
2924
|
|
C=O
|
1643
|
1643
|
1651
|
1651
|
1651
|
|
C-O
|
995
|
1010
|
1002
|
1010
|
1018
|
The
bioplastic mostly used as the commodity for daily use, its life span in water
is also important part to know the water uptake properties. The water uptake properties
was investigated by the water absorption test. The water absorption of glycerol
plasticized bioplastic is shown in Figure 3.
Figure 3: Water
absorbability of different concentrations of glycerol plasticized film
The
plot of weight gain v/s time revealed weight gain of glycerol-plasticized film
at various concentrations. In 6, 28, and 50 hours, the weight of pure
glycerol-plasticized bioplastic reached 1.23 g, 1.33 g, and 1.48 g,
respectively. After 6 hours of exposure to moisture, the weight of
glycerol-plasticized bioplastics at concentrations of 97.2, 48.6, 36.5, 24.3,
and 2.4 percentage w/w increased by 74%, 67%, 60%, 48%, and 30%, respectively;
after 50 hours of exposure, their weight reached to 79%, 78%, 70%, 59%, and 37%
respectively. The plot of the water absorption test revealed that the 97.2%
glycerol plasticized bioplastic absorbed more water than the 2.4% glycerol
plasticized bioplastic. Since the hydroxyl group of glycerol has a great
affinity with water molecules to form hydrogen bonds[39,40],
glycerol-plasticized starch-based bioplastics become more hydrophilic as
glycerol concentration increases[32].
The
biodegradability of starch-based bioplastic as measured by its weight loss in
grams after 64 days of soil burial was depicted in Figure 4. The plot of weight
loss vs time indicated that the weight of all bioplastics raised for the first
four days before beginning a slow decline after eight days. The increase in
weight over the first four days is due to the bioplastic's ability to absorb
water.
Figure 4: Soil burial test
of different concentration of glycerol-plasticized bioplastic
Due to the change in the amount of glycerol
contained in the plasticized bioplastic, there is a considerable difference in
the weight increase percentage of the material at different glycerol
concentrations. Different quantities of glycerol in bioplastic decomposed at
different times, but all followed a similar degradation pattern. Bioplastics
with a high concentration of glycerol, such as 97.2 % w/w, 48.6 % w/w, and 36.5
% w/w, deteriorated in 52 and 56 days, whereas bioplastics with a lower
concentration of glycerol, such as 24.3 % w/w and 2.4 % w/w, took over 64 days
to breakdown entirely. The deterioration of bioplastics may be caused by the
attack of numerous bacteria, germs, fungus, etc. present in the soil and water[31],
which initiated the hydrolysis reaction that caused potato starch to break down
into microscopic fragments. Whereas the difference in breakdown rate is
primarily attributable to the ability of different bioplastics to absorb water.
The pace of bioplastic degradation will increase as the amount of water
absorbed by the material increases the microbial process[41].
Due to the hydrolysis of the polymer chain of starch-containing hydroxyl (-OH),
carbonyl (C=O), and ester (-COOR), biodegradation occurred[42]
3.5 Acid-base
resistance test
The
acid-base resistance test was conducted to determine the bioplastic's
disintegration time in acidic and basic environments. The acid resistance test
of glycerol-plasticized bioplastic at various concentrations is shown in Figure
5.
Figure 5: Acid resistance test of different concentrations of
glycerol-plasticized bioplastics.
Figure 5 depicts the relationship between
weight gain and bioplastics' breakdown time. The different concentrations of
glycerol-plasticized bioplastics were resist to decompose in acid solution upto
42 hours. As the concentration of glycerol in the bioplastic used from 2.4% to
97.24%, the weight gain of each sample gradually with the percentage of
glycerol increased in the bioplastics. Due to the presence of hydroxyl group of
glycerol in bioplastics, which undergoes condensation reaction with the removal
of the water molecule, a three-dimensional network with a covalent bond is
formed that gives resistance to acid assault[32].
Thus, it can be inferred that both the concentration of glycerol and the
barrier against acid were raised in bioplastic. Figure 6 depicts the base
resistance test of glycerol-plasticized bioplastics at various glycerol
concentrations. Figure 6 depicts the base resistance test of
glycerol-plasticized bioplastics at various glycerol concentrations. The
glycerol-plasticized bioplastics with a weight gain of 2.4% and 24.3% glycerol
began to disintegrate within 3 hours and entirely disintegrated within 6 hours.
However, bioplastic containing 36.5% glycerol remained stable for 45 hours. During
this test, pure glycerol plasticized film and 48.6% plasticized bioplastics did
not degrade in basic solution for up to 45 hours, despite absorbing a greater
quantity of base solution.
Figure
6: Base resistance test of different
concentrations of glycerol-plasticized bioplastics
The glycerol-plasticized film with a greater concentration has a greater
base resistance[32].
The nature of the plot for the base resistance test was nearly identical to
that of the acid resistance test, with a little difference.
4. Conclusion
The
starch was successfully extracted from potato peels for the fabrication of
bioplastics by using different concentrations of glycerol as a plasticizer and
the synthesized starch-based bioplastic was characterized by using FTIR
spectroscopic technique. The formation of starch based bioplastics was
confirmed by the presence of amylopectin, amylose and axial or equatorial C-H
bonds to the ring in pyranose sugar. Further, the plasticizer i.e. glycerol
showed the good interaction with the starch in the bioplastics. The
physicochemical properties i.e. water
absorption, soil burial test, and acid-base resistance of glycerol-plasticized
starch-based bioplastics were investigated in different concentration (% w/w)
of glycerol. The bioplastic plasticized using pure glycerol (97.2 %) absorbed a
higher amount of water (i.e. 79%) and
gradually decreased the amount of water absorbed in each concentration of
glycerol-plasticized bioplastics in water absorption test. Similarly, higher
concentration of glycerol (97.2%) plasticized bioplastic was degraded earlier
at about 52 days and other concentration (48.6%-2.4%) of glycerol-plasticized
bioplastics gradually extended the day of decomposition upto 64 days in soil
burial test. Further, the concentration (97.2%) of glycerol-plasticized
bioplastics showed higher resistance to both acid and base in acid-base resistance
test.
Conflict of Interest
Author(s)
declared that on any potential conflict of interest. All the others proclaims
that the work presented here is a genuine research and the results as well as
conclusion drawn here are based on the experimental verifications. The
objectives of this research work are not motivated by any external factors.
Acknowledgement
The authors would like
to acknowledge the Department of Plant Resources Thapathali, Kathmandu for FTIR
spectroscopic analysis.