J. Pac .Sci. Tech.

Vol.23No.2（2014）

Original Paper～～～～～～～～～～～～～～～～～～～～～～～～～～～～

Preparation and Characterization of Soya Film CoatedPaper as Food Packaging:
A Case Study of the Packaging for Shelf Life Extensionof Dried Banana
Sukrit Tantrawong*

Paper is used as packaging for various consumable products, especially food and fresh produce. The
paper may come in the form of plain, waxed or polyethylene-coated paper (PE-coated paper). The latter
two are either brittle or present difficulty in managing waste from the packaging. In this report, soya film
coated paper (SF-coated paper) was produced by casting from soya milk. Physical properties of thecoated paper were investigated, and compared with those of PE-coated and plain paper. Additionally, thelyotropic textures of the soya milk used in the casting of the soya film were investigated under opticalpolarizing microscopy. The results show that the textures of soya milk are comparable to those ofcommercially available lecithin, which is composed of bilayer structures of Maltese cross and Myelin
finger. The bilayer structure plays an important role in the properties of the SF-coated paper. The
SF-coated paper exhibited better physical properties than the plain paper, though, not as good as the
PE-coated paper. As for the shelf life of dried banana, the SF-coated paper exhibited the best performanceof all three types. Thus, the soya film coated paper might be an eco-friendly choice for the packaging ofdried banana and other food produce.
Keywords：soya film coated paper, shelf life, food packaging, lyotropic phases

Introduction
Dried bananas are one of the most popular local products of Pathumthani province, in the northern
suburb of Bangkok, Thailand. Usually, bananas are best consumed fresh. However, when fresh bananas
are abundant, the favorite way to preserve the excess is by drying. Normally fresh bananas are best notlater than a week after harvesting. On the other hand, dried bananas can be kept on the shelf for 4-5 weeks
before decomposition occurs under ambient conditions. Shelf life extension enables dried banana
producers to expand their market. There are several ways to lengthen their shelf life, e.g., improving
production processes, adding food preservatives, or better packaging. This report focusses on the last
option, better packaging.
The dried banana producer selected is Vilailuck Dried Banana in Pathumthani. The packaging usedin this factory is polyethylene coated paper (PE-coated paper). A thin sheet of polyethylene is coated on
one side of 70g/m2 paper and the product detail is printed with soya ink on the other side. A dried banana
is packed inside the paper (polyethylene on the inner surface) and heat sealed, and several dried bananas
are put into a cardboard box ready for shelving. As mentioned above, the shelf life of this dried banana is*

Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumthani 12121, Thailand
Email: sukrittan@hotmail.com OR sukrit@tu.ac.th－ 123 －

Preparation and Characterization of Soya Film Coated Paper as Food Packaging

around 4-5 weeks. To extend the shelf life of the dried bananas, the PE-coated paper should be replaced,by changing from polyethylene film to another type of film.
From prior observation, soya film can be kept for a long time without any decomposition. It has beenpreviously reported that soya film could extend the shelf life of fruits and prevent micro-organism growththat causes rotting by dipping the fruit into a solution containing soya extract and leaving it to dry (1).
Consequently, soya-film-coated paper (SF-coated paper) might be used to replace PE-coated paper.
The main ingredients of soya milk are various types of oil, phospholipids (e.g., lecithin) and someother proteins. Lecithin has been reported to form colloidal and lyotropic textures, Maltese cross andMyelin finger forms (2), which possess bilayer structures similar to those of the bacterium membrane (3).
This similarity, while not the same, may cause the slow-down of micro-organism reproduction as the
micro-organisms need more time to aggregate enough proper molecules to form a cell membrane andother organs (3). This would extend the shelf life of the products that are packaged with the SF-coatedpaper.
Additionally, this report also investigated the bilayer behavior of the soya milk using an opticalpolarizing microscope to investigate the lyotropic phase textures, together with those of commerciallyavailable lecithin. These lyotropic textures possessing bilayer structure might explain the properties of thesoya film coated on the paper.
Preliminary study of the decomposition process of the dried bananas was investigated beforehand.
When left inside its original packaging (sealed PE-coated paper) under ambient conditions in a laboratoryfor 6 days after Expiry Date, white fungi were observed on the surface of the bananas without any surface
deformation. The deformation, together with traces of fluid and bad odors appeared after 10 days. On theother hand, when left uncovered on a petri dish for 5 days after Expiry Date, no white fungi wereobserved but surface deformation with traces of fluid and bad odors occurred, and the white fungi
appeared after 8 days. It is concluded at this point that, in an air-tight container, the decomposition
process of dried bananas starts with fungi and is followed by bacteria, and in the open-air condition, theprocess starts with bacteria and is followed by fungi.
The objective of this research is to study the production and properties of the SF-coated paper as analternative to the PE-coated paper currently in use as dried banana packaging. First, SF-coated paper was
produced by casting paper with soya milk and the coated paper was used as packaging for dried bananas.
The results were compared with those from plain and PE-coated paper. Furthermore, the physicalproperties of the 3 types of paper were investigated (4), i.e., water and oil resistivity, water and oil
resistivity at fold mark, tensile strength when dry and wet, perception testing and finally the shelf life (5)of dried bananas.

Materials and Methods
1. Chemicals and samples
All chemicals (Sigma-Aldrich) used in this work were purchased from retailers (Bangkok, Thailand).
Commercially available plain paper (70g/m2) and PE-coated paper (PE sheet coated on 70g/m2 paper)- 124 -

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were obtained from the Bangpa-in Paper Factory Company Limited. An Olympus BX50-P opticalpolarizing microscope, equipped with Olympus DP20 microscope digital camera, was employed toinvestigate the lyotropic textures (transparent mode) and paper surfaces (reflection mode). Skinned soyabeans were bought from a supermarket (Tesco Lotus Superstore) in one batch to prevent quality variationfrom different cultivation. To compare the phase behavior, commercially available lecithin (MegaFood,
Expiry Date: 12 January 2014) was used as a solute in a solvent of water and ethanol. Dried bananas insealed plastic bags were bought from Future Park, Rangsit (Vilailuck Dried Banana, Production Date: 2
January 2011, Expiry Date: 31 January 2011) in Pathumthani.
2. Soya milk production
Skinned soya beans (100g) were washed with distilled water (100ml x 2) and then soaked in distilledwater (300ml) for 5 hours. After the water was discarded, the swollen beans were placed in a blenderequipped with filter, water was added (600ml) and the mixture was crushed and filtered to form soya milk.
The crude milk was heated at 90ºC for 2 hours and then left to cool to room temperature. The soya milk
was freshly made in each experimental step.
3. Casting of solid soya film
The soya milk (10mL) was poured onto a petri dish ( 15cm) and left to dry overnight to form asoya film. The thickness of the film was measured using a micrometer and the film was investigatedunder an optical polarizing microscope.
4. Lyotropic textures of soya milk, solid soya film, and lecithin
The lyotropic textures of aqueous soya milk, solid soya film, and commercially available lecithinwere investigated using an optical polarizing microscope. To prepare a sample slide of soya milk, a dropof soya milk was placed on a glass slide and covered with a cover slip. Then, for soya film, a small pieceof soya film was nipped out from the very thin part of the film and placed between a glass slide and acover slip. At the same time, a drop of lecithin was mixed with 2 drops of ethanol and 2 drops of water. A
drop of the final mixture was placed on a glass slide and covered with a cover slip. Finally, each of the
prepared sample slides was placed under an optical polarizing microscope objective at 100x
magnification and crossed polarization, where lyotropic textures were elucidated from transmission
arrangements and the surface textures were obtained from reflection arrangements.
5. Coating of soya film onto paper
After the soya milk was freshly prepared, it was coated onto a piece of paper in the form of a thinfilm for use as packaging for dried bananas. The coating was introduced when the heating of the freshsoya milk was passing into the second hour. A piece of paper was placed on the surface of the hot milk
which was partially absorbed into the paper surface. After 10 seconds of absorption, the paper was
removed from the liquid surface and was left to dry in air for 24 hours to obtain the SF-coated paper. To
flatten the SF-coated paper, a cylindrical roller, similar to that used in cooking, was rolled over the paper.
Finally, the surface and physical properties of the SF-coated paper were investigated and the packaging
ability was evaluated.

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Preparation and Characterization of Soya Film Coated Paper as Food Packaging

6. Physical properties of three types of paper
Together with the plain and PE-coated paper, the properties of all 3 types of paper were investigated(4), within the categories of Grammage, thickness, water resistivity, oil resistivity, tensile strength at dry
and wet, and perception testing. Each factor was averaged from 10 samplings, except the perception test.
For testing resistivity, a drop of either water or cooking oil was placed on the coated side of the paper
sheet (10cm x 10cm) and the time-to-wet (resistivity time) was recorded. Furthermore, the paper (10cm x10cm) was folded twice to make a cross mark and a drop of water or a drop of oil was positioned on themark and the time-to-wet of the paper was recorded. To test the tensile strength of the paper, either dry or
wet, a thin strip (1cm x 5cm) was hung under an increasing weight until it was completely torn apart.
Additionally, the perception test of the paper was carried out by rubbing the paper between thumb andindex fingers.
7. Storage test
Freshly produced dried banana (20g) was packaged using each paper type (10cm x 10cm, 10
samples for each paper type, total 30 samples). Storage conditions were 30C and 70% humidity, in an
air-conditioned room. Once a week, a sample of each type was visually checked. If a sample was rotten or
fungi were found, the rest of the packed samples of that paper type were opened and the number of clean
samples recorded. Additionally, 3 pieces of dried banana without any wrapping were left on a bench in theroom for use as the control group.

Results and Discussion
1. Soya milk production
The extracted soya milk was pale yellow in color and almost odorless. It had a weak scent of cookedsoya bean and a hint of fresh soya bean. At first there was no aggregation at the bottom of the beaker
However, after cooling down to room temperature for 2 hours, some small aggregation was observed.
Consequently, the soya milk was stirred thoroughly before use. It is common for colloidal material toaggregate on cooling.
2. Casting of solid soya film
The concentration of the soya milk was calculated to be 23.2% w/v. After 10mL of soya milk wasdried, the weight of solid film was 2.32g. The thickness of the soya film was 125m. The
photomicrograph of the film in reflection mode is shown in Figure 1.
3. Lyotropic textures of soya milk, solid soya film, and lecithin
The lyotropic textures of commercial lecithin in water-ethanol mixture are shown in Figure 2. The
conventional lyotropic textures, Maltese cross and Myelin finger, are clearly seen.
On the other hand, the soya milk shows only some areas of Myelin finger as in Figure 3. The photos
show clearly that in soya milk the lecithin and some other phospholipids and proteins form lyotropic- 126 -

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phases in aqueous solvent. The bright short lines in Figure 3 are cell walls and some carbohydrate lumpsof the crushed soya beans.

ML

MF
MC
LT 0.5 mm

0.5 mmFigure 1. Photomicrograph of soya film with crossed
polarizers. The lyotropic texture (LT) is clearly visible
inside the film and the multilayer composition (ML)of the film is also shown.

MF

CW

Figure 2. Photomicrograph of lecithin in water-ethanol
solvent mixture with crossed polarizers. Most of the
textures are Maltese cross (MC), with some Myelin finger(MF) texture at the top right-hand corner. The big circle
is a bubble of air.

SS

0.5 mmFigure 3. Photomicrograph of soya milk with crossed
polarizers. The photo shows some area of Myelin finger(MF), together with cell wall (CW) and soya solid (SS).

In Figure 1, the multiple layer structure of the solid soya film is clearly seen at the rim of the film,
while some Myelin finger texture is partly visible and the lyotropic texture is frozen inside the solid soya
film. This evidence shows that the soya film is composed of a number of very thin films aggregated toform a thick film, while trapping and encapsulating the lyotropic texture inside. This aggregation isstabilized by the bilayer structure of the lyotropic phases.
4. Coating of soya film onto paper
The surfaces of the plain paper, SF-coated paper and PE-coated paper are shown in Figure 4(a)-(c),respectively. In Figure 4(a), the roughness of the surface of the plain paper is clearly visible. The paper
fibers are woven tightly together creating a flattened surface. However, the roughness arises from thesmall lumps of pulp scattered in the thickness of the paper and exposed at the surface of the paper.
We found that the solid coating filled the roughness thus forming a much smoother and flatter－ 127 －

Preparation and Characterization of Soya Film Coated Paper as Food Packaging

surface of soya coated film (Figure 4(b)) and of PE-coated film (Figure 4(c)). The production methods ofthe two types of paper are different. The PE-coated paper is produced by hot pressing of a thin PE film
onto the surface of the paper, thus the surface might contain some non-attached fractions as air might not
be fully expelled out of the interface. On the other hand, in the production of SF-coated paper, the plain
paper is placed on the surface of the liquid soya milk thus the milk is better absorbed into the fibrous
structure of the paper, as seen in Figure 5. Consequently, the soya film could adhere better to the paper
surface than PE film.

(a)

(c)

0.5 mm(b)

0.5 mm

0.5 mmFigure 4. Photomicrographs of the surfaces of (a) plain paper, (b) SF-coated paper and (c) PE-coated paper, at reflection
mode. The SF-coated paper is the smoothest, followed by the PE-coated paper and the plain paper.

5. Physical properties of three types of papers
The physical and packaging properties of the 3 types of paper were evaluated. Table 1 shows theGrammage, thickness, resistivity (time-to-wet) of the paper to water or oil, tensile strength of the paper
strips at dry and wet, perception of the papers and the shelf life of the dried banana wrapped in each typeof paper packaging.
Table 1 shows the mass of the coated paper per square meter. The mass of the soya film on the paper
surface is around 56g/m2 while the mass of the PE film on the paper surface is around 64g/m2. The
thickness of the paper shows a similar trend, thicker on the PE-coated paper. It is also shown that plain
paper is unable to resist water and oil, with both being absorbed straight into the paper. On the other hand,both of the coatings could withhold water and oil for a much longer time, with the PE-coated paper- 128 -

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Vol.23No.2（2014）

having highest resistivity. However, when there was any fold mark on the paper, water and oil could
penetrate through the broken texture at the coated film surface and the resistivity time was much lower.
The PE-coated paper was found to be the strongest as a result of the coated polyethylene film, either when
wet or dry.

Figure 5. SEM photo of the surface of the SF-coated paper.
The surface of paper is covered with soya film containing some porous structure.
Table 1. Properties of plain paper, SF-coated paper and PE-coated paper on Grammage, thickness, water
resistivity, oil resistivity, tensile strength, perception and packaging.

Properties

Plain paper

SF-coated paper

PE-coated paper2

Grammage (g/m )Thicknessa (m)
Water resistivitya (sec)

70±3
91±2
1

126±5
125±4
542±12

134±4
143±3
>600

Oil resistivitya (sec)Water resistivity, fold marka (sec)
Oil resistivity, fold marka (sec)Tensile Strength, drya (g)Tensile Strength, weta (g)Perception
Shelf life (dried banana) (weeks)
Number of clean samples

11
1
720±19350±10
Rough
5
4

350±9
182±6
175±4
1080±20620±20
Smooth
7
3

422±7
260±6
251±4
1310±11830±14
Smooth (sticky)
4
4

a

a

10 Replications, average ± standard

deviation

6. Storage test
Finally, the 3 types of paper were used as packaging for dried banana. Each paper type was used topack 10 replications of 20g dried banana. Each week, one sample from each paper type was taken andchecked.
At the end of week 4, the expiry date of the dried banana, white fungi were found on the sample
using PE-coated paper. All the remaining 5 samples were opened and it was found that 4 samples wereclean and 1 sample had white fungi. At the end of week 5, white fungi were observed on the sample
packaged in plain paper, and the control group that was left uncovered was rotten. All the remaining 4
samples packed in plain paper were opened and all were clean. On the other hand, at the end of week 7,
the remaining 3 samples of the SF-coated paper were opened and it was found that no sample exhibited－ 129 －

Preparation and Characterization of Soya Film Coated Paper as Food Packaging

any white fungi and none were rotten. Consequently, the results show that the best paper for this purposeis the SF-coated paper, followed by plain and PE-coated paper. This might be due to the air permeating
properties of the plain paper and the SF-coated paper. While PE film on the PE-coated paper prevents any
air to pass through, the plain paper and the SF-coated paper allow the air to permeate through thus
providing better ventilation (1). In this experiment, the moisture inside the PE-coated paper packaging
would be in saturation which encouraged fungi growth on the dried banana, followed by bacterial growth.
In contrast, the SF-coated paper packaging allowed proper air and moisture permeation through the soya
film, therefore the growth of fungi and bacteria were inhibited, thus the decomposition rate of the dried
banana was slowest. Consequently, the SF-coated paper can extend the shelf life of dried banana and thepaper may be a preferable choice for use as packaging to the PE-coated paper, as mentioned in theobjective.

Conclusions
The organic components of soya milk exhibit lyotropic textures under optical polarizing microscopy.
Soya film can be coated on paper and SF-coated paper can be used as a substitute for PE-coated paper,even though the quality (resistivity and strength) of the former is somewhat lower. It would be beneficial
for food producers to apply this option to their products, i.e., Japanese mochi or daifuku sweets.
Further study is needed on mass production of SF-coated paper, with modifications to suit specificapplications. One of the ideas is to use a paper roll in production, by continuously passing the surface ofthe paper across the hot soya milk surface combined with blow drying. After the paper is dried, it is rolled
ready for cutting and storing. Additionally, detailed study on the mechanism of food decomposition
should be carried out.

Acknowledgement
This work was supported by the Higher Education Research Promotion and National Research
University Project of Thailand, Office of the Higher Education Commission.

References
1) Graham-Rowe, D.; New Scientist, 2321, 24 (2001).
2) Demus, D., Goodby, J., Gray, G.W., Spiess, H.-W., and Vill, V.; Handbook of Liquid Crystals, V.1(Vill V. ed.), Wiley-VCH, Weinheim, Germany, pp.408-411 (1998).
3) Campbell, N.A. and Reece, J.B.; Biology, 7th ed., Pearson and Benjamin Cummings, San Francisco,
U.S.A., pp.119-129 (2005).
4) Scott, W.E., Abbott, J.C. and Trosset, S.; Properties of Paper: An Introduction, 2nd ed., TAPPI Press,
Atlanta, Georgia, USA, (1995).
5) Lima, A.W.O. and Cal-Vidal, J.; J. Stored Prod. Res., 24(2), pp. 73-78 (1988).
（Received: January 16, 2013）（Accepted: February 17, 2014）- 130 -