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Effects of Citric Acid on Starch-Based Barrier Coatings
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With growing environmental concerns, efforts are made to replace petroleum based products with renewable alternatives. This is particularly evident in the packaging industry, where replacing synthetic polymers with renewable materials is of considerable interest. Materials for food packaging need to give protection, acting as a barrier against substances that can adversely affect the food quality such as water and oxygen.

In this work, barrier dispersion coatings based on starch were used to produce coated papers which act as barrier against water and oxygen. However, since starch is both a hydrophilic and hygroscopic material, this barrier material becomes problematic to use at high relative humidity. In order to reduce this problem and improve the barrier properties enabling starch based barrier materials to be used in food packaging applications, two approaches were studied.

Citric acid was utilized as a cross-linker of the starch and it was found to reduce the moisture sorption, the molecular movement and swelling at high relative humidity. It was seen that cross-linking and hydrolysis due to the low pH both affected the barrier properties significantly, but in opposing directions. By controlling these two reactions it was seen that this could lead to reduced gas permeability. It was also seen that cross-linking of starch by citric acid occurs at low temperatures, 70 °C at pH as high as 6.5.

Starch nano-composites were produced by incorporating montmorillonite, to the barrier dispersion to improve the barrier properties. It was seen that the suspension viscosity was reduced by poly(ethylene glycol) and citric acid adsorption on the montmorillonite particles. Also, a tendency for improved barrier properties with reduced aggregate volume fraction and reduced swelling was observed. It was also seen that up scaling this formulation to pilot scale was possible and that promising results were achieved.

Abstract [en]

Baksidestext

With growing environmental concerns, efforts are made to replace petroleum based materials with renewable alternatives such as starch. In this work, dispersions based on starch were used to produce coated papers which act as barrier against substances that can adversely affect the food quality such as water and oxygen. However, since starch is both a hydrophilic and hygroscopic material, this barrier material becomes problematic to use at high relative humidity.

Citric acid was utilized as cross-linker for starch and it was found to reduce the moisture sorption, diffusion and swelling at high relative humidity. Both cross-linking and hydrolysis due to the low pH affected the barrier properties significantly, but in opposing directions. By controlling these two reactions it was possible to achieve reduced gas permeability.

Starch nano-composites were produced by incorporating montmorillonite clay, to the barrier dispersion. It was seen that the suspension viscosity was reduced by poly(ethylene glycol) and citric acid adsorption on the clay. Also, a tendency for improved barrier properties with reduced aggregate volume fraction and reduced swelling was observed. It was also seen that up scaling this formulation to pilot scale was possible and promising results were achieved.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2013. , p. 80
Series
Karlstad University Studies, ISSN 1403-8099 ; 2013:25
Keyword [en]
Starch, Citric acid, Cross-linking, hydrolysis, Montmorillonite, WVTR, OTR, Barrier properties
National Category
Polymer Technologies
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-27127ISBN: 978-91-7063-502-1 (print)OAI: oai:DiVA.org:kau-27127DiVA: diva2:618709
Public defence
2013-06-13, 9C203, Nyquistsalen, Karlstads universitet, 10:15 (English)
Opponent
Supervisors
Projects
Renewable Functional Barriers
Funder
Vinnova
Available from: 2013-05-23 Created: 2013-04-29 Last updated: 2013-05-23Bibliographically approved
List of papers
1. Influence of citric acid and curing on moisture sorption, diffusion and permeability of starch films
Open this publication in new window or tab >>Influence of citric acid and curing on moisture sorption, diffusion and permeability of starch films
2013 (English)In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 94, no 2, p. 765-772Article in journal (Refereed) Published
Abstract [en]

Starch films with different amounts of citric acid produced by solution casting were subjected to differ-ent curing temperatures and compared with films plasticized with glycerol. The films were tested in a controlled moisture generator, which enabled the moisture sorption to be measured and the diffusion coefficient and water vapor permeability to be calculated. It was shown that increasing the amount of citric acid added led to a reduction in the equilibrium moisture content, diffusion coefficient and water vapor permeability of the films, the values of which were all considerably lower than the values obtained for the films plasticized by glycerol. It was also seen that curing the film with 30 pph citric acid at 150

◦C led to a significant reduction in the equilibrium moisture content, the diffusion coefficient and the water vapor permeability at high relative humidity which suggests that crosslinking occurred. The calculated water vapor permeability data were comparable with the value obtained with direct measurements.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2013
Keyword
starch, citric acid, cross-linking, diffusion
National Category
Polymer Technologies
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-27071 (URN)10.1016/j.carbpol.2013.02.006 (DOI)000317946200007 ()23544631 (PubMedID)
Projects
Renewable functional barriers
Funder
VINNOVA
Available from: 2013-04-26 Created: 2013-04-26 Last updated: 2017-12-06Bibliographically approved
2. Molecular structure of citric acid cross-linked starch films
Open this publication in new window or tab >>Molecular structure of citric acid cross-linked starch films
Show others...
2013 (English)In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 96, no 2, p. 270-276Article in journal (Refereed) Published
Abstract [en]

The effect of citric acid (CA) on starch films has been examined. A new method to detect cross-linkingof starch by CA in solution-cast films by molecular weight measurements is described. Furthermore, wemanaged to distinguished between free, mono- and di-esterified CA and quantify di-ester content withinstarch films by using a modification in the method of complexometric titration with copper(II)-sulfate.Cross-linking of starch by CA occurred at low temperature, 70

◦C, which we assumed is so far the lowesttemperature reported where cross-linking reaction occurred. This is essential for starch coating applica-tions within paper industry since no high temperatures for curing will be required. However, curing at150◦C and high CA concentrations, 30 pph, increased cross-linking reaction. Furthermore, the physicalproperties like water solubility, gel content and glass transition temperature, were highly reflected bychanges in the molecular structure i.e. cross-linking and hydrolysis, as well as CA content and curingtemperature.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2013
Keyword
Starch-based film, Citric acid, Cross-linking, Molecular weight, Hydrolysis, Amorphous starch, Plasticization
National Category
Polymer Technologies
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-27073 (URN)10.1016/j.carbpol.2013.03.044 (DOI)000320486000034 ()23688480 (PubMedID)
Projects
Renewable functional barriers
Funder
VINNOVA
Available from: 2013-04-26 Created: 2013-04-26 Last updated: 2017-12-06Bibliographically approved
3. The effect of pH on hydrolysis, cross-linking and barrier properties of starch barriers containing citric acid
Open this publication in new window or tab >>The effect of pH on hydrolysis, cross-linking and barrier properties of starch barriers containing citric acid
Show others...
2013 (English)In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 98, no 2, p. 1505-1513Article in journal (Refereed) Published
Abstract [en]

Green cross-linking of thermoplastic starch for food packaging applications has been intensely studied during the last decade as a method of producing water-insensitive renewable barriers. This work has shown how the barrier properties of paper coated with a barrier dispersion containing starch and citric acid were affected by the solution pH and the drying temperature. The barrier properties of the coated paper were linked to molecular properties showing both hydrolysis and cross-linking reaction of starch in the presence of citric acid at different solution pH and different reaction temperatures (curing) on cast films. Hydrolysis was shown to be almost completely hindered at solution pH ≥4 at curing temperatures ≤ 105 °C and at pH ≥ 5 at curing temperatures ≤150 °C, whereas cross-linking still occurred to some extent at pH ≤ 6.5 and drying temperatures as low as 70 °C. The water vapor transmission rate was significantly affected by the competition between these two reactions. Coated paper showed a minimum in water vapor transmission rate at pH was kept around 4 in the starch coating solution, corresponding to the point where hydrolysis was effectively hindered but where a significant degree of cross-linking still occurred.

Keyword
starch; cross-linking; hydrolysis; molecular structure; barrier properties; citric acid
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kau:diva-27081 (URN)10.1016/j.carbpol.2013.07.040 (DOI)000326313600034 ()24053833 (PubMedID)
Projects
Renewable Functional Barriers
Funder
VINNOVA
Available from: 2013-04-26 Created: 2013-04-26 Last updated: 2017-12-06Bibliographically approved
4. Montmorillonite clay for starch-based barrier dispersion coating: Part 1 The influence of citric acid on viscosity and barrier properties
Open this publication in new window or tab >>Montmorillonite clay for starch-based barrier dispersion coating: Part 1 The influence of citric acid on viscosity and barrier properties
2014 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, p. 160-166Article in journal (Refereed) Published
Abstract [en]

Starch-based coating formulations were draw-down coated on a paper substrate and the effect on the water vapor transmission rate, WVTR, upon montmorillonite addition was compared to that of the pure the starch matrix without added plasticizer and for poly(ethylene glycol), PEG, and citric acid, CA, as potential plasticizers. Both potential plasticizers were added at a ratio of 3 to 10 to starch. Addition of CA to the starch solution substantially lowered the WVTR of the coated papers compared to coating with a pure starch or PEG-plasticized coating. The WVTR of the papers coated with starch and CA was lower than that obtained for paper coated with non-plasticized montmorillonite-starch dispersions, and approximately similar to paper coated with montmorillonite-starch dispersions plasticized with PEG or with citric acid. Addition of montmorillointe was effective in reducing the WVTR in the case of PEG-plasticized starch but not in the case of CA. These differences in WVTR were indicated to be due to differences in the viscosity and the clay swelling indicating that CA and PEG affected the state of dispersion differently. CA was also investigated as a dispersing agent by addition in small quantities to the montmorillonite dispersion at different pH values. When this clay dispersion was added to a suspension already containing PEG, CA gave slightly reduced plastic viscosity and there was also a weak correlation between low plastic viscosity and WVTR.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Starch; Montmorillonite; Citric acid; Poly(ethylene glycol); Rheology; Barrier properties
National Category
Polymer Technologies
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-27110 (URN)10.1016/j.clay.2014.04.040 (DOI)000340302200022 ()
Projects
Renewable Functional Barriers
Funder
VINNOVA
Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved
5. Montmorillonite clay for starch-based barrier dispersion coating: Part 2 Pilot trials and PE-lamination
Open this publication in new window or tab >>Montmorillonite clay for starch-based barrier dispersion coating: Part 2 Pilot trials and PE-lamination
2014 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 97-98, p. 167-173Article in journal (Refereed) Published
Abstract [en]

Montmorillonite clays have been shown to improve the barrier properties of thermoplastic starch coatings. In this paper, it is shown that it is possible to scale-up a recipe containing poly(ethylene glycol)-plasticized starch and a citric acid-dispersed montmorillonite to pilot scale and to achieve water vapor barrier properties which are comparable to those achieved in laboratory scale. The results are compared with those obtained with two commercial synthetic barrier dispersions. The combination of a polyethylene film laminated on top of a 3 g/m2 starch-based coating showed the potential to give also oxygen barrier properties to the multilayer structure when applied on a paper substrate.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2014
Keyword
Starch; Montmorillonite; Poly(ethylene glycol); Citric acid; Pilot coating; Barrier properties
National Category
Polymer Technologies
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-27111 (URN)10.1016/j.clay.2014.04.039 (DOI)000340302200023 ()
Projects
Renewable Functional Barriers
Funder
VINNOVA
Available from: 2013-04-28 Created: 2013-04-28 Last updated: 2017-12-06Bibliographically approved

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