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PEER-REVIEWED REVIEW ARTICLE
hemicellulose-based films including birch xylan, GGM and konjac glucomannan (KGM), with promising results being achieved (birch xylan coated paper < KGM < GGM)
(Mikkonen and Tenkanen 2012). Hemicellulose/mineral composites
Several studies have been conducted with different hemicellulose composites to obtain enhanced barrier properties (Johansson et al. 2012; Mikkonen and Tenkanen 2012). Talja et al. (2011) produced glycerol plasticized birch xylan/nanoclay (bentonite) composite coating, which was found to enhance WVTR and aroma barrier properties due to the addition of nanoclay. Nanoclays have the potential to improve barrier properties against gas, water vapor, and aromas due to their feature of making rambling diffusion paths for the molecules. Excellent oxygen barrier performance (< 0.18 cm 3 μm/m 2 dkPa) was achieved by using oat spelt arabinoxylan (AX) mixed with sorbitol and 50 wt% cellulose nanocrystals. Spruce galactoglucomannan (GGM) displays resistance towards oxygen, although the oxygen resistance did not improve with the addition of cellulose nanocrystals. The same phenomenon was noticed in water vapor properties. The moisture resistance of oat spelt AX films increased by adding cellulose nanocrystal, whereas the cellulose nanocrystal addition did not improve moisture resistance of GGM-based films. Nanofiller additions have also been studied in the perspective of enhancement of mechanical properties (Mikkonen and Tenkanen 2012). Hemicellulose/biopolymer blends The mechanical properties of hemicelluloses depend on the chemical structure, relative humidity, and possible additives, e.g. , plasticizers (Mikkonen and Tenkanen 2012). Many studies have investigated the mechanical properties of hemicellulose-based films and their improvement through the addition of another polymer (Mikkonen et al. 2008; Mikkonen and Tenkanen 2012), fillers, or by crosslinking. Mikkonen et al. (2015) achieved improved oxygen and water vapor resistance of hydroxypropylated birch xylan (HPX) with an addition of an external plasticizer (sorbitol). Likewise, Gröndahl et al. (2004) reported high oxygen barrier properties with aspen glucuronoxylan with 35 wt% of sorbitol. Hemicellulose-based coatings, while blended with a crosslinking agent or a hydrophobizing agent, improved their resistance to oxygen, aroma, and grease in parallel to the improvement of their water vapor resistance. However, due to their hydrophilic nature, hemicellulose-based films are sensitive to changes in RH. An increase in the amount of plasticizer, such as glycerol or sorbitol, reduced the moisture uptake of GGM films at lower RH (Mikkonen and Tenkanen 2012). Crosslinked (5% citric acid) and plasticized HPX films achieved promising grease, mineral oil, oxygen, and water vapor barrier properties. The WVTR was comparable with that of commercial PLA films (Vartiainen et al. 2014). A light barrier is an important property in food packaging to maintain the quality of the packaged good. The GGM was found to act as an UV light barrier (Mikkonen and Tenkanen 2012). Improved tensile strength of GGM was achieved by blending with KGM (Mikkonen et al. 2008; Mikkonen and Tenkanen 2012). Lignins Lignin is an amorphous polyphenolic macromolecule with functional groups (Mousavioun et al. 2010; Gordobil et al. 2014), including hydroxyl phenylpropane
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Helanto et al. (2019). “ Bio-based barriers ,” B io R esources 14(2), Pg #s to be added.
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