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PEER-REVIEWED REVIEW ARTICLE
other hand, are glucomannans, while a considerably lower amount consist of xylans (Jääskeläinen and Sundqvist 2007; Mikkonen and Tenkanen 2012). In kraft pulping, hemicelluloses are isolated from hardwood chips and from hardwood black liquor (Talja et al. 2011). Hemicelluloses can also be isolated as a co- product from several production processes, such as the production of dissolving pulp, nanocrystalline cellulose (CNC), nanofibrillated cellulose (NFC), and sugar for biofuels (Mikkonen et al. 2015). Hemicelluloses could be recovered also by filtrating wastewater streams (Edlund et al. 2010). However, hemicelluloses are less-commonly used in industrial scale relative to starch and cellulose (Mikkonen and Tenkanen 2012). A couple of examples from hemicellulose isolation processes are presented in Fig. 3. Hemicelluloses as such are hydrophilic. The hydrophilicity enables good resistance towards oil and grease (Mikkonen and Tenkanen 2012; Laine et al. 2013). Challenges in hemicellulose-based films involve their hygroscopicity and mechanical properties (Saadatmand et al. 2013; Chen et al. 2016). The high internal cohesion leads to the film-forming properties of xylan to be relatively poor (Talja et al. 2011; Vartiainen et al. 2014). Nevertheless, hemicelluloses are chemically modifiable due to their free hydroxyl groups. Plasticizers are typically used to improve the flexibility of hemicellulose-based films (Hansen and Plackett 2008; Laine et al. 2013; Chen et al. 2016). Suitable plasticizers for this purpose include xylitol, sorbitol, glycerol (Hansen and Plackett 2008; Vartiainen et al. 2014) and bio-based polymers such as carboxymethyl cellulose (CMC), lignin (Chen et al. 2016), and alginate (Vartiainen et al. 2014). Hemicellulose-containing pulp has been a preferred conventional practice in the paper industry to provide a higher yield and to enhance mechanical properties. Hemicelluloses have also been utilized in the food industry, e.g. as a sweetener, thickener, and an emulsifier (Gröndahl et al. 2004). For example, xylan has been used industrially as a raw material for chemicals (Talja et al. 2011). Hemicelluloses have a great potential to be utilized also as bio-based barrier coatings: the raw material is easily available, and suitable application methods include air brush, bar, and curtain coaters (Gröndahl and Bindgård 2014). Due to the encouraging oxygen, grease, and taint resistance of hemicellulose-based films, dry food packaging could be a suitable application. Nevertheless, further improvements with respect to the mechanical properties are required in order to utilize hemicellulose-based barrier materials in application fields, such as food packaging. Mikkonen and Tenkanen (2012) have published a comprehensive article on the use of xylan and mannans in food packaging materials. Researchers have reported promising oxygen barrier properties with hemicellulose-based films (Hansen and Plackett 2008; Edlund et al. 2010; Laine et al. 2013; Saadatmand et al. 2013). Laine et al. (2013) achieved a significantly lower oxygen permeability level by using xylan-based barrier material than with a coating of polyethylene terephthalate (PET). In addition, the same xylan-based barrier functioned as a mineral oil barrier. Acetylated galactoglucomannan (AcGGM) has a promising resistance level towards oxygen, yet it is sensitive to moisture. Hartman et al. (2006) produced benzyl-galactoglucomannan (BnGGM) films, which were less sensitive to high- moisture conditions than unmodified AcGGM films, consequently maintaining their oxygen barrier properties. Moreover, galactoglucomannan (GGM) has been shown to have a low resistance towards carbon dioxide, which is desirable in some packaging applications (Mikkonen and Tenkanen 2012). However, there is a need to improve GGM’s tensile strength and elongation at break properties (Mikkonen et al. 2008). There have been studies about the permeability of aroma compounds into different
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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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