The barrier properties of packaging films are important for the enhancement of storage shelf life of food products by prevention of deteriorative changes due to microorganisms, moisture, and oxidation. These properties of edible films or coatings indicate the extent of resistance for permeation of gases or water vapours through it. The most extensively studied barrier property of starch-based biodegradable antimicrobial films is water vapour permeability as moisture level is an important parameter for keeping the physiochemical quality of the product as well as retarding microbial growth on the surface. Generally, the low water vapour permeability of packaging him is required to avoid the transfer of moisture between the product and its neighbouring environment for increasing storage life and maintaining product quality. However, the specific requirement for the level of water vapour permeation of the him is based on various factors like the properties of food, expected storage life, and conditions of the surrounding environment.
The integration of NPs of starch gives a positive impact on barrier properties of starch hlms. Liu et al. (2016) reported the decrement in water vapour permeability of starch hlms on the incorporation of SNPs up to 15%. Cassava starch hlms reinforced with SNCs (2.5%) showed a 40% decrement in permeability for water vapours as compared with controls samples (Garcia et al., 2009). The improvement in barrier capacity nanocomposite for water vapours and oxygen by adding SNCs was reported by Angellier et al. (2005). Added nanocrystals in a him matrix cause a curved and complicated passageway for the vapours to diffuse through the hlms resulting in decreased penetration and permeability of water molecules through nanocomposite hlms. However, the concentration of nanohllers is important for the alteration of barrier properties of starch nanocomposite hlms. A higher concentration of SNCs in the him can cause the aggregation of starch particles and decrease the barrier capacity for water vapours as compared to hlms unfilled or tilled with lower level of starch nanohllers. Pea starch nanocomposite hlms showed a rise in water vapour permeability on the addition of SNCs at a level exceeding 5% (Li et al., 2015).
Alteration of the barrier capacity of starch hlms on inclusion of antimicrobial compound depends on the starch's properties, type and the concentration of compound. and its interaction with other ingredients of the him matrix. The added antimicrobial agents may shrink the volume of free space in polymer, weaken the water binding capacity, or cause the blockage of diffusive pathway throughout the him, which ultimately results in decreased water vapour permeability value of the him (Rawdkuen et al., 2012; Hashemi et al., 2017; Kaewprachu et al., 2017; Xu et al.,
2018). The enhancement in water vapour permeation of hlms occurred when added antimicrobial compounds act as a plasticizing agent, lead to increased intermolecular interactions, and raise the mobility of macromolecules in the him (Xu et al., 2018). Besides, the barrier properties of hlms also depend on the structural and chemical properties of macromolecules, other added compounds, and the hydrophilicity of the hlms. Mohan et al. (2017) prepared biohlm based on corn starch filled with nanoclay and compared its properties with unfilled cornstarch biopolymer. Results showed decreased water absorption capacity, lower moisture uptake, and less swelling while increasing the oxygen barrier for the biohlm containing a nanoclay filler as compared to the biohlm without nanohller (Sadegh-Hassani and Nafchi et al., 2014). Increased mechanical strength and reduced water vapour permeability were noticed on the addition of nanoclay up to 5% in cassava starch-based organically modified nanocomposite (Muller et al., 2012). Slavutsky et al. (2012) reported that the incorporation of NPs of clay below the level of 10% improved the water barrier capacity of a corn starch-clay nanocomposite and found it suitable for the production of biopolymer hlms (Park et al., 2002). The incorporation of nanoclay and silver NPs in corn starch-based nanocomposites improved barrier capacity for water vapours and oxygen gas (Abreu et al., 2015).
Optical properties of food packaging are very vital properties as these directly influence the appearance of products and also consumer’s acceptance. The colour traits are generally expressed in terms of L* value (lightness to darkness), a* value (red to green), and b* value (yellow to blue). Starch films are appreciably transparent and types and concentrations of fillers or additives significantly affect its transparency. SNCs are composed of amorphous and crystalline zones. The crystalline regions decrease transmittance of light by causing reflection and refraction phenomenon. Therefore, the incorporation of SNPs changes the appearance of nanocomposite films by increasing opacity. The integration of SNPs up to the level of 25% reduced light transmittance by about 14% in the case of corn starch nanocomposite films (Liu et al., 2016).
The addition of antimicrobial compounds influences the appearance of packaging material by altering the transparency and colour values. The type and source of antimicrobial agent added into the biopolymer matrix and pigments inherent in it are mainly responsible for changes in the colour attributes of the final packaging material. Generally, the degree of change in appearance is directly related to the concentration of the antimicrobial agent added. The biopolymer-based films with less transparency are unsuitable as packaging material when transparent packaging and clear shine on product’s surface are required. Although the colour attributes of starch-like biopolymer-based antimicrobial film may influence the consumers’ opinion, there are also several advantages, such as protecting the packaged product to some extent from ultraviolet and visible light, causing discolouration, loss of nutrients, and having antimicrobial properties which are not present in the conventional packaging.
The antimicrobial activity of starch films may be an inherent character of its base material or due to the addition of some antimicrobial compounds during preparation. The antimicrobial compound immobilized into the film performs its function either by contacting the food surface or by slowly releasing itself into the food matrix. Commonly, the agar disc diffusion method is follow'ed to evaluate the antimicrobial effect of films. In this method, a portion of the film is positioned on the surface of agar already inoculated by the target microbes, subjected to the optimum environment for required periods, and then developed in an inhibition zone around the film’s portion is observed. The activity of the antimicrobial biopolymer-based films observed against a broad range of microbes showed that the characteristics of the target microorganism, nature, and concentration of incorporated antimicrobial agents, as well as the chief active compounds present, are the major factors influencing it. Generally, efficacy is directly related to the concentration of active compound used. Different microorganisms show' unique responses to any particular antimicrobial compound; for example, gram-positive bacteria are more sensitive to a majority of antimicrobial agents as compared to gram-negative bacteria. The reason behind it is the unique features of different microorganisms, like the strong protective covering of cell wall, allow the limited diffusion of an antimicrobial agent in case of gram-negative bacteria. In addition to the nature of the target microorganisms, there are some other factors such as film preparation method, storage conditions, composition, and the hydrophilic- hydrophobic balance of the antimicrobial agent, that influence the release or diffusion of antimicrobial material through the film. The phenomena of action of added active compound also depend on its interaction with the biopolymer molecules and food components. All these factors play important role and are able to be modified for production of an efficient antimicrobial packaging material. Various studies on starch- and clay-based nanocomposites incorporated with NPs of silver and copper reported significant antimicrobial activity, particularly antibacterial effects, of these biopolymers (Shankar et al„ 2015,2017; Carbone et ah, 2016). However, copper is generally not added in polymers used for food packaging as it may cause the deterioration of food products by accelerating the reaction of chemical oxidation.
Abreu et ah (2015) prepared a nanostructured antimicrobial starch film using corn starch incorporated w'ith NPs of montmorillonite and silver NPs. The film showed an antimicrobial effect for Staphylococcus aureus, Escherichia coli, and Candida Albicans with no major differences due to variation in concentration of silver NPs in the starch matrix. Negligible migration of components indicated that nanostructured material produced by incorporation of nano-silver and nanoclay particles in starch is suitable for packaging applications. Amjad et ah (2018) utilized high amylose starch (80% amylose) incorporated with pomegranate peel for film preparation and evaluated it for antibacterial effects. Results showed that the film inhibited the growth of 5. aureus and Salmonella, indicating its potential to be used as a biodegradable antimicrobial biopolymer for packaging.