Properties of Nano-Starch-Based Antimicrobial Biopolymers

The fundamental functions for antimicrobial packaging should be the same as for the traditional food packaging, along with antimicrobial effect. The important parameters which determine the suitability of antimicrobial film to any particular application include mechanical properties (tensile strength and elongation at break), permeability (water vapour and gases permeability), and optical quality (colour and transparency). The quality of edible coating or film mainly depends on nature and concentration of raw material, ratio and type of additives, technique, and conditions used for film preparation (Hernandez et al., 2008). The addition of NPs of starch greatly affects the mechanical, barrier, appearance, and biodegradability of a starch- based nanocomposite. The nano-size of inserted starch particles increases the compactness in the matrix and decreases the rate of diffusion of vapours or gases, causing a reduction in the permeability of films. SNPs as fillers exhibit better compatibility in the starch matrix, due to similar inherent characters from starch granules, give a reinforcement effect in starch nanocomposite films. Production processes alter the crystallinity of SNPs and hence affect the light transmittance of films when incorporated in the film matrix. The incorporation of SNPs in films enhances the biodegradable due to the smaller size and more surface area of NPs than native granules of starch. The incorporation of antimicrobial substance in the biopolymer helps to prevent the growth of microorganisms while it may positively or negatively alter the mechanical strength, permeation capacity, and thermal stability of the ultimate packaging material. So these factors must be considered while making antimicrobial biopolymer- based packaging material. The data on mechanical and barrier properties of biodegradable antimicrobial films based on starch from different sources have been summarized in Table 15.1.

Mechanical Properties

The protection of food from physical harms like bruising, breaking, and denting is the primary function of packaging. The performance of this function of packaging film depends mainly on two mechanical parameters, namely tensile strength,

TABLE 15.1

Mechanical Properties and Water Vapour Permeability of Starch Based Antimicrobial Films

Base material

Antimicrobial

Compounds

Mechanical Properties

Water Vapour ' Permeability (x10-10 g/m2 s Pa)

References

Tensile Strength (MPa)

Elongation at Break (%)

Tapioca starch

Grape pomace extracts

2.58-3.55

22.5—45.8

13.42-20.22

Xu etal.,2018

Taro starch

saponin

24.10-28.18

29.77-92.74

-

Assefa and Admassu. 2013

Corn starch

Silver nanoparticles

-

-

1.30±0.16

Abreu et al., 2015

Corn starch + Montmorillonite

Silver nanoparticles

1.01 ±0.21

Abreu et ah, 2015

High amylose starch film

Pomegranate peel

21.32-22.84

5.64-28.32

-

Ali et ah. 2018

Starch

Potassium sorbate

20.92-36.25

38.74—43.67

-

Raigond et ah. 2019

representing the highest strength that a film can bear up, and elongation at break, indicating the stretchability of film. The study of the mechanical characteristics of antimicrobial films gives the knowledge about the elasticity and the resisting capacity of film to the applied force that is helpful in predicting the efficacy of film throughout the handling and storage time. The smaller size and shape of incorporated NPs play an important role in the enhancement of the mechanical properties of starch films. Owing to their nanometric size, SNPs become able to occupy inter- and intramolecular sites, causing a densification of the matrix. Moreover, the higher surface area of NPs induces more interfacial interactions between additives or fillers and polymeric molecules, as well as within polymer complex, and results in improved mechanical strength of the matrix. Therefore, nanocomposite films filled with SNPs exhibit enhanced mechanical strength. The effects of adding SNPs to the mechanical properties of films are profoundly depend on the quantity or level of NPs filled in the matrix. SNCs incorporated at a high concentration in the matrix get aggregated and decrease the interactions between the molecules of filler and polymer, causing the weakening of the ultimate nanocomposite. Liu et al. (2016) reported improved tensile strength of corn starch films on the incorporation of SNPs 15% while the reverse effect on tensile strength was observed on the addition of a higher concentration (up to 25%). The incorporation of SNCs at the level of 5% improved tensile strength of a pea starch-based nanocomposite while the decrement in the tensile strength and increment in elastic modulus were observed on increasing the concentration of starch nanofillers (Li et al., 2015). Generally, the addition of NPs to films reduces the elongation at break value of nanocomposite films. Studies showed that the reinforcement of a rice starch-based film with the addition of SNCs up to 20% improved tensile strength and elongation at break (Piyada et al., 2013).

The mechanical properties of starch-based edible films may or may not be improved by incorporation of antimicrobial compounds, although the concentration of antimicrobial compounds strongly influences these properties. Poor elongation or low tensile strength of the film may cause the untimely breakdown or cracking of the film during preparation, application, or storage. Therefore, for production of antimicrobial films with sufficient mechanical strength, the quantity of antimicrobial agent must be taken into consideration prior to its preparation. The addition of nano-silicon (Si02) in starch-based nanocomposite films increased the mechanical strength and transparency while decreasing the water uptake capacity of biopolymers (Xiong et al., 2008). The reason behind it could be the decrement in intermolecular hydrogen bonding in the starch matrix.

 
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