**3. Edible film manufacturing and coating application method**

The edible film is commonly wrapped around a food product surface as a solid matrix and can act as primary packaging deprived of any sensory or nutritional appeal. Edible film can be utilised in the shape of pouches or sachets specifically for energy drinks and meal replacement shakes [8]. Aqueous solutions are transformed into edible films during the coating operation using specialised equipment. Only two basic converting methods – casting on steel belt conveyors and casting on a disposable substrate (such as release paper) on a coating line – are commonly employed to make edible films. However, a wide range of technologies are available for making thin coatings and films [114].

#### **3.1 Casting**

Casting (**Figure 1**) is a manufacturing process by which a liquid material is usually poured into a mould and then allowed to solidify [115]. The most popular method for forming films, known as solvent casting, is typically used at laboratory scales. Three

fundamental steps are involved in casting a biopolymer-based movie: the biopolymer is dissolved in a suitable solvent, the solution is cast in a mould and the casted solution is dried [116]. To dissolve soy protein isolate polymer, the chosen polymer is dissolved or dispersed in an appropriate solvent (ethanol) [117]; this process is known as solubilisation. The resulting solution is poured into a glass plate with Teflon coating or a predetermined mould. The drying procedure gives the solvent enough time to evaporate, resulting in a polymer coating that adheres to the mould. For the casting of films to facilitate solvent removal and easy peeling of the film, air driers such as hot air ovens, tray dryers, microwaves and vacuum driers are used [118]. To improve the intramolecular interaction between the polymer chains and achieve a proper microstructure for the film, the air-drying process for casting edible film is crucial [119]. Quick-drying casting techniques have had a negative impact on the physical and structural qualities of the film [120]. The primary benefit of the casting method of film formation is its low cost and ease of production without the need for specialist equipment [121]. However, the film formation in solvent casting depends on the solubility of the polymer rather than melting [122].

In the casting process, following are the key drawbacks: (i) Limiting the forms (the simplest forms that frequently arise are simple sheets and tubes). (ii) Possibility of harmful solvent being trapped inside the polymer. (iii) Denaturation of proteins and other molecules that are incorporated into polymers using solvents [121]; vacuum drying of films can be used to remove the hazardous solvent [123]. (iv) A cap on the quantity of films that can be made [124]. (v) Films with various features can be created when evaporation levels and temperatures are variable [125]. (vi) Casting requires long drying time, which is not possible for commercial production [117].

### **3.2 Steel belt conveyors**

Wherein the solutions are cast or spread uniformly on a continuous steel, then the moisture is removed by passing through a drying chamber. After being removed from the steel belt, the dry film is wound into mill rolls for subsequent processing. Additionally, the steel belt revolves around sizable drums that are located at either end of the line. With the aid of traditional coating equipment, the solution is evenly applied at one end of the line before being dried in a chamber. To avoid sticking or blocking, a thin, secondary dusting of starch powder may be applied to the dried film as it leaves the drying chamber. It is also decorated or marked with edible inks; given a variety of other treatments; and then taken from the steel belt and wound into enormous master rolls. In general, the steel belt conveyor lines (**Figure 2**) are 50–100 feet long when measured from the centre of the two drums. The widths of steel belts range from 20 to 60 inches. One of the highly desired characteristics of steel belt conveyors is the ability to directly cast aqueous solutions on the belt surface. This reduces the cost of a separate carrier web-like polyester film or coated paper while increasing uniformity, heat transfer and drying efficiency. However, some coating formulas could adhere to the steel belt too firmly. The coated substrate is then stripped from the belt and wound into a master roll [114].

#### **3.3 Extrusion method**

Extrusion is alternative technique used for producing polymeric films, and a pictorial view is given in **Figure 3**. This technique is preferred over a casting technique because it requires less energy and takes less time to remove water for making film [128]. *Application of Edible Packaging in Dairy and Food Industry DOI: http://dx.doi.org/10.5772/intechopen.107850*

#### **Figure 2.**

*Steel belt conveyor. Source: Gamboni et al. [126].*

#### **Figure 3.**

*Extrusion machine for polymeric films. Source: Suhag et al. [127].*

It is one of the foremost polymer processing techniques currently in use at a commercial scale [129]. The extrusion process, commonly, is divided into three zones: (i) the feeding zone, (ii) the kneading zone and (iii) the heating zone at the final part/exit from the machine [130]. This technique best works with a minimum content of water or solvents; therefore, it is also called a dry process. However, to increase film flexibility, plasticisers are needed [131]. The mechanical (specific mechanical energy) and thermal energies (extruder barrel temperature) are involved in this method to yield an extruder-based edible film [132]. Certain parameters such as moisture content of the feed, screw speed, temperature of the barrel, the diameter of die, pressure at the die, energy input, etc. are critical for the extrusion process to influence the final products [127]. If high temperature is produced during the process, it affects the sensorial and nutritional properties of biopolymer of edible film which restrict its usage for high temperature with low moisture content FFS [133]. Co-extrusion is a method that can be used to create multi-layer films and gives flexibility in determining the required film properties. In addition to enhancing the produced film's functionality and processing capabilities, the multilayer also benefits from the inventive structure of the multilayer film [134]. For preparing the edible film from pectin/starch blends added with pasticiser and glycerol, Fishman [128] used extrusion method in which extruder has nine heating zones and two screws. Liu [135] give optimal parameters/conditions for preparation of pectin film that must be 225 rpm and temperature should be 125°C in third zone and 110°C in fourth zone. These conditions were fixed on the basis of film's physical and mechanical properties such as elongation, puncture strength, colour, thickness, etc. [136]. A short processing time with low energy consumption, better mechanical and optical qualities, such

as elongation and transparency of edible film, are the key benefits of extrusion film formation over casting technique [129, 137]. It is a high-performance, inexpensive and efficient method utilised in the commercial food manufacturing industry [138, 139].

## **3.4 Electrospinning**

## *3.4.1 Principle*

The process is chiefly carried out in three phases such as (1) jet initiation, (2) elongation and (3) solidification of solution (Masoud Aman [140]). An electrospining (**Figure 4**) having components such as syringe or capillary tube which transfer solution to which high voltage is given by using high-voltage battery followed by producing the nanofiber and solution is collected with the help of collector [142].

A widely used technique for processing biopolymer-based film-forming solutions worldwide is electrospinning [143]. It is an economical method that can create thin films that could increase a material's solubility and improve an application [144, 145]. In the electrohydrodynamic process used in electrospinning, a liquid droplet is electrified to create a jet, which is then stretched and lengthened to create fibres [146, 147]. For producing thin film, strong electric field is applied with small size orifice to spinneret (usually, a hypodermic needle with blunt tip). The diameter size of electrospun polymer fibre made by this technique generally is from 10 to 1000 nm and hence performs better electrical, mechanical and thermal properties than the synthetic packaging ([148] BG book). This electrospun fibre acts as an adhesion; hence, this can be used to combine two layers of biopolymer which increases the barrier properties of EF [149]. Electrospining technique helps to tightly hold two layers which maintain the thickness of film, and due to nanometric size of fibre, it enhances the mechanical property without affecting the optical property of the film [150]. The modified spinning process has plate die in place of spinneret which can produce flat film from casein [151]. Direct current (DC) or alternating current (AC) power supplies are both acceptable (AC). Surface tension causes the liquid to protrude from the spinneret and form a pendent droplet during electrospinning. When a droplet is electrified, the electrostatic attraction between surface charges with the same sign causes them to repel one another, transforming the droplet into a Taylor cone from

#### **Figure 4.**

*Electrospinning machine for making biopolymer-based film. Source: Ebrahimi et al. [141].*

*Application of Edible Packaging in Dairy and Food Industry DOI: http://dx.doi.org/10.5772/intechopen.107850*

which a charged jet is released. Due to bending instabilities, the jet first extends in a straight line before undergoing ferocious whipping motions. The jet is then compressed to smaller diameters, where it quickly hardens and deposits solid fibres on the grounded collector [152, 153]. Cui et al. [154] produced the nanofibers based on chitosan, gelatin and clove oil by electrospinning. The experiment of wrapping cheese by the obtained nanofiber for inhibiting microbial contamination proved that electrospinning was a creative strategy to fabricate nanofibers applied onto food [155]. Ebrahimi et al. [141] introduced a nozzle-less electrospinning device as a favourable technology to produce food-grade nanofibers on large scale with a high production yield in comparison to a nozzle-based instrument.

#### **3.5 Thermoplastic method**

The thermoplastic method uses shear pressures, high temperatures and little water to continuously shape the materials, allowing for industrial-scale large-scale manufacture of films [156]. It has been claimed that this technique can expand applications and get beyond the drawbacks of conventional techniques such as casting [157]. For chitosan films and gelatin films, the thermoplastic method – also known as the 'dry process' – involves extrusion strategy [158], blown approach [159], compression moulding [160] and the aforementioned combination [138]. The thermoplastic method, which is typically used for synthetic polymers, bio-plastics made from proteins, polysaccharides and other biopolymers, can be carried out as a continuous unit operation using control of temperature, size, shape and moisture [161]. For the most part, thermoplastic processing is an effective way to create chitosan or gelatin films for commercial uses in a wide variety of food products. However, there is no information available regarding thermoplastically manufactured chitosan-gelatin composite films [155].
