Milk Protein Based Edible Films and Coatings

Day by day consumers’ demand is increasing towards better quality, healthy, safe and long shelf-life of food without compromising its nutritional quality and sensory characteristics. Consumers’ demand plays a major role for the processing industry to implement novel and innovative technologies (Fortuin and Omta, 2009) for preserving food with renewable and biodegradable materials (Avramescu SM et al,2020). Recently, edible films and coatings are becoming an attractive concept not only because of their benefits on food products but also in novel product differentiation.

Edible films have potential to extend shelf-life and improve food quality by controlling mass transfer between the environment and food or between various food product components (Tulipano G, 2020).Food ingredients such as antimicrobials, antioxidants, flavors, etc. can be carried by edible films, which also have the ability to improve the mechanical and rheological properties of the food. All these benefits can be achieved by limiting the migration of moisture, CO₂, O₂, lipids, colors, and flavours of food components (López-Barreto et al, 2018; Pirsa S and A ghbolagh Sharifi K, 2020).

Edible films and coatings are produced using natural, biodegradable, edible biopolymers as well as food grade additives (Han JH,2014; Roy S et al, 2023).The main three biopolymer groups are carbohydrates, lipids and proteins that are used either alone or together for edible film. These are decomposed naturally without making any toxic or harmful substances into the environment (Šuput D Z et al, 2015).The eco-friendly nature of these biopolymers with excellent keeping quality and safety adds value to edible films and coatings(Shah U et al, 2016).

Edible Film and its Properties

Edible films are thin preformed layers of polymers that are applied on the surface of fresh fruits, vegetables and other foods. They provide protection against adverse environmental conditions (light, temperature, humidity), acting as physical barriers to mass transfer and preserving food features such as freshness, firmness, color, nutritional and microbiological quality.

Mechanical protection, mass transfer regulation and sensory properties are some of their key properties.Among these adequate mechanical strengthis very much important to maintain the integrity of the packaging during distribution(Chen H,1995).  The type of milk protein has an impact on mechanical, optical, hydrodynamic and antioxidant properties of edible films and coatings. Moreover, the concentration of biopolymer and use of plasticizer also affect the film characteristics.

Milk Protein Based Edible Filmand Coatings

Edible films based on proteins are more interesting since they possess higher nutritional, barrier features and better mechanical properties than those made with either carbohydrates or lipids (Moghadam M et al,2020). Milk is one of the most significant sources of protein and this protein is more nutritious in nature as compared to other food proteins (Bourtoom T,2008). Milk proteins are the excellent source for preparation of edible films and coatings due to its high nutritional and functional properties. In addition, to their nutritional qualities, milk proteins contribute numerous physical attributes that are crucial for their efficacious behavior in edible films and coatings such as water solubility and emulsification properties (Shendurse A et al,2018).Moreover, the simple milk protein films are good barriers to gas transfers due to their complex intermolecular bindings (Chen H, 1995). They also possess antioxidant, antihypertensive, immuno modulatory, mineral binding, antimicrobial, antithrombotic, opioid-like activities and similarly modulate the digestive, cardiovascular, immune, endocrine and central nervous systems. Further, their bioactive peptides regulate functions related to food intake, body weight gain and glucose homeo stasis (Tulipano G,2020).

Milk proteins are divided into two main proteins namely casein and whey proteins. Casein is the principal protein, which represent about 80% of the total milk protein, while whey proteins contribute around 20%. Whey protein concentrate (WPC), whey protein isolate (WPI) and milk protein concentrate (MPC) can be obtained from milk which may have promising application in the manufacturing of edible films and coatings (Pirsa S and Aghbolagh Sharifi K, 2020).

Total Milk Proteins Based Edible Filmand Coatings

            The edible films and coatings can be prepared from total milk proteins (TMP), extracted from non-fat dry milk. Smooth casting surfaces are often utilized for drying the film solution at room temperature.The TMP film formation process becomes more complex due to presence of lactose. This lactose gets crystallized during the process resulting in the formation of non-homogeneous film and adhesion of film to surfaces.Crystallization is prevented by the addition of potassium sorbate. Film using TMP obtained from non-fat dry milk by removing lactose by ultrafiltration (UF) or suspension in ethanol followed by filtration, has a higher moisture barrier and more flexibility. However, denaturing by ethanol reduces the water solubility of film. TMP solution is heated at 135°C to prepare insoluble films and the prepared films are stronger and less brittle due to dissociation of micelles and formation of intermolecular bonds (Khwaldia et al, 2004).The edible films also formed from TMP concentrate extracted by UF, has lowest water vapor permeability and the highest tensile strength at break. Commercial TMP concentrates produced films more ductile than those from the UF‐TMP or retentate from ethanol extraction (Maynes and Krochta, 1994;Phillips LGet al,1994).Casein and whey protein concentrate (WPC) films are prepared through casting by plasticizing with sorbitol and glycerol independently. Increasing the plasticizer content resulted in an increase in film thickness, water vapor permeability and tensile strain while decreased the elastic modulus and tensile strength (Wagh et al, 2014).

Numerous investigations have been conducted to comprehend the impact of protein composition and structure on the features of edible film barriers.

Casein Based Film 

Principal milk proteins, casein is important due to their beneficial qualities and industrial surplus (Khwaldia et al, 2004).Edible casein films can be obtained using different caseinates by solubilizing them in water, casting and drying.Caseinates film are manufactured from aqueous solutions without heat treatment due to its random coil nature. The interactions in the matrix of the caseinates film are likely to involve through hydrogen bonding, hydrophobic bonding and ionic bonding. Caseinate films are flexible and transparent but their water barrier properties are limited. The production of free-standing films and coatings on food products have been studied in relation to casein. Dried fruits and vegetables were shielded against oxidation and moisture absorption by laminated film containing casein (Avena-Bustillos et al, 1993).

Whey Proteins Based Edible Film

Whey proteins are a valuable food ingredient with high nutritional value and can also be used as gels, emulsifiers and foams (Asdagh Aet al, 2021). Therefore, it is necessary to evaluate its suitability for use in the production of edible films and coatings in order to meet the Food and Agriculture Organization of the United Nations (FAO) "zero waste" objectives. Moreover, the increase in environmental awareness has contributed towards the growth of whey proteins based films and coatings. Whey proteins based edible films have suitable mechanical, sensory, optical properties and resistance to oxygen permeability but are sensitive to moisture(Tulipano G, 2020). Along with edible packaging materials, whey based film performs other functions such as carriers of nutrients, antioxidants and antimicrobial agents (Ramos OLet al, 2012).

Food Applications

Milk protein based edible films and coatings have drawn interest in food application because milk protein have the ability to regulate the transfer of oil, oxygen, moisture, fragrance, and flavor components in the food system though its depends on the type of edible-film forming material. It acts as a moisture barrier ,aroma barrier, gas barrier, antimicrobial, and nutraceutical application for fresh fruits, vegetables, frozen foods, etc.Several food applications of milk protein based edible films and coatings are depicted in Table 1.

Table 1 : Food Applications of Milk Protein Based Edible Films and Coatings

Milk proteins used in edible film formation	Function performed	Food application
Sodium caseinate	Reduce water loss	Cherries
Calcium caseinate	Prevent oxidative browning	Potato slices
	Reduce moisture loss	Cut carrots
	Reduce browning	Cut potatoes
	Reduce oil migration	Peanut
Casein–stearic acid	Moisture retention	Peeled carrots
Caseinate–acetylated monoglyceride	Moisture barrier	Celery sticks
Casein–acetylated monoglyceride	Reduce moisture loss	Frozen fish
Potassium caseinate–rennet casein	Improve sensorial properties	Frozen fish fillets
Whey protein isolates (WPI)	Microencapsulation	Milk fat
	Reduce weight loss	Asparagus
	Reduce respiration	Apples
	Reduce rancidity	Roasted peanuts & walnuts
	Reduce rehydration	Strawberry pieces
	Moisture barrier	Breakfast cereal, raisins
Whey protein isolates - acetylated monoglyceride (WPI–AM)	Reduce rancidity and moisture loss	Frozen salmon
Whey protein concentrates- Bee wax (WPC–BW)	Reduce enzymatic browning	Cut apple slices
	Reduce browning and weight loss	Cut persimmons

(Source: Shendurse A et al, 2018)

Merits and Demerits of Edible Films and Coatings

Milk proteins are suitable ingredients for obtaining functional edible films and coatings. The main advantages of edible films and coating are to perform comparable tasks to traditional packaging including acting as strong barriers against gasses, water vapor and flavoring compound.In addition, they can enhance the mechanical handling and structural integrity of coated foods which is crucial for extending the shelf-lifeof foods (Fematt-Flores GEet al, 2022).Beyond these, they act frequently as transporters of various active substances that have antibacterial, antioxidant and nutraceutical properties (Rodriguez-Aguiler and Oliveira, 2009). Edible packaging is cost effective as it utilizes less of the conventional packaging materials and also offering superior quality and maintaining freshness.

The primary drawback of edible films and coatings is that they cannot be utilized as the only packaging, additional non-edible packing is also needed for proper handling of foodstuffs. Protein based edible films and coatings are not as widely used in food packaging due to their low mechanical strength and poor resistance to water vapour.

Conclusion

Nowadays, milk protein based edible films and coating are very popular due to their eco-friendly behavior, inherent edibility, biodegradability properties, etc. These are showing great potential for enhancing food stability, preventing surface contamination during processing and storage. But this process yet to replace the conventional packaging system totally. Further research is needed to assess the method of edible films and coatings formation from milk protein for improving their properties and better applications.

References

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