Eco-friendly Packaging:Today's Need

     Usage of non-biodegradable materials for the various packaging applications has raised environmental pollution concerns. Food packaging accounts for the biggest growing sector within the synthetic plastic packaging market domain. Large amounts of different materials, like paper, glass and plastics, are used globally to manufacture packaging materials and more than two thirds are used in the food sector alone. This amount is growing unceasingly as a result of changes occurring in habits of food preparation and consumption, as well as the positive development of various areas and markets in the world. The packaging industry consumes the highest volumes of plastics produced globally and is the main source supplying waste plastics into the environment at an alarming rate. This can be attributed to single use plastics and the increase of on the go snacks and readymade meals that imply the once-off use of durable plastic packaging material. As a result, there is increasing need for eco-friendly sustainable packaging materials with the desired physical, mechanical and barrier properties for food packaging.

 

  

Biodegradable Food Packaging

The food packaging industry is now in pursuit of biodegradable packaging that is lightweight  for reducing materials use, waste and as well as transportation costs. Plastics from biopolymers are promising to fulfill this requirement. Biodegradation is a process of defragmentation, initiated by heat, moisture, and/or microbial enzymes, which transforms larger molecular substances into smaller compounds. It can also be simply defined as a process by which substances are broken down by living organisms. 

Due to the increasing environmental concerns created by excessive plastic accumulation, interest has shifted towards the development of such packaging materials that not only improve performance but are also easy to recycle and reuse i.e., “bio-plastics”.  According to the European Bioplastics organization, bioplastics can be defined as plastics based on renewable resources or as plastics which are biodegradable and/or compostable polymers. Bioplastics are derived from different renewable sources such as vegetable oil, corn starch, potato starch, fibers obtained from pineapple, jute, hemp, henequen leaves and banana stems and also from used plastic bottles and other containers using microorganisms.

 

Natural polymers or polymers derived from biomass

The natural polymers are derived from animal, marine, and agricultural sources, which include the polysaccharides, such as starch, cellulose, chitosan, gums etc., proteins like plant derived proteins (zein, gluten, soy, etc.) and animal extracted proteins (casein, collagen, gelatin, etc.) and lipids including cross linked triglycerides. By nature most of these polymers are hydrophilic and crystalline in nature, which create several problems while processing in moist food packaging. However they have excellent gas barrier properties which make them acceptable for their utilization in food packaging.

a)       Starch: Starch is the most abundant commonly used renewable raw material and easy biodegradable natural resource. It is obtained from seeds, corn, wheat, rice, potato, sweet potato, and cassava. Starch is usually used as a thermoplastic and constitutes a substitute for polystyrene (PS). It is plasticized through destructuration in presence of specific amounts of water or plasticizers (glycerol, sorbitol) and heat and then it is extruded. Starch is an attractive material for packaging applications because of its relatively low cost, availability, and biodegradability. Starch having poor resistance to moisture and their poor mechanical property restricts their use. Therefore to improve these properties starch is blended with various biopolymers and certain additives.

b)      Cellulose: Cellulose is the most abundant natural polymer and is derived by a delignification from wood pulp or cotton linters. Cellulose is very difficult to use in packaging because it is hydrophilic and crystalline in nature possessing poor mechanical properties in its raw form. Therefore, it must be treated with chemicals like NaOH, H2SO4, CS2, etc. to produce cellophane having excellent mechanical characteristics. Cellulose derivatives can be produced by derivatization of cellulose from the solvated state, via esterification or etherification of hydroxyl group. Cellulose derivative forms are used for films or edible coatings are Hydroxypropyl cellulose, hydroxypropyl methylcellulose, Carboxymethyl cellulose or Methyl cellulose.  Incorporation of hydrophobic compounds is one method for increasing the moisture barrier, such as fatty acids into the cellulose ether matrix to develop a composite film.

c)       Chitosan or chitin: Chitosan or chitin, is the second abundant polysaccharides resource after cellulose found in nature. It naturally appears in the exoskeleton of arthropods and in the cell walls of yeasts and fungi. It is produced commercially by chemical extraction processes from prawns and crabs wastes. Chitosan is obtained from deacetylation of chitin, and different factors (e.g. alkali concentration, incubation time, and chitin to alkali ratio, temperature and chitin source) can affect its properties. Chitosan forms films without the addition of additives, exhibits good carbon dioxide and oxygen permeability, as well as excellent mechanical properties and antimicrobial properties which reduces the oxidation process and is beneficial for increasing the shelf life and quality of food products.

d)      Proteins: Proteins are complex structures made up of amino acids and can be obtained from plant (wheat gluten, corn, zein, soy protein etc.) and animal (casein, whey, keratin, gelatin, etc.) sources. They are highly desirable to modify the required characteristics of packaging materials due to the presence of unique side chain in their structure. Due to the renewable nature, biodegradability and their excellent gas barrier properties proteins and protein based materials find their use in many industrial applications. But they are adversely affected by their hydrophilic nature like starch-based polymers. Therefore, they need to be blended with other polymers or must be chemically or micro-biologically modified.

i.         Casein is a milk derived protein, when processed with suitable plasticizers at temperature of 80-100 ⁰C, form materials with mechanical performance varying from stiff and brittle to flexible and tough performance. Casein films have an opaque appearance. Irrespective of its relatively high price, it is used today for bottle labeling because of its excellent adhesive properties.

ii.       Gluten plastics exhibit high gloss and show good moisture resistance under certain conditions. They do not dissolve in water, but absorb some water on immersion.

iii.    Soya proteins are commercially available as soya flour, soya concentrate and soya isolate. Soya protein isolate (SPI) may be used to prepare edible and biodegradable packaging films. The films obtained from SPI exhibit excessive friability, so their performance is limited. In order to improve them, they must be modified by the addition of a plasticizer, such as glycerol.

The cheapest protein, keratin extracted from waste streams such as hair, nails and feathers. Keratin the most difficult protein to process due to its structure and a high content of cysteine groups. On the other hand, whey proteins, byproducts from the cheese industry, are widely employed as edible films and coatings. Several lipid components like fatty acids, natural waxes, resins and vegetable oils are generally incorporated in the films to provide hydrophobicity so that moisture barrier properties can be improved.

 Synthetic polymers

They are produced from classical chemical synthesis from bio-based monomers. In this category, polylactic acid (PLA) is one of the most commercially available and exploited bioplastics.

a)       Polylatic acid (PLA): PLA one of the most promising and biodegradable polyester made from renewable resources such as corn, sugar beets, and potato starch for commercial use as a substitute for high density polyethylene (HDPE) and low density polyethylene (LDPE), polystyrene (PS) and polyethylene terephthalate (PET). It is obtained by conversion of corn, or other carbohydrate sources, into dextrose, followed by fermentation into lactic acid. Through direct polycondensation of lactic acid monomers or through ring-opening polymerization of lactide, PLA pellets are obtained. The processing possibilities of this transparent material are very vast, ranging from injection molding and extrusion over cast film extrusion to blow molding and thermoforming. PLA is becoming an advancing alternative as a green food packaging material because it was found that in many circumstances its performance was better than synthetic plastic materials. PLA comes in the form of films, thermo-formed cups and trays, containers and coatings for paper and paper boards etc.

Applications of bioplastics in food packaging

Among the extensively used bio-based plastics, PLA is widely used. Moreover, the bioplastics nowadays have found applications for both short-shelf life products like fresh fruits and vegetables and long shelf life products, like potato chips and pasta.