Pullulan – A Valuable Edible Coating For Fruit & Meat

Most of us are not that familiar with pullulan but we may have eaten it at some point because it is nowadays used as an edible coating of fruit to prolong their shelf-life. The coating is labelled as E1204 on any ingredient list. It is safe to use showing no mutagenicity or carcinogenic effects.

A fungus called Aureobasidium pullulans produced the polysaccharide from starch (Diab et al., 2001; Cheng  et al., 2011). The fungus avoids drying and dessication by producing pullulan which reduces moisture loss but still permits movement of materials into and out of the cell.

It is a polysaccharide based on linearly linked alpha-trisaccharide units of maltotriose. It is also known chemically as α-1,4- ; α-1,6-glucan whereby three glucose sugar units are connected by an α-1,4 glycosidic bond. Consecutive maltotriose units are connected to each other with an α-1,6 glycosidic bond. The degree of pullulan polymerization varies between 100 to 5000 alpha-glycopyranoside units which means its molecular weight can vary from 10³ to 10⁶ a.m.u.

There is a similarity in structure with beta-glucan but they have widely different properties.

Compared to other edible films it has a number of distinct advantages:- its colourless, tasteless, resists oils, highly soluble in water and has a number of barrier properties including low oxygen permeability (Singh et al., 2008). In commercial terms, the barrier properties are exploited in the prevention of rancidity of fat based food products (Duncan, 2011) and it is in the field of nanotechnology that some of the most pertinent applications are to be found.

Preparation Of A Pullulan Coating

A pullulan coating is prepared by creating a 10% w/v solution of the polysaccharide in water at 80 °C. Glycerol is added to 3% w/v/ plasticize the coating with xanthan coating at 0.1% w/v to help stabilise the matrix. Whan all the components are dissolved and mixed together at ambient for up to 6 hours, the whole solution is autoclaved at 121 °C for 15 minutes. Sterilised components are added to the solution.

Applications

• Antimicrobial

Pullulan can be infused with nanoparticles and various antimicrobial essential oils which have been demonstrated to be effective against a range of microbes. Pullulan film containing a partially purified lysozyme that showed antimicrobial activity against Escherichia coli (Kandemir et al., 2005).  Sakacin A–containing pullulan films were used to control the growth of L. monocytogenes which demonstrated an additional benefit, the applicability of active pullulan films in delivering a bacteriocin directly to a food surface (Trinetta et al., 2010). Recently, a pullulan coating infused with nanoparticles and antimicrobials was effective at killing various micro-organisms including Staphylococcus aureus, Salmonella Typhimurium, L. monocytogenes, and Escherichia coli O157:H7 on meat surfaces and poultry inoculated with these bacteria (Morsy et al., 2014). The same type of coating works equally well at protecting eggs  during storage at 25 °C.  The egg’s shelf-life was increased by 2 weeks without impacting other measureable parameters (Morsy et al., 2015).

• Delayed Ripening

Ripening can be delayed by coating fruit with pullulan for example. Chemical modification of pullulan by esterification with n-octenyl succinic anhydride (Nirali et al., 2016) can reduce water permeability by up to 30% with improvements to the coating properties by reducing the tensile strength and extensibility. In this example, the researchers coated Sapota (Manilkara zapota) fruit with various edible films including unmodified pullulan and a coating with emulsified palm oil and pullulan. The chemically modified coating delayed both ripening and senescence which would be a major benefit when bringing sensitive fruits to market

References

Cheng, K., Demirci, A., Catchmark, J.M. (2011) Pullulan: biosynthesis, production, and applications. Appl. Microbiol. Biotechnol. 92: pp. 29–44

Diab, T., Biliaderis, C.G., Gerasopoulos, D., Sfakiotakis, E. (2001) Physicochemical properties and application of pullulan edible film and coatings in fruit preservation. J. Food Sci. Agric. 81 pp. 988–1000.

Duncan, T.V. (2011) Applications of nanotechnology in food packaging and food safety: Barrier materials, antimicrobials and sensors. J. Colloid Interface Sci. 36: pp. 1–24

Gniewosz, M., Synowiec, A. (2011) Antibacterial activity of pullulan films containing thymol. Flavor Frag. J. 26 pp. 289–395.

Kandemir, N., Yemenicioglu, A., Mecitoglu, C., Elmaci, Z.S., Arslanoglu, A., Oksungur, Y. (2005) Production of antimicrobial films by incorporation of partially purified lysozyme into biodegradable films of crude exopolysaccharides obtained from Aureobasidium pullulans fermentation. Food Technol. Biotechnol. 43: pp. 343–50.

Morsy, M.K., Khalaf, H.H., Sharoba, A.M., El-Tanahi, H.H., Cutter, C.N. (2014) Incorporation of essential oils and nanoparticles in pullulan films to control foodborne pathogens on meat and poultry products. J. Food Sci. 79(4):M675–84

Shah, N.N., Vishwasrao, C., Singhal, R.S., Ananthanarayan, L. (2016) n-Octenyl succinylation of pullulan: Effect on its physico-mechanical and thermal properties and application as an edible coating on fruits. Food Hydrocolloids, 55, pp. 179

Singh, R.S., Saini, G.K., Kennedy, J.F. (2008) Pullulan: Microbial sources, production and applications. Carbohydr. Polym. 73: pp. 515–31

Trinetta, V., Floros, J.D., Cutter, C.N. (2010) Sakacin A-containing pullulan film: an active packaging system to control epidemic clones of Listeria monocytogenes in ready-to-eat foods. J. Food Safety 30 pp. 366–81