NEW ACTIVE PACKAGING MATERIALS BASED ON PROPOLIS

Abstract

The use of propolis as an active compound, successfully exploited primarily in the pharmaceutical field, was intended in this work to set up natural packaging materials. To achieve this goal, coating techniques were used for the deposition of active layers on plastic films and cellulosic sheets. The stability of the so obtained active coatings was evaluated according to the polyphenols degradation over time. [PUBLICATION ABSTRACT]

Headnote

ABSTRACT

The use of propolis as an active compound, successfully exploited primarily in the pharmaceutical field, was intended in this work to set up natural packaging materials. To achieve this goal, coating techniques were used for the deposition of active layers on plastic films and cellulosic sheets. The stability of the so obtained active coatings was evaluated according to the polyphenols degradation over time.

Key words: Active packaging, antioxidant activity, coating, propolis.

INTRODUCTION

Propolis, a natural resinous substance collected by honeybees, is well-known for its several properties such as antimicrobial (Katircioglu and Mercan, 2006), antioxidant (Jasprica et al., 2007) and anti-inflammatory (Burdock, 1998). These properties are due to its unique composition: propolis is composed by a pool of molecules that act synergistically. It usually contains resins (50%), flavonoids and phenolic acids, essential oils (10%), pollen (5%) and various inorganic compounds (5%) such as Fe and Zn, vitamins (B1, B2, B3 and B6), fatty acids, esters, quinones, steroids and sugars as well (Pastor et al, 2010). Recent studies have paved the way for potential applications of propolis also within the food and food packaging fields, e.g. to control primary factors of antimicrobial degradation and oxidation (Pastor et al., 201 1; Mascheroni et al., 2010). The use of propolis as an active compound could be helpful to extend the shelf life of perishable foods. Towards this goal, innovative packaging characterized by 'active features' provided by propolis were developed.

MATERIALS AND METHODS

Three active coatings were prepared according with the following different formulations: a) hydroalcoholic (60 wt% ethanol) solution of propolis (81.5%), gelatin (10%), glycerol (7.5%) and lipids (1%); b) hydroalcoholic (60 wt% ethanol) solution of propolis (96.5%) and cellulose nanocrystals (3.5%) and c) hydroalcoholic (60 wt% ethanol) solution of propolis.

The coating technique used is described hereinafter: an aliquot of the a) and b) solutions were deposited separately by roll-coating onto the corona-treated side of 12 ± 0.5 µp? thick poly(ethylene terephthalate) (PET) films (Toray Saehan, Kyungbuk, South Korea) and another aliquot of the c) solution onto a commercial paper samples by use of an automatic applicator (ref 1 137, Sheen Instruments, Kingston, UK) equipped with a steel horizontal wire-wound rod, at a constant speed of 150 mm-s-1, according to ASTM D823-07 - Practice C. Coatings were firstly dried using a constant and perpendicular flux of mild air (25.0 ± 0.30C) at a distance of 40 cm from the applicator for 2 min, followed by overnight storage at room temperature.

Finally, the samples were stored in darkness at room temperature for two months in air and in 100% nitrogen conditions. Both antioxidant activity and polyphenols concentration were assessed by means of the DPPH* assay and HPLC method, respectively, at regular time spans (10, 30, and 45 days).

For the determination of the antioxidant activity of the samples, a UV-Vis spectrophotometer Lambda 25 (PerkinElmer, USA) was used. The decrease in DPPH* concentration was measured by monitoring the decrease in the absorbance continuously at 517 nm during 30 min. Aliquote of 40 µL of each sample extracted with methanol and 2.96 mL of the DPPH* 90 µM solution were placed in each test tube in triplicate. A blank with methanol and the DPPH* solution was included.

Polyphenols were extracted with methanol and detected using a HPLC-LC Module I plus (Waters Corporation, Milford, Massachusetts, USA) system equipped with a Waters Symmetry C 18 Column, W486 Tunable Absorbance Detector as UV detector and a W715 Ultra Wisp autoinjector. Quantification was performed by integration of peak areas, with reference to calibrations done while using known amounts of pure compounds. The gradient profile was formed using solvent A (Water plus 0. 1% trifluoracetic acid) and solvent B (Acetonitrile plus 0. 1% trifluoracetic acid) in the following program: the initial proportion of B (20%) remained for the first six minutes, increased from 20% to 30% over the next 4 min, increased to 40% over the next 30 min, increased to 60% over the next 5 min, increased to 100% over the next 5 min, returned to the initial conditions over the next 3 min and finally remained at the initial conditions for five minutes. The flow rate was 1.2 ml/min, and the column temperature was 30°C.

RESULTS AND CONCLUSIONS

The analyses revealed the same results for plastic films coated with gelatin and cellulose nanocrystals: no significant degradation of polyphenols occurred when propolis was loaded as coating on commercial plastic films (PET) and stored in air and in 100% nitrogen conditions for 45 days in darkness at room temperature (Table 1). In the case of commercial paper coated with propolis (Table 1) there was found a degradation of 50% of polyphenols. The correlation between the concentration of polyphenols that remain stable and the antioxidant activity (evaluated by means of a linear regression) (Fig. 1) resulted in good linearity with a R2 coefficient higher than 0.92. Therefore, it can be stated that polyphenols are responsible for the antioxidant activity of this coating with propolis. Moreover, no differences in losses between samples stored in air and in 100% nitrogen conditions (Table 1) apparently indicate that the oxygen is not the primary factor of polyphenols degradation for these active materials.

These results confirm that propolis can be used successfully in a coating of gelatin or cellulose nanocrystals onto plastic films, whereas coating of propolis directly on cellulosic materials is not suitable as a packaging material because of lack of stability of the polyphenols fraction. In fact, in PET coatings there were no significant losses of polyphenols, while in paper coating samples high degradation of polyphenols occurred. The main reason could lie in the interaction between polyphenols and the chemical compounds of the paper sheets.

References

REFERENCES

Burdock, G.A. 1998. Review of the biological properties and toxicity of propolis. Food Chemistry and Toxicolology 36: 341-363.

Jasprica, I., Bojic, M., Mornar, A., Besic, E., Bucan, K., Medic-Saric, M. 2007. Evaluation of antioxidative activity of Croatian propolis samples using DPPH· and ABTS·+ stable free radical assays. Molecules 12: 1006-1021.

Katircioglu, H., Mercan, N. 2006. Antimicrobial activity and chemical compositions of Turkish propolis from different region. African Journal of Biotechnology 5: 1151-11 53.

Mascheroni, E., Guillard, V., Nalin, F., Mora, L., Piergiovanni, L. 2010. DlffusMty of propolis compounds in Polylactic acid polymer for the development of anti-mlcrobial in packaging films. Journal of Food Engineering, volume 98(3): 294-301.

Pastor, C, Sánchez-González, L., Chafer M., Chiralt A., González-Martínez, C. 2010. Physical and antifungal properties of hydroxypropylmethylcellulose based films containing propolis as affected by moisture content. Carbohydrate Polymers 82: 1 174-1 183.

Pastor, C, Sánchez-González, L., Marcilla, ?., Chiralt, A. Chafer, M., Gonzalez-Martinez, C. 201 1. Quality and safety of table grapes coated with hydroxypropylmethylcellulose edible coatings containing propolis extract. Postharvest Biology and Technology 60: 64-70.