Investigation of mechanical, physical and antibacterial gelatin film incorporated by Turmeric (Curcuma longa) hydrosol

Nowadays, edible packaging founds great attention due to environmental issues related to synthetic packaging materials. Gelatin is one of the most commonly ingredients used in edible packaging formulation due to its good barrier properties against gases and UV irradiation. Addition of plant extracts or essential oils is one approach for confer antimicrobial activity to edible films. The aim of this study was to use different percentages (5-100%) of turmeric hydrosol in the formulation of edible gelatin films and assessing the various properties of the resulted film.

Extraction of turmeric hydrosol was done by Clevenger apparatus. Separation of the essential oil from hydrosol was done by a separating funnel. For preparation of gelatin films, 4g gelatin was dissolved in 100ml distilled water, and glycerol as plasticizer was added at ratio of 25w/w gelatin. In gelatin films included turmeric hydrosol, substitution of turmeric hydrosol instead of distilled water in the ratio of 5-100% was performed. Then, the mixture was poured in plexiglass plates (80×120 mm) and dried at 22ºC, 50% RH for 48h.Texture analysis was carried out using the texture analyzer TA-XT plus. The water vapor permeability (WVP) was measured using gravimetrically method. Light transmission was determined by spectrophotometer jenway 6305 at 200-800 nm wavelength. Morphological assessment was performed by scanning electron microscope (phenom proX). For Fourier-transform infrared spectroscopy analysis, Perkin-Elmer (Spectroma2 model) at 4000-4500 cm-1 frequency, used. Antimicrobial test was done by measurement inhibitory zone (mm) of edible films against Staphylococcus aureus (PTCC 1112), Staphylococcus saprophyticus (PTCC 1440) and Staphylococcus epidermium (PTCC 1435).
The studied bacteria (Staphylococcus aureus, Staphylococcus saprophyticus and Staphylococcus epidermium) were purchased from the Iranian Scientific and Industrial Research Center and transferred to the suitable medium in sterile condition and incubated at 37°C for 32 hours. Microbial cells were harvested by centrifugation at 4000 rpm. For the estimation of microbial population, McFarland turbidity method was applied. First, the results from comparing turbidity showed the population of microorganisms was equal to 0/5 McFarland solution (approximately 1/5×108 CFU/ml), then for reaching to the desired microbial population (1/5 × 106 CFU/ml), dilution with physiological saline was done.  Agar disc diffusion method was used for assessing the antimicrobial effect of the gelatin films.  
Gelatin films were cut into a 10mm diameter in aseptic condition using a circular knife and then placed on agar plates inoculated with 100µL of tested bacteria (with approximately 106 CFU/mL).  The plates were then incubated (37ºC, 24h). The diameter of the inhibition zone was precisely measured using a digital micrometer (Guanglu model 701-211). Each experiment was performed triplicate.

 The results of this study showed that the use of turmeric hydrosol causes significant difference on mechanical and physical properties of related gelatin films (p≤0.01). The stress tension was increased by adding hydrosol to the edible film formulation, but the elongation percentage of the edible film decreased. The highest the stress tension was observed in sample containing 100% hydrosol (33.3 Mpa) but the lowest elongation percentage also was belong to this sample (96%). Addition of hydrosol to film formulation cause increasing of UV-radiation barrier property and reducing the transparency of the film. FTIR analysis of gelatin films included turmeric hydrosol, exhibited the existence of aromatic bonds according to appearing peak at 680cm-1. 
Structural studies by SEM method, showed uniformity in the structure of different gelatin films and hydrosol addition cause minor changes in the structure of films. The water vapor permeability was influenced by the edible film formulation and by adding different percentages of turmeric hydrosol, the permeability to water vapor, decreased (p≤0.01).Thickness was also affected by gelatin formulation and hydrosol caused reduction the thickness. Antimicrobial assessments showed that the addition of turmeric hydrosol cause inhibitory effects against studied staphylococcus strains. Staphylococcus aureus and Staphylococcus saprophyticus showed the highest susceptibility to hydrosol included films ( inhibition zone equal to 37 mm for gelatin film included 100% turmeric hydrosol).