Antimicrobial potato starch — based films incorporating lactic acid

The aim of the work was to produce and assess properties of biodegradable antimicrobial films based on potato starch, glycerin and lactic acid as functional additives for food packaging. Films were produced by the casting method from the solution: 2 % (w/v) starch dispersion was gelatinized (70 ± 1 °C, 30 min), glycerin (0.4 %) and lactic acid (1.0 %) were added, the mixture was degased, poured and dried (60 °C, 24 h), and then conditioned (48 h, 23 ± 2 °C). Thickness, mechanical and barrier properties as well as the antimicrobial activity were measured. The antimicrobial activity was determined by the disc diffusion method on the LB nutrient medium with Escherichia coli and Bacillus subtilis as test cultures, 30 µg tetracycline as a positive control and sterile filter paper disc as a negative control (comparison with free lactic acid). The films obtained were optically homogeneous, without macro defects, and had a thickness of 0.09–0.11 mm. Films demonstrated high tensile strength of 74.8 ± 7.4 MPa and elongation of 23.7 ± 4.6 % (n = 10). Water vapor transmission rate was 1290.7 ± 60.8 g · m^–2 · day, vapor permeability coefficient was 2.05 ´ 10^–4 g · m · m^–2 · day^–1 · Pa^–1 (90 % relative humidity of air). With that, in dry conditions (relative humidity of about 0 %), quite low gas permeability in terms of O₂/CO₂ was recorded (lower than the threshold of the calculation of the stationary permeability). Analysis of the antimicrobial activity by the disc diffusion method showed that the films formed inhibition zones (E. coli: 10.67 ± 1.53 mm; B. subtilis: 10.67 ± 0.58 mm) that were comparable with free lactic acid (9.67 ± 0.58 and 12.00 ± 1.00 mm, respectively). The combined results confirm that starch films with lactic acid have high barrier properties (in the dry phase) and antimicrobial activity (upon contact with the moist surface), which make them promising for using as active packaging for chilled meat and dairy products and as a functional layer in multi-layer films.

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