*3.3. Grammage, Thickness, Mechanical and Optical Properties of The Films*

The grammage and thickness were significantly higher (*p*-value < 0.05) in the films incorporating licorice EO casted on the lotus negative template (Table 1), which is probably due to the imprisonment of air microbubbles during the casting. The film-forming solution must fill all the holes of the lotus negative template, which are full of air that can become entrapped in the film matrix. Moreover, the incorporation of the EO in the films most likely increases their apparent density, which is also reflected in higher values of grammage.



(Results expressed as mean ± SD) a, b, c and d correspond to each type of film; Significant *p*-values are highlighted in bold.

Concerning the mechanical properties, zein films are brittle, and thus, plasticizers are needed to improve their flexibility [30]. In general, tensile strength and elongation are key factors in packaging materials, which keep their integrity during packaging, transport, storage and sales [7]. The tensile strength of the films casted on lotus negative template was significantly lower (*p* < 0.05) when compared to the films casted in simple polystyrene Petri dishes (Table 1), incorporating (or not) licorice EO. This means that it is not the licorice EO that affects tensile strength, but the support material where the films were casted, probably because of the same effect of air microbubble entrapment, as explained above.

Nevertheless, elongation and elastic modulus were similar within all the films (Table 1). Finally, the mechanical properties of the zein-based films developed in this work, using glycerol as plasticizer, are very close to the ones obtained previously for zein films using oleic acid as the plasticizer agent [29,30], but improved upon than the ones obtained using other plasticizers (buriti oil, macadamia oil and olive oil) [29].

The optical properties (L\*, a\* and b\*) of the films with EO were only slightly affected (Table 1), since zein is a yellow powder and the licorice EO is yellow-brownish. On a previous work, the licorice EO was incorporated in carboxymethyl xylan films (white) and the optical properties were significantly changed by the EO, particularly transparency [13]. In the present work, the color of the EO is not a problem since zein is also yellow.
