Agriculture Reference
In-Depth Information
The chemical nature of the essential oils also plays an important role in the barrier
properties of edible fi lms. Differences observed in these properties can be explained
by the hydrophobicity of the plant essential oils. Carvacrol, a phenolic compound
containing an alcohol group in its chemical structure, seems to be a good barrier
compared to aldehyde compounds (e.g., cinnamaldehyde, citral) because the hydroxyl
group has less affi nity for water than for the carbonyl groups. Carvacrol then offers
the possibility not only to enhance antimicrobial effi ciency but also to improve water
barrier properties of edible fi lms.
McHugh and Senesi (2000) suggested that lipids with lower melting points, such
as vegetable oil, oleic acid, and myristyl alcohol, exhibit superior barrier properties
presumably due to their smooth structure and lack of channels between crystalline
platelets through which water could migrate easily. The incorporation of emulsion
droplets in the fi lm increases the distance traveled by water molecules that diffuse
through the fi lm, thereby decreasing water vapor permeability (McHugh and Krochta
1994c ).
McHugh and others (1996) demonstrated that apple-based edible fi lms are excellent
oxygen barriers, particularly at low-to-moderate relative humidities. An apple purée
edible fi lm was a good oxygen barrier, exhibiting low oxygen permeability values of
22.6
m/m
2
-d-kPa. The oxygen permeability values of this fi lm increased as
higher amounts of plant essential oils were incorporated. McHugh and Krochta (1994a)
indicated that fi lms containing lipids exhibit relatively poor oxygen barrier properties.
The oil chemical nature plays a major role in the barrier properties of edible fi lms.
Lower oxygen permeability was observed in fi lms that contained oregano, lemongrass,
and cinnamon oils than in those that contained its antibacterial compounds carvacrol,
citral, and cinnamaldehyde, respectively (Rojas - Gra ü and others 2006 , 2007a ).
Tensile strength is one of the most common indicators of the mechanical property
of an edible fi lm. It expresses the maximum stress developed in a fi lm specimen during
tensile testing (Gennadios and others 1994). The incorporation of plant essential oils
in apple-based edible fi lms caused a signifi cant increase in tensile strength, % elonga-
tion, and elastic modulus of the fi lm. These differences could be related to differences
in their polarities. These results are in agreement with those obtained by Pranoto and
others (2005), who studied the physical and antibacterial properties of alginate edible
fi lm with garlic oil. Elongation at break is a measure of the fi lm stretchability prior
to breakage (Krochta and De Mulder-Johnston 1997). Zivanovic and others (2005)
studied the antimicrobial and physicochemical properties of polysaccharide (chitosan)
fi lms enriched with essential oils. They observed a decrease in tensile strength and an
increase in elongation percentage when the essential oils were introduced into the
fi lms. This behavior also was observed by Bégin and Van Calsteren (1999).
1.3 cm
3
±
μ
Evaluation of Antimicrobial Activity of Volatile Components
The growth of microorganisms on the surface of a food is a key factor affecting the
safety and/or spoilage of food products (Padgett and others 1998). The direct addition
of an antimicrobial additive into foods might reduce its effectiveness, due to the pres-
ence of substances that interact with it, to reduce its antimicrobial effect (Durango
and others 2006). The use of antimicrobial fi lms could be more effi cient than adding
