Agriculture Reference
In-Depth Information
antimicrobials directly to the food. The antimicrobials migrate selectively and gradu-
ally from the fi lm surface toward the surface of the food, and therefore maintain a
high concentration of antimicrobial at the food surface for extended exposure (Ouattara
and others 2000). Antimicrobial substances incorporated into edible fi lms can control
microbial contamination of fruits and vegetables by reducing the growth rate of target
microorganisms, or by inactivating microorganisms by direct contact.
Most of the existing methods for testing the antimicrobial activities of substances
require direct contact between the active agent and the microorganism (i.e., food), and
thus are not relevant to many commercial products in which there is little or no direct
contact between the food and the packaging material (RodrÃguez and others 2007).
Vapor phase tests, which are not direct contact assays, can be used to assess the pro-
tection provided by the antimicrobial volatile materials under no direct contact
conditions.
One advantage of essential oils is their bioactivity in the vapor phase, a character-
istic that makes them useful as possible fumigants for stored commodity protection.
The antimicrobial activity of essential oils by vapor contact was fi rst reported by
Kellner and Kober (1954). They studied the effect of 175 essential oils in the gaseous
state against eight airborne bacteria and fungi using an inverted petri plate technique
(Maruzzella and Sicurella 1960). A volatile compound contained in a cup or on a paper
disc was exposed to the inverted agar medium inoculated with a test organism. The
size of the growth inhibitory zone after incubation is used as the measure of vapor
activity. This technique is convenient for qualitative analysis, but not for quantitative
comparison of the vapor activity of essential oils (Inouye and others 2003).
For components to evaporate and be classed as volatile it is imperative that there
is a loss of weight over a time or temperature course (Fisher and Phillips 2008). The
evaporation of the essential oils is effected by external factors such as temperature,
humidity, concentration, and pressure (Aumo and others 2006). Volatile compounds
from plants usually have a relatively high vapor pressure and are capable of interacting
with an organism through the liquid and the gas phase (Fries 1973).
Storage temperature also infl uences the antimicrobial activity of chemical preserva-
tives. Generally, increased storage temperature can accelerate the migration of the
active agents in the fi lm/coating layers, and refrigeration slows down the migration
rate (Quintavalla and Vicini 2002).
Methods to Measure the Antimicrobial Activity of Edible Films
Plant essential oils are a potentially useful source of antimicrobial compounds that
can be incorporated into edible fi lms. Factors such as the composition and solubility
of the oil, bacterial strain, the sources of antimicrobial samples used, and the method
of growing and enumerating the surviving bacteria can infl uence the determination of
the antimicrobial activity of a plant oil (Friedman and others 2002; Zaika 1988).
Zone of inhibition assay (agar diffusion assay) is a commonly used method for the
measurement of antimicrobial activity of edible fi lms on solid medium. A recent study
on the contribution of the vapors to the antimicrobial effects in direct disc diffusion
method indicated that only the water-soluble components diffused across the agar
while the redeposition of the vaporized components on the surface of the agar
