Agriculture Reference
In-Depth Information
possible to conclude that the food in the package underwent a significant pH
change.
7.2.3.2 Detection of Gases
However, the most promising use of nanosensors for food packaging applications is
for the detection of gases. Excess of oxygen and moisture are the main causes of
food spoilage. Normally, due to the necessity of the destruction of the package to
analyze the vapor or gas content inside, packaged foods are tested randomly in
processing facilities, typically one in every 300-400 (Mills
2005
). This is costly and
time consuming and yet does not ensure that unsampled packages meet the required
standards (Luechinger et al.
2007
). The use of noninvasive gas sensing methods
based on nanoparticles permits to monitor the gas content of a package headspace
easily during their whole life span.
Many studies have focused on the development of this kind of nano-based
sensors with many different nanoparticles. One example of a sensor for moisture
content (Luechinger et al.
2007
) is based on copper nanoparticles with a carbon
coating dispersed on a tensile film. The swelling of the polymer matrix in humid
environments results in different degrees of separation between nanoparticles,
which cause sensor strips to absorb or reflect different colors. Also, composite
thin films with iron oxide nanoparticles have been used as conformable humidity
sensors (Taccola et al.
2013
). In this case, their sensitivity to humidity increased
with increasing nanoparticle concentration.
Other authors (Lee et al.
2005
; Mills
2005
; Mills and Hazafy
2008
) developed a
photoactivated indicator ink based on nanosized TiO
2
or SnO
2
particles and a
redox-active dye (methylene blue) for in-package oxygen detection. The detector
changes the color (from white to blue or black) in response to even small amounts
of oxygen. Quantification of the oxygen content is not possible with this sensor, but
it would provide the customers an easy and visual method to detect possible fails in
the seal integrity in modified atmosphere packages (MAPs). Another noninvasive
method for measuring CO
2
content in MAPs has been developed by von
B¨ ltzingsl¨wen et al. (
2002
). It is based upon the lifetime analysis of luminescent
dyes standardized by fluorophore-encapsulated polymer nanobeads with a wide
detection range of 0.8-100 % and a resolution of 1 %.
Other gases related to food safety and quality can also be detected with nano-
based sensors, like a series of electronic sensors with SnO nanobelts or ZnO-TiO
2
nanocomposites to detect volatile organic compounds (VOCs) like ethanol, ace-
tone, or carbon monoxide (Comini et al.
2006a
,
b
; Barreca et al.
2007
); detection of
gaseous amines (indicators of meat and fish spoilage) using fluorescence quenching
of nanofibrils of perylene-based fluorophores (Che et al.
2008
; Che and Zang
2009
)
or using conductance changes in composites of SnO
2
nanoparticles and TiO
2
nanorods (Zhang and Zhang
2008
). In addition, ethylene gas (indicator of the
fruit ripening) can be detected by WO
3
-SnO
2
nanocomposites (Pimtong-Ngam
