Note: Descriptions are shown in the official language in which they were submitted.
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FOOD FRESHNESS SENSOR
Filed of Invention
The present invention generally relates to pathogen detection devices and
methods, and, in particular, to devices and methods for detecting food-borne
pathogens and spoilage.
Background
Food borne diseases as well as food spoilage remain a significant burden in
the global food supply. In the U.S. alone there are 76 million cases of food-
borne
illnesses annually, which is equivalent to one in every four Americans,
leading to
approximately 325,000 hospitalizations and over 5000 deaths annually.
According
to the United States Government Accounting Office (GAO) and United States
Department of Agriculture (USDA), food-borne pathogens cause economic losses
ranging from $7 billion to $37 billion dollars in health care and productivity
losses.
Hazard Analysis and Critical Control Point (HACCP) regulations state that a
hazard
analysis on a food product must include food-safety analyses that occur
before,
during, and after entry into an establishment. There is a clear need to ensure
that
food transported from the processor to the consumer is as safe as possible
prior to
consumption. For example, the development of antibiotic resistance in food
borne
pathogens, the presence of potential toxins, and the use of growth hormones,
all
indicate a need for further development of HACCP procedures to ensure that
safer
food products are delivered to the consumer. There is also a need to monitor
foods
being handled by a consumer even after such food is purchased, partially used,
and
stored for future use.
Meat, for example, is randomly sampled at a processor for food borne
pathogens. Generally, no further testing occurs before the meat is consumed,
leaving the possibility of unacceptable levels of undetected food-borne
pathogens,
such as Salmonella spp. and Listeria spp., as well as spoilage bacteria, such
as
Pseudomonas spp. and Micrococcus spp. being able to multiply to an undesirable
level during the packaging, transportation, and display of the product.
Subsequently,
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the food product may be purchased by the consumer, transported, and stored in
uncontrolled conditions that only serve to exacerbate the situation, all these
events
occurring prior to consumption.
Retailers generally estimate shelf life and thus freshness with a date stamp.
This method is inaccurate for at least two reasons: first, the actual number
of
bacteria on the meat at the processor is typically unknown, and second, the
actual
time-temperature environment of the package during its shipment to the
retailer is
typically unknown. As an example, a temperature increase of less than 3 C can
shorten food shelf life by 50% and cause a significant increase in bacterial
growth
over time. Indeed, spoilage of food may occur in as little as several hours at
37 C
based on the universally accepted value of a total pathogenic and non-
pathogenic
bacterial load equal to 1x10' cfu/gram or less on food products. Food safety
leaders
have identified this level as the maximum acceptable threshold for meat
products.
While many shelf-life-sensitive food products are typically processed and
packaged at a central location, this has not been typical for the meat
industry. The
recent advent of centralized case-ready packaging as well as "cryovac"
packaging
for meat products offer an opportunity for the large-scale incorporation of
sensors
that detect both freshness and the presence of bacteria.
A number of devices are known that have attempted to provide a diagnostic
test that reflects either bacterial load or food freshness, including time-
temperature
indicator devices. To date, none of these devices has been widely accepted
either in
the consumer or retail marketplace, for reasons that are specific to the
technology
being applied. First, time-temperature devices only provide information about
integrated temperature history, not about bacterial growth. Thus it is
possible,
through other means of contamination, to have a high bacterial load on food
even
though the temperature has been maintained correctly. Wrapping film devices
typically require actual contact with the bacteria. If the bacteria are
internal to the
exterior food surface, then an internally high bacterial load on the food does
not
activate the sensor. Ammonia sensors typically detect protein breakdown and
not
carbohydrate breakdown. Since bacteria initially utilize carbohydrates, these
sensors typically have a low sensitivity in most good applications, with the
exception
of seafood.
Further, known devices and methods for detecting bacteria in food
substances typically integrally incorporate the device in to a package at
manufacture.
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Neither the provider nor the consumer is able to continue the monitoring with
a
repackaging of the food product. It is desirable to provide a device, food
packaging,
and associated methods for detecting at least a presence of bacteria in a
perishable
food product. Further, it is desirable for a consumer to be able to detect a
presence
of bacteria throughout the handling of the food product by the consumer.
Summary of the Invention
The present invention may be directed to detecting at least a presence of
bacteria in a perishable food product carried within a container or package
prepared
by a supplier of the food product or by a consumer handling the food product
after
purchase. Embodiments of the invention may provide a quantitative measure of
bacterial load and detect the presence of bacteria in or on the food product.
In
addition, a sensor according to the teachings of the present invention may be
safely
consumed if mistakenly eaten.
One sensor for detecting a presence of bacteria in a perishable food may
include a pH sensitive solution of bromothymol blue and methyl red mixed with
an
alkaline solution, by way of example, resulting in a pH value and a generally
green
color changing to a generally orange color responsive to exposure to a
concentration
of carbon dioxide. The solution is packaged in a gas permeable container using
a
TPX (PMP) thin film that allows an effective diffusion of carbon dioxide
through the
container. The pH level drops when acidic carbon dioxide comes into contact
with
the solution resulting from a formation of carbonic acid making the solution
an
indicator of carbon dioxide concentration and thus bacterial growth.
Another embodiment may include a sensor for detecting a presence of
bacteria from a perishable food product, wherein the sensor may include a
sealed
container having a gas permeable wall formed from a TPX (PMP) thin film and a
transparent portion for viewing its contents. A pH sensitive solution is
carried within
the container and may have a generally green color changing to a generally
orange
color responsive to a 0.5% concentration of an acidic gas generated outside
the
container in a bacteria detection range between one million and ten million
bacteria.
The pH sensitive solution may be carried between first and second gas-
permeable
wall portions of the container for permitting a desirable diffusion of the
carbon dioxide
between the wall portions.
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A sensor may also include a pH sensitive mixture carried within a container
with the mixture including bromothymol blue and methyl red mixed with an
alkaline
resulting in a pH value between 6 and 8. Yet further, the sensor may include
the pH
sensitive mixture of bromothymol blue and methyl red mixed with an alkaline
resulting in a generally green color changing to a generally orange color
responsive
to exposure to a 0.5% concentration of an acidic gas, wherein the bromothymol
blue
comprises a%wt/volume between 0.02 and 0.08, the methyl red comprises a
%wt/volume between 0.001 and 0.005, dissolved in an alkaline amount ranging
between 0.5 mM and 1.5 mM.
One embodiment of the invention may comprise an aqueous pH indicator in a
gas permeable envelope such that C02 gas (produced by bacteria as they grow)
diffuses into the container and reacts with the solution to reduce the pH:
CO2 +H2O H H2CO3 H H+ + CO3 --
As the pH of the aqueous solution drops, due to the formation of carbonic
acid, the pH indicator changes color thereby providing a visual indication of
the drop
in pH and therefore the presence of bacteria.
Extensive research and development has resulted in a desirable format for
one embodiment of the invention including a sensor. In order to maximize the
diffusion of carbon dioxide into the sensor, a two-sided design was selected
that
permits diffusion of gas from both sides of the sensor. This permits a rapid
color
change that minimizes the time a sensor is in an "uncertain zone," where color
changes are gradual and not produced in a step-styled change as is the case
for
embodiments of the present invention. To further improve free diffusion of gas
to
both sides of the sensor, it may also be desirable to place the sensor in a
spaced
relation to a wall of a food package in which the food product is carried.
Each component was selected and optimized to achieve the highest
performance and longest shelf life at the lowest cost to manufacture. The
sensor
may comprise:
1. pH indicators and an initial pH of the sensor solution;
2. A thin permeable film to enclose the solution; and
3. Manufacture of the sensor through a sealing of the solution between two
layers of the film.
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Brief Description of the Drawings and Photographs
Features and benefits of the present invention will become apparent as the
description proceeds when taken in conjunction with the accompanying drawings
5 and photos in which:
FIG. 1 is a diagrammatical cross section view of embodiments of the invention
useful in detecting spoiling of a food product;
FIG. 2 is a partial cross sectional view of one embodiment of a sensor in
keeping with the teachings of et present invention;
FIG. 3 includes a spectrum (360-720nm) of a solution of a pH formulation at
room temperature at day one (hashed plot) and day sixty (solid plot)
reflecting
excellent shelf life of the formulation; and
FIG. 4 is a table illustrating an effect of incubation of skinless chicken
that had
been cooked or was raw then stored at 10 C on biochemical and microbiological
parameters.
Detailed Description of the Preferred Embodiments
The present invention will now be described more fully hereinafter with
reference to the accompanying drawings, in which embodiments of the invention
are
described. This invention may, however, be embodied in many different forms
and
should not be construed to be limited to the embodiments set forth herein.
Rather,
these embodiments are provided so that this disclosure will be thorough and
complete, and fully convey the scope of the invention to those skilled in the
art. Like
numbers refer to like elements throughout.
Referring initially to FIGS. 1 and 2, and by way of example, a sensor 10 in
keeping with the teachings of the present invention for detecting a presence
of
bacteria from a perishable food product 12 includes a sealed container 14
having
opposing gas permeable walls 16, 18 formed from a TPX (PMP) transparent thin
film
for viewing a pH sensitive solution 20 carried by the container 14. For one
embodiment, the pH sensitive solution 20 has a generally green color changing
to a
generally orange color responsive to a 0.5% concentration of an acidic gas
generated outside the container 14 by a spoiling of the food product 12 for a
bacteria
detection range between one million and ten million bacteria. With continued
reference to FIGS. 1 and 2, the pH sensitive solution 20 is carried between
the
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opposing walls 16, 18 of the container for permitting desirable gas diffusion
22 of
carbon dioxide gas 24 emitted from the food product 12 to pass through the
container 14 and solution 20. While not required, it is expected that the
sensor 10
may be placed in a package 26 with the food product 12 being monitored. As
above
described, in order to maximize the diffusion of the carbon dioxide gas 24
into the
sensor 10, a two-sided design was selected that permits diffusion of gas from
both
sides of the sensor. This permits a rapid color change that minimizes the time
a
sensor is in an "uncertain zone," where color changes are gradual and not
produced
in a step-styled change as is the case for embodiments of the present
invention. To
further improve free diffusion of gas to both sides of the sensor 10, it may
also be
desirable to place the sensor in a spaced relation to walls 27 of the package
26
carrying the food product 12 or surfaces 13 of the food package itself, as
illustrated
with reference again to FIG. 1.
As herein described by way of example for one embodiment of the invention,
carbon dioxide is used as a generic indicator of bacterial growth and for
quantitatively estimating a level of bacterial contamination present in the
food
product 12. As is well known, when carbon dioxide comes into contact with a
solution, the pH drops as a result of a formation of carbonic acid, making a
pH value
an indicator of carbon dioxide concentration and thus of a bacterial load.
For embodiments of the invention as herein described, the sensor 10 includes
the solution 20 having a pH value between 6 and 8. Further, an embodiment
includes the pH sensitive solution having bromothymol blue and methyl red
mixed
with an alkaline solution of sodium hydroxide. One embodiment includes the
bromothymol blue in a 0.05 %wt/volume and the methyl red in a 0.0035wt/volume
dissolved in 1 mM sodium hydroxide for providing a pH value of approximately
6.8.
By way of example, test results have resulted in effective solutions 20 with
the
bromothymol blue having a%wt/volume between 0.02 and 0.08, the methyl red
having a%wt/volume between 0.001 and 0.005, dissolved in an alkaline solution
of
sodium hydroxide ranging between 0.5 mM and 1.5 mM for providing the pH value
of
the solution ranging between 6 and 8.
For the embodiment of the sensor 10 illustrated with reference again to FIG.
2, the walls 16, 18 are made from the thin film having a thickness dimension
28 of
approximately 0.001 inches. As will come to the mind of those skilled in the
art now
having the benefit of the teachings of the present invention, an antifreeze
agent such
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as ethylene glycol may be added to the solution 20 with an appropriate
modification
of the mixture to achieve the desired pH value. One embodiment for which test
data
is herein presented included a 1.4 mil thick transparent film with the TPX
(PMP) film
as opposing sheets sealed about a periphery 30. One embodiment included the
container 14 having a dimension 32 of approximately one inch by one inch, as
illustrated with reference again to FIG. 1. For the embodiments herein
presented by
way of example, heat was applied for sealing the periphery 26 of the opposing
film
sheets.
With regard to the solution 20, studies involved a pH range finding to yield a
product with an initial color of rich green (similar to traffic light green)
while also
producing an orange-red color (typically accepted danger color) at a relevant
microbial load. By way of example, while potentially useful for some
situations, an
initial formulation proved to be too sensitive and thus not desirable for a
practical
application of interest as a freshness detector (color change at 0.5% CO2 and
approximately 5x105 CFU/g). One desirable embodiment including a formula
containing 0.05% bromothymol blue, 0.003% methyl red dissolved in 1 mM NaOH
provides a starting pH of 6.8 and yielded a green to orange color change
occurring at
a 0.5% C02 concentration. Of course modifications to the formulation may be
required for certain applications (e.g. antifreeze agents such as ethylene
glycol may
be added to the active formulation to prevent freezing at lower temperatures).
Further, the Material Safety data Sheet (MSDS) of the chemicals used at the
concentrations herein presented, by way of example, indicate that such
formulations
at the concentrations presented would not be harmful to a human if consumed in
error. By way of example, and as illustrated with reference to the plot of
FIG. 3, a
spectrum (360-720nm) of a solution 20 of a pH formulation at room temperature
at
day one (hashed plot) and day sixty (solid plot) resulted in an excellent
shelf life for a
desirable formulation.
With regard to the container 14, a wide variety of transparent thin films were
available in the marketplace. However, requirements for a film that will hold
the
aqueous solution are very specific and a substantial regimen of research and
experimentation into optimal material for the sensor was undertaken. Desirable
requirements included features selected from: a high gas permeability; thin
film
available (< 2/1000 inch); relatively high carbon dioxide gas permeability; a
high
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transparency; high flexibility; a heat sealable material; high flexibility;
unstained by
the pH indicator formulation; and a relatively low cost for manufacturing.
After extensive evaluation, it was determined that a TPX film thickness of 1.4
one thousandths of an inch with a high transparency rating meets all the above
criteria. One embodiment of the sensor 10, and as above described, includes
the
manufacture of a square sensor, by way of example, by cutting two squares of
TPX
1.4 mil thick, transparent film 1" square, placing one square on top of the
other, using
a pulsed heat sealer to seal three sides, adding 0.5 ml of formulation to the
formed
container 14, and sealing the final side. If leaks occur at the corner, double
seals on
each side will solve the leaking issue.
The sensor 10 is now ready for use and has stability for at least two months
at
room temperature and a predicted shelf life in excess of one year at
refrigerated
temperatures. Naturally many parameters described in the manufacturing process
may be varied dependent of application such as shape, size, volume of
indicator
added. The method of sealing may be heat as described above alternatively glue
or
other bonding agent may be applied.
Wiffi reference to FIG. 4, a table illustrates data that reflect performance
of the
sensor manufactured, as above described. Bacterial concentration is presented
in
colony forming units per gram (CFU/g). By way of example, the sensor 10
described
above reflects one embodiment of the invention for which data were collected.
Cooked chicken was handled following cooking to introduce a microbial
population to
the surface. The cooked chicken required approximately 1.5-times more time to
reach a high microbial load, but the sensor performance was good for both
fresh and
cooked chicken.
Many modifications and other embodiments of the invention will come to mind
of one skilled in the art now having the benefit of the teachings presented in
the
foregoing descriptions. By way of example, this invention may also be applied
to
preparing a sensor responsive to ammonia with the color change being green to
blue. Alternative pH indicators may be selected that would provide alternative
color
changes as the pH increased to the alkaline as a result of the formation of
hydroxide
ions. Therefore, it is understood that the invention is not to be limited to
the specific
embodiments disclosed, and that modifications and embodiments are intended to
be
included within the scope of claims supported by this disclosure.
