Carbon Monoxide in Meat Packages – Overview
It has been a practice in Norway since 1986 to introduce a small (0.400 %) concentration of Carbon Monoxide (CO) into the headspace of case-ready meat package headspace – as is solves the problem of off-coloring due to the reduced oxygen present in case-ready meat package headspace – which are generally provided with an Oxygen –free mix of Carbon Dioxide (CO2) and Nitrogen to improve shelf life. The addition of small levels of Carbon Monoxide restores meat coloring without degrading shelf life – and has been recently approved by the FDA for use in the US as a safe chemical additive in these small doses.
The typically approved headspace gas mix for case-ready meats is 0.400 % CO, 30.0% CO2, Balance N2 – which has been generally adopted directly from the 20 years of experience in Norway. Headspace gas analyzers have been historically measured Oxygen (O2), or a combination of O2 and CO2. The inclusion of CO in the headspace gas mix means increased complexity for MAP gas mixing, filling, and fill-mix confirmation applications. In all cases, the headspace and fill gas analyzers have to measure small quantities of CO in a CO2 and N2 -rich environment – as well as O2 to confirm adequacy of fill ratio and oxygen reduction in the package headspace.
There are two technologies that are applicable to the CO gas measurement issue, Chemical Sensor and NDIR. This white paper discusses both the design and application trade-offs for these two approaches.
Chemical Sensor Carbon Monoxide Measurement
Chemical sensors have been used for toxic gas measurement, including CO, in personnel safety monitors for many years. They are relatively simple devices that react to the presence of small amounts of CO in ambient air and produce a small current which is an analog of the ambient CO concentration. They are zero stable – as no current output will be produced unless CO is present. However, as the output is produced by a chemical reaction to CO gas, they are intrinsically temperature and time unstable. Typically, chemical sensors are specified by their manufacturers as having about 5% span sensitivity increase for every 10 Deg C temperature change, 2% sensitivity reduction per month from the date of manufacture– and lifetimes of 12-24 months. In addition, their accuracy and longevity can only be assured by frequent calibration with test gas. This effect is referred to as span instability – and is typically the case with chemical sensors. – Zero Stable, but Span Unstable. A case in point is the Oxygen chemical sensor found in most MAP headspace gas analyzers.
By far the most popular method to measure Oxygen is the use of a chemical sensor similar to the CO sensor, but responsive only to Oxygen. It has typical chemical sensor time and temperature instability, but in this case, room air containing a stable level of O2 is readily available for frequent sensor calibration. However, this ready availability of natural calibration gas is not the case for MAP headspace gas measurement instruments which measure CO with a chemical sensor.
For the case-ready meat packaging headspace application, this means that the user should expect frequent (monthly or weekly) gas calibration of gas analyzers using a chemical sensor to make the CO gas measurement. Additionally, this will not assure that the instrument will remain stable when exposed to a temperature change. This situation is unlike the present CO2 and O2 analyzers MAP headspace gas analyzers, which use NDIR for CO2 measurement and a chemical sensor for O2 measurement. The ready availability of ambient air to stabilize the CO2 and O2 channels has made these very practical instruments in the MAP headspace gas measurement application. However that is primarily due to the choice of gas measurement technologies which take advantage of the characteristics of room air to zero-stabilize the NDIR CO2 gas measurement channel and span-stabilize the chemical sensor O2 measurement channel.
The new generation of CO or CO/CO2/O2 gas analyzers using chemical sensors to measure CO will no longer be able to do that. Frequent gas calibration using test gas will be required to maintain CO measurement accuracy.
NDIR (Infrared) Carbon Monoxide Measurement
As discussed earlier, NDIR (Infrared) technology has been used commonly in MAP headspace gas analyzers for measuring CO2. It has been proven to be accurate, stable, and trouble free in this application. Because CO is also a relatively strong infrared absorbing gas, NDIR is a good choice for CO measurement as well. In fact, while MAP headspace gas analyzer users have had little experience with NDIR measurement of CO, automotive exhaust gas analyzers have measured CO, CO2 and HC gases using NDIR technology for more than 25 years – and in similar gas concentration levels. While, the presence of high levels of CO2 in the headspace gas mix makes the measurement of low levels of CO gas somewhat more difficult, this is quite similar to the situation found in exhaust gas measurement, where high levels of CO2 also exist in the presence of low levels of CO – especially in catalytic converter equipped vehicles. Techniques have been developed to overcome these gas-mix issues, and a well designed analyzer using NDIR technology for CO and CO2 measurement will prove just as stable and trouble-free in the CO/CO2/O2 MAP headspace gas measurement application as the present generation of O2 and O2/CO2 MAP headspace gas analyzers.
In essence, the same approach that is used currently – zero-stabilizing the NDIR channels for CO and CO2 and span-stabilizing the chemical sensor channel for O2 using ambient air – will yield a practical, stable, and long-life CO/CO2 and O2 MAP headspace gas analyzer.