I would like to detect the level of odour in my fridge. I was looking in aliexpress to find cheap sensors for this purpose. I found some, but I am not sure which one to use. The list I found is:
MQ-2 smoke gas sensor
MQ-3 alcohol sensor
MQ-4 methane sensor
MQ-5 Liquefied Petroleum Gas, Natural Gas, City Gas Sensor
MQ-6 isobutane propane sensor
MQ-7 Carbon Monoxide Sensor Module
MQ-8 Hydrogen Sensor
MQ-9 Carbon Monoxide Combustible Gas Sensor
MQ-135 air quality detection sensor
.........etc.
Any advice or comment is appreciated.
Methane I supposed since flatulence (fart) is composed of methane...
Not an answer but there are many reasons why a fridge can stink... there are tons of hints online on why or what to do. here is an example https://www.sos-parts.com/fridge-smells-bad.html
Not sure methane will cut it.
Could you share why?
There was a well past it tub of yougurt found in my fridge the other day.
I never heard it fart once.
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Really your fridge shouldn’t smell - detecting the fact it does , seems an odd way of dealing with it .
Take the stuff out and clean the fridge is my advice .
Anyhow .. if you don’t want to do that , you need to find out what chemical creates the smell by way of some research ( as per earlier post ) then see how , if possible , to detect it .
The first step is to define "bad odor".
you would need to know which gases are bring produced by the various items in the fridge then get sensors to detect those items.
I'd start with seeing if a hot wire gas resistance reading would suffice. I'm guessing as the gas levels rise the gas would coat the hotwire sensor to cause an increase in gas resistance.
A particle sensor may work,
I suggest starting with this sensor, since it is reacting to the additive to gas that smells like rotting flesh.
If the OP is one of the rare person with limited ability to small like our #1 son has and my wife's father also had, this would be the most smell most apt to be a problem with stored food with meat products in it.
Good luck. IF it works, I might build one for my son.
Paul
Does one often fart in the fridge?
Sorry I couldn't help myself 
Thank you All for your comments and advices. I could download a Food Microbiology book from the internet that, maybe, will answer my question in more details. I will post the answer if I find it in the book.
Alas I doubt any of them is really sensitive enough. Animal olfaction (ie the sense of smell) is extremely sensitive (parts per billion of some molecules can be detected - this extreme sensitivity is evolved specifically to avoid rotting materials which pose a threat of infection). The typical bad smells from decay include hydrogen sulfide (rotten eggs), putrescene (deeply unpleasant), iso-valeric acid (smelly feet! - although this is produced by cheese too), and various other specific products of decay. Most of the gas sensors in the list are for odorless or low-odour gasses. We can only smell natural gas because a sulfur-containing molecule is added to it so it can be detected.
So the best sensor for the job is your nose I suspect. However at bad levels of odour some of these sensors will probably be usable - but unlikely to be highly sensitive.
Note that methane is unlikely to be produced in the presence of air so I not be hopeful for that. Alcohols are definitely produced in some decay processes in air.
Bananas particularly and many other fruit naturally produce ethene gas, which may well trigger some of these sensors erroneously too.
Its complicated!
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Thank you MarkT. I understand that it is complicated to detect odour as it can be a result of many kinds of gases.
Searching in my little "warehouse", I found a sensor (TGS2450) that is supposed to detect odour.
Here is the whole description of the sensor (in japanese 
)
tgs2450_data.pdf (567.2 KB)
I will try it and feed this post with the results.
I found a video that tests the sensor: 【Arduino】においセンサーに「くさい」って言わせてみた～そして悲劇は生まれる～ - YouTube
Interesting. Have fun!
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