Dear George,
I agree but in my experience, Colour is often the most immediate issue for oil being used for deep-fried, coated goods. Used oil of ffa 1% can have a significantly different colour to 3% (and similarly the original output 
). Some oils also change colour more rapidly with ffa than others. One consequence is that customers' specs. tend to minimise the acceptable ffa level (eg1%) while the producer the opposite logic (cost-oil rejection rate)(eg 2-3%).
Generally AFAIK there are few explicit international regulatory values for these parameters since they are mostly not interpreted as safety related although oil ffa levels of 6-10% can become somewhat chemically challenged IMO (and smokey).
From Rudra's POV, no idea if pets have any great colour sensitivity. I guess differences may be obscured by the containing matrix although used oil of ffa 1% is IMO likely to have a superior flavour compared to, say, 3%. I have a suspicion smell is often the first doggie rejection factor perhaps followed by estimated cost. 
As far as rancidity is concerned, my output was all frozen which probably contributed to the observation that with ffa in 1-3 % range, there seemed little tendency to significant (tasteable) rancidity in a 6 month period.
Rgds / Charles
PS (added) @Rudra - You mentioned that you do tests on received oils, what range of ffa / PV levels are you mostly finding ?? Hopefully ffa not all 6% UP ? Is your product stored at ambient temperature ?
Charles,
Colour is unlikely to be an issue in petfood, unless you are trying to coat a light coloured biscuit / kibble with a dark coloured oil. Whilst the pet might not object from a colour perspective, the marketing people, the petfood retailer and the pet owner might object if something that should be light in colour is darker than expected.
However, the key question is what is causing the darkening of the oil? Could this be due to polymerisation during the heating / frying process that could give rise to the presence of coloured and potentially toxic compounds? Could it be due to the presence of (relatively) innocent colourants from the food that has been fried (assuming the oil has already been used)?
Rancidity is
not always an issue in petfood with respect to serious food safety issues eg those requiring vet treatment.
Whilst this seems strange the reason is in the pet's in-built defence mechanisms in that the off-flavours and smells associated with rancidity might mean that the animal rejects the feed.
In doing so it doesn't consume enough to become ill. However, rancidity is still undesirable as it can cause vomiting in the pets and diarrhea and also pet owner rejection. In this context, petfood producers initially look at rancidity in the first instance as an undesirable
quality issue ie off odour rather than specifically as a food safety issue.
To address this quality issue extensive control is applied throughout the supply chain from the edible oil producer / supplier through petfood manufacture, shelf-life and into the pet feeding bowl. As indicators of the effectiveness of these controls, parameters like PV, Anisidine Value etc are used, as indicated in other posts on this thread.
Regarding PV please note that this can be miss-leading. For sure a "high" PV means you do have a problem. However, as PV can cycle ie change from low to high to low etc during storage, a low value doesn't tell you a lot! For this reason you need other measures like Anisidine Value.
Regarding the use of "used" oil, please also note that there might also be changes in the nutritional value eg fatty acid composition of "used" oil. Some fatty acids are temperature sensitive and levels can drop during heating.
Hope this helps!
DP2006
