There is a lot of conflicting information debating this question about fermented cod liver oil. We have investigated this issue in great detail, and have spent much time looking into it and understanding the science revolving around this.
Tell-tale signs of rancidity include the presence of aldehydes, peroxides (and other oxidative species), and loss of omega-3 fatty acids (such as DHA and EPA) which are naturally abundant in cod liver oil. Scientific analysis of fermented cod liver oil has repeatedly shown that there are no aldehydes, or oxidative species, and that there is a large amount of the omega-3 fatty acids EPA and DHA, all of which indicate that the oil is not rancid. You can see one such lab report, which was done by a 3rd party lab, here.
Confusion around this topic seems to have arisen from a combination of malicious marketing by competitors combined with a poor understanding of the unique processing methods that are used to make fermented cod liver oil. The processing principles of fermented cod liver oil, along with how the oil is protected from rancidity is explained in detail here, and can help you to understand the subject in greater detail.
There is still much to learn about the oil. The bacteria which ferment it may produce beneficial nutrients, the oil may contain beneficial phenolic nutrients (like chlorophyll) and there is little known about the variety of vitamin D related compounds found in the oil. However, what we do know is that the oil is not rancid, contains omega-3 fatty acids, vitamin A & D, and the majority of people report beneficial effects.
Having reviewed a lot of the scientific evidence and health benefits of fermented cod liver oil, we are confident in its health benefits and are proud to stock the entire range of Green Pasture products. If you are in any way concerned about the oil, please feel free to contact us, as we are always happy to answer your questions.