I ground some sesame seeds for tahini because I wanted to make a tahini sauce. My first version was horribly bitter and I had to assume that it was because hulled sesame seeds seem to go rancid at a rate of knots.
I started reading up about it, however, and it seems it's a bit more complicated than that.
It is true that hulled sesame seeds go rancid very quickly (even more than most nuts and seeds, they benefit from being kept in the freezer but - good luck finding out how long the seeds have been stored before you bought them) but it also seems that the variety of sesame seed matters.
According to
Hummus Bros: Levantine Kitchen, good tahini is made with:
'Humera' sesame seeds, usually grown in Ethiopia, and sometimes Paraguay. Some inferior tahini is made from Nigerian seeds, but we only buy tahini with Humera seeds for our restaurant...
The ultimate test for tahini is to taste it pure, without any addition of water. If there is an aftertaste (which is usually bitter), then this will certainly come out in the taste of the finished hummus. When you have high quality tahini made from the rights seeds there is no aftertaste and the consistency is almost quicksand like, which sticks to the top of your mouth. If you can easily eat a spoonful of raw tahini that means it's not good enough.
I've no idea how accurate this opinion is
but it has set off one of my usual rants which is that packaging in the UK rarely informs you when something was harvested rather than when it was packed. My package label has a section for 'Country of Origin' but no country was identified.
Sometimes, I think we dislike nuts because they're dry or rancid because, unbeknownst to us, they were harvested up to two years previously rather than in the past few months. My tahini is a lot more bitter than I like - I've no idea if it's my technique at fault (tho' I don't see how, it's just grinding), the storage of the sesame seeds en route to my purchase of them, or the seed variety.
Unless nuts and seeds are purchased unhulled and with a harvest date then I don't know how else we can have any confidence in their freshness. It would add enormously to the cost of purchasing them if they had to have a verified cold chain or to be sold refrigerated/frozen but this present hit and miss system is far from satisfactory.
I often wonder how rancid some of the food that we regularly consume is - oils, nuts/seeds etc. and why it doesn't seem to be possible to agree on packaging that isn't open to abuse but does protect contents properly. Clear bottles/packets don't seem at all sensible for oils/nuts/seeds etc.
[/Rant] As you were.
NB: it's probably obvious that I feel strongly about this subject. It's particularly more so because my sense of smell and taste were impaired after an accident and they've not fully recovered so I can't rely on smell to detect that something is 'turning' or rancid.