Ah yes, I know this, it used to be "I can't believe it's not butter" and they rebranded it. The original is a 59% fat spread, butter and (true) margarine are legally >80%. Butter is naturally 82% fat, iirc. There is a low fat version of ICBINB, sorry ICBISG, probably about 40%. This will be thinner again.The runny shite in question is: ‘I can’t believe it’s so good’ (honest!) can’t remember where we purchased said item.
These products are made with vegetable oils that are softer than most animal fats. This means that they have a lower melting point and are softer at any given temperature. This gives a very rapid melt upon eating and makes them less creamy and unctuous in the mouth. In addition there is less fat, 59% plays 82. The rest is water, held together with dairy protein and maybe some gels and gums. As soon as this gel warms up it melts, like jelly. Remember jelly melting from being a kid, it doesn't go creamy, it just melts.
You can improve the melt characteristics of any spread by using harder fats (higher melt point). However these are by their nature more saturated, longer carbon chains, fewer polyunsats. You could also use hydrogenated oils, these are now absolutely out of fashion. This runs against the claims that the marketing dept want to make. Low fat, low saturates, high PUFAs, you know the score. It's very hard (as in physically impossible to date) to match butter's unique melt characteristics while maintaining low saturates, reduced fats, whatever else. "Ye cannae fight the laws o' physics, Cap'n...she'll blow!"
I could make a synthetic butter substitute that would have a melt characteristic indistinguishable from that of butter. How? By fractionating a load of vegetable fats and making a blend at 80% fat that matched the fatty acid profile of butter. So many triglycerides, so many di, so many mono. So many saturates, so many unsats. So many C6, C8, C10, so it goes on. It's only chemistry. However this would obviously cost a fortune and the whole point of yellow fat spreads is that you blend a number of inexpensive and widely available ingredients to get close enough. A bit of palm oil, some rapeseed, maybe some hero ingredients like olive, make your emulsion and pack it at £1 or £1.50 a tub. It's a bit thin? Ah well, call it "easily spreadable" and they won't care. Mostly, they don't. It's cheap, it's palatable. Oh good, BakeOff starts in 5 minutes, I can watch people cooking without having to do any of the actual work myself.