For a Blanch & Shock event two weeks ago, I made a batch of butter to be served infused with fried mushrooms and seaweed, to make an umami spread served with butter - a distant relative of dripping served with bread.
I inoculated 500 grams of Helsett Farm cream with 200 grams of their live crème fraîche, and left it to colonise the cream for three days and nights, at room temperature (around 21C during the day, dropping to 19C at night.
I recently bought an incredibly cheap plate warming blanket thing from Lidl, which heats up fast, but lacks a variable control and claims to hit 70C, which would destroy the bacteria ripening the cream. Until I have put together a PID controller to regulate it, the temperature of my kitchen will have to suffice.
I whipped the butter after chilling it briefly, draining the first and second waves of buttermilk that broke (to be used in an unrelated sauce) and then whipping the mass, with some of the buttermilk and 0.5% salt. The smell was extremely buttery, rich in what I have learned to recognise as diacetyl, an aromatic compound in butter. The taste was rich without being especially grassy or herbaceous like it might be in summer. I packed it and cooled it before it was later infused with powdered dulse and fried girolle and chestnut mushrooms.
Two days later, and left with a decent amount of the butter, I added three large scoops to a container of live yoghurt whey to try and instigate a kind of bacterial showdown. It had four days in the fridge and then three days at room temperature
The whey treatment has added a satisfactorily identifiable extra note, at once related and foreign to the butter, and an umami flavours have developed.
The butter has no significant role to play at the moment, it having been an experiment, but will inspire me to look at the interaction of yoghurt and cream and whether they can collaborate. For now, it has been steadily disappearing, mainly spread onto bread from Brickhouse Bakery in Peckham. The final spoonful is around sixteen days old now, and, as has been the case with most of the butters I have subjected to such tests, it has started to drift towards being like a cheese.
I will most probably end up in a bowl of scrambled eggs.