Why is my sourdough too sweet? I’ve been feeding my 40g sourdough starter with 40g water and around 45g wholemeal flour (for a firmer consistency, as I’ve had bad experience with liquid accumulating on top) each day. It initially started smelling quite acidic and yeasty and started bubbling nicely.

From what I had read online, I was afraid that my first bread would be way too sour. But in fact the opposite was the case: after a few days, my starter stopped smelling sour altogether and started smelling almost unbearably fruity-sweet instead. The starter was still good however, so I made bread from it, using a variation of the beginner’s sourdough bread from The perfect loaf. The bread came out alright but rather than tasting sour, it tasted fairly sweet.

I don’t mind the taste that much but I’m still wondering what’s causing my nicely active sourdough to smell and taste distinctly sweet and not the least bit sour. From what I’ve read, the opposite should be the case: my firm sourdough should be more acidic.

For reference, the sourdough jar is standing next to a radiator but my house isn’t heated during the working hours, nor between midnight and 7am, and will get down to 16 °C. To compensate, I’ve wrapped a kitchen towel around the jar.
Lack of sour indicates more yeast activity over the souring lactic acid bacteria (LAB), and using a firm starter from that link favors yeast and not LAB. For more sour, increase the water content. There's a bunch of other dials you can fiddle with, Ken Forkish in "Flour Water Salt Yeast" indicates that a "warmer levain cultures encourage lactic acid production", your 16 °C favors acetic acid production. Keep the temperature around 26C to 32C to favor LAB. (Perhaps use an insulated box to help avoid the nighttime temperature swings?) Another point would be what temperature water are you hitting it with; tap water during winter could be quite cold (I use room temperature water that has stood to evaporate off the municipal chlorine). Other things to experiment with might be adding small amounts of salt (though this will slow down the ferment) or changing the flour or mix of flours used; whole grain vs. not, rye vs. wheat, etc.

A notebook may help; track ambient temperatures (min/max), temperature of the water used, amount, type of flour, plus any notes on what the starter looks and smells like.