Poop smells fermented

So I ended up taking I this neglected neighbours cat of on an informal basis - last time I took her home to them they said to me that they thought she had been with me and I told them that I would hate to take their cat but their response was "but she likes you better" as they have been leaving her out in the freezing cold I did take her home with me I have flead and wormed her and she seems very happy in my house. She used to be an indoor cat until about 8 months ago.

The only problem is that when she poops although her stools are generally firm and well formed there are always a couple of spots of very soft stool - often just outside the litter box. And her poop smells kind of fermented - like beer. She is on a diet of mostly high quality dry food as either my mum or myself have been feeding her for some time now (she had gotten VERY skinny a few months back) but who knows what other neighbours may have been feeding her as she would disappear every now and then for a few days.

I just gave her a teaspoon of natural yoghurt to see if she tolerates it - if it helps. I'm a bit hesitant to take her to the vet in case these people ask for her back. I don't think even they know what they want - I think she is too much trouble for them now they have kids and think it is fine to leave a cat outdoors and let neighbours feed her (as she refuses to go home even if hungry)

If I need to I will take her to the vet but any suggestions about what may be going on with her?

First of all, thank you for taking in this poor neglected cat! I cannot imagine how she would've survived without your help! Hats off to you and your efforts!

For your cat's stool, I honestly have no clue. I'd suggest you take a fresh sample to a vet to get a more definitive diagnosis. It could be her food, but I am not sure. Have you tried feeding her a different type of high quality dry? If you feed her the same quality type of food, there shouldn't be too much digestive upset. I'd try a different brand, grain-free, and one with lots of protein in the top 4+ ingredients if possible. Or you could try some high quality canned food, one that is again grain-free and without starches from vegetables too (potatoes or peas, for example). You can also try adding human grade probiotics if you have some and adding it to her wet food and some water to see if it helps.

I hope others can come on board to help you with more tips! Good luck!