Apple cider vinegar is definitely acidic when it goes in. HOWEVER...once the body digests it, it turns the body alkaline. So it has an alkalizing effect on the body. At first the ACV may not alkalize on contact, but over a day or so, it is starting to alkalize the crop via digestion. And by adding ACV to a chickens water once a week keeps the crop in a healthy state all the time with less chances of souring.
I don't do the yogurt either. As Pwand has talked about, it produces a lot of mucus. So while there is ACV in the water, I add probiotics to the feed. Attack with good probiotics on all fronts. And by the way, ACV with the mother in it, has probiotics as well.
If the yeast infection goes unnoticed for a long period of time, it can get completely out of control and need medications. I have tried vet recommended Nystatin with absolutely no luck. The problem with Nystatin is, that it MUST come directly in contact with the yeast/fungal infection. So if there is ANY food in the crop, Nystatin just does not work. And because the chicken needs to eat all through out treatment, it seems to me to be a useless drug.
And as Pwand has mentioned and used, I have had SUPERIOR luck with vaginal cream. The Clortrimazole 2% works fabulous. I have not have much luck with Miconazole (monistat). I use a syringe (without the needle) and load in 1 cc of cream and dose the bird down the throat 3 times a day for the first few days. If the sour crop is not too far gone, 4 days later I can clear it all up.
I have a bird that is prone to sour crop. She will get it if someone even mentions raisins! (too sugary for her crop) Her first case of it was at 6 months old and it took me 2 full months to clear it up. She nearly died of not only of sour crop toxicity, but of starvation. I did not catch this sour crop until it was too far gone. She has had many more cases of it over the years, although because my sensitivity to her needs, she gets it very little any more. Although I now have a "crop obsession", LOL and everybody gets their crops fondled regularly.
So...the moral of the story is...add ACV to your chickens water once a week for good health, don't over do the acidic foods and be careful with free rangers. Check their crops on a regular basis first thing in the morning. If you catch it early on, sour crop can be cleared up in a few days.