What is the science behind the phenomenon when only after you scoop out some yoghurt from a fresh tub that you will find the whey has separated by the next day.
WAG [wild ass guess] the whey is bound in the mass created by the fat and protein coagulating due to microbial action. When you break the surface tension by spooning out the yoghurt, or in the process of cheese making by cutting the curds you force the whey out of the bonded glop. I know that the process of ‘cheddering’ is heating the mass of whey and curd gently which compresses the protein structure and forces more whey out of the curds so when I make a cheddered cheese I get more whey than when I make something with a different forming process.
you’ve made a depression where it can collect and be evident.
I’ve always thought it was because I was eating a few spoonfulls from the tub and the spoon transfered some saliva into the yoghurt. The saliva contains digestive enzimes which break down some of the yoghurt.
Does this happen at all?
I’ve seen whey result after scooping out yogurt without adding any saliva in any way. So I doubt the phenomenon is due to the addition of a few salivary enzymes.
i saw what you did.
No way! That weighs on me. 
See quesiton 4 form this biology test.
PDF
If the link doesn’t work then google “saliva enzymes yogurt” and it’s the only PDF link on the results page, close to the bottom.
For those of you who don’t want to open the link, the 4th question:
“While eating a container of yogurt, you have to leave, so you store the yogurt in the refrigerator. A day later you return and
find the surface of the yogurt is no longer smooth but has broken into several liquified products. You correctly guess that
enzymes from your saliva, via the spoon, have continued digesting the yogurt in your absence. What will happen over time?”
Greek yogurt contains no whey (the whey has been removed). That’s why it has a creamier taste. It does contain more protein than regular yogurt, probably from some other milk protein. Yet I do see some liquid in the yogurt after I’ve broken the surface.
This is overwhelmingly likely to be a question testing the basic conceptual understanding of an enzyme (hastens spontaneous reactions, lowers activation energy, is not consumed etc.) and thus hypothetical, rather than a description of what actually happens.
Eyeballing the exam, it seems to be from a very basic biology course.
It’s mostly casein but obviously yes, the question is not actually about whey but about the fact that liquid appears. Agreed it is not salivary enzymes as I do not eat out of my tub; only a clean spoon hits it. It seems to happen even if the surface left flat as well.
I’m betting on a variant of the first answer: yogurt is made of loosely bound hydrophobic casein micelles suspended in a hydrophylic colloid which is what prevents the fluid from separating from the solids. Scooping some out breaks up that lattice structure some, freeing the water.
You mention that enzymes “hasten spontaneous reactions”.
Is it possible that salivary enzymes accelerate or otherwise contribute to whatever other process causes the phenomenon?