Some Theory …
The yoghurt structure consists of a three-dimensional network forming by casein micelles. This network is ”alive”.
Depending on the practice I use when making yogurt, deficiencies may be created when the 3-D network is formed. This means that the 3-D network is weak and unstable.
Two critical factors: temperature changes and pH change cause rearrangement of the 3-D network. This rearrangement or shrinking, in general, is called syneresis.
The rearrangement rate firstly, and how perfect the 3-D network is will determine the whey outlet from the gel, or as it is called, the serum expulsion.
Let us start:
caseins
So whenever I have milk with a few caseins percentage, then we start already with serum expulsion in yoghurt. In this case the 3-dimensional network is weak.
homogenised milk or not
In homogenised milk yoghurt, the new fat globules resulting from homogenization have a much smaller size that can approximate that of caseins and have also protein features. They participate more effectively in the three-dimensional network. In this case the 3-dimensional network is more powerful.
In yogurt from non-homogenized milk, the fat globules interrupt the 3-dimensional network, weakening it. In this case, the 3-dimensional network has imperfections.
heat treatment
I want intense heat treatment [high T for a sufficient amount of time] to evaporate water, i.e. to increase 
the casein content.
Intense heat treatment also results in denaturation of whey proteins. Serum proteins change structure, interact with caseins and enhance the 3 dimensional network. In this case the 3-dimensional network is more powerful.
slow rate of acidification
At a fast rate of acidification [pH drop], the 3-dimensional network can not smoothly be rearranged. In this case the 3-dimensional network will have imperfections.
what time I should remove the yogurt from the oven [how acidic it should be]
Ideal pH value is 4.50. A pH value of 4.40 to 4.60 is satisfactory. pH values below 4.40 or above 4.60 result in defects in the 3-D network.
slow cooling rate
The temperature from 42-43 degrees should be decreased to 4 degrees. Fast temperature rate change causes abnormal rearrangement of the 3-dimensional network. In this case the 3-dimensional network will have imperfections.
any mechanical disorder
Even a shake, since from the point yogurt has formed the 1st gel structure [calculate after 1.5 hours in the oven] can cause a crack in the 3-dimensional network.
dimensions of the manufacturing container
If you use cups with a small cross-sectional area and a high height will serum elimination will be promoted. Whey expulsion is also promoted whenever the yoghurt is half-consumed and then stored with a large area of exposure.
when the lids will be placed in cups
Lids onto the cups should be placed when the yoghurt has a low temperature, i.e., when the yoghurt is in the refrigerator and has reached a temperature of 4 degrees Celsius. Otherwise, moisture concentration on the surface of yoghurt, due to condensation, will promote serum excretion.
You tube: Athanasios Kouris
https://www.youtube.com/channel/UCfh1gAHj-6BKNGPKYSvscJA
fb: Traditional Homemade Dairy Products Manufacture
