Dear Mahomud,

According to Dalgleish and Law (1988), the amounts  of casein dissociation from the micelles is depends on both temperature and pH. More caseins are dissociate from micelles into the serum as temperature decreases from 30 to 4°C (Lee et al., 2010).

Gel strength also depends on the size of fat globules because size of milk fat globules is inversely proportional to size of gel of yogurt. (Chavan et al., 2014)

Ref: 

Dalgleish, D. G. and A. J. R. Law. 1988. pH-induced dissociation of bovine casein micelles. 1. Analysis of liberated caseins. J. Dairy Res. 55:529-538.

Effect of microfluidization on mango flavoured yoghurt: rheological properties and pH parameter” (2014). Rupesh Chavan, Anit Kumar, Vijendra Mishra and Prabhat K. Nema, International journal of food and nutritional sciences. Volume 3, No 4, 84-90.

Formation and Physical Properties of Yogurt. W. J. Lee and J. A. Lucey. Asian-Aust. J. Anim. Sci. Vol. 23, No. 9 : 1127 - 1136 September 2010.

Yes heating to above 90 oC is vital for gel strength in yogurt, reducing syneresis etc. This is due mainly to the increased covalent cross-linking between the denatured whey and casein micelles. This increases gel strength and prevents excess whey expulsion. The below paper is excellent:

Lee, W.J., & Lucey, J.A. (2010).  Formation and physical properties of yogurt.  Asian-Australasian Association of Animal Societies, 23, 1127-1136

Agree with Dr. Naim O'Sullivan As heating milk between 85-90 c help in water evaporation from milk and improve water binding capacity so reduce syneresis

My interest is that skim milk is heating at high temperature and different pH. At this condition, kappa casein and beta-lactoglubolin make disulfide bond and remain in the serum phase...Rather than what things happen that will affect yoghurt properties? 

To form the gel yogurt casein is necessary to denature the whey proteins by heat (85-90°)

If you have milk at normal pH and it begins to heat, denaturation of proteins occurs, the S-S bridges are broken and molecules unfold, especially intermolecular hydrophobic interactions are facilitated. pH action over caseins could cause coagulation. The effect of temperature could destabilize casein and it may fail to coagulate.

Heating at higher temperature results in protein denaturation and formation of covalent bonds and hydrophobic interactions. This results in formation of small crevices thus prevents synerisis hence gell strength is increased.

It is obvious that heating leads to denaturation of the milk proteins, hence increasing there surface for bonding and further interactions. This will result in an increase in gell strength of the yoghurt.