The firmness of the yoghurt depends largely on 2 factors:
Yoghurt culture:
Lactobacillus bulgaricus (considered the original and sometimes ‘the only real one’) produces the firmest yoghurt. Streptococcus thermophilus produces a soft, creamy yoghurt. Lactobacillus acidophilus produces an even softer yoghurt.
Protein:
The casein - ideally the αs1 casein, which is the main component of cow's milk protein - coagulates during fermentation and forms so-called casein micelles (tiny lumps). These bind water, forming the perfect yoghurt structure and allowing the yoghurt to ‘set’. Sheep's milk is therefore also very suitable for yoghurt production. With goat's milk, however, the yoghurt always remains a little more liquid. The addition of milk powder or milk protein makes the yoghurt firmer. Protein milk’ can also be used, to which additional milk protein is simply added. This increases the protein content from the typical 3.5 per cent to up to 8 per cent.
The fat content plays an absolutely subordinate role. Additives such as inulin ensure a creamier consistency.