2. Fat content: The higher the fat content, the weaker the gel because the
fat globules interrupt the network.
3. Homogenizing: Homogenization of the milk leads to a much enhanced
firmness because the fat globules then contain fragments of casein
micelles in their surface coat by which they can participate in the
network upon acidification (see also Section 9.6). The volume fraction
of casein is thus effectively increased. (Homogenization of skim milk
makes no difference.)
4. Heat treatment: Heat treatment of the milk considerably enhances
firmness. The deposition of denatured serum proteins increases the
volume fraction of aggregating protein; it also may alter the number
and the nature of the bonds between protein particles. Milk is generally
heated for 5 to 10 min at 85°C to 90°C.
5. Yogurt cultures: These vary in the firmness they produce (at a given
acidity), but as a rule, the differences are small.
6. Acidity: Generally, the yogurt is firmer at a lower pH. The preferred
pH is between 4.1 and 4.6.
7. Incubation temperature: The lower it is, the longer it takes before a
certain pH, and thereby a certain firmness, is reached, but the finished
product is much firmer.
8. Temperature of the yogurt: For the same incubation temperature, a
lower measuring temperature gives a greater firmness. The effect is
quite strong (see Figure 22.5). The explanation is, presumably, that the
casein micelles swell when the temperature is lowered (and vice versa);
because the particles are essentially fixed in the network and the network
cannot swell, this would imply that the contact or junction area
between any two micelles is enlarged, by which a greater number of
bonds are formed per junction.