The diagram shows one method of yogurt production - Edexcel - GCSE Biology - Question 4 - 2014 - Paper 1

The milk is maintained at a temperature of 40 °C during fermentation to provide optimal conditions for the growth of beneficial bacteria, such as Lactobacillus bulgaricus and Streptococcus thermophilus, which are essential in yogurt production. This temperature promotes a favorable environment that allows these bacteria to thrive and produce lactic acid effectively.

Furthermore, at 40 °C, the enzyme activity is heightened, which accelerates the fermentation process. The production of lactic acid lowers the pH of the milk, resulting in the coagulation of proteins, which is critical for achieving the desired texture and flavor of yogurt. Thus, keeping the milk at this temperature is crucial for both the fermentation speed and the quality of the final yogurt product.