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

The milk is kept at a temperature of 40 °C during fermentation to optimize the activity of the starter culture bacteria. This temperature is ideal for the growth and metabolic activity of beneficial bacteria such as Lactobacillus bulgaricus and Streptococcus thermophilus, which are responsible for converting lactose in the milk into lactic acid.

Higher temperatures may denature the enzymes and kill the bacteria, while lower temperatures would slow down the fermentation process, leading to inadequate yogurt production. Therefore, maintaining a steady temperature of 40 °C ensures efficient fermentation, resulting in the desired texture and flavor of the yogurt.