How could you investigate the impact of lactose concentration on pH in yogurt?

Investigating the impact of lactose concentration on pH in yogurt involves directly manipulating the lactose levels in the yogurt culture. By adding more lactose to the culture, you can observe how it affects the fermentation process carried out by the bacteria present. As these bacteria ferment lactose, they produce lactic acid, which in turn lowers the pH of the yogurt. Thus, a higher concentration of lactose would likely lead to more lactic acid production and a subsequent decrease in pH. This direct relationship between lactose concentration and pH change is fundamental in understanding the fermentation process and the quality of yogurt.

In contrast, the other options do not directly pertain to the investigation of lactose concentration and its effects on pH in yogurt. Introducing sugar substitutes may alter sweetness without affecting lactose levels, increasing incubation time could impact overall fermentation but not specifically the lactose concentration effect, and cooling the culture would slow down or halt the fermentation process, countering the objective of studying lactose impacts.