Final answer:
Cornstarch should not be used to thicken acidic liquids or dairy-based liquids.
Step-by-step explanation:
Cornstarch should not be used to thicken acidic liquids like lemon juice or vinegar as it breaks down the cornstarch and prevents it from thickening. The acid in these liquids interferes with the structure of cornstarch, resulting in a runny consistency.
Moreover, cornstarch should not be used to thicken dairy-based liquids like milk or cream because it can cause lumps. Cornstarch needs to be dissolved in a cold liquid first before adding it to a hot mixture to avoid lumps, but this method doesn't work well with dairy products. Instead, cornstarch can be replaced with other thickeners like flour or arrowroot in these cases.