I added amylase to a cornstarch suspension.

I thought that amylase breaks down starch into oligosaccharides.

But then I heated up the amylase + cornstarch solution and added Benedict's reagent to it (which tests for the presence of monosaccharides). The solution changed colour to orange, indicating the positive presence of monosaccharides, not oligosaccharides.

So why did amylase break down starch into monosaccharides? Or am I mistaken in thinking that amylase only breaks down starch into oligosaccharides?

Amylase is an enzyme that hydrolyzes or breaks down starch into smaller carbohydrate molecules. Specifically, it cleaves the glycosidic bonds between glucose units in starch. Amylase works by catalyzing the hydrolysis of the alpha-1,4-glycosidic bonds in starch, which results in the release of oligosaccharides and ultimately monosaccharides.

Now, let's explore the reaction you described. When you added amylase to a cornstarch suspension, the amylase started breaking down the starch molecules. Initially, this process produces oligosaccharides, as you correctly mentioned. However, when you heated up the amylase and cornstarch solution and added Benedict's reagent, a positive color change to orange occurred. This indicates the presence of monosaccharides.

The reason for this is that heating the solution accelerates the hydrolysis reaction, breaking down the oligosaccharides even further into smaller carbohydrate molecules, namely monosaccharides. Benedict's reagent, which is a test for reducing sugars including monosaccharides, detects the presence of these monosaccharides in the solution and produces an orange color change.

In summary, amylase initially breaks down starch into oligosaccharides, which are intermediate-sized carbohydrate molecules. However, when the reaction is further driven by heat, these oligosaccharides get broken down into smaller molecules, specifically monosaccharides. This explains why the addition of Benedict's reagent resulted in an orange color change, indicating the presence of monosaccharides in the solution.