When bread turns black in a toaster oven, it is likely due to a combination of two main factors: the Maillard reaction and caramelization.
Maillard reaction: The Maillard reaction is a chemical reaction that occurs between amino acids (proteins) and reducing sugars (sugars that can donate electrons) when exposed to heat. When bread is toasted, the heat causes the amino acids and sugars present in the bread's proteins and carbohydrates to undergo the Maillard reaction. This reaction produces a range of flavorful compounds, giving the bread its characteristic toasted taste and aroma.
Caramelization: Caramelization is another chemical process that takes place when sugars are heated. As the bread's sugars are exposed to the high heat of the toaster oven, they start to break down and undergo caramelization. This process creates a brown color and imparts a sweet and nutty flavor to the bread.
Together, the Maillard reaction and caramelization cause the bread to turn brown and develop a crispy texture on its surface. In some cases, if the bread is left in the toaster oven for too long or at very high temperatures, it may become excessively brown or even black. This is because the sugars and proteins continue to undergo further browning and may eventually burn, giving the bread a darker color and potentially a bitter taste.
It's important to find the right balance when toasting bread in a toaster oven to achieve the desired level of browning and crispiness without overdoing it. Different types of bread and toaster ovens may require varying toasting times and settings, so some experimentation may be needed to get the perfect toast.