The browning of apples and bread are both caused by the same browning reaction, but the mechanisms are different. Apples turn brown from the reaction with oxygen due to enzymatic browning, while bread turns yellow due to the chemical reaction of sugars and proteins in the Maillard reaction, a non-enzymatic browning process.
If you leave an apple out at room temperature for a while, you’ll notice that it soon turns an unappealing brown color. Similarly, if you coat flour with an egg and bake it in the oven, you’ll notice that only the coated part will turn a nice golden brown color. Both of these phenomena have one thing in common: the browning reaction. This reaction can have an undesirable effect on the quality of the food, or it can contribute to its improvement.
However, the browning process doesn’t work the same way in both cases. First, let’s take a look at how apples change color. As we know, it’s caused by the reaction between the apple and oxygen. This is called enzymatic browning. Enzymatic browning occurs when enzymes such as the polyphenols oxidase and tyrosinase react with oxygen and phenols in the fruit to produce the brown pigment melanin. Enzymatic browning is desirable in red ginseng, black tea, and other products, but it reduces the texture of fruits and vegetables. To inhibit enzymatic browning, you need to lower the activity of the enzyme. This can be done by lowering the pH with lemon acid, lowering the temperature, or packaging with an inert gas such as nitrogen.
Next, the process of browning bread is different from that of apples. Leaving flour at room temperature for a long time doesn’t cause it to brown. Bread browns because of non-enzymatic browning reactions, most notably the Maillard reaction. The Maillard reaction is named after Louis Camille Maillard, who first described it. The Maillard reaction occurs when a highly reactive sugar, such as glucose, reacts with protein. This browning reaction usually occurs when food is subjected to heat, resulting in the production of melanoidin, a brown polymeric pigment, as well as several aromatic compounds. This is why bread, sesame seeds, biscuits, etc. have a toasty aroma when fried or baked. The Maillard reaction is so important to food quality that it has been studied extensively, but the exact reaction pathway is still not fully understood.
In addition to the Maillard reaction, another non-enzymatic reaction is caramelization. Caramelization occurs when sugars are heated to high temperatures. The reaction is accelerated by small amounts of certain acids. Although proteins don’t participate in this reaction, caramelization is similar to the Maillard reaction. Caramelization reactions also produce brown polymeric pigments and volatile compounds with a distinctive aroma, and the intermediates produced during the reaction are the same. Examples of caramelization reactions include caramel candy and brown sugar.
As such, browning reactions are broadly divided into enzymatic and non-enzymatic browning reactions, and non-enzymatic browning reactions are further divided into the Maillard reaction and caramelization reactions. While browning can degrade the quality of food, such as fruit spoilage, it can also enhance the appearance and texture of food. It’s important to note that foods that have been browned by the Maillard reaction at high temperatures, such as French fries or onion rings, should be consumed in moderation. In the case of the Maillard reaction, frying foods at high temperatures produces acrylamide, which is known to harm the nervous system. This substance has also been implicated in causing cancer, so you should avoid eating too much of it.
