In this blog post, we will look at the reasons why apples and bread turn brown from the perspective of their browning reactions, and find out the differences and similarities between them.
If you leave an apple that looked fresh just a moment ago at room temperature, you will soon see that it turns into an unappetizing brown color. Also, if you coat flour with egg and bake it in the oven, you can see that only the part coated with egg turns a delicious golden brown. These two phenomena have one thing in common: they both undergo a “browning reaction.” As such, the browning reaction can have both undesirable effects on the quality of food and, conversely, contribute to improving its quality.
However, the browning reactions in the two cases do not occur using the same principle. First, let’s look at the process of how apples change. As is commonly known, this phenomenon occurs when apples react with oxygen. This is called the “enzymatic browning reaction.” The enzymatic browning reaction occurs when enzymes such as polyphenol oxidase and tyrosinase, which are present in fruits, react with oxygen and phenol to produce brown melanin pigment. The enzymatic browning reaction is desirable in red ginseng and black tea, but it reduces the texture of fruits and vegetables. To inhibit the enzymatic browning reaction, the activity of the enzyme must be lowered. Methods for this include lowering the pH using citric acid, lowering the temperature, and packaging with an inert gas such as nitrogen.
Next, the process of bread turning brown is different from that of apples. Leaving flour at room temperature for a long time does not turn it brown. The reason why bread turns brown is due to non-enzymatic browning reactions, which are caused by the Maillard reaction. The Maillard reaction is named after Louis Camille Maillard, who first published this reaction. The Maillard reaction occurs when a highly reactive sugar such as glucose reacts with a protein. This browning reaction usually occurs when food is heated, resulting in the formation of not only melanoidind, a brown polymer pigment, but also various aromatic compounds. This is why bread, sesame seeds, and biscuits have a savory aroma when they are fried or baked. The Maillard reaction is very important for food quality, so much research has been conducted on it, but the exact reaction pathway has not yet been fully elucidated.
In addition to the Maillard reaction, another non-enzymatic reaction is the caramelization reaction. The caramelization reaction occurs when sugar is heated to a high temperature. This reaction is accelerated by a small amount of a specific acid. Although proteins do not participate in this reaction, the caramelization reaction has similarities to the Maillard reaction. The caramelization reaction also produces brown polymer pigments and volatile compounds that give off a peculiar aroma, and the intermediate substances produced during the reaction are the same. Examples of caramelization reactions include caramel candy and brown sugar.
The browning reaction is largely divided into enzymatic browning and non-enzymatic browning, and non-enzymatic browning is further divided into Maillard reaction and caramelization. Browning can deteriorate the quality of food, like the deterioration of fruit, but it also enhances the appearance of food by making it look ripe and giving it a pleasant texture. One thing to note is that foods that have been browned through 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, acrylamide, which is known to harm the nervous system, is produced when foods are fried at high temperatures. This substance has also been identified as a cancer-causing factor, so excessive consumption should be avoided.
