In this blog post, we’ll explore why apples and bread turn brown from the perspective of their respective browning reactions, and interestingly examine their differences and similarities.
You can observe that an apple, which looked fresh just moments ago, quickly turns an unappetizing brown when left at room temperature for a short time. Similarly, when you brush flour with egg and bake it in the oven, only the egg-coated parts turn a delicious golden brown. These two phenomena share one commonality: they both involve a ‘browning reaction’. Such browning reactions can sometimes negatively impact food quality, while other times they contribute to enhancing it.
However, the browning reactions in these two cases do not occur through the same mechanism. First, let’s examine the process of apple browning. As commonly known, this phenomenon occurs when apples react with oxygen. This is called ‘enzymatic browning’. Enzymatic browning happens when enzymes present in fruit, such as polyphenol oxidase and tyrosinase, react with oxygen and phenols to produce brown melanin pigments. While enzymatic browning is desirable in products like red ginseng or black tea, it degrades the texture of fruits and vegetables. To inhibit enzymatic browning, enzyme activity must be reduced. Methods include lowering pH using citric acid, reducing temperature, or packaging with inert gases like nitrogen.
Next, the process by which bread turns brown differs from that of apples. Leaving flour at room temperature for a long time does not cause it to brown. Bread browns due to non-enzymatic browning reactions, specifically the Maillard reaction. The Maillard reaction is named after Louis Camille Maillard, who first described it. The Maillard reaction occurs when highly reactive sugars, such as glucose, react with proteins. This browning reaction typically happens when heat is applied to food, resulting in the formation of brown polymeric pigments called melanoidins, as well as various aromatic compounds. This is why toasting or roasting bread, sesame seeds, biscuits, etc., produces that savory aroma. The Maillard reaction is critically important for food quality and has been extensively studied, yet its precise reaction pathway remains incompletely understood.
Beyond the Maillard reaction, caramelization is another non-enzymatic reaction. Caramelization occurs when sugars are heated to high temperatures. This reaction is accelerated by the presence of small amounts of specific acids. Although proteins do not participate in this reaction, caramelization shares similarities with the Maillard reaction. Caramelization also produces brown polymeric pigments and volatile compounds that impart distinctive aromas, and the intermediates formed during the reaction are the same. Examples of caramelization include caramel candy and brown sugar.
Thus, browning reactions are broadly categorized into enzymatic browning and non-enzymatic browning, with non-enzymatic browning further divided into the Maillard reaction and caramelization. While browning reactions can degrade food quality, like spoilage in fruit, they also enhance appearance and texture by cooking food attractively. One important note is that foods browned through the Maillard reaction at high temperatures, such as French fries or onion rings, should only be consumed in moderation. During the Maillard reaction, acrylamide—a substance known to harm the nervous system—is produced when foods are fried at high temperatures. Furthermore, this substance is also identified as a potential carcinogen, so excessive consumption should be avoided.
