General

Enzymatic and Non-enzymatic Browning of Foods

Browning is the process of food turning brown due to the chemical reactions that take place within. The browning process is one of the chemical reactions that take place in food chemistry. Browning has a variety of significant consequences for the food industry in regards to safety, technology and economic costs. While there are several different ways in which food chemically changes over time, browning in particular falls into two major categories: enzymatic and non-enzymatic browning processes. The browning of food can be either non-enzymatic (caramelized or maillard reactions) or enzymatic.

Non-Enzymatic Browning

Caramelization is a non-enzymatic reaction that happens as carbohydrates or sugars are heated in food. It is the process of separating water from sugar, followed by isomerization and polymerization. The Maillard reaction is a chemical reaction between the amino acid and the reduced sugar, typically involving the addition of heat. Like caramelization, it’s a kind of non-enzymatic browning. The sugar reactive carbonyl group interacts with the amino acid nucleophilic group, resulting in intriguing yet poorly characterized odor and taste molecules.

This mechanism accelerates in the alkaline atmosphere because the amino groups do not neutralize it. This reaction is the basis of the flavoring industry, as the form of amino acid determines the resulting taste. Hundreds of different flavor compounds are produced in the process. These compounds, in essence, break down to form even more new flavor compounds, and so on. Each type of food has a very distinctive collection of flavor compounds that are produced during the Maillard reaction. These are the same compounds that have been used by flavor scientists over the years to produce artificial flavours.

Enzymatic Browning

Enzymatic browning happens as the enzyme polyphenol oxidase or other enzymes catalyze the degradation of phenols in the fruit to produce compounds called quinones. Quinones can then polymerize to form melanins that cause brown pigments. Enzymatic browning of fruit and vegetables is typically not beneficial and causes significant economic losses for farmers. In the other hand, enzymatic browning is essential for the production of taste in tea (here the reaction is inappropriately called fermentation), the development of color and flavor in dried fruit such as figs and raisins.

Methods of Controlling Enzymatic Browning

For the food industry, managing enzymatic browning has always been a problem. To avoid or slow down the enzymatic browning of foods, a number of methods are used, each method aimed at targeting particular steps of the chemical reaction. It is possible to divide the various kinds of enzymatic browning regulation into two broad groups: physical and chemical methods.

Physical Methods

Heat Treatment: Treating food with heat, such as blanching or roasting, denaturates enzymes and kills the reagents responsible for browning. Blanching is used, for example, in wine making, in the production of tea, in the handling of nuts and bacon, in the preparation of vegetables for freezing. Meat is often partly browned under high heat before being introduced into a larger recipe to be baked at a lower temperature that causes less browning.

Cold Treatment: Refrigeration and freezing are the most effective methods of preserving food, stopping it from rotting. The activity of browning enzymes, i.e. the reaction rate, decreases at low temperatures. Thus, refrigeration helps to preserve the original appearance, colour and taste of fresh vegetables and fruit. Refrigeration is also used for distribution and retailing of fruit and vegetables.

Oxygen Elimination: The availability of oxygen is essential for enzymatic browning, so removing oxygen from the atmosphere helps to slow down the browning reaction. Removing or combining air with other gasses (e.g. N₂ or CO₂) during storage, such as vacuum packing or changed atmosphere packaging, bottled wine or juice, impermeable glass or edible coatings, dipping into salt or sugar solutions, prevents food free from direct oxygen interaction. Impermeable coating consisting of plastic or other materials protects food from being exposed to oxygen in the air which prevents lack of moisture.

Irradiation: Food irradiation using UV-C, gamma rays, x-rays and electron beams is another tool for enhancing food shelf life. Ionizing radiation inhibits the vitality of microorganisms responsible for food spoilage and slows down the maturation and growth of vegetables and fruits.

Chemical Methods

Acidification: Browning enzymes, like other enzymes, are active at a particular pH range. Acidifying agents and acidity regulators are commonly used as food additives to preserve the optimal pH in food products. Acids such as citric acid, ascorbic acid and glutathione are used as anti-browning agents.

Antioxidants: Many antioxidants are used as food additives in the food industry. These compounds react with oxygen and inhibit the mechanism of browning. They also interact with the intermediate products of the subsequent reactions and hinder the production of melanin. Ascorbic acid, N-acetylcysteine, L-cysteine, 4-hexylresorcinol, erythorbic acid, cysteine hydrochloride, glutathione are examples of anti-browning antioxidants.

Chelating Agents: Polyphenol oxidase requires copper as a cofactor for its functionality, which is why copper chelating agents inhibit the action of this enzyme. Many chelating agents have been researched and used in different fields of the food industry, such as citric acid, sorbic acid, polyphosphates, hinokitiol, kojic acid, EDTA, porphyrins, polycarboxylic acids, and various proteins.

Advantages of Enzymatic Browning

The final product, melanins has antimicrobial properties that prevent any infection and inflammation to the plant or fruits.
Melanin also has antioxidant, anticancer and antibacterial properties.
Enzymatic browning in some food enhance color and flavors in tea, coffee, cocoa, raisins and prunes.

Disadvantages of Enzymatic Browning