Lesson Enzymatic Browning

Student Learning Objectives 1. Explain how enzymatic browning occurs and how it affects food products. 2. Explain a variety of methods that can be used to stop or slow the process of enzymatic browning.

Terms antioxidant buffer chelate chelating agent enzymatic browning melanin pH polyphenol oxidase

What is enzymatic browning and how does it affect food products? Pealing, cutting, bruising, or natural deterioration of vegetables and fruits causes enzymes from the tissues to be released. Additionally, using copper or iron pans, length of exposure to air, and humidity increase this process. When these enzymes come in contact with oxygen a reaction occurs creating a brown pigment or discoloration. This browning cannot only make the food look unappealing, soften it and cause it to lose flavor, but it can also severely limit the shelf life of the product decreasing its value. Some foods can benefit from enzymatic browning. Foods that have a more desirable flavor or color as a result of enzymatic browning include cocoa, tea, coffee, raisins, and prunes.

A. Enzymatic browning is a biochemical process in which plant (fruit or vegetable) tissues take on a brown color when exposed to oxygen. Other types of browning in foods include maillard browning (proteins and sugars at high temperatures), carmelization (sugars at high temperatures), and lipid browning (fats at high temperatures). B. Enzymatic browning occurs when plant tissue is exposed to air resulting in a brown colored pigment, melanin, being produced as a result of a series of biochemical reactions. Phenolases which are enzymes found outside the cell wall come in contact with colorless phenols which are found inside the cell causing the brown color to appear.

C. Melanin is any of a group of brown or black pigments occurring in plants and animals. Melanin provides the color in the skin of humans. D. A naturally occurring enzyme called polyphenol oxidase acts as a catalyst to speed up the process which can occur rapidly at warm temperatures when the pH is between 5.0 and 7.0. Polyphenol oxidase is a copper-containing enzyme that catalyzes or causes the oxidation of phenol compounds contained in plant tissues.

How can enzymatic browning be stopped or slowed? The polyphenol oxidase enzyme can be inactivated by certain organic acids. Acids such as ascorbic acid and citric acid are commonly used in the food industry to prevent browning of fruits and vegetables. A. Ascorbic acid acts as an antioxidant to prevent enzymatic browning. An antioxidant is a substance that bypasses oxidation or inhibits reactions promoted by oxygen; often used as a preservative. Oxygen preferentially reacts with the ascorbate, rather than the phenolic compound in the fruit until the ascorbic acid is completely used up. The melanin can also be bleached by the ascorbic acid.

B. Citric acid acts as a chelating agent to prevent the fruit sample from browning. Citric acid inhibits the polyphenol oxidase enzyme by reducing copper ions which are necessary for the enzyme to be active. A chelating agent is a substance that causes a metal ion (eg. copper, iron) to react with another molecule to form a chelate. A chelate is a chemical compound in which the metal ion (eg. copper) is attached to neighboring atoms by at least two bonds in such a way as to form a ring structure. In addition, citric acid also lowers the pH inactivating the polyphenol oxidase.

C. Acetic acid lowers the pH of the fruit tissues, and browning is reduced or prevented. If the pH is reduced below 3.0, the polyphenol oxidase enzyme is inactivated. 1. The pH is a measure of the relative acidity or alkalinity of a substance. The pH scale is used to measure the hydrogen ion (H + ) concentration in a solution. B. The scale starts at 0 and ends at 14. A pH of 7.0 is neutral; the concentration of H + and OH - (hydroxide ions) is equal. A number lower than 7.0 indicates that the solution is acidic and a number higher than 7.0 indicates that the solution is basic or alkaline. Each step from one number to the next on the pH scale indicates a logarithmic increase. For example, a solution of 6.0 is ten times more acidic than a solution with a pH of 7.0; a solution of 8.0 is ten times more basic or contains 10 times as many hydroxide ions as a solution with a pH of 7.0.

2. A buffer is a weak acid or base that reacts with a stronger acid or base to help neutralize the substance or bring it closer to 7.0. Buffers are needed because strong acids and bases can disrupt chemical reactions causing the body to not maintain homeostasis (maintaining stable internal conditions in the body). 3. See pH scale on following page.

D. Fresh fruits placed in a water bath will not turn brown D. Fresh fruits placed in a water bath will not turn brown. Water restricts the amount of oxygen in contact with fruit tissues, and oxygen is necessary in the enzymatic browning process. E. Cooking foods (blanching) inactivates the polyphenol oxidase. However, this can be unappealing to many people. F. Minimal handling decreases browning. G. Improved cultivars, storage conditions, and atmospheric conditions are also being studied.

Review/Summary What is enzymatic browning and how does it affect food products? How can enzymatic browning be stopped or slowed?