Unit
Food Science
Problem Area
Handling and Storing Plant
Products
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
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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. 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?