Lab Activity 1
IUG, Fall 2012
Dr. Tarek M Zaida
1
Nonenzymatic Browning
• Background
Brown colors often develop during:
1. processing,
2. storage, and
3. preparation
of foods and food ingredients.
2
Types of Browning
• 1. Enzymatic
Enzymes (e.g. Oxidases) responsible for reactions that
produce brown colors in food.

Example:
The enzyme polyphenoloxidase acts on phenols and
oxygen in plant foods to form brown colors. It only
occurs in fresh cut plants where the enzyme is still
active and has access to its substrates.
Note:Polyphenoloxidase requires Cu as a cofactor, so
copper chelators can limit enzymatic browning.
3
2. Nonenzymatic
In such browning cases, enzymes are not
required for the production of brown colors in
food.
There three types of nonenzymatic browning:
1. Caramelization
2. Lipid Browning
3. Maillard reaction
4
1. Caramelization
• The process in which brown colors and
pleasing aromas are resulted during the
heating of sugars.
Caramel colors are widely used to color cola
beverages and other foods.
• Sugars in solution are quite stable to heat in
the pH range of 3-7.
• Melting dry sugar or heating its solutions in
the presence of acidic or basic catalysts,
causes the sugar to caramelize.
5
2. Lipid Browning
• The polymerization of lipids after extensive
heating (e.g., in old frying oils).
6
3. Maillard Reaction
•
•
•
•
•
Also known as the browning reaction,
Is the phenomenon responsible for
Turning meat brown,
Converting bread to toast, etc.,
The Maillard reaction is named for LouisCamille Maillard, a French chemist who
studied the science of browning during the
early 1900s.
7
• Certain foods contain carbohydrates in the form
of sugars, while others contain amino acids in the
form of proteins.
• Sugars and amino acids often exist side-by-side,
as in the case of raw meats.
• They may also be blended together, as in the case
of bread dough.

As long as there is no outside catalyst, or
cause for change, the meat remains red and the
bread dough remains white.
8
• The Maillard reaction is the catalyst for change, primarily
by the addition of heat.
• When bread dough or meat is introduced to a hot oven, a
complex chemical reaction occurs on the surface.
• The carbon molecules contained in the sugars, or
carbohydrates, combine with the amino acids of the
proteins.
• This combination cannot occur without the additional heat
source.
 The end result of this chemical recombination is the Maillard
reaction. The surface of the heated bread dough is now brown,
as is the outer layer of the roasted meat.
9
Everyday-life applications of maillard reaction
1. Maillard reaction responsible for the brown color as
well as for the change of flavor (meat & bread).
2. Recipes containing both eggs, which contain protein,
and flour, which contains carbohydrates, benefit from the
Maillard reaction to achieve a pleasing browned
appearance.
3. Self-tanning products also rely on the reaction
between amino acids and sugars to create a brown skin
tone.
4. The Maillard reaction is also used to create artificial
flavorings, based on the hundreds of complex amino
acid/sugar combinations formed after the process.
10
The Maillard reaction occurs in three main
steps:
1. The initial step: formation N glycoside
11
2. After formation of N glycoside the immonium ion
is formed and then isomerize, this reaction is called
Amadori rearrangement and forms a compound
called ketosamine:
12
3. The ketosamine products then either dehydrates into
reductones and dehydro reductones, which are caramel, or
products short chain hydrolytic fission products such as
diacetyl, acetol or pyruvaldehyde which then undergo the
Strecker degradation.
13
Inhibition of Maillard Reaction
The Maillard reaction can be inhibited by:
1. keeping the temperature low,
2. reducing the accessibility of reagents or
3. adding sulfite ions.
14
• Sulfite ions are strong nucleophiles that can add
to aldehyde groups to form an intermediate that
does not take part in browning.
 The mechanism of action of a sulfite is believed
to be as follows:
• the browning intermediate DH reversibly binds a
sulfite ion to form a sulfonated intermediate
(DSH) which is not capable of browning.
15
DH= 3-deoxyhexosulose
16

Because sulfite ion is readily volatile as
sulfur dioxide gas and sulfur dioxide gas can
cause attacks in a proportion of asthmatics,
the additive has been prohibited by the FDA in
foods that are meant to be eaten raw.
17
• Sulfites are allowed to a defined limit in some
cooked foods because the additive escapes as
gas during the cooking process.
• There is no widely accepted replacement for
sulfites in foods although deprotonated thiols
(R-S-) are capable of acting as nucleophiles in
a similar manner to sulfite and, if they react
suitably with DH, may form the basis for an
anti-Maillard browning additive if flavor
problems can be overcome.
18
Experiment
• Materials
5% w/v solutions of the following:
Sugar
Amino Acid
Fructose
Lysine
Galactose
Methionine
Glucose
Serine
Maltose
Valine
Sucrose
19
Procedure
• 1. Combine 3 mL of each sugar solution with 3 mL of
each amino acid solution, in screw cap test tubes.
2. Cap the tubes loosely.
3. Autoclave at 121º C for 60 min (or prepare a boiling
water bath by bringing a beaker of water to a boil on a
hotplate), and heat the tubes for 60 min.
4. Remove tubes and allow cooling before handling.
5. Describe the aromas produced and the degree of
browning for each solution.
 Record the color as 0 = none, 1 = light yellow, 2 = deep
yellow, and 3 = brown.
20