Enzymatic Browning in Food Chemistry

Browning

Dr.Anamika Dixit

•

Browning is the process of food turning brown due to

the chemical reactions that take place within. The

process of browning is one of the chemical reactions

that take place in food chemistry and represents an

interesting research topic regarding health, nutrition,

and food technology. Browning is the process

of food turning brown due to the chemical reactions

that take place within. The process of browning is one

of the chemical reactions that take place

in food chemistry and represents an interesting

research topic regarding health, nutrition,

and food technology.

Type of browning

•

Enzymatic BrowningLight colour Fruits and

vegetables darken when exposed to air as a

result of the presence of oxidative enzymes.

•

Enzymatic browning occurs in those fruits and

vegetables when the cellular organization is

disrupted by cutting,...

How does enzymatic browning

happen?

•

Enzymatic browning affects mostly fruits and

vegetables. When you peel or cut open a fruit

with a knife, oxygen is introduced to the

damaged part. The presence of oxygen

triggers the enzymes, polyphenol oxidase

(PPO) in apples for example, to oxidize

phenolic compounds in the tissues. This

results in formation of quinones,

which 

polymerize to form brown pigments

called melanins

.

•

Not only PPO in present in fruits and

vegetables. PPO is also responsible for the

black spots in crustaceans like shrimps and

prawns. The black spots begin to manifest

hours or days after harvest.

The ways to control browning

•

To increase the yield and inhibit PPO in foods, many

methods are used. These methods are classified into

two groups: chemical methods and physical methods.

•

These physical methods work by the use of high or low

temperature, by removing oxygen, and irradiation.

They work by destroying the enzymes, slowing down

the enzyme activity or by oxygen removal. While

chemical methods include the use of antioxidants,

acidifying agents and/or chelating agents. Also, a new

way to fight browning is also starting to gain a

following—genetic engineering.

Blanching

•

One common way of using high temperature to stop enzymatic browning is

blanching. Through this method, 

the fruit or vegetable is heated in a short time (a

few minutes)

. And then treated with cold air or iced water to immediately bring

the temperature down. Not only 

it slows enzyme activity down

. It also helps clean

fruits and vegetables of dirt and microorganisms, and retain nutrients.

•

As a part of the pre-treatment process, 

blanching is widely used in many parts of

the food manufacturing industry

. For example, when preparing vegetables for

freezing, blanching is necessary to stop the enzyme activity that may affect the

color, texture, and flavor during storage.

•

One common way of using high temperature to stop enzymatic browning is

blanching. Through this method, 

the fruit or vegetable is heated in a short time (a

few minutes)

. And then treated with cold air or iced water to immediately bring

the temperature down. Not only 

it slows enzyme activity down

. It also helps clean

fruits and vegetables of dirt and microorganisms, and retain nutrients.

•

As a part of the pre-treatment process, 

blanching is widely used in many parts of

the food manufacturing industry

. For example, when preparing vegetables for

freezing, blanching is necessary to stop the enzyme activity that may affect the

color, texture, and flavor during storage.

REFRIGERATOR

•

One common way of using high temperature to stop

enzymatic browning is blanching. Through this method, 

the

fruit or vegetable is heated in a short time (a few

minutes)

. And then treated with cold air or iced water to

immediately bring the temperature down. Not only 

it slows

enzyme activity down

. It also helps clean fruits and

vegetables of dirt and microorganisms, and retain

nutrients.

•

As a part of the pre-treatment process, 

blanching is widely

used in many parts of the food manufacturing industry

.

For example, when preparing vegetables for freezing,

blanching is necessary to stop the enzyme activity that may

affect the color, texture, and flavor during storage.

Oxygen removal

•

Enzymatic browning can only take place in the

presence of oxygen. When the oxygen removed,

the browning process slows down. There are a

few ways to achieve this. Today, 

Vacuum

packaging and modified atmosphere packaging

(MAP) are widely used

. Both processes started in

the 20th century, the time of many innovations in

food science.

•

Oxygen can turn apples brown in just 2 minutesWondering why the part of the apple you had taken a bite of became brown in just minutes? No longer appealing to eat, right? In food

science, there are two types of browning: 

enzymatic and non-enzymatic browning

. Years of research has been done to better understand these two.

•

This is 

to take advantage of non-enzymatic browning

 in many food items like nuts, baked products, fried foods and beans. And secondly, 

minimize and control the enzymatic

browning

 in foods especially fruits and vegetables.

•

Let’s discuss the two further.

•

Table of Contents

•

Enzymatic browning

–

How does enzymatic browning happen?

–

The ways to control browning

•

Blanching

•

Refrigeration and Freezing

•

Oxygen removal

•

Use of antioxidants and acidifying agents

•

Irradiation

•

Use of copper-chelating agents

•

Genetic modification

–

Benefits of enzymatic browning

•

Non-enzymatic browning

–

Caramelization

•

How does caramelization happen?

–

Maillard reaction

•

How does Maillard reaction happen?

•

Enzymatic browning

•

Enzymatic browning is an oxidation process that occurs in foods. It usually results in undesirable quality changes—in color, taste, texture and even nutrition.

•

How does enzymatic browning happen?

•

Enzymatic browning affects mostly fruits and vegetables. When you peel or cut open a fruit with a knife, oxygen is introduced to the damaged part. The presence of oxygen triggers the

enzymes, polyphenol oxidase (PPO) in apples for example, to oxidize phenolic compounds in the tissues. This results in formation of quinones, which 

polymerize to form brown pigments

called melanins

.

•

Not only PPO in present in fruits and vegetables. PPO is also responsible for the black spots in crustaceans like shrimps and prawns. The black spots begin to manifest hours or days after

harvest.

•

Most enzymatic browning is not desirable

. To a consumer, it is maybe simple. Just remove the browning part of a banana and that’s it. But looking at the bigger picture, enzymatic

browning is not a simple predicament. Through the years, the global food industry has experienced economic losses due to enzymatic browning. In fact, research shows that

 over half of

produce is lost as a result of enzymatic browning

.

•

The ways to control browning

•

To increase the yield and inhibit PPO in foods, many methods are used. These methods are classified into two groups: chemical methods and physical methods.

•

These physical methods work by the use of high or low temperature, by removing oxygen, and irradiation. They work by destroying the enzymes, slowing down the enzyme activity or by

oxygen removal. While chemical methods include the use of antioxidants, acidifying agents and/or chelating agents. Also, a new way to fight browning is also starting to gain a

following—genetic engineering.

•

Blanching

•

blanching can delay enzymatic browningOne common way of using high temperature to stop enzymatic browning is blanching. Through this method, 

the fruit or vegetable is heated in a

short time (a few minutes)

. And then treated with cold air or iced water to immediately bring the temperature down. Not only 

it slows enzyme activity down

. It also helps clean fruits

and vegetables of dirt and microorganisms, and retain nutrients.

•

As a part of the pre-treatment process, 

blanching is widely used in many parts of the food manufacturing industry

. For example, when preparing vegetables for freezing, blanching is

necessary to stop the enzyme activity that may affect the color, texture, and flavor during storage.

•

Refrigeration and Freezing

•

The 

low temperature involved in refrigeration and freezing slows down reaction rates

. As a result, foods retain their qualities: color, texture, and flavor. Furthermore, 

foods like fruits

and vegetables, in their frozen state preserve their nutrients

. And in some frozen fruits and vegetables, they contain more vitamins and minerals than their counterparts.

•

Further reading: 

What Is Chilling in the Food Industry?

•

Oxygen removal

•

Enzymatic browning can only take place in the presence of oxygen. When the oxygen removed, the browning process slows down. There are a few ways to achieve this. Today, 

Vacuum

packaging and modified atmosphere packaging (MAP) are widely used

. Both processes started in the 20th century, the time of many innovations in food science.

•

Vacuum packaging

•

Vacuum packaging involves 

removing oxygen before sealing the pack, providing an airtight environment

. The lack of oxygen also helps the food from spoiling quickly since

microorganisms require oxygen to survive. Today, a wide variety of foods come vacuum packed like cured meats, cheese, cereals, and chips.

•

Modified atmosphere packaging (MAP)

•

As the name suggests, 

MAP involves altering or changing the composition of gases inside a sealed food package

. Basically, MAP aims to lower the level of oxygen in the pack while

increasing the level of another gas like carbon dioxide and nitrogen. This ability to customize the proportion of gases in the packaging enables the food to maximize its shelf life.

•

Another benefit of MAP is the reduced ethylene production in fresh produce

, which retards the ripening process. This is beneficial particularly to farmers who raise climacteric fruits.

Read more about 

climacteric and non-climacteric fruits

.

•

Today, MAP is more popular as a result of demand for fresh products and foods that are minimally processed.

•

Use of antioxidants and acidifying agents

•

As proven by studies, 

antioxidants and acidifying agents contain anti-browning properties

. Ascorbic acid, an antioxidant (also called vitamin C) prohibits the conversion of phenols to

quinones. Another thing is that 

enzyme activity declines as acidity lowers

. 

Enzymes like PPO have an optimum pH level of 6.5 to 7.0

. pH is a measure of how acidic or basic a substance is.

This is why food items that contain

 vitamin C like lemon juice are effective to prevent browning fruits and vegetables

. Crushed vitamin C is also an option.

•

Glutathione is an antioxidant that reduces melanin production. You may have probably heard of this antioxidant as a component in skin whitening. But glutathione does help prevent

browning in food.

Antioxidant

•

As proven by studies, 

antioxidants and acidifying agents

contain anti-browning properties

. Ascorbic acid, an

antioxidant (also called vitamin C) prohibits the conversion

of phenols to quinones. Another thing is that 

enzyme

activity declines as acidity lowers

. 

Enzymes like PPO have

an optimum pH level of 6.5 to 7.0

. pH is a measure of how

acidic or basic a substance is. This is why food items that

contain

 vitamin C like lemon juice are effective to prevent

browning fruits and vegetables

. Crushed vitamin C is also

an option.

•

Glutathione is an antioxidant that reduces melanin

production. You may have probably heard of this

antioxidant as a component in skin whitening. But

glutathione does help prevent browning in food.

IRRADIATION

•

Food irradiation

 is the 

application of ionizing radiation

to prolong the shelf life of foods

 by destroying

microorganisms. To date, 3 sources of radiation are

used as approved by FDA: gamma rays, x-rays, and

electron beam. Irradiation can also fight enzymatic

brown in fruits and vegetables by delaying sprouting

and maturation.

•

You might ask: is it safe to consume irradiated foods?

Yes, definitely. In fact, astronauts of National

Aeronautics and Space Administration (NASA) eat

irradiated meat during their trips to space.

•

Non-enzymatic browning

•

The other type of food browning is non-

enzymatic browning. Unlike enzymatic

browning, the 

brown pigment in foods

manifest without enzyme activity

, as the

name suggests. There are 

two forms of non-

enzymatic browning: caramelization and

Maillard reaction

.

Caramelization

•

Caramelization is a form of 

non-enzymatic

browning that occurs when the

carbohydrates

are heated

. It is the process of

browning of sugar in foods, to simply put. As

you may already know, 

caramelization results

in desirable flavor (caramel or nutty) and

brown color in foods.

Maillard Reaction

•

The Maillard reaction is another form of non-enzymatic

browning. However, unlike caramelization, 

Maillard

reaction requires sugars and amino acids in proteins to

take place

. Foods such as biscuit, steak, french fries, and

roasted coffee owe their flavor, taste and especially color to

Maillard reaction.

•

This form of non-enzymatic browning was first observed in

1912. That year, French chemist 

Louis Camille Maillard

investigated the reaction between amino acids and

sugars

 when heated. Then in 1953, American chemist, John

E. Hodge established the mechanisms in the Maillard

reaction pathway. However, even up to this day, food

scientists still do not fully understand this complex reaction

•

There are many factors that affect Maillard

reaction. These include:

•

Time

•

Temperature

•

Water activity

•

pH

•

Reducing sugar

•

amino acid

•

food composition