UNIT 10 - BROWNING

INTRODUCTION

Browning is the process of food turning brown due to the chemical reactions that take place within. The process of food browning is one of the most important reactions that take place in food chemistry and represents an interesting research topic regarding health, nutrition, and food technology.
Though there are many different ways food chemically changes over time, browning in particular falls into main 2 categories,
1.    Enzymatic and
2.    Non-enzymatic processes.
The browning process of foods may yield desirable or undesirable results, depending on the type of food.

Enzymatic Browning

• Enzymatic browning is one of the most important reactions that takes place in most fruits and vegetables as well as in seafood.
• These processes affect the taste, color, and value of such foods. 
• Generally, it is a chemical reaction involving polyphenol oxidase, catechol oxidase, and other enzymes that create melanin's and benzoquinone from natural phenols. 
• Enzymatic browning (also called oxidation of foods) requires exposure to oxygen. 
• It begins with the oxidation of Phenols by Polyphenol oxidase into Quinones.

Examples:
Developing color and flavor in Coffee, Cocoa beans, and tea.
Developing color and flavor in dried fruit such as figs and raisins.

Non-enzymatic browning

• It is a process that also produces the brown pigmentation in foods, but without the activity of enzymes. 
• The two main forms of non-enzymatic browning are caramelization and the Maillard reaction. 
• Examples non-enzymatic browning:
• Fresh fruit and vegetables, including apples, potatoes, and black spots on peels bananas and avocados.
• Polyphenols oxidases is the major reaction in the formation of Melanosis in crustaceans such as shrimp. 

Caramelisation

• Caramelisation is the oxidation of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color.
•  Caramelization is a type of non-enzymatic browning reaction. 
• As the process occurs, volatile chemicals are released producing the characteristic caramel flavor. 
• The reaction involves the removal of water (as steam) and the break down of the sugar. 
• The caramelization reaction depends on the type of sugar. Sucrose and glucose caramelize around 160 degree C (320 degree F) and fructose caramelizes at 110 degree C (230 degree F).
• The highest rate of the color development is caused by fructose as caramelization of fructose starts at 110 degree C. Baked goods made from honey or fructose syrup will therefore give a darker color.

Maillard reaction

• The Maillard reaction, creates flavor and changes the color of food. 
• Maillard reactions generally only begin to occur above 285°F (140°C). 
• Until the Maillard reaction occurs meat will have less flavor.
• The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat.
•  Like caramelization, it is a form of non-enzymatic browning. 
• The Maillard reaction occurs between reducing sugars and principally free amino acids and peptides (usually from proteins) when heated.
•  The reaction is also known as the browning reaction.
• Browning by the Maillard reaction occurs more quickly in alkaline than in acid conditions and also at intermediate water activities. The reaction is also time/temperature related. 
• Thus baking at low temperatures slowly gives the same colour results as baking at high temperatures quickly provided that the atmosphere around the product does not become too dry.
• The Maillard reaction is most important for the production of brown hues on the surface of baked biscuits. The inclusion in biscuit dough of glucose or invert syrups is to ensure that the Maillard reaction occurs as required. If there is excessive Maillard reaction it may be difficult to dry the biscuit without too much colour formation. Sometimes proteins are added as milk powders. Milk contributes lactose which is a reducing sugar. The Maillard reaction contributes flavours to baked products.
• It reduces the nutritional value of food.

Prevention of Enzymatic Browning

Enzymatic browning is the second largest cause of quality loss in fruits and vegetables.
Chemical, physical (blanching, freezing), controlled atmosphere and coating methods, can be used to prevent enzymatic browning .

1. Treatment with antioxidant agents

 Antioxidants can prevent the initiation of browning by reacting with oxygen.

2. Treatment with agents of firmness 

Calcium salts are the best known; they are used in the strengthening of cell walls. The cell walls are more stable to different treatments.

3. Treatment with acidifying agents 

PPO is sensitive to pH variations. The fruit is a naturally acidic environment, additional acidification may reduce the PPO activity or inactivate it below pH 3.

4. Blanching 

Blanching food is a heat treatment. Blanching treatments are presented according to the heat medium used: blanching in boiling water and/or in steam; blanching by using microwave was also developed the last years. The blanching time varies depending on the technique used, the type of product, size or maturity status.
This process inactivates the enzymatic systems responsible for sensory and vitaminic alterations . In addition, the colours of plants are heightened, for better presentation.

5. Freezing 

Freezing is a technique often used to stop browning reactions in fruit. Indeed, freezing causes a decrease in available water for enzymatic reactions.

6. Conservation in modified atmosphere 

Oxygen is essential for the oxidation reaction and PPO activity, a solution to control enzymatic browning reactions would be to change the oxygen content of the storage atmosphere. The studies dealt with modified atmosphere packaging, by modifying the composition of atmosphere, showed that the enzymatic systems are delayed without altering product quality

7. Coating 

The coating agents are usually used to extend the shelf-life of fruits during their storage. It consists on the application of a layer of any edible material on the surface of fruit. Actions of these agents deal with the decrease of moisture and aroma losses, the delaying of colour changes and gas transfer, and the improvement of the general appearance of the product through storage.