Chapter 10 :- Browning

 Introduction:- Browning is a common colour change seen in food during pre-preparation, processing, or storage of food. It occurs in varying degrees in some food material. The colour produced range from cream or pale yellow to dark brown or black, depending on the food item and the extent of the reaction. Browning reactions may be desirable or undesirable. In some food, the brown colour and flavour developed during browning is highly desirable and associated with a delicious, highly acceptable, and quality product. Browning reactions contribute to the aroma, flavour and colour of the product such as the browning crust of bread, and all baked goods, potato chips, roasted nuts, roasted coffee beans and many other processed foods.

The undesirable effects of browning reactions are seen in dehydrated food such as milk, eggs, dry fruits, in cut fruits and citrus fruit juice concentrates, in canned milk and in coconut.

The colour varies from light cream to black while coconut develops a saffron colour. The off-colour and off odour developed in foods depends on the extent to which the browning reaction has progressed. Off flavours may vary from mild flavour changes to stale and very bitter.

Controlled browning is necessary even in foods where browning is desired because excessive browning can produce an undesirable product.

(A.) Types (enzymatic and non-enzymatic):-

Enzymatic Browning 

The colour change that takes place in fruits & vegetables, etc due to the presence of enzymes is called as enzymatic browning.

• Light 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, bruising or other injuries to the tissues.
• This is due to the action of oxidative enzymes on the presence of phenolic substances present in the fruit and vegetable tissues.
• Apples, Banana, Pears, Brinjals and potatoes undergo enzymatic browning.
• Enzymatic browning takes place only in fruits and vegetables which contain phenolic compounds. These phenolic compounds act as the substrate, and in the presence of oxygen and by the action of enzymes, the following oxidative reaction is observed:

 

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Non-Enzymatic Browning

1. Maillard Reaction– Maillard was the first to describe the development of a brown colour in mixtures containing amino acids and reducing sugars. The reaction between certain free groups of amino acids, such as the NH2 group and a carbohydrate, affects the product in many ways in addition to the colour change. The aroma and flavour of the ready product is also affected. The reaction is also known as carbonylamine reaction or protein sugar reaction. 

The brown pigment formed contributes to the aroma, flavour, and colour of many ready to eat cereals, toffees, malted barley, and bakery products such as bread, cakes, and biscuits. If the dough contains less reducing sugar then the colour of the crust is light. The more the percentage of reducing sugar,  the darker the crust in bakery products such as dinner rolls.

2. Caramelization- Sugars are caramelized at 163 to 170 °C because of action of heat. Darkening of syrups, brown colour of candies such as caramels, taffy, and brittle are because of caramelization reaction. Caramelization or sugar browning reaction occurs with sugar alone. Sugar is broken down into a number of compounds because of intensive heat. 

             Caramel is used to flavour and colour alcoholic beverages, soups,gravies, sauces,soup cubes, cake mixes, coffee products, canned meat products, etc.

3. Ascorbic acid browning- Ascorbic acid present in fruits undergo oxidation with the formation of a compound which produces a brown pigment and causes discolouration. This type of browning is seen in preserves. Low storage temperature and addition of  bisulphites retard the reaction.

4. Lipid browning- This type of browning is seen in fats and fatty phases of food. The amino groups of phospholipids and lipoproteins can react with aldehydes and reducing sugars resulting in browning. 

              This type of browning is quite uncommon and may be observed in fats stored for long periods. It is an undesirable reaction. 

(B.) Role in food preparation:-

Browning reactions may be desirable or undesirable affecting the appearance, flavour, and aroma of food. All browning reactions can be controlled and hence it is essential that the caterer is aware of all factors which influence these reactions. 

           Enzymatic browning is considered unsightly and undesirable, especially while preparing salads and fruit salads. Natural browning is usually undesirable but normal cooking processes ,such as roasting, grilling, or baking, bring about brown colouration. This is expected and desirable and contributes to the acceptance and palatability of food. Cooking brings about chemical breakdown which along with colour, adds flavour to food. The aroma of freshly baked bread and roasted coffee beans is due to desirable Maillard reaction. 

             Food components interact chemically and give rise to a brown colour accompanied by modifications in flavour, aroma, and taste. Because food components are involved in the formation of the brown colour, there is some loss of nutritive value. 

(C.) Prevention of undesirable browning:-

It can be done by following methods:

By inactivating enzymes: Enzymes can be inactivated by any of the following measures.

1. Application of heat: Blanching or cooking fruits and vegetables which are prone to browning prevents discolouration. Enzymes are protein in nature and heat denatures proteins thereby inactivating the enzymes.
2. Addition of salt: Vegetables may be immersed in a solution of sodium chloride (NaCl) to retard enzymatic browning. The chloride ion of NaCl retards the reaction but this is a temporary measure as it would make food unpalatable.
3. Lowering the pH: Enzymatic browning is prevented by lowering the pH to 2.5-2.7 by addition of acid. Acids used to prevent browning are- Ascorbic acid, Malic acid, Citric acid and lime juice. Ascorbic acid or Vitamin C acts as an antioxidant and retards enzymatic browning.
4. Chilling of food below temperature optimum for enzymatic activity. The optimum temperature for enzymes to act is 43°C. In cold storage, the browning reactions slow down, but even fruits stored in frozen storage brown unless they are treated. 

By avoiding contact with oxygen  Oxygen should not come in contact with the substrate. This can be achieved by any of the following measures:

1. Coating fruit with sugar or covering it with syrup keeps atmospheric oxygen away from the surface.
2. Immersing vegetables in water Contact with atmospheric O2 can be avoided by immersing cut vegetables in water. Since water contains dissolved oxygen, it is more effective if it is first boiled to remove dissolved air.
3. Vacuum packaging It prevents the food from coming in contact with oxygen and thus prevents enzymatic browning since it is an oxidation reaction. 
4. By elimination of substrate This method of prevention is not practical. A variety of peach called 'sunbeam' is an exception because it is deficient in substrate. 

Sulphuring of fruits prior to dehydration Sulphur prevents oxidative browning due to enzyme activity. Fruit is treated with sulphur fumes prior to drying. Treatment with SO2 gas or sulphurous acid solution or 0.25% sodium sulfite for 45 seconds prevents browning. Bisulphites & metabisulfites are also used. Sulphurous acid is a strong reducing agent and prevents discolouration.

Key Terms:-

• Carbonyl group:- Free group of reducing sugars.
• Enzyme:- Catalyst made up of protein and responsible for most reactions in living tissues. Destroyed by heat as protein gets coagulated. 
• Maillard reaction:- A reaction between proteins or amino acids and sugars resulting in a brown colour, often accompanied by aroma and flavour when food is cooked. 
• Oxidation:- A reaction in which oxygen is gained or hydrogen is lost or loss of electrons. 
• Reducing sugars:- Sugars that contain the aldehydic or ketonic reducing group,e.g., glucose, fructose, lactose, and pentoses (five carbon groups).