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.
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:
Non-Enzymatic Browning
1. Maillard Reaction– it is a protein-sugar interaction. It leads to the development of brown colour in a mixture containing amino acids and reducing sugars. It is also called carbonyl-amine reaction. The brown colour formed contributes to the aroma, flavor, and colour of many ready to eat cereals, baked food, malted barley etc. various factors which affect this reaction are-type of sugar- the more percentage of reducing sugar, the darker the crust in the bakery products, type of amino acid, temperature, moisture.
Maillard reaction also leads to undesirable changes like unfavourable brown colour eg. In dried milk. Detrimental flavour changes eg. Burnt flavour.
Condensation of amino group of protein brown colour+ carbonyl group of sugar
2. Caramelization– when sugar is heated at high temperature(160 deg C) in the absence of water and amino acids, it turns brown and this sugar-sugar interaction is known as caramelization.sugar is broken down into a number of compounds due to intense heat.
sucrose caramel + acid
development of caramel flavour and colour in caramel custard is a desirable change, whereas uncontrolled heating of sugar can lead to objectionable burnt colour and flavour.
3. Reaction of oxidation products-
- ascorbic acid browning
- lipid browning
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 temp can help in retarding the reaction.
Lipid browning-many fatty foods during processing and storage exhibit browning which can be attributed to the reaction of lipid oxidation products with amino acids, amines and proteins. For eg.white fish containing PUFA and high concentration of protein shows discolouration in muscle during frozen storage.