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Browning in Food: Browning reactions taking place during food processing
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When we cut apples or bananas, they turn brown in colour with time. This makes it unpleasant to eat. During roasting coffee beans, they turn brown which enhances its quality. Starchy or sugary food becomes brown and has a bitter taste. All these brown pigments produced are due to specific reactions taking place during food processing. There are negative impacts as well as positive impacts are observable. Food constituent present and processing conditions are equally important for these interactions. These include Enzymatic browning, Mailard browning and Caramelization.

Phenolic compounds; monopheols and diphenols present in fruits and vegetables act as the substrate to enzymatic browning. These phenolic compounds are important as antioxidants and the degradation of these results in loss of nutritional quality. Enzymatic browning occurs mostly during handling and preparation of food items and during long term storage. In the reaction phenolic compounds are converted into melanin pigments. Enzymatic browning can take place at refrigeration temperatures. Enzymatic browning is catalyzed by a group of enzymes called oxidoreductases which includes polyphenol oxidase, phenolase, tyrosinase and catecholase. Oxygen acts as the Hydrogen acceptor in the reaction. End product, melanin can bind with proteins which can be a problem in digestion. Enzymatic browning spoils the quality of dehydrated products, blanched products etc. Prevention can be done by inactivation of the enzyme system of the fruit or vegetable, decreasing oxygen concentration by dipping in water or using lime or salt.

Melanodin pigments are one of the major products of Maillard Browning which includes pyrazines, imidazoles, reductonoses, and hydroxymethyl furfurals. Maillard browning is also known as non-oxidative browning. These pigments are reddish brown in colour and insoluble in water at room temperature. Other products include volatile food flavours and aroma compounds, UV absorbing compounds. Aldehydes, Ketones and amines serve as the starting material for Maillard browning. pH and the amino acid composition matter a lot in this reaction. The structure of the end products depends on whether the substrate is an aldose or a ketose. These compounds undergo different arrangements (Amadori and Heyns) in the formation of the final product which can be further degraded via aldol condensation and polymerization. Nutritive value of foods is lost when amino sugars are formed. Essential amino acids are lost and lysine is the most susceptible. For Maillard browning to take place favourable pH should be above the isoelectric point of constituent amino acids. Usually in the range of pH 7.8-9.2, the reaction proceeds vigorously. In strongly acidic food, Maillard browning does not take place to a significant extent. Intermediate moisture levels are more suitable for this reaction. Metal ions increase the rate of these reactions. And pentoses, monosaccharides and aldoses are known to react faster than hexoses, disaccharides and ketoses respectively. Maillard browning can be controlled by reducing the moisture content, removal of substrates, limited use of heating steps and using chemicals such as Potassium metabisulfite (KMS) or Sodium metabisulfite (SMS). It is found that some products of Maillard browning are mutagenic. Some degradation products formed before Melanoidin pigment formation may contribute to nitrosoamine formation. Melanoidin production is a significant issue in case of roasting of cereals as it leads to loss of essential amino acids.

Stecker degradation is another process where essential amino acids are destroyed during heat processing and long term storage. Reactants include α-dicarbonyl compounds and amino acids. If Maillard browning has taken place in food, it is assumed that stecker degradation has occurred. This reaction does not contribute to colour, but important for flavour and aroma. Products from this type of browning include aldehydes, pyrazines, and sugar fragmentation products. Favourable conditions for this type of browning are almost similar to Maillard browning.

Caramelization occurs during direct heating of carbohydrates which results in thermolysis of sugars. Temperature is crucial for this process. In Caramelization, wide range of flavour compounds and brown pigments are formed. This process involves number of reactions resulting in anomeric shifts, ring size alterations, breakage of glycosidic bonds, dehydration etc. Products have conjugated double bonds which can absorb colour and are polymers which are not dissolved in water. Caramel pigments are commercially available and are used in products like coca cola, puddings etc. Rate of Caramelization is increased with high temperatures and high pH.
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Browning in Food: Browning reactions taking place during food processing00