Food browning | Enzymatic and Non Enzymatic browning

What is Food browning?

Food browning is the process in which the food changes brown due to the chemical reactions that take place within the food.

Food browning reactions occurs very widely in food materials. The colours produced with range from pale yellow to dark brown or black, depending on the type of product and the extent of the reaction.

In some food browning is consider desirable, e.g., honey, chocolate, brown crust of baking products, etc. While in other foods it is detrimental, as in darkening of dehydrate fruits, vegetables, etc.

Browning reactions may be either enzymatic or nonenzymatic. Many of the enzymatic reactions are seen in fruits and vegetables, and involve the oxidation of polyphenolic compounds by oxidative enzymes in plant cells.

The nonenzymatic browning reactions frequently involve sugars or sugar-related compounds.

(1) Enzymatic food browning

Food browning

Many fruits and vegetables have a tendency to turn brown when damaged or when cut surfaces are expose to air, e.g., apples, bananas, potatoes, etc.

The forma tion of brown colour is due to the action of the enzyme phenolase (also known as polyphenol oxidase, tyrosinase or catecholase) on phenolic substances. Normally, the phenolic substrates are separate from phenolase in intact tissues and browning does not occur.

When foods containing such substrates are cut and expose to air continue browning of cut surface takes place.

The enzymatic browning is due to the oxidation of phenols to orthoquinones, which in turn rapidly polymerize to form melanin (the brown pigment).

When the substrate is a phenol, it is first converted by hydroxylation into orthodiphenol and then oxidized to orthoquinone. Tyrosine is the major phenolic substrate for phenolase action in foods.

Other phenolic substances are caffeic acid, protocatechuic acid and chlorogenic acid. The reactions occur in several steps and are catalyse by several enzymes, e.g., phenolases, peroxidases and others.

Melanin form by unusual during the processing of fruits and vegetables. Pigment formation can be eliminate by inhibiting enzyme action.

Heat treatment, addition of sulphur dioxide or sulphites and ascorbic acid are the commonly used methods of inactivating these enzymes. Both sulphur dioxide and ascorbic acid are strong reducing agents.

(2) Nonenzymatic food browning

Nonenzymatic browning reactions are respon sible for the colour and flavour of foods. Roasting of potatoes, like toasting of biscuits and baking of breads and cakes, produces a golden-brown colour.

Sometimes this reaction produces a desirable flavour (the chocolate flavour of cocoa beans), at other times the reaction is undesirable (dark brown of potato chips). Because this does not involve enzymes, it is distinguish from enzymatic browning by being name nonenzymatic browning.

Hence the presence of reactive reducing sugars is responsible for browning in foods. On heating the sugars undergo ring opening, enolisation, dehydration and fragmentation.

(a) Maillard reaction

food browning

Maillard reaction also known as Maillard browning is a colour, flavour, odour, and sometimes texture changes. Which results from a chemical reaction between proteins and carbohydrates. It is named after the Frenchman Maillard who discover it.

The Maillard reaction denotes a group of many complex reactions between

(i) nitrogenous compounds and sugars,
(ii) nitrogenous compounds and organic acids,
(iii) sugars and organic acids, and
(iv) among organic acids themselves.

The carbonyl group of acyclic sugars readily combines with the basic amino groups of proteins, peptides and amino acids, resulting in sugar amines.

Maillard browning is responsible for the desirable browning of most heated foods such as bread crusts, roasted meats.

This browning is accelerate by heat and occurs very quickly in ovens, slowly at room temperature, and very slowly at refrigerator temperatures.

Maillard browning is also accelerate by low moisture content. Commercially dried milk and home dried foods which are both low moisture and stored at room temperature.

And frequently discarded due to this reaction. Maillard browning becomes more pronounced with increased storage time.

Another nutrient which is affect vitamin B₁ (thiamine). Because it has an amino group, it acts in the same way as an amino acid in combining readily with sugar in the browning reaction.

Hence, Maillard reaction products predominate in browned foods. The condensation product of sugar and amine undergoes enoli zation and rearrangement.

Then condensation and polymerization to form red-brown and dark-brown compounds. The brown to black, amorphous, unsaturate heterogeneous polymers are “melanoids”.

(b) Caramelization

food browning

Sugar in dry conditions, when heated beyond their melting point decompose and form a brown mass known as caramel, Which has a bitter, astringent taste. The process of caramelization occurs at a high temperature, while Maillard reactions develop brown colour at low temperature.

With the use of suitable catalysts it is possible to carry out caramelization to provide either flavouring. For flavouring purposes, sucrose as concentrated syrup is caramelisation.

Besides reaction of sugars breakdown of ascorbic acid during storage of the products may be another possible reason for the development of browning.

Browning may also be due to metallic contamination, mainly by iron and copper salts. The tannin in fruits and vegetables reacts with the iron of the tinplate to form ferric tannate, which is black in colour and spoils the appearance.

Some times hydrogen sulphide gas is liberate (due to reaction between fruit acids and tinplate) from the canned product which, in turn, reacts with the iron of the can and forms black iron sulphide.

Besides, browning is also caused by unsuitable vessels, e.g, traces of copper (1 ppm) coming in contact with hydrogen sulphide form black copper sulphide.

Metallic contamination can be avoid by using glass containers. Normly coating the interiors of cans with suitable lacquers of eliminating use or iron and copper vessels.

Conclusion:

I hope you may get all the information concerning about food browning, when reading my article. After this, if you got any doubt then i will be able to help you tell me the comments section.

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