Nonenzymatic browning

These are chemical reactions that lead to a brown colour when food is heated. The relevant chemistry is known as the Maillard reaction, which actually comprises a sequence of reactions that occurs when reducing sugars are heated with compounds that contain a free amino group, for example, amino acids, proteins and amines (Fig. 1.22, Table 1.7). In reflection of the complexity of the chemistry, there are many reaction intermediates and products. As well as colour, Maillard reaction products have an impact on flavour and may act as antioxidants. These antioxidants are mostly produced at higher pHs and when the ratio of amino acid to sugar is high. It must also be stressed that some of the Maillard reaction products can promote oxidative reactions. Other Maillard-type reactions occur between amino compounds and substances other than sugars that have a free carbonyl group. These include ascorbic acid and molecules produced during the oxidation of lipids.

The Maillard reaction should not be confused with caramelisation, which is the discoloration of sugars as a result of heating in the absence of amino compounds.

In the primary Maillard reaction, the amino compound reacts with the reducing sugar to form an N-substituted glycosylamine that rearranges to 1-amino-1-deoxy-2-ketose (the so-called Amadori rearrangement product). This goes forward in a cascade of reactions in various ways depending on the pH. At the pH of most foods (4-6), the primary route involves melanoidin formation by further reaction with amino acids. Other products are Strecker aldehydes, pyrazines, pyrolles and furfurals. The substances produced in these reactions have flavours that are typical of roasted coffee and nuts, bread and cereals. The pyrolle derivatives afford bitter tastes. The Maillard reaction may

1,2-Eneaminol

3-Deoxyosone

Furfurals pyrroles

Strecker aldehydes

Heterocyclic amines

Reducing sugar Amino compound

N-substituted glycosylamine

N-substituted glycosylamine

Strecker aldehydes

Heterocyclic amines

Carbonyl Amine Browning

Cyclic flavour compounds

Low molecular weight coloured compounds and melanoidins

Amadori rearrangement product

2,3-Enediol

1-deoxyosone, 4-deoxyosone, 1-amino-1,4-dideoxyosone

reductones

Cyclic flavour compounds

Low molecular weight coloured compounds and melanoidins

Fig. 1.22 The Maillard reaction.

Table 1.7 Some products of the Maillard reaction.

Type of compound Example

Flavour descriptors

Products derived from interactions of sugars and amino acids

Pyrolle Pyridine

Pyrazine Oxazole Thiophene

2-Acetyl-1-pyrroline 2-Acetyl-1,4,5,6-tetrahydropyridine Methylpyrazine Trimethyloxazole 2-Acetylthiphene

Products derived from the sugar Furan Furaneol

Carbonyl Diacetyl

Products derived from the amino acid Cyclic polysulphur 5-Methyl-5-pentyl-

1,2,4-trithiolane Sulphur-container Methional Thiazole 2-Acetylthiazole

Newly baked crust of wheat bread Cream crackers

Green, nutty, sweet Onion, mustard

Caramel, strawberry Butterscotch

Fried chicken

Mashed potato Popcorn also lead to aged or cooked characters in products such as processed orange juice and dried milk products.

The early products in the Maillard reaction are colourless, but when they get progressively larger, they become coloured and responsible for the hue of a wide range of foods. Some of these coloured compounds have low molecular weights, but others are much larger and may include complexes produced by heat-induced reactions of the smaller compounds and proteins.

The exact events in any Maillard-based process depend on the proportion of the various precursors, the temperature, pH, water activity and time available. Metals, oxygen and inhibitors such as sulphite also impact. The flavour developed differs depending on the time and intensity of heating for instance -high temperature for a short time gives a different result when compared with low temperature for a long time. Pentose sugars react faster than do hexoses, which in turn react more rapidly than disaccharides such as maltose and lactose. With regard to the amino compounds, lysine and glycine are much more reactive than is cysteine, for instance, but more than that, for the flavour also depends on the amino acid. Cysteine affords meaty character; methionine gives potato, while proline gives bready.

As water is produced in the Maillard reaction, it occurs less readily in foods where the water activity is high. The Maillard reaction is especially favoured at Aw 0.5-0.8.

Finally sulphite, by combining with reducing sugars and other carbonyl compounds, inhibits the reaction.

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