Some topics are of general significance for many of the foodstuffs considered in this book and, accordingly, reference is made to them here.
Non-enzymatic 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 com-prises 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 sub-stances 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
Reducing sugar Amino compound
N-substituted glycosylamine
Fragmentation products
Low molecular weight coloured compounds and melanoidins
1,2-Eneaminol Amadori rearrangement
product
3-Deoxyosone 2,3-Enediol
Furfurals pyrroles
1-deoxyosone, 4-deoxyosone, 1-amino-1,4-dideoxyosone Strecker
aldehydes reductones
Heterocyclic amines
Cyclic flavour compounds
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 2-Acetyl-1-pyrroline Newly baked crust of wheat bread Pyridine
2-Acetyl-1,4,5,6-tetrahydropyridine
Cream crackers
Pyrazine Methylpyrazine Nut
Oxazole Trimethyloxazole Green, nutty, sweet Thiophene 2-Acetylthiphene Onion, mustard Products derived from the sugar
Furan Furaneol Caramel, strawberry
Carbonyl Diacetyl Butterscotch
Products derived from the amino acid Cyclic polysulphur
5-Methyl-5-pentyl-1,2,4-trithiolane
Fried chicken Sulphur-container Methional Mashed potato
Thiazole 2-Acetylthiazole 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 avail-able. 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 lac-tose. 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 Aw0.5–0.8.
Finally sulphite, by combining with reducing sugars and other carbonyl compounds, inhibits the reaction.
Enzymatic browning
This arises by the oxidation of polyphenols to o-quinones by enzymes such as polyphenol oxidase (PPO) and peroxidase (Fig. 1.23). A day-to-day example
OH
OH Polyphenol
oxidase
O2 H2O
O O
Polyphenol Quinone
Melanin Polymerisation
H2O2 2H2O
Peroxidase 1
2
Fig. 1.23 Polyphenol oxidation.
CH3 Maltol
OH O
O
O
O H
O O Isomaltol
Fig. 1.24 Some flavour compounds produced in caramelisation reactions.
would be the browning of sliced apple. In other foods, the reaction is wanted, for example, in the readying of prunes, dates and tea for the marketplace.
Whereas heating boosts non-enzymatic browning, the converse applies to enzymatic browning, as the heat inactivates enzymes. The alternative strate-gies to avoid the reaction are to lower the levels of polyphenols (the agent polyvinylpolypyrrolidone (PVPP) achieves this) or to exclude oxygen.
Caramel
This is still produced to this day by burning sugar, but in very controlled ways. The principal products are produced by the polymerisation of glucose by dehydration. The process is catalysed by acids or bases and requires tem-peratures in excess of 120◦C. In some markets, the word caramel is retained for materials that are produced in the absence of nitrogen-containing compounds and these products are used for flavouring value. Where N is present, then
‘sugar colours’ are produced and these are used for colouring purposes.
Caramel is polymeric in nature, but also contains several volatile and non-volatile lower molecular weight components that afford the characteristic flavour compounds, such as maltol and isomaltol (Fig. 1.24).
α-Tocopherol
β-Carotene
Catechin
Caffeic acid
Rutin HO
O
HO OH OH
OH
OH
OH HO
HO
HO
HO
HO OH OH
O
OH OH OH
OH
HO O
O
O O
O O
O N
Fig. 1.25 Some antioxidants.
Antioxidants
There is much interest in antioxidants from the perspective of protecting foodstuffs from flavour decay, but increasingly for their potential value in countering afflictions such as cancer, rheumatoid arthritis and inflammatory bowel diseases. Figure 1.25 presents a range of these antioxidants. Many are phenolics and act either by scavenging or by neutralising (reduction) the rad-icals that effect deterioration or by chelating the metal ions that cause the production of these radicals.
The tocopherols are fat soluble and are found in vegetable oils and the fatty regions of cereals, for example, the germ. The carotenoids (e.g. lycopene) are water soluble and are found in fruits and vegetables. The flavonoids are water-soluble polyphenols found in fruits, vegetables, leaves and flowers. Such molecules have particular significance for some of the products discussed in this book, notably wine, beer and tea. The phenolic acids, for example, caffeic and ferulic acids and their esters, are abundant in cereal grains such as wheat and barley.