**6. Flavour generation** *via* **Maillard reaction**

The Maillard reaction is incredibly complex. For instance, a simple example such as the reaction of glucose with ammonia gives evidence, using simple methods, of the formation of more than 15 compounds and the reaction of glucose with glycine gives more than 24. Using HPLC and TLC on solvent-soluble material only [0.1% (w/w) of reactants], about 100 components are detectable as reaction products of xylose and glycine (Hodge, 1953).In order to understand something so complex, it is necessary to draw up a simplified scheme of the reactions involved. This has been done most successfully by Hodge in 1953 (**Fig. 2**).

The discussion here is based on this.

24 The Development and Application of Microwave Heating

selectively heat some portions more.

*4.1.2. Surface area* 

*4.1.3. Specific heat* 

specific in nature.

somewhere in the interior.

metals or aluminum foils to reflect microwave energy away from corners and thus

In microwave heating, the product temperature rises above its ambient temperature due to volumetric heating. Higher product surface area therefore results in higher surface heat loss rate and more rapid surface cooling. During microwave heating, the highest temperature is not at the surface of the product (despite the higher intensity of power absorbed there) but

How much a food product will heat given a specific amount of energy depends on its heat capacity. The implication of this for microwave heating is that different food products heated together have different temperature histories. To control this, some microwave food packages are sealed tight to allow heat transfer between hotter and colder foods, thus giving

Microwave energy is transported as an electromagnetic wave in certain frequency bands in the range between about 0.3 GHz and 300 GHz. When microwaves impinge on a dielectric material, part of the energy is transmitted, part reflected and part absorbed by the material where it is dissipated as heat. Heating is due to `molecular friction' of permanent dipoles within the material as they try to reorient themselves with the oscillating (electrical) field of the incident wave. The power generated in a material is proportional to the frequency of the source, the dielectric loss of the material, and the square of the field strength within it. A material is subjected to microwave energy in a device known as an applicator or cavity. Considering all these features, it is possible to identify those candidate materials and processes that can use microwave heating effectively and understand microwave ingredient interaction mechanisms. Only after such a step is taken can microwave heating be exploited fully in terms of its unique characteristics, which include the facts that no contact is required between the energy source and the target and that heating is volumetric, rapid and highly

International convention dictates that microwave ovens (and other industrial, scientific and medical microwave applications) operate at specific frequencies, the most favoured being 2.45 GHz. At this frequency the electric field swings the orientation of water molecules 109 times every second, creating an intense heat that can escalate as quickly as 10 0C per second (Lew et al., 2002). Water being the predominant component of biological materials, its content directly influences heating. However, there are minor contributions from a host of other factors (Schiffmann, 1986): heating is accelerated by ionic effects (mostly salt content) and specific heat of the composite material (Decareau, 1992). Specific heat is an important property in the

similar temperature history for different foods in the same package.

**5. Microwave heating and the dielectric properties of foods** 

Hodge subdivides the Maillard reaction as follows:


Reaction C: Sugar dehydration Reaction D: Sugar fragmentation Reaction E: Amino acid degradation (Strecker degradation)

iii. Final stage: products highly coloured. Reaction F: Aldol condensation Reaction G: Aldehyde–amine condensation and formation of heterocyclic nitrogen compounds.

It is worth noting that Mauron (1981) calls the three stages Early, Advanced, and Final Maillard reactions, respectively. The way these reactions fit together is outlined in **Fig.2**. The

final products of nonenzymic browning are called melanoidins to distinguish them from the melanins produced by enzymic browning. Theoretically, the distinction is clear; however, in practice, it is very difficult to classify the dark brown products formed in foods, since they tend to be very complex mixtures and are chemically relatively intractable. Reaction H has been inserted into **Fig.2**. It represents the much more recently discovered free-radical breakdown of Maillard intermediates. Oxygen plays an essential role in enzymic browning, but is not essential for nonenzymic browning. It may help in fact, for example, in the formation of reductones, such as dehydroascorbic acid, but it may also hinder the progress of the reaction, for example, in oxidising 2-oxopropanal to 2-oxopropanoic acid.

**Figure 2.** Maillard reaction.

In relation to the flavours produced on exposure to microwave radiation, Yaylayan et al.(1994) examined many combinations of sugars and amino acids, grouping the latter into aliphatic, hydroxylated, aromatic, secondary, basic, amide, acid, and sulfur-containing ones. The odours observed were grouped into eight and they have been assigned to the above groups, as far as possible, below:

Caramel (1); Meaty (4); Nutty (9); Meaty + vegetable Fragrant (6); Baked potato (5) and Baked (3).

Shibamoto and Yeo (1994) have compared microwave (700 W, high setting, 15 min) and thermal treatment (reflux, 100 0C, 40 h) for a glucose–cysteine system. The conditions used were determined by the onset of browning and aroma formation. The two sets of conditions gave samples with similar popcorn and nutty flavours, but the microwaved samples also gave pungent, raw, and burnt aromas, absent from the conventionally heated ones. The sample prepared conventionally at pH 9 contained much higher amounts of methylpyrazine and 2,6-dimethylpyrazine, whereas the microwaved one gave a much higher amount of 4,5 dimethyloxazole and was the only one to produce 2,3-dihydro-3,5-dihydroxy-6-methyl-4Hpyran-4-one. Such data, to some extent, explain the differences in acceptability of the two types of heating. For browning to occur in microwaving, a minimum of 10% moisture is required. Surprisingly, microwaving at pH 2 gave about twice the absorption at 420 nm than at pH 9 (about 1 AU), the absorption for pH 5 and 7 samples being nearly 0 (0.1 AU).

26 The Development and Application of Microwave Heating

**Figure 2.** Maillard reaction.

Baked (3).

groups, as far as possible, below:

final products of nonenzymic browning are called melanoidins to distinguish them from the melanins produced by enzymic browning. Theoretically, the distinction is clear; however, in practice, it is very difficult to classify the dark brown products formed in foods, since they tend to be very complex mixtures and are chemically relatively intractable. Reaction H has been inserted into **Fig.2**. It represents the much more recently discovered free-radical breakdown of Maillard intermediates. Oxygen plays an essential role in enzymic browning, but is not essential for nonenzymic browning. It may help in fact, for example, in the formation of reductones, such as dehydroascorbic acid, but it may also hinder the progress

In relation to the flavours produced on exposure to microwave radiation, Yaylayan et al.(1994) examined many combinations of sugars and amino acids, grouping the latter into aliphatic, hydroxylated, aromatic, secondary, basic, amide, acid, and sulfur-containing ones. The odours observed were grouped into eight and they have been assigned to the above

Caramel (1); Meaty (4); Nutty (9); Meaty + vegetable Fragrant (6); Baked potato (5) and

of the reaction, for example, in oxidising 2-oxopropanal to 2-oxopropanoic acid.

As the Maillard reaction is a series of chemical transformations (Yaylayan, 1997) factors that influence a chemical reaction also affect the Maillard reaction. In general, the rates of chemical reactions depend primarily on temperature, pressure, time, and concentration of reactants. High temperature, pressure, and superheating of reaction solvent associated with microwave irradiation can accelerate simple or single-step chemical reactions such as esterification, hydrolysis and cyclization reactions (Richard et al., 1988; Bose et al., 1994). If the microwave heating is performed under a closed system, then the rate of the microwave reaction accelerates up to 1000 times. However, the time factor plays a crucial role in influencing the product distribution of more complex reactions when carried out under microwave heating. The influence on competitive and consecutive reactions is an important consequence of fast rate of heating under microwave irradiation that is especially pertinent to the propagation of Maillard reaction.
