Caramelization degradation reactions

If a nitrogen source is added, or if residual nitrogen-containing compounds are present, classical non-enzymatic browning (MaiHard) along with caramel ization can occur. Although the Maillard reaction is not the major emphasis of this review, it is difficult not to include it in discussions of carbohydrates and their degradation products. 

With alkaline degradation, the formation of 1,2-enols is also the initial step. This reaction in turn can produce three-carbon compounds which yield a series of intramolecular reactions involving condensation and polymerization. Both acidic and alkaline caramelization produce numerous volatile and nonvolatile compounds that significantly contribute to aroma, taste, and color. 

Furans are the most abundant products of the Maillard reaction and they account for the caramel-like odor of heated carbohydrates (8). Some sugar degradation compounds, such as maltol, isomaltol, 4-hydroxy-5-methyl-3(2H)-furanone, 2,5-dimethyl-4-hydroxy-3(2H)-furanone, and 2-hydroxy-3-methyl-2-cyclopentene-l-one (cyclotene), have odors usually described predominantly... 

The flavors produced by the Maillard reaction also vary widely. In some cases, the flavor is reminiscent of caramelization. The Strecker degradation of a-amino acids is a reaction that also significantly contributes to the formation of flavor compounds. The... 

The flavors of foods such as wheat, peanuts, and sesame, after being cooked, are quite different from those of the raw materials. Flavor formation from flavor precursors in the processed foods is primarily via the Maillard reaction, caramelization, thermal degradation, and lipid-Maillard interactions. 

The products of caramelization are distributed between volatile and nonvolatile fractions. The composition of the volatile firaction is pretty well characterized, contrary to that of the nonvolatile fraction. Thus, neither is the structure of all compounds formed precisely known, nor are the processes which occur understood in detail (see, for instance, a review by Orsi ). The composition of the volatile fraction from the thermolysis of sucrose is the best recognized. The profound decomposition products from the decomposition in vacuo of sucrose arc water, carbon monoxide, carbon dioxide, formaldehyde, acetaldehyde, methanol, and ethanol. The detailed rates and temperature relationships suggest that, with the possible exception of ethanol, the other products result from secondary reactions of dehydration products. The low-molecular-weight portion of the nonvolatile fraction of the thermal degradation of sucrose contains D-fhictose, D-glucose,... 

Even if the water is rapidly removed by vacuum, at this stage the reactions will inevitably tend to follow the course of those occurring in very concentrated solution. The sugar anhydrides readily form dimers, levulosan in particular being converted into diheterolevulosans. It is now known that similar products are formed by refluxing fructose in 80% solution (24). The reactions of caramelization cannot be limited to the initial stages without simultaneous further dehydration and polymerization, as well as extensive degradation to produce hydroxymethylfurfural (13). 

The degradation of sugars in the absence of amino acids and proteins by heating them over their melting point and thereby causing color and flavor changes is called caramelization. If this reaction is not carefully controlled it could lead to the production of unpleasant, burnt, and bitter products. If the reactions are carried out under controlled conditions, pleasant qualities of caramel are obtained. 

For chocolate production, the raw cocoa is stored, shipped, and processed. The processing steps are roasting and liquor production. The heat treatment induces Maillard reactions, caramelization of sugars, protein degradation, and formatiiMi of volatile aroma components [85, 89]. An often applied step to cocoa is the dutching, the alkali treatment of cocoa powder in order to modify the color, and other physiochemical properties. The pH values of cocoa powders are adjusted from ph 5.3-5.8 in natural powders to higher than 7.6 in heavily dutched materials. The total flavanol contents are reduced from more than 34 to 3.9 g kg In the same way, the antioxidative properties of the powders are diminished [90]. 

The most important reactions which provide volatile carbonyl compounds were presented in sections 3.7.2.1.9 (lipid peroxidation), 4.2.4.3.3 (caramelization) and 4.2.4.4.T (amino acid decomposition by the Strecker degradation mechanism). 

Starch degrades to dextrins, mono- and disaccharides at the relatively high temperatures to which the outer part of the dough is exposed. Caramelization and nonenzymatic browning reactions also occur, providing the sweetness and color of the crust. The thickness of the crust is dependent on temperature and baking time (Table 15.51) and type of baked product (Table 15.54). The composition of some types of bread is presented in Table 15.55. 

During acid hydrolysis, a number of side reactions occur (cf. 4.2.4.3.1). Reversion products are formed in amounts of 5—6% of the glucose used. These are predominantly isomaltose (68—70%) and gentiobiose (17—18%), and, in addition, other di- and trisaccharides. Furthermore, degradation products of glucose are formed, e. g., 5-hydroxymethylfurfural and other compounds typical of caramelization and the Maillard reaction (cf. 4.2.4.4). 

The mutual condensation of some degradation products of sugars yields important alicychc substances called cyclopentenolones, which are characterised by a caramel flavour similar to maltol, and other secondary reaction products of sugars. For example, condensation of hydroxyacetone with lactic aldehyde yields the basic member of the homologous series 2-hydroxy-3-methylcyclopent-... 

Furanones have a planar enol-oxo-configuration and a caramel-like odor. Among the many products from carbohydrate degradation, 3(2H)-furanones belong to the most striking aroma compounds. The conversion of sugars into furanone seems to involve a series of rearrangement and tautomerization, but whether the furanones detected in citrus products are the favored reaction products at low pH of citrus and are obtained exclusively by a nonenzymic reaction is not yet clear. 

---