Maillard water activity

The rate of aspartame degradation in dry mixes is more dependent on the water activity than on the temperature (23). In dry mixes, aspartame may also engage ia Maillard reactions with the aldehyde moieties of flavoting agents, resulting ia the loss of sweetness and flavor. Use of the corresponding acetals of the flavor compounds to avoid this reaction has been reported (24). 

The influence of the carrot variety on Maillard reaction Samples of six freeze-dried carrot varieties were equilibrated at room temperature to a water activity of 0.33 (J 7), corresponding to an average water content of 6.3 % (related to wet matter). Then the samples were heated to 55 °C for 30 h and the concentrations of Amadori compounds as well as the corresponding sensory changes were determined. The results are listed in Table IV. The amount of Amadori compounds formed by the heating process seems to be correlated... 

The importance of Maillard reaction products to the flavor of foods has received considerable attention. One group of Maillard products, the alkylpyrazines, are thought to contribute roasted, toasted and nutty flavor notes to a variety of foods. Several reviews have detailed the presence of pyrazines in a wide variety of foods (1-7). Considerable work has previously focused on mechanisms of formation and the effects of various parameters on pyrazine formation (8-17). Part one of this study reported on the effects of type of amino acid and type of sugar on the kinetics and distribution pattern of pyrazines formed (18). The current study investigates the effect of pH and water activity on the kinetics of alkylpyrazines formation. 

By thermal and shear forces during extrusion which cause protein rearrangement/degradation, the Maillard reaction progresses, and a wide variety of potent flavoring compounds can result. Reaction rates in turn are influenced by the types of sugars and amino acids present, temperature, water activity, duration of heating, and pH. 

There are various chemical reactions that proceed and may be accelerated at low values of water activity. Maillard reactions leading to lysine loss and brown color develop peaks at aw values around 0.5-0.8. Nonenzymatic lipid oxidation increases rapidly below aw = 0.4. Enzymic hydrolysis decreases with water activity to aw= 0.3, after which, it is negligible. 

The Maillard reaction between reducing sngars and amino acids under specific conditions (pH, water activity, and temperature) is primarily responsible for the production of heterocyclic volatile compounds such as pyrazines, pyridines, pyrroles, furans, and the Strecker aldehydes. Maillard reactions produce many potent aroma compounds identified in some roasted tree nuts, including 3-methylbutanal, 2,3-butanedione, methional, phenylacetaldehyde, 2-ethyl-3,5-dimethylpyrazine, and 2,5-dimethyl-4-hydroxy-3(2//)-furanone, among others. 

Cereals are more stable than other foods because they are low in total fat (2-5%) and contain relatively high levels of natural tocopherols (20-50 ppm a-, 13- and /-tocopherols). Cereals are also stabilized by antioxidant products formed during baking by the browning or Maillard reaction. Natural compartmentalization within plant cells and low water activity also contribute to the low susceptibility of cereal lipids to develop rancidity. As in other foods, since flavor deterioration in cereals is caused by minor amounts of lipid decomposition products, the amount of lipid in a product is much less important than its susceptibility to oxidation. 

Cuzzoni, M.T. Stoppini, G. Gazzani, G. Influence of water activity and reaction temperature of ribose-lysine and glucose-lysine Maillard systems on mutagenicity, absorbancy, and content of furfurals. Food Chem. Toxicol. 1988, 26,815-822. 

Many biochemical reactions can be induced by temperature increase in foods Maillard reactions, vitamin degradation, fat oxidation, denaturation of thermally unstable proteins (resulting in variation of solubility or of the germinating power of grains, for example), enzyme reactions (which can either be promoted or inhibited), and so on. Some of these biochemical reactions generate components suitable, for example, for their sensory properties (flavor development) others may be more or less undesirable for nutritional or potential toxicity reasons (vitamin losses, changes in color, taste or aroma, formation of toxic compounds). All the reactions are linked to the simultaneous evolution of product composition, temperature and water content (or chemical potential, or water activity), these factors varying diflferently from one point to another, from the center to the surface of the products. 

One of the most important parameters influencing flavor formation via the Maillard reaction is processing temperature. This effect is obvious if one considers, for example, the sensory quality of roasted vs. stewed meats. Stewed meat lacks flavor notes characteristic of the roasted product. This is primarily because the stewed product has a water activity of approximately 1.0 and never exceeds a temperature of ca. lOO C. The roasted meat, however, dries on the surface so water activity is substantially less than 1.0. Also, since the surface dries, surface temperature may exceed lOO C. The lower water activity and higher surface tanperatures favor the production of flavor compounds giving the meat roasted notes from the same basic reactants rather than stewed notes. 

Leahy and Reineccius [22] found the influence of water activity on the rate of formation of alkyl pyrazines in a model system to parallel that of classical Maillard reaction. Briefly, the accumulation of pyrazines reached a maximum when heated at an Aw of ca. 0.75 and decreased with either increasing or deCTeasing Aw. This observation suggests that the rate of pyrazine formation is controlled by the initial stages of the Maillard reaction as opposed to an effect on specific pathways of pyrazine formation. 

The rate of fat oxidation in foods depends greatly on water activity (see Section 7.9). In dry foods, oxygen can more easily penetrate into the material than in normal foods. The minimum rate of oxidation occurs in foods with water activity values around 0.3. This situation is explained by the decrease in catalytic activity of metals, by quenching free radicals and by formation of antioxidants in the Maillard reaction. In foods with higher water activity, the rate of oxidation is again higher, probably due to increased mobility of metal ions, which catalyse the autoxidation. 

Owing to the complexity of the Maillard reaction, its optimisation is not an easy task. Optimisation is also impeded by the fact that the individual factors, such as temperature, water activity and pH, do not act in isolation, but can influence each other. The study of the influence of individual factors on the reaction is considerably more difficult, and the interdependence of the various factors is often the cause of conflicting and contradictory results obtained in different situations. 

The Maillard reaction rate increases with gradually decreasing water activity and the maximum reaction rate is at a water activity of about 0.7-0.8, which is attributed to the increase in the concentration of reactants (amino compounds and reducing sugars). With further decrease of water activity, the reaction rate decreases. The Maillard reaction does not proceed at all when the water activity decreases to less than 0.2-0.3, because the mobility of reactants is too low. ... 

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