The distinctive flavor of grilled, roasted and baked foods develops as a result of the Maillard reaction. This is a specific type of chemical reaction that occurs in hot and dry conditions between the nucleophilic amino groups of amino acids and the reactive carbonyl groups of reducing sugars.
In this case study, scientists used NMR spectroscopy to uncover an inhibitor of the Maillard reaction, which prevents the formation of visible color and flavor. The study may be used in further research to enhance the taste of food.
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Products of the Maillard reaction give foods that have been grilled, roasted or baked the characteristic taste that is quite different from if they had been boiled, poached or steamed.
A wide range of foods undergo the Maillard reaction, from seared steaks and fried fish, to biscuits, bread, and toasted marshmallows. The differences in cooking aromas arising from the different food types are caused by the reaction occurring between different sugars and amino acids.
The Maillard reaction is recognized as a key factor in developing the flavor of food, and chefs have devised numerous techniques to optimize this process. Since it is the products of the Maillard reaction that impart the desired flavor, researchers have been focusing on the identification of Maillard reaction products which could be used for flavor enhancement in the food industry.
Maillard reaction products
The Maillard reaction results in the formation of a wide range of compounds that contribute to both the flavor and appearance of food. These compounds are thus of interest to the food industry as additives that can be used to improve and preserve the appearance and flavor of processed foods1.
However, Maillard reaction products can undergo cross-linking and polymerization when subjected to heat, which makes them unsuitable for use as food additives. Once the food is heated during meal preparation the Maillard reaction products are likely to be lost, with detrimental effects on flavor2.
For this reason, stable Maillard reaction intermediates formed during the initial stages of the Maillard reaction are used instead3. When heated, such intermediates, for example, the Amadori rearrangement product, generate thermal decomposition products that cause desirable changes in food color and flavour4.
Maillard reaction intermediates are physicochemically stable and thus are considered ideal substitutes to Maillard reaction products for use as food additives that provide a fresh and desirable flavor on exposure to heat during subsequent food preparation5.
Isolating Maillard reaction intermediates
Since the final product will be destined for human consumption, it is preferable to use aqueous solution for the purification of Maillard reaction intermediates, rather than organic solvents.
However, it is difficult to determine the formation conditions of stable intermediates at a high reaction rate in aqueous conditions. Fortunately, the formation of Maillard reaction intermediates can be indicated indirectly using cysteine as a marker3,5.
Researchers have recently used cysteine as an indicator to show the critical formation conditions of Maillard reaction intermediates at varying temperatures6. The Maillard reaction was monitored in aqueous solution using L-cysteine as the indicator. Stepwise increases in temperature were used to identify the temperature at which the Maillard reaction intermediate 2-threityl-thiazolidine-4-carboxylic acid (TTCA) was produced.
Maillard reaction intermediates, TTCA and the Amadori rearrangement product, were purified and identified by ultrahigh performance liquid chromatography with mass spectroscopy and nuclear magnetic resonance (NMR) spectrometry. All NMR experiments were performed on a Bruker DRX 400 MHz spectrometer (now replaced by the Bruker AVANCE series).
Cysteine and TTCA were mixed into a solution and treated under various thermal conditions to investigate the mechanism of browning and flavor enhancement.
During the low-temperature reaction stage, the intermediates were gradually formed and result in the greatest accumulation of Maillard reaction intermediates. In addition, low-temperature thermal reaction reduced the decomposition of Maillard reaction intermediates and increased its reacting potency with added cysteine.
The browning of the final Maillard reaction products and concentration of downstream degradation products indicates that added cysteine reacts with TTCA to inhibit the formation of visible color. This was made possible by preventing the generation of dicarbonyl compounds derived from the Maillard reaction intermediates.
The addition of cysteine was shown to affect the normal reaction pathway from TTCA to Amadori rearrangement products and other downstream products by restoring TTCA to sugar and amino acid under heat treatment.
This study demonstrates that the addition of cysteine to TTCA can inhibit the progression of the Maillard reaction to a later stage, preventing the formation of visible color (and flavor).
References
- Nooshkam M, et al. Food Chem. 2019;275:644-660.
- Kaitano TE. Food additives: Flavors and flavor enhancers. In Encyclopedia of Food Safety, Motarjemi, Y.; Moy, G.; Todd, E., Eds. Academic Press: Cambridge, 2014:466-470.
- Cui H, et al. Food Chem. 2019;271:47-53.
- Haase PT, et al. J. Agric. Food Chem. 2017;65(9):1924-1931.
- Cui H, et al. RSC Advances 2017;7:45442-45451.
- Zhai Y, et al. J. Agric. Food Chem 2019. Epub ahead of print 16 Jul 2019. DOI: 10.1021/acs.jafc.9b04374
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