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Lipid oxidation , maillard reaction and thiamin degradation into flow chart

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Jan 09, 2024 Jan 09, 2024

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  • Lipid Oxidation

Over the years, numerous researchers have evaluated the contribution of carbonyl compounds produced from lipids to the flavor of poultry. Intramuscular glycerol and structural fatty acids are present in lean meat. Thus, meat can develop both desired and unwanted flavors as a result of lipid oxidation. Desirable aroma components and odour are produced in cooked meats by mild thermal oxidative modifications of lipids (Aaslyng & Meinert, 2017).

Like other meats, poultry meat's flavor develops mostly because of its lipid content. Cooking meat releases hundreds of volatile chemicals as a result of lipid degradation, specifically the oxidation of lipids' fatty acid components. Aliphatic hydrocarbons, alcohols, ketones, esters, carboxylic acids, certain aromatic hydrocarbons, and oxygenated heterocyclic compounds like lactones and alkylfurans are examples of these molecules(Wojtasik-Kalinowska et al., 2023). Lipid-derived aldehydes account for 41 of the 193 total chemicals identified in the flavor of roasted chicken. The most prevalent aldehydes found in chicken flavor are hexanal and 2,4-decadienal, which are recognized as the main oxidation products of linoleic acid. But because 2,4-decadienal has a far weaker smell than hexanal, it's thought to be a more significant odorant for chicken flavor (Baptista et al., 2022).

Because cooked meat undergoes more rapid reactions than uncooked meat, which results in rotten off-flavours, a distinct composition of volatiles essentially gives meat its attractive flavor forms. Nevertheless, it is thought that the primary cause of the deterioration of meat flavor and the development of undesired "warmed over flavor (WOF)" in chicken meat products is a deficiency of α-tocopherol in chicken meat(Baéza et al., 2022). It has been observed that a number of volatile chemicals, such as 2-methyl-3-furanthiol, 2-furfurylthiol, methionol, 2,4,5-trimethyl-thiazole, nonanol, 2-trans-nonenal, and others, play a key role in the flavor of chicken. On the other hand, 2-methyl-3-furanthiol is believed to be the most important chemical component for the development of chicken flavor (Takahashi, 2018).

Lipid oxidation is a process that can produce a variety of chemicals that have unfavorable tastes. Fishy smells and unfavorable textural changes are caused by lipid oxidation, which is also a key contributor to poor quality. Lipid-oxidation products interact with proteins to produce these effects(Chu et al., 2023).Fish lipid oxidation is intimately associated with phospholipids that contain polyunsaturated fatty acids in their cell membranes because these lipids have a high degree of unsaturation and a large surface area that is vulnerable to oxidation(Amaral et al., 2018).

Prior research has demonstrated that, due to the high level of unsaturation and significant surface area rate of oxidation, fish exhibit lipid oxidation incidence that is dependent on a variety of factors, including the quantity of lipids in the sample, the oxidative conditions, the lipoxygenases' enzymatic activity, and the quantity of antioxidant compounds present(T. Wu et al., 2022) . The production of volatiles derived from lipids in fresh fish requires enzymatic involvement. Nevertheless, studies show that heme protein autooxidation is the primary nonenzymatic initiator of lipid peroxidation in nonliving fish tissue (Geng et al., 2023).  In reality, a number of catalysts, including hemoglobin, iron, and lipoxygenases, cause lipid oxidation in fish muscle. Moreover, lipoxygenases cause significant fishy smells, whereas hemoglobin is linked to a strong oxidized oil smell(H. Wu et al., 2022).

Volatile compounds derived from lipids are quantitatively dominant in beef, even when it is cooked without any subcutaneous, intermuscular, or intramuscular fat. Maillard-derived volatiles are primarily present in beef that is grilled under extreme conditions, usually over 300 °F (149 °C).
Aroma and flavor thresholds are key ideas to grasp when talking about these volatile chemicals(Gardner & Legako, 2018) .Parts per million (ppm) or parts per billion (ppb) of the volatile molecule are typically used to quantify the aroma threshold, which is the lowest concentration at which the component can be detected (generally dissolved in water or another solvent). This measurement is helpful in describing and comprehending how each of these volatiles adds to the overall flavor of beef, as approximately 75% of beef is water (Kerth & Miller, 2015). For beef volatiles formed from Maillard processes, the thresholds for lipid-derived volatile compounds are often greater than those for heterocyclic compounds containing sulfur and nitrogen. This indicates that a stronger concentration is needed for the human nose to identify them as aromas. This implies that a greater quantity of lipid-derived compounds must be generated in order for them to be identified.  Generally speaking, heating raises the quantity of lipid-derived compounds produced in beef since it has a higher lipid content than some other protein sources(Diez-Simon et al., 2019) .

Hexanal is typically the most prevalent volatile chemical present in cooked beef, and the bulk of these compounds are six to ten carbon-saturated and unsaturated aldehydes. Nonanal and hexanal are two common volatile chemicals found with fatty acid heat hydrolysis, as one might predict given that oleic acid (omega-9 monounsaturated fatty acid) is the most prevalent fatty acid and that omega-6 fatty acids make up around 20% of phospholipids(Ramalingam et al., 2019) . Significant contributions are also made by heptanal, pentanal, and 2,4-decadienal. This illustrates how species preferences are strongly associated with fatty acid profiles and highlights the role lipids play in the evolution of the fundamental meat preference(Mwangi et al., 2019).

  • Maillard reaction

One of the primary chemical reactions that occur when meat is cooked is the Maillard reaction.  Food experts are particularly interested in the Maillard reaction, a kind of non-enzymatic browning.  The Maillard reaction is essential for food since it produces color, flavor, and off-flavor, lowers nutritional value, may be hazardous (due to the potential synthesis of nitroso derivatives and imidazoles), and ultimately has antioxidant characteristics (Tamanna & Mahmood, 2015).
This usually happens between decreased sugars and amino compounds, and the end product is an immense quantity of chemicals that give meat its flavor. The carbonyl group of a reducing sugar condenses with amino molecules to create Amadori products during the early stages of this reaction via glycosylamine. Rearranging and dehydrating the resultant product via deoxyosones forms a variety of sugar dehydration and degradation products, including derivatives of furfural and furanone, hydroxyketones, and dicarbonyl compounds. The aroma compounds are produced by these chemicals interactions with other reactive substances like amines, amino acids, aldehydes, hydrogen sulfide, and ammonia(Yu et al., 2020).

A significant similar event is the Strecker breakdown of amino acids by dicarbonyl compounds generated by the Maillard reaction. During this reaction, an amino acid's decarboxylation and deamination result in the formation of an aldehyde, and the dicarbonyl compound produces an α-aminoketone or amino alcohol. When cysteine is used as the amino acid, Strecker degradation additionally produces hydrogen sulfide, ammonia, and acetaldehyde(Ferreira & Guido, 2018) . Carbonyl substances from the Maillard reaction interact with these molecules to produce intermediates that are then involved in additional flavor-forming reactions. In the end, this results in the production of a variety of important types of flavor compounds, such as heterocyclic compounds, furans, pyrazines, pyrroles, oxazoles, and thiophenes (J. Liu et al., 2015).

The main ingredients that give meat its flavor are carbonyl compounds and sulfur compounds that come from ribose and cysteine. In chicken, ribose is recognized as the most significant flavor precursor. Furthermore, it has been established that thiamine acts as a key precursor for a variety of sulfur compounds. Ribose is produced as a result of the breakdown of nucleotides, like IMP. It subsequently participates in several secondary processes that result in the production of a vast number of volatile chemicals. As a result, IMP is frequently seen as the main nucleotide in muscle that gives meat its flavor. Generally speaking, chicken flesh has an IMP concentration of 75–122 mg/100 g; however, breed-specific variations do occur (Zhou et al., 2019).

During the Maillard reaction, ribose interacts with sulfur-containing amino acids (cysteine or cystine) or peptides (glutathione) to form 2-methyl-3-furanthiol in chicken broth. While cysteine generates hydrogen sulfide after prolonged heating, glutathione releases hydrogen sulfide quickly during the first phases of cooking. Furfural and cysteine react to generate 2-furfurylthiol, another significant odorous substance found in chicken broth. Furthermore, the main ingredients in meat that is grilled under extreme conditions are the volatile substances produced by the Maillard reaction(Hui et al., 2012a) .

Although the browning of beef at higher temperatures is commonly linked to the reaction of carbonyl groups with free amino acids, this reaction can also happen at room temperature or in the refrigerator, for example, when dehydrated foods like beef jerky darken and take on distinct flavors. Furthermore, the caramelization of sugar during heating—possibly without the presence of proteins or amino acids—should not be confused with browning, which is a hallmark of the Maillard reaction(Xia et al., 2021) . An aldehyde is formed when the amino acid undergoes decarboxylation and deamination, whereas an aminoketone or aminoalcohol is produced when the dicarbonyl is transformed. Hydrogen sulfide, ammonia, and acetaldehyde can also be produced by the Strecker breakdown of an amino acid, specifically cysteine. Furans (containing O), pyrazines (containing N), pyrroles (containing N), oxazoles (containing N), thiophenes (carrying S), thiazoles (containing S), and other heterocyclic compounds are among the numerous significant groups of flavor chemicals that these lead to. The extremely complex Maillard and related processes provide a wide range of compounds that go into generating the flavors that are commonly characterized as roasted, browned, meaty, caramelized and fishy etc (Kang et al., 2019).

  • Lipid-Maillard interactions

The interaction of these products is inevitable, as would be predicted given the hundreds of chemicals that may potentially be formed from the lipid breakdown and maillard processes. Sometimes the presence of products from one reaction process completely or partially blocks the formation of certain products found in the other, and other times it results in the formation of additional volatile compounds(Sun et al., 2022) . The Maillard reaction may be triggered by aldehydes produced during thermal lipid degradation at the start as well as the end of thermal processing. One can produce volatile alkyl side-chained pyridines, pyrazines, thiophenes, thiazoles, and oxazoles (Chansataporn et al., 2019).

In fried chicken and heated chicken, respectively, thiazoles with C4 to C8 n-alkyl substituents in the 2-position and additional alkylthiazoles with a greater number of 2-alkyl substituents (C13 to C15) have been stated(Jayasena et al., 2013) .

Longer alkyl chains have been found in the volatiles of roasted beef, but other thiazoles with four- to eight-carbon alkyl substituents in the 2 position have been described in roast beef and other alkylthiazoles. While the volatile chemicals produced in each of the major reactions have stronger olfactory thresholds and stronger odor intensities, those from lipid-Maillard interactions often have weaker odors . Furthermore, it's possible that the volatiles created by these two systems' interactions will indirectly affect the production of volatile taste components(Vilar et al., 2022). Important processes in the Maillard reaction that lead to the synthesis of heterocyclic fragrance compounds are inhibited by phospholipids and the products of their breakdown. Consequently, this inhibition may lessen the production of hetercyclics containing sulfur during the thermal processing of beef. Interestingly, this inhibition may act as a check and balance in the balanced creation by maintaining appropriate amounts of numerous sulfur compounds that may smell bad at larger concentrations(Zhang et al., 2020) .

  •  Thiamin degradation

One most important ingredient thought to contribute to the flavor of cooked meat is thiamin. It has been discovered by researchers that thiamin's heat breakdown results in several ending and intermediate taste compounds(Güntert et al., 1992). It was believed that thiamin's thermal breakdown is a rather complicated reaction with multiple pathways of degradation that result in flavor compounds that are intriguing to taste, most of which have one or more atoms of sulfur and/or nitrogen and many of which have heterocyclic structures(Kaczmarska et al., 2021) .

According to a report, 4-methyl-5-(2-hydroxyethyl)thiazole is one of the main products of thermally-degraded thiamin. This compound then reacts to form thiazoles and other sulfur compounds, like 5-hydroxy-3-mercaptopentan-2-one, which in turn yields some sulfur-containing compounds, including thiophenes and furans (DWIVEDI et al., 1972). It has been demonstrated that heating temperature and pH levels have an impact on thiamin degradation products. The predominant volatile chemicals with meaty aromas at pH 5.0 and 7.0 were 2-methyl-3furanthiol and bis (2-methyl-3-furyl) disulfide, along with thiophenes. However, when the pH is raised to 9.0, the amounts of these meaty compounds drop. A few other compounds that were found to have aromas were 4, 5-dimethylthiazole (earthy, skunky), 3-thiophenethiol (cooked, meaty), 2-methyl-4, 5-dihydro-3(2H)-thiophenone (sour-fruity, mushy, green), 2-acethylthiophene (burnt), 2-formyl-5-methylthiophene (meaty), and 2-methyl-3- (methyldithio) furan (meaty)(Hui et al., 2012b).

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