Pyrazines flavour

The oceurrence of a large number of pyrazines as flavouring or aroma eonstituents and as pheromones in extremely low coneentrations has led to mass spectrometry being the method of ehoiee for determining the gross struetural details of a pyrazine nueleus. The method appears to be generally applieable and relatively specifie and sensitive. 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

The MHLW is currently evaluating several individual flavouring substances not covered by the aforementioned groups but that are of commercial interest. Most of these substances are lower alcohols, aldehydes and pyrazines. As soon as the evaluation has finished, the result will be published and in positive cases the substances will by added to the list of permitted substances. 

Fig. 7.7 Some pyrazines that are important contributors to the flavour of fruits and vegetables...

Several mechanisms have been proposed for the formation of pyrazines in food flavours [18, 23, 25], but the major route is from a-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation (Scheme 12.3). Self-condensation of the aminoketones, or condensation with other aminoketones, affords a dihydropyrazine that is oxidised to the pyrazine. 

Pyrazines are mainly used in roasted, peanut and chocolate flavours. Alkyl pyrazines can be obtained by the reaction in an autoclave at high temperature (120-160 °C) of reducing sugars like rhamnose and ammonia. 

As for S-containing heterocycles, many N-containing heterocycles are also found in heat-treated foods as secondary flavours as a result of Maillard-type reactions between reducing sugars and amino acids. Pyrazines are N-heterocycles important contributors to the taste and aroma of roasted and toasted foods as well as vegetables and fermented foodstuffs. In cultures of Pseudomonas perolens ATCC 10757, amino acids such as valine, glycine and methionine were shown to... 

Scheme 23.22 Some nitrogen-containing flavour compounds produced by microorganisms, a Methylanthranilate formation from N-methyl methylanthranilate 1 Trametes sp., Polyporus sp. b Different pyrazines produced with microorganisms in optimised media 2 mutant strain from Pseudomonas perolens ATCC 10757 3 Bacillus subtilis, Brevibacterium linens 4 mutant strain of Corynebacterium glutamicum...

As the final pH of meat increases above the normal range of 5.6-5.8, there is a decrease in meat flavour.101102 The reaction between norfuraneol and cysteine, which plays a crucial role in meat flavour, is very sensitive to pH.103 At pH 4.5, the major volatile products are mercaptoketones, furan- and thiophenethiols, 2-methyltetra-hydrothiophenone, and 3,5-dimethyl-l,2-dithiolan-4-one, whereas, at pH 6.5, these compounds were detected, if at all, only in traces, except for the thiophenone however, pyrroles, pyrazines, thiazoles, and oxazoles had achieved prominence. The sensory observations are a clear reflection of such differences. Similar effects of pH had been found for the ribose-cysteine and other systems.104... 

The dipeptide camosine, /J-alanyl-i.-histidinc, is one of the most abundant N compounds present in the non-protein fraction of vertebrate skeletal muscles. It constitutes, for example, 50, 150, and 276 mg per 100 g of muscle tissue from chicken leg, bovine leg, and porcine shoulder, respectively. Chen and Ho138 examined its effects on volatile generation in a model system of ribose and cysteine (180 °C, 2 h, pH 5 and 8.5). These were complex the levels of thiophenes and some meaty compounds, such as 2-methyl-3-furanthiol, 2-furfurylthiol, and their associated dimers, were generally lowered, but those of important N compounds, such as pyrazines and thiazoles, which are known to elicit roasty and nutty flavours, were enhanced. 

Chilli or paprika is used for flavour, not heat, in some cuisines. Flavour is a complex sensation determined in the mouth. One of the most potent volatiles known to humans is found in chilli, the pyrazine 2-methoxy-3-isobutyl-pyrazine, the green bell pepper smell. Reports indicate that humans can detect this odour at two parts per trillion. Luning et al. (1994) reported the presence of more than 80 odour compounds in C. annuum. C.frutescenscv. Tabasco contained 125 compounds whose relative abundance changed with the season of harvest. The composition of aroma compounds of Tabasco differed significantly from that of the green... 

Chemists produce synthetic flavourings such as smoky bacon and even chocolate . Meaty flavours come from simple heterocycles such as alkyl pyrazines (present in coffee as well as roast meat) and furonol, originally found in pineapples. Compounds such as corylone and maltol give caramel and meaty flavours. Mixtures of these and other synthetic compounds can be tuned to taste like many roasted foods from fresh bread to coffee and barbecued meat. 

Other fused heterocycles have very attractive flavour and odour properties. Pyrazines, in general, are important in many strong food flavours a fused pyrazine with a ring junction nitrogen atom is one of the most important components in the smell of roast meat. You can read about the simple pyrazine that provides green peppers with their flavour in the Box on the next page. 

For analysis of free compounds, the headspace analysis with a multiphase fiber is even more interesting and less time-consuming. This approach can replace the majority of the quantification of free compounds measured by SPE techniques, considering also the possibility of the sampling automation using a GC-MS system which can be coupled to a statistical treatment of fragments abundance (Kinton et al., 2003 Cozzolino et al., 2006). Moreover, HS-SPME/GC-MS is a very effective and efficient method to analyze specific compounds present in trace levels at about ppt level, because they can be better and selectively enriched in the headspace. This method is employed nowadays to quantify some important and peculiar sensory compounds such as ethyl and vinylphenols, pyrazines, cork off-flavour substances (TCA, etc.) and other contaminants such as geosmine (Riu et al., 2002 Chatonnet et al., 2006) and, as shown below, sulphur volatiles. 

CIC The typical creamy-fatty note is produced by delta-octalactone and delta-deca-lactone, balanced with free fatty acids C8-C12 and 2-nonanone. The flavouring industry mainly uses gamma-nonalactone (the so called coconut lactone ) to impart the typical coconut flavour. The roasted coconut meat contains in addition pyrazines. 

CIC The flavour of freshly roasted filberts, is a balanced composition of alkyl pyrazines and aldehydes, derived from the Maillard reaction in combination with 4-methyl-5-hepten-2-one. 

Heat transforms ketoacids to methylketones like 2-heptanone, 2-nonanone with a green fatty metallic blue cheese note. Hydroxyy acids form the corresponding lactones. The creamy, buttery, coconut-like 5-decanolide, 4-dodecanolide, 5-dodecano-lide contribute to the sweet creamy buttery flavour in cream and butter. Lactose undergoes a caramelisation reaction to develop sweet, caramelic maltol and 4-hy-droxy-2,5-dimethyl-furan-3(2H)-one. Lactose and milk proteins react in a Maillard reaction to roasted, nutty, burnt notes such as 2,5-dimethyl pyrazine. 

CIC the typical sulphurous flavour is represented by the high concentration of dimethyl sulphide, combined with traces of 1,2-dithia-cyclo-pentene. The vegetable-green note results from 2-isopropyl-3-methoxy pyrazine, resembling raw potatoes, and 2-sec-butyl-3-methoxy pyrazine, a green bell pepper note. 

CIC the earthy odour of fresh potatoes is represented by 2-isopropyl-3-methoxy pyrazine. This earthy note is supported by the mushroom character of l-octen-3-ol. The key component of boiled potatoes is 3-(methylthio)-propanal, balanced with dimethyl sulphide. The high reaction temperatures in baked and fried potatoes start the Maillard reaction to form mainly heterocyclic components 2-ethyl-3,5-dimethyl pyrazine, 2-ethyl-6-vinyl pyrazine, 5-methyl-6,7-dihydro-(5H)cyclopenta-pyrazine, 2-acetyl-l,4,5,6-tetrahydro-pyridine are responsible for the roasted, nutty cracker-like flavour. The heat-induced degradation of the potato lipids and the frying oil imparts a fatty, tallowy character to the french fried potatoes. (E,E)-2,4-Decadienal, 2-octenal, octanoic acid and decanoic acid are main contributors to this fatty note. 

The model tests were carried out both iso-thermally (i.e. at constant temperature) and in temperature gradients (from warm to cold). Alcohols (e.g. ethanol, hexanol, octa-nol, decanol), aldehydes (e.g. decanal), ketones (e.g. diacetyl), acids (e.g. caproic acid), esters (e.g. ethyl acetate), terpenes (e.g. menthol, menthone, 6-pinene, limonene), amines (e.g. butyl amine), pyrazines and other classes of substances have been investigated as flavouring substances [6,9,11,13-18]. Fig. 5.6 shows the forma-... 

Pyrazine derivatives [27, R = CHMe2, CH(Me)Et, and CHjCHMcj] may be responsible for the flavour of this species (green peas). ... 

Maga J.A. (1982b) Pyrazines in flavour. In Food Flavours. Part A. Introduction. Morton I.D. and McLeod A.J., Eds, Elsevier Scientific Publishers, 283-323. 

Hawksworth, G. Scheline, R.R. (1975) Metabolism in the rat of some pyrazine derivatives having flavour importance in foods. Xenobiotica 5, 389-399. 

Pyrazines have low vapour pressure and an intense smell and contribute to desirable flavours and aroma of fresh vegetables (Midler Rap>p)ert, 2010). 2,3-Diethyl-5-methyl pyrazine has a nutty, meaty, roasted hazelnut odour (Burdock, 2002). 

Our results showed that the flavour profile of peas was affected by market class, cultivar location, and crop year. The highest total volatile compound (TVC) was observed in cultivars from marrowfat-market class. Crops grown in Meath Park location had the highest TVC. Furthermore, different volatile compounds were identified in pea cultivars. In both crop years, cultivars from the green-market class had the highest mean values of esters and hydrocarbons, whereas the highest value of alcohols was observed for the marrowfat-market class, and the dun-market class had the highest mean values of ketones and pyrazine. 3-Methyl-butanol, 1-propanol, 2-ethyl-hexanol, 3-methyl-butanal, trichloromethane, 2-butanone, dimethyl sulfide, ethyl acetate and 2,3-diethyl-5-methyl pyrazine were the most abundant volatile compounds observed in the pea cultivars. 

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