Nutty flavours

The taste of the product includes nutty flavours that have been produced by Maillard reactions between reducing sugars produced by the malt and proteins present. These flavours and the included grains give the product its distinctive character. 

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. 

Bitter, off-tasting, cyclopeptides can also arise during the storage of cold pressed vegetable oils. For example, fresh linseed oil provides a nutty flavour, but on storage at room temperature it develops a bitter off-taste. The key bitter compound responsible for the bitter taste is the cychc octapeptide cyclo-(Pro-Leu-Phe-Ile-MetO-Leu-Val-Phe), known as cyclolinopeptide E, which contains methionine sulfoxide. 

In the second pathway, dehydration of 3-deoxyhexosones leads to hydroxymethyl furaldehydes. The reactions which follow the formation of these intermediates are complex and result finally in the formation of dark brown nitrogen-containing pigments and also N-heterocyclic compounds such as pyrazines and pyrroles, which appear to be mainly responsible for the roasted, bready and nutty flavours of heated foods. 

Oxazoles have been found in relatively few cooked foods, although over 30 have been reported in coffee and cocoa, and 9 in cooked meat. Oxazolines have been found in cooked meat and roast peanuts, but not to any extent in other foods. 2,4,5-Trimethyl-3-oxazoline has been regularly detected in cooked meat [26], and when it was first identified in boiled beef [27] it was thought that the compound possessed the characteristic meat aroma however, on synthesis it was shown to have a woody, musty, green flavour with a threshold value of 1 mg/kg [28]. Other 3-oxazolines have nutty, sweet or vegetable-like aromas and the oxazoles also appear to be green and vegetable-like [28]. The contribution of these compounds to the overall aroma of heated foods is probably not as important as the closely related thiazoles and thiazolines. 

At low levels (5 mg/L), diacetyl is considered to add complexity to wine aroma since it can impart positive nutty or caramel characteristics, although at levels above 5 mg/L it can resuit in spoilage, creating an intense buttery or butterscotch flavour, and is perceived as a flaw. Microbial formation of diacetyl is a dynamic process and its concentration in wine depends on several factors bacterial strain, pH, wine contact with lees, SO2 content (Martineau and Henick-Kling 1995 Nielsen and Richelieu 1999). The sensory threshold for the compound can vary depending on the levels of certain wine components, such as sulfur dioxide. It can also be produced as a metabolite of citric acid when all the malic acid has been used up. However, diacetyl rarely taints wine to levels where it becomes undrinkable. 

The most significant ketone produced by yeast is diacetyl (2,3-butanedione), a vicinal diketone, although malolactic fermentation is a more important source, when it is used in wine production. Having a sensory threshold of 0.2-2.9 mg/L, according to the type of wine, it is characterised by a nutty , toasty or buttery aroma depending on concentration (Martineau et al. 1995). Dry white wines tend to contain lower concentrations (0.1-2.3 mg/L) than red wines (0-7.5 mg/L) (Bartowsky et al. 2002 Martineau et al. 1995). Acetoin, which produces a buttery flavour, is formed by partial reduction of diacetyl, and is itself reduced to 2,3-butanediol. Acetoin is usually present at concentrations (<80 mg/L) much lower than its sensory threshold of 150 mg/L (Romano and Suzzi 1996). 

Under basic conditions thiamine degrades to 5-(2-hydroxyethyl)-4-methylthiazole (45, sulfurol) and the pyrimidine derivative 46 [71]. Sulfurol is used in compounded flavourings in the flavour industry. It is almost odourless as such [73] however, it can decompose giving rise to thiazole and its derivatives such as 4-methylthiazole (47), 4,5-dimethylthiazole (48), 4-methyl-5-ethylthiazole (49) and 4-methyl-5-vinylthia-zole (50) ]71, 74], which possess nutty, green notes [75]. 

Fresh coconut milk has a sweet juicy, fruity, slightly creamy, fatty flavour. Roasted coconut meat develops a nice creamy, nutty characteristic odour and taste. 

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 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. 

Several sulfur compounds have been detected as volatile constituents of truffles. 2,4-Dithiapentane is a major component of the volatile aromatic compounds of the Italian white truffle, Tuber magnatum. Over 120 compounds have been detected in the black Perigord truffle, T. melanosporum. These include dimethyl sulfide, 2-methylbutanol, 2-methylpropanal and2-methylpropan-l-ol. The nutty and earthy flavour is attributed to anisoles and polymethoxybenzenes. Truffles also produce a volatile steroid, androst-16-en-3-one (7.67), which when more concentrated has an unpleasant smell. The combination of these compounds produces an odour that is a powerful animal attractant. The capacity of animals to detect the presence of underground black truffles by these substances has been evaluated by burying samples of the different compounds. The animals located the dimethyl sulfide lure as well as the black truffle flavouring. 

Flax seed has a warm, earthy and subtly nutty, butter flavour. The seed can be eaten entirely, and has traditionally been used as an add-on to cereal or bread. Today flaxseed is experiencing a renaissance among nutritionists, the health conscious public, food and feed processors (see Chapter 6) and chefs alike. It is one of the richest sources of lignan and a-linolenic acid (> 50%), one type of fatty acid in the polyunsaturated co-3 fatty acid (PUFA) family, considered essential fatty acids. Lignans, like isoflavones, are one of the major classes of phytoestrogens, which are estrogen-like chemicals and act as antioxidants. 

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). 

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