Metallic flavour

Stark, W. and Forss, D. A. 1962. A compound responsible for metallic flavour in dairy products. I. Isolation and identification. J. Dairy Res. 29, 173-180. 

Methional is also the key compound responsible for the defect of alcohol-free beers described as worty flavour. The other off-flavour in milk is a cardboard-like or metallic flavour, which develops in milk with a prolonged exposure to light. Compounds responsible for this off-flavour are secondary lipid oxidation products, such as hexanal, pentanal and some other substances formed by riboflavin-catalysed photooxidation of fatty acids. 

The coatings have a number of criteria to meet in order to satisfy the can manufacturers requirements, particularly in the two piece beer can market, as the flavour of beer is easily impaired. A 5-10 ppm iron contamination of a beer packed in a two piece electro tin plated steel can will impart a metallic flavour to the product It is therefore very important that the internal coating provides a protective dry film of high integrity. The following criteria are general requirements which must be met by the coating formulaton... 

The coated can is filled with a bottled or draught lager beer, left to stand, and then compared against a standard. Points of reference are changes in acidity, bitterness, and any metallic flavour compared to the standard. A trained and skilled flavour (taste) panel is normally used to assess this property. 

The corrosive effects to be considered (mainly simple corrosion of metals) are, as would be expected from the edible nature of foodstuffs which are not excessively either acidic or basic but which may contain sulphur, less severe than those often encountered with inedible materials containing reactive substances. The importance of corrosive efiects where foodstuffs are concerned lies not so much in the action of the foodstuffs on the metal involved as in the resultant metal contamination of the foodstuff itself, which may give rise to off-flavours, in the acceleration of other undesirable changes (by the Maillard reaction for example), and in the possible formation of toxic metallic salts. Metal ions generally have threshold values of content for incipient taste effect in different liquid foodstuffs. Except in the case of the manufacture of fruit juices and pickles, process plant failure through corrosion must be rare. Nevertheless all foodstuffs, particularly liquid ones, should be regarded as potentially corrosive and capable of metal pick-up which may be undesirable. 

The important commercial feature of these juices, especially significant with blackcurrant and tomato juices, is their ascorbic acid (or vitamin C) content, of which loss by oxidation is known to be accelerated both by heat and by metal (particularly copper) contamination. The effect of copper has been carefully investigated for pure ascorbic acid", and more recently ascorbic acid in blackcurrant juice and model systems. There are, however, oxidation inhibitors of different kinds (which may themselves be heat-sensitive) present in various fruits, which give differing results. The presence of metals will also affect flavours", may cause discoloration, and may give rise to clouding effects, as in apple juice. ... 

Crockery preferably is made from ceramic materials, although it is brittle and can break rather easily. Properties of ceramics, such as resistance to absorb flavours and low heat conductivity, however, are superior compared to metals and plastics. Therefore, we defined a task to design crockery with improved mechanical strength. 

Polysubstituted 3-thiofiirans which are receiving a great interest as flavour and odour chemicals have been obtained by mono-//J5osubstitution and ortho-metallation from 3,4-dibromofuran <96T4065>. Dihydrofuran is used in a new synthesis of ketones from acids via acyl hemiacetals (Scheme 8, <96JOC6071>). 

Lipid hydroperoxides are either formed in an autocatalytic process initiated by hydroxyl radicals or they are formed photochemically. Lipid hydroperoxides, known as the primary lipid oxidation products, are tasteless and odourless, but may be cleaved into the so-called secondary lipid oxidation products by heat or by metal ion catalysis. This transformation of hydroperoxides to secondary lipid oxidation products can thus be seen during chill storage of pork (Nielsen et al, 1997). The secondary lipid oxidation products, like hexanal from linoleic acid, are volatile and provide precooked meats, dried milk products and used frying oil with characteristic off-flavours (Shahidi and Pegg, 1994). They may further react with proteins forming fluorescent protein derivatives derived from initially formed Schiff bases (Tappel, 1956). 

Asymmetric catalysis involving metal catalysed hydrogenations and isomerisations is becoming increasingly important in the production of pharmaceuticals, agrochemicals and flavours and fragrances. More examples of... 

Certain chloropyrimidines can be metaUated even with BuLi " and the metallation of methoxy- and chloropyrazines is useful in the synthesis of flavouring compounds (Scheme 118) and the antiarrhythmic agent arglecin (Scheme 119) . ... 

The previous chapters have described the bond valence model and shown how it can be used to understand many aspects of the crystal chemistry of inorganic compounds, but the model has found application in many other fields ranging from metals to proteins. This chapter does not pretend to be a comprehensive review of the uses to which bond valences have been put. Rather it is intended to give a flavour of the wide range of problems that can be treated using the model, presented from the point of view of the scientific issues that need to be addressed rather than from the point of view of the model itself It is apparent that these applications extend well beyond the inorganic systems within which the model was developed, but the common feature is that all involve some form of acid-base bonding. 

In addition to the principal constituents listed in Table 1.1, milk contains several hundred minor constituents, many of which, e.g. vitamins, metal ions and flavour compounds, have a major impact on the nutritional, technological and sensoric properties of milk and dairy products. Many of these effects will be discussed in subsequent chapters. 

The first isolable product of oxidation of the triglyceride is a tasteless and odourless hydroperoxide. The hydroperoxide can undergo homolysis, usually catalysed by a metal ion, to form an alkoxy radical whose decomposition may follow either ( > or T scission, ( > Scission gives a free aldehyde with flavour and/or odour along with an intermediate free triglyceride radical that can react further. Alternatively the alkoxy radical decomposes according to () scission, leading to a bound, odourless aldehyde and an R° radical that can react with other neutral molecules, or with itself, to form R-R. 

Whey is the by-product of cheese-making. The traditional form of whey in confectionery is whey powder, which has been used as an ingredient in some toffees. Apart from this example, whey has not been much used in sugar confectionery. The reasons for this are hard to see, except that in toffees the flavours imparted by whey are not that pleasant. As the major ingredient is lactose this places another restriction on its use - lactose has its limited solubility compared with other sugars, when used to excess it imparts the unpleasant metallic taste, and it is not tolerated in the diets of certain consumers. 

On a small scale the dissolving and vacuum boiling are carried out in a steam-heated vacuum pan. An advance on this would be to use an open pan to dissolve the sugar and then to feed this to two vacuum pans. The vacuum-cooked mass is then poured onto a cooled metal slab and the mass is turned in on itself to cool the middle (Figure 6.4). Glucose syrup-based boiled sweets form a cool skin on the outside. This means that they can be manipulated by hand. Because of this skin it is necessary to fold the inside of the mass to the outside to cool it. When the mass has cooled sufficiently the colour and flavour may be added and kneaded in,... 

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