During roasting

Roasting occurs between temperatures of 530—650°C. Virtually no volatilisation of selenium or tellurium takes place during roasting. Conversion of both elements to the hexavalent form is complete. 

It is generally unacceptable to emit sulfur dioxide, thus the scmbber effluent must be treated for sulfur dioxide removal. If the plant aheady possesses faciUties for the production of sulfuric acid, this rather concentrated sulfur dioxide stream can be easily fed into the wet gas cleaning circuit and disposed of in the sulfuric acid plant. The quantity is so small that it does not put any additional burden on the sulfuric acid plant. Because no tellurium is carried over with the selenium dioxide during roasting, it is possible to produce a selenium product which can be purified to commercial grade (99.5-99.7%). 

The ore is ordinarily ground to pass through a ca 1.2-mm (14-mesh) screen, mixed with 8—10 wt % NaCl and other reactants that may be needed, and roasted under oxidising conditions in a multiple-hearth furnace or rotary kiln at 800—850°C for 1—2 h. Temperature control is critical because conversion of vanadium to vanadates slows markedly at ca 800°C, and the formation of Hquid phases at ca 850°C interferes with access of air to the mineral particles. During roasting, a reaction of sodium chloride with hydrous siUcates, which often are present in the ore feed, yields HCl gas. This is scmbbed from the roaster off-gas and neutralized for pollution control, or used in acid-leaching processes at the mill site. 

Reaction 3 also occurs on cooling since the concentration of SO is very low at roaster temperatures of 950°C and approaches zero at 1000°C. Another important reaction that occurs during roasting is the formation of zinc ferrite, Zn0-Fe2 03 above 650°C (see Ferrites). Zinc ores contain 5—12% iron. Zinc ferrite forms soHd solutions with other spinels, such as Fe0-Fe203, and therefore the zinc—iron compositions formed are of indefinite stoichiometry. Ferritic zinc is difficult to solubilize in hydrometaHurgical leaching but several recovery processes are discussed below. 

The natural moisture of the cocoa bean combined with the heat of roasting cause many chemical reactions other than flavor changes. Some of these reactions remove unpleasant volatile acids and astringent compounds, partially break down sugars, modify tannins and other nonvolatile compounds with a reduction in bitterness, and convert proteins to amino acids that react with sugars to form flavor compounds, particularly pyrazines (4). To date, over 300 different compounds, many of them formed during roasting, have been identified in the chocolate flavor (5). 

About 50—80% of the trigonelline is decomposed during roasting. Trigonelline is a probable source for niacin [59-67-6] but also a source of some of the aromatic nitrogen compounds such as pyridines, pyrroles, and bicycHc compounds found in coffee aroma (16). Certain acids, such as acetic, formic. 

Sulfide ores usually contain small amounts of mercury, arsenic, selenium, and tellurium, and these impurities volatilize during the ore treatment. All the volatilized impurities, with the exception of mercury, are collected in the dust recovery systems. On account of its being present in low concentrations, mercury is not removed by such a system and passes out with the exit gases. The problem of mercury contamination is particularly pertinent to zinc plants since the sulfidic ores of zinc contain traces of mercury (20-300 ppm). The mercury traces in zinc sulfide concentrates volatilize during roasting and contaminate the sulfuric acid that is made from the sulfur dioxide produced. If the acid is then used to produce phosphatic fertilizers, this may lead to mercury entering the food chain as a contaminant. Several processes have been developed for the removal of mercury, but these are not yet widely adopted. 

Another economy sometimes practiced in this process is that of spraying enough water to cool the beans, as well as to compensate for some of the weight loss that occurs during roasting. 

The chaff, which is the bean silverskin caught in the fold of the bean, flies off the bean during roasting and can be collected, pelletized, and mixed with roasted beans which are to be ground, even though it may impart a bitter flavor to the final beverage. 

Chlorogenic acid loss is also correlated with its incorporation in browning products.3 During roasting, the diphenols, 4-ethylpyrocatechol and pyrocatechol are formed from the caffeic acid moiety and the quinic acid moiety yields phenol and benzoic acid as well as all the di- and trihydroxybenzenes.39... 

Pyridine is formed from trigonelline during roasting.3 It is presumed to contribute to the flavor, especially in the darker coffee roasts.15... 

Roasted Robusta coffee contains 16-O-methylcafestol, mostly in its esterified form 128 and it has been recognized as useful for detecting the addition of Robusta to Arabica coffee because of its stability during roasting.129... 

However, 5-hydroxytryptamides themselves do not seem to ever become significant in the coffee beverage because they are partly decomposed during roasting and the rest remains in the spent grounds.148 Any small amount present in a brew is emulsified and could be retained by a filter paper in a drip brew of coffee.148... 

Primary zinc processing activities generate air emissions, process wastes, and other solid-phase wastes. Air emissions are generated during roasting, which is responsible for more than 90% of... 

Czerny M and Grosch W. 2000. Potent odorants of raw arabica coffee. Their changes during roasting. J Agric Food Chem 48(3) 868-872. 

Coffee has been the focus of many studies regarding formation of ochratoxin A, its destruction during roasting, and its presence in brews. Mechanical color sorting leads to minor reductions in ochratoxin A levels, however steaming produces approximately 25% reduction. The effect of heat on stability is considerable, 80% of ochratoxin is destroyed during bean... 

Biedermann-Brem, S., Noti, A., Grob, K., Imhof, D., Bazzocco, D. and Pfeiferle, A. (2003). How much reducing sugar may potatoes contain to avoid excessive acrylamide formation during roasting and baking Eur. Food Res. Technol., 271, 3, 185-194. 

Shin, H.K. and Crouzet, J., Recovery of coffee aroma during roasting 1. Trapping emitted gases by condensation and absorption. Cafe Cacao The, 25 (1981) 127-136. 

Cadmium is obtained as a byproduct in zinc recovery processes. The metal volatdizes during roasting of zinc concentrates and collected as dust or fume in bag houses or electrostatic precipitators. The dusts are mixed with coal (or coke) and zinc chloride and calcined. The cadmium chloride formed volatihzes upon calcination and thus separates out from zinc. The chloride then is treated with sulfuric acid in the presence of an oxidizing agent. This converts lead, present as impurity in cadmium ore, to lead sulfate which precipitates out. Cadmium is finally separated from copper by the addition of zinc dust and... 

Contrary to soybeans which are rich in isoflavones (Mazur et al., 1998), no isoflavones were found in the morama bean samples. The level of secoisolariciresinol and lariciresinol is retained during roasting, which is an important characteristic since the beans usually are eaten roasted (Holse et ah, 2010). 

Fig. 15.22 On-line REMPI-TOFMS (at 266 nm) analysis of roast gas while roasting 80 g Ara-bica coffee, a The full-time-mass-intensity three-dimensional plot as recorded during roasting, b A time-intensity cross-section from a at a fixed time (medium roast level). The three phenolic VOCs, phenol (m/z 94), giraiacol (m/z 124) and 4-vinylguaiacol (150 m/z), are efficiently ionised at 266 nm. In addition, firrfurylacohol (m/z 96), dihydroxybenzene (m/z 110), indol (m/z 117) and caffeine (m/z 194) were also detected. (Adapted from [203])...

Similar reactions may lead to HDF formation during roasting of coffee, cocoa beans or sesame seeds. However, this has to be proven by further experiments. 

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