Bisulfite bleaching

Cationic FBAs are used to brighten polyacrylonitrile fibres. The brightening of acrylic fibres is carried by the exhaust method at an acid pH, usually in the presence of sodium chlorite or bisulfite bleach. Pyrazolines produce very high whites but are unstable to bleach and therefore the quaternary benzimidazoles are the preferred class, e.g. (3.65) and (3.66). 

Calculate the polymeric color of the bisulfite bleached sample as follows ... 

Polymeric color was determined using the bisulfite-bleached sample using the following formula ... 

Fractionation of Red Wine Polymeric Pigments by Protein Precipitation and Bisulfite Bleaching... 

The Hagerman and Butler method for tannin analysis was combined with bisulfite bleaching of monomeric anthocyanins to give estimates of polymeric... 

Figure 4. Correlation between the amount of polymeric pigment as determined in the Somers assay at wine pH and the sum of LPP and SPP as determined by combined protein precipitation and bisulfite bleaching. 2003 American Society for Enology and Viticulture. AJEV 54 301-306...

As red wines age, the monomeric forms of anthocyanins undergo condensation reactions with other anthocyanin or tannin molecules to form polymeric pigments. The polymeric pigments are less sensitive to bisulfite bleaching and changes in pH (10). The color analysis method of Somers and Evans (10) as described above takes advantage of these differences to follow color changes as wines age. 

Anthocyanin may also dimerize yielding A-A dimers. Such dimers were tentatively identified by ESI-MS in grape skin extracts (Vidal et al, 2004), as well as in red wine fractions obtained by high-speed countercurrent chromatography (Salas et al, 2005) and by fractionation on Toyopearl after bisulfite bleaching (Alcalde-Eon et al, 2007). Two structures have been proposed for A-A+ A-type bond (C4-C8 and C2-0-C7) flavan-fiavylium dimers and B-type bond (C4-C8) flavene-flavylium (Vidal et al, 2004). Oligomers consisting of a flavanol residue directly linked to an anthocyanin dimer (F-A-A" ) have also been recently identified in red wine fractions (Alcalde-Eon et al., 2007). 

Several works have shown that, in general, pyranoanthocyanins exhibit a higher color intensity and stability up to pH 6 and higher resistance to bisulfite bleaching than anthocyanins. The substitution at the carbon 4 of the pyranic ring C protects the chromophore group from the nucleophilic attack of water (hydration) and bisulfite anion (Bakker ... 

Calcium bisulfite bleach liquor S% tote) SOt 5S free) PTH Kl PPl SI ... 

Calcium bisulfite, bleach liquor 253 Calcium bromide 231,481,659, 674, 750 Calcium carbide - 614... 

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