Sugars colorimetric procedures

Copper. In the presence of sulfur dioxide, copper-protein cloudiness may develop in white wines. Only small amounts of copper (about 0.3 to 0.5 mg/liter) cause cloudiness. Widespread use of stainless steel in modern wineries has reduced copper pickup, but many wineries routinely test their wines for copper. Atomic absorption spectrophotometry is the method of choice (51) although reducing sugars and ethanol interfere, and correction tables must be used (107). To reduce this interference, chelating and extracting with ketone is recommended (108). Lacking this equipment colorimetric procedures can be used, especially with di-ethyldithiocarbamate (3, 4, 6, 9,10, 22,109). Neutron activation analysis has been used for determining copper in musts (110). 

Because of their speed and reasonable instrumental requirements, there is continuing interest in colorimetric procedures for lactose determinations. Most procedures are based on the reducing properties of lactose samples that contain only one sugar are easily measured colori-metrically, but samples with three or four sugars may require several different colorimetric assays to determine the composition accurately. 

Two kits are available from Englyst Carbohydrate to help ensure that accurate analytical results are obtained. The kit for the colorimetric procedure contains color reagent and a second kit contains a solution of allose as internal standard for the GC procedure. Both kits contain the required enzymes, sugar solutions, and reference materials and are rigorously tested in-house. 

Hunkdr s (1938) procedure for oxidation of methyl alcohol to formaldehyde by bromine in the presence of sugar is capable of detecting only 5% of the alcohol. A colorimetric procedure for methyl alcohol in vinegar was developed by Piccoli (1933). Jauker (1937) tested various procedures and used a semiquantitative method said to be sensitive to 0.0001%. 

Malic acid was determined colorimetrically with Pinerua s reagent (i8-naphthol in sulfuric acid) by Ghimicescu (1935a). The malic acid was separated in 70% alcohol. Nitschk5 (1952) also proposed a colorimetric procedure based on the red color produced in alkaline solution with a-naphthol. A standard curve is prepared the color must be extracted with isooctyl alcohol and is sensitive to light. Sugars, tannins, and tartaric and lactic acids should be absent. 

Spectrophotometry is any technique that uses light to measure chemical concentrations. A procedure based on absorption of visible light is called colorimetry. The most-cited article in the journal Analytical Chemistry from 1945 to 1999 describes a colorimetric method by which biochemists measure sugars.4... 

Iron. Excess iron in wines causes cloudiness, interferes with the color, and can impair flavor. The mechanism of ferric phosphate precipitation has been intensively studied, and numerous colorimetric methods have been developed. For routine purposes the color developed with thiocyanate is adequate (6,9), but many enologists prefer the orthophenanthro-line procedures (3, 4, 6, 22). Meredith et al. (Ill) obtained essentially the same results for iron using 2,4,6-tripyridyl-s-triazine (TPTZ) to develop the color. Atomic absorption spectrophotometry can be used but, as with copper, corrections for reducing sugar and ethanol are necessary (51). 

Number-average molecular weights, M, were obtained from colorimetric data from end-group analysis (reducing sugar) according to procedure described earlier (19) ... 

For individual nucleotides isolated by chromatographic procedures, the ultraviolet absorption spectra provide facile identification and quantitation (/). Well-known chemical (colorimetric) methods are available to identify and quantitate the sugar component, and to measure the relative amounts of sugar and phosphate in isolated nucleotide fractions (34). Enzymatic methods are also used in characterizing and in quantitation of nucleotides (35). 

There is not much to be said, regarding the analytical procedures used to make these determinations, because these are well-known. May I mention, however, that the percentage of soap is obtained by direct potentiometric titration of sucroglyceride dissolved in 2-propanol and water the free sugar is determined by the thin layer chromatographic method in comparison with a standard and the dimethylformamide content is obtained colorimetrically by the copper dimethyldithio-carbamate method. 

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