Desenvolvimento de procedimentos automáticos para determinação de etanol, glicerol e ácido tartárico em vinho empregando multicomutação em fluxo.
Resumo
In the present work, flow systems for the determination of ethanol, glycerol and
tartaric acid in wine without previous sample treatment are described. The flow
system were based on multicommutation and controlled by microcomputer allowing
that the analytic procedures were accomplished automatically without the intervention
of the operator. For ethanol determination the proposed procedure employed
detection by chemiluminescence using equipment developed in the Centro de
Energia Nuclear na Agricultura. The detection device comprised a glass flow cell that
was installed between two photodiodes forming a compact unit that was packed into
a metallic box. The procedure for ethanol determination was based on the enzymatic
reaction using alcohol oxidase producing acetaldehyde and hydrogen peroxide. The
hydrogen peroxide reacted with luminol catalyzed by potassium hexacianoferrate (III)
producing chemiluminescence. The luminescence intensity presented a direct
relationship with ethanol concentration. The system allowed wine sample analysis
without any prior treatment, presenting linear response between 2.5 and 25% (v/v) of
ethanol characterized by the equation Signal (mV) = (20 ± 1) + (7.8 ± 0.3) % of
ethanol (r = 0.997), a coefficient of variation of 1.8% for a typical wine sample
presenting 11 % (v/v) of ethanol and a sampling frequency of 28 determinations for
hour. For glycerol determination it was based on the reaction with glycerol
dehydrogenase in presence the cofactor NAD+, resulting in oxidation of NAD+ to
NADH that was monitored at 340 nm. After system optimization, a linear response
between 2.0 and 10.0 g l-1 of glycerol characterized by the equation Abs = (0.1320 ±
0.003) + (149x10-4 ± 4x10-4) g l-1 of glycerol (r = 0.998), a coefficient of variation of
1.6% for a typical wine sample presenting 5.3 g L-1 of glycerol and a sampling
frequency of 33 determinations for hour were obtained. For tartaric acid
determination a single line module of analysis was designed, based on the reaction
with vanadate and detection was carried out by spectrophotometry at 490 nm after
reaction with sodium vanadate. The procedure presented a linear relationship
between 0.50 and 10.0 g l-1 of tartaric acid characterized by the equation Abs =
(0.206 ± 0.001) + 0.0404 ± 2.76x10-4 g l-1 of tartaric acid (r = 0.999), a coefficient of
variation of 2.1% for a typical wine sample presenting 1.84 g L-1 of tartaric acid and a
sampling frequency of 28 determinations for hour. Comparing the reagent
consumption with earlier works it was observed that reduction about 65% was
obtained. The three systems that comprise this work when applied to analysis wine
samples presented results, which compared with results obtained using official
methods no significant difference at 90 % confidence level were observed.