| Summary: | Thesis (MSc Food Sc )--Stellenbosch University, 2003. === ENGLISH ABSTRACT: This study shows the potential of near infrared spectroscopy for both
qualitative and quantitative analyses of wine and brandy in the distilling
industry.
Wines intended for further processing by the distillation industry have to
meet certain specifications to ensure an efficient and cost-effective distilling
process with a suitable product yield. Certain compounds have, therefore,
been recognised as significant quality parameters and through their
regulation, a product of consistent quality can be ensured. These quality
parameters include alcohol, total acid, volatile acid, total sulphur dioxide, total
phenolics, reducing sugars and acetaldehyde concentrations and pH. The
effective control of all these compounds in the wines prior to distillation, is
critical to maintain the feasibility and efficiency of the process.
Fourier transform near infrared spectroscopy (FT-NIRS) can be used
as a rapid and accurate analytical method for the determination of alcohol
concentrations in distilling and brandy base wine. The choice of path length
for the liquid sampling cell is an important consideration. In this study, alcohol
with its strong absorption of OH bands in the near infrared region was not
significantly affected, but difference in path length showed a significant
influence on the prediction of the volatile acid concentration in distilling wine.
Very strong correlations were found between the spectral data and the alcohol
concentration data in distilling wine (r = 0.99, SEP = 0.18% v/v, RER =30) and
brandy base wine (r = 0.92, SEP = 0.18% v/v, RER = 11.1). Good predictions
were obtained for the volatile acid content of brandy base wine (r = 0.85, SEP
= 0.04 g.L-1
) but better accuracy could be obtained by incorporating a wider
concentration range and more sensitive, although not the accepted reference
methods, such as HPLC and GC, into the modelling. In contrast to the brandy
base wine, the much cruder, turbid distilling wine (r = 0.67, SEP = 0.33 g.L-1
)
yielded poor calibration results for volatile acid.
FT-NIRS can also be used as a rapid screening method to measure the
total acid content, pH and total phenol levels in brandy base wine samples.
Satisfactory predictions were obtained for the total acid content (r = 0.89,
SECV = 0.38 g.L-1
), pH (r = 0.84, SEP = 0.09) and total phenol levels (r = 0.71, SEP = 16.4 mq.L" GAE). The SEP of all the parameters compared
well with the SEL and were within acceptable limits. The range of the
concentration measurements were, however, very narrow and the accuracy of
these models (measured as the ratio of the range to the standard error of
prediction) were all below 10, indicating that modelling of these parameters in
brandy base wine should be attempted with reference values covering a wider
range. Poor correlations and predictions were obtained between the wine
spectral data and the acetaldehyde (r = 0.39 , SECV =1.45 rnq.L"), residual
sugar (r = 0.58, SECV =0.49 mq.L" ) and total sulphur dioxide (r = 0.35, SEP
= 10.9 mq.L" ) concentration data.
The unique character of potstill distillates, originating from the grape
cultivar, type and age of the oak casks, lends itself to the development and
blending of different brandies. The process of blending and correcting the
brandy is necessary to obtain a product and brand of established and
recognised character. The brandy is generally divided into different classes
prior to blending based on the degree of "smoothness" or "hardness" of the
matured distillate as determined by a sensorial panel. The ideal would,
however, be to replace this subjective evaluation method with a more
objective approach, such as an instrumental method like NIRS.
Near infrared spectroscopy showed potential to successfully
discriminate and classify brandy samples in terms of their sensorial
classification status. The near infrared spectra of unblended three-year old
brandies were measured and discriminant analysis performed using the
subjective sensory classification data of the brandy. SIMCA (soft independent
modelling of class analogy) models were developed by calculating separate
peA (principal component analysis) models for each of the classes for which
qualitative analysis were required. An assessment of class membership of an
unknown sample was made on the basis of the distance to the cluster
centroid. Two types of NIRS instruments, a Fourier transform near infrared
system and a diode array spectrophotometer were employed to examine the
discriminatory abilities of NIRS. Excellent predictions (90.9%-100%) were
obtained for the "smoothest" and "hardest" classes within the 2000 and 2001
seasons with FT-NIRS. For the diode array spectra reasonable classification
data were obtained for all the classes within the 2000 and 2001 seasons, as well as an excellent classification rate (100%) of the smoothest class in the
1999 season. Prediction results for class membership of the brandy samples
from the three season's combined data did not deliver satisfactory
discriminatory results. By applying NIRS and SIMCA, rapid and objective
classification of brandy samples within seasonal context can be established to
enhance the blending processes.
Having accurate reference data in the calibration set is essential in
near infrared spectroscopy (NIRS). The precision (usually decided by blind
duplicate determination) of the reference data serves as the performance
target for correlation-based near infrared analysis. The differences between
reference values obtained from two independent laboratories and the Foss
Winescan FT 120, which is also a predictive method, for the alcohol and
volatile acid determinations in distilling wine and between two independent
laboratories for the measurement of the total sulphur dioxide content, were
investigated. ANOVA and post-hoc analysis (Bonferroni testing) revealed
statistically significant differences (p<O.OS)between all three measurements
for the volatile acid analysis. One laboratory differed significantly from the
other two measurements in its measurement of the alcohol content in distilling
wine. A paired t-test performed on the two laboratories' values obtained for
total sulphur dioxide revealed statistically significant differences (p<O.OS)
between the two sets of results. This study restated the importance of
obtaining reliable reference data for calibration purposes to maintain the
efficiency and accuracy and therefore the reputability of NIRS as an accurate
and reliable analytical method. === AFRIKAANSE OPSOMMING: Hierdie studie dui op die potensiaal van naby infrarooi spektroskopie (NIRS)
vir beide die kwalitatiewe en kwantitatiewe analise van wyn en brandewyn in
die distillasie industrie.
Sekere spesifikasies word gestel waaraan wyn met voldoen voor dit
verdere prosessering tydens distillasie kan ondergaan. Die regulering van
wynkwaliteit is noodsaaklik om die distilleringsproses ekonomies volhoubaar
te maak. Sekere komponente in die wyn het 'n direkte invloed op die kwaliteit
van die eindproduk en word beskou as betekenisvolle parameters wat beheer
moet word om 'n produk van konstante hoë kwaliteit te lewer.
Hierdie parameters sluit alkohol, titreerbare suur- en vlugtige
suurinhoud, asook die pH, totale swaweidioksied, totale fenol vlakke,
reduserende suikers en asetaldehied konsentrasies in. Die regulering van al
hierdie komponente in die wyne is essensieel voor die aanvang van distillasie
om die effektiwiteit en produktiwiteit van die proses te verseker.
Fourier transformasie naby-infrarooi spektroskopie (FT-NIRS) kan
benut word as 'n vinnige en akkurate analitiese metode om die alkohol
konsentrasie in stook- en rabatwyn te bepaal. 'n Geskikte padlengte kuvet
moet gekies word vir die ontleding van vloeistof monsters met NIRS. In
hierdie studie is gevind dat die voorspelling van alkoholkonsentrasie in wyn,
nie soseer beïnvloed is deur die keuse van padlengte nie. 'n Vlugtige
komponent soos asynsuur, wat die maatstaf van die vlugtige suurinhoud van
die wyn is, word egter regstreeks beïnvloed deur die keuse van padlengte. 'n
Sterk korrelasie is gevind in die datastel vir die alkoholinhoud van die
stookwyn (r = 0.99, SEP = 0.18% v/v, RER = 30) en rabatwyn (r = 0.92, SECV
= 0.18% v/v, RER = 11.1). Baie goeie voorspellings is ook gevind vir die
vlugtige suurinhoud van rabatwyn (r = 0.85, SEP = 0.04 g.L-1). Die
akkuraatheid van die metode (uitgedruk as die RER waarde) het egter
aangedui dat 'n wyer konsentrasie omvang en meer akkurate
verwysingswaardes gebruik moet word om 'n geskikte model vir vlugtige suur
kwantifikasie in rabatwyn te bewerkstellig. In teenstelling met die resultate
verkry vir die rabatwyn, het die ruwer, turbiede stookwyn swak gekalibreer vir
die vlugtige suurinhoud (r = 0.67, SEP = 0.33 g.L-1
). FT-NIRS kan ook gebruik word as 'n vinnige seleksie metode om
titreerbare suur vlakke, pH en totale fenolinhoud van rabatwyn monsters te
bepaal. Sterk korrelasies is in die datastel gevind vir die titreerbare
suurinhoud (r = 0.89, SECV = 0.38 g.L-1
), pH (r = 0.84, SEP = 0.09) en totale
fenol vlakke (r = 0.71, SEP = 16.4 rnq.L" GAE) van rabatwyn. Die SEP
waardes verkry vanaf die NIRS bepalings het baie goed vergelyk met die
standaard foute wat verkry is vir die chemiese bepalings. Die konsentrasie
omvang van die verwysingswaardes was, egter, beperk en die akkuraatheid
van die NIRS modelle (uitgedruk as die verhouding tussen die konsentrasie
omvang en die standaard fout van voorspelling) was laer as 10. Dit het
daarop gedui dat die NIRS kalibrasie van die parameters eerder uitgevoer
moet word met monsters wat 'n wyer konsentrasie omvang dek en gemeet is
met meer sensitiewe analitiese metodes soos HPLC en GC. Die kalibrasie en
voorspelling van die asetaldehied inhoud (r = 0.39 , SECV =1.45 rnq.L"),
reduserende suiker konsentrasie (r = 0.58, SECV =0.49 mq.L" ) en totale
swaweldioksiedinhoud (r = 0.35, SEP = 10.9 rnq.L" ) het onaanvaarbare
voorspellings gelewer.
Die unieke karakter van potketel distillate wat bepaal word deur die
druifkultivar, tipe en ouderdom van die eikehoutvate, bied die potensiaal vir
die ontwikkeling en versnyding van verskillende style brandewyn. In 'n poging
om 'n produk en handelsnaam van erkende en gevestigde karakter te verkry,
moet die brandewyn eers versny word na veroudering. Die brandewyn word
geklassifiseer in verskillende style op grond van sensoriese gehalte. NIRS is
gebruik as 'n vinnige tegniek om te onderskei tussen verskillende style
onversnyde drie jaar-oue brandewyn.
SIMCA (soft independent modelling of class analogy) modelle is
ontwerp deur aparte PCA (hoof komponent analise) modelle vir elke klas
waarvoor kwalitatiewe analise vereis is, te ontwikkel. 'n Skating van klas
lidmaatskap van 'n onbekende monster is gemaak deur die afstand van die
monster na die groepsmiddelpunt te bereken.
Twee soorte NIRS instrumente, 'n FT-NIR en 'n diode array
spektrofotometer is gebruik tydens die studie om die diskrimineringsvermoë
van elk te ondersoek. Uitstekende voorspellings (90.9%-100%) is gekry vir
die "sagste" en "hardste" klasse binne die 2000 en 2001 seisoene met FTStellenbosch NIRS. Met die diode array spektra is redelike klassifikasie resultate verkry vir
al die klasse binne die 2000 en 2001 seisoene, asook 'n uitstekende
klassifikasie (100%) vir die sagste klas binne die 1999 seisoen. Voorspellings
vir klas lidmaatskap van die brandewyn monsters vir die drie seisoene se
gekombineerde data het egter onaanvaarbare diskriminasie resultate gelewer.
NIRS en SIMCA sal 'n vinnige en objektiewe klassifikasie proses van
onversnyde brandewyn monsters kan meebring wat tot voordeel van die
versnydings- en produksieproses sal wees. Hierdie voorspellings sal egter
binne seisoensverband moet geskied.
Akkurate verwysingsdata in die kalibrasie datastel is 'n essensiële
fasset van naby infrarooi spektroskopie. Die presisie (bepaal deur blinde
duplikaat bepalngs) van die verwysingsdata dien as 'n prestasie teiken vir
korrelasie-gebaseerde naby infrarooi voorspellings. Die verskille tussen die
data verkry vir die alkohol- en vlugtige suurinhoud van stookwyn deur twee
onafhanklike laboratoriums en 'n instrumentele metode (Foss Winescan FT
120) asook die verskille tussen die data verkry van twee laboratoriums vir die
totale swaweidioksied metings, is ondersoek. ANOVA en post-hoc toetse het
aangedui dat al die data verkry vir die vlugtige suur bepalings, statisties
betekenisvol van mekaar verskil het (p<0.05). Vir die alkohol bepalings in
stookwyn, het die data verkry van een laboratorium betekensvol verskil van
die ander twee datastelle. Gepaarde t-toetse uitgevoer op die data verkry vir
totale swaweldioksiedinhoud, het ook betekinisvolle verskille aangedui tussen
die twee laboratoriums. Hierdie studie het die belangrikheid van betroubare
verwysingsdata vir kalibrasie doeleindes beklemtoon, veral waar 'n tegniek
soos NIRS se werksverrigting afhang van die akkuraatheid van dié waardes.
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