The colour and phenolic content of Robertson Red grape cultivars : distribution, correlation with wines and analyses

Van der Merwe, Hanneli (2013-03)

Thesis (PhD)--Stellenbosch University, 2013.


ENGLISH ABSTRACT: South African red wine is often acknowledged world wide as being full bodied and deep in colour. This is often the result of high temperatures that is experienced during the important growth stages of grapes especially post véraison. In the Robertson area in South Africa however, temperatures often exceeds the range for optimal anthocyanin development during these growth stages. The distinction between grapes being technologically ripe and being ripe on a phenolic level is also accepted as an important determining factor for the perfect time to pick grapes. In co-operative wineries such as Robertson Winery (RW) where grapes are delivered from a large area and different producers, it is difficult to individualise grape blocks when it comes to ripeness level in terms of sugar or phenolic ripeness. In most circumstances a generalised set of parameters for deeming grapes ripe or acceptable for delivery is the best substitute. The levels of these parameters are based on research literature that is available for the area as well as data collected through years of maintaining the vineyards of that area. The grape parameters that are currently being used by RW for ripeness and quality are pH, titratable acidity (TA) and sugar level. In recent years RW in conjunction with the Department of Viticulture and Oenology, Stellenbosch University, decided to investigate more parameters to determine the quality of grapes at the time of harvest. Most importantly for the grape growers this quality is connected to a price point and therefore compensation. Two important quality parameters of red wine are the red colour and mouth feel of wine. Anthocyanin and tannins are respectively connected to these two quality attributes and are both widely accepted as quality indicators. Wine with high anthocyanin and tannin content often originates from grapes with a high colour and phenolic profile. The existence of a correlation between grape and wine anthocyanin and tannin content is therefore the basis of attempting to use these parameters in the grape to predict end wine’s colour and phenolic quantity. Determination of anthocyanin and tannin content of grapes has already become part of some private owned wineries’ standard set of determinations. However, sample preparations, extractions and consumables needed are all factors that need to be reduced to make the measurement and therefore the use of these parameters more viable in a co-operative cellar laboratory, where large volumes of grapes are received during harvest. The first objective of this work was to determine the levels of anthocyanin and tannin in red grapes from different vineyard blocks from the producers of RW from three successive vintages. This would give insight as to what can be seen as a low and high anthocyanin and tannin content for grapes received at the cellar. For this purpose, blocks of the most important red wine cultivars for RW was selected and analysed for these compounds. The ranges and average levels of anthocyanin and tannin content were determined using measurement techniques that could be used by any winery. The average mono flavanol and total colour level of the grapes were found to be lower than those often reported in literature, with total grape flavanols being higher. However, a wide range of values for these compounds were found that correlated with those found in other studies. The possible reasons for differences in levels of occurrence of these compounds were discussed and mostly pertain to differences in cultivar, micro climatic and season. The second objective was to determine the correlation between levels of colour and phenolic compounds in grapes and their corresponding wines. Such correlations will form the foundation for the use of phenolic content to predict the colour and phenolic potential of the wine and possibly wine quality as well. When the grape and wine colour and phenolic data were correlated for all seasons and cultivars inclusive it was found that grape and wine colour showed better correlations than for instance total phenols and tannins. This was especially true for total colour pigments in red grapes, measured with HPLC, when correlated with certain spectrophotometric analysis of wine colour. Cultivar and season as well as the synergism between the two were further investigated for its role in affecting correlations. When these relationships were further differentiated by season and by cultivar the resulting correlations varied. This work contributed a great deal of information to support the use of grape colour and phenolic compounds for the prediction of end wine colour and phenolic composition. The third objective was to investigate near infrared spectroscopy (FT-NIR) as a viable option to rapidly measured anthocyanins, tannins and total phenolics in red grapes. If proven successfully, this could be employed by a large cellar such as RW. FT-NIR has been used with success on grape extracts and in this instance the focus was to establish a calibration on the grape homogenate itself. Preliminary results showed that FT-NIR could be applied for the use of determination of anthocyanin and tannin levels in red grapes originating from RW. The prediction of total phenols was not found to be as accurate, but this could also be due to the reference method that was used. This work brought some interesting, practical information not only of importance for RW, but all wineries that are concerned with improving the basis on which grape quality is determined. The use of aerial data mapping for indicating areas regarding important grape colour and phenolic parameters was used in this study and is a very visual way of showing the distribution of certain ripeness parameters over a large area. Correlations between the grape and wines of such a large amount of red grape blocks for a specific area have not also been reported in South Africa before. The use of FT-NIR to determine anthocyanins and tannin concentrations in grape homogenates is also novel for its use in South African wineries. This work may assist grape and wine producers as well as analysts on the phenolic and colour profile of grapes and wines from RW.

AFRIKAANSE OPSOMMING: Suid-Afrikaanse rooiwyn word wêreld-wyd geken aan ‘n dieprooi kleur en vol struktuur. Die grootste rede vir hierdie verskynsel is hoë temperature wat ervaar word tydens rypwording en veral na véraison. In die Robertson wynstreek is temperature egter tydens rypwording dikwels vêr bo dit was as optimaal vir antosianien ontwikkeling beskou word. Die gepaste tyd om druiwe te pluk word nie net gedryf deur die tegnologiese rypheidsvlak nie, maar ook deur fenoliese rypheid. In ‘n koöperatiewe kelder omgewing soos Robertson Wynkelder (RW) word ‘n hoë lading druiwe elke dag ontvant vanaf verskillende produsente oor ‘n breë streek. Dit maak dit moeilik om te bepaal watter druiwe werklik beide tegnologies en fenolies ryp is. Die beste manier om hiervoor te vergoed is om ‘n standaard te stel vir ‘n reeks voorafbepaalde parameters. Die vlakke van die gekose parameters is, word bepaal deur navorsinguitsette sowel as die geskiedkunde data wat ingesamel is vanaf elkeen van die bepaalde blokke. Die parameters wat tans in gebruik is by RW om oesdatum en kwaliteit by inname te bepaal is pH, titreerbare suur (TA) en suiker vlak. Die tekortkoming hier is dat kwaliteit van druiwe beswaarlik met slegs hierdie informasie kan bepaal word, maar dat dit die betaling van die produsent by aflewering wesenlik kan beïnvloed. Dit het RW genoop om in samewerking met die Departement van Wingerd en Wynkunde, Universiteit van Stellenbosch nog parameters te ondersoek wat hierdie rypheid- en kwaliteitsbepaling by inname sou kon versterk. Twee belangrike faktore wat kwaliteit van rooiwyn bepaal is die kleur en struktuur. Antosianiene en tanniene is onderskeidelik verantwoordelik vir hierdie kwaliteits eienskappe van wyn. Wyn wat bestempel word as hoog in kleur en tannien inhoud word dikwels verbind met druiwe wat hoog is in hierdie faktore. Die moontlike korrelasie tussen die antosianien en tannien inhoud van druiwe en die wyn wat daarvan berei word is dus die basis waarop die potensiële toepassing van hierdie parameters berus. Die bepaling van antosianien en tannien vlakke word reeds in sommige laboratoriums gedoen. Die monster voorbereidings tyd, ekstraksies, toerusting en verbruikbare items nodig om hierdie tipe analieses te doen is egter hoog. Die analiese van hierdie komponente is meer lewensvatbaar in groot laboratoriums (soos in ‘n koöperatiewe kelder) waar groter volume druiwe ingeneem word gedurende parstyd. Die eerste doelwit van hierdie studie was om te bepaal teen watter vlakke antosianiene en tanniene in druiwe voorkom, spesifiek van die Robertson area. Die het behels ‘n wye verskeidenheid van blokke, verspreid oor die hele streek wat oor 3 seisoene gemonitor is in terme van veral kleur en tanniene maar ook ander belangrike parameters. Die idee hier is om insig te kry rakende watter vlakke bestempel kan word as laag en hoog in terme van antosianien en tanniene vir die Robertson streek. Daarvoor is slegs die mees aangeplantste rooi kultivars gebruik. Die verspreiding en gemiddelde vlakke waarteen antosianien en tanniene voorkom was bepaal deur gebruik te maak van metodes wat as relatief algemeen in laboratoria gebruik word. Die gemiddelde mono-flavonoïed en totale kleur pigment inhoud van die druiwe was laer as van die vlakke wat in die literatuur beskikbaar is, met totale flavanole wat hoër was. Die wyer verspreiding van die waardes het egter beter gekorreleer met die waardes soos beskryf in die literatuur. Die moontlike redes vir die verskillende vlakke word in die studie bespreek en word waarskynlik bepaal deur verskille in kultivar, mikro-klimaat en seisoen. Die tweede doelwit was om te bepaal of daar ‘n korrelasie te vinde is tussen die kleur en tannien inhoud van die druiwe en ooreenstemmende wyne. Sulke tipe korrelasies sal die basis vorm om antosianien en tannien inhoud van wyn reeds in die druiwe fase te kan voorspel. Nadat die ingesamelde druif en wyn data as ‘n geheel beskou was, was dit sigbaar dat die wynkleur parameters beter korrelasies bied as meeste tannien en totale fenole. Dit was veral waar in die geval van totale kleur pigmente soos gemeet met die HPLC teenoor die wynkleur parameters gemeet met spektrofotometriese metodes. Verdere ondersoeke in terme van die impak wat die kultivar en seisoenale kan hê het tot variërende korrelasies gelei.. Hierdie werk het ‘n groot bydrae gelewer om voorspellings van wyn kleur en fenoliese inhoud reeds met sukses vanaf die druif te bepaal. Derdens het die werk fourier transformasie naby infrarooi skandering (FT-NIR) ondersoek as ‘n lewensvatbare metode vir die bepaling van antosianien, tannien en totale fenoliese inhoud van druiwe en wyn. FT-NIR word reeds oor ‘n wye reeks wyne en druiwe ekstraksiemonsters toegepas en die doelwit hier was om druiwe homogenaat as matriks te kalibreer. Voorlopige resultate het bevind dat antosianien en tannien vlakke in druiwe van RW gemeet kan word met die FT-NIR, maar dat die kalibrasie vir totale fenole nog verbeter kan word. Hierdie werk het ‘n wye reeks interessante en prakties bruikbare informasie na vore gebring wat van onskatbare belang is vir RW en ander kelders wat besorgd is oor die verbetering van algemene druifkwaliteit. Geografiese kaarte wat belangrike druifkleur en fenoliese parameters aandui is in hierdie studie gebruik en wys hoe data visueel voorgestel kan word om die geheelindruk van gekose parameters oor ‘n groot area te vergelyk. Korrelasies tussen druiwe en wyn van so ‘n groot hoeveelheid druiwe blokke is nog nooit voorheen in Suid-Afrika getoon nie. Dieselfde geld vir die gebruik van FT-NIR vir die meet van kleur en fenoliese parameters in druiwe homogenate. Hierdie werk kan druiwe- en wynproduseerders sowel as analiste assisteer in terme van die kleur en fenoliese profiel van druiwe en wyn van RW.

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