WINETECH Technical Yearbook 2020

Grape flavan-3-ol composition under altered light and temperature conditions in Cabernet Sauvignon (PART 2)

in grape skins and seeds where applicable. The proanthocyanidin cleavage products were determined by RP-HPLC using a method adapted from Kennedy and Taylor (2003). RESULTS AND DISCUSSION Seed tannin compositional data by phloroglucinolysis revealed that the terminal seed flavan-3-ol subunits were (+)-catechin, (−)-epicatechin and (−)-epicatechin- 3-O-gallate (table 1). The proportional composition of terminal subunits changed throughout berry development in both seasons (data not shown). The seasonal impact on the seed proanthocyanidin terminal subunit composition was larger than the treatment impact. This is primarily due to the higher light intensities observed in the 2010/2011 season and lower light intensities in the 2011/2012 season. (−)-Epicatechin was the main constituent of the seed extension subunits with (+)-catechin and (−)-epicatechin-3-O- gallate being present in lower proportions in both seasons (table 2). The proportional composition of extension subunits changed throughout berry development in both seasons. The results of this study agree with that of Fujita et al. (2007) and Cohen et al. (2008), who reported minimal variation in the seed proanthocyanidin composition with shading, heating and cooling of berries.

Furthermore, a higher degree of poly­ merisation occurs in grape skins (Adams, 2006). In berry skin, (+)-catechin has been identified as the main terminal and extension subunit skins (Escribano-Bailón et al. , 1995; Souquet et al. , 1996; De Freitas et al. , 2000; Kennedy et al. , 2001; Downey et al. , 2003; Obreque-Slier et al. , 2010). The main flavan-3-ol subunits present in grape seeds are (+)-catechin, (−)-epicatechin and (−)-epicatechin-3- O-gallate. The main terminal subunit is (+)-catechin and the main extension subunit is (−)-epicatechin (Romeyer et al. , 1986; Prieur et al. , 1994; Downey et al. , 2004). Grape skins differ from seeds, because (−)-epigallocatechin and a lower proportion of galloylated units are present in the skins (Escribano-Bailón et al. , 1995; Souquet et al. , 1996; De Freitas et al. , 2000; Kennedy et al. , 2001; Downey et al. , The site and methods have been di S­ cussed in Part 1 of this series. Addi­ tionally the compositional analysis of proanthocyanidins was carried out following acid-catalysed cleavage in the presence of excess phloroglucinol (phloroglucinolysis) (Kennedy & Jones, 2001a). The method provided information regarding the subunit composition, mean degree of polymerisation (mDP), percentage of galloylation (%G) and the percentage of prodelphinidin units (%P) 2003; Obreque-Slier et al. , 2010). MATERIALS AND METHODS

OCTOBER 2020

ERNA BLANCQUAERT: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: Flavonoids, flavan-3-ol composition, grape ripening

This study aimed to investigate flavan-3-ol composition under altered microclimatic conditions in Cabernet Sauvignon grown in the Stellenbosch Wine of Origin District. INTRODUCTION Proanthocyanidins or condensed tannins are mostly situated in the solid parts of the cluster (skins, seeds and stems) and to a lesser degree in the pulp (Sun et al. , 1999; Jordão et al. , 2001; Ó-Marques et al. , 2005). Proanthocyanidins are transferred from the solid parts of the grape (skins, seeds and stems) into the must during winemaking operations

(crushing, maceration and fermentation). They are partly responsible for the wine organoleptic properties (Monagas et al. , 2003). The quantity, structure and degree of polymerisation of grape proanthocyanidins differ, depending on their localisation in the grape tissues (seeds, skin and pulp). Seeds have the highest concentration of procyanidins (Ricardo da Silva et al. , 1992a). Within the grape berry, proanthocyanidins or condensed tannins are situated in the hypodermal layers of the skin and the soft parenchyma of the seed between the cuticle and the hard seed coat (Adams, 2006).

WINETECH TECHNICAL YEARBOOK 2020 60