Tuesday, April 28, 2015

Characteristics of high-quality wine grapes

As the starting point for the production of a quality wine, it is imperative that high-quality grapes be delivered to the cellar door. I posit that high quality grapes are a funcntion of both what is within and what is without. Further, what is within can be divided into two broad classes: objective and subjective.

The objective criteria revolve around the components of grape juice at maturity, with soluble solids (major component being sugar), acid, and pH being the primary indicators based on (i) abundance and (ii) ease of measurement.
  • Soluble solids -- 6-carbon sugars (glucose and fructose; 90 - 95% of total), non-fermentable sugars (inclusive of 5-carbon pentose sugars), pectins, acids and their salts, tannins, pigments, and dry extract.
  • Acid – primarily tartaric acid (5 - 10 g/l in grapes) but also includes malic (2 - 4 g/l), citric, and other acids.
  • pH – measure of the amount of hydrogen ions in a solution. Regarded as the acidic strength of the solution.. Increases with increases in sugar concentration. According to Wynboer, not a reliable measure on its own. Should be between 3.1 and 3.3 for white grapes and 3.3 - 3.5 for red grapes.
  Table 1. Summary of objective grape quality measures (Source: UCDavis)
Wine Type
Soluble Solids (%)
Tartaric Acid (g/L)
18 - 20
7 - 9
2.8 – 3.2
White Table
19 - 23
7 - 8
3 – 3.3
Red Table
20 - 25
6 – 7.5
3.2 – 3.4
Sweet Table
22 - 25
6.5 - 8
3.2 – 3.4
23 - 26
5 – 7.5
3.3 – 3.7

Soluble solids are reported in Brix, specific gravity, Baume, or Oeschle, depending on preferences, and are measured by refractometry (juice in the vineyard) or hydrometry (during fermentation). Anthocyanin and total phenolic levels are measured using Near-infrared and Spectrophotometry, equipment that is not readily available to most wineries. Titratable acidity (TA) is measured by acid-base titrations while pH is measured using a pH meter.

It should be noted that, depending on winemaker preferences or climatic conditions, some of the soluble solids and acidity level metrics may be over- or under-shot and is then adjusted during the winemaking process by acid/sugar addition or subtraction.

The subjective criteria for quality are as follows: color; ease of removal of berries from pedicel; texture; aroma; and flavor. These tests should be conducted by the winemaker during vineyard walk-throughs. Of the objective tests mentioned, the test for flavor is the most important. Bisson (UCDavis) sees optimal maturity as assessable only by monitoring flavorants themselves but such a task is laborious and expensive and has to be approximated through tasting.

But all of the quality elements associated with the grape are not subcutaneous. Grapes in a vineyard are hosts to what Gourrand (Using non-Saccharomyces yeasts during alcoholic fermentations: taking advantage of yeast biodiversity) calls native microflora -- molds, lactic bacteria, acetic bacteria, Saccharomyces spp, and non-Saccharomyces yeasts (Pichia, Metchnikowia, Kloeckera, Kluyveromyces, Candida, Zygosaccharomyces, Torulaspora, Cryptoccus, Brettanomyces, and Hanseniaspora). According to Loureiro and Malfeito-Ferreira (Spoilage yeasts in the wine industry, International Journal of Food Microbiology 86, 2003), mature, healthy grapes harbor microbial populations at levels of 103 - 105 CFU/g (colony forming unit -- a measure used in microbiology that indicates the number of micro-organisms present in a water sample (www.legionella.com/cfu)), levels that vary based on environmental conditions (rainfall, temperature, grape variety, the application of chemicals in the vineyard).

It is the yeast element of this microflora that natural-yeast winemaking adherents seek to exploit. Wild yeasts accumulate on the grapes from flowering through harvest with the presence of SC being pegged at 1 in 1000 grapes (Robert Mortimer, Vineyard Theory of Wild Yeast, UC Berkeley). At harvest, SC is the least prevalent of the grape-resident yeast strains.

Wine-associated yeasts are identified in the table below.

©Wine -- Mise en abyme

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