A Complete Guide to Quality in Small-Scale Wine Making

By John Anthony Considine and Elizabeth Frankish

A Complete Guide to Quality in Small-Scale Wine Making, Second Edition is the first and only book to focus specifically on the challenges relevant to non-industrial scale production of optimal wine with a scientifically rigorous approach. Fully revised and updated with new insights on the importance of all aspects of the production of consistent, quality wine, this book includes sections on organic wine production, coverage of the selection and culturing of yeast, and the production of sparkling, ‘methode champenois’ and fortified wines. The new edition includes insights into the latest developments in flavor chemistry, production protocols, NIR and FTIR for multipurpose analysis and microplate and PCR procedures, and IR methods for essential analysis among others.

Written by an expert team with real-world experience and with a multi-cultural approach, this text will provide a complete guide to all the stages of the winemaking process and evaluation, and clearly explains the chemistry that underpins it all.

• Fully revised and updated, each chapter includes new insights and latest information
• Presents fully referenced, tested and proven methods
• Elaborates on the chemistry to enable understanding of the processes and the impact of variation

• Language: English
• Publisher: Academic Press
• Release date: Aug 17, 2023
• ISBN: 9780323992886

John Anthony Considine

John Considine has had a lifetime of association with viticulture and for most of his professional life has been associated with viticulture. He has served on national R&D boards and on state authorities. His particular interest in wine grew from a large collaborative research project on the interface between viticulture and the composition and quality of Chardonnay for wine in the Margaret River Region of Western Australia. This text grew out of a course he developed for undergraduate and mature-age postgraduate students.

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Chapter 1

Introduction to the winemaking process

Abstract

The production of common wine styles is outlined to give the reader a framework on which to build additional knowledge. We consider the process from vineyard to palate and the concept of quality and its assessment in grapes and in wine. The role of scientific methods in making these assessments is discussed and placed in context with human sensory evaluation. Primary and secondary fermentations by yeast and bacteria are introduced with role of hygiene in minimizing the risk of microbial faults. The role of yeast and grapevine nutrition is discussed along with the vital role of organic acids and sulfur in aiding hygiene and controlling cultured and wild organisms during and after fermentation. Emphasis is placed on developing personal sensory skills as a key element to determining quality control throughout the winemaking process.

Keywords

Wine; process; production; quality; history; grape quality

1 Introduction

Winemaking is a blend of art and science: art because we use human sensory analysis and personal judgement and science because we use scientific methods to underpin those judgements and provide consistent quality control analyses. The process is complex and enduringly, intellectually, challenging. Producing consistently, high quality wine—vintage after vintage—from fruit that differs from one year to the next is an unending source of motivation and pleasure. But how do we assess the quality of the fruit leading up to vintage, when do we pick and how do we process the fruit to maintain wine quality or add value to the fruit? This book aims to guide the aspiring winemaker in the production of technically sound wine. Your challenge is to go from producing fault free wines to producing high quality wines consistently and perhaps even exceptional wines.

Wines of all styles have much in common but producing wine of a specific style from a parcel of fruit may demand unique conditions. Let’s deal first with producing red and white ‘table wines.’

Broadly speaking, red wines are exposed to oxygen during the primary ferment while white wines are not (Figure 1.1). Red wine fermentation is carried out at a higher temperature, 20°C–25°C, but perhaps briefly higher to aid extraction of tannins and color, while white wines are fermented at lower temperatures to preserve aromatic volatiles, commonly, 15°C–17°C but as low as 8°C. Red wine ferments include all berry parts, skin, pulp and seeds, but white wine ferments include the juice or ‘must’ pressed only from the pulp in order to minimize the extraction of bitter phenolics and tannins from the skins and seeds.

Figure 1.1 Outline of the winemaking process for red and white wines illustrating the flow of the processes and their distinguishing features. Note especially the regions that are fully aerobic, micro-aerobic and anaerobic. In red wine production, the anaerobic may be managed as micro-aerobic through a process known as micro-oxidation.

Red table wines are usually fermented twice; first with yeast to convert sugar to alcohol and then with bacteria to convert malic acid to lactic acid. This latter process—malolactic fermentation—reduces acidity and adds flavors, especially in conjunction with oak maturation, and promotes stability against unwanted further fermentation in the bottle even if the wine is not sterile.

White table wines, with some full-bodied exceptions, such as Burgundy style Chardonnay, are only yeast fermented then cold stabilized and filtered before bottling. They do not have any oak maturation. Careful filtration is required to prevent fermentation of malic acid once bottled. This is a much quicker process than the red wine process and suits the acid-driven, aromatic style generally sought.

Winemaking practices common to both red and white table wines include cellar hygiene, fermentation of grape sugars, management of acidity, and nutrient status and steps to protect and stabilize the wine. There are important distinctions between the two processes; it is possible to produce an acceptable red wine in your ‘back yard’ but challenging to produce a sound white wine under the same circumstances. This is because much greater control is required of oxygen exposure, hygiene, yeast nutrition and temperature.

Typically, a commercial yeast culture is added to red or white crushed berries or ‘musts.’ Wild yeasts present on the grapes may ferment to completion and may produce exceptional wine, but, may also result in an undrinkable wine. Few wild yeast taxa are able to complete fermentation to dryness, that is, to ferment all the available sugar. Unfermented sugar, termed residual sugar, may not only impair the palate of a wine but also its stability against in-bottle fermentation and contamination by spoilage microorganisms. Only commercial yeast genera (Saccharomyces) usually complete fermentation reliably. These yeasts may occur naturally in the vineyard (however are common in a winery).

Sulfur dioxide (SO2) is usually added at the vineyard after picking, and/or at the crusher in the winery, to reduce the risk of oxidation and to suppress wild yeasts and bacteria that may spoil the wine. Sulfur dioxide is an essential part of modern winemaking practice. Its addition is especially important once fermentation has finished to reduce the risk of oxidation and spoilage. It is usually added as the salt, potassium metabisulfite (K2S2O5)—PMS.

Red winemaking is an ‘extractive’ process that is carried out in the presence of the skins, seeds and possibly even some stems (Figure 1.1). It is less prone to oxidation because of the presence of high levels of antioxidants: tannins and anthocyanins. Indeed, some mild oxidation may be beneficial—it is part of the process. The skins, which naturally float and form a ‘cap,’ are buoyed by CO2 produced during fermentation. The cap is mixed with the fermenting wine regularly (plunged or bathed by pumping the liquid over the top) to encourage the extraction of anthocyanins and tannins. Failure to do this may lead to the growth of spoilage yeasts in the cap. For reviews of red winemaking processes see (Sacchi et al., 2005; Setford et al., 2017).

Red wines and some white wines undergo a secondary fermentation that is accomplished by a bacterium, Lactobacillus oeni. This is termed a malolactic fermentation. It reduces wine acidity by converting malic acid to lactic acid. This may be especially important in wines produced from grapes grown in a cool climate and which may have a high level of malic acid at harvest. It also increases stability against spoilage and fermentation in bottle by removing a remaining fermentable substance, malic acid. The bacterial fermentation may also broaden the spectrum of aromas and flavors. In large commercial wineries, this fermentation is conducted in tanks, but for premium wine and in small wineries, it is often performed in oak barrels.

White wine on the other hand is sensitive to oxidation—especially after fermentation, has little skin contact, and must be stabilized by sterile filtration (Figure 1.1). Part of the stabilization process for white wine involves chilling before bottling to precipitate excess potassium bi-tartrate salts (as used in baking powder), which otherwise might form unsightly crystalline deposits in the bottle when refrigerated. White wines are also ‘fined’ with a clay (bentonite) to remove excess protein that might coagulate and form an unsightly haze should the wine get too hot during storage and transportation. Finally, they may be treated with copper sulfate to remove hydrogen sulfide (H2S, ‘rotten egg gas’ aroma) that may be formed when nitrogen starved yeast metabolize grape proteins (this may also be true of red wines but they usually contain more nutrients). Other ‘fining’ processes to remove bitter tannins may include the use of natural products, such as proteins from eggs, fish, or gelatin, or synthetic, such as polyvinylpolypropylene. The fining precipitates are removed before bottling.

Aging in ‘toasted’ oak is a standard practice in red winemaking to broaden and balance the sensory characters of the final wine. Selection of particular oak sources and of the level of toast is a major part of fine winemaking and an expensive aspect. Oak may also be used with some white wines, for example, Chardonnay and some Sauvignon blanc and Semillon wine styles (e.g., Fumé blanc).

The processes are simple conceptually. The ‘art’ lies however in the science of analyzing the raw ingredients; in monitoring the process, in the skill and judgement of the winemaker in managing the details of the process. Vital also are those of the viticulturist in matching cultivar to site and in devising appropriate vineyard management strategies (and luck in the season!).

The final product is judged on its sensory appeal and being aware of this throughout the process is at the heart of becoming a good winemaker. Once a wine becomes spoiled its value is limited or nil. The chemical and the quality assurance (QA) processes described in this text serve to assist in that goal but do not guarantee it. Sensory alertness to ‘off’ flavors and aromas in vessels, pumps, tubing, etc., is vitally important as the human nose can detect some aromas down to pico- or even femto-gram levels (10−12–10−15 g/L). As in all things, prevention is far better than cure: be alert and avoid problems. Care needs to be taken however to check one’s palate outside the winery for it is easy to become adapted to particular off-aromas (e.g., Brettanomyces is a problem requiring constant management in even the best of commercial-scale wineries).

2 Great wines begin with great grapes

Quality is an issue that may make or break the smaller vineyard owner and determine the profitability of even the largest vineyard company. This is the case because the value of the fruit depends on the winemaker’s judgment (and experience) regarding the value of the wine that may be made from that fruit. Fruit destined for superpremium wine may be valued from two- to ten-fold $ per ton above that for standard commercial fruit.

Defining grape quality and selecting appropriate maturity indices and quality measures are issues that are still widely debated among viticulturists and winemakers. Commonly, fine wines are associated with regions that inherently produce low vigor and low yields in a climate that enables full ripening under mild climatic conditions (Gladstones, 1992; Jones et al., 2010; White et al., 2006).

The best indicator of potential quality is the historical record. This is the basis on which regional QA labels are allocated: viz. the Grand and Premier Cru classifications in France. Experience tends to be the benchmark on which site selection is determined. For example, the premium viticultural areas of the ‘New World’ in southern Western Australia were identified by comparison with the best viticultural regions in France and California. These regions had a long and reliable history and their climate and soils were used as benchmarks (Gladstones, 1965; Olmo, 1956).

Natural factors are predominant in determining wine quality: those that are based on location and the attributes of the vine itself, and its suitability for a particular location (Figure 1.2). This statement is not intended to diminish the importance of the human factor in viticultural practice and enology. It is, however, indisputably the case that all great wine comes from particular regions. The French have coined a term for this, ‘Terroir,’ simply, the ecology of the site, its soils, aspect, and climate (Figure 1.2, see also Gladstones, 1992; White, 2003; Wilson, 1998). Whether a great wine is achieved, however is influenced by the human factor and by chance, season.

Figure 1.2 Outline of factors and activities that influence fruit quality for winemaking in a vineyard.

While the French terminology may seem to emphasize the physical environment, the soil and aspect, the factor that determines site suitability, cultivar selection and wine style is temperature (Gladstones, 1992; Jones et al., 2010). Grapevines, Vitis vinifera L., appear to have originated in the Near East and interbred naturally with local V. vinifera ssp. sylvestris populations in western Europe (Myles et al., 2011). Wine grapes are almost unique in agriculture in that the commercial cultivars that predominate today are those that arose naturally in historical times. They are adapted to Mediterranean and nearby temperate climatic zones.

The seasonal cycle of growth and dormancy, that is their phenology, the timing of bud burst, flowering, ripening and leaf fall, is tightly linked to an annual cycle of seasonal changes in photoperiod and temperature. Indeed, grapevine phenology has been used to gauge historical trends in climate (Chuine et al., 2004). From the viewpoint of quality, temperature during the ripening period, from veraison (softening) to harvest is critically important; however, extremes at any period are detrimental (White et al., 2006).

Cultivar selection is included in the Natural/Enduring section of Figure 1.2 because selection of a cultivar and wine style is intimately associated with climate and while mankind has intervened in its selection, it is essentially natural. Figure 1.3 illustrates the impact of climate on two important measures of wine quality. Table 1.1 represents one attempt at classification. A broader range is canvassed in other studies (Parker et al., 2020; Tonietto & Carbonneau, 2004; White et al., 2006). A warming climate will impact on this classification because not all cultivars respond equally (Morales-Castilla et al., 2020; Wolkovich et al., 2017).

Figure 1.3 Impact of region classified by growing degree day (see Table 1.1) on two measures of grape quality for winemaking for two premium red grape cultivars (Petit Sirah is syn. Durif). (A) Color and (B) acidity (Winkler, 1963). Source: Data from Winkler, A. J. (1963). General Viticulture (1st ed.). Jacaranda Press.

Table 1.1

Heat sums calculated on the basis of temperature above a minimum of 10°C (50°F), during the growth and ripening period only, termed growing degree days (GDD or Huglin Index).

Source: Compiled from Winkler (1963), Huglin and Schneider (1998), and Gladstones (1992) but interpolated to fit within Winkler’s classification. Note each author used a different method to calculate GDD so the table is approximate only.

While many elements are at work in determining quality, several of these are given expression through one particular aspect that seems to have a profound influence, vigor (Figure 1.2). Vigor is the expression of a plant’s response to its environment: to water supply, nutrition, and temperature to consider three of the more important elements. Soil type and depth interact strongly with water and nutrition and play a substantial role in the expression of vine vigor.

Soils provide not only support for the vine and a medium for roots but their physical characteristics determine water holding capacity in the root zone and their chemical properties control nutrient availability, quantity and balance. Viticultural soils are typically free draining and open and shallow to moderate in depth, limiting the availability of water and nutrients (and often rocky). However, these characteristics also expose the vine to risk of stress if water supply becomes deficient (a drought) or nutrition is neglected (White, 2003). Thus it is not surprising that soils rank highly in the minds of those who search for quality in viticultural production and that contemporary viticultural practice is to add infrastructure to supplement natural supplies as necessary.

The outcome of excessive vigor is a dense leaf canopy that shades the fruit and buds and causes poor color development, overly large berries, low fruitfulness, bud senescence, and senescence of shaded leaves. This latter factor, in turn, leads to unpalatable, vegetative characters in the fruit (Bureau et al., 2000; Perez & Kliewer, 1990; Razungles et al., 1998; Ristic et al., 2007). A dense leaf canopy is also associated with poor color and tannin development (Cohen et al., 2012; Cortell & Kennedy, 2006).

The outcome of inadequate vigor is low yield and, if caused by water deficit stress, then unripe and seriously flawed fruit characters and unbalanced fruit chemistry (high pH, R. Bowen, Pers. Comm.). If, however, through nutrient deficiency, then unbalanced fruit chemistry arises that affects not only the physiology of the fruit, but that of yeast, which depend largely on the fruit for their nutrient supply. These are important areas of research and the role of water and nutrient status has been studied and reviewed by a number of authors (Chapman et al., 2005; Matthews et al., 1990; Mpelasoka et al., 2003; Treeby et al., 2000).

The great vine regions of the world enable viticulturists to maintain vines readily in their ‘Goldilock’s’ region, not too much water and nutrients and not too little—just right. Viticulture can and does succeed in other regions, but in those the viticulturist must intervene more intensely and, even then, there is only so much that can be achieved. Aspects that do not require constant intervention are those of planting density (and rootstock) and trellis design. It is essential that these are appropriate for the region and the cultivar (see, e.g., Figure 1.4).

Figure 1.4 Grapevines known for quality fruit production showing a little of the diversity of vine training and planting density that characterizes particular vineyard production regions. (A) Burgundy (high density low vigor, vertically shoot positioned, VSP), (B) Châteauneuf-du-Pape (low density, bush vine), (C) Coonawarra, mechanically hedged (low density) and (D) Margaret River (low density VSP).

The role of the viticulturist lies principally in the ‘Management’ tasks with the goal of ensuring that practices enable the attributes of the cultivar and sites to be fully and appropriately expressed. The winemaker, the end-user, usually plays a significant part in guiding viticultural practice, so it is not usually enough just to understand the winemaking process—the best winemakers also master viticulture.

Thus, in principle, a superior grapevine should be treated in a manner similar to that which would apply to a fine athlete—all the care and nutrition required, no more, no less. That is the essence of the science and art of viticulture. The ease with which this is achieved is the art of site selection.

3 Measuring quality

Few hard and fast rules can be provided to measure quality and industry relies heavily on ‘experience.’ Viticultural researchers are yet to adequately analyze the maturation process at a level that is fully meaningful for winemakers because the winemaking process itself is required to unmask the flavors and aromas. Advances in molecular biology and biotechnology could see this situation change quite soon. ‘Sensory’ assessment or taste remains the best guide; chemical measures are important but those presently available are ultimately inadequate (but nonetheless are vitally important). The definitive test of fruit quality is wine quality—how much would a discerning public pay for this wine (especially if ‘blind’ tasted, label covered and perhaps transferred to a nondescript container).

Small scale ferments can provide an economical and effective way of assessing the differences in the winemaking quality of fruit from different vineyards, from different parts of a vineyard or the impact of changing a management system. Be aware however that it will take an expert to judge the finer differences and that ferments may vary markedly, by chance, one from another. Therefore fermentation trials should be replicated (Chapter 10).

Regional differences are important in determining quality and style. Guidelines for a particular cultivar growing in a cool region may vary significantly from that growing in a warm region. Ripeness for end use is important. For example, grapes for light, ready to drink, and rose style, red wines are picked earlier (less mature) than those intended for full-bodied premium, ‘investment’ or ‘gift’ wines (those worthy of ageing). The practice of blending wines of differing maturity or from diverse regions to obtain a wider spectrum of sensory characters (or to obtain year by year consistency as with Champagne) provides a further complication. Sometimes, blends are made at the winemaking stage (cofermented, common in Europe) while Australian practice is to blend before bottling, once fermentation is complete. This requires separation of batches and following them through the process independently until bottling. Blending requires an exceptionally well-trained palate to capture the diversity of varietal and site differences to maximize the value of the wine and achieve the winemaker’s goals.

3.1 Measuring maturity

Ripeness is the first guide to quality. As a grape ripens, sugars accumulate and organic acids, notably malic acid, decline. Thus measures of sugars and acids are the primary measures of ripeness. Note however, that tartaric acid is the major acid in the ripe berry. It does not decline. The loss of titratable acidity is due largely to metabolism of malic acid. Malic acid loss is cultivar dependent as well as temperature-dependent, the higher the temperature, the greater the loss (see, e.g., Figure