Wine Fermentation Tanks Mail

Grapes

The Merriam-Webster's Dictionary defines wine as the usually fermented juice of a plant product (as a fruit) used as a beverage. While in rural communities in countries such as Great Britain wines have from time immemorial been produced from all manner of plant materials (and not only fruits), I restrict discussion in the present chapter to the products of commercial entities furnishing wines based on the grape (Fig. 3.1).

The importance of sound viticulture as a precursor to wines of excellence is unequivocally accepted as a truth in wine making companies worldwide. More so than for beer is the belief held that it is not possible to make an excellent product unless there is similar excellence in the source of fermentable carbohydrate. Most wineries tend to grow their own grapes or buy them from nearby vineyards.

The ideal climate for growing wine grapes is where there is no summer rain, it is hot or at least warm during the day, there are cool nights and little risk of frost damage. The great grape-growing and wine regions are listed in Table 3.1. A benchmark figure for the yield of wine from one metric ton of grapes would be around 140-160 gal. As red grapes are fermented on the skins and therefore are less demanding in the pressing stage, the yield is some 20% higher than for whites.

Vine

| Picking Grapes

Sulphur dioxide

Crush

Must

Sulphur dioxide

Primary/secondary fermentation

Primary/secondary fermentation

Newly fermented wine I Stabilisation, maturation

Stabilised wine j Bottling ± maturation

Fig. 3.1 An overview of wine making.

Table 3.1 Major wine grape-growing regions (1998).

Wine production Grape production Country (thousand litres) (thousand tons)

Table 3.1 Major wine grape-growing regions (1998).

Wine production Grape production Country (thousand litres) (thousand tons)

Italy

5.42

9.21

France

5.27

6.88

Spain

3.03

4.88

United States

2.05

5.36

Argentina

1.27

2.00

Germany

1.08

1.41

South Africa

0.82

1.30

Australia

0.74

Chile

0.55

Romania

0.50

Data derived from Dutruc-Rosset, G. (2000).

Data derived from Dutruc-Rosset, G. (2000).

Fig. 3.2 Scion buds grafted on to the rootstock. Courtesy of E & J Gallo.

Wine grapes belong to the genus Vitis. Within the genus, the main species are vinifera (by far the most important), lubruscana and rotundifolia. Commercial vines tend to be Vitis vinifera grafter onto rootstocks of the other Vitis species. Of course within the species is a diversity of varieties (cultivars) - for example, V. vinifera var. Cabernet Sauvignon.

It takes approximately 4-5 years from the first planting to yield the first truly good crop of grapes. The scion (top) of the vine and the rootstock to which it is grafted (Fig. 3.2) must be selected on the basis of compatibility, one with the other and the combination with the local soil and climate. Other key issues that come to bear in viticulture are the availability of sunlight, depth of the soil, its nutrient and moisture content and how readily it drains.

Table 3.2 Some varieties of grape.

Type

Example

Comments

White cultivars

Messiles

Sauvignon blanc

Bordeaux. Green pepper and herbaceous notes

Muscats

Muscat blanc

Raisin notes. Prone to oxidation, so often made into

dessert wines

Noiriens

Chardonnay

Widespread use globally; use in champagne

production. Wines have apple, melon, peach notes

Parellada

Catalonia. Apple/citrus notes

Rhenans

Gewurztraminer

Cooler European regions. Lychee characters

Riesling

German origin. Rose and pine notes

Viura

Rioja. Butterscotch and banana

Red cultivars

Carmenets

Cabernet Sauvignon

Bordeaux. Tannic. Blackcurrant aroma Lighter in

Merlot

character

Nebbiolo

Italy. Acid, tannic. Truffle, tar and violet notes

Noiriens

Pinot Noir

Beet, cherry, peppermint notes when optimal

Sangiovese

Chianti. Cherries, violets, liquorice

Serines

Syrah

France (n.b. Shiraz in Spain). Tannic, peppery

aromas

Tempranillo

Spain, especially Rioja. Also grown in Argentina.

Jam, citrus, incense notes

Zinfandel

California. Also used for light blush wines

Some regions are especially susceptible to diseases such as Pierce's disease and phylloxera (an insect that attacks rootstock and which is prevalent, for instance, in the Eastern United States but now also in California).

Vines should go dormant in order to survive cold winters. Cool autumn conditions with light or medium frosts allow the vine to store enough carbohydrate for good growth in the ensuing spring. There may be 500-600 or more vines per acre. New vines are trained up individual stakes in the first growing season. Only one shoot is trained in each instance with the others being pinched off. Pruning of vines takes place in winter months after the vines have proceeded to dormancy and the canes have hardened and turned brown.

It is important to match grape variety to the location and to the style of wine. A variety may develop certain characteristics earlier depending on how warm the growing region is. Accordingly, when that grape achieves full maturity, it may have lost some of that character. Table 3.2 summarises varietal issues.

There is some understanding (though far from complete) of the chemistry involved in varietal differences. For instance, methyl anthranilate is found in Lambrusca, 2-methoxy-3-isobutylpyrazine in Cabernet Sauvignon, dama-scenone in Chardonnay (Fig. 3.3). For muscats there are terpenes such as linalool and geraniol and there are terpenols in White Riesling. Some of these are found in the form of complexes with sugars known as glycosides (Fig. 3.4). Yeast produces enzymes called glycosidases that sever the link between the flavour-active molecule and the sugar over time, illustrating the time dependence of flavour development in this type of wine.

cc cooch3

Methyl anthranilate

och3

2-Methoxy-3-isobutylpyrazine

Fig. 3.3 Some compounds responsible for varietal differences in wines.

Damascenone

Fig. 3.3 Some compounds responsible for varietal differences in wines.

Glycosidase

Glycoside

Examples of aglycones: terpenes and terpenols For example, geraniol " OH

Fig. 3.4 Glycosides and glycosidases.

Unless a soil is extremely acidic or alkaline or suffers from deficient drainage, the soil type per se is unlikely to be a major issue with regard to grape quality. Any deficiencies in nitrogen level will need to be corrected by adding N, avoiding excess so as not to promote wasteful growth of non-grape tissue or increase the risk of spoilage and development of ethyl carbamate.

The local climate also influences the susceptibility of the vines to infestation. If there are rains in summer months or if the vineyard is afforded excessive irrigation, there is an increased risk of powdery or downy mildew. Excess water uptake by grapes can also cause berries to swell and burst, which in turn enables rot and mould growth. Over-watering leads to excessive cane growth and delays the maturation of the fruit. In regions where infestation and infection are a particular problem, it is likely that some form of chemical treatment will be necessary. Botyritis cinerea is common where summer rains are prevalent. Winemakers refer to it as grey mould when regarded in an unfavourable light but as 'noble rot' when deemed desirable. The contamination leads to oxidation of sugars and depletion of nitrogen, as well as reduction of certain desirable flavours. However, the character of certain wines depends on this infection, for example, the Sauternes from France.

Of particular alarm in some grape-growing regions is Pierce's disease. This is caused by the bacterium Xylella fastidiosa and is spread by an insect known as the glassy-winged sharpshooter. It is prevalent in North and Central America, and is of annual concern in some Californian vineyards. It appears to be restricted to regions with mild winters. The sharpshooter feeds on xylem sap and transmits bacteria to the healthy plant. The water-conducting system is blocked and there is a drying or 'scorching' of leaves, followed by the wilting of grape clusters.

Harvesting of grapes is usually in the period from August through September and October. The time of harvesting has a significant role to play in determining the sweetness/acid balance of grapes. Grapes grown in warm climates tend to lose their acidity more rapidly than do those in cooler environs. This loss of acidity is primarily due to respiratory removal of malic acid during maturation. The other key acid, tartaric, is less likely to change in level.

Ripe fruit should be picked immediately before it is to be crushed. If white grapes are picked on a hot day, they should be chilled to less than 20° C prior to crushing, but it may be preferable to pick them by night. However, this is not the same for red wine grapes as the fermentation temperature is higher. Fruit destined for white table wine is picked when its sugar content is 23-26°Brix. Grapes for red table wine have a longer hang time. These values are selected such that there is an optimal balance between alcohol yield, flavour and resistance to spoilage. The pH values in these grapes will be 3.2-3.4 and 3.3-3.5, respectively.

Harvesting is increasingly mechanical. While more physical damage occurs, it can be performed under cooler night-time conditions which is desirable, especially for white cultivars. Sulphur dioxide may be added during mechanical harvesting.

Payment is made on the basis of the measured Brix content of the fruit, measured by a hydrometer or, more usually, by a refractometer. A commercial specification will also state the maximum weight of non-grape material that can be tolerated (perhaps 1-2%) and that the berries should be free from mould and rot. For many winemakers, it has been decided that growing their own grapes is prudent. However, the buying in of some material from other suppliers does allow financial flexibility.

The structure of the grape is illustrated in Fig. 3.5. The main features are the skin and the flesh. The skin comprises an outer 1-cell deep epidermis and an inner 4-20-cell deep hypodermis, which is the origin of the colour and most of the flavour compounds in the grape. Sugar and acid are concentrated in the flesh. The sugar content may reach as high as 28%. Tartaric and malic acids account for 70% of the total acids in the grape.

Grape processing

Nowadays the vessels used for extracting grapes and fermenting wine are fabricated from stainless steel and are jacketed to allow temperature regulation.

Endosperm Seed Embryo

Teste

Endosperm Seed Embryo

Teste

Pericarp

Fig. 3.5 The structure of the grape.

These tanks are subject to in-place cleaning, usually a caustic regime incorporating sequestering agents, followed by the use of sanitisers.

Grapes are moved by screw conveyors from the receiving 'bin' to the stemmer-crusher. They pass from there either to a drainer, a holding tank or (in the case of red grapes) directly to the fermenter.

Stems are not usually left in contact with crushed grapes so as to avoid off-flavours. This is not uniformly the case. Pinot noir, then, is sometimes fermented in the presence of stems in order to garner its distinct peppery character.

Stemmer-crushers frequently employ a system of rapidly spinning blades, but may have a roller-type design (Fig. 3.6). In either case, there is an initial crushing into a perforated drum arrangement that separates grape from stem.

The aim is even breakage of grapes. If grapes are soft or shriveled, they are tougher to break open. Excessive force will lead to too much skin and cell breakage, and in turn in the release of unwanted enzymes and buffering materials that maintain too high a pH. There will also be problems later on in the clarification stage. It is also important to avoid damaging seeds in order that tannins are not excessively released.

It is not necessary to separate the juice from skins immediately for red wine, but is so for white or blush wines. The colour is located in the skin as polyphenolic molecules called anthocyanins. Blush wines are lighter than rose wines. For the latter, overnight contact between juice and skin with a modest fermentation (perhaps a fall in Brix of 1-5) allows the appropriate extraction of anthocyanins. After rose or blush juice has been separated from the skins, it should be protected from oxidation by the addition of sulphur dioxide (SO2). SO2 addition to the crusher depends on several factors, notably whether mould or rot is present and also what the surface area to volume ratio is in the tank (i.e. the likelihood of air ingress). If the grapes are not infected

Stemming and crushing

Fig. 3.6 (a) Grape receiving area, Livingston Winery, California; (b) destemmer and (c) crush pit receiving grapes from gondolas. All photographs courtesy of E & J Gallo.

and the area to volume is low, then SO2 may perhaps be avoided. However, in this instance, the juice should be settled at a low temperature (< 12°C). The rapid separation of skin and juice for white wines also minimises the pick-up of astringent tannins. The process may also impact other flavour compounds, for example, the flavours that impact Muscat. For certain grapes/wines, therefore, there is a balance to be maintained in terms of oxygen availability, SO2 use, contact time and temperature.

Although seldom used for wines of quality, 'thermovinification' may be used to enhance colour recovery in some wines. The technique involves rapid heating and cooling of crushed grapes. The heat kills the cells, allowing pigments to be released, which may result in undesirable flavours.

Botrytis (see earlier) produces an enzyme called laccase that oxidises red pigments, developing a brown colouration (see Enzymatic browning in Chapter 1). In these circumstances, heating before vinification may be used to destroy the enzyme. Another enzyme that oxidises polyphenols - PPO - is located in the grape per se, but it is inhibited by SO2.

During fermentation, the pH should be maintained below 3.8. Wines then tend to ferment more evenly, there is a reduced likelihood of malolactic fermentation and the wine develops better sensory properties. Furthermore, at higher pH, SO2 is less inhibitory to wild yeast. Maintaining this low pH is especially important for white wines. Prolonged contact with the grape skin causes lower total acidity through precipitation of potassium acid tartrate. The pH may be lowered to 3.25-3.35 by the addition of tartaric acid.

Drainers and presses

Drainers are basically screen-based systems (Fig. 3.7). Presses differ according to the severity of their operations (Fig. 3.8). Membrane or bag presses are very

Fig. 3.7 Inside a Diemme Millenium 430 Bladder press, showing drain channels. Courtesy of E & J Gallo.
Fig. 3.8 Diemme Millenium 430 Press. Courtesy of E & J Gallo.

CO2H

CO2H

Fig. 3.9 Caftaric acid.

Fig. 3.10 The repeating unit of pectin: lengthy sequences of anhydrogalacturonic acid partly esterified with methanol.

COOH

COOH

O COOH

Fig. 3.10 The repeating unit of pectin: lengthy sequences of anhydrogalacturonic acid partly esterified with methanol.

gentle and leave little sediment. By contrast, bladder presses are often used on account of their rapidity, but the juice tends to contain higher solids levels.

The extent to which Maillard reactions can occur during processing is controlled by attention to temperature, pH and the type of sugar. These reactions occur for the most part at around 15% moisture.

Oxidative reactions may occur, with the major substrates being caffeoyl tartaric acid (caftaric acid; Fig. 3.9), ^-coumaroyl tartaric acid and feruloyl tartaric acid. These are the precursors in PPO-catalysed browning reactions for those wines that have minimum skin contact.

To accelerate juice settling so as to obtain a clearer product, pectic enzyme is frequently added at the crushing stage to minimise the level of pectin, which originates in the wall material of the grape (Fig. 3.10). The enzyme also allows easier pressing and affords higher yields.

CO2H

Fermentation

Juice

Once the juice has separated from the skins, it is held overnight in a closed container. Thereafter it is racked off (or centrifuged), prior to the addition of yeast. Winemakers generally aim to leave some solids as a surface for the yeast to populate (or perhaps as a nucleation site to allow CO2 release, as is the case for the residual cold break in brewery fermentations, see Chapter 2). Failing this, they may add diatomaceous earth or bentonite.

In locations where the grapes do not ripen well owing to a short growing season, it may be necessary to add sugar (sucrose), but only up to a maximum of 23.5°Brix. Such a practice is illegal in some locales, for example, California.

The typical composition of the grapes from which the juice is derived is given in Table 3.3.

Diverse sugars, notably glucose and fructose, are present in essentially equal quantities in mature grapes. Sucrose is hydrolysed at the low pH values involved and this is further promoted by invertase. Total reducing sugars will usually amount to <250gL-1.

The organic acids are predominately tartaric acid in grapes grown in warmer climates and malic acid in grapes from colder climates (Fig. 3.11).

Amino acids and ammonia are present, together with lesser amounts of proteins (<20 to >100mgL-1 in the juice). The latter presents a risk to the colloidal stability of wine.

Although vitamins are present in only small amounts, they are generally sufficient for yeast.

A diversity of phenolic compounds is present, and these can be classified as catechins, flavonols and flavanones (Fig. 3.12).

Table 3.3 Composition of grapes (percentage of the fresh weight).

Component

Range

Water

70-85

Glucose

8-13

Fructose

7-12

Pentoses

0.01-0.05

Pectins

0.01-0.1

Tartaric acid

0.2-1.0

Malic acid

0.1-0.8

Citric acid

0.01-0.05

Acetic acid

0.0-0.02

Anthocyanins

0.0-0.05

Tannins

0.01-0.1

Amino acids

0.01-0.08

Ammonia

0.001-0.012

Minerals

0.3-0.5

Information from Amerine et al. (1980)

Information from Amerine et al. (1980)

Fig. 3.12 Some polyphenolic species: (a) catechin, (b) flavonol and (c) flavanone. Table 3.4 Yeasts for fermenting wine.

Saccharomyces cerevisiae Saccharomyces bayanus Zygosaccharomyces bailii Schizosaccharomyces pombe Torulaspora delbrueckii (flor yeast)

The main inorganic cation in juice is potassium, from <400 to >2000 mgL-1.

Yeast

The relevant species are Saccharomyces cerevisiae and Saccharomyces bayanus (Table 3.4).

Contrary to commercial-scale brewing, dried yeast is extensively employed in wine making, where the precise nuances of yeast strain seem to be deemed less important than is the case for beer.

Pesticides employed on the grapes can inhibit yeast. Clarification of the must eliminates most of them, but bentonite or carbon treatment may also be employed. However, ironically, the most common inhibitor of fermentation is SO2.

The chief limiting factor in wine fermentations is nitrogen, that is, the amino acid level in the must. Accordingly, it is frequently the case that the level of assimilable nitrogen is increased by the addition of diammonium phosphate.

As for the fermentation of brewer's wort, O2 is introduced to satisfy the demands of the yeast. However, for wine fermentations, aeration is customarily after the introduction of yeast so as to avoid the scavenging of the oxygen by PPO.

White wines are fermented at 10-15°C whereas reds are produced at 20-30°C. Fermentation is inherently more rapid at higher temperatures, with the attendant increase in production of flavour-active volatiles such as esters. Rose and blush wines are fermented akin to white wines.

Fermentation tends to be progressively inhibited as the ethanol concentration rises, especially at higher temperatures. Naturally there is also more evaporative loss of alcohol at higher temperatures.

The varietal character of certain wines is better preserved at lower fermentation temperatures. Thus, for example, the terpenols in White Riesling are retained better. As in the case of beer, high levels of the undesirables such as hydrogen sulphide can arise if fermentations are sluggish.

In all cases, fermentation should be complete within 20-30 days. The progress of fermentation is monitored by measuring the decline in Brix value.

Wine is usually racked off the yeast once fermentation is complete. However, some winemakers leave the wine in contact with the yeast for several months, perhaps with intermittent rousing, in order that materials should be released from yeast, beneficially impacting flavour.

Colour and flavour extraction from red grapes is maximised by mixing -either by pumping or by stirring. Usually pumping over (of half of the total vessel contents) is performed twice per day. Extraction is also greater at higher temperatures and increased ethanol concentrations.

A technique traditional for Beaujolais wines is Maceration carbonique, which leads to wines with distinct estery, 'pear drop' characteristics. Whole grape clusters are exposed to an atmosphere of CO2. The sugar converts to ethanol (about 2.5% ABV), with the accompanying production of several phenolic compounds. The initial phase of fermentation in the whole grapes is conducted at 30-32°C. The weight of the berries, together with the action of the developed ethanol and carbon dioxide, break down the grape cells and colour is extracted. After 8-11 days, the grapes are pressed and the juice obtained is combined with that which is free running. The whole is fermented to dryness at 18-20° C. Then SO2 is added and the wine is clarified.

Clarification

White wines are either centrifuged or treated with bentonite, which will also adsorb protein. Bentonite is a clay that contains high levels of aluminium and silica. Sometimes it is substituted by silica gels of the type extensively used in brewing.

Casein may be added to remove phenols, which can also be achieved by PVPP. Isinglass is also sometimes used as a fining agent.

Red wines are primarily fined in order to reduce their astringency. Fining agents include gelatin, egg white and isinglass.

Filtration

Contrary to most beers, this is relatively uncommon and only performed on an as-needs basis, either to recover wine from lees (i.e. the residual solid material) after cold stabilisation treatments or immediately before bottling. Microbial threats may be eliminated by membrane filtration.

Stabilization

One of the biggest threats to wine is oxidative browning (see Chapter 1). The ingress of oxygen after fermentation should be minimised. Sometimes 'pinking' of white wines in bottle is prevented by adding ascorbic acid. But the chief antioxidant is SO2, by reacting with the active peroxides in wine

Metal ions, such as iron, which potentiate the conversion of oxygen into activated forms such as peroxide (see Chapter 1), are removed by casein or citrate.

The sulphur dioxide must be in a free, unbound form at concentrations between 15 and 25 mg L-1.

Any hydrogen sulphide present in wine may be eliminated by the addition of low levels of copper

Making Your Own Wine

Making Your Own Wine

At one time or another you must have sent away for something. A

Get My Free Ebook


Post a comment