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 envi-rons. 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 desir-able, 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.
Endosperm Embryo Seed
Pericarp
Testa
Mesocarp (flesh) Exocarp
(skin)
0 2 mm
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.
Stemming and crushing
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 some-times 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). SO2addition 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
(a)
(b)
(c)
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 SO2may 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, SO2use, 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 fer-mentation and the wine develops better sensory properties. Furthermore, at higher pH, SO2is 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.
OH OH
HO H O
H O
CO2H CO2H
Fig. 3.9 Caftaric acid.
OH
COOH OH
O O
COOCH3
OH OH O
O O
OH
COOH OH
O
OH
COOH OH
O O
COOCH3
OH OH
O
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 con-trolled 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), p-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.