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Sweet Wine: The Science of Sweetness

Sweet Wines

Part 1: Introduction

Part 2: Must Weight

Part 3: Botrytis

Part 4: Late Harvest Wines

Part 5: Eiswein

Part 6: Dried Grape Wines

Part 7: Vin Doux Naturel

In my introduction to this series I outlined the two main categories of sweet wine; those made through dehydration, and those made by mutage. Both processes achieve the desired outcome; the creation of a wine with a high concentration of natural grape sugars, producing wines which range from off-dry to an intense, heady level of sweetness. Over the coming weeks I will be exploring the varied methods employed in producing sweet wines, dealing with Botrytis, late harvest, eiswein, dried-grape and mutage styles in turn. Before we start, however, I think we should be equipped with a little of the lingo of sweetness, an exploration which should give us our first insight into the making of a sweet wine. And we shall also give a passing mention to the methods that can be employed by winemakers on a budget; inexpensive methods of producing wines which, although not of interest to those who look for complexity and cellaring potential, are nevertheless sweet.

Must Weight

Any winemaker, whether turning out dry, sweet, sparkling or fortified wines, needs to know exactly just how much sugar exists prior to fermentation; this indicates the ripeness of the grapes and, when measured in the vineyard, can be used to set a harvest date. For the sweet winemaker, however, knowing the concentration of grape sugars will also indicate the potential of the grapes; will this be a year for dry wines only, or are the grapes sufficiently rich in sugars to permit the production of a great sweet wine?

All methods of measuring sugar concentration - known as the must weight - depend on measuring the quantity of solids in the wine. Of these, about 90% are grape sugars, the remaining 10% consist of acids and other ions and minerals taken up by the grapes. So in truth, the must weight is an approximation, as one can never be certain exactly what proportion of the measured solids is sugar. Nevertheless, the process is tried and tested, and knowledge of the must weight is vital for the vigneron. There are two methods for elucidating the must weight; by use of either a refractometer or a hydrometer.

$Refractometer & hydrometer$ The refractometer is precision laboratory equipment; or, more conveniently, it is a small hand-held device which can be used by the winemaker in the vineyard. The juice of a ripening grape is squeezed onto the device, which provides a sugar reading by the process of refraction. This process depends on how light changes direction when it passes between two media, in this case air and grape juice. The concentration of the solids in the juice affects the angle of change; measure the angle, and you have an indication of the concentration of the solids, which are mostly sugars. A quite different method, however, is the hydrometer; this device is more likely (although certainly not exclusively) to be employed in the winery. The hydrometer is simply a hollow glass tube, bulbous and weighted at the bottom, so that it floats upright. As the concentration of solids in the grape juice affects its density, it will also affect how the hydrometer floats in the juice. The denser the juice, the higher the hydrometer floats; on the side of the stem is a scale, which allows the winemaker to make a reading. Just what the scale shows, however, depends on where he or she is working; there are many different scales for must weight.

Scales for Must Weight

The number of different scales for must weight is testament to just how vital this measurement is to the winemaker. Distant wine regions have developed their own systems long before the era of globalisation and international conformity. Each one describes, in subtly different ways, the amount of solids in the wine.

Specific Gravity
Although not commonly used in winemaking, it is worth getting to grips with specific gravity; it is the basis for at least one method of describing must weight. Specific gravity is essentially density; the greater the density of the substance, the greater its specific gravity. In the case of grape juice, increased density is due to increased concentration of solids, which are mostly sugars. The specific gravity of water is 1; grape juice with specific gravity of 1.075 is 7.5% more dense than water, the increased density entirely due to the dissolved solids.
Oechsle
A uniquely German scale, the Oechsle value reflects specific gravity. The aforementioned grape juice, with a specific gravity of 1.075, is 75º on the Oechsle scale. Must weights are of obvious importance to German winemakers, as it determines the Prädikat; regulations differ from region to region, but as an example Mosel Auslese must be above 83º Oechsle.
KMW
Just over the border, Austrian winemakers happily discuss must weight in terms of KMW, or Klosterneuberg Mostwage. Developed in Austria's centre for oenology, Klosterneuberg, each point on the KMW scale is equivalent to about 5º Oechsle. A wine that is 75º Oechsle is 15º KMW.
Brix/Balling
Both the Brix and Balling scales, commonly used in Australia and the USA, describe the percentage of the grape juice accounting for by dissolved solids. Juice that is 75º Oechsle is 18º Brix; that is, the solids (of which 90% are sugars) account for 18% of the solution. This has nothing to do with percentage of alcohol that may be achieved with fermentation, although this is described by the Baumé scale.
Baumé
This scale, used throughout much of Europe, describes the potential alcoholic strength of the juice if it were fermented to total dryness. The potential alcohol is based on the sugar concentration; it takes approximately 16.5g of sugar to produce 1% alcohol by volume. Our 75º Oechsle wine is 10º Baumé, and thus has a potential alcohol of 10% by volume. Sadly, not a very commonly found wine today!

Key points

Must weight is a vital concern for all winemakers

Its measurement is based on either refraction or density

Its value is indicated by the variety of scales that exist

It determines the potential alcoholic strength, and the final sweetness, of the wine

In the Winery

The winemaker dedicated to producing a fine sweet wine does most of his work in the vineyard, guided in part at least by the forecast must weight of his crop. The handsome must weight of botrytised, late-harvest and eisweins is the result of dehydration prior to harvest, whereas other styles - dependent upon post-harvest dehydration by drying, or on mutage - obviously depend on some manipulation of the grapes before or during fermentation. These methods I will discuss in much more detail in coming weeks. Nevertheless, less admirable vignerons may produce a sweet wine by other methods, as I hinted in my introduction to this article. It is worth knowing of these, although these wines should not be of interest to anyone reading this article! Such methods include the addition of sweet grape juice, a practice not unfamiliar to many German winemakers, where the additive is known as Süssreserve. Rest assured that this practice is illegal for wines with a Prädikat; your precious bottles of Prüm, Loosen and Haag Auslese are not made by this method! Of course, the real villain could add non-grape sugar to sweeten his wine; although legal as Chaptalisation, where the aim is to potentiate the final alcoholic strength, this is an illegal method for producing sweet wines in most countries. One other legal method that may be employed, however, is the use of sulphur. The winemaker ferments grapes of high must weight, and when the alcoholic strength is satisfactory, and the fermenting juice still sweet, an unhealthy slug of sulphur serves to arrest fermentation and stabilise the wine. Rather a heavy handed approach, and not one that I will be discussing any further.

None of these unscrupulous methods interest us. Of far greater importance are the world's fine eisweins and botrytised wines; grapes picked rich in sugar, with an Oechsle reading well into the hundreds, which ferment gracefully, yielding a nectar like no other. We start, next time, with what else but Botrytis: the Noble fungus. (13/9/05)

Next instalment: Botrytis, the Noble fungus.