Cellar Dweller Lingering sweetness...


Some sweet wines retain grape flavours and others conceal them. By Gary Strachan


A


bout 25 years ago I tried my hand at icewine for the first time. What would happen if I destemmed the frozen berries instead of doing a whole cluster press? Imagine the sound of a bushel full of marbles in a clothes dryer. Picture a destemmer firing marbles across the research station pilot plant. Machine harvesting now removes berries from the stems, so this experiment may never be repeated.


There are many ways to make wines with residual sweetness. If you ferment must with less than about 30 Brix, the fermentation will move very slowly, but eventually you will get a dry wine. At sugar levels above 30 Brix, the fermentation will likely stop and leave residual sugar. Some yeast strains (osmophiles) are more tolerant of sugar than others, so there is no hard endpoint. To make sweet wines you can also arrest fermentation by chilling, cross flow filtration, or you can complete the fermentation for part of the batch and add back unfermented juice.


Osmotic pressure is created by a concentration difference between the dissolved compounds inside yeast cells and outside the cells. The gradient tends to equilibrate by dehydrating the yeast cells. At high concentration, yeast cells can no longer resist this pressure, metabolism declines and the cells may die, but a level this high (over 70 Brix) is only obtained during the production of grape concentrate.


One of the ancient ways of dehydrating grapes by osmosis was to immerse them in sea water.


We can classify sweet wines into several broad categories. We have those that retain grape flavours and those that conceal them. For example, a late harvest or sun-dried muscat variety can be made into a moscato style of wine. For the most part, the dominant flavour will still be the intense fruitiness of the terpenes of muscat grapes.


Winemaking will develop complexity from oxidation products, a wide variety of alcohols and aldehydes which impart notes such as honey, tea, almond or tobacco.


British Columbia FRUIT GROWER • Winter 2013-14 25


The oxidized products are those that form the dominant nutty character of sherry, a product with concealed berry character. Moderate oxidation frees terpenes from glycosidic links and intensifies the fruity aroma. Terpenes chemically bound to a glucose molecule have no aroma.


These reactions also occur to a greater or lesser degree during icewine production. The amount of change varies according to the point at which the crop is harvested. When ice crystals form, the cell constituents (mainly sugars and organic acids) are excluded from the crystals. Water crystals exclude dissolved substances, thus concentrating them in the remaining free water.


Sugars dissolved in water decrease the freezing point, like antifreeze in a car radiator. Ice crystals puncture berry cell membranes and enzymes are released that initiate oxidation of cell components. During a typical winter, there will be many freeze-thaw cycles before the temperature drops low enough for harvest. Oxidative changes and berry dehydration proceed a little further with each cycle.


The bottom line of this process is that icewine harvested in early winter will have fruitier character and less of a “sherry” character than icewine harvested later. The extent of water crystallization is directly related to temperature. At (say) -14 it is difficult to press juice because most water is tied up in crystals. At -10 a highly concentrated juice will be released at (say) 50 Brix. During pressing the temperature will slowly rise and the sugar will become less concentrated. The winemaker must decide when to stop pressing. A typical press cycle may take six hours of drowsy patience.


Sweet wines are typically more expensive than table wines, a reflection of their higher cost of production. Here is a selection of the smaller bottles used for their packaging.


Port style wines are harvested at the normal date but fermentation is arrested by adding high proof alcohol mid fermentation. The most complex ports are produced with alcohol produced by a pot still from the same grape variety used to make the wine. A typical port fermentation is arrested by draining the sweet free run must into a tank when it has reached (say) 5 per cent alcohol and high proof sprit is added. The remaining wine is pressed and the press wine, higher in tannins, is added.


The amount of alcohol to add can be estimated with a Pearson’s square. Traditional ports are fermented in shallow tanks which contribute aeration and develop higher aldehyde content than conventional table wines. Along with the aldehydes present in pot distilled alcohol, aeration contributes to colour stabilization.


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