Wild Fermentation

The other night we opened a bottle of wild fermented New Zealand Pinot Noir from the Marlborough region.
It wasn't expensive and tasted excellent and quite different from most other Pinots in the same price bracket.
It had the depth of flavour and earthiness that is expected from more expensive wines of that variety.
Why would wild fermentation contribute to that difference?
Fermentation of wine is the conversion of sugar in grape juice by yeast into alcohol and carbon dioxide.

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

Normally the yeast added to grape juice is a culture of a Saccharomyces cerevisiae strain.
Different S. cerevisiae yeast strains have differing physiological and fermentation properties which can have a direct impact on the flavour and complexity the finished wine.
Winemakers purchase one of the many strains of this yeast, rehydrate it and add it to the must.
They can be fairly certain that fermentation will proceed in a predictable way and produce a satisfactory result.
But there is another method ie. wild fermentation.
Wild or indigenous yeasts are the naturally existing yeasts in the air, on vegetation and on inert surfaces. Therefore they are found in the vineyard, in the winery and on winery equipment. 

So when grapes are picked and crushed and let sit, eventually they will ferment as the indigenous yeasts on the grapes and those hanging around the winery colonize and begin to consume the natural sugar in the must turning it into wine.

For this reason many winemakers add sulphur dioxide at this early stage of production to 'kill off" the wild yeasts or add a higher selected Saccharomyces yeast inoculum so they predominate in the fermentation process.

There are thousands of different types of wild yeast.

Many do not make good wine so they are broken down into two sub categories: wine yeasts and spoilage yeasts.

The most common wild yeasts found in the vineyard are from the genera Kloeckera, Candida and Pichia with Kloeckera apiculture being the most dominant species.

Saccharomyces cerevisiae is actually quite rarely found in the vineyard or on the surface of freshly harvested wine grapes unless the winery frequently returns winery waste eg. pomace, to the vineyard.

A close up look at Saccharomyces cerevisiae yeasts

How do you know what yeast is there and whether it is good or bad?

Without a laboratory, there is no way of knowing what yeast is in the must.

So what are the dangers of wild yeast fermentation?
Many wild yeasts have a low resistance to alcohol (< 4%) so fermentation can become 'stuck'  ie. stops before all the sugar is consumed, producing flabby wines and unwanted residual sugar problems.
Many are slow to colonize meaning fermentation is delayed and the grape juice is left open to oxidation and infection by spoilage organisms.
Fermentation by-products can produce off aromas and esters which can taint the finished wine.
Then why do many wine makers proceed with wild fermentation?
Pretty much for some of same reasons considered as risks.
Delayed onset of fermentation allows longer contact with the grape skins developing better colour, depth of character and more body.
By-products can impart add complexity eg. as in our wine, earthiness.
By the time wild yeasts have reached their critical alcohol survival levels, they have already done their thing and any Saccharomyces present may have reached a level where they can take over and finish the fermentation. If not, monitoring of fermentation progress (by specific gravity based tests) allows the winemaker to inoculate the wine with the desired Saccharomyces strain at the right stage.
Wineries in the old world have been using this method for centuries with great success.
The new world is just catching on.
Is this something I might try?
All I can say is it's worth thinking about.