Subtle's the key to a good wine
Winemaking is about keeping good bugs happy and bad bugs out, said Rich DeScenzo of ETS Labs in Blenheim yesterday.
Some people would rather cut their arm off than filter wine before it went into a bottle but this opened the risk of unwanted bugs having a party in there, he said.
"Once you put a cork or a cap on you lose control of what will happen."
Dr DeScenzo was speaking to about 50 wine company staff at a roadshow hosted by Hill Laboratories at the Marlborough Research Centre.
Winemakers also took a risk relying on indigenous rather than commercial yeasts, Dr DeScenzo said. The complexity this brought had a dark side, such as wild yeasts producing a lot of ethyl acetate, which gave the wine the unacceptable aroma of nail polish remover.
Wine spoilage by microbes could happen during cold soaking of fruit, fermentation, barrel ageing and in the bottle.
Winemakers all knew about in-your-face overt spoilage of wines that never hit the market, he said. They should also worry about subtle, covert spoilage that detracted from a wine's potential.
If winemakers learned what could go wrong at particular points and what to look for, they had a better chance of controlling spoilage, Dr DeScenzo said.
Grapevine physiology specialist Glen Creasy of Lincoln University said vineyard managers played a vital part in ensuring that grapes came into the winery containing everything yeasts needed to grow, divide and make quality wine.
In a sense, winemakers were making yeast feed, he said.
Sugar and yeast-available nitrogen (YAN) were vital ingredients of a good ferment. Between 140 and 150 milligrams per litre of YAN was ideal but levels naturally varied from almost zero to 500mg, he said.
Winemakers could easily fix low YAN levels by adding diammonium phosphate for example, but the market was moving away from additives.
In the vineyard, grape variety, water availability, soil fertility and competition from inter-row species all influenced YAN. Viticulturists could maximise levels by ensuring grapes had plenty of nitrogen and water and removing cover crops, for example. This was especially important at high grape yields. Grape skins and seeds contained a lot of YAN, which was why stuck ferments were more common in red wine than white.
Research in New York suggested a strong correlation between YAN levels in fruit two weeks before and at harvest, helping predict possible fermentation problems, Dr Creasy said.
Plant & Food scientist Dion Mundy said research in Marlborough showed YAN levels one season were a good indicator of levels the next.
Steve Price of ETS Labs described the different influence of ripe and over-ripe grapes on phenolics, which provide mouth feel rather than taste.
Hill Laboratories, which hosted a winemaking roadshow in Blenheim yesterday, has wine analysis laboratories around New Zealand and works in partnership with California-based specialist wine lab ETS Labs.
The Hill laboratory at the Grove Business Park north of Blenheim offers export certification and label analyses.