Harvest has started in the southern hemisphere which for me means phone calls and queries from winemakers. These questions, I am starting to realise, provide ample blogging material. Here goes one I got yesterday: 
Q: I used "Yeast X" last year and it worked beautifully on my Sauvignon blanc. I therefore used it again in 2009 and got nothing. The wines were very bland. Did you change the yeast?
A: Yeast companies don’t generally “change” their yeasts – not the well-known established brands anyway. I can’t speak for distributor brands. The OIV requires that yeast strains are identified on the packaging especially if the original strain name is not the same as the brand name. There can, however, be quality differences in the manufacturing of a specific strain. Reputable yeast producers have strict manufacturing standards to ensure product performance repeatability.
In some cases when one company steals another company’s strain and manufactures and sells it under a different name – the copied strain can have decreased fermentation performance. This is simply because the company that originally commercialised the strain has perfected its production process. The company who stole the yeast does not have the in depth knowledge of the specific strain’s metabolism since they did not do the research to develop the strain. I have seen published scientific results from a reputable institute on fermentation performance of two strains that I know to be the same strain. The original outperforming the copy. Sadly, very good copies also exist. Why do these copies exist? Well its simple – they are cheaper than the original since the copying company did not have to pay for four to five years of research and neither do they pay royalties to the research institute who owns the strain.  
So if the yeast strain does not change? What does? How can you have a good wine one year and a bland one the next – from the same vineyard? Well, the answer is quite simple. No two years are the same in terms of weather patterns and if you add changes in viticulture to that, you increase the effect on grape composition even more.
Yeasts cannot perform miracles. They can only work with the raw material you supply. Grapes contain aroma and flavour precursors. If, in some years, these precursors do not exist in adequate concentrations as a direct result of a change in climate, yeasts cannot transform them to flavour active compounds. Simple as that. So it’s not the yeast supplier that is to blame, more likely the viticulturist, who should have adapted his/her viticultural practices to accommodate the change in climate form the previous year. Unfortunately the effect of some climatic conditions cannot be corrected through a change in viticultural practices and one has to resort to changes in winemaking practices to try and save the day. This can include a change in the choice of yeast, lower fermentation temperature in the case of white wines and higher juice clarity. The best thing to do if you are uncertain as how to handle lower quality fruit than what you normally have is to contact a fermentation / winemaking consultant (phone a friend) and ask for advice. There is no point in doing what you normally do and hoping for the best. This “hoping for the best” could have serious financial implications.