MOG and the Grape Harvest

When I was taking the UC Davis Online Course, I was happy to leave my former world full of acronyms but I learned that there are a few acronyms in Viticulture and Enology as well. One that comes up at harvest is MOG. The Wikipedia defines MOG as "A winemaking abbreviation for Material Other than Grapes and usually refers to debris like leaves, dirt and stems that can be unintentionally harvested with the grapes."¹ There are other unseen MOG that also comes in with the harvest and in an article in Practical Winery and Vineyard written by Lisa Van de Water called Monitoring Microbes During Fermentation, she explains that the most prevalent yeast on grapes is Kloeckera apiculata and it's sporiferous form Hanseniaspora uvarum.^2,3
Characteristics of Kloeckera:

• SO₂ sensitivity is similar to that of Saccharomyces
• Cold tolerant so can readily grow in musts at temperatures between 10-15 ^oC
• Can survive much further into the fermentation, sometimes all the way to the end
• Can sometimes form thick, slimy scum, or stringy clumps that fall out later as fluffy lees
• Can make prodigious amounts of ethyl acetate (odor of airplane glue or nail plish) and amyl acetate (odor of bananas)
• Efficient in quickly depleting nutrients, particularly thiamine and other vitamins
• Microscopically, it resembles bowling pins or thin lemons with a knob on each end

Lisa Van de Water advises using a phase constrast microscope to examine the must for microbes. Before fermentation begins, if more than 5 or 10 non-Saccharomyces yeasts are seen in a 40x field, a problem is developing. Read about Kloeckera and more in the excellent online article Monitoring Microbes During Fermentation.

1. Wikipedia Glossary of Winemaking Terms
2. Lisa Van de Water, Monitoring Microbes During Fermentation
3. P. Ribéreau-Gayon, D. Dubourdieu, B. Donèche, and A. Lonvaud, Handbook of Enology, Volume 1, The Microbiology of Wine and Vinifications, Second Edition, 2006, John Wiley & Sons, Ltd, pg. 41.