<p>Mr Herzog was, of course correct.&nbsp; Yeast consume the sugar in the grape juice and excrete alcohol.&nbsp; To make grape juice you must kill the yeast. In order to kil the yeast you can 1) boil the juice or 2) add potassium or sodium bisulfate.&nbsp; The knowledge of bisulfates is from, at least. the middle ages (afor lavan).</p> <p>Sulfates dissipate, hence in modern winemaking they are commonly useed to kill the natural yeast found on the grapes (that whitish powder that you can see) and then after, say 24 hours, a specific type of yeast (thats best for the type of wine you are making) is added.</p> <p>Once the yeast are killed by boiling or bisulfates, they can be reintroduced into the juice&nbsp;either directly or by letting it stand and accumulating sufficent amounts from ambient yeasts in the air and it may turn into wine.&nbsp;The&nbsp;new yeasts will then do their thing with the sugar in the juice. &nbsp;Of course, if some vinegar producing bacteria get to it first it will then become vinegar.&nbsp;</p> <p>The real halachic question is: what is the definition of "potential to make wine".&nbsp; It the above senario sufficent or not.</p> <p>Unfortunately, the is another example of some poskim not understanding the real metzios before they clair arein and cause all sorts of greif and macklokos.</p> <p>Zev Wrotslavsky</p> <p>a home winemaker in Southfield MI</p> <p>&nbsp;</p>