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Conventional wisdom states that dissolved oxygen is driven out of wort when it is boiled. I can certainly understand that after 60-90 minutes of boiling the oxygen would be all gone. What about when boiling wort for 10 minutes for making a yeast starter? How much of the oxygen is still there and how much oxygenation is needed?

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In principle boiling water for just a few minutes will reduce the oxygen levels to "near zero". Boiling wort for a similar time will probably have improved results - as wort boils at a slightly higher temperature than water. Oxygen dissolves less in hot water and will leave the liquid via the surface. As the water heats up, the solubility of oxygen can decrease quickly and and air/oxygen bubbles can form in the water before it actually boils. If the water is actually boiling with steam bubbles then the steam will assist the rest of the oxygen to escape from the volume of the boiling liquid as well as from the surface.

To make an efficient yeast starter from boiled wort one would have to cool the wort and then shake it vigorously after exposing the cooled wort to air a few times. I don;t tend to use wort for making starters any more but when I did I put the cooled wort into a plastic 1 litre "soft drink" bottle. I screwed the top on a shook the bottle vigorously for (say) 20 seconds. Then I open the cap and squeezed the bottle to drive out old air and put in new air, then I closed the cap again and shock again. Repeating this (say) 3 times was shown to put 7ppm dissolved oxygen back into the wort (measured with a membrane O2 probe). Even if I "went crazy" with the aeration technique I couldn't get it much above 10ppm. I understand that anything at or above 7ppm dissolved oxygen is good for initiating primary fermentation.

Now I use glucose for preparing a starter (not a "culture" for keeping). I sterilise a capped glass bottle by filling it (and the cap) with boiling water. After a few minutes I drain the water (careful!) and add 50g dextrose/sugar to the bottle and dissolve it in a minimum of boiled water (about 1/2 - 1 cup). leave this for a minute or two and the heat will pasteurise the sugar. Then (careful - heat stress)) add cold water to the bottle until it is "baby milk hot"(test it!). At this point add the yeast to the solution and shake vigorously for (say) 20 seconds with the top on the bottle. Allow some fresh air into the bottle and repeat (say) 3 times. Leave in a warm place for an hour. The yeast should by now show good signs of activity. If so it is safe to pitch. If not wait and recheck later. If still no sign of vigorous activity then discard and make another (preferably using a different batch of yeast). The method is quicker and less costly - more so if the yeast is inactive!

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