Starch Test
It will be seen in the recipes that we must keep the wort at a certain temperature for a certain period. This is because during this period starches that would cloud the beer are converted by enzyme action into sugar which is later fermented out. It sometimes happens, no matter how careful we are, that not all the starch is converted during the time stipulated and it is not always possible by looking at the wort to decide whether or not the changes are complete. If conversion is not complete boiling will ‘fix’ the starch and removal later on will become a problem.
There is a simple test we can carry out to ascertain whether the changes have taken place or not at the end of the time given in the recipes. If changes are not complete the wort may be left for half an hour or an hour at the stipulated temperature until the changes are complete. Do not carry out this simple test until the end of the time given in the recipe, unless you feel sure that the changes have taken place earlier than expected.
Take about a tablespoonful of the wort into a glass or white cup or basin; to this add a few drops of medicinal iodine which has been diluted with an equal quantity of water. If the sample turns blue, starch is still present. If it does not, then the changes are complete. Do not worry if first tests show the presence of starch - just maintain the temperature for a little longer as already suggested and all will be well. A second test may be carried out if doubt still remains after extending the period in the mash tun.
Do not return tested sample to the bulk - throw it away.