If you tell the dost.jar file of DITA OT which source DITA file to transform and the output format you want, it can do the rest.

If it can't find any specialized handling for the ingredients element, DITA OT needs to do the necessary lookups to find out whether it to use the prereq processing on the ingredients element.

You have to have it installed when you use the DITA OT anyway, so for large projects it's worthwhile to learn Ant and then build some Ant scripts around the ones included with the DITA OT.