Impvoving the loaf with Sourdough Starter--Help needed
I hope I don't double post--my first try seems lost.
I like to make King Arthur's Multi-Grain Loaf (http://www.kingarthurflour.com/recipes/multi-grain-loaf-recipe  ) but it is still a bit heavy and stales quickly. Dan Leopard suggests using sourdough starter along with the yeast to help the loaf stay moist and enhance the flavor. He suggests that you add 150 g starter for every 500 g of flour in the recipe (approx. 30%).
Does that mean ADDING 150 g starter to the recipe for every 500g of flour or does it mean REPLACING 150 grams for every 500 g. flour. My inclination is the latter. In the above recipe, for example, there are 14 oz of flours in the recipe. 4.2 oz is 30%, so I would add 4.2 oz of starter, replacing 2.1 g. flour and 2.1 g. water. But is that right, or should I just be adding 4.2 g. starter to the recipe as it stands????
My other question is that this recipe calls for "Whole Grain Bread Improver". I make my own by adding vital wheat gluten plus a little vinegar. Do I still need to do this if I'm adding sourdough starter, or does the starter replace the "improver"??? My inclination is to continue to add the vital wheat gluten to help the structure, but the vinegar to acidify it won't be necessary.