Hops question on brewing day!

My recipe calls for 0.5 oz. (2 AAU) Hallertau for boiling, 0.5 oz. (2 AAU) Hallertau for flavor, and 0.25 oz. (1.25 AAU) Saaz for aroma.

The problem is I don't have enough Hallertau. The recipe assumes a 4% aa rating, but mine are only 2.8%.

I have enough for the boiling stage, so would it be possible to make up the remaining AAU with Saaz in the flavor stage? That would be 50-60% Saaz instead of Hallertau.

One more question...is it even important to consider AAUs except for the boiling hops? For example, for aroma should I go with 0.25 oz. or 1.25 AAU of Saaz? Does a weaker aa rating mean only the alpha acids in that batch are low, or does it also say something about the general "power" of the hops (oil content and so on)?

The table I have says that the conversion rate to iso-alpha acids is 30% in the boiling stage, 21% flavor stage and 10% aroma stage. I didn't realize that the flavor and aroma hops added that much to the bitterness. I thought it was negligable.

This is such a low IBU beer, I wouldn't worry about it. But, you could make up the bittering AAU with Saaz and not worry about the AAU for flavor & aroma as they go by go by weight, not AAU. The flavor and aroma oils are more or less independent of the AAU, so using a little extra Saaz in for bittering would have the least impact on the overall flavor.

Utilization is very time dependent, so the exact numbers depend on when you make the flavor & aroma adds.

^^^That's a good point. This recipe says to add the flavor hops 30 minutes into the boil and leave in for 30 min. I always add flavor hops in the last 15 minutes.

Maybe that is why the AAUs for the boiling hops are so low - the recipe is counting a little bittering power from the flavor hops ???

I thought the bitter oils were only extracted after 50 min. or longer, so maybe that is not the purpose.

Right.

At 60 mins you get 30% bittering -- that's the max you get out of any hop.

At 30 mins you only get 15% bittering.

Anything less is flavor and/or aroma because it's not boiled long enough to extract the bittering oils.

In your recipe you would need 1.4 oz of Hallertau to give you the 4%, or .7 oz (2.8) each addition versus .5 (at 4%). Just divide the larger AA by the smaller AA and you'll get how much of the smaller AA you'll need.