Muffins not fluffy

[IceBus]

Hey all... I was wondering if any other pastry chefs could diagnose where I'm going wrong.

I'm using this recipe for Blueberry muffins.

First time I cooked them, great success, light fluffy yogurty blueberry muffins. However, subsequently they've turned out quite dense and heavy (second time was very rubbery).

I'm mixing the wet and dry ingredients with only 5-6 strokes (flour is well sifted) so I don't think I'm overworking the mixture. Possible things I can think of:

1) Melting the butter in the microwave it always ends up bubbling (even when done on a low heat) - could the fat be separating?

2) First time round I mixed the sugar with the wet ingredients (to dissolve it before folding on) - something I haven't done since.

Any ideas would be much appreciated

 

[IceBus]

My SR flour was only opened a couple of weeks ago, but I was wondering about adding some raising agent too - I'll give that a shot and see how it goes. I'll report back!

 

[IceBus]

What makes me scratch my head is that she says to fill them 85% full, yet those muffins are barely peeking over the top of the wrapper. When I do mine, I put 2 dessert spoons in each so it's certainly less than 50% and mine end up 1-2 cm above the wrapper easily.

First time round I filled them 3/4 full and they only just peeked about 1cm above the top, so I'm guessing it may be something to do with the acidity in the yoghurt knocking out some of the raising power of the flour.

Out of interest, have you got your recipe to hand? I'd be interested to compare the ingredient breakdown.