Loaf is too dense

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Hi! My husband has been making bread for several months now but continues to have an issue with the loaves being way too dense. What causes that?

What type of bread is it? If it is sourdough, a couple of possibilities: The sourdough starter may not have been active enough. The sourdough starter must be at its peak of ripeness and active. Also, under fermenting and under proofing the dough can also result in heaviness. More detail about his process and formula, particularly time and temperature at each step would help nail the issue in more detail. 

Cheers,

Gavin.