Fats
Adding a small proportion of fat can very slightly enrich dough and help it to stay fresh longer. Personally I find butter is best of all, but vegetable fat or lard if you prefer can be used instead. Proper enriched doughs are those that have quite a lot of fat in them, sometimes plus eggs and dried fruit (e.g. hot cross buns, mall loaves). The extra fat retards the growth of the yeast, so an extra quantity of yeast is used in this type of recipe.