Unrefrigerated oils may produce botulism. Refrigerated, they should keep two days to a week. Using a microwave is acceptable for making a roux. Melt the butter and flour together first, then stir in liquid. Sauce will cook around the edges first.
Lumps are riskiest in starch-bound sauces.
To produce a velvety sauce, make sure liquids are hot before you add them and stir until blended. If a sauce separates, the problem is usually overheating, which evaporates the liquid. To save it, remove from heat, add an additional tablespoon or so of liquid and whisk like crazy. All rights reserved.