Our hen lays an egg a day. I have used her eggs in cakes, and find that the eggs makes cakes that are too light -- so light that they are hard to cut. How do I compensate? Use fewer eggs? It seems silly to buy stale store eggs when I have fresh ones on hand.
Most store eggs are just days old, so they are probably only a bit older than those you've been saving. Eggs do change with age, however. Because their shells are porous -- perforated by thousands of holes that allow the free interchange of gasses -- they tend to lose moisture and carbon dioxide.
When first laid, an egg has the same pH (acidity) as the hen's blood. As the egg sits, carbon dioxide -- which is dissolved in the egg white and which maintains the egg's pH right around 7, neither acidic nor basic -- evaporates from the egg white and passes through the shell to the surrounding atmosphere. Because carbon dioxide forms carbonic acid when dissolved in water, its leaving the egg white (white is 80 percent water) causes the pH to rise. With the rise in pH, the egg white proteins don't beat into a foam as quickly as do those of fresh egg white.
If it is the pH of the fresh eggs that is making the difference, you could artificially age the eggs. Measure 1/16 teaspoon of baking soda per egg and mix that with the eggs. You should see a big difference in their foam potential.
I have tried to melt three brands of low-fat or low-salt cheese to make a cheese sauce. I shred the cheese first and then heat it in milk. The cheese turns into a gummy blob. Is it the fat or the salt that makes a cheese melt?
It is neither fat nor salt that is responsible for a cheese's ability to melt. Mozzarella is a superb melting cheese, and it contains comparatively little of either.
What makes a melting cheese is a chemical change in the curd. With mozzarella, the curd "acid ripens" before it is heated and formed. Acid ripening causes a change in the milk protein molecule, making it much more soluble. With aged cheddars, enzymes split the milk proteins into smaller molecules, and the curd softens when heated. Young cheddars melt poorly. Old cheddars melt readily.
The cheeses you are buying are probably very similar to young cheddars. Their molecules are large, and they don't melt well. I would recommend buying a skim-milk mozzarella. It might contain a little more sodium than the cheeses you are buying, but at least it will melt.