Baking – subsitute grapefruit for either lemon or orange in baking

A 13" cake is large enough to be a major problem -- the center just won't set correctly before the edges overcook.

You generally want to cook large cakes at a lower temperature than you'd cook a smaller cake at; they won't rise as much, but as they're typically used for stacking, a denser cake may be preferred.

There are also two gadgets that you may wish to consider when you're dealing with cakes over 10 inches:

• Baking Strips : They cool the edge of the cake, so that it cooks slower. Wilton sells them, but you can also make your own from an old towel
• Heating Core : They conduct heat to the middle of the cake, so it'll cook faster. Wilton sells them, too, but if it's not too deep of a cake, you may be able to use a flower nail. If you have a heat safe metal cup, that could work as well; I've heard of people using a cocktail shaker. (but it's important to grease it liberally, as you want what's inside to come out cleanly so it can be used to plug the hole).

I can't help with the gas mark, as I don't know the British system.

Cooking temperature and time are determined by a number of factors. The idea is to get the inside of the product properly cooked before the outside dries out, becomes tough, or becomes unpleasantly dark or even burned. At the same time, you usually want the product to get nicely browned (adds flavor and looks nice) before the inside is overcooked. So it's a balance.

Factors which influence appropriate temperature and time include:

• ingredients: High protein ingredients (like meat or eggs) easily become tough when overcooked. High sugar or starch recipes will tend to brown or burn more easily.

• moisture level: For some products, such as popovers or many kinds of pastry, steam is an important leavening agent, and a high temperature is called for. In other products, like cookies, one of the goals of baking is to drive off excess moisture. And in still others, moisture is absorbed into the other ingredients.

• shape: A fat, round loaf will usually need a longer cooking time and lower temperature than a thin, flat pizza or a long, skinny baguette because it takes longer for the center of the loaf to heat up.

• pH: Changing the pH of the product will change how it browns.

• leavening: Some chemical leavening, like double-acting baking powder, activates at a certain temperature.

• personal preference: At the end of the day, the most important factor is whether you like the way the product turned out. If you like a crispier crust, change the temperature and/or cooking time to suit your taste.

Any baking recipe should specify the temperature and cooking time, unless perhaps it's from a book that specifies those things for a number of recipes at once. If not, find a similar recipe and use the temperature specified there, but keep a close eye on the product during the baking process. Learn how to tell when the product is done. For cakes, you usually go by color for the outside, and by temperature or using a toothpick or wooden skewer for the inside. (Briefly: poke a wooden toothpick into the center of a cake; if it comes out with wet batter, keep cooking; if it comes out clean and dry, it's probably overcooked; if it comes out with a few crumbs stuck to it, it's probably perfect.)