Evenly Baked Cookies

When one looks into baking sheets for cookies, one hears plenty about "even baking". One of the things I have heard several times is that one wants baking sheets with very low or no edges so that air can circulate over the pan to provide even baking. Another thing is that heavier pans help with even baking. One should not place cookies too close together in order to promote even baking. I think I remember hearing years ago that one should not place cookies too close to the edge of the pan because they will become overbaked.

It would take a lot of time to thoroughly investigate these issues about even baking. However, I have baked numerous batches of cookies on various baking pans, and I have made some interesting observations.

First about edges of baking sheets. There are two issues - how close should you place cookies to the edge of a baking sheet, and how high can an edge be and still produce good cookies.

I have six baking sheets with very low or no vertical edges. Three of them seem to be the very best baking sheets of the eight I own. Three of them are the very worst. There is no obvious correlation of low edges with high quality cookies. I bought one baking sheet with a very high edge - a "jelly roll pan". For the several batches of cookies I baked with it, the cookies were remarkably evenly-baked. Of the three sheets without edges which produce the nicest-looking cookies, two of them are insulated and I attribute the high quality of those cookies to the air between the layers of the pan, not to the lack of vertical edges. In addition, the three best have a traditional finish rather than a non-stick finish, and that also seems to have a positive effect.

I have so far not seen any reason to believe that a vertical edge as high as 1" causes unevenly-baked cookies due to inadequate air flow or that a low edge prevents unevenly-baked cookies. On the other hand, I would certainly avoid baking cookies in a cake pan! I'd say that would be asking for trouble. A baking sheet with no vertical edge on at least one side is easy to slide parchment paper off of after pulling a batch of cookies from the oven, so it's desirable from that point of view. I believe that it is important for air to circulate sufficiently well over the cookies in the oven, and I can easily imagine that pan edges have some effect on that. However, the concrete effect of high edges on air flow seems to be slight enough that it is not observable in the baked product, and we need only consider whether we want to be able to slide parchment paper off the pans and any advantage a rim provides in being able to grip the sheet to remove it from the oven.

Two caveats - First, I have not tested in various ovens. I have a typical inexpensive electric oven. Conceivably the height of the rims makes more difference in some other type of oven. Also, I baked with one sheet in the oven at a time, on the center rack. Perhaps the height of the edge makes more of a difference if a baking sheet is on the top or the bottom rack because there are two sheets baking at the same time. However, one gets the most reproducible results easiest by baking with one sheet in the oven at a time.

The other edge question is how close cookies should be placed to the edge. I noticed something interesting - cookies at the edges of pans do tend to get baked more on the part of the cookie next to the edge. I noticed early on that my two Good Cook/Oneida slide-off pans have horizontal handles at two opposite edges, and the cookies at the edge of the sheet where the handles are get overbaked, while the cookies at the edge of the sheet without handles or rim do not get overbaked. A-ha! A clue! What's going on there? The handles are horizontal so do not impede air flow over the pan - so what makes the cookie next to the handles bake more?

I formed a hypothesis - that a large amount of metal next to the cookies, and which is unoccupied by other cookies, causes the cookies next to the bare metal to take up excessive heat from the large expanse of bare metal. Metal takes up and gives up heat very easily - baking sheets are designed to be good heat conductors, for the most part. If there is a cookie over a portion of the metal, the heat gets radiated into the cookie, which takes up heat easier than the air does (air being a poor heat conductor, which is why insulated pans work). If there is no cookie over a portion of metal, the metal has a harder time getting rid of its heat so it gets especially warm - but the one direction in which it can lose its heat easily is to the side, right into an adjacent portion of the baking sheet which has a cookie on it. The portion of metal in the center of the baking sheet is in a sort of equilibrium with the cookies. Heat tends to radiate upwards into the cookie on each section, and the same is true of the sections of pan all around it because those sections have their own cookies on them. A little heat comes in through the pan metal from adjacent sections of the pan, but the section itself gives away some heat in the same direction. All the center sections behave the same. But the section at the very edge, near the large expanse of bare metal, radiates just as much heat up into its cookie, but receives more heat from the bare metal next to it than it sends in that direction - so that section of the pan gets more heat and that cookie gets baked a little more. Hence, overbaking at the edge.

To verify that, I tried an experiment. Using the William-Sonoma Commercial Baking Sheet, which has two edges that are angled upwards for about a half-inch to form a grip all along the edge, I baked two batches of cookies on the same pan. For the first batch, I placed the outermost rows of cookies along those edges right next to the angle bends, to minimize the amount of bare metal next to those cookies. For the second batch, I baked the same number of cookies but placed the outermost rows about 1.5 inches from the angle bend. Photos of the two batches are attached to the post. If it is a large bare expanse of metal next to the cookies that causes a problem, the cookies closest to the angle bends will be right next to the edge and not be next to a large bare expanse, so they should bake nicely. The ones I place a distance away from the angle bends will be next to large expanse of bare metal and will overbake.

The result - yes, the batch with cookies right up to the angle bend showed no overdone cookies, and the batch with cookies kept away from the angle bend showed an overbaked effect on the sides nearest the expanse of bare metal. The hypothesis becomes a theory! Which I will operate by from now on.

When placing cookie near an edge, they should be reasonably close to the edge. Cookies that spread shouldn't be so close to the edge that they drip to the bottom of the oven! But ideally there should be no more bare metal around cookies at the edge than there is around cookies in the center.

This takes us back to the "how high should an edge be" question. If there is any vertical edge, a spreading cookie can't spread over it. If it spreads right to it and it is held back by the edge, then the cookies will be misshapen - we don't want that! But here's the thing - any vertical edge can be looked at as an expanse of bare metal at the edge of the pan. To a point, the higher the edge is, the larger the expanse of bare metal. The higher the edge is, the closer to the vertical edge you want your cookie. But you can't stand a cookie on its side to be close to the very edge, which is actually the top of the rim, can you? So you put the cookie right next to the edge and hope the expanse of bare metal doesn't massacre your edge cookies. If all the edge cookies are brown, you might think, "Oh, gosh, the edge of the pan is interfering with air circulation!" No, the edge of the pan is absorbing too much heat and passing it on to the adjacent cookies. You might think, "Oh, the edge of the pan is too hot, I have to move the cookies farther from the edge!" But no, that really won't help. It may even make it worse because now there's an even larger expanse of bare metal next to the edge row of cookies. If there's a problem you can't overcome by putting the cookies closer to the edge, get a different baking sheet.

This effect of vertical edges, however, is greatly minimized by using heavy pans. Denser pans - pans made of thicker metal - have their heat content affected much less by the cookies on them. With the thicker metal, the heat on all portions of the pans is in better equilibrium with all other portions of the pan, whether there is a cookie on it or not. So if you have two pans with high edges, one heavy, one light, cookies placed at the edge of the heavier pan will show much less edge effect than the cookies at the edge of the lighter pan. If you make a point of placing cookies at edges as close as you can to the rising edge, then a heavy pan may bake without any edge effects at all, or the effects will be barely apparent. This is what I observed with my pans - the jelly roll pan is extremely heavy and has pretty high edges relatively speaking, and there is just a little bit of edge effect. The Wilton EverGlide pan has moderate edges and is pretty heavy, and shows a rather slight edge effect. My lightest pan has a fairly low rim, but the edge effect is quite pronounced. However, with that pan, because the rim is so low, if I place the cookies right up next to the rim the mass of bare metal can be made to be only the same as between cookies in the center of the pan, so I should be able to make that edge effect disappear. I didn't experiment with placing cookies right next to the edge of the light pan.

To demonstrate the effect of metal density on the effect of pan edges on cookies, I attached a third photo of three rows of three cookies each. The three cookies on the left were from a left-most row on their respective pans. The three cookies on the right were from a right-most row on their respective pans. The three cookies in the center vertical column were from the centers of the respective pans.

Observe that the midle row shows almost no edge effect. There is a glare in the center of the picture, but the center cookie has the same color as the cookies to its left and right. At the very left edge of the left cookie and the right edge of the right cookie in the center row it looks like there might be a slight shadow, but that is actually a slight bit of extra browning at the very edge of the pan. Those cookies are from the pan arranged as shown in the pan photo beneath the other at the top of the post.

The cookies in the top row of the nine-cookie rectangle are from the pan arranged as shown in the pan photo on the top. Observe these cookies show a much stronger edge effect on the left side of the left cookie and the right side of the right cookie. You can see that the so-called edge effect is increased when the cookies in the outer rows on the sheet are further from the edge, as I described a few paragraphs earlier.

The cookies in the bottom row are from one of the two Good Cook/Oneida slide-off pans. The left cookie is obviously baked much darker than the cookie from the center of the pan. The cookie on the right isn't too much darker than the center cookie, but you can see that the brown around the outside of the cookie is darker at the right edge. The appearance was like this for all the edge cookies on that pan. It's interesting that the edge effect spreads over the whole of the outer cookies rather than only showing up at the very edge of the cookies nearest the wide horizontal handles at the edge of the pan! These pans have much lighter density. That accentuates temperature differences and not only affects how much effect there is at the edge, but also how far towards the center of the pan the effect spreads. (The overall difference in color between the cookies in the bottom row and in the other rows is partly because the Good Cook/Oneida slide-off pans simply have poor heating qualities, but possibly also because the cookies on the bottom didn't spread as much, so there was more bare metal next to them and so baked differently. The cookies in all three rows were baked to equal doneness in calibration batches.)

Of the four points I mentioned in the first paragraph, I haven't said anything about the effect of closeness of cookies on even baking. I haven't investigated that at all so far. Possibly I will comment on it in the future. However, it makes sense that spacing is important. If large adjacent expanses of bare metal at the edge of a pan cause a cookie at the edge but "not close enough" to bake faster than other cookies, then obviously the amount of bare metal near any cookie affects how that cookie bakes. I said before that heat across a pan reaches an equilibrium together with the cookies on the pan. If every cookie is identical and every cookie is exactly the same distance from every other cookie, then every cookie will bake equally, except possibly for at the very edges. But if some cookies are farther apart from each other than others, then the pattern of the equilibrium has a funny shape. The cookies with more bare metal around them will bake a little more than the cookies close together. If you have two pans, one with cookies closer together and one with cookies farther apart, the one with cookies closer together isn't going to deliver quite as much heat to each cookie, so those cookies will take longer to bake.

Even beyond that, though, cookies bake from both the top and the bottom. Heat moves across the tops differently from the way it moves across the bottoms. We know that extra bare metal around a cookie causes it to draw additional heat from the larger space around it because the metal is such a good heat conductor. Will extra air space around a cookie cause it to draw the same degree of extra heat from the air into the top of the cookie? The air is not a good heat conductor, so it probably won't be affected to the same degree. In other words, the spacing of cookies on the baking sheet, close together versus farther apart, affects the balance between bottom heat and top heat going into the cookie. That means that it's more than just the baking time that's affected - one can adjust the baking time but still not come out with exactly the same result.

That last is just conjecture. I don't feel like spending time testing that. I'm just going to try to always space my cookies reasonably and try to make them the same size so that there is the same amount of bare metal around all the cookies on a baking sheet.