- underbaking
-- change oven temperature - high + less time
-- lower by 25 degrees + few more minutes
- too much air in batter
-- meringue too stiff, mix until just stiff
-- batter not mixed enough
- resting time too long
- oven temperature too low
-- try base temperature 135 celcius or over 300F
You do not want to go lower than 300 degrees as that is when the hollow action occurs. This is because they are baking too slowly.
You can always give a higher oven temp as well however, you want to make sure your macaronage technique is perfected first as well as waiting for the macaron to firm up BEFORE putting into oven. These two steps are crucial to the French method way.
proper mixing is 90% of the struggle of macarons. you want to fold the meringue and almond flour/10x mix till it becomes shiny again, like your meringue should be. I fold 3 or 4 times after everything is combined and check and adjust the consistency. Some people say it should flow like lava...great. I like to mix till it ribbons off the spatula. not big fluffy globs dripping off, but a 'flat' batter when it comes off the spatula.
do not overmix the meringue. I shoot for a 'bec d'oiseau', or 'birds beak'
If you whip too much air into the meringue, you need to get it back out. That results in over mixing. Tapping the trays on the countertop helps to eliminate any extra air inside the shells before they dry. dried shells that are ready for the oven should not stick to your fingers, should not be tacky.
It is important to note that I am at high altitude as well. I bake around 350 to get good oven spring in a larger commercial oven. 5 minutes, rotate, 5 more and pull them. at home i will go down to 300, and bake 8, turn then bake an additional 3.
https://cheftalk.com/threads/hollow-macaron-shells-please-help.87318/
I'm not sure about that recipe, I have the book, but haven't tried it. But, most of the ones I've used have more sugar than almond meal. I know you are shooting for less sweet, but it could effect it. I use the French method because to me it's less sweet than the Italian. And, I feel the Italian makes a harder shell.
You could try double panning, starting the oven temp higher for the first 5 mins, then lower it. I used to do that in one oven I used. And we would prop the door open slightly too when we lowered the temp. This was all a big process the pastry chef I worked with then came up with so they would work. And for the most part it did.
This is a recipe I've used recently:
130 g almond flour
160 g confectioner's sugar
80 g granulated sugar
100 g egg whites, room temp (you can "age" them if you want, I usually have egg whites hanging around at work that are old, so I don't worry about it)
pinch of salt
Color
Grind almond flour and conf. sugar in a cuisinart for about 3 mins. Sift to remove any larger bits of almonds. If you sift out too much almond flour, grind again and sift.
Whip whites and salt until foamy, gradually add sugar and whip to stiff peaks. Add color if using.
Fold in almond mixture.
Pipe on silpat and let dry 10-20 mins. Double pan and bake at 350 for 5 mins. Prop oven door open slightly and bake until done.
The oven I use now at work doesn't give me any trouble when I make them. I bake everything at 300 because it runs really hot. But, I just throw a double pan in, and let them go until they are done.
It takes a lot of trial and error with these stupid things, along with a lot of wasted almond flour (which is the worst part!) Maybe some of what I've mentioned will help.
In my experience:
- For commercial convection oven, I get better result when I bake them around 285-300 degrees, definitely not under 285 degrees.
- Too much air will cause hollow shells. Making sure to deflate all the air in the batter when folding in the meringue.
- The thiner the mat, the higher the foot. This means parchment paper will give a higher foot, compare to silicone mat.
- For 1.5" macarons, I normally bake them at 300 degrees for 10-13 minutes. For bigger macarons, I stay with the same oven temperature but bake them longer. If they're 3", I'd say bake for 13-15 minutes.
Try adding more salt as this will mask some of the sweetness.
I also add a tiny amt of a vanilla/butter flavoring emulsion.
My macs taste sweet and buttery.
https://cheftalk.com/threads/french-macarons-driving-me-insane-please-help.79549/
I had this problem as well because every source I found on the internet said to bake at a lower temperature (300 degrees). I finally decided to divide a batch into 4 and then tested baking at different temperatures.
I found that higher temperatures, baking for a shorter time, finally got rid of the hollow. 10-12 minutes at 360 degrees was my magic number but it's best to experiment with your oven. My oven is electric with the heating element on the bottom.
I think most people say to bake at a lower temp to prevent browning, so that is something you have to watch out for if you up the temp. I put another rack two positions above the sheet and then put an empty sheet pan to try to delay the browning process. I really don't mind having them a little browned since the insides turn out well.
Hey thank you so much for experimenting and sharing your results. I was getting really frustrated with the hollow shells last night (and they'd stick to the parchment too!) and increased the temperature today, which fixed the hollow shell problem.. but yeah, now I have protruding feet too. The problem I had today though was that mine (strawberry shells) over browned too. I'll have to try your technique next time.The only recipe that has always worked out well for me is David Lebovitz's chocolate macaron recipe and he bakes his at 350 for 15-18 minutes.
the batter may be undermixed which is causing them to puff up but then the innards will retract a little, so mix a little more but its a fine line. Try overmixing some just to see the difference.
severe overmixing will result in a wrinkly dense cookie.
the higher temps may help as it will make them "set" harder quickly so they don't get time to retract, if you notice the feet etc always retract a little.
also leave the whites out a day or so to lose some moisture which will also control the puffyness as well.
https://www.chowhound.com/post/fix-macarons-767665?page=2
Why is my macaron hollow?
Often the macarons have a mind of their own that is next to impossible to read. There could be different reasons for why the macarons are hollow, but one of the easiest or simplest causes to eliminate hollow macarons is to bake them through properly. Just like the meringue will have a hollow inside if they’re removed from the oven too quickly (the unbaked part of the meringue just collapses once taken out of the oven), so will the macarons.
The difference could be just 1 minute, so you really need to know your oven and your macaron recipe. It is better to overbake your macarons, which will give you a crispy throughout macaron shell. But once paired with the filling and allowed to mature, the macarons will go back to being soft and moist on the inside. You can even quickly dip or brush the bottom of each macaron shell into some simple syrup (1:1 ratio of water to sugar) before piping the filling to help with the maturation. In the end, no one will know that the macarons were overbaked.
https://letthebakingbeginblog.com/2018/03/basic-macarons-italian-meringue-method/
I figured I would make a couple of small changes to see if they make a difference. Firstly, I tried resting the macarons before I baked them. Though it is something many French meringue macaron recipes call for, it’s not something I have ever had the need to do. My macs have never suffered. I figured that if my batter was correct, they wouldn’t be going anywhere anyway (i.e. spreading). I also cut down the time I whip the egg whites, since it appeared the problem was too much air, I wouldn’t whip as much into them.
Yields makes 40 macarons
Ingredients
115g ground almonds (store bought or home ground in a spice/coffee grinder and sifted before you weigh)
230g icing sugar
15g cocoa powder for chocolate macarons or 15g freeze-dried raspberries, ground in a spice grinder
144g egg whites, separated, covered in plastic wrap and left at room temperature for a few hours. You can separate them up to 3 days before you use them - just keep them covered in the fridge and bring them to room temperature for a few hours before you use them
72g granulated or caster sugar
(food colouring powder – about 2 teaspoons for this amount of macarons)
Combine almond flour, powdered sugar and either the cocoa powder or freeze dried fruit powder in a food processor, pulsing about 10 times for a few seconds, until all ingredients thoroughly incorporated.
Sift dry ingredients twice using a fine sieve and pressing the mixture through with your hands and set aside.
Using a stand mixer, beat the egg whites and sugar at a low speed (Kitchen Aid speed four) for 2 minutes, medium speed (Kitchen Aid six) for 2 minutes and a high speed (Kitchen Aid eight) for 2 minutes. The egg whites will be very stiff at this point.
Add the colouring powder and mix for one minute at the highest speed (Kitchen Aid ten).
Add the dry ingredients to the egg whites.
Fold the mixture, pressing it against the sides of the bowl to deflate the mixture. Fold about 40 times (counting single strokes), stopping every couple of strokes after 25 to check the consistency. It should be lava-like, flowing in ribbons off the spatula.
Transfer half the mixture to the piping bag, sealing the open end with a twist and holding firmly with the hand that will not be actively piping.
Pipe four tiny dots of mixture under the corners of the parchment paper to make sure it stays put.
Pipe your macarons, holding the piping tip at an angle to the baking sheet, about 3cm in diameter (they will spread during cooking), and quickly removing the tip when you have finished piping, making a shape like a comma.
Rap the tray 3-4 times on a hard surface. You'll see air bubbles coming to the surface of the unbaked shells.
Fill the bag with the rest of the mixture and pipe and rap the second tray.
REST the trays of macaron shells for 30 minutes before baking. At this point you should heat the oven to 300˚F.
Place one tray of macarons on an empty baking tray and bake on the middle rack of the oven for 16 minutes at 300˚F, turning the tray from back to front halfway through.
Remove from oven and let the tray sit for a few minutes.
Remove the parchment from the tray and allow to sit on a cool surface for at least 30 minutes, then remove macaron shells to a cooling rack.
Store in an airtight container overnight.
https://www.eatlivetravelwrite.com/2012/03/why-do-my-macarons-have-hollow-shells-a-work-in-progress/
https://www.eatlivetravelwrite.com/category/macarons/
Underbaking will result in dramatically hollow macarons. The meringue inside an underbaked macaron hasn’t fully set, so when it’s removed from the oven prematurely, the meringue collapses as it cools, leaving a hollow shell behind.
http://bravetart.com/blog/TheTenCommandments
Underbaked macarons have a soft interior that doesn’t have enough structure to support itself as it cools, so make sure the macarons have baked through and will cleanly peel away from the parchment paper. I don’t like baking macarons on silicon because even underbaked macarons will slip off easily, which makes judging doneness more tricky and hollows more common
Try a splash of oil. Macarons are fairly lean by nature and sometimes a little extra fat is all they need. Any odorless, flavorless oil will work. Add in 4 teaspoons of oil along with the dry ingredients before grinding and/or sifting. Don’t worry about the slight dampness, just sift as usual; use a spatula or whisk to make sifting easier if you need to. The oil seems to minimize hollows in a variety of circumstances, but most especially when using less-than-fresh nuts (or nuts without an expiration date, like some purchased from a bulk bin).
http://bravetart.com/blog/HollowPursuits
The feet, or the spongy, airy ruffle at the bottom of the cookie, are what give it away. Here I can give you some examples of what gives away the hollow.
This macaron above is hollow. As you can see, there are no air spaces that let the cookie rise up and become fluffy. All of the fluffiness has sank to the bottom while baking, and that is why the foot is straight with no holes. So when you bite into it, the insides are chewy since there is no fluffiness.
Tall feet means that most of the interior of the cookie has exploded out of the shell, causing the insides to be hollow. Also, there is no air space between the shell and the foot, so that means it will be hollow, Lopsided macarons are definitely a sign of hollowness.
So, what makes a good macaron then?
Short, ruffly feet indicate that you have made a fluffy macaron. The insides have not spewed outside the shell nor have sank to the bottom.
Air space between the foot and the shell also indicate that you've made a good macaron. You can look inside this air space and see that the meringue has turned fluffy.
I noticed that most macarons that are hollow are usually undercooked. This can easily be fixed by cooking it a bit more, and making sure you deflate the air in the batter correctly.
This makes a big difference in whether your macarons turn out hollow or not. Air in the batter contributes to hollows. Make sure you get every single area of the batter while scratching the bottom.
To get rid of hollows aside from the batter making, you need to make sure your oven is the right temperature. Raising up the temperature a little higher than 300 (my recipe) if your oven thermometer says it is not actually 300 will help. I bake on the middle-high rack, since my heat is mostly from the top. I use a convection oven as well. You also need to bake them a little longer, like 13 minutes. This will prevent hollows and ensure a stable inside. However, the trade off is that you will have crispier macarons than normal. If you have a high-moisture filling, it will disappear with a day or two of maturation. If you have a low-moisture filling like buttercream, you can additionally brush the bottoms of the macarons with milk or whatever will go with your flavor, like jam or coffee. This will help maturation proceed faster, and your once-cripsy macarons will turn into fluffy, soft, chewy confections.
http://loveandmacarons.blogspot.gr/2013/03/how-to-get-rid-of-hollow-macaronsfor.html
Pressing into the back of the shells in order to push the body back up to the top – which some bakers do to hide hollows – may be a temporary solution.
...although hollow macarons may taste the same as non-hollow ones, severely hollow macarons are more fragile and lack the distinctly chewy texture of a full and fluffy macaron. Those are two of the reasons why you might want to work on your hollow macarons.
Meringue Stiffness
Over-beating the meringue can cause your shells to deflate while baking, resulting in a hollow shell. A good stiff meringue is required for my recipe, however, over-beating it can be the culprit to your hollow macaron problem. This is especially true if your macaron looks almost perfect in every way except for a little gap inside the shell.
If your macarons are looking fabulous in every way already(dainty feet, smooth tops), then this fix is for you. Beat the meringue up to stiff peaks and stop immediately once that is reached.
How stiff is stiff? Beat the meringue until a small peak points upwards when you pull the whisk out of the meringue. The peak can be small! If you beat until a big clump of egg whites can point upwards, you are probably over-beating. This is worth mentioning as some of you may have been trying to test the peaks with a lot more meringue than is needed.
Another way to scale back on how much you have been beating your meringue is to test the bowl by tipping it upside down. Once the egg whites can cling securely onto the bowl without slipping and you have a SMALL stiff peak, stop beating the meringue.
Macaronage & Batter Consistency
The batter should be properly deflated. If you tend to over-beat the meringue, this is important for you. In the beginning of our macaron baking journey, we were told to be careful with the batter. “It’s better to under-fold than over fold,”…etc. But that’s where a lot of hollow macaron problems begin – from an under deflated batter. When you’re folding, you don’t need to baby it as much as you think.
On the flip side, over-folding can also cause hollows and widely spread feet. When you are folding, constantly test the batter with the Figure-8 test in the video. The batter should flow steadily without breaking but it should not be too runny either. Aim for a honey-like consistency.
Rap Tray
Rapping the tray after piping will further eliminate the small gaps in your shells. Make sure you are tapping it hard enough against a hard surface. In my macaron class, the instructor literally dropped the baking trays from waist level down onto the floor. It’s quite a violent action and sometimes the shells do change in shape after tapping, I would recommend piping on Silpat mats which are great for retaining the macaron’s shape. Pop any remaining bubbles with a toothpick.
Bake Time & Temperature
Meringues and by extension, macarons, love heat! Give it as much oven love as you can and you’ll see less hollow macarons in your kitchen. You can do this by either using a:
a.) higher temperature OR
b.) lower temperature with a longer bake time
325F for 12-14 minutes is a great place to start. (I know of a baker who likes to go even higher at 350F for 10 minutes.) Most readers have found 325F or higher can cause their shells to become browned so another temperature/time combo I can recommend is 275F for 18 minutes. You can play with the time/temperature combinations. For every 25F, compensate with a 2-3 minutes change in the inverse direction.
Some Recommended Baking Temperature and Times:
350F for 10 minutes – suitable for spacious large ovens
325F for 12-14 minutes – standard starting point for my recipe
300F for 16-18 minutes
275F for 18-20 minutes – suitable for smaller ovens with small compartments
https://www.indulgewithmimi.com/what-causes-hollow-macaron-shells/
If the macaron shells are hollow, the egg whites were overbeaten. Beat them at low, then medium speed, and don’t overdo it.
http://www.chocoparis.com/macaron-troubleshooting-tips/
If the oven is too hot, you might set the outside crust too fast. So the crust will form but you'll have to remove the shells from the oven before the inside is fully set. Hence a too-moist, too-sticky inside.
Maybe lower the oven temp by 10 - 20 C and add a few minutes of baking time.
If you over-beat the dough, it will also collapse. I use a French meringue and mix the powders just enough to incorporate all ingredients and for the dough the become shiny.
Over-beating leads to "flat" macarons that are denser, and moister. Not sure it would lead to a "hollow part"
https://www.quora.com/Why-is-the-shell-of-my-macaron-hollow-in-upper-part-and-chewy-sticky-in-lower-part
-- change oven temperature - high + less time
-- lower by 25 degrees + few more minutes
- too much air in batter
-- meringue too stiff, mix until just stiff
-- batter not mixed enough
- resting time too long
- oven temperature too low
-- try base temperature 135 celcius or over 300F
You do not want to go lower than 300 degrees as that is when the hollow action occurs. This is because they are baking too slowly.
You can always give a higher oven temp as well however, you want to make sure your macaronage technique is perfected first as well as waiting for the macaron to firm up BEFORE putting into oven. These two steps are crucial to the French method way.
proper mixing is 90% of the struggle of macarons. you want to fold the meringue and almond flour/10x mix till it becomes shiny again, like your meringue should be. I fold 3 or 4 times after everything is combined and check and adjust the consistency. Some people say it should flow like lava...great. I like to mix till it ribbons off the spatula. not big fluffy globs dripping off, but a 'flat' batter when it comes off the spatula.
do not overmix the meringue. I shoot for a 'bec d'oiseau', or 'birds beak'
If you whip too much air into the meringue, you need to get it back out. That results in over mixing. Tapping the trays on the countertop helps to eliminate any extra air inside the shells before they dry. dried shells that are ready for the oven should not stick to your fingers, should not be tacky.
It is important to note that I am at high altitude as well. I bake around 350 to get good oven spring in a larger commercial oven. 5 minutes, rotate, 5 more and pull them. at home i will go down to 300, and bake 8, turn then bake an additional 3.
https://cheftalk.com/threads/hollow-macaron-shells-please-help.87318/
I'm not sure about that recipe, I have the book, but haven't tried it. But, most of the ones I've used have more sugar than almond meal. I know you are shooting for less sweet, but it could effect it. I use the French method because to me it's less sweet than the Italian. And, I feel the Italian makes a harder shell.
You could try double panning, starting the oven temp higher for the first 5 mins, then lower it. I used to do that in one oven I used. And we would prop the door open slightly too when we lowered the temp. This was all a big process the pastry chef I worked with then came up with so they would work. And for the most part it did.
This is a recipe I've used recently:
130 g almond flour
160 g confectioner's sugar
80 g granulated sugar
100 g egg whites, room temp (you can "age" them if you want, I usually have egg whites hanging around at work that are old, so I don't worry about it)
pinch of salt
Color
Grind almond flour and conf. sugar in a cuisinart for about 3 mins. Sift to remove any larger bits of almonds. If you sift out too much almond flour, grind again and sift.
Whip whites and salt until foamy, gradually add sugar and whip to stiff peaks. Add color if using.
Fold in almond mixture.
Pipe on silpat and let dry 10-20 mins. Double pan and bake at 350 for 5 mins. Prop oven door open slightly and bake until done.
The oven I use now at work doesn't give me any trouble when I make them. I bake everything at 300 because it runs really hot. But, I just throw a double pan in, and let them go until they are done.
It takes a lot of trial and error with these stupid things, along with a lot of wasted almond flour (which is the worst part!) Maybe some of what I've mentioned will help.
In my experience:
- For commercial convection oven, I get better result when I bake them around 285-300 degrees, definitely not under 285 degrees.
- Too much air will cause hollow shells. Making sure to deflate all the air in the batter when folding in the meringue.
- The thiner the mat, the higher the foot. This means parchment paper will give a higher foot, compare to silicone mat.
- For 1.5" macarons, I normally bake them at 300 degrees for 10-13 minutes. For bigger macarons, I stay with the same oven temperature but bake them longer. If they're 3", I'd say bake for 13-15 minutes.
Try adding more salt as this will mask some of the sweetness.
I also add a tiny amt of a vanilla/butter flavoring emulsion.
My macs taste sweet and buttery.
https://cheftalk.com/threads/french-macarons-driving-me-insane-please-help.79549/
I had this problem as well because every source I found on the internet said to bake at a lower temperature (300 degrees). I finally decided to divide a batch into 4 and then tested baking at different temperatures.
I found that higher temperatures, baking for a shorter time, finally got rid of the hollow. 10-12 minutes at 360 degrees was my magic number but it's best to experiment with your oven. My oven is electric with the heating element on the bottom.
I think most people say to bake at a lower temp to prevent browning, so that is something you have to watch out for if you up the temp. I put another rack two positions above the sheet and then put an empty sheet pan to try to delay the browning process. I really don't mind having them a little browned since the insides turn out well.
Hey thank you so much for experimenting and sharing your results. I was getting really frustrated with the hollow shells last night (and they'd stick to the parchment too!) and increased the temperature today, which fixed the hollow shell problem.. but yeah, now I have protruding feet too. The problem I had today though was that mine (strawberry shells) over browned too. I'll have to try your technique next time.The only recipe that has always worked out well for me is David Lebovitz's chocolate macaron recipe and he bakes his at 350 for 15-18 minutes.
the batter may be undermixed which is causing them to puff up but then the innards will retract a little, so mix a little more but its a fine line. Try overmixing some just to see the difference.
severe overmixing will result in a wrinkly dense cookie.
the higher temps may help as it will make them "set" harder quickly so they don't get time to retract, if you notice the feet etc always retract a little.
also leave the whites out a day or so to lose some moisture which will also control the puffyness as well.
https://www.chowhound.com/post/fix-macarons-767665?page=2
Why is my macaron hollow?
Often the macarons have a mind of their own that is next to impossible to read. There could be different reasons for why the macarons are hollow, but one of the easiest or simplest causes to eliminate hollow macarons is to bake them through properly. Just like the meringue will have a hollow inside if they’re removed from the oven too quickly (the unbaked part of the meringue just collapses once taken out of the oven), so will the macarons.
The difference could be just 1 minute, so you really need to know your oven and your macaron recipe. It is better to overbake your macarons, which will give you a crispy throughout macaron shell. But once paired with the filling and allowed to mature, the macarons will go back to being soft and moist on the inside. You can even quickly dip or brush the bottom of each macaron shell into some simple syrup (1:1 ratio of water to sugar) before piping the filling to help with the maturation. In the end, no one will know that the macarons were overbaked.
https://letthebakingbeginblog.com/2018/03/basic-macarons-italian-meringue-method/
I figured I would make a couple of small changes to see if they make a difference. Firstly, I tried resting the macarons before I baked them. Though it is something many French meringue macaron recipes call for, it’s not something I have ever had the need to do. My macs have never suffered. I figured that if my batter was correct, they wouldn’t be going anywhere anyway (i.e. spreading). I also cut down the time I whip the egg whites, since it appeared the problem was too much air, I wouldn’t whip as much into them.
Yields makes 40 macarons
Ingredients
115g ground almonds (store bought or home ground in a spice/coffee grinder and sifted before you weigh)
230g icing sugar
15g cocoa powder for chocolate macarons or 15g freeze-dried raspberries, ground in a spice grinder
144g egg whites, separated, covered in plastic wrap and left at room temperature for a few hours. You can separate them up to 3 days before you use them - just keep them covered in the fridge and bring them to room temperature for a few hours before you use them
72g granulated or caster sugar
(food colouring powder – about 2 teaspoons for this amount of macarons)
Combine almond flour, powdered sugar and either the cocoa powder or freeze dried fruit powder in a food processor, pulsing about 10 times for a few seconds, until all ingredients thoroughly incorporated.
Sift dry ingredients twice using a fine sieve and pressing the mixture through with your hands and set aside.
Using a stand mixer, beat the egg whites and sugar at a low speed (Kitchen Aid speed four) for 2 minutes, medium speed (Kitchen Aid six) for 2 minutes and a high speed (Kitchen Aid eight) for 2 minutes. The egg whites will be very stiff at this point.
Add the colouring powder and mix for one minute at the highest speed (Kitchen Aid ten).
Add the dry ingredients to the egg whites.
Fold the mixture, pressing it against the sides of the bowl to deflate the mixture. Fold about 40 times (counting single strokes), stopping every couple of strokes after 25 to check the consistency. It should be lava-like, flowing in ribbons off the spatula.
Transfer half the mixture to the piping bag, sealing the open end with a twist and holding firmly with the hand that will not be actively piping.
Pipe four tiny dots of mixture under the corners of the parchment paper to make sure it stays put.
Pipe your macarons, holding the piping tip at an angle to the baking sheet, about 3cm in diameter (they will spread during cooking), and quickly removing the tip when you have finished piping, making a shape like a comma.
Rap the tray 3-4 times on a hard surface. You'll see air bubbles coming to the surface of the unbaked shells.
Fill the bag with the rest of the mixture and pipe and rap the second tray.
REST the trays of macaron shells for 30 minutes before baking. At this point you should heat the oven to 300˚F.
Place one tray of macarons on an empty baking tray and bake on the middle rack of the oven for 16 minutes at 300˚F, turning the tray from back to front halfway through.
Remove from oven and let the tray sit for a few minutes.
Remove the parchment from the tray and allow to sit on a cool surface for at least 30 minutes, then remove macaron shells to a cooling rack.
Store in an airtight container overnight.
https://www.eatlivetravelwrite.com/2012/03/why-do-my-macarons-have-hollow-shells-a-work-in-progress/
https://www.eatlivetravelwrite.com/category/macarons/
Underbaking will result in dramatically hollow macarons. The meringue inside an underbaked macaron hasn’t fully set, so when it’s removed from the oven prematurely, the meringue collapses as it cools, leaving a hollow shell behind.
http://bravetart.com/blog/TheTenCommandments
Underbaked macarons have a soft interior that doesn’t have enough structure to support itself as it cools, so make sure the macarons have baked through and will cleanly peel away from the parchment paper. I don’t like baking macarons on silicon because even underbaked macarons will slip off easily, which makes judging doneness more tricky and hollows more common
Try a splash of oil. Macarons are fairly lean by nature and sometimes a little extra fat is all they need. Any odorless, flavorless oil will work. Add in 4 teaspoons of oil along with the dry ingredients before grinding and/or sifting. Don’t worry about the slight dampness, just sift as usual; use a spatula or whisk to make sifting easier if you need to. The oil seems to minimize hollows in a variety of circumstances, but most especially when using less-than-fresh nuts (or nuts without an expiration date, like some purchased from a bulk bin).
http://bravetart.com/blog/HollowPursuits
The feet, or the spongy, airy ruffle at the bottom of the cookie, are what give it away. Here I can give you some examples of what gives away the hollow.
This macaron above is hollow. As you can see, there are no air spaces that let the cookie rise up and become fluffy. All of the fluffiness has sank to the bottom while baking, and that is why the foot is straight with no holes. So when you bite into it, the insides are chewy since there is no fluffiness.
Tall feet means that most of the interior of the cookie has exploded out of the shell, causing the insides to be hollow. Also, there is no air space between the shell and the foot, so that means it will be hollow, Lopsided macarons are definitely a sign of hollowness.
So, what makes a good macaron then?
Short, ruffly feet indicate that you have made a fluffy macaron. The insides have not spewed outside the shell nor have sank to the bottom.
Air space between the foot and the shell also indicate that you've made a good macaron. You can look inside this air space and see that the meringue has turned fluffy.
I noticed that most macarons that are hollow are usually undercooked. This can easily be fixed by cooking it a bit more, and making sure you deflate the air in the batter correctly.
This makes a big difference in whether your macarons turn out hollow or not. Air in the batter contributes to hollows. Make sure you get every single area of the batter while scratching the bottom.
To get rid of hollows aside from the batter making, you need to make sure your oven is the right temperature. Raising up the temperature a little higher than 300 (my recipe) if your oven thermometer says it is not actually 300 will help. I bake on the middle-high rack, since my heat is mostly from the top. I use a convection oven as well. You also need to bake them a little longer, like 13 minutes. This will prevent hollows and ensure a stable inside. However, the trade off is that you will have crispier macarons than normal. If you have a high-moisture filling, it will disappear with a day or two of maturation. If you have a low-moisture filling like buttercream, you can additionally brush the bottoms of the macarons with milk or whatever will go with your flavor, like jam or coffee. This will help maturation proceed faster, and your once-cripsy macarons will turn into fluffy, soft, chewy confections.
http://loveandmacarons.blogspot.gr/2013/03/how-to-get-rid-of-hollow-macaronsfor.html
Pressing into the back of the shells in order to push the body back up to the top – which some bakers do to hide hollows – may be a temporary solution.
...although hollow macarons may taste the same as non-hollow ones, severely hollow macarons are more fragile and lack the distinctly chewy texture of a full and fluffy macaron. Those are two of the reasons why you might want to work on your hollow macarons.
Meringue Stiffness
Over-beating the meringue can cause your shells to deflate while baking, resulting in a hollow shell. A good stiff meringue is required for my recipe, however, over-beating it can be the culprit to your hollow macaron problem. This is especially true if your macaron looks almost perfect in every way except for a little gap inside the shell.
If your macarons are looking fabulous in every way already(dainty feet, smooth tops), then this fix is for you. Beat the meringue up to stiff peaks and stop immediately once that is reached.
How stiff is stiff? Beat the meringue until a small peak points upwards when you pull the whisk out of the meringue. The peak can be small! If you beat until a big clump of egg whites can point upwards, you are probably over-beating. This is worth mentioning as some of you may have been trying to test the peaks with a lot more meringue than is needed.
Another way to scale back on how much you have been beating your meringue is to test the bowl by tipping it upside down. Once the egg whites can cling securely onto the bowl without slipping and you have a SMALL stiff peak, stop beating the meringue.
Macaronage & Batter Consistency
The batter should be properly deflated. If you tend to over-beat the meringue, this is important for you. In the beginning of our macaron baking journey, we were told to be careful with the batter. “It’s better to under-fold than over fold,”…etc. But that’s where a lot of hollow macaron problems begin – from an under deflated batter. When you’re folding, you don’t need to baby it as much as you think.
On the flip side, over-folding can also cause hollows and widely spread feet. When you are folding, constantly test the batter with the Figure-8 test in the video. The batter should flow steadily without breaking but it should not be too runny either. Aim for a honey-like consistency.
Rap Tray
Rapping the tray after piping will further eliminate the small gaps in your shells. Make sure you are tapping it hard enough against a hard surface. In my macaron class, the instructor literally dropped the baking trays from waist level down onto the floor. It’s quite a violent action and sometimes the shells do change in shape after tapping, I would recommend piping on Silpat mats which are great for retaining the macaron’s shape. Pop any remaining bubbles with a toothpick.
Bake Time & Temperature
Meringues and by extension, macarons, love heat! Give it as much oven love as you can and you’ll see less hollow macarons in your kitchen. You can do this by either using a:
a.) higher temperature OR
b.) lower temperature with a longer bake time
325F for 12-14 minutes is a great place to start. (I know of a baker who likes to go even higher at 350F for 10 minutes.) Most readers have found 325F or higher can cause their shells to become browned so another temperature/time combo I can recommend is 275F for 18 minutes. You can play with the time/temperature combinations. For every 25F, compensate with a 2-3 minutes change in the inverse direction.
Some Recommended Baking Temperature and Times:
350F for 10 minutes – suitable for spacious large ovens
325F for 12-14 minutes – standard starting point for my recipe
300F for 16-18 minutes
275F for 18-20 minutes – suitable for smaller ovens with small compartments
https://www.indulgewithmimi.com/what-causes-hollow-macaron-shells/
If the macaron shells are hollow, the egg whites were overbeaten. Beat them at low, then medium speed, and don’t overdo it.
http://www.chocoparis.com/macaron-troubleshooting-tips/
If the oven is too hot, you might set the outside crust too fast. So the crust will form but you'll have to remove the shells from the oven before the inside is fully set. Hence a too-moist, too-sticky inside.
Maybe lower the oven temp by 10 - 20 C and add a few minutes of baking time.
If you over-beat the dough, it will also collapse. I use a French meringue and mix the powders just enough to incorporate all ingredients and for the dough the become shiny.
Over-beating leads to "flat" macarons that are denser, and moister. Not sure it would lead to a "hollow part"
https://www.quora.com/Why-is-the-shell-of-my-macaron-hollow-in-upper-part-and-chewy-sticky-in-lower-part