Troubleshooting a soufflé that collapses during baking

Question

I am trying to make a cheese soufflé to surprise my girlfriend on her birthday (24 Jan). After several cooking trials, I still face a key problem: my soufflé collapsed after baking for 25-30 minutes.

What did I do wrong? Here are the steps to the recipe;

• boil equipment
• grease butter to the renakin, place into the freezer
• separate egg whites (no yolk mixed)
• add a little salt to egg whites
• add a little sugar to egg whites
• using a hand mixer, whip the egg whites adding more sugar during whip
• melt 40g butter (100C)
• Whisk 40g all purpose flour into melted butter
• pour 250g milk slowly and whisk util it mixed
• wait until it cools down
• re-grease butter to the renakin, place into the freezer
• beat the egg yolks to a creamy consistency, then pour into the milk mixture
• add 1/3 of the mixture to the base, continue to add the whites folding very gently.
• Preheat oven to 200C (5-10 minutes)
• pour the mixture into the renakin
• add the cheese on the top
• bake at 170'C (with heat select="lower")

After about 25-30 minutes and a 2cm rise, the soufflé collapses prior to removing from oven.

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updated 19 Jan Evening :

I do

• buy better hand mixer
• use more flour (50g)
• move egg whites whipping process to last steps
• reduce whipping time to 5 minutes
• bake with heat select="lower" about 15min after that change to "upper and lower"
• sit in front of oven and see the soufflé every moment.

result:

• at 20mins: the soufflé raise about 3.5 to 4cm (from 5cm height renakin)
• around 25mins: it collapse about 0.5cm, so I immediately remove it from oven (T-T)
• after than it collapse very fast. (I guess it is common)

Thank you very much @jefromi and @KatieK for your comments and links. I plan to buy oven thermometer this morning because I have no it yet.

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updated 21 Jan Lunch :

Yesterday I found problem on the oven temperature. When I set temperature to 170 C, real temperature is 150C or below (because it keep going down). So today I try to keep in range of 160-200C and no collapses now.

Thanks you very much to everybody again for kindly help.

Answer 1

There are two likely causes of soufflé problems.

1) Egg foam. Be sure that you're whipping them to peaks (soft peaks, as I recall) when you first whip them. Then, when adding the 2 mixtures together in 3rds, be sure that you're properly folding in the whites; this is a very specific technique, and easy to over-do.

2) Oven heat. Use an oven thermometer to check your oven's temp - it's probably wrong. Put some extra mass in that oven (like a baking stone or brick) to reduce wild temperature swings during baking.

But, as in the linked questions in the comments, all soufflés fall to some degree.