How does the amount of sugar added to egg white affect the height of Soufflé? (Kristy)

Soufflé is a lightly baked cake with a combination of egg yolks and a soft peak meringue, made out of egg white and sugar (So Soufflé).  The first recorded recipe of soufflé appeared in a French chef's cookbook -- Vincent La Chapelle’s Le Cuisinier Moderne in 1742 (Mobile Cuisine). It is one of the representational dishes in French cuisine and it can be both a savory main dish or a dessert (Aftabchloe). In French, the word soufflé is the past participle of the French word soufflér, which means to ‘blow up’ or 'puff up' (Mobile Cuisine). 

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Recipe of Vanilla Souffle (For four 3-inch ramekins)

Required time： 40 minutes　　　　　

Ingredients:

1 tsp butter (for brushing ramekins)

2 tbsp caster sugar (for coating ramekins)

10g bread flour

80g milk

1 tsp vanilla extract

1 egg yolk

1 egg white

20g caster sugar

Steps:

1. Preheat the oven at 180‎°C.
2. With a pastry brush, grease the ramekins with softened butter in upward strokes (this will help the soufflé rise evenly). 
3. Coat the ramekins in caster sugar and keep the prepared ramekins in the fridge to set (this will help the butter to solidify) until ready to use.
4. Whisk egg yolk with caster sugar until even.
5. Add vanilla extract into the milk. Heat the milk in a small pan over low heat to a simmer (just when it starts bubbling around the edges). Remove it from heat. 
6. Add flour and stir until thickened. Let cool and add egg yolk mixture. Mix well.
7. Whisk the egg white until foamy. Add 20g caster sugar little by little. Whisk until the egg white becomes stiff. Scrape out 1/3 egg white with a plastic spatula and combine with the egg yolk batter.
8. Pour egg yolk batter in 2 or 3 smaller portions into whisked egg white. Mix well swiftly with a spatula.
9. Pour into cups until each cup is 90% full.
10. Bake in the oven in a tray with water at 180°C for 25 minutes.
11. Remove from the oven and serve immediately.

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The science behind sugar added into egg white:

When making soufflés, sugar is added into the egg white for whisking. Let’s first talk about why we need to whisk the egg white for making soufflé. Egg white are mostly moisture and proteins (about 10%) without any fat (Food Crumbles). Beating allows air to blend into egg white. During the whisking process, the protein in egg white — conalbumin unwinds and denatures as they knock against the air bubbles (Bramen), thus forming loose protein chains (Corriher; Food Crumbles). The unwound protein then link loosely together around the air bubbles (Corriher) and form a layer around the bubbles of air (Palca), resulting in a foamy texture. 

For sweet soufflés, caster sugar is added slowly into the egg white (usually for three to four times) not only to give soufflé a sweet taste but also to aid the white in forming tight protein bonds (SBS). When egg white are whipped together with sugar, it is called meringue (Food Crumbles). This process makes sure the finished soufflé rise evenly in tall and firm cylinders (Hesser). To explain why, sugar help stabilize the structure of egg white as it dissolves the protein on the surface of the air bubbles  (Corriher; Food Crumbles) This sugary syrup prevents the protein from drying out and tightening up too quickly (Corriher). Sugar also increases the viscosity of the liquid egg white that makes it more difficult for the air bubbles to merge together and escape so the air bubbles in egg white will not go away so quickly and the meringue stays foamy for a longer time (Food Crumbles).

When beating sugar into the egg white, sugar draws the water out of the white at the same time. When there is more sugar inside the white, the more water will be drawn out and evaporated, resulting in a drier and stiffer egg white peaks (Corriher). If there is not enough sugar, the soufflé may not hold its shape and may collapse quickly.

Some may ask why caster sugar, instead of granulated sugar is added into egg white. This is because caster sugar is smaller sugar crystals, meaning a smaller particle size that can be dissolved more easily during whisking when compared to the coarse, granulated sugar (SBS; Food Crumbles).

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References:

-       Aftabchloe. “What is Soufflé?: It’s Characteristics and Brief History’ https://aftabchloe.wordpress.com/2013/12/05/what-is-souffle-its-characteristics-and-brief-history/
-       Bramen, Lisa. “Souffles: The Original Molecular Gastronomy?” By Smithsonian Magazine https://www.smithsonianmag.com/arts-culture/souffles-the-original-molecular-gastronomy-104704476/
-       Corriher, Shirley. “How Fragile Egg-White Foams are Transformed into Firm, Airy Meringues’ by Fine Cooking https://www.finecooking.com/article/how-fragile-egg-white-foams-are-transformed-into-firm-airy-meringues
-       Food Crumbles. “The science of meringue: egg whites and sugar” https://foodcrumbles.com/a-blog-on-meringue-a-literature-study/
-       Hesser, Amanda. “The Modern Souffle: Bastion of Strength” by The New York Times https://www.nytimes.com/2000/03/08/dining/the-modern-souffle-bastion-of-strength.html
-       Kae. Jolly Kitchen Series: Handmade Cake. Next Publications Ltd.
-       Manning, Anneka. “How to: Make Meringue” by SBS Australia https://www.sbs.com.au/food/explainer/make-meringue
-       Mobile Cuisine. “Soufflé Fun Facts” https://mobile-cuisine.com/did-you-know/souffle-fun-facts/
-       Palca, Joe. "The Science of the Perfect Souffle” by National Public Radio  https://www.npr.org/templates/story/story.php?storyId=19004631
-       So Soufflé. ’About Us” http://www.sosouffle.com/en/
-       The Passionate Sisters "Vanilla Souffle Recipe" http://thepassionatesisters.blogspot.com/2016/09/vanilla-souffle-recipe.html