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Heat and Mass transfer mechanisms behind baking a cake
Temperature and moisture fields during and after baking [1]
‘Fingerprints’ of snow
Effects of temperature and moisture on snowflake shape [3]
‘Tis the Season for Science
By Gayathri Hariharan
Sugar, Spice and Everything nice…!
If you have ever binged on episodes of the Great British Bake Off (guilty pleasure of mine and I am sure I am not alone), you know baking isn’t a ‘piece of cake’. The process is akin to making a magic potion; fetching and whisking the right ingredients, artfully constructing each layer and most importantly baking at the right temperature. Leave it for too long ,you have char and open it too often you have a collapsed cake .As we ring in the season’s spirits and tuck away our diet plans, we thought it would be interesting to cut through the layers and discover the science behind a good bake . At the bottom of all that decadence is simultaneous heat and moisture transfer: convective, radiative and conductive type of heat transfer or their different combinations from the oven cavity towards the cake followed by an internal heat diffusion, and an internal moisture diffusion (either liquid or vapor), internal (evaporation through the pores of the cake) and surface evaporation and convective surface moisture removal mechanisms. Take a moment to digest all that (pun intended)!!
Heat and Mass transfer mechanisms behind baking a cake
Baking not only serves to heat the final product but is also key in inducing important biochemical reactions such as inactivation of certain enzymes and microorganisms and decrease in moisture content- both important for the shelf life of the baked goods. The baking process itself can be divided into two sequential steps- ‘heating up’ when the core temperature of the cake reaches a plateau and the ‘crust-crumb period’, when the texture gradient across the cake develops. An important parameter that determines the ‘sponginess’ of your cake is the moisture diffusion. Generally both heat and moisture distribution are modelled through the diffusion equations (time to dust out the Advanced heat transfer notes!!) and are coupled by their respective gradient terms. If you thought solving this was an easy task, think twice!! The thermo-physical properties of the cake such as heat capacity, bulk density, effective moisture diffusivity and thermal conductivity are temperature and moisture dependent, leading us to a set of non-linear partial differential equations with appropriate initial and final boundary conditions. A team of researchers [1] ‘hungry’ for understanding the heat and mass transfer simulated the process through 5 coupled differential equations ,assuming a viscoelastic model to predict the swelling. Their results provided the temperature and moisture fields during and at the end of the baking. Thanks to the evaporation-condensation phenomena, your cake has a crisp outer ‘crust’ and a warm, spongy inner ‘crumb’.
Temperature and moisture fields during and after baking [1]
Baking ovens are designed to optimise the heat transfer through the 3 modes: Radiation, conduction and convection. For baking, radiation is the predominant mode, contributing to about 50-80 % of the total heat transfer. Radiation sources are the electric resistances, vapor or hot gas tubes, gas-fired burner flames, while conduction is through hot surfaces directly in contact with the food like the racks. Convection ovens work by increasing the convective part of heat transfer through forced air circulation to accelerate the baking process and allow for more even cooking. In addition, they create a dry atmosphere unlike ovens where the moisture in the air can’t escape. Dry air helps create that crisp caramelised brown sugar coating over your roast, while the interior still remains succulent. If you made a lopsided muffin, do not be hard on yourself. Ask any baker and they will tell you not to bake sponge cakes , souffles or tarts in the convection ovens. Improper design can lead to poor airflow and inhomogenous product quality, with reports of partially cooked goods and flying cookies hitting the oven door.
We have all sat down in the Advanced Heat Transfer lectures dreading the impending exams and the gloomy grades that might arrive at our Christmas doorstep. On the brighter side, this Christmas, you have an intelligent conversation to strike at the dinner table when the cake emerges out of the oven!
A Winter Wonderland
Fascinated by the nature around him, Wilson “Snowflake” Bentley, a farmer from Vermont satiated his curiosity about snowflakes and pioneered the technique of photomicrography. In his words, “Under the microscope, I found that snowflakes were miracles of beauty; and it seemed a shame that this beauty should not be seen and appreciated by others. Every crystal was a masterpiece of design and no one design was ever repeated., When a snowflake melted, that design was forever lost. Just that much beauty was gone, without leaving any record behind.” Through his extensive work of capturing more than 5000 snowflakes, he observed that no two snowflakes are the same. The ‘fingerprint’ of snow remains a mysticism to more than half of the world who has not seen snow. So , what exactly are snowflakes, what makes each of them unique and if you do come across one, can you preserve it?
‘Fingerprints’ of snow
Snowflakes form when water vapor travels through the air and condenses (changes from a gas to a solid) on a particle. Dust particles play an important role in kick starting the process, because water can remain in a supercooled state even below freezing point. In the absence of a nuclei or seed crystal (dust in this case), water will remain a liquid till about -40 oC. Thanks to dust particles, snowflakes start to form at about -6 oC. As a droplet freezes, water vapour in the air condenses on the surface making the crystal lattice structure of the snowflake grow. The formation of the ice crystals are highly sensitive to temperature and moisture giving rise to the their numerous astounding shapes. Since the path taken by each crystal is ridden with chaotic whirls and eddies (turbulence, anyone? Watch out for the interview with Prof. Jerry this month!), the chance that each crystal has the same growth journey is very slim. The distinctive route taken by each snowflake gives it its unique shape- literally one in a million. Keep in mind that this is a very simplistic explanation of a complex phenomenon. There are still many secrets that need unravelling and with the help of molecular dynamics, surface physics, growth instabilities, pattern formation and statistical mechanics, we can get closer to deciphering nature’s artistic creation. And just in case you are lucky enough to see snowflakes, you can create your own snowflake fossils. All you need is cardboard, superglue, glass slides, a brush and most importantly snow and immense patience.
Effects of temperature and moisture on snowflake shape [3]
[1]Le Bideau, P., Cutté, R., Glouannec, P., & Le Page, J. F. (2016). Numerical Model for Predicting Heat and Mass Transfer Phenomena During Cake Baking. In Proceedings of the 2016 COMSOL Conference in Munich.
[2]Zhou, W., & Therdthai, N. (2008). Heat and mass transfer during baking of sweet goods. Food engineering aspects of baking sweet goods, 173-190.
[3] https://www.americanscientist.org/article/the-formation-of-snow-crystals