Biochemistry of a cake

Today my wife and me decided to prepare "Banana Walnut Cake". It was mainly to salvage two over ripened bananas. We were very much disturbed to read that 25,000 people die every day of hunger or hunger related causes (read it as one person in every three and a half seconds, ref. http://www.poverty.com/). As our humble contribution to curtail the poverty we have decided to save as much food as possible, hoping that a negative feed back loop may eventually set in and decrease the overall food consumption/wastage in long run.

Coming to the Banana Walnut Cake; we got the taste of it from Sudha and Binoj and recipe from "my pleasure my treasure" ( a wonderful cooking blog frequently visited by my wife). I did the cooking under the supervision of my wife. The details can be found the above mentioned blog. Here I am going to discuss the biochemical principle underlying cake making.

Someone has said that biochemistry is nothing but glorified cooking !! (Or vice versa depending on your inclination). I consider myself as a student of biochemistry (trying get a taste of molecular biology). One basic requirement for a biochemist is the inquisitiveness to understand the role of various components in a chemical reaction. Otherwise the interpretation or troubleshooting of the experiment may not be possible.

Alright. The major components of the reaction mix are, all purpose flour, Egg, baking powder/baking soda, butter, and butter milk. The function of these components are,

1. All purpose flour: It is a blend of high and low gluten wheat (or wheat flour with medium gluten level). Gluten is a mixture of water insoluble proteins gliadin and glutenin.

- Gliadin: is a glycoprotein. Different isoforms are found in nature including alpha/beta, gamma and omega. There are a number of allergic conditions associated with it, Eg: Coeliac disease (Gluten sensitive enteropathy (GSE) or gluten allery), baker's asthma etc.

-Glutenin: Composed of 20% high molecular weight subunits (low in sulfur) and 80% low molecular weight subunit (high in sulfur). During the kneading process it can form inter and intramolecular disufide bonds resulting in a three dimensional network. This reaction is responsible for the firmness of the dough. The carbon dioxide formed in one of following described reactions will be trapped in this network resulting in the "rising of dough".

2.Egg: I hope egg does not need any special introduction. Its an elliptical round object........

I believe everyone is aware of the nutritive value and draw backs of egg. So I will skip that part.

Egg serves more than 20 different functions in baking process. Some biochemically interesting ones are,
-Coloring agent: The golden yellow color in the banana walnut cake comes from the carotenoids present in the egg yolk. ( It might have got its name from Carrots, which are rich source of them). More that just coloring, vitamin -A can be synthesized from beta-carotenes. It also act as an anti-oxidant.

- Coagulating agent: is the solidification of a dissolved solid. (Most famous is blood coagulation). When you boil egg, it is coagulation that is happening, resulting in solidification of egg albumin. Egg help in coagulation of various ingredients in baking mix, and bind them together.

- Emulsifying agent: An emulsion is a stable blend of materials that are generally very difficult to mix and for a uniform suspension. Eg: Oil and water do not mix. One of the major emulsifying agent present in egg is lecithin. -->
Egg yolk is a rich source of lecithin,
which is a phospholipid with both water loving (hydrophilic) and water fearing (Hydrophobic subunits). So it can form spherical structures called micelles which help in dispersal of fat in more hydrated environment. It coats the liquid and lipds (butter) in the cake mix to give a smooth uniform texture to the cake. (Lecithin structure from http://www.scientificpsychic.com/fitness/fattyacids2.html).

3. Baking soda/baking powder: This is most biochemically active ingredient in our cake mix.

Baking soda is sodium bicarbonate (NaHCO3). The solution of Soda bicarb is mildly alkaline, thus it can also interact with acids resulting in the formation of a salt and carbonic acid. Carbonic acid is highly unstable (most of you favourite sodas are nothing but carbonic acids) and dissociates to carbon dioxide and water.

For Eg: Your stomach has concentrated Hydrochloric acid. If you happen to take in a spoon of baking soda the following reaction can occur in your stomach.

NaHCO3 + HCl → NaCl + H2CO3

H2CO3 → H2O + CO2.

You can well imagine the discomfort associated with accumulated carbon dioxide, which is nothing but gas, in the stomach. Since the cake that we are going to eat has plenty of fats (butter, yolk ...) it can form a layer over the aquous content of stomach resulting in froathing and thus preventing the escape of this gas through your mouth (Brrrr....). You may need some antifroathing agents (Eg: active ingredients of most of the antacids that you may rely on such occasion) to get rid of this gas (Sorry if I am scaring you, but dont worry this happens only if you add excess amount of baking soda, that is beyond the nuetralizing capacity of the acid in our recipe.

The acid component in our recipe is nothing but buttermilk !! Yes. Butter milk is nothing but the fermented milk. Fermentation happens as result of the action of lactobacillus (mostly) in the culture. One of the byproduct is lactic acid which is the driving force behind our delicious cake !! \
So what happens inside the cake mix,
Lactic acid + Soda.bicarb ---> Sodium lactate + Carbonic acid ----->CO2.

This CO2 get trapped in the fine gluten net from all purpose flour. As the reaction proceeds the pressure builds up resulting in the rising of dough !!

Another alternative to baking soda is baking powder.

What is the difference between baking soda and baking powder ?
Baking soda is pure Soda bicarbonate. So you will have to acidify your recipe with an acid component for the reaction to occur. On the other hand baking powder is a mixture of Soda bicarb and an acidifying agent (Eg: Sodium aluminium sulfate or mono calcium phosphate).

So why the alkali-acid reaction does not happen inside the bottle ? Answer: the reaction do not take place until you provide water/moisture. Because ionization of the components do not happen in solid phase, it requires a "dissociating agent" like water. Take home message (THM) : Keep your baking poweder bottle dry !!

To give you more options: there are two types of baking powder available.
i) Single acting baking powder: Here the reaction will start upon exposure to moisture. So rising of dough can happen even at room temperature ( THM: Cook immediately upon adding a single acting baking powder !!).
ii) Double acting baking powder: Reaction happen in two stages. The reaction begins at a minimum level in room temperature as in single acting powders, but for maximum activity the temperature has to be increased , which happens inside the oven.

Now, we used baking powder alone. But still added butter milk. Why? I do not know some time certain thing become rituals. I was not sure about the principle while I was making the cake. Anyway it did not hurt !!.

And finally butter: Butter is an emulsion of tryglyceride and water. It give richness, softness to the cake in addition to adding to the flavor.

And finally here is the net result of all the chemical reactions that I have described here ...!!