Saturday, April 28, 2012

Understanding Leaveners - Yeast, Baking Soda, and Egg Whites




To the home baker, this is often a conundrum “Shit, I ran out of baking soda what else can I use?” We have all been there, and this is all a matter of understanding what is actually going on inside your recipe. Some of this believe it or not is grade school science, and I am here simply to reiterate something that seems elaborate but is not. Leaveners are the things that make our baked goods rise, in short.

Fundamentally, all of these things achieve one thing: they produce (usually carbon dioxide, or incorporate gas.) This is can be achieved three different ways: microbial, chemical, or mechanical.

Microbial

Microbial leavening is achieved through yeasts. The same things that produce two of our favourite things: alcohol, and bread. This is fermentation. Yeast leavening can be complicated by going down a baking aisle, and seeing fast acting yeast, cake yeast, brewers yeast, sourdough etc. I will delve into that more complicated bit after. What is important is to understand that yeasts eat sugars. These can be sugars from starches (carbohydrates), table sugar, honey, etc. Some of these sugars are more digestible then others, but for our purposes, that is not important. When a yeast eats a sugar it farts, vomits, and procreates (much like a frat house). It farts carbon dioxide, vomits alcohol, and produces more yeast.

If left in a liquid with a sugar, it will produce alcohol and the carbon dioxide will escape. This is how we get beer. Bread is much like beer, and bread dough is even mildly alcoholic. The different is that when we knead a dough together we create gluten, and this gluten traps the carbon dioxide which causes the bread to “rise,” much like blowing up a balloon. When the bread is baked, the alcohol is burned off.

This brings us to the next topic -types of yeast. There are different strains of yeast, some are better are producing alcohol, some better at producing carbon dioxide. There are also wild yeasts (yes they watch you while you are sleeping). For baking purposes we are interested in the ones that produce carbon dioxide. Now we have a selection to choose from, and without getting into all of them it fundamentally comes down to how fast they wake up and do their thing. Think normal yeast as sleepy factory workers, and thing of super active yeast as the Incredible Hulk. Our Hulk is the same as our Average Joe yeast, except he is packed with beneficial nutrients that wake him up and get him going faster. If we gave our factory yeasts Red Bull (ie yeast nutrients) the same thing would happen.

Having a Hulk yeast is not always a beneficial thing. This is an important principle in bread making. Time equals Taste. The longer your bread ferments, the more flavour it will have. This is the reason store bought white bread tastes so different than a sourdough (which we will get into soon).

So hypothetically, we want a flavourful bread but all we have is the Hulk. How can we achieve this?

A big factor in microbial action, and fermentation is temperature. The higher the temperature (that they can survive happily,) the faster they will work. The reverse is also true. If we put our dough the fridge, it will take longer to rise, and produce more flavour. If we need it to rise faster we put it in a warmer spot in the house. Simple non?

So what then is sourdough, and why is it so tasty. Sourdough is the homeless yeast, the teenager, or the Iggy Pop, it is wild and fairly untamed. It is produced by creating a substance that will attract the ambient yeasts living in your house, and cause them to colonize it. This is usually accomplished by a mixture of flour and water, left out in the open for several days until it either molds (in which case it was colonised by other microbials,) or begins to bubble and ferment. When it begins to do this you basically have another pet. You have to feed it, nourish it, and give it a place to sleep. I will not go too in depth to the art of sourdough. Because this yeast really does not “want” to work, the dough takes a long time to ferment, and produces a very fermented flavour. This flavour is unique because more then likely this yeast is unique to your specific area. This is why San Francisco sourdoughs are so popular, they produce a different strain of yeast! Not to say “Your House” sourdoughs cant be better.

To add to the confusion, often you will see the word starter, biga, poolish, or sponge. These are all basically the same thing, they are “pre-ferments”, it is a middle of the road for flavour. How this is achieved it by starting a “dough” ahead of time, letting the yeasts age and establish themselves and then using this “dough” to ferment your actual dough. If we wanted to make a pre-ferment we would take a small portion of yeasts, and water out of our recipe, put our yeasts in that, and let it do its thing overnight on the counter or in the fridge. Where all these strange names differ is in what ratio of flour to water that is in the pre-ferment. Some bakers prefer it more liquid, some prefer it more dough-like in consistency. Another way to achieve this is to make your initial dough, and save a tiny piece for the next batch of dough; they downside however is the reliability.

The things we can take away from this is: Yeasts produce carbon dioxide, that carbon dioxide gets trapped in our dough and causes it to rise, the longer it takes to rise the more flavour it will have.

Chemical

Chemical leavening is usually achieved by using either baking soda or baking powder. I find the easiest way to explain this is to go back to a grade school science fair where without a doubt, someone made a “volcano.” If you remember the reaction was created by taking baking soda, and vinegar putting them together and watching the mess ensue. This is what happens when you mix an acid (vinegar), and a base (sodium bicarbonate) This is exactly what happens when you use them in baking. If a recipe calls for baking soda, it will always have an acid accompanying it. This is usually lemon juice, buttermilk or cream of tartar (which is actually tartaric acid). What happens is our volcano reaction happens in our mixture creating carbon dioxide which is trapped and baked. This can be accomplished using baking soda and any acid you can think of (well as long as it is food friendly). The chemical responsible for this is sodium bicarbonate.

The difference between baking soda, and baking powder is that baking powder contains both sodium bicarbonate, and tartaric acid (cream of tartar). It often uses an inert starch as a drying agent as well.

So we have a recipe that calls for baking powder, but all we have is baking soda. How can we create our own baking powder?

1 part baking soda
1 part cornstarch
2 part cream of tartar

Ok, but I am out of cream of tartar, and cornstarch. What else can I use? If we require one teaspoon of baking powder we can use these substitutions:

1/2 tsp baking soda +1/2 cup buttermilk, yogurt, or soured milk (1/2 cup milk + 1 1/2 tsp vinegar/lemon juice)
1/2 tsp baking soda +1 1/2 tsp lemon juice, or vinegar
1/2 tsp baking soda + 1/3 tsp citric acid

More often then not you will use 1 tsp of baking powder per 1 cup of flour. When we substitute using liquid you must also adjust your recipe by taking out other liquids. The problem with substitutions is although some can be straightforward others can change the texture/flavour of the recipe. Technically it is an exact science, realistically not so much.

There are also single acting, and double acting baking powders. Single acting only activates when applied with heat (ie being baked), whereas double acting will react at room temperature AND when applied with heat. Also note that self rising flour is normal flour with the addition of baking powder. To make your own take normal flour and add 1 1/4 tsp of baking powder, and a 1/4 tsp of salt.

What we can take away from chemical leavening is this: Baking soda is a base, when this base is combined with acid it will produce carbon dioxide and cause our recipe to rise (with the application of heat), baking powder is baking soda combined with acid already. Now that you understand the “why” it should be much easier to understand what is going on in your recipes, and how to substitute the various ingredients.

Mechanical

Mechanically leavening something is probably the simplest of all processes. We use mechanical movement to introduce air, and then gently heat it to cement it together. Mainly what we are talking about are egg whites. Whipped egg whites are responsible for many delicacies such as souffles, macarons, meringues, sponge cake, etc. Cream is not generally baked while whipped, however ice cream is an example where air is whipped into the ice cream mixture which increases its volume and then frozen. We will concentrate on egg whites.

Without going too scientific (if you wish to, buy On Food and Cooking by Harold Mcgee) when egg whites are whipped two chemicals in the egg whites are attracted to each other and form the framework through molecular bonds. If baked right away (usually very low gentle heat,) this framework is solidified, and leaves us with a light airy product. Although it is not exactly that simple, the same forces that create this network can also become too tightly bonded and cause the whites to collapse. To further complicate things whites have water, and if contaminated by yolk, fat. Both of these things will have a negative effect on the end product, and work against your desired results.

Commonly it is advised both most French pastry chefs that in order to get the best egg whites, the eggs must be older, room temperature and aged. Harold McGee notes that your eggs should be in a chilled bowl, and if needed a tiny bit of acid added to them -1/2 tsp cream of tar tar. This is kind of contradictory and unfortunately I do not have an answer. As eggs age, they actually become less acidic, and warm eggs have a harder time bonding together. Take what you will from that, I am not going to argue with neither a Pastry chef, nor the Godfather of food science.

One thing that sounds like an old wives tale which does have scientific merit, if the belief that a copper bowl should be used when whipping eggs. This is true.

“It turns out that along with a few other metals, copper has the useful tendency to form extremely tight bonds with reactive sulphur groups: so tight that the sulphur is essentially preventing from reacting with anything else. So the presence of copper in foaming eggs whites essentially eliminates the strongest kind of protein bond that can form, and makes it harder for the proteins to embrace each other too tightly.” On Food and Cooking, Harold McGee

In a nut shell, the copper makes it very hard for the whites to collapse. McGee also notes that silver has the same effect as copper (for those of us that can afford silver, let alone copper bowls). I've only briefly touched on this subject because it is either a very simple straightforward topic, or a highly scientific one. My copy of On Food and Cooking, is currently lent out, so perhaps ill post one on the science of it when I get it back.

I hope this answers more questions, then it makes. Hopefully never again will someone ask me about baking soda substitutions, or at least I will have a place to steer them. Cheers.

1 comments:

Anonymous said...

Very detailed and useful! Thanks for the effor! ;)

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