The Chemistry in Your Kitchen: Part One


Quite a few people seem to have an aversion to chemistry in the same way that some people also have an aversion to math. However, since many Filipinos seem to also love food and cooking, food chemistry becomes a good avenue to introduce the average person to its concepts.

Here are three chemical reactions, out of many others, that take place in your food. Watch out for these the next time you cook or prepare anything fancy in the kitchen.

Milk Coagulation

The process of making cheese is actually much simpler than one might imagine. There are three methods you can utilize when doing so, but however you choose to do it, all three lead to the process of milk coagulation. The method you choose, however, ultimately determines the type of cheese you end up with.

[Photo credit: Chef JJ's website]

For instance, cottage cheese is easily made with the following basic recipe from Alton Brown (2007):

1 gallon Pasteurized skim milk
3/4 cup white vinegar
1 1/2 tsp salt
1/2 cup heavy cream

1. Pour the skim milk into a saucepan over medium heat; heat this to 49˚C.
2. Afterward, gently pour in the vinegar, and stir for 1-2 minutes. You will start to see the curd separating from the whey.
3. Cover this and allow it to sit for 30 minutes. Afterward, filter the curds from the whey using a cheesecloth. The dry curds will be mixed with salt and heavy cream.

This recipe utilizes acid coagulation. Milk is normally slightly acidic: pH 6.5-6.7 compared to water, which has a neutral pH of 7.0. At this pH, tiny aggregates (called micelles) of the milk protein casein are negatively charged and soluble in water. Thus, the negative charges repel each other.

When you add acid, the pH lowers to 4.6. Once it reaches this point, the positive charges equal the negative charges, and these aggregates will clump together into the curds that you see separating from the clear liquid called whey. Cream cheese can also be made in this manner. When bacteria that produce lactic acid are also introduced into the milk, this also lowers the pH and can result in the formation of cheese.

[Photo credit: Eventbrite website]

On the other hand, cheeses like paneer and queso blanco can be made using heat-acid coagulation, wherein you use a higher temperature (82˚C). These result in firmer cheeses, and the higher temperature kills off more microbes.

The last method uses a complex of enzymes called rennet, obtained from the stomachs of young calves. Neufchatel and the more challenging blue cheese are made using rennet, and here is a sample recipe from David Fankhauser (2005):

In a sterilized pot, buttermilk is added to milk, and 1/4 of a tablet of rennet is dissolved in water and added to the mixture.

Take note that the temperature of milk is lower than the previous methods, being only 20˚C. This is because when it's too hot, the rennet enzymes will denature and no longer work. This also explains why the pot and the materials used have to be sterile - non-sterile pot will cause unwanted microbes to grow in your cheese!

After it's left to sit overnight, the curd will have formed and should be separated from the whey. These enzymes work by breaking up portions of the casein protein, causing the globules to become unstable and interact with one another, forming the curds.

Maillard Browning

You may have wondered why meats are more flavorful when they are first seared in the pan before they're turned into stew - or cooked all the way through in a broth and roasted for the last few minutes. Or, why food prepared with a lot of wet heat like steamed Hainanese chicken will tend to taste differently from food also prepared with a lot of dry heat, like roasted Hainanese chicken.

[Photo credit: Andrew Scrivani, The New York Times website]

Watch any cooking show on television where the chef prepares steak, and he or she may also likely mention that the browning on the surface of the meat where we derive a lot of flavor from is due to the Maillard reaction.

This complex series of reactions is ultimately the reaction between reducing sugars like glucose and galactose, and an amino acid in the presence of heat. Depending on the amino acid you have in your food (and there are many to choose from), the flavor will change. The wide variety of combinations therefore lead to a variety of different flavors. The reaction will also produce melanoidins, polymers that give food like toasted bread their golden-brown color.

[Photo credit: Terence Ong, Wikipedia website]

Moisture level is another factor that can actually slow down the Maillard reaction. To illustrate, steamed Hainanese chicken is prepared in a way that requires a lot of water. The highest temperature you can reach when cooking using methods like boiling and steaming is naturally 100˚C - you can still achieve the Maillard reaction at this temperature, but it will take much, much longer. Steaks on the other hand, sear and brown in as little as a couple of minutes due to the high temperatures.

Lastly, you can also speed up or enhance the Maillard reaction - most of us do it without even being aware of it! Remember, we need amino acids and reducing sugars as components. Whenever we brush egg yolks onto the surface of a bun, we probably do it to give the end product a glossy sheen. However, by adding egg proteins, we're also adding to the Maillard reaction and we get a browner coating on top.

[Photo credit: David Monniaux, Wikipedia website]

Denaturation

When you crack an egg into hot water to poach it, the egg will gently simmer over time until you get a finished product with a runny yolk and now opaque, firm whites. Previously colorless and viscous, the egg white is largely composed of the albumin protein, a molecule that denatures or unravels with high heat.

[Photo credit: Anthony Blake, The Guardian website]

The process of denaturation is not unique to albumin; in fact, proteins will denature or change their native molecular structures due to heat, acidity, or a variety of other reasons. This is biologically unfavorable since denatured proteins will lose their functionality, and usually irreversibly.

As the egg proteins unfold, they will form weak bonds with the amino acids of other unfolded proteins, forming a protein network. The egg proteins therefore coagulate, and with more heat they will solidify further and turn opaque.

To a cook, denaturation and coagulation are what give cooked eggs the firm texture you look for when biting into a hard-boiled egg. Even whipping egg whites into a foam when making meringue denatures them, due to heat and surface forces.

[Photo credit: Vanjo Merano, Panlasang Pinoy website]

Ceviche (or better known here as kilawin) is another example of protein denaturation, where the cook uses acid instead of heat. However, fish prepared in this manner should still be prepared fresh. After all, microbes can still easily thrive in meat that hasn't been cooked all the way through.

In Part Two of The Chemistry in Your Kitchen, we will tackle other interesting chemical reactions such as caramelization, and also share the author's personal method (using liquid nitrogen!) for making the smoothest, creamiest coconut milk ice cream.


REFERENCES:
2.   Brown, A. (2007). "Quick cottage cheese." Retrieved from: http://www.foodnetwork.com/recipes/alton-brown/quick-cottage-cheese-recipe/index.html#!
3.   Baltes, W. (1982). Chemical changes in food by the Maillard reaction. Food Chemistry 9(1-2): 59-73.
4.   Fankhauser, D. (2005). "Neufchatel: An unripened cheese." Retrieved from: http://biology.clc.uc.edu/Fankhauser/Cheese/neufchatel/neufchatel.htm
6.   deMan, J. (1999). Principles of food chemistry. 3rd ed. Springer.
7.  Lersch, M. (2012). "Maximizing food flavor by speeding up the Maillard reaction." Retrieved from: http://blog.khymos.org/2012/06/04/maximizing-food-flavor-by-speeding-up-the-Maillard-reaction/
8.  Society for Food Science and Technology. (n.d.) "IFT experiments in food science series: Food chemistry exp eriments." Retrieved from: http://www.ift.org/Knowledge-Center/Learn-About-Food-Science/K12-Outreach/Food-Science-Experiments/~/media/Knowledge%20Center/Learn%20Food%20Science/Experiments/TeacherGuidePROTEINS.ashx


No comments:

Post a Comment