Author Archives: Jessica

Life Experimental has moved!

If you’re interested in finding me on the web these days you can find me writing and blogging over at my permanent home: http://jessicaruprecht.com.

I look forward to hearing from you!

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The building blocks of food

So now that we’ve talked a little about what it means to do science and some of the basic principles of food chemistry, the next question that might be reasonable to ask is: what is food? The answer to this question is complex, but it turns out it is also deceptively simple. Because most food is comprised almost entirely of four substances: water, fats, proteins, and carbohydrates. And if we understand what these four components of food include, it turns out that a lot of basic recipes can be interpreted as simple ratios of these four ingredients. So, what sort of properties can be attributed to these substances?

Water

As discussed in the last post, an important part of understanding how recipes work is determining whether something is acidic or basic, a characteristic which requires the presence of water. Another important feature of water is that water is a polar molecule, which means that one side of the water molecule (the side nearest the oxygen) has a slight negative charge (red) and the other side (nearest the two hydrogen molecules) has a slightly positive charge (blue). This is illustrated below and has important consequences for the chemical properties of water:

The polarity of the water molecule means that water is good at dissolving other substances. Because many proteins and carbohydrates have similarly uneven distributions of charge, the polarity of water molecules causes them to cluster around the charged regions of  these larger molecules. This mechanism works to pull apart the larger molecule, resulting in the breakdown of the molecule when exposed to water.

Fats

Something we all know about fats (even if we may never have thought about it much before) is that they don’t mix well with water. This you know if you’ve ever tried putting olive oil in your pasta water or tried to emulsify a vinaigrette. (Emulsification is the process of mixing two liquids which are normally unmixable, vinegar and oil in the case of a vinaigrette). The oil and the water don’t mix readily and if you want to emulsify your vinaigrette you’d best be prepared for some vigorous whisking. (You may know that there are tricks that you can utilize to help emulsify your vinaigrette and we’ll talk about the science of emulsification at some point).  The difficulty in mixing water and oil is due to the structure of the fat molecule. A fat or oil molecule is non-polar (unlike water which is polar) and therefore the two substances repel each other.

Another thing to note about fats is that some fats are what we call saturated fats, while other fats are unsaturated. A fat molecule is comprised of essentially two parts, a carboxylic acid  (\text{COOH}) bonded to a fatty acid chain comprised of carbon and hydrogen. The number of hydrogen molecules bonded to the carbon determines how saturated the fat is (saturated with hydrogen, that is). As you may know, the saturation of the fat effects its properties: because of the regular structure of the saturated fatty acid chains, saturated fats form a regular (solid) structure much more readily than unsaturated fats. Hence saturated fats are solid at room temperature (butter, vegetable shortening, etc.) while unsaturated fats remain liquid (olive oil, fish oil, etc.). Rancidity occurs when the carbon chain of the fatty acid is broken by a reactive molecule (such as oxygen in the air), producing small fragments. In saturated fat, the carbon backbone is protected by its bonds with the hydrogen atoms and is therefore less easily broken. This is why unsaturated fats go rancid much more quickly than saturated fats.

Proteins

Proteins are some of my favorite food molecules because they tend to change the most dramatically during cooking. A protein is a polymer, a long  chain (or possibly several connected chains) of molecules (in this case amino acids). So what’s an amino acid? An amino acid is comprised of an amine group (\text{NH}_2) and a carboxylic acid group (\text{COOH}) which are bonded to another structure, called the side-chain. The structure of the side-chain distinguishes between the different amino acids and is generally composed of carbon, oxygen, and nitrogen. Because there is variety in the structure of the amino acid side chains, the proteins they make up have interesting properties. A single protein will be comprised of a variety of amino acids and the way the side-chains interact determines the structure of the protein. This structure has three levels: the order of the amino acids themselves, the spiral structure formed by the bonding between the amino acids, and folding caused by bonding between amino acid side-chains located at different places in the amino acid chain. So what’s important about protein for the home cook? Proteins are important in the browning reactions which give foods flavor when cooked (we’ll discuss this in detail in the next post), also many amino acids and short chains of amino acids have flavors of their own and contribute to the taste of foods that have had their proteins partially broken down (say due to curing or fermentation processes). Furthermore, in addition to flavor, proteins are also important for texture. Many proteins can absorb at least some water, but only some proteins are water soluble (i.e. they dissolve in water). And as we discussed last time, proteins can denature (i.e. lose their structure) when heated, exposed to acid, or agitated which can be very important in cooking. And in addition to these interesting behaviors, some proteins are even more interesting and they’re called enzymes. Enzymes are a group of proteins that serve to catalyse specific reactions (i.e. help them to occur) and sadly most of these reactions in cooking have to do with spoilage. It’s enzymes which cause the oxidation (discoloration) of cut fruits (think apples or avocados), cause fish to turn mushy, and green vegetables to brown with age. However, enzymes also do some good things such as tenderizing meat or aiding  in fermentation.

Carbohydrates

Carbohydrates encompass a large family of molecules which we know colloquially by a variety of names: sugar, starches, and gums are all examples of carbohydrates. So named because they were initially thought to be made up of carbon and water, this later proved to be untrue. While carbohydrates are comprised of carbon, hydrogen, and oxygen, the hydrogen and oxygen do not form water in the carbohydrate structure. Now there’s a lot that could be said about carbohydrates, but I just want to stick to the basics for now, so let’s start with some terminology. A sugar is the simplest carbohydrate and is formed of a single molecule. There are many different types of sugar: sucrose, fructose, glucose, deoxyribose, etc. and when we eat them, sugars taste sweet. One step up from sugars we have oligosaccharides (i.e. “several-unit sugars”). “Saccharide” is another word for “sugar”, which you may have guessed if you know that “saccharine” is a synonym for “sweet”. These sugars are short groups of only a handful sugar molecules; however, they are too large to trigger our taste buds, and therefore don’t taste sweet to us. Interestingly, the human digestive system doesn’t break down oligosaccharides itself, instead they pass through to the large intestine where they’re broken down by bacteria. Next up we have polysaccharides (i.e. “many sugars”) or sugar polymers. Whereas the oligosaccharides were comprised of a handful of sugar molecules, a polysaccharide may contain as many as several thousand individual sugar molecules. Polysaccharides are pervasive in both biology and in cooking and are produced in plants (amylose, amylopectin, cellulose, etc.) and in animals (glycogen), and as the home cook is probably aware polysaccharides are commonly used for thickening. Flour, corn starch, pectin, and plant gums are all polysaccharides.

Putting it all together:

And now we get to the interesting part. It turns out that many recipes can be reduced to simple ratios of these four building blocks of food. For example lets consider different doughs, here are some simple ratios that could be used to make a variety of different baked goods:

  • Bread: 5 parts flour (carbohydrate) : 3 parts water : (teeny bit of yeast and salt)
  • Pasta: 3 parts flour (carb) : 2 parts egg (protein + fat)
  • Pie crust: 3 parts flour (carb) : 1 part water : 2 parts fat
  • Biscuits: 3 parts flour : 2 parts liquid : 1 part fat
  • Cookies: 3 parts flour (carb) : 2 parts fat : 1 part sugar (carb)

Note that these ratios generally omit small ingredients, such as chemical leaveners, which can have a big effect on the character of the final product. Though not all starches and sugars are made equal (you couldn’t go replacing the flour with cornstarch) these ratios are a good place to start and a good way to think about proportions in cooking. And a fun way to experiment! You have to admit, it’s pretty fantastic that simply changing the ratios of the same ingredients can have such a varied result.

References:

  • McGee, Harold. “The Four Basic Molecules of Food.” On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. 792-809.
  • Ruhlman, Michael. Ratio: The Simple Codes behind the Craft of Everyday Cooking. New York, NY: Scribner, 2009.

Back to basics

Any scientific investigation into the world of cooking and chemistry must begin with a discussion of acids and bases. Armed with an understanding of this simple chemical principle a home cook can understand a variety of topics including how the use of chemical leaveners serves to create airy baked goods (as well as the difference between baking powder and baking soda – and why they’re not interchangeable!), how to harness science to prevent botulism when canning food at home, and much more. Indeed, a basic understanding of simple chemistry will prove invaluable in the future as we continue to explore the science of everyday life.

So, what are acids and bases?

Simply put, solutions that are acidic have a high concentration of \text{H}^{+} ion and solutions that are basic have a high concentration of \text{OH}^-. However, the salient point for most home chefs is that when an acidic substance (such as vinegar) and a basic substance (such as baking soda) are combined the result is a chemical reaction (in this case, a vigorous bubbling that you may be familiar with). And, if you combine the correct amount of acid with the correct amount of base you can use them to neutralize each other, so that the resulting solution will be neither acidic or basic. This is because when \text{H}^{+} and \text{OH}^- are mixed, the following reaction occurs:

 \text{H}^{+} + \text{OH}^- \rightarrow \text{H}_2 \text{O}

Thus, if I mix the correct amount of acid with the correct amount of base I can create a solution which has a neutral pH.

Now as with any useful scientific principle, we must have a scale with which to measure and quantify, and for acids and bases the scale is called the pH scale (because it is a measure of the proportion of \text{H}^+ ion present in the solution). A neutral solution (one that is neither acidic nor basic, such as water) is given a pH of 7 on a scale from 0-14. Solutions which are acidic have a pH < 7 and those which are basic have a pH > 7.

Applying the science of acids and bases to cooking:

One way in which acids are important to cooking is in home canning projects. Acidic solutions are often relatively safe from microbial growth, it’s the reason why pickling is such an effective method of food preservation. Thus, if you happen to be interested in home canning projects, having an acidic pH in your canned goods is an important safeguard against dangers such as botulism (since Clostridium botulinum, the bacteria responsible for botulism cannot grow at a pH below 4.6).

Second, as I mentioned at the beginning, chemical interactions between acids and bases are used for chemical leavening in many baked goods such as quick breads, muffins, and biscuits. In this reaction a base (usually baking soda, i.e. sodium bicarbonate, \text{NaHCO}_3) reacts with an acid present in the dough (from buttermilk, vinegar, lemon juice, or a chemical acid such as cream of tartar) and produces carbon dioxide (a gas) as a result of this reaction. The resulting gas bubbles are preserved in the cooked dough and are responsible for the texture of your baked goods. Too little chemical leavener, and you’ll find yourself with a very dense bread indeed. For those scientists in the audience, here are the relevant reactions:

\text{NaHCO}_3 +\text{H}^+ \rightarrow \text{Na}^+ + \text{CO}_2 +\text{H}_2 \text{O}

and

\text{2NaHCO}_3 +\text{heat} \rightarrow \text{Na}_2 \text{CO}_3 + \text{CO}_2 +\text{H}_2 \text{O}

And have you ever wondered why you cannot switch out baking soda for baking powder in a recipe? The science of acids and bases strikes again! Baking powder is a mixture of acid and base whereas baking soda is a purely basic compound. Thus, if you substitute baking soda for baking powder in your recipe you may find yourself with no acid present to react with the baking soda and leaving you with an unleavened result.

Another area in which acids interact with cookery is in the science of proteins. If you’ve ever substituted buttermilk in a recipe with a mixture of milk and acid (lemon juice, vinegar, cream of tartar, etc.) you’ll have noticed that introducing the acid into the milk causes it to curdle. This is because milk contains protein, and protein structures can become disrupted in an acidic environment. This disruption causes the proteins to lose their structure (to become denatured), and the result is curdled milk. And if you ever tried to skip the acid and substitute regular milk for buttermilk in a recipe you may have been disappointed because the lactic acid in buttermilk is caused by fermentation (the souring process) and unsoured milk therefore contains less acid.

While protein denaturation isn’t always desirable (unless you’re trying to substitute for buttermilk, curdled milk is rarely the goal in a recipe), it can also be used to great effect. If you’ve ever struggled to poach an egg, you could try cracking the egg into a bowl of white vinegar and letting it stand for a few minutes before pouring the egg into your poaching water. The acid in the vinegar will begin to coagulate the proteins on the outside of the egg and this will help to prevent the egg becoming disrupted when introduced into the poaching water.

The point I’ve hopefully convinced you of is that acids and bases are fundamental to the science of cooking, and as we move into discussions of specific types of recipes and how and why they work, an understanding of acids, bases, and the effects they can have on ingredients will be inescapable. Plus, a basic grasp of the science of cooking will enable you to make recipe substitutions with confidence, and even to create your own recipes from scratch.

References:

  • Edwards, W. P. The Science of Bakery Products. Cambridge: Royal Society of Chemistry. 2007.

Doing science at home

As a scientist, I’m used to investigating things quantitatively. I try to answer questions with numbers and facts – with evidence, and this scientific way of thinking bleeds over into my everyday life. For example, when I noticed that the house seemed to feel colder than the thermostat claimed it was, I didn’t just trust that what I thought was true. Instead, I tried to answer the question quantitatively.

In this case, I compared the temperature displayed on the thermostat with the temperature reported by a kitchen thermometer I have for baking bread. And because good science requires multiple samples for good statistics, I didn’t just look at the temperature once and decide that I knew the answer. Instead, I compared the temperatures many times over the period of several days, at different times of day, and at different temperatures in the house. The result of this informal study seemed to indicate that the temperature difference is around 4 degrees.

While it seems likely to me that the 4 degree offset I’ve measured in this very informal “experiment” is probably approximately correct, the result is really not scientifically meaningful because I haven’t done things rigorously enough. To make my measurements truly scientific, I would need to properly examine the possible sources of error in my experimental system. Here’s a brief list of improvements I could have made to my simple experiment which would allow me to have more confidence in my answer.

  1. I should place the kitchen thermometer next to the thermostat. Because I left the thermometer in the pantry (where it lives, since it is used in the kitchen) instead of putting it in the hallway near the thermostat, it’s possible that I am simply measuring a 4 degree temperature gradient between the hallway and the pantry which actually exists. My own experience tells me that this is unlikely, since I would almost certainly notice a 4 degree difference in temperature as I move about the house. However, good science requires that I measure the temperatures in the same place for proper comparison.
  2. I have not determined that the kitchen thermostat is reporting an accurate temperature. This is important, since I have no scientific evidence that the thermometer is any more accurate than the thermostat. There are arguably reasons why the thermometer is more likely to be accurate than the thermostat, primarily that I bought it recently and so far it doesn’t seem to have led me astray. However, good science requires that I measure known temperatures with my thermometer to determine its accuracy. Boiling water and ice water would be good candidates here, since their temperatures (212F and 32F, respectively) are known and they can be easily made in my kitchen.
  3. I have not quantified the uncertainty in my measurements. The thermometer is analog and reports temperature on a dial with tick marks every 2 degrees. This means that at best I can only determine the temperature on the thermometer plus or minus 1 degree (i.e. I know it is a temperature greater than 62F but less than 64F, which should be reported as 63F plus or minus one degree). In fact, this error is even worse, since viewing angle has some effect on the apparent position of the needle on the temperature dial. So when I say the temperature difference between the thermometer and the thermostat is roughly 4 degrees, I’m not acknowledging that a better way to say this is that the temperature difference is between 3 and 5 degrees.

In the spirit of scientific rigor, I took my own advice and tested the accuracy of the thermometer, since this seemed the most likely source of significant error. In a small pot of boiling water, the thermometer read 211F (with error 1 degree). So I can conclude that the thermometer is accurate (to within error) as expected. To make this even more certain I could have tested the ice water, too. But even with just the one data point, I am quite confident in saying not just that the thermostat reports temperatures that are 4 degrees warmer than the thermometer, but also that the thermostat is in fact reporting a temperature that is 4 degrees warmer than the actual temperature in the house.

This sort of reasoning makes up what is known as the scientific method, and represents a way of investigating the world using quantitative evidence to discover reproducible facts about the world around us. Which is, at the most fundamental level, exactly what science is.

In this blog I will be attempting to share some of the science that we use every day, often without realizing it. I’ll explain such things as the chemistry of cooking, the biology of bread, the thermodynamics of air conditioning, and investigate simple experiments that can be done at home to answer age old questions: What’s the best way to remove those pesky red wine stains?  What odors is baking soda most effective at neutralizing and why? And how clean is my counter if I clean it with vinegar instead of with bleach?